Micromachined sensors, silicon

We expect these micromachined sensors to become more and more important in the household industry, in many domestic applications of silicon pressure sensors, acceleration sensors, tilt sensors, infrared detectors and thermopiles, flow meters, as well as gas sensors and liquid constituent sensors. 

Cutting-edge Silicon-based Micromachined Sensors for Next Generation Household Appliances... 

D Processing Technology 200 Sensors, Actuators and Passive Components 201 Bulk Micromachining Technology 201 Surface Micromachining of Silicon 205 Summary 205... 

Mass-produced cantilever sensors, however, have the potential to satisfy the conditions of selectivity, sensitivity, miniature size, low power consumption, and real-time operation [5, 6], Microcantilevers are micromachined from silicon or other materials and can easily be fabricated in multiple-element arrays. They resemble miniature diving boards measuring 100 to 200 pm long by about 20 to 40 pm wide by 0.3 to 1 pm thick and having a mass of a few nanograms. Their primary advantage originates from their sensitivity, which is based on the ability to detect their motion with subnanometer precision. 

Silicon microcantilever sensors that can be mass-produced using currently available microfabrication techniques, however, have the potential to satisfy the conditions of sensitivity, miniature size, low power consumption, and real-time operation [2], Microcantilevers are generally micromachined from silicon wafers using conventional techniques. Typical dimensions of a micromachined cantilever are 100 p,m in length, 40 p,m in width, and 1 xm in thickness. The primary advantage of a cantilever beam originates from its ability to sensitively measure displacements with sub-nanometer precision. Sensitive detection of displacement leads to sensitive detection of forces and stresses. 

O. Tabata, pH-controlled TMAH etchants for silicon micromachining, Sensors Actuators A 53, 335, 1996. 

R.M. Langdon, Micromachining of silicon for sensors, Proc. NATO ASI on Novel Si-Based Technologies 1989, 143-172. 

The term polysilicon arises from the structure of these silicon layers, which are essentially polycrystalline in contrast to single-crystalline silicon substrates, due to the growth of these films on amorphous starting layers (in silicon surface micromachining, a silicon oxide layer is usually used as both a seed layer and a sacrificial layer). Polysilicon is widely used in sensor technology it can be used as part of a membrane layer, as an electrical connector, or as a part of a thermopile structure. In this contribution, we focus on its most important function - as the functional layer in surface-micromachined structures. In surface micromachining basically two approaches for producing polysilicon films are used ... 

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. 

Most silicon accelerometers are based on a micromachined variable capacitance element (g-cell) that is converted to a voltage using a C-V converter and then amplified, filtered, and buffered to provide an analog output as shown in Fig. 7.1.4. To date, open-loop implementations for capacitive read-out circuits are more widely employed than closed-loop systems, primarily as a result of the stability of such systems [16]. Interface electronics for micromachined sensors depend not only upon the transduction technique (input specification) and the product requirements (output specification) but also on the packaging approach, as parasit-ics are introduced when a multiple-die packaging technique is used. 

Yu Dun, Hsieh H Y and Zemel J N 1993 MicroChannel pyroelectric anemometers for gas flow measurements Sensors Actuators 39 29-35 Pfahler J, Harley J C, Bau H H and Zemel J N 1990 Liquid transport in micron and submicron channels Sensors Actuators A 21-23 431 Wilding P, Pfahler J, Bau H H, Zemel J N and Kricka L J 1994 Manipulation and flow of biological fluids in straight channels micromachined in silicon Clin. Chem. 40 43-7... 

Diagram of the MGS 1100 sensor from Motorola. Micromachined sensor element is illustrated on the left, and the sensor housing on the right. The sensitive films were obtained by rheotaxial growth and thermal oxidation of tin layers (RGTO) deposited on the silicon oxide-nitride membrane. From Simon etal. (2001). 

Other substrates that are used for MEMS include quartz, glass, and polymers. Quartz is attractive primarily because it is piezoelectric, so that it may be used as a sensor and an actuator. Quartz is, however, more difficult to micromachine than silicon. Pyrex glass is used in conjunction with silicon wafers, primarily for packaging, because of its optical transparency and close match in coefficient of thermal expansion (GTE). Polymers such as polycarbonate have been adopted as substrates for microfluidics because of their low cost channels require large areas, which makes silicon too expensive, and channels can be fabricated in polymers inexpensively by hot embossing. The advantages offered by silicon substrates are not required because microfluidic devices typically do not have complex... 

Baratto C, Fagha G, SbervegUeri G, Boarino L, Rossi AM, Amato G (2001) Front-side micromachined porous silicon nitrogen dioxide gas sensor. Thin Sohd Films 391 261-264 Barber M, Sharpe P, Vickerman JC (1974) An investigation of the SO /Ag surface reaction using secondary ion mass spectrometry. Chem Phys Lett 27 436-438... 

Miyoshi Y, Miyajima K, Saito H, Kndo H, Takenchi T, Karube I, Mitsnbayashi K (2009) Flexible hnmidity sensor in a sandwich confignration with a hydrophilic porons membrane. Sens Actnators B 142 28-32 Mlcak R, Tnller HL, Greiff P, Sohn J, Niles L (1994) Photoassisted electrochemical micromachining of silicon in HF electrolytes. Sens Actnators A 40 49-55... 

One way to make microvolume chambers is with micromachining in silicon. We have chosen this method for several reasons it is the material for which the greatest variety of micromachining methods are available and it is also a good sensor material, so that it is possible to place a chemical sensor in the chamber. 

Kovacs, G. T. A., Petersen, K., and Albin, M. 1996. Silicon micromachining—Sensors to systems. Anal. Chem. 68 A407-A412. 

Welter O, Bayer T and Greschner J 1991 Micromachined silicon sensors for scanning force microscopy J. Vac. Sc/. Technol. B 9 1353... 

The latest development are micromechanical sensors. Their development began with the large-scale introduction of silicon micromachined pressure sensors to the automotive industry in the nineties, which entailed a massive price reduction. Then acceleration sensors for airbag firing, yaw rate sensors and more were introduced. Many devices are still being discovered. The next step is product evolution, with introduction times between a few years and over a decade, as shown in Tab. 2.2. Once customers in the industry have accepted a product, investment in large-scale production can go ahead. It helps to find more applications for the product The time scale for the product evolution process varies from about five... 

CO Resistive sensors pellistors, metal-oxide sensors Optical sensors micro-spectrometer, IR-sources, IR-detectors, IR-filters Hybrid or integrated, surface micromachining Sn02 sintered thick film (Figaro, FIS,. ..), Sn02 thin and thick film on silicon (MiCS, Microsens) IR spectroscopy (Vaisala, Honeywell,. ..)... 

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. 

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