Micromachining hotplates

To fabricate the hotplates one can use either silicon bulk or surface nucromachining (Mlcak et al. 1994 Lang 1996 Simon et al. 2001). The features of membranes developed using these two approaches are shown in Figs. 7.3 and 7.4. Surface micromachining creates free-standing membranes using 

Sacrificial layer Mechanical layer Sacrificial etchant 

Silicon dioxide Polysilicon, silicon nitride, silicon carbide HF-based 

Polysilicon Silicon nitride, silicon carbide KOH-based 

Resist, polymers Aluminum, silicon carbide Acetone, oxygen plasma 

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B. Margesin, F. Giacomozzi, M. Zen, G.U. Pignatel, A.A. Vasiliev, and A.V. Pisliakov. Development of a low-power thick-film gas sensor deposited by screen-printing technique onto a micromachined hotplate , Sensors and Actuators B77 (2001), 95-99. 

Obverse of CMOS silicon micromachined hotplate (a) optical picture (b) diagram. Courtesy of Dr Steve Semancik, NIST, USA. 

In the main, two types of metal-oxide gas sensitive films have been integrated into micromachined hotplate transducers thin and thick films. The different developments will be presented in this section. The integration of a third type of structure - nanowires, into which considerable efforts are... 

Nanowires are seen as a solution with which to improve the sensitivity, selectivity, stability and response time of metal-oxide gas sensors. Meier et al (2007) grew Sn02 nanowires of 100 nm in diameter by the vapor-solid growth method. For testing, they were deposited onto micromachined hotplates with a focused ion beam scanning electron microscope (FIB-SEM), as shown in Fig. 6.19. Due to their diameter being similar to the Debye length, a completely depleted conduction channel can be obtained. Maximum response to CO and NH3 occurred at about 260°C. 

Ryu, K., Zhang, D. and Zhou, C. (2008), High-performance metal oxide nanowire chemical sensors with integrated micromachined hotplates , Appl. Phys. Lett, 92,093111. 

Recently the advantages of using hybrid sensors id est. combinations between thick film sensing layers and micromachined hotplates when one tries... 

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. 

It then addresses the micro-hotplates concept that has led to the development of different types of micromachined gas sensor devices. The different reahzations of micromachined semiconductor gas sensors are presented thin- and thick-film metal-oxide, field effect, and those using complementary metal-oxide semiconductors (CMOSs) and silicon-on-insulator (SOI) technologies. Finally, recent developments based on gas sensitive nanostructures, polymers, printing and foil-based technologies are highlighted. 

Key words silicon micromachining, micro-hotplates, semiconductor, metal-oxide, field-effect, gas sensors, CMOS and SOI, nanowires, printing, polymeric, plastic. 

This chapter therefore focuses on silicon micromachined semiconductor gas sensors. After a brief history of silicon hotplates and metal-oxide... 

Micro-hotplates are made using a combination of thin-fihn and silicon micromachining processes. There are two main kinds of micromachined silicon substrates closed-membrane and bridge-membrane. They consist of a suspended thin dielectric membrane, made of silicon nitride and/or silicon oxide, that is released using silicon micromachining on either the obverse or... 

The dielectric membrane of the micro-hotplates will be composed of CMOS dielectric films. It can be formed through a silicon micromachining post-process either on the back or the front. 

Cerda Behnonte, J., Pnigeorbe, J., Arbiol, J., Vila, A., Morante, J. R.,Sabate, N., Gracia, I. and Cane, C. (2006), High-temperature low-power performing micromachined suspended micro-hotplate for gas sensing appheations . Sens. Actuators... 

Houlet, L. F.,Tajima, K., Shin, W, Itoh,T., Izu, N. and Matsubara, I. (2006) Platinum micro-hotplates on thermal insulated structure for micro-thermoelectric gas sensor. lEEJ Transactions on Sensors and Micromachines, 126,568-572. 

Bulk Micromachining, Fig. 4 (a) Micro gas sensor array comprising micro-hotplates, (b) Etched mesa structures without convex comer compensation mask, (c) Etched mesa structures with convex comer compensation mask... 

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