D. Lange

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 MEMS - present and future

The paper reviews the state-of-the-art in the field of CMOS-based microelectromechanical systems (MEMS). The different CMOS MEMS fabrication approaches, pre-CMOS, intermediate-CMOS, and post-CMOS, are summarized and examples are given. Two microsystems fabricated with post-CMOS micromachining are presented, namely a mass-sensitive chemical sensor for detection of organic volatiles in air and a 10-cantilever force sensor array for application in scanning probe microscopy. The paper finishes with a look into the future, discussing key challenges and future application fields for CMOS MEMS.

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.

Integrated atomic force microscopy array probe with metal–oxide–semiconductor field effect transistor stress sensor, thermal bimorph actuator, and on-chip complementary metal–oxide–semiconductor electronics

A microfabricated 2×1 array of active and self-detecting cantilevers is presented for applications in atomic force microscopy (AFM). The integrated deflection sensor is based on a stress sensing metal–oxide–semiconductor transistor. Full custom complementary metal–oxide–semiconductor amplifiers for signal readout are combined on the same chip. A sensor sensitivity of 2.25 mV/nm, or a change in current ΔId/Id=2.8×10−6/nm, was obtained at the final output stage. Three Al–Si thermal bimorph actuators are integrated on each cantilever for self-excitation and feedback actuation. The efficiencies of the heaters are 2.4–4.7 K/mW. In the experimental setup, a maximum displacement of 8 μm was achieved at 45 mW input. A pair of parallel AFM images in the constant height mode, a typical tapping mode image, and a constant force image with 1.3 μm high features have been successfully taken with the array probe.

CMOS resonant beam gas sensing system with on-chip self excitation

We present a single-chip CMOS resonant beam gas sensing system with on-chip self excitation for the detection of volatile organic compounds (VOC). For the first time, a mass-sensitive resonant cantilever is cointegrated with a feedback circuit yielding a sensor system operating in a self-excitation mode. Analyte gases are absorbed by a sensitive layer deposited on the cantilever. The corresponding mass increase leads to a decrease of the beams resonance frequency. In this way, concentrations of various VOCs in synthetic air are measured. The influence of polymer thickness on resonance frequency, quality factor, and vibration amplitude is investigated. The frequency stability and sensitivity of the cantilever sensor is tested and an optimal polymer thickness of 3-4 pm determined. A limit of detection of 0.8 ppm for octane is calculated. This corresponds to a mass resolution of less than 0.4 pg. The on-chip integration of the feedback circuit makes it possible to abandon external driving circuitry. Thus, it drastically reduces the system size and number of components, and facilitates mass production.

CMOS Cantilever Sensor Systems: Atomic Force Microscopy and Gas Sensing Applications

1. Introduction.- 2. Design Considerations.- 3. Cantilever Beam Resonators.- 4. Resonant Gas Sensor.- 5. Force Sensors for Parallel Scanning Atomic Force Microscopy.- 6. Conclusions and Outlook.- Appendices.- A.1 Process Sequence Resonant Gas Sensor.- A.2 Process Sequence Resonant Gas Sensor (Maskless).- A.3 Process Sequence AFM Sensor Arrays.- A.4 Material Properties of Thin Film Materials.- References.

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.

Parallel scanning AFM with on-chip circuitry in CMOS technology

We present the first force sensors for application in Atomic Force Microscopy (AFM) fabricated with industrial CMOS technology. Sensing is based on two different detection schemes: a piezoresistive Wheatstone bridge and stress-sensing MOS transistors. The system combines on a single chip (i) two cantilevers for parallel scanning, (ii) thermal actuators for independent deflection of the two cantilevers, (iii) sensors to measure the deflection, and (iv) offset compensation and signal conditioning circuitry. The AFM probes were tested in contact and dynamic mode. In the dynamic mode, images with a resolution of better than 20 /spl Aring/ were recorded. Moreover, we successfully took parallel scanning images in contact mode.

A gas detection system on a single CMOS chip compromising capactive, calorimetric, and mass-sensitive microsensors

A single-chip chemical microsensor system fabricated in industrial 0.8 /spl mu/m CMOS technology includes three different polymer-coated micromachined transducers. The chip forms an integral part of a handheld unit to detect volatile organics. On-chip circuitry includes signal conditioning, A/D-converters, filters, digital controller, and serial bus interface (I/sup 2/C).

Nanochemical surface analyzer in CMOS technology.

We have developed an atomic force microscopy (AFM) cantilever system, fabricated using a standard CMOS process and a few post-processing steps, capable of detecting the difference between hydrophilic and hydrophobic samples for the purpose of nanochemical surface analysis. The fully integrated cantilever comprises a thermal actuator for cantilever deflection and a Wheatstone bridge to sense cantilever bending, thus obviating the need for cumbersome laser detection and external piezoelectric drives. Glass microspheres have been affixed to the cantilevers and, were either modified with a self-assembled monolayer to form hydrophobic tips, or left unmodified for hydrophilic tips. Force-distance curves have been used to measure the force between the functionalized/unfunctionalized tips and hydrophobic/hydrophilic sample surfaces. In an optimization step three different Wheatstone bridge sensors have been designed and characterized; best Wheatstone bridge sensitivity is 8.0 microV/nm with a 713 nm/mW actuator efficiency.