CA2064230A1 - Holographic lithography - Google Patents

Abstract

First and second coherent beams illuminate a common area on an exposure station (17). A phase detector, comprising left and right photodiodes (26), senses the relative phase between the first and second beams (31 and 32) to provide a control signal. There is at least one phase shifter (19) in the path of at least one of the coherent beams (31 and 32). The control signal is coupled to the phase shifter (19) to adjust the phase imparted thereby so that the relative phase between the first and second coherent beams (31 and 32) is substantially constant.

Description

WO 91/01517PCr/US90/04028 ` ' ` 20~4230 , IMPROVING HOLOGRAPHIC LITHOGRAPHY
The present invention relates to holographic lithography. In holographic lithography a periodic or quasi periodic pattern is exposed in a photosensitive film (usually called a resist) by overlapping two beams, typically with mirrors, from a laser or other coherent source. In one particular implementation of holographic lithography, termed "achromatic holographic lithography", gratings are used to split and recombine the beams. (see E. Anderson, K. Komatsu, and H. I. Smith, "Achromatic Holographic Lithography in the Deep W ," J. Vac. Sci.
Technol. B6, 216, (1988)). As a result, the source need not have the high degree of temporal coherence (i.e., narrow bandwidth) or spatial coherence commonly seen in laser sources. The minimum period, p, (i.e., center-to-center distance between adjacent lines) obtainable in holographic lithography is given by - p = ~/2 sin e, (1) where ~ is the wavelength of the exposing radiation and is half the angle between the intersecting beams. It is relatively easy to make this angle as large as 62 degrees, in which case p = 0.57 ~. The limit is, of course, p = ~/2.
Holographic lithography has been known for many years (H. I. Smith, "Fabrication Techniques for Surface-Acoustic wave and Thin-Film Optical Devices," IEEE Proc 62, 1361-1387 (1974)). It is used commercially to produce large-area periodic diffraction gratings for spectroscopy. Prior to the advent of the laser, which made holographic lithography more practical, large area gratings were made by mechanical ruling engines, which are expensive, costly to operate, and produce well-known systematic errors, called ghosts.
Holographic lithography is also used in research, in areas ranging from distributed feedback lasers, and .,' ;

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:'..,''-. '. 20g~2~0 quantum-effect devices, to x-ray imaging and spectroscopy. competitive means of creating periodic and quasi-periodic patterns in a resist, such as scanning-electron-beam lithography, suffer from two main shortcomings: extremely long writing times and small field-of-view. It is generally agreed that in those applications where the area of the grating exceeds about one square millimeter, and the period is finer than about one half micrometer, holographic lithography is the preferred method of exposing resists.
If one overlaps two beams from a single coherent source, such as a laser, and if environmental vibrations are sufficiently low that the beams do not move relative to one another by more than about p/2, it is nearly guaranteed that one will be able record in a resist film a recognizable dif~raction grating. However, to achieve ~ reliable and repeatable results in holographic - lithography, and to do so over large area, has proven to - be an extremely difficult task, and for many investigators an elusive goal.
An important object of this invention is to provide a novel means of conducting holographic lithography in which the stability in space and time of the intensity distribution at the recording place is ensured by means of a feedback loop. This invention greatly enhances the contrast in the exposing intensity distribution, as well as the reproducibility of holographic lithography. It also enables one to increase the area of exposure.
This invention results from the recognition that the quality of the "image" (i.e., the pattern of fringes and the depth of modulation) in holographic lithography is critically dependent on the phase relationship between the two tor more) beams that form the image. In this .

