A. Flöter

Diamond electro-mechanical micro devices — technology and performance

Abstract For Mikro Electro Mechanical System (MEMS) based on chemical vapour deposition (CVD) diamond many key steps in growth and processing have been developed and work reliably. Devices can be tailored depending on their specific application. Static and dynamic performance data become available as specifically discussed for the electromechanical microrelay. However, although steady progress is observed the variety of devices is still limited and a number of key elements still needs to be developed for systems including fully free moving parts.

Photoconductivity of undoped, nitrogen- and boron-doped CVD- and synthetic diamond☆

Abstract Nitrogen-doped CVD- and synthetic type IIa and Ib diamonds were investigated by the constant photocurrent method (CPM). Nominally undoped CVD-films containing nitrogen show broad absorption bands with threshold energies at 1, 2.3, 3 and 4.2 eV. The typical nitrogen donor absorption band with a threshold at 1.7 eV is partially masked by the 1 eV band in CVD-films. The absorption bands are too broad to be described by simple theories based on photoionization of single unbroadened impurity levels. Boron-doped CVD- and type IIb synthetic diamond was studied by photoconductivity and photothermal ionization in the near infra-red. The large electron-phonon coupling in diamond gives rise to oscillatory photoconductivity minima due to fast capture of holes by the excited states of boron acceptors. In CVD-films with boron concentrations around 1019 cm−3, the oscillation pattern inverts at low temperatures and sharp minima were found in the spectrum.

Influence of the microstructure on the thermal properties of thin polycrystalline diamond films

Highly oriented and columnar grained diamond layers only a few microns thick, deposited at different substrate temperatures (500, 550, and 800 °C) on silicon using microwave-plasma-assisted chemical vapor deposition, are investigated by special photothermal techniques and high-resolution transmission electron microscopy (HRTEM). Small effective diamond–silicon boundary resistances of <4×10−9 m2 K/W are determined for thermal conduction normal to the interface. Thermal conductivities normal to the interface, k⊥, are found to be about an order of magnitude greater than the conductivities parallel to the interface, k∥ (k⊥/k∥=9–18). The boundary resistances measured are in good agreement with limits estimated from the interface structure observed by HRTEM, which indicate a low near-interfacial disorder for the layers.

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.

Electron microscopy of interfaces in chemical vapour deposition diamond films on silicon

Abstract The structure of interfaces in diamond films grown on Si(100) has been investigated by transmission electron microscopy for the early stages of microwave-assisted chemical vapour deposition. Using conditions optimized for achieving so-called highly-oriented diamond films the depositions were performed in two steps, a bias-enhanced nucleation step and a subsequent growth step. Characteristic for the early deposition stages is the self-organized formation of regular arrays of predominantly {111}-facetted Si substrate surface grooves and islands elongated along [110] and [110] directions. Subsequently, an interlayer of nanocrystalline β-silicon carbide islands forms, followed by the formation of epitaxially oriented diamond nanocrystals with high fractions of {111} interfaces. High-resolution electron microscopy of the interface regions depicts arrays of terminating {111} diamond planes at an average ratio of five diamond to four SiC lattice planes which corresponds to a remaining lattice mismatch of 2.3%. The orientation relationships between the lattices may be described by a coincidence site lattice model if the elastic lattice distortions are taken into account. Only small fractions of amorphous inclusions are present near interfaces, essentially consisting of amorphous carbon as could be deduced from analyses of the C K edge fine structure in electron energy loss spectra. The observations are compared with cases for which diamond nucleation directly on silicon has been obtained.

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.

Thermal resistance and electrical insulation of thin low-temperature-deposited diamond films

Abstract Diamond layers only a few microns thick were deposited by microwave plasma-assisted chemical vapour deposition (CVD) on silicon at different substrate temperatures (500, 550 and 800°C) using different methods for nucleation enhancement (ex situ mechanical pretreatment or in situ substrate biasing). The thermal resistance was measured for conduction normal to these thin layers, which span a wide range of structural properties from random small-grained over columnar to highly oriented grain structures. It was shown that the thermal resistance normal to thin CVD diamond layers depends strongly, for a given layer thickness, on the grain size and the degree of grain orientation in the direction of growth. The smallest thermal resistances were observed for bias-nucleated, highly oriented films deposited at 800°C with pronounced fibre textures. An upper limit for the effective thermal resistance of the diamond-silicon boundary of 1.8 × 10−9 m2K/W was determined for mechanically pretreated, columnar-grained films deposited at low temperatures, which suggests a small interfacial disorder for these films. Furthermore, the electrical insulation of the low-temperature deposited films was shown to be comparable with that of high-temperature-deposited diamond.

Characterization of textured polycrystalline diamond by electron spin resonance spectroscopy

Electron spin resonance (ESR) is shown to be a useful and versatile technique for the detection and characterization of preferred orientation effects in polycrystalline diamond films. A nitrogen related center known as P1 is used for this purpose. The ESR signal coming from this center is composed of a central line and hyperfine satellite lines. It is found that crystallite disorientation causes a linewidth broadening of the satellite lines, which can thus be used to quantitatively characterize the diamond film texture. It is shown that the method is able to separate contributions of disorder induced by rotations of the crystallites around the growth direction from other contributions. The general conditions in which the method can be applied, and its applicability to other materials, are discussed.