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Chapter 3 System Design Considerations

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1. are ideally suited to this type of measurement because they provide parallel reference and measurement paths which are offset vertically by 19 mm 0 750 inch For such an application a user supplied reference plane mirror is required in addition to the measurement reflector on the X Y stage The Agilent 10715A interferometer shown in Figure 7G 1 also permits differential measurements between two plane mirrors However instead of having an offset spacing as in the Agilent 10719A or Agilent 10721A the Agilent 10715A permits the reference beams and the measurement beams to be aligned essentially coaxially A spedially shaped reference plane mirror is supplied with the Agilent 10715A Customized differential configurations are possible with several other interferometers However considerable care should be exercised during design and layout to avoid introduction of alignment errors thermal or mechanical instabilities and potential deadpath problems When making differential measurements both reflectors reference and measurement should be of the same type cube corner or plane mirror this minimizes thermal drift problems with ambient temperature changes User s Manual 3 21 Chapter 3 System Design Considerations Optics To use the Agilent 10702A Agilent 10705A or Agilent 10766A in a differential configuration the reference cube corner can simply be detached from the interferometer housing and attached to the reference surface
2. 10715A Agilent 10700A 6796 path Agilent 10701A Agilent 10715A Agilent 10700A 33 path Agilent 10707A Agilent 10717A Assuming a minimum laser power of 120 microwatts you can calculate the worst case power at the X Y and Wavelength Tracker receivers by multiplying the product of component efficiencies by thelaser output power Power at X 0 61 x0 39 x 0 25 x 120 27 1 Power at Y 0 61 x0 39 x 0 25 x 120 27 1 Power at WT 0 27 x 0 98 x 0 25 x120 7 9 This system has a power safety factor of 4 7 at worst case based on use of the Agilent 10780C Receiver which requires 1 5 microwatts for each axis resulting in reliable operation and easy alignment 3 26 User s Manual Chapter 3 System Design Considerations Beam Path Loss Computation You can also calculate this safety factor using the typical optical efficiency values listed in the following table Axis component efficiencies typical Component Component Efficiencies Typical Agilent 10700A 67 path Agilent 10701A Agilent 10715A Agilent 10700A 67 path Agilent 10701A Agilent 10715A Agilent 10700A 33 path Agilent 10707A Agilent 10717A Using thetypical laser power of 400 microwatts you can calculate the typical power at the X Y and Wavelength Tracker receivers by multiplying the product of each component efficiency by the laser output power for each axis Power at X 0 63 x0 45 x 0 36 x 400 240 8 Power at Y 0 63 x0 45 x 0 3
3. Receiver Splitter Note Beam from laser passes under the receivers The return beams return to the receivers after being offset in the interferometers Figure 3 12 Two axis system using two Agilent 10715A differential interferometers YAW MEASUREMENT OF X Y STAGE Agilent 10706A Plane Mirror Interferometer X Y STAGE YAW U Y Axis Receiver Agilent 10706A Plane Mirror Interferometer Agilent 10701A 50 Beam Splitter EE A wai s Y Axis A 67 Receiver MIRRORS n AA Agilent 10700A 33 L1 Agilent 10706A 33 Beam Splitter gt Plane Mirror Interferometer A X Axis Receiver Laser Figure 3 13 Yaw measurement of x y stage using discrete plane mirror interferometers 3 36 User s Manual Chapter 3 System Design Considerations Example Configurations Theresulting angular measurement will only be as accurate as the measurement distance D However even if D is not known precisely this technique can provide extremely high resolution or relative angular changes The resolution depends on D and with electronic resolution extension can be well under 0 01 arc seconds For applications in which the stage is servo controlled to its initial angle THETA 0 this high resolution is the key measurement consideration and the accuracy of D is not critical For applications in which accuracy and resolution are both critical D may be determined precisely by rotating the stage through a known
4. Splitter no housing Agilent 10726A Beam Bender no housing Agilent N1203C Precision Beam Translator Agilent N1204C Precision Horizontal Beam Bender User s Manual 3 5 Chapter 3 System Design Considerations Determining What Equipment is Needed Table 3 1 Equipment choices Continued Component Comment s Beam Directing Optics Continued Agilent N1207C Measurement Optics Precision Vertical Beam Bender One Interferometer plus Reflector pair required per axis Agilent 10702A Agilent 10702A 001 Agilent 10703A Agilent 10704A Agilent 10705A Agilent 10706A Agilent 10706B Agilent 10715A Agilent 10715A 001 Agilent 10716A Agilent 10716A 001 Agilent 10717A Agilent 10719A Agilent 10721A Agilent 10724A Agilent 10735A Agilent 10736A Agilent 10736A 001 Agilent 10737L Agilent 10737R Agilent 10766A Agilent 10767A Agilent 10767B Agilent 10770A Agilent 10771A Agilent 10774A Agilent 10775A 3 6 Linear Interferometer Same as above but with wedge windows required if interferometer is the moving component Reflector paired with Agilent 10702A Reflector paired with Agilent 10705A Single Beam Interferometer Plane Mirror Interferometer High Stability Plane Mirror Interferometer Differential Interferometer Differential Interferometer turned configuration High Resolution Interferometer High Resolution Interferometer turned configuration Wavelength Tracker
5. The Agilent E1709A receiver was designed for systems that have more than six measurement axes Minimum laser output power is 120 microwatts for the laser heads Thetypical laser output power is about 400 microwatts The output power is relatively constant over the life of the tube and tends to drop off immediately at the end Higher laser output power is available upon request User s Manual 3 23 Chapter 3 System Design Considerations Beam Path Loss Computation INTRODUCING AN OFFSET A Offset Only pe To Other Optics From Laser Head gt B Offset Plus Direction Change To Other Optics lt 90 90 From Laser Head p Figure 3 6 Introducing an offset into the laser beam e Minimum required power at the Agilent 10780C Receiver is 1 5 microwatts The Agilent 10780F Remote Receiver and Agilent E 1708A Remote Dynamic Receiver require 2 2 microwatts with its standard 2 meter fiber optic cable more with longer cables The Agilent E 1709A Remote High P erformance Receiver requires a minimum of 0 20 to 0 80 microwatts depending on the AC DC ratio with standard 2 meter plastic fiber optic cable Adjustment of the receiver s gain is required to obtain this sensitivity See the alignment and gain adjustment procedures in Chapter 8 Receivers of this manual 3 24 User s Manual Chapter 3 System Design Considerations Beam Path Loss Computation The beam splitters have worst case as well as typical tra
