High Precision MEMS Digital    Temperature Sensor

In this work, we present a dual-resonator design which, not only enables temperature sensing of the resonators but also acts as a general-purpose temperature sensor. The frequency stability of the temperature compensated resonator depends on the accuracy with which the temperature of the resonator is measured. The dual-resonator design, described here, produces temperature-dependent beat frequency which is inherent to the resonator and thus eliminates any spatial and temporal thermal lag associated with the use of an external temperature sensor.   Furthermore, this design can also be used as a CMOS-compatible digital temperature sensor. We achieved the sensor resolution of approximately 0.008°C which is comparable to that of the best CMOS temperature sensors available today.

 
Figure 1 - Dual-resonator with composite beams of Si and SiO2. The thickness of thermally grown SiO2 coating is approximately 0.33 μm. The cross-section of the beams is designed to achieve two different temperature coefficients of frequency (TCf) for the two resonators, while keeping the two frequencies close together.

 

                                                       (a)                                                        (b)
Figure 2 - (a) SEM view of the composite Si resonator beam with thermally grown SiO2 coating (b) Enlarged view of the oxide coated resonator beam.

 
 
Figure 3 - Schematic of technique for generating the beat frequency from a dual-resoantor.

 

 
Figure 4 - Temperature dependence of f1 and f2 of dual-resonator.

 
 
Figure 5 - Comparison of the temperature dependence of the beat frequency with that of the dual-resonator frequencies.

 
 
Figure 6 - Temperature dependence of fbeat for various designs having resonator frequencies in the range of 1.0MHz, 1.5MHz and 2.5MHz.

 
 
Figure 7 -  Resonator f-T characteristic in rapid-temperature cycling (slew rate ~ 6°C /min) using (a) an external temperature sensor – Pt. RTD (b) beat frequency as a temperature sensor.
 
Figure 8 - Time-history plot of the beat frequencies (correlation coefficient ~ 0.9) from the two different devices at constant temperature (TCf of fbeat ~ -360 ppm/°C).
Figure 9 - Evaluation of the Allan deviation of the measured beat frequency data and its noise. We find that the resolution of the beat-frequency thermometer is 0.008 °C for a one-second measurement and as low as 0.0023 °C for a ten-second measurement.
 

 

The beat-frequency thermometer is probably the best temperature sensing technique for temperature compensation or temperature control of a MEMS-based reference oscillator. This technique eliminates the effect of thermal lag and static temperature gradients as the temperature signal comes directly from the resonator, and it provides a method that does not rely on analog signal processing, which might bring in added temperature coefficients. It is also important to point out that this device is a potentially interesting CMOS-Compatible digital thermometer for ordinary circuit applications. By using the compensated resonator to count the beat-frequency, the temperature can be determined to milli-degree accuracy, which makes this device competitive with the best CMOS thermometers available today.

 

Supported by the DARPA HERMIT Program

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