OFET measurement manual:
Contents
1. Probe Station Overview The optic probe station allows for quick and easy testing of individual substrates on a variety of OFET electrode designs thanks to the interchangeable probe board The unit is composed of two main sections the XYZ position stages and the low noise probe board The positioning stage is controlled through three micrometer screws 0 5 mm accuracy and can be easily operated in a glove box The substrate is loaded in to the substrate holder mounted on top of the vertical stage and is kept firmly in position by a steel bar acting as top gate pad contact The bottom and the edge of the holder provide an alternative route to electrically connect the gate to the SMU The probe itself uses POGO connectors which are rounded and spring loaded to allow robust non destructive electrical connection with the electrode of the Device Under Test DUT Note The probe station has been tested to allow for non destructive contact in case of metal adhesion layer electrodes only Au Cr Pt Ti etc Metal with poor adhesion properties on silicon or glass such as Au are prone to be damaged by the POGO unless extreme care is taken during the positioning of the probe on the electrodes The test board has a double ground plane acting as a Faraday Cage which minimises the effect of the environmental Electro Magnetic noise However the user must consider that measurement of currents in the range of tens of nA or lower can still be ea2. 2014 15 enabling organic electronics In Table 1 we reproduce the correction factors for thin rectangular samples of side d and d where d52 gt d a oss7 orus onus m0 os95s 0 9957 0 9957 Table 1 Four point probe correction factors for square rectangular thin sheets of side length d and d where d 2d When it is not possible to look up the correction factor for the exact dimensions of your sample we recommend using cubic spline interpolation on the data from the tables in order to estimate the precise correction factor This can be done quite easily using the interpolation function within MATLAB or by writing your own code For example using MATLAB to find the correction factor for a sample with d s 11 d2 d 3 you simply need to enter x 3 45 7 5 10 15 20 40 y 0 5958 0 7115 0 7888 0 8905 0 9345 0 9696 0 9830 0 9957 x_new 11 y_new interp1 x y x_new spline and the output should show the interpolated value of y_new as 0 9453 Cubic spline interpolation involves the creation of a separate cubic polynomial between each 2 points nodes ensuring that the function passes through both nodes and that the 1 and 2 derivatives of adjacent polynomials match at each node Ossila Ltd Copyright 2009 2014 16 enabling organic electronics Guidance on Sample Positioning and Probe Usage In order to ensure accurate results samples should be positioned so that the probes touch do
3. The 1620 2008 IEEE Standard for Test Methods for the Characterization of Organic Transistors and Materials recommend settling time of 10 ms to 100 ms per voltage change Note Additional stray capacitance can play an important role in slowing down the response of OFETs For example extra stray capacitance arises from the overlap between gate dielectric and drain source electrodes Coaxial cable Low RC shielded coaxial cables are required for fast and accurate measurement Fig 1 The cables must be kept away from ES EM noise sources and vibrations Vibrations of the cable can in fact induce carriers in the dielectric due to the rubbing of the dielectric against the external jacket of the cable triboelectric effect or as effect of the mechanical stress on the dielectric itself see Fig 2 The noise induced by vibration generated charges in the dielectric become non negligible when measuring low few nA currents Teflon or PTFE insulated coaxial cables are ideal for measuring currents less than 10 Amp Cables with polyethylene insulation also have low leakage resistance and will give good performance In any case the coaxial cable should be chosen to have low capacitance per unit length and kept as short as possible L Outer Centre jacket Shield Insulation conductor S25 SOO 252959 iesssseneseenees OK 5250 5 lt gt amp S25 ee OO 25090 SSS 2525 OSS L9 2525 25 SCOP Figure 1 Right side Coaxial cabl
4. factors including setup and measurement settings can severely degrade the quality of the acquisition In particular any test fixture such as coaxial BNC cables substrate holder etc introduces additional noise and leakage currents mainly due to the internal leakage resistances parasitic capacitances and EM ES pickup making high precision measurement of low value current voltage challenging and expensive To complicate matters some organic semiconductor compounds require high voltage 100 V and higher to yield current of the order of a few hundred nana Ampere a condition that imposes stringent requirements on the insulating properties of the test board coaxial cable and switches On the other end of the spectrum high performance semiconductors with large ON OFF ratio requires the capability of maintaining high accuracy while acquiring current varying across several measurement ranges of the SMU In the following we will briefly outline some of the more common factors affecting the accuracy and reliability of OFET characterisation while the section Four probe measurement will deal with one of the most effective strategies for measuring low value quantities such as resistance or OFET mobility Kelvin four wire probe Ossila Ltd Copyright 2009 2014 29 enabling organic electronics High Accuracy Measurement Let us first introduce a few concepts and definitions that are commonly used when describing the precision
