Page 96 - Power Electronics Handbook

Smartpower 89

The various components making up the smart power device may be
assembled using different techniques. For example they may be built into the
same silicon dice or they may consist of separate die which are assembled
into the same physical package using techniques such as hybrid circuits or
multichip modules.
Smart power devices are often targeted at high volume applications with
relatively low levels of power requirements, such as automotive, small motor
control, and consumer. Examples are given in later chapters.

3.5.1 Hybrid Circuits
Hybrid integrated circuits can be considered to be miniature printed circuit
boards in which the bare semiconductor die are connected onto the substrate
and most passive components are formed directly on the substrate rather than
added as discrete components (Sergent and Harper, 1995). Hybrid circuits
are one of two types, thin film and thick film. Although these look very
similar they differ in their production techniques, which gives thin films
better performance characteristics but makes them more expensive than thick
films.
A variety of substrate materials can be used for thin film circuits, such as
glass or pure glazed alumina. Thin films may also be formed on to silicon
dioxide and can therefore be combined with monolithic circuits as a single
dice. The surface of the substrate must be flat and free from deformations and
it must also be chemically stable, so as not to effect the film
characteristics.
Gold is the commonest conductor material, used to interconnect the
components on the substrate. Gold has very low resistance but it does not
adhere well to glass. It is usual to cushion the gold with a layer of nicrome,
which is an alloy of about 80 per cent nickel and 20 per cent chromium.
Aluminium is also used as the conductor material. Although it is cheaper than
gold, aluminium has a higher resistance and reacts chemically with the gold
wires which are sometimes used to connect semiconductor chips to the
tracks.
Nicrome is the popular material for thin film resistors. It has excellent
adhesion to glass and gives resistors with a low temperature coefficient. For
high valued resistors, material consisting of a compound of dielectric and a
metal, known as cermets, are used.
Capacitors can be added to thin film circuits in chip form or built onto the
substrate as layers of conductor and dielectric materials. Both tantalum oxide
and aluminium oxide are used and they are formed by first putting down a
layer of the metal, oxidising it to give the dielectric, and then adding the top
conductor layer.
The film can be deposited on the thin film substrate by several techniques,
such as evaporation and sputtering. Evaporation is the most direct and
consists of placing the source material and the substrate in a partial vacuum.
The source is heated to vaporise it and this material then settles on the
substrate. The disadvantage of the evaporation system is that it gives a film
which has low adhesion to the substrate and a low density.
An alternative technique, known as sputtering, overcomes these dis-
advantages but is slower in forming a film of a given thickness. In sputtering