Method for patterning mask, method for producing insert or mold, and optical article having surface microstructure

Abstract

The present disclosure relates to a method for preparing an object having a curved surface to be etched, the resulting etched surface being useful as a mold for making an optical article. The method includes illuminating the curved surface of the object through a patterned mask placed over or on the curved surface. The patterned mask includes a curved surface configured to conform to a curved surface of the object. The curved surface of the patterned mask includes at least one micron-sized region that differs in transmission function from an adjacent region. The curved surface is made of a material whose chemical resistance to the developer may locally change when irradiated in this way. The disclosure further relates to a method for patterning a mask having a curved surface and to an optical article having a curved surface comprising microstructures that are consistently normal to the curved surface.

Description

Technical Field

[0001] The present invention relates to the field of ophthalmic optics.

[0002] More particularly, the present invention relates to a method for patterning a mask having a curved surface, a method for preparing an object having a curved surface to be etched, and an optical article having a curved surface. Background Art

[0003] Myopia has become a major public health problem worldwide due to its increasing prevalence in recent decades. In recent years, a variety of options have been evaluated to prevent or slow the progression of myopia in children.

[0004] One of these options is the daily use of corrective eyeglass lenses that also slow myopia progression by exhibiting a specific arrangement of surface microstructures.

[0005] Stellest TM The lenses are examples of polycarbonate single-focal lenses that include a spherical front surface with an aspherical lenslet arrangement. These lenses can be successfully manufactured by injection molding using concave nickel-phosphorus plated steel inserts with microstructures machined by diamond turning.

[0006] However, such steel inserts are very expensive to manufacture. Furthermore, only a limited number of microstructures can be produced by diamond turning. A more economically feasible method is needed to produce inserts or molds with surface microstructures for injection molding or casting.

[0007] Further, glass inserts are required for injection molding to produce negative finished single vision polycarbonate lenses without defects such as weld lines and center deformation. The reason is that glass has poor thermal conductivity compared to steel inserts, which slows down the cooling of molten polycarbonate. However, it is currently impossible to produce precise microstructures on the surface of typical borosilicate glass. A manufacturing process is needed that is compatible with an insert material that has a thermal behavior similar to that of borosilicate glass and further allows easy formation of microstructures on its surface. Summary of the invention

[0008] The present invention has been made in view of the above problems.

[0009] According to a first aspect of the proposed technique, there is provided a method for patterning a mask having a curved surface by inducing local changes in optical properties on the curved surface, wherein:

[0010] The method includes irradiating a selected micrometer-scale area of ​​a curved surface; and

[0011] The curved surface is made of a material which, when so illuminated, undergoes local transformations in the illuminated area, these local transformations being associated with local changes in the optical properties.

[0012] A first aspect of the proposed technique allows providing a curved patterned mask exhibiting local variations of optical properties, such as transmittance.

[0013] Further, since the transformation of the curved surface is local, on a micrometer-scale area, the first aspect of the proposed technology allows the patterned curved mask to significantly diffract visible light. This allows the patterned curved mask to be used as a grayscale curved mask for visible light by exploiting the wave interference phenomenon.

[0014] Optionally, illuminating the selected micron-scale regions of the curved surface comprises: for each selected micron-scale region, shifting the mask so that the selected micron-scale region is at a set XYZ position, and illuminating the set XYZ position.

[0015] This allows for greater control over the shape of the illuminated areas, regardless of their location on the curved surface.

[0016] Optionally, the material of the curved surface is photoresist.

[0017] Photoresists are photosensitive materials. In the case of positive photoresists, the photosensitive materials are degraded by light. In the case of negative photoresists, the photosensitive materials are enhanced (polymerized or cross-linked) by light.

[0018] Optionally, the method further comprises:

[0019] After irradiating selected micrometer-scale areas of the curved surface, locally dissolving the material of the curved surface to locally expose the coating, such as by applying a developer; and,

[0020] Optionally, after locally dissolving the material of the curved surface, the exposed coating is etched, for example by reactive ion etching.

[0021] In the case of a positive photoresist, the developer dissolves away the areas exposed to light. In the case of a negative photoresist, the developer only dissolves away the areas not exposed to light.

[0022] A coating is one or more layers or films that cover a substrate. The coating and the substrate have different optical properties.

[0023] Typically, the coating is opaque and the substrate is transparent. Some areas can then be made transparent by locally removing the coating while keeping other areas opaque. In the case of a conventional chrome-on-glass mask, the coating left behind the dissolved areas of the photoresist can be, for example, a chrome layer or a first chrome oxide layer covering a second chrome layer. However, various other types of coatings are suitable.

[0024] In addition to forming a mask by irradiating a photoresist covering a coated substrate, a mask can also be formed by irradiating certain types of substrates without a photoresist and a coating.

[0025] In this regard, optionally, the material of the curved surface is high energy beam sensitive (HEBS) glass.

[0026] For example, some known HEBS glasses may darken when exposed to a high energy beam, such as an electron beam or a laser beam.

[0027] Optionally, irradiating the selected micron-scale region of the curved surface includes: irradiating the first selected micron-scale region with a first exposure dose; and irradiating the second selected micron-scale region with a second exposure dose different from the first exposure dose.

