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Nanoindentation and Nanoscratching with SPMs For NanoScope

1. Force Calibration Phot Brterdi ng Aetracting Setpoind 2 position 2 00 Uldiv Bigitel instruments Hanoscore Z soam size 452 0 ni Z scan rate 0 9854 HE Bata seale 2 500 V Force Calibration Plat Extending Retracting bef lection 0 50 Urdiv Setpoint Z position 1 50 Wdis Bigital Instresents Haroscope 2 scan sine 38 0 m Z scan rate 0 3864 Hz Bala scale 2 500 Y 225 17 Top Force plot showing drifted zero deflection level Bottom Force plot resulting when Trigger threshold is out of the photodetector s range 225 42 Support Notes Support Note No 225 Nanoindentation If the user tries to execute an indent with a Trigger threshold out of the photo detector s range the software will not perform an indentation Also the force plot will appear as a flat horizontal line with no sloped portion visible see figure 225 17 bottom plot A message should appear on the screen near the force plot show ing the word Limit in red If this occurs it is necessary to adjust the zero deflec tion level to a position lower on the photo detector preferably in the center This can be done by withdrawing the microscope and setting the Vertical deflection to zero As done previously prior to engaging the microscope set the vertical deflec tion to zero using the photo detector adjustment knobs located on the left side of the Dimension or Multimode Head Dimension users can use the four quadr
2. The following is a quick start procedure for first time nanoindenting and nanos cratching users for more details about nanoindenting refer to the latter sections It is assumed that the reader is familiar with operation of TappingMode AFM imag ing If not the reader should refer to the TappingMode AFM sections of the Instruc tion manual and practice the engaging and imaging procedures using standard imaging cantilevers It is also suggested that first time nanoindent nanoscratch users should use a soft sample such as the 1um pitch gold ruling provided with the nanoindentation package to practice making indentations or scratches with an indentation probe Review all steps below before attempting nanoindentation or nanoscratching NOTE It may be necessary to switch to the nanoindenting nanoscratching version of the NanoScope software which can be accessed by typing z from within the C indent directory To perform nanoindentation nanoscratching do the following 225 2 1 Engage surface in TappingMode e Mount indentation probe and load sample onto into the microscope First time nanoindentation users should use the soft 1um pitch gold ruling sample to prac tice indenting scratching e Align the laser beam on the top of the cantilever Position the laser beam on the reflector located on the free end of the cantilever The reflector will be visible in the optical image when focused on the tip Center the laser spot on the photode
3. Y offset Scratch length H Rotate Scratch rate Scratch angle Units 225 7 Default Indent Controls panel settings for scratching Set Trigger threshold Before making an scratch it is necessary to set the force applied to the tip This is set using the Trigger threshold parameter abbrevi ated Trig threshold located in the Indent Controls panel The Trigger thresh old specified in volts is the cantilever deflection at which the controller stops pushing the tip into the surface and begins the lateral movement for the scratch For scratching a good starting value for the Trigger threshold is 0 1 V Execute a scratch To make an scratch in the surface click on the Probe Run Single command or click on its icon The tip will be forced into the sample until the Trigger threshold is reached whereupon the tip will be moved laterally for the required distance Then the tip will be retracted from the surface to its initial position above the surface A force plot will appear on the display monitor when the scratch is complete The force plot is a plot of the cantilever deflection ver sus Z displacement which is recorded while the tip is moving towards and away from the sample at the start and end of the scratch respectively To capture the force plot generated to the capture directory click on the Capture icon or select Capture from the Capture menu Return to image mode and image the scratch Return to image mode by select
4. tector to zero the vertical and horizontal deflection signals e Execute Cantilever Tune to find the resonance peak of the cantilever The reso nant frequency is in the range of 35 60 kHz for indentation probes Set the Drive frequency at the center of the resonance peak Adjust the Drive ampli tude until the RMS amplitude of the cantilever is from 0 25 0 3 Volts This is significantly lower than the RMS amplitude typically used with standard Tap pingMode probes 225 6 Support Notes Support Note No 225 Nanoindentation e Set Real time imaging parameters Set the Scan Controls and Feedback Con trols to values appropriate for TappingMode imaging The Scan rate should be limited to about 1Hz for indentation cantilevers Engage with a small Scan size about 1 3 um and increase the Scan size after engaging if necessary The stan dard Integral gain and Proportional gain is 0 5 and 1 0 respectively e Dimension Users Execute Locate Tip and Focus Surface commands before engaging the surface Always execute the Locate Tip command first and the Focus Surface command last Always re focus on the surface if the tip focus position has been changed Make sure that the tip and surface are in focus each time you engage The tip may crash into the surface and cause damage if the tip or surface focus positions are incorrect e Dimension Users Be very cautious when lowering the SPM head toward the sample when executing the Focus Surface comm
5. impossible to push with sufficient force to produce a comparable indentation In this case it would be better to either use a harder tip or rerun samples using the softer tip to obtain consistent results across all samples Support Notes 225 29 Nanoindentation Support Note No 225 225 5 Wear Testing Indentation cantilevers can also be used for wear testing using contact mode AFM Wear tests can be performed using the existing Nanoscope software to wear the sample simply by scanning the sample in contact mode using indentation cantile vers This is possible since indentation cantilevers have spring constants more than 100 times greater than standard contact mode imaging cantilevers This is the rea son that indentation cantilevers cannot be used to image in contact mode Some users are interested in wearing sample surfaces with a scan box pattern which can be done by scanning a square pattern using the indentation cantilevers in contact mode The worn areas can be imaged afterward using TappingMode with the same tip 225 14 Wear testing 225 5 1 Procedural summary The basic wearing procedure consists of the following e Load the sample and indentation probe into the microscope e Align the laser on the reflector atop the cantilever e Set Real time Stage Parameters SPM safety to 200 um 225 30 Support Notes Support Note No 225 Nanoindentation Dimension Users Engage on surface in contact
6. the Trigger threshold to values less than 0 10 volts to prevent unnecessary damage to the tip The sensitivity should be measured in this way prior to or immediately after per forming hardness tests on the desired samples In particular the cantilever should not be moved or the laser repositioned between measuring the cantilever sensitivity and performing the hardness tests If the tip is moved or the laser repositioned the cantilever sensitivity will change 225 4 1 Notes on comparing hardness data Nanoindentation is best suited for making relative rather than absolute hardness measurements When making hardness comparisons it is imperative that conditions be replicated as best as possible preferably using the same diamond tip If possible the tests should be performed with the same diamond tip without re positioning the tip or changing the position of the laser spot on the cantilever during the tests If it is necessary to compare many samples or many locations within a sample or if the tip is replaced note the following Support Notes 225 27 Nanoindentation Support Note No 225 1 All things being equal cantilever spring constant nanoindentation parameters etc a sharp diamond will make a deeper indentation than a blunt diamond This means that the same amount of force applied to two different diamonds will not make the same size indentation Therefore indentation size alone should not be used to calculate hard
7. 2 At each location a indentation or scratch is per formed Similarly the Row step specifies the distance between each row For example a Row step value of 700 nm will move the tip from row 1 a distance of 700 nm to row 2 Again an indentation or scratch is performed at each location The array is performed by stepping the tip in the X direction from right to left by the Column step amount and performing an indentation or scratch at each location After each row is complete the tip is returned to its initial X position and stepped down in the Y direction by the Row step amount Thus the array is performed row by row in a downward direction with the first indentation scratch made at the top right and the last at the bottom left 225 3 4 3 Threshold step The Threshold step parameter is the increment used to vary the indentation or scratch force When a row of indents or scratches is executed the Trigger thresh old is incremented by the Threshold step amount at each position along the row Each row in the array is executed the same way Thus the force is varied in the X direction and held constant in the Y direction The Threshold step can be set to both positive and negative values in order to increase or decrease the force respectively A positive negative Threshold step will increase decrease the force from right to left along each row The Trigger threshold should be set to the starting value of the cantilever deflection desi
8. Deflection sensitivity is the ratio of the Z motion of the piezo measured in nm to the cantilever deflection measured in volts Its units are nm volt It has also been called the detector sensitiv ity cantilever sensitivity or tip sensitivity since it depends on the photo detector and the cantilever tip For indentation this value is used to convert the Trigger threshold from Volts to Nanometers In general the Deflection sensitivity is used convert the cantilever deflection in volts to the cantilever deflection in nanometers which is necessary to calculate the force applied to the sample See section 225 4 on Interpreting Hardness Data for more information The Deflection sensitivity must be measured for each cantilever used since it var ies from cantilever to cantilever It also depends on the laser position on the cantile ver and the position of the probe within the cantilever holder Thus to be accurate the Deflection sensitivity should be re measured each time the probe is reposi tioned in the cantilever holder or the laser is repositioned on the cantilever or the probe is replaced or exchanged Support Notes 225 25 Nanoindentation Support Note No 225 225 4 Interpreting hardness data By measuring the size of an indentation made by a measured force it is possible to estimate the hardness of the sample surface This is analogous to industrial hardness tests e g Rockwell and Brinnell however at the
