Micro Structures & Sensors Lab
Kenny Group @ Stanford

Ovenized Dual-Mode Clock (ODMC)

Yunhan Chen

:: Introduction

The Ovenized Dual-Mode Clock (ODMC) is an epi-sealed dual-mode silicon MEMS resonator operational over a large ambient temperature range. We maintain a localized, elevated operating temperature of the resonator by utilizing the temperature coefficient of frequency (TCf) difference between two excitation modes of the same resonant body as a thermometer, and by integrating a micro-oven in the encapsulation layer. Compared with other ovenization schemes that involve an extra thermometer or two separate resonant bodies, the dual-mode ovenization method eliminates temperature gradients between resonators and thermometers, which in turn increases the overall stability over a large ambient temperature range.

Figure 1. Schematic of the fully-encapsulated Lamé resonator              Figure 2. Temperature dependence of frequency of ambient
 which incorporates the functions of out-of-plane sensing and                 temperature for unheated devices. Dashed lines are simulat-
micro-ovenization in the encapsulation layer.  And a top view               -ion results for n-type 6.2e19cm-3 doping; circles are experi-
 SEM image of the Lamé resonator design.                                            -mental data.                                                                             

:: Design and Method

The design of the device is illustrated in Figure 1. A square shaped Lamé resonator with a circular center anchor hanging from the top is fulled encapsulated in an high vacuum, ultral clean environment. Top electrodes are integrated in the cap layer. The Lamé resonator, in this case, can be simultaneously operated as a plate-bending-mode resonator. Due to the n-type doping and its high concentration, two distinct TCf characteristics for the Lamé mode and out-of-plane plate-bending mode can be observed as shown in Figure 2.
Both modes are driven into resonance simultaneously. The resonant frequencies are tracked and the frequency difference between the two modes is used as a thermometer and provides feedback to the control loop. The Joule heating current passing through the micro-oven in the encapsulation layer is controlled to maintain a constant elevated device temperature.

:: Real-Time Frequency Stability Measurements

Real-time frequency measurements of the ODMC subject to 6℃/min ambient temperature ramps over -20 ~ +80℃ are shown in Figure 3. Measured frequency deviates within a range of ±250ppb. No frequency drift or hysteresis is observed over a testing period of 16 hours.

Figure 3. Real-time (a) frequency; (b) heating voltage; (c) ambient temperature;
(d) frequency difference between two modes measurement of the ODMC subject
to 6℃/min ambient temperature ramps.

:: Publications

[1] Yunhan Chen, Eldwin J. Ng, Dongsuk D. Shin, Chae H. Ahn, Yushi Yang, Ian B. Flader, Vu A. Hong and Thomas W. Kenny, "Ovenized Dual-Mode Clock (ODMC) Based on Hightly Doped Single Crystal Silicon Resonators", 2016 29th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), pp. 91-94, Jan 2016.  (Best Paper Award) [LINK]
[2] Yunhan Chen, Eldwin J. Ng, Yushi Yang, Chae Hyuck Ahn, Ian Flader, and Thomas W. Kenny, "In-situ ovenization of Lamé-mode silicon resonators for temperature compensation," 2015 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), pp. 809-812, Jan 2015.   [LINK]
Last updated on: Apr 5 2016 12:59pm