handbook of modern sensors

9.2 Tactile Sensors 333
Fig. 9.9. Tactile sensor with a polymer
FSR.
Fig. 9.10. Micromachined silicon threshold switch
with trapped gas. (From Ref. [10].)
sealed cavity formed by wafer bonding is used to plastically deform a thin silicon
membrane bonded over the cavity, creating a spherically shaped cap. The structure
shown in Fig. 9.10 is fabricated by the micromachining technology of a silicon wafer.
At normal room temperature and above a critical force, the upper electrode will buckle
downward, making contact with the lower electrode.
Experiments have shown that the switch has a hysteresis of about 2 psi of pressure
with a closing action near 13 psi. The closing resistance of the switch is on the order
of 10 k, which is usually low enough for the micropower circuits.
In another design, a vacuum, instead of pressurized gas, is used in a microcavity.
This sensor, shown in Fig. 9.11 [11], has a silicon vacuum configuration, with a cold
field-emission cathode and a movable diaphragm anode. The cathode is a sharp silicon
tip. When a positive potential difference is applied between the tip and the anode, an
electric field is generated, which allows electrons to tunnel from inside the cathode to
the vacuum, if the field exceeds 5 × 10 7 V/cm [12]. The field strength at the tip and the
quantity of electrons emitted (emission current) are controlled by the anode potential.
When an external force is applied, the anode deflects downward, thus changing the
field and the emission current.
The emission current can be expressed through the anode voltage V as
I = V 2 a exp
(
− b
βV
)
, (9.7)