PORTLAND, Ore. MEMS oscillators harness electrostatic forces rather than relying on the piezoelectric effect used in traditional quartz crystals. But silicon resonators are less stable than quartz, forcing MEMS makers to include temperature compensation using a power-hungry phase-locked loop (PLL).
By adding a piezoelectric thin-film atop its silicon resonator, and using a switched capacitor method for performing temperature compensation (instead of a PLL), the Swiss Center for Electronics (CSEM) is claiming a 1,000-fold decrease in power consumption in a paper presented this week during the International Solid-State Circuits Conference.
CSEM's piezoelectric silicon resonator consumes 3 microamps while achieving a native accuracy for its real-time clock of 5 parts per million/degree Celsius compared to 15 to 30 PPM/degreeC for typical MEMS resonators that consumes 3 or more milliamps.
"Our MEMS resonators consumes microamps instead of milliamps," said David Ruffieux, the designer of the CSEM circuit. "That makes our MEMS oscillator three orders of magnitude lower in power than any currently available commercial product."
MEMS oscillator makers are seeking ultra-low power performance. For instance, SiTime Corp. (Sunnyvale, Calif.) recently announced the lowest power commercial MEMS oscillator. Epson Toyocom also recently claimed it had lowered the power consumption of its QMEMS real-time clock chip using a MEMS-sized resonator made from a quartz crystal.
Silicon Clocks Inc. (Fremont, Calif.) also recently claimed to have designed a low-power MEMS oscillatorusing a proprietary material wrapped around its resonator. The design puts stress on the device as temperature changes, but does not require piezoelectric materials on the silicon resonator itself.
CSEM's approach takes the middle road, retaining the silicon resonator but coating it with a thin film of aluminum nitride to make it a piezoelectric device. When a voltage is applied, the piezoelectric material contracts, permitting the resonator to oscillate in synchronization with the applied voltage in a manner similar to conventional quartz crystal oscillators (which are also piezoelectric).
"Silicon itself is not piezoelectric, so we added a layer of aluminum nitride," said Ruffieux. "We add a two-micron layer of aluminum nitride on top of a 20 to 100 micron silicon resonator on SOI wafers."
The second innovation claimed by CSEM is the use of a switched capacitor to provide precise temperature compensation to its real-time clock without a PLL running all the time, resulting in microamp current consumption. When the oscillator needs to supply an RF time base along with the real-time clock, a PLL is engaged to generate that frequency. When only a real-time clock is needed, the switched capacitor can handle the task.
"The principle is the same for all oscillators, namely that if you change the capacitance, you change its frequency," said Ruffieux. "We could have used a fine bank of capacitors, but since tuning is nonlinear, a better idea was to use a single big capacitance that we switch on and off in a variable duty cycle, allowing us to interpolate the desired frequency."
Since capacitive switching operates over a small range, it is used only for fine-tuning the frequency of oscillation. For coarse adjustments, CSEM used fractional division similar to that used in a PLL without the extra circuitry.