Patent Publication Number: US-7713684-B2

Title: System and method for absorbance modulation lithography

Description:
The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/653,766 filed on Feb. 17, 2005, and is a continuation-in-part application of U.S. Ser. No. 11/154,352 filed on Jun. 16, 2005. 
    
    
     BACKGROUND 
     The present invention relates generally to lithography, and in particular, to zone plate array lithography. 
     In a zone plate array lithography system, an array of diffractive lenses such as Fresnel zone plates may be used to form an array of tightly focused spots on a photosensitive layer that is on top of a substrate. For example, U.S. Pat. No. 5,900,637, the disclosure of which is hereby incorporated by reference, discloses a mask-less lithography system and method that employs a multiplexed array of Fresnel zone plates. The light incident on each diffractive lens may be controlled, for example, by one pixel of a spatial light modulator. The spatial light modulator for use in such a system should provide a high refresh rate, be able to operate at short wavelengths such as under 200 nm, and be able to perform gray-scaling or intensity modulation in real time. 
     One commercially available spatial light modulator that may satisfy the above requirements is the grating light valve (GLV) spatial light modulator made by Silicon Light Machines of Sunnyvale Calif. The GLV consists of a linear array of pixels, and each pixel consists of six metallic ribbons that form a diffraction grating. Alternate ribbons may be moved by electrostatic actuation to provide either a reflective surface or a grating. 
     The lithographic resolution, however, of such a system may be limited by the contrast of the aerial image. The image contrast is dependent on the printed pattern. The optical performance may be quantified by calculating the aerial image contrast of a dense grating as a function of the half-pitch of the grating. The image contrast, K is defined as: 
                   K   =         I   max     -     I   min           I   max     +     I   min                 (   1   )               
where I max  and I min  are defined as the maximum and minimum intensities of an illumination signal that may be employed to provide a desired pattern. For example, as shown in  FIG. 1 , a desired pattern  10  that includes alternating imaged regions (as shown at  12 ) and non-imaged regions may be created using an illumination signal  14 . Note that the pitch p of the desired grating pattern  10  corresponds to the pitch p of the illumination signal  14 .
 
     The intensity profile of the illumination signal  12 , however, results in an imaging pattern on a photo-resist layer  16  when imaged by illumination source signals  18  (again having a pitch p). The photo-resist layer  16  is supported by a wafer  20 , and includes marked regions in the photo-resist layer that have been exposed by the sources  18 . After exposure, the photo-resist layer is developed, and the marked regions are removed, leaving exposed portions  22  of the underlying wafer  20 . Efforts to increase resolution (e.g., decrease the pitch p), however, may result in a degradation in image contrast, due at least in part to the intensity profile of the illumination signal  14 . 
     In particular, the aerial images for gratings of different periods may be simulated assuming a zone plate array lithography system of numerical aperture (NA)=0.7 and λ=400 nm. The cross-section through each grating may then be averaged over several line-scans, and the image contrast may be calculated using Equation (1) above. The image contrast may be plotted as a function of k 1 , where k 1  is a measure of the lithographic resolution (normalized to the wavelength and NA), and is given by: 
                     k   1     =       p   2     ⁢       N   ⁢           ⁢   A     λ               (   2   )               
where NA is the numerical aperture and λ is the exposing wavelength. For example, a system of the prior art may provide that k 1 =0.32, which corresponds to an image contrast of about 18%. As the pitch p becomes smaller, the image contrast will be negatively affected, due in pail, to the spatial extent of each illumination source  18 .
 
     Contrast enhanced lithography may be employed in an effort to improve image contrast. In particular, as shown in  FIGS. 2A-2D , a contrast-enhancement material  24  that is spin coated on top of a photo-resist layer  26  on a wafer  28 . The contrast-enhancement material  24  may, for example, be a photo-bleachable polymer, whose absorption decreases (i.e., becomes more transparent) with increasing exposure dose. The intensity of transmitted light may be plotted as a function of time for an ideal contrast-enhanced system. Prior to exposure of the contrast-enhancement material  24 , the material  24  is opaque, and almost no light passes through the material  24 . After sufficient exposure by illumination beams  30 , the material becomes transparent and light is transmitted in areas indicated at  32 . Light is let through into the photo-resist layer only where the exposure dose is high enough to bleach the contrast-enhancement material  24  completely. This increases the contrast of the image recorded in the photo-resist. An antireflective coating between the photo-resist and the wafer may also be employed. 
