CMOS-compatible

Many devices have been denoted to be CMOS-compatible , this term, however, not being clearly defined. In most cases CMOS-compatible means, that CMOS materials have been used, or the design can be used within a modified CMOS-process. As modifications in industrial CMOS-processes are difficult to implement, two main approaches have been pursued so far. One approach relies on an open process window... 

The basics of the paste preparation were explained in Sect. 2.3.3. For the devices presented in this book, the paste was deposited onto cleaned chips using a dropcoating method [48,61]. The deposition was performed by the company Applied-Sensor (Reutlingen, Germany). A metal-wire loop is immersed in the paste and the tin-oxide suspension adhering to the loop forms a droplet, which is accurately positioned in the membrane center. After the drop deposition, the whole chip is put in a belt oven and annealed for 20 min at a temperature of 400 °C. This temperature is close to the elevated-temperature steps at the backend of the CMOS process. Consequently, we never observed a significant difference of the circuitry performance between coated and uncoated chips. The whole deposition process is, therefore, fully CMOS compatible, and no additional on-chip annealing is necessary. 

M.Y. Afridi, J.S. Suehle, M.E. Zaghloul, D.W Berning, A.R. Hefner, S. Semancik, and R.E. Cavicchi. A Monolithic Implementation of Interface Circuitry for CMOS Compatible... 

Moreover, since the process is completely complementary metal oxide semiconductor (CMOS) compatible, more complex systems can be developed, with a large variety of components as different as valves, coolers, and photodetectors. 

A third application for pTAS is in the biomedical field. Gumbrecht et al. [46, 47] developed a monolithically integrated, ISFET-based sensor system for (bedside) monitoring of blood pH, p02 and pC02 in patients. Here the successful introduction on the market mainly depends on the price of the system, for which reason a CMOS-compatible design of the silicon part is needed. Evidently, such a development is only possible in the case of a high volume market. 

Two challenges are uniform thinning of the top wafer and high aspect ratio etching of the thinned layer. An approach to high-density TSV interconnection using a variety of CMOS-compatible fabrication steps (such as CVD tungsten and polysilicon, BCD copper, and solder micro-bumps) is presented by Knickerbocker et al. in References 86 and 87. 

S van der GroenRosmeulen M, Jansen P, Baert K, Deferm L. CMOS compatible wafer scale adhesive bonding for circuit transfer. Proceedings of The International Conference on Solid-State Sensors and Actuators 1997. p 629-632. 

Surface micromachining does not usually require two-sided processing of the silicon, rendering it often more CMOS-compatible. A structural material is patterned over the top of a sacrificial material. Subsequently, the sacrificial material is etched, leaving the anchored structural material free to move. Capacitance is used almost exclusively as the transduction technique with surface micromachined devices. An example of a three-layer polysilicon surface micromachined accelerometer is shown in Fig. 7.1.12f. 

In comparing different sensor technologies it should be mentioned that 60-80% of sensor fabrication cost consists of packaging costs, an aspect not addressed frequently enough. In comparison with CMOS-compatible sensors, the packaging of thick-film sensors is often easy. Since packaging expenses overshadow all other costs, this is a very important decision criterion. 

One IC-based sensor array approaching commercialization is the Wide Range Ha Sensor [63, 64], which combined CMOS-compatible metal-insulated semiconductor (MIS) diode sensors and resistive sensors (Pd-Ni alloy) with on-chip control electronics. The wide-range claim relies on the MIS sensor for sensitivity to low concentrations of hydrogen gas (0.1%), and the resistive sensor for higher concentrations (1-100%). Non-standard IC processes for deposition of the Pd-based structures were devised at the Sandia foundry. Tests indicated little variability among sensors, e.g. 1-2% error. It was found that sensor exposures... 

Ster, E., Klemic, J. F., Routenberg, D. A., Wyrembak, P. N., Turner-Evans, D. B., Hamilton, A. D., LaVan, D. A., Fahmy, T. M., Reed, M. A. (2007). Label-free immunodetection with CMOS-compatible semiconducting nanowires. Nature 445, 519-522. 

The heater material is a crucial point for the stability of this type of device during operation. Driven by CMOS compatibility, Poly-Si was first used but it suffers from an inappropriate drift of its electrical resistivity at high temperature (Ehmann et al, 2001). Platinum is the material that has been implemented for the heater for improved reliability. It is used in most micromachined metal-oxide sensors on the market at the time of writing, not only for the heater, but also for the electrodes. Courbat et al (2008) showed that adding a small amount of another refractory metal (such as iridium) to the platinum can improve its resistance to electromigration. However, Mo exhibited superior performances to platinum, allowing higher operational temperatures (Mele et a/., 2012). 

Complementary metal-oxide semiconductor (CMOS)-compatible metal-oxide gas sensors... 

Four main concerns need to be addressed when integrating metal-oxide sensors in a CMOS-compatible process ... 

The post-deposition of the metal-oxide sensing layer needs to be CMOS-compatible, and its post-deposition annealing is limited in terms of temperature and time. 

Laconte, X, Dupont, C., Flandre, D. and Raskin, J.-P. (2004), SOI CMOS compatible low power microheater optimization for the fabrication of smart gas sensors , IEEE Sensors 7,4(5), 670-80. 

M. Agirregabiria, F. Blanco, J. Berganzo, M. Arroyo, A. Fullaondo, K. Mayora, and J. Ruano-Lopez, Fabrication of SU-8 multilayer microstructures based on successive CMOS compatible adhesive bonding and releasing steps. Lab on a Chip, 5(5), 545-552, 2005. 

The microfabrication of electrode arrays built with silicon micromachining techniques illustrates an positive approach towards future Cl electrode array development in respect to the traditional manufacturing method used now days. Also lithography and MEMS technology facilitates the addition of enhanced functionality to the microelectrode arrays. There is, however, still a long way to go until these devices can be used in real Cochlear Implants. The fabrication possibilities and characterization of different CMOS compatible metals (Ti, TiN and Al) provides a strong base to go ahead with further research in this direction. In our electrical tests done we conclude that TiN is able to withstand a high current density 2.8, while aluminium failed... 

The surface micromachining technique is a CMOS-compatible process and has some advantages over the bulk micromachining one. The most popular microstructure is the so-called comb-drive structures as shown in Fig. 7. Several commercially available sensors with integrated circuit on chip have been developed by using this technique such as accelerometers, gyroscopes, microphones, etc. 

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