Novel approaches to the design of high-sensitivity CMOS humidity sensors for miniature biomedical implants

There remains a need for satisfactory integrated sensors that can monitor the humidity inside miniature implanted devices in which delicate electronics are operating. This thesis describes a novel design of a capacitance-to-digital readout circuit system with high-sensitivity for ultra-small medical implants. The proposed approach has improved the entire system, including the sensitivity of the humidity sensor and an innovative CMOS integrated readout circuit using XFAB XC06 technology. The humidity sensor has been modelled using finite element method software (COMSOL) to simulate the nominal capacitance and humidity sensitivity, which is dominated by the sensing material, sensor size, finger gap and finger width. The modelling and optimisation of the humidity sensor has been designed as a generic model. By using the parameters from a CMOS technology, the model predicts the capacitance performance of the technology. In this thesis, the modelling improved the accuracy by increasing the mesh density of the region of interests (ROIs) such as finger gap, finger width, finger thickness and dielectric thickness. Furthermore, the modelling also saved computation by narrowing the geometry size of the surrounding boundary, simplifying the air's and substrate's thickness and innovating a size scaling method of the interdigitated structure. The modelling prediction has been verified with the sensor measured results exhibiting good agreement. The modelling suggests the highest sensitivity design for the interdigitated capacitive sensor and benefits the readout design, which avoids over-engineering and reduces at least 20% sensor area while achieving the same humidity sensitivity and saves power consumption no less than 50% based on the prediction of the optimised capacitance performance. The novel readout uses charge sharing to cancel the parasitic capacitance from the measured capacitive input, which provides a broader dynamic range than conventional capacitance readout circuits, and consequently increases the readout sensitivity. The entire system features space-saving by overlaying the readout with the sensor and requires no post-processing fabrication such as passivation etching or polyimide thickening; the sensor capacitor is placed on the top of the readout. The system was designed in XFAB's 0.6-μm CMOS technology, achieving a measured resolution of 0.1 fF per count and can detect 0.034±0.007%RH change. The circuit consumes 15.7 µW, including a sensor, and occupies an area of 0.42 mm2 where the sensor uses the top metal layer, and the beneath metal layer are available for circuitry.