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VACUUM TECHNOLOGY

7.2 SEMICONDUCTOR FUNDAMENTALS AND BASIC MATERIALS

TABLE 7.1 Atmospheric Pressure Expressed in a Set of Units
Psi torr mBar Pa Kg/cm 3 inches of Hg
14.7 760 1013 101,300 1.033 29.92

7.1.3 The Composition of Atmospheric Gases
The air we breathe is composed of approximately 78 percent nitrogen and 20.1 percent oxygen, the
balance being argon, carbon dioxide, water vapor, and other trace gases. Dalton’s law of partial pres-
sure describes the relationship between the total pressure and the partial pressures of gases in a
gaseous mixture.

tot ∑
P = i P i
1
where P = total pressure of all gases in the mixture
tot
P = partial pressure of gas i
i
At sea level the approximate pressure the atmosphere exerts on all surfaces it contacts is 14.7
pounds per square inch (psi). Other units of measurement used include the torr, mBar, and pascal (Pa).
Unit conversion calculators for pressure are available on the following website: http://www.avs.org/ and
in Table 7.1.

7.1.4 Typical Applications of Vacuum Technology
Vacuum technology is essential to the microelectronics industry as many of the processes involved
in the fabrication of microcircuits require a controlled environment in which thin films can be
deposited with minimal interference from atmospheric gas molecules. Other industries that rely on
vacuum technology include the food processing and pharmaceutical industries (products are freeze
dried under vacuum).

7.2 METHODS FOR MEASURING SUBATMOSPHERIC PRESSURE

7.2.1 Force Displacement Gauges

These pressure gauges rely on the physical displacement of a solid or liquid surface by an applied
pressure as a means to measure the magnitude of the applied pressure. This family of pressure gauges
will give an accurate reading of pressure within their range of operation for all gases independent of
the composition of the gas. As such these gauges are referred to as being gas species insensitive.

Liquid Level Barometer. One of the earliest pressure gauges developed, the water barometer and
later the mercury barometer were used to measure the pressure of the earth’s atmosphere. Early
barometers were made by filling a long glass tube open at only one end (the tube being approxi-
mately 33 ft long if water was the fluid being used and approximately 30 in long for mercury), cap-
ping the open end of the tube, inverting the tube and placing the capped end under the surface of the
same liquid used to fill the tube in a secondary container, and then removing the cap. The column of
liquid inside the inverted tube would drop under the force of gravity until the pressure on the top sur-
face of the liquid in the secondary container was equal to the gravitational load of the column of liq-
uid in the inverted tube. As the external atmospheric pressure changed, the height of the supported

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