A new bias-enhanced nucleation method based on an AC-bias step to form highly oriented diamond (HOD) nuclei on silicon substrates is presented. The uniformity of the nucleated film and the bias time strongly depended on the substrate temperature and the substrate holder. In our case the shortest bias time and highest nucleation densities were achieved at ~ 850°C while using a graphite susceptor. Following this nucleation enhancement step the diamond films were grown out using conditions employing an α-parameter slightly greater than 2. This ultimately leads to extremely smooth and well-faceted (100) textured HOD films which could be used as substrates for the fabrication of electronic devices. Schottky diodes with high rectification ratios and high breakdown voltages have been fabricated for the first time via selective growth of the active boron doped diamond layers on these HOD films. Results of the growth procedure and diode performance will be given.

Diamond cantilever actuators show high resonance frequencies but need also high actuation forces, pointing towards piezoelectric actuation by a PZT/diamond unimorph. In this study lead zirconate titanate (Pb(Zr,Ti)O-3, PZT) layers have been deposited onto nanocrystalline diamond films by sol-gel deposition, to realize high-speed MEMS actuators. The fabrication technology is based on self-aligned patterning and on optical lithography. A mechanical resonance frequency of 3.9 MHz has been obtained for 30 mu m cantilever length dominated by the nanodiamond Young's modulus of approximately 1000 GPa. (C) 2008 Elsevier B. V. All rights reserved.

In this work we discuss growth and properties of nanocrystalline diamond films grown with a HFCVD system on 4 in. silicon (100) wafers. Nucleation was performed by an in situ bias pretreatment with nucleation densities of more than 10(10) cm(-2). Growth of nanocrystalline films has been accomplished by reduction of the concentration of atomic hydrogen. This is obtained by an increase in the recombination of hydrogen radicals either by addition of nitrogen to the gas phase or by an increase of the total pressure during growth. The concentration of nitrogen to carbon atoms in the gas phase can reach values of 10 and more and pressures varies from 1 to 9.5 kPa.In this nanocrystalline morphology Young's moduli of the diamond films between 800 and 980 GPa as well as fracture strengths of more than 3 GPa can be obtained. The renucleation process which produces this nanosized diamond material with a grain size of less than 60 nm creates also a homogeneous growth pattern, which leads to an almost vanishing vertical stress gradient across the film thickness.Variations of growth parameters such as pressure and substrate temperature result in different compressive stress values between 0 and 390 MPa. The possibility to control and adjust the absolute value of the stress inside the diamond film with respect to the silicon substrate allows to use the built-in stress as design parameter of MEMS devices and engineer for example bistable membrane configurations. (c) 2005 Elsevier B.V. All rights reserved.

Here, we present a planar multi-patch-clamp-system based on a nano-crystalline diamond patch-membrane used for the investigation of ion channels in cell-membranes. In such a system the cell-membrane is electrically stimulated through a potential difference in the electrolyte across the patch-membrane. Usually, the patch-membrane material is a highly insulating dielectric. The use of diamond allows to add highly localized probes in close proximity to the clamping area for a localized analysis of the stimulation mechanism. The technological concept, realization and first electrical characteristics of the principle structure are discussed.

Electronic properties of hydrogen terminated diamond have been investigated by comparing the DC and frequency-dependent characteristics of metal–diamond and liquid–diamond interfaces. For this purpose various electronic device structures including Surface Channel FETs and electrodes for investigation in the liquid are fabricated on single crystal diamond substrates with H-induced surface conductive channels. In both experimental configurations (metal and liquid junctions) it has been shown that the p-type H-induced conductive channel is separated from the diamond surface by a thin insulating layer (so-called ‘lossy dielectric’). The measurements only allow a description of the surface layer with electrical parameters. An identification of its physical/chemical nature needs still further analysis.

