WO2013029985A2

WO2013029985A2 - Method for manufacturing periodic structures on a surface of a substrate

Info

Publication number: WO2013029985A2
Application number: PCT/EP2012/065868
Authority: WO
Inventors: Carl Schlockermann; Katrin SCHINDLER; Sven Hansen
Original assignee: Suss Microtec Lithography Gmbh
Priority date: 2011-09-02
Filing date: 2012-08-14
Publication date: 2013-03-07
Also published as: WO2013029985A3; TW201327063A

Abstract

The present invention relates to a system and method for manufacturing periodic structures onto a wafer surface. In particular, a wafer surface is coated with a photoresist, and the wafer is positioned so that the distance between the coated wafer surface and a mask having a periodic structure is approximately the Talbot distance. Then, the mask is illuminated with light to obtain a photoresist image on the wafer surface, wherein the incidence angular spectrum of the light has a defined but small collimation angle. For example, an aperture, through which the light illuminates the mask, can define the incidence angular spectrum of the light.

Description

Method for Manufacturing Periodic Structures on a Surface of a Substrate The invention provides a method for manufacturing periodic structures on a surface of a substrate, e.g., a wafer, for example, for fabricating LEDs.

For effectively fabricating LEDs (light emitting diodes), it is necessary to mass-produce periodic structures on sapphire substrates. These so-called patterned sapphire substrates (PSS) are advantageous, since the structure on the wafer surface can prevent problems occurring due to a mismatch of lattice parameters in a subsequent epitaxial growth of, for example, GaN. Hence, epitaxial growth on a structured wafer surface can result in a higher efficiency, since lattice defects can be avoided or at least be controlled. Moreover, the layer may function as an anti-reflection coating or as a photonic crystal and may avoid light reflection between GaN and sapphire so that the emission of the light and thus the LED efficiency is further enhanced.

Recently, random structures, for example caused by an anisotropic wet chemical process, or periodic structures formed by plasma etching have been used. In particular, if using plasma etching, a photoresist is structured and then further processed so as to form a three- dimensional structure in the sapphire substrate.

In order to apply such a plasma etching process, it is necessary to mass-produce substrates coated with a patterned photoresist, wherein the pattern of the photoresist has a low defect density.

In order to achieve the necessary resolution of the patterned photoresist, it is possible to carry out contact exposure or a "step&repeat" or "scan" exposure or to use a nanoimprint process. However, contact exposure requires a hard, soft or vacuum contact between mask and wafer so that the mask can easily be contaminated and might thus not be used for many further illuminations or the mask may get worn-out by many cleaning cycles. With respect to step&repeat or scan exposures, these methods are usually very expensive and slow; further, fine structures at the boundaries of subsequent scanning steps are often difficult to produce precisely, thus resulting in defects. The application of nanoimprint processes for providing very small structures are presently too expensive and prone to defects, and therefore not useful for mass-production. A good starting point for imaging a periodic structure onto a wafer surface may be provided by the Talbot effect (i.e. a self-imaging of gratings), according to which a precisely defined distance (Talbot distance) between the lattice structure (i.e. the periodically structured mask) and the wafer surface must be observed and the structure must be illuminated with a monochromatic parallel light beam.

In order to evaluate the applicability of the Talbot effect for methods for manufacturing periodic structures on a wafer surface, experiments were carried out with 5 μπι periodicity in a hexagonal lattice reducing the incidence angular spectrum to a collimation angle of ±0.3° by using e.g. an aperture at the pupil plane of the optical system of a mask aligner. Further, only the i-spectral line (365 nm) of a Hg lamp passed through a particular filter, to fulfill the requirements of monochromatic light. Then, the distance between the mask and the wafer surface was set to the Talbot distance of 103 μιη for α 5 μηι periodicity of the hexagonal lattice structure of the mask, according to the formula dj = 3/2 · a²/ , wherein dx is the Talbot distance, a is the periodicity constant, and λ is the wavelength of the monochromatic light. In case of a square lattice, the Talbot distance is dx = 2 · a²/ = 137 μηι for a 5 μπι periodicity.

However, it was found that the reduction of the incidence angular spectrum resulted in long exposure times due to less efficient use of the light source (e.g. Hg lamp), thus making such a process uneconomical. Further, the depth of focus, i.e. the variation of the Talbot distance at which a reasonable imaging is still possible, was only ± 2,5 μιη. If considering the typical thickness of the photoresist being 3 μη , a tolerance of only ± 1 μπι was left. But, taking variations of, for example, the mask aligner, the unevenness of the mask, and the waviness of the wafer surface into consideration, these sources of error result in a necessary tolerance of about ± 10 μηι.

In view of this, the technology using the Talbot effect may be applied for typical lithography applications in a mask aligner; however, as described above, the original effect using parallel monochromatic light cannot be used in a production environment due to its limited depth of field.

