3D lithography with laser beam writer for making hybrid surfaces

A method of etching a feature in a surface of a substrate. The substrate is provided. A photoresist layer is formed on the surface of the substrate. A thickness profile of the formed photoresist layer is determined. A grayscale scanning pattern is determined based on the feature and the thickness profile of the photoresist layer. The determined grayscale scanning pattern is laser written on the photoresist layer to expose a portion of the photoresist layer. The exposed portion of the photoresist layer is removed to form a grayscale pattern in the photoresist layer. The photoresist layer and the surface of the substrate are etched to form the feature in the surface of the substrate.

FIELD OF THE INVENTION

The invention relates to methods of lithography using a laser beam writer to produce features in a substrate and, more particularly, to a method of producing desired three dimensional features in a substrate.

BACKGROUND OF THE INVENTION

Direct e-beam (electron-beam) milling has been used conventionally to produce grayscale features in a substrate by directing an electron beam towards the substrate in a scanning pattern and either modulating the intensity of the electron beam or the scan rate across the substrate such that a grayscale pattern is produced in the substrate as grayscale features. However, direct e-beam writing is both slow and costly, and also the precision of the grayscale features (e.g., the height of each step of the grayscale features) is based on how precisely the scan rate and/or the intensity of the e-beam are set. The use of lower intensities and commensurately lower scan rates may improve the precision with which these parameters may be controlled, but at a cost in throughput.

Given these potential disadvantages of direct e-beam milling, an alternative method of producing precise grayscale features in a substrate with a reduced time to completion and a reduced cost may be desirable. Improved precision in the height of these grayscale features (e.g., the overall height and the height of each step) may also be desirable. Three dimensional (grayscale) lithography may offer such an alternative approach to forming grayscale features. The present invention involves the use of a laser beam writer for grayscale lithographic applications.

SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention is a method of etching a feature in a surface of a substrate. A photoresist layer is formed on the surface of the substrate. A thickness profile of the formed photoresist layer is determined. A grayscale scanning pattern is determined based on the feature and the thickness profile of the photoresist layer. The determined grayscale scanning pattern is laser written on the photoresist layer to expose a portion of the photoresist layer. The exposed portion is removed to form a grayscale pattern in the photoresist layer. This patterned photoresist layer and the surface of the substrate are etched to form the feature in the surface of the substrate.

Another exemplary embodiment of the present invention is a method of etching a feature in a mold surface of a mold part. A photoresist layer is formed on the mold surface of the mold part. A thickness profile of the formed photoresist layer is determined. A grayscale scanning pattern is determined based on the feature and the thickness profile of the photoresist layer. The determined grayscale scanning pattern is laser written on the photoresist layer to expose a portion of the photoresist layer. The exposed portion is removed to form a grayscale pattern in the photoresist layer. This patterned photoresist layer and the mold surface of the mold part are etched to form the feature in the mold surface of the mold part. A release layer is formed on at least a portion of the mold surface of the mold part to reduce adherence by a mold product to the mold part during molding.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a flowchart illustrating a method of etching a feature on a surface of a substrate in accordance with an exemplary embodiment of the present invention.FIGS. 2-7are cross-sectional side views of an exemplary substrate illustrating the steps the exemplary method shown inFIG. 1.

Referring now toFIGS. 2-7, in exemplary embodiments of the invention, a feature270of a non-planar substrate200(as shown inFIG. 7) may be realized, for example, by: (1) laser-writing-a grayscale scanning pattern corresponding to feature270to be etched on the surface of substrate200on a photoresist layer210(as shown inFIG. 4) to expose a portion of photoresist layer210; (2) removing the exposed portion of photoresist layer210to form grayscale pattern255(as shown inFIG. 5) in photoresist layer210; and (3) etching photoresist layer210and the surface of non-planar substrate200to form feature270. That is, for example, a laser writing device (not shown) may laser-write grayscale scanning pattern220onto photoresist layer210, and by removing the portion of photoresist layer210exposed by the laser writing device, and etching of photoresist layer210and the surface of non-planar substrate200, desired feature270may be formed.