5. negligible time constant Tm less than 1 ps The device time constant Tpyr mainly depends on the dielectric capacitance of the gate and other parasitic capacitance see below For dielectric the time constant is more conveniently expressed in term of the cutting off frequency f defined as 1 Laen 2TET Indicative fe values for common dielectrics are 1250 KHz for SiO 7530 for PMMA and 9 KHz for Al O3 High fe values translate into faster dielectric which therefore require shorter charging time The RC value for a typical SMU Ossila probe station with and without coaxial cable and the three dielectrics above are summarised in the table below Number of RC RC Typical SMU AV 1V 300 us Ossila Probe Station Ossila Probe two meters Coaxial cable 15 ps 127 ns PMMA 18 ys Here we assume that the resistive and capacitive elements of both DUM and measurement apparatus are in series and parallel respectively Ossila Ltd Copyright 2009 2014 32 enabling organic electronics Often however the most important limiting factor to the speed of OFET characterisation is the transistor ON OFF frequency given by f LVps E 2TL Where u is the mobility Vps the drain voltage and L is the channel length For low mobility organic semiconductor and long channel transistor t 1 f can be of the order of few ms value that significantly limit the acquisition speed of l V curves for organic based devices
6. of a measurement apparatus For more details please refer to Low Level Measurements Handbook 6 Ed and Keighley The content of this paragraph is mainly based on this publication Accuracy defines how close the measured value is to the actual value Generally SMU data sheet explicitly states the accuracy as the sum of a constant offset plus a term depending on the measured quantity of measured quantity These values are determined under some precise operational conditions temperature time elapsed from the last factory calibration etc and most importantly depend on the measurement range Sensitivity is the minimum quantity that the SMU can detect Again this depends on the measurement range temperature etc Resolution is the smallest change that the SMU can output or measure As above the resolution can depend on the measurement range and is guaranteed if and only if certain conditions are met temperature calibration etc For a SMU the output and input resolution can be different Reproducibility refers to the capability of a measurement apparatus to yield the same value over time Ageing poor maintenance temperature variations are detrimental to the reproducibility of the measurement outcomes Uncertainty is the estimated error that accompanies any measurement Let us suppose that a SMU is used to estimate the unknown electrical resistance R by applying a voltage V and measuring both voltage V and current across the r
7. position secure it on a work bench far away from Electromagnetic and Electrostatic noise sources 2 Using low capacitance coaxial cables connected the gate and drain BNC connectors to the gate and drain channel of a multi channel SMU respectively or to two single channel SMUs 3 Remove the push fit bracket and insert the substrate in substrates holder Make sure that the gate and drain contact pads are facing the drain and gate POGO pin Ossila Ltd Copyright 2009 2014 27 enabling organic electronics 4 Secure the substrate with the push fit bracket and make sure that both left and side gate switches are ON 5 Set the drain and source switches of the devices to measure to the ON position and make sure that all the other drain sources switches are OFF 6 Run the measurement of choice During operation keep a safe distance from the test board do not operate the switches when voltage and or current are applied to the test board 7 Do not apply voltage in excess of 100 V or current in excess of 100 mA 8 Refer to the High Accuracy Measurement of this section for further information on safety and measurement accuracy Ossila Ltd Copyright 2009 2014 28 enabling organic electronics Measurement Error and Noise Measurement accuracy defines how close the measured quantity current voltage is to the actual value While the accuracy primarily depends on the intrinsic accuracy of the SMU several external
8. 27 0 15 mm Pogo diameter 1 07 mm Substrates holder 15 15 mm x 20 15 mm length 21 44 mm gext 7 92 0 08 mm gint 4 34 0 08 mm Drain Source electrical contact gold coated spring loaded pogo Gate electrical contact substrate holder 304 Steel 3D translation stage material Aluminium Screw and washer material Nylon RC values indicative values a 100 tolerance is understood data measured using TeNMA 72 7730 Vichy VC6013 Resistance and capacitance values for the probe station board Drain to Pin Resistance Source to Pin Resistance Total Drain Source Resistance 25 pF 17 pF Resistance and capacitance values for the probe station board with a 2 metre long coaxial cable The coaxial cable can introduce a significant amount of capacitance For example the table below shows the capacitance and resistance of the probe station connected to a two metre long RG58 coaxial cable with Ceabie 56 pF m and resistance R apie 0 1 Q m Drain to Pin Resistance Source to Pin Resistance Total Drain Source Resistance 136 pF 128 pF Ossila Ltd Copyright 2009 2014 9 enabling organic electronics Open circuit l V current Data acquired with NI PXI 4132 indicative values only 100 tolerance is understood Data depends on the accuracy and resolution of the SMU measurement setting and EM environmental noise Iya By Wa Ned 100 80 60 40 70 O 20 40 60 80 100 Vos V Transfer Characteristics Open circu