US-A-2011/0199598 describes methods making use of the periodicity of the Talbot light distribution in the direction of the distance between mask and substrate. In particular, the fact is used that the light distribution is always the same between adjacent Talbot planes and a sum of the intensity distributions over a full Talbot length is created by using different means. For example, WO2012/004745 suggests a method of creating a sum of the intensity distributions over a full Talbot length by using a cone of illumination angles.

However, the above procedures using a sum of distributions over a full Talbot length require that the average must be performed over exactly a full Talbot length to be independent from the start position of e.g. a displaced wafer. However, for many features, it is impossible to design a mask pattern that creates the desired pattern on the wafer with the contrast necessary for a production process. In particular, for a feature size and pitch size in the vicinity of or larger than the applied wavelength, the Talbot light distribution exhibits many sub-planes with self-images with higher spatial frequency. In this application range, averaging over a full Talbot distance creates an extremely low contrast image or no feasible image at all. Since the averaging distance, the Talbot length, is proportional to a²/ (with "a" being the periodicity constant, i.e. the pitch and λ being the wavelength), the practical implementation of the averaging becomes more difficult if the target pitch is larger compared to the applied wavelength. For example, for an about 5μπι pitch structure, a perfectly parallel wafer displacement over exactly 100 μιη would have to be realized, comparable to the technical challenge of realizing an exact wafer position of 100 μηι distance from the mask.

US 3,697,178 describes a method of reducing dirt-effects as spurious diffraction-associated secondary patterns, created by a non-perfect experimental setup by varying the oblique orientation of the incident light on the mask during exposure. This is achieved by rotating the mask-wafer setup versus the illumination on a slightly tilted turntable (about 0.05°).

In view of the foregoing, it is an object of the invention to provide a method and system enabling illumination of periodic structures on a surface of a substrate, e.g. a wafer, resulting in a precise pattern of the photoresist on the substrate surface and which allows high tolerances of the applied distance between the mask and the surface of the coated substrate. This object is achieved with the features of the claims. Basically, the invention makes use of the Talbot effect with an average distance between the possibly uneven mask and the possibly wavy surface of the coated substrate corresponding tathe calculated Talbot distance or a multiple thereof. In order to avoid side lobes of the illuminating light at that surface created due to varying distances, i.e. deviation, between the mask and that surface across the extended substrate, and which may produce unwanted structures at displaced positions of the substrate, for example, the incidence angular spectrum of the illuminating light is increased compared to parallel light typically used in conjunction with the application of the Talbot effect.

The gist of the present invention is that a more z-independent image is designed by averaging over x-y-shifted (aerial) images for distances between mask and coated substrate within a z-independent range. For example, the x-y-shift may be created by a slight tilt in the illumination angle, particularly by applying a circular aperture resulting in an illumination by a cone of angles. This may result in an averaged light distribution over the individual x-y-shifted images corresponding to each angle.

Compared to the above discussed prior art setups, it is thus possible to process patterns with a pitch of or larger than the used wavelength, also in 2D ("2 dimensions") and also with a mean illumination angle of 0°. For example, a 3μηι pitch structure with 2μηι circular features in a hexagonal lattice can be printed with an aperture producing 1° collimation angle and with an exposure gap between mask and wafer of about the Talbot distance of 37μιη. The corresponding x-y-shift of the individual images is up to +/- 0.65μηι (calculated from: ta (l⁰)/37 m) and is thus a technically reasonable design parameter. The optical path difference due to the illumination tilt is l/cos(l°)-l = 0.015% of the Talbot length (5.6 nm), thus z-averaging is neglectable in the setup according to the concept of the present invention.

Hence, according to the present invention, the optical path difference of the light from the periodically structured mask to the coated substrate may be less than the Talbot length. According to the present invention, periodic structures are manufactured on the surface of a planar substrate by coating the surface of the substrate with a photoresist, positioning the coated substrate in an x-y-plane and - in parallel thereto - a mask having a periodic structure, wherein the distance between the mask and the substrate is half of the Talbot distance or a multiple thereof in a z-direction perpendicular to the x-y-plane, and illuminating the mask so that the image resulting from the illumination is identical to a superposition of copies of the intensity distribution resulting from an illumination with parallel light, wherein the copies are shifted in the x-y-plane. For example, the x-y-shift corresponds to a (large) collimation angle spectrum of, e.g., 1°. In particular, the x-y-shift range may depend on the pitch of the periodic structure of the mask: For example, the x-y-shift may be in a range of -0.64 μτΆ to +0.64 μιη for a 5 μιη pitch which corresponds to a cone of angle with 1⁰ collimation angle. Such an exemplarily explained adaption of the collimation angle can generally be carried out be using, for example, aerial image simulations for the respective pitch size and feature shape/size and is based on a focusing of the light to interception points of the lattice.

In view of the above described basic principle, a periodic structure is applied onto a substrate surface coated with a photoresist by focusing (monochromatic) light to interception points of the periodicity of the structures.

By using the concept behind the present invention, it is advantageously possible to blur the side lobes of the light that appear when the whole or part of the coated substrate surface deviates from the Talbot distance, or to smear out light intensity maxima within areas which should intentionally be unexposed, in particular smear out light intensity maxima belonging to the half of the grating pitch.

