Thermal Isolation of MEMS Resonators

An in-chip thermal isolation technique for a micro-ovenized MEMS resonator is presented here. Resonators with a micro-oven are used for high precision feedback control of temperature to compensate for the temperature dependence of resonator frequency over a wide temperature range of -40°C to 85°C. However, ovenization requires power consumption for heating, and the thermal time constant must be minimized for effective temperature control. This work demonstrates an efficient local thermal isolation mechanism which can reduce the power requirement to a few milliwatts and the thermal time constant to a few milliseconds. In this method, the mechanical suspension of the resonator is modified to provide thermal isolation which also includes an integrated resistive heater. This combination provides mechanical suspension, electrical heating and thermal isolation in a compact structure that allows reduction in heating power with a small thermal time constant. A power consumption of approximately 12 milliwatts for a 125°C temperature rise and a thermal time constant of approximately 5 milliseconds is reported here, which is orders of magnitude lower than that of commercially available ovenized quartz resonators. All this is achieved without any modification to the fabrication process of the resonator. A CMOS-compatible wafer-scale encapsulation process is used to fabricate this device.

 
Figure 1 - Schematic of a typical MEMS resonator chip attached to a package with adhesive

 
Figure 2 - Isometric view of the device layer of the chip which contains resonant structure with input-output electrodes and a resistive heater. The thickness of the device layer is 20mm. The resistive heater is used to ovenize the device for temperature control to compensate the temperature dependence of the resonator frequency. However this type of heater is not thermally isolated and hence leads to high power consumption.

Figure 3 - Top view of double ended tuning fork (DETF) type resonant structure. FEM simulation of flexural vibration mode of a DETF (exaggerated view).
Figure 4 - Temperature profile along the length of a current carrying resistive heater having thermal resistance of Rth and electrical resistance of Re. The above temperature profile shows that if the resonator is attached at the center of the heater, it will maximize heating and minimize heat loss, thereby leading to reduced power consumption for the same temperature rise.
Figure 5 - Resonator design with local thermal isolation. The heater is in-built to the DETF such that the resonant structure is attached at the center of the heater. The entire structure is released except at the four anchors.
Figure 6 -  (a) Optical image of the top view of the fabricated device before the deposition of the encapsulation layer. (b) SEM view of the cross-section of a resonator after the deposition of the encapsulation layer.
Figure 7 - Isometric view of the device layer schematic showing the DETF with the in-built heater. A stimulus signal is applied to the input electrode. Heating voltages V1 and V2 are controlled using feedback control loop to maintain a constant bias for the resonator.
Figure 8 - Schematic of the test setup for frequency measurement. The device after being attached into the package is soldered in a PCB to conduct sweep measurement experiment to get a frequency output response.
Figure 9 -   Variation of resonator frequency due to joule heating of the in-built heater. The decrease in frequency (right y-axis) in the above plot corresponds to a temperature rise (left y-axis, pre-calibrated) with increasing input power. Experimental results are compared with theoretical estimation. 1-D analytical results give an upper bound as it is a measure of maximum temperature at the center of the in-built heater whereas the FEM output corresponds to the temperature at the center of the resonator beams.
Figure 10 -  Thermal response of resonator with in-built heater. Time constant of approximately 5 ms was measured using wheatstone bridge with an input pulse of 4V at 10Hz and a DC offset of 0.5V. Y-axis represents change in heater resistance due to cooling.
Figure 11 -  A drop test resulted in a temporary change in frequency at the time of drop. This test is done to check the flexibility of the in-built heater. In order to increase the thermal isolation, the thermal resistance of the heater should be as large as possible, however this also leads to a flexible structure which reduces the mechanical stability of the resonator. The above drop test confirms that in spite of large thermal resistance, this structure is mechanically stable.
 

An efficient heat delivery and thermal isolation mechanism for a MEMS resonator has been demonstrated. The in-built heater based thermal isolation technique serves a dual purpose of localized heating and thermal isolation, thereby providing maximum heating with reduced input power. At the same time the device has a small thermal time constant and high impact resistance because of its miniature design. Compared to the commercially used quartz crystals (1-10 Watts and around 30 minutes warm-up time), this work has demonstrated orders of magnitude improvement in power dissipation and thermal time constant with a potential for further improvement. Furthermore this method is simple enough to implement it into any existing MEMS fabrication process. The described design of micro-oven is highly suitable for temperature stabilization of micro-resonators and for very precise control of frequency (< 1.0 ppm) over a large temperature span.

Supported by the DARPA HERMIT Program

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