[0028] Since the transparency of some known HEBS glasses is a function of exposure dose, illuminating different micrometer-scale areas with different exposure doses allows providing a curved grayscale photomask by precisely controlling the transmittance of the curved mask over its curved surface with a resolution in the micrometer range.

[0029] According to a second aspect of the proposed technology, there is provided a method for preparing an object having a curved surface to be etched, the resulting etched surface being useful as a mold for manufacturing an optical article, wherein:

[0030] The method includes illuminating a curved surface through a patterned mask placed over or on the curved surface;

[0031] The patterned mask includes a curved surface configured to conform to a curved surface of an object;

[0032] The curved surface of the patterned mask includes at least one micron-scale region that differs in transmissive function from adjacent regions; and

[0033] The curved surface is made of a material whose chemical resistance to the developer changes locally when irradiated in this way.

[0034] Due to its characteristics, the material forming the curved surface of the object is a photoresist, and the object is a stack comprising at least two layers: a photoresist and a substrate. The substrate can be made of any material that can be physically and / or chemically etched. This includes dielectric materials (SiO 2 、Si 3 N 4The substrate may be selected from materials such as silicon-based materials (Si, amorphous silicon (a-Si), polycrystalline silicon (poly-Si), group III-V materials (GaAs, InP, GaN, etc.), and metals (Al, Cr, Ti, etc.). The choice of substrate may be based on its thermal conductivity, which is preferably within two orders of magnitude of the thermal conductivity of the intended material of the optical lens to be molded against the etched curved surface of the object.

[0035] The curved surface of the patterned mask may comprise a plurality of micrometer-scale regions, each micrometer-scale region differing in transmission function from the corresponding adjacent region.A possible way to obtain such a patterned mask is by patterning a curved mask according to the first aspect of the proposed technique.

[0036] The second aspect of the proposed technology allows the creation of a rich variety of microstructures on the curved surfaces of objects. The objects can then be used as inserts or molds, respectively, for injection molding or casting of optical lenses with surface microstructures.

[0037] Since the mask has a curved surface that conforms to the curved surface of the object, the resulting insert or mold further allows for a finished single vision polycarbonate lens without weld lines, whose surface microstructure is geometrically uniform and structurally normal to the curved surface. The resulting insert or mold can further be used as a master for creating a microstructured stamp for embossing.

[0038] Optionally, the patterned mask is a binary mask, such as a chrome-on-glass binary mask.

[0039] Here, a binary mask is to be understood literally as a mask having, for example, at least one opaque micrometer-scale region adjacent to a transparent region, or having at least one transparent micrometer-scale region adjacent to an opaque region.

[0040] The patterned mask may also be an alternating phase-shift mask or an attenuated phase-shift mask, both possible implementations being similar to a binary mask.

[0041] Since the region or regions under consideration are on the micrometer scale, and since the light source illuminating the mask in common lithography systems typically emits ultraviolet light at wavelengths of 248 nm and 193 nm, the binary mask is functionally a grayscale mask due to diffraction at the region under consideration.

[0042] Optionally, the curved surface of the binary mask includes a plurality of micron-scale regions that are different in transmission function from adjacent regions, and the micron-scale regions are arranged according to a pattern that includes the micron-scale regions of a first zone having a first density and the micron-scale regions of a second zone having a second density different from the first density.

[0043] Optionally, the patterned mask is a grayscale mask, such as a grayscale high energy beam sensitive glass mask.

[0044] This means that the curved surface of the patterned mask is not simply opaque in some areas and transparent elsewhere, but has a transmittance value that varies continuously or in a ramping manner between one or more intermediate levels, between a minimum value corresponding to the most opaque areas and a maximum value corresponding to the most transparent areas.

[0045] Optionally, the curved surface of the grayscale mask includes a first region having a first transmittance and a second region having a second transmittance different from the first transmittance.

[0046] Optionally, irradiating the curved surface of the object through the patterned mask comprises:

[0047] irradiating a first region of a curved surface of the object through the first region of the curved surface of the patterned mask so that the first region of the curved surface of the object receives a first exposure dose; and

[0048] A second region of the curved surface of the object is illuminated through the second region of the curved surface of the patterned mask so that the second region of the curved surface of the object receives a second exposure dose different from the first exposure dose.

[0049] In an example, one of the first exposure dose and the second exposure dose is not zero, while the other is equal to 0. As a result, the curved surface of the object may have a microstructure only on a region irradiated by a single pattern region of the first region and the second region.

[0050] According to a third aspect of the proposed technology, there is provided an optical article having a curved surface including microstructures;

[0051] Therein, the microstructures are uniformly normal to the curved surface.

[0052] Optionally, the optical article is used to slow the progression of myopia.

[0053] When surface microstructures are specified as part of the optical design of an optical article, their specified orientation is typically normal to the surface on which the microstructures are formed. For various purposes, including optical quality, it is desirable that the microstructures actually formed on the surface of the optical article be as close to their specified orientation as possible.

