Method and apparatus for improving exposure uniformity of periodic patterns

Abstract

A method for improving the uniformity of exposure of a first periodic pattern in a photomask by a collimated beam of monochromatic light in a photolithography system for printing a second periodic pattern in a photosensitive layer on a substrate adjacent to the photomask, the method comprising: providing a plate made of a material transparent to the beam and having opposing surfaces that are parallel and separated by a thickness; positioning the plate in the photolithography system such that the beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates the photomask; and rotating the plate by at least one angle about at least one axis of rotation during exposure such that the transmitted beam is translationally displaced across the photomask.

Description

[Technical Field]

[0001] Photolithographic exposure of periodic patterns using the Talbot effect or Talbot imaging is well known in the art. For example, in Zanke et al., "Large-area patterning for photonic crystals via coherent diffraction lithography," J. Vac. Sci. Technol. B22(6), 2004, a chrome mask with a hexagonal lattice of elliptical holes with a spatial period of 583 nm is illuminated at normal incidence with a plane-wave illumination of 193 nm wavelength. The periodic pattern in the mask causes the beam to diffract at predetermined periodic distances from the mask, separated by the so-called Talbot distance, and the diffraction orders interfere to reproduce the pattern on the mask. By placing a photoresist-coated substrate at one of these "self-image" planes, the periodic pattern of the mask is printed into the photoresist.

[0002] U.S. Patent No. 8,841,046 discloses two different but related photolithography methods based on Talbot imaging for printing one-dimensional or two-dimensional arrays of periodic or quasi-periodic features onto a photoresist-coated substrate. Quasi-periodic refers to a periodic pattern with a period that varies slowly over the entire area of ​​the pattern, such that the local period is substantially constant (over distances comparable in dimension to the spacing between the mask and the substrate). Furthermore, it is disclosed that the above method can be applied to periodic patterns of curved and linear features. In the first of these two methods, a periodic or quasi-periodic pattern on a mask is illuminated with a beam of collimated light from a source with a broad spectral bandwidth, and the substrate is positioned such that the Talbot effect produces a lateral intensity distribution that is "static," i.e., a lateral intensity distribution that is independent of further increasing distance from the mask. Solak et al., "A chromatic spatial frequency multiplication; A method for production of nanometer-scale periodic features", J. Vac. Sci. Technol.B23(6),2005) states that such distances are

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[0003] In the second method, a periodic or quasi-periodic pattern on a mask is illuminated with a collimated beam of monochromatic light, and the distance between the mask and the substrate is varied during exposure by a distance corresponding to an integer multiple of the Talbot distance. This results in printing an average of the lateral intensity distribution formed between successive Talbot image planes, resulting in a periodic distribution with a virtually infinite depth of focus. The above disclosure further teaches that the mask-to-substrate distance can be varied continuously over a required range during exposure, or discretely by exposing the plate at multiple positions.

[0004] These two methods are commonly referred to as "achromatic Tarbo lithography" (ATL) and "displacement Tarbo lithography" (DTL), respectively.

[0005] US Pat. No. 8,525,973 discloses a variation of the DTL technique that allows multiple periodic patterns, each with a different period value in the mask, to be printed simultaneously onto the photosensitive layer.

[0006] To uniformly print periodic patterns using any of the above Talbot-based methods, it is generally important that the intensity of the beam illuminating the mask be uniform across the entire area of ​​the pattern (typically varying by less than ±3%). Even more important is that the beam be well collimated, i.e., that the light rays illuminating each localized area of ​​the mask be precisely parallel. Without good collimation, light rays at different angles will project Talbot images in different directions behind the mask, causing smearing or blurring in the composite image formed on the substrate, thereby reducing the resolution of the printed pattern. The required degree of collimation depends on the period of the pattern and the distance between the mask and the substrate. For a submicron period and a mask-to-substrate distance of approximately 0.1 mm, the required degree of collimation is typically approximately 1 mR. Examples of optical systems that have been devised to generate such beams with uniform intensity and good collimation from monochromatic and broadband sources suitable for DTL and ATL applications, respectively, are disclosed, for example, in U.S. Pat. No. 8,525,973, U.S. Pat. No. 10,365,566, and U.S. patent application Ser. No. 14 / 123,330.

[0007] A drawback of high collimation requirements, especially when using monochromatic light, is that the intensity uniformity of the beam illuminating the mask is sensitive to small imperfections or defects, typically about 0.1 mm to 1 mm in size, in or on the optical components of the illumination system that form the beam. Such imperfections—for example, microbubbles inside a lens, digs or particles on a lens surface, or tiny scratches (small in one dimension)—can cast shadows or other perturbations on the beam illuminating the mask, resulting in localized areas of significantly different intensity. In other words, the high collimation requirement of the Talbot effect not only prevents smearing from the periodic pattern used to illuminate the photoresist, but also prevents smearing of intensity nonuniformities in the illumination beam due to small defects in the beam-forming optics. Depending on the nature of the defect, for example, whether the defect directly affects the amplitude and / or phase of the transmitted light, a dark or bright spot or a set of concentric rings of alternating high and low intensity may form in the beam illuminating the mask. The dimensions of the unwanted non-uniformities in the mask typically range from 0.1 mm to 5 mm, depending on the defect size and the degree of collimation, and the contrast of the defect relative to the intensity of the surrounding beam typically ranges from 1% to 20%. These non-uniformities in the intensity distribution of the mask illumination cause corresponding non-uniformities in the energy density distribution of the mask exposure during exposure, and consequently, in the dimensions of periodic features in the DTL printed pattern. Depending on the application involved, such local non-uniformities in the printed pattern may be unacceptable.

[0008] Another drawback of the highly collimated, monochromatic exposure beam required by both conventional Talbot imaging lithography and DTL is that the intensity distribution of the beam illuminating the mask is further degraded by general scattering from the optics of the beam-shaping system, particularly scattering from micro- and nanoscale roughness on lens surfaces, which persists even after high-quality polishing. The general scattering from the optics, combined with light with a high degree of temporal and spatial coherence, results in a ubiquitous speckle pattern in the beam illuminating the mask. Speckle can also be described as a distribution of localized intensity nonuniformities, but present throughout the entire beam. The size of individual speckles is determined by the beam collimation and typically ranges from 0.5 mm to 5 mm, and the contrast of individual speckles relative to the average intensity of the surrounding beam typically ranges from 0.5% to 10%.

[0009] Of course, these local intensity non-uniformity problems can be avoided by using perfect optical elements without surface or internal defects, and by ensuring that no particles are deposited on any of the optics, but this can be impractical and / or result in prohibitively expensive optics, especially when there are many large diameter elements in the beam shaping system.

[0010] Another photolithography method that requires a collimated monochromatic beam of light with high intensity uniformity to illuminate a periodic pattern in a mask and is therefore similarly sensitive to imperfections in or on the optical elements of the illumination system is near-field holographic lithography. This method is employed, for example, in Tennant et al., "Characterization of near-field holography grating masks for optoelectronics fabricated by electron beam lithography," J. Vac. Sci. Technol. B10(6), 1992. According to this method, a collimated monochromatic beam of light illuminates the mask not at normal incidence but at an oblique angle selected in relation to the period of the mask's linear diffraction grating and the illumination wavelength, preferably such that only the zeroth and first diffraction orders propagate to the rear of the mask at diffraction angles symmetrically disposed on either side of the mask normal. The diffraction grating in the mask is preferably a phase grating designed and fabricated so that the intensities of the two transmitted orders are substantially equal. In this way, the two diffraction orders interfere in the near-field region behind the mask, thereby producing a periodic, high-contrast intensity distribution with planes of high and low intensity perpendicular to the mask, whose lateral distribution reproduces the mask's grating pattern. By placing a photoresist coating layer on the substrate adjacent to the mask, the mask's grating pattern is essentially copied into the layer.

