Peter Gluche

DIAMOND MEMS : A NEW EMERGING TECHNOLOGY

Abstract Diamond is a superhard, wide bandgap, semiconductor material of high mechanical strength and thermal stability and therefore an ideal candidate for micro electromechanical devices. Using these properties in a diamond-on-Si technology, a number of sensors and actuators have been attempted. However, their industrial implementation lags far behind that of silicon microelectromechanical systems (Si-MEMS) technologies. In this study, highly oriented chemical vapor deposition (CVD) diamond films were deposited on large area Si-substrates, micromachined into structured membranes and applied to two demonstrators: a seismic mass membrane acceleration sensor and a liquid ejector based on a diamond microspot heater. In this technology, the outstanding and extreme diamond material properties are already widely reflected in the performance of the demonstrators. The technology may be scaled and implemented into existing Si-based microsystem technologies and may therefore open up the possibility to integrate diamond MEMS technology into the Si-based mainstream.

Diamond surface-channel FET structure with 200 V breakdown voltage

An enhancement mode diamond FET using a hydrogen-terminated surface as hole conductive channel has been fabricated with 200 V gate to drain breakdown voltage. At the 8.5-/spl mu/m gate length the maximum drain current was 22 mA/mm. 90 mA/mm maximum drain current was obtained at a gate length of 3.0 /spl mu/m. Scaling to below 1 /spl mu/m gate length assuming undegraded breakdown conditions will result in a projected RF power handling capability above 6 W/mm.

High temperature, high voltage operation of diamond Schottky diode

Abstract Very high temperature operation of homoepitaxial diamond Schottky diodes is demonstrated with rectifying behaviour up to 800 C. The Schottky material is p+-Si with a chemically stabilized Si-diamond interface, leading to significantly reduced thermal activation of the reverse currents. On diamond films with low surface doping concentration 150 V breakdown voltage at room temperature is observed.

Very high temperature operation of diamond Schottky diode

For the first time, the operating temperature of a Schottky diode structure has been pushed to 1000/spl deg/C. The diode structure consists of a Si-based Schottky material deposited onto a homoepitaxial boron doped diamond surface. At high temperatures, the forward I-V characteristics are dominated by the thermionic emission (n/spl ap/1.01) across a barrier of 1.9 eV height. The reverse characteristics are still dominated by thermally activated defects. The series resistance shows thermal activation associated with the boron doping.

High-voltage Schottky diode on epitaxial diamond layer

Abstract Homoepitaxial diamond Schottky diodes with leakage current less than 10−7 A cm−2 up to the breakdown voltage of 90 V are discussed. Under forward voltage an exponential current increase over six orders of magnitude and an ideality factor of n

High-temperature, high-voltage operation of pulse-doped diamond MESFET

The fabrication and operation of a pulse-doped diamond metal-semiconductor field-effect transistor (MESFET) is presented showing a usable source drain voltage of 70 V and no breakdown up to 100 V at 350/spl deg/C operating temperature. A channel sheet concentration of 8.5/spl times/10/sup 12/ cm/sup -2/ could be fully modulated leading to a maximum transconductance of 0.22 mS/mm, although full activation of the boron acceptor had not been reached. For an optimized device structure, with reduced gate length below 0.25 /spl mu/m and full activation, more than 10 W/mm RF-power density can be predicted.

Surface micromachined diamond microswitch

Abstract The realization of an electrostatically actuated all-diamond microswitch is presented. Diamonds mechanical properties are superior to those of most of the common MEMS materials combined with the widest range of electrical conductivity offering the possibility for stacks of insulating and conductive layers of essentially identical thermal conductivity, Youngs modulus, fracture strength and thermal expansion coefficient. Thus, microswitch devices for high current/high temperature operation as well as structures with high mechanical switching frequency are possible. Experimental results of the static properties as well as simulation approaches to the dynamic device behavior are presented.

Analysis of piezoresistive properties of CVD-diamond films on silicon

Abstract The piezoresistive properties of CVD-diamond are still very much in discussion since not only the materials energy band structure properties have to be considered but also the grain boundaries and internal stress distribution. Here, the experimental piezoresistive properties of CVD-diamond-on-silicon layers for free standing structures have been investigated comprehensively. The longitudinal gauge factor kl has been extracted using freestanding diamond cantilevers on silicon. The piezoresistors have been grown selectively onto the surface of diamond cantilevers near the mechanical suspension and doped with boron (acceptor). The electrical contacts are based on the tunneling mechanism with a silicon-based multilayer metalization leading to a linear IV-characteristic. Gauge factor values, kl, have been extracted on various structures with different doping concentrations and diamond film quality (highly oriented and textured, textured, randomly oriented), depending on temperature (room temperature, −350°C) and intrinsic stress. Highly oriented and textured films with grain sizes between 3 and 10 μm have been used to realize ‘single grain’ resistor structures enabling the investigation of grain boundaries in the electrical current path of the piezoresistor. Raman measurements have been performed to measure the intrinsic stress in the diamond grains. Gauge factors, kl of between 4 and 28 have been extracted. Largest kl values were observed on piezoresistors on highly oriented and textured diamond (HOD) films. Results of this work have been used in piezoresistive sensor applications.

Application of highly oriented, planar diamond (HOD) films of high mechanical strength in sensor technologies

Abstract Diamond possesses many characteristics of an ideal material for microsensors, and has indeed emerged as a promising candidate. In comparison to its competitors Si and SiC, large area diamond films are still polycrystalline and inhomogeneous in grain size and orientation. This still determines the material properties, and thus the sensor technology and device performance. However, highly oriented diamond films of high quality have been developed recently, using a modified bias enhanced nucleation method [1]. These films can be described by highly planar, textured surfaces, mirror like backsides, low internal stress and high mechanical strength. Conventional semiconductor processing schemes can now be fully implemented, allowing one to scale high performance micromechanical sensor structures into th lower micrometer range. In this paper, a novel concept based on selective area epitaxy (SAE), pulse doping, reactive ion etching, multilayer contacts and wet chemical backside patterning with micron resolution is presented. The elastic properties and the piezoresistive characteristics of boron doped diamond have both been investigated from diamond cantilever beam deflection measurements. For 15 μm thin HOD-films, a Youngs modulus of approximately 830 GPa has been extracted from resonance frequency measurements and nanoindentation measurements. From this data a fracture strength of σfr=2.72 GPa is calculated. To our knowledge, these data represent the highest values reported up to now for such thin films.

Highly rectifying Au-contacts on diamond-on-silicon substrate

Au Schottky diodes have been fabricated on highly oriented diamond (HOD) films, grown on silicon using AC-bias nucleation and microwave plasma chemical vapor deposition. The active layers were selectively grown and doped by solid boron source. High rectification ratios were obtained up to 500/spl deg/C in a bias voltage range up to /spl plusmn/15 V. A current density of more than 1 A/cm/sup 2/ and a breakdown field strength up to 2.0/spl middot/10/sup 6/ V/cm for point contacts has been demonstrated.