[0054] In particular, it is known that some microstructure arrangements on the surface of an optical article inherently contribute to slowing myopia progression by intentionally forming a partially blurred image on the wearer's retina when the optical article is worn. It is of utmost importance to accurately control the orientation of the microstructures so that the optical article forms a partially blurred image on the wearer's retina as desired when the optical article is worn.

[0055] Optionally, the optical article is free of weld lines and / or free of other types of defects, such as central deformation defects. Both types of defects are known and well described in the art.

[0056] When molding optical articles, weld line defects occur if the thermal conductivity of the material of the optical article is significantly different from that of the mold. Typically, weld lines can form during injection molding when an advancing polymer melt front encounters another melt front, which can result from filling the mold cavity from multiple gates. This can result in head-on collisions of separate flow fronts of polymer melt, or splitting and converging of flow fronts due to the presence of obstructions or excessive lateral part thickness variations.

[0057] Alternatively, the optical article may be made of organic glass, for example, polycarbonate, isosorbide polycarbonate, polymethyl methacrylate, polyamide, copolyester, thermoplastic polyurethane, polysulfone, polyphenylsulfone or cyclic olefin copolymer. The thermal conductivity of polycarbonate is 0.2 Wm -1 .K -1 The thermal conductivity of polymethyl methacrylate is 0.18Wm -1 .K -1 The thermal conductivity of polyamide is 0.2Wm -1 .K -1 The thermal conductivity of polyphenylsulfone is 0.35Wm -1 .K -1 The thermal conductivity of cycloolefin copolymer is 0.16Wm -1 .K -1 .

[0058] Further considering the root cause of the above-mentioned weld line, in the method according to the second aspect, the obtained etched surface can be a thermal conductivity greater than 0.02Wm -1 .K -1 , preferably greater than 0.1Wm -1 .K -1 , and less than 50W.m -1 .K -1 , preferably less than 25W.m -1 .K -1 materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] For a more complete understanding of the description provided herein and the advantages thereof, reference is now made to the following brief description taken in conjunction with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

[0060] Figure 1 A to Figure 1 F schematically shows the photolithography process used to produce rectangular microstructures on a substrate.

[0061] Figure 2 A to Figure 2 D schematically shows the grayscale lithography process used to produce gradient microstructures on a substrate.

[0062] Figure 3 Schematic showing a chrome-on-glass mask patterned with opaque pixels.

[0063] Figure 4 Schematic representation of the intensity distribution of the pattern obtained by single-slit diffraction.

[0064] Figure 5 A to Figure 5 C schematically illustrates the intensity distribution of the pattern obtained by diffraction from different possible patterned masks.

[0065] Figure 6 The results of the photolithography process are schematically shown.

[0066] Figure 7 and Figure 8 Exemplary results of the photolithography process are shown.

[0067] Fig. 9 A to Fig. 9 D schematically shows the chrome-on-glass photomask fabrication process.

[0068] Fig.10 An electron beam lithography system is schematically shown.

[0069] Fig.11 A surface comprising 2 μm concentric lines as obtained by electron beam lithography is shown.

[0070] Fig.12 A to Fig.12 D schematically shows a glass prism array obtained by grayscale lithography using a high-energy beam-sensitive glass mask.

[0071] Fig.13 A and Fig.13 B schematically shows a 600 nm Fresnel lens obtained by grayscale lithography.

[0072] Fig.14 A method for producing an insert according to an embodiment of the invention is schematically illustrated.

[0073] Fig.15 Schematic representation of a microlens array on a polycarbonate lens and a corresponding microlens array on an insert made of fused silica bonded to stainless steel, which has been used to obtain a polycarbonate lens by injection molding. DETAILED DESCRIPTION

[0074] The proposed technology aims to produce inserts for thermoplastic injection molding or molds for thermoset casting of lenses with surface microstructures. More precisely, the proposed technology aims to manufacture inserts or molds from silicon wafers or photosensitive or photostructured glass (PSG) using very mature wafer-based semiconductor manufacturing technology. An additional benefit of such inserts is the ability to produce weld-line-free finished single-vision polycarbonate lenses with surface microstructures due to the low thermal conductivity of the insert material.

[0075] The proposed technique allows the formation of surface microstructures that extend precisely over specific micrometer-scale areas of a curved surface of an insert or mold. As will be described in detail, the formation of the surface microstructures by etching relies on the use of a curved mask to precisely illuminate micrometer-scale areas.

[0076] Patterning a curved mask is inherently challenging, so a patterning technique has been proposed.

[0077] Definitions are now provided for various terms and expressions used herein.

[0078] The curved surface of the mask, object or optical article is the opposite of a flat surface. The curved surface can be spherical, aspherical or cylindrical. The shape and size of the curved surface of the mold or insert are the same as the shape and size of the curved front or back surface of the ophthalmic optical lens to be manufactured by molding with such an insert.

