Page 14 - Power Electronics Handbook
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Fabrication process   7
                    again  avoided.  Since  in  both  the  floating-zone  and  pedestal-pulling
                    methods the molten silicon is held in place by surface tension, the size of
                    the melt, and therefore of the silicon bars (and wafers) produced by these
                    methods,  cannot be  very large. These techniques are  used  for making
                    relatively low-power components.
                      During preparation of  pure silicon, impurities of  p or n type can be
                    added to the melt, to give the final silicon ingots the required resistivity.
                    The ingots are then cut into slices, using a diamond-impregnated saw. The
                    saw cuts are made along the <111> or <loo> planes of the crystal. These
                    cuts usually damage the crystal lattice near the silicon surface, resulting in
                    poor resistivity and minority carrier lifetime. The damaged area, which is
                    about 20pm deep, is removed by etching in a mixture of hydrofluoric and
                    nitric acids, and the surface is then polished to give a strain-free, highly flat
                    region.

                    1.2.2  oxide growth
                    Silicon oxide, also called silica or SiO,,  may be grown and removed several
                    times  from  the  suxface  of  the  silicon  slice during manufacture of  the
                    semiconductor device. The oxide layer is used for diffusion masking, for
                    sealing and passivating the silicon surface, and for insulating the metal
                    intemnncctions  from  the  silicon.  Although  the  oxide  layer  may  be
                    deposited onto the slice, as done for the epitaxy layer described in the next
                    section, it is more usual to grow it using dry or wet oxygen or steam.
                      Figure 1.2(a) shows  a  typical arrangement of  the apparatus used  for
                    oxidation (and diffusion). The silicon slices are stacked upright in a quartz
                    boat and inserted into a quartz tube. The tube is heated to between 1ooo"C
                    and 1200°C by zoned heaters, so that the boat is located in an area having a
                    uniform temperature along a length of the tube. Nitrogen, dry oxygen, wet
                    oxygen (which  is oxygen bubbled into water at 95OC)  or  steam can  be
                    passed over the slices to grow the oxide layer. A thickness of  about 1 pn
                    takes about 4 h to grow and consumes between 0.4 pn and 0.5 pm of  the
                    silicon. The colour of the silicon surface changes with the thickness of the
                    oxide layer, due to the shift in the wavelength of  the reflected light. This
                    effect is used as an indication of  the layer thickness.


                    1.2.3 Epitaxy pwtb
                   Epitaxy means growing a single-crystal silicon structure on the original
                   slice, such that the new structure is essentially a molecular extension of the
                   original silicon. Epitaxy layers  can be closely controlled regarding size and
                   resistivity,  to about  210%.  This compares favourably with  the  f30%
                   resistivity control obtained when pulling from the silicon melt.
                     Epitaxy apparatus is similar to the oxide growth arrangement shown in
                   Figure 1.2(a). However, r.f. heating coils are normally used and the silicon
                   slices are placed in a graphite boat, which may be coated with quartz to
                   prevent  the  graphite  contaminating the  silicon.  The  bubbler  usually
                   contains silicon tetrachloride (Sit&)  to which may be added a controlled
                   amount of an impurity, such as Pa3. Hydrogen gas is bubbled through this
                   mixture before entering the quartz epitaxy tube.
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