High resolution imagery systems and methods

What is claimed is:

1. A system for producing a highly corrected optical image comprising:

substantially monochromatic light source means providing a plurality of light sources having limited spatial coherence and substantial temporal coherence within a distributed light beam area;

an optical system within the path of the light beam and having an aperture stop; and

microlithographic phase transmission grating means at the aperture stop, the grating means including a microstructure comprising a plurality of sets of plateaus, the light beam being disposed to provide light distributed through the optical system and the sets of plateaus of the grating means.

2. The invention as set forth in claim 1 above, wherein the sets of plateaus provide incremental amounts of phase retardation in the light energy from the light source means at the aperture stop.

3. The invention as set forth in claim 2 above, wherein the sets of plateaus comprise concentric circular patterns having non-linearly varying radial widths.

4. The invention as set forth in claim 3 above, wherein the optical system has spherical type aberration and the non-linear patterns of the grating means produce a varying phase retardation as a function of radius, the phase retardation opposing and compensating the spherical type aberration of the optical system.

5. The invention as set forth in claim 4 above, wherein the optical system has a known chromatic dispersion and the chromatic dispersion in the grating means opposes that in the optical system.

6. The invention as set forth in claim 4 above, wherein the sets of non-linear patterns are configured with at least one 180.degree. phase change at some fraction of the dimension of the aperture stop.

7. The invention as set forth in claim 4 above, wherein the sets of patterns are configured with changes in the sets of plateaus at various incremental radii to produce phase changes in the light, providing a pupil function comprising more than one annular zone.

8. The invention as set forth in claim 4 above, wherein selected ones of the patterns are partially transmissive or opaque in accordance with a selected pupil function.

9. The invention as set forth in claim 1, wherein the grating means bends the light of the wavefront passing through the aperture stop at angles limited to less than approximately 5.degree..

10. The invention as set forth in claim 1 above, wherein the system includes means incorporating the light source for providing spatially dispersed point sources of light throughout an area of illumination.

11. The invention as set forth in claim 1 above, wherein the optical system and phase transmission grating means are configured to bend the light into converging planar wavefronts generating narrow elongated illumination along a central axis.

12. The invention as set forth in claim 11 above, wherein the optical system comprises at least one spherical refractive element and the phase transmission grating has a plurality of concentric multi-plateau rings of substantially periodic radial width and spacing.

13. The invention as set forth in claim 12 above, wherein the optical system comprises a microscope for viewing a specimen, the microscope including refractive elements having a degree of tolerated spherical type aberration and the grating means compensating for the spherical type aberration.

14. The invention as set forth in claim 13 above, wherein the optical system further comprises transparent cover means and the grating means further compensates for spherical aberration from the cover means.

15. The invention as set forth in claim 1 above, wherein the optical system comprises a cylindrical lens means and the phase transmission grating comprises a plurality of substantially parallel multi-plateau tracks substantially parallel to the axis about which the cylindrical lens means curves.

16. An optical system for providing a desired light distribution at a chosen surface, the system comprising:

light source means providing a wavefront of light that is temporally coherent to a selected number of waves and of limited spatial coherence;

optical lens means disposed in the path of the light for refracting the light to form a wavefront providing an approximation of the desired light distribution at the chosen surface, the wavefront having local phase variations therein arising from aberrations in the lens means; and

phase plate means including multi-level transmission grating means disposed with the optical lens means for locally phase retarding the wavefront to a substantially smaller degree than the number of waves of temporal coherence to obtain the desired wavefront and light distribution at the chosen surface.

17. The invention as set forth in claim 16 above, wherein the light source means comprises coherent light source means and means responsive to the degree of spatial coherence in the wavefront light for controllably decreasing the spatial coherence in the light from the coherent light source means.