[0079] The surface microstructures are microlenses or tiny lenses or any other type of structure or element with a physical Z-deformation / height or depth between 0.1 μm and 50 μm and a width / length between 0.5 μm and 1.5 mm. The Z-deformation / height represents the local deviation of the curved surface from the original spherical, aspherical or cylindrical shape. These structures preferably have a periodic or pseudo-periodic layout, but can also have random positions. The preferred layout of the microstructures is a grid with a constant grid step size, a honeycomb layout, multiple concentric rings, adjacent, such as without space between microstructures. These structures can provide light wavefront modification in terms of intensity, curvature or light deviation, wherein the intensity of the wavefront is configured so that the structure can be absorptive and can locally absorb the wavefront intensity in the range of 0% to 100%, wherein the curvature is configured so that the structure can locally modify the wavefront curvature in the range of + / -20 diopters, and the light deviation is configured so that the structure can locally scatter light in the angular range of + / -1° to + / -30°. The distance between structures can range from 0 (adjoint) to 3 times the distance between structures in the "X" and / or "Y" dimensions (separated microstructures).

[0080] In addition, the microstructures forming the microstructured main surface of the ophthalmic lens substrate may include small lenses. The small lenses may form protrusions and / or recesses (i.e., convex or concave small lens structures) on the main surface to which they are arranged. The outline of the small lens may be circular or polygonal, such as a hexagon. More specifically, the small lens may be a microlens. The microlens may be a spherical, toric, cylindrical, prismatic or aspherical shape, or any combination to form a multi-element shape. The microlens may have a single focus, or a cylindrical power, or a multifocal power, or a non-focused point. The microlens may be used to prevent the progression of myopia or hyperopia. In this case, the base lens substrate includes a base lens that provides a power for correcting myopia or hyperopia, and respectively, if the wearer is myopic, the microlens can provide a power greater than the power of the base lens, or if the wearer is hyperopic, the microlens can provide a power less than the power of the base lens. The lenslet may also be a Fresnel structure, a diffractive structure (such as a microlens defining each Fresnel structure), a permanent technical protrusion (protruding structure), or a phase shifting element. The lenslet may also be a refractive optical element such as a microprism and a light diffusing optical element such as a small protrusion or a recess, or any type of element that produces roughness on a substrate. The lenslet may also be a π-Fresnel lenslet described in US2021109379, i.e., a Fresnel lenslet having a phase function with a π phase jump at the nominal wavelength, as opposed to a single-focus Fresnel lens having a phase jump of multiple values ​​of 2π. Such a lenslet includes a structure having a discontinuous shape. In other words, the shape of such a structure may be described by a height function, which exhibits discontinuity or a derivative exhibits discontinuity in terms of the distance from the reference plane of the main surface of the optical lens to which the lenslet belongs. Within the available range, the microstructure may be a brand mark, a holographic mark, a metasurface, or the like.

[0081] The lenslets may have an outer shape that can be inscribed in a circle with a diameter greater than or equal to 0.5 micrometers (μm) and less than or equal to 1.5 millimeters (mm). The lenslets have a height measured in a direction perpendicular to the main surface on which they are arranged, which is greater than or equal to 0.1 μm and less than or equal to 50 μm. The lenslets may have a periodic or pseudo-periodic layout, but may also have random positions. One layout of the lenslets is a grid with a constant grid step size, a honeycomb layout, a plurality of concentric rings, contiguous, for example without space between the microstructures. The structures may provide a light wavefront modification in terms of intensity, curvature or light deviation, wherein the intensity of the wavefront is configured such that the structure may be absorptive and may locally absorb the wavefront intensity in the range of 0% to 100%, wherein the curvature is configured such that the structure may locally modify the wavefront curvature in the range of + / -20, 500 or 1000 diopters, and the light deviation is configured such that the structure may locally scatter light in the angular range of + / -1° to + / -30°. The distance between structures can range from 0 (adjoint) to 3 times the distance between structures (separated microstructures).

[0082] The microstructures on the mold or on the insert can be used to mold, for example, refractive microstructures and / or diffractive microstructures / nanostructures.The microstructures on the mold or insert can be the inverse of the desired final pattern formed on the resulting lens after molding.

[0083] The micrometer-scale region of the curved surface is a region where the outer shape can be inscribed within a circle having a diameter greater than or equal to 0.5 micrometers (μm) and less than or equal to 2.0 millimeters (mm).

[0084] Reference now Figure 1 A to Figure 1 F, These figures show a simplified photolithography process to produce rectangular microstructures on a substrate. Wafer-based semiconductor manufacturing techniques such as photolithography, resist processing, and reactive ion etching have proven to allow very accurate shaping and precise positioning of surface contours.

[0085] The stack is basically provided with three layers, namely a substrate (10) (such as a fused silica substrate), a coating (12) (such as a chrome layer covering the substrate), and a photoresist (14) covering the coating.

[0086] like Figure 1 As shown in A, a binary photomask (16) is placed over or on the photoresist, and then Figure 1 As shown in B, areas of the photoresist are exposed to UV light through a photomask. Then, a developer is applied over the photoresist. The result is a Figure 1 As shown in C, the areas exposed to UV light will dissolve, revealing the coating underneath.

[0087] Once the pattern on the photomask has been successfully transferred to the stack by exposing the corresponding areas of the coating, known chemical and / or mechanical etching techniques (e.g., reactive ion etching) can be applied to further transfer the pattern to the substrate. Figure 1 As shown in D, the exposed areas of the coating can be first etched to expose the underlying substrate and then Figure 1 As shown in E, the exposed area of ​​the substrate is etched to form the desired rectangular microstructure in the substrate, and finally as shown in Figure 1 As shown in F, any remaining coating material is removed leaving only the substrate with the transferred surface microstructure.

