EP1588122A1

EP1588122A1 - Single point laser measurement reference

Info

Publication number: EP1588122A1
Application number: EP03816399A
Authority: EP
Inventors: John P. Flowers; Lee C. Kalem
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2003-01-28
Filing date: 2003-08-21
Publication date: 2005-10-26
Also published as: EP1588122A4; JP2006513436A; WO2004083775A1

Abstract

A laser interferometer positioning system includes a first board having a first phase meter and a second phase meter. The first phase meter outputs a digital reference phase signal based on an analog electrical reference signal and a clock signal. The digital reference phase signal is the only digital reference signal for a laser head in the system. The second phase meter outputs a digital phase measurement signal based on a electrical measurement signal and the clock signal. The first board is inserted into a backplane. The backplane includes a bus that carries the digital phase reference signal and the digital phase measurement signal to a second board. The second board subtracts the digital phase reference signal from a digital phase measurement signals it receives from multiple first boards.

Description

SINGLE POINT LASER MEASUREMENT REFERENCE John P. Flowers Lee C. Kalem FIELD OF INVENTION

[0001] This invention relates to laser interferometer positioning systems.

DESCRIPTION OF RELATED ART

[0002] Typical applications for a laser interferometer positioning system include integrated circuit (IC) manufacturing equipment (wafer steppers, step and scan tools, and E-beam lithography systems), precision machine tools, and custom stages. The precision and accuracy of positioning measurements are vital to the performance of these systems. When built into these types of equipment, the positioning system measures the position and controls the motion of the platform with great precision and accuracy.

SUMMARY

)3] In accordance with one embodiment of the invention, a laser interferometer positioning system includes a first board having a first phase meter and a second phase meter. The first phase meter outputs a digital reference phase signal based on an analog electrical reference signal and a clock signal. The digital reference phase signal is the only digital reference signal for a laser head in the system. The second phase meter outputs a digital phase measurement signal based on an electrical measurement signal and the clock signal. The first board is inserted into a backplane. The backplane includes a bus that carries the digital phase reference signal and the digital phase measurement signal to a second board. The second board subtracts the digital phase reference signal from digital phase measurement signals it receives from multiple first boards.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Fig. 1 illustrates a laser interferometer positioning system.

[0005] Figs. 2 and 3 illustrate a computer system in the system of Fig. 1. [0006] Fig.4 is a block diagram of a computer system in a laser interferometer positioning system in one embodiment of the invention.

[0007] Fig. 4A illustrates a laser interferometer positioning system in one embodiment of the invention.

[0008] Fig. 5 is a block diagram of a phase measurement board in the system of Fig. 4 in one embodiment of the invention.

[0009] Fig. 6 is a block diagram of a combiner board in the system of Fig. 4 in one embodiment of the invention.

[0010] Fig. 7 is a block diagram of a phase measurement and combiner board in a computer system of a laser interferometer positing system in one embodiment of the invention.

DETAILED DESCRIPTION

[0011] Fig. 1 illustrates a typical laser interferometer positioning system 10 used with IC manufacturing. A power supply 12 provides power to a laser head 14 that generates a light beam 16. An optical element 18 splits light beam 16 into beams 20 and 22. A reference optical receiver 24 converts beam 20 to an analog electrical reference signal and transmits it to a computer system 26 (e.g., a Versa Modular European bus mainframe).

[0012] An optical element 28 splits beam 22 into beams 30 and 32. Using an interferometer 33, beam 30 is reflected off a mobile stage 34 along a first axis (e.g., the X-axis) and then received by an optical receiver 36. Similarly, using an interferometer 37, beam 32 is reflected offstage 34 along a second axis (e.g., the Y-axis) and then received by an optical receiver 38. Optical receivers 36 and 38 convert beams 30 and 32 to analog electrical measurement signals, respectively, and transmit them to computer system 26. Of course, additional axes of measurement can be taken.

