I-SECTION ANALYST TUTORIAL REVIEW THE www.rsc.org/analyst CMOS-based chemical microsensors Andreas Hierlemann* and Henry Baltes Physical Electronics Laboratory, ETH Zurich, Hönggerberg, HPT H 4.2, CH-8093 Zurich, Switzerland. E-mail: hierlema@iqe.phys.ethz.ch; Fax: +41 1 633 1054; Tel: +41 1 633 3494 Received 2nd September 2002, Accepted 14th November 2002 First published as an Advance Article on the web 4th December 2002 1 Introduction 2 CMOS technology 3 Micromachining techniques 3.1 Bulk micromachining 3.2 Surface micromaching 4 CMOS-based chemical sensors 4.1 Chemomechanical or mass-sensitive sensors 4.1.1 Flexural-plate-wave or Lamb-wave devices 4.1.2 Resonating cantilevers 4.2 Thermal sensors 4.2.1 Catalytic thermal sensors (pellistors) 4.2.2 Thermoelectric or Seebeck-effect-based sensors 4.3 Optical sensors 4.3.1 Bioluminescent bioreporter integrated circuits (BBIC) 4.4 Electrochemical sensors 4.4.1 Voltammetric/amperometric sensors 4.4.2 Potentiometric sensors (chemotransistors) 4.4.3 Conductometric sensors 4.4.3.1 Chemoresistors 4.4.3.2 Chemocapacitors 4.5 Monolithic integration of different transducers 4.5.1 CMOS multiparameter biochemical sensor 4.5.2 CMOS single-chip gas microsensor system 5 Outlook 6 Acknowledgements 7 References 1 Introduction The rapid development of integrated circuit (IC) technology during the past decades has initiated many initiatives to fabricate chemical sensors on silicon or CMOS substrates (CMOS: Complementary Metal Oxide Semiconductor). 1,2 The largely two-dimensional integrated circuit and chemical sensor structures processed by combining lithographic, thin film, etching, diffusive and oxidative steps have been recently extended into the third dimension using micromachining or MEMS (MEMS: MicroElectro-Mechanical Systems) technolo- gies—a combination of special etchants, etch stops and sacrificial layers. 3–9 A variety of micromechanical structures including cantilever beams, suspended membranes, freestand- ing bridges, etc. have been produced. 3–9 The realization of microelectronics and micromechanics (MEMS-structures) on a single chip allows for on-chip control and monitoring of the mechanical functions as well as for data preprocessing such as signal amplification, signal conditioning, or data reduc- tion. 3–12 CMOS- or CMOS-MEMS-technology, therefore, provides excellent means to meet some of the key criteria of chemical sensors such as miniaturization of the devices, low power consumption, rapid sensor response characteristics, or batch fabrication at industrial standards and low costs. Additional advantages come from the possibility of monolithic co- integration of circuitry and transducers. These include improved sensor signal-to-noise characteristics due to on-chip signal processing and analog/digital conversion, or the realization of smart features on the sensor chip. Drawbacks of using CMOS technology encompass a limited selection of materials (see CMOS technology section) and a predefined fabrication process for the CMOS part. Sensor-specific or transducer-specific materials and fabrication steps have to be introduced as post- processing after the CMOS fabrication. 13 After a short introduction to CMOS and basic micro- machining technologies, we will give an overview on different types of chemical sensors fabricated in CMOS and CMOS- MEMS technology. Andreas Hierlemann received a Diploma in Chemistry in 1992 and a PhD in Physical Chemistry in 1996 from the University of Tübingen, Germany. After working as a Postdoc at Texas A&M University, College Station, TX (1997), and Sandia National Laboratories, Albuquerque, NM (1998), he moved to ETH Zurich in Switzerland where he is currently a member of the technical staff. The focus of his re- search activities is on CMOS transducers, chemical sensors and interfacial design. Henry Baltes received a DSc degree from ETH Zurich in 1971. He has held faculty positions at Freie Universität Berlin and University of Düsseldorf, Germany, University of Waterloo, Canada and EPF Lausanne, Switzerland. He worked for Landis & Gyr Zug, Switzerland (1974–1982), directing the Solid- State Device Laboratory; he held the Henry Marshall Tory Chair at the University of Alberta, Edmon- ton, Canada (1983–1988), where he directed a research program in microsensors; he was a Director of LSI Logic Corporation of Canada (1986–1988), and since 1988 he has been Professor of Physical Electronics at ETH Zurich and Director of the Physical Electron- ics Laboratory active in silicon integrated micro-systems. This journal is © The Royal Society of Chemistry 2003 DOI: 10.1039/b208563c Analyst, 2003, 128, 15–28 15