18. An optical system for providing a desired light distribution at a chosen surface, the system comprising:

light source means comprising means for providing pulse sequences of light and providing a wavefront of light that is temporally coherent to a selected number of waves and of limited spatial coherence;

optical lens means disposed in the path of the light for refracting the light to form a wavefront providing an approximation of the desired light distribution at the chosen surface;

phase plate means disposed with the optical lens means for locally phase retarding the wavefront to a substantially smaller degree than the number of waves of temporal coherence to obtain the desired light distribution at the chosen surface;

means responsive to the light for sensing the degree of spatial coherence and;

means responsive to the sensed degree of spatial coherence for controlling the means for decreasing the same.

19. The invention as set forth in claim 18 above, wherein the means for controllably decreasing the spatial coherence comprises a pair of surfaces having randomized phase transmissive patterns in the path of the light and means for moving one of the surfaces relative to the other during the pulse sequences.

20. An optical system for providing a desired light distribution at a chosen surface, the system comprising:

coherent light source means providing a wavefront of light that is temporally coherent to in excess of about 10,000 waves and of limited spatial coherence, the light source means providing of the order of 10.sup.2 point sources of light on each point at the image plane;

means for controllably decreasing the spatial coherence in the light from the light source means;

optical lens means disposed in the path of the light for refracting the light to form a wavefront providing an approximation of the desired light distribution at the chosen surface, the spherical aberration of the optical lens means being limited to about 75 waves or less maximum difference from a spherical aberration-free condition; and

phase plate means disposed with the optical lens means for locally phase retarding the wavefront to a substantially smaller degree than the number of waves of temporal coherence to obtain the desired light distribution at the chosen surface.

21. A system for precision imaging of light energy comprising:

means providing illumination in the form of a multiplicity of independent light sources of substantially the same wavelength throughout a beam area;

a plurality of refractive lens elements together providing light imaging with a predetermined spherical type aberration; and

transmission grating means cooperative with the refractive lens elements for phase delaying and redirection of waves from the multiplicity of independent light sources to compensate by locally varying phase retardation for the spherical type aberration of the refractive lens elements such that a compensated composite wavefront is provided.

22. The invention as set forth in claim 21 above, wherein the independent light sources are substantially spatially incoherent and temporally coherent to about the order of 10,000 waves.

23. The invention as set forth in claim 21 above, wherein the transmission grating means comprises a system of concentric multi-plateau rings, each providing incremental amounts of phase delay in the illumination passing therethrough to provide of the order of about 1/20th wave precision throughout the composite wavefront.

24. The invention as set forth in claim 23 above, wherein the multi-plateau rings have varying slopes and widths and provide progressively varying phase delays in increments of a maximum that is an integral number.

25. A system for precision adjustment of the shape of the wavefront of a beam of light energy comprising:

means providing illumination in the form of a multiplicity of light sources of substantially the same wavelengths throughout a beam area;

a plurality of refractive lens elements in the path of the beam illumination and together providing light imaging with predetermined aberration characteristics which vary symmetrically as a predetermined function of the ray height at the aperture stop, but providing a high degree of correction for aberration components which vary as the image height or as the angular orientation of the ray meridian at the aperture stop; and

transmission grating means at the aperture stop in the path of the beam illumination and cooperative with the refractive lens elements for phase delay and redirection of the illumination in accordance with a second predetermined function of the ray height at the aperture stop and compensating for the predetermined aberration characteristics of the refractive lens elements which vary symmetrically as a predetermined function of the ray height.

26. The system as set forth in claim 25 above, wherein the aperture stop symmetrical variations which are corrected by the phase plate are spherical type aberrations.

27. A system for providing a precisely compensated monochromatic image on a semiconductor wafer comprising:

a series of refractive lens elements disposed along an optical axis and having an aperture stop, the cumulative spherical type aberration being no greater than about 75 waves and the cumulative chromatic dispersion being positive;

a transmission grating disposed at the aperture stop, the grating having a plurality of rings each with multiple plateaus providing local wavefront compensation for the spherical type aberration and a compensating negative chromatic dispersion for light of a predetermined wavelength; and

illuminating means directing substantially monochromatic light along the optical axis, the substantially monochromatic light being of the predetermined wavelength and having temporal coherence of the order of 10,000 waves and being substantially spatially incoherent.