Nanocrystalline diamond films have been grown in a hot filament CVD (HFCVD) system allowing BEN, textured growth and HOD growth of diamond on 4" silicon (100). The growth process of nanocrystalline diamond discussed here consists of a BEN process for diamond nucleation on silicon, with a density of more than 10(10) cm(-2) across the entire wafer surface. The subsequent growth of the nanocrystalline diamond films is achieved by addition of nitrogen into the gas phase with a relative concentration to carbon of 1:1. At substrate temperatures between 680 degreesC and 740 degreesC, closed diamond films are grown with thicknesses between 1 mum and 20 mum. At low nitrogen concentrations the nanocrystalline diamond films exhibit still a columnar structure with a lateral grain size below 200 nm whereas the vertical grain size can reach several microns. Higher nitrogen concentrations lead to a nanocrystalline diamond growth mode with no columnar structure and a grain size is no longer detectable by SEM. The advantages of diamond in MEMS technology rely on its outstanding mechanical and thermal properties. Additionally, the stress distribution inside the diamond films due to thermal expansion as well as the polycrystalline nature of the film has to be taken into account. Applying different test structures we could measure fracture strength of more than 4.0 GPa and elasticity moduli of up to 1020 GPa for these nanocrystalline films. The intrinsic vertical and horizontal stress remains in most cases below the detection limit of 5 MPa, so that freestanding structures with a length of several millimeters can be fabricated without noticeable bending. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Stacking faults containing microtwins in (111)-oriented diamond layers grown on a high-pressure high-temperature (HPHT)-synthesized diamond substrate by chemical vapor deposition start to form just on the substrate. The microtwins in the stacking faults form on the {1̄11} plane, not on the (111) substrate plane. To explain these results, we propose an atomic-scale model in which a foreign atom remains on the HPHT substrate surface and a C atom on the foreign atom cannot form a covalent bond with it. Therefore, twinning of the C atom occurs on the {1̄11} plane. The next C atoms bond with the twinned C atom in an untwinned (normal crystalline) relation. Consequently, the formation of stacking faults that contain microtwins occurs.

A setup for bias enhanced nucleation inside a hot filament CVD system is described. It consists of a combination of two separate voltages for independent generation of ions in the gas phase as well as the acceleration of the ions toward the substrate surface. Using the bias current as a monitor for the dynamics of diamond nucleation, the pretreatment can be terminated at a definite condition and the diamond growth process can be continued under normal deposition conditions. With suitable deposition parameters we are able to obtain diamond nucleation on an entire 4″ Si (100) substrate with a density of more than 109 cm−2 as confirmed by secondary electron microscopy. Furthermore, the area of oriented diamond nucleation on a 4″ Si wafer was investigated. Terminating the pretreatment at the threshold of the bias current rise leads to oriented nucleation of diamond. With the present homogeneity of the filament-substrate distance we are able to extend the area of oriented diamonds up to 50 cm2.

Over the past few years a variety of diamond electron devices have been fabricated, analysed and simulated. This includes Schottky diodes on boron-doped p+ diamond substrates, boron/nitrogen pn-junction diodes, bipolar transistors based on this pn-junction and field effect transistors (FETs) with boron delta-doped channels and hydrogen-related surface conductive layers. Many of the fabricated devices considered here represent the current state-of-the-art in this field. This includes the operation of diamond Schottky diodes at temperatures of up to 1000 °C, as well as diamond FET devices with a cut-off frequency of 30 GHz and channel current densities of 300 mA mm−1. Simulations show that diamond boron delta-doped FETs might yield an RF-output power density of up to 30 W mm−1.

Diamond has been considered a material with extreme mechanical properties for a long time, but is also a wide bandgap semiconductor, potentially enabling electronic and microsystems device characteristics which are currently out of reach. Recent progress has however allowed verification of many ideal diamond materials properties in devices. Two basic electronic components are discussed (FETs and microswitches), which may be viewed as building blocks for future ultra high power high temperature electronics.

S-parameters measurements were carried out on diamond-based FET devices with a p-type channel induced by hydrogen surface termination extracting f(T) and f(max) for devices with gate lengths ranging from 5 mum to 0.2 mum. For the 0.2 mum gate length FET f(T) = 11.55 GHz and f(max) (MAG) = 33.3 GHz values were obtained. High f(max)(MAG)/f(T) ratios of above 2.5 were obtained for all devices. Further downscaling may result in an f(T) above 20 GHz and in addition, an f(max) (MAG) above 50 GHz.

Diamond hydrogen-induced surface channel field effect transistors (FETs) were fabricated with gate lengths down to 0.2 μm in part using electron beam lithography. Down-scaling of the gate-length resulted in both improved DC- and RF characteristics, especially for a 0.2-μm gate length in a maximum output current of IDmax=360 mA/mm with a peak transconductance of 148 mS/mm. The optimum cut-off frequencies were fT=11.5 GHz, fmax(MAG)=31.7 GHz and fmax(U)=40.2 GHz. A maximum drain voltage of 68 V was obtained before pattern-related destructive breakdown occurred. This allows estimation of the RF power handling capability to above 3.0 W/mm. The data are the highest reported for diamond FETs. Scaling of the device parameters with gate length allows to estimate a velocity-limited maximum output current to 750 mA/mm and an fT above 20 GHz at 0.1-μm gate length. At high drive, current drift and current compression is observed in the quasi-DC output characteristics as well as in first microwave large signal measurements. These instabilities seem at present to be the main hurdle of hydrogen-induced surface channel FETs in high power microwave applications.