According to the invention, the side lobes may be blurred by using, for example, an increased incidence angular spectrum of the illuminating light compared to parallel light, and/or by sequentially illuminating the mask at different illumination angles, and/or by moving the coated substrate within the x-y-plane, and/or by moving the mask parallel to the x-y-plane. In an embodiment, the response of the photoresist to the light converts a photoresist image obtained after applying the light to the photoresist into a binary pattern. In particular, the interception points in the photoresist image exposed to the main maximum of the light have a different value compared to areas in the photoresist image exposed to the (blurred) side lobes or (smeared-out) light intensity maxima. Therefore, the photoresist and the exposure time should be selected according to its threshold for effective exposure so that the light intensity in the main maxima is above and in the (blurred) side lobes or (smeared- out) light intensity maxima is below said threshold for effective exposure. The concept of the present invention is based, for example, on the findings that light appears to be easier focusable to interception points (e.g.: for the generation of pillars (by means of a negative photoresist) or of holes (by means of a positive photoresist)) of the periodicity of structures instead of focusing to the remaining areas (for example, around the pillars or holes, respectively). Due to the fact that the area to be illuminated is smaller in case of focusing to the interception points, a high contrast can easier be achieved. Further, carrying out a Fourier analysis essentially reveals the fundamental frequency and only less harmonics, and the fundamental frequency is less sensitive if varying the illumination distance. A negative photoresist can be co-polymerized if it is illuminated so that - after developing - the co-polymerized areas of the photoresist (e.g. pillars) remain on the wafer surface. In case of a positive photoresist, the illuminated areas become soluble to the photoresist developer so that holes are produced on the wafer surface. Hence, the concept provided by the present invention is advantageous, since the light is focused only to the interception points of the periodicity (instead of focusing the remaining areas around the interception points) so that - if using, e.g., a negative photoresist - only the interception points remain covered with the photoresist on the wafer surface after developing.

The focusing to the interception points can be supported by using a mask having a (periodic) structure, which is transparent at the interception points (for example, at the pillars in, e.g., a hexagonal or linear or two-dimensional periodic structure). In an embodiment, the substrate to be coated with a photoresist is a round wafer or a rectangularly shaped or square substrate made of e.g. a semiconductor, glass, ceramics, plastics or sapphire material. In an embodiment, the light may be of limited spectral width, preferably be monochromatic light and may be obtained by using an i-line filter. However, if using a photoresist which is only sensitive to one spectral line of e.g. an Hg lamp, it is not necessary to provide monochromatic light. For example, the photoresist nLOF™ 2020 is not sensitive to other Hg spectral lines besides the i-line (365 ran).

If, for example, using a structured mask to focus to interception points, the light passing through the transparent areas of the mask, i.e. the interception points (for the generation of e.g. pillars or holes), does not show a sinusoidal intensity curve so that intensity side lobes are present, as soon as the Talbot distance is not exactly realized. As a result from such an inaccuracy, undesired areas of the photoresist may be exposed to the light, thus resulting in less precise structures on the wafer surface. However, it was found that the intensities of the side lobes appear quite smaller than the main maximum so that it is possible to blur/suppress the side lobes, for example, by increasing the angular spectrum, e.g. by increasing the aperture through which the light passes to be focused to the interception points. In doing so, deviations of the gap between the mask and the substrate from the Talbot distance do not result in or at least diminish any inaccuracies in the desired structure on the wafer surface.

In other words, the step of focusing light to interception points of the periodicity of the structures can be achieved by positioning the substrate (or wafer) so that the distance between the coated surface of the wafer (the wafer coated with a suitable photoresist) and a mask having a periodic structure is the Talbot distance. And, the step of blurring the side lobes of the light can be achieved by illuminating the mask with light to obtain a photoresist image on the wafer surface, wherein, for example, the incidence angular spectrum of the light is modified, e.g. increased compared to fully parallel light of normal Talbot lithography.

The invention relates, thus, also to a method for manufacturing (periodic) structures on a wafer surface, wherein the wafer surface is coated with a negative or positive photoresist for producing e.g. pillars or holes, respectively. The wafer is positioned so that the distance between the coated wafer surface and a mask having a periodic structure is the Talbot distance. The mask is illuminated with, for example, monochromatic light to obtain a photoresist image on the wafer surface, and the incidence angular spectrum of the light is specifically chosen, e.g. increased, to blur the side lobes while keeping the intended maxima at the interception points.

