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The BoronPlus and PhosPlus sources change
dimensions when they are inserted into the diffusion furnace
because of their thermal expansion coefficients. These dimensional
changes could cause the sources to be damaged during use if
the diffusion carrier or boat in which they have been placed
is not designed properly. This bulletin therefore provides
the diffusion engineer with the appropriate guidelines necessary
for him to prepare acceptable boats for his diffusion system.
The BoronPlus and PhosPlus sources use the same boat design.
boats of various designs have been successfully used with
BoronPlus and PhosPlus sources. Since quartz boats tend to
deform at high temperatures, polysilicon or silicon carbide
boats are often used above
Figure 1 shows a typical boat which holds
27 sources and is designed for manual push/pull systems. Longer
boats which hold more sources can be used, but they become
quite heavy when fully loaded making them difficult to handle
and difficult to push in and pull out of the diffusion furnace.
Figure 2 is a picture of a four-rail boat
designed for automatic transfer systems. These boats usually
hold 13 sources and fit end-to-end on standard paddles and
cantilever systems. Since boats that fit on automatic transfer
systems also require specific external dimensions, the diffusion
engineer should contact the manufacturer of his transfer equipment
to obtain the specific boat design details.
Dimensions that are of particular concern
when designing a carrier for the BoronPlus or PhosPlus sources
are the diameter and width of the slot for the source and
the distance between the silicon surface and the source surface.
These dimensions are given in Table
I for sources ranging from 2" to 150mm in diameter.
Generally the width of the slots for the
sources should be about 0.010" (.25mm) wider than their
thickness. The sources should fit loosely in the boat, allowing
room for expansion of at least 0.020" (0.5mm) per inch
of diameter. Also, the boats are normally designed so that
the spacing between the silicon surface and the source surface
is 0.100" (2.5mm). Since some automatic transfer systems
require a closer spacing than this because of the thickness
of the BoronPlus and PhosPlus sources, the dimension can be
decreased to as low as 0.060" (1.5mm). Whatever spacing
is selected, however, it should remain constant across the
The diffusion engineer must ultimately
choose the appropriate materials and boat design for his specific
application. This bulletin contains information to assist
him in making the correct choices.