Borosilicafilm C_o=1x10²⁰ yields an SiO₂ layer doped with boron in sufficient concentration to yield a surface concentration in silicon of 1x10²⁰ atoms for diffusion penetrations up to 5 microns. Thereafter, the source no longer adds additional boron to the silicon and further diffusion into the silicon proceeds via a Gaussian process. This situation holds for wafers which are stacked vertically in the diffusion boat. when the wafers are coin stacked, the diffusion process will be of the erfc type for penetrations up to 20 microns.

Borosilicafilm C_o=1x10²⁰yields, after heat treating as described below, a pure glassy film consisting only of B₂O₃:SiO₂. No surface pitting will be observed on the silicon or in the SiO₂ layer which may be used as a diffusion mask. The glassy films will be very uniform in thickness and of extremely low porosity. Diffusion fronts which result will be flat with no diffusion spikes or non-uniformities. Under proper processing conditions the sheet resistivity over the wafer surface will be uniform to -2%. In addition, the silicon wafers when subjected to suitable etches, will exhibit no slip lines or etch pits characteristic of large concentrations of boron in silicon.

Processing Recommendations

When the solution is fresh it may be used without filtration. Should there be an accumulation of particulate matter in the solution, it may be filtered through a 1 micron membrane filter in the same manner as photo-resist is filtered. However, caution is advised to be sure that the membrane filter is insoluble in ethyl alcohol which is the solvent for the solution.

Borosilicafilm C_o=1x10²⁰ is designed to be applied by spinning. When spun at 3000 rpm and baked at 100^oC for 1 hour, a film will result which is approximately 2000 angstroms thick. After diffusion in a non-oxidizing atmosphere a film thickness of 1200-1500 angstroms will result. While there is great latitude in the baking temperature, after spinning it is best to subject the wafers to 100^oC heat treatment to prevent pinholes form appearing in the film. After spinning the film will be soft and this mild heat treatment will remove the solvent slowly and eliminate pinholes.

Diffusion

The diffusions should be carried out in nitrogen or argon with a few percent of oxygen. The following diffusion results are typical of what will be observed. The films were applied to 10 ohm cm."N" type silicon wafers oriented 1-0-0, the spin speed was 3000 rpm, and the wafers were heated at 100^oC for 1 hour in air prior to diffusion.
　

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