Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-09-14
2003-09-23
Anderson, Bruce (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C252S492000, C252S398000
Reexamination Certificate
active
06624429
ABSTRACT:
FIELD OF THE INVENTION AND DESCRIPTION OF PRIOR ART
The present invention relates to lithographic patterning of a resist layer on a curved substrate, in particular with spherical concave curvature. More specifically, the invention relates to a lithographic method for producing an exposure pattern on a curved substrate field of a substrate, the substrate field comprising material sensitive to exposure to an energetic radiation. In a pattern transfer system a wide, substantially parallel beam of the energetic radiation is produced, and by means of the collimated beam a planar mask having a structure pattern, namely, a set of transparent windows to form a structured beam, is illuminated and the structure pattern is imaged onto the substrate by means of the structured beam, the substrate being positioned after said mask as seen in the optical path of the beam, producing a pattern image, namely, a spatial distribution of irradiation over the substrate.
In manufacturing semiconductor devices, one important step for structuring the semiconductor substrates is lithography. The substrate, for instance a silicon wafer, is coated with a thin layer of photosensitive material, called photo-resist. By means of a lithographic imaging system, a pattern is imaged onto the photo-resist, and the subsequent development step removes from the substrate either the exposed or the unexposed portion of the photo-resist. Then, the substrate is subjected to a process step such as etching, deposition, oxidation, doping or the like, the photo-resist pattern on the substrate covering those portions of the surface that shall remain unprocessed. The photo-resist is stripped, leaving the substrate with the new structure. By repeating this sequence, multiple structure layers can be introduced to form the semiconductor micro-circuits.
There has been a growing interest in patterning of curved surfaces at sub-micron dimensions, in particular with optical sensor arrays on concave surfaces, so-called focal surface arrays (FSA). Applications lie in the field of imaging technology, such as infra-red cameras and wide-field optical sensors. To date, the ability to reduce the size, or even miniaturize, infra-red cameras is limited by the size and weight of the optical components; these could be reduced by an order of magnitude if a spherical imaging array is used instead of flat one. Moreover, spherical imaging arrays enable simple, compact optical designs with ultra-wide fields-of-view. In comparison with flat designs, however, the patterning of curved surfaces on the one-micron scale is a challenge because of the large depth of field that is needed for the topographical variation of the substrate.
The inventors, in J. Vac. Sci. Technol B 17(6) pp. 2965-2969, 1999, have shown that shadow printing lithography, in particular ion beam proximity (IBP) lithography, has the resolution and depth-of-field required for the task of patterning FSAs. Lithographic printing methods as well as lithographic devices using electron or ion beams are discussed, for instance, by H. Koops in ‘Electron beam projection techniques’, Chapter 3 of ‘Fine Line Lithography’, Ed. R. Newman, North-Holland, 1980, pp. 264-282. Electrons and in particular ions have the advantage of very low particle wavelengths—far below the nanometer range—which allow of very good imaging properties, as e.g. discussed by Rainer Kaesmaier and Hans Löschner in ‘Overview of the Ion Projection Lithography European MEDEA and International Program’, Proceedings SPIE, Vol. 3997, Emerging Lithography Technologies IV, 2000. Proximity printing using stencil masks is, however, not restricted to particle beam systems, but also possible with lithography systems based on photons, like EUV (Extreme UV) or X-ray lithography.
In shadow printing lithography (and likewise in projection lithography), the pattern to be imaged onto the photoresist-covered substrate is produced by using a mask or reticle having the desired pattern. For particle lithography systems, stencil masks are used in which the patterns to be projected are formed as apertures of appropriate shape in a thin membrane, i.e., a few micrometers thick. The mask pattern is built up from a number of apertures in a thin membrane through which the particle beam is transmitted to expose the resist-coated wafer in those areas required for device fabrication.
Lithographic patterning of curved substrates suffers from a complex of problems arising from the projection of the mask pattern onto substrate areas inclined with respect to the mask, which causes not only a distortion of the mask pattern as compared to the original mask pattern, but also a reduced local exposure dose density. Blur in shadow printing, such as IBP lithography, is due to imperfect collimation of the ions and so depends on the gap between the mask and the substrate surface. Thus, in the particular case of a concave substrate field, the blur, linewidth and exposure latitude all depend upon the radial distance from the center of the substrate field.
It is an aim of the present invention to overcome the above-mentioned problems and, in particular, to show a way to correct for the distortions incurred from the projection of the mask pattern onto the curved substrate field while avoiding other problems as, e.g., deterioration of pattern reproduction due to non-uniformity of dose density.
SUMMARY OF THE INVENTION
The invention provides a solution to the above task by a method as mentioned in the beginning wherein the center of curvature of the substrate field is positioned on a line as defined by a ray of the beam running through a pattern center defined on the mask within the area of the structure pattern, the windows of the structure pattern being arranged in a manner that along each radius from the pattern center, the radial spacing of said windows decreases with increasing radius from the pattern center, and wherein the direction of incidence of said beam onto the mask is varied through a sequence of inclinations with respect to the normal axis to the mask, the sequence of inclinations being adapted to merge those exposure pattern components which result from neighboring windows of the structure pattern, the exposure with respect to the sequence of inclinations superposing into a spatial distribution of exposure dose on the substrate, said distribution exceeding the specific minimum exposure dose of said resist material within only one or more regions of the substrate field, said region(s) forming the exposure pattern.
By virtue of the invention the design of self-supporting masks is possible which allow of a patterning of curved, in particular concave spherical, substrates with a flat mask. The decreasing spacing of the windows with increasing radial distance from the pattern center can suitably be employed to compensate for the effect of distortion due to the curvature of the substrate field.
Devices which are in particular suitable for the above method according to the invention are the lithography apparatus according to claim 14 and the lithography mask according to claim 15. In a preferred embodiment of the invention, the radial spacing of the windows follows the projected distances of uniformly-spaced points on the substrate field projected onto the mask plane. Thus, the imaging distortions upon projection onto the curved substrate are taken into account, simplifying the design of the respective mask pattern. In particular, the structure pattern can be a subset of an array of windows, the position of the windows determined by a two-dimensional array obtained from a regular two-dimensional array of uniformly-spaced points deformed by a transformation corresponding to a projection from the substrate field onto the mask plane. Preferably, the inclination range corresponds to the inclination range used to image an array of windows positioned on said regular two-dimensional array into a full-field exposure pattern on a planar substrate positioned at a distance equal to the radius of curvature of the substrate field.
In a further preferred embodiment, the substrate
Ruchhoeft Paul
Wolfe John Charles
Anderson Bruce
Ladas & Parry
University of Houston
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