PhosPlus (R) Brochure
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Doping Properties of PhosPlus Sources

Polysilicon: The minimum sheet resistivity that can be obtained from polysilicon wafers that are saturated with phosphorus partially depends upon the thickness of the polysilicon layer as shown in Figure 9. These sheet resistivities are about 12 ohm/sq for 5000Å of polysilicon and about 32ohm/sq for 2500Å, and they occur when the deposited phosphorus glassy film exceeds about 500 to 600Å as shown in Fig. 10.

Glassy films that are less than 500Å can also be uniformly deposited on the polysilicon wafers from the PhosPlus sources to produce higher sheet resistivities for special applications. Figure 10 can be used as a guide to determine the approximate thickness that is required for different sheet resistivities.

Doped polysilicon layers with smooth surfaces are important in the manufacture of certain devices. Very smooth surfaces can be obtained when the polysilicon layer is first doped with the TP-250 sources to a level slightly below saturation (400-500Å as shown in Fig. 10.). When this glass is etched off and the silicon wafers are annealed near 950°C for about 15 min, smooth surfaces are maintained and the silicon exhibits the minimum sheet resistivity characteristic of its thickness (Fig. 9).

Sheet resistivities at or above the saturation of phosphorus in polysilicon can be obtained from either phosphorus source. The appropriate deposition cycle can be selected from the curves shown in Figures 11 and 12 (see below).




Uniformities: When the various processing conditions are optimized, uniformities of 2% across the silicon, 3% across the boat and 4% run-to-run or better can generally be obtained on single-crystal silicon. A total variation of about 3% can be achieved on high-quality polysilicon wafers doped to their minimum sheet resistivities (saturation with phosphorus).

These uniformities are quite typical of the planar diffusion system and tend to be independent of the diameter of the wafer and the number of silicon wafers being processed during a run. This independence can result in an increase in silicon throughput compared to the number of silicon wafers often processed in gas systems. It can also significantly increase production yields by improving process control as demonstrated by the decrease in beta variation when TP-470 PhosPlus sources are used for an emitter diffusion instead of POCI3 (Figure 13).

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