Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-05-30
2003-02-25
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S296000, C430S321000, C430S325000
Reexamination Certificate
active
06524756
ABSTRACT:
BACKGROUND OF THE INVENTION
“High efficiency diffractive coupling lenses by three-dimensional profiling with electron lithography and reactive ion etching,” by A. Stemmer et al, J. Vat. Sci. Technol. B 12 (6), November/December 1994, teaches three dimensional profiling of a photoresist and transferring the three-dimensional microstructures of photoresist into the substrate using reactive ion etching. Three-dimensional profiling of photoresist with electron beam direct write on photoresist however is not cost effective for production quantities. “Fabrication of diffractive optical elements using a single optical exposure with a gray level mask,” Walter Daschner, et al, J. Vat. Sci. Technol. B 13 (6), November/December 1995 teaches generating a gray level mask with eight discrete gray levels by means of cycles of evaporation of Iconel and a following lift-off step. This gray level mask allowed to expose a multi-level DOE in a single optical exposure step for three-dimensional profiling of photoresist. CAIBE was used to transfer the analog resist structure into the substrate. The tight thickness control necessary in the Iconel evaporation steps makes this method of fabricating the gray level mask economically undesirable.
“Gray scale microfabrication for integrated optical devices,” George Gal et al., U.S. Pat. No. 5,480,764, issued Jan. 2, 1996, teaches the fabrication of three-dimensional microstructures including photonic waveguide surface, lens surface, and inclined planar surface for use as a beam splitter in a photonic device, using a half tone gray scale photomask for three-dimensional profiling of photoresist and reproducing the photoresist replica in the substrate with differential ion milling. However, a half tone gray scale mask is not desirable due to limited resolution.
Other gray level mask fabrication methods have been demonstrated and show potential for mass fabrication. See for example: H. Andersson, M. Ekberg, S. Hard, S. Jacobson, M. Larsson, and T. Nilsson, Appl. Opt. 29, 4259, 1990; and Y. Oppliger, P. Sixt, J. M Mayor, P. Regnault, and G. Voirin, Microelectron. Eng. 23, 449, 1994. After the mask fabrication only a single-exposure step is necessary to generate a multilevel resist profile. These approaches, however, have limited resolution since silver halide-based photographic emulsion is used and the grayscale mask is a halftone mask, that is not a true gray scale mask.
The fabrication of microoptical elements such as refractive microlens arrays, diffractive optical elements, prism couples, and three-dimensional microstructures in general can be realized with the existing micro-fabrication methods normally used for the production of microelectronics. The well-established microfabrication technologies include photolithographic process and reactive ion etching. Photolithographic processes are employed to print a mask pattern in a photomask onto photoresist film, which is typically coated on a silicon wafer or a glass wafer. Commercially available photolithographic printers for microfabrication include contact and proximity printers, 1× projection printer, 1× steppers and 5× as well as 10× reduction steppers. Reactive ion etching is employed to transfer and/or replicate patterns in photoresist into the underlying substrate material. Commercially available systems include plasma etchers, inductive coupled plasma (ICP) and chemically assisted ion beam etchers (CAIBE).
For the fabrication of integrated circuits (IC) in microelectronics industry, a set of binary masks is used in the photolithographic process. The binary masks typically have IC patterns defined in a chrome film which is coated on a silicate glass plate, typically a fused silica glass plate. However, for the fabrication of microoptical elements, a gray scale photomask is needed to define the three dimensional microstructures.
A gray scale photomask carries patterns with areas of different transmittance. When the pattern is printed on photoresist, areas of different transmittance in the gray scale mask create areas of different thickness in photoresist after development. Therefore, a gray scale pattern in a gray scale photomask can be used to create predetermined 3D microstructures in photoresist film, which are then transferred and/or replicated into the underlying substrate material in a reactive ion etcher.
Instead of using a gray scale photomask a varied exposure in a photoresist can also be generated by directly exposing the photoresist with an e-beam writer or a laser beam writer. The developed 3D resist structure can then be transferred into underlying substrate material to produce microoptical elements. However, in this case no mask is created. Each element must be written one at a time, with no benefit from economies of scale. Namely, it is not cost effective for making microoptical elements in production quantities using this direct write method.
There have been several methods of making gray scale photomasks in the past, but each of them have a major shortcoming as described below.
U.S. Pat. Nos. 5,480,764 and 5,482,800 of Gal et al and an article by W. W. Anderson et al “Fabrication of Micro-optical Devices” conference on Binary Optics 1993, pp. 255-269, teach half tone gray scale masks. According to this technique, the mask is created by constructing a plurality of precisely located and sized openings, the frequency and size of these openings produce the desired gray scale effect provided the mask pattern is blurred in the photolithographic process to print on photoresist. The smallest features of this mask are binary, either open or closed, i.e. on or off. A group of a large number of on and off spots is needed to create a gray scale resolution element. The gray scale resolution element appears for example as 80% transmittance or as 20% transmittance depending on the ratio of the number of the on-spots and off spots. Therefore, the resolution of a half-tone gray scale pattern is much reduced from that of a binary pattern in a chrome mask.
Photographic emulsion has been used to provide gray scale masks. A gray area consists of a number of silver grains and openings. The silver grains are totally opaque and the openings are totally transparent. Therefore, the photographic gray scale mask is also a half-tone mask. The gray scale resolution element of a photographic film is in general larger in size than that of the halftone gray scale chrome mask. This is because the silver grains in a developed photographic emulsion film are in general larger than an opening or a chrome spot that can be made in a chrome mask.
One improvement in the production of gray scale masks for use in fabricating microoptical elements has been realized with the provision of a gray scale mask wherein different thicknesses of a light absorbing material, such as Inconel are coated on a glass plate to form the gray scale mask (see U.S. Pat. No. 6,071,652 of Feldman, et al, Jun. 6, 2000). This gray scale mask could have the high resolution required for fabricating microoptical elements. However, one disadvantage of this technique is the cost of the mask generation, wherein multiple direct write steps on photoresist are required to provide the lift off process of the light absorbing material for each discrete thickness desired. The tight thickness control necessary in the material evaporation steps makes this technique economically undesirable.
The gray scale photomasks described above cannot be utilized for the fabrication of high quality micro-optical elements in production quantities, because of their failure to satisfy either one or both of the following requirements: 1.) a sub-micrometer gray scale resolution element, 2.) acceptable cost in the mask generation.
High Energy Beam Sensitive (HEBS) glasses were described in U.S. Pat. Nos. 4,567,104, 4,670,366, 4,894,303, 5,078,771, and 5,285,517 (Wu patents herein after) by Che-Kuang Wu who is also known as Chuck Wu, the latter name is used in some of his publications in technical journals. The size of a gray scale resolution element in an HEBS-gla
Canyon Materials, Inc.
Fish & Richardson P.C.
Young Christopher G.
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