For illustrative purposes, an example of producing a first exemplary mold part for molding an optical device (e.g., a convex hybrid lens having a diffractive section) is described. However, this example is not intended to be limiting, it is contemplated that substrate200may be any number of other structures. For example, it may be: (1) a different type of mold for optical or non-optical use; (2) machined parts, such as pistons or other metal, ceramic, dielectric, or plastic mechanical structures; (3) deflective/reflective/spiral lenses; (4) lens with micro-arrays; (5) elliptical or cylindrical mirrors; (6) micro-electrical mechanic structure (MEMS) sensors and devices; (7) micro-machine devices or nano-machine devices; and/or other mechanical devices, among others. Furthermore, the method according to an exemplary embodiment of the invention may produce a hybrid lens directly.

In step100, non-planar substrate200is provided. Non-planar substrate200, shown inFIG. 3, may be formed by any number of materials, such as tungsten carbide, silicone carbide, sapphire, glass, ceramic, nickel, and/or stainless steel, among others. Non-planar substrate200refers to any substrate having a surface which is not planar or is not substantially planar (i.e., a surface having a three dimensional (3D) feature either therein or projecting therefrom). The 3D feature may be provided for only a portion of the surface of non-planar substrate200or may be provided for the entirety of the surface of non-planar substrate200.

FIG. 2illustrates an exemplary planar substrate205, this exemplary substrate may be machined to form an exemplary non-planar substrate200, using any machining standard technique, including molding, single point turning (with a diamond, sapphire, tungsten carbide, etc. tip), vibration assisted milling, milling with a diamond or other wheel, ion milling, laser ablation, and/or various etching techniques. It is noted that, alternatively, outward projecting features may grown on the surface of exemplary planar substrate205by conventional methods such as selective area growth to form an exemplary non-planar substrate200.

At optional step110, the surface of non-planar substrate200may be profiled by any number of profilometry techniques including, for example, by: (1) contact stylus profilometry techniques; (2) optical profilometry techniques; and/or (3) other non-contact profilometry techniques such as using an atomic force microscope or a scanning force microscope, among others.

Contact stylus profilometry techniques may include, for example, passing a stylus probe across the surface of non-planar substrate200and following its motion to produce a surface profile.

Optical profilometry techniques may use either a low coherent source or a coherent/monochromatic source, among others. Low coherent source techniques include, for example, white light interferometry, coherence radar, coherence scanning, correlation microscopy and/or interference microscopy. Coherent/monochromatic source techniques include, for example, laser profilometry, phase-shifting interferometry, speckle and holographic techniques, fringe projection and depth of focus methods, among others.

Profiling step110may be optional and if completed may enable a detection of abnormalities on the surface of non-planar substrate200. This may allow grayscale scanning pattern220to be compensated for at step140based on the anomalies of the non-planar surface of substrate200. Such compensation will be described below with reference to step140.

A photoresist layer210may be formed on the surface of non-planar substrate200, step120(as shown inFIG. 3). Photoresist layer210desirably may be positive photoresist, and may be formed from any number of known photoresist agents. Positive photoresist refers to a photoresist layer that, after having portions exposed, is developed (possibly using a chemically etch) to remove the exposed portions of the positive photoresist layer, while the unexposed portions of the photoresist layer remain.

Photoresist layer210may be formed by any number of coating techniques, including, for example, dip coating, film coating, spin coating, vapor deposition, and/or simply spreading the photoresist with a scalpel, etc., among others. Dip coating refers to dipping at least one portion of non-planar substrate200in a photoresist agent to form photoresist layer210by surface tension between the photoresist agent and non-planar substrate200. Film coating refers to coating at least one portion of the surface of non-planar substrate200with a film which is made to adhere to the at least one portion of the surface of non-planar substrate200. Spin coating refers to coating the at least one portion of the surface of non-planar substrate200with photoresist layer210by depositing photoresist agent at a desired location (e.g., approximately centered) on the surface of non-planar substrate200and spinning non-planar substrate200to cause the photoresist agent to spread (e.g., in a substantially uniform manner) across the at least one portion of the surface of non-planar substrate200. Typically, vapor deposition refers to placing non-planar substrate200in a vacuum chamber and adding a gas (i.e., vapor) including the photoresist agent at low pressure which after a period of time deposits (accumulates) on non-planar substrate200as photoresist layer210.

A thickness profile of photoresist layer210may be determined, step130. The thickness profile refers to the thickness of photoresist layer210in a direction of light230incident on photoresist layer210for laser writing the grayscale scanning pattern220at step150. The method of forming photoresist layer210may affect its uniformity. For example, applying a film to the surface or spreading the photoresist (e.g., spin-coating) may lead to thickness variations based on the application or spreading means. Further, because photoresist layer210is formed on the non-planar surface of non-planar substrate200, the thickness profile of photoresist layer210may not be constant in thickness in this incident direction. It is noted that the topography of the surface may cause non-uniformities in the photoresist thickness, even for spin coating and vapor deposition techniques.