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According to the invention, a beam-splitting optical element is above and behind a plate used to hold substrates during holographic lithography. This beam splitter intercepts portions of two beams used in holographic lithography, and produces at least two interference patterns whose variations reflect variations in the relative phase difference between the two overlapping beams. The at least two interference patterns are monitored by two photodetectors, and the difference in the output signals from the two detectors is used in a feedback loop to adjust the potential on a pockel-cell phase shifter so as to minimize this differençe.
Numerous other features and advantages of the invention will become apparent from the following description when read in connection with the accompanying drawings in which:
; 20 FIG. l is a pictorial representation of a holographic lithography system;
FIG. 2 is a diagrammatic representation of an exposure station;
FIG. 3 is a top view of an assembly according to 25 the invention including elements in FIG. 2; ~ `
FIG. 4 is a pictorial representation illustrating the function of the beam-splitting optical flat;
FIG. 5A is a pictorial representation of overlapping beams illustrating the principle of -holographic lithography;
FIGS. 5B and SC illustrate the intensity as a function of distance of high contrast and low-contrast images, respectively, produced by the interference i pattern;

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FIG. 6 is a diagrammatic representation of structure for strongly attenuating reflected beams;
FIGS. 7A and 7B show relief structures for high and low contrast images, respectively;
FIG. 8 is an electronmicrograph of a grating pattern in resist obtained by holographic lithography without the feedback loop in operation;
FIG. 9 is an electron micrograph of a grating pattern in resist obtained by holographic lithography with the feedback loop activated according to the invention.
FIG. 10 illustrates a process for obtaining a relief grating of higher aspect ratio; and FIGS. llA and llB are partial pictorial perspective and front views, respectively, of structure in FIGS. 2-4 helpful in understanding the relationship of the system elements.
With reference now to the drawing and more particularly FIG. 1 thereof, there is shown a pictorial representation of a holographic lithography system.
A laser 11, such as an argon ion laser operating at the W wavelength 351 nm, emits a beam which strikes shutter 12. Shutter 12 can be opened electronically to commence the operation of holographic lithography by directing the beam to the remainder of the apparatus, comprising an interferometer. It is typically fixed to a table of low vibration amplitude.
An advantage of this invention, because it involves active stabilization of the phase difference between interfering beams, is that it relaxes the constraints on the stability of the table to be achievable in typical lab or shop environments with a low cost table.
The interferometer comprises the beam splitter 14 which creates two beams, a variable attenuator 15 for - . , - . ~ ,`. : ~
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adjusting the irradiance in one of the two beams until both match, and mirrors 16a, 16b and 16c for redirecting the beams onto the exposure station 17. In some cases, mirror 16c may be omitted. Spatial filter 18 includes a lens which focuses the laser beam onto a plane that has a single pinhole. The pinhole is chosen to be equal to the diffraction-limited focal spot of the lens. In this way, essentially pure spherical waves emanate from spatial filter 18. A collimating lens 20 (shown in dotted lo outline) may be placed downstream from spatial filter 18 to yield a parallel beam. However, the distance between spatial filter 18 and exposure station 17 may be sufficiently great so that lens 20 is unnecessary. The pinhole of spatial filter 18 is adjusted in position by means of a commercially available piezoelectric displacer attached to a micrometer drive (not shown) to allow for remote adjustment. Phase shifter 19 operates on the principle of a Pockell cell: application of a voltage across the cell causes a change in index of refraction and hence shifts the phase of the optical beam that is traversing the cell.
Exposure station 17 is shown in greater detail in FIG. 2. FIG. 2 shows an arrangement of photocells 22 built into the vertical plate 21 of the exposure station.
Photocells 22 are used for centering the two interfering beams on vertical plate 21. Rotary table 23 can be swung so that photocells 22 face either the right or the left beam. After such centering is completed, the substrate to be exposed is held in front of the array of photocells 22 by means of a low pressure vacuum holder or "chuck"
(not shown).
FIG. 3 is a top view of the vertical plate 21 showing substrate 24 held onto the plate 21, a cover 25 , which prevents exposure of substrate 24 during adjustment 35 of the photodiodes 26 and the feedback loop. A beam- -;', . , :
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20~42~0 -splitting optical flat 27 is positioned on top of vertical plate 21. In place of the optical flat one could instead use a beam splitting cube as item 27.
The function of the beam-splitting optical flat 27 is illustrated in FIG. 4. The beam 31 coming in from the left is partially reflected from the flat 27 and partially transmitted (shown as dotted lines.) The beam 32 from the right is also partially reflected and partially transmitted. The interference patterns due to overlap of transmitted and reflected beams are sketched below the left and right photodiodes 26. These sketches show left and right "bulls-eye" patterns 33 and 34, respectively, with a dark center (left) and a bright center (right), respectively. The character of the - 15 bulls-eye pattern depends on the phase difference between left and right beams 31 and 32 and can be adjusted by applying a voltage to phase shifter 19. The photodiodes 26 are typically centered on the two bulls-eye patterns.
The bulls-eye pattern is characteristic of the ~ 20 interference of two spherical waves of large but slightly ; different radii. If the left and right beams are not ; spherical waves, but, for example, plane waves, the - interference pattern would consist of parallel fringes ~ -and can be processed as well accordingly to the invention. The relationship of system elements is helpfully illustrated in FIGS. llA and llB showing pictorial perspective and front views respectively of certain elements described above.
In the absence of feedback, the bulls-eye interference patterns 33 and 34 oscillate rapidly due to relative phase shifts between left and right beams.
These oscillations produce brightening and darkening of the central spot and the successive rings of the bulls-eye pattern. For example, a relative phase shift of radians between left and right beams causes the central , . .