[0088] For more complex microstructures (such as microlenses, prisms, Fresnel lenses), it is necessary to form the gradient structure in the photoresist before transferring it to the substrate by etching (e.g. by reactive ion etching). This can be achieved by using multiple binary masks (i.e. by repeatedly providing different binary masks in succession). Figure 1 A and Figure 1 B) to achieve, but this method is very time-consuming. In practice, grayscale lithography (GSL) is preferred because it enables the formation of gradient structures in photoresist through a one-step lithography process.

[0089] Reference now Figure 2 A to Figure 2 D, These figures simply illustrate the grayscale lithography process for producing gradient microstructures on a substrate as redrawn based on Non-Patent Document 1.

[0090] like Figure 2 As shown in Figure A, a photoresist layer (24) covering a substrate (20) is exposed to UV light through an optical gradient grayscale mask (26) associated with the desired gradient microstructure. Figure 2 After exposure and development as shown in B, part of the photoresist layer is dissolved, and the surface of the photoresist layer has a 3D profile corresponding to the desired gradient microstructure. Figure 2 C, the desired gradient microstructure can be transferred to the substrate, for example, by reactive ion etching using one or more etchants (28). After etching the substrate and removing any remaining photoresist, as shown in FIG. Figure 2 As shown in D, the desired gradient microstructure is obtained on the surface of the substrate.

[0091] One of the main techniques for making grayscale optical masks is to make a conventional chrome-on-glass (COG) binary mask (30), such as Figure 3As shown, the binary mask contains sub-resolution opaque pixels (32) that cause diffraction. In projection lithography using a COG mask patterned with opaque pixels, diffraction occurs when the size (34) and pitch (36) of the pixels are close to or below the resolution of the projection lithography system.

[0092] In a typical projection lithography system used in today's semiconductor industry, light is projected through a patterned mask. With a pattern encoded in the light, a reduction lens system reduces and focuses the pattern onto the photosensitive surface of a wafer. After the pattern is printed, the system moves the wafer slightly and replicates the blueprint again.

[0093] In case of a binary mask with sub-resolution opaque pixels, the superposition of the wavefronts of light scattered by the sub-resolution opaque pixels allows different target areas on the photosensitive surface to be illuminated with different, non-zero, target exposure doses. Figure 4 A single slit diffraction pattern (40) and a corresponding intensity graph (42) are shown. Figure 4 As shown, the light diffracted by the sub-resolution pixels is resolved into spatial diffraction orders (0, +1, -1, +2, -2, +3, -3, etc.), forming a diffraction pattern. The resolution of a photolithography system is limited by the wavelength of the light used and the ability of the reduction lens system to capture enough diffraction orders from the illuminated mask.

[0094] like Figure 5 As shown in FIG. 1 , the reduction lens system (54) collects one or more spatial diffraction orders in the diffraction pattern (52) according to its numerical aperture and allows the reconstruction of the pixels of the binary mask (50) in the aerial image (56) projected toward the photosensitive surface if at least the 0th and 1st orders (+1 or -1) of the diffraction orders are collected. However, when the size and spacing of the pixels are sufficiently small, such as Figure 5 As shown in B, the reduction lens system collects only the 0th order and focuses it onto the photosensitive surface.

[0095] The intensity that passes through the mask depends on the fill area of ​​each pitch. Figure 6As shown, if the mask (60) is designed with square pixels (62) and a set spacing (66) between the pixels, the intensity of the light passing through the objective lens and projected toward the photosensitive surface (68) depends on the percentage of opaque area in each spacing area. When the spacing is selected to be lower than the resolution of the projection system, the distance between each pixel remains lower than the resolution. A simple way to modulate the intensity of the light passing through the objective lens is to keep the spacing constant and modulate the pixel size. Other suitable methods include keeping the size of the pixel constant and modulating only the spacing, or modulating both the size and the spacing. The number of gray levels depends on the resolution and magnification of the projection lithography system, together with the minimum pixel size and the increment between subsequent pixel sizes. The number of gray levels determines the roughness or high definition of the contours in the photoresist.

[0096] For a given projection lithography system, there is a resolution limit on the minimum pixel size, which sets an upper limit on the area and spacing on the mask. Since the diagonal spacing between two pixels is the maximum spacing, an approximation for the minimum pixel size can be shown by the following equation:

[0097]

[0098] Among them, W min is the minimum pixel width, P is the pitch chosen for the mask, and P C is the resolution of the lithography system.

[0099] For example, when the resolution of the photolithography system is 3.5 μm and the pitch is set to 3.5 μm, the minimum pixel width is about 1 μm.

[0100] If the size of a pixel is below the minimum pixel width defined above, the light diffracted by this pixel has maximum intensity. The maximum pixel size is up to the pitch size. The mask can be patterned by modulating the pixel size between these minimum and maximum values.

