Formation of microstructures using a preformed photoresist sheet

#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.

#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., Jun. 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.

#In the drawings:

#FIG. 1 is a simplified illustrative side view of a substrate with existing micromechanical structures formed thereon.

#FIG. 2 is a view of the substrate of FIG. 1 with a coating of photoresist thereon.

#The present invention may be utilized in the formation of microstructures as carried out in LIGA processes and extensions thereof. Exemplary processes for carrying out the production of micromechanical metal structures is described in U.S. patent application Ser. No. 07/874,116 by Henry Guckel, filed Apr. 24, 1992, now U.S. Pat. No. 5,190,637, entitled Formation of Microstructures by Multiple Level Deep X-Ray Lithography With Sacrificial Metal Layers, the disclosure of which is incorporated herein by reference. The present invention may also be utilized for producing patterned photoresist which may be used for other applications, as where the photoresist is used directly for purposes unrelated to providing a mold for the electro-deposition of metal. The process allows the formation of thick photoresist structures which enjoy substantially no internal strain after adherence to the substrate, good adhesion to the substrate, and generally ready removability from the substrate without substantial additional processing, with photoresist thicknesses up to 1,000 microns being readily achievable. Films cast directly onto a substrate in comparable thicknesses inevitably experience internal strain which detracts from the integrity of the films, increases the likelihood of poor adhesion to the substrate, and leads to crazing of the films. The present invention may be utilized with relatively thin layers of spun on film as part of a multiple layer structure, as described below, with the spun on film thicknesses typically being in the range of 5 .mu.m or less, and in multilayer structures formed from two or more preformed photoresist sheets.

#In the present invention, a preformed photoresist sheet, preferably one formed of linear (non-cross linked), high molecular weight (>2 million) polymethylmethacrylate (PMMA), may be adhered to a substrate before or after patterning in a manner which firmly bonds the preformed sheet to the substrate without introducing internal strain in the substrate. The preformed sheet may be commercially obtained, and generally will have little or no internal strain associated with it in its free form. As used herein, a strain free preformed photoresist will not substantially mechanically distort after portions of the photoresist are exposed and removed or when the remaining photoresist is released from the substrate. The preformed sheet is adhered to the surface of the chosen substrate utilizing a suitable adhesive. In the present invention, a preferred adhesive layer for use with a preformed PMMA sheet is a relatively thin spun on film of PMMA which is applied to the substrate in a conventional manner and cured and forms a strong bond to the substrate. Because the initial spun on film of PMMA is relatively thin (1 .mu.m to 5 .mu.m or less), the initial spun on film is not subject to the adhesion problems, internal strain problems, and crazing which is commonly associated with the much thicker (25 .mu.m to 200 .mu.m) films which are utilized in LIGA processing. Because the initial PMMA film layer will be covered by the preformed PMMA sheet, and is relatively thin, the initial PMMA layer will not suffer crazing during the electro-deposition process and does not need to have a cross-linking agent added to it to maintain the integrity of the film. Consequently, by using a non-crosslinked preformed PMMA sheet of high molecular weight PMMA, the entire photoresist may be readily removed using a suitable solvent after the patterning and deposition of the metal have occurred, without requiring a blanket exposure to X-rays or UV of the entire surface of the substrate on which the photoresist film is formed. This eliminates a time consuming and relatively expensive final step which has been heretofore an essential step in LIGA processing. Adhesion between the preformed PMMA sheet and the initial PMMA layer may be readily obtained by utilizing methylmethacrylate (MMA) monomer liquid applied to the interface between the preformed sheet and the initial layer. The monomer wicks through the interface between the two layers by capillary action, causing a bond between the preformed sheet and the initial layer, and is released by diffusion through the preformed sheet after a sufficient period of time. The preformed sheet adhered to the substrate may be mechanically worked, as by milling, to reduce the thickness of the layer to a precise height above the surface of the substrate, e.g., by using commercially available micromilling equipment.

#After the photoresist has been applied and milled to the desired thickness, it may be patterned in a conventional manner, such as by using X-ray radiation from a synchrotron and an X-ray mask, to render areas of the photoresist susceptible to removal by a developer liquid. High molecular weight, linear PMMA exhibits good selectivity to conventional developers so that the unexposed PMMA is not substantially affected by the developer, while the exposed PMMA, which consists of PMMA molecules of substantially shorter length as a result of molecular chain scission from the radiation exposure, is readily dissolved by the developer.