6. can be used Since either the interferometer or the reflector is moving during the measurement protecting the laser beam and the moving components requires a telescoping cover or a cover that is self sealing A wide variety of commercially available protective covers are suitable for this purpose User s Manual 3 11 Chapter 3 System Design Considerations Adjustment Considerations Figure 3 2 illustrates techniques for protecting the laser beam and optical components with different types of protective covering Note that the cover for the retroreflector allows the retroreflector to be moved very close to the interferometer This helps minimize the deadpath errors Chapter 15 Accuracy and Repeatability in this manual has more details on minimizing deadpath PROTECTIVE COVERS Protective Cover Moving XV Interferometer a 20 xc Xo lt gt x 3 1 Se lt Fixed Conduit SS A AW For Laser Beam Fixed Interferometer Protective Cover Receiver To Electronics Laser Beam Moving Interferometer Flexible Seal Protective Cover Sl e Machine Slide 3 1 Fixed Table VIEW A Figure 3 2 Protective covers for optics and laser beam 3 12 User s Manual Chapter 3 System Design Considerations Adjustment Considerations Figure 3 3 shows a different type of protective cover Again the mechanical arrangement allows the retroreflector to be close to the interferometer at the closest point of
7. measure all linear motions that is 3 degrees of the 18 degrees of freedom defined in the Glossary Small angle measurements may be made by multiple measurements on the same axis The measurement system is relatively insensitive to all other motions as briefly described below See Figure 3 1 3 2 User s Manual Chapter 3 System Design Considerations Introduction POSSIBLE COMPONENT MOTIONS Laser Head Retroreflector Interferometer Receiver 2 Yaw x cx lt ec Figure 3 1 Possible component motions 1 6 Motion of the receiver or laser head along the beam path X has no effect on the measurement since both f4 and fp would exhibit Doppler shift Small motions of the laser head receiver interferometer or retroreflector in a direction perpendicular to the beam path Y or Z have no effect on the measurement The only restriction is that sufficient light returns to the receiver Angular motion pitch or yaw of the laser head about the Z or Y axis has the effects described below a It introduces a measurement error cosine error b It may displace the laser beam so that insufficient light returns to operate the receiver Although the laser head or the receiver may be rotated in 90 increments about the beam axis roll other roll deviations from the four optimum positions degrade the measurement signal If either the laser head or receiver is rotated 45 about the beam axis all position infor
8. 0F receiver mounting is isolated from ground by using the nylon screws supplied A system using VME electronics Agilent 10898A Agilent 10897B and Agilent 10895A axis boards PC electronics Agilent 10885A axis board or PCI electronics N 1231A axis board should be grounded through the electronics power line 3 14 User s Manual Chapter 3 System Design Considerations Laser Head Laser Head Orientation An Agilent laser head may be mounted in any orientation as long as it is positioned to direct the beam into the optical system parallel to or orthogonal with the machine axes being measured See Chapter 5 Laser Heads in of this manual for more information about laser head orientation Mounting plane tolerance The plane defined by the three mounting feet on the laser head must be parallel to either the bottom or sides of the beam splitters and of the beam bender housings to within 3 and to the bottom or sides interferometers to within 11 This ensures that the polarization axes of the interferometers are oriented properly relative to the polarization vectors of the laser beam Figure 3 4 The laser head can be rotated in 90 increments about the beam axis roll without affecting the system performance but the measurement direction sense will change with each 90 rotation Allow 50 mm 2 inches clearance around the laser head for easy servicing Allow at least 100 mm 4 inches clearance at the back of the las
9. 10726A Beam splitters are supplied by Agilent without mounting hardware When you attach these optical pieces to their mounting hardware use an attachment method that will not damage or distort them AGILENT 5517C 009 MOUNTING LOCATION Nd Pin 4004 0 008 1 Jo o1 A 1 024 x 1 575 Pin 4 004 9 008 Ld a 170 077 A 0 1576 9 pia k B 0 0003 5 47 mm A 0 1576 9003 Dia A 1 850 4 lt 4 A Beo 3xp 46 1100 THRU i G o 3 w A B c a 26 X 40 pote cumin 1 024 X 1 575 rOH ve A 7 087 0 020 189 mm 26 X 40 7 441 4 024 X 1 575 0 0 2X 5 1 000 e 0 0 197 5 039 A 0 05 0 002 Section A A Notes 1 Dowel Pin steel 04 004 4 012 2 Places 2 Dimensioned in Accordance with ANSI Y14 5M 1982 Figure 3 16 Agilent 5517C 009 Mounting Location Dimensions Site preparation for referenced interferometers Referenced interferometers currently available from Agilent are listed in the following table User s Manual 3 41 Chapter 3 System Design Considerations Site Preparation For information about See subchapter Agilent 10719A One Axis Differential Interferometer Agilent 10721A Two Axis Differential Interferometer Agilent 10735A Three Axis Interferometer Agilent 10736A Three Axis Interferometer Agilent 10736A Option 001 Three Axis Interferometer with Beam Bender The opti
10. 6 x 400 240 8 Power at WT 0 30 x 0 99 x 0 36 x 400 42 8 By using the typical efficiencies of the component a safety factor greater than 28 is achieved based on use of the Agilent 10780C Receiver which requires 1 5 microwatts The Agilent 10780F 2 2 microwatts Agilent E 1708A 2 2 microwatts and Agilent E 1709A 0 20 to 0 80 microwatts receivers are more sensitive Hence can operate with more axes User s Manual 3 27 CAUTION Chapter 3 System Design Considerations Receivers Receivers General When determining the receiver mounting locations and positions keep the following points in mind 1 Ata 45 position roll the signal will go to zero 2 Thereceiver typically dissipates 2 0 Watts with a maximum dissipation of 2 7 Watts Plastic pads keep an air gap around the receiver and act as thermal and electrical isolators Use nylon screws only Agilent 2360 0369 The receiver housing must be electrically isolated from the mounting fixture 3 The remote sensor in the Agilent 10780F Remote Receiver Agilent E1708A Remote Dynamic Reciever and Agilent E1709A Remoter High Performance Reciever does not dissipate any power The remote sensor does not require a nylon screw 4 Allow a 5 cm space at the rear of each receiver housing for each cable connection 5 The fiber optic sensor head of the Agilent 10780F E1708A and E 1709A receivers may be mounted directly to certain interferometers Agilent 10719A Agile
11. Mirror Agilent 10772A Turning Mirror Agilent 10773A Flatness Mirror High Performance Laser Head Cable for Agilent 5517B C D Laser Head used with the Agilent 10897B and 10898A VME Axis boards and N1231A PCI Axis board cable has a DIN connector for connecting to the Agilent 10884A Power Supply to provide power to the laser head one cable per system Agilent N1251A 3 meters 9 8 feet Agilent N1251B 7 meters 23 0 feet High Performance Receiver Cables for use with Agilent 10897B and 10898A VME Axis boards and N1231A PCI Axis board one cable per receiver Agilent N1250A 5 meters 16 4 feet Agilent N1250B 10 meters 32 8 feet 3 8 User s Manual Chapter 3 System Design Considerations Electronic Components Electronic Components Transducer Systems There arethree different types of electronics for Agilent laser transducer systems These electronics use different backplanes and have different performance and outputs Full details are given in the appropriate electronics system manuals PC Based Electronics The Agilent 10885A PC Axis Board is compatible with PC ISA backplanes Up tosix Agilent 10885As may be used in a single system VME Compatible Electronics The Agilent 10898A High Resolution VM E bus Dual Laser Axis Board Agilent 10897B High Resolution VMEbus Laser Axis Board and Agilent 10895A VME bus Laser Axis Board are compatible with VME backplanes The Agilent 10896B VME Laser Compensatio