[0101] The maximum value of the pixel size is associated with the maximum intensity of the diffracted light. This maximum intensity can be chosen, for example, to cause complete degradation of a positive photoresist placed behind the mask, while any intermediate value of the pixel size is associated with an intermediate intensity or "gray level" of diffracted light that only causes partial degradation of the positive photoresist. Thus, after development, a grayscale microstructure corresponding to the mask pattern is formed on the positive photoresist. The same principle can be applied to a negative photoresist, which is fully or partially enhanced depending on the intensity of the diffracted light that illuminates the area of ​​the negative photoresist.

[0102] The height of the gray levels depends on many factors, mainly the initial photoresist thickness, the intensity of the aerial image at each gray level, the absorption coefficient giving the concentration of both the residual photoactive compound and the exposure product, the development time, and the contrast of the photoresist. Therefore, a calibration mask with multiple gray patterns of different pitches and with all available pixels can be used to determine the design limits and thicknesses.

[0103] The pixel pitch can also be modulated to adjust the size of the pattern encoded by the light transmitted by the mask when the mask is placed in a projection lithography system. However, when the pitch is slightly higher than the resolution of the projection lithography system, such as Figure 5 As shown in C, oscillations in the intensity of the aerial image may occur.

[0104] Figure 7 Shows Example of a P4620 photoresist (70) which was initially 5 μm thick and 25 μm long and which has first been illuminated by light diffracted by pixels of a mask and then developed.

[0105] The arrangement of pixels on this mask is as follows. The pitch is uniformly equal to 3.5μm. The pixels are arranged in 30 rows, one for each pixel size. The minimum pixel size is 0.4μm, and the size increases in successive increments of 0.1μm up to 3.3μm between successive rows of pixels. It can be seen that the photoresist is fully developed below a pixel width threshold of about 1μm.

[0106] Figure 8 Shows Another example of a P4620 photoresist (80) which has been first illuminated by light diffracted by pixels of another mask and then developed. The pixels on this other mask are arranged as follows: the pitch is uniformly 3.5 μm, 220 rows of pixels, and every 10 rows correspond to one pixel size.

[0107] Can be used with the already Figure 1 A to Figure 1 The photomask is made by a process similar to the photolithography process described in F for producing microstructures on the substrate. The main difference is the way the photoresist is exposed: in the case of making a photomask, the exposure is typically by an electron beam, while in the case of photolithography, the exposure is typically by a light beam. Fig. 9 A to Fig. 9 D is an illustration of the chrome-on-glass photomask manufacturing process.

[0108] A photomask blank is provided, comprising a transparent substrate (90) and an opaque chrome coating (92) and a resist (94). Fig. 9 As shown in A, electrons or laser beams irradiate specific areas of the resist to form a pattern. Fig. 9 As shown in B, the resist layer is developed to expose specific areas of the chromium coating. Fig. 9 C, the exposed area is etched, and then Fig. 9 The remaining photoresist is removed as shown in D. The end result is a chrome-on-glass photomask with the chrome coating forming opaque pixels on the otherwise transparent surface of the substrate.

[0109] The key to grayscale lithography based on COG binary mask is to encode a specific 3D profile into a dot density pattern, which can be done with the help of computer software. Then, the pattern is written into the photoresist with an electron beam or laser beam using the above method to form a COG photomask.

[0110] Fig.10 The schematic diagram shows an electron beam lithography system that can be used to pattern a common flat COG. First, a COG sheet (106) coated with photoresist is mounted on an XY linear motorized stage. In operation, the system:

[0111] - setting the beam position to the position of a first desired exposure pixel (108) having a certain shape,

[0112] - using a beam blanker (102), "un-blanking" the beam emitted by the electron source (100), i.e. starting to bombard the photoresist with electrons,

[0113] - Pause or dwell for a defined period of time to fully expose the resist at that pixel location;

[0114] - using the beam deflector (104) to reposition the beam and / or using the motor of the motorized stage to reposition the photoresist coated COG sheet, stepping to the next pixel in the shape, and pausing to expose that pixel,

[0115] - Repeat the previous two steps for each exposed pixel in the shape, and

[0116] - Using a beam blanker, the beam is "blanked", ie exposure of the sample is stopped, so that the beam can then be repositioned at the starting pixel of the next shape to be exposed.

[0117] For example, the beam blanker may be controlled by a signal output by an electron beam pattern generator using a dot density pattern provided, for example, in the form of a CAD pattern file. Further, the beam deflector may be controlled by a scanning amplifier that receives an analog signal obtained by converting a digital signal output by the electron beam pattern generator.

[0118] Curved COG can be easily produced by typical glass shaping and chrome coating processes. However, producing specific patterns on a curved COG surface is more challenging than patterning a flat COG surface. A similar electron beam lithography system is needed, which has an XY stage (or XYZ stage) with Z height travel combined with accurate surface mapping. The working principle is similar to 3D printing. Researchers at Pennsylvania State University (PSU) have used a Raith EBPG5200 electron beam lithography system to produce concentric lines (112) with a width of 2μm on a curved surface (110), such as Fig.11 As shown, as redrawn based on non-patent document 2.