9. Dev 10 Dev 20 R 0 50 0 2 0 R 0 50 0 2Q0 R 0 80 0 2 0 C 85 3 pF C 88 1 pF C 90 4 pF Dev 4 Dev 9 Dev 14 Dev 19 R 0 60 0 2 0 R 0 50 0 3 0 R 0 50 0 3 0 R 0 60 0 2 0 C 85 9 pF C 83 9 pF C 83 pF C 90 4 pF Dev 3 Dev 8 Dev 13 Dev 18 R 0 7 Q 0 3 Q R 0 70 0 3 0 R 0 60 0 3 0 R 0 60 0 2 0 C 87 3 pF C 82 9 pF C 85 8 pF C 90 pF Dev 2 Dev 2 Dev 12 Dev 17 R 0 70 0 3 0 R 0 70 0 3 0 R 0 80 0 3 0 R 0 70 0 2 0 C 88 1 pF C 86 pF C 87 3 pF C 90 2 pF Dev 1 Dev 16 R 0 80 0 3 R 0 90 0 4Q0 C 88 7 pF C 92 6 pF Open circuit l V current Data acquired with NI PXI 4132 indicative values only 100 tolerance is understood Data depends on the accuracy and resolution of the SMU measurement setting and EM environmental noise Resistance from input to pogo from pogo to GND Ossila Ltd Copyright 2009 2014 25 enabling organic electronics ae T i t 4 L 100 80 60 40 20 O0 20 40 60 80 100 Vos V Transfer Characteristics Open circuit current Data acquired with NI PXI 4132 indicative values only 100 tolerance is understood Values depend on the accuracy and resolution of the SMU measurement setting and EM environmental noise los Ves i R rey nm An APL een be VS Pte peer Ree 150 100 80 60 40 270 0 20 40 60 80 100 Ves V Ossila Ltd Copyright 2009 2014 26 enabling organic electronics OFET Measurement 1 Screw the nylon spacer on the board and
10. Ossila enabling organic electronics OFET measurement manual e Probe Station E451 e Low Density OFET Test Board E222 e High Density OFET Test Board E481 Ossila Ltd Copyright 2009 2014 1 enabling organic electronics Contents Safety Standards Statement 3 Declaration of Conformity 4 Probe Station Overview 6 Technical Data 9 OFET Measurement 11 4 point Probe 13 Equations Sheet Resistance 14 Mobility 17 Maintenance 18 Low Density OFET Test Board Overview 19 Technical Data 20 OFET Measurement 22 High Density OFET Test Board Overview 24 Technical Data 25 OFET Measurement 27 Measurement Error and Noise 29 High Accuracy Measurement 30 Ossila Ltd Copyright 2009 2014 2 enabling organic electronics Safety Warning To avoid safety hazards obey the following Do not leave devices with applied bias or current unattended as a power failure may result in board damage or device damage and potentially hazardous situations The maximum allowed driving voltage is 100 V and the maximum current is 100 mA Operating the probe station or OFET test board outside the operational range can cause damage to the device and greatly increase the risk of electrical shock The electrical components under high voltage are exposed The user must therefore exert extreme caution when operating the probe station and maintain a safety distance of at least 1 m during high voltage operations Do not o
11. e the shield is connected to the LOW or to the ground output of the SMU while the central conductor is connected to the HIGH output Left side equivalent circuit of coaxial cable A coaxial cable can be modelled as a sequence of RLC circuit in series where Ra R and C stand for resistive inductive and capacitive element of the circuit Ossila Ltd Copyright 2009 2014 33 enabling organic electronics Frictional motion at boundary due to cable motion Inner conductor Coaxial cable fanducive lubricant in low noise cable Figure 2 Vibration and mechanical stress exerted on the dielectric of a coaxial cable can induce charges generated at the Jacket Dielectric interface tribolectric effect or inside the dielectric itself piezoelectric current Negligible for high current measurement these spurious currents can degrade the accuracy of low level current acquisitions Metal Metal Heterojunction Any temperature difference between two parts of a conductive material results in a potential drop across the material itself Seebeck effect The Seebeck effect is quantified by the Seebeck coefficient which has the unit of uV C and expresses the potential drop for a 1 C of temperature difference The coefficient depends on the type of metal s and on the temperature of the conductor itself Typical values range from few uV C for Al Au and Cu to hundreds of uV C for Si For standard OFET characterisation with dr