12. System Design Considerations Chapter 3 System Design Considerations Introduction Introduction Although there are many possible configurations of the laser and optics all Agilent laser measurement systems have these basic parts in common A laser source to produce the two optical frequencies f and fz and generate the reference signal In discussions in this manual f is the lower frequency and f is the higher e Beam directing optics to direct all or part of the laser beam to each measurement axis of the system using right angle bends Measurement optics to separate the two optical frequencies direct them over the reference and measurement paths and recombine them Onereceiver per measurement axis to detect the difference in optical frequencies and produce the measurement signal for that axis Electronics to convert the measurement and reference signals into displacement data Twoimportant characteristics of Agilent interferometers must be emphasized e Only the change in relative position of the optics is detected Either optical component may move as long as optical alignment is maintained If the interferometer is fixed and the retroreflector is the moving component toward or away from the interferometer motion with respect toits original position is detected Conversely if the retroreflector is fixed the interferometer can bethe moving component Agilent laser position transducers can detect and
13. angle THETA and solving the above equation for D When installing this type of yaw measuring system take careto ensure the parallelism of the adjacent linear measurements to minimize cosine errors Angular rotation of the measurement mirror is limited tothe Alignment Requirement vs Distance value for the interferometer used See the Specifications section of subchapter 7C Plane Mirror Interferometer in this manual When yaw control of a stage must be done at high speeds using a closed loop control system the Y Y value needs to be obtained quickly If the difference is calculated in software in the controller it may be too slow There are two methods to achieve a high speed Y Y output Electronically e Optically Electronic yaw calculation method This difference calculation can be done in hardware for both the Y and the Y axes A custom servo board could be designed to accept position information from both Y and Y and perform a fast angular calculation yielding an input for the yaw servo Seethe appropriate electronics documentation for servo loop interfacing Optical yaw calculation method There are optical configurations that will allow direct output of the difference between Y and Y for example on the Y axis receiver This is shown in Figures 3 14 and 3 15 both using the Agilent 10706A Plane Mirror Interferometer User s Manual 3 37 Chapter 3 System Design Considerations Example Configuration
14. cs Agilent 10719A Agilent 10721A Agilent 10735A Agilent 10736A elsewhere in this manual Other optics require you to fabricate your own mounts In general the alignment procedures are performed with all optical components in place Your measurement system design should allow for adjustment of the laser optics and receivers during alignment Laser beam and optics protection The laser measurement system requires protection against unintentional laser beam blockage and air turbulence problems In some applications such as machine tools protection should be provided to prevent metal chips or cutting fluid from interfering with the measurements Also the optical components usually require protection to prevent contamination of the optical surfaces by oil or cutting fluid In applications which are considered clean protection may not be needed If protection of the laser beam and optical components is required there are two general types moving component protection and stationary component protection In many applications the onl y moving component is theinterferometer or the reflector Many of the beam benders are stationary and only direct the laser beam to the measurement axis n these cases it is only necessary to provide fixed tubing for the laser beam and some type of sealed enclosure for the optics Since only onelaser beam of approxi mately 6 mm 0 24 inch in diameter is involved relatively small diameter tubing
15. cs in a referenced interferometer are referenced to points on the outside of the case This allows the interferometer to be installed in a predefined position and minimizes any alignment required with respect to the measurement mirror s used with it Refer to Chapter 7 Measurement Optics in this manual for information that can help you design the mounting location for an Agilent referenced interferometer Product specifications and descriptions in this document subject to change without notice Copyright C 2002 Agilent Technologies Printed in U S A 07 02 This is a chapter from the manual titled Laser and Optics User s Manual For complete manual order Paper version p n 05517 90045 CD version p n 05517 90063 This chapter is p n 05517 90103 3 42 User s Manual
16. eams travel to external mirrors Any motion of the interferometer itself that is common to both beams will not appear as a measurement Of course any vibration between the reference and measurement mirrors will constitute real measurable displacements Adjustable mounts for optics The optical elements inside several of the Agilent laser measurement system optics are not precisely referenced to their housings In most applications involving these optics a few simple alignments during system installation can usually provide equal or better alignment than referencing the optics to their housings Therefore slight positioning adjustments of the unreferenced interferometers beam splitters and beam benders are needed for proper system alignment User s Manual 3 19 Chapter 3 System Design Considerations Optics Positioning adjustments for most optics can be provided by using Agilent 10710B or Agilent 10711A Adjustable Mounts as appropriate These mounts allow adjustment of pitch and yaw of any attached optic Roll adjustment is typically not required and can usually be avoided by careful optical system layout For a listing of which Adjustable Mount supports which optic see the Chapter 9 Accessories in this manual In some applications referenced housings can provide significant advantages For example the alignment requirements for certain multiaxis applications can be difficult or impossible to achieve without referenced hou
