H. Baltes

Micromachined thermally based CMOS microsensors

An integrated circuit (IC) approach to thermal microsensors is presented. The focus is on thermal sensors with on-chip bias and signal conditioning circuits made by industrial complementary metal-oxide-semiconductor (CMOS) IC technology in combination with post-CMOS micromachining or deposition techniques. CMOS materials and physical effects pertinent to thermal sensors are summarized together with basic structures used for microheaters, thermistors, thermocouples, thermal isolation, and heat sinks. As examples of sensors using temperature measurement, we present micromachined CMOS radiation sensors and thermal converters. Examples for sensors based on thermal actuation include thermal flow and pressure sensors, as well as thermally excited microresonators for position and chemical sensing. We also address sensors for the characterization of process-dependent thermal properties of CMOS materials, such as thermal conductivity, Seebeck coefficient, and heat capacity, whose knowledge is indispensable for thermal sensor design. Last, two complete packaged microsystems-a thermoelectric air-flow sensor and a thermoelectric infrared intrusion detector-are reported as demonstrators.

Process-dependent thin-film thermal conductivities for thermal CMOS MEMS

The thermal conductivities /spl kappa/ of the dielectric and conducting thin films of three commercial CMOS processes were determined in the temperature range from 120 to 400 K. The measurements were performed using micromachined heatable test structures containing the layers to be characterized. The /spl kappa/ values of thermally grown silicon oxides are reduced from bulk fused silica by roughly 20%. The /spl kappa/ of phosphosilicate and borophosphosilicate glasses are 0.94/spl plusmn/0.08 W m/sup -1/ K/sup -1/ and 1.18/spl plusmn/0.06 W m/sup -1/ K/sup -1/, respectively, at 300 K. A plasma-enhanced chemical-vapor-deposition silicon-nitride layer has a thermal conductivity of 2.23/spl plusmn/0.12 W m/sup -1/ K/sup -1/ at 300 K. This value is between published data for atmospheric-pressure CVD and low-pressure CVD nitrides. For the metal layers, we found thermal conductivities between 167 W m/sup -1/ K/sup -1/ and 206 W m/sup -1/ K/sup -1/, respectively, at 300 K, to be compared with 238 W m/sup -1/ K/sup -1/ of bulk aluminum. The temperature-dependent product /spl kappa//spl rho/ of /spl kappa/ with the electrical resistivity /spl rho/ agrees better than 8.2% between 180-400 K with that of pure bulk aluminum. The /spl kappa/ values of the polysilicon layers are between 22.4 W m/sup -1/ K/sup -1/ and 37.3 W m/sup -1/ K/sup -1/ at 300 K. They are reduced from similarly doped bulk silicon by factors of between 2.0-1.3. The observed discrepancies between thin film and bulk data demonstrate the importance of determining the process-dependent thermal conductivities of CMOS thin films.

Microfabrication techniques for chemical/biosensors

Microfabrication processes for chemical and biochemical sensors are reviewed. Standard processing steps originating from semiconductor technology are detailed, and specific micromachining steps to fabricate three-dimensional mechanical structures are described. Fundamental chemical sensor principles are briefly abstracted and corresponding state-of-the-art examples of microfabricated chemical sensors and biosensors are given. The advantages and disadvantages of either fabricating devices in IC fabrication technology with additional microfabrication steps, or of using custom-designed nonstandard microfabrication process flows are debated. Finally, monolithic integrated chemical and biological microsensor systems are presented, which include transducer structures and operation circuitry on a single chip.

Application-specific sensor systems based on CMOS chemical microsensors

Abstract We report on results achieved with three different types of polymer-coated chemical microsensors fabricated in industrial CMOS technology followed by post-CMOS anisotropic etching and film deposition. The first and most extensively studied transducer is a microcapacitor sensitive to changes in dielectric properties of the polymer layer upon analyte absorption. An on-chip integrated ΣΔ-converter allows for detecting the minute capacitance changes. The second transducer is a resonant cantilever sensitive to predominantly mass changes. The cantilever is electrothermally excited; its vibrations are detected using a piezoresistive Wheatstone bridge. In analogy to acoustic wave devices, analyte absorption in the polymer causes resonance frequency shifts as a consequence of changes in the oscillating mass. The last transducer is a microcalorimeter consisting of a polymer-coated sensing thermopile and an uncoated reference thermopile each on micromachined membranes. The measurand is the absorption or desorption heat of organic volatiles in the polymer layer. The difference between the resulting thermovoltages is processed with an on-chip low-noise differential amplifier. Gas test measurements with all three transducer principles will be presented. The goal is to combine the three different transducer principles and vary the polymers in an array type structure to build a new generation of application-specific microsensor systems.