28. The system as set forth in claim 27 above, wherein the individual plateaus provide incrementally varying amounts of optical wave retardation in the locally impinging areas of light, and wherein the illuminating means provides a plurality of spatially incoherent, phase random, light sources having a bandwidth of the order of 0.03 nm in the ultraviolet region of the spectrum.

29. The system as set forth in claim 28 above, wherein the rings are varied in width and slope to provide a wavelength of optical retardation to compensate locally for wavefront variations in the composite wavefront, wherein the phase relation of the waves is varied in .pi. phase fashion, and wherein the transmission grating bends the light to a maximum angle of about 5.degree..

30. The system as set forth in claim 28 above, wherein the plurality of rings each have progressively varying plateaus and are arranged in selected subsets in which the phase relation in the progressions of the plateaus vary at least one radial region relative to the rings to define a pupil function of at least two zones, and wherein the illuminating means comprises a pulsed laser source.

31. The system as set forth in claim 29 above, wherein the illuminating means comprises an excimer laser, etalon tuning cavity, and phase randomizer means comprising a pair of quasi-random phase plates in the path of the illumination and means for varying the spatial relationship of the light sources relative thereto.

32. The system as set forth in claim 31 above, wherein the phase randomizer means comprises means for sensing the degree of spatial coherence in the light sources, and means responsive to the sensed degree of coherence for introducing varying relative movement of the light sources between the quasi-random phase plates, and wherein the illuminating means is controllable and the system further comprises means responsive to the light energy of the illuminating means for controlling the duration of operation of the illuminating means.

33. An optical system for providing precise aspheric correction of a composite wavefront having aberrations from idealized specifications comprising:

a transmissive blazed grating plate having a microlithographic pattern of incrementally varying narrow tracks, adjacent tracks varying differentially in height, the maximum differential height of the tracks on the plate being proportioned to the ratio of one wavelength of incident monochromatic light at a selected wavelength divided by the difference of the index of refractive of the plate from unity, the widths of the tracks through the pattern being proportioned to provide local phase delay of the wavefront according to a predetermined function for the composite wavefront to effect redistribution of the microstructure of the wavefront, the widths being precise to within about 1/20th wavelength of the monochromatic light; and

means for illuminating the pattern on the plate with monochromatic light of the selected wavelength, the light having predictable periodicity and constituting multiple phase random sources.

34. The invention as set forth in claim 33 above, wherein the monochromatic light is in the ultraviolet region, wherein the tracks are concentric and arranged in periodic sequences in which heights vary progressively, wherein the minimum track widths at the minimum are of the order of 1 micron, and wherein the plate bends the light to a maximum of about 3.degree..

35. The invention as set forth in claim 34 above, wherein the system further comprises a number of spherical optical elements in optical combination with the grating plate, the optical elements providing cumulative aberrations and the plate compensating for such aberrations.

36. The invention as set forth in claim 35 above, wherein the system has an aperture stop and the plate is disposed at the aperture stop, wherein the progressive patterns are arranged in subsets of different phase to provide a pupil function comprised of at least two annular zones, and wherein the cumulative aberrations in the spherical lenses that are compensated are principally spherical type aberration and chromatism.

37. The invention as set forth in claim 36 above, wherein there is a pupil function comprised of six annular zones for increase of depth of focus, and wherein the periodicity of the monochromatic light is such that the waves have temporal coherence of at least about 50 times greater than the maximum phase delay, and wherein the multiple phase random sources are such as to provide approximately 10.sup.2 sources for each point being illuminated.

38. The invention as set forth in claim 37 above, wherein the wavelength of the monochromatic light is at about 248 nm, and wherein the maximum height differential between adjacent plateaus is about 0.427 microns.

39. The invention as set forth in claim 38 above, wherein the tracks are arranged in rings having 8 tracks per ring varying incrementally in 1/8th wavelength optical retardation steps from 0 to 7/8th wave height for rings within each zone.

