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GALLIUMSILICAFILM WITH ARSENIC

Galliumsilicafilm with Arsenic is designed for use as a diffusion source for the simultaneous diffusion of gallium and arsenic into silicon. As shown in the table below, gallium diffuses more rapidly through silicon than arsenic does. In addition, gallium is not masked as easily by a thermal oxide as is arsenic. If windows are photo-etched in an oxide masked "N" type wafer, it is possible to carry out a simultaneous base and emitter diffusion. Of the several applications where this type of diffusion source would be particularly advantageous, the use for the preparation of high voltage power transistors is particularly apparent. In this application where deep diffused bases and emitters are required, one can realize considerable savings in furnace time as a result of the simultaneous diffusion process.

Diffusion Process

The diffusion coefficients and the surface concentrations of the gallium and the arsenic are shown in the table for one hour diffusion at the temperatures shown.


  1200oC 1250oC
  D (cm3/sec) Co (atoms/cm3) D (cm3/sec) Co (atoms/cm3)
Gallium 3.3x10-12 5x1018 6x10-12 3x1018
Arsenic 5x10-13 1x1020 1.5x10-13 7x1019



From this data one will obtain as a result of the simultaneous diffusion, a heavily doped "N" region near the surface and a less heavily doped "p" region beneath it.

A one hour deposition is employed to provide the base and emitter doping. A making oxide 4000 angstroms thick is sufficient to retard the arsenic and permit the doping of the base with gallium. In this diffusion process it is necessary to coin-stack the wafers to prevent loss of the gallium due to volatilization. The ambient should be N2 or argon since the presence of oxygen interferes with the gallium diffusion.

A typical diffusion process is as follows:

1. 4000 angstroms of SiO2 is grown on the silicon wafer.

2. Emitter windows are photo-etched.

3. Galliumsilicafilm with Arsenic is spun on the wafer at 3000 rpm.

4. The wafers are baked at 200oC for 15 minutes in air.

5. The wafers are diffused at 1200oC for 1 hour in N2.

6. The following results are observed:

Gallium:
Sheet resistivity---------------150 ohms/sq.
Junction depth-----------------5 microns


Arsenic:
Sheet resistivity---------------50 ohms/sq.
Junction depth-----------------1.3 microns


7. After the deposition the emitter windows are deglassed in 10% HF or buffered HF for 30 seconds to remove the diffusion source from the windows. This is necessary to prevent pitting in the emitter windows and in the base due to the precipitation of gallium oxide from the diffusion source during long diffusions.

After the deglassing the wafers are further diffused to achieve the deep emitter and base penetrations desired. For example, after 16 hours of diffusion at , one will observe a base penetration of 20 microns and on emitter penetration of 5.5 microns.

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