P.J. French

Polysilicon: a versatile material for microsystems

The initial attraction of polysilicon was the ability to deposit semiconductor layers on a wide range of substrates. This leads to the development of polysilicon gate MOS, polysilicon emitters and a range of passive devices. In the field of sensors and actuators, the strain sensors, based on piezoresistive effect, was one of the first successful applications. However, it was probably the development of surface micromachining, which received the most attention. The ability to deposit polysilicon on oxide meant that free-standing structures could be fabricated and this has lead to a wide range of successful devices. Polysilicon has a structure comprising small single-crystal grains with many orientations separated by thin grain boundaries. The size of these grains is highly dependent upon processing and therefore a wide range of electrical and mechanical properties can be achieved. This yields greater flexibility but this sensitivity to process parameters can lead to problems in obtaining a stable process. This paper will look back at the development of polysilicon, its structure, fabrication and both mechanical and electrical properties.

Sensors with digital or frequency output

Abstract Most sensors are based on elements of which one of the parameters (resistivity, dielectric constant, Hall voltage, signal frequency, etc.) shows a small change in response to one or more measurands. To apply sensors is a system, an intimate knowledge of the working of the sensor element is usually necessary. The new age of sensors has come about with the development of planar silicon (and GaAs) technology, permitting the integration of sensor elements and signal conditioning circuitry on one chip. This approach might eventually lead to a family of sensors with a microprocessor-compatible output as shown in Fig. 1, which can easily be applied in bus-organized data-acquisition systems [1, 2]. An intermediate and necessary step to reach this ambitious goal is the design of sensors with a digital or frequency output. In this paper the most important techniques and structures are reviewed.

Polycrystalline silicon strain sensors

Abstract A new theoretical model for piezoresistance in both n- and p-type polycrystalline silicon is described. This model considers piezoresistance in both the grains and in the Schottky-type barrier regions around the grain boundaries. Transport across the grain boundary is assumed to be dominated by thermionic emission. The effect of strain is to change the relative contribution to thermionic emission from each band minimum. Grain size, orientation trap density and doping dependence effects are included. By combining the bulk piezoresistive effects within the grain and the boundary effects, this model has given reasonable agreement with experimental results. This theory now enables optimum processing parameters for highest gauge factor production to be predicted.

Polycrystalline silicon as a strain gauge material

A theoretical model for piezoresistance in both n- and p-type polycrystalline silicon is described. This model considers the contribution to piezoresistance from the grain and the Schottky-type barrier regions around the grain boundaries. Comparison between theory and experiment shows reasonable agreement for both longitudinal and transverse strain measurements. The difference in magnitude between longitudinal and transverse gauge factors depends on texture and is found to be explained by the anisotropy of piezoresistance in silicon. Experimental results for the temperature coefficients of resistance and gauge factor in conjunction with the model for piezoresistance may be used to optimise sensor characteristics within the confines of available processes.

Polysilicon strain sensors using shear piezoresistance

Abstract The conventional piezoresistive strain sensor uses either the longitudinal or transverse piezoresistive effect. However, in addition to the change in resistance, an electric field is developed perpendicular to the current flow. This yields a voltage output, similar to that found in Hall measurements, which is proportional to strain instead of magnetic field. The magnitude of this voltage is also determined by the angle of the applied strain. This paper examines the use of this effect in producing a compact pressure sensor in polysilicon films. A theoretical model is described for both the change in resistance and the perpendicular voltage, due to a shear strain, for a polysilicon device and a comparison with experimental results shows reasonable agreement.

Frequency output piezoresistive pressure sensor

Abstract The sensitivity of silicon to strain and its application to the fabrication of integrated circuits enable sensors to be designed which provide the primary sensing and digital encoding of the signal within the same piece of material. One possible configuration, originally described by Reichl, uses a resistor as the basic sensor to vary the injector currents in an I 2 L ring oscillator. The change in value of the resistance causes a change in the delay of the I 2 L gates and hence changes the frequency of the oscillator. If two such oscillators are used with the resistors associated with them being subjected to opposite strains, a differential frequency is provided which incorporates a first-order cancellation of temperature effects. This frequency difference may be detected digitally and encoded for transmission within a microprocessor-based measurement system. An NMOS circuit to carry out this function has been designed.

SOI pressure sensor

Abstract A single-crystal silicon piezoresistor has a high sensitivity to strain but has the problem of leakage at the pn junction at elevated temperatures. This usually limits the applications of these devices to less than ≈ 150 °C. This paper presents a technique for fabricating a single-crystal silicon-on-oxide (SOI) pressure sensor for high-temperature applications. The SOI structure is formed from the substrate by a combination of trench etching and anisotropic etching techniques. The desired SOI island is under-etched using an anisotropic etch and then separated from the substrate by an oxidation step. The problems associated with etching convex corners have been avoided by using two trench etchings. The remaining cavities are refilled by polysilicon deposition. By using this technique, a good-quality single crystal can be achieved, without requiring lengthy or expensive processing.

Stress analysis of SiO2/Si bi-metal effect in silicon accelerometers and its compensation

Abstract Stress in cantilever beams of silicon accelerometers, caused by the SiO2/Si bi-metal effect, can result in a large temperature drift of offset. This has been simulated using a structure analysis program. The distribution of the stress within the beam shows a sharp rise at the SiO2/Si interface, in contrast to an acceleration-induced stress. The dependence of this stress on the beam structure is investigated, and the calculated value verified experimentally. Using a drift compensation method, involving an additional beam without a seismic mass and subtracting the output from that of the other beam, a reduction of the offset drift up to 20 fold is obtained for a simple beam structure.

Modelling of the flip-flop sensor and an optical pattern detector☆

Abstract The modelling of the flip-flop sensor operating in the unstable state is derived. The formulae obtained are verified both experiments and SPICE simulations, showing a good agreement. The thermal behaviour is calculated using the model. The limitations of the model are discussed. As an application of the flip-flop sensor, a sensor array containing 64 elements organized in an 8 × 8 matrix is presented. Each element is a flip-flop consisting of two resistors and two phototransistors, one of which is covered with aluminium. When the flip-flop is brought into the unstable state, it will go to ‘one’ or ‘zero’ depending on the light. The device functions as a two-dimensional optical signal pattern detector, yielding a ‘yes’ or a ‘no’ depending on whether the light level is above or below a threshold value.

An angle-of-rotation sensor using flip-flop photodetecting techniques

Abstract A simple, effective and compact method for detecting angle of rotation has been developed. This has been achieved by incorporating flip-flop photosensing techniques. A single sensing element yields a one or zero output, which when connected, in a one-dimensional array can be used to give a pattern corresponding to the position of the light beam. In order to avoid the need for a synchronized clock, latch circuitry has been included, which yields a clock-independent output from each sensing cell. Further latch circuitry is used to give hysteresis of the switching levels. This reduces the parasistic effects of mechanical vibration or light fluctuations. Additional signal processing has been developed that yields two outputs, giving the rate and direction of rotation.