Portable phones, pocket calculators, laptop computers and even Nintendo
Gameboys all rely on the compactness of silicon chips to make them light
and portable. But the way chips are packaged at the moment, only one tenth
of the package is actually taken up by the chip.
Simply by changing the way chips are connected together, the amount
of space they need could shrink up to fivefold in the next three years.
This will make them faster, cheaper and more reliable, predicts Norman Stockham,
manager of a £13-million pan-European research project called Pepite,
which is due to start this month.
Today, most silicon chips are encased in solid plastic with metal legs
sticking out to connect them onto a circuit board. It would be much cheaper
and easier to solder the bare silicon chips onto a large piece of silicon
or ceramic, which has circuits printed on it, and then package them all
together into what is known as a multichip module. An MCM package only takes
twice the area of the silicon itself. Today’s MCMs are very expensive and
are only made in small quantities for satellites and other specialised uses.
As well as saving space, MCMs will let circuits run faster. Fast chips
are sometimes slowed down by the time it takes data to travel from neighbouring
chips. Stockham, who works at the Welding Institute near Cambridge, says
the Pepite project aims to make MCMs which are 400 times as fast as current
fast chips, so the chips in the MCM must be as close together as possible.
Dave Pedder, a researcher at GEC Plessey Semiconductors, which makes
MCMs at its Lincoln factory, says that they are seen as an alternative to
the ultimate in chips – wafer-scale integration – in which all the circuitry
for something like a laptop computer is made on a single chip. The first
experimental computers-on-a-chip have been made but they are difficult to
design and build. They must have spare circuits built in routinely, in case
an error in manufacturing causes a particular circuit to fail.
MCMs are put together with chips that are generally quite small and
cheap to make. Any chip that does not work is simply discarded, so few spares
are needed. Also, several chips made by different manufacturing processes
can be combined.
Connecting the tiny silicon chips to an MCM substrate is quite a challenge
for a production line. The chips can be stuck down, then connected with
very fine wires which are pressed onto the substrate and soldered in place
by the heat from a fine laser beam. An alternative, which could be much
faster, is called flip-chip bonding.
A flip chip has blobs of solder around the edges of the top of the chip,
instead of on the bottom. The substrate has corresponding lumps of solder.
To connect the two, the chip is flipped over and positioned, upside down,
on top of the solder lumps which are then melted. The two sets of solder
lumps only have to be lined up approximately, says Pedder: as long as they
half overlap, the surface tension of the molten solder will pull them into
Pepite, which is part of the Eureka programme, involves 12 companies
and one university, who will spend three years and £13 million on
the project. They will not be looking at silicon substrates, but are investigating
other materials, including ceramics.
The collaborators have already shown two new ways of making ceramic
substrates. Martin Lockyer, a researcher at the University of Warwick, has
developed a machine similar to a crepe maker, called a tape caster. He pours
a mixture of glass particles – which look like icing sugar – and an oily
solvent onto a plastic sheet on a platform, making a thin sheet of ceramic
which hardens as the solvent evaporates. The ceramic is known as green tape.
It can be cut and punched, and connections printed on it, before it goes
into the kiln, for firing at up to 900 °C.
Researchers at the Welding Institute have developed a device which spray-coats
ceramic onto metal plates with a gun rather like an oxyacetylene cutter.
Acetylene, oxygen and powdered ceramic are mixed in a combustion chamber
and ignited, shooting a white-hot flame out of the end of the gun. The particles
melt at about 2000 °C in the flame, where they stay for a microsecond.
The design of the gun is 35 years old, but Harvey believes it has never
been used to deposit ceramic on metal before.