[0119] Although making conventional chrome-on-glass (COG) binary masks with sub-resolution pixels is a possible technique for making optical masks suitable for transferring grayscale patterns on photoresist surfaces, an alternative technique is to make "true" grayscale masks, i.e., masks that can be zoned, with adjacent zones having different absorbance values.

[0120] To this end, one possibility is to manufacture an all-glass photomask from a high energy beam sensitive (HEBS) glass sheet whose transmittance is locally modified using an electron beam pattern generator or a laser beam pattern generator.The mask pattern can be written into the surface glass layer of the all-glass photomask.

[0121] HEBS glass is a photosensitive glass made from a base glass, such as low expansion zinc borosilicate glass, also known as white crown glass. The base glass can be produced from a glass melt like conventional white crown optical glass. After the glass is melted, drawn, ground and polished, the base glass sheet is ion exchanged in an acidic aqueous solution containing soluble ionic silver. The ion exchange process is carried out at a temperature of more than 320°C and for a duration sufficient to diffuse the silver ions into the glass sheet, thereby forming a layer of about 3 μm wide and containing silver-alkali metal halide (AgX) m (MX) n The HEBS glass may darken when exposed to high-energy particles (electrons or ions) due to the chemical reduction of silver ions in the silver-alkali halide complex crystals, producing colored silver atomic spots. The transparency of the photosensitive glass is a function of the exposure dose. The higher the exposure dose, the darker the photosensitive glass becomes. Thus, a continuous change in the exposure dose, for example, produces a continuous change in the optical intensity of the mask.

[0122] Likewise, HEBS glass can be formed into a variety of curves using typical glass forming and shaping processes. Curved HEBS glass can be used to produce curved grayscale photomasks using the Raith EBPG5200 electron beam lithography system mentioned above.

[0123] Fig.12 A to Fig.12 D shows a prism array (126) as redrawn based on non-patent document 3, which is produced by a grayscale photolithography process using a substrate (120) covered with a photoresist layer (122) and irradiating the photoresist layer through a flat HEBS glass mask (124). Prisms with a sag height ranging from 6μm to 20μm and a base area of ​​100μm x 100μm are produced.

[0124] Fig.13 A and Fig.13 B shows a 600 nm Fresnel lens (130) and a corresponding height profile (132) as redrawn based on Non-Patent Document 4, the Fresnel lens being produced by a grayscale lithography process.

[0125] Of course, there are many types of microstructures that can be produced by photolithography in addition to the examples above. Using these established techniques, inserts and molds with complex surface microstructures can be precisely made. However, a particular challenge in using photolithography to produce microstructures on curved insert or mold surfaces is ensuring that all resulting microstructures are uniformly normal to the surface. This problem can be solved by using a photomask whose shape closely matches the target surface.

[0126] Fig.14 A process for producing inserts or molds having curved surfaces with surface microstructures is demonstrated.

[0127] First, a wafer (140) having a curved surface is provided. The wafer may be an all-glass wafer made of photosensitive glass, or may include any transparent substrate and one or more layers including a photoresist (142) located on the curved surface. The photoresist may be deposited on the curved surface by spin coating or by any other suitable known process.

[0128] The microstructure is then formed on the surface of the curved wafer by photolithography using a curved mask (144), followed by etching. More precisely, when the wafer comprises a photoresist, a developer is applied to the photoresist and etching, such as reactive ion etching, enables the pattern to be transferred to the substrate. In contrast, when the wafer is made of photosensitive glass, the areas that have been exposed and become colored can also be etched away by known etching techniques.

[0129] The key to forming the desired microstructures on the substrate normal to the curved surface of the wafer is to use a curved mask whose shape conforms to the curved surface of the wafer.

[0130] The term "conformal" is used to mean that the curved surfaces of the mask and the wafer can be placed against each other with the space between them being below a given threshold at all points, for example below 0.1 mm.

[0131] The photomask can be a rigid glass mask having a surface profile matching the target surface as described above. The photomask can also be a mask made of a soft material (e.g., a polymer material such as PDMS), and the mask either originally has the same shape as the curved surface of the wafer surface, or is adapted to conform to the surface when in contact.

[0132] To ensure that the resulting microstructures are normal to the target surface, the photomask must be very close to the curved surface of the wafer (eg, <0.1 mm) or in direct contact with it.

[0133] The curved wafer can then be bonded to a substrate (146) (e.g., a stainless steel substrate) using a process similar to typical glass bonding inserts used for injection molding finished single vision polycarbonate lenses to form a composite insert (148) having microstructures on the surface. Alternatively, the curved wafer with surface microstructures can be used as is as a mold for casting a thermoset lens.

[0134] An additional benefit of using fused silica or photosensitive glass inserts for injection molding is that due to the low thermal conductivity of these inserts (very close to the thermal conductivity of borosilicate crown glass), it is possible to produce finished single vision polycarbonate lenses with microstructures that are free of weld line and center deformation defects.

[0135] In addition to making inserts for injection molding or using them as molds for casting, the microstructured wafers can also be used as masters to produce stamps using soft polymer materials (such as PDMS). The stamps can then be used to produce additional microstructured silicon wafers by imprint lithography, which are typically much cheaper than photomasks.

