Formation of microstructures using a preformed photoresist sheet

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FIELD OF THE INVENTION

This invention pertains generally to the field of semiconductor and micromechanical devices and processing techniques therefor, and particularly to the photoresist used in formation of microminiature structures such as those formed of metal.

BACKGROUND OF THE INVENTION

Deep X-ray lithography involves a substrate which is covered by a thick photoresist, typically several hundred microns in thickness, which is exposed through a mask by X-rays. X-ray photons are much more energetic than optical photons, which makes complete exposure of thick photoresist films feasible and practical. Furthermore, since X-ray photons are short wavelength particles, diffraction effects which typically limit device dimensions to two or three wavelengths of the exposing radiation are absent for mask dimensions above 0.1 micron. If one adds to this the fact that X-ray photons are absorbed by atomic processes, standing wave problems, which typically limit exposures of thick photoresists by optical means, become a nonissue for X-ray exposures. The use of a synchrotron for the X-ray sources yields high flux densities--several watts per square centimeter--combined with excellent collimation to produce thick photoresist exposures without any horizontal runout. Locally exposed patterns should therefore produce vertical photoresist walls if a developing system with very high selectivity between exposed and unexposed photoresist is available. This requirement has been satisfied using polymethylmethacrylate (PMMA) as the X-ray photoresist, and an aqueous developing system. See, H. Guckel, et al., "Deep X-ray and UV Lithographies for Micromechanics," Technical Digest, Solid State Sensor and Actuator Workshop, Hilton Head, S.C., June 4-7, 1990, pp. 118-122.

Deep X-ray lithography may be combined with electroplating to form high aspect ratio structures. To do so requires that the substrate be furnished with a suitable plating base prior to photoresist application. Commonly, this involves a sputtered film of adhesive metal, such as chromium or titanium, which is followed by a thin film of metal which is suitable for electroplating the metal to be plated. In appropriate cases, the use of an initial layer of adhesive metal is not necessary. Exposure through a suitable mask and development are followed by electroplating. This process results, after cleanup, in fully attached metal structures with very high aspect ratios. Such structures were reported by W. Ehrfeld and co-workers at the Institute for Nuclear Physics (KFK) at Karlsruhe in West Germany. Ehrfeld termed the process "LIGA" based on the first letters for the German words for lithography and electroplating. A general review of the LIGA process is given in the article by W. Ehrfeld, et al., "LIGA Process: Sensor Construction Techniques Via X-Ray Lithography," Technical Digest, IEEE Solid State Sensor and Actuator Workshop, 1988, pp. 1-4.

A crucial factor in the production of microminiature devices, such as those formed by the LIGA process, is the photoresist that is used. As noted, PMMA has been successfully used as the photoresist for formation of LIGA structures. The PMMA films have been produced by casting of liquid MMA directly on the substrate, with the film being reduced to the desired thickness, generally not more than two to three hundred microns, with a casting jig. The cast film is then solidified, typically by utilizing a polymerization agent or initiator in the casting solution and a cross-linking agent which results in cross-linking upon curing. There are several disadvantages and limitations of the PMMA films formed in this manner. The casting procedures require special equipment and fixturing, which adds to the time and cost of the process. As with almost all casting operations, a heat cycle is necessary to produce the solidified film. Typically, annealing cycles up to 110.degree. C. are required. These heat cycles build up strain in the film due to a significant mismatch in thermal expansion coefficients between the PMMA photoresist and the substrate. Internal strain in the photoresist also occurs due to the shrinking of the film during curing, which has been observed to result in up to a 20% shrinkage of the film from its as-cast state. As a consequence, the cast film, after curing, often has poor adhesion to the substrate and can buckle off the substrate. Even where adhesion to the substrate is retained, the internal strain that is built into the film can result in distortion of the walls formed in the film after patterning of the photoresist by X-ray exposure and development.

PMMA photoresist films are typically cross-linked through the addition of a cross-linking agent to the casting solutions to minimize crazing of the films. Because of this cross-linking, it is necessary to have an additional X-ray exposure step, a blanket exposure of the entire photoresist film, followed by development of the film to remove the resist when its use is complete. Even with the use of cross-linking agents, the maximum thickness of resists which have been successfully cast, exposed, and developed have been in the range of about 300 microns. Most samples of PMMA photoresist films having thickness greater than 200 microns have unacceptable amounts of crazing and adhesion loss. Typically, the cast PMMA films may only be used once because such films are found to exhibit significant crazing after the microplating of metal into the patterned openings in the film during the electroplating step of the LIGA process.