12. e accuracy of the minimum range required to measure the large ON current PLC rejection The PLC Power Line Cycle is the oscillation of the electrical current supplied through the power grid which is 50 or 60 Hz depending on the location Make sure that the SMU is capable of efficiently rejecting the PLC oscillations of the power supply otherwise especially for low currents PLC can manifest itself into an oscillating behaviour of the current voltage reading Aperture Time is the duration of the acquisition for a single data point it is usually expressed as unit of PLC 1 PLC corresponds to 20 or 16 6 ms for 50 or 60 Hz PLC respectively For high precision low noise measurement it is good practice to run measurement at 1 PLC or multiple integer of PLC so to allow the SMU to effectively reject PLC noise Zero Drift AutoZero For V 0 a SMU should read zero current however a small offset current is always present If available activate the zero function of the SMU to automatically subtract this current from the measured data Settling Time Any physical system requires a finite time to respond to an external input and reach a new equilibrium state In the contest of current voltage measurement this transient time is often referred to as settling time and it depends on the resistivity and capacitive components of the measurement apparatus SMU cable and test fixture and on speed of the device under test The SMU settl
13. esistor If 6V and l are the independent and random errors associated to the voltage and current measurements respectively the uncertainty of the resistance R V lis given by In general for dependent and or non random error SR lt R Z aA V IZ The formulas above can be easily generalised to any quantity Q calculated as the product of the measured quantities x y l m as in Q xyz Imn Factors affecting measurement accuracy SMU and Test fixture Whenever possible it is good practice to measure the output of the SMU as well Nominal and actual output value can in fact differ of a small but non negligible quantity Introduction to error analysis JR Taylor 2 edition Ossila Ltd Copyright 2009 2014 30 enabling organic electronics Measurement Setting Generally a SMU allows the user to set the appropriate measurement range i e the maximum current that the SMU is expected to measure Currents exceeding the measurement range are not allowed and the SMU will therefore stop increasing the voltage and output a compliance warning Each measurement range corresponds to a different SMU circuitry optimised to generate and measure signal falling in the chosen range Usually the higher the range the lower the accuracy and the resolution of the SMU Therefore for a fixed range measurement of an I V sweep spanning several orders of magnitudes the accuracy of the OFF current is limited by th
14. ing time are specified in the SMU User s guide for any measurement range Note however that the SMU can require a longer time to settle if the load current is close to the upper limit of the chosen range For example for the National Instruments PXI 4132 the settling time of the SMU at 50 of the current load is 300 us per one volt change Let us suppose that the capacitive and resistivity component of the measurement apparatus and of the DUT are Cm Cour and Rm Rpur respectively and that a constant voltage Vo is applied at the time t 0 The voltage on the DUT will then obey the following equation Voum t 1 enc Eq A1 Ossila Ltd Copyright 2009 2014 31 enabling organic electronics Where Vpumlt is the voltage at the DUT at a time t gt 0 with R Rm Rov and C Cm Cour total resistance and capacitance of the measurement apparatus and DUT RC has the unit of time and is usually referred to as the time constant t of the system t RC If a voltage V is applied at t O Vouu t 0 63 V at t t It is good practice to allow several time constants t at least 4 5 before acquiring a measurement The table below shows the value of the voltage V at the DUT for 8 Number of RC a 7 2 different time constants 0 865 a 0 982 6 0 998 so 1 000 For the Ossila probe station with a two meter long coaxial cable typical values of resistance and Capacitance are Rm lt 1 Q and Cm lt 150 pF which gives
15. it current Data acquired with NI PXI 4132 indicative values only 100 tolerance is understood Values depend on the accuracy and resolution of the SMU measurement setting and EM environmental noise L Fd ay ed UT ta ht La A ocala 100 80 60 40 20 0 20 40 60 80 100 Ves V Ossila Ltd Copyright 2009 2014 10 enabling organic electronics FET Measurement 1 2 3 Using the Z position knob lower the vertical positioning stage Using the X position knob move the test board off the substrate holder Loosen the screws of the top pad gate contact 4 With the help of a pair of tweezers or a screwdriver gently incline the top pad contact and 5 slide the substrate into the substrate holder Make sure that the substrate fits perfectly in the rectangular holder and secure the electrical connection by tightly screwing the plastic screws Ossila Ltd Copyright 2009 2014 11 enabling organic electronics 6 Connect the gate inner and drain outer socket on the probe station to a double channel SMU or alternatively to two single channel SMUs using two shielded coaxial cables Switch the common ground to the ON position For FET measurements only if you are using long coaxial cables make sure that the drain and gate SMU grounds are at the same potential by avoiding using cables with different lengths and or specifications see Ground loop in High Accuracy Measurement section For fou
