Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2000-01-04
2002-12-24
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C347S241000
Reexamination Certificate
active
06498685
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates primarily to microlithography, and more specifically to extreme ultraviolet (EUV) lithography. The invention is applicable to semiconductor lithography and lithographic patterning of microstructures such as micromechanical systems, micro-optics, porous membranes, etc.
EUV lithography is a leading candidate technology for next-generation semiconductor lithography, which will require sub-100-mn printing resolution. An industry consortium (the EUV Limited Liability Company) is focusing development efforts on an engineering approach that is basically an evolutionary extension of LW and DUV projection lithography, the main difference being that the EUV system requires all-reflective optics due to the lack of EUV-transmitting lens materials (Ref. 1). The basic system design has nine reflective surfaces between the EUV source and the wafer, including four condenser mirrors, the photomask, and four projection camera mirrors. Seven of the reflectors (including the photomask) comprise multilayer coatings that operate at near-normal incidence and are optimized for peak reflectance at a wavelength of 13.4 nm.
Mirror coatings are one of the key enabling technologies that have made EUV lithography possible. Presently, multilayer Mo/Si (molybdenum/silicon) multilayer mirrors routinely achieve peak reflectance efficiencies of 67.5% at the 13.4-nm operating wavelength, with a spectral reflectance bandwidth of 0.56-nm FWHM (Refs. 2, 3). Nevertheless, the lithography system's optical efficiency is significantly limited by the compound reflectance losses of seven multilayer mirrors operating in series.
The mirror surface figure tolerances scale in proportion to the operating wavelength, which is an order of magnitude smaller than DUV wavelengths at the 13.4-nm EUV wavelength. Furthermore, tolerance requirements for mirrors are generally much more stringent than lenses. Thus, the four EUV projection camera mirrors which image the mask onto the wafer must be fabricated to extremely tight (i.e., atomic-scale) absolute figure tolerances over large aperture areas.
The system uses multilayer reflective masks, which operate at 4:1 reduction. Mask fabrication presents significant technical challenges due to factors such as the short operating wavelength and small feature dimensions, lack of EUV pellicle materials, and very stringent registration requirements. EUV masks are expected to cost approximately $50,000 at 100-nm print resolution (Ref. 4).
Two research groups at the University of California at Berkeley and Stanford University are currently investigating maskless EUV lithography approaches (Refs. 5, 6). Their basic approach is to replace the mask with a spatial light modulator (SLM) comprising a programmable array of micromirrors whose reflectance properties can be varied by modulating the mirrors' tilt or translational positions. Although these are maskless systems, they nevertheless require EUV projection optics to image the micromirror array onto the wafer. Due to minimum size limits of the SLM pixel elements, the projection optics must operate at a very high reduction ratio, (e.g., 20 to 40× demagnification), which precludes the use of current-generation (4×) EUV projection optics.
Another limitation of micromirror systems is that the SLM must operate at a very high frame rate (e.g., multi-megahertz) in order to achieve requisite throughput requirements. (The system's total data throughput requirement is at least 10 bits per second.) This requires a continuous EUV source such as a synchrotron, and precludes the use of a pulsed EUV source such as a high-power, laser-produced plasma (LPP) source that is currently being developed by Sandia National Laboratories for EUV lithography (Refs. 7, 8).
Several recent patents and publications describe a variety of other SLM-based maskless lithography design concepts, some of which are applicable to EUV (Refs. 9-18). Most of these systems employ projection optics, but one of these inventions, a Fresnel zone plate microlens system (Refs. 16-18), eliminates the need for projection optics as well as the mask. The system is designed primarily for x-ray lithography at a wavelength of 4.5 nm, but the basic design principle could be applied equally well to EUV. The device comprises an array of zone plate microlenses that focus an incident x-ray (or alternatively, EUV) beam onto an array of diffraction-limited points on a wafer surface. Each microlens transmission is modulated by a micro-actuated shutter as the wafer is raster-scanned across the focal point array to build up a synthesized, high-resolution image.
Since the zone plate system does not use projection optics, it could conceivably use a very large number of zone plate lenses in multiple, large-area arrays, all operating in parallel. Based on printing throughput requirements, the required modulation frame rate would scale in inverse proportion to the number of lenses, so with a sufficiently large number of lenses the system could conceivably use an LPP EUV source operating at a moderate frame rate (e.g., 6 kHz, Ref. 8). However, the proposed method for manufacturing the zone plates lenses (spatial-phase-locked e-beam lithography) is not very practical for large-quantity production, and there are also other complicating factors that preclude the use of an LPP source with the zone plate system.
The plasma source in the LPP has a fairly large spatial extent (approximately 200 &mgr;m diameter, Ref. 7), and since each focus point on the wafer is a diffraction-limited image of the source, optical resolution would be limited by the source's geometric image size. Furthermore, the LPP would be used in conjunction with EUV mirrors that have a 2% combined spectral bandwidth, and the zone plate's chromatic dispersion within this band could also significantly limit focus resolution. The source's geometric image size on the wafer and the image's chromatic spread should preferably both be significantly smaller than the diffraction-limited image of an ideal monochromatic point source. This condition could be achieved, in principle, by using very small microlenses with short focal lengths; but the zone plate lens elements must be sufficiently large to accommodate the shutter mechanism, data paths, and supporting framework, while also maintaining an acceptably high aperture fill factor. Due to this aperture size limitation, a practical zone plate system would probably perform poorly with an LPP, and a more costly synchrotron source would thus be required.
Another limitation of zone plate lithography is that the printing resolution and contrast can be significantly degraded by extraneous diffracted orders. Continuous-profile, refractive microlenses such as those shown in Ref. 13 would not have this limitation, but for EUV application such microlenses would be either too highly absorbing or too small to be of practical utility when used in the mode described in Ref. 13.
SUMMARY OF THE INVENTION
The present invention overcomes the limitations of prior-art microlens printing systems by using multiple microlenses in series to focus exposure illumination onto a printing surface such as a semiconductor wafer. In a preferred embodiment of the invention, the microlenses are used in pairs, each pair consisting of first- and second-stage microlenses, respectively designated as lens L
1
and lens L
2
, wherein L
1
focuses illuminating radiation onto L
2
, and L
2
thence focuses the radiation to a focal point on the printing surface. Each such microlens pair forms a printer pixel, and multiple such pixels are combined to form a printhead, which focuses the radiation onto multiple focal points on the printing surface. A modulator mechanism modulates the focal points' exposure intensity levels, and as the points are modulated, the printhead is scanned relative to the printing surface (either the surface or the printhead, or both, can be moved for the scanning) to build up a synthesized, high-resolution exposure image on the print su
Mack Ricky
Townsend and Townsend / and Crew LLP
LandOfFree
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