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5. Theory of Operation
The Model 2002 transducer is comprised of two very different sensors which provide a span of measurement
extending from 1000 Torr down to less than 1x10-4 Torr. The piezoresistive device is a direct force sensor
which provides pressure indication from 1000 Torr down to less than 1 Torr. The thin film Pirani device is
a thermal conductivity sensor that provides pressure indication from 100 Torr down to less than 1x10-4
Torr. The two decade overlap in measurement range is convenient for smooth transition either descending
or ascending in pressure. Both sensors are small micro machined die that are bonded to a Au coated Al2O3
preform (stress isolation) which in turn is bonded to a TO-8 header. The header is resistance welded into a
316 stainless steel envelope as shown in Figure 5.1.
5.1. Piezoresistive Sensor
Dual Sensor Emodiment
Figure 5.2 shows a typical schematic of a B implanted Wheatstone bridge network in a Si diaphragm inverted
box type geometry. The inside of the box is evacuated during anodic bonding to a Pyrex substrate. The
membrane has maximum deflection at atmosphere (or higher pressure) and the membrane resistances
change value as the differential pressure is decreased during pump down. The resulting differential output is
Vo = SPV+V1
where
S is the sensitivity
P is the pressure
V is the applied bridge voltage
V1 is the no load output voltage
Since the sensitivity changes so dramatically with temperature, some correction is required for compensation.
The change in output voltage
Electronics
Board
(EEPROM)
PIRANI
SENSOR
Piezoresistive
Senso
r
TO-8
HEADER
To
p
Ca
p
Cable
Connector
Fig 5.1