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BOROFILM TYPE B
Borofilm type B is a polymer solution containing boron which
is applied to silicon wafers by spinning. When the silicon wafer
coated with Borofilm type B is subjected to diffusion
temperatures, the polymer film decomposes to yield CO2,
H2O and B2O3. The B2O3
is the source of boron for the diffusion.
Specifications for Borofilm Type B
|Viscosity||115cp at 25oC|
|Boron Content||5.5% as B2O3|
|Metallic Impurities||Na < 1 ppm|
|Fe < 1 ppm|
|Cu < 1 ppm|
|Mn < 1 ppm|
Wafer treatment prior to spinning.
While Borofilm type B is not particularly sensitive to surface condition of the wafer, more uniform results will be obtained with a hydrophillic surface as is obtained by treating the wafer with hot
H2SO4 + H2O2 or hot nitric acid etch prior to coating.
The recommended spin speed for coating the wafer is 6000 thick films will exhibit cracking at the diffusion temperatures. Should such cracking be observed, higher spin speed which produces a thinner film should be employed.
After spinning the wafer should be bake at 150 - 200oC in air.
Borofilm type B is intended for applications requiring a rpm. Excessively deep diffusion with low sheet resistivity and high C0. Diffusion temperatures are in the range of 1200 to 1300oC.
A typical diffusion process is as follows:
|Results: Rs||0.5 ohms/square|
After diffusion the films are removed in conc HF solution. Should a boron stain be exhibited, this may be avoided by use of a thinner film, or by increasing the oxygen content of the ambient during diffusion.