Reliable Contacts Research Articles

Porous silicon-based humidity sensor with interdigital electrodes and internal heaters

A novel design of a one wafer side processed porous silicon-based humidity sensor with interdigital electrodes is presented. An integrated heater element over the porous layer provides the effective heating and the low power consumption of the device. Reliable contacts between metal and porous Si are formed via crystalline n-Si islands within the porous layer, formed by exploiting the selectivity of the electrochemical etching process. The effects of the electrode and heater geometry and also the parameters of the porous matrix are investigated with special emphasis on response and recovery time. To ensure the adequate thermal conditions sensor structures and packaging techniques were also investigated. The applied heater geometry results in faster recovery at a cost of reduced power consumption.

Extending the pressure and temperature limits of hydrothermal diamond anvil cells

A modified hydrothermal diamond anvil cell (HDAC) has been developed. The cell is designed primarily for use with anvils incorporating internal heaters, but also solves several technical problems limiting the pressures and temperatures that can be reached with conventional HDACs. The essential features of the cell are (i) the pressurizing mechanics ensure strictly parallel alignment and preclude accidental collision of the anvils, (ii) the mechanical anvil holders provide both robust fixation of the anvils without any glue and reliable electrical contacts with the anