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GALLIUMSILICAFILM WITH BORON
Galliumsilicafilm with Boron is an ethyl alcohol
solution of a silica forming compound which yields on drying pure
SiO2 doped with gallium and boron. This solution is
designed for use as an open tube diffusion source to replace the
older ampoule diffusions. By including boron in the SiO2
layer, one obtains in one diffusion step, the high surface
concentration of boron atoms (> 1020) and the more
rapid penetration of gallium to reduce the long diffusion times
required for deep penetrations.
Galliumsilicafilm with Boron is applied to the surface of the silicon wafer by placing several drops of solution in the approximate center of the wafer and allowing the solution to wet out over the surface. When the silicon surface is hydrophilic, a drop of the dopant solution will spread in a uniform circle approximately one inch (1") in diameter. Four drops will cover a two inch (2") wafer. The time required to spread over the surface in 1-5 seconds. After the wafers are coated, the films may be hardened by heating at 200oC for 10-15 minutes in air. If the wafer surface is not hydrophilic, it may be made so by heating to temperatures in excess of 300oC for several minutes, or by soaking the wafer in NH4OH:H2O2 or H2SO4:H2O2 for 10-15 minutes.
Diffusion is carried out in either N2 or argon. The wafers should be coin stacked. It has been found that it is not necessary to coat both surfaces to achieve boron-gallium diffusion into both sides of the wafers. The uncoated side in contact with the coated surface will be doped by volatilization from the coated surface. Typical diffusion results are illustrated in the table below which shows the penetration achieved with Galliumsilicafilm with Boron compared with the penetration observed with Galliumsilicafilm without boron and with Borosilicafilm Co=1x1021.
DIFFUSION RESULTS WITH VARIOUS SOURCES
|Diffusion Source||Temperature oC||Time (HOURS)||Ambient||Xj (MICRONS)||Rs (Ohms/Sq.)|
|Galliumsilicafilm with Boron||1250||16||N2||44||1.5|
Removal of Films
After diffusion, the films may be removed with HF solution. If there is any boron staining, this is removed by oxidizing the wafers for 10-15 minutes at 900oC or higher to grow more SiO2. The boron stain is then removed in HF. Boron staining may be prevented by replacing the nitrogen atmosphere with air or oxygen in the last hour of the diffusion process.
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