Page 299 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

MAGNETIC SENSORS 279

i i i i i

75 150 225 300
Magnetic flux density, B, (mT)

Figure 8.46 Response of a SAW delay-line magnetic sensor. After Hanna (1987)

(RF SQUID) or two (DC SQUID) Josephson junctions. In a DC SQUID, a bias current
I bias is applied, with currents passing through each Josephson junction. The magnetic flux
inside the loop 0j nt must be quantised in units of the quantum flux 0o, which takes a
value of h/2e or 2.07 fWb. Thus, the internal flux is a measure of the external flux <£ ext
and the difference in phase S at the two junctions,

sin 5j) (8.49)
'int — Vext
where L is the self-inductance of the ring. The corresponding oscillation in junction
voltage can be averaged and the typical V-I characteristic of a DC SQUID is shown in
Figure 8.47(a), where I 0 is the peak current flowing through the junctions and R is the
junction resistance.
An integrated DC SQUID has been reported (Koch 1989) and the basic layout is shown
in Figure 8.47(b). The SQUID ring is made of a thin superconducting film that is shaped
like a square washer, a large pickup coil and a smaller input coil. The washer acts as a
ground plane to the input coil and hence improves the coupling between the spiral input
coil and the SQUID ring. The input coil has an inductance of 100 nH, whereas the SQUID
loop is about 1000 times less at only 100 pH. The integrated SQUID can then be run in
an AC bridge with the deflection as a measure of the external magnetic flux and hence
the flux density. Flux densities as low as 10 -14 T can be measured, which is comparable
with those emitted from the human heart or brain. Moreover, as the SQUID can be run
at radio frequencies, it is possible to power and measure the flux density in a wireless
fashion.
To sum up, magnetic microsensors are currently employed in a range of other mechan-
ical sensors, such as proximity sensors, speed sensors, flow sensors, and so on. Therefore,