The semiconductor industry uses masks for photolithography techniques to form microscopic or sub-microscopic circuit elements such as integrated circuits. In photolithography, a semiconductor substrate is covered with a photoresist that reacts to exposure to radiation. Radiation from a source is focused onto the photoresist through a patterned mask. The mask typically has a mask substrate, a patterned later and a protective covering layer known as a pellicle. The pattern on the mask corresponds to a portion or layer of the desired integrated circuit. Portions of the photoresist that are exposed to the radiation react with light such that they are either easily removed (for a positive resist) or resistant to removal (for a negative resist), e.g., by a solvent. After removal of portions of the resist, a reduced image of the mask pattern is transferred to the photoresist. Portions of the substrate may then be etched through openings in the pattern on the photoresist. Alternatively, material may be deposited on the substrate through the openings in the photoresist. The size of the features on the photoresist pattern is limited by diffraction. As successive generations of integrated circuits require smaller and smaller circuit features, shorter wavelengths of radiation must be used. The use of shorter wavelengths can have an undesirable impact on the material used as the mask substrate.
Nano-imprint lithography is based on embossing adapted to the needs of semiconductor processing. Nano-imprint lithography is essentially a micromolding process in which the topography of a template patterns a photoresist on a wafer. In photolithography, by contrast, the resist is patterned by optical exposure and development. Unlike photolithography, imprint lithography does not use reduction optics. Instead, the size of the template determines the size of the pattern. Thus masks for nano-imprint lithography are often referred to as 1× masks. One advantage of nano-imprint lithography is that the parameters that limit resolution in classic photolithography (including wavelength and numerical aperture) do not apply. Nano-imprint lithography resolution is typically limited only by the resolution of the template fabrication process.
Calcium Fluoride (CaF2) is a crystalline material that has been proposed for future 157-nm photolithography applications due to its preferable transmission characteristics at that wavelength. Specifically, the presence of water in the mask substrate interferes with the transmission of the 157-nm radiation. CaF2 masks were proposed since they provide a substantially water-free environment. Unfortunately, photo masks for 157-nm photolithography require a quartz pellicle since the 157-nm radiation tends to destroy the polymers commonly used as a pellicle material. Thus CaF2 masks for 157-nm photolithography are relatively expensive compared to conventional photo masks. Development programs for 157-nm lithography have been stopped and engineering development resources focused on 193-nm immersion photolithography using fused silica mask substrates.
Amorphous fused silica is widely used today for mask substrates for photo lithography and is used currently in develop of 1× masks for nano-imprint lithography. The primary advantages of fused silica are its cost and transmission properties for photolithography down to 193-nm, wavelengths. However, when fused silica is etched, the etch depth can lack uniformity due to the amorphous structure of fused silica. When fused silica masks are etched at 193-nm etch depths, as they are for 193-nm photo-lithography alternating phase shift (altPSM) or chromeless phase lithography (CPL) mask designs, the non-uniformity of etch depth will change the phase characteristics of the mask and cause non-uniformity of the line width on the resist wafer. This in the end will change device performance because the transistor level gate lengths will have non-uniformity across the layout of the device. Also, fused silica is used as a 1× mask substrate for nano-imprint lithography. At 1× lithography the mask features are typically four times smaller than photo masks used in 4× reduction photolithography. These nano-imprint masks are patterned by etching the features onto the mask surface. These features are much smaller than traditional photomask features, and thus are susceptible to etch non-uniformity caused by voids and nano-fractures in the amorphous fused silica. This results in high densities of point defects on line edges and edge roughness on these small 1× structures on nano-imprint masks.
Thus, there is a need in the art, for lithography masks that overcome these disadvantages.