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spot to change from maximum darkness to maximum brightness. A relative phase shift of 2 radians will further shift the central spot of the bulls-eye pattern back to maximum darkness. The relative phase shift .5 between left and right beams can arise from any of several environmental factors: vibration of the interferometer table, the mirror or other components;
changes in temperature; changes in the density or pressure of the air through which the beams propagate.
These environmental variations are typically radom and uncontrolled. Thus, changes in the interference patterns "at the locations of the two photodiodes 26 reflect changes in the relative phases of the left and right interfering beams at the location of the substrate.
FIG. 5 depicts structure helpful in understanding a basic principle of holographic lithography. Beam 31' coming from the left overlaps with beam 32' coming from the right. In the region of overlap 35, there is an optical standing wave whose spatial period is given by equation (l) above. The vertical hatching depicts this standing wave, which consists of sinusoidally alternating dark and liqht fringes (i.e., regions of high and low irradiance), as plotted in the graphs of FIGS. 5B and 5C
showing high-contrast and low-contrast interference patterns or holographic images, respectively. The interference pattern is recorded in the photosensitive film or resist 36.
The left and right incident beams 31' and 32' are partially reflected from substrate 37, and the interference of these reflected beams can lead to an orthogonal standing wave, as discussed in the paper ~y N.
N. Efremow~ N. P. Economou, K. Bezjian, S. S. Dana, and -H. I. Smith, "A Simple Technique for Modifying the Profile of Resist Exposed by Holographic Lithography, "J.
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206~2~ - 8 -- vac. sci. Tech. lg, 1234 (1981). It is generally desirable to strongly attenuate these reflected beams.
To accomplish this attenuation reference is made to the configuration shown in FIG. 6. The antireflection coating 38 is a commercial product, a film which contains a dye that strongly attenuates the beams 31' and 32' as they pass through it. In this way, the resist 36 records primarily the interference due to the incoming beams 31' and 32', and only a very small remnant of the beams reflected from the substrate 37. XL, a commercial product of Brewer Scientific has been used as the ARC 38, and Microposit 1400-17, as resist 36.
The proper exposure of the resist with a high contrast holographic image, and the subsequent development, yields a relief structure such as depicted in FIG. 7A. If the image has low contrast, a relief ~-structure such as depicted in FIG. 7B can result. An important function of the present invention is to . reliably provide a high contrast image. The contrast of the image can be degraded by many factors, including:
(1) imbalance in the power density in the left and right beams; (2) a variations, over the course of the exposure, of the relative phase of the left and right beams. The former problem is easily rectified. An object of this invention is to solve the second problem in a manner that is widely applicable. Stabilizing the relative phase of the two beams greatly improves the image contrast in holographic lithography. This stabilization of the relative phase occurs according to the invention by monitoring the signals from photodiodes 26, and using these signals to control the voltage applied to the phase shifter 19 in FIG.1. This is accomplished by a feedback loop circuit, a method well-known to anyone familiar with control electronics. The essential purpose of this circuit is to apply a voltage to phase shifter 19 such .'' ' :