16. iving voltage larger than 1 V the error introduced by Seebeck is negligible On the contrary for low voltage driving potential lt 0 1 V the Seebeck effect can affect the measurement accuracy especially for study of the temperature dependence semiconductor properties such as the field effect mobility Such low voltage measurements are common for graphene based devices Temperature and Humidity The precision of the SMU is stated with respect to a specific calibration temperature to this respect the User is advised to carefully read the SMU manual and familiarise with the concept of temperature coefficient for current voltage sweep In general if the SMU temperature is more than five degree centigrade lower or higher than the factory specifications the User should correct the reading using the appropriate equation and temperature coefficients as stated in the SMU User s manual In addition the temperature effects are responsible for variations in the resistance of any resistor R used in the measurement setup Humidity can increase spurious current by reducing the dielectric capacitance and increasing leakage resistance Ossila Ltd Copyright 2009 2014 34 enabling organic electronics Ground Loop A ground loop is an unwanted potential difference between two points of the measurement set that are supposed to be at the same potential For example if the ground of the DUT and the ground of the SMU are not the same a ground loo
17. linear four point probe setup the current at the outer probes is 2l therefore the total resistance is ral 2I p E Since sheet resistance R is commonly defined as R t In z 7 and we find Geometrical Correction Factors Whilst the above equation is correct for a semi infinite and thin sheet deviation from either of these characteristics requires the use of a geometrical correction factor These correction factors can be looked up for a variety of shapes using tables in the following publications F M Smits Measurement of Sheet Resistivities with the Four Point Probe Bell Syst Tech J May 1958 p 711 and Haldor Topsoe Geometric Factors in Four Point Resistivity Measurement 1966 As long as the ratio between the film thickness and the inner probe thickness t s is less than 0 5 no correction factor needs to be applied to take account of the thickness of the sample For a larger ratio it is necessary to correct resistance using the table that can be found in the publications mentioned above For finite area sheets a geometrical correction factor is also required This factor depends upon the dimensions of the sample For circular samples with diameter d or rectangular samples of side d and dv d2 d and a probe with tip spacing s these can easily be looked up in the tables reported in the aforementioned papers Note that for the Ossila Four Point Probe s 1 27mm Ossila Ltd Copyright 2009
18. nderstood data measured using TeNMA 72 7730 Vichy VC6013 Resistance and capacitance values for the probe station board Total Drain Source Resistance 0 210 Gate Resistance Capacitance Drain 47 pF Capacitance Gate 37 pF Open circuit l V current Data acquired with NI PXI 4132 indicative values only 100 tolerance is understood Data depends on the accuracy and resolution of the SMU measurement setting and EM environmental noise CA PL w Eh Aah j h Ba i m 7 yay a a p Ze w alee i l4 a0 A 100 80 60 40 70 O0 20 40 60 80 100 Vos V Ossila Ltd Copyright 2009 2014 20 enabling organic electronics Transfer Characteristics Open circuit current Data acquired with NI PXI 4132 indicative values only 100 tolerance is understood Values depend on the accuracy and resolution of the SMU measurement setting and EM environmental noise los Ves a Can 150 100 80 60 40 270 0 20 40 60 80 100 Ves V Ossila Ltd Copyright 2009 2014 21 enabling organic electronics OFET Measurement 1 Screw the nylon spacer on the board and position secure it on a work bench far away from Electromagnetic and Electrostatic noise sources 2 Using low capacitance coaxial cables connected the gate and drain BNC connectors to the gate and drain channel of a multi channel SMU respectively or to two single channel SMUs 3 Remove the push fit lid and insert the substra
19. ough the board is shielded using a double ground plane the POGO pins are exposed It is therefore good practice to position the DUT at least 1 meter away from electrically charged body In addition to DC electrostatic noise AC noise due to PLC frequency and RF noise can degrade the accuracy of low current voltage measurement The double ground plane is designed to reject EMC noise however the source of intense AC noise such as pumps and mobile phone screens must be kept at least 1 meter away from SMU coaxial cables and DUT Test set up conditions Care must be taken to ensure that the bottom of the board is well isolated from any conductive surface both for safety reason and to minimize leakage currents To this purpose the POGO connections on the bottom of the board are covered with a conformal coating to minimize leakage currents due to grease particles and humidity Ossila Ltd Copyright 2009 2014 35
20. p will exist between the DUT and the SMU A ground loop can also be found between two SMUs grounded to the building ground usually there is a small difference of potential between any power sockets therefore two SMUs grounded to two different sockets can present a ground loop This potential difference generates a spurious current which in turn introduces an error in the measurement For OFET measurements make sure that the ground of the drain and the ground of the gate are at the same potential On the probe station and the manuals board both the gate and drain are grounded through the ground plane thus minimising any ground loops However if you use long BNC cables it is good practice to connect the ground low together In addition SMU with an internal ground LOW output independent of the building ground minimise ground loop DUT Experiment source Ground 1 Electrostatic Interference and RF A charged body such as human being or charged plastic object can interfere with the measurement apparatus and introduce errors of the order of nA in particularly when carrying out a high resistance measurement as the externally induced charge does not dissipate quickly These errors are usually negligible for current in the range of pA or larger however they can potentially disrupt the measurement of the low OFF transistor currents where the simple movement of a human hand close to the DUT can result in detectable spikes Alth
