Andreas Koll

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.

The electronic nose in Lilliput

Electronic noses today are handy enough systems for detecting gaseous chemicals used in industrial cleaning or fabrication processes. Available from a half-dozen manufacturers, the instruments are desktop or laptop in size, depending on their features. The gases, called volatile organic compounds (VOCs), are toxic, carcinogenic and quick to evaporate in combination, a danger to their environs. Here, the authors describe how a miniature experimental system based on a CMOS chip is being readied to detect a range of gaseous compounds.

CMOS chemical microsensors based on resonant cantilever beams

We present a chemical gas sensor based on a resonating cantilever beam in CMOS MEMS technology. The sensor is actuated employing electrothermal actuation. Thus, for a 300 micrometers long beam vibration amplitudes of 6.5 nm per mW heating power are achieved. The vibrations are detected with piezoresistors in a Wheatstone bridge scheme. Detection sensitivities above 200 (mu) V per mW heating power are measured with the bridge biased at 5 V. The beams have quality factors of up to 600. The static power dissipation that goes along with the electrothermal actuation scheme leads to a small temperature elevation of 0.3 K/mW of the sensitive area. The beams are coated with poly(etherurethane) as the sensitive layer. The layer thickness was determined by the change of the initial resonance frequency. Concentrations of octane, ethanol and toluene in synthetic air were measured. For toluene, concentrations as low as 250 ppm can be detected.

Micromachined CMOS calorimetric chemical sensor with on-chip low noise amplifier

We report on a micromachined calorimetric chemical sensor for volatile organic compounds in air fabricated in industrial CMOS technology. The sensor measures enthalpy changes during absorption and desorption of a gaseous analyte into a chemically selective polymer. The enthalpy changes cause temperature variations of a thermally insulated micromachined membrane. The temperature variations are detected using thermopiles integrated on the membrane. The microsystem consists of a polymer coated sensor structure, an uncoated reference structure, and a low noise differential amplifier amplifying the difference of the two thermovoltages. The system provides a sensitivity of 51-1 mV//spl mu/W, resolving enthalpy changes of the order of /spl mu/J at sampling intervals down to 45 ms. To demonstrate the sensitivity and selectivity of the calorimetric chemical microsystem, we present results of different volatile organic compounds (VOCs).

N-well based CMOS calorimetric chemical sensors

New micromachined calorimetric chemical sensors based on an n-well island structure have been designed, fabricated in industrial CMOS technology, and tested. The suspended island structure is covered with a polymer and changes its temperature upon absorption or desorption of analyte. The temperature change is recorded by integrated polysilicon/aluminum thermopiles. A polysilicon or metal heating resistor covers the n-well structure which allows a more accurate calibration compared to our previous design . The system provides a physical sensitivity of 34 and 26.5 mV//spl mu/W for the square and rectangular shaped membrane devices, respectively. Sensitivity and performance of the calorimetric chemical microsystem are shown by measurements for different volatile organic compounds. The system has a sensitivity of 0.045 and 0.049 mV/ppm to ethanol and 0.209 and 0.229 mV/ppm to toluene for the square and rectangular membrane devices, respectively.

Discrimination of volatile organic compounds using CMOS capacitive chemical microsensors with thickness-adjusted polymer coating

We present a new method for discriminating organic vapors based on the variation of capacitance changes of an interdigitated CMOS capacitor with the thickness of the sensitive polymer layer. By carefully adjusting the thickness of the polymer layer, discrimination potential in addition to the chemical selectivity of the polymer is provided by the fact that the interdigitated capacitor signals depend on the layer thickness. At polymer thicknesses small compared to the center-to-center spacing of the electrodes, an increase in capacitance is observed for all analytes, whereas for thick layers, the direction of the capacitance changes depends on the dielectric constant of the analyte. Sensors can be designed to be insensitive towards a certain analyte by varying the polymer thickness. Measurements for volatile organic compounds using CMOS capacitors coated with different polymer thicknesses are presented to demonstrate the new way of increasing sensor selectivity.

CMOS-based chemical microsensors: components of a micronose system

We report on results achieved with three different types of polymer-coated chemical microsensors fabricated in industrial CMOS technology. The first and most extensively studied transducer is a microcapacitor sensitive to changes in dielectric properties of the polymer layer due to analyte absorption. An on-chip integrated (Sigma) (Delta) -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 consequent of changes in the vibrating 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. Enthalpy changes on the order of (mu) J have been detected.

Flip-chip packaging for smart MEMS

The cointegration of IC microsensors, actuators and readout circuit leads to smart Micro Electro Mechanical Systems (MEMS) which are superior in many aspects to their conventional discrete counterparts. However, the packaging of such device is still a challenge and a major factor of the overall production cost. On one hand MEMS need protection against mechanical contact and media. On the other hand, the encapsulation of the transducer must be partially permeable to the environment. We developed a packaging method which successfully addresses these challenges. Thereby the number of steps needed to electrically contact and partially seal the MEMS are reduced by combining them using flip-chip technology. An opening in the substrate is aligned with the transducer, and enables the interaction with external media. Concurrently with the electrical connections, a frame plated onto the microsystem is soldered to a corresponding structure on the substrate. This frame seals the rest of the chip from the medium interacting with the transducer. Using passive test chips we evaluate the performance of the new packaging method. Various underbump metal and solder deposition techniques were investigated. Both ceramic and flexible organic substrate materials were used. The combination Ni/Au bumps/InPb40 solder/ceramic substrate showed the following mechanical and electrical parameters: For 98% of the tested chips, the helium leakage rate of the sealing frame surrounding the sensor is below the threshold of the used mass spectrometer (5 X 10-7 Pa l s-1). For flip-chip pads ranging from 200 to 300 mm square, the bump resistances are smaller than 2 m(Omega) . The approach, is illustrated with three successfully packaged MEMS for the measurement of humidity, gas flow, and volatile organic compounds, respectively. They all contain integrated readout circuitry providing digital output.

IC microtransducers --- new components with old materials ?

In the first part of this paper, we review the integration of MEMS and CMOS microtransducer technology and out-line the related IC MEMS CAD tools SOLIDIS and ICMAT. The IC microtransducer approach is illustrated by deflectable micromirrors, infrared sensors, and a thermally isolated n- well CMOS device. In the second part, we report two novel chemical microsensors based on CMOS MEMS technology: (i) a piezoresistive resonating beam with hydrocarbon-sensitive polymer layer and (ii) a microsystem chip including a piezoresistive and capacitive chemical sensors with co- integrated heating deice and circuitry.