Silicon micromachined thin-film

Bulk-micromachined membranes are usually formed from dielectric materials like silicon oxide or silicon nitride combined with additional materials for example, in pressure sensors, silicon is used to increase the membrane thickness to the required values and in thermal sensors, platinum or other metals are needed for the sensing elements. The overall stress state of the membranes has to be controlled well to prevent buckhng (under high compressive stress) or fracture (under high tensile stress). With proper processing control, silicon oxide and silicon nitride thin films meet this requirement, making them ideal candidates for membrane-type devices. 

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. 

Micro electrode arrays can also be produced by thin film technology and silicon micromachining. Electrochemical analysis using planar thin film metal electrodes as transducer can be done with high performance in vitro [59]. 

For air monitoring a complete miniaturized system made by silicon micromachining has been proposed [86]. Valves,gas fluidics, filters, thin film sensors and pumps are integrated in silicon and mounted on a printed circuit board (Fig. 3). The application of such systems will become apparent in the future. 

The mechanical properties of thin films used as membranes or moving structures and produced either by bulk or surface silicon micromachining are of crucial importance. In either type of application, the mechanical stress and the fracture properties of the applied thin films determine the behavior and long-term reliability of the device. In automotive applications, the environmental constraints (e.g., high ambient temperatures and temperature variations) and the required long life expectancy of the sensors require the use of high-quality thin films with well-controlled properties. 

The CVD method is very versatile and can work at low or atmospheric pressure and at relatively low temperatures. Amorphous, polycrystalline, epitaxial, and uniaxially oriented polycrystalline layers can be deposited with a high degree of purity, control, and economy. CVD is used extensively in the semiconductor industry and has played an important role in past transistor miniaturization by making it possible to deposit very thin films of silicon. CVD also constitutes the principal building technique in surface micromachining (see below). 

Microfabrication and micromachining techniques have also been used in the manufacture of electrochemical sensors. This includes po and pco sensors. Zhou et al [9] describe an amperometric CO2 sensor using microfabricated microelectrodes. In this development, silicon-based microfabrication techniques are used, including photolithographic reduction, chemical etching, and thin-film metallization. In Zhou s study, the working electrodes are in the shape of a microdisk, 10 pm in diameter, and are connected in parallel. In recent years, silicon-based microfabrication techniques have been applied to the development of microelectrochemical sensors for blood gases, i.e. P02. Pcoj and pH measurements. 

A diagram of a typical cross-sectional view of a silicon micromachined metal-oxide (MOX) sensor is presented in Fig. 6.2. Their development has evolved towards silicon substrates to produce devices suitable for commercialization due to their low-cost, low-power consumption and high reliability. To lower the resistivity of the gas sensitive film, as well as to improve the kinetics of the chemical reactions, the metal-oxide layer is heated with a micro-heater. The heated area is usually embedded in a thin dielectric membrane to improve the thermal insulation and to reduce the power consumption of the device, which is typically in the order of a few tens of milliwatts at 300°C, and its thermal time constant (few to tens of milliseconds). Thermal programming allows kinetically controlled selectivity. 

By adding small amount of O2 gas, the etch rate can be accelerated. In general, in this approach, the etch rate is relatively small and in the range of few nanometers to tens nanometers per minute. The RIE process is not applicable for bulk material micromachining, and therefore, it is suitable only to etch thin films with thickness of few micrometers. However, RIE is a suitable process to etch shallow microchannels in silicon substrate with good etch-depth control. 

Instead of deposition of different thin films, silicon-on-insulator (SOI) micromachining may... 

The cavity can be made of silicon, quartz, or glass substrates or any other appropriate materials. Because of the convenience of silicon micromachining techniques, silicon is commonly used to fabricate the cavity and flexible membrane. Normally a thin-film heating resistor is integrated onto one inner side of the cavity, with a flexible diaphragm wall sealing the opposite inner side of the small cavity. When an electrical current is passed through the resistor, the trapped fluid is heated to evaporate and expand. The expansion causes the flexible wall to flex outward. This outward flex movement is then used to close or open the flow. An SCE microactuator can function as a normally opened microvalve, which opens the channel in its normal... 

Mescheder U (2004) Porous silicon technology and applications for micromachining and MEM-S. In Yurish SY, Gomes MTS (eds) Smart sensors and MEMS, vol 181, Nato science series, series II mathematies, physics and chemistry. Kluwer, Dordrecht, pp 273-288 Misra SCK, Bhattacharya R, Angelucci R (2001) Integrated polymer thin film macroporous silicon microsystems. J Ind Inst ScTi 81 563-567... 

There are now three top-down techniques developed to realize porous silicon membranes from solid silicon electrochemical etching (anodization), micromachining, and thin film deposition/annealing. These techniques create different pore morphologies and are suited to different membrane thiek-nesses and porosity ranges. In this regard they are quite complementary. 

---