Advantageously, due to the combination of, for example, a modified incidence angular spectrum and the effective exposure of only the interception points of a periodic structure, a tolerance of almost ± 15 μπι for the light exposure distance (for 5 μπι hexagonal periodicity and 3 μηι diameter pillars or holes) can be achieved. Hence, any variations (e.g. unevenness of the mask and/or waviness of the wafer surface) and system errors as discussed above can be absorbed. In an embodiment, e.g. the diameter of the aperture, through which the light illuminates the mask, is varied in order to vary the incidence angular spectrum of the light for the intended blurring of the side lobes. An aperture may be used in the optical system, e.g. at the pupil plane, in order to limit the diameter of the light beam at the position of the aperture and/or the divergence of the light beam illuminating the mask. In another embodiment, instead of a simple aperture, other optical elements can define the collimation angle (e.g. diffractive optical elements, that define the angular spectrum without reducing the overall light intensity at the wafer plane). The incidence angular spectrum of the light is increased to thereby preferably smearing out, blurring or suppressing any side lobes of the light. For Talbot lithography the angular spectrum of e.g. a mask aligner is narrowed down to light which is as parallel as possible, for example to a collimation angle of ±0.1°. In contrast, in an embodiment of the current invention, the incidence angular spectrum of the light is set to at least ± 0.25° to ± 2°, particularly to more than ± 0.3° to less than ± 1.5°, or more than ± 0.7° to ± 1.5°, more particular to ± 0.5° to ± 1°, more particular to between ± 0.7° and ± 0.8°. In a particular embodiment, the incidence angular spectrum of the light is set to ± 0.7°.

In an embodiment, the incidence angular spectrum of the light is increased so that side lobes are suppressed such that a response of the photoresist to the illumination converts the image into a binary pattern, wherein - due to the threshold for effective exposure of the photoresist - an area of the photoresist exposed to the main maximum of the light has a different value than an area of the photoresist exposed to the side lobes. For example, the side lobes may be smeared out so that the image of the photoresist on the wafer surface after illumination can be interpreted as a kind of binary pattern, meaning that the contrast between areas exposed to the main maximum of light and areas exposed to the suppressed side lobes is sufficiently high, for example in an aerial image. That means, if considering a binary pattern, an area exposed to the main maximum may have the value "1" or "0", whereas the other areas may have the value "0" or "1", respectively. If such a binary pattern can be achieved by increasing the incidence angular spectrum of the light, light exposure of undesired areas of the photoresist on the wafer surface can be avoided or at least controlled by blurring the side lobes. Hence, for example, the angular spectrum may be adjusted so as to provide a particular contrast between the main maximum and the side lobes. In this case, it is possible that the response of the photoresist to the illumination provides a photoresist image, wherein areas exposed to the suppressed side lobes are not covered by the photoresist after developing. In an embodiment, the spectral width of the light is varied in order to increase the effect of blurring the side lobes. For example, additionally or alternatively to defining a specific angular spectrum, e.g. by varying the aperture or the characteristic parameter of another optical element, through which the light illuminates the mask, the spectral width of the incident light may be adjusted by e.g. filters or other elements in the optical system. In an embodiment the elements defining the angular spectrum and the elements defining the spectral width (i.e. the wavelength distribution) may be varied to reach the best pattern on the wafer. Varying may be automatically or manually, independently or performing a full system optimization. In an embodiment, the periodic structure of the mask is inverse to the patterned photoresist image. For example, the mask has transparent areas at the interception points of the photoresist pattern on the wafer surface after illumination and has opaque areas around the interception points. In view of the above considerations, a further advantageous method, for example, for manufacturing pillar structures (with diameter of e.g. 0.5-10 μπι, e.g. 0.5-5 μηι, e.g. 1-2 μπι, e.g. 2-4 μιη, or e.g. 3 μιη; and a height of e.g. <10 μπι, e.g. 0.3-5 μιη, e.g. 0.5-4 μπι, e.g. 0.5-1.5 μπι, or e.g. 2-3 μπι, or e.g. 3 μηι), has been provided in accordance with the present invention.

In an embodiment, the photoresist on the wafer surface is developed, and optionally a reflow step is carried out to the wafer surface at a temperature above the melting temperature of the developed photoresist.

For example, the shape of particular structures, such as the shape of the photoresist coating on the pillars or around holes (in case of a negative or positive photoresist, respectively) after developing, may depend on the actual light exposure distance. Further, specific profile shapes may occur in connection with the photoresist properties e.g. negative flank angles may occur in connection with the negative photoresist. Hence, it is advantageous to adjust the resist profile by shortly heating the wafer surface to a temperature above the melting temperature of the photoresist. In doing so, the photoresist partly "flows" also to the sides/flanks of the structures due to the gravitational force and/or the surface tension in order to cover the whole structure. Finally, the photoresist has an appropriate shape for a subsequent etching step.

The invention also relates to a system for carrying out any one of the methods as described above. In particular, the system comprises an apparatus for illuminating a mask with, for example, monochromatic light or e.g. light with a limited spectral bandwidth and e.g. a mask aligner for positioning the mask between the apparatus and a wafer surface to be structured. The apparatus may, for example, comprise an aperture or other optical element for limiting/enlarging the incidence angular spectrum of the light illuminating the mask.