[0011] Therefore, it is an object of the present invention to provide a method for improving the uniformity of exposure of a collimated beam of a first periodic pattern on a photomask in a photolithography system that uses the Talbot effect to print a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask. In particular, it is an object of the present invention to provide a method for reducing the contrast of local non-uniformities in the energy density distribution of the exposure of a first periodic pattern on a photomask caused by small imperfections or defects in or on optical elements of the beam forming system.

[0012] A second object of the present invention is to provide an apparatus for improving the uniformity of exposure by a collimated beam of a first periodic pattern on a photomask in a photolithography system that uses the Talbot effect to print a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask. In particular, an object of the present invention is to provide an apparatus that reduces the contrast of local non-uniformities in the energy density distribution of the exposure of the first periodic pattern on the photomask caused by small imperfections or defects in or on optical elements of the beam forming system.

[0013] A third object of the present invention is to provide an apparatus for improving the uniformity of exposure of a first periodic pattern on a photomask by a collimated beam of monochromatic light in a photolithography system that prints a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask. In particular, an object of the present invention is to provide an apparatus that reduces the contrast of local non-uniformities in the energy density distribution of the exposure of the first periodic pattern on the photomask caused by small imperfections or defects in or on optical elements of the beam forming system.

[0014] According to a first aspect of the present invention, there is provided a method for improving uniformity of exposure of a collimated beam of a first periodic pattern on a photomask in a photolithography system that utilizes the Talbot effect to print a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask, the method comprising: a) providing a plate made of a material transparent to the beam, the plate having opposing surfaces that are parallel and separated by a thickness; b) placing the plate in a photolithography system such that a beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates a photomask; c) rotating said plate by at least one angle about at least one axis of rotation during exposure so that the transmitted beam is displaced translationally across the photomask; A method is provided, comprising:

[0015] Preferably, the plate is rotated about a single axis of rotation that is perpendicular or parallel to the direction of the beam.

[0016] Advantageously, the plate is arranged so that the beam illuminates it at an oblique initial angle of incidence and the plate is rotated through an angle about an axis of rotation parallel to the direction of the beam, preferably the oblique angle of incidence is greater than 5°, most preferably greater than 10°, and the angle by which the plate is rotated is preferably an integral number of revolutions of 360°.

[0017] Preferably, the transmitted beam is translationally displaced across the photomask by a distance greater than the maximum spatial extent or dimension of the exposure energy density non-uniformity, most preferably by a distance greater than five times the maximum spatial extent or dimension of the non-uniformity.

[0018] Preferably, the plate is positioned within the photolithography system such that it is stationary at the start of exposure, whereby the beam illuminates the plate at a particular or selected initial angle of incidence, or alternatively, such that at the start of exposure the beam illuminates the plate at a selected initial angle of incidence or at an initial angle of incidence within a selected range of angles, and the plate is rotated about at least one axis of rotation prior to exposure.

[0019] Advantageously, the rotation of the plate about at least one axis of rotation is a continuous movement throughout the exposure period, or alternatively, there may be stepwise movement in which the plate is stationary or at least slow between successive steps of the series of rotation steps.

[0020] Optionally, the step of rotating the plate by at least one angle about at least one axis of rotation during exposure is repeated one or more times during exposure so that the transmitted beam is translationally displaced across the photomask.

[0021] Preferably, the thickness, initial angle of incidence, at least one rotation angle, and at least one rotation axis are selected so that the resulting beam translational displacement across the photomask is larger than at least one dimension of the local intensity non-uniformity of the exposure beam, most preferably at least five times larger, so that the contrast of the time-integrated intensity distribution caused by the displacement of the local non-uniformity when illuminating the photomask is correspondingly reduced. For example, if the non-uniformity is an elongated stripe, the resulting translational displacement should be at least as large as the width (but not the length) of the elongated stripe, and the contrast of the time intensity distribution will be reduced by at least half the width, most preferably by one-fifth of the width, so that the contrast of the time intensity distribution will be reduced by at least one-fifth of the width.

[0022] Preferably, the transparency of the plate is such that the internal transmittance of the plate (ie, the transmittance ignoring surface reflections) is greater than 90%, most preferably greater than 98%.

[0023] The parallelism of the opposing surfaces of the provided plate is determined so that variations in the angle of incidence of the transmitted beam at the photomask caused by rotating the plate by at least one angle about at least one rotation axis during exposure are sufficiently small so that lateral deviations in the intensity distribution exposing the photosensitive layer do not result in unacceptable blurring and thus a reduction in the contrast of the second periodic pattern printed in the layer. Preferably, the lateral deviations in the intensity distribution when illuminating the photosensitive layer (ignoring variations in the shape of the intensity distribution in the layer caused by variations in the spacing between the photomask and the substrate performed according to DTL) are less than 1 / 10 of the period of the second periodic pattern, and most preferably less than 1 / 20 of the period of the second periodic pattern.

[0024] The first periodic pattern in the photomask and the corresponding second periodic pattern printed in the photosensitive layer on the substrate can be a one-dimensional periodic pattern, an array of alternating line and space features, or alternatively a two-dimensional array of features, such as features arranged in a hexagonal grid or features arranged in a square grid.

[0025] The first periodic pattern in the mask is preferably formed as a phase mask, in which individual features of the periodic pattern change the phase of locally transmitted light, or alternatively, the first periodic pattern may be an amplitude mask, in which individual features change the transmission amplitude of locally transmitted light.

[0026] It will be appreciated that the photomask may further include other first periodic patterns that are used to print other corresponding second periodic patterns into a photosensitive layer on a substrate, where the other first periodic patterns may not necessarily be the same as the first periodic pattern or may not necessarily be identical to each other, and may have periods, dimensions and / or orientations that are different from the first periodic patterns or from each other.

[0027] It should further be understood that the period of the first periodic pattern and the resulting second periodic pattern printed on the photosensitive layer need not be strictly periodic, but may be quasi-periodic, i.e., have a period that varies gradually across the pattern area in accordance with prior art displacement Tarbo lithography. Similarly, the linear features of the one-dimensional periodic grating pattern need not be purely linear, but can have a gradual curvature such that the lines are substantially linear across the local area of ​​the diffraction grating.

[0028] The above paragraphs apply not only to the first aspect of the present invention, but also to the second, third and fourth aspects described below.

[0029] According to a second aspect of the present invention, there is provided an apparatus for improving uniformity of exposure of a first periodic pattern on a photomask by a collimated beam in a photolithography system that utilizes the Talbot effect to print a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask, the apparatus comprising: a) a plate made of a material transparent to the beam, the plate having opposing surfaces that are parallel and separated by a thickness; b) means for positioning said plate within a photolithography system such that a beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates a photomask; c) means for rotating said plate through at least one angle about at least one axis of rotation during exposure so as to cause translational displacement of the transmitted beam across the photomask; An apparatus is provided comprising:

[0030] Preferably, the area of ​​each surface allows complete beam transmission through the plate without beam truncation or beam scattering at the edges of the plate.

[0031] Advantageously, the positioning means positions the plate so that the beam illuminates the plate at an oblique initial angle of incidence, and the rotating means rotates the plate about an axis of rotation parallel to the direction of the beam.

[0032] Advantageously, at least one of the two opposing faces of the plate is provided with an anti-reflection coating, which reduces power losses in the transmitted beam, in order to reduce the time required for photolithographic exposure.

[0033] Preferably, the beam is monochromatic and the distance between the photomask and a photoresist-coated substrate placed in close proximity to the photomask is varied during exposure according to the method of displacement Tarbo lithography.

[0034] Alternatively, the beam has a desired spectral bandwidth and the photoresist coated substrate is positioned at a minimum distance from the photomask according to the method of achromatic Tarbo lithography.