     As shown in  FIG. 2B , illumination is able to reach defined regions  34  of the wafer  28  only in areas where the contrast-enhancement material  24  has become transparent (as shown at  32 ). The contrast-enhanced material  24  is then removed as shown in  FIG. 2C  using a suitable medium in (such as water) in which the contrast-enhancement material  24  will dissolve. The defined regions  34  are then removed through photo-resist development, leaving openings  36  in the photo-resist layer  26  through which portions of the wafer  28  may become exposed as shown in  FIG. 2D . 
     By employing a contrast-enhancement material and by controlling the photo-bleaching rate of the contrast-enhancement material, as well as the clearing dose of the photo-resist, one may enhance the contrast of the aerial image that is recorded in the photo-resist. The contrast enhancement material behaves, in essence, as a contact mask, which increases the contrast of the image recorded in the photo-resist. The contrast enhancement material is removed from the resist prior to development. If the contrast-enhancement material is incompatible with the photo-resist, a barrier layer is needed between the contrast-enhancement material and the photo-resist. This also incurs an additional step for removal of the barrier layer after exposure. There are several commercially available contrast-enhancement materials, some of which are water-soluble. 
     Ideally, such contrast-enhancement materials would become bleached by the illumination signal  14  in an on/off step pattern that provides an instantaneous step at the edge of each illumination beam. Since the beams, however, provide an intensity profile as shown in  FIG. 1B , the contrast-enhancement material bleaches in varying amounts with distance from the center of each illumination beam. This limits resolution. Moreover, repeated illumination near a non-imaged area may accumulate over time, and may eventually reach a threshold within the material for becoming transparent. 
     Contrast-enhancement may also be achieved by diluting the developer or by using thin photo-resist layers, but such systems may also involve difficulties such as increased line-edge roughness, as well as difficulties with pattern transfer respectively. 
     There is a need therefore, for an imaging system that more efficiently and economically provides increased image contrast in mask-less lithography. 
     SUMMARY 
     In accordance with an embodiment, the invention provides a lithography system that provides an array of areas of imaging electromagnetic energy that are directed toward a recording medium. The reversible contrast-enhancement material is disposed between the recording medium and the array of areas of imaging electromagnetic energy. 
     In accordance with another embodiment, the invention provides a lithography system that includes a first interference system for providing an interference pattern of a first electromagnetic field of a first wavelength on a surface of a recording medium, as well as a reversible contrast-enhancement material being disposed between said recording medium and the first interference system. 
     In accordance with a further embodiment, the invention provides a method of forming a lithographic image on a photo-resist material. The method includes the step of illuminating at least a first portion of a reversible contrast-enhancement material and an associated portion of a photo-resist material with a first electromagnetic energy field having a first wavelength. The illumination of the reversible contrast-enhancement material causes the reversible contrast-enhancement material to change from a first state to a second state in the first portion. The method also includes the step of illuminating at least a second portion of the reversible contrast-enhancement material that with a second electromagnetic energy field having a second wavelength causing the reversible contrast-enhancement material to remain in said first state in the second portion. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
       The following description may be further understood with reference to the accompanying drawings in which: 
         FIG. 1A  shows an illustrative diagrammatic view of a desired image pattern; 
         FIG. 1B  shows an illustrative diagrammatic view of an illumination signal for forming an image pattern as shown in  FIG. 1A ; 
         FIG. 1C  shows an illustrative diagrammatic view of the formation of the desired image pattern shown in  FIG. 1A  using the illumination signal shown in  FIG. 1B  in accordance with the prior art; 
         FIGS. 2A-2D  show illustrative diagrammatic views of a process for forming the desired image pattern shown in  FIG. 1A  using contrast-enhanced lithography in accordance with another system of the prior art; 
         FIGS. 3A-3F  show illustrative diagrammatic views of a process for forming the desired image pattern shown in  FIG. 1A  in accordance with another system of the prior art; 
         FIG. 4  shows an illustrative diagrammatic view of a reversible contrast enhancement material in accordance with another embodiment of the invention; 
         FIG. 5  shoes an illustrative diagrammatic exploded view of an array of energy sources and an array of diffractive elements for use in a system in accordance with an embodiment of the invention; 
         FIG. 6  shows an illustrative diagrammatic view of a lithography system in accordance with an embodiment of the invention; 
         FIG. 7  shows an illustrative diagrammatic view of a lithography system in accordance with another embodiment of the invention; 
         FIG. 8  shows an illustrative diagrammatic view of a lithography system in accordance with a further embodiment of the invention; 
         FIGS. 9A-9E  show illustrative diagrammatic views of a system in accordance with a further embodiment of the invention that may be employed for forming marks having a scale smaller than the resolution of the imaging system; 
         FIG. 10  shows an illustrative diagrammatic view of a lithography system in accordance with a further embodiment of the invention; 
         FIGS. 11A-11F  show illustrative diagrammatic representations of a lithography process in accordance with an embodiment of the invention; 
         FIG. 12  shows an illustrative diagrammatic view of a system in accordance with a further embodiment of the invention; and 
         FIG. 13  shows an illustrative diagrammatic view of a system in accordance with a further embodiment of the invention 
     
    
    
     The drawings are shown for illustrative purposes only and are not necessarily to scale. 
