Conductivity of a liquid solution provides valuable information about the solution, but commercially available conductivity probes are costly and have diameters greater than 1cm, limiting their potential applications. Microscale thin-film liquid conductivity sensors are a novel application of semiconductor processing methodology and are capable of measuring ion concentrations in nanoliter size samples of liquid. The devices can be readily commercialized for small volume sample analysis or for in-vivo measurement in chemical or biological processes and mesoscale machines. Sensors can be fabricated as part of an integrated circuit and may be integrated with other sensors for multiparameter measurements. These robust, low-cost sensors also have potential for a wide range of commercial and industrial monitoring and control applications.
Current, commercially available liquid conductivity sensors are bulky, expensive and unable to measure small sample volumes or to be used for in-vivo and in-situ applications. A newly developed surface and bulk micromachining technique enables the fabrication of low cost, robust sensor probes with temperature compensation and improved packaging on silicon wafers.
From 200 to 2000 devices (depending on desired size) can be fabricated on a 4" silicon wafer, greatly reducing fabrication cost. Pt-black electroplating reduces the electrode/solution interfacial impedance, maintaining measurement accuracy while significantly reducing operation frequency. Reduction of operation frequency eliminates dependence on expensive measurement electronics.
With the new process and 4-electrode measurement, sensor probes can be fabricated with tip widths much less than 100 microns. To improve device accuracy, an RTD temperature sensor can be integrated on the sensor probe tip to provide local temperature compensation. The bulk micromachining method also allows the thermal mass of the sensor to be reduced, thus improving the response time of the probe.
- Analysis of chemical and biological fluids.
- In-vivo and in-situ measurements for medical testing.
- Controls for washing machine and dishwasher rinse cycles.
- Monitors to improve the performance and safety of water and ice dispensers and water purifiers.
- Devices for monitoring ultrapure water and measuring seawater salinity.
- Sensors can measure electrical conductivity in nanoliter size samples over a wide range of solution conductivity.
- The sensor tip accommodates an integrated temperature sensor that enhances sensor accuracy by measuring local temperature.
- A four-electrode design provides independent measurement of current and voltage.
- The device is hundreds of times smaller than commercially available conductivity probes; future devices may be miniaturized to 100 micron tip width.
- Sensors can be operated at very low current and power consumption and are ideal for low-power or battery-operated applications.
- Sensors can be driven at relatively high electric currents, with the resulting high temperature acting to de-foul the electrodes.
- Because of low fabrications costs, sensors can be disposable.