[0035] According to a third aspect of the present invention, there is provided a method for improving the uniformity of exposure of a first periodic pattern on a photomask by a collimated beam of monochromatic light in a photolithography system that prints a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask, the method comprising: a) providing a plate made of a material transparent to the beam, the plate having opposing surfaces that are parallel and separated by a thickness; b) placing the plate in a photolithography system such that a beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates a photomask; c) rotating said plate by at least one angle about at least one axis of rotation during exposure so that the transmitted beam is translationally displaced across the photomask; A method is provided, comprising:

[0036] Preferably, the beam illuminates the mask at normal incidence, and the photolithography system utilizes the Talbot effect to print the second periodic pattern from the first periodic pattern on the mask. Alternatively, the beam illuminates the mask at an oblique angle selected in relation to the period of the mask pattern and the wavelength of the monochromatic illumination, such that only the zeroth and first diffraction orders propagate behind the photomask in accordance with the method of near-field holography.

[0037] According to a fourth aspect of the present invention, there is provided an apparatus for improving the uniformity of exposure of a first periodic pattern on a photomask by a collimated beam of monochromatic light in a photolithography system for printing a second periodic pattern on a photosensitive layer on a substrate adjacent to a photomask, the apparatus comprising: a) a plate made of a material transparent to the beam, the plate having opposing surfaces that are parallel and separated by a thickness; b) means for positioning said plate within a photolithography system such that the beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates a photomask; c) means for rotating said plate through at least one angle about at least one axis of rotation during exposure so as to cause translational displacement of the transmitted beam across the photomask; An apparatus is provided comprising:

[0038] In all of the above aspects of the present invention, it is understood that the wavelength of the monochromatic light may be in any part of the spectrum, including near ultraviolet, deep ultraviolet and extreme ultraviolet, and not just visible wavelengths.

[0039] It should also be understood that the methods and apparatus disclosed herein are applicable to lithography systems requiring illumination of the mask surface with a collimated light beam, even when the pattern on the mask or the pattern to be printed on the substrate is not periodic in nature.

[0040] It should be understood that in all of the above aspects of the present invention, a photosensitive layer on a substrate does not necessarily refer only to a photosensitive layer disposed directly on the surface of the substrate, but may instead refer to a photosensitive layer disposed indirectly on the substrate, with at least one intermediate layer of another material, such as a metal or dielectric, between the substrate and the photosensitive layer, or to a photosensitive layer deposited or coated over a previously printed and / or otherwise processed pattern or structure, made of the same material as the substrate or at least one other material on the surface of the substrate.

[0041] The substrate material is not limited and may be, for example, glass, silicon or other semiconductor materials.

[0042] These and other aspects of the present invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0043] [Figure 1a] 1 is a front view showing a first embodiment of the present invention. [Figure 1b] FIG. 1 is a right side view showing a first embodiment of the present invention. [Figure 2a] FIG. 10 is a front view showing a second embodiment of the present invention. [Figure 2b] FIG. 10 is a right side view showing a second embodiment of the present invention. [Figure 2c] FIG. 10 is a top view of a second embodiment of the present invention, with the lighting module omitted to promote clarity. [Figure 3a] FIG. 10 is a front view showing a third embodiment of the present invention. [Figure 3b] FIG. 10 is a right side view showing a third embodiment of the present invention. [Figure 4a] FIG. 10 is a front view showing a fourth embodiment of the present invention. [Figure 4b] FIG. 10 is a left side view showing a fourth embodiment of the present invention. [Figure 5] 10 is a graph showing the time dependence of the components of the angle of incidence of the beam on the plate in the xz and yz planes, respectively, during exposure. [Figure 6] FIG. 10 is a front view showing a fifth embodiment of the present invention.

[0044] Referring to FIGS. 1a and 1b, which show front and right-side views, respectively, of a first embodiment of the present invention, an illumination module of a photolithography system 1 designed to perform displacement Tarbo lithography projects a collimated, monochromatic beam of light 2 having a substantially uniform intensity across its circular cross-section. The illumination source, beam-shaping optics, and beam-homogenization optics (not shown) within module 1 are described, for example, in U.S. Pat. No. 8,525,973. Here, beam 2 has a diameter of slightly over 4 inches and a wavelength of 363.8 nm, making it suitable for printing patterns on a 4-inch diameter substrate or wafer coated with standard i-line sensitive photoresist. Beam 2 is directed toward a photomask 3 having a periodic pattern of features 4 positioned closely above a photoresist-coated wafer 5. Both photomask 3 and wafer 5 are mounted on a vacuum chuck (not shown) in the DTL photolithography system and are aligned parallel to each other and perpendicular to the direction of illumination beam 2 using standard alignment tools and procedures.

[0045] While Beam 2 has good intensity uniformity on the macroscale, small imperfections in or on the lens, such as microbubbles within the lens and microdigestions and dust particles on the lens surface, cause localized intensity nonuniformities in the form of spots typically 1-2 mm in diameter and with low intensity up to 5% contrast. Performing DTL exposure with such a beam can, in certain applications, result in undesirable nonuniformities in the periodic pattern printed in the photoresist, specifically unacceptably small lines or features whose dimensions are larger or smaller than those of the surrounding pattern.

[0046] Between the illumination module 1 and the photomask 3, a rectangular plate 6 made of fused silica, through which the beam 2 passes, is interposed in accordance with the present invention. The plate 6 is shown tilted at an angle relative to the x-axis, causing the beam 2 to illuminate the plate 6 at an oblique angle of incidence in the xz plane and perpendicular to the beam in the yz plane. The length and width of the plate 6 are sufficient to ensure that the angle of incidence of the beam 2 on the plate 6 is 20° in the xz plane and perpendicular in the yz plane, allowing the entire beam 2 to pass through the plate 6 without obstruction. The plate is 25 mm thick. The top and bottom surfaces of the plate 6 are polished to provide good λ / 4 flatness and good 40 / 20 scratch / dig quality, so that the collimation and intensity uniformity of the transmitted beam 7 are not substantially affected by the presence of the plate 6.

[0047] The upper and lower surfaces of plate 6 are further polished to be sufficiently parallel so that the angle of incidence of transmitted beam 7 on photomask 3 remains sufficiently constant as plate 6 is rotated around the angular range used in this embodiment. Just as a large range of beam angles would cause each point on the mask to be instantaneously illuminated (i.e., insufficient collimation) and result in unacceptable smearing or blurring of the periodic pattern features printed on the photosensitive layer, a large wedge angle between the upper and lower surfaces of the substrate, resulting in a large temporal change in the beam angle on the mask during exposure, can result in unacceptable blurring of the periodic pattern features printed on the wafer. A change in the angle of incidence ΔΦ of the illumination beam causes the intensity distribution illuminating the photoresist to shift laterally by a distance of approximately dΔΦ, where d is the spacing between the mask and wafer. Therefore, this distance is preferably less than 1 / 10 of the period of the pattern to be printed, and most preferably less than 1 / 20 of this period. For example, when the period of the printed pattern is 250 nm and the spacing between the mask and wafer is 0.1 mm, the angle of incidence of the beam on the photomask is most preferably kept constant at less than 0.12 mR during exposure. The resulting parallelism required between the plate surfaces can be readily determined by those skilled in the art of basic optics using Snell's law. If the plate is rotated during exposure so that the angle of incidence of the beam on the plate varies over a range from -θ1 to θ1, the angle of incidence of the beam on the photomask, φ, is given by:

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[0048] Evaluating the above equation at θ=20°, n=1.47 and ω=3 arcmin, we get

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[0049] The top and bottom surfaces of the plate 6 are provided with anti-reflection coatings, which minimize power losses and exposure times of the beam.

[0050] Plate 6 is mounted by a fixed bracket 8 to a motorized rotation stage 9 that allows plate 6 to rotate about a rotation axis 10 parallel to the y-axis. Plate 6 does not need to be rotated at high speed or with high angular resolution (1° / s and 0.1°, respectively, are sufficient), so suitable stages are readily available from a number of suppliers, for example Newport Corporation, Irvine, USA (www.newport.com). Plate 6 is shown tilted at an angle relative to the horizontal, so that beam 2 is incident at an oblique angle of incidence θ i irradiates plate 6 with a ray. Refraction of beam 2 as it passes through plate 6 produces a transmitted beam 7 that is parallel to the incident beam 2 but is laterally offset from beam 2 by a distance Δx (shown in FIG. 1 ), where Δx is

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[0051] θ i For small values ​​of , the dependence of the lateral offset on the angle of incidence is Δx=Tθ i (n-1) / n formula (3) approximates the linear relationship given by

[0052] Thus, when the plate 6 is rotated at a constant angular velocity over a range of small angles of incidence (eg ±20°), the transmitted beam 7 is scanned across the photomask 3 at a substantially constant velocity.