9. Scan rate for the test The Scan size is set to the desired size of the wear pattern The Scan rate and Scan size should be set for the desired tip velocity The tip veloc ity um s or nm s is calculated by multiplying the Scan size um or nm by twice the Scan rate Hz The Number of samples parameter should be set to obtain the desired resolution Since the Number of samples parameter deter mines the number of scan lines made during the test its value will effect the out come of the wear test NOTE When performing wear tests on multiple samples to compare properties the following parameters should be remain be fixed for all tests Scan rate Scan size Number of samples Integral gain Proportional gain and Setpoint e Increase the Deflection setpoint to the required value and immediately select Frame Up or Frame Down from the Real time menu As soon as the Deflec tion setpoint is increased by a sufficient amount the tip will return to the sur face and start to wear Thus it is important to execute a Frame Up or Frame Down immediately to begin a fresh scan The force applied normal to the sam ple is set using the Deflection setpoint In contact mode the Deflection set point is a measure of the cantilever deflection when scanning the surface The force also depends on the free air vertical deflection which should be set near zero prior to engaging For further information refer to the next section Force calculations for
10. cantilever Spring constant k of 100 N m and a Deflection sensitivity of 200 nm V If the objective is to apply the same amount of force to the sample the Trigger threshold must be adjusted to com pensate for the differences in spring constant and cantilever sensitivity In this case we will solve for the Trigger threshold a Force N lnm Trigger th eshold Sas sensitivity nm V x Spring constant Tm sa 0 6 16 0x10 N N 1 nm J 08V J 200 nm V x 100 N m 1x 10 m mies 225 28 Support Notes Support Note No 225 Nanoindentation That is by setting the Trigger threshold of tip B to 0 8 Volts the same amount of force should be delivered to the surface as was using tip A By observing dents creating by the two tips on the same sample surface it is possible to obtain a sub jective comparison For this reason it is useful to keep a touchstone nanoindenta tion standard Recall that indentation sizes vary depending upon the sharpness of the diamond even for tips mounted on cantilevers having the same spring constant If the observed dents are grossly different however recheck the calculations the Trigger threshold may be set incorrectly 4 It may prove difficult to apply the same force using a new tip having a much higher or lower spring constant For example if switching from a harder tip or more stiff cantilever to a much softer tip or less stiff cantilever it may be
11. for the array The Columns and Rows parameters are the number of scratches to perform in the X and Y direction respectively Auto Panel Columns Rows Column step Row step Threshold step Capture 225 9 Auto Panel settings for a scratch array 225 12 Support Notes Support Note No 225 Nanoindentation Set the spacing between rows and columns in the array Set the distance between the scratches in the array using the Column step and Row step parameters in the Auto Controls panel The Column step and Row step parameters are the X and Y offsets used for the array The Column Row step defines the distance between two neighboring scratches in the X Y direction Automated scratch ing uses the beginning of the scratches as the reference for the array Thus after a X and or Y offset is performed the scratch is made in the specified direction This needs to be considered if it is desired to do a two dimensional scratch array Typically the Rows parameter is set to 1 and the Scratch angle is set to 90 degrees This will result in a row of scratches which are parallel to the Y axis Set the forces used for the array The forces used for the array are set using the Trigger threshold parameter abbreviated Trig threshold located in the Indent Controls panel and the Threshold step parameter located in the Auto Controls panel For automated scratching the force can be
12. incremented in the X direction using these two parameters The Trigger threshold is incremented by the value of the Threshold step to vary the scratch force Set the Trigger threshold to the initial value required for the deflection of the cantilever This will be used as the Trigger threshold for the first scratch in any row Set the Threshold step to the required increment for the Trigger threshold For exam ple if the Trigger threshold is 0 1 V and the Threshold step is 0 1 V then a row of scratches in the array will be made using a cantilever deflection varying from 0 1 0 2 0 3 0 4 volts The force is always incremented from right to left To execute an array of scratches at the same force set the Trigger thresh old to the desired force and set the Threshold step to 0 0 V Set all other Indent Controls as desired typical values shown in section 225 2 3 Execute a scratch array After setting the Auto Controls parameters execute the array by selecting the Probe Auto Scratch command from the Scratch mode menu The array will be executed and the force plot for each scratch will appear on the display monitor A message box will display the X and Y offsets as the array is performed To capture the force plots it is necessary to set the Capture parameter located in the Auto Controls panel to Enabled before executing the Auto Scratch command If Capture is set to Off the force plots will be lost Return to image mode Real time Vie
13. ing View Image Mode or click on its icon The system will return to Tapping Mode imaging and the scratch may be viewed If the scratch is not visible on the sample surface return to Scratch mode and execute another scratch with a larger Trigger threshold Increase the Trigger threshold by 0 05 V increments until scratches are visible on the sample surface 225 10 Support Notes Support Note No 225 Nanoindentation 225 2 4 Automated indentation e Using an indentation probe engage the surface in TappingMode and enter Indent mode as described in the previous sections 225 2 1 and 225 2 2 above Review these two sections including all precautions before proceeding e Open Auto Controls panel Once in Indent mode open the Auto Controls panel which contains the parameters necessary to perform an indent array using automated indentation See panel in Figure 225 8 Auto Panel Columns 5 Rows 3 Column step 500 nm Row step 500 nm Threshold step 8 100 U Capture Enabled 225 8 Auto Panel settings for indenting e Set the number of indents in the array Using the Columns and Rows parame ters in the Auto Controls panel set the number of columns and rows for the array The Columns and Rows parameters are the number of indents to perform in the X and Y direction respectively e Set the spacing between rows and columns in the array Set the distance between the i
14. to the sample corresponding to the upper leftmost point on the force plot See example force plot shown in Figure 225 10 The Trigger threshold is specified in volts Force Calibration Phot Bhensdi na Ketraoting Deflection D10 Urdiw he art bso Ti J inn a aai t J Trigfger threshold Setpoint 2 position 2 00 Urdiu Bogttal Instrosents Hanosoore Z soan size 452 0 nw Zo scan rate 2 035 Hz Bata seale 1 000 U 225 10 Force plot showing the location of the Trig ger threshold The Trigger threshold controls the amount of force applied to the sample When indenting or scratching the tip is pushed into the sample surface until the cantilever deflection equals the Trigger threshold At this point the tip is either lifted up away from the sample to remove the load if indenting or the tip is moved laterally in a prescribed direction if scratching The cantilever deflection is measured rela tive to the value of the deflection at the surface contact point located somewhere on the flat portion of the force plot where the force is zero The Trigger threshold is only specified in Volts since the nanometer representation is incorrect unless the cantilever sensitivity is measured The cantilever sensitivity labeled Deflection sensitivity or Deflection sens on software panels located in the Channel 1 panel is the conversion from volts to nanometers for the cantilever deflection Refer to section 225 3 2 1 or 225 4 1 for mo
15. vertical deflection Volts The following equation may be used to deter mine the cantilever deflection Deflection V Deflection setpoint V Free air vertical deflection V Then the cantilever deflection x nm can be determined as x nm Deflection V x Deflection sensitivity nm V This value is used to determine the force in the above equation F kx The free air vertical deflection is the value of the deflection when no force is on the cantilever thus it is the reference point to determine the force The value of the free air vertical deflection is set when the laser is positioned on the photodetector prior to engaging the microscope Typically the laser spot is centered on the photodetec tor resulting in a free air vertical deflection near 0 V If this is the case then the Deflection setpoint used in contact mode is the cantilever deflection If a non zero value is used this should be accounted for in the above equations The value of the free air vertical deflection is determined prior to engaging the surface and is dis played as follows MultiMode SPMs The vertical deflection is displayed on the top meter or LED of the MultiMode base Dimension Series SPMs The vertical deflection is displayed on the screen prior to engaging Support Notes 225 35 Nanoindentation Support Note No 225 225 6 Troubleshooting This section contains troubleshooting notes specific to nanoindentation and n
16. within the X Y plane at which the scratch is executed This angle is measured relative to the conventional positive X axis see figure below A rotation of zero degrees will result in a scratch made along the X axis from left to right A rotation of 90 degrees will execute the scratch along the y axis beginning at the bottom and preceding to the top Generally a Scratch angle of 90 degrees is recommended A rotation of 180 degrees will result in a scratch made along the X axis like the zero degree setting but the scratch is made from right to left A general rule is that the scratch is made with angular orientation as measured on the conventional unit circle in a direction pointing away from the center of the unit circle see diagram below 90 180 0 270 Diamond cantilevers are available with their diamonds mounted at various angles to accommodate the intended scratch angle The diamond can be mounted so that one edge of the diamond is oriented to scratch along the preferred direction For more information contact Digital Instruments Support Notes 225 21 Nanoindentation Support Note No 225 225 3 2 Display Controls panel 225 3 2 1 Spring constant The Spring constant parameter is used to record the spring constant of the indenta tion cantilever that is currently being used This parameter is input by the user and is recorded along with each force plot captured It is used for off line analysis of the f
