Method and apparatus for improving exposure uniformity of periodic patterns - Patents.com

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 is fabricated with a hexagonal lattice of elliptical holes with a spatial period of 583 nm and 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 predefined periodic distances from the mask, separated by the so-called Talbot distance, and the diffraction orders interfere to reproduce the pattern in the mask. By placing a photoresist-coated substrate in one of these “self-image” planes, the periodic pattern in the mask is printed into the photoresist.

[0002] US Patent No. 8,841,046 discloses two different but related photolithography methods based on Talbot imaging for printing one- 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 area of ​​the pattern such that the local period is substantially constant (over a distance of a dimension comparable 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, the periodic or quasi-periodic pattern on the mask is illuminated by a beam of collimated light from a source with a wide spectral bandwidth, and the substrate is placed in a position where the Talbot effect produces a lateral intensity distribution that is "static", i.e., 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 the mask is illuminated with a collimated beam of monochromatic light, and the distance between the mask and the substrate is varied during the exposure by a distance corresponding to an integer multiple of the Talbot distance, which prints the average of the lateral intensity distributions formed between successive Talbot image planes, resulting in a periodic distribution with a substantially infinite depth of focus. The above disclosure further teaches that the distance between the mask and the substrate can be varied continuously over the required range during the exposure, or can be varied 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 periodic value in the mask, to be printed simultaneously onto the photosensitive layer.

[0006] To print periodic patterns uniformly using any of the above Talbot effect based methods, it is generally important that the intensity of the beam illuminating the mask is uniform over the area of ​​the pattern (typically varying less than ±3%). More importantly, the beam is well collimated, i.e. the light rays illuminating each local area of ​​the mask are exactly parallel. If the beam is not well collimated, the light rays at different angles will project their Talbot images in different directions behind the mask, causing smearing or blurring of the composite image formed on the substrate, thus reducing the resolution of the printed pattern. The degree of collimation required depends on the period of the pattern and the distance between the mask and the substrate. For a period of submicron and a mask-to-substrate distance of about 0.1 mm, the degree of collimation required is typically about 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 the high collimation requirement, and especially of using monochromatic light, is that the intensity uniformity of the beam illuminating the mask is sensitive to small imperfections or small defects, typically of about 0.1 mm to 1 mm in size, in or on the optical components of the illumination system forming the beam. Such imperfections, e.g., microbubbles inside a lens, digs or particles on a lens surface, or microscopic scratches (small in size in one direction), 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 for illuminating the photoresist, but also prevents smearing of intensity non-uniformities in the illumination beam due to small defects in the beam-forming optics. Depending on the nature of the defect, e.g., whether the defect affects the amplitude and / or phase of the light directly transmitted, a dark or bright spot or a set of concentric rings of alternating high and low intensity may be formed in the beam illuminating the mask. The dimensions of the resulting undesired non-uniformities in the mask depend on the defect dimensions and the degree of collimation but are typically in the range of 0.1 mm to 5 mm, and the contrast of the defect relative to the intensity of the surrounding beam is typically in the range of 1% to 20%. Such non-uniformities in the intensity distribution of the mask illumination give rise to corresponding non-uniformities in the energy density distribution of the mask exposure during the exposure period and, consequently, in the dimensions of periodic features of the DTL printed pattern. Depending on the application involved, such local non-uniformities in the printed pattern may be unacceptable.

[0008] Another drawback of exposure beams containing highly collimated monochromatic light 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, especially from micro- and nano-scale roughness of the lens surfaces that is present even after high quality polishing. The general scattering from the optics combined with light having both 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 non-uniformities, but present throughout the entire beam. The dimensions of individual speckles are determined by the collimation of the beam and are typically in the range of 0.5 mm to 5 mm, and the contrast of individual speckles relative to the average intensity of the surrounding beam is typically in the range of 0.5% to 10%.