[0053] Stage 9 initially orients plate 6 so that the initial angle of incidence of beam 2 on plate 6 is selected as -20° at the start of exposure. During exposure, stage 9 rotates plate 6 by an angle of 40° using a constant rotation speed so that the angle of incidence of beam 2 on plate 6 reaches +20° at the end of exposure. Because the exposure time required to print the desired pattern on the wafer is 40 seconds, the selected rotation speed is 1° / s. This rotation of stage 9 during exposure results in a slow displacement or scanning of transmitted beam 7 across photomask 3. Using equation (2), the scanning distance is calculated to be approximately 5.9 mm from the rotation angle (-20° to +20°), the plate thickness (25 mm), and the plate's refractive index (n = 1.47). Therefore, this scanning of transmitted beam 7 during exposure causes a blur or smear of nonuniformity in the time-integrated intensity distribution when illuminating photomask 3. The contrast reduction of nonuniformity can be estimated as follows: That is, a defect of about 2 mm in size displaced over a distance of about 5.9 mm produces a smeared region of about 6.9 mm in length, so the contrast of the defect here is about

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[0054] The lateral displacement of beam 7 in the x-direction during exposure causes some non-uniformity at the edges (in the ±x-direction) of the time-integrated intensity distribution as it illuminates the photomask; specifically, the exposure energy density at the edges of the beam decreases over the distance of the associated lateral displacement, approximately 5.9 mm in this case, which also reduces the width of the uniform region of the time-integrated distribution by the same amount. The x-direction width of pattern 4 on photomask 3 must then be smaller than the width of this region in order to print pattern 4 uniformly on wafer 5.

[0055] Meanwhile, the rotation speed of stage 9 is preferably selected to reach the desired final angle at the end of exposure, although in other embodiments it may be selected so that stage 9 reaches the final angle sometime before the end of exposure, or equivalently, begins rotation sometime after the start of exposure and reaches the final angle at the end of exposure. However, selecting such a non-optimized rotation speed is generally undesirable because it will generally result in non-uniform smearing of the defect, resulting in higher contrast at the edges of the smeared region compared to the contrast obtained using an optimized rotation speed.

[0056] In the embodiment described above, plate 6 is initially stationary at the start of exposure and is tilted at a predetermined angle so that beam 2 initially illuminates plate 6 at the desired initial angle of incidence. In a variation of that embodiment, plate 6 is initially positioned at a different tilt angle and then rotated before exposure at a selected speed so that beam 2 initially illuminates plate 6 at the desired initial angle of incidence at the start of exposure. In this case, plate 6 continues to rotate at the same or a different speed during exposure to produce the required displacement of the transmitted beam at photomask 3.

[0057] In another variant, plate 6 is repeatedly scanned over the angular range described in accordance with the first embodiment before exposure begins, so that at the start of exposure the initial angle of incidence of the beam on the plate is somewhat arbitrary across the desired angular range. Plate 6 then continues to be repeatedly scanned over the same angular range during exposure, preferably at an optimized scanning speed and with negligible delay between successive scans, so that by the end of exposure an integer number of cycles of the angular range are preferably completed, thereby minimizing the contrast of smeared features in the time-integrated distribution when illuminating photomask 3.

[0058] Rotation of plate 6 through the selected rotation angle is preferably accomplished using a constant rotation speed to achieve uniform smearing of defects throughout the smeared region, but may alternatively be accomplished using a variable rotation speed, although such a variable speed strategy will typically result in non-uniform smearing of defects, which will result in locally higher non-uniform contrast (compared to that produced at a constant speed) anywhere along the smeared region, which is generally undesirable.

[0059] Using the apparatus of this embodiment, smaller or larger contrast reduction factors of smear defects can be obtained by scanning the plate 6 over a smaller or larger angular range, respectively, so that the defects are scanned a smaller or larger distance across the photomask 3. The angular range should preferably not be so large that the AR coating on the plate surface becomes insufficient or that the exposure beam 2 is clipped by the edge of the plate 6.

[0060] Although the angular scanning of plate 6 described in this embodiment is symmetrical about normal incidence, in other variations of this embodiment, plate 6 can be scanned asymmetrically, such that the angle of incidence of the beam on plate 6 varies, for example, between -15° and 25° or between 0° and 30°.

[0061] Furthermore, while the angular scanning of plate 6 described above is a single scan, the same apparatus can also be used to perform double scanning of plate 6 during exposure. That is, plate 6 can be rotated so that the angle of incidence of the beam on plate 6 is scanned from -20° to +20° in the first half of the exposure, and then scanned back again from +20° to -20° in the second half of the exposure. In this embodiment, the scanning speed should be substantially constant in each of the forward and reverse scans (although not necessarily the same in the two scans), and it is preferable that the time required to switch scan directions between the forward and reverse scans is negligible with respect to the total exposure time. It is clear that this double scanning variation of the first embodiment can be extended to other multiple scanning variations. In the case of multiple scanning, it is less important that the final scan is completed precisely at the end of the exposure, because the accumulated exposure dose from the previous scans ensures a relatively high level of uniformity across the smeared area.

[0062] Rather than angularly displacing the plate 6 through a desired or selected rotation angle at a constant scanning speed, the apparatus of the first embodiment can achieve the same angular displacement using a stepping motion of the stage 9, which approximates displacement at a constant speed. For example, the stage 9 can be rotated from -20° to +20° in small angular increments of 0.5°, with a constant stepping frequency and short dwell times between steps. This also achieves good uniformity across smeared features.

[0063] The rotation axis 10 of the stage 9 used in the first embodiment is arranged to be parallel to the y-axis and therefore exactly perpendicular to the direction of the beam 2, although this is not essential. However, to maximize the beam displacement at the photomask 3 caused by a particular angle of rotation of the plate about the rotation axis, it is desirable for the rotation axis 10 to be substantially perpendicular to the beam direction, preferably within ±20°, and most preferably within ±10°.

[0064] In the first and other embodiments, the rate of change of the gap between the photomask used in DTL exposure and the photoresist-coated wafer is preferably selected relative to the rate of rotation of the plate about at least one axis of rotation, such that two or more gap change scans are performed during or per rotation of the plate, or conversely, two or more gap change scans are performed during or per rotation of the plate. In this manner, by synchronizing the DTL scan with the plate rotation scan, the uniformity of the DTL printed pattern is further optimized in areas of local non-uniformity in the intensity distribution when illuminating the photomask.

[0065] Referring to FIGS. 2a, 2b, and 2c, which illustrate a second embodiment of the present invention, a beam 12 from an illumination module of a lithography system 11 designed to print periodic patterns using DTL technology is directed toward a photomask 13. The photomask 13 defines a periodic pattern of features 14 and is positioned above and in close proximity to a photoresist-coated wafer 15. Between the illumination module 11 and the photomask 13 is a 25 mm thick circular plate 16 made of fused silica tilted at a predetermined angle so that the beam 12 illuminates the plate 16 at an initial angle of incidence of 15°. The diameter of the plate 16 is large enough to allow the complete beam 12 to pass through without obstruction at that angle of incidence. The top and bottom surfaces of the plate 16 are polished to a flatness and scratch / dig quality comparable to that of the plate 6 in the first embodiment.