[0013] Computer system 26 may be coupled to air sensor 40 and temperature sensor 42 to compensate the dependency of the laser wavelength on its environment. Computer system 26 uses the phase differences between the reference signal from receiver 24 and the measurement signals from receivers 36 and 38 to determine the position of stage 34 along the measurement axes. Computer system 26 then uses an actuator 44 to move stage 34. [0014] Fig. 2 illustrates a front panel of a typical computer system 26A used in system 10. Computer system 26A includes multiple measurement axis boards 60 to 70. Each board receives the analog electrical reference signal and one or more measurement signals to determine the position of stage 34 along a measurement axis. The analog electrical reference signal is received directly from receiver 24 by one of the measurement axis boards. A front panel ribbon cable 71 then routes the analog electrical reference signal from that board to the other boards in parallel.

[0015] Fig. 3 illustrates a front panel of another typical computer system 26B used in system 10. Computer system 26B includes multiple measurement axis boards 80 to 90. One of the boards (e.g., board 85) receives the analog optical reference signal (e.g., beam 20) from an optical fiber and converts it to the analog electrical reference signal. Board 85 then routes the analog electrical reference signal to adjacent boards 84 and 86 using cables 102 and 104. Similarly, each board propagates the analog electrical reference signal to its adjacent board in a daisy chain fashion.

[0016] Both computer systems 26A and 26B suffer several disadvantages. First, the circuitry required to convert the reference signal from its analog form to its digital form is duplicated for every measurement axis board. Second, the reference signal may not be replicated accurately as it is passed from board to board due to changes in loading from the addition of more boards, buffering between the boards, and electrostatic discharge (ESD) on the cables. Third, the cabling takes up precious space on the front panels of the boards. Fourth, the reference signal may age differently on the way to the boards due to varying signal paths. Fifth, the cabling makes it difficult to change the routing of the reference signal. Sixth, the system requires predetermined wiring if multiple reference signals are required. Thus, what is needed is a laser interferometer positioning system that addresses these disadvantages.

[0017] Fig. 4 is a block diagram of a computer system 120 in a laser interferometer positioning system 121 (Fig.4A) in one embodiment of the invention. System 120 includes phase measurement boards 124-1, 124-2 . . . and 124-i plugged into a backplane 125 in an electronic rack. As represented by board 124-i, each phase measurement board ("PMB") accepts multiple fiber optic input signals 126-1, 126-2 . . . and 126-j. Each fiber optic input signal can be an analog optical reference signal from laser head 14 or an analog optical measurement signal reflected from stage 34 along a measurement axis. For example, input signal 126-j is a reference optical signal from laser head 14 (e.g., beam 20 in Fig. 4A), input signal 126-1 is a measurement optical signal reflected from stage 34 (e.g., beam 30 in Fig. 4A) along the first measurement axis, and input signal 126-2 is a measurement optical signal reflected from stage 34 (e.g., beam 32 in Fig. 4A) along a second measurement axis. In this embodiment, receivers 24, 36, and 38 (Fig. 1) are replaced by optical lenses 24A, 36A, and 38A (Fig. 4A) coupled to optical fibers that transport beams 20, 30, and 32 to system 120.

[0018] The PMB measures the phase of each input signal, whether reference or measurement, relative to an internal clock signal that is synchronized between all the PMBs in system 120. PMBs 124-1, 124-2 . . . and 124-i then report the measured phases in digital form over buses 128-1, 128-2 . . . and 128-i, respectively, to a combiner board 130. Buses 128-1 to 128-i also carry the internal clock signal from combiner board 130 to PMBs 124-1 to 124-i, respectively.

[0019] Combiner board 130 subtracts a phase reference signal from each of the phase measurement signals of PMBs 124-1 to 124-i. From the phase differences, combiner board 130 determines the position of stage 34. Combiner board 130 then reports the position of stage 34 over a bus 132 (e.g., a P2 bus) to a controller 134. Controller 134 can issue commands over a bus 136 (e.g., a Versa Modular European bus) to combiner board 130. Buses 134 and 136 can be implemented on a standard VME (Versa Modular European) backplane 138. Combiner board 130 then passes these commands over a bus 140 on backplane 125 to PMBs 124-1 to 124-i.