20~42~0 that the bulls-eye interference pattern remains unchanged. This, in turn, guarantees that the relative phase difference between beams 3l and 32 i5 unchanged.
FIG. 8 is an electron micrograph of a grating pattern in resist obtained by holographic lithography without the feedback loop in operation, and FIG. 9 shows the result of activating the feedback loop.
Once the relief grating is obtained in the resist, several processes can be brought to bear to obtain a : lO relief grating of higher aspect ratio. One such process is shown in FIG. lO. The relief structure in resist is ~ coated at an oblique angle (so-called shadowing) with ; titanium, SiO or other suitable material. The substrate is then placed in a reactive-ion-etching system (commercially available) supplied with oxygen gas.
Activation of the reactive-ion-etching leads to rapid etching of the polymeric ARC 38, below those areas not protected by the Ti or sio.
There is attached as Appendix A Decription of ZO Technology helpful in further understanding the invention.
Other embodiments are within the claims.

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WO 91/01517 PCr/US90/04028 ., . ~
20~'230' 10 l~PP~D~
Description of Technology 1.1 Introduction A simple and effective feedback system has been deYeloped to stabilize the interference pattern used in producing holographic graSings. Gratings have a very large number of scientific and engineering applications (see [1] for example) and can be produced by a Yariety of different techniques. One 5 of the most effecti~e techniques is to expose a photosensiti~e material to the interference pattern of two optical fields. The photose~;itive material changes in some useful way when exposed to light and then, perhaps, further processed. U~Iolographic" gratings are produced by taking adYantage of the change in the photosensitive material either directly or as part of a multi-10 step process to make a grating structu~e.
; In order to successfully fabricate holographic gratings the interference pattern must be stable in space to within a small fraction of the grating period during the entire time required to expose the photosensitive mate-rial. This requirement becomes more and more difflcult to achieYe as the 15 grating period becomes smaller, as the exposure time becomes longer, and when the optical system must be in a less than ideal enYironment such as a clean room or production area where noise and Yibration leYels are high. If the interference pattern carmot be held steady because of Yibration, noise, thermal drift, and other instabilities the cantrast recorded in the photo 20 sensitiYe material decreases quickly. This makes the hbrication of usefiil gratings difflcult and unre!iable and, when the contrast is smaU enough, impossible. .
The purpose of the inYention is to counteract the urlwanted fringe mo-tion with a simple feedback system so that the resulting interference pattern 25 is stable in space even when large amplitude Yibration, thermal, and other instabilities are present. With the stable fringe pattern resulting from ac-ti-~e feedback the grating fabrication process is made more reliable and less sensitive to small process Yariations such as exposure dose a~d deYel-opment time. Further, the feedback system makes possible holographic 30 grating exposure in unfaYorable enYironments such as clean rooms where large amounts of noise and Yibration exist.
The inYentiOn is a particular implementation of the fringe stabiliza-. . .

