M.J. Vellekoop

Glass-to-glass anodic bonding with standard IC technology thin films as intermediate layers

Glass-to-glass wafer bonding has recently attracted considerable interest. Especially for liquid manipulation applications and on-chip chemical analysis systems, all-glass sealed channels with integrated metal electrodes are very attractive. In this paper, we present a novel anodic bonding process in which the temperature does not exceed 400°C. This is a crucial requirement if metal patterns are present on the wafers. A number of thin film materials available in most conventional IC processes deposited on the glass wafers have been tested as intermediate bonding layers. Successful bonding is obtained for various layer combinations and an explanation of the bonding mechanism is given.

Integrated-circuit-compatible design and technology of acoustic-wave-based microsensors

Abstract A combination of different technologies is used for the realization of smart acoustic-wave-based microsensors, where electronic circuitry and an acoustic delay line are integrated on a single silicon chip. For a monolithic Lamb wave oscillator, the three required technologies are described: bipolar IC technology, thin-film piezoelectric zinc oxide technology, and micromachining technology. The last two technologies have to be IC compatible and are not allowed to affect the IC processing in order to prevent malfunction of the electronic circuitry. The realized monolithic Lamb wave sensor has several applications. Examples are the determination of the density, viscosity and sound speed of liquids, and (bio)chemical applications, where mass adsorption at the sensor surface is monitored. The sensor described in this paper demonstrates the determination of the weight percentage of water in water—ethanol solutions.

Wafer-to-wafer fusion bonding of oxidized silicon to silicon at low temperatures

Abstract In this paper a silicon wafer-to-wafer bonding process is presented where silicon dioxide is used as an intermediate layer. Because the process temperature is very low (120 °C) and because the chemical treatment of the surface before bonding does not damage aluminium patterns, wafers containing electronic circuity can be bonded. The oxide layer gives an electrical insulation between the two wafers. High bond strengths (over 20 MPa) are obtained.

A novel molecularly imprinted thin film applied to a Love wave gas sensor

Novel molecularly imprinted materials (MIMs) have been deposited as thin films on acoustic Love wave devices. MIMs offer high selectivity towards target molecules while Love wave devices offer one of the highest sensitivities with respect to surface mass changes that can be achieved with microacoustic devices. Hence this specific sensor configuration promises high sensitivity and specificity at the same time. In this contribution we outline the device configuration and present experimental results obtained with this sensor.

Downscaling aspects of a conductivity detector for application in on-chip capillary electrophoresis

Abstract We are presenting the experimental results of capacitively coupled 2- and 4-electrode conductivity detector fabricated as a glass dip-stick. These results are used as a starting point for investigating the consequences of downscaling the device for on-chip capillary electrophoresis (CE) applications, where the detector is located in a separation microchannel. Our studies show that, it is possible to do accurate and sensitive conductivity sensing in a 150xa0μm wide channel, by making use of 4-electrode set-up with silicon carbide (SiC) thin insulating layers on top of the electrodes.

Evaluation of liquid properties using a silicon lamb wave sensor

Abstract The design and realization of a Lamb wave oscillator sensor system are described. Both the oscillation frequency and the amplifier gain level are monitored, giving information about the Lamb wave velocity and amplitude, respectively. The system is fabricated in two separate silicon chips (hybrid set-up) or in one chip (monolithic set-up), and can be used for different sensor applications, in liquid or gas environment. We demonstrate the sensor for detection of the density and viscosity of a liquid, and for sensing of the adsorption of human serum albumin (HSA) at the sensor surface.

All-silicon plate wave oscillator system for sensors

Abstract We present the design and realization of an all-silicon acoustic plate wave oscillator system for sensors. The system consists of a plate wave sensor device and an amplifier, together forming an oscillator. The plate wave device and the electronic circuitry have been fabricated in two separate silicon chips. The system can be used in several sensor applications, such as biosensors and gas sensors, by applying a suitable (bio) chemical thin film on the sensor device. The adsorption of molecules in this thin film causes a velocity change of the plate wave, which results in a frequency shift of the oscillator. The antisymmetric lowest-mode plate wave sensor devices have low phase velocities, which permits their use in liquids and opens up the way for the development of biosensors. The plate wave device consists of a Si x N y / ZnO/SiO 2 /Si layered structure. Flexural plate waves of the antisymmetric and symmetric lowest mode are generated and detected by interdigital transducers at the interface of the zinc oxide layer and the silicon nitride layer. For the fabrication of the thin membrane (plate) we used a time-stopped KOH etching process. The device dimensions are 4 × 10 mm and the membranes measure 3 × 9mm. The membranes are between 10 and 20 μm thick. The devices have centre frequencies between 10 and 25 MHz, depending on the membrane thickness. The electronic circuitry is designed for use with antisymmetric lowest-mode plate wave sensors and consists of a transimpedance amplifier with automatic gain control (AGC). This type of amplifier shows an optimal signal-to-noise ratio and minimizes regeneration of acoustic waves at the transducer of the plate wave device. The gain (transimpedance) of the amplifier amounts to 20 kΩ, and the AGC range is 30 dB. The chip has dimensions of 3 × 4 mm and has been realized in a BICMOS process, using solely bipolar transistors. The design and fabrication technology of the devices are discussed and experimental results of each separate device (transfer characteristics) and the total system (oscillator performance) are given.

Measuring liquid evaporation from micromachined wells

Abstract The evaporation process of liquids from sub-nanoliter wells is studied using both optical microscopy and electrical impedance measurement utilizing integrated electrodes. The wells are manufactured on silicon wafers using conventional photolithography and etching techniques. The volumes of the wells vary from 15 to 540 pl. The evaporation rate of glycol from wells of various sizes is determined to vary from 0.2 (15-pl well) to 1.7 pl/s (540-pl well). Measurements show that the evaporation rate of glycol depends linearly on the well width, not on the well surface area. The impedance measurement between two integrated electrodes located in the well is an accurate method for monitoring the liquid evaporation in sub-nanoliter wells, and can be applied in automated high-speed screening (HSS) systems.

High-resolution liquid volume detection in sub-nanoliter reactors

Abstract Miniaturized reactors with volumes of 60 and 540 pl with integrated contact-less liquid volume detectors are fabricated on silicon. Aluminum electrodes are fabricated in the bottom of the reactors and covered with silicon nitride for electrical insulation. The reactors are formed in a 6-μm thick SiO2 layer by dry etching. The liquid volumes in the reactors are determined by using an impedance measurement and compared to the optical fluorescence microscopy measurements for reference. The applicability of the measurement device for different liquid compositions and reactor size is studied. The presented contact-less measuring method can be used to measure sub-nanoliter liquid volumes with a resolution better than 1 pl. The integrated volume measurement method can be employed in miniaturized automated arrays in High Speed Screening (HSS) systems to determine the dispensed liquid volume.

A temperature-controlled smart surface-acoustic-wave gas sensor

Abstract This paper deals with the thermal and temperature-control aspects of a gas sensor based on surface-acoustic-wave (SAW) devices in a dual delay-line configuration. Application of smart temperature sensors combined with a micro-controller in a temperature-control system provides for a high-performance low-cost A/D conversion. An anti-windup control mechanism has been implemented to solve the problem of actuator saturation.