In an embodiment, the apparatus is adapted, for example, to vary the incidence angular spectrum of the light. In an embodiment e.g. an aperture diameter is enlarged or reduced, or the characteristics of another optical element are varied. By defining the angular spectrum it is possible to suppress or blur any side lobes of the illuminating light so that only desired areas of the photoresist are effectively exposed by the illuminating light. In an embodiment, the system is adapted to vary the spectral width of the incident light independently or e.g. depending on an adjustment of the defined incident angular spectrum. Alternatively or additionally, the system is adapted to enlarge the collimation angle or to make the collimation angle smaller depending on the incident spectral/intensity distribution of the light. Hence, the system can control the correlation between the enlarging/reduction of the collimation angle and the spectral/intensity distribution of the incident light to optimally coordinate the respective system configurations.

The invention also relates to a computer program product comprising one or more computer readable media having computer executable instructions for performing the steps of any one of the methods as described above. For example, the computer program product may be configured to control the system described above, so that the system can, for example, be adjusted and/or operated to carry out the method according to the invention. Features discussed in connection with the method of the invention may also be applied to the system of the invention and vice versa.

In view of the foregoing, the method and/or system of the present invention realizes manufacturing of a periodic structure onto a substrate with a process window large enough for a practicable production process.

Features of particular embodiments and considerations of the inventor are discussed in the following. Fig. la is a schematic view of a typical illumination system for mask aligner lithography; Fig. lb a schematic representation of the optical set-up for explaining the invention; Fig. 2 shows three images of pillars of a periodic structure;

Fig. 3 shows a simulation of aerial images with varying gap between mask and wafer surface and with varying ratio "periodicity/wavelength"; Fig. 4 shows a simulation of aerial images if using a bright field mask and a dark field mask and if varying the distance between mask and wafer surface;

Fig. 5 shows a simulation of aerial images if varying the distance between mask and wafer surface and if varying the incidence angular spectrum;

Fig. 6 shows a simulation of aerial images if varying the distance between mask and wafer surface; Fig. 7 shows images of a structure after light exposure and after a subsequent reflow step for different exposure times;

Fig. 8 shows the simulated aerial images through the gap for different aperture diameters; Fig. 9 shows simulations for different collimation angle spectra and the effect on the aerial image in different cut plane directions;

Fig. 10 shows SEM images of printed resist patterns according to different sets of specifications; and

Fig. 11 shows further SEM images of printed resist patterns according to different sets of specifications.

Fig. la sketches a schematic view of a typical illumination system for mask aligner lithography comprising an ellipsoidal reflector, two optical integrators, a condenser and a front lens. The latter is a typical apparatus for illuminating a mask 5 and a substrate or wafer 7 coated with a photoresist 6 with light. According to the invention the angular spectrum in such an apparatus or in a similar system for illuminating a mask is defined, i.e. the angular spectrum of the light between the front lens 4 and the level of the mask 5. In an embodiment, the angular spectrum may be defined by e.g. an aperture or a diffractive optical element, resulting in a small but non-zero collimation angle at the mask level. Such an aperture or optical element can be positioned e.g. at the pupil plane of the specific optical system. The relation between aperture and collimation angle of the light depends on the actual set-up of the machine. As shown in Fig. lb, a light source 1 of finite diameter di illuminates a lens 2. This lens 2 focuses the light onto an aperture 3 of diameter d₂, that blocks the light partly and thus, reduces the diameter of the imaginary light source. The diverging light passing the aperture 3 is then parallelized through a second lens 4. As the aperture 3 is positioned in the first focal point of lens 4, the ratio d₂/L of the aperture diameter d₂ and distance L between the aperture 3 and the lens 4 defines the angular spectrum of the light which illuminates the mask 5 and the substrate 7 coated with the photoresist 6; this principle known from the Fourier optics. Generally, the substrate 7 is arranged in an x-y-plane, the mask 5 is arranged parallel to the x-y-plane, and the direction of the distance between the mask 5 and the substrate 7 is the z-direction which is perpendicular to the x-y-plane.

A suitable angular spectrum may also be achieved through a simpler setup, e.g. with fewer lenses or directly using a point light source of diameter d₂ or defocused laser beam or a more complex configuration including additional lenses, diffractive elements, apertures or microlens arrays forming e.g. Kohler integration optics or an MO Exposure Optics setup supplied by Siiss MicroTec AG. The angular spectrum may also be achieved by moving an optical configuration with small angular light distribution with respect to the substrate or wafer and the mask during the exposure of the photoresist.

Fig. 2 shows three images of pillars of a periodic structure on a wafer surface after light exposure. In particular, Fig. 2 shall demonstrate that it is of great importance not to deviate from the exact Talbot distance if merely relying on the Talbot effect strictly requiring a small collimation angle as discussed above. As it is apparent from the differences in the images (a), (b), (c), the gap (i.e. the distance between mask and wafer surface) should be precisely set in order to obtain accurate structures as shown in image (a). Images (b) and (c) already show that a gap shift of only 3 μηι (between image (a) and (c)) results in a less precise pillar shape of the photoresist. Hence, merely relying on the self-imaging effect of gratings (periodic structures), if monochromatic parallel light is used to form images of the grating at periodic distances, does not provide a depth of focus suitable for mass- production.