40. A system for high resolution imagery on an object being illuminated comprising:

an optical system having an aperture stop;

first substantially monochromatic light source means of a given wavelength transmitting through the optical system;

light bending means including a first transmission grating structure disposed at the aperture stop for providing a really distributed varying wave delays in light of the first wavelength transmitted through the optical system to provide an adjusted composite wavefront, said light bending means also including a second transmission grating structure displaced from the first transmission grating structure; and

second monochromatic light source means of a second wavelength illuminating the second transmissive grating structure for alignment of the object being illuminated relative to the optical system.

41. The system as set forth in claim 40 above, wherein the first transmission grating structure is defined by a plurality of concentric tracks in an interior region of the light bending means, and wherein the second transmission grating structure is an annular pattern disposed about the interior region.

42. The system as set forth in claim 41 above, wherein the first and second transmission grating structures each comprise a plurality of concentric rings each defined by a progression of varying height plateaus on a transmissive substrate.

43. The system as set forth in claim 42 above, wherein the first monochromatic light source means operates in the ultraviolet region, and wherein the second monochromatic light source means operates in the red region, and wherein the second monochromatic light source means and the second transmission grating structure provide a finely focused reference beam at an object plane for the image defined by the first monochromatic light source means and the first transmission grating structure.

44. The system as set forth in claim 43 above, wherein the optical system further comprises aspheric means disposed along the optical path thereof for shaping the light from the second light source into an annular pupil of diameter corresponding to that of the second transmission grating structure.

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BACKGROUND OF THE INVENTION

The use of high resolution optical imagery systems, as in photolithographic systems for the semiconductor industry and microscope systems for a wide variety of applications, has continued to grow despite the availability of other technologies, such as high resolution systems based upon atomic particle matter typified by electron beams or X-rays. The greater cost and lower operating convenience of these latter systems, as well as the longer times required for the formation of images, establish that the optical imagery systems will remain preferable for many applications for the foreseeable future. However, constantly increasing requirements for more precise technology have taken the optical imagery systems virtually to the limits of resolution values which can be achieved with refractive optics. For example, very large scale integrated circuits are constantly being reduced in size and made with higher component density, an objective measure of which is the minimum linewidth specification. Whereas one micron linewidths were suitable until recently, present objectives in the industry contemplate linewidths well down into the submicron category, of less than 0.5 microns and even 0.3 microns. This requires a line resolution for a refractive optical system of the order of several thousand lines per millimeter, which has not heretofore been achievable with an optical imaging system of suitable aperture and depth of field.

In response to these problems the optical industry has devised progressively more sophisticated multi-element lens systems using advanced lens design computer programs. The advanced level of the state of the art is exemplified by the so-called "i-line" lens system, which utilizes a complex configuration of some twenty refractive elements of highest quality glasses. The best that this system can achieve, however, is in the range of 0.7 micron linewidth resolution, because the multiple factors involved in complex lens design (chromaticism, coma, astigmatism, spherical aberration being included), together with the problems of achieving sufficient uniformity and adequate wave energy at the target, establish ultimate limitations that are presently at about 0.7 micron linewidth. There are also inherent limitations on manufacture when dealing with this order of precision. For example, the best diamond turning procedures still leave optical surfaces too rough for operation at short wavelengths (e.g. ultraviolet).

The semiconductor industry, however, has devised many production and inspection procedures based upon optical imaging systems, and would prefer to use these for the specific advantages they provide. For example, in preparing the successive layers on a silicon or other wafer, a "wafer stepper" system is employed incorporating the high resolution refractive optics. There is a different precision photomask for each layer to be formed. The wafer is first covered with a layer of photosensitive material of the type on which an image can be fixed by exposure to a suitable amount of light energy. The wafer stepper mechanism then precisely and sequentially places the wafer at chosen matrix positions relative to an optical axis. At each position in the matrix pattern on the wafer, an exposure is made through the photomask, with the optical system typically reducing the image a selected amount, usually five or ten times. Inherent requirements for this type of system are that the light energy be adequate for each exposure, that the exposed image be uniform across the image, that the depth of field be sufficient and that the resolution meet the specifications of the design. These requirements are not easily met in combination, because the very small size of the images and the extreme precision that are required greatly restrict the design alternatives that are available. Once the exposure is made at all positions in the matrix and the unfixed material is washed off, the images can be checked for accuracy and uniformity of reproduction. Optical microscopes are usually employed for checking, on a statistical basis, the characteristics of the various images. The inspection may comprise one or more of a combination of procedures involving automatic or operator measurement of linewidths or other characteristics, but all of these procedures entail precise and high resolution magnification of the image.