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The invention provides an absorbance modulation lithography system in which a contrast-enhancement material may be used that is reversible in accordance with an embodiment of the invention. The exposure dose due to the background in each focused illumination beam may then be completely eliminated from the contrast-enhancement material and therefore the recorded image. After reversal, the photo-resist will have no remaining recording of the previous exposures. Two spots, therefore, may be placed closer together than would otherwise have been possible using conventional contrast-enhancement material. Such imaging systems may be particularly suitable, for example, in dot-matrix printing systems in which each spot is printed independently of other spots. The smallest distance between exposed spots may, therefore, be much smaller than that dictated by the diffraction limit. 
       FIGS. 3A-3F  show a lithography process in accordance with an embodiment of the invention. As shown in  FIGS. 3A and 3B , a reversible contrast-enhancement material  40  on a photo-resist material  42  on a wafer  44  is imaged with illumination beams  46 , causing imaged regions  48  to be formed in the photo-resist material  42 . Once the illumination beams  46  cease, the reversible contrast-enhancement material  40  then reverts back to being opaque as shown in  FIG. 3C . 
     Then either the stage supporting the wafer is moved, or the illumination beams are moved. The composite is then imaged with illumination fields  50  at different locations with respect to the wafer than the image beams  46  as shown in  FIG. 3D . This causes imaged regions  52  to be formed in the photo-resist material  42 . The imaged regions  52  may be very close to the imaged regions  48  in the photo-resist material  42 . When the reversible contrast-enhancement material is removed (as shown in  FIG. 3E ), the photo-resist material  42  may be developed, leaving the desired pattern of unexposed portions  54  in the photo-resist material  42  as shown in  FIG. 3F . 
     A photochromic organic compound, which includes reversible dyes under photochemical control, may be used for the reversible contrast-enhancement material. The recovery in this case occurs predominantly by thermal mechanism as is the case with spiropyrans, spirooxazines and chromenes. The thermally driven recovery may be aided by photochemical processes as well. In other compounds, the recovery may be predominantly photochemical (for, e.g., fulgides or arylethenes). In this case, exposure with a different wavelength (that does not affect the underlying photo-resist), may be used to induce the recovery. Such recovery is also present in the saturable absorbers used in mode locked lasers, although the contrast may not be particularly high. The recovery is preferably spontaneous. The ideal material for the reversible contrast enhancement will have a high contrast between the transparent and the opaque states. It will also have very fast photobleaching and recovery kinetics. It should also be easily spin-coated into a thin film on top of the photoresist on a flat substrate. Finally, it should be easily removed after exposure without affecting the underlying photoresist. Other potential candidates for this material are: nanoparticles dispersed in polymer matrices, photochromic dye molecules (described above) dispersed in polymer matrices, thin films of Antimony, or semiconductor saturable absorbers used in mode-locked lasers, and carbon nanotubes. For example,  FIG. 4  shows a substrate material  56  (e.g., a polymer matrix) that includes photochromic dye molecules  58 . 
     A lithography system in accordance with an embodiment of the invention may be used with arrays of a variety of focusing elements, such as Fresnel zone plates as disclosed in U.S. Pat. Nos. 5,900,637 and 6,894,292, the disclosures of which are hereby incorporated by reference. As shown in  FIG. 5 , an array of focusing elements  53  may be arranged on a substrate  55 , wherein each zone plate defines a unit cell. The array is supported on a thin membrane with vertical, anisotropically-etched silicon (Si) joists  57  for rigid mechanical support that divide rows of unit cells. In alternative embodiments of the invention, the joists may not be necessary, and the substrate need not be formed of silicon. The membrane is formed of a material that is transparent to the beam source. If the source is 4.5 nm x-ray, then the membrane may be formed of a thin carbonaceous material. If deep UV radiation is used, the membrane may be made of glass, and the zone plates may be made from phase zone plates, e.g., grooves cut into a glass plate or membrane. 