This determination of the thickness profile of photoresist layer210in the direction incident to light230used for laser writing may be used to determine a grayscale scanning pattern220, step140. By measuring the thickness profile of photoresist layer210, any deviation (anomalies) in this thickness may be compensated for by grayscale scanning pattern220. That is, by changing the exposure pattern in photoresist layer210and either by exposing a portion of the photoresist layer at the deviation/anomaly for a longer duration/or with a higher power (for overly thick portions) or, otherwise, exposing photoresist layer210at the deviation/anomaly for a shorter duration or with a lower power (for overly thin portions), anomalies may be mitigated to produce the desired feature270in substrate200. Grayscale scanning pattern220refers to an exposure mapping and the subsequent exposed of proportions of the surface of photoresist layer210, which are desirably to be removed to form grayscale pattern255on photoresist layer210.

More particularly, by determining the thickness profile of photoresist layer210and using predetermined information of desired feature270(for example, as shown inFIG. 7) to be etched into non-planar substrate200, grayscale scanning pattern220may be calculated in step130. The etching rate of the photoresist in photoresist layer210and the etching rate of the material of substrate200are desirably taken into account in determining grayscale scanning pattern220. It is noted that the etching rate of the photoresist is often faster than the etching rate of the material. Thus, grayscale scanning pattern220may often have greater relief than the desired feature270, which may allow for increased precision in the height of desired feature270, but may impose a reduced maximum feature aspect ratio due to the aspect ratio that may be achieved in the photoresist without excessive undercutting.

The thickness profile of photoresist layer210may be determined by profiling non-planar substrate200prior to forming photoresist layer210(alternative step110) and after forming photoresist layer210. In this case, since the profiles are conducted separately, it is desirable to perform registration of these profiles as part of the thickness profiling. Although profiling photoresist layer210may be performed using any of the profilometry technique discussed above, a non-contact technique may be desirably, depending on the durability of the photoresist used. It may be desirable to use the same profilometry technique for both measurements.

One alternative may be to calculate the difference between the profiles prior to and after forming photoresist layer210. A second alternative may be to estimate the thickness profile of photoresist layer210by profiling (i.e., measuring) photoresist layer210after photoresist layer210is formed and calculating the thickness profile of photoresist layer210based on this measured profile of photoresist layer210after formation and an estimated profile (i.e., based on predetermined milling parameters) of the surface of non-planar substrate200. A third alternative may be to profile the boundary of photoresist layer210and non-planar substrate200and the exposed surface of photoresist layer210simultaneously, for example, by using white light interferometry or another optical profilometric technique (by selecting photoresist layer210such that it is substantially transmissive/transparent to at least one of the wavelengths of light used in the optical profilometric technique). By such simultaneous profiling, any difficulties with registration of the profiles may be eliminated.

By determining the profile of the surface of non-planar substrate200, grayscale scanning pattern220may be adjusted to further compensate for abnormalities (e.g., changes with respect to an expected profile) of the non-planar substrate200.

Grayscale scanning pattern220may be laser-written on photoresist layer210, step150, to expose a portion or portions of photoresist layer210. Laser-writing refers to scanning a laser beam of a particular wavelength for which photoresist layer210is susceptible to being exposed in a scan pattern over the photoresist layer. For example, a laser beam with a wavelength in the visible light range may be scanned in either a raster scan pattern across or a circular fashion around the surface of photoresist layer210, which is responsive to the visible wavelength of the laser beam, thereby, exposing the portion or portions of photoresist layer210.

As shown inFIG. 4, laser light230from a laser source (not shown) is directed towards photoresist layer210via focusing member240(e.g., a lens) and incident on photoresist layer210to form a beam spot250. By controlling a scan rate of beam spot250, an intensity of beam spot250and/or a width of beam spot250, grayscale scanning pattern220on photoresist layer210may be produced. The scan rate and beam width of beam spot250may be controlled by moving substrate200and/or focusing member240, or be manipulating laser beam230using optics included within the laser source of the laser writer. The intensity of beam spot250may be controlled by controlling the output power of the laser source directly or with a variable optical attenuator (not shown), and/or by varying the width of beam spot250.

