Page 234 - Electrical Properties of Materials

216 Principles of semiconductor devices

(c) Surround

(a) Cr/Au
Si
SiO 2 Mirror
Si
Resist
(b)
Torsion
suspension
(e)

Cavity
(d) Mirror
(h)
Suspension Cavity
Electrodes
(f)
Fig. 9.58
Fabrication sequence for a single-axis (g)
electrostatically driven MEMS torsion
Drive
mirror. After Aksyuk et al. Proc.
electrode
SPIE, 4178, 320, 2000.
You will realize that the aim was to show the basic principles by giving an
example of practical significance. There are better solutions but the present one
will also work well for a limited angular range. Turn angles of a few degrees
may be achieved with drive voltages of 100–200 V. Two-axis mirrors may be
constructed using similar principles, by mounting the mirror in a gymbal with
two orthogonal elastic suspensions and two orthogonal sets of drive electrodes.

9.26.2 A mass spectrometer on a chip
Our next example is a mass spectrometer. As the name implies, it measures
the range of mass in a particular assembly of molecules. The problem is to
find out how many molecules are between two limits, say between 150 and
160 atomic units. In principle, we could weigh each molecule on a sensitive
balance and, having measured their weight/mass, we could choose the right
ones and put them in a box marked ‘150–160’. In practice, this is not a feasible
way to proceed. Instead of boxes, we should have detectors and the selection
should be done by some sort of filter. How can we do that? The means at
our disposal are electric and magnetic fields. Unfortunately, they cannot affect
neutral molecules.