17. er head for cable connections User s Manual 3 15 Chapter 3 System Design Considerations Laser Head LASER POSITION TRANSDUCER MOUNTING Beam Bender Figure 3 4 Laser position transducer mounting Pointing stability Tomaintain good pointing stability it is good practiceto use kinematic mounting principles Refer to Chapter 5 Laser Heads in this manual for more information about laser head pointing stability Thermal isolation Because there is some heat dissipation from the laser heads you should choose the mounting method and location with care Where possi ble mount the laser head away from the measuring area to avoid any thermal effects Vibration isolation Since the system measures only the relative motion between the interferometer and reflector measurements are not affected by vibration along the beam axis of the laser source or the receiver 3 16 User s Manual Chapter 3 System Design Considerations Optics When vibration of the laser head causes displacement of the beam perpendicular to beam axis at an interferometer or receiver the beam signal power can fluctuate If this fluctuation is too great insufficient beam signal will arrive at the receiver causing a measurement signal error Magnetic shielding Agilent laser heads contain a permanent magnet When installing an Agilent laser measurement system in an application sensitive to magnetic fields shielding around thelaser head may be requ
18. g Agilent 10790C 20 meters long Receiver Cables for use with Agilent 10885A PC Axis Board or Agilent N1231A PCI Three Axis Board one cable per receiver Agilent 10880A 5 meters long Agilent 10880B 10 meters long Agilent 10880C 20 meters long Laser Head Cables for Agilent 5517A B C D Laser Head used with Agilent 10885A 10889B or N1231A axis boards cable has a DIN connector for connecting to the Agilent 10884A Power Supply to provide power to the laser head one cable per system Agilent 10881A 3 meters long Agilent 10881B 7 meters long Agilent 10881C 20 meters long Laser Head Cables for Agilent 5517A B C D Laser Head used with Agilent 10885A 10889B or N1231A axis boards cable has spade lugs for connection to a power supply to provide power to the laser head one cable per system Agilent 10881D 3 meters long Agilent 10881E 7 meters long Agilent 10881F 20 meters long Laser Head Cables for Agilent 5519A B Laser Head used with Agilent 10887P Programmable PC Calibrator Board in the Agilent 5529A system one cable per system Agilent 10882A 3 meters long Agilent 10882B 7 meters long Agilent 10882C 20 meters long User s Manual 3 7 Chapter 3 System Design Considerations Determining What Equipment is Needed Table 3 1 Equipment choices Continued Component Comment s Accessory Reflectors Order as required for your application Agilent 10728A Plane Mirror Agilent 10769A Beam Steering
19. iameter The 9 mm lens can replace the 6 mm lens if replacement becomes necessary be sure to order the 9 mm Alignment Target also The standard Agilent 10780C input aperture is designed for use with a 6 mm laser beam soit is not recommended for use in a 9 mm laser system Site Preparation Site preparation for laser head Generally Agilent laser heads require no special site preparation other than providing appropriate mounting holes The Agilent 5517C 009 Laser Head s laser beam output is referenced to locations on its base You can install this laser head simply by providing appropriate mounting holes or you can create a specially prepared site to take advantage of its referenced output capability specifications for a site for this latter use are given in Figure 3 16 Site preparation for optical devices Beam Benders such as the Agilent 10726A are used to create the laser path from the laser head tothe interferometer The Agilent 10726A Beam Benders are supplied by Agilent without mounting hardware When you attach these optical pieces to their mounting hardware use an attachment method that will not damage or distort them 3 40 User s Manual Chapter 3 System Design Considerations Site Preparation In a measurement system having morethan one interferometer unit a Beam Splitter such as the Agilent 10725A is used to create a second laser path to deliver the laser beam from the laser head to the second interferometer Agilent
20. ility before troubleshooting other parts of the system The Agilent 10735A and Agilent 10736A interferometers are designed to use a 9 mm nominal diameter laser beam The required 9 mm beam is available from an Agilent 5517C 009 laser head The laser tube in this laser head is referenced to the base of the laser head Thelaser head baseis different from that of the standard Agilent 5517C Laser Head and requires a special mounting site confi guration as shown in Figure 3 16 User s Manual 3 39 Chapter 3 System Design Considerations Site Preparation The standard Agilent beam directing optics are designed for use with a 6 mm maximum nominal diameter laser beam For usein 9 mm installations Agilent offers the Agilent 10725A 9 mm L aser Beam Splitter and the Agilent 10726A 9 mm Laser Beam Bender These two optical devices do not include mounting hardware The 9 mm laser measurement system user designer or installer must devisea mounting method that will hold the required optics in position without causing stress that may distort the optic The recommended receiver for 9 mm work is an Agilent 10780F Remote Receiver with a 9 mm lens on the fiber optic cable input If you have an Agilent 10780F Remote Receiver with a 6 mm lens you can order a 9 mm Replacement Lens Kit Assembly Agilent part number 10780 67003 and a 9 mm Alignment Target Agilent part number 10780 40009 The 9 mm lens can be used with any laser beam having a smaller d
21. ir and material temperature sensors and your PC The boards convert the analog electrical voltages from the sensors to digital forms that the PC uses to calculate the compensation factors These factors adjust for changes in the systems operating environments Typical sensors used with each Agilent 10886A PC Compensation board are the Agilent 10751C D Air Sensor and one to three Agilent 10757D E F Material Temperature sensors The Agilent 10887B PC Calibrator Board enablethe PC to perform laser calibrator related functions with the Agilent 5529A calibrator software An Agilent Two Axis 5529A 5529A Dynamic Calibrator and an Agilent 55292A USB Expansion Module are also available The USB software hosts up to five axes on one computer 3 10 User s Manual Chapter 3 System Design Considerations Adjustment Considerations Adjustment Considerations In general when aligning the Agilent optics it will be necessary to adjust most or all of the optical components Most optics are not referenced to their housings since simple adjustments by the user can usually provide opti mum alignment The Agilent 10710B and Agilent 10711A Adjustable Mounts should be used to provide the adjustment capability for most optical components There are a few exceptions however Certain optics designed for multiaxis systems provide referenced housings Installation and alignment of these optics depends on the optic refer to specific instructions for these opti
22. ired Mounting See Chapter 5 Laser Heads in this manual for laser head installation and mounting instructions The laser source in Agilent 5517C 009 9 mm Laser Head is referenced to locations on the outside of the laser head allowing the laser head to beinstalled in a predefined mounting location minimizing the need for laser head alignment A diagram of the mounting location for this laser head is presented in Figure 3 16 Optics Plane of orientation with respect to laser head The mounting plane tolerance of the optics to the laser head is the same as discussed in the paragraph titled Mounting plane tolerance above That is the bottom or sides of the interferometers should be parallel to within xl of the plane defined by the laser head s three mounting feet Effect of optics on measurement direction sense The orientation and configuration of the interferometers affects the measurement direction sense The direction sense depends on which frequency is in the measurement path of the interferometer F or example if f4 lower frequency is in the measurement path and f higher frequency is in the reference path and the optics are moving away from each other the fringe counts will be INCREASING This corresponds to using an Agilent 5517A Agilent 5517B or Agilent 5517C Laser H ead mounting feet in horizontal plane with an Agilent 10702A Linear Interferometer mounted with labels facing up and down see Figure 3 5 I
