Page 16 - Power Electronics Handbook

Fabrication process 9
Initially the slices are heated to about 12000C and pure hydrogen and
hydrochloric acid vapour are passed over them to etch away any oxide or
impurities which may exist on the surface of the silicon. HCl vapour is then
turned off and hydrogen bubbled through the Sick, the vapour passing
over the silicon slices. When this reaches the hot silicon it dissociates and
silicon atoms are deposited on the slice, where they rapidly establish
themselves as part of the original crystal structure. It is essential to saturate
the tube with SiCl, vapour, to ensure a uniform layer thickness over the
whole slice.
The epitaxy layer may be doped with p- or n-type impurities by
introducing these, in the required concentration, into the vapour stream.

1.2.4 Photolrthagraphy
The main use of photolithography, in the manufacture of semiconductor
components, is to selectively remove areas of oxide or metal from the
surface of the silicon slice (Levenson, 1995; Jeong et al., 1994). To do this
the surface is first covered with a thin uniform layer of liquid photoresist.
This is best obtained by holding the silicon slice in a vacuum chuck and
placing a fixed amount of photoresist onto its centre. The slice is then rotated
at very high speeds, to distribute the resist, the excess flying off due to
centrifugal forces. The amount left on the slice is clearly a function of the
oxide properties and the viscosity of the resist. The slice is then heated for a
few minutes in an oven to harden the resist.
The mask, containing the pattern of the area which is to be selectively
removed, is then placed in contact with the surface. This may consist of a
glass or chrome mask, or, more commonly, a projection of the image onto the
surface of the silicon. Once in place, the surface is exposed to ultraviolet
light. If negative resist material is used the exposed areas become hardened
by the light. The slice is now covered with developer, which dissolves the
unexposed resist areas. The slice is then placed in a bath of etchant, such as
hydrofluoric acid, which dissolves the exposed areas of the surface, but not
the silicon. For applications which require most of the covering surface
material to be removed it is more convenient to use a positive resist material,
such that the areas exposed to ultraviolet light are dissolved in the
developer.
The alternative to the use of photolithography is ion beam lithography in
which an ion source is used to form the required pattern directly onto a
silicon dice. This system is currently economic only when dense circuits,
with very fine lines, are required (Finkelstein and Mondelli, 1995).

1.2.5 Dif?wbn
In epitaxy a large area is doped by a closely controlled amount of impurity.
Diffusion, on the other hand, enables selective areas to be doped. These
areas represent those which are not covered by an oxide layer, so that the
photolithographic stage is normally followed by diffusion.
The diffusion furnace resembles the arrangement shown in Figure 1.2.
The bubbler contains the impurity and nitrogen is passed through it, on its