[0066] Plate 16 is mounted by fixtures 18 to a rotary stage 19 whose axis of rotation 20 is parallel to the direction of exposure beam 12 and is approximately centered on beam 12. Unlike stage 9 used in the first embodiment, stage 19 has an open aperture centered on its axis of rotation 20, which allows beam 17 transmitted through plate 16 to pass unimpeded through stage 19 and illuminate photomask 13. The passage of the beam through the aperture of rotary stage 19 can be clearly seen in the top view of the apparatus of the second embodiment shown in FIG. 2c. For clarity, illumination module 11 has been omitted from FIG. 2c.

[0067] The top and bottom surfaces of plate 16 are polished to provide good parallelism between these two surfaces, so that the angle of incidence of transmitted beam 17 at photomask 13 remains substantially constant as tilting plate 16 is rotated by stage 19 about rotation axis 20. The required parallelism can be readily determined by those skilled in the art of basic optics using Snell's law. The residual wedge angle ω between the top and bottom surfaces changes the angle of incidence ω of the beam at the photomask as the plate rotates. Therefore, a residual wedge angle of 20 arcsec maintains the angle of incidence of transmitted beam 17 at photomask 13 at a constant value of approximately 0.1 mR, which is sufficient to print a 0.5 μm periodic pattern when the distance between photomask 13 and wafer 15 is 0.1 mm. The top and bottom surfaces of plate 16 are provided with an anti-reflective coating to substantially maximize the intensity of transmitted beam 17.

[0068] As in the first embodiment, stage 19 does not need to rotate at a high rotational speed or with high angular resolution, and therefore is available from many suppliers, such as Newport Corporation, as one of their DC Motor Rotation Stage series. As in the first embodiment, the oblique angle of incidence of beam 12 on plate 16 and the refraction of the beam as it passes through plate 16 result in a lateral offset of transmitted beam 17 relative to incident beam 12. Note that, using the apparatus of this embodiment, because the tilt angle of plate 16 relative to the rotation platform of stage 19 is fixed and the stage rotation axis is parallel to the beam direction, the magnitude of the beam's angle of incidence on the plate remains constant as the stage rotates, and therefore the magnitude of the lateral offset of the transmitted beam also remains constant. However, because the plane of incidence of the beam on plate 16 rotates with the stage, the direction of the lateral offset similarly rotates about axis 20, resulting in a circular scan trajectory of beam 17 across photomask 13. Thus, local intensity non-uniformities in transmitted beam 17 cause blurring or smearing over an annular region in the time-integrated intensity distribution when exposing photomask 13 .

[0069] Stage 19 is initially stationary and then rotates at a constant speed selected to be 6° / s when exposure begins, causing stage 19 to complete one full 360° rotation at the end of the required 60 second exposure time. The full rotation of stage 19 during exposure ensures that any intensity non-uniformities in the instantaneous beam 17 illuminating photomask 13 result in a uniform smear across the entire annular region in the time-integrated intensity distribution illuminating photomask 13. The mean radius of the annular region is the distance of the lateral offset of transmitted beam 17 relative to incident beam 12 formed by tilting plate 16, i.e., θ = 1 / (θ ⁢ ... i= 15°, plate thickness 25 mm, and n = 1.47), is calculated to be 2.15 mm. The circumference of the annular region at that average radius is therefore approximately 13.5 mm. The contrast reduction of a defect of a given size can be estimated: a 1 mm sized defect will be smeared over a (annular) 13.5 mm long region, reducing its contrast by a factor of approximately 13.5; thus, a defect with approximately 10% contrast in instantaneous beam 17 will be reduced to a defect with less than 1% contrast in the time-integrated distribution when illuminating photomask 13, which can be ignored for most applications.

[0070] The rotation rate in this embodiment is preferably selected so that one full rotation of the stage 19 occurs during exposure; however, the rotation rate can alternatively be selected to result in a greater number of integer rotations (i.e., 720°, 1080°, etc.), achieving the same result of uniformly smearing defects throughout the associated annular region. Alternatively, rotation rates resulting in non-integer rotations, preferably greater than half a rotation, i.e., greater than 180°, can also be used, but such rotation rates have the disadvantage that non-uniformities do not result in uniform smearing throughout the annular region, resulting in higher-than-normal contrast values ​​elsewhere in the annular region. The difference between optimized and non-optimized contrast values ​​resulting from integer and non-integer stage rotations decreases as the number of rotations increases, and can be made negligible, for example, by adjusting the rate to result in at least two rotations during exposure.

[0071] Although the start and stop of the stage rotation described in the above embodiment are synchronized with the start and end of exposure, this is not required. What is important is that an integer number of rotations is preferably performed during the exposure time. Therefore, the stage rotation can be turned on before the exposure starts and / or switched off after the exposure ends. However, it is still preferable that the rotation speed be calculated from the exposure time so that the desired integer number of rotations is completed during the exposure. Alternatively, a different or non-optimized speed can be used, but the disadvantages mentioned above will prevent uniform smearing across the entire annular region due to non-uniformity.

[0072] As with the first embodiment, the rotary stage 19 can alternatively be rotated at a variable speed, although this may not be desirable as it would generally result in higher contrast somewhere in the smeared area.

[0073] As with the first embodiment, the rotational stage 19 may alternatively be configured to displace using a sequence of incremental steps through the desired rotation angle rather than using a constant or variable speed. For example, a 360° angular displacement can be achieved using 18 incremental steps of 20° each with a constant stepping frequency and a constant dwell time between steps. However, for short exposure times of less than 60 seconds, this is not an optimal choice because such short exposure times require significant acceleration and deceleration of the stage 19 for each step, which can introduce undesirable vibrations between the photomask and wafer and degrade the quality of the periodic pattern printed on the wafer 15.

[0074] The rotation axis of the stage used in the second embodiment is preferably aligned parallel to the beam direction; however, in one variation of this embodiment, the rotation axis is alternatively aligned at a 5° tilt relative to the beam direction. This asymmetric alignment results in a change in the magnitude of the beam's angle of incidence on the plate as the plate is rotated about its axis, resulting in an elliptical scan trajectory that is slightly different from the circular scan trajectory of the beam illuminating the photomask, and a slight change in the contrast of the time-integrated intensity distribution of each defect around this elliptical region. However, the results obtained are substantially equivalent to those obtained with the circular scan trajectory of the second embodiment. Note that, to minimize the contrast of the time-integrated intensity distribution over the smear region for a particular tilt angle of the plate (relative to the plate's rotation axis during exposure) and plate thickness, it is preferred that the stage rotation axis be aligned substantially parallel to the beam direction, particularly preferably within ±10° of the beam direction, and most preferably within ±5° of the beam direction.

[0075] Referring to FIGS. 3a and 3b, for a third embodiment of the present invention, an illumination module 21 in a DTL-based photolithography system emits a collimated beam 22 of monochromatic light at a near-ultraviolet wavelength. The beam 22 is directed toward a mask 23, which defines a periodic pattern 24 and is positioned above and closely adjacent to a photoresist-coated wafer 25. Between the illumination module 21 and the mask 23 is a 15 mm-thick plate 26 made of fused silica with parallel upper and lower surfaces and an anti-reflection coating for the wavelength. The surface parallelism requirements can be determined as in the second embodiment. The plate 26 is tilted obliquely relative to the beam 22 and is sized to allow the entire beam 22 to pass through without being clipped. The plate 22 is mounted by a fixture 28 to the platform of a first motorized rotary stage 29, which has a rotation axis 30 perpendicular to the direction of the beam 22. The base of the rotation stage 29 is attached by a bracket 31 to the platform of a second motorized rotation stage 32 having a rotation axis 33 parallel to the direction of the beam 22 and centered approximately on the beam 22 in the output plane of the illumination module 21. The second stage 32 has a central through aperture centered on the stage's rotation axis 33 and large enough to allow the beam 27 transmitted through the plate 26 to pass unimpeded through the stage 32 as it is rotated.

[0076] As in the first and second embodiments, refraction of beam 22 as it passes through tilted plate 26 causes a lateral offset of transmitted beam 27 relative to incident beam 22, with the magnitude of this lateral offset given by equation (2) and the direction of the offset parallel to the plane of incidence of beam 22 on plate 26. That is, angular displacement of first rotation stage 29 changes the magnitude of the lateral offset, while angular displacement of second rotation stage 32 changes the direction of the offset.