23. lable through Agilent Technologies n both figures a small 5096 non polarizing beam splitter is required This beam splitter must be very small to avoid blocking or clipping the adjacent beam This is also true for the beam bender required in the configuration shown in Figure 3 15 Multiaxis systems using Agilent 10719A and Agilent 10721A inteferometers Multiaxis systems using Agilent 10719A and Agilent 10721A interferometers are described in subchapter 7J Agilent 10719A One Axis I nterferometer of this manual Multiaxis systems using Agilent 10735A and Agilent 10736A three axis inteferometers Multiaxis systems using Agilent 10735A and Agilent 10736A interferometers are described in subchapter 7N Agilent 10735A 10736A and 10736A 001 Three Axis I nterferometers of this manual Optical Device Troubleshooting Problems with the optical devices are usually caused by their misalignment Refer to the alignment procedures in Chapter 4 System Installation and Alignment of this manual for further information Air turbulence caused by ventilation equipment or temperature gradients near the laser beam path can also cause measurement problems If this is suspected shield the area around the laser beam and optical devices with cardboard tubing plastic sheet or other suitable material Some problems with sporadic counting and drift can be traced to air turbulence around the measurement path This should be considered as a possib
24. mation will be lost because the receiver will not be able to distinguish between the two frequencies Angular motion of the receiver about the Y or Z axis has no effect on the measurement within alignment limits specified for the receiver Receiver specifications are given in Chapter 8 Receivers of this manual Angular motions of the interferometer and retroreflector depend on the particular components for limitations User s Manual 3 3 Chapter 3 System Design Considerations Accuracy Considerations Accuracy Considerations Several factors outside the laser measurement system can affect system accuracy These factors the measurement environment machine and material temperature and the optics installation and their interrelationships must be understood in order to predict the performance of the system Detailed descriptions and methods of compensation are given in Chapter 15 Accuracy and Repeatability of this manual Generally Agilent laser measurement systems offer automatic compensation for air environments and also for temperature changes of the work material For a temperature controlled environment 20 30 5 C typical system accuracy using air sensor automatic compensation is 1 5 ppm Using the Agilent 10717A Wavelength Tracker for compensation the measurement repeatability is on the order of 30 2 ppm depending on the environment Determining What Equipment is Needed First sketch out your optical configurati
25. n Board is also compatible with VME backplanes and works with the Agilent 10895A Up to six Agilent 10895As and several Agilent 10896As up to one for each Agilent 10895A may be used in a single system PC Based PCI Electronics The Agilent N 1231A PCI Three Axis Board is optimized for connection to a PMAC mation control system from Delta Tau It is a full size Universal 3 3V and 5 0V signaling compatibility 32 bit 33 MHz PCI Rev 2 2 compliant card for use in PC compatible controllers as part of an Agilent laser interferometry position measurement system User s Manual 3 9 Chapter 3 System Design Considerations Electronic Components Calibrator System Electronics Agilent 5529A Dynamic Calibrator The Agilent 5529A Dynamic Calibrator is a laser system used to ensure the accuracy of a machine s motion and positioning Controlled through your PC with Microsoft Windows installed the system is able to collect and analyze measurement data for a number of measurements The Agilent 5529A Dynamic Calibrator typically indudes the following electronic components e Agilent 5519A B Laser Head e Agilent 10886A PC Compensation board optional for automatic compensati on Agilent 10887B PC Calibrator Board e Agilent 10751C D Air sensor and Agilent 10757D E F Material Temperature sensor s optional as required e Agilent 10888A Remote Control units optional The PC compensation boards provide the interfaces between the a
26. neAgilent 5517C Laser Head onelaser head cable Usean Agilent N 1251A B High Performance Laser Head Cable to connect to Agilent 10895A Agilent 10897B or Agilent 10898A VME electronic boards Usean Agilent 10881A B C Laser Head Cable to connect to Agilent 10885A Agilent 10889B or Agilent N1231A PC compatible electronics MULTIAXIS SYSTEM a i Agilent 10706B High Stability Plane Agilent 10706B Mirror Interferometer Agilent 10780C F Receiver we Agilent 10706B Sy High Stability Plane x Mirror Interferometer Y Axis High Stability Plane Mirror Interferometer X Axis Agilent 10780C F S Receiver G lt Agilent 10780C F Receiver Agilent 10701A Agilent 10717A 50 Beam Splitter Wavelength Tracker Agilent 10780C F Receiver Agilent 10701A Agilent 10707A 50 Beam Splitter Beam Bender Agilent 5517C Laser Head Figure 3 8 Multiaxis system for a precision x y stage three Agilent 10701A 5096 Beam splitters three Agilent 10706B High Stability Plane Mirror interferometers e oneAgilent 10707A Beam Bender User s Manual 3 31 NOTE NOTE Chapter 3 System Design Considerations Example Configurations oneAgilent 10717A Wavelength Tracker e four Agilent 10780F Remote receivers four receiver cables UseAgilent 10790A B C Receiver cables to connect to Agilent 10898A Agilent 10897B or Agilent 10895A VME electronic boards UseAgilent 10880A B C Receiver cables to connect
27. nsmission and reflection specifications Refer to the paragraphs titled Axis component efficiencies worst case and Axis component efficiencies typical on the following pages for these specifications e n addition all optics have small reflection and absorption losses that occur at each internal interface or component which is taken into account in their efficiency value Fingerprints dirt or oil on a glass surface significantly reduce optical efficiency by increasing both reflection and absorption losses System misalignment alsoreduces the amount of light reachingthe receiver Thermal gradients in the beam path can bend the beam and distort the wave front both of which reduce optical signal strength at the receiver Calculation of signal loss In order to assess the signal loss in a measurement system each optical component has been characterized by both worst case and typical optical efficiencies These efficiency values for each optical component are listed in the Specifications section for each optic that is the specifications section in Chapter 6 Beam Directing Optics for beam splitters and Chapter 7 Measurement Optics for interferometers Optical efficiency is defined as m _ Optical Power Out Efficiency Optical Power In The optical efficiencies for the interferometers are given with the respective measurement reflector efficiency included For example the Agilent 10702A Linear Inte
