Planarization and fabrication of sacrificial membranes across deep grooves in silicon by glass soot deposition

A new technique is presented that provides planarization after a very deep etching step (tens of micrometers) in silicon. This offers the possibility for film deposition, resist spinning, and film patterning across deep grooves or holes. Flame-hydrolysis-deposited fine glass particles first fill the grooves and extend onto the external surface of the silicon wafer. After the subsequent consolidation process (950°C), the glass soot is transformed into a transparent layer. Unlike the general expected result for films deposited on deep grooves, the densified glass layer was found to float upon the grooves with large voids near the bottom. The behavior can be interpreted by the sintering characteristics of glass soot deposition, where a large volume shrinkage factor of more than 95% (e.g., layer thickness changing from 420 to 20 μm) can be obtained due to the viscous flow of porous glass driven by surface energy reduction. Finally, the deposited glass layer can be polished or etched back to the silicon surface while the remaining layer forms a sacrificial membrane and planarizes the deep grooves without filling them.