Etching silicon dioxide in HF vapor is a quasi dry process. Due to the humidity in the HF vapor atmosphere a very thin water film is present on the wafer. HF is absorbed and etches the silicon dioxide (SiO2). During the reaction, Silane and water is produced. The Silane escapes in the gas phase. It is interesting to see that in this reaction water acts as an initiator and is produced by the process itself. In heating the substrate, the etch rate can be adjusted by controlling the amount of water on the surface. At etch rates of 4-6 um/hrs most structures can be released without sticking. The etching progress and homogenity are shown in the following pictures. Before test: Silicon wafer with 1 um thermal oxide After 25min: Typical rings appear. The large inner part of the same color indicates a homogenous etching After 30min: Some oxide is left After 35min: All oxide is removed on 80 % of the wafer diameter.
The etching progress and homogenity are shown in the following pictures.
|Before test: |
Silicon wafer with 1 um thermal oxide
|After 25min: |
Typical rings appear. The large inner part of the same color indicates a homogenous etching
|After 30min: |
Some oxide is left
|After 35min: |
All oxide is removed on 80 % of the wafer diameter