The problems of obtaining higher resolution optical imagery systems of practical application thus appear to have approached a limit. Whether or not such limit ultimately is found to be insurmountable with more complex multi-element lens systems remains to be seen. Some substantially different approach appears to be needed, however, that would free optical imagery systems from the constraints on design and manufacture that are inherently imposed in reconciling many higher order terms involved in optical design equations. Tentative movements in this direction were made a number of years ago in proposals that an aspherical element of a special character be introduced into the lens system. These proposals are best evidenced in an article by Kenro Miyamoto entitled "The Phase Fresnel Lens", presented at the November 1960 meeting of the Optical Society of America and subsequently published in the Journal of the Optical Society of America, January 1961, pp. 17-20. In that article, Miyamoto also referenced earlier articles of philosophically similar nature. He principally suggested that a "phase Fresnel lens" be inserted in the pupil plane of an optical system to deform the wavefront surface passing therethrough so as to compensate, for example, for spherical aberration. His proposals were general in nature with no consideration being given to problems of achieving high transmission efficiency, high resolution such as would approach the needs of the semiconductor industry, or adequate depth of field. Miyamoto, in an example, suggested the use of single layer thin film rings of a minimum radial dimension of 0.63 mm. He did not address the difficulties involved in fabricating a more precise system, i.e. a blazed transmission grating.

Miyamoto asserts that a phase Fresnel lens can be made to deform a wave surface by an amount:

.phi. (u,v) - (k-1)

where K = 1, 2, . . . m, where the amount of deformation in all zones is smaller than .lambda., by depositing a (single) thin film covering various circular zones. He then asserts that a wave surface thus deformed is "quite equivalent" to a lens which deforms the wave surface by the amount .phi. (u,v).

His equations describe a perfectly blazed phase grating yet his description of his method using a single thin film leads to the creation of a binary phase grating, which might also be called a "phase reversal zone plate". This type of grating can only function to provide alternation of phase delays between two values.

The phase reversal zone plate was studied by Melvin H. Horman in an article entitled "Efficiencies of Zone Plates and Phase Zone Plates", published in Applied Optics, November 1967, pp. 2011-2013. Horman defines the efficiency of a zone or phase plate as the "percentage of the flux in the illuminating wavefront which goes to their principal images", and using this definition he gives the first order efficiency of the phase reversal zone plate as 40.5%. Horman notes that the efficiency of a phase Fresnel lens, if it could be built, would approach 100%. Fabrication of a phase Fresnel lens of high efficiency, working in conjunction with highly corrected optics, however, has apparently not been attempted or reported in the intervening years. Triangular profile plates for independent use as microlenses have been made for certain applications.

The Miyamoto proposal thus is recognized as offering the possibility for greater freedom of lens design, but as far as is known from the literature was never implemented. This was probably due to a combination of reasons including limitations perceived as to the advantages to be derived, the difficulty of fabrication of the phase Fresnel lens in the form described, other advances in optical design using solely refractive optics, and a lack of appreciation of much more complex factors inherent in the problem. For example, there can be a substantial variation in efficiency between the parallel and orthogonal components of incident light, with grating blaze angle. Also, Miyamoto failed to appreciate, or at least discuss, the important role that temporal coherence of the individual spectral components plays in maintaining the resolution or space-bandwidth product of a phase Fresnel lens. It is shown hereafter that by properly considering, in the manipulation of wavefront aberration, factors including wave component distributions, the precise distribution of the illuminating energy, and local, temporal and spatial rearrangement of phase relationships, together with a coactive refractive lens configuration, the resolution of an optical imagery or readout system can be increased beyond levels previously thought unattainable, with useful depth of field and high efficiency.