[0009] Of course, these local intensity non-uniformity problems can be avoided by using perfect optical elements with no surface or internal defects, and by ensuring that there are no particles on any of the optics, but this may 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 light beam with high intensity uniformity to illuminate the periodic pattern in the mask and is therefore also sensitive to imperfections present 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 light beam does not illuminate the mask at normal incidence, but at an oblique angle that is preferably selected in relation to the period of the linear diffraction grating of the mask and the illumination wavelength, so that only the zeroth and first diffraction orders propagate to the rear of the mask, with diffraction angles preferably symmetrically located on either side of the normal to the mask. The diffraction grating in the mask is preferably a phase grating designed and fabricated such 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] It is therefore a first object of the present invention to provide a method 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, 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, it is an object of the present invention to provide an apparatus 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.

[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 for printing 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 an apparatus 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.

[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 through at least one angle about at least one axis of rotation during exposure such 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 such 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 non-uniformity of the exposure energy density, and 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 at the start of exposure, the beam is stationary so that 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 motion throughout the exposure period, alternatively there may be a stepping motion in which the plate is stationary or at least slow between successive steps of the series of rotating steps.

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

[0021] Preferably, the thickness, the initial angle of incidence, at least one rotation angle, and at least one rotation axis are selected such that the resulting translational displacement of the beam across the photomask is greater than the dimension in at least one direction of the local intensity non-uniformity of the exposure beam, most preferably at least 5 times greater, 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 must be at least as large as the width (but not the length) of this elongated stripe, and the contrast of the time intensity distribution is reduced by at least 1 / 2 of the width, most preferably by 1 / 5 of the width, so that the contrast of the time intensity distribution is reduced by at least 1 / 5.

[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 facing surfaces of the provided plate is determined such that the variation in the angle of incidence of the transmitted beam at the photomask caused by rotating said plate by at least one angle about at least one axis of rotation during exposure is sufficiently small so that the lateral displacement of the intensity distribution exposing the photosensitive layer does not result in unacceptable blurring and thus in a decrease in the contrast of the second periodic pattern printed on the layer. Preferably, the lateral displacement of the intensity distribution when illuminating the photosensitive layer (ignoring the change in the shape of the intensity distribution in the layer caused by the change in the spacing between the photomask and the substrate performed according to DTL) is less than 1 / 10 of the period of the second periodic pattern, 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 into the photosensitive layer on the substrate are one-dimensional periodic patterns, arrays of alternating line and space features, or alternatively, two-dimensional arrays 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 the locally transmitted light, or alternatively, the first periodic pattern may be an amplitude mask, in which individual features change the transmission amplitude of the 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, but may have a different period, dimension and / or orientation from the first periodic pattern 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 slowly 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 may have a gradual curvature such that the lines are substantially linear across the local area of ​​the grating.

[0028] The above paragraphs apply not only to the first aspect of the 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 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 apparatus comprising: a) a plate made of a material transparent to the beam and 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 such that 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 that the transmitted beam is translationally displaced 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 scattering at the edges of the plate.

[0031] Advantageously, the positioning means positions the plate such 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, thereby reducing 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 in accordance with 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 in accordance with 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 for printing 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 through at least one angle about at least one axis of rotation during exposure such 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 exploits 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 and 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 such that 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 that the transmitted beam is translationally displaced across the photomask; An apparatus is provided comprising:

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

[0039] It should also be appreciated that the methods and apparatus disclosed herein are applicable to lithography systems requiring illumination of a 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 invention described above, 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, having at least one intermediate layer of another material, such as a metal or dielectric, between the substrate and the photosensitive layer, or may refer 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 substrate surface.

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

[0042] The above 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 description of the drawings]

[0043] [Figure 1a] FIG. 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. 4 is a front view showing a second embodiment of the present invention. [Figure 2b] FIG. 4 is a right side view showing a second embodiment of the present invention. [Figure 2c] FIG. 2 is a top view of a second embodiment of the present invention with the lighting module omitted for clarity. [Figure 3a] FIG. 11 is a front view showing a third embodiment of the present invention. [Figure 3b] FIG. 11 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. [Diagram 5] 13 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 respectively show a front view and a right side view of a first embodiment of the present invention, an illumination module of a photolithography system 1 designed to perform displacement Talbot lithography projects a collimated monochromatic beam 2 of light having a substantially uniform intensity across its circular cross section. The illumination source, beam shaping optics, and beam homogenization optics (not shown) in module 1 are described, for example, in U.S. Pat. No. 8,525,973. The beam 2 here has a diameter of just 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 a standard i-line sensitive photoresist. The beam 2 is directed towards a photomask 3 having a periodic pattern of features 4, positioned closely above a photoresist coated wafer 5. Both the photomask 3 and the 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 the illumination beam 2 using standard alignment means and procedures.

