Microhotplates in CMOS technology

The following chapter includes the description of different types of microhotplates that feature resistor and transistor heating elements. Three of them were specifically designed to be monolithically integrated with circuitry, and one was a testing device that was used for the assessment of temperature distributions on the microhotplates. 

The first device is a circular microhotplate (Sect. 4.1). One important guideline was to implement the microhotplate in CMOS technology with a minimum of post-CMOS micromachining steps. Additionally the hotplate had to be optimized for drop-coating with nano crystalline tin-oxide layers. This microhotplate was cointegrated with circuitry, and the respective monolithic sensor system will be discussed in Sect. 5.1. 

The second microhotplate design is derived from this circular microhotplate, in contrast to the first device, it does not feature a silicon island underneath the heated area, but exhibits a network of temperature sensors in order to assess the temperature distribution and homogeneity (Sect. 4.2). The measured temperature distribution was compared to simulations, and the model described in Chap. 3 was validated. Furthermore, the influence of the tin-oxide droplet on the temperature distribution was studied. A microhotplate without silicon island is much easier to fabricate, though the issue of sufficient temperature homogeneity has to be evaluated. 

The third microhotplate introduced in Sect. 4.3 was designed to extend the operation temperature limit imposed by the CMOS-metallization contacts in the heated area. A new heater design was devised, and a microfabrication sequence that enables the realization of Pt temperature sensors and Pt-electrodes was developed. This microhotplate was also monolithically integrated with circuitry as presented in Sect. 5.2, and operating temperatures of up to 500 °C have been achieved. 

Finally, a transistor-heated hotplate will be described (Sect. 4.4), which offers the advantage of lower power consumption, since there is no additional power transistor needed on the chip. Moreover, a transistor hotplate can be digitally controlled and addressed so that new operation modes can be realized (Sect. 4.5). The integration of the transistor hotplate with accompanying, mostly digital circuitry will be described in Sect. 6.3. 

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M. Graf, R. Jurischka, D. Barrettino, and A. Hierlemann. 3D nonlinear modeling of microhotplates in CMOS technology for use as metal-oxide-based gas sensors . Journal of Micromechanics and Micro engineering 15 (2005), 190-200. 

D. Barrettino, M. Graf, M. Zimmermann, A. Hierlemann, and H. Baltes. A Smart Single-chip Microhotplate-based Chemical Sensor System in CMOS Technology , Proc. IEEE International Symposium on Circuits and Systems (ISCAS), Phoenix, AZ, USA (2002) Vol. 2, 157-160. 

R. Jurischka. Microhotplate-basierte Metalloxid-Gassensoren in CMOS-Technologie, Diploma thesis, Institute for Microsystem Technology, University of Freiburg, Germany (2002). 

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