     An array of individually selectable sources  59  is also provided on a support substrate  51  such that each source is aligned with one of the focusing elements  53 . The sources may be semiconductor lasers, diode lasers, light emitting diodes (LEDs), vertical cavity surface emitting lasers (VCSELs) or a variety of other sources such as x-ray sources or electron beam sources. These may be microfabricated in arrays, and may provide extremely high modulation frequencies (about 1 GHz), which translates to very high patterning speeds. In further embodiments, the each source  59  may include a micro-lens and/or phase-shift mask that provides a de-focused pattern (e.g., a ring phase shifted to π/2) at the corresponding focusing element  53  to narrow the point spread function at the image plane. 
     The focusing elements may be any of a variety of diffractive and/or refractive elements including those disclosed in U.S. patent application Ser. No. 10/624,316 filed Jul. 22, 2003, (the disclosure of which is hereby incorporated by reference) which claims priority to U.S. Provisional Applications Ser. Nos. 60/397,705 and 60/404,514, including, for example, amplitude and/or phase Fresnel zone plates, blazed Fresnel zone plates, bessel zone plates, photon sieves (e.g., amplitude photon sieves, phase photon sieves, or alternating phase photon sieves), and the diffractive focusing elements may be apodized. These may be microfabricated in large arrays as well, and may be designed to compensate for wavefront characteristics in the radiation output from the source array to achieve, for example, the smallest possible focal spot. 
       FIG. 6  shows an example of a lithography system  60  in accordance with an embodiment of the invention. The system  60  includes a laser source  62 , an imaging lens  64 , a spatial filter  66 , a collimating lens  68 , a spatial light modulator  70 , telescoping lenses  72 ,  74 , a diffractive element array  76  including a plurality of diffractive elements  78 , and a photo-resist material  84  that is covered by a reversible contrast-enhancement material  82  and is supported by a stage  86 . The reversible contrast-enhancement material  82  is used as a top coat on the photo-resist material  84 . The diffractive elements may be zone plates (such as Fresnel zone plates) or photon sieves. 
     Illumination from the laser source  62  is directed toward the spatial light modulator  70 , and light is selectively reflected from the spatial light modulator  70  onto specific zone plates  78  of the zone plate array  76  for forming the desired imaging pattern. The illuminated zone plates  78  direct focused illumination onto desired locations of the photo-resist material  86  as shown at  80 . The scanning stage may be moved to provide the desired image pattern over the entire photo-resist material  84 . In this example, the exposing wavelength bleaches the reversible contrast-enhancement material. When the exposing illumination is turned off (either by the laser turning off or the spatial light modulator moving the illumination beam away from the zone plates  78 ), the reversible contrast-enhancement material relaxes spontaneously. The timing of the on and off states of the exposing light is controlled to achieve the appropriate exposure of the photo-resist. 
     In accordance with another embodiment, a system  90  (shown in  FIG. 7 ) may include a laser source  62 , an imaging lens  64 , a spatial filter  66 , a collimating lens  68 , a spatial light modulator  70 , telescoping lenses  72 ,  74 , a zone plate array  76  including a plurality of zone plates  78 , and a photo-resist material  84  that is covered by a reversible contrast-enhancement material  82 ′ and is supported by a stage  86  similar to those shown in  FIG. 6 . As also shown in  FIG. 7 , however, the relaxation of the reversible contrast-enhancement material  82 ′ of the system  90  may be assisted by exposure with infra-red (IR) illumination (if, for example, the relaxation is thermal), or by exposure with illumination having a second wavelength (λ 2 ) that is different than the wavelength (λ 1 ) of the laser source  62  (if, for example, the relaxation is photo-initiated). 
     The exposure of the IR or λ 2  illumination may be provided by a source  92 , shutter or acousto-optic modulator (AOM)  94 , an imaging lens  96 , a spatial filter  98 , a collimating lens  100  and a beam splitter/combiner  102 . This system  90  provides that the exposure of the IR or λ 2  illumination for relaxing the reversible contrast-enhancement material  82 ′ floods the reversible contrast enhancement material  82 ′ after each time that the source  62  and modulator  70  are used for imaging a spot on the photo-resist material  84 . The AOM  94  may be used to switch the exposure for relaxation in rhythm with the exposing wavelength. The relaxation exposure is toned on when the exposing wavelength is turned off. 