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11 20642.30.. `.5- -tion feedback concept in which the method of detectir~g the motion of the fringes is di~erent from previous holographic stabilization systems. The new method of detecting the fringe motion uses the fact that a grat~ng is being exposed and not a general hologram. Because of this, a simple 5 and easy to adjust fringe motion detector can be used, resulting in several advantages.
In surnmary, a feedbac~ system, using a new and simple fringe motion detection method is described which canceles large amplitude vibration, noise, and thermal drift in the optical interference pattern used for the 10 production of holographic gratings. This system results in reliable, large process latitude, holographic grating fabrication even in high noise envi-ronments such as clean rooms.
1.2 Technical Description The feedback system consists of four elements: an electro-optic crystal 5 (Poc~els cell), high-voltage amplifier to dri~e the crystal, a beamsplitter, and a pair of photodiodes. Figure 1 shows the basic configuration. The beamsplitter is placed above and behind the substrate (to be out of the way but still within the o~erlap of the two beams) forming a Mach-Zehnder interferometer. In the Mach-Zehnder interferometer, one beam is transmit-20 ted by the beamsplitter and interferes with the other beam that is reflectedby the bea~splitter. For example, in.Fig. 1 the interference intensity mea-sured by diode "1" is proportional to the magnitude squaret of the sum of the amplitude of beam "a" reflected by the beamsplitter and bearn ~b"
transmitted by the bearnsplitter. A complementary pattern (i.e. at diode 25 "2") is formed from the interference of the two beams with the role~ of transmitted and re~ected bearns re- ersed. A pair of photodiodes is used to measure the intensity signal of both interference patte~s. By power conserYation, the sum of the signals measured in each arm is proportional to the laser output power, which is approximately constant. The diodes 30 are connected to a differential amplifier which subtracts one diode signal from the other. With this configuration, a signal proportional to the sine of the phase dif~erence bet~een the two interferometer arms is produced and the com~Qon mode signals such as the room lights are effectively canceled.
The output of this di~erential amplifier modulates a high-~oltage power .. . .

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mplifier Photodiodes Figure 1: This shows the holographic configuration with feedback stabi-Iization. A bearnsplitter is placed in the overlap of the two coherent bearns but out of the way of the substrate so that no spurious scattering onto the substrate will occur. Two photodiodes measure the intensity of the large s width fri~ges (not to be confused with the fine period fringes forrned at the substrate) formed orl both sides of the beamsplitter. The difference of these two intensities is proportional to the sine of the path phase difference between the two arrns. This signal is amplified aud used to dri~e the Pock-els cell in one arm. The Pockels cell introduces a phase shift (proportional to the applied ~roltage) that almost completely cancels the noise introduced by mechanical Yibration and other disturbances.

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W~ 91/01517 PCr/lJS90/04028 supply which in turn drives an ADP-crystal Poc~els cell. The Poc~els cell modulates the phase of one of the interferometer arrns thereby closing the feedback loop. The high gain of the feedback loop keeps the dif~erence between the two photodiode signals very small resulting in a stable fringe 5 pattern. An advantage of using two diodes and taking the difference is that the fringe pattern is lar~ely independent of the arnbient light level and the Iaser output power. A change in the laser output power will only produce a change in the loop gain and the fringe position is insensiti~e to the loop gain if it is large enough.
10 1.2.1 Details of Feedback Theory The dif~erence of the signals from the two photodiodes is proportional to the sine of the phase difference in the path lengths of the two interferometer arms. This phase difference,~sO~, as a function of time consists of two terms: a term representing the phase of the Poc~els cell and a phase term 15 due to the noise (vibration, air currents, etc) of the system.
~So~l = ~ou~ + ~od~b (1) Let V~ be the ~oltage of the amplifier electronics when one diode is dark and the other one is light, the feedback equation becomes:
V.
¢~Pochb =--7r V Sin(95Poeh~b + q~Noi~) (2) 20 where Vr is the 7r phase shifting ~oltage for the Poc~el~ cell. If the feedbadc system is wor~ing properly then the Poc~els cell phase, ~pO~ will be al-most equal and opposite to~the noise phase disturbance, ¢Nou<. Under these conditions the sine function can be expanded in a Taylor series. Keeping only the first order terms we find:
~Pock-b = lr V (~Podul~ + ~Nou~) (3) Sol~ring this equation for the total phase d~erence giYes ~So~ 5PocJul~ + 9SNou~ = I + K ~Nou~ (4) For a moderate arnount of loop gain, 1rV~/~r, the total noise term can be k substantially reduced.