21. perate the switches or touch the probe station or OFET test board when voltage and or current are applied Caution To avoid damaging devices or equipment obey the following Avoid electrostatic discharge ESD as this may damage the device To avoid damage use Static discharge and prevention equipment where necessary Only use the device for the purposes intended described in this document Do not expose the device to any inappropriate cleaning fluids or solvents Follow good practice when setting up the test system Avoid placing mobile phones on top of the system as this can cause interference Ossila Ltd Copyright 2009 2014 enabling organic electronics EC Declaration of Conformity In line with directive 2004 108 EC of the European Parliament and of the Council and directive 2006 95 EC of the European Parliament and of the Council Manufacturer Name Ossila Limited Manufacturer Address Kroto Innovation Centre North Campus Broad Lane Sheffield S3 7HQ Item OFET Probe Station Low Density OFET Test Board High Density OFET Test Board Model number E451 E222 E481 Specifications of product under harmonised standards 2004 108 EC EN 61326 1 2006 Electrical equipment for measurement control and laboratory use EMC requirements Part 1 General requirements IEC 61326 1 2005 EN 61326 2 1 2006 Electrical equipment for measurement control and laboratory use EMC requirements Part 2 1 Particular req
22. r glass such as Au are prone to be damaged by the POGO unless extreme care is taken during the positioning of the probe on the electrodes The double ground plane of the manual board provides partial isolation from external Electromagnet noise However the User must consider that measurement of currents in the range of tens of nA or lower can still be easily affected by a number of factors including leakage non optimal measurement settings stray capacitive currents temperature variations and EM ES Electromagnetic Electrostatic noise Refer to the section High Accuracy Measurement for a brief description of the most common causes of measurement error and noise for I V measurements The switches insert switch type has been carefully selected to guarantee almost no resistance in the ON state with nominal infinite resistance when OFF Ossila Ltd Copyright 2009 2014 24 enabling organic electronics Technical Data Weight 81 g Size 100 mm x 100 mm Distance between pogos Drain to Source 17 8 0 3 mm Distance between pogos device to device 2 54 0 1 mm Pogo diameter 1 07 mm Substrates holder 15 mm x 20 mm length 21 44 mm gext 7 92 0 08 mm gint 4 34 0 08 mm Drain Source electrical contact gold coated spring loaded pogo RC values indicative values 100 tolerance is understood data measured using TeNMA 72 7730 Vichy VC6013 e 1 e Resistance and capacitance Gate 1 R 0 20 C 41 4 pF Dev 5
23. r probe measurements only do not ground the inner with the outer coaxial cables together Inner and outer probes must remain completely independent of each other during the acquisition 7 Using the X Y Z position micrometers move the pogo to contact the drain and source of the transistor to measure 8 During operation keep a safe distance from the probe station do not operate the switches when voltage and or current are applied to the probe station 9 Donotapply voltage in excess of 100 V or current in excess of 100 mA Ossila Ltd Copyright 2009 2014 12 enabling organic electronics 4 Point Probe The accuracy of resistance measurements can be affected by the spurious contribution of the test fixture used to connect the DUT with the SMU ohmeter cables crocodile clips switches POGOs etc and the resistance arising from the probe DUT electrical contact As a result Rm Rpur Re where Rn is the measured resistance while Roy and R are the resistance of the DUT and the test fixture respectively R is commonly referred to as contact resistance and represents all the measurement equipment dependent contributions while R is the intrinsic material value For low resistance material such as conductive thin film the value of R is no more negligible and therefore Rm should be corrected accordingly if a standard two point probe is used Four point probes allow accurate measuring of both sheet and bulk re
