Resist-less patterning on SiO2 by combination of X-ray exposure and vapor HF etching

We succeeded in a resist-less patterning of SiO₂/Si substrates by a combination of X-ray exposure and vapor hydrogen fluoride (HF) etching. A 2 μm thick SiO₂ layer was formed on a Si substrate by employing a thermal oxidation process. An X-ray mask consisted of a 1 μm thick Ta absorber on a 2 μm thick Si₃N₄ membrane mounted on a 1 mm thick Si frame, and a honeycomb pattern where 640 nm diameter circle dots arranged in the corners of a hexagon with a pitch of 960 nm was processed. X-ray exposure experiments were carried out on a beamline BL-4 with a peak photon energy of 2 keV at the TERAS synchrotron radiation (SR) facility. When a dose energy was 750 mAh, the transfer of the patterns was confirmed, although irradiations with different dose energy were also conducted. Moreover, heating temperatures and total etching times of SiO₂/Si substrates in vapor HF etching were changed, and the shapes of etched patterns were observed by scanning electron microscope. It was learnt that an appropriate etching time existed between 30 and 60 min. Moreover, we observed discoloration of irradiated area by SR; and this seemed to be caused by changes in the etching rate of SiO₂/Si substrates that led to the development of resist-less patterning technique.