Excimer lasers recently have been used to perform materials processing in a variety of applications in order to produce novel structures and devices previously difficult or impossible to manufacture. For example, the high powered, high repetition rate and short wavelength excimer lasers have been employed in semiconductor etching as disclosed by F. A. Houle in the article entitled "Basic Mechanisms in Laser Etching and Deposition", Applied Physics A, 41 315 (1986), M. D. Armacost et al. in their article entitled "193-nm Excimer Laser Assisted Etching of Polysilicon", Materials Research Society Symposium Proceedings, 76 147 (1987) and in the article by Y. Horiike et al. "Excimer-Laser Etching on Silicon", Applied Physics A, 44 313 (1987). In addition to semiconductor etching, excimer lasers also have been used in laser doping as reported in the article by P. G. Carry et al. "Fabrication of Submicrometer MOSFET's Using Gas Immersion Laser Doping (GILD)", IEEE El Dev. Lett., EDL-7 440 (1986) and to stimulate alloy growth as discussed in the article by J. R. Abelson et al. "Epitaxial Ge.sub.x Si.sub.1-x /Si(100) Structures Produced by Pulsed Laser Mixing of Evaporated Ge on Si(100) Substrates", Applied Physics Letters 52 230 (1988). Furthermore, certain metalization processes have benefited from the use of excimer lasers as discussed by M. Rothschild et al. in their article "Visible-Laser Etching of Refractory Metals by Surface Modification", J. Vac. Sci. Technol. B, 5 1400 (1987) to name a few applications. A common requirement for these processes is to overcome the non-uniform intensity profile of the excimer beam. These non-uniformities tend to produce detrimental effects in the end product such as rough edge profiles, non-uniform doping profiles and non-uniform alloy concentrations.
Typically, an excimer laser beam without homogenization has an intensity profile much like that shown in FIG. 1. A top view of this intensity profile has, generally speaking, a rectangular spatial pattern, see FIG. 2. This rectangular spatial pattern when combined with the non-uniform intensity profile makes such a composite beam pattern incompatible with numerous materials processing applications.
Several schemes are currently in use which bring the intensity profile into a, more or less, more spatially uniform distribution. One scheme utilizes a segmented mirror which chops the light into roughly 35 pieces and overlaps the images. This produces a uniform central region with a high intensity border that requires cropping. Several additional optical elements are required to fill the effective area of the segmented mirror and image the light after passing through a cropping aperture. Unfortunately, this procedure produces very high energy losses due to the large number of optical elements and its sensitivity to variations in the excimer laser beam divergence to deliver, homogenize, reshape and focus the excimer beam. Using the segmented mirror scheme requires six mirrors and an aperture. Assuming the most efficient coating (UV enhanced aluminum) at 248 nm, results in approximately 12% absorption losses at each mirror. Divergence losses have been measured higher than 10% at each mirror due to the angular dependence of the optical coatings. The aperture contributes roughly 10% losses in such a set up so that the overall loss is at least 78% of the original excimer laser beam energy.
An alternate scheme utilizes a light tunnel with a square aperture into which the excimer beam is focused. The multiple reflections from each internal surface of the light tunnel cuts the beam into many segments which overlap at the exit aperture. The number of reflections in the light tunnel is determined by the focal length of the launching optics. In this case there is a trade off between the high optical homogeneity requiring many reflections and intensity losses. However, to obtain satisfactory intensity uniformity, at least four reflections are required within the light tunnel resulting in at least a 12% loss at each reflection off the alluminized interior. Adding these losses to an additional 0.5% loss at each AR coated surface of the launching and imaging optics, the total laser energy loss amounts to at least 42%. Again, additional optical elements are required to launch and extract the light from the homogenizer.
Neither of these homogenization schemes easily accommodates custom shaped spatial patterns. As a consequence, additional efforts and optical devices must be relied upon to give a designer the necessary latitude to perform a particular materials process application.
Thus, there is a continuing need in the state of the art for an apparatus and method to homogenize the intensity profile of an excimer laser which relies upon the collection of the excimer laser beam by an optical fiber bundle array and the intermixing of the light from the individual fiber cores to produce the uniform intensity profile without a multiplicity of lenses and precisely arranged reflection surfaces with coatings.
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