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1.2.2 Experimental Configuration Figure 1 shows the experimental configuration. A conventional holographic lithography configuration is modified by placing an ADP Pockels cell phase modulator in one arrn. A beamsplitter is placed above and behind the su~
strate to be out of the way and thus not scatter light onto the substrate but still be in the o~erlap of the two bearns. The bearnsplitter forrns a Mach-~ehnder interferometer whose output intensity is monitored by two photo-diodes. The photodiodes are connected to a differential cu~rent arnplifier to produce a signal proportional to the sine of the phase difference between the two arrns. This signal is fed into a high-voltage audio-frequency-bandwidth amplifier which drives the Pockels cell, closing the feedback loop.
Figure 2 shows the oscilloscope traces when the fiinges are locked and not locked for an interferometer configuration that uses the entire 8 foot length of the optical table set up to produce 200nm period gratings. The noise in the fringes without feedback is effectively larger than the grating period resulting in contrast much too small for useful grating exposure.
With the fringes locked, good gratings can be ge~erated. Figure-. 3 and 4 show the difference between exposed grating profiles with and without feed-back, respectiYely.
1.3 Ad~ra~tages and Improvements 1.3.1 Existing methods . . - .:
The use of feedback to stabilize holographic fringes, of which a grating is a special case, goes back to Naumarm and Rose ir~ 1967 [2l. In this ~vor~
they describe a feedbac~ control system that uses a piezoelectric crystal translator, attached to a mirror, as the phase modulator and a method to detect the motion of the fringes consisting of a lens to magnify the fringes onto a slit connected to a photomultiplier tube (PMT). The PMT translates the optical intensity into an eiectrical signal which is then used to driYe the piezoelectric translator. The next adlraDce in holographic feedback ` 30 stabilization was to use a preYiously recorded almos~identical hologram to detect the fringe motion. This scheme was anticipated by MacQuigg in 1974 [33 and proposed by Johansson et al. [41 in 1976. This reference hologram is placed near the hologram to be exposed and produces a moire . .

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Figurc 2: The top oscilloscope trace, ~hows a sigrlal proportional to the sine of the phase noise, withOue the feedbac~ loop, for a interferometer configuration that uses the entire 4 by 8 foot are~L of an optic~ table in a clean room enYirOnment. The noise is much larger than the period of S the grating to be exposed and therefore the contrast is well below a usable leYel. The bottom trace shows the signal with a loop ga~n of about 15. The residual noise is e~sentially zero, resulting in high contrast fri~ges that can be ~ed to expose a large area grating and allow a large process latitude.

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Figure 3: An SE~I micrograph of exposed photoresist profile using feed- .
bacl~. The feedback greatly increases the ringe contrast.

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200nm k ~1 : Figure 4: An SEM micrograph of exposed photoresist ~ithout using feed-bacl~. The contrast is so poor that there is no hope of producing a good grating from this exposure.