[0045] Although beam 2 has good uniformity of intensity on the macroscale, small imperfections in or on the lens, such as microbubbles within the lens and microdigests and dust particles on the lens surface, cause local non-uniformities of intensity 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 result in undesirable non-uniformities in the periodic pattern printed in the photoresist in certain applications, specifically unacceptably small lines or features whose dimensions are larger or smaller than the surrounding pattern lines or features.

[0046] Between the illumination module 1 and the photomask 3, a rectangular plate 6 made of fused silica and through which the beam 2 passes is interposed according to the invention. The plate 6 is shown tilted at an angle with respect to the x-axis, so that the beam 2 illuminates 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 hindrance. The thickness of the plate is 25 mm. The upper and lower surfaces of the plate 6 are polished to provide good flatness of λ / 4 and good scratch / dig quality of 40 / 20, 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 the plate 6 are further polished to be sufficiently parallel so that the angle of incidence of the transmitted beam 7 on the photomask 3 remains sufficiently constant as the plate 6 is rotated about the range of angles used in the embodiment. Just as a large range of incidence angles of the beam would cause each point on the mask to be illuminated instantaneously (i.e. insufficient collimation) and would cause unacceptable smearing or blurring of the features of the periodic pattern printed on the photosensitive layer, a large wedge angle between the upper and lower surfaces of the substrate and a large change in the incidence angle of the beam on the mask over time during exposure may cause unacceptable blurring of the features of the periodic pattern printed on the wafer. A change in the incidence angle of the illumination beam ΔΦ, where d is the spacing between the mask and the wafer, shifts the intensity distribution when illuminating the photoresist laterally by a distance of about dΔΦ. This distance is therefore 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 the wafer is 0.1 mm, most preferably the incidence angle of the beam on the photomask is kept constant at less than 0.12 mR during exposure. The resulting parallelism required between the plate surfaces can be easily determined by those skilled in the art of basic optics using Snell's law. If the plate is rotated during exposure so that the incidence angle of the beam on the plate varies over a range from -θ1 to θ1, the incidence angle 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 gives:

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[0049] The top and bottom surfaces of the plate 6 are provided with an anti-reflective coating, which minimizes power losses and exposure time of the beam.

[0050] Plate 6 is mounted by a fixed bracket 8 to a motorized rotation stage 9 which 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) and 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 to the horizontal, such that beam 2 is at an oblique incidence angle θ i 1 illuminates plate 6. 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 it by a distance Δx (as 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) We approximate the linear relationship given by

[0052] Thus, as 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] The stage 9 initially orients the plate 6 such that at the beginning of the exposure, the initial angle of incidence of the beam 2 on the plate 6 is selected as -20°. During the exposure, the stage 9 rotates the plate 6 by an angle of 40° using a constant rotation speed such that at the end of the exposure, the angle of incidence of the beam 2 on the plate 6 reaches +20°. Since the exposure time required to print the desired pattern on the wafer is 40 seconds, the rotation speed selected is 1° / s. This rotation of the stage 9 during the exposure causes a slow displacement or scanning of the transmitted beam 7 across the photomask 3. Using equation (2), the scanning distance is calculated to be about 5.9 mm from the rotation angle (-20° to +20°), the thickness of the plate (25 mm) and the refractive index of the plate (n=1.47). This scanning of the transmitted beam 7 during the exposure therefore causes a blurring or smearing of the non-uniformity in the time-integrated intensity distribution when illuminating the photomask 3. The contrast reduction of the non-uniformity 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 smear 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 the 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 when illuminating the photomask, in particular the exposure energy density at the edges of the beam decreases over the relevant lateral displacement distance, which in this case is about 5.9 mm, and hence the width of the uniform region of the time-integrated distribution also decreases by the same amount. The width in the x-direction of the pattern 4 on the photomask 3 must then be smaller than the width of the region here in order to print this pattern 4 uniformly on the 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 some time before the end of exposure, or equivalently, begins rotation some time after the start of exposure and reaches the final angle at the end of exposure. However, selecting such a non-optimized rotation speed is usually undesirable as it will generally result in non-uniform smearing of the defect, resulting in higher contrast at the edges of the smeared regions 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 arranged tilted at a given 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 arranged at another 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, the 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 more or less arbitrary over the desired angular range. The 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 the 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 area, 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, resulting in locally higher non-uniform contrast (compared to that produced at a constant speed) anywhere along the smeared area, which is typically undesirable.