17. a Digital 8 instruments Support Note No 225 Rev F Nanoindentation and Nanoscratching with SPMs For NanoScope Version 4 32 Software 225 1 Overview This support note describes nanoindentation and nanoscratching procedures with Digital Instruments SPMs using NanoScope software Both methods are useful for measuring mechanical characteristics of materials on a nanometer scale using a dia mond tip mounted to a metal foil cantilever These indentation cantilevers are useful for indenting wearing scratching and also imaging sample surfaces using Tapping Mode AFM The forces involved when indenting are typically in the range of 1 100 uN micro Newtons for a standard cantilever A variety of samples can be tested using the same cantilever with the same laser alignment to obtain equal forces during each test Then the samples can be imaged and the relevant data com pared Advantages of using cantilevered tips include ability to image the surface with the diamond tip excellent repeatability and low noise for relative measures of hardness and wear testing Each probe is individually tested to determine the tip sharpness lt 25 nm tip radius and the spring constant 10 accuracy The basic steps described below may be used as a guide for performing nanoindentation using Digital Instruments Dimension Series BioScope and MultiMode SPMs 225 1 1 Indentation Probes An indentation probe consists of a diamond tip moun
18. and If the head is lowered below the surface focus position the tip may crash into the sample damaging the expensive indentation probe When focusing on the surface the user should not only watch the video image but should also watch the SPM head as it is lowered toward the sample The surface should be come into focus when the tip is 1 millimeter away from the sample If the tip appears to be closer than 1 mil limeter from the sample the optics are likely below the surface focus position and the SPM head should be raised to find the surface e Dimension Users Set SPM Parameters to appropriate values for indentation cantilevers Parameters in the Real Time Stage SPM Parameters can be set to the values in Figure 225 4 It is important to set the SPM safety to at least 200 um or the tip may be damaged during the engage SPM Parameters Sample clearance 1000 um SPM safety 200 um SPM engage step 0 500 um Load Unload height 2000 um 225 4 Default SPM Parameters panel settings for indentation e MultiMode Users Position the tip close to sample surface before engaging Manually lower the tip to a position near the sample prior to engaging Use cau tion when lowering the tip since the tip may be damaged When lowering the tip using an optical microscope to monitor the tip position be aware that the dia mond tip extends 100 um beneath the underside of the cantilever Thus the tip is about 100 um
19. anos cratching Troubleshooting of TappingMode AFM is also described in your micro scope s Instruction Manual and may be consulted 225 6 1 Fails to indent or scratch surface Failure to indent or scratch the surface may be caused by a variety of problems The following is a list of possible problems and proposed solutions Insufficient Force For indentations If the indent force is not sufficiently large an indentation will not be visible on the sample surface Try indenting the surface using a larger Trig ger threshold increase the Trigger threshold by 0 1 0 2 V increments until a indent is visible on the sample In general indentations will not be visible on most samples for Trigger threshold values lt 0 2 V Also a Trigger threshold of 1 0 V should almost certainly result in an indentation on the sample surface If not see below for other causes of this problem Note that successful indentations should be visible when imaged using Scan sizes from 1 3 um For scratches Similarly if the force used for a scratch is too low the scratch may not be visible on the sample surface For scratching start with a Trigger threshold of 0 10 V and increase the Trigger threshold by 0 05 V increments until a scratch is visible on the sample surface Like indentations successful scratches should be visible when imaged using Scan sizes from 1 3 um NOTE The force necessary to make an indentation scratch on a particular sample will vary w
20. ant photodetector graphic and vertical deflection meter located on the Image monitor to set the vertical deflection value MultiMode users need to use the vertical deflection meter located on the front of the microscope base Next reengage the microscope on the surface and indent The zero deflection level should now be near the center of the force plot NOTE The standard engage process involves setting the vertical deflection to zero prior to engaging the surface This is equivalent to setting the zero deflection level to zero on the photo detector Typically the zero deflection level will be near zero after engaging the microscope but can drift after scanning for a period of time Insufficient Z Travel An indentation attempt may also fail if the Z Scan size is too low The Z Scan size needs to be sufficiently large to provide enough Z movement to cause the cantilever to deflect the required amount Indenting on a soft sample will require more Z travel than indenting on a hard sample since the tip will penetrate deeper into the soft sample before the Trigger threshold is reached Also larger Trigger threshold values will require larger Z Scan size values Thus the Z Scan size should be set to different values depending on the sample and the indent force used If the Z Scan size is insufficient the force plot will appear as seen in Figure 225 18 Simply increase the Z Scan size until a typical force plot is obtained If necessary disabl
21. arge forces on very hard samples may cause damage to the diamond tip On softer samples such as the gold ruling provided with the nanoindentation pack age use a larger Trigger threshold such as 2 0 V Performing indentations with large forces on a very soft sample may knock off the tip debris If the tip has been successfully cleaned the image resolution should improve If the hardness of the sample is not known use the gold sample provided The gold sample is soft enough to insure that the diamond tip is not damaged even with the largest indent force during the tip cleaning procedure The gold sample also has good topographic fea tures to determine if the image resolution has improved NOTE The gold sample provided with the nanoindentation nanoscratching option is a lum pitch grating used at Digital Instruments to test each indentation cantilever for tip sharpness orientation and indentation ability The print out con tained in the nanoindentation package is an image of the gold ruling which was imaged using your indentation tip The image contains indentations made using Trigger thresholds of about 0 4 0 6 0 8 V This image can be used by the user to test the indentation cantilever s imaging and indenting ability Simply engage on the gold ruling and check if the image and indents are comparable to the print out supplied Support Notes 225 37 Nanoindentation Support Note No 225 B Indent on Soft Gold Sample In g
22. below the focus position on the surface of the cantilever Support Notes 225 7 Nanoindentation Support Note No 225 Execute the Engage command Select the Real Time Motor Engage com mand or the engage icon Dimension users should verify that the SPM safety is set to 200 um before engaging The SPM safety is located in the Real time Stage Parameters panel Image the surface After engagement lower the Setpoint by about 10 to ensure that the tip is tracking the surface Locate the feature or particular site at which indenting scratching is desired Position this feature in the center of the scan Note that indentations scratches will be made in the center of the current scan 225 2 2 Execute an indentation Enter Indent mode Select the Real Time View Force Mode Indent com mand The display monitor will show the plot grid The control monitor will dis play a new menu containing new panels including the Indent Controls Feedback Controls Display Controls and Auto Controls panels and three data channels Open the Indent Controls panel using the Panels menu option if it is not already open First time users can use the Indent Controls panel parameter values as shown in Figure 225 5 as a guide for parameter settings for nanoindentation These settings are a good starting point whenever indenting on a new and unknown sample Indent Controls Indent Controls Other Controls Z scan size Indent setpoint 6 900 Z
23. deflection curve Then click the left mouse button again to fix the line at the desired position The second click on the mouse causes the computer to calculate the slope of the line segment and display the value as the TM Deflect sens parameter in the Channel 1 panel This is the value necessary to calculate the indentation force using the above equa tions See Chapter 11 on force imaging in the Instruction Manual for more informa tion It is also possible to measure the cantilever sensitivity of a captured force plot in Off line To measure the cantilever sensitivity select the captured force plot in Off line and select View Graph from the menu This will display the captured force plot on the screen along with a menu of options above the force plot Select Sensi tivity from this menu and two red arrows will appear on the curve The slope of the line connected by these two arrows is calculated and displayed in the Graph panel as the Deflection sens parameter Move each of the arrows by clicking the left 225 26 Support Notes Support Note No 225 Nanoindentation mouse button on them Drag each of them in turn to the desired location and click the right mouse button to fix the arrow location The displayed value is the required cantilever sensitivity One arrow should be placed near the top of the deflection curve and the other near the bottom Both arrows should be placed on the loaded region of the curve NOTE The cantileve
24. dth and thickness of about 350 100 and 13 um respectively and a resonant frequency of 50 kHz Digital Instruments measures and supplies the customer with the spring con stant of each probe purchased The typical indentation force range available with our instrument is 1 100 uN with a resolution of less than 0 5 uN Larger forces up to about 300 uN can be accommodated using custom cantilevers with higher spring constants while maintaining good imaging ability The diamond tip mounted to the end of the cantilever has a tip radius less than 25 nm to ensure good imaging resolution and nanometer scale indents and scratches The diamond tip is a three sided pyramid plus the bottom with an apex angle of about 60 degrees The apex angle being the angle between a face and an edge of the pyramid To provide more symmetric dents the diamond is mounted such that the vertical axis of the pyramid is approximately normal to the sample when mounted on the microscope 225 2 Support Notes Support Note No 225 Nanoindentation 225 1 2 Nanoindentation Summary Using indentation cantilevers it is possible to dent a variety of materials Dents can be made at various forces and rates using the deflection of the cantilever as a mea sure of the force A dent is made by forcing the tip into the sample surface until the required cantilever deflection is reached The tip is then lifted to its initial Z posi tion above the sample surface For each d
