Page 97 - Power Electronics Handbook

90 Power semiconductor control components
a glow discharge is formed in argon at between 0.01 to 1.0 torr, by a high
voltage applied between the source to be sputtered (cathode) and the
substrate (anode). Argon ions are formed and these strike the source releasing
molecules which are negatively charged. These molecules bombard the
substrate, giving a film which has good adhesion and density. The track
pattern can be formed on the thick film substrate by either covering the
whole surface with the track material and then using photolithography to etch
out the areas not required, or by placing a mask in contact with the substrate,
prior to film deposition, with cut outs in the regions where the film is to be
deposited on the substrate.
The materials used for thick film production are called inks or pastes.
Many different ink compositions are available, depending on the supplier and
whether conductor, resistor or dielectric material is required. Basically all
inks are made from fine metal and glass powders which are mixed with an
organic solvent. The substrate must be pure and flat with a high mechanical
strength. It should have good electrical resistance and low thermal resistance.
The thick film process involves very high temperatures, above 1OOO"C and
the substrate must remain chemically stable throughout. A good general
purpose substrate material is a compound of 96 per cent alumina and 4 per
cent glass, although 99.5 per cent alumina substrates are also used. Beryllia
is preferred for power circuits since it has good thermal conductivity.
Thick film tracks are formed by printing the inks through a polyester or
stainless steel mesh. The areas where the ink is not required are blocked off
by emulsion. Figure 3.13 illustrates the print operation. After the ink has been
printed onto the substrate it is allowed to stand for a few minutes, for the ink
to coalesce. It is then dried in an oven or under infrared heaters to remove the
volatile components of the paste. Following the drying process the circuit is
fired in a zoned oven. This first removes the remaining volatile elements
from the ink and carbonises and oxidises the organic binders. Then the glass
content of the paste is melted and this forms a seal around the metallic
particles as well as fixing the track to the substrate.
Printed resistors have an accuracy between batches of about 30 per cent.
For tighter tolerance, trimming must be used which removes a portion of the
track area and so adjusts the resistance value upwards. Two techniques exist
for trimming. The first is air abrasive trimming and uses a high velocity
stream of fine abrasive powder to wear away the film. It does not result in
any appreciable temperature rise or shock or vibration, but it does give an
overspray action onto adjoining components. The trimming operation also
removes the vitreous parts of the resistor paste and exposes the resistor to
subsequent contamination. In laser trimming a high energy laser beam is
directed onto the film, raising its temperature and vaporising the required
area. It uses more expensive equipment than air abrasive trimming and the
laser needs to be re-tuned each time a different colour resistor is trimmed.
However, there is no overspray and the high temperature results in a flow of
the vitreous contents of the film so that the cuts are re-sealed.
There are many techniques in use for connecting semiconductor die onto
hybrid substrates. First the die must be fixed to the substrate and then
conducting bonds made to its terminals. Eutectic bonding may be used for
connecting the chip to the substrate. It relies on the fact that an eutectic alloy
such as gold-silicon has a lower melting point than both gold and silicon. The