Because the substrate200may be non-planar, the beam spot250may preferably be continuously focused or focused in a stepwise manner depending on the profile of non-planar substrate200and grayscale scanning pattern220. Desirably, an autofocusing mechanism (not shown) may be included in the laser writing device to ensure that beam spot250may be continuously focused on the photoresist surface.

It may be desirable to focus and scan beam spot250on photoresist layer210by moving non-planar substrate200on a table (not shown) configured to move with up to six degrees of freedom, i.e., three translation directions (X, Y and Z directions) and three rotational directions (rotation in the X-Y plane, rotation in the X-Z plane and rotation in the Y-Z plane) to produce the desired grayscale pattern255. It is contemplated, however, that either non-planar substrate200or focusing member240may be moved for focusing and/or scanning of the beam spot.

The surface of the substrate200and or desired feature270may be such that at step150, laser writing of grayscale scanning pattern220includes scanning beam spot250of the laser writing device across or around at least one portion of the non-planar surface of substrate200while focusing or auto-focusing beam spot250and modulating an intensity of laser light230from the laser writing device according to grayscale scanning pattern220.

The scanning of beam spot250across photoresist layer210, the focusing of beam spot250and the modulating of the intensity of laser light230include adjusting the scan rate of the scanning of beam spot250, adjusting a focus or autocorrecting a focus of beam spot250on the non-planar surface of substrate200and/or adjusting the intensity of laser light230from the laser writing device to provide a desired exposure level for each scan point in the determined grayscale scanning pattern220.

Grayscale pattern255may be formed by removing the portion of photoresist layer210exposed by laser-writing of grayscale scanning pattern220on photoresist layer210, step160. That is, for example, by using a positive photoresist layer and developing the portion or portions of photoresist layer210that are exposed during the laser writing operation to remove these portions, grayscale pattern255may be formed in photoresist layer210. Any number of other known developing techniques may be used to form grayscale pattern255, including both dry etch and wet etch techniques. It is noted that wet etches are usually isotropic, thus, dry etching techniques may be desired for this application so that similar techniques may be used for developing photoresist layer210and for forming the desired feature on the substrate surface.

Photoresist layer210and the surface of non-planar substrate200are etched, step170, using either a wet or dry etch technique to form desired feature270. The etching technique is selected to etch both photoresist layer210and substrate200, although the etching technique may etch these two material at different rates. This is different from most standard semiconductor device etching processes in which the etchant is chosen to etch only the exposed substrate and not the photoresist. It is desirable to use an anisotropic etching technique such as ion beam milling or reactive ion etching, among others to perform step170. In either of these exemplary anisotropic etching technique, the ion, or etchant, being used is desirably able to etch both photoresist layer210and substrate200. It is contemplated, however, that an isotropic, or semi-isotropic, etching technique may be used by compensating for such isotropic etching during the determination of the grayscale scan pattern220at step140.

The exemplary method ofFIG. 1may also include an optional step (not shown) of the selection of an etchant, photoresist layer210and/or substrate200according to respective etching rates of the etchant on the photoresist of photoresist layer210and the material of substrate200. This optional selection step may allow the etching of photoresist layer210may occur at a first etching rate and the etching of the surface of substrate200may occur at a second etching rate to form desired feature270in substrate200as a substantially scaled version of grayscale pattern255in photoresist layer210in the etching direction.

By selecting different etching rates for photoresist layer210and substrate200, a scaled version of grayscale pattern255formed in the photoresist may be realized in substrate200. That is, when the etching rate in photoresist layer210for a particular etchant is less than that of substrate200, feature270is proportionally larger in the etching direction than that of grayscale pattern255. Moreover, when the etching rate in photoresist layer210is greater than that of substrate200, feature270is proportionally smaller in the etching direction than that of grayscale pattern255. Thus, it may be preferable to adjust etching rates, for example, to produce precisely expanded or contracted features in substrate200from grayscale pattern255. Moreover, improved controllability of the height of various features may allow smaller feature sizes to be achievable.

It is contemplated that this method may be used to produce any number of different features such as a plurality of concentric Fresnel grooves and/or a plurality of linear grooves, among others, on various materials such as tungsten, tungsten carbide, sapphire, plastics, ceramics, dielectrics, and/or metals including stainless steel and nickel, among others. Also, although substrate200is shown as non-planar inFIGS. 2-7, it is contemplated that substrate200may be a planar substrate.

Once desired feature270is produced in substrate200, any portion of photoresist layer210remaining on substrate200may be removed by any number of known techniques. However, it is noted that the selected technique desirably should not substantially affect the material of substrate200.

