Development and study of nano-imprint and electron beam lithography materials for semiconductor devices
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Step and Flash Imprint Lithography (SFIL) is a next generation lithography option that has become increasingly attractive in recent years. Elimination of the costly light sources and optical elements in current exposure tools makes SFIL a serious candidate for large-scale commercial patterning of sub-50 nm features. The imprint resist material is one of the key components in the SFIL process, and it has several design requirements, including low viscosity, low volatility, rapid reaction rate, high mechanical strength, low adhesion to the template, high adhesion to substrate, and high oxygen etch resistance. It is quite challenging to find materials that meet all the material requirements. Traditionally, acrylates have been the monomers of choice for use for Step and Flash Imprint Lithography (SFIL) etch barrier formulations, in part because of the commercial availability of silicon-containing acrylates (necessary for etch resistance), together with their low viscosities and capability for rapid photopolymerization. However, despite many desirable properties, the polymerization of acrylates via radical chain propagation causes some potential issues in the SFIL process as a result of the inhibition of these processes by oxygen. Vinyl ethers are prime candidates to replace acrylates. Their curing proceeds by a cationic mechanism that is insensitive to oxygen and very rapid, while the vinyl ether group contribution to viscosity is significantly lower than that of an acrylate. Silicon-containing vinyl ethers are not widely commercially available and so were synthesized for this study. As expected, formulations based on these vinyl ethers were lower viscosity and faster curing than the acrylate etch barrier formulations presently employed, while the tensile strength of cured vinyl ether formulations were found to be higher than their acrylate counterparts. The throughput of SFIL can be improved by lowering the viscosity of the imprint material and reducing the drop size. Decreasing viscosity generally increases the volatility of a material, and decreasing drop size increases the area available for evaporation. The rate of evaporation can be predicted based on the methods of LeeKesler and Joback-Reid that employ group contributions. These predictions were used to explore the effect of drop size (200 nl, 1 nl, 80 pl) on evaporation rate at 20 ºC for various acrylate and vinyl ether monomers. The predicted rates correlate well with experimental values. The photopolymerization of acrylate and vinyl ether monomer systems is exothermic when they are photocured. If the imprint process were adiabatic, the heat generated during polymerization could increase the temperature of the material to greater than 300 oC, possibly resulting in material degradation and image distortion. A finite element method was used to analyze the temperature profile during photopolymerization for non-adiabatic conditions. The heat from UV absorption is negligible because the polymers are transparent at the exposure wavelength and the loading of photo acid generator and photo radical initiator is low. This model indicates that the temperature increase from polymerization is very small (less than 0.05 oC) due to the rapid heat transfer from the curing material to the silicon wafer. Vinyl ethers generally require larger separation forces to remove the template after photocuring than do acrylates, and to reduce the separation force, various reactive and nonreactive fluorinated additives were introduced into the vinyl ether formulations. With a 2 wt% addition of methyl perfluorooctanoate (FA), the separation force of the cured vinyl ether was reduced to half that of cured vinyl ether without FA. BVMDSO (1,1,3,3-tetramethyl-1,3-bis(vinyloxymethyl)-disiloxane) has the lowest viscosity of the synthesized silicon containing vinyl ethers that meet the volatility requirement for an 80 pL dispense volume. A formulation comprised of BVMDSO, CHDVE (cyclohexanedimethanol divinyl ether) and TEGDVE (triethylene glycol divinyl ether) containing a small amount of surfactant shows good tensile strength and modulus and accurately prints 30 nm lines with low separation force. Resist materials with a thermally activated solubility switch offer a potential method to eliminate such proximity effects, and Hydrogen Silsesquioxane (HSQ) has been studied both as an electron-beam resist and as a thermally cured low dielectric material. However, as beam spot size shrinks, resist heating also drops significantly. At small feature sizes, it becomes increasingly difficult to obtain the temperature rise required for thermal switching to occur.