[0077] As with the previously described embodiment, the beam 22 generated by the illumination module 21 has good uniformity of intensity on the macroscale, but on the millimeter scale has multiple undesirable non-uniformities in some local radial regions up to about 1 mm, with intensity deviations from surrounding regions of up to about 10%.

[0078] Before the DTL-based photolithography exposure proceeds, the first rotational stage 29 is adjusted so that the beam 22 illuminates the plate 26 at an initial angle of incidence of 10°. At the start of the exposure, the second rotational stage 32 is actuated to rotate the plate 26 about the axis 30 at a constant rate of 18° / s, selected so that the stage 32 completes two full rotations during the desired exposure time of 40 seconds. Approximately halfway through the exposure, i.e., after approximately 20 seconds, the first rotational stage 29 adjusts the tilt angle of the plate by 13° in a time period less than 1 second (short compared to the exposure duration) so that the angle of incidence of the beam 22 on the plate is substantially 23° during the latter half of the exposure. The magnitude of the adjustment to the tilt angle is preferably selected so that the lateral offset of the transmitted beam 27 is magnified by at least the largest dimension of the intensity nonuniformity, thereby minimizing the contrast of the nonuniformity in the time-integrated intensity distribution as it illuminates the photomask 23. As the tilt angle of a 25 mm thick plate 26 made of fused silica with a refractive index of approximately 1.47 changes from 10° to 23°, the lateral offset (using Equation 2) changes from 1.5 mm to 3.6 mm, i.e., is increased by 2.1 mm, a distance at least equal to the largest dimension of the inhomogeneity, 2 mm.

[0079] Rotating second stage 32 during each of the first and second halves of the exposure creates a circular scanning motion of each non-uniformity in the plane of photomask 23. The radius of the scanning path during each half of the exposure corresponds to the lateral offset of transmitted beam 27 caused by the respective tilt angle of plate 26. By arranging for this lateral offset to be expanded by the maximum dimension of the non-uniformity between the first and second halves of the exposure, the composite area scanned by each non-uniformity during exposure becomes substantially annular, with a width (the difference between the inner and outer radii) approximately twice the maximum dimension of the non-uniformity. The total length of the scanning path of each non-uniformity during exposure is the sum of the circular paths formed by the two halves of the exposure, i.e.,

number

number

[0080] However, the contrast reduction factor estimated above is an average value over the entire annular region, because the radius of the track scanned by each non-uniformity in the first half of the exposure is approximately 40% of the radius scanned in the second half of the exposure, and the contrast reduction factor obtained over the entire area scanned in the first half of the exposure is approximately 40% of the contrast reduction factor obtained over the entire area scanned in the second half of the exposure. The average value calculated above is obtained more uniformly over the entire annular region by using exposure times for the first and second half of the exposure that are proportional to the respective lateral offsets of the transmitted beams (to ensure that the total exposure time corresponds to the desired value of 40 seconds). Thus, the desired effect is achieved from exposure times of approximately 12 seconds and approximately 28 seconds for the first and second half of the exposure, respectively. Of course, the rotation speed of the second stage 32 during the first and second half of the exposure must be selected accordingly, so that one full rotation is preferably performed during each half.

[0081] Alternatively, many variations of the procedure described above can be used to suppress local non-uniformities in the exposure beam using the apparatus of the third embodiment. For example, instead of the case described above, the exposure could be divided into three or more parts instead of just two, and a different tilt angle of the plate 26 could be used for each part, and the same methodology described above could be extrapolated. In another variation, rather than continuing to expose the photomask 23 during the short time between the first and second parts of the exposure while the first rotary stage 29 adjusts the tilt angle of the plate 26, the exposure beam 22 could instead be switched off during this time and switched on again after the tilt angle has been adjusted. The resulting contrast reduction would be substantially the same. In yet another variant, the first rotary stage 29 is not adjusted substantially in the short period between two (or more) portions of the exposure, as described in the third embodiment above, but instead may be rotated continuously simultaneously with the rotation of the second stage 32, preferably at a rotation speed selected so that the angular change of the first stage after a full rotation of the second stage 32 corresponds to the angular change recommended in the third embodiment, i.e., so that the lateral offset of the transmitted beam is magnified by at least the maximum dimension of the intensity non-uniformity. By rotating the two stages 29, 32 simultaneously as described, each local non-uniformity in the exposure beam is scanned in a spiral trajectory over the photomask 23, with the distance between successive loops of the spiral corresponding to the maximum dimension of the non-uniformity.

[0082] As with the above-described embodiment, the plate can be positioned so that it is stationary at the start of exposure and the beam initially illuminates the plate at an initial angle of incidence. In a variation of that embodiment, one or both of the stages rotate about their respective axes before exposure so that the beam illuminates the plate at the desired initial angle of incidence or within a desired range of angles of incidence at the start of exposure, and then the stages rotate sequentially and / or simultaneously during exposure to produce the desired displacement of the beam across the photomask.

[0083] Although the change in the tilt angle of plate 26 in the third embodiment is achieved using a motorized rotation stage 29, in another variation of this embodiment, the rotation of plate 26 can be achieved by other means, for example by arranging one side of plate 26 to be hinged and the other side of plate 26 to be displaced using a linear actuator.

[0084] The rotation axes of the first and second rotary stages used in the third embodiment are preferably arranged so that they are orthogonal and parallel to the beam direction, respectively. However, as with the previous embodiment, this is not required, and in a variation of this embodiment, the axes are arranged obliquely to the beam direction. By arranging the rotary stages in this oblique manner, if the length of the non-scanning trajectory of the beam illuminating the photomask is the same as the trajectory length formed using the third embodiment, the contrast of the time-integrated non-uniformity when illuminating the photomask is also reduced. Note that the rotation axes of the first and second rotary stages are preferably arranged so that they are substantially orthogonal and parallel to the beam direction (with the same accuracy as specified in the previous embodiment).

[0085] 4a and 4b, for a fourth embodiment of the present invention, an approximately 10 cm diameter beam 42 of collimated light at a near-ultraviolet wavelength is projected from an illumination module 41 of a DTL exposure system. As in the previous embodiment, the beam 42 is directed toward a photomask 43 defining a periodic pattern, which is positioned above and closely adjacent to a photoresist-coated wafer 45 in the DTL system. The beam illuminating the photomask 43 is uniform on a macroscale, but has local non-uniformities with spatial dimensions of approximately 1 mm, resulting in a contrast of approximately 10%.

