Polysilicon Heater Research Articles

Operation and short-term drift of polysilicon-heated CMOS microstructuresat temperatures up to 1200 K

The results of this paper are pertinent to micromachinedintegrated chemical sensors, infrared sources and hotplates, among others.We report on the operation and characterization of micromachinedcomplementary metal-oxide semiconductor (CMOS) microstructures withintegrated CMOS polysilicon heaters at temperatures up to 1200 K. The newresults concern the stability of integrated CMOS polysilicon heaters undersuch extreme conditions. Two resistance drift phenomena appearing abovethe polysilicon recrystallization temperature Tcr≈870 Kwere identified. The first is a reversible resistance relaxation leadingto resistance changes of several tens of per cent. Relaxation times are ofthe order of minutes and point to a thermally activated process. Thesecond drift mechanism leads to slow resistance changes, for example, 3%after six hours above Tcr.Temperature calibration of such devices supported by finite elementsimulations is proven to be feasible and reliable. Despite the largetemperature gradients in the heated microstructures, natural convection inthe surrounding gas was found to be ineffective in comparison with heatconduction.

Fabrication and calibration of a gas sensor based on chemically vapor deposited WO 3 films on silicon substrates : Application to H 2 sensing

The electrical resistance of low pressure chemically vapor deposited (LPCVD) WO 3 films exhibits reversible changes when gases, such as hydrogen, are present in their environment. Within the frame of this work, WO 3 films activated with some gold monolayers were integrated, together with a polysilicon heater and two temperature-sensing resistors, on the same silicon substrate. For the fabrication, processes typically used in the manufacturing of silicon-based integrated circuits were used. After fabrication, dies with dimensions of 5 mm×5 mm have been packed and wire-bonded in classic dual in-line (DIL) packages. The temperature-sensing resistors were calibrated by external heating and monitoring their electrical resistance. Devices were tested in H 2 and were found to be sensitive to concentrations down to 100 ppm. Sensitivity was dependent on temperature and H 2 concentration. The maximum sensitivity (approximately 50%) was observed at temperatures around 120°C.

A high-speed capacitive humidity sensor with on-chip thermal reset

This paper reports a high-speed capacitive humidity sensor integrated on a polysilicon heater. A response time of 1.0 s and a sensitivity of 30.0 fF/%RH have been obtained. High speed is achieved using multiple polyimide columns having diameters of a few microns and allowing moisture to diffuse into them circumferentially. Using structures that eliminate the air-gap capacitance between the columns, the simulated sensor output drifts by only 1% when the relative dielectric constant in the air region changes from 1 to 10. A polysilicon heater is used to measure relative humidity levels >80% RH. An accuracy of /spl plusmn/3% RH has been obtained using this method, with measurement errors of /spl plusmn/0.5/spl deg/C and /spl plusmn/2% RH in temperature and relative humidity, respectively. The heater also reduces the recovery time after wetting, enables the sensor to recover from contamination and aging, and allows the sensing film to be reset on demand during self-test protocols.

Integrated flow sensor for in situ measurement and control of acoustic streaming in flexural plate wave micropumps

This paper presents the design, fabrication, and characterization of a micromachined flow sensor, which is integrated onto the flexural plate wave (FPW) micropump. The flow sensor and the FPW micropump represent a complex microfluidic system that is able to control the fluid flow in the device. The system was designed using a commercial software package. The microfluidic system of a size of 10×10 mm was fabricated using common fabrication techniques. The micropump is made of an aluminum, piezoelectric zinc oxide, polysilicon, and low-stress silicon nitride membrane with a typical thickness of 1–3 μm. The thermal flow sensor consists of a polysilicon heater and polysilicon–aluminum thermopiles as temperature sensors. The cold junctions of the thermopiles are located in a new design that will avoid the drift effect of the flow sensor. The results show expected flow velocity–drive voltage characteristics.

A micro-electro-mechanical-system-based thermal shear-stress sensor with self-frequency compensation

By applying the micro-electro-mechanical-system (MEMS) fabrication technology, we developed a micro-thermal sensor to measure surface shear stress. The heat transfer from a polysilicon heater depends on the normal velocity gradient and thus provides the surface shear stress. However, the sensitivity of the shear-stress measurements in air is less than desirable due to the low heat capacity of air. A unique feature of this micro-sensor is that the heating element, a film 1 µm thick, is separated from the substrate by a vacuum cavity 2 µm thick. The vacuum cavity prevents the conduction of heat to the substrate and therefore improves the sensitivity by an order of magnitude. Owing to the low thermal inertia of the miniature sensing element, this shear-stress micro-sensor can provide instantaneous measurements of small-scale turbulence. Furthermore, MEMS technology allows us make multiple sensors on a single chip so that we can perform distributed measurements. In this study, we use multiple polysilicon sensor elements to improve the dynamic performance of the sensor itself. It is demonstrated that the frequency-response range of a constant-current sensor can be extended from the order of 100 Hz to 100 kHz.