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W(~ )1517 , ~ . PCI~/US90/04028 20~30 like interference pattern. This moire pattern results from the interference between an incoming wave and its holographic reconstruction. A shift in this pattern corresponds to a proportional shift in the interference pattern.
Therefore a shift from light to dark to light again represents a full 27r phase 5 shift in one of the arms. This innovation greatly increases the signal level and therefore the signal to noise ratio since not just one fringe, as used by Rose et al., but approximately 103 to 10~ fringes contribute to the detected signal. In 1977 MacQuigg contributed another innovation by Lntroducing a small ~dithern signal which allows the use of loclc-in amplifier technology 10 to measure the error signal (51. A lock-,n amplifier can recover a small - signal with a definite frequency, fo, (i.e: the dither frequency) eYen in the presence of a large noise component. Recent papers on holographic fringe stabilization for applications such as Fourier synthesis of blazed gratings include ~61 and ~71-:
1.3.2 Ad~antages of the new system A serious shortcoming of the previously de~eloped schemes is that a good 4uality, almost-identical hologram must f~rst be produced. But thi~ is pre-cisely the problem that we are trying to sol~e with feedbaclt control. Under conditions where the grati~lg period is ~ery sm~ll and there i~ ~ollsiderable envirorl~nental Iloise disturbing the sy5tem it is not possible to rnake a good quality hologram (i.e. grating). The new phase detector scheme that we propose eliminates this problem for the special case of a grating. The new scheme may not work for a general holog~am but works ver~ well for a grating.
The new scheme uses an ordinary beamsplitter instead of an almost-identical hologram (i.e. grating) placed near the substrate to be exposed and in the overlap of the tvo interfering beams (but out of the way so that light is not scattered from the beamsplitter or its holder onto the sub-strate). Further, the beamsplitter is usually a very efficient optical element so that almost all the light incident on it is converted into a useful signal bythe photodiodes. ~ecause of this the signal detected by the photodiodes is sufflciently strong so that a loc3~-in amplifier is not needed to separate the - signal from the noise and, further, wide bandwidth operation is possible.
- When two photodiodes are used in a differential mode, as in our system, .
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the "common mode" signals such as the room lights and variations in the laser power are canceled out and do not affect the fringe position. There-fore, the significant advantages of this system over prior systems is that a good quality grating does not need to be fabricated in order for the feed-5 back system to work and that the feedback signal level is very strong andfree of common mode interference simpl~fying the electronics and allowing feedback compensation over wide bandwidths.
1.4 Commercial Applications Gratings have a wide range of scientific and engineering applications. For 10 many of these applications, with relatively large periods, active feedbac~
of the hologTaphic exposure system is not needed. ~Iowever, there are important applications, in general where the period is small, where feedbacl~
is critical. For example, the perrneable base transistor (PBT) is a high speed microwaYe power transistor with considerable economic appeal. One 15 of the critical steps in the fabrication of this deYice is producing the f~neperiod grating which i3 used as the Ubase.'' For this step reproducible and high contrast grating exposures are needed. Another application is in the fabrication of distributed feedback laser~. These solid state lasers use a grating structure to pro~ride the feedback. For both of these applications, 20 the final device, (i.o. a permeable base transistor or a distributed feedbac~laser), sells for several hundred to thollsands of dollars. There LS a large market demand for DFB lasers in the telecommunications industry and PBTs in the microwave industry.
In fact, Carl Bozler from Lincoln Lab, whose project is to cornmercial-2s ize the PBT technology~ has visited our lab and we have done experiments together. These experiments show that the feedback system on our holo-graphic set up can make a substantial improvement in the quality and re-producibility of his process. Another visitor to our lab, Kiyoshi Fujii, from NEC corporation in Japan mentioned to me that his group had tried to 30 make DFB lasers using holographic technique and had given up because the process was too unreliable. I believe that if they used the feedback scheme described here that they would have been successful. These two events convinced me that there should be significant economic interest in this technology to warrant patenting and licensing it.

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WO 91/01517 . PCI/US90/04028 j, 20~230 20 Refer enc es T.K. Gaylord and M.G. Moharam U.~nalysis and Applications of Op-tical Dif~raction by Gratings" EE;~ Proc., vol 73, pp. 89~937, ~ay 1985.
~21 D.B. Naumann and H. W. Rose, "Improvement of Recorded ~ol~
graphic Fringes by Feedback Control," Appl. Opt. vol 6, pp. 1097-1104, June 1967.
~3~ D.R. MacQuigg, ~The Modulated Grating ~Iologra~, Proc. Soc.
Photo-Opt~ Irtstrum. Eng. vol 48, pp. 9~100, 1974.
[4~ S. Johansson, L.-E. ~ilsson, K. Biedermann, and K. Kle~eby, 4Hol~
graphic Dif~raction Gratings with Asymmetric Groove Profiles," in Procccdtng~, ICO Jcrusalcm 1976 ConJcrcncc on ~ologr~phy and Op-tical Data Proccssing Pergar on, New York, pp. 521, 1976.
5] D.R. MacQuig~, "Hologram ~rillge stabilization method,~ Applic~ Op-tics, /ol 16, pp. 291-292, Feb. 1977.
6] Lars-Eri!~ Nilsson and Hans Ahlén, "Stabilization of the exposi~g in-terference pattern in holographic grating production, SPI~ ~ol. 240 Per;odic Structures, Gratings, Moiré Patterns alld Diraction Phe-nomena, pp. 22-26,1980.
71 Jaime Frejlich, Lucila.Cescato, and Geraldo F. Mendes, "Analysi~t of an active stabilization system for a holographic setup," A pplicd Optic~, vol. 27, pp. 1967-1976, May 1988~