[0086] Between the illumination module 41 and the photomask 43 is a 20 mm thick plate of fused silica 46 through which the complete beam 42 passes. The top and bottom surfaces of the plate are parallel and are anti-reflective coated for the relevant wavelength. Surface parallelism requirements can be determined similarly to the second embodiment. The plate 46 is mounted on a motorized two-axis gimbal system 48, which includes a first stage 49 that rotates the plate 46 about a first axis 50 perpendicular to the direction of the beam 42 (and the y-direction), and a second stage 51 that rotates the plate 46 about a second axis 52 perpendicular to the first axis 50. The second stage of the gimbal system is mounted to the first stage, so that the orientation of the second axis rotates with the rotation of the first stage but remains substantially perpendicular to the beam direction during exposure. Plate 46 is attached to second stage 51 by adapter components (not shown), which itself is attached to first stage 49 by yoke 53, such that rotation of first stage 49 results in rotation of axis 52 of second stage 51. Suitable motorized two-axis gimbal systems are available, for example, from Newmark Systems Inc., California, USA (www.Newmarksystems.com). As with the previous embodiment, tilting plate 46 relative to the direction of incident beam 42 results in a lateral offset of transmitted beam 47, and changing the tilt angle of plate 46 during exposure results in a translational displacement of beam 47 across photomask 43. The tilt of plate 46 is initially adjusted using first rotation stage 49 and second rotation stage 51 at the start of exposure so that the components of the angle of incidence of beam 42 in the x-z and y-z planes on plate 46 are +15° and 0°, respectively. The tilt of the 20 mm thick plate 46 results in a lateral offset of the transmitted beam in the xz plane of +1.7 mm, using equation (2).When exposure begins, a control system (not shown) initiates angular scanning motions of two rotation stages 49 and 51. The tilt angle of the plate 46 in the x-z plane is varied from +15° to -15° by the first stage 49 with a cosine-like time dependence shown in Figure 5, and the tilt angle of the plate 46 in the y-z plane is varied from +15° to -15° by the second stage 51 with a sinusoidal time dependence shown in Figure 5. The rates of change of the angular motion are selected so that each stage 49 and 51 completes a single oscillation of its motion at the end of exposure. As a result of these scanning motions in the x-z and y-z planes, the angle of incidence of the beam 42 on the plate 46 remains constant throughout the exposure, but the plane of incidence of the beam 42 on the plate 46 rotates at a constant speed, completing one rotation at the end of exposure. As a result, the beam 47 illuminating the photomask 43 is displaced translationally in a single circular orbit of radius 1.7 mm, i.e., with no or negligible change in the angle of incidence of the beam. Thus, a local inhomogeneity of 1 mm diameter has an area approximately 13.6 times larger than the area of ​​this inhomogeneity.

number

[0087] Thus, the circular scanning motion of beam 47 at mask 43 obtained using the two-axis gimbal system 48 of the fourth embodiment is similar and substantially equivalent to the circular scanning motion obtained using the rotation of a tilting plate about a single axis obtained using the apparatus and procedure of the second embodiment, although the apparatus of the second embodiment is mechanically simpler and therefore generally preferred.

[0088] The apparatus of the fourth embodiment can be used with alternative multiple angular scanning or stepping schemes of the rotary stages 49, 51 to similarly suppress the contrast of local non-uniformities in the exposure beam 42. For example, faster oscillations of the rotary stages 49, 51 can be used to produce two or more integer number of stage oscillations during the exposure time, thereby allowing the exposure beam 47 to scan more times around the circular orbit during exposure. Alternatively, faster (or slower) oscillations of the rotary stages 49, 51 can be used, preferably resulting in more than one non-integer number of stage oscillations during the exposure time.

[0089] In another alternative variation of this embodiment, the amplitudes of the oscillation tilt angles in the xz and yz planes have different values, so that the scanning trajectory of the transmission beam 47 illuminating the photomask 43 follows an elliptical trajectory rather than a circular trajectory. In another variation, multiple oscillations of the two rotation stages 49, 51 are used, with the amplitude of the oscillations gradually increased (or decreased) over time, thereby forming a spiral scanning trajectory of the exposure beam 47 on the photomask 43. In another variation, the movement of the two rotation stages 49, 51 is not an oscillation, but a sequence of linear scans that form, for example, a raster scanning trajectory or a square or any other shaped trajectory around the edge of the beam 47 illuminating the photomask 43.

[0090] An advantage of the two-axis gimbal system used in the fourth embodiment is that the axes of the two rotation stages intersect and can be positioned to coincide with the center of the plate, allowing for a relatively compact system. In other variations of the fourth embodiment, other configurations of the rotation stages can alternatively be used to rotate the plate about mutually orthogonal rotation axes that do not intersect or are near the center of the plate. In a two-axis gimbal system, the second rotation axis rotates along with the rotation about the first axis. Preferably, the second axis remains substantially orthogonal to the beam direction during exposure, particularly within ±25°, and most preferably within ±15°; otherwise, the magnitude of the lateral displacement of the beam illuminating the photomask caused by the rotation of the plate about the second axis may be unacceptably reduced.

[0091] Another variation of the fourth embodiment uses an alternative two-axis gimbal system in which the two axes are mutually orthogonal but not coupled, i.e., one stage axis is not rotated by the other stage. The system is configured so that both axes of the gimbal system are orthogonal to the beam direction (and remain orthogonal to the beam) as the plate rotates about each axis in a DTL exposure system. Such decoupled two-axis gimbal systems generally have negligible advantages over the off-the-shelf gimbal systems described above in terms of the resulting contrast of the time-integrated intensity distribution when illuminating a photomask.

[0092] While each of the above-described embodiments and variations thereof uses a single plate to provide angular displacement about at least one axis of rotation, other embodiments may alternatively use two or more plates to provide angular displacement. For example, referring to FIG. 6 , which illustrates a sixth embodiment of the present invention, the single plate of the first embodiment is replaced by a pair of plates 6a and 6b, each of which has the same surface area as the surface of a single plate and a combined thickness equal to the thickness of the single plate. The plates are arranged in series, i.e., stacked, in the beam, preferably parallel to each other, with a spacing preferably small relative to the plate thickness, facilitating mounting of each plate to the rotation stage. The two plates are separated by a distance of 0.5 mm using a spacer 8a in the form of a sheet of material with a large central aperture inserted between the plates. The modified system is used in the same manner as in the first embodiment, and the lateral offset of the beam passing through a pair of plates can be calculated in the same way using equation (2), except that the parameter T refers to the combined thickness of the plates (excluding the spacing between the plates). This replacement of a single plate with a pair of plates or a larger number of plates can also be applied to any of the other preceding embodiments. For multi-plate embodiments, it becomes even more important to provide anti-reflective coatings on the surfaces of the plates to avoid excessively long exposure times.

[0093] In other multi-plate embodiments, the additional plates may alternatively be mounted on additional rotation stages that rotate the additional plates during exposure about the same or different rotational axis as the first plate.

[0094] While in each of the above embodiments the application of the present invention to the particular photolithography technique of displacement Talbot lithography has been described and illustrated, in other embodiments the same methodology for reducing local intensity non-uniformities in a beam can be used with other Talbot effect based lithography techniques such as achromatic and conventional Talbot lithography, or alternatively with other photolithography techniques such as near-field holographic lithography, which require illumination of a periodic pattern on a mask with a well-collimated monochromatic light beam.

[0095] In the former case, since the exposure beam illuminates the photomask at normal incidence in Talbot effect-based techniques, the device for reducing local intensity non-uniformities can also be similar.

[0096] It should be noted that for near-field holographic lithography, the exposure beam must illuminate the mask at an oblique angle so that only two diffraction orders propagate behind the mask in the desired direction (as discussed above). In this case, the apparatus of various embodiments must be reconfigured so that the beam transmitted through the rotating plate illuminates the photomask at the required angle of incidence. In particular, the orientation of the photomask, photoresist-coated substrate, and associated mechanical subsystems must be adjusted relative to the orientation of the combined illumination module and rotating plate module so that the beam transmitted through the rotating plate illuminates the photomask at the required oblique angle of incidence.

[0097] In near-field holography, the spacing between the photosensitive layer on the substrate and the periodic pattern on the photomask must be sufficiently small so that the range of incident angles of light in the collimated beam illuminating each point on the photomask does not cause unacceptable blurring or smearing of the periodic intensity distribution when illuminating the photosensitive layer, i.e., due to the same considerations as those discussed above in accordance with Talbot imaging-based photolithography. Thus, the photosensitive layer on the substrate must be positioned in close proximity to the periodic pattern on the photomask, relative to the degree of collimation of the illumination beam and the period of the pattern on the photomask, such that the periodic pattern printed on the photosensitive layer has the desired contrast.

[0098] In a variant of this embodiment based on near-field holography, the diffraction grating in the mask may be an amplitude grating in the form of chrome lines on a transparent mask substrate such as fused silica, or advantageously a phase grating as is often used in the prior art, or alternatively a volume grating in which the lines and spaces are formed as regions of different refractive index in a suitable photosensitive recording material.

