Silicon island

The doping dependence of the PS formation process and the possibility of transforming PS to oxide at relatively low temperatures has been used to form dielectrically isolated silicon islands, as shown in Fig. 10.23. 

Fig. 10. 23 Fabrication of a silicon island using the FIPOS process. Redrawn from [Im 1 ].

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. 

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 electrochemical etch-stop technology that produces the silicon island is rather complex, so that an etch stop directly on the dielectric layer would simplify the sensor fabrication (Sect. 4.1.2). The second device as presented in Fig. 4.6 was derived from the circular microhotplate design and features the same layout parameters of heaters and electrodes. It does, however, not feature any sihcon island. Due to the missing heat spreader, significant temperature gradients across the heated area are to be expected. Therefore, an array of temperature sensors was integrated on the hotplate to assess the temperature distribution. The temperature sensors (nominal resistance of 1 kfl) were placed in characteristic locations on the microhotplate, which were numbered Ti to T4. 

Instead of a silicon island underneath the dielectric layer, a polysilicon plate can be placed in the membrane center. Such a device was not fabricated, but the effect of a heat spreader that is integrated in the dielectric membrane was demonstrated by simulations. The results of the simulations are discussed in Sect. 4.2.2 [115,116]. 

Fig.4.6. Close-up of the circular microhotplate with temperatiu e sensor array and without silicon island...

Although this device has no feature for improving the temperature homogeneity (silicon island etc.), the temperature gradient at T3 at 300 °C hotplate temperature is only... 

In conclusion, a simple KOH-etching process without ECE is applicable for future microhotplate designs, although the best temperature homogeneity is achieved with the silicon island heat spreader. The island remains an important design feature, especially for the use of thin-film sensitive layers, where the additional heat spreading effect of the sensor materials is small. 

The third microhotplate design included a MOS-transistor heater embedded in a silicon island (Sect. 4.4). One advantage of this configuration is the reduction of the... 

D. Briand, S. Heimgartner, M.A. GretiEat, B. van der School, and N.R de Rooij. Thermal optimization of micro-hotplates that have a silicon island. . Journal of Micromechanics and Microengineering 12 (2002), 971-978. 

Fig. 6.11 When the electrode potential is reduced to —1800 mV vs. Fc/Fc+ silicon islands grow above the surface and merge laterally leading to agglomerates (a). The in situ i/U tunneling spectrum shows that both the layer and the islands exhibit a band gap of 1.1 0.2eV, typical for mycrocrystaline semiconducting Si (b).

Micro- and Nanoscale Anemometry Implication for Biomedical Applications, Figure 10 (a) Top view of transparent parylene skin with An wires connected to a single silicon island, (b) Side view of flexed patylene skin with Au wires and silicon island, (c) Flexed parylene skin with platinum (Pt) wires... 

Kyushu, Japan s southern island (Silicon Island)... 

Figure 2 Lithographic surface modification (A) photolithographic towers and (B) indented square posts (Oner and McCarthy, 2000), (C) diced silicon wafer (Yoshimitsu etal., 2002), (D) photolithographic towers (Zhu etal., 2006a) and (E) silicon nanowires grown on those silicon islands (Cao et al., 2007). Images reprinted with permission from (A, B, C, and E) American Chemical Society, Copyright 2000,2002 and 2007 respectively, (D) Elsevier, Copyright 2006. Adapted with permission of The Royal Society of Chemistry.

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