CMOS Schmitt trigger design

CMOS Schmitt trigger design with given circuit thresholds is described. The approach is based on studying the transient from one stable state to another when the trigger is in linear operation. The trigger is subdivided into two subcircuits; each of them is considered as a passive load for the other. This allows the relations governing the deviations of the circuit thresholds from their given values to be obtained. The trigger device sizes are thus determined by the threshold tolerances. >

CMOS as sensor technology

Abstract Sensor design and fabrication using industrial IC technologies has the advantages of batch fabrication and on-chip interface circuitry. Sensors made by CMOS or bipolar IC technology have been demonstrated for magnetic, temperature and radiation measurands. Certain thermal, mechanical and chemical sensors can be realized by combining IC technologies with additional, compatible processing. We distinguish the methods of multiple project wafers, single project wafers, post-processing IC chips or wafers, and IC fabrication merged with sensor processing performed before, after, and in-between the regular IC processing steps. This paper is focused on sensor prototypes realized by industrial CMOS IC technology with post-processing micromachining. Examples include thermally excited acoustic resonators, thermoelectric gas flow, infrared, and power sensors, and a thermal conductivity sensor.

Two-dimensional magnetic microsensor with on-chip signal processing for contactless angle measurement

The reported CMOS microsystem is the key element for accurate angle measurements. In combination with a permanent magnet, it is used for various wear free angular positioning control systems for automotive and industrial applications covering the full 360/spl deg/ range. The integrated system includes a two-dimensional (2-D) magnetic microsensor (30/spl times/30 /spl mu/m/sup 2/ active area), offset compensation, and signal conditioning circuitry. A novel approach for the angle calculation is presented using an on-board incremental ADC. A bitstream representing the angular position of the applied permanent magnet is provided at the system output. The system achieves a 1/spl deg/ angular resolution with 9 mW power consumption and a permanent magnet of 100 mT. The chip is fabricated in a generic 2-/spl mu/m, double-poly, double-metal CMOS process and covers an area of 2.6/spl times/4.1 mm/sup 2/.

Silicon gas flow sensors using industrial CMOS and bipolar IC technology

Abstract We report two silicon gas flow sensors fabricated by combining silicon micromachining with either standard CMOS or standard bipolar technology. Both sensors are based on the Seebeck effect. An integrated thermopile is placed on a free-standing cantilever beam and measures the temperature difference between the heated tip of the beam and the bulk silicon, which depends on the gas flow. For the CMOS sensor we report an output voltage sensitivity to the gas flow velocity of 1.78 V(m/s)−1/2 W−1. The sensitivity of the bipolar sensor is 0.26 V (m/s)−1/2 W−1.

Mechanical properties of thin films from the load deflection of long clamped plates

A plane-strain load-deflection model for long plates clamped to a rigid support is developed. The analytical model describes the nonlinear deflection of plates with compressive or tensile residual stress and finite flexural rigidity under uniform load. It allows for the extraction of the residual stress and plane-strain modulus of single-layered thin films. Properties of compressively and weakly prestressed materials are extracted with an accuracy achieved previously only with tensile samples. Two approximations of the exact model are derived. The first reduces the plates to membranes by neglecting their flexural rigidity. Considerable errors result from this simplification. The second approximation provides an exact expression for the linear plate response. Using the model, mechanical properties were extracted from two plasma-enhanced chemical-vapor deposition (PECVD) silicon nitride films with weakly tensile and compressive prestress, respectively. Measured residual stresses are 1.3/spl plusmn/3.8 and -63/spl plusmn/12.4 MPa, respectively. Corresponding plane-strain moduli are 134.4/spl plusmn/3.9 and 142/spl plusmn/2.6 GPa, respectively.

Thermoelectric AC power sensor by CMOS technology

The authors report the development of a thermoelectric AC power sensor (thermoconverter) realized by industrial CMOS IC technology in combination with postprocessing micromachining. The sensor is based on a polysilicon heating resistor and a polysilicon/aluminum thermopile integrated on an oxide microbridge. The thermopile sensitivity is 9.9 mV/mW and the burn-out power of the sensor is 50 mW. The time constant is 1.85 ms and the SNR (signal-to-noise ratio) is 8*10/sup 9//W. The linearity error with respect to frequency is less than 0.1% below 400 MHz and less than 1% up to 1.2 GHz.<<ETX>>