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Microtechnologies for Science Instrumentation Applications 133

traditional foundries. In addition, all magnetometers, which use ferromagnetic
materials have a limited dynamic range and the variation in magnetization requires
a calibration process. An interesting approach for space application, where large
current-carrying supply lines can change the magnetic environment around the
magnetic boom, would be the use of such a magnetometer with remote interroga-
25
tions. Lorentz force-based magnetometers promise a high dynamic range with a
zero offset and wide linearity. They are based on the measurement of the deflection
of a MEMS structure with an AC or DC current flowing in it. One example is the
JPL device, 26 that uses DC current and measures the static deflection of a mem-
brane with conductors using a tunneling current as the transduction method. The
sensitivities of this device are in the order of mT. A more sensitive magnetometer
has been designed at JHU/APL, 2,24 based on a resonating ‘‘xylophone’’ bar, a few
hundred microns long and supported at the nodes where an AC current is supplied.
At the resonance frequencies, Qs for these devices in vacuum are in the order of 50–
100k, and small fields can generate a large magnitude of deflection. Devices etched
photolithographically from CuBe with lengths of a few millimeters have been
used to measure magnetic fields with sensitivities as low as 100 pT/Hz 1/2 using
optical beam deflection as the transduction method. Figure 7.5 shows a device
surface micromachined in polysilicon using the MUMPs process. The sensitivity of
theses devices is limited by the current-carrying capability of the polysilicon
supports as well as the integration of the transduction into the device. An improve-
ment has been achieved by using a complementary metal oxide semiconductor
(CMOS) process or a silicon on sapphire (SOS) CMOS process. 27 While the
mechanical properties of the resonating device are somewhat degraded, the use of
multiple metal layers and the integration of the control electronics as well as the
capacitive readout onto the same die improve the performance. Major advantages of
the Lorentz force magnetometers are the wide dynamic range, since the signal is the

FIGURE 7.5 Surface micromachined ‘‘xylophone’’ magnetometer. (Source: JHU/APL.)

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