[0059] Using the apparatus of the embodiment, a smaller or larger contrast reduction factor of the smear defect can be obtained by scanning the plate 6 over a smaller or larger angular range, respectively, such that the defect is 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] While the angular scanning of plate 6 described in this embodiment is symmetrical about normal incidence, in other variations of this embodiment, plate 6 may 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, although the angular scanning of the plate 6 described above is a single scan, the same device can also be used to perform a double scan of the plate 6 during exposure, i.e., the plate 6 can be rotated so that the angle of incidence of the beam on the 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 (but is not necessarily the same in the two directions), and it is preferable that the switching time of the scanning direction between the forward and reverse scans is negligible with respect to the total exposure time. It is clear that such a double scan variant of the first embodiment can be extended to other multi-scan variants. In the case of multi-scanning, it is not so important that the last scan is completed exactly at the end of the exposure, because the accumulated exposure dose from the previous scans ensures a relatively high level of uniformity over the entire smear 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 use a stepping motion of the stage 9 to approximate a displacement at a constant speed to achieve the same angular displacement. For example, the stage 9 can rotate from -20° to +20° in small angular steps of 0.5°, with a constant stepping frequency and short dwell times between steps. This also achieves good uniformity across the smeared features.

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

[0064] In the first and other embodiments, the rate of change of the gap between the photomask used for DTL exposure and the photoresist coated wafer is preferably selected in relation 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 for each rotational scan of the plate, and conversely, two or more gap change scans are performed during or for each gap change scan of the plate. In this way, by synchronizing the DTL scan with the rotational scan of the plate, the uniformity of the DTL printed pattern is further optimized in areas of local non-uniformity of the intensity distribution when illuminating the photomask.

[0065] Referring to Figures 2a, 2b and 2c, which show 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 towards 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 such 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 similar to that of the plate 6 in the first embodiment.

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

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

[0068] As in the first embodiment, the stage 19 does not need to rotate with high rotational speed or high angular resolution and is therefore available from a number of suppliers, such as Newport Corporation, as one of their series of DC Motor Rotation Stages. As in the first embodiment, the oblique incidence angle of the beam 12 on the plate 16 and the refraction of the beam as it passes through the plate 16 results in a lateral offset of the transmitted beam 17 relative to the incident beam 12. Note that, using the apparatus of this embodiment, because the tilt angle of the plate 16 relative to the rotation platform of the stage 19 is fixed and the rotation axis of the stage is parallel to the direction of the beam, the magnitude of the incidence angle of the beam on the plate remains constant as the stage rotates, and therefore the magnitude of the lateral offset of the transmitted beam is also constant. However, because the plane of incidence of the beam on the plate 16 rotates with the stage, the direction of the lateral offset also rotates about axis 20, which results in a circular scanning trajectory of the beam 17 on the photomask 13. Thus, local intensity non-uniformities in transmitted beam 17 result in 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, such that stage 19 completes one full rotation of 360° at the end of the required exposure time of 60 seconds. The full rotation of stage 19 during exposure ensures that any intensity non-uniformities in the instantaneous beam 17 illuminating photomask 13 will result in a uniform smear over the entire annular region in the time-integrated intensity distribution as it illuminates photomask 13. The average 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., calculated using equation (2) and associated values ​​(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 mean radius is therefore approximately 13.5 mm. The reduction in contrast of a defect of a given size can be estimated, i.e. a defect of 1 mm size will be smeared over an area of ​​(annular) length 13.5 mm, and its contrast will be reduced by a factor of approximately 13.5, so a defect with a contrast of approximately 10% in the instantaneous beam 17 will be reduced to a defect with a contrast of less than 1% in the time-integrated distribution when illuminating the photomask 13, which can be neglected for most applications.