24. sily affected by a number of factors including leakage non optimal measurement settings stray capacitive currents temperature variations and EM ES Electromagnetic Electrostatic noise Refer to the section High Accuracy Measurement for a brief description of the most common causes of measurement error and noise for l V measurements Ossila Ltd Copyright 2009 2014 6 enabling organic electronics Ow Z positioning micrometer X positioning micrometer Y positioning micrometer Vertical positioning stage Z direction Substrates holder lateral and back electrical gate connections Top pad electrical gate connection top substrate holder Translation stage XY plane Interchangeable electronic probe board O CO NOW BWYN e Switches 10 BNC connectors Drain and Source Ossila Ltd Copyright 2009 2014 7 enabling organic electronics Interchangeable probe board Ossila Ltd Copyright 2009 2014 Tip spacing 1 27mm Probe Tip Diameter 1 07mm Tip spacing 2 45mm Probe Tip Diameter 1 07mm Tip spacing 18mm Probe Tip Diameter 1 07mm enabling organic electronics Technical Data Weight 1 9 kg Size 175x125x105mm XY travel 25 mm 0 5 mm accuracy Z travel 5 mm 0 5 mm accuracy Distance between pogos high density 2 54 0 15 mm Distance between pogos low density 18 0 3 mm Distance between pogos four probe 1
25. sistance by almost completely removing the contact resistance term from the measured value i e Rm Rouz even for very low Rm This is achieved by using an external 2 probe to deliver a current to the DUT while an inner 2 probe is used to measure the voltage across the material under test Since a very low current ideally zero flows across the inner probes the measured voltage drop AV is solely given by the material resistance with almost no contribution from the contact resistance see Fig 1 To justify this assertion it must be noted that in order for the contact resistance to manifest itself in the final measurement a current should be allowed to flow through the inner probe which is the typical working condition for a 2 point probe This current will result in a potential drop AV I R across the probe which in turn will increase the measured voltage drop AV by the quantity AV or AV m AVpyt AV and the measured resistance can then be written as Rm Rout Re However with no current flowing through the inner probe AV AVpuz and therefore R Rout 4 point probes come with tips arranged either in a square or linearly fashion the Ossila probe system uses the latter arrangement see Fig 3 Figure 1 Circuit diagram of a linear four point probe Ossila Ltd Copyright 2009 2014 13 enabling organic electronics 4 Point Probe Equations Sheet Resistance The sheet resistance R of a 4 point probe is gi
26. te into the bracket Make sure that the gate and drain contact pads are facing the drain and gate POGO pin Ossila Ltd Copyright 2009 2014 22 enabling organic electronics 4 Secure the substrate with the push fit lid and make sure that both left and side gate switches are ON 5 Set the drain and source switches of the devices to measure to the ON position and make sure that all the other drain sources switches are OFF 6 Run the measurement of choice During operation keep a safe distance from the test board do not operate the switches when voltage and or current are applied to the test board 7 Do not apply voltage in excess of 100 V or current in excess of 100 mA 8 Refer to the High Accuracy Measurement of this section for further information on safety and measurement accuracy Ossila Ltd Copyright 2009 2014 23 enabling organic electronics High density Manual Board Overview The high density manual test board is designed to provide an easy to use and low noise electromechanical interface to measure high density Ossila OFETs 20 devices per substrates or any other design compatible with the electrode layout The gold coated spring loaded POGO allows for non destructive low resistance connection with the DUT Note The manual board has been tested to allow for non destructive contact in case of metal adhesion layer electrodes only Au Cr Pt Ti etc Metal with poor adhesion properties on silicon o
27. the actual potential drop across the channel As shown in Fig 4 this is achieved by depositing an extra pair of electrodes which probe the potential in between the drain and source The voltage Va is therefore independent of the electrodes semiconductor injection barrier Refers to Organic Field Effect Transistor by Bao and Locklin Eds chapter 2 1 Ossila Ltd Copyright 2009 2014 17 enabling organic electronics Bao and J Locklin Organic Field Effect transistors 2007 CRC Press Redraw Probe station maintenance Store the probe station in a dry and clean environment Regularly clean the substrates holder gate connections POGO pin and BNC connection using DI water and or clean IPA Do not use normal water or any electrolytic solution Make sure the probe station is completely dry before using as the liquid trapped inside the connectors and switches can cause a significant amount of leakage current and constitute a major safety hazard Ossila Ltd Copyright 2009 2014 18 enabling organic electronics Low Density OFET Test Board Overview The low density OFET test board is designed to provide an easy to use and low noise electromechanical interface to measure low density Ossila OFETs or any other design compatible with the electrode layout The gold coated spring loaded POGO allows for non destructive low resistance connection with the DUT Note The manual board has been tested to allow for non des