[0020] As described above, the phase reference signal is calculated once by a PMB and then centrally processed by combiner board 130 in its digital form. System 120 eliminates the need to duplicate the reference phase circuitry on each board and the associated front panel cabling. With the front panel cables eliminated, the associated ESD problem of the cabling is also eliminated. Furthermore, the phase reference signal is not affected by the load of additional boards. The position calculations are also now more accurate because the same reference phase signal is compared with phase measurement signals from multiple axes.

[0021] Fig. 5 illustrates one embodiment of PMB 124-i. PMB 124-i includes measurement channels 160-1, 160-2 . . . and 160-j receiving input signals 126-1, 126-2 . . . and 126-j, respectively. Measurement channels 160-1 to 160-j generate phase signals 161-1, 161-2 . . . and 161-j. In one example, signal 161-1 and 161-2 are phase measurement signals, and signal 160-j is a phase reference signal. [0022] As represented by measurement channel 160-j, each measurement channel includes an optical receiver 162 and a phase meter 164. Optical receiver 162 converts analog optical input signal 126-j to an analog electrical input signal. Phase meter 164 measures the phase of signal 126-j relative to a clock signal Timebase. Clock signal Timebase is provided to PMB 124-i (and the other PMBs) by backplane 125 (Fig. 4). In response to a clock signal Framesync, phase meter 164 periodically transmits measured phase signal 161-j over backplane 125 to combiner board 130.

[0023] Fig. 6 illustrates one embodiment of combiner board 130. Combiner board 130 includes a multiplexer 180 that routes the measured phase signals from the measurement channels in the PMBs to any of subtraction blocks 182-1, 182-2 . . . and 182-k. For example, multiplexer 180 routes all the measured phase signals on bus 128-i from PMB 124-i (Figs. 4 and 5) to subtraction block 182-k. Multiplexer 180 can route any one of the measured phase signals as a phase reference signal to be subtracted from the other measured phase signals. Typically, multiplexer 180 routes the measured phase signal from optical lens 24A (Fig. 4A) as the phase reference signal while the measured phase signals from optical lenses 36A and 38A (Fig. 4A) as the phase measurement signals. However, in certain applications, the measured phase signal from optical lens 36A or 38A may be used as the phase reference signal. Multiplexer 180 may be programmed on the fly to route the measured phase signals.

[0024] As represented by subtraction block 182-k, each subtraction block includes subtraction circuits 184-1, 184-2 . . . and 184-m. Each subtraction circuit subtracts a reference phase signal from a measurement phase signal. For example, subtraction circuit 184-2 subtracts reference phase signal 161-j from measurement phase signal 161-2. Each subtraction circuit then provides the phase difference to a position processor 186. Using the phase differences, position processor 186 determines the position of stage 34 along the measurement axes.

[0025] Combiner board 130 also includes a clock 188 that generates clock signals Timebase and Framesync. In one embodiment, the lines on backplane 125 (Fig. 4) that carry clock signals Timebase and Framesync to each PMB are of the same length so that clock signals Timebase and Framesync arrive at each PMB at the same time. This ensures that the phases are measured relative to the same clock signal. [0026] In one embodiment, combiner board 130 is implemented with a field programmable gate array (FPGA) that is configured with the above-described blocks and circuits. The FPGA can be programmed to route any of the measured phase signals as a phase reference signal or a phase measurement signal to any of the subtraction blocks. The FPGA can also be programmed with any method to determine the position of stage 34 from the phase differences.

[0027] Fig. 7 illustrates one embodiment of a phase measurement and combiner board 210 that includes the features of the PMB and the combiner board. Board 210 includes measurement channels 160-1 to 160-j that receives analog optical input signals 126-1 to 126- j, respectively. Measurement channels 160-1 to 160-j measure the phases of the inputs signals relative to clock signal Timebase from clock 188. Measurement channels 160-1 to 160-j output the phases to multiplexer 180 in response to clock signal Framesync from clock 188. Multiplexer 180 routes one of the phase signals as a reference phase signal to subtraction circuits 184-1 to 184-m, and the remainder of the phase signals as input signals to subtraction circuits 184-1 to 184-m. Instead of multiplexer 180, the reference phase signal and measurement phase signals can be hardwired to predetermined subtraction circuits. Subtraction circuits 184-1 to 184-m calculate the phase differences and output them to position processor 186. Using the phase differences, position processor 186 determines the position of stage 34 along the measurement axes.