[0070] The rotation speed in this embodiment is preferably selected so that one revolution of the stage 19 occurs during the exposure, but alternatively the rotation speed can be selected to cause a larger number of integer revolutions (i.e. 720°, 1080°, etc.), which will achieve the same result of defects being smeared uniformly throughout the relevant annular region. Alternatively, rotation speeds that cause non-integer revolutions, preferably more than half a revolution, i.e. more than 180°, can also be used, but such rotation speeds have the disadvantage that non-uniformities do not cause uniform smearing throughout the annular region, which will result in higher than normal contrast values ​​somewhere in the annular region. The difference between optimized and non-optimized values ​​of contrast caused by integer and non-integer stage revolutions respectively decreases as the number of revolutions increases, and can become negligible, for example by adjusting the speed so that at least two revolutions occur during the exposure.

[0071] Although the start and stop of the stage rotation described in accordance with the above embodiment is synchronized with the start and end of the exposure, this is not essential. What is important is that preferably an integral number of rotations are performed during the exposure time. Thus, the stage rotation can be turned on before the exposure starts and / or switched off after the exposure is finished. However, it is still preferred that the rotation speed is calculated from the exposure time such that the desired integral 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 over the entire annular area due to non-uniformity.

[0072] As with the first embodiment, the rotation stage 19 could alternatively be rotated at a variable speed, but this would generally result in higher contrast anywhere in the smeared area and may be less desirable.

[0073] As in the first embodiment, the rotation stage 19 may alternatively be configured to be displaced using a sequence of incremental steps through the desired rotation angle rather than using a constant or variable speed. For example, 18 incremental steps of 20° each with a constant stepping frequency and constant dwell time between steps may be used to perform a 360° angular displacement. However, for short exposure times of less than 60 seconds, this is not the optimal choice, as such short exposure times would require large acceleration and deceleration of the stage 19 for each step, which could cause undesirable vibrations between the photomask and the wafer, degrading 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 arranged parallel to the direction of the beam, but in a variant of this embodiment, the rotation axis is alternatively arranged tilted at 5° to the direction of the beam. This asymmetric arrangement causes a change in the magnitude of the angle of incidence of the beam on the plate when the plate is rotated about its axis, resulting in an elliptical scanning trajectory that is slightly different from the circular scanning trajectory of the beam illuminating the photomask, and also causes 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 annular scanning trajectory of the second embodiment. Note that, in order to minimize the contrast of the time-integrated intensity distribution over the smeared region for a particular tilt angle of the plate (relative to the rotation axis of the plate during exposure) and plate thickness, it is preferable for the rotation axis of the stage to be aligned substantially parallel to the direction of the beam, particularly preferably within ±10° of the direction of the beam, most preferably within ±5° of the direction of the beam.