28. nt 10721A Agilent 10735A Agilent 10736A Agilent 10737L R Alignment pins are provided for easy installation and alignment This eliminates the need for any other user supplied mount for the sensor head 6 Maintain a bend radius not less than 35 mm 1 4 inches to prevent signal attenuation in the receiver s fiber optic cable Clearance for laser beam Figure 8 2 shows the Agilent 10780C and Agilent 10780F receivers and the proper beam spacing Alignment adjustment required The Agilent 10780C Agilent 10780F Agilent E 1708A or Agilent E 1709A receiver requires an alignment relative to the input beam to maximize measurement signal strength See the alignment and gain adjustment procedures in Chapter 8 Receivers of this manual 3 28 User s Manual Chapter 3 System Design Considerations Example Configurations Example Configurations Single axis system for servo track writing Figure 3 7 shows a single axis system to control servo track writing This system uses one each of Agilent 5517A 5517B or 5517C L aser Head laser head cable Usean Agilent N 1251A B High Performance Laser Head Cable to connect to Agilent 10895A Agilent 10897B or Agilent 10898A VME electronic boards Usean Agilent 10881A B C Laser Head Cable to connect to Agilent 10885A Agilent 10889B or Agilent N1231A PC compatible electronics Agilent 10705A Single Beam I nterferometer Agilent 10704A Reflector Agilent 10780C Receiver Agilen
29. nterchanging f and f e g rotating interferometer 90 in this example will result in the fringe counts DECREASING User s Manual 3 17 Chapter 3 System Design Considerations Optics The optical schematics for the interferometers in Chapter 7 M easurement Optics show which frequency polarizations are in the measurement path DIRECTION SENSE Reference Cube Corner f2 p my Agilent 10703A f E y A f4 Retroreflector NI a gt gt gt ne NET 7 Agilent 10702A Linear Interferometer LEGEND m qm m m gt fq and fz Figure 3 5 Direction sense fringe counts increase as optics move apart As with the laser heads when the interferometers are rotated 90 the measurement direction sense will change This rotation causes switching of frequencies in the measurement path Configuration effects Many of the distance measuring interferometers can be configured to turn the beam at right angles When configuring the linear single beam and plane mirror interferometers to turn the beam the measurement di recti on sense will be changed This is because the measurement and reference paths are switched on the interferometers therefore changing the direction sense For more information seethe Chapter 7 Measurement Optics in this manual 3 18 User s Manual Chapter 3 System Design Considerations Optics Vibration isolation for optics Vibration of the optics along the beam can cau
30. of interest This is shown in Figure 7A 7 Be aware that all installation and alignment requirements for the measurement reflector now apply alsoto the reference reflector Tousethe Agilent 10706A or Agilent 10706B interferometer in a differential configuration a plane mirror is recommended as the reference reflector Simply replace the reference cube corner or high stability adapter with the Agilent 10722A Plane Mirror Converter and attach the reference plane mirror to the reference surface of interest This is shown in Figure 7C 4 Again install and align the reference reflector the same as you would the measurement reflector Moving interferometer instead of reflector When moving the interferometer instead of the measurement reflector is required the Agilent 10702A 001 or Agilent 10766A should be used In practice for alignment reasons these arethe only interferometers except the straightness interferometers that can be moved while making measurements For a detailed explanation of why this option is required see Figure 7A 2 Introducing an offset into the laser beam There may be an occasion when you will want to simply introduce an offset into your laser beam to get it from the laser head tothe interferometer without having to relocate either one of them Figure 3 6 shows two ways in which this can be done To simply translate the beam you can use two reflectors such as the Agilent 10726A Beam Bender as a periscope a
31. on Remember Each measurement axis except for the Agilent 10717A Wavelength Tracker requires an interferometer and associated retroreflector e Each measurement axis after the first one requires a beam splitter The number of beam splitters required is n 1 where n is the number of measurement axes e If an Agilent 10717A Wavelength Tracker is used it counts as a measurement axis famultiaxis interferometer such as the Agilent 10721A Agilent 10735A Agilent 10736A or Agilent 10737L R is used be sure the beam directing optics you select will provide enough laser beam power to drive the receivers through the multiple measurement paths of the interferometer Beam benders should be arranged so their exiting beams are perpendicular to one polarization plane of the incoming laser beam Rotation of the beam during bending can result in problems due to the effects of polarization Beam splitters should be arranged so 3 4 User s Manual Chapter 3 System Design Considerations Determining What Equipment is Needed oneexiting beam is along the axis of the incoming beam and the second beam is perpendicular to one polarization of the incoming beam as described above for beam benders Each measurement axis requires an interferometer The nature of the measurement s to be made influences the interferometer choice Each measurement axis including the Agilent 10717A Wavelength Tracker requires a receiver The inte
32. ons FOUR AXIS CONFIGURATION Single Beam Retroreflector Interferometer Beam Bender 25 Linear Retroreflector Interferometer 50 Beam 25 Receiver Splitter gt a a Receiver A 50 A Laser Beam Splitter Linear ye M Interferometer Y 50 25 N y 25 Receiver Single Beam Interferometer Retroreflector Receiver Figure 3 9 Four axis configuration Two axis plane mirror Figure 3 10 shows an X Y stage measurement configuration using the Agilent 10706B High Stability Plane Mirror Interferometer The X Y stage has plane mirrors mounted at 90 to each other these are the reflectors for the plane mirror interferometers The advantages of this configuration are discussed in Chapter 15 Accuracy and Repeatability of this manual The Agilent 10706A Plane Mirror Interferometer is used to bend the laser beam Two axis plane mirror in a vacuum In an application where the X Y stageis installed in a vacuum chamber the configuration in Figure 3 10 may not be suitable Figure 3 11 shows a configuration using the Agilent 10567A Dual Beam Beam Splitter which allows the laser beam to enter and exit the chamber through one window This allows the receivers to remain outside the chamber and leaves only the optics inside For window specifications refer to the Vacuum Applications subsection under the Optics section of Chapter 3 System Design Considerations in this manual If the Agilent 10567A is not used two window