The thicknesses of photoresist utilized for micromechanical processing is typically a few hundred microns or less, which is below the typical thicknesses of preformed photoresist sheets. Ehrfeld and co-workers have reported attempts to adhere a preformed PMMA photoresist sheet to a substrate, with the photoresist being calendered before adherence to the substrate to reduce the sheet to the desired thickness for carrying out the LIGA process. However, such attempts were reported to be unsuccessful, apparently because the strain fields in the calendered photoresist were excessive.

SUMMARY OF THE INVENTION

In accordance with the present invention, production of microstructures is facilitated by utilizing a photoresist structure which comprises a preformed sheet which can be adhered to a substrate before further processing so as to yield essentially no or very little strain within the photoresist. The preformed sheet is of conventional thickness, which allows convenient handling of the sheet as it is adhered to the substrate, with the thickness of the sheet being reduced to the desired thickness for the formation of microstructures by mechanically removing a portion of the sheet, as by milling by a micromill. A preferred photoresist sheet is formed of substantially non-crosslinked linear polymethylmethacrylate which has a very high average molecular weight and which is essentially strain free. Either before or after the sheet of photoresist is reduced to the desired thickness, the photoresist may be patterned by exposure through a mask to radiation which will affect the susceptibility of the exposed photoresist material to a developer. Depending on the photoresist used, the radiation may be X-ray radiation such as from a synchrotron or, in appropriate cases, deep UV light. The exposed portions of the photoresist (or unexposed portions, depending on the type of photoresist) may then be removed in a suitable developer.

For carrying out the formation of electroplated microstructures utilizing the present invention, a plating base is applied to a substrate prior to the photoresist. A non-exposed photoresist may then be adhered onto the plating base, and the photoresist then exposed. The exposed portions are then removed using a developer, and metal is then electroplated onto the exposed plating base to fill the area defined by the void(s) in the photoresist. The remaining photoresist may then be removed. Where the photoresist is formed of non-crosslinked PMMA, removal can take place by utilizing a solvent which dissolves the non-crosslinked PMMA. Where a cross-linked PMMA sheet is utilized, an additional blanket exposure to synchrotron X-rays is required before the photoresist is removed. By utilizing the preferred non-crosslinked PMMA photoresist sheet, this additional exposure step can be avoided, which produces a significant savings in time and expense over procedures which require a blanket exposure.

Where the photoresist is to be applied over existing structures formed on a substrate, relatively thin coats of conventional photoresist, such as PMMA, may then be spun onto the substrate to cover the structures. The photoresist sheet is then placed on top of the spun on photoresist and the interface between the two is wetted with a monomer. For example, with high molecular weight PMMA being utilized as the material of the photoresist sheet, a lower molecular weight PMMA dissolved in a solvent is spun on to the substrate to cover the mechanical structures, and the monomer, liquid methylmethacrylate, is then applied to the interface between the two which results in a solvent bonding of the materials at the interface. The preformed photoresist sheet is generally much thicker than the spun on photoresist, which need only be thick enough to cover the existing structures, typically a few microns or tens of microns in height.

The adherence of a preformed photoresist layer onto the substrate, and the mechanical milling of the photoresist sheet to a desired thickness, results in a photoresist of any desired thickness which can be precisely controlled using the mechanical milling process. Good adhesion of the photoresist sheet to the substrate is obtained and with very low internal strain within the photoresist. Consequently, much thicker photoresist structures can be formed than has heretofore been possible utilizing conventional photoresist materials without substantial distortion of the walls of the photoresist during the process of exposing the photoresist and removing the exposed resist with developer. The photoresist of the invention may be utilized during more than one electrodeposition process, inasmuch as the preformed PMMA sheet exhibits much less crazing and other damage during the electro-plating process than is observed in conventional cast PMMA photoresist layers.