The prospects of two classes of diamond devices are reviewed, namely electronic devices on single-crystal substrates and Microsystems devices on Si substrate. The transistor structures on single-crystal diamond represent still proof-of-concept experiments; however, they already allow us to extract their potential. The Microsystems actuator and sensor devices already reflect the materials properties in their characteristics. Two of the most complex structures and future trends are discussed.

Diamond is an intriguing wide band-gap semiconductor material with extraordinary mechanical, thermal, chemical and electronic properties. This combination makes it an ideal material for a large variety of microsystems. This review focuses on electromechanical, electrothermal and electrochemical sensor and actuator devices fabricated on highly oriented diamond (HOD) films on Si in our laboratory.

In this paper a novel concept for diamond power FETs is presented. This concept is based on a delta -doped active channel using homoepitaxial CVD-layers on 100-oriented single crystals. The channel is controlled by a recessed sub-mum pn-junction gate. Based on technological building blocks developed previously, the structure has been simulated and 13 W/mm RF power are predicted. First fabricated FETs show that the concept is feasible.

An electrical model of the surface conductive layer in diamond induced by hydrogen plasma treatment is proposed. The model is discussed by analysing the characteristics of Schottky diodes and resistors built on the hydrogen terminated diamond. It is proposed that the hydrogen induced acceptors in diamond are separated from the surface by a separation layer characterised by dielectric constant ε∼50 and thickness of about 30–50 nm. This layer allows the complete depletion of the hydrogen induced acceptors by a 0.5–1 eV Schottky barrier, but prevents the tunneling through the barrier at forward bias. Also, this model allows charges on the surface of the proposed separation layer. The charging/discharging of surface states can explain the degradation of the device performance during testing.

Diamond junction field effect transistors (FETs) utilizing δ-boron-doped diamond films were fabricated and analyzed. In order to allow full charge modulation by the gate, the total channel sheet charge must not exceed the order of 1013cm−2. However, boron doping shows full activation only for concentrations above ca 1020cm−3 [1]. This yields a thickness for a fully activated channel in the range of ca 1nm. To approach such narrow doping spikes any parasitic boron doping tails need to be eliminated. One possible way of achieving this is to compensate boron doping with nitrogen doping, an extremely deep donor. This results in the formation of a pn-junction, where the nitrogen doped part is not activated at room temperature and which therefore represents a semi-insulating (lossy) dielectric at low temperature and high frequency. At elevated temperature and low frequency the nitrogen doped layer becomes conducting acting as a series resistor to the interfacial pn-junction. Using this concept of a lossy dielectric pn-junction in the δ-doped channel FET, two gate diode configurations were investigated. In the first the nitrogen doped (Ib) synthetic diamond substrate served as a large area back gate, while in the second the nitrogen doped gate layer was grown on top of the δ-channel. The devices show high drain currents of up to 100mAmm−1 and full channel modulation even at moderate operation temperatures of 200–250°C. By extrapolation a current density of 1Amm−1 is expected for a 0.25μm gate length device.

Capacitors using diamond membranes as the dielectric were fabricated and evaluated up to 600°C using various MPCVD grown films. The films were grown on Si and were randomly oriented or highly oriented with different grain sizes. In part they were nitrogen doped. Admittance measurements at room temperature (RT) show a dissipation factor <10−5 at 10kHz, which was the resolution limit of the measurement system. The loss increases with the temperature with an activation energy of 1.0–1.3eV. This activation energy was found to be independent of the diamond CVD growth configuration. The level of the leakage is influenced by the grain size but not by the nitrogen doping concentration.

The temperature-handling capability of diamond diode and field effect transistor structures is discussed and compared with recent results on GaN. The main parameters limiting the high-temperature performance are identified and evaluated. A diamond high-temperature technology is presented which has allowed 1000 degrees C operation of a diamond Schottky diode, the highest temperature of operation of any semiconductor diode yet.

Thermal microheaters as used for example in thermal ink print heads contain a variety of materials, the heater itself is commonly based on refractory metals. Here, a novel concept based on diamond as a multifunctional material is proposed and demonstrated. In this concept, diamond serves as an insulating substrate, a metal-like heater and as a temperature sensor. Superheating of water could be achieved after 7.5 μs heating of a 60×60 μm element. Bubble nucleation was visualized using a high speed stroboscopic technique. The influence of the diamond thermal properties and geometry on the thermal response were investigated by dynamic 3D-simulations.