SILICON CAPACITIVE ACCELEROMETER.docx
- 1. SILICON CAPACITIVE ACCELEROMETER PIEZORESISTIVE PRESSURE SENSOR CONDUCTOMETRIC GAS SENSOR ELECTROSTATIC COMB-DRIVE Category Sensor Sensor Sensor Actuator and sensor. Purpose Measures the acceleration of the body on which the sensor is mounted Measures the pressure, typically of gases or liquids It detects and quantifies the presence of a gas, that is, its concentration As an actuator: sets something into motion or applies force on something. As a sensor: measures displacements by quantifying capacitance change Key words Proof mass, suspension, capacitance Piezoresistivity, diaphragm Adsorption, desorption Linear electrostatic actuator, suspension, fixed and moving comb fingers Principle of operation Converts displacement caused by the inertial force on the proof-mass to a voltage signal via a change in capacitance between movable and fixed parts. External pressure loading causes deflection, strain, and stress on the membrane. The strain causes a change in the resistance of the material,which is measured using the Wheatstone bridge configuration. The principle is that a suitable catalyst, when heated to an appropriate temperature, either promotes or reduces the oxidation of the combustible gases. The additional heat released by the oxidation reaction can be detected. By applying external voltage between fixed and moving combs, an electrostatic force is generated that provides the actuation along the length of the comb fingers Application Automotive, aerospace, machine tools, biomedical, consumer products, etc Automotive, aerospace, appliances, biomedical, etc Environmental monitoring, automotive application, air conditioning inairplanes, spacecraft, living spaces,sensor networks, breath analyzers, food control applications, etc. 1. In closed-loop capacitive sensors where a feedback force is to be provided. 2. In opto-mechanical systems to move optical elements such as mirrors and lenses.
- 2. 3. In resonant sensors and gyroscopes to set a component into motion. Material used 1. Single-crystal silicon to form the physical structure. 2. Silicon dioxide sandwiched in a silicon-on-insulator (SOI) wafer gives electrical isolation. 3. Handle-layer of the SOI wafer is the substrate. 4. Gold for electrodes. The material used to form this device is single-crystal silicon The materials used for making these are films of metal oxide such as SnO2 and TiO2. 1. Polysilicon and single-crystal silicon to form the physical structure. 2. Silicon dioxide as a sacrificial layer material. 3. Silicon nitride for electrical insulation. 4. Gold for electrodes. Fabrication process This is a bulk micromachining. In general, almost any microfabrication process can be used to make an accelerometer. Bulk micromachining with bipolar circuitry plus glass frit wafer- bonding is the fabrication process. Gas sensors are fabricated using single-crystalline SnO2 nanobelts. Nanobelts are synthesized by thermal evaporation of oxide powders under controlled conditions in the absence of a catalyst. Fabrication is mainly by surface micromachining. Sandia National Laboratories uses a very sophisticated surface micromachining process called SuMMiT (Sandia Ultra Multilayer Microfabrication Technology) that has five movable and released polysilicon structural layers with four gap layers created by the sacrificial oxide Advantages 1. Very low sensitivity to temperature-induced drift. 2. Higher output levels than other types. 3. Amenability for force-balancing and hence for closed-loop operation. 4. High linearity. 1. Compact size, making them suitable for a variety of applications, including those that use an array of such sensors to measure pressure distribution. 2. Good thermal stability, since thermal compensation can be built into the sensor. 3. Good market potential due to low Micromachining simplifies this through miniaturization and also reduces the cost
- 3. cost. Typical Applications 1. Consumer: airbag deployment systems in cars, active suspensions, adaptive brakes, alarm systems, shipping recorders, home appliances, mobile phone, toys, etc. 2. Industrial: crash-testing robotics, machine control, vibration monitoring, etc. 3. High-end applications: military/space/aircraft industry navigation and inertial guidance, impact detection, tilt measurement, high-shock environments, cardiac pacemaker, etc 1. Direct pressure-sensing applications: such as weather instrumentation, combustion pressure in an engine cylinder or a gasturbine, appliances such as washing machines, etc. 2. Altitude-sensing applications: such as in aircraft, rockets, satellites, weather balloons, where the measured pressure is converted using an appropriate formula. 3. Flow-sensing applications, manifold pressure sensing: in automobiles, etc. 1. Environmental monitoring. 2. Exhaust gas sensing in automobiles. 3. Air conditioning in airplanes, spacecrafts, houses, and sensor networks. 4. Ethanol for breath analyzers. 5. Odor sensing in food-control applications, etc Actuators: resonators, microgrippers, force balancing accelerometers and microscanners. Sensors: automotive, aerospace, machine tools, biomedical, space applications, etc. Principle of Operation 1. Proof mass: the inertial mass used in the accelerometer whose displacement Diaphragm: a thin sheet of a flexible material, anchored at its circumference, over which 1. Conductivity: a material property that quantifies the material’s ability to conduct 1. Electrostatics: the phenomena arising from stationary or slowly moving electric charges. Electrostatic phenomena arise
- 4. Key Definitions relative to a rigid frame is a measure of the influence of external acceleration. 2. Suspension: the compliant structure by which the proof mass is suspended fromthe frame. 3. Capacitance:the capacity of a body to hold an electrical charge. Capacitance is also a measure of the amount of electric charge stored for a given electric potential. Differential capacitance arrangement: in this arrangement, there are three plateswith a movable middle plate. As the plate moves, the capacitance between one of the pairs will increase while that of the other decreases. This gives a signal that is linearlyproportional to the applied acceleration. Quality factor: a system’s quality factor, Q, describes the sharpness of the system’sdynamic response. differential pressure is applied. Piezoresistivity: the dependence of electrical resistivity on mechanical strain. Polysilicon shows substantive piezoresistivity electric current when an electric potential (difference) is applied. It depends on the number of free electrons available. 2. Adsorption: the process of collection and adherence of ions, atoms or molecules on a surface. This is different from absorption, a much more familiar term. In absorption, the species enterinto the bulk, that is,the volume;in adsorption, they stay put on the surface. 3. Desorption: the reverse of adsorption; species (ions, atoms or molecules) are given out by the surface. 4. Combustion: a technical term for burning: a heat-generating chemical reaction between a fuel (combustible substance) and an oxidizing agent. It can also result in light (e.g. a flame). from the forces that electric charges exert on one another, as described by Coulomb’s law. 2. Parallel-plate capacitor: a capacitor consists of two conductors separated by a nonconductive region that may be dielectric medium or air gap. The conductors contain equal and opposite charges on their facing surfaces, and the medium contains an electric field. 3. Folded-beam suspension: a planar suspension to suspend a body so that it can move freely with low stiffness in one planar direction but is stiff in the other planar direction as well as in the out-of- plane direction. This is a compliant- design equivalent of a sliding joint.