The present invention may further be used to produce photoresist structures which can be released from the substrate. The strain free photoresist sheet, e.g., PMMA, either linear or cross-linked, is adhered to a release layer on a substrate. After the photoresist sheet is patterned, the release layer may be removed by a remover which etches or dissolves the release layer without substantially affecting the remaining photoresist. Because the photoresist sheet is strain free, the photoresist parts which are thus freed from the substrate are dimensionally stable and will not distort or curl.

The present invention also allows multiple layers of patterned photoresist to be constructed which can be applied to a substrate or utilized as a separate product. Such multiple layer structures allow the formation of metal structures by electrodeposition which can have a variety of shapes which vary in all three dimensions. For example, the metal structures may be formed having upper portions which are wider or which extend outwardly from the lower portions, and which undergo several variations in geometry from the top to the bottom of the structure. Such multiple layer photoresist structures can be formed in various manners in accordance with the present invention. In an exemplary multiple layer process, a second layer of photoresist is bonded to the upper surface of the first layer after the first layer has been exposed but before it has been developed. The second layer is then milled to the desired thickness and an X-ray exposure of the second layer photoresist takes place. The exposed photoresist is then developed (removed) and metal structures may be electroplated in the voids vacated by the exposed photoresist. Structures having more than two layers may be built up in this manner. Generally, it is necessary when utilizing this process that the exposure for the second layer (or subsequent layers) either lie within the exposed regions in the first layer (or all underlying layers) or that the incident X-rays be sufficient to fully penetrate and expose all layers of photoresist.

In another exemplary multilayer process, a layer of preformed photoresist (e.g. linear or cross-linked PMMA) which is not adhered to a substrate is exposed on one side. The photoresist sheet may be quite thick, e.g. 1 mm to 3 mm, so that the incident X-rays do not necessarily cause sufficient exposure of the photoresist in all exposed areas to allow the photoresist sheet to be fully exposed all the way through. The exposed photoresist may then be developed with a developer to remove it, and then the preformed sheet may be bonded to a substrate on the side which had been exposed and developed. The free side of the photoresist is then milled down to a desired thickness which is below the level at which all of the areas exposed to X-rays were sufficiently exposed so that they will be completely removed by the photoresist to leave a pattern of voids therein. This provides an initial single layer structure which is equivalent to a structure produced by bonding a photoresist to a substrate first, and then a milling to a desired thickness and then exposing to X-rays and developing the exposed photoresist. However, multiple photoresist layers may be formed by treating a free sheet (unbonded to a substrate) of photoresist in this manner, whether the initial layer is formed in this manner or is formed after being adhered to the substrate. For example, the second sheet of photoresist may be exposed to X-rays on one side in a desired pattern, the exposed photoresist developed to remove it, and then the surface of the photoresist which has been exposed may be bonded to the free surface of the first layer, with milling of the free surface of the second layer carried out to reduce the thickness of the second layer to below the level at which the exposed photoresist has been developed and fully removed to leave a pattern of voids. Third and additional layers may be formed in a similar manner.

The preformed photoresist sheet may also be exposed on one side and a first layer bonded to a substrate (or to a previously applied photoresist layer) at the surface which had been exposed to X-rays, but without developing the exposed photoresist. The sheet is then milled down to a level which is below the level at which the exposed photoresist would be fully removed from the developer. The photoresist may then be removed immediately. However, it is not necessary to do so, and a second preformed photoresist sheet may be treated in a similar manner, exposing one side to X-rays partially through, adhering the exposed side to the prior layer, and then milling down the second layer to a thickness such that all areas of the photoresist which were exposed to X-rays are sufficiently fully exposed so that they will be fully developed. When the desired number of layers have been built up, the entire laminated structure may be developed with a liquid photoresist developer to remove all of the exposed photoresist. In this process, it is necessary that the areas of exposed photoresist in each layer overlap one another so that the developer can work through the layers to remove all of the exposed photoresist. Alternatively, the exposed regions must be accessible at the side edges of the laminate.

In either of the above-processes, it is not essential that the substrate be a metallic substrate. For example, the substrate may comprise a thick photoresist sheet, or a variety of other materials, including a semiconductor wafer with or without electronic circuitry thereon. This initial thick photoresist sheet may also have been previously processed so that it contains either developed structures or undeveloped X-ray exposed regions. Additional photoresist layers are then bonded to the first layer and milled in sequence to form a laminate, with the various layers either being developed before or after they are bonded together. The initial thick