RESOLUTION LIMITS AND PROCESS LATITUDE OF X-RAY MASK FABRICATION

This article reports on the process latitude and resolution limits of x-ray mask fabrication by electron beam lithography for both additive (gold plated) and subtractive (etched tungsten) masks. The experiments were performed using a vector scan thermal field emission electron beam lithography machine with a maximum beam voltage of 100 kV and a minimum beam diameter of 27 nm. Features as small as 0.075 mum have been patterned on x-ray masks. Exposure latitude was studied as a function of beam diameter and acceleration voltage. The results indicate that forward scattering in the resist is a major cause of degradation in process latitude, and that going to a higher beam energy or thinner resist provides substantial benefits. For subtractive processes where a high-Z material (e.g., tungsten) underlies the resist, backscattered electrons are also a significant factor in limiting exposure latitude and resolution. For both substrate types, increasing the accelerating voltage from 50 to 100 kV can improve the exposure latitude by a factor of 2. At 100 kV the resolution limits are caused by the high aspect ratios found in the sub-tenth micron structures. As the aspect ratio exceeds 5:1 three major problems are encountered. First, the resist tends to fall over due to a lack of mechanical strength. Second, a noticable decrease in linewidth control occurs. Finally, the plating rate becomes dependent on the feature size, causing absorber thickness nonuniformity.