Photoresist process latitude optimization for synchrotron x-ray lithography

Abstract

Acid catalyzed positive tone APEX photoresist has been examined for exposure using the Helios compact synchrotron x-ray source at the IBM Advanced Lithography Facility (ALF) in East Fishkill, New York. A four factor Taguchi optimization was implemented to test the effects of post exposure bake (PEB) temperature, PEB time, post apply bake (PAB) and develop time. The experiment was split into two tests; one for maximum process latitude and the other for attaining the target critical dimension (CD) of 350 nm. CD line widths were measured in resist using a top down scanning electron microscopy. The experimental analysis of variance indicated that 79% of the CD variation could be attributed to the PEB (temperature + time) and 12% from the PAB temperature/develop time interaction. Both tests converged on the same process conditions for maximum process latitude and target CD. These parameters reduced the thermal dose (PEB) in exchange for higher x-ray exposure. Line width uniformity data across 8' wafers will be shown for 200 nm structures and some additional results down to 125 nm using a 40 micrometers gap. Thermal infrared analysis of PEB hot plate uniformity was shown to be within specification but sensitive to backside wafer contamination. A novel 'net dose' exposure wedge which is built on the x-ray mask provided a record of the combined PEB/x-ray dose and did track with the thermal IR data.