25. e cantilever response may be improved by cleaning the surface on which the substrate of the indentation cantilever rests First remove the tip carefully an put in a safe place Second clean the cantilever holder using one or more of the following methods e Clean surface using a cotton swab and alcohol e Blow clean with available air source After cleaning the cantilever holder re install the indentation tip carefully Again it is necessary to re align the laser and adjust the laser position on the photo detector before checking if the cantilever response has improved Support Notes 225 45 Nanoindentation Support Note No 225 Resonant Frequency Shift Another problem which occurs frequently with indentation cantilevers is when the resonant frequency changes or shifts during operation of the microscope This typically occurs after engaging the microscope and operating for a period of time If the resonant frequency shifts enough the Drive frequency set prior to engaging the surface may not coincide with the resonance of the cantilever This will cause problems imaging the surface If the tip is not tracking the surface topography well the resonance peak may have shifted One possible solution is to lower the Ampli tude setpoint slightly While adjusting the Amplitude setpoint look at the scope traces in Scope Mode to determine how well the tip is tracking the surface If low ering the Amplitude setpoint does not impro
26. e directory The force plots are saved under the current filename xxx as with Real time images If Capture is set to Off the force plots will not be saved to the Capture directory The force plots will be lost There are also various Capture commands located under the Capture menu item within indent mode including Capture Abort and Continuous The Capture command will capture either the previous or the next force plot generated The Continuous command will capture all force plots generated while the Continuous command is active The Abort command will halt any of the active capture com mands 225 3 5 Channel 1 2 3 panels Within the various force modes there are three channels available to display the var ious data types Each panel contains the following parameters to select the desired data and specify the scale of the display For Indent and Scratch modes the only accessible channel is Channel 1 Also the data type for Channel 1 is always set to TM Deflect since this is required for nanoindentation nanoscratching applica tions 225 3 5 1 Data type Typically within the various force modes the Data type for each channel may be assigned to Deflection Amplitude and a variety of auxiliary data inputs Aux B C D The auxiliary inputs are accessed using the Signal Access Module or SAM The selected data type is displayed on the screen when viewing the various force plots Since the cantilever oscillation is halted during nan
27. e the Auto ramp size parameter which sets the Z Scan size to a value 50 times the Trigger threshold Support Notes 225 43 Nanoindentation Support Note No 225 Force Colibratien Flat Ex tend i rey Retracting Def laeti an 0 15 div Bie ES Setpoint I ie aad Z position O 75 Udi Digital Astriments Kanceeope 2 pcan size 16 5 ne 2 span rate 0 S264 Hz Data stale 2 300 V 225 18 Force plot having an insufficient Z Scan size NOTE An indentation attempt made with too low a Z Scan size may still result in a visible indent on the sample surface but the cantilever deflection or Trigger threshold will be incorrect Thus the indent force will be unknown 225 6 2 Poor Image Quality Poor image quality or resolution can be caused by a variety of problems the most common problems encountered with indentation are listed below For more infor mation on problems related to general TappingMode imaging see the Instruction Manual Diamond Tip is Contaminated It is not uncommon for the diamond tips to pick up debris from the sample when indenting and in particular when scratching A dirty tip can cause not only prob lems with indenting the surface but also imaging the surface In particular a dirty tip will not resolve fine features as well as a clean tip A description of this problem and a variety of solutions are discussed above 225 44 Support Notes Support Note No 225 Nanoindentation Poo
28. ection and rate The tip is then lifted to its initial Z position above the sample surface 225 3 Nanoscratching consists of moving a diamond tip through material at a specified depth and with a measured force As with nanoindentation it is also possible to execute arrays of scratches automati cally using the Auto Scratch command Automated scratching also includes the ability to increment or step the scratch force simply by specifying an initial force and a force increment Again the force is incremented in the X direction and held constant in the Y direction The number of scratches and the spacing between the scratches within the array in both X and Y directions can be preset by the user prior to executing the scratch array 225 1 4 Procedural summary The basic nanoindentation nanoscratching procedure consists of the following e Load the sample and indentation probe into the microscope e Align the laser on the reflector atop the cantilever e Set Real time Stage Parameters SPM safety to 200 um Applies to Dimension users only Support Notes 225 5 Nanoindentation Support Note No 225 e Engage on surface in TappingMode with RMS amplitude of 0 25 0 3 Volts e Image sample to locate area of interest e Transfer to Indent mode or Scratch mode and indent scratch the surface e Return to image mode to view the indentations just created 225 2 Quick Start Procedure for Nanoindenting and Nanoscratching
29. eneral cleaning the tip by indenting is more successful on a soft sample with large forces than on a harder sample using moderate forces Thus if the sample is relatively hard it may be helpful to engage on a soft sample such as the gold ruling provided with the nanoindentation package and try indenting as described above Use a large Trigger threshold about 2 0 V and indent multiple times in the same location If the tip has been successfully cleaned the image resolution should improve If the first attempt fails to clean the tip offset to a new location on the sample and repeat above This method is not always successful on the first attempt C Indent on Soft Gold Sample with Increased Force If the previous methods A and B are unsuccessful it may prove useful to make an indentation on the gold sample with even more force than possible in Indent mode The following procedure can be used Review all steps before attempting This should only be attempted if the previous two methods fail to clean the tip Step 1 Engage the microscope on the gold sample in TappingMode This proce dure should only be attempted on the gold sample provided with the nanoindenta tion package or a sample equally soft Step 2 Within real time set the Scan size to 0 V This halts the scanning and holds the diamond tip at a fixed location on the sample Step 3 Next set the Amplitude setpoint to 0 0 V This causes the tip to be forced into the sam
30. ent a plot of the cantilever deflection ver sus the displacement in the Z direction called a force plot is recorded It is also possible to execute indentation arrays automatically using the Auto Indent command Automated indentation includes the ability to increment or step the indentation force simply be specifying an initial force and a force increment The force is incremented in the X direction and held constant in the Y direction Both the number of dents and the spacing between them in both X and Y directions can be preset by the user prior to executing the indentation array For each dent a force plot is recorded Support Notes 225 3 Nanoindentation Support Note No 225 225 2 Nanoindentation sequence 1 engagement in TappingMode 2 4 indentation 5 imaging in TappingMode 225 4 Support Notes Support Note No 225 Nanoindentation 225 1 3 Nanoscratching summary Indentation cantilevers can also be used for scratch testing on a variety of materials Scratches can be made at different forces rates angles and lengths Nanoscratch ing is essentially the same process as nanoindentation except that the tip is moved laterally by a prescribed amount after the sample surface is penetrated A scratch is made by forcing the tip into the sample surface until the required cantilever deflec tion is reached Then with the Z feedback turned off the tip is moved laterally using the preset distance dir
31. for the upcoming indentation or scratch The Lift height is the height at which the fast approach changes to the slower approach The Lift height parameter should always be set to 200 nm this parameter is not a user set parameter A value less than 200 nm may cause the diamond tip to crash into the surface during the fast approach process For this reason this parameter is normally not shown in the Indent Controls panel 225 3 1 10 Units The Units parameter allows the user to switch the units displayed for certain parameters The Units parameter has two settings e Volts Displays parameters in Volts where applicable e Metric Displays parameters in microns or nanometers where applicable 225 20 Support Notes Support Note No 225 Nanoindentation 225 3 1 11 Scratch length Length of the scratch in microns or nanometers The length of the scratch is limited by the maximum Scan size of your scanner Scan Controls panel Typically the Scratch length is set to a value between 1 3 um but depends on the application 225 3 1 12 Scratch rate Speed of the tip during a scratch in Hz Use caution when selecting the Scratch rate The user should consider scratch force and length when selecting the Scratch rate Although the Scratch rate can be set to values from 0 1Hz to 122Hz sug gested Scratch rates are from 0 5 5Hz 225 3 1 13 Scratch angle The Scratch angle is the angle specified in degrees
32. if the tip is clean 225 40 Support Notes Support Note No 225 Nanoindentation Photo Detector at Limit If the photo detector does not have enough range to allow for the specified cantile ver deflection the indentation will fail This commonly occurs because the zero vertical deflection level has drifted within the photo detector s range The level of zero vertical deflection is the level of the flat horizontal portion of the curve in the force plot existing rightward of the sloped portion This zero deflection level can drift upward on the photo detector causing decreased range for indentation Since the Trigger threshold is measured relative to this zero deflection level drift can cause decreased range for indenting The full range of the photodetector is 2 50 V to 2 50 V An increase in cantilever deflection causes the laser beam to move up on the photo detector which in turn causes a positive voltage change If the zero deflection level has drifted to a value of 1 0 V then the cantilever will only be able to deflect by 1 5 V 2 5 minus 1 0 V before the laser beam moves out of the photo detector s range Hence even though the Trigger threshold can be set to a maximum of 2 50 V this amount of deflection may not be possible if the zero deflection level has drifted Figure 225 17 top plot shows a force plot where the zero deflection level has drifted Support Notes 225 41 Nanoindentation Support Note No 225