Multi-electrode substrate for selectivity enhancement in air monitoring

A new microstructured silicon substrate for thin gas sensitive layers has been developed composing a thermoresistor at the chip surface and a buried polysilicon heater. The process flow of the largely MOS-compatible fabrication is described. The two pairs of planarized interdigitated electrodes with both a width and spacing of 2 microns were manufactured by a special lift-off process. A second interdigitated electrode is placed above the sensing tin oxide layer and can also serve as a measuring electrode. As a further possibility, we suggest the application of a transverse electric field to the sensing layer by using the upper interdigitated electrode as a gate electrode and the heater as a counter electrode. This additional electric field can influence the effective signal values caused by desorption and adsorption of gas specimen thus enhancing the selectivity of resistive gas sensors. In order to detect oxidizing gases like nitrogen oxides with a metal oxide layer we have developed a reactive sputter process for tin oxide with a homogenous unstoichiometry across the complete grains. Measuring the d.c. resistance of these strongly reduced layers at relatively low temperatures of about 175°C we have achieved a sensitivity in the sub-ppm range with regard to nitrogen dioxide in dry air.

A high-resolution silicon monolithic nozzle array for inkjet printing

This paper describes a high-resolution monolithic nozzle array for inkjet printing. The nozzles are fabricated using a bulk anisotropic etching technology to undercut a network of highly-boron-doped silicon support ribs, forming an array of microchannels which are then sealed using thermal oxidation and LPCVD dielectrics. Closely-spaced trapezoidal or triangular nozzles are realized after cutting the wafer perpendicular to the microtubes. With a 21-/spl mu/m nozzle width and a 4-/spl mu/m nozzle-to-nozzle separation, a resolution of 1016 dots per inch (d/in) can be achieved. Polysilicon heaters are integrated on top of each microchannel so that when activated, the underlying ink is vaporized and a drop of ink is expelled to impinge on the paper. The fabrication of this device requires only five masks and is compatible with the addition of on-chip circuitry for multiplexing the heater control signals. Heat transfer efficiency to the ink is enhanced by the high thermal conductivity of the silicon ribs in the channel ceiling, while the bulk silicon maintains high interchannel isolation. Current pulses 20 /spl mu/s wide with a power density of 3.35/spl times/10/sup 8/ W/m/sup 2/ have been used in initial printing tests, resulting in 20-/spl mu/m diameter ink dots on a piece of paper which was set 2 mm away from the nozzle. The energy required to fire an ink drop is 11.5 /spl mu/J.

Thermo-optically controlled ring resonator on silicon

A temperature controlled integrated optical ring resonator processed on a silicon substrate is studied using integrated polysilicon heater and thermistor elements. The response speed of the optical 2/spl pi/ phase shift is evaluated as the surface temperature of the device is measured with the thermistor. The effect of an applied heat sink on the device operation is studied.

A wideband monolithic RMS-DC converter using micromachined diaphragm structures

This paper reports a monolithic rms-dc converter for measuring wideband rms signals. Two identical thermoelements for thermal rms-dc conversion are formed using planar diaphragm structures and micromachined dielectric windows. Each thermoelement consists of two polysilicon heaters and a thin-film temperature sensor located on a window and operated at constant temperature using on-chip control feedback circuitry. The thermoelements achieve a thermal efficiency greater than 7/spl deg/C/mW with a thermal time constant less than 5 ms. The on-chip control circuitry is realized using a standard 3 /spl mu/m p-well CMOS process with minor modifications for process compatibility with the dielectric window formation. This converter measures rms signals over a full scale range of 1 Vrms, handles crest factors in excess of 5, exhibits a typical nonlinearity of less than 1%, and achieves a 3 dB bandwidth greater than 20 MHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A CMOS thermally-isolated heater structure as a substrate for semiconductor gas sensors

A heater structure for gas sensors is realized by utilizing standard CMOS technology together with post-process etching. The heater, fully suspended over an ≈100 μm deep cavity, is capable of supplying temperatures necessary for the operation of most semiconductor gas sensors. Various heater structures are characterized and results show that the power requirement to achieved a temperature of 100°C for a polysilicon heater of 3 μm 2 cross-section and 300–500 μm length is on the order of 10 mW.

A fully integrated silicon-based 40-V thermal ink jet IC

A fully-integrated silicon-based thermal ink jet printhead which produces high speed, laser-quality printing at 300 spots per inch resolution is described. Monolithic integration of the 5-V logic circuitry, 13-V predriver circuitry, 40-V MOSFETs, and 192 n + polysilicon heater elements requires multiple process and device design trade-offs. The TIJ printhead is customer replaceable and must be protected from far greater static discharges than normal VLSI circuits. Design issues and failure mechanisms of the power MOSFETs and ESD protection devices are discussed. Two-dimensional simulations which include parasitic bipolar action and lattice heating illustrate the design concepts. Electrical characteristics of the 40-V power MOSFETs and 16-kV ESD protection devices are presented.