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THE CLEANEST room on campus at the University of Cambridge is nearly
40 years old. Converted from traditional laboratories in the engineering
department, the room is equipped for modern researchers competing in the
race to make microchips work faster by reducing the size of the components.
They are dealing with components smaller than a millionth of a millimetre
in chips that can be wrecked if unwanted particles lodge in the tiny gaps
of their circuits.

The designers have achieved the ultra-clean atmosphere by using a laminar
flow system. Fans push air, of a constant temperature and humidity, through
a battery of filters that form the ceiling of the room. The air passes vertically
down the room and through a perforated floor. This air flow is akin to heavy
rain falling. The air falls in sheets, dragging unwanted particles down
with it and trapping them below the floor. Under the floor, ducts return
the air to an air-conditioning plant that removes particles and regulates
temperature and humidity.

The system circulates the air at about 0.46 metres per second, resulting
in about 600 changes of air every hour. The room complies with a standard
in the computer industry that specifies that, in 270 cubic centimetres of
air, there should be no more than one particle larger than 0.5 micrometres
in size.

A vibration-free environment is another crucial aspect of making state-of-the-art
microchips. A silicon wafer sits in a transmission electron microscope for
long periods, waiting for a computer-controlled electron beam to etch the
circuits directly onto the wafer. The size of these circuits means that
even the slightest vibration could cause the electron beam to cut the wafer
in the wrong place.

When the engineers carried out a survey, they found that a large vehicle
passing by could cause a movement of 1.5 micrometres. Even at night, there
is a natural vibration of about 0.3 micrometres – incidentally, about the
wavelength of light – but still about 500 times too much movement for the
work.

The answer, developed by Arthur Timbs from the department, was to sit
the microscope on an isolation system. This is a combination of a table
with air valves built into it and, below, air springs. The pneumatic circuitry
is straightforward: it uses PVC tubing and threaded compression fittings.
Compressed air feeds into a filter/regulator unit housed in a control cabinet
in the wall of the laboratory. From this, air at a set pressure passes into
inlet chambers in the table via the valves. These valves can act independently
of each other. Sensors monitor the valves: if one opens, gauges in the control
cabinet alter the air supply to the other valves. This makes the table self-levelling
during transient vibrations, even when someone steps on it.

Another step in reducing movement was to lower the centre of gravity
of the microscope. A high centre of gravity means the system is sensitive
to disturbances in the air and to floor vibrations. It is similar to trying
to hold a ruler upright when gripping it close to the bottom; it nods vulnerably.
By lowering the centre of gravity below that of the air springs, the system
becomes inherently stable. The ruler is effectively held at the top, acting
like a pendulum.

As there was no way of raising the ceiling of the room, the solution
was to build a pit and to attach weights to the underside of the microscope.
This brought the centre of gravity below the level of the air springs. To
look at the end result, you would not know the technology behind the set
up. All you can see is a microscope sitting on a platform at floor level.