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To achieve the lowest surface concentration with greatest uniformity a two-step diffusion process has been found to be the most feasible. This process consists of a deposition at low temperature followed by a deglassing of the phosphorous doped glass dopant film and a drive-in at higher temperature. It can be shown that the Co resulting from this process is given by the following expression:

Co2 =

Co1 1.13 (D1t1) 1/2
(3.14 D2t2) 1/2

Co2 = final Co
D1 = Diffusion coefficient at low temperature
t1 = time of diffusion at low temperature
D2 = Diffusion coefficient at high temperature
t2 = time of diffusion at high temperature
Co1 = Co of low temperature diffusion

As a typical example the following process was carried out:
Doping Solution ------- Phosphorosilicafilm Co = 5x1017
Spin Speed ------------ 3000 rpm
Bake Temperature ------ 200oC 15 minutes in air
Deposition Diffusion -- 1000oC
Atmosphere ------------ N2
Time ------------------ 1 Hour

Results: Sheet Resistivity----- 9000 ohms/square
Deglass in 10% HF solution ---- 2 minutes
Drive-in Temperature----------- 1200oC
Atmosphere------------- N2:O2 4:1
Time ----------------- 1 Hour

Results: Sheet Resistivity----- 4000 ohms/square
Junction Depth-------- 2.6 microns
Co---------------- 1x1016 atoms/ cm3

The following table lists the temperature below which the various Phosphorosilicafilm solutions will not dope:

Concentration Minimum Temperature (oC)
Phosphorosilicafilm Co = 5x1016 1150
Phosphorosilicafilm Co = 5x1017 1000
Phosphorosilicafilm Co = 5x1018 900

The reason there exists a minimum doping temperature is believed to reside in the inability of phosphorous to penetrate the thin SiO2 layer which always exists on the wafer surface while standing in air.

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