Development of Low Power Micro Preconcentrator (μPC) for Chemical Sensing Applications

Abstract

In recent years, there is a growing need for environmental monitoring systems that can reliably trace contaminants that affect human health and environmental safety. These systems comprise of three main sub-systems: the detector(s), separation columns, and pre-concentrators. The pre-concentrator is the vital part of any systems. The pre-concentrator is used to purify a sample that increases the concentration of the targeted analytes in a range of detection limit of the detector. The ideal pre-concentrator has high adsorption capacity and desorption efficiency [1]. In this work, we have optimized the pre-concentrator’s geometry by maximizing the active surface to volume ratio while improving the desorption efficiency. These optimizations reduce the power consumption, pressure drop across the channel while shortening desorption time. The fabrication process starts with transfer of mask pattern on to a silicon wafer using photolithography and etching the silicon using Deep Reactive Ion Etching (DRIE) for 360 µm. Next, a double sided polished silicon wafer is used to seal the pre-concentrator by gold eutectic fusion bonding [2]. The last step is to deposit the heater on the backside of the wafer using the lift-off process. The advantage, of DRIE over wet etch, is to have a high aspect ratio structure and low surface roughness level. The low surface roughness is essential for the uniform coating of the adsorbing material. The positive photoresist SPR 220 is used as the soft mask for DRIE process to reduce fabrication steps. The pre-concentrator has pillars with 50 µm in diameter and 25 µm in spacing. Other critical dimensions have been shown in Figure 1(a). The total surface to volume ratio of the pre-concentrator is 58,000 m-1 with the total volume of 13.8 µL. To optimize the geometry of the pre-concentrator, a COMSOL model was developed to calculate the pressure drop across the preconcentrator while evaluating the heater performance. The preconcentrator ramp rate is 47.5 °C/second and the average power consumption is, Figure 1(b). The pressure drop across the pre-concentrator is 1.2 kPa for the air flow of 40 sccm. Voiculescu, I., M. Zaghloul, and N. Narasimhan, Microfabricated chemical preconcentrators for gas-phase microanalytical detection systems. TrAC Trends in Analytical Chemistry, 2008. 27(4): p. 327-343.Navaei, M., et al., Micro-Fabrication of All Silicon 3 Meter GC Columns Using Gold Eutectic Fusion Bonding. ECS Journal of Solid State Science and Technology, 2015. 4(10): p. S3011-S3015. Figure 1