The same principles upon which high resolution optical imagery or readout can be achieved by combinations of phase gratings and optical refractive elements can also be used in other optical applications. These include microscopy and optical transform functions, conical axicon phase gratings in combination with a spherical object lens, cylindrical phase gratings in combination with conventional cylindrical lenses, and toroidal aspheric grating lenses. Conical axicon phase gratings can be particularly useful in combination with optical refractive elements to provide a narrow line of light of predetermined length for an optical disk recording or readout device, eliminating the need for an autofocus system. The ability to precisely correct wavefront aberrations can in other words be of potential benefit wherever refractive optics limits are approached provided that the particular spectral characteristics of phase plates are recognized and accounted for in the system design.

SUMMARY OF THE INVENTION

Systems and methods in accordance with the invention dispose at least one holographic areally distributed transmissive grating element in the optical train of a refractive optical system, one of the elements usually being at the aperture stop. The grating element and other components are illuminated monochromatically by a multiplicity of distributed spatially incoherent but temporally coherent sources in such manner as to introduce incrementally varying phase retardation. These incremental variations vary nonlinearly but in controlled fashion throughout the illuminating field, forming a composite wavefront which compensates for selected aberrations. In an optical imaging system the compensation is not only for predetermined spherical aberration but also predetermined chromatism in the refractive optics. Wave retardation is effected by a transmission grating having segments defined by multiple plateaus varying by fractional wavelength increments that provide high efficiency diffraction. Areal organization of the segments may include phase reversals and transmissivity changes to modify the wavefront component interactions so as to create a number of interrelated pupils whose composite effect can be, for example, increased depth of field, better contrast, and improved resolution.

In one general example of an imaging system, an illuminator is employed that comprises a monochromatic light source, means for distributing the beam evenly throughout an extended beam area and means for establishing temporal coherence of the waves at above a predetermined minimum, but with spatial coherence effectively eliminated. The phase plate in this instance comprises a light transmissive element having a plurality of concentric rings, each having multiple plateaus varying by incremental wavelength fractions, the plateaus of a ring together providing low angle bending of the local wavefront. The phase plate is disposed at the aperture stop of a refractive optics system, the design of which is integrated with the phase plate and thereby simplified. The refractive optics may, for example, include a collimating lens portion and an objective lens portion but is typically designed with relatively few elements having known but limited aberrations acceptable within overall limits for the system. The phase plate is fabricated by microlithographic techniques so as to provide rings of varying radii with successive plateau levels within each ring. By changing the step relationship of plateaus in different ring groupings the phase relationship of waves passing through different subdivisions of the phase plate is selectively reversed to provide a number of pupils. Some rings or groups of rings may be rendered opaque or partially transmissive so that light from certain areas can be blocked or attenuated. Thus the spatial distributions and phase relationships of the illumination from the multiplicity of light sources are restructured so that the composite waveform is precisely reconstituted to cancel out the permitted aberration in the refractive optics. By this system and method resolution of the order of 2,500 lines per millimeter, high efficiency transmission, high depth of field and excellent contrast are achieved. The refractive optics used in the system not only require substantially fewer elements but the design procedure can accommodate greater tolerance for specific characteristics, such as spherical aberration and chromaticity.

To achieve high beam intensity, evenness of intensity distribution, and achromatism a pulsed laser is a preferred illuminator source for semiconductor fabrication operations. However other sources, such as mercury arc systems, can be used in conjunction with conventional methods for overcoming intensity distribution, filtering and chromatism problems.

