Axel Berthold

Capillary electrophoresis with on‐chip four‐electrode capacitively coupled conductivity detection for application in bioanalysis

Microchip capillary electrophoresis (CE) with integrated four‐electrode capacitively coupled conductivity detection is presented. Conductivity detection is a universal detection technique that is relatively independent on the detection pathlength and, especially important for chip‐based analysis, is compatible with miniaturization and on‐chip integration. The glass microchip structure consists of a 6 cm etched channel (20 νm×70 νm cross section) with silicon nitride covered walls. In the channel, a 30 nm thick silicon carbide layer covers the electrodes to enable capacitive coupling with the liquid inside the channel as well as to prevent interference of the applied separation field. The detector response was found to be linear over the concentration range from 20 νM up to 2 mM. Detection limits were at the low νM level. Separation of two short peptides with a pI of respectively 5.38 and 4.87 at the 1 mM level demonstrates the applicability for biochemical analysis. At a relatively low separation field strength (50 V/cm) plate numbers in the order of 3500 were achieved. Results obtained with the microdevice compared well with those obtained in a bench scale CE instrument using UV detection under similar conditions.

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.

Fabrication of a glass-implemented microcapillary electrophoresis device with integrated contactless conductivity detection

Glass microdevices for capillary electrophoresis (CE) gained a lot of interest in the development of micrototal analysis systems (νTAS). The fabrication of a νTAS requires integration of sampling, chemical separation and detection systems into a microdevice. The integration of a detection system into a microchannel, however, is hampered by the lack of suitable microfabrication technology. Here, a microfabrication method for integration of insulated microelectrodes inside a leakage‐free microchannel in glass is presented. A combination of newly developed technological approaches, such as low‐temperature glass‐to‐glass anodic bonding, channel etching, fabrication of buried metal interconnects, and deposition of thin plasma‐enhanced chemical vapour deposition (PECVD) silicon carbide layers, enables the fabrication of a CE microdevice with an integrated contactless conductivity detector. The fabrication method of this CE microdevice with integrated contactless conductivity detector is described in detail. Standard CE separations of three inorganic cations in concentrations down to 5 νM show the viability of the new νCE system.

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.

IC-compatible silicon wafer-to-wafer bonding

Abstract In this paper a fully IC-compatible silicon wafer-to-wafer fusion-bonding process is described. Before the bonding, the silicon surfaces are treated by chemicals which do not attack the electronic circuits or aluminium patterns on the silicon. The prebonding of the two silicon wafers is performed at room temperature. after which bonding takes place through annealing at temperatures between 120 and 400°C without affecting the performance of the electronic circuitry. The bonding is sufficiently strong for microsensor applications.

Backwafer optical lithography and wafer distortion in substrate transfer technologies

A method has been developed by which, after removal of the bulk silicon in a substrate transfer process, the backside of a wafer can be processed with the same lithography as the front side of the wafer. To achieve an accurate front-to-backwafer alignment accuracy, mirror symmetric alignment markers for an ASML PAS5000 waferstepper have been developed and applied in a Silicon-on- Anything process. In this manner minimum dimension low-ohmic contacts were fabricated on the backwafer. The mirror symmetric alignment markers are used in combination with standard overlay test procedures to characterize the front-to backwafer overlay accuracy. The measured overlay errors are divided up in non- mirror symmetric lens distortions and wafer distortion as a result of the substrate transfer process. The practical minimum device feature that can be realized on the backwafer is limited to 0.9-1.2 micrometers as a result of front-to-backwafer overlay errors.

Micromachined wet cell for a Love-wave liquid sensor

In this paper we present a silicon micromachined wet cell for use with a Love-wave liquid sensor. The Love-wave sensor is composed of an electronic amplifier and an acoustic Love- wave delay-line on a piezoelectric substrate. Together they form an oscillator. Liquid is placed in intimate contact with the Love-wave sensor; corresponding to its viscosity the acoustic wave velocity changes, which is observed through a change in the oscillation frequency. An issue that arises in a sensor of this type is that the input impedance of the interdigital transducers (IDTs) of the delay-line changes dramatically due to the dielectric properties of the liquid above them. This adds electrical load to the amplifier and affects the oscillators performance by reducing its resolution and sensitivity. The electric loading of the IDTs by the liquid also leads to unwanted sensitivity with respect to the electrical properties of the liquid. The wet cell was designed to overcome this disadvantage. By virtue of this cell the liquid is directed only over the wave propagation path, and so the transducers are protected from the liquids influence. In designing the cell, bubble formation in the liquid, chemical inertness, bonding aspects and temperature effects were all considered. The design utilizes a silicon micromachined channel that guides the liquid between the transducers. Furthermore a heater for controlling the temperature of the liquid has been incorporated. Experiments have shown that placing thin side walls of a silicon micromachined channel in the propagation path of the wave adds little to the insertion loss. Losses of only 6 dB or less were recorded, which confirms the suitability of this configuration. In addition to viscosity sensors this design can be applied to a broad range of Love-wave liquid sensors, including those in the biochemical area.

Low-temperature quartz-to-silicon bonding for SAW applications

A new process for the fabrication of piezoelectric quartz thin films on silicon is investigated. With this process, new silicon-implemented acoustic wave delay lines for sensor applications can be realized. An acoustic-wave delay-line consists of two interdigital thin film metal transducers fabricated on a piezoelectric crystal. In order to realize acoustic-wave devices on (non-piezoelectric) silicon, the use of piezoelectric thin films such as zinc oxide, aluminum nitride or PZT has been reported. However, these films often exhibit stress, aging, pinholes, or poor reproducibility which affects the performance of the device. The bonding of piezoelectric quartz (with its known and fixed mechanical and piezoelectric properties) to silicon improves the performance of silicon-implemented acoustic-wave devices. The process used, consists of a wet chemical treatment after which the wafers are prebonded at room temperature. Annealing at 140 degree(s)C for 3 hours yields a sufficient high bond strength.

Influence of processes and selective bonding technology

The influences of surface characteristics, including adsorptive states led by different chemical treatments and surface roughness, on direct bonding between dissimilar CVD materials were investigated. The bonding procedures were carried out at temperature lower than 400 degrees Celsius. In this temperature range, LPCVD poly-silicon, PECVD oxide, and LPCVD silicon-nitride showed highly process dependent bonding behaviors, i.e., bondable or not bondable to another material under certain experimental conditions. Based on these facts, a selective bonding conception for Si-based CVD material is proposed and applied to fabricate new fluid structures and devices.

Quartz-to-Silicon Fusion Bonding for Micro Acoustic Wave Applications

A new process for the fabrication of piezoelectric quartz thin films on silicon is investigated. With this process, new silicon-implemented acoustic wave delay lines for sensor applications can be realized. The process consists of low temperature fusion bonding of a piezoelectric quartz wafer to a silicon wafer followed by a back-lap of the quartz to a thickness of between 10 and 50 μm. In this contribution we report on the first step: the fusion bond process.