[0075] 3a and 3b, for a third embodiment of the invention, an illumination module 21 of a DTL-based photolithography system emits a collimated beam 22 of monochromatic light at near-ultraviolet wavelengths. The beam 22 is directed towards a mask 23, which defines a periodic pattern 24, positioned in close proximity above 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 anti-reflection coated for the wavelengths mentioned above. The requirements for parallelism of the surface can be determined as in the second embodiment. The plate 26 is inclined obliquely with respect to the beam 22 and its dimensions are such that the complete beam 22 passes through it 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 centred closely to the beam 22 in the output plane of the illumination module 21. The second stage 32 has a central through aperture centred 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 the 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, whereas 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 rotation stage 29 is adjusted so that the beam 22 illuminates the plate 26 at an initial angle of incidence of 10°. At the beginning of the exposure, the second rotation stage 32 is actuated to rotate the plate 26 about the axis 30 at a constant speed of 18° / s, selected so that the stage 32 completes two full rotations during the desired exposure time of 40 seconds. About halfway through the exposure, i.e., after about 20 seconds, the first rotation stage 29 adjusts the tilt angle of the plate by 13° in a time period of less than 1 second (short compared to the exposure period) 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 expanded by at least the largest dimension of the intensity non-uniformity, thereby minimizing the contrast of the non-uniformity in the time-integrated intensity distribution when illuminating 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 expanded by 2.1 mm, a distance at least equal to the maximum dimension of the inhomogeneity of 2 mm.

[0079] Rotating the 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 the photomask 23. The radius of the scanning trajectory during each half of the exposure corresponds to the lateral offset of the transmitted beam 27 caused by the respective tilt angle of the plate 26. By arranging such that the lateral offset is 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 the exposure becomes substantially annular, with a width (difference between the inner and outer radii) of approximately twice the maximum dimension of the non-uniformity. The total length of the scanning path of each non-uniformity during the exposure is the sum of the circular trajectories formed by the two halves of the exposure, i.e.

number

number

[0080] However, the contrast reduction coefficient estimated above is an average value over the entire annular region, since the radius of the track scanned by each non-uniformity in the first half of the exposure is about 40% of the radius scanned in the second half of the exposure, and the contrast reduction coefficient obtained over the entire area scanned in the first half of the exposure is about 40% of the contrast reduction coefficient 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 proportional to the respective lateral offsets of the transmitted beams (as it is guaranteed that the total exposure time corresponds to the desired value of 40 seconds), so that the desired effect is achieved from exposure times of about 12 s and about 28 s for the first and second half of the exposure, respectively. Of course, the rotation speed of the second stage 32 between 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 of the exposure beam using the apparatus of the third embodiment. For example, instead of the above case, the exposure can be split into three or more parts instead of only two, and a different tilt angle of the plate 26 for each part can be used, and the same methodology described above can be extrapolated. In another variation, instead of continuing to expose the photomask 23 during the short time when the first rotating stage 29 adjusts the tilt angle of the plate 26 between the first and second parts of the exposure, the exposure beam 22 can instead be switched off during this time and switched on again after the tilt angle has been adjusted. The magnitude of the contrast reduction obtained will be substantially the same. In yet another variant, the first rotating 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 such 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. such that the lateral offset of the transmitted beam is magnified by at least the maximum dimension of the intensity inhomogeneity. By rotating the two stages 29, 32 simultaneously as described, each local inhomogeneity in the exposure beam is scanned in a spiral trajectory over the photomask 23, where the distance between successive loops of the spiral corresponds to the maximum dimension of the inhomogeneity.

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

[0083] While changing the tilt angle of plate 26 in the third embodiment is achieved using a motorized rotation stage 29, in another variation of this embodiment, rotation of plate 26 can be achieved by other means, for example with one side of plate 26 attached to a hinge and the opposite side of plate 26 arranged 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 to be orthogonal and parallel to the direction of the beam, respectively. However, as in the previous embodiment, this is not essential, and in a variant of the embodiment, the axes are arranged obliquely to the direction of the beam. By arranging the rotary stages in this oblique manner, the contrast of the time-integrated non-uniformity in illuminating the photomask is also reduced, 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. Note that the rotation axes of the first and second rotary stages are preferably arranged to be substantially orthogonal and parallel to the direction of the beam (with the same accuracy as specified in the previous embodiment), respectively.