Further, the Phable™ principle of the Eulitha AG was evaluated, according to which a wafer is moved by the Talbot distance with respect to the mask during the exposure to record an integral or average image. A blurring of side lobes can be achieved due to the ratio between the periodicity of the structures (for example, of the pillars and holes) and the wavelength if using parallel, monochromatic light by getting close to the diffraction limit.

However, as shown in the simulated aerial images of Fig. 3, the Phable™ principle may be used for small ratios of the periodicity per wavelength, see the first line of images for the ratio 1.25, which might provide a sufficient contrast depending on the used resist material (see images indicated by the arrows (B)). It should further be noted that nearfield effects modify the Talbot distance d_Taibot (see (A)). As it is apparent from the second last and last line of the images, the arrows (C) indicate "hotspots" in the images destroying the applicability of the Phable™ method for larger ratios of periodicity per wavelength. Hence, the Phable™ principle does not appear to be advantageously applicable if using image periodicities of, for example, 0.5-10 μπι, e.g. 1-5 μιη, or e.g. 3 μιη if using, for example, wavelengths of 365 nm (i.e. the i-line of an Hg lamp).

Further, while evaluating an improved method, it was found that light appears to be easier focusable on interception points of the periodicity of structures instead of focusing on the remaining areas, as can be seen from Fig. 4. Fig. 4 shows a simulation of aerial images if using a bright field mask (with transparent hexagonal structure) and a dark field mask. The images are taken by varying the distance between mask and wafer surface from the Talbot distance to twice the Talbot distance, at both of which the structure is imaged precisely (see the first and last image in each column of the images). As can be seen from the encircled areas, intense spots appear always in the middle of the hexagonal structure if the distance varies from the Talbot distance. Hence, focusing to interception points, in the case of Fig. 4 focusing to the middle of the hexagon, i.e. to the pillar, should be a promising alternative to usual methods rather illuminating the remaining photoresist areas around the pillars. In view of the above, a method has been developed based on the Talbot effect, but modifying its setup conditions by using an increased incidence angular spectrum, instead of a parallel light beam. For example, the angular spectrum of the light illuminating the mask is enlarged in order to suppress/smear out side lobes while nevertheless achieving a suitable contrast. Fig. 5 shows aerial images for different gaps (distances between mask and wafer surface) between 10 μηι and 250 μπι and for different divergence angles of ± 0.2°, ± 0.4°, ± 1°, and ± 2° (and thus for different incidence angular spectra). As can be seen in Fig. 5 for a divergence angle of ± 0.2°, side lobes are still present and are indicated by the encircled areas and arrows (X). Consequently the gap range in which good structures can be produced is rather small (e.g. depicted in Fig. 5 by only a single image). Given typical machine tolerances, lithography with ± 0.2° would not be feasible for volume production. A good contrast of the main maximum as well as a suitable "smearing out" of the side lobes over a large gap region were achieved by using a divergence angle of ± 1°, as indicated by the encircled area and arrow (Y). The large applicable gap region is illustrated in Fig. 5 by five good images in the encircled area. With such an angular spectrum the pattern on the wafer is insensitive to typical machine tolerances of the gap and thus the process is applicable for volume production.

As an additional advantage, by increasing the incidence angular spectrum compared to parallel light, the intensity of the light source (e.g. an Hg lamp) can be used more efficiently and the exposure time can significantly be reduced. Fig. 6 shows a simulation of an aerial image if varying the distance (gap) between mask and wafer surface and if using an increased incidence angular spectrum, proving that the image, particularly the contrast of the image, is not essentially diminished if varying the light exposure distance up to ± 15 μιη (for 5 μπι hexagonal periodicity and 3 μηι diameter pillars). Hence, any variations (e.g. unevenness of the mask, waviness of the wafer surface) and system errors can be absorbed by the method according to the present invention.

Fig. 7 shows images of a structure (pillars) after light exposure and development (see first line images (a)) and after a subsequent reflow step (see second line images (b)). In particular, the reflow step can be carried out by placing the wafer substrate onto a hotplate at a photoresist melting temperature of 150°C for 10 seconds. As it is apparent from a comparison between the images in (a) and (b), melting of the photoresist causes a flow of the photoresist to also cover the sides/flanks of the structures. As a consequence, and as shown in the images in line (b), the whole structure is covered by the photoresist (not only the "heads" of the pillars as shown in each of the images in line (a)) due to the gravitational force and/or the surface tension. Hence, the occurrence of negative flank angles can be compensated, so that the photoresist has an appropriate shape for a subsequent etching step. As apparent from Fig. 7, this method can adjust the photoresist profile according to the needs of the subsequent process step, e.g. etching.

