Polysilicon interdigitated electrodes as impedimetric sensors

Abstract

The suitability of polysilicon as a material for the fabrication of interdigitated electrodes and their application to the development of sensors is studied in this work. The main interest in using this material lies in the possibility of obtaining integrated sensors with commercial CMOS technologies and simple post-processing steps. Electrodes with 3μm finger width and 3, 10, and 20μm spacing were fabricated and characterised. Conductivity measurements in the range from 4 to 50μS/cm yielded a linear response with cell constants of 0.04cm prostate-specific antigen (PSA) [11], bacteria [12], red blood cells (hematocrit) [13], and detection of specific sequences of DNA [14]. In some applications, the measurement of a physical magnitude such as dielectric constant or conductivity of the media provides direct information of the analyte concentration [5,13]. In other applications, the deposition of a membrane or functional monolayer on top of the electrodes as a recognition element is necessary to obtain sensitivity and selectivity to particular species [1][2][3][4][6][7][8][9][10][11][12][13][14]. Different arrangements of electrodes (i.e. different number of electrodes and geometries) may be used for impedimetric measurements. Among them, interdigitated electrodes have significant advantages for certain applications: 1) their low cell constant (resistance to resistivity ratio) permit the measurement of very low conductivity solutions, 2) measurement of dielectric properties is possible thanks to a high inter-electrode capacitance (higher than stray capacitance even for conductive substrates), 3) the short penetration depth 3 of electric fields make them less dependent on the measurement cell geometry and allow the use of thin membranes for tailoring selectivity.In this paper the suitability of polysilicon as a material for the fabrication of interdigitated electrodes and their application to the development of sensors is studied. For example, the feasibility of measuring solution conductivity and permittivity with polysilicon electrodes is demonstrated. The main interest in using this material lies in the possibility of fabricating the electrodes with commercial CMOS technologies and simple post-processing steps, and therefore being able to integrate them as sensors in a more complex system-onchip. Another interesting advantage is that this material can be easily modified following a one-step silanisation reaction. In this way, robust immobilisation of either biomolecules or membranes can be carried out on to both the electrode fingers and the area in between them with the aim of developing a sensor device. As an example, a fully functionalised biosensor for the detection of urea is also presented.The basic structure of the interdigitated electrodes is depicted in Fig. 1. The polysilicon traces forming the interdigitated geometry are separated from the silicon substrate by a thick silicon dioxide layer. represented by R sol and C so l . From these components, the resistivity ρ (or the conductivity σ) and th...

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