Instrumentation pour la nanocaractérisation
Since MEMS devices are presenting very high impedances and in the trends towards miniaturisation and increased frequency of operation of MEMS resonators, we initiated cooperation with Agilent Technologies for the investigation of RF measurement techniques of very high impedances. By high impedances, we consider impedances much larger than 50 Ω where the sensitivity of network analysers becomes low. In our case we are typically interested on small capacitances in the range of the fF the impedance of which is above 100 kΩ at 1 GHz. For measuring such small capacitances, we are working on microwave interferometers and reflectometers. Two main notable results were achieved. First, we achieved the measurement of the vibration amplitude of MEMS resonators with a resolution of 10-14 to 10-15 fm/√Hz.
With this resolution, the electrical measurement of the vibration of a MEMS resonant cantilever is similar or better than the optical detection [IEEE Transducers 2013 (accepted) / Appl. Phys. Lett., (submitted)]. This detection scheme is actually employed for experiments with MEMS based AFM sensors described above (Fig. 1). Secondly the interferometry technique is used for electrical measurement of small capacitances with an AFM coupled with a vectorial network analyzer. First results show the calibrated detection of 100 fF capacitances (Fig. 2) and is limited mainly by the parasitic capacitance of the measurement probe. This opens the way for RF characterisation of extreme impedances.