EMULSITONE COMPANY
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BOROFILM

PHOSPHOROFILM

Borofilm and Phosphorofilm are solutions of boron and phosphorus containing polymers in water. When these solutions are applied to the silicon surface as described below, a film forms in intimate contact with the silicon. When the films are heated to the doping temperature, a thin glass-like oxide forms consisting of the doping element dispersed in an Si0² matrix. This oxide is the source of the doping element for surface doping. Where an Si0² film separates the boron or phosphorus from the silicon, the silicon surface will receive no doping elements. Oxides as thin as 1000A are sufficient to shield against surface doping.

CONTROL OF SURFACE CONCENTRATION

The surface concentration and sheet resistance achieved with Borofilm and Phosphorofilm depend only upon the deposition temperature. Table I lists typical sheet resistances obtained in a fifteen (15) minute soak at various temperatures from 850^oC in air. Higher sheet resistances will be obtained with Borofilm in an oxygen ambient since the Si0² layer which grows on the silicon wafer acts as a sink for boron atoms.

APPLICATION OF BOROFILM AND PHOSPHOROFILM

Borofilm and Phosphorofilm can be applied with a spinner as is common practice in photo-resist operations. For a production process, the doping solutions may be applied after the required windows have been etched in the masking oxide. Films of Borofilm and Phosphorofilm can be stored for extended periods of time with no deterioration in doping characteristics. Of course, protection from dust and mechanical abrasion should be provided.

An alternative method of application is the spraying of the film with an atomizer.

The sheet resistances obtained with Borofilm and Phosphorofilm are independent of spin speed over a wide range. However, at slower spin speeds an excessively thick film will result which will leave a residue which etches slowly. At excessively high speeds, too thin a film will result and lower sheet resistances will be obtained. Two types of Borofilm and Phosphorofilm are available. The type A materials have a nominal viscosity of 30cp. The type B materials have a nominal viscosity of 100cp. The type A Borofilm and Phosphorofilm are recommended for spin speeds of 3,000 to 6,000 rpm. Type B materials are recommended for spin speeds in excess of 6,000 rpm.

TABLE I

TYPICAL SHEET RESISTANCE OBTAINED WITH
BOROFILM AND PHOSPHOROFILM

Borofilm (15 minutes soak in air)

Phosphorofilm (15 minutes soak in air)

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