Page 83 - Semiconductor Manufacturing Handbook

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VACUUM TECHNOLOGY

7.4 SEMICONDUCTOR FUNDAMENTALS AND BASIC MATERIALS

Pirani Gauge. The Pirani gauge uses two heated electrodes, each made from platinum, to sense
the pressure inside the gauge tube. One of the electrodes is the reference and is encapsulated in an
evacuated glass tube, and the other is the sensor enclosed in a similar glass envelope that is open to
the interior of the Pirani-gauge tube volume. These two electrodes are connected to a Wheatstone
bridge circuit. As the pressure in the Pirani gauge tube decreases, the temperature of the sensor fila-
ment increases due to fewer collisions with gas molecules. The electrometer in the Wheatstone
bridge circuit senses the change in resistance of the sensor filament and increases the current to that
leg of the circuit to maintain the current balance in the Wheatstone bridge. The current applied to the
sensor filament is used to infer pressure. The typical operating range for a Pirani gauge is from
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approximately 1 torr down to 10 torr.
Convection-Enhanced Pirani Gauge. In this gauge technology, the range of a typical Pirani gauge
is extended by adding a resistively heated coil that creates an environment in which induced con-
vection occurs in the viscous flow regime. Under these conditions the range of operations of the
gauge is from atmospheric pressure down to approximately 1 mtorr.

7.2.4 Ionization Gauges
Gauges that infer pressure from ionization events are gas species sensitive.

Hot Cathode Ionization Gauge. The basic hot cathode ionization gauge has three electrodes—an
electron emitter (often referred to as the filament), an electron collector (referred to as the grid), and
the ion collector (referred to as the cathode). Following the evacuation of the internal volume of the
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hot cathode ionization gauge tube to a pressure of less than 10 torr, current is provided by the gauge
controller to the filament that becomes heated to a temperature of approximately 2000°C. At this
operating temperature the tungsten or thoria coated iridium filament glows white hot and emits elec-
trons. These electrons are electrostatically attracted to the electron collector that has a positive bias
of approximately 150 to 180 V dc. En route to the electron collector, these electrons are likely to col-
lide with neutral gas molecules inside the gauge tube body. These collisions often result in the ejec-
tion of an electron (or two) from the gas molecules, resulting in the creation of ionized gas
molecules. These ionized gas molecules are electrostatically attracted to the ion collector by a bias
applied to it by the gauge controller. As ionized molecules impinge upon the ion collector, they
extract an electron to become neutral once again. This extracted current (ion current) is used to infer
pressure. Two factors that influence the observed reading for a specific gas species are the physical
size of the gas molecule (cross section) and its ionization potential.

Cold Cathode Ionization Gauge. As with the hot cathode ionization gauge, the cold cathode gauge
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must be internally evacuated to a pressure of less than 10 torr prior to operation. In the cold cath-
ode gauge, a high voltage (up to approximately +6 kV) is applied to the anode inside the gauge. This
bias causes spontaneous electron emission from the cathode inside the cold cathode gauge tube body.
These electrons traversing the space between the anode and cathode can strike neutral gas molecules
and may create gas-phase ions. These ions will move under the applied bias toward the cathode.
Once the ions impact the cathode, they will extract an electron and result in a net ion current that
may be used to infer pressure inside the gauge tube. Often a strong permanent magnet is placed out-
side the cold cathode gauge tube body to cause the electrons to travel in a helical path, thus increas-
ing the probability of an impact with a gas-phase molecule. A variety of designs for cold cathode
gauge tubes are commercially available including an inverted magnetron design.

7.2.5 Partial Pressure Measurement
It is often useful to know not only the total pressure of gases remaining in a vacuum vessel after evacu-
ation, but also the composition of the gas mixture and the relative amounts of each gas species in the
mixture. Partial pressure analysis (PPA), also known as residual gas analysis (RGA), is the method used

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