33. ith tip sharpness A duller tip will require more force than a sharper tip Thus the force used will depend on the particular indentation cantilever being used at the time 225 36 Support Notes Support Note No 225 Nanoindentation Diamond Tip is Contaminated It is not uncommon for the diamond tips to pick up debris from the sample when indenting and in particular when scratching Like a dull tip a dirty tip may require more force to successfully indent or scratch Also a dirty tip can cause irregular shaped indents not triangular to be made Or it may be that no indentation can be made even using the maximum force available If the tip is dirty the user should also notice a loss in the image resolution A dirty tip like a dull tip will not resolve fine features This becomes most apparent when comparing images before and after a nanoindentation nanoscratching operation If the image has degraded noticeably it is probably contaminated To clean the tip try the following procedures in the order shown Tip Cleaning Procedures A Indenting on Sample Position the tip over a new section of the sample Try to clean the tip by performing multiple 3 5 indentations at the same location on the sample On harder samples such as DLC diamond like carbon films limit the Trigger threshold to about 0 5 V Hopefully these moderate force indentations will knock the debris off the tip Do not use larger forces than are necessary l
34. ly occurs because the wear test results in a large amount of debris in and around the worn area The debris is sometimes is pushed around or picked up by the stiff indentation cantilevers resulting in poor imaging quality Before switching to other imaging tips the user should first attempt to image using the indentation cantilever in TappingMode This is best accomplished by using the lowest imaging force possible Use the maximum Amplitude setpoint possible while still tracking the surface Also increasing the Integral gain may increase the imaging quality by increasing the feedback response It may help to scan the worn area in the same scan direction up down in which the wear test was performed since most of the debris is usually located at the end of the scan This will prevent debris from ruining the entire scan because the tip will not contact the debris until the end of the scan 225 5 4 Force calculations for wear testing To calculate the force between the tip and surface during a wear test it is necessary to calculate the cantilever sensitivity referred to as Deflection sensitivity in soft ware The Deflection sensitivity is the cantilever deflection signal versus the volt age applied to the Z piezo determined from the slope of the force plot obtained using force calibration Real time View Force Mode Calibrate The Deflec tion sensitivity is a conversion factor from cantilever deflection in volts to cantile ver deflecti
35. meters within Indent or Scratch mode since they are usually set to appropriate values previously in image mode 225 3 4 Auto Controls panel The Auto Controls panel provides an easy way of generating an array of indenta tions or scratches on sample surfaces The indentation force can be varied automat ically to obtain a range of forces within the array To execute an array of indentations or scratches set the required parameters in the Indent Controls and Auto Controls panels and select the Probe Auto Indent or Probe Auto Scratch command from the menu Note that each indentation scratch is executed using the 225 22 Support Notes Support Note No 225 Nanoindentation same Indent Controls parameters excluding the Trigger threshold which can be varied to change the force and the X and Y offsets used to position the tip The fol lowing is a description of all parameters within the Auto Controls panel 225 3 4 1 Columns and Rows The Columns parameter specifies the number of columns to be indented or scratched Columns are numbered along the X axis from right to left Similarly the Rows parameter specifies the number of rows to be indented or scratched Rows are numbered along the Y axis from top to bottom 225 3 4 2 Column step and Row step The Column step parameter specifies the distance between each column For example a Column step value of 500 nm will move the tip from column 1 a dis tance of 500 nm to column
36. method C except that the vertical stepper motor is used to maximize the range of the Z piezo prior to the tip cleaning indentation Do the following in conjunction with method C above Step 1 After engaging the microscope on the gold sample using TappingMode manually step the motor down until the Z piezo is almost fully retracted This can be accomplished by selecting the Motor Step Motor command from the Real time menu which opens the Motor Control panel From within this panel set the SPM step size to approximately 0 5 um and click on the Tip Down button until the scanner is almost retracted The motor may not move the SPM until multiple steps of the motor are executed Continue stepping the motor until the line on the Z Cen ter Position bar image monitor moves to a position near the Retracted side The Z Center Voltage displayed near the Z Center Position bar should decrease to approximately 200 V Note that the fully retracted Z Center Voltage is 220 V This step provides the Z piezo with the largest range with which to push the tip into the sample during the tip cleaning indentation Step 2 Perform steps 2 5 in method C above E Scan in Contact Mode on Rubber Sample Another possible way to clean the diamond tip is by scanning the rubber sample provided with the nanoindentation nanoscratching package using contact mode AFM This procedure may be used as a last resort if above methods fail to clean
37. mode with minimal force and zero Scan size Perform wear test by scanning the sample surface Withdraw from the surface and switch to TappingMode Engage on surface in TappingMode with RMS Amplitude of 0 25 Volts Image the worn area The following procedure is a general guide for wear testing 225 5 2 Engage surface in contact mode Mount indentation probe and load sample onto into the microscope Align the laser beam on the top of the cantilever Position the laser beam on the reflector located on the free end of the cantilever The reflector will be visible in the optical image when focused on the tip Center the laser spot on the photode tector to zero the vertical and horizontal deflection signals For wear tests it is critical to set the vertical deflection as close to zero as possible since it will effect the force calculation see sections 225 5 3 and 225 5 4 below Set Real time imaging parameters Set the Scan Controls and Feedback Con trols to values appropriate for contact mode imaging Also set the Scan size to 0 0 so that the tip does not begin to scan immediately after engaging Set the Deflection setpoint to about 0 3 0 5V which will minimize the engage force but should ensure proper engagement Set both the Integral gain and Propor tional gain to a value of 2 0 Dimension Users Execute Locate Tip and Focus Surface commands before engaging the surface Execute the Locate Tip command first and the Focus Surface c
38. nanometric level precise one to one comparisons are elusive Force Calculations for Indentation To calculate the force applied to the sample during indentation it is necessary to first measure the cantilever sensitivity The cantilever sensitivity called the Deflec tion sensitivity or TM Deflect sens in the software is the cantilever deflection sig nal versus the voltage applied to the Z piezo determined from the slope of the force plot during indentation The cantilever sensitivity is a conversion factor from canti lever deflection in volts to cantilever deflection in nm Its units are nm volt Once the sensitivity is known the force F of indentation is determined from Hooke s Law F kx where k is the spring constant in N m and x is the cantilever deflection for this par ticular case The cantilever deflection is determined from the cantilever sensitivity and the maximum deflection used for the indentation Trigger threshold Measuring Cantilever Sensitivity The cantilever sensitivity can be measured from within indentation mode or from within Off line analysis after the force plot is captured To measure the cantilever sensitivity in indentation mode simply indent the surface and obtain a force plot Next click the left mouse button on the force plot near the topmost part of the deflection curve A line will appear on the screen By dragging the mouse around orient the line so that it is parallel to the sloped part of the
39. ndents in the array using the Column step and Row step parameters in the Auto Controls panel The Column step and Row step parameters are the X and Y offsets used for the array The Column Row step defines the distance between two neighboring indentations along the X Y direction e Set the forces used for the array The forces used for the array are set using the Trigger threshold parameter abbreviated Trig threshold located in the Indent Controls panel and the Threshold step parameter located in the Auto Controls panel For automated indentation the force can be incremented in the X direction using these two parameters The Trigger threshold is incremented by the value of the Threshold step to vary the indentation force Set the Trigger threshold to the initial value required for the deflection of the cantilever This will be used as the Trigger threshold for the first indentation in any row Set the Threshold step to the required increment for the Trigger threshold For exam ple if the Trigger threshold is 0 2 V and the Threshold step is 0 1 V then any row of indentations in the array will be made using a cantilever deflection vary ing from 0 2 0 3 0 4 0 5 volts The force is always incremented from right Support Notes 225 11 Nanoindentation Support Note No 225 to left To execute an array of indentations at the same force set the Trigger threshold to the desired force and set the Threshold step to 0 0 V Set all
40. ness data between two or more tips Although the same sam ple may be used as a relative standard between two or more tips bear in mind that the diamonds employed may differ significantly 2 When changing diamond tips or when reinstalling the same tip the cantilever sensitivity must be checked and reset each time See section 225 4 1 above or Chap ter 11 in the Instruction Manual for instructions on sensitivity measurement 3 When comparing hardness results between two or more tips mathematical mod eling provides the best approximation of force and therefore hardness Applying the F kx formula described in Section 225 4 1 above one may use the following example as a template e Assume that tip A has a cantilever Spring constant k of 200N m and a Deflection sensitivity of 160 nm V If the Trigger threshold is set to 0 50 V the force F brought to bear on the surface would be F kx Force N Spring constant N m x Trigger threshold V x Sensitivity nm V x units conversion factor 9 F 200 N m x 0 50 V x 160 am V x 2 e 16 0x 10 N Here k 200N m is multiplied by Trigger threshold times the Deflection sensi tivity The conversion factor 1 x 10 m lnm is multiplied to convert the units properly to Newtons It is absolutely critical that the cantilever s sensitivity be cali brated as part of the force measuring procedure otherwise results will be wrong e Next assume that tip B has a
