Development of 248nm AR Coatings for Photolithography Applications

Abstract

A key application for thin film coatings, minimizing Fresnel reflections from each surface is critical to achieving high throughput for multi-component, transmissive optical systems. Assuming small, plano optics, reflection from a single laser-line operating near normal incidence can be significantly recovered using a relatively simple antireflection design. Limited in low-reflection bandwidth however, these simple designs increase quickly with angle and/or small amounts of non-uniformity, and become inadequate for large-diameter, fast systems. At ultraviolet wavelengths, the demand for throughput becomes even more challenging, driving system designs to larger apertures and faster, steeper elements. Unfortunately, many thin film materials begin to absorb strongly in the UV, limiting the number of high index options in particular. In addition to inherent material absorption, UV wavelengths are considerably more sensitive to other sources of loss such as scatter and contamination, which must therefore be minimized in combination with reflection. In this study, we studied the trade-offs between low-reflection bandwidth and absorption. By maximizing index contrast, a wider band could be achieved, but often at the expense of absorption from high index materials. A survey of materials was also conducted, where the influence of process energy on contamination was examined. Secondary attributes such as mechanical durability, UV laser damage resistance, and homogeneity were also considered. Once optimized, attention would turn toward multi-layer design and manufacturability, where development results were scaled into a robust and stable multi-layer process. In an effort to reduce the amount of bandwidth consumed by the process, sources of both random and systematic errors were addressed, uniformity minimized, and process repeatability maximized. Conversely, once angle shift and non-uniformity were accounted for, AR bandwidth determined the amount of error the design could tolerate, and thus the level-of-effort required to stabilize the process. The final combination of materials and processes achieved a preliminary loss (reflection + absorption) of 25% through 42 spherical and aspherical surfaces, exhibiting very high laser damage resistance and mechanical durability. Once demonstrated, the process was then applied to two additional systems, improving in performance with each iteration, and ultimately achieving a minimum loss of <23%.