Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making named article
Reexamination Certificate
1998-06-09
2002-06-25
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making named article
C430S323000, C430S396000, C430S945000
Reexamination Certificate
active
06410213
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for producing micro-optical structures, for use as optical elements, such as lenses and gratings, with arbitrary (essentially any) surface-relief profile and, if desired, with optomechanical alignment marks.
Briefly, the invention is carried out by varying the exposure dose, spatially, based upon predetermined contrast curves of a low-contrast photosensitive material. Arbitrary one-dimensional (1-D) or two-dimensional (2-D) surface contours including spherical, aspherical, toroidal, hyperbolic, parabolic, and ellipsoidal can be achieved. The final medium for the fabricated microstructures can be a wide range of materials through the use of etching and replication technology. The method is adapted to mass production. Applications for this invention include, but are not limited to, and particularly the manufacture of microstructures for use in the fields of optical communications, optical data storage, optical interconnects, telecommunications, and in displays, and focal-plane arrays.
This invention is related to the invention described in U.S. patent application, Ser. No. 09/0905,300, filed Jun. 10, 1998 now U.S. Pat. No. 6,075,650, issue Jun. 13, 2000.
BACKGROUND
A microlens or microcylinder imparts a given phasefront to incident radiation. Similar to macroscopic optical systems, these micro-optical elements can focus incident radiation, diverge the radiation, as well as impart a phase function to correct aberrations from previous elements in the system, or precorrect for aberrations occurring downstream of the microelements. There are numerous methods of fabricating surface-relief micro-optics, and these include multi-step mask and etching, resist thermal transport, e-beam exposure, laser ablation, stamping, etc. The present invention offers advantages over all of these described processes, due to the precision control one has over the final surface shape of the micro-optical element, and in providing optical microstructures having profiles with heights exceeding 15 &mgr;m (microns) which in lenses is called a deep sag.
Previous methods of fabrication of elements using exposures of photoresist either concentrated upon fabricating binary (two-level) structures, such as in the case of micro electro-mechanical systems (MEMS), or exposed photoresist to a varying dosage of exposure radiation using thin photoresist. Photoresist has been exposed to create continuous-relief photoresist profiles of optical devices (e.g., microlenses, diffractive phase plates, and diffraction gratings). Exposure methods have included laser pattern generation, see, T. R. Jay and M. B. Stern M. T. Gale, M. Rossi, J. Pedersen, H. Schütz, “Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresist,”
Opt. Eng
. 33, 3556-3566 (1994), grayscale mask exposures, see, H. Andersson, M. Ekberg, S. H{dot over (a)}rd, S. Jacobsson, M. Larsson, and T. Nilsson, “Single photomask, multilevel kinoforms in quartz and photoresist: manufacture and evaluation,”
Appl. Opt
. 29, pp. 4259-4267 (1990); D. C. O'Shea and W. S. Rockward, “Gray-scale masks for diffractive-optics fabrication: I. Commercial slide imagers,”
Appl. Opt
. 34, 7507-7517 (1995); D. C. O'Shea and W. S. Rockward, “Gray-scale masks for diffractive-optics fabrication: II. Spatially filtered halftone screens,”
Appl. Opt
. 34, 7518-7526 (1995); W. Daschner, P. Long, R. Stein, C. Wu, and S. H. Lee, “Cost-effective mass fabrication of multilevel diffractive optical elements by use of a single optical exposure with a grayscale mask on high-energy beam-sensitive glass,” Appl. Opt. 36, 4675-4680 (1997), and holographically, M. C. Hutley, “Optical techniques for the generation of microlens arrays,”
J. of Mod. Opt
. 37, 253-265 (1990); H. Ming, Y. Wu, J. Xie, and Toshinori Nakajima, “Fabrication of holographic microlenses using a deep UV lithographed zone plate,”
Appl. Opt
. 29, 5111-5114 (1990). But the present invention overcomes limitations of prior processes in affording arbitrary micro-structure profiles, especially with heights (sag in case of lenses) exceeding 15 &mgr;m.
