Vaisala PTB220 User Manual
Page 11
C
HAPTER
2___________________________________________________ PRODUCT DESCRIPTION
V
AISALA
__________________________________________________________________________ 5
Thin film metallization
Silicon
Vacuum gap
Silicon
Glass
Silicon diaphragm
F
IGURE
2-1
The BAROCAP
®
pressure sensor
The BAROCAP
®
pressure sensor consists of two layers of single
crystal silicon having a layer of glass between them. The thinner
silicon layer is etched on both sides to create an integrated vacuum
reference chamber for the absolute pressure sensor and to form a
pressure sensitive silicon diaphragm. The thicker silicon layer is the
rigid base plate of the sensor and it is clad with a glass dielectric. The
thinner piece of silicon is electrostatically bonded to the glass surface
to form a strong and hermetic bond. Thin film metallization has been
deposited to form a capacitor electrode inside the vacuum reference
chamber; the other electrode is the pressure sensitive silicon
diaphragm.
The coefficients of thermal expansion of silicon and glass materials
used in the BAROCAP
®
pressure sensor are carefully matched
together in order to minimize the temperature dependence and to
maximize the long-term stability. The BAROCAP
®
pressure sensor is
designed to achieve zero temperature dependence at 1000 hPa and its
long-term stability has been maximized by thermal ageing at an
elevated temperature.
The BAROCAP
®
capacitive pressure sensor features a wide dynamic
range and no self-heating effect. The excellent hysteresis and
repeatability characteristics are based on the ideal spring
characteristics of single crystal silicon. In the BAROCAP
®
pressure
sensor, the silicon material is exerted to only few percent of its whole
elastic range.