Balanced Dual Platinum Micro-Cantilever Thermal Conductivity Gas Sensor Using 3-Omega Technique

Introduction Thermal Conductivity Detector (TCD) is one of the commonly used sensors in Gas Chromatography (GC) systems. The principle behind TCD sensor is joule heating which heats the surrounding gas and the thermal-physical properties of the sensor changes based on the injected sample thermal conductivity [1, 2]. This technique can detect any gas that has different thermal conductivity with respect to the carrier gas. Therefore, TCD which is a universal gas detector have variety of applications including agriculture and environmental monitoring. TCD can also detect ammonia gas which has important role in agriculture [3]. There have been considerable efforts to miniaturize gas sensors, to achieve portable systems which require low power consumption and simple to operate outside the laboratory. Among these sensors, TCD has several advantages such as simple operation principle, short response time, high corrosion resistance and CMOS compatible fabrication process. One of the limitations of this sensing method is Limit of Detection (LOD), therefore, the TCD sensors geometrical design and electrical measurement method have been continuously engineered to improve it. The concept of cantilever-based platinum balanced TCD was introduced by our group to increase the sensors working temperature and ability to tolerate high thermally induced stresses. This advancement allows us to use the 3-Omega [3-5] technique for electrical measurement of the sensor while not sacrificing the high operating temperature. Method The conventional TCD sensor used a single layer of thin film for both heating and sensing. In this work, a new design for the cantilever-based platinum balanced TCD is proposed that has two active layers, one for heating and another for sensing. The TCD structure composed of ALD aluminum oxide layer sandwiched between platinum layers, and these three layers are sandwiched between two aluminum oxide layers. In designing double active layers TCD, the heating layer of the sensor needed to be thick enough to be able to pass a sufficiently high current. On the contrary, for the sensing layer to be sensitive to the change of temperature, it needs to be thin enough to have a high resistance value. A lumped thermal model was developed to study the time delay between platinum layers assuming constant properties and steady-state response. Results and Discussion The new design had two active layers which can use different types of excitation current or voltages. It w