     In accordance with a further embodiment of the invention, a system  110  may provide for relaxation of selective regions of the reversible contrast-enhancement material as shown in  FIG. 8 . In particular, the system  110  may include a laser source  62 , an imaging lens  64 , a spatial filter  66 , a collimating lens  68 , a spatial light modulator  70 , telescoping lenses  72 ,  74 , a zone plate array  76  including a plurality of zone plates  78 , and a photo-resist material  84  that is covered by a reversible contrast-enhancement material  82 ′ and is supported by a stage  86  similar to those shown in  FIG. 6 . As also shown in  FIG. 8 , however, the relaxation of the reversible contrast-enhancement material  82 ′ of the system  110  may be assisted by exposure with IR illumination, or by exposure with illumination having a second wavelength (λ 2 ) that is different than the wavelength (λ 1 ) of the laser source  62 . The exposure of the IR or λ 2  illumination may be provided by a source  112 , an imaging lens  114 , a spatial filter  116 , a collimating lens  118 , a second spatial light modulator  120 , a mirror  122 , and a beam splitter/combiner  124 . This system  110  provides that the exposure of the IR or λ 2  illumination for relaxing the reversible contrast-enhancement material  82 ′ is selectively directed to desired locations of the reversible contrast enhancement material  82 ′ immediately after imaging of the associated locations on the photo-resist material  84 . The focusing of the IR or λ 2  illumination may be achieved by using the same zone plates  78  of the zone plate array  76 , either by providing that the zone plates  78  are sufficiently focused at the desired locations to achieve relaxation of the reversible contrast-enhancement material, or by providing that the zone plates  78  are designed to focus illumination of wavelengths λ 1  and either IR or λ 2  at the same focal distance. 
     In accordance with yet another embodiment, the invention may provide that a shaped-beam may be used to create a null in the reversible contrast-enhanced material at the desired exposed location. For example and as shown in  FIGS. 9A-9E , a photo-resist material  130  may be top coated with a reversible contrast-enhancement material  132  (and may also include an anti-reflective coating between the photo-resist material and the wafer  134 , as well as a barrier layer if desired). The reversible contrast-enhancement material in this example, however, is initially transparent to exposing illumination having a wavelength λ exp . The reversible contrast-enhancement material becomes opaque when exposed to illumination having a transforming wavelength λ tr . 
     In particular, as shown in  FIG. 9B , an annular ring spot is first created using the illumination λ tr  as diagrammatically shown at  136 . The illumination  136  causes the reversible contrast-enhancement material  132  to develop opaque regions  140  while leaving transparent regions  138  as shown in  FIG. 9B . The transparent region  138  that is in the center of the ring spot is then used as a mask for illuminating the photo-resist material  130  at that location  142  using exposure illumination  144  (λ exp ) as shown in  FIG. 9C . The imaged region  142  may be smaller than the smallest imaging element size possible using conventional imaging, and is made possible by using the reversible contrast-enhanced material in accordance with an embodiment of the invention. 
     The system may then either wait until the reversible contrast-enhancement material relaxes and becomes transparent again, or may apply a relaxation illumination  146  (e.g., IR or another wavelength illumination λ ret ) to cause the reversible contrast-enhancement material to relax and become transparent again as shown in  FIGS. 9D and 9E . The wavelength λ ret  may be equal to λ tr . 
     As shown in  FIG. 10 , a system  150  for providing the annular ring spot used in  FIGS. 9A-9E  may include the laser source  62 , the imaging lens  64 , the spatial filter  66 , the collimating lens  68 , the spatial light modulator  70 , telescoping lenses  72 ,  74 , the diffractive element array  76  including a plurality of diffractive elements  78 , and the photo-resist material  84  that is covered by the reversible contrast-enhancement material  82 ″ and is supported by the stage  86  as discussed above with reference to  FIG. 6 . The system also includes a source  152  (of λ tr  illumination), a shutter or acousto-optic modulator (AOM)  154 , an imaging lens  156 , a spatial filter  158 , a collimating lens  160 , a phase plate  162 , and a beam splitter/combiner  164 . The phase plate  162  provides a phase shift in the λ tr  illumination (for example a spiral phase shift) to provide the annular ring spot illumination  136  shown in  FIGS. 9B and 9C . 
