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BOROFILM 100

Borofilm 100 is a formulation which yields very uniform boron diffused layers in silicon. It can be used as a direct replacement for BBr₃, etc. in the conventional deposition-deglaze-drive-in-diffusion process for base-resistor structures. For isolation diffusions, Borofilm 100 can be used in a simple one-step process. When properly applied, one will obtain uniformity in sheet resistivity over the wafer surface and from run to run of +/- 1-2%. Borofilm 100 is free of heavy metal contaminants and will yield excellent p-n junctions.

Borofilm 100 will form a thin very uniform film, free of ridges or other structures. Variations in relative humidity will not affect the results one obtains with Borofilm 100. The solution may be applied over wafers with windows in the SiO₂ diffusion mask that vary widely in dimension. One will observe no difference in junction depth or sheet resistivity from the smallest aperture to the largest.

APPLICATION

Borofilm 100 is supplied free of particulate matter. However, continual use with frequent opening and closing of the bottle will allow particles to accumulate in the solution. These may be removed by filtering with membrane filters or by centrifuging for 15 - 20 minutes at 3500-5000 rpm. If filtration is used for removal of particulate matter, Solvinert membrane filters of 0.45 micron pore size supplied by the Millipore Corporation or silver filter membranes supplied by the Selas Corporation should be used.

For best results the silicon surface should be hydrophilic. Any of the conventional surface treatments commonly used in silicon wafer processing to achieve a hydrophilic surface may be used, i.e., hot nitric acid, hydrogen peroxide + H₂SO₄, or chromic acid-sulfuric acid. It is advisable not to expose the silicon surface to HF solutions after the hydrophilic treatment and prior to coating.

Borofilm 100 may be applied by spinning with any commercially available spinner. Since Borofilm 100 is not corrosive it will adapt easily to any automatic applicator device supplied with these spinners. To coat the wafers, several drops of solution are applied to the center of the wafer and the spinner is started. One will observe film formation starting from the center of the wafer and extending outward as spinning progresses. Spin time is sufficient when a stable non-varying interference color is observed in the film. This time is approximately 20-30 seconds. About 4 drops of solution will coat a 2" wafer, 10 drops will coat a 3" wafer and 15 drops is sufficient for a 4" wafer.

Baking
Excess solvent may be removed from the film by baking at 200^oC for 15 minutes in air. Alternatively, one may allow the wafers in the diffusion boat to dwell in the mouth of the diffusion furnace for 15-20 minutes prior to diffusion. With large diameter wafers, where it is common practice to slowly insert the wafers into the diffusion zone of the furnace to minimize wafer warpage, a baking interval may not be necessary depending upon the insertion rate. Since there is great variation in thermal gradients for various furnaces and diffusion temperatures, a few experiments will quickly determine if baking prior to diffusion is necessary.

Two criteria will determine this:

1. Smoking of the wafers as they enter the diffusion zone.

2. Sheet resistivity variation across the wafer greater than plus or minus 2%.

The former indicates that excess solvent has not been removed. The second criterion indicates that there are still remnants of solvent in the film which are not apparent by smoking, but are sufficient to yield a wider spread in sheet resistivity over the wafer than will be obtained with proper solvent removal.

DFFUSION

Wafers may be diffused with their surface either parallel or perpendicular to the gas flow. Table I lists sheet resistivity and junction depths one will obtain for the temperatures and times shown.

Table II indicates the variation of sheet resistivity values obtained when various N₂:O₂ mixtures were the diffusion ambients. Note that here is some sensitivity to oxygen concentration in the ambient and that oxygen concentrations in excess of 20% should be avoided in that the uniformity of the sheet resistivity is impaired.

Film Removal and Clean-up
Films, applied to wafers, and prior to diffusion may be removed by soaking in H₂O. The film observed on the wafer after diffusion is removed in 10% HF solution.

Apparatus used in the application of Borofilm 100 may be cleaned by washing in warm water.

TABLE I

SHEET RESISTIVITIES AND JUNCTION DEPTHS YIELDED BY BOROFILM 100

TABLE II

EFFECT OF AMBIENT ON SHEET RESISTIVITY

Diffusion Temperature-950oC
Time - 60 Minutes
Spin Speed - 3000 rpm

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