Properties of PhosPlus Sources
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.
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.
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
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).
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).
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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