     In accordance with another embodiment, the system may provide an absorbance modulation lithography system in which a reversible contrast-enhancement material  174  deposited on a photo-resist  172  on a substrate  170  as shown in  FIG. 11A  is illuminated with a standing illumination waveform  176  as shown in  FIG. 11B . The waveform  176  is provided at an imaging frequency of λ im . Another standing waveform  178  may also be provided at a reversing frequency λ rev  at which the reversible contrast-enhancement material may be actively reversed. The waveforms  176  and  178  are provided  180  degrees out of phase from one another and may be sufficiently close in frequency that they remain synchronous over the imaging region. The photo-resist is chosen such that it may be imaged by an electromagnetic field at a frequency λ im  but not by a field at a frequency of λ rev . 
     The standing waves may be formed by interference of monochromatic coherent sources as discussed below with reference to  FIGS. 12 and 13 . As shown in  FIG. 11B , both the waveforms  176  and  178  may be provided at the same time such that the nodes of the grating at λ im  coincide with the anti-nodes of the grating at λ rev . The intensities of the illumination at the frequencies λ im  and λ rev  may be adjusted independent of one another to provide that the resist  172  is exposed in the desired amount for specific applications. As shown in  FIG. 11C , following exposure, the entire material  174  is reversed either by flood exposure to illumination at the wavelength λ rev  or by waiting for the material  174  to reverse on its own or due to application of thermal energy as discussed above. The wavelength of the reversing illumination λ rev  may be greater than that of the imaging illumination wavelength λ im  but less than twice the imaging illumination wavelength (&lt;2λ im ). 
     The substrate  170  may be stepped by a small amount as determined by the desired pitch of the final grating. In further embodiments, rather than stepping the substrate, the fringes of the incident gratings may be moved by changing the phase of the incident illumination. These steps may be implemented without requiring the removal of the substrate from the lithography tool. As shown in  FIG. 11D , the photo-resist  172  may then again be imaged (optionally at the same time as the π/2 shifted reversing illumination) at different areas of the photo-resist  172 . The process may then be repeated as necessary to provide a photo-resist  172  with very finely developed features in the photo-resist  172  as shown in  FIG. 11E . The reversible contrast-enhancement material  174  may then be removed as shown in  FIG. 11F . 
     As shown in  FIG. 12 , a system in accordance with an embodiment of the invention includes in imaging laser source  180  that is directed via imaging optics toward a reversible contrast-enhancement material  182  on a photo-resist  184  on a substrate  186 . The imaging optics include a beam splitter  190 , an adjustable delay unit  192 , mirrors  194  and  196 . The system may also include a reversing illumination laser source  200  that directs reversing illumination toward the surface of the material  182  via a beam splitter  202 , an adjustable delay unit  204 , mirrors  206  and  208 . The optical path lengths from the laser  180  are controlled to ensure that the two portions of the imaging illumination provide interference at the material  182  (from +/− angle α) that yields a standing waveform of λ im  on the material  182 . Similarly, the optical path lengths from the laser  200  are controlled to ensure that the two portions of the imaging illumination provide interference at the material  182  (from +/− angle β that is less than α) to yield a standing waveform of λ rev  on the material  182 . Since the period is defined as P=π/sin θ, the period of each of the waveforms may be set to be equal to one another by requiring that λ im /sin α=λ rev /sin β. 
     A lithography system in accordance with a further embodiment of the invention includes a diffraction grating  220  that receives incident imaging and optional reversing illumination (at wavelengths λ im  and λ rev ). The grating and incidence angle are adjusted to provide positive and negative first order diffraction as shown, and the first order diffracted illumination is received by additional gratings  222  and  224  as shown in  FIG. 13 . Each of the gratings  222  and  224  provides positive and negative first order diffraction as well, and the positive first order diffraction from one grating is interfered with the negative first order diffraction from the other grating as shown in  FIG. 13 . The interference of the fields occurs at a reversible contrast-enhancement material  226  on a photo-resist  228  on a substrate  230 . One of the gratings  224  includes a phase delay unit that comprises a substrate  232  (such as glass) having a thickness (d) such that the Difference in phase shifts is provided by the substrate  232  to both wavelengths of illumination that pass through the substrate  232 . In particular, the phase shift (φ) in the material  232  is provided by the optical path length through the material  232  divided by the wavelength of the illumination in the material  232 . The difference in phase shift φ im −φ rev  is set to equal π, and the thickness d is determined knowing λ im , λ rev , and the indices of refraction of the material  232  at the two wavelengths solving for the above equations. The system, therefore, provides an achromatic interference lithography system. 
     Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.