FIG. 8Ais a cross-sectional side view illustrating a mold for producing a concave hybrid lens with converging diffractive sections that may be formed using the exemplary method ofFIG. 1.

FIGS. 8B-8Dare top plan views illustrating alternate exemplary embodiments of the feature in the substrate shown inFIG. 8A.FIG. 8Billustrates an exemplary mold base200A which may be used to form a circular lens.FIG. 8Cillustrates an exemplary mold base200B which may be used to form an elliptical lens.FIG. 8Dillustrates an exemplary mold base200C which may be used to form a cylindrical lens.

Referring now toFIGS. 8A-8D, after desired feature270is formed in non-planar substrate200, non-planar substrate200may be used as a first mold part for molding applications such as compression molding, extrusion molding, or injection molding, among others. A further optional step may include forming release layer280on at least a portion of the surface of substrate200to prevent adherence thereto by the material being molded. Release layer280may be formed, for example, by a vapor deposition process and may include a thin film metallization layer. For example, thin films including one or more of: platinum; palladium; iridium; nickel; tungsten; titanium; chromium; molybdenum; scandium; vanadium; or aluminum, among others, may be used to form release layer280. When non-planar substrate200is used as a mold part, release layer280may both prevent adherent by a mold product by providing reduced adhesion to ease part release from the mold. It may also provide for improved corrosion resistances to protect non-planar substrate200during the molding process.

The exemplary mold ofFIG. 8Aincludes second mold part290. It is contemplated that the shape of second mold290may be flat, as shown inFIG. 8A, or may include a 3D pattern on its surface as well. In the case of a mold for an optical device, the mold product produced by the exemplary mold ofFIG. 8Amay be, for example, a hybrid lens with a Fresnel groove pattern or an optical grating thereon.

FIGS. 9-11are cross-sectional side views illustrating other exemplary molds for producing hybrid lens with diffractive sections which may be produced by the exemplary method ofFIG. 1. In particular,FIG. 9is a cross-sectional side view illustrating an exemplary mold for producing a convex hybrid lens with diverging diffractive sections,FIG. 10is a cross-sectional side view illustrating an exemplary mold for producing a convex hybrid lens with stair step diffractive sections,FIG. 11is a cross-sectional side view illustrating a mold for producing a convex hybrid lens with diffractive grating sections, andFIG. 12is a cross-sectional side view illustrating a mold for producing a concave hybrid lens with diverging diffractive sections.

Referring now toFIG. 9, the exemplary mold includes a first mold part300that may be formed from a substrate using the exemplary method ofFIG. 1and a second mold part310. First mold part300includes at least one grooved section330formed in a portion thereof and a concave section320. For example, by compression molding, a mold product may be formed in a cavity formed between first and second mold parts300and310. Grooved section330may form a variable width sawtooth pattern to produce a diverging diffractive section in the mold product.

Referring now toFIG. 10, the exemplary mold includes a first mold part400that is formed from a substrate using the exemplary method ofFIG. 1and a second mold part410. First mold part400includes at least one stepped section430formed in a portion thereof and a concave section420. For example, by compression molding, a mold product may be formed in a cavity formed between the first and second mold parts400and410. Stepped section430may form a variable width step pattern to produce a stair step diffractive section in the mold product.

Referring now toFIG. 11, the exemplary mold includes a first mold part500that is formed from a substrate using the exemplary method ofFIG. 1and a second mold part510. First mold part500includes a plurality of slots530formed in a portion thereof and a concave section520. For example, by compression molding, a mold product may be formed in a cavity formed between the first and second mold parts500and510. The plurality of slots may form a variable depth slot pattern to produce a diffractive grating section in the mold product.

Referring now toFIG. 12, the exemplary mold includes a first mold part600that is formed from a substrate using the exemplary method ofFIG. 1and a second mold part610. First mold part600includes at least on sawtooth section630formed in a portion thereof and a convex section620. For example, by compression molding, a mold product may be formed in a cavity formed between the first and second mold parts600and610. The sawtooth section may form a variable width groove pattern to produce a diverging diffractive section in the mold product.

As is apparent from the exemplary optical molds illustrated in FIGS.8A and9-12, any number of other features (i.e., grayscale scan patterns) may be formed in a mold to produce a mold product using a method according to exemplary embodiments of the present invention. Alternatively, such features may be formed directly on a product using a method according to exemplary embodiments of the present invention.