Coatings for Ultraviolet Lithography

Research into an optical thin film coating that enhances the current design of lithographic excimer laser gratings operating at 193 and 157 nm has been performed. The goal of such research is to improve durability and reflective efficiency. Possible coating materials include the fluorides, magnesium fluoride, barium fluoride, lanthanum fluoride and calcium fluoride. Each fluoride material was deposited on calcium fluoride and silicon substrates and tested for a variety of physical, optical and coating properties. 'The results fiom percent transmission tests and ellipsometric analysis were then used to characterize the materials and determine the index of refraction and extinction coefficients for the coating materials. Finally, a system calibration was developed in order to coat a final mirror with an enhanced aluminum reflector. Background The driving forces behind optical lithography improvements lie in shorter wavelengths of the light source and increasing numerical apertures of the projection lens. When such improvements are made, microprocessor features can be reduced in size by great orders of magnitude. Moore's Law, which predicts that semiconductor linewidth will halve every two to three years is currently in danger of becoming obsolete. Unless new technologies are developed which allow lasers to operate under extreme ultraviolet wavelengths, an alternative to optical lithography may soon become necessary. Currently, lasers operating under 193 nm are emerging from research labs and are beginning to operate within industry. 157 nm lasers are currently in development and pose an even greater challenge because of the millions of pulses of ultraviolet energy that each grating must endure (3). Such a high volume of pulses can cause failures in laser parts such as the gratings used in narrowing the spectrum to 193 or 157 nm. Gratings used in excimer lasers for lithographic purposes are exposed to millions of pulses in relatively short periods of time. The challenge is to build a grating that can withstand continuous pulses of high energy with a minimum of absorption. Introduction The challenge in creating an effective optical thin film coating lies in the fact that relatively few coating materials exist which can withstand the continuous bombardment of ultraviolet light that is required in lithographic applications. A layer of aluminum, protected by a layer of magnesium fluoride, currently coats the grating in excimer lasers. While the current grating is substantially effective as an ultraviolet reflector, a greater level of durability and reflectivity is desired. Zn order to increase grating durability and reflective efficiency, optical thin film coatings will be applied to the grating in alternating layers of materials of high and low refractive indices. Fluorides have been selected as possible materials to test because of their low absorption at 193 and 157 nrn. An additional desirable quality lies in their low ultraviolet cut-off, which is at or below 140 nm (11. Fluorides also posses relative durability when exposed to continuous high-energy pulses. Disadvantages to using fluorides lie in their poor deposition properties. It is often advantageous to use heat or ion assistance to pack down the coating, thereby reducing porosity and increasing physical durability. Unfortunately, this process creates dificulties in coating gratings for industrial applications. As a result of this, the methods described above were not utilized. It should be noted that oxides were considered as a possible coating material because of their desirable refractive indices and better deposition properties, however, because of their high absorption at ultraviolet wavelengths, they were disregarded. After the final coatings are characterized, they will be deposited on a substrate in alternating quarterwave layers of high and low indices of refraction. A quarterwave is defined as the index of refraction, n of a coating, multiplied by its physical thickness, d, or