[0136] Injection molding of a PC lens was performed using an insert having an array of microlenses on a fused silica substrate bonded to a stainless steel substrate. In this example, each microlens is a hemisphere with a diameter of approximately 100 μm. The spacing between adjacent microlenses is also consistent, at approximately 110 μm. Fig.15 As shown, the microlens array has been successfully transferred from the insert (152) to the molded polycarbonate lens surface (150).

[0137] For all intents and purposes, the following non-patent literature is cited.

[0138] Non-Patent Document 1: Scrymgeour, David, Kemme, Shanalyn A., Boye, Robert R., Ellis, A. Robert, Carter, Tony Ray, Samora, Sally, and Hunker, Jeffrey D., 2013, “Micro-optical grayscale excitation lenses for atom and ion trapping,” U.S., https: / / www.osti.gov / servlets / purl / 1297085 .

[0139] Non-patent document 2: https: / / www.mri.psu.edu / nanofabrication-lab / capabilities / lithography / electron-beam-lithography-0 .

[0140] Non-patent document 3: Christiane Gimkiewicz, Detlev Hagedorn, Jürgen Jahns, Ernst-Bernhard Kley, and Frank Thoma, "Fabrication of microprisms for planar optical interconnections by use of analog gray-scale lithography with high-energy-beam-sensitive glass", Appl. Opt. 38, 2986-2990 (1999), https: / / opg.optica.org / ao / abstract.cfm? URI=ao-38-14-2986 .

[0141] Non-patent literature 4: Nachmias, Tali & Ohayon, Avi & Meltzer, Shefer & Kabla, Meni & Louzon, Elie & Levy, Uriel, (2009), Shallow Fresnel lens fabrication using grayscale lithography made by high energy beam sensitive mask (HEBS) technology and reactive ion etching [Shallow Fresnel lens fabrication using grayscale lithography through high energy beam sensitive mask (HEBS) technology and reactive ion etching], Proceedings of SPIE-The International Society for Optical Engineering [Proceedings of the International Society for Optical Engineering (SPIE)], 7205.10.1117 / 12.809376.

Claims

1. A method for patterning a mask having a curved surface by causing a localized change in an optical property on the curved surface, wherein: The method includes irradiating selected micrometer-scale areas of the curved surface; and The curved surface is made of a material which, when so illuminated, undergoes a local transformation in the illuminated area, the local transformation being associated with a local change in the optical properties.

2. The method according to claim 1, in, Illuminating selected micron-scale regions of the curved surface includes, for each selected micron-scale region, shifting the mask so that the selected micron-scale region is at a set XYZ position, and illuminating the set XYZ position.

3. The method according to claim 1 or 2, wherein: The material of the curved surface is photoresist.

4. The method of claim 3, wherein: The method further comprises: After irradiating selected micrometer-scale areas of the curved surface, locally dissolving the material of the curved surface to locally expose the coating, such as by applying a developer; and, Optionally, after locally dissolving the material of the curved surface, the exposed coating is etched, for example by reactive ion etching.

5. The method according to claim 1 or 2, wherein: The material of the curved surface is high energy beam sensitive glass.

6. The method of claim 5, wherein: Irradiating the selected micron-scale region of the curved surface includes: irradiating the first selected micron-scale region with a first exposure dose; and irradiating the second selected micron-scale region with a second exposure dose different from the first exposure dose.

7. A method for preparing an object having a curved surface to be etched, the resulting etched surface being capable of being used as a mold for the manufacture of an optical article, wherein: The method includes: illuminating a curved surface of the object through a patterned mask placed above or on the curved surface; The patterned mask includes a curved surface configured to conform to a curved surface of the object; The curved surface of the patterned mask includes at least one micron-scale region that differs in transmission function from adjacent regions; and The curved surface is made of a material whose chemical resistance to the developer changes locally when irradiated in this way.

8. The method of claim 7, wherein: The patterned mask is a binary mask, such as a chrome-on-glass binary mask.

9. The method of claim 8, wherein: The curved surface of the binary mask includes a plurality of micrometer-scale regions, the micrometer-scale regions being different in transmission function from adjacent regions, the micrometer-scale regions being arranged according to a pattern, the pattern including the micrometer-scale regions of a first zone having a first density and the micrometer-scale regions of a second zone having a second density different from the first density.

10. The method of claim 7, wherein: The patterned mask is a grayscale mask, such as a grayscale high-energy beam sensitive glass mask.

11. The method of claim 10, wherein: The curved surface of the grayscale mask includes a first region having a first transmittance and a second region having a second transmittance different from the first transmittance.

12. The method according to claim 9 or 11, wherein: Illuminating the curved surface of the object through the patterned mask comprises: irradiating a first region of the curved surface of the object through a first region of the curved surface of the patterned mask so that the first region of the curved surface of the object receives a first exposure dose; and A second region of the curved surface of the object is irradiated through the second region of the curved surface of the patterned mask so that the second region of the curved surface of the object receives a second exposure dose different from the first exposure dose.

13. An optical article having a curved surface including microstructures; in, The microstructures are uniformly normal to the curved surface.

14. The optical article of claim 13 for use in slowing myopia progression.

15. The optical product according to claim 13 or 14, wherein: The optical article is free of weld lines.

Images