33. requires measurement receiver and cable One axis Differential Interferometer requires 3 mm beam from Agilent 5517C 003 Two axis Differential Interferometer requires 3 mm beam from Agilent 5517C 003 Plane Mirror Reflector Three axis Interferometer Three axis Interferometer Three axis Interferometer with Beam Bender Compact three axis Interferometer left Compact three axis Interferometer right Linear Interferometer Linear Retroreflector paired with Agilent 10766A Lightweight Retroreflector Angular Interferometer Angular Retroreflector paired with Agilent 10770A Short Range Straightness Optics matched set Long Range Straightness Optics matched set User s Manual Chapter 3 System Design Considerations Determining What Equipment is Needed Table 3 1 Equipment choices Continued Component Comment s Optic Mounts Adjustable mounts simplify installation and alignment Agilent 10710B Use with Agilent 10700A 10701A 10705A 10707A Agilent 10711A Use with Agilent 10702A 10706A 10706B 10715A 10716A Measurement Receivers One required per axis one required with Agilent 10717A Wavelength Tracker if used Agilent 10780C Receiver Agilent 10780F Remote Receiver Agilent E1708A Remote Dynamic Receiver Agilent E1709A Remote High Performance Receiver Receiver Cables for use with Agilent 10895A VME Axis board one cable per system Agilent 10790A 5 meters long Agilent 10790B 10 meters lon
34. rferometer used can influence the receiver choice Note that the Agilent 5519A and Agilent 5519B laser heads includea built in rece ver Then from your layout determine your optics needs Choose the Agilent laser head optical and electronic components accordingly Decide on a compensation scheme and finally select cables Table 3 1 summarizes the equi pment choices For advice and help contact Agilent Technologies Table 3 1 Equipment choices Component Comment s Laser One required per system Agilent 5517A Lowest velocity largest size 6 mm beam Agilent 5517B 25 higher velocity than Agilent 5517A small size 6 mm beam Agilent 5517C Higher velocity than Agilent 5517A and 5517B small size Std 6 mm beam diameter 5517C 003 3 mm beam diameter 5517C 009 9 mm beam diameter Agilent 5517D Highest velocity small size 6 mm beam Agilent 5519A Largest size built in receiver and power supply used in the Agilent 5529A Dynamic Calibrator System and Metrology applications Agilent 5519B Largest size built in receiver and power supply higher velocity than Agilent 5519A used in the Agilent 5529A Dynamic Calibrator System and Metrology applications Beam Directing Optics Order as required to manipulate beam path to your configuration Agilent 10567A Dual Beam Beam Splitter useful in vacuum applications Agilent 10700A 33 Beam Splitter Agilent 10701A 5096 Beam Splitter Agilent 10707A Beam Bender Agilent 10725A 5096 Beam
35. rferometer efficiency includes the efficiency of the Agilent 10703A Retroreflector The combined optical efficiency of a given measurement axis is the product of the efficiencies of the individual optics in the beam path This combined efficiency times the minimum laser output power in microwatts yields the worst case optical power at the receiver This value must be at least 1 5 microwatts for the Agilent 10780C Receiver or 2 2 microwatts for the Agilent 10780F Remote Receiver and Agilent E1708A Remote Dynamic Receiver or 0 20 to 0 80 microwatts for Agilent E1709A Remote High Performance Receiver A beam power User s Manual 3 25 Chapter 3 System Design Considerations Beam Path Loss Computation safety factor of at least three is recommended even though worst case laser and optics are assumed Creating a system with five or more axes of measurement may result in a beam power safety factor that is less than three As an example consider a typical installation with two measurement axes and a Wavelength Tracker axis Figure 71 3 Assume differential interferometers good optical alignment 98 efficient plane mirrors on the stage comparable path lengths and use of any Agilent laser head Thethree axes X Y and Wavelength Tracker WT havethe components listed in the following table Axis component effi ciencies worst case Component Component Efficiencies Worst Case Agilent 10700A 67 path Agilent 10701A Agilent
36. s OPTICAL METHOD FOR YAW MEASUREMENT Agilent 10706A Plane Mirror Interferometer X Y STAGE Y Axis YAW U Receiver Agilent 10725A 50 Beam Splitter D 4 R Axis Bare Optic M eceiver Fa La 7 Laser Beam From ay Pak a 33 Beam Splitter pa 67 Path Agilent 10706A Agilent 10700A Plane Mirror Interferometer Figure 3 14 Optical Method for Yaw Measurement OPTICAL METHOD FOR YAW MEASUREMENT Note A 1 2 wave plate is note needed when the interferometer is rotated 90 as shown Agilent 10706A A Y Axis Plane Mirror Receiver Interferometer ae RE gt lt 4 YAW U Pd Agilent 10726A Bare Optic Agilent 107254 D Y Y Axis Bare Optic Receiver y M lt Laser Beam From a gt all m 33 Beam Splitter mi 6796 Path Agil g gilent 10706A Agilent 10700A Plane Mirror Interferometer Figure 3 15 Optical Method for Yaw Measurement Similar techniques can be used with the Agilent 10715A Differential Interferometer This is done by splitting off part of the Y axis combined measurement signal after going completely through the interferometer and using this as the input beam to the Y axis interferometer This technique outputs Y Y information directly on the Y axis receiver 3 38 User s Manual Chapter 3 System Design Considerations Optical Device Troubleshooting Both of these optical configurations require some special optical components not avai
37. s and possibly additional beam splitters and benders will be required User s Manual 3 33 Chapter 3 System Design Considerations Example Configurations TWO AXIS PLANE MIRROR INTERFEROMETER CONFIGURATION Agilent 10706B Plane Mirror Interferometer X Y STAGE X Y Agilent 10780C Receiver ee Laser Agilent 10706B gt Plane Mirror Ini Agilent 10701A Interferometer 50 Beam Y Agilent 10780C Splitter Receiver Figure 3 10 Two axis plane mirror interferometer configuration X Y STAGE INSTALLED IN A VACUUM CHAMBER Agilent 10706B Plane Mirror Interferometer X Y STAGE Vacuum Chamber x Y Agilent 10780C Y Axis Receiver a Agilent 10706B ii a Plane Mirror O Interferometer Agilent 10780C X Axis Receiver Window Figure 3 11 X Y stage installed in a vacuum chamber Laser 3 34 User s Manual Chapter 3 System Design Considerations Example Configurations Two axis measurement system using two Agilent 10715A differential interferometers In X Y stage applications where maxi mum measurement accuracy and stability are required the Agilent 10715A Differential Interferometer can be used instead of the Agilent 10706A B Plane Mirror Interferometer n Figure 3 12 an X Y stage using Agilent 10715A s is illustrated As with plane mirror interferometers the reflectors are plane mirrors mounted at 90 to each other on the stage Using the Agilent 10715A Differential In
38. s shown in Figure 3 6 A Changing the spacing between the reflectors or rotating the device can change the amount of offset Toreverse the direction of the beam you can usetworeflectors in a retroreflector arrangement as shown in Figure 3 6 B 3 22 User s Manual Chapter 3 System Design Considerations Beam Path Loss Computation Beam Path Loss Computati on Multiaxis positioning systems must be designed to allow sufficient optical power to reach each Agilent 10780C Agilent 10780F Agilent E1708A or Agilent E1709A Receiver in the system Since all optics have an efficiency of less than 100 an optical power loss budget must be created as a part of any multiaxis system design This chapter defines optical efficiency as it relates to the signal loss through components A method for computing the optical power loss in a system is described Considerations The following considerations are important in designing a reliable multiaxis measuring system When usingthe Agilent 10780C F or Agilent E1708A receivers typically up to four measurement axes can be easily implemented without optical power loss imposing significant constraints A system of five or six axes is usually feasible although closer attention to the power loss budget is required A system having more than six axes is possible under certain circumstances with PC or VM E based electronics but the optical power loss budget quickly becomes the limiting constraint
