Matthias Werner

Review on materials, microsensors, systems and devices for high-temperature and harsh-environment applications

The considerable investment in silicon technology has rarely addressed device use in harsh environments such as high temperatures, aggressive media, and radiation exposure. A clear future requirement is to save weight, volume, and reduce costs in unfriendly environments like high temperatures. This can be achieved either by cooling systems or by electronic microsystem components suited to withstand high temperatures. The current status of cooling systems, harsh-environment sensors, and microsystems in view of markets, realized devices, material, properties, process maturity, and packaging technologies are reviewed. Possible semiconductor candidates for high-temperature applications are discussed. The main obstacles for the future of high-temperature and harsh-environment microsystems is highlighted.

Diamond metallization for device applications

Nearly any diamond electronic or sensor device needs at least one ohmic contact. These contacts play a key role in the overall device performance. This paper reviews the dependence of the Schottky barrier height on the surface termination and the impact of annealing of carbide-forming metals on the specific contact resistivity to diamond. It is concluded that carbide patches dominate the specific contact resistivity after annealing. Furthermore, the doping dependence of the specific contact resistivity and suitable diffusion barriers, which avoid interdiffusion of the contact scheme, are briefly discussed.

Electrical properties of lithium-implanted layers on synthetic diamond

Lithium implantation (40 and 50 keV; doses of 2 × 1016 and 4 × 1016 cm−2) has been performed in several synthetic and natural diamond crystals at room temperature (RT) and 850–900 °C (high temperature (HT) implantation). In contrast with the case of the RT implantation, the HT implantation did not result in radiation-induced surface graphitization. The samples implanted at RT and 850 °C were subsequently annealed at 900 °C. Comparison of the electrical properties of the doped crystals shows the dependence of the electrical activation on the annealing temperature. Thermal emf measurements have established that the conductivity is n-type.

The effect of metallization on the ohmic contact resistivity to heavily B-doped polycrystalline diamond films

Three metallization schemes, namely Al/Si, Ti-Au and TiWN-Au contacts on B-doped polycrystalline diamond films have been compared. After annealing at 450/spl deg/C in nitrogen Al/Si contacts show the lowest contact resistivity in the order of /spl sim/10/sup -7/ /spl Omega/ cm/sup 2/. TiWN-Au contacts were found to be the most stable contact system in view of interdiffusion and oxidation. Ti-Au contacts show a catastrophic interdiffusion at moderate annealing temperatures and strong oxidation at the very surface. High surface boron doping concentrations lead to low contact resistivities. At sufficient high doping levels current transport through the metal-diamond barrier is due to field emission. >

Physical properties of diamond for thermistors and pressure transducers

The semiconducting and mechanical properties of diamond thin films have been investigated in the context of thermistors and pressure transducers for high-temperature environments. This paper considers the conduction mechanisms which contribute to the temperature coefficient of resistance of boron-doped diamond. The influence of temperature on the Youngs modulus of diamond films is discussed with respect to microstructure and the limits imposed by high-temperature oxidation.

Processing and electrical characterization in intrinsic conducting polymers for electronic and MEMS applications

Electronic polymer devices and test structures based on PEDOT/PSS were fabricated in a fully CMOS compatible process. The resistivity of PEDOT/PSS polymer films is dependent on film thickness. The resistivity increases with decreasing film thickness for polymer film thicknesses between 190 nm and 380 nm. The resistivity differs by a factor of ~3 depending on film thickness. The evaluation of the specific contact resistivity depending on the choice of the metallization leads to a difference of the specific contact resistivity by a factor of 190. The specific contact resistivity does not follow the Schottky-Mott law and thus indicates a non-ideal behavior of the metal PEDOT/PSS interface. The lowest average specific contact resistivity was obtained for silver with an average value of 0.14 Ωcm2 and the highest specific contact resistivity was obtained for platinum. Even the lowest specific contact resistivity for silver is still very high when compared with low resistivity ohmic contacts to silicon. However, the specific contact resistivity is expected to have a significant drawback for overall device performance. Possible future applications of MEMS and electronics based on polymers will be for simple devices like transistors, ID tags, thermistors, acceleration and pressure sensors as well as radiation and UV detectors.

Temperature Sensor On Boron Ion Implanted Diamond

p-type semiconducting boron doped layers have been fabricated on diamond substrates by ion implantation and subsequent annealing. A number of the related published experimental data and theoretical models on electrical properties of boron doped diamond are analyzed with regard to the temperature coefficient of resistance (TCR) of temperature sensors. The dependencies of the conductivity and activation energy on three parameters: (i) boron doping level NA, (ii) electrical compensation ratio ND/NA- C and (iii) duration of the postimplantation annealing time ta are studied. By variation of NA, C and t, an optimized technological regime for the temperature sensor fabrication can be obtained. One can summarize that: 1) the TCR value is not remarkably reduced with the boron concentration up to NA -10 19 cm -3 , 2) an increase of the electrical compensation decreases the activation energy and consequently the TCR coefficient,3) 1 h annealing at 1500°C is sufficient to remove the compensating radiation defects, 4) the variation of the ta from 1 min to 1 h changes the TCR value by 20% to 30%. Technological steps of the fabrication of a micro temperature sensor are given.

Nondestructive characterization and application of doped and undoped polycrystalline diamond films

In this overview the mechanical, thermal and electrical properties of CVD (Chemical Vapor Deposition) diamond, determined by various non-destructive techniques, are highlighted and compared with calculations. In the case of Youngs modulus the measurement results of high quality samples leads to an average value of 1126 GPa which is in good agreement with the calculated value of 1143 GPa and close to the Young+s modulus of single crystalline diamond. However, values as low as 242 GPa were determined on 300 +m thick bulk CVD diamond. The differences in the measurement results can be traced back to extended voids in the sample. A traditional heated bar technique was used to measure the temperature dependent thermal conductivity of CVD-diamond. High quality polycrystalline diamond films reached a room temperature thermal conductivity of 20.5 W cm-1 K-1. This value is comparable to the thermal conductivity of the best single crystal diamonds available. For the lower quality samples, boundary scattering and point defects are most likely responsible for the lower thermal conductivity. The electrical properties of B-doped polycrystalline diamond films were characterized by temperature dependent Hall and conductivity measurements. These measurements together with a semi-empirical model give insight in to the current transport mechanism. The model indicates, that the electrical mobility in diamond thin films is lower compared with single crystal diamond. However, the current conduction mechanism are essentially the same when compared with single crystal diamond.