Completely monolithic high-Q micromechanical signal processors constructed of polycrystalline silicon and integrated with CMOS electronics are described. The signal processors implemented include an oscillator, a bandpass filter, and a mixer+filter-all of which are components commonly required for up- and down-conversion in communication transmitters and receivers, and all of which take full advantage of the high Q of micromechanical resonators. Each signal processor is designed, fabricated, then studied with particular attention to the performance consequences associated with miniaturization of the high-Q element.
The fabrication technology which realizes these components merges planar integrated circuit CMOS technologies with those of polysilicon surface micromachining. The technologies are merged in a modular fashion, where the CMOS is processed in the first module, the microstructures in a following separate module, and at no point in the process sequence are steps from each module intermixed. Although the advantages of such modularity include flexibility in accommodating new module technologies, the developed process constrained the CMOS metallization to a high temperature refractory metal (tungsten metallization with TiSi2 contact barriers) and constrained the micromachining process to long-term temperatures below 835degC. Rapid-thermal annealing (RTA) was used to relieve residual stress in the mechanical structures. To reduce the complexity involved with developing this merged process, capacitively transduced resonators are utilized.
The prototype high-Q oscillator uses a 16.5 kHz folded-beam, capacitive-comb transduced polysilicon microresonator in a series resonant oscillator architecture. The quality factor of the reference resonator has been measured to be 50,000 under a vacuum at 10 mTorr pressure. Its temperature coefficient is measured at -10 ppm/degC, linearly and monotonically decreasing with increasing temperature over a 270K to 370K temperature range. Equivalent circuits for general multi-port capacitively transduced micromechanical resonators are proposed for design and noise analysis purposes. The oscillator sustaining amplifier is transresistance and is equipped with variable gain controls for future amplitude-level control.
The 16.5 kHz prototype oscillator generates an output signal with visibly low phase noise. Theoretical analysis shows that phase noise contributions due to superposed electronic noise (from the sustaining amplifier and Brownian noise of the resonator) are comparable to those in macroscopic high-Q oscillators, for similar carrier powers. However, theoretical considerations also predict that as the dimensions of the mechanical resonator shrink (i.e., as the frequency increases), the short-term stability of the resonator becomes increasingly susceptible to mass loading effects, where instantaneous differences in the rates of adsorption and desorption of contaminant molecules give rise to mass fluctuations, hence, frequency noise. At 100 MHz, theory predicts that mass loading noise dominates over superposed electronic noise at certain pressures and temperatures. One of the keys to avoiding mass loading-derived phase noise is to operate the resonator at pressures which minimize its effect-generally, extremely low pressures (< 1 uTorr).
High-Q single resonator and spring-coupled micromechanical resonator filters are also investigated, with particular attention to noise performance, bandwidth control, and termination design. The noise in micromechanical filters is found to be fairly high due to poor electromechanical coupling on the micro-scale with present-day technologies. Poor coupling leads to high series motional resistance in resonators, which implies large amounts of Brownian motion noise at resonance. Solutions to this high series resistance problem are suggested, including smaller electrode-to-resonator gaps to increase the coupling capacitance. Despite the high series resistance, micromechanical filters still have enough dynamic range to satisfy IF applications.
Active Q-control techniques are demonstrated which control the bandwidth of micromechanical filters and simulate filter terminations with little passband distortion. Noise analysis shows that these active techniques are relatively quiet when compared with other resistive techniques.
Modulation techniques are investigated whereby a single resonator or a filter constructed from several such resonators can provide both a mixing and a filtering function, or a filtering and amplitude modulation function. These techniques center around the placement of a carrier signal on the micromechanical resonator. Mixing and filtering of signals from 200 MHz (RF) down to 20 kHz (IF) has been demonstrated.
Finally, micro oven stabilization is investigated in an attempt to null the temperature coefficient of a polysilicon micromechanical resonator. Here, surface micromachining procedures are utilized to fabricate a polysilicon resonator on a microplatform two levels of suspension equipped with heater and temperature sensing resistors, which are then imbedded in a feedback loop to control the platform (and resonator) temperature. Since excellent thermal isolation can be achieved on the micro-scale, only 2 mW of power was required to set the platform temperature at a reasonable bias point and control the temperature to stay there. Using micro oven control, the temperature coefficient of the resonance frequency is reduced from -10 ppm/degC to -2 ppm/degC
December 31, 1994
Nguyen, C. T. (1994). Micromechanical Signal Processors. United States: University of California, Berkeley.