39. sethe fringe count in the laser measurement system electronics to fluctuate rapidly Vibrations along this axis constitute real measurable displacements you will have to decide if these fluctuating measurements are acceptable in your application In extreme cases however the velocity of the optics may momentarily exceed the velocity limitation of the laser system causing an error When vibration occurs perpendicular to the beam the beam signal power can fluctuate If this fluctuation is too great insufficient beam signal will arrive at the receivers causing a measurement signal error Loose mounting can cause the optics to move inappropriately during a measurement causing a measurement error or loss of beam power Elastic mounting can have the same effect as loose mounting It can also be responsible for a sag offset in the optics positions If thereis vibration in the machine an elastic mounting can transmit and amplify the vibration to the attached optic possibly causing more errors Y ou should anticipate these effects and minimize them if necessary during the laser measurement system design process Certain interferometers are inherently less susceptible to vibration effects than others This is particularly true of differential style interferometers such as the Agilent 10715A Agilent 10719A and Agilent 10721A Thestability of theseinterferometers is duetothe fact that both their reference beams and their measurement b
40. sings In those cases interferometers such as the Agilent 10719A Agilent 10721A Agilent 10735A and Agilent 10736A should be considered These products have referenced housings and prealigned optical elements Because they have individual mounting requirements these products are not intended for use with the adjustable mounts described above F or more information about these optics refer to Chapter 7 Measurement Optics in this manual Fasteners for optics Any optical component that fits an adjustable mount is supplied with mounting screws to mount it on the appropriate adjustable mount Vacuum applications There are vacuum options for Agilent optical components which are compatible with vacuum environments Contact Agilent Call Center for information telephone numbers of various call centers are listed on the Service and Support page at the back of this manual The housings of these components are made of stainless steel and the optical elements are attached to these housings using a low volatility space grade adhesive See the Specifications information for each optic for a list of materials used in the optic If the laser beam has to go through a window for example into a vacuum chamber the window must meet the following requirements 1 A minimum window aperture of 25 4 mm 1 inch with a minimum thickness of 8 mm 0 3 inch If a larger window is used it must be proportionally thicker to assure no distortion in
41. t 10790A Receiver Cable Usean Agilent 10790A B C Receiver Cable to connect to Agilent 10895A Agilent 10897B or Agilent 10898A VME electronic boards Usean Agilent 10880A B C Receiver Cable to connect to Agilent 10885A P C compatible electronics Agilent laser axis of measurement electronics Agilent 10885A 10897B or 10898A User s Manual 3 29 Chapter 3 System Design Considerations Example Configurations SINGLE AXIS SYSTEM a Agilent 10704A gt 4 UEM gt d Retroreflector iiia Agilent 10705A Single Beam Interferometer d Agilent 5517B C D n and Head Agilent 10780C F Receiver Figure 3 7 Single axis system for servo track writing Multiaxis configurations The maximum number of independent axes of displacement that can be measured using one laser head depends on 1 the measurement system electronics 2 the strength of the beam from the laser head and 3 the sensitivity of the receivers used By using the proper combination of beam splitters beam benders and interferometers the measurement axes can be established with a minimum number of components The following paragraphs provide examples of routing the laser beam for multiaxis measurement configurations 3 30 User s Manual Chapter 3 System Design Considerations Example Configurations Multiaxis system for a precision x y stage Figure 3 8 shows a multiaxis system for a precision X Y stage This system uses o
42. terferometer also requires mounting the reference mirror supplied with the Agilent 10715 between the interferometer and measurement reflector Mounting instructions for the reference mirror are given later in this chapter The Agilent 10715A 001 interferometer turns the beam as shown in Figure 3 12 This configuration requires use of opposite input apertures for each interferometer resulting in reversed direction senses for the X and Y axes The reversed direction sense must be corrected in the electronics or by software Note that the receiver for each axis is above the input beam Three axis measurement system using discrete plane mirror interferometers X Y YAW Some X Y stage applications require measurement or control of the stage yaw Y aw is angular rotation of the stage about an axis the Z axis perpendicular to the plane of the stage With two interferometers on one axis of the stage angular motion can be calculated Figure 3 13 the yaw angle THETA is measured using axes Y and Y and is calculated as follows THETA arctan Y User s Manual 3 35 Chapter 3 System Design Considerations Example Configurations TWO AXIS MEASUREMENT SYSTEM X Axis E X Y STAGE Differential Interferometer 001 X Reference Mirror Y X Axis X Axis Receiver MIRRORS Laser Beam lr Reference Laser A Y Axis Mirror Laser Beam AA Gin Y Axis Differential Ln A Interferometer 001 50 Y Axis L Beam
43. the window when under differential pressures 2 Transmitted wavefront distortion less than A 10 peak valley single pass over a 23 mm 0 9 inch diameter 3 Parallelism of faces less than 32 arc minutes to reduce beam steering 3 20 User s Manual Chapter 3 System Design Considerations Optics 4 Surface quality 60 40 or better per Mil 0 13830 5 The window must be strain free Differential measurements with interferometers Several interferometers have the capability to make differential measurements A differential measurement is onein which both the reference beam and the measurement beam travel to external mirrors outside the interferometer housing This allows measurement of the relative positions of the two external mirrors either or both of which may be moving Viewed another way this allows measuring the motion of onereflector relativeto a reference datum el sewhere in the machine external tothe interferometer itself This is unlikethe typical interferometer configuration because usually the reference beam path length does not change in differential configurations it can One useful example of a differential measurement in a lithography application is for measuring the motion of the X Y stagerelativetothe optical column The Agilent 10719A One Axis Differential Interferometer shown in Figure 7 I of subchapter 7 and the Agilent 10721A Two Axis Differential nterferometer shown in Figure 7K 1 of subchapter 7K