In the multi-step mask and etch process, as described in M. B. Stern, “Chapter 3: Binary Optics Fabrication,” in
Micro-Optics: Elements, Systems, and Applications
, ed. H. P. Herzig, (Taylor & Francis, Bristol, Pa., 1997), pp. 53-85, one exposes a photosensitive material (typically photoresist developed for the semiconductor industry) using a binary amplitude mask. This mask consists of a series of optically opaque and clear features (e.g., Chromium on glass) that is used to selectively expose selected areas of a photoresist-coated substrate to electromagnetic radiation. After development, one achieves a binary structure in the photoresist that can be transferred into the underlying substrate material through an etching process. This exposure, development and etching process can be repeated with a series of different photomasks in order to achieve a multi-level surface-relief pattern. Although of practical use for the fabrication of diffraction gratings and phase plates, the multi-step masks and etch process has disadvantages when the fabrication of deep-sag or high numerical aperture (NA) micro-optical elements is required. For a diffraction grating fabricated with a multi-level process operating at the order m, the maximum theoretical diffraction efficiency (&eegr;
m
) attainable is given by
η
m
=
sin
c
2
(
m
/
p
)
=
[
sin
(
π
m
/
p
)
π
m
/
p
]
2
,
(
1
)
where p is the number of levels of the structure. Therefore, one notes that if a structure is blazed for 1
st
order (phase depth of 2&pgr;n and constructed with 16 levels, the diffraction efficiency one can theoretically achieve is 98.7%. Likewise if a structure is blazed for 2
nd
order (phase depth of 4&pgr;) and constructed with 32 levels, the diffraction efficiency one can achieve is also 98.7%. In other words, if a multi-level profile is such that each level represents &pgr;/8 phase, the efficiency is 98.7%. Similarly, if each level of the structure translates to &pgr;/4 or &pgr;/2 phase, the efficiency drops to 95.0% and 81.1%, respectively. To first-order, one can determine the efficiency of a multi-level microlens structure using Eq. (1) for a diffractive structure. For a surface-relief structure with a refractive index of 1.5 operating at the telecommunications wavelength of 1.3 &mgr;m, a physical depth of 2.6 &mgr;m corresponds to a phase depth of 2&pgr; according to
φ
=
2
π
λ
d
(
n
-
1
)
,
(
2
)
where &phgr; is the phase depth, d is the relief depth, &lgr; is the operating wavelength and n is the index of refraction of the substrate material (air is assumed to be the second medium). Therefore, if one has a microlens that requires a sag of 10.4 &mgr;m, the phase depth is 8 &mgr;n. With a 16-level structure, the element will only be 81% efficient at best (assuming no fabrication errors).
Such a microlens with additional levels in order to increase the optical efficiency, is impractical to attain. For a 99 percent efficient f/3 microlens operating at &lgr;=1.3 &mgr;m, the critical dimension (CD) of the surface features required is approximately 0.5 &mgr;m. This CD offers an extreme challenge in terms of achieving such features, let alone the mask-to-mask alignment tolerances required (generally one quarter of the CD). Since microlenses required for telecommunications and optical data storage can require f/2 and f/1 optical speeds, the use of multi-level mask and etch technology is therefore impractical for achieving the majority of the optical elements required.
Laser pattern generators, which have been proposed, have emphasized the production of binary masks. See, for example, S. Charles Baber, “Application of high resolution laser writers to computer generated holograms and binary diffractive optics,”
Holographic Optics: Optically and Computer Generated
, SPIE Proc.1052 66-76 (1989); E. Jäger, J. Ho&bgr;feld, Q. Tan
Emmel Peter M.
Morris G. Michael
Raguin Daniel H.
Barreca Nicole
Cole Thomas W.
Corning Incorporated
Huff Mark F.
Nixon & Peabody LLP
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