Maskless, microlens EUV lithography system

#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.

#A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.

#FIG. 1 is a cross-sectional schematic view of a printer pixel comprising two EUV microlenses and a modulator;

#FIG. 2 is a cross-sectional schematic view of a printhead comprising multiple pixels;

#Following is a general overview of the system design concept, in a preferred embodiment. Later sections provide more detail on the system architecture, operational principles, methods of manufacture, and alternative embodiments.

#FIG. 1 is a cross-sectional, schematic illustration of a printer pixel 101. The pixel comprises a first-stage microlens L1, a second-stage microlens L2, and a modulator 102, which may be a digitally controlled, micromechanical shutter 103a. The lenses are preferably composed of molybdenum. Although the lenses function as focusing elements, each lens has a meniscus form (i.e., it is thinner at the center than at the edge) because the refractive index of molybdenum is less than 1 at EUV wavelengths. A collimated incident EUV beam 104 is focused by lens L1 toward a point 105 at the center of lens L2's aperture, but the diffraction-limited focus spot (illustrated by the amplitude profile 106) actually overfills the lens aperture. (In this design the illumination source's geometric image at point 105 is much smaller than the diffraction-limited spot size; although the geometric image could dominate the diffraction limit in alternative embodiments.) Lens L2 further condenses the beam onto a point 107a on the surface of a wafer 108. The diffraction-limited, wafer-plane focused amplitude distribution is illustrated as profile 109. The respective clear aperture diameters D1 and D2, and nominal focal lengths F1 and F2 of lenses L1 and L2 are D1=5 μm, D2=1 μm, F1=171 μm, and F2=3.25 μm. (More detail on the lens design is provided in §2.)

#The “focusing” action of the microlenses is more analogous to that of a condenser lens than of an imaging lens, in that each microlens functions to condense incident illuminating radiation onto a target illumination spot. Unlike conventional imaging systems, the microlenses do not necessarily have well-defined focal planes, or if they do, they need not operate at exactly their design focal lengths. For example, the wafer plane is positioned slightly above lens L2's design focal plane in the present design because the diffraction-limited focus is not actually optimum at the design focal plane (as defined by geometric optics). Furthermore, lens L1 is not operated at its best (diffraction-limited) focus position because this would (according to optical simulations) significantly increase sensitivity to lens aberrations. (Tolerance issues are addressed in §11.)