Steven G. Colbern

Porous silica pellicle frame

Two main issues with current pellicle frames are: (1) thermal expansion mismatch between the anodized aluminum frame and the photomask, and (2) the lack of porosity for purging and contamination control. Both issues can be addressed by using a sol-gel-derived porous silica frame. The silica frame has essentially the same thermal expansion coefficient as the fused silica photomask substrate. The porous nature of the silica frame provides contamination control by N2 purging and scavenging capability. The porosity characteristics and mechanical properties of the frame material were determined. Porous silica frame was successfully mounted onto quartz plate by a commercial process, suggesting the suitability of using porous silica as pellicle frame material. The sol-gel derived porous silica represents the first proof-of-concept for an alternative frame material with a potentially significant impact on the photomask industry.

Sol-gel fabrication of high-quality photomask substrates for 157-nm lithography

Modified fused silica is a strong candidate material for photomask substrates in 157-nm lithography. Such a material must possess improved transmission and birefringence characteristics compared to conventional photomask substrates. Although there has been some success in producing modified silica using chemical vapor deposition, substantial improvements in quality and cost are desirable. A novel sol-gel based technique to inexpensively produce high quality 157-nm photomask substrates is being developed to address these issues. The complex relationships between glass properties and glass forming parameters were determined. Methods to improve the ultraviolet transmission at 157-nm were established, and modified silica without striae or optical defects were developed. Glasses produced using the sol-gel process displayed low birefringence and good homogeneity. With further improvement in transmission at 157 nm this glass may demonstrate technical and cost superiority to commercially available 157-nm photomask substrates.

Sol-gel fabrication of high-quality photomask substrates

Synthetic silica photomask substrates are currently manufactured by cutting from glass boules, which are prepared using a flame hydrolysis process. An alternative technique based on sol-gel processing demonstrates several potential advantages in fabricating high quality substrates. This new approach allows near net shape fabrication of synthetic silica photomask substrates, eliminating the need for cutting and grinding. The complex relationship between glass properties and process parameters in the formulation, drying, and sintering steps has been determined, and a repeatable process has been established. The resulting substrates meet all SEMI specifications for ultra-low thermal expansion (ULTE) photomasks for 248-nm lithography. The technology may also be extended to 193-nm and 157-nm photomask substrates. This sol-gel-based process may represent a unique and cost-effective alternative for manufacturing photomask substrates for deep UV lithography.