The deposited thin films in surface micromachining have a lot of residual stress, and it is essential to measure this for both process development and monitoring. We estimate residual stress by electrical measurements on a series of fixed-fixed polysilicon beams designed to deflect laterally due to stress. To minimize errors in estimation during parameter extraction, the device dimensions also have to be measured accurately. Surface micromachining of an oxide-anchored polysilicon cantilever beam can result in beam undercut, reduction in beam thickness, and increase in the gap between the beam and the substrate. The undercut in the beam is estimated from the resonance frequency of the cantilever beam and also by using polysilicon resistors. Final device thickness is obtained by measuring the resistance in fixed-fixed beams.
In a surface micromachined cantilever the beam thickness, width and the gap get modified during fabrication. Since
performance of sensors and actuators strongly depend on the final geometry, it is essential to measure these parameters.
Electrical measurements have several advantages over microscopy and we present here two different electrical
techniques to measure undercut due to etching. In the resistance based approach, two polysilicon resistors of different
length and width are used and the undercut is extracted from the ratio of the measured resistances. In the second
technique, resonance frequency is measured for two sets of oxide anchored polysilicon cantilever beams with 20 and 30
µm widths. For a given length, the ratio between the measured frequencies for the two widths gives the value of
undercut. Measurements are done on both wet and dry etched polysilicon.
In surface micromachined structures, many parameters like geometry and Young's modulus depend on the process steps and need to be measured for accurate prediction of their functionality. This work discusses simple electrical measurement techniques on surface micromachined cantilever beams to determine Young's modulus, the gap between the beam and the substrate, and the thickness of a deposited aluminum layer on the beam. Cantilevers are ubiquitous in most microelectromechanical system (MEMS) sensors and actuators, and hence are ideal test structures. Pull-in, and a novel resonance frequency measurement based on the pull-in technique, are done on oxide anchored doped polysilicon beams at the wafer level, and some of the device and material properties are extracted from these measurements. The extracted values are compared with those determined from established methods like vibrometry and surface profiler measurements, and show good agreement. Since the measurements are all electrical, they can be part of standardized testing and are also suitable for packaged devices.
This paper discusses a simple electrical measurement technique to determine resonance frequency of surface
micromachined cantilever beams that is also suitable for packaged devices. Measurements are done on oxide anchored
doped polysilicon beams. If the beam is driven by an AC signal riding on the DC bias, the beam starts vibrating. When
the drive frequency matches the natural frequency of the beam, the oscillation amplitude is maximum. In this
measurement, the DC bias is fixed at a value lower than the pull-in voltage. A small AC bias is then applied such that the
sum of the DC and the maximum amplitude of the AC is less than the pull-in voltage. The frequency of the AC is then
swept and at resonance, because of large displacement, the beam is pulled in and this is detected by a current flowing
between the beam and the substrate. By iteratively adjusting the DC bias it is possible to make sure that pull-in occurs
only due to resonance and the frequency setting at this point gives the natural frequency of the beam. Measured values
for different beam lengths were compared with Doppler Vibrometry results and gave an excellent match.