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Claims (8)

1. A method of holographic lithography which method includes the steps of illuminating a substrate with coherent radiant energy from first and second beams, intercepting radiation from the first and second beams with a first beam splitter that provides at least a first transmitted beam from one of said first and second beams and a reflected beam from the other of said first and second beams that are essentially parallel to provide at least a first interference pattern, photodetecting at least said first interference pattern to provide a control signal representative of at least said first interference pattern, and using said control signal to control the relative phase shift between said first and second coherent beams to maintain the phase difference therebetween substantially constant.

2. Holographic lithographic apparatus comprising, a source of first and second coherent beams of radiant energy, an exposure station illuminated by the first and second coherent beams, at least one phase shifter in the path of at least one of said coherent beams, a first beam splitter arranged to intercept at least a portion of said first and second beams to provide at least a first transmitted portion from one of said first and second beams and a first reflected portion from the other of said first and second beams propagating in essentially the same direction to provide at least a first interference pattern, at least a first radiant energy detector for detecting at least said first interference pattern which is representative of the phase difference between energy in the first and second coherent beams illuminating said exposure station to provide a control signal representative of said phase difference, and a feedback path intercoupling at least said first radiant energy detector with the phase shifter, whereby the control signal applied to the phase shifter establishes the phase imparted by the phase shifter so that the relative phase between the first and second coherent beams at the exposure station remains substantially constant.

3. A method of holographic lithography in accordance with claim 1 and further including the step of illuminating a second beam splitter with radiant energy from a laser to form said first and second beams, establishing the intensity of said first and second beams to be of substantially equal intensity on said substrate, sensing the interference pattern established by said first and second coherent beams with said first beam splitter and at least a first photodetector to provide said control signal, and maintaining the relative phase shift between said first and second coherent beams with said control signal to maintain said interference pattern substantially stationary.

4. Holographic lithographic apparatus in accordance with claim 2 wherein said source of first and second coherent beams of radiant energy comprises, a second beam splitter, a laser illuminating said beam splitter to provide said first and second coherent beams, said first beam splitter coacting with at least said first radiant energy detector for providing said control signal, a variable attenuator in the path of at least one of said first and second coherent beams, and at least said first radiant energy detector comprises at least a first photocell.

5. A method of holographic lithography in accordance with claim 1 wherein said intercepting step also includes providing a second interference pattern and further comprising, photodetecting said first and second interference patterns to provide first and second photodetected signals, and combining said first and second photodetected signals to provide said control signal.

6. A method of holographic lithography in accordance with claim 5 wherein said step of combining said first and second photodetected signals includes differentially combining said first and second photodetected signals to provide said control signal.

7. Holographic lithographic apparatus in accordance with claim 2 wherein said first beam splitter also provides a second interference pattern and further comprising, a second radiant energy detector illuminated by said second interference pattern for providing a second detected signal, and a combiner for combining said first and second detected signals to provide said control signal.

8. Holographic lithographic apparatus in accordance with claim 7 wherein said combiner is a differential combiner that differentially combines said first and second detected signals to provide said control signal.