Thermal oxide is a building block film used to make
both simple and complex semiconductors. It is a good dielectric
or insulating film. Silicon Dioxide (SiO2) is one silicon atom and
two oxygen atoms. Silicon wafers enter the diffusion furnace and
are exposed to oxygen gas. After a layer of 500 Angstroms is grown,
the oxygen is no longer in direct contact with the silicon surface.
The oxide vapor migrates through the already existing layer of oxide
until it reaches silicon and the layer keeps growing from the bottom
up, contrast to CVD or PECVD process.
Two primary method of growing oxide is dry and wet oxide at 800-1200
deg. C. Wet oxides are grown using Hydrogen and Oxygen gases in
a internal or external torch. Thick oxides are usually found as
a "field oxide" isolating polysilicon, metals or other
conductive thin film(s) from the silicon wafer. Dry oxide is preferred
for layers less than 500 Angstroms in an RTP (Rapid Thermal Processor)
system. In dry oxide process only pure Oxygen is used. Dry oxidation
produces more uniform and denser thermal oxide with higher dielectric
strength than wet oxide, however it is a much slower growth process.
Dry oxides are primarily used for thin gate and capacitor oxides.
All oxides growth depend on the quality of the starting wafer
material. Resistivity (or dopant) uniformity across a wafer affects
the uniform growth of oxide. Thermal oxide grows faster in areas
of low resistivity (more dopant) and slower in areas of high resistivity
(less dopant). Uniform oxides means terrific "gate oxides"
and more stable and reliable devices.
