PROCESS TECHNOLOGIES FOR TA/SIC X-RAY MASKS

This paper describes process technologies for an x-ray mask using Ta as the x-ray absorber and SiC as the x-ray membrane. SiC can be grown heteroepitaxially in a coldwall LPCVD reactor at 1000°C and 3.5 torr using a gas mixture of SiHCl3, C3H8, and H2. SiC has a stress of 4 × 109 dyn/cm2 and a Young's modulus of 4.7 × 1012 dyn/cm2. Pure Ta with Ar ions implanted by 150 keV is more stable and dense than Ta alloys and their nitrides. The film density is about 16 g/cm3 and the stress change due to annealing of 200°C is below 2 × 108 dyn/cm2. The yield of stress control is 68% for 2 × 108 dyn/cm2. The temperature of the membrane during Ta RIE is controlled by He cooling from the back. It is held between 45° and 100°C at a power density of 0.8 W/cm2. A mixture of Cl2 and CC14 (1:1) is used as the etching gas. The gas pressure is from 0.18 to 0.2 torr. The etch rate of Ta is 1.2 µm/min and the selectivity of Ta to a resist (AMS-1) is 7. A 0.8 µm thick Ta pattern of 0.15 µm lines and spaces is obtained by using a 0.5 µm thick single-layer resist. Electron beam lithography on a thin membrane requires a high-contrast resist (AMS-1) and a well-controlled correction of the proximity effect because of the high electron backscattering on Ta. Si etchback is done by spray etcher using a mixture of HF and HNO3 (1:3) with a high speed of 8 min per mask. The total fabrication process is designed to minimize mask distortion. The maximum mask distortion due to the absorber stress is 0.11 µm (3σ) including the measurement errors of Nikon 21. The mask used for the measurement had a circular membrane of 60 mm diam and a 32 × 32 mm field window. The SiC was 2 µm thick and the Ta was 0.8 µm thick with a stress of 1.4 × 108 dyn/cm2. © 1990, The Electrochemical Society, Inc. All rights reserved.