41. nk Z scan rate 2 035 Hz Bata geale 1 000 U 225 6 Typical nanoindentation force plot e Return to image mode Real time View Image Mode or use its icon and image the indentation The system will return to TappingMode imaging and the indentation may be viewed Use a Scan size of 1 3 um to image the indenta tion If the indentation is not visible on the sample surface return to Indent mode and execute another indentation with a larger Trigger threshold Increase the Trigger threshold by 0 1 0 2 V increments until indentations are visible on the sample surface 225 2 3 Execute a scratch e Enter Scratch mode Select the Real time View Force Mode Scratch com mand The display monitor will show the plot grid The control monitor will dis play a new menu containing new panels including the Indent Controls Feedback Controls Display Controls and Auto Controls panels and three data channels Open the Indent Controls panel by using the Panels menu option if it is not already open First time users can use the Indent Controls Support Notes 225 9 Nanoindentation Support Note No 225 panel shown in Figure 225 7 as a guide for parameter settings for nanoscratch ing These settings are a good starting point whenever scratching on a new and unknown sample Indent Controls Indent Controls Other Controls Z scan size Indent setpoint Z scan rate Start mode Trig threshold Auto ramp size H offset Lift height
42. ntilever deflection measured relative the deflection at the surface contact point is equal to the Trigger threshold Figure 225 13 The Indent setpoint is useful in cases where the free air pre contact part of the deflection curve is not flat If this is the case the maximum deflection and force during indenting will vary depending on where the indentation was triggered The Indent setpoint allows the user to move the surface contact point closer to the point where cantilever deflection begins See figure 225 13 A typical range for Indent setpoint is 0 5 1 0 a good default value to use is 0 9 CAUTION If the Indent setpoint parameter is set too low the tip may be extended too far into the sample surface possibly destroying the tip Since the RMS amplitude is typically about 0 25 V any Indent setpoint below 0 5 may cause a crash This is due to the fact that the noise level of the RMS amplitude approaches but cannot ever reach 0 0 V Thus the tip is pushed into the surface the full extent of the Z piezo Also if the Indent setpoint parameter is set too high the tip will be retracted from the surface as the control loop attempts to attain an RMS amplitude which is higher than the free air amplitude During the tip s descent to the surface the graph will reveal the surface contact point as a vertical yellow line near the ramped sloped portion of the plot see below 225 18 Support Notes Support Note No 225 Nanoindentati
43. oindentation and nanos 225 24 Support Notes Support Note No 225 Nanoindentation cratching Amplitude data is not meaningful For this reason data types other than Deflection are not available in Indent and Scratch mode but can be accessed in other force modes 225 3 5 2 Data scale The Data scale is the total voltage range to be scaled along the vertical axis of the force plot The Data scale should be set large enough to display the entire force plot prior to indenting the surface In most cases setting the Data scale about two times the Trigger threshold should display the force plot well The maximum Data scale will change depending upon the Data type being viewed For deflection TM Deflect the maximum Data scale is 5 00 V The Data scale can be adjusted after the force plot is captured 225 3 5 3 Data center The Data center is the vertical offset in units of volts or nm used to shift the data up or down within force plot window Its range of settings depends on the Data type selected for each particular channel For indentation scratching with the Data type set to Deflection the Data center can be adjusted from 2 50 V to 2 50 V 225 3 5 4 Deflection Sensitivity TM Deflect sens The Deflection sensitivity also called cantilever sensitivity is the conversion factor to convert the cantilever deflection from volts to nanometers It is calculated from the slope of the force plot in the contact region The
44. ommand last Always re focus on the surface if the tip focus position has been changed Make sure that the tip and surface are in focus each time you engage The tip may crash into the surface and cause damage if the tip surface focus positions are incorrect Also be careful when lowering the head toward the sample when focusing on the surface If the head is lowered below the sur face focus position the tip may crash into the sample Support Notes 225 31 Nanoindentation Support Note No 225 Dimension Users Set SPM Parameters to appropriate values for indentation cantilevers Parameters in the Real Time Stage SPM Parameters can be set to the values in Figure 225 15 It is important to set the SPM safety to at least 200 um or the tip may be damaged during engagement SPM Parameters Sample clearance 1000 um SPM safety 200 um SPM engage step 0 500 um Load Unload height 2000 um 225 15 Default SPM Parameters panel settings for indentation MultiMode Users Position the tip close to sample surface before engaging Manually lower the tip to a position near the sample prior to engaging Use cau tion when lowering the tip since the tip may be damaged When lowering the tip using an optical microscope to monitor the tip position be aware that the dia mond tip extends 100 um beneath the underside of the cantilever Thus the tip is about 100 um below the focus position on the surface of
45. on Changes to the Indent setpoint will cause shifts in the surface contact point on the force plot increasing the Indent setpoint will shift the surface contact point right ward on the plot decreasing the Indent setpoint will shift the surface contact point leftward Figure 225 13 Generally an Indent setpoint of 0 9 is recommended Tip Defi h 0 25 Wdiv A Trigger point Setpoint erp a rep prep pecs 1 a a iea point Tip Defi 0 25 Udiv 1 r L k E 4 4 4 x L 1 1 1 a x k i a w i 7 k k 1 1 1 1 1 1 1 ieme pee pees eee epee es pees a eer i i 1 1 1 1 1 p TF 1 Surface contact point shifted left 2 position 1 00 Vdiv 225 13 Changes in Indent setpoint will shift the surface contact point left or right 225 3 1 7 Start mode Start mode allows the user to switch between the various force modes without returning to image mode Start mode may be switched between the following set tings e Indent The normal start mode to use for nanoindentation This uses Tapping Mode to find the surface e Scratch The normal start mode to use for nanoscratching This uses Tapping Mode to find the surface Support Notes 225 19 Nanoindentation Support Note No 225 e Calibrate Produces standard Force Mode force plots Includes the ability to continuously cycle the tip up and down e Step Produces standard Force Mode force plots with added control
46. on in nm Its units are nm volt To determine the sensitivity use Force Calibration as outlined for contact mode See sections regarding force calibration and or sensitivity in your Instruction Manual 225 5 4 1 Notes The sensitivity of the cantilever must be determined before the conditions of the wear test are changed since the sensitivity is not a constant The sensitivity will change if any of the following are changed e Position of laser spot on the cantilever e Position of the photodetector relative to the laser beam e Tip position in the cantilever holder 225 34 Support Notes Support Note No 225 Nanoindentation If any of the above are changed between a wear test and the determination of the sensitivity then the sensitivity will be incorrect for force calculations It is sug gested that the sensitivity be determined right before or right after the correspond ing wear test Once the sensitivity is known the force F of indentation is determined from Hooke s Law F kx where k is the spring constant in N m and x is the cantilever deflection for this par ticular case The cantilever deflection is determined from the sensitivity and the deflection of the cantilever during the wear test Determining the Cantilever Deflection For wear testing the cantilever deflection is the difference between the Deflection setpoint Volts used when wearing the sample in contact mode and the value of the free air
47. orce plot only This parameter does not effect the real time indentation or scratch process It is not critical to set the Spring constant since it can be altered in the off line analysis of the captured force plot 225 3 2 2 Number of samples The number of data points collected during a downward extension or an upward retraction travel cycle of the piezo when capturing a force plot during indentation This parameter defines the resolution of the force plot The Number of samples can be set to discrete values from 4 to 512 The higher the value the greater the detail shown on the force plot Typically it is set to 256 or 512 Higher values will increase the amount of disk space necessary to store the force plot and vice versa 225 3 2 3 Average count Sets the number of indentations used to average the display of the force plot For example if the Average count is set to 10 and the Probe Run single command is executed 10 indentations will be made in the same location and only 1 force plot will be displayed This force plot is the average of the force plots for each indenta tion The Average count may be set between 1 and 1024 but is usually set to 1 225 3 3 Feedback Controls panel The Feedback Controls panel allows the user access to a few important parameters used in image mode including Integral gain Proportional gain Amplitude set point Drive frequency and Drive amplitude It is not necessary to set or adjust these para
48. other Indent Controls as desired typical values shown in section 225 2 2 Execute an indent array After setting the Auto Controls parameters execute the array by selecting the Probe Auto Indent command from the Indent mode menu The array will be executed and the force plot for each indentation will appear on the display monitor A message box will display the X and Y offsets as the array is performed To capture the force plots it is necessary to set the Capture parameter located in the Auto Controls panel to Enabled before executing the Auto Indent command If Capture is set to Off the force plots will be lost Return to image mode Real time View Image mode or use the icon and image the indentations The system will return to TappingMode imaging and the indentation array may be viewed The array should appear centered within the current scan 225 2 5 Automated scratching e Using an indentation probe engage the surface in TappingMode and enter Scratch mode as described in the previous sections 225 2 1 and 225 2 3 above Review these two sections including all precautions before proceeding Open Auto Controls panel Once in Scratch mode open the Auto Controls panel which contains the parameters necessary to perform an scratch array using automated scratching See panel in Figure 225 9 Set the number of scratches in the array Using the Columns and Rows parame ters Auto Controls panel set the number of columns and rows
