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Aluminumsilicafilm is a solution which yields a film of SiO2 doped with
aluminum. As a diffusion source, the film yields a surface concentration of 1x1017
Al atoms/cm3 in silicon.
1. Aluminumsilicafilm is applied by spinning at 3000 rpm for 10-15 seconds.
2. After spinning the film is densified by heating the wafers at 150oC for 45 minutes in air.
The most critical factor in achieving reproducible results with Aluminumsilicafilm, as with all Al diffusions, is the exclusion of O2 from the ambient during the diffusion process.
It is well known that the presence of O2 during diffusion results in the growth of an SiO2 film between the source and the silicon surface. This nascent SiO2 film acts as a sink for Al and interferes with the diffusion process to the extent that poor reproducibility will result. The preferred ambient is dry, oxygen free, argon. Forming gas is also adequate. For open tube diffusion, the tube must be well purged with argon, and the access port of the diffusion tube must be designed to prevent the random introduction of air when the wafers are pushed into the diffusion zone. When care is taken to exclude O2 or H2O, excellent reproducibility in diffusion characteristics will result.
Listed below are typical diffusion results obtained with Aluminumsilicafilm. The silicon wafers were phosphorous doped with a resistivity of 20 ohms-cm.
|Diffusion Temp. oC||Time-min.||Rs(Ohms/square)||Xj (microns)|
4. After diffusion the film is removed in dilute HF solution, which leaves a clean surface with no insoluble residues.
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