28. tructive contact in case of metal adhesion layer electrodes only Au Cr Pt Ti etc Metal with poor adhesion properties on silicon or glass such as Au are prone to be damaged by the POGO unless extreme care is taken during the positioning of the probe on the electrodes The double ground plane of the manual board provides partial isolation from external Electromagnet noise However the User must consider that measurement of currents in the range of tens of nA or lower can still be easily affected by a number of factors including leakage non optimal measurement settings stray capacitive currents temperature variations and EM ES Electromagnetic Electrostatic noise Refer to the section High Accuracy Measurement for a brief description of the most common causes of measurement error and noise for l V measurements The switches insert switch type has been carefully selected to guarantee almost no resistance in the ON state with nominal infinite resistance when OFF Ossila Ltd Copyright 2009 2014 19 enabling organic electronics Technical Data Weight 81 g Size 100 mm x 100 mm Distance between pogos Drain to Source 17 8 0 3 mm Distance between pogos device to device 2 54 0 1 mm Pogo diameter 1 07 mm Substrates holder 15 mm x 20 mm length 21 44 mm gext 7 92 0 08 mm gint 4 34 0 08 mm Drain Source electrical contact gold coated spring loaded pogo RC values indicative values 100 tolerance is u
29. uirements Test configurations operational conditions and performance criteria for sensitive test and measurement equipment for EMC unprotected applications IEC 61326 2 1 2005 2006 95 EC EN 61010 1 2010 Safety requirements for electrical equipment for measurement control and laboratory use Part 1 General requirements IEC 61010 1 2010 EN 61010 2 030 2010 Safety requirements for electrical equipment for measurement control and laboratory use Part 2 030 Particular requirements for testing and measuring circuits IEC 61010 2 030 2010 EN 61140 2002 Protection against electric shock Common aspects for installation and equipment IEC 61140 2001 EN 61187 1994 Electrical and electronic measuring equipment Documentation IEC 61187 1993 Modified Ossila Ltd Copyright 2009 2014 4 enabling organic electronics EN 61010 2 081 2002 Safety requirements for electrical equipment for measurement control and laboratory use Part 2 081 Particular requirements for automatic and semi automatic laboratory equipment for analysis and other purposes IEC 61010 2 081 2001 Declaration hereby declare that the equipment named above has been designed to comply with the relevant sections of the above referenced specifications The unit complies with all applicable Essential Requirements of the Directives Signed Name Dr James Kingsley Date 07 05 2014 Ossila Ltd Copyright 2009 2014 5 enabling organic electronics
30. ven by r ra with V and voltage and current read form the SMU measurement Demonstration The resistivity p and resistance R are related by A pa where A is the cross sectional area of the sample through which the current is flowing while is the length along which it flows The differential resistance dR is thus given by Eq1 In the following discussion we assume homogenous and isotropic material i e material for which the resistivity 9 constant across the material If these conditions are fulfilled a carrier current injected in a bulk sample through the POGO needle travels in a isotropic manner across the conductors as a result the propagation front can be modelled as a hemisphere of area A given by 27r For a thin film sample of thickness t the propagation front of the current is a ring of area A 2rtrt with r distance between the propagating rings and the probe Figure 2 Current emanating from a tip travels through a thin sample in rings Ossila Ltd Copyright 2009 2014 14 enabling organic electronics To calculate the resistance in the case of a linear probe set up it is convenient to set the position of the tip injecting the current at 0 and the two inner probes at s and 2s respectively Since the voltage is sampled between the two inner probes the integration of the differential resistance dR Eq 1 over the inner tip distance s gives the total resistance R Ref d E n In 2 For a
31. wn in the centre of the sample area as far as possible from the sample edge With a rectangular sample this means aligning the probes along the longer side of the sample Figure 3 Alignment of probes should ensure that tips are as far as possible from the edge of the sample A 2 channel source measure unit is required to operate the probe The outer probes should be connected to one channel set to deliver current whilst the inner probes should be connected to the second channel set to sense voltage We have found good results using a magnitude of current which corresponds to measuring a voltage in the 10 100mV region Ensure that the probe tips have fully engaged with the sample before readings are taken and it is worth zeroing your voltage reading before each measurement after the probes have contacted the sample and before the current is applied Care should also be taken to ensure that the current channel reads the correct current if the current reading is lower than applied value it may be indicative of a poor contact and the readings should not be trusted OFET Mobility with 4 point probe For OFET mobility measurement the injection barrier caused by the misalignment of the electrode workfunction with the HOMO LUMO level of the organic semiconductor can lead to underestimation of the mobility Four probe measurement combined with two appropriate channel design can alleviate this source of errors by allowing a direct measurement of
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