[0099] Furthermore, in the above and previous embodiments, the photosensitive layer onto which the second periodic pattern is printed during exposure is preferably a positive or negative photoresist, which is subsequently developed after exposure to remove the exposed or unexposed areas, respectively. Alternatively, the photosensitive layer may be made of another type of material, for example, a material that records periodic variations in exposure energy density as periodic variations or modulations in refractive index, as is typically the case with holographic recording materials.

[0100] In another variation of the preceding embodiment, each of the two surfaces of the plate is alternatively coated with a partially reflective coating, for example, providing approximately 50% reflectivity and approximately 50% transmittance per surface. When such a coating is used, the plate transmits approximately 25% of the directly transmitted beam, and a portion of the incident beam's power is transmitted in the same direction as the directly transmitted beam after being doubly reflected between the two surfaces of the plate, contributing an additional approximately 6% to the plate's total transmittance. Due to the oblique incidence angle of the beam on the plate, the doubly reflected component of the transmitted beam is laterally offset (by a distance proportional to the incidence angle and the plate thickness) from the directly transmitted component. The superposition of the two laterally offset components reduces the instantaneous contrast of local intensity uniformity, further reducing the contrast of non-uniformities in the time-integrated intensity distribution when illuminating a photomask. Although the illumination wavelengths used in the above embodiments are in the near ultraviolet part of the spectrum and the photoresist used is sensitive to this wavelength, in other embodiments of the invention the illumination wavelength may be in another part of the spectrum, for example visible light, deep ultraviolet, in particular 248 nm or 193 nm or extreme ultraviolet, and the photoresist used should be appropriately selected to have the required sensitivity to the wavelengths in question.

[0101] While the parallel plates described in the above embodiments are all made of fused silica, it should be understood that in other embodiments, the plates may be made of other materials that are similarly transparent or partially transparent to the wavelengths of the relevant illumination beam. For example, certain borosilicate glasses with high transmittance up to about 360 nm are good substitutes for fused silica for near-UV exposure wavelengths, and UV-grade calcium fluoride is a substitute material for deep-UV exposure wavelengths. Alternatively, glasses with low transmittance to the illumination wavelengths can be used, but this has the disadvantage of longer exposure times.

Claims

1. 1. A method for improving uniformity of exposure by a collimated beam of a first periodic pattern on a photomask in a photolithography system that utilizes the Talbot effect to print a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask, the method comprising: a) providing a plate made of a material transparent to said beam, said plate having opposing polished surfaces that are parallel and separated by a thickness; b) placing the plate in the photolithography system such that the collimated beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates the photomask; c) rotating the plate through at least one angle about at least one axis of rotation during exposure so that the transmitted beam is translationally displaced across the photomask; A method comprising:

2. The method of claim 1 , wherein the plate is rotated about a rotation axis perpendicular to the direction of the beam.

3. 2. The method of claim 1, wherein the plate is positioned such that the beam illuminates the plate at an oblique initial angle of incidence and the plate is rotated about an axis of rotation parallel to the direction of the beam.

4. 2. The method of claim 1, wherein the plate is positioned such that the beam illuminates the plate at an oblique initial angle of incidence and is rotated a first angle about a first axis of rotation parallel to the direction of the beam in a first stage, a second angle about a second axis of rotation perpendicular to the beam in a second stage, and a third angle about the first axis of rotation in a third stage.

5. 2. The method of claim 1, wherein the plate is positioned such that the beam illuminates the plate at an oblique initial angle of incidence and is simultaneously rotated by a first angle about a first axis of rotation parallel to the direction of the beam and by a second angle about a second axis of rotation perpendicular to the beam.

6. The method of claim 1 , wherein the plate is rotated about first and second axes of rotation that are orthogonal to each other and substantially perpendicular to the direction of the beam.

7. the plate is rotated about first and second axes of rotation that are orthogonal to each other and substantially perpendicular to the direction of the beam; the time dependence of rotation about the first axis of rotation and the second axis of rotation is described by a sine function and a cosine function, respectively, whereby the magnitude of the angle of incidence of the beam on the plate remains substantially constant during exposure; The method of claim 1.

8. the initial angle of incidence is selected or within a selected range of angles of incidence; The method of claim 1.

9. 2. The method of claim 1, wherein the thickness of the plate, the initial angle of incidence, and at least one angle about the at least one axis of rotation are selected to produce a desired displacement of the beam transmitted across the photomask.

10. The method of claim 1 , wherein rotation of the plate is initiated prior to exposure of the photomask.

11. The method further comprises: providing at least one additional plate made of a material transparent to said beam; positioning the at least one additional plate within the photolithography system such that the transmitted beam illuminates the at least one additional plate and such that the beam transmitted through the at least one additional plate illuminates the photomask; rotating the at least one additional plate about at least one axis of rotation during exposure; 2. The method of claim 1, comprising:

12. 1. An apparatus for improving uniformity of exposure of a periodic pattern on a photomask by a collimated beam in a photolithography system that uses the Talbot effect to print a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask, comprising: a) a plate made of a material transparent to said beam, said plate having opposing parallel polished surfaces separated by a thickness; b) means for positioning the plate within the photolithography system such that the collimated beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates the photomask; c) means for rotating said plate through at least one angle about at least one axis of rotation during exposure so that the transmitted beam is translationally displaced across said photomask; An apparatus comprising:

13. 13. The apparatus of claim 12, wherein the rotating means rotates the plate about an axis of rotation perpendicular to the direction of the beam.

14. the positioning means positions the plate so that the beam illuminates the plate at an oblique initial angle of incidence; the rotating means rotates the plate about a rotation axis parallel to the direction of the beam; 13. The apparatus of claim 12.

15. the positioning means positions the plate so that the beam illuminates the plate at an oblique initial angle of incidence; the rotating means rotates the plate through a first angle about a first axis of rotation parallel to the direction of the beam in a first stage, through a second angle about a second axis of rotation perpendicular to the beam in a second stage, and through a third angle about the first axis of rotation in a third stage.

13. The apparatus of claim 12.

16. 13. The apparatus of claim 12, wherein said rotating means simultaneously rotates said plate about a first axis parallel to the direction of said beam and a second axis perpendicular to said beam.

17. 13. The apparatus of claim 12, wherein said rotating means rotates said plate about first and second axes of rotation that are orthogonal to each other and substantially perpendicular to the direction of said beam.

18. the rotating means rotates the plate about first and second axes of rotation that are orthogonal to each other and substantially perpendicular to the direction of the beam; the time dependence of rotation about the first axis of rotation and the second axis of rotation is described by a sine function and a cosine function, respectively, whereby the magnitude of the angle of incidence of the beam on the plate remains substantially constant during exposure; 13. The apparatus of claim 12.

19. the beam is monochromatic; the distance between the photomask and a photoresist-coated substrate placed in proximity to the photomask is varied during exposure according to a displacement Tarbo lithography method; 13. The apparatus of claim 12.

20. 1. A method for improving the uniformity of exposure of a first periodic pattern on a photomask by a collimated beam of monochromatic light in a photolithography system that prints a second periodic pattern on a photosensitive layer on a substrate adjacent to the photomask, the method comprising: a) providing a plate made of a material transparent to said beam, said plate having opposing polished surfaces that are parallel and separated by a thickness; b) placing the plate in the photolithography system such that the collimated beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates the photomask; c) rotating the plate through at least one angle about at least one axis of rotation during exposure so that the transmitted beam is translationally displaced across the photomask; A method comprising:

21. 1. In a photolithography system for printing a second periodic pattern on a photosensitive layer on a substrate adjacent to a photomask, an apparatus for improving uniformity of exposure of a first periodic pattern on the photomask with a collimated beam of monochromatic light, the apparatus comprising: a) a plate made of a material transparent to said beam, said plate having opposing parallel polished surfaces separated by a thickness; b) means for positioning the plate within the photolithography system such that the collimated beam illuminates the plate at an initial angle of incidence and the beam transmitted through the plate illuminates the photomask; c) means for rotating said plate through at least one angle about at least one axis of rotation during exposure so that the transmitted beam is translationally displaced across said photomask; An apparatus comprising:

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