ENHANCING MICRO-OVEN POWER AND STIFFNESS IN ENCAPSULATED DEVICES FOR TIMING REFERENCE APPLICATIONS

In this paper, we investigate the tradeoff between power-usage and stiffness to find an in-chip device layer micro-oven design with optimal yield. A variety of micro-ovenized double-ended tuning forks (DETFs) anchored via a serpentine heater or micro-oven are fabricated in an ultra-clean vacuum seal encapsulation process. The micro-ovens contain beams of varying lengths, widths, and thicknesses. We compare the power required to raise the microovens to a fixed temperature and the yield of the devices. These results are the first report of yield statistics for in-chip device-layer ovenized devices. We show that a serpentine micro-oven located in the device layer of an encapsulated device with a thermal conductance of 0.17 mW/C and a 25% yield, has 4X improvement over the power required for earlier, stiffer micro-oven designs. INTRODUCTION MEMS timing references are replacing quartz clocks in industrial applications due to their improved stability, small size, and low cost [1]. To achieve the high frequency stability required for MEMS sensors in industrial applications, temperature compensation methods are essential. Previous work has shown that a combination of passive and active temperature compensation methods can achieve the frequency stability required in timing reference applications [1-11]. Active temperature compensation can be achieved using a resistor or micro-oven embedded within the device layer, which heats only the suspended resonating element that is thermally isolated within the die [1,3-5]. This “in-chip ovenization” enables a significant size and power reduction that can heat the resonating element to a specific temperature and allow its frequency to remain constant with drastic changes in the temperature of its surroundings. Although power is significantly reduced in the device layer micro-ovenization in comparison to a cap layer micro-oven (3mW/C) [2], the stiffness of the mechanical support may be sacrificed. Thermal conductance depends on the length, width, and thickness of the micro-oven beams. Generally, operating power can be reduced by increasing the length of the beams and decreasing the width and thickness of the beams. As these parameters are changed to increase thermal resistance, the micro-oven becomes more flexible and therefore more prone to fatal in-process stiction, which occurs during the fabrication process [12]. It is necessary to investigate the relationship between the microoven stiffness and thermal conductance to determine the limits of the thermal properties that can be achieved with a micro-oven embedded in the device layer in our encapsulation process. In this paper, we study a series of devices with different micro-oven designs to find this relationship and find the tradeoff between microoven power and yield. DESIGN AND FABRICATION We study the properties of a set of micro-ovens attached at the anchor of a double-ended tuning fork (DETF), as shown in Figure 1. We model the eigenfrequency mode for the DETF inplane Figure 1: Diagram of micro-ovenized DETF with parameters studied. bending mode using the COMSOL Structural Mechanics module. The frequency of the bending mode is 1MHz at room temperature, as shown in Figure 1. We model the thermal conductance of the micro-oven with the COMSOL Joule Heating and Thermal Expansion module. We apply a split heater voltage of 1V at the micro-oven terminals (+VH and -VH) and set the temperature at the terminals to 25C, as shown in Figure 3. The voltage applied to the micro-oven induces an electric potential across the micro-oven beams, which elevates their temperature to approximately 100C. The DETF structure is heated via conduction from the micro-oven. The parameters used in the simulations are defined in Table 1. We designed and fabricated a range of micro-ovens with varying beam widths, lengths, and thicknesses to compare the thermal conductance of micro-ovens with varying stiffness. Additionally, we designed and fabricated micro-ovens with geometric variations to compare the effect of differences in the beam geometry. The variations include serpentine heaters with beam holes, round edges, and variable beam widths, as shown in Figure 2. These structures contain two suspension structures, aligned with the X and Y axis to isolate the thermal gradients in these axes, such as the ones described in [3]. We model the effective stiffness of each device using the COMSOL Solid Mechanics module. We apply a 100mN force at the tip of the DETF along the Y-axis and measured the inplane deformation of the device along the Y-axis, we similarly model the out-of-plane stiffness (Z-axis) of each device. The parameters for beam variations and simulation results are summarized in Table 2. The devices were fabricated on p-type SOI wafers with a 40μm and a 60μm device layer using an epitaxial vacuum encapsulation Beam Length (L)