[0085] 4a and 4b, for a fourth embodiment of the 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 towards a photomask 43 that defines a periodic pattern, which is positioned in close proximity above 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, and 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 upper and lower surfaces of the plate are parallel and are provided with an anti-reflection coating for the relevant wavelength. The requirements for parallelism of the surfaces can be determined similarly to the second embodiment. The plate 46 is mounted on a motorized two-axis gimbal system 48 with a first stage 49 for rotating 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 for rotating the plate 46 about a second axis 52 perpendicular to the first axis 50. The second stage of the gimbal system is mounted on 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 direction of the beam during exposure. The plate 46 is attached by an adapter part (not shown) to the second stage 51, which itself is attached to the first stage 49 by a yoke 53, such that rotation of the first stage 49 results in rotation of the axis 52 of the 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 the plate 46 with respect to the direction of the incident beam 42 results in a lateral offset of the transmitted beam 47, and changing the tilt angle of the plate 46 during exposure results in a translational displacement of the beam 47 across the photomask 43. The tilt of the plate 46 is initially adjusted using the first rotation stage 49 and the second rotation stage 51 such that the components of the incidence angle of the beam 42 in the xz and yz planes on the plate 46 are +15° and 0°, respectively, at the start of the exposure. 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 the exposure is started, a control system (not shown) initiates an angular scanning movement of the two rotation stages 49, 51 in which the tilt angle of the plate 46 in the xz plane is changed by the first stage 49 from +15° to -15° with a cosine-like time dependence shown in Figure 5, and the tilt angle of the plate 46 in the yz plane is changed by the second stage 51 from +15° to -15° with a sinusoidal time dependence shown in Figure 5, the speed of change of the angular movement being selected such that each of the stages 49, 51 completes a single oscillation of its movement at the end of the exposure. As a result of such scanning movements in the xz and yz planes, the angle of incidence of the beam 42 on the plate 46 has a constant magnitude during the exposure, but the plane of incidence of the beam 42 on the plate 46 rotates at a constant speed and completes one rotation at the end of the 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., such that no or negligible change in the angle of incidence of the beam occurs. Thus, a local inhomogeneity of 1 mm diameter is approximately 13.6 times larger in area than the area of ​​this inhomogeneity, area.

number

[0087] Thus, the circular scanning motion of the beam 47 at the 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 rotation of a tilt 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 rotating stages 49, 51 to similarly suppress the contrast of local non-uniformities in the exposure beam 42. For example, faster oscillations of the rotating stages 49, 51 can be used to cause two or more integer number of stage oscillations during the exposure time, which allows the exposure beam 47 to scan more times around the circular orbit during the exposure. Alternatively, faster (or slower) oscillations of the rotating stages 49, 51 can be used, which preferably results in more than one non-integer number of stage oscillations during the exposure time.

[0089] In another alternative variant of the 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 instead of a circular trajectory. In another variant, multiple oscillations of the two rotating stages 49, 51 are used, the amplitude of the oscillations being gradually increased (or decreased) in time, thereby forming a spiral scanning trajectory of the exposure beam 47 at the photomask 43. In another variant, the movement of the two rotating stages 49, 51 is not an oscillation, but a sequence of linear scans forming, for example, a raster scanning trajectory or a square-shaped or any other shaped edge surrounding trajectory of the beam 47 illuminating the photomask 43.

[0090] The 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 located near the center of the plate. In a two-axis gimbal system, the second rotation axis rotates with the rotation about the first axis. Preferably, the second axis remains substantially orthogonal to the direction of the beam during exposure, in particular within ±25°, 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] In another variation of the fourth embodiment, an alternative two-axis gimbal system is used, 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 such that in a DTL exposure system, 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. Such a decoupled two-axis gimbal system generally has 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 in each of the above-mentioned embodiments and variations thereof a single plate provides angular displacement about at least one axis of rotation, in other embodiments two or more plates may alternatively provide angular displacement. For example, referring to FIG. 6, a sixth embodiment of the invention is shown, in which the single plate of the first embodiment is replaced by a pair of plates 6a, 6b, each plate having the same surface area as the surface of the single plate and the combined thickness of the plates being the same as the thickness of the single plate. The plates are arranged in succession, i.e. in a stack, in the beam, preferably parallel to each other and with a spacing that is preferably small relative to the plate thickness, which allows easy mounting of the plates 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 way as in the first embodiment, and the lateral offset of the beam transmitted through the 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). Such replacement of a single plate by 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 specific 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 in Talbot effect based techniques the exposure beam illuminates the photomask at normal incidence, the apparatus for reducing local intensity non-uniformities can be similar.

