M. Adamschik

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.

Prospects of diamond devices

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 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.

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.

Diamond microwave micro relay

Abstract Based on the technology of the electrostatically actuated cantilever micro switch, entirely made of diamond on a Si-baseplate (except for the final top interconnect metal level) [reported previously] and based on the static and dynamic switching characteristics [reported previously], in coplanar arrangement was designed, fabricated and successfully operated. Due to the DC-coupling the new MEMS-relay allows switching of bias and signal simultaneously.

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.

On the ion-sensitivity of H-terminated surface channel devices on diamond

The transient behavior of ungated ion-sensitive FET-like structures with no reference gate electrode on H-terminated diamond surfaces is studied in various pH solutions. The surface conductive channel of these devices is formed by hydrogen termination of the diamond layers selectively grown on single crystal and polycrystalline diamond substrates. Two types of the ISFET-like structures are used in the experiments: (a) large area devices, where the as-formed surface channel is not subjected to further processing and (b) lithography-patterned (micron-size) ISFET-like structures with the surface channel formed by openings in polyimide passivation via chemical etching. The response of the devices to solutions with different pH was found to be strongly dependent upon the type of substrate and the channel fabrication technology. Reproducible response is observed only for large area devices on single crystal substrates. The response of the large area devices on polycrystalline diamond is less reproducible and is characterized by a large (several hours) relaxation time to restore the initial conductivity. The structures patterned by lithography on polycrystalline diamond undergo fast degradation, once immersed into solution. Then, no ion-sensitivity and no recovery are observed.

Design of high-speed diamond microswitch

In this work, microswitches based on polycrystalline diamond films are investigated. Device properties such as switch-on and switch-off times are calculated and compared to measurement results obtained from fabricated structures. Pre-stress effects, common in free-standing diamond structures, have been included in the model. The influence of various geometric parameters on the transient behaviour of the switches is calculated. Diamond is shown to improve the transient behaviour of the devices in comparison with other materials such as silicon.

Diamond technology for electronics and MEMS: review of status and perspectives

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.