Polycrystalline diamond (poly-C) technology and piezoresistive sensor application for cochlear prosthesis

Polycrystalline diamond (poly-C), with high piezoresistivity and unique mechanical, chemical and thermal properties, becomes a promising material for piezoresistive sensor application in particular in harsh environments and high temperature. However, due to the technology limitation and integration difficulty, the application of poly-C is far from being a standard technology like silicon process, which limited the mass production and commercialization of diamond based sensors.; The research of this dissertation developed several optimum poly-C technologies for better integration with microsystems and analyzed the dependence of poly-C piezoresistivity on film properties. Then, the application of poly-C piezoresistive sensor was successfully demonstrated by integrating it into the silicon thin film cochlear implant probe as the position sensor for its high piezoresistive gauge factor (GF).; A new diamond seeding method, spin-coating of diamond powder loaded water (DPLW), was developed with uniform, nondestructive and repeatable high seeding density (2x1010/cm2) on 4 inch oxidized wafer. Uniform poly-C growth, with less than 20% thickness variation, was realized on 4 inch size wafer using MPCVD method with 3 kW microwave power. Low-resistance contact between poly-C and titanium was realized by adding a thin highly doped poly-C interlayer.; The dependence of poly-C piezoresistive GF on film resistivity and grain size were studied in detail using cantilever beam method. Optimized poly-C process, with resistivity in the range of 20 to 80 O*cm and average grain size of 0.8 mum, was chosen for piezoresistive sensor application. This process yielded high GFs in the range of 30-70.; Cochlear probe is a device that can be implanted into a person's cochlea and deliver sound signal to the auditory nerves of deaf people. The position sensors on the probe are critical to determine the probe insertion position accurately. Poly-C position sensor was successfully integrated into two generations of cochlear probes and demonstrated its high piezoresistive GF of 44 for the first time. Design, fabrication and characterization of the poly-C sensors were accomplished with several improvements, such as high microwave power, contact interlayer, short resistor and large grain size. This work demonstrated a successful integration and promising application of poly-C technology with the silicon based microsystems.