[0096] It should be noted that for near-field holographic lithography, the exposure beam must illuminate the mask at an oblique angle such that only two diffraction orders propagate behind the mask in the desired direction (as discussed above). In this case, the apparatus of the 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 such that the beam transmitted through the rotating plate illuminates the photomask at the required oblique angle of incidence.

[0097] In the case of near-field holography, the distance between the photosensitive layer on the substrate and the periodic pattern of the photomask must be small enough so that the range of incidence angles of light in the collimated beam illuminating each point of 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 described above in accordance with Talbot imaging-based photolithography. Therefore, the photosensitive layer on the substrate needs to be positioned in close proximity to the periodic pattern of the photomask in terms of the degree of collimation of the illumination beam and the period of the pattern of the photomask so 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 for 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 a reflectivity of about 50% and a transmittance of about 50% per surface. When such a coating is used, the transmittance of the directly transmitted beam at the plate is about 25%, and a portion of the power of the incident beam is transmitted in the same direction as the directly transmitted beam after being doubly reflected between the two surfaces of the plate, resulting in an additional contribution of about 6% to the total transmittance of the plate. Due to the oblique incidence angle of the beam at the plate, the doubly reflected component of the transmitted beam is laterally offset (by a distance proportional to the incidence angle and the thickness of the plate, respectively) with respect to the directly transmitted component. The superposition of the two laterally offset components reduces the instantaneous contrast of the local intensity uniformity, which further reduces the contrast of non-uniformities in the time-integrated intensity distribution when illuminating the photomask. While 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 wavelengths may be in other parts of the spectrum, for example visible light, deep ultraviolet, in particular 248 nm or 193 nm or extreme ultraviolet, and the photoresist used is desirably appropriately selected to have the required sensitivity to said wavelengths.

[0101] Although the parallel plates described in the above embodiment 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 transmission up to about 360 nm are good replacements for fused silica for near UV exposure wavelengths, and UV grade calcium fluoride is a replacement material for deep UV exposure wavelengths. Alternatively, glasses with low transmission to the illumination wavelengths can be used, but this has the drawback of longer exposure times.

Claims

1. 1. 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 proximate to the photomask, the method comprising: a) providing a plate made of a material transparent to said beam, said plate having opposing surfaces that are parallel and separated by a thickness; b) placing the plate in the photolithography system such that the beam illuminates the plate at an initial angle of incidence and such that 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 such that a 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 a direction of the beam.

3. 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 a rotation axis 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 a first angle about a first axis of rotation parallel to a direction of the beam and 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 rotational axes that are mutually orthogonal and substantially perpendicular to a direction of the beam.

7. the plate is rotated about first and second rotation axes that are mutually orthogonal and substantially perpendicular to a direction of the beam; the time dependence of rotation about the first axis and the second axis 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 a 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; placing 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 said at least one additional plate about at least one axis of rotation during exposure; The method of claim 1 , comprising:

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

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

14. the positioning means positions the plate such 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 such 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 a 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 mutually orthogonal and substantially perpendicular to a direction of said beam.

18. the rotating means rotates the plate about respective first and second rotation axes that are mutually orthogonal and substantially perpendicular to a direction of the beam; the time dependence of rotation about the first axis and the second axis 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 disposed proximate 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 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 proximate to a photomask, the method comprising: a) providing a plate made of a material transparent to said beam, said plate having opposing surfaces that are parallel and separated by a thickness; b) placing the plate in the photolithography system such that the beam illuminates the plate at an initial angle of incidence and such that 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 such that a 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 exposure uniformity of a first periodic pattern on the photomask with a collimated beam of monochromatic light, comprising: a) a plate made of a material transparent to said beam, said plate having opposing surfaces that are parallel and separated by a thickness; b) means for positioning the plate within the photolithography system such that the beam illuminates the plate at an initial angle of incidence and such that 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 such that a transmitted beam is translationally displaced across said photomask; An apparatus comprising:

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