Optimization of a Suspended, Hot Micro-Structure for Better Energy Efficiency and Improved Performance
Authors: Mahdavifar Alireza , Navaei Milad , Hesketh Peter 
Published: February 2020

Joule heating is utilized in many MEMS devices that serve as thermal sensors or actuators. The main participating element of these devices is a structure, suspended from the substrate to achieve high temperatures without wasting much electric power. Metal oxide micro gas sensors are a significant category of this type, where an embedded microheater raises the sensor temperature to facilitate interaction of the target gas with the sensing film surface. More importantly, micro thermal conductivity gas detectors (TCD) depend solely on the heat transfer from a suspended micro heater to detect concentration variations in the gas medium. In all these examples energy loss could be considerable and a significant concern for low power demanding applications, such as portable compact devices. Many applications in this field, such as microGC detector, require a new generation of fast micro electro-thermal detector that aims at lower mili-watt power consumption range while having superior sensitivity. We have tried developing 100×2 µm microbridge heaters made of doped polysilicon that offer a great thermal performance as a TCD.

Design improvements via try-and-error fabrication approach for MEMS devices is expensive, therefore computer simulation can be considered a better pathway towards optimizations in this field. The challenge, however, is the lack of accuracy in simulations due to complex three dimensional geometry, multi-physics phenomena, and simplifications involved in the molding. We have developed a novel comprehensive computer model that addresses all these issues by directly solving coupled system of governing equations in a large domain that includes all solid and gas participating medium. It was shown that the model was able to predict the response of a thermal conductivity detector microbridge with less than 10% error in nitrogen gas. Here we present the insight provided by the modeling and apply it towards optimization of the microbridge design.

Simulation results show that temperature distribution in a heated suspended micro element is highly non uniform, also a considerable amount of heat is conducted to the cool substrate through solid material. Solid constituents have relatively high thermal conductivity and solid pathways of heat don’t contribute to gas sensing. Therefore, a design that provides more thermal isolation for the heated element will further improve the efficiency.

As there are higher limits for the temperature of a material and, it means the whole temperature distribution is limited by the maximum values of the non-uniform temperature distribution. From this aspect it will be beneficial if the temperature distribution can be more uniform over the heater, although a fully uniform distribution cannot be achieved because of fast conduction in solid material.

To improve the above issues, we suggest geometrical optimizations while keeping all strong aspects of the current technology such as small dimensions, low power consumption, great surface to volume ratio and high aspect ratio. The recommendation includes having wider anchors made out of same electrically conductive material as the microbridge center, and suspended from the substrate as shown in figure 1.

The idea is that this wider part will have several folds less electrical resistance than the thin bridge due to the larger cross section. Therefor it provides electrical connection while doesn’t contribute to the power consumption; almost all the heat will be dissipated on the thin microbridge. The thermal resistance of the solid structure in this design is much higher and the wider anchor are suspended from substrate and don’t participate in heating. Thus a larger portion of heat will transfer through gas medium. The performance and energy consumption of the new design was verified through the developed simulation too.  Figure 2 shows the resulting temperature distribution and heat generation on the suspended element, which shows significant improvement over the previous design.

Figure 1