Monolithic gas sensor systems

This chapter includes two different sensor system architectures for monolithic gas sensing systems. Section 5.1 describes a mixed-signal architecture. This is an improved version of the first analog implementation [81,91], which was used to develop a first sensor array (see Sect. 6.1). Based on the experience with these analog devices, a complete sensor system with advanced control, readout and interface circuit was devised. This system includes the circular microhotplate that has been described and characterized in Sect. 4.1. Additionally to the fabrication process, a prototype packaging concept was developed that will be presented in Sect. 5.1.6. A microhotplate with a Pt-temperature sensor requires a different system architecture as will be described in Sect. 5.2. A fully differential analog architecture will be presented, which enables operating temperatures up to 500 °C. 

---

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

M.Y. Afridi, J.S. Suehle, M.E. Zaghloul, D.W. Berning, A.R. Hefner, R.E. Cavicchi, S. Semancik, C.B. Montgomery, and C.J. Taylor. A monolithic CMOS microhotplate-based gas sensor system , IEEE Sensors Journal 2 (2002), 644-655. 

D. Barrettino, M. Graf, S. Taschini, M. Zimmermann, C. Hagleitner, A. Hierlemann, and H. Baltes. Hotplate-based conductometric monolithic CMOS gas sensor system , Proc. IEEE VLSI Symposium, Kyoto, Japan, (2003), 303-306. 

Microfluidic concepts can be used to develop an integrated total chemical analysis system (TAS) [40], which include sample preparation, separation, and detection. The microminiaturization of a TAS onto a monolithic structure produces a //-TAS that resembles a small sensor. The first /(-TAS was a micro-gas chromatograph (GC) fabricated on a 5-in. silicon wafer in 1979 by a group at Stanford University [41]. Since then, developments in micromachining has led to the development of microsensors, microreactors,... 

On-chip analyte transport can be realized by integrating microfluidic flow systems or miniaturized gas chambers with the sensor device [69]. Hu et al. demonstrated a microfluidic sensor device monolithically integrated with planar Ge-Sb-S ChG waveguides [70]. Quantitative chemical sensing via evanescent wave absorption spectroscopy was demonstrated using the microfluidic device. 

---