In an embodiment of the present invention, if a target resist structure comprising round features with a diameter D and a pitch p in a 2D hexagonal lattice is needed, for example, the side wall angle limits of the resist structures, the necessary CD ("critical dimension") uniformity, and the resist thickness may be pre-specified. In this connection, the preferred resist material may mainly be characterized by its absorption characteristic and its refractive index. According to this example, the round dots in the hexagonal lattice are the mask features resembling the target resist structure. In an embodiment, the main design parameter to reach a large depth of field is the collimation angle, as discussed above. Further degrees of freedom for the design procedure are the choice of the gap, i.e. the distance between the mask and the resist-coated substrate, which should be a multiple of the half Talbot length, and the choice of a bright-field or a dark-field mask. Generally, in order to adapt the target resist structure, a small bias may be applied to adapt the feature size accordingly. For example, a simulation program is used to optimize the collimation angle to obtain printed target resist features through the largest possible gap range, i.e. the largest possible distance range between mask and coated substrate. Fig. 8 shows the simulated aerial images through the gap for different aperture diameters, i.e. for different collimation angles.

With an expanded collimation angle, smear effects in the aerial image can be created, for example, to enlarge the depth of field compared to almost parallel light. The aerial image simulated with parallel light under perpendicular incidence as shown in Fig. 8(b) is more inhomogeneous in contrast and dot diameter through the gap than the aerial image shown in Fig. 8(c) simulated with an 1.0° collimation angle as shown in Fig. 8(a). The inhomogeneity of the aerial image in Fig. 8(b) obtained with a 0° collimation angle might not fulfill the pre-specifications regarding CD uniformity and resist material through a large gap range, whereas the aerial image simulated with the larger collimation angle spectrum (Fig. 8(c)) is more homogeneous with respect to the exposure gap, and the structures printed into the resist with such an aerial image might fulfill the pre- specifications. The homogenous aerial image shown in Fig. 8(c) may be considered as being a superposition of aerial images with different collimation angles, particularly, a two dimensional set of angles. Compared to a fully collimated light image, strong contrast features are averaged and a more homogenous z-dependence is advantageously created.

Fig. 9 shows exemplarily simulations for different collimation angle spectra and their effect on the aerial images in different cut plane directions. In particular, a slit aperture in a given spatial direction as shown in the top row of Fig. 9 results in an intensity superposition of the aerial image of all corresponding angles. A slit in x-direction (Fig. 9(a)) thus results in an averaging in x-direction, a slit in y-direction (Fig. 9(b)) results in an averaging in y-direction, and a slit in the x-y-plane is shown and simulated in Fig. 9(c). The above described procedure can be considered as a superposition of a series/plurality of shifted copies of the original aerial image (created by parallel light). In particular, the shift for the x-direction slit is an x-shift, whereas the y-directional slit results in a y-shift, and the shift for the slit in the x-y-plane is an x-y-shift. The effect is illustrated in the x-y-intensity plot at the Talbot gap (2^nd figure in the top row of each of the Figs. 9(a), (b), (c)) and the x-z and y-z aerial images (middle and bottom row of each of the Figs. 9(a), (b), (c)).

In a first embodiment, Figs. 10 and 11 show SEM images of printed resist patterns according to different sets of specifications. In particular, the used negative resist had a 2 μιτι thickness, the collimation angle was 1.0°, and the mask had a 3 μΐΉ pitch and a 2 μπι feature size, i.e. 2 μπι diameter round features in a two-dimensional hexagonal pattern. Further pre-specifications were that the dot diameter tolerance was given with +/- 10% of the feature size and that the side wall angle should be greater than 70°.

Figs. 10(a)-(c) show the SEM images for distances of 30 μηι, 31 μιη, and 32 μηι between the structured mask and the coated substrate. Figs. 10(d)-(f) show the SEM images for distances of 33 μιη, 35μπι, and 37 μηι, Figs. l l(a)-(b) show SEM images for distances of 39 and 41 μπι, and Figs. 1 l(c)-(f) show SEM images for distances of 42 μιη, 43 μηι, 44 μηι, and 45 μηι. The Figs. 10 and 11 show that finally good prints can be achieved for gaps between 32 to 44 μηι, i.e. a depth of field of about 13 μηι was reached for the shown example. All measured side wall angels were well within the pre-specification, i.e. higher than 70°. For example, a larger depth of field may be reached if more effort is spent for resist process optimization and/or dose calibration.

Further experiments were carried out, for example according to the following pre- specifications: In a second embodiment, the same pre-specifications as in the first embodiment were used, but processed with a cost effective not chemically amplified positive resist (e.g. AZP41 10). The whole design process is done for the positive resist and its absorption characteristics. Experiments show that a depth of field of at least 7 μτη can be reached with this setup. In a third embodiment, the same pre-specifications as in the first embodiment were used, but with a 2 μηι pitch and a 1 μιη feature size. Further, a negative resist with 1 μηι thickness was realized with a 5 μηι depth of focus.