[0028] Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. As described above, a single optical reference input signal is necessary for embodiments that use one laser head. All the other input signals can be optical measurement signals. In embodiments that use multiple laser heads, one optical reference input signal is required for each laser head. The optical reference input signals can be received by any of the PMBs as long as multiplexer 180 routes them to the appropriate subtraction circuits. In some embodiments, a measurement channel may not include the optical receiver. Thus, a phase meters may receive an analog electrical reference or measurement signal from optical receiver 24, 32, or 36 (Fig. 1) that are located off the phase measurement board or the phase measurement and combiner board. Numerous embodiments are encompassed by the following claims.

Claims

CLAIMSWhat is claimed is:

1. A laser interferometer positioning system, comprising: a first board, comprising: a first phase meter receiving an analog electrical reference signal and a clock signal, the first phase meter outputting a digital phase reference signal based on the analog electrical reference signal and the clock signal, wherein the digital phase reference signal is the only digital reference signal for a laser head in the system; and a second phase meter receiving an analog electrical measurement signal and the clock signal, the second phase meter outputting a digital phase measurement signal based on the analog electrical measurement signal and the clock signal.

2. The system of claim 1, wherein the first board further comprises: a first optical receiver receiving an analog optical reference signal and outputting the analog electrical reference signal to the first phase meter; and a second optical receiver receiving an analog optical measurement signal and outputting the analog electrical measurement signal to the second phase meter.

3. The system of claim 1 , further comprising: a second board, the second board comprising: an arithmetic circuit receiving the digital phase reference signal and the digital phase measurement signal, the arithmetic circuit outputting a phase difference signal; and a processor receiving the phase difference signal and outputting a position signal; a backplane for receiving the first and the second boards, the backplane comprising: a clock line carrying the clock signal to the first board; a data bus carrying the digital phase reference signal and the digital phase measurement signal from the first board to the second board.

4. The system of claim 3, wherein: the first board further comprises a plurality of phase meters receiving analog electrical measurement signals and the clock signal, the phase meters outputting digital phase measurement signals based on the electrical measurement signals and the clock signal; and the second board further comprises: a plurality of arithmetic circuits outputting phase difference signals based on the digital phase reference signal and the digital phase measurement signals; a multiplexer selectively routing the digital phase reference signal and the digital phase measurement signals to the arithmetic circuits; wherein the processor receives the phase difference signals and outputs position signals.

5. The system of claim 3, wherein the second board further comprises a clock generating the clock signal, the clock line carrying the clock signal from the second board to the first board.

6. The system of claim 5, further comprising a plurality of first boards outputting digital phase measurement signals, wherein: the second board further comprises: a plurality of arithmetic circuits outputting phase difference signals based on the digital phase reference signal and the digital phase measurement signals; a multiplexer selectively routing the digital phase reference signal and the digital phase measurement signals to the arithmetic circuits, wherein the processor receives the phase difference signals and outputs position signals; and the backplane receiving the first boards, the backplane further comprising a plurality of data buses and a plurality of clock lines, the data buses carrying the digital phase measurement signals from the first boards to the second board, the clock lines carrying the clock signal from the second board to the first boards.

7. The system of claim 6, wherein distances of the clock lines from the clock to each of the first boards are the same.

8. The system of claim 3, wherein the second board further comprises: a second arithmetic circuit outputting a second phase difference signal based on the digital phase measurement signal and a second digital phase measurement signal; a multiplexer selectively routing the digital phase measurement signal and the second digital phase measurement signal to the second arithmetic circuit; wherein the processor receives the second phase difference signal and outputs a second position signal.