In accordance with further features of the invention, the illuminator in one specific example comprises an excimer laser and etalon tuning cavity combination operating in the ultraviolet range, as at 248 nm, to provide bursts of light energy that have temporal coherence to in excess of 50,000 waves. The bursts of illuminating light energy are passed through a phase randomizer comprising a pair of spaced apart random phase plates and an intermediate beam shifting device which distributes spatially incoherent multiple light sources in a statistically uniform manner at the photomask or object plane. The phase plate is configured to provide on the order of 3.degree. of light bending, with high efficiency transmission of first order waves, and in a preferred configuration includes a six zone-pupil configuration defined by segments of alternating phase, achieved by selective reversal of the plateau progressions in the phase plate subdivisions. More than one phase plate can be arranged in a system, with one being disposed at the aperture stop and the others adjacent along the beam path to provide particular aspheric characteristics. With a 248 nm source, the maximum thickness of the phase plate plateau regions is limited to approximately 0.427 microns, the individual plateaus being only of the order of 1.5 microns wide at the narrowest ring. The temporal coherence in the wavelets is maintained at approximately 50 times greater, or more, than the maximum phase delay introduced by the multi-plateau regions.

A number of different systems in accordance with the invention illustrate the versatility of the concept. In a microscope system, for example, light directed from an illuminator system onto an object under examination is imaged with higher resolution than heretofore by using a phase plate which is at the critical aperture and compensates for aberrations of refractive elements in the system, as well as spherical aberrations introduced by a transparent cover plate over the specimen under examination. In an axicon type of system, a phase plate in accordance with the invention is configured to cooperate with one or more spherical elements so as to bring planar waves into conical focus. The converging waves create the relatively long needle of light along the optical axis that characterizes the axicon design. In the cylindrical lens system, wavefront compensation is effected by parallel rather than concentric plateaus, again for higher resolution and precision.

Further, the phase plate may advantageously include an outer annular region of concentric rings defining separate light bending gratings and fiducial patterns. Coherent light of a different wavelength (e.g. a red wavelength) than the image wavelength can be transmitted through different portions of this outer annular region for use in alignment of the target surface relative to the projected image, without affecting the photosensitive surface.

Phase plates in accordance with the invention are fabricated by preparing photomasks or direct writing to define ring patterns of desired character for each of a succession of deposition or etching steps, preferably arranged in binary progression. For example, three successive steps can be used to define deposition layers for one, two and four plateau heights, to provide cumulatively a progression of from zero level to seventh level plateaus, using a series of three photoresist, wash and deposition subsequences. Each deposition subsequence may, for example, add an incremental fraction of a wavelength difference of high purity silica, with predetermined radial variations between the rings. Thus a phase plate may be constructed having approximately 1,600 rings, each of eight plateau levels, on an element of the order of 10 centimeters in diameter. This size is of the range needed for current wafer stepper equipment that can produce the large wafers and high resolution required for modern semiconductor products. A complementary binary sequence may alternatively be used, but based upon etching instead of deposition of the layers.

The outer annular rings used with a second wavelength source for target alignment are similarly formed, recorded from the phase mask or written directly at the same time as the rings in the image area. The outer rings are however deposited separately because the wavelength and consequently the required layer thicknesses are different.

A slightly smoother and more efficient blaze angle may be formed on the grating by creating a succession of 16 plateau heights using four binary masks to define a progression from 0 to 15 levels sized to create optical phase delays from 0 to 15/16 wavelength. Similarly, coarser and less efficient gratings may alternatively be created for specific applications using only four plateau levels.

Different sets of diffractive or reflective rings are also advantageously deployed on the phase plate. For this purpose the element is initially overlaid, at least in certain regions, with a base (e.g. chrome) layer. The rings are defined by scribing during rotation or by photo-etch techniques.

One set of rings forms a number of groups of lens centering and spacing gratings. These gratings are positioned and specially configured with respect to different individual lens elements, or subgroups of the lens elements. They provide a focused beam at the optical axis when collimated light is directed through them at the critical aperture and the lens elements in the chosen lens subgroup are properly positioned. Thus the centration and axial position of each lens can be precisely referenced as it is added into the assembly.

A second set of reflective rings is initially written as an outer peripheral grouping concentric with a nominal axis that is to serve as the center for subsequent patterns. This ring set serves as a fiducial reference for photomasks or for compensating for eccentricity of the phase plate when separate tracks are being directly written during fabrication of the phase plate in a rotating system.

The precision required in placing the multi-plateau rings on the phase plate in order to achieve sub-micron resolution im