In a fourth embodiment, the same pre-specifications as in the first embodiment were used, but with a 5μπι pitch and 3μιη feature size structures exposed in 3 μιη thick negative resist. The reached depth of field was larger than 20 μιη. In this range all pre-specifications were fulfilled including a dot diameter tolerance of ±10% and measured side wall angles of >75°. In view of the foregoing, a method and a corresponding system and computer program product are provided, combining the advantages of focusing light to interception points of a periodic structure, and blurring the side lobes of the light so that variations of the Talbot distance between the resist-coated surface of a substrate and the periodic structure (for example, a periodically structured mask) do not negatively affect the accuracy of the image. This may be achieved by illuminating the mask so that the aerial image resulting from the illumination is identical to a superposition of copies of the intensity distribution resulting from an illumination with parallel light, wherein the copies are shifted in the x-y- plane. Hence, the present invention allows for a mass-production of substrates/wafers coated with a patterned photoresist, wherein the pattern of the photoresist is reproducible and promises high yield in volume production, because it is insensitive to typical machine tolerances.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and non-restrictive. The invention is thus not limited to the disclosed embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word 'comprising' does not exclude other elements or steps, and the indefinite article 'a' or 'an' does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage.

Claims

1. Method for manufacturing periodic structures on the surface of a planar substrate, comprising:

(a) coating the substrate surface with a photoresist,

(b) positioning the coated substrate (6, 7) in an x-y-plane and - in parallel thereto - a mask (5) having a periodic structure, wherein the distance between the surface (7) of the coated substrate (6, 7) and the mask (5) is at or close to half of the Talbot distance or a multiple thereof in a z-direction perpendicular to the x-y- plane, and

(c) illuminating the mask (5) with light having an incidence angular spectrum to obtain a photoresist image of the mask structure on the surface of the coated substrate (6, 7),

wherein step (c) comprises increasing the incidence angular spectrum of the light compared to parallel light, and/or sequentially illuminating the mask at different illumination angles, and/or moving the coated substrate within the x-y-plane, and/or moving the mask parallel to the x-y-plane.

2. Method of claim 1, wherein an image resulting from the illumination of the mask in step (c) is identical to a superposition of copies of the intensity distribution resulting from an illumination with parallel light, wherein the copies are shifted in the x-y- plane. Method of claim 2, wherein the maximum shift in the x-y-plane is at least 1% of the pitch size and/or the feature size of the periodic structure of the mask.

Method of claim 1, 2, or 3, wherein an optical element (3) is provided for defining the incidence angular spectrum of the light illuminating the mask (5) and wherein the properties of the optical element are defined depending on the periodic structure to be printed.

Method of claim 4, wherein the optical element comprises an aperture arranged e.g. at the pupil plane of the optical system or a diffractive optical element.

Method of claim 4 or 5, wherein the position of the collimation-angle-defining optical element (3) and/or the mask (5) is varied in order to adjust the incidence angular spectrum of the light.

Method of any one of claims 1 to 6, wherein the increase of the incidence angular spectrum of the light and/or the sequence at different illumination angles and/or the moving of the coated substrate within the x-y-plane and/or the moving of the mask parallel to the x-y-plane, the intensity of the illuminating light, and the photoresist having a certain threshold for exposure are selected such that a response of the photoresist to the illumination converts the image into a binary pattern, wherein an area of the photoresist exposed to the main maximum of the light has a different value than an area of the photoresist exposed to side lobes.

Method of any one of claims 1 to 7 , wherein the incidence angular spectrum of the light is increased by at least ± 0.25° to ± 2°, particularly by more than ± 0.3° to ± 1.5°, or more than ± 0.7° to ± 1.5°, more particular by ± 0.5° to ± or ± 0.7° to ± 0.8°, or particularly ± 0.7°.

Method of any one of the preceding claims, comprising the following step after step (c):

(d) developing the photoresist (6) on the surface of the substrate (7).

10. Method of claim 9, comprising the following step after step (d):

(e) reflowing of the developed photoresist on the substrate surface at a temperature above the melting temperature of the developed photoresist.

11. Method of claim 9 or 10 with the following step after step (d) and/or (e), respectively:

(f) etching the substrate (7) comprising the developed photoresist.

12. Computer program product comprising one or more computer readable media having computer executable instructions for performing the steps of the method of any one of the preceding claims.

13. System for carrying out the method of any one of claims 1 to 11 , comprising:

(a) an apparatus for illuminating a mask (5) with light,

(b) a mask aligner for positioning the mask (5) between the apparatus and a coated substrate to be structured, wherein the system is adapted to enlarge and/or to reduce the collimation angle depending on the intensity distribution of the incident light, and/or enlarge and/or reduce the distance between the mask and the substrate surface, and/or sequentially illuminate the mask at different illumination angles, and/or move the coated substrate within the x-y-plane, and/or move the mask parallel to the x-y-plane. System of claim 13, wherein the apparatus comprises a system, e.g. an aperture or a diffractive optical element, for limiting and/or enlarging the incidence angular spectrum of the light illuminating the mask (5).

System of claim 13 or 14, wherein the apparatus is adapted to adjust one or more of the parameters of the optical element that defines the collimation angle, e.g. the diameter of an aperture or the parameters, e.g. geometry, of a diffractive optical element, and/or the apparatus is adapted to control the spectral and/or intensity distribution of the incident light.