Introduction

Impedance Spectroscopy (IS) is emerging as a powerful technique with broad applications in the field of micro-scale sensing. In short IS is the process of measuring a sensor's impedance (complex resistance) response over a wide range of frequencies. A great deal of information can be gathers from the impedance spectrum, and this has opened up many exciting applications. These applications include: estimation of skeletal muscle ischemia, detecting properties of blood, particle detection, measurement of human erythrocytes, measurement of bacterial metabolism, preparation of neuroelectronic cultured probes, cell positioning on microarrays and DNA identification.

It is clear that many applications exist for on-chip IS, and a lot of work has been done in testing and expanding these applications. However, very little work has been done in the development of systems to actually measure IS on-chip. Nearly all of the applications above attach on-chip probes to bench-top instruments to do the actual impedance measurement. But making the measurements on-chip is rapidly becoming necessary for a number of reasons. First, many of the above applications tend toward a high density array of sensors. It is not possible to bring probes from a hundred or more sensors out to any external instrumentation. Multiplexing a hundred signals is also not possible because leakage current in the transistors will swamp out the signal being measured. To enable high density arrays, the sensors must be measured on-chip and the data summarized before being sent off chip. Second, probes run from a chip to external circuitry increase the amount of noise that is coupled onto the signal. With sensors being fabricated on the surface of these chips, the chip's electronics will have better a signal to process. Lastly, fully on-chip IS measurements are necessary for practical applications of this technology. Bench-top equipment greatly limits the portability and economy of these sensors, so in most applications where these senors would be used, it would not be practical to require bench-top equipment for usage. By fabricating this functionality on-chip it will be possible to reduce total expense, size and power consumption of the sensors.

Goals

Some research is being done into on-chip IS, but the potential challenges and limitations are not yet well known. The goal of this project was to develop a baseline on-chip IS system to which more advanced future IS can be compared. It is also necessary to provide some basic specifications for the system.

The advantage of bench-top instrumentation is that it is very flexible and can make measurements in many applications. It is not possible to duplicate all of this functionality in on-chip IS systems, so the specific needs of the IS measurements must be identified. Biological systems usually respond relatively slowly, so most interesting information is in the lower frequency ranges. A lower limit of 1Hz was chosen for this system. Many biological systems are measured in ionic solutions. Between the solution and the metallic probes there is always a capacitance, known as the ionic double layer capacitance. This capacitance effectively acts as blocking capacitor, eliminating all DC from the sensor response. Also, it is typical that these systems exhibit very large resistances on the order of 100kΩ to 1MΩ. This translates into very small currents on the order of 50nA. For this project a current range of 10nA to 100nA was selected for input range. Finally, high density arrays impose their own requirement. The small signal currents and transistor leakage mean that it is difficult to multiplex hundreds of sensors down to one readout circuit, because the signal is not much bigger than the total leakage. Therefore, at least the front end IS measurement circuitry must be small enough to be instantiated many times on the die.