9. The system of claim 1, wherein the first board further comprises: an arithmetic circuit receiving the digital phase reference signal and the digital phase measurement signal, the arithmetic circuit outputting a phase difference signal; a processor receiving the phase difference signal and outputting a position signal.

10. The system of claim 9, further comprising: a third phase meter receiving a second analog electrical measurement signal and the clock signal, the phase meter outputting a second digital phase measurement signal based on the second analog electrical measurement signal and the clock signal.

11. The system of claim 10, wherein the first board further comprises a clock generating the clock signal, and clocks lines of equal distance from the clock to the first and the second phase meters.

12. A method for a laser interferometer positioning system to determine a position, comprising: generating a digital phase reference signal on a first board, the digital phase reference signal being based on an analog electrical reference signal and a clock signal, the digital phase reference signal being the only digital reference signal for a laser head in the system; and generating a digital phase measurement signal on the first board, the digital phase measurement signal being based on an analog electrical measurement signal and the clock signal.

13. The method of claim 12, further comprising: optically detecting an analog optical reference signal on the first board; generating the analog electrical reference signal on the first board, the analog electrical reference signal being based on the analog optical reference signal; optically detecting an analog optical measurement signal on the first board; and generating the analog electrical measurement signal on the first board, the analog electrical measurement signal being based on the analog optical measurement signal.

14. The method of claim 12, further comprising: transmitting the digital phase reference signal and the digital phase measurement signal from the first board to a second board; generating a phase difference signal on the second board, the phase difference signal being based on an arithmetic operation of the digital phase reference signal and the digital phase measurement signal; and generating a position signal on the second board, the position signal being based on the phase difference signal.

15. The method of claim 14, further comprising: generating digital phase measurement signals on the first board, the digital phase measurement signals being based on analog electrical measurement signals and the clock signal; transmitting the digital phase measurement signals from the first board to the second board; routing the digital phase measurement signals to arithmetic circuits on the second board, the arithmetic circuits performing arithmetic operations on the digital phase reference signal and the digital phase measurement signals; generating phase difference signals on the second board, the phase difference signals being based on the arithmetic operations; and generating position signals on the second board, the position signal being based on the phase difference signals.

16. The method of claim 14, further comprising: transmitting the clock signal from the second board to a plurality of first boards; generating digital phase measurement signals on the first boards, the digital phase measurement signals being based on analog electrical measurement signals and the clock signal; transmitting the digital phase measurement signals from the first boards to the second board; routing the digital phase measurement signals to arithmetic circuits on the second board, the arithmetic circuits performing arithmetic operations on the digital phase reference signal and the digital phase measurement signals; generating phase difference signals on the second board, the phase difference signals being based on the arithmetic operations; and generating position signals on the second board, the position signals being based on the phase difference signals.

17. The system of claim 16, wherein said transmitting the clock signal comprises transmitting the clock signal along a same distance so the clock signal arrives at a same time on the first boards.

18. The system of claim 14, further comprising: generating a second digital phase measurement signal, the second digital phase measurement signal being based on a second analog electrical measurement signal and the clock signal; transmitting the second digital phase measurement signal to the second board; routing the digital phase measurement signal and the second digital phase measurement signal to a second arithmetic circuit on the second board, the second arithmetic circuit performing an arithmetic operation on the digital phase measurement signal and the second digital phase measurement signal; generating a second phase difference signal on the second board, the second phase difference signal being based on the arithmetic operation; and generating a second position signal on the second board, the second position signal being based on the second phase difference signal.

19. The system of claim 12, further comprising: generating a phase difference signal on the first board, the phase difference signal being based on an arithmetic operation of the digital phase reference signal and the digital phase measurement signal; and generating a position signal on the first board, the position signal being based on the phase difference signal.

20. The system of claim 19, further comprising: generating a second digital phase measurement signal on the first board, the second digital phase measurement signal being based on a second analog electrical measurement signal and the clock signal; generating a second phase difference signal on the first board, the second phase difference signal being based on a second arithmetic operation of the digital phase reference signal and the second digital phase measurement signal; and generating a second position signal on the first board, the second position signal being based on the second phase difference signal.