Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-09-25
2002-04-16
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S296000, C430S396000, C430S942000
Reexamination Certificate
active
06372391
ABSTRACT:
FIELD OF THE INVENTION AND DESCRIPTION OF PRIOR ART
The invention relates to lithographic patterning of a plurality of identical structures, in particular large areas of periodic nanostructures, onto a target substrate.
One application of periodic nanostructures is the fabrication of near infrared (IR) mesh filter arrays. Photovoltaic energy conversion generally has poor efficiency with thermal—i.e., non-solar—energy sources due to the incongruity between the very broad photo-emission spectrum of thermal radiators with the narrow energy band of photovoltaic conversion. A solution to this problem has been described by Home et al. in U.S. Pat. No. 5,611,870. In that approach the photovoltaic cell is coupled with an infrared bandpass filter which transmits only those photons that can be efficiently converted to electricity by the photocells and reflects those with either shorter and longer wavelengths back into the source where they are reabsorbed. As infrared bandpass filter a metal-mesh filter array consisting of cross-shaped openings in a thin gold film is used; the cross-shaped openings have a length of 450 nm and an arm width of only 50-80 nm. The use of these filters is expected to increase the efficiency of GaSb photovoltaic cells from less than 1% to near 30% for operation with a 1500 K black-body radiator.
The challenge in forming patterns like the cross-shaped openings of the IR filter array lies in the resolution needed to reproduce the fine comers at the center of the cross. M. D. Morgan et al., in J. Vac. Sci. Technol. B 14 (6), 1996, pp.3903-3906, discuss the fabrication of IR filters using electron-beam lithography (EBL) and ion beam proximity printing (IBP) techniques, which were prepared with equivalent spectral response. The EBL approach uses a finely focused, high-energy beam of electrons to expose resist on a substrate. Deflectors are used to scan the beam across the substrate and so write the desired pattern with high accuracy, but the serial nature of this approach makes the fabrication process extremely time consuming and expensive.
FIG. 1
shows the principle of IBP patterning. Sensitive material 101, such as a photo-resist covering a substrate
102
which is to be structured according to the pattern defined in a pattern mask
103
, is exposed by transmitted beamlets
104
that are formed when the mask
103
is illuminated by a broad beam
114
of light ions which pass through the transparent regions
105
to expose the underlying resist. In IBP, the mask
103
is positioned in proximity to the substrate
102
; the distance G between the mask and the substrate is small, e.g. a few mm or less, and depends on the optical properties of the ion beam system. As illustrated in
FIG. 1
, the transparent regions
105
are typically openings. The openings of the stencil mask are, for instance, openings corresponding to the desired pattern, and the mask
103
is typically realized as a silicon membrane with etched openings. The parallel printing technique realized with the help of lithography masks replicates the pattern in the mask with a single exposure and so the cost to replicate a mask is effectively independent of array size up to a maximum determined by the size of the beam, which is typically 2 to 8 inches in diameter. Lithographic patterning methods, including printing and projection techniques, 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 Loschner in ‘Overview of the Ion Projection Lithography European MEDEA and International Program’, Proceedings SPIE, Vol. 3997, Emerging Lithography Technologies IV, 2000. Lithographic patterning using stencil masks is not restricted to particle beam systems, but also possible with lithography systems based on photons, like EUV (Extreme UV) or X-ray lithography. Also instead of transmission masks, reflection masks can be used, in particular in connection with EUV systems; in this case the mask has regions of higher reflectivity in place of transparent regions.
The stencil masks for IR filter arrays are conventionally fabricated using EBL to define the mask structures on a thin silicon membrane substrate. The high cost of the EBL process makes the fabrication of large-area IBP masks—i.e., greater than 1 cm
2
—uneconomical. To overcome this limitation, one can take advantage of the periodicity of the pattern to print step-and-repeat copies of a small mask onto a second mask substrate to form a second-generation replica with a much larger area. It would be desirable to repeat this replication process to generate subsequently larger generations of the original mask; however, in practice it proves difficult to maintain the fidelity of the original mask structure in even the first copy. This difficulty is due to the inherent blur of an ion beam system. Because of the blur the ion beam system acts as a low-pass spatial filter that attenuates the high-frequency information of a pattern of the original mask when imaged onto the substrate (e.g., a secondary mask).
The blur attenuates the high-frequency information that describes the corners of the original shape to the point where they are significantly rounded. In the case of a cross-shape pattern such as used with an IR filter array, for instance, the center of the structure is enlarged, while the width of the arms varies along their length. Moreover, the rounding of the corners and the widening of the center accumulates over multiple mask generations. Experiments done with 0,49 cm
2
stencil masks confirmed that the quality of the mask pattern degrades from first to second generation. Therefore, the reproduction of high-frequency spatial information in multi-generational masks, in particular structures comprising comers and/or elbows of lines, is problematic.
The degradation of an IBP mask image is shown in FIG.
2
. The graph shows the resist foot-print of five generations of IBP mask copies, where the resist footprint of the previous generation is used as the mask to print the next generation, according to a simulation calculation where the blur was chosen to be 70 nm FWHM. Clearly, the final, fifth generation mask pattern does not resemble the original mask pattern; rather, the initial cross pattern is considerably blotted.
It is an aim of the present invention to overcome the above-described problems with the production of patterns, in particular in the context of multi-generation reproduction of mask patterns.
SUMMARY OF THE INVENTION
This aim is met by a method for lithographic patterning of a plurality of identical structures onto a target substrate wherein, according to the invention, a template mask bearing a template structure pattern, comprising a plurality of identical template structures each consisting of a set of at least one structure element of circular shape, is used for lithographic patterning of the target substrate, wherein by means of a broad beam of energetic radiation the template mask is illuminated to form a structured beam and the template structure pattern is imaged onto the target substrate by means of the structured beam, the target substrate being positioned after the mask as seen in the optical path of the beam and comprising material sensitive to exposure to said energetic radiation, producing a pattern image on the target substrate, the pattern image thus produced comprising a plurality of identical target structures.
A template mask according to the invention, that is, a mask bearing a template structure pattern comprising a plurality of identical template structures each consisting of a set of at least one structure element of circular shape, is suitably produced by a method wherein starting from a primary mask bearing a primary structure pattern consisting
Ruchhoeft Paul
Wolfe John Charles
Ladas & Parry
The University of Houston
Young Christopher G.
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