49. ple by the full extent of the Z piezo causing an large indentation The indentation force will be approximately five times larger than the maximum force possible in Indent mode When the Z piezo is fully extended the Z Center Posi tion bar located on the image monitor should display Limit Also the line on the Z Center Position bar will move to the Extended side to show that the scanner is fully extended Step 4 Reset the Amplitude setpoint to its original value after the scanner has extended This will cause the Z piezo to lift the tip away from the sample surface and return to TappingMode imaging Also the Z Center Position should return to a position somewhere in the middle of its range Step 5 Next offset to a new location on the sample and check if tip cleaning was successful Offset until the indentation that has just been made does not appear in the scan area Use an offset of about 5 10 um in the X and or Y direction Again if the tip has been successfully cleaned the image resolution should improve Com pare the image resolution before and after this procedure It may be useful to cap ture images of the surface before and after cleaning the tip 225 38 Support Notes Support Note No 225 Nanoindentation D Indent on Soft Gold Sample with Maximum Force If method C above is not successful it may be helpful to further increase the indent force used for cleaning the tip This procedure is similar to
50. r Cantilever Response In some cases indentation cantilevers can have unfavorable amplitude response which may cause poor imaging quality The signs of poor cantilever response can be seen in the cantilever tune or frequency sweep some of which are listed below e Amplitude peak is not well defined The frequency sweep shows multiple resonance peaks The resonant peak is not as steep and sharp as usual The optimum frequency sweep consists of a single symmetric amplitude peak which is only a few kHz wide e Requires unusually high Drive amplitude to obtain required RMS Amplitude Typically the Drive amplitude is below 500 mV If any of the above is true the cantilever is not responding optimally It is still pos sible to engage the microscope and image the sample but the image quality and resolution may or may not be as good as usual If any of the above are true try the following to improve the cantilever response A Re position the indentation tip within the cantilever holder The cantilever response can sometimes be improved by moving the tip to a different position in the cantilever holder Simply remove the cantilever holder from the head and re posi tion the tip with some tweezers After moving the tip it will be necessary to re align the laser and to adjust the laser spot on the photo detector Finally check the canti lever tune for an improved response B Remove the indentation tip and clean the cantilever holder Th
51. r sensitivity will vary slightly depending on the location of the arrows or the location of the line for the previous method It is more accurate to measure the cantilever sensitivity on a very hard sample since there is little surface penetration Since the cantilever sensitivity is a measurement of the change in cantilever deflection per change in the Z position it is important to have negligible sample penetration If a change in Z position causes not only canti lever deflection but also sample penetration the sensitivity measurement is inaccu rate For a soft sample more Z motion is required to obtain the same deflection as on a hard sample Thus the sensitivity is greater when measured on a harder sample The ideal sensitivity would be obtained on an infinitely hard sample The ideal sen sitivity is approached as the sample penetration approaches zero or as the hardness becomes infinite It is suggested that a hard sample such the sapphire sample pro vided with the nanoindentation package is used to measure the cantilever sensitiv ity before or after indenting the desired sample This is especially important if it is desired to calculate the indentation force accurately To measure the cantilever sen sitivity on a hard sample simply engage on the sample in TappingMode indent a single time capture the force plot and measure the sensitivity in the Off line CAUTION When indenting hard samples for sensitivity measurements restrict
52. re details on cantilever sensitivity Support Notes 225 15 Nanoindentation Support Note No 225 225 3 1 4 X offset and Y offset The X offset and Y offset are used to adjust the position of the tip in the X Y plane Units are volts or microns In Indent mode the X and Y offsets are used to offset the tip to execute a indentation or scratch at a different location Their values are limited to a range of 220 to 220 Volts The X and Y offsets are further limited by the Scan size since they are used to offset the X Y scan in image mode A large Scan size will reduce the range of the X Y offsets since they define the center of the scan Similarly large X Y offsets will reduce the maximum Scan size possible 225 3 1 5 X Rotate X Rotate allows the user to move the tip laterally in the X direction during inden tation This is useful since the cantilever is at an angle relative to the surface One purpose of X Rotate is to prevent the cantilever from plowing the surface laterally typically along the X direction while it indents in the sample surface in the Z direc tion Plowing can occur due to cantilever bending during indentation or due to X movement caused by coupling of the Z and X axes of the piezo scanner When indenting in the Z direction the X Rotate parameter allows the user to add move ment to scanner in the X direction X Rotate causes movement of the scanner oppo site to the direction in which the cantilever points Wi
53. red This value will be used for the first indent or scratch in any row If the Threshold step is positive this will be the minimum force used to indent or scratch For exam ple if Threshold step is set to 0 2V and the Trigger threshold is set to 0 2 V then the indentations scratches will range as 0 2 0 4 0 6 0 8 volts Support Notes 225 23 Nanoindentation Support Note No 225 The Threshold step can be set to values between 2 50 V and 2 50 V but is realis tically limited by the maximum Trigger threshold of 2 5 V For example if the Trigger threshold is set to 0 5 V and the Threshold step is set to 0 5 V then the indentations scratches will range from 1 5 2 0 2 5 2 5 Volts Thus the Trigger threshold of 2 5 V will be repeated if more than three indentations or scratches are performed with this particular Threshold step When choosing the Threshold step the user should consider the array size and the Trigger threshold limitations For indentation the Threshold step is typically set to values from 0 1 V to 0 4 V Whereas for scratching the Threshold step should be limited to values from 0 02 V to 0 10 V To execute an array of indentations or scratches all at the same force set the Threshold step to OV 225 3 4 4 Capture The Capture parameter specifies whether or not to save the force plots for the indents or scratches in the array When set to Enabled all force plots are saved automatically to the Captur
54. scan rate 2 03 Hz Start mode Trig threshold 0 2000 U Auto ramp size Enabled H offset id Lift height 200 nm Y offset Units H Rotate 22 0 deg 225 5 Default Indent Controls panel settings for indentation Set Trigger threshold Before making an indentation it is necessary to set the force applied to the tip This is set using the Trigger threshold parameter abbreviated Trig threshold located in the Ramp Controls panel The Trigger threshold specified in volts is the cantilever deflection at which the controller stops pushing the tip into the surface It is a measure of the force applied to the sample during indentation A good starting value is 0 1 0 2 V Execute an Indentation To make an indentation in the surface click on the Probe Run Single command or click on its icon The tip will be plunged into the surface and a force plot will appear on the display monitor To capture the 225 8 Support Notes Support Note No 225 Nanoindentation force plot generated to the capture directory click on the Capture icon or select Capture from the Capture menu An example force plot is shown in Fig ure 225 6 for a Trigger threshold of 0 4 V The force plot is a graph of the can tilever deflection versus the Z movement of the scanner F orce Calibratian Plot Bietendi ng Retracting Def lect ion D10 Yrdivy Setpoint 2 position 2 00 Urdiwu Bigiteal Instroxents Hanosoore 2 soan size 452 0
55. ted to a metal foil cantilever which is used to image indent scratch and wear surfaces Indentation probes are thicker wider and longer than standard AFM cantilevers and are composed of stain Document Revision History 09FEB98 All Version 4 32 220 220 163 23JAN98 All Version 4 23 w py Digital Instruments 1998 112 Robin Hill Road Santa Barbara CA 93117 805 967 2700 225 1 Nanoindentation Support Note No 225 less steel as compared with silicon or silicon nitride The typical ranges for the spring constant of contact mode TappingMode and indentation probes are 0 01 1 0 N m 20 100 N m and 100 300 N m respectively The resonant frequency for indentation probes is generally in the range of 35 60 kHz depending upon the dimensions of the cantilever and the size of the diamond For comparison the reso nant frequency for standard TappingMode probes is about 300 kHz Unlike contact mode imaging DI s patented TappingMode technique allows the use of the high spring constant cantilevers required for nanoindentation while still imaging the surface with minimal damage A silicon reflector is mounted on the top side of the cantilever to obtain a well focused laser spot on the photodetector 1 1 1 or 1 1 1 I microns J 350 75 microns 100 microns wide 225 1 Typical indentation probe A typical indentation cantilever has a spring constant of 150 N m length wi
56. th the same force Support Notes 225 17 Nanoindentation Support Note No 225 225 3 1 6 Indent setpoint The Indent setpoint parameter on the panel serves as a multiplier to the Amplitude setpoint value located in the Real time Feedback Controls panel and affects the point at which the indentation or force plot is triggered It is in effect only for nanoindentation nanoscratching and is disabled at all other times Nanoindentation and nanoscratching use the TappingMode function to find the surface when execut ing an indentation or scratch Before penetrating the surface the cantilever is oscil lated using the Drive frequency and Drive amplitude previously set in image mode Then the tip is moved toward the sample until the amplitude of oscillation of the cantilever measured on the photodetector has been reduced to a predeter mined amount This predetermined amount is specified by the Amplitude setpoint previously set in image mode and by the Indent setpoint and is equal to the Indent setpoint times the Amplitude setpoint Figure 225 23 For example if the Feedback Controls Amplitude setpoint parameter is set to 0 2 V and the Indent setpoint is set to 0 9 then the predetermined amount would be 0 18 V 90 of the Amplitude setpoint The Z position at which the amplitude is reduced to this predetermined amount is the surface contact point Finally with the oscillation off the tip is forced into the sample until the ca
57. the tip The following is a step by step procedure Step 1 Before engaging position the tip over a flat region on the rubber sample It is important not to engage on the circular bubble like regions on the sample see Figure 225 16 If possible position the tip so it will engage between two of the cir cular bubbles as shown in the figure Support Notes 225 39 Nanoindentation Support Note No 225 EI Tr Cleaning site inital irairssmia tenors wubb land 225 16 A rubber surface may be used to clean nanoindentation tips Step 2 Engage on the surface in contact mode Prior to engaging set the vertical deflection to about 2 V as usual for contact mode imaging Also set the Deflection setpoint to 5 V which will provide a high tip sample force while scanning the sur face Use a Scan rate of about 1 0 Hz and a Scan angle of 0 degrees Set other parameters to values appropriate for contact mode imaging and engage on the sam ple surface Step 3 After the microscope has engaged increase the Scan size to its maximum value The optical image should show the cantilever bending as it scans across the rubber sample This high force scanning should remove some debris from the end of the tip It is only necessary to scan the sample for about 30 seconds before with drawing the microscope Step 4 Withdraw the microscope and load a new sample Engage on desired sam ple now using TappingMode and check
58. the cantilever Execute engage command Select the Real time Motor Engage command or the Engage icon Dimension users should verify that the SPM safety is set to 200 um before engaging The SPM safety is located in the Real time Stage Parameters panel Immediately lower the Deflection setpoint Once engaged decrease the Deflec tion setpoint by about 1 2 volts to lift the tip off the surface This will prevent damaging the surface prior to the wear test To verify that the tip is off the sur face check that the Z piezo is retracted by looking at the image monitor The Z Center Position should move to the retracted side of the Z Center Position bar located on the image monitor Also the word Limit should appear in place of the Z center voltage value It this does not occur the Z piezo is not fully retracted and the tip may still be on the surface The user should decrease the Deflection setpoint by 1 volt increments until the Z is retracted NOTE In general engaging in contact mode using indentation cantilevers will result in an small indentation made at the first point of contact If desired the user should offset to fresh location on the sample to perform the test 225 32 Support Notes Support Note No 225 Nanoindentation 225 5 3 Wear the sample surface e Set parameters for wear test Once engaged on the surface using the procedure above set the required parameters for the wear test Set the Scan size and
59. thout X Rotate control the tip may be prone to pitch forward during indentation Normally it is set to about 22 0 degrees This parameter typically ranges between 15 and 25 degrees ea p Tapping mode w v gt Contact mode 225 11 The tip engages the surface in TappingMode then begins indentation At engagement 1 the tip is oriented normally however as the tip is pressed into the surface it tends to pitch forward 2 By applying a slight X axis offset 3 the tip is brought normal again 225 16 Support Notes Support Note No 225 Nanoindentation The images shown in Figure 225 12 all use the same Trigger threshold value and demonstrate the effect of X Rotate at various settings Notice that the indentation is larger for a value of 0 0 degrees and less for an X Rotate value of 20 0 degrees Notice also that there is material deposition pileup on the left outboard side of each indentation depending upon the amount of correction The pitching forward of the cantilever during nanoindentation tends to move the laser spot in a direction opposite to normal deflection This produces a counter effect that may result in less deflection at the photodetector but higher forces The end result is deeper larger dents for lower X Rotate values X Rotate 0 00 deg X Rotate 12 00 deg X Rotate 20 00 deg 225 12 The effect of various X Rotate values for dents made in the same material 1 um gold ruling sample wi
60. to step the tip towards the surface 225 3 1 8 Auto ramp size The Auto ramp size may be Enabled or turned Off e Enabled If the Auto ramp size is Enabled the Z scan size is automatically adjusted to a value 50 times the value of the Trigger threshold For example if the Trigger threshold is set to 1 0 V then the Z scan size will be 50 V Note that the Z scan size remains at 10 V for all Trigger threshold values below 0 2 V e Off Turns off automatic adjustment of the Z scan size parameter In this case the Z scan size must be adjusted by the user depending on the indentation force or Trigger threshold used in order to optimize the force plot 225 3 1 9 Lift height The Lift height parameter is used to speed up the indentation process Its definition will become clear through an explanation of the indent process Prior to executing an indentation or scratch the indentation probe is held above the surface at a height equal to the current Z scan size value During indentation the tip is then moved quickly towards the sample surface a fast approach until the distance between the tip and sample is equal to the Lift height When the tip reaches the Lift height the amplitude feedback is enabled causing the probe to approach the surface a slower approach The Z position at which the oscillation amplitude of the cantilever reaches a predetermined setpoint Amplitude setpoint x Indent setpoint is used as a reference for the surface
61. too small the indentation may be incomplete because of the scanner s limited vertical movement Be sure to select a value which best displays the force plot along the horizontal axis this scaling cannot be changed after inden tation If in doubt set the Auto ramp size parameter in the Indent Controls panel to Enabled This will automatically adjust the Z scan size parameter according to the value of the Trigger threshold described below If the Auto ramp size is Enabled the Z Scan size is automatically adjusted to a value 50 times the value of the Trigger threshold For example if the Trigger threshold is set to 1 0 V then the Z Scan size is automatically adjusted to 50 V Note that the Z scan size remains at 10 V for all Trigger threshold values below 0 2 V 225 3 1 2 Z Scan rate The Z Scan rate is the speed in Hertz at which the cantilever sample is loaded and unloaded during indentation and scratching If set to 1Hz an indentation will take 1 second to execute Although the Z Scan rate is limited to a range of 0 01 Hz to 260 Hz values from 0 5 Hz to 10 Hz are typically used for indentation and scratching 225 14 Support Notes Support Note No 225 Nanoindentation 225 3 1 3 Trig ger threshold The Trigger threshold abbreviated as Trig threshold is the value of the cantile ver deflection as measured by the photodetector desired for the indentation or scratch The Trigger threshold defines the maximum force applied
62. ve the image withdraw the micro scope from the surface and view the frequency sweep in cantilever tune If necessary adjust the Drive frequency so its centered on the shifted amplitude peak Then adjust the Drive amplitude if necessary to obtain the appropriate RMS amplitude for the cantilever Finally re engage the microscope and check if image quality has improved This procedure may be repeated periodically if the tip has problems tracking the surface features 225 46 Support Notes
63. w Image mode or use its icon and image the scratches The system will return to TappingMode imaging and the scratch array may be viewed Support Notes 225 13 Nanoindentation Support Note No 225 225 3 Nanoindentation Nanoscratching Parameters The various force modes including Indent and Scratch mode may be accessed via the Real time View Force Mode menu The following is a description of the parameters within the control panels used for Indent and Scratch mode The pan els include Indent Controls Display Controls Feedback Controls Auto Con trols and 3 data channels Within Indent or Scratch mode all panels can be accessed via the Panels menu option The following is a description of all panels within Indent and Scratch mode 225 3 1 Indent Controls panel Parameters in the Indent Controls panel are the central controls used for indenta tion and scratching In Indent mode these parameters control the forces rates and position of the indentation In Scratch mode they control the forces rates posi tion and also the length and orientation of the scratch The following is a descrip tion of all parameters within the Indent Controls panel 225 3 1 1 Z scan size The Z Scan size sets the total range for the Z piezo movement which is scaled along the horizontal axis of the force plot The units are volts or nanometers The Z scan size is limited by the full range of the Z piezo If the Z Scan size is set
64. wear testing section 225 5 4 e Execute the Withdraw command immediately after one scan is complete or when the desired number of full or partial scans are complete This prevents more wear from occurring than is desired e Re engage the surface in TappingMode and image the worn area Refer to sec tion 225 2 1 for TappingMode engage guidelines using indentation cantilevers Do not reset the X and Y offsets or the tip will engage in a different location Once engaged set the Scan size above the value used for the wear test and image the new features Comments on Wear Testing If the application requires non standard indentation cantilevers with lower or higher spring constants than standard indentation cantilevers imaging the worn area may require switching to standard TappingMode or contact mode tips If this is the case it may be necessary to reference the location of the worn areas using a spe cific feature on the sample prior to performing the test This is necessary since the tip position relative to the sample will not be the same after switching tips The Support Notes 225 33 Nanoindentation Support Note No 225 above feature can be used to re locate the worn areas after switching tips If the worn areas are visible through the optics used to view the sample this will not be necessary Switching to other tips may also be necessary if the indentation cantilever is not imaging the worn area sufficiently This usual

Nanoindentation and Nanoscratching with SPMs For NanoScope

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