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Sources Show Long Lifetimes
Industrial experience has shown that PhosPlus sources normally
exhibit lifetimes in the range of 100-500 use hours. The
actual lifetimes of PhosPlus sources used in typical plant
production, however, depend upon many factors, such as temperature
of use, care in handling, device being manufactured, the
sensitivity of the process to the eventual decrease in the
P2O5 evolution rate, etc.
potential lifetime of a PhosPlus source can be estimated
by periodically doping a silicon wafer with the source being
held in a diffusion furnace and observing how the resulting
sheet resistivity varies with time. Figure 1 shows the average
sheet resistivity obtained on the silicon wafers doped for
60 minutes at 1020°C with TP-470 sources when used in
a typical production environment. Little change in sheet
resistivity was observed for the first 600 hours of use.
The sheet resistivity then began to slowly increase as the
P2O5 evolution rate was gradually
second method of estimating the lifetime of a source is
to measure the amount of weight a source loses at a use
temperature as the P2O5 evolves. Weight
loss data for the TP-470 sources (Figure 2) indicate a continuing
process of P2O5 evolution over hundreds
of use hours.
3 and 4 show similar sheet resistivity and weight loss data
for the TP-250 sources. These data also indicate that hundreds
of hours of use can be obtained from a set of TP-50PhosPlus
Properties of PhosPlus Sources
Crystal Silicon: Typical sheet resistivity versus
deposition time curves for the two PhosPlus sources are
plotted in Figures 5 and 6. The curves are different for
each source because the sheet resistivity of the silicon
wafer for a given deposition cycle depends somewhat upon
the thickness of the deposited glassy film. The thicker
the glassy film, the lower the sheet resistivity. Figure
7 shows how the deposited
film thickness varies with the type of source being used.
deposition time for a solid-source diffusion system:
A typical deposition time for a solid-source diffusion system
is about 45 minutes. This time is usually long enough for
the sources to uniformly dope the silicon wafers. At the
same time, it is short enough to be compatible with most
semiconductor process parameters.
Figure 8 shows the sheet resistivity and junction depth
that is obtained from a 45-minute deposition with the TP-470
PhosPlus sources at various deposition temperatures.
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