EMULSITONE COMPANY
19 Leslie Court
Whippany, New Jersey 07981
TEL. (973)386-0053
FAX (973)503-0256

RECOMMENDED PROCESS FOR USE OF BOROSILICAFILM AND PHOSPHOROSILICAFILM IN SIMULTANEOUS DIFFUSION

For the production of silicon rectifiers and diodes, especially where deep diffusions are required at long soak times, it is economical to diffuse phosphorous and boron simultaneously. Lapped rather than polished wafers are usually used for this process. Borosilicafilm and Phosphorosilicafilm offer two advantages over the usual paint-on solutions--there is almost total absence of wafer staining and there is no yield reduction due to cross diffusion of dopant to the opposite surface when phosphorous vapor is evolved at high temperature.

WAFER CLEANING

No unusual cleaning steps are required. After lapping, the wafers should be cleaned to remove all traces of grease or cutting oils. The wafers should be soaked in concentrated nitric acid for ten minutes prior to application of the dopant. The wafers are removed from the acid and rinsed in DI water followed by a rinse in alcohol and/or acetone and blown dry.

APPLICATION OF DOPANT

The dopant may be applied by painting or spraying. Spinning the material over the wafer will cause some of the solution to wet on to the opposite surface due to pumping action which holds the wafer on the chuck. A camel's hair brush is wet with dopant and spread over the wafer surface in a thin layer. If the materials are sprayed, the wafers should be supported on a pedestal so that liquid does not wash onto the reverse surface. After coating the wafers are baked in air on a hot plate for 15 minutes at 150^oC.

With a clean brush the uncoated side is painted with the second dopant. Again the wafers are baked at 150^oC in air for 15 minutes.

DIFFUSION

The wafers may be stacked vertically with boron sides facing or they may be stacked as coins, again with similar doped sides touching. In this latter instance, there will be a tendency for the boron doped sides to stick together after diffusion. This may be prevented by sprinkling the boron sides with alumina powder.

The atmosphere in which the diffusion is carried out is important, especially with the boron diffusion. Excess oxygen will yield a thick SiO₂ layer which acts as a sink for boron, reducing the surface concentration. Insufficient oxygen will result in brown stains on the boron diffused side which may be difficult to remove and may interfere with the contact plating process. A diffusion atmosphere consisting of 5% oxygen in nitrogen will prevent staining and result in limited oxide growth to minimize boron gettering.

Any diffusion temperature may used consistent with the time required to achieve the desired penetration. Over the temperature range form 900^oC to 1280^oC the surface concentration for both phosphorus and boron will be nearly independent of temperature. These products yield surface concentration in excess of 5X10²⁰.

The penetration into the silicon will be erfc (i.e. the sheet resistivity will decrease with the square root of the time) for a period of time which depends upon the thickness of the dopant layers. A single layer, e.g. a single drop spread over a 1 1/2" wafer, will remain erfc for penetrations into the silicon of 10 microns. Thereafter the sheet resistivity will remain constant although additional penetration into the silicon will continue, with some lowering of the surface concentration. In Table I are shown the sheet resistivities obtained with Borosilicafilm and Phosphorosilicafilm at 1100^oC,1200^oC, and 1250^oC for fifteen minute heat soaks in 5% O₂ in N₂, and for 4 hour heat soaks. Lower sheet resistivity may be obtained by applying a thicker coating, so that the diffusion remains erfc longer.

Minority Carrier Lifetime

The simultaneous diffusion process will yield wafers with minority carrier lifetimes as high as 30 microseconds due to the gettering capabilities of the boron and phosphorous doped glasses.

TABLE I

DIFFUSION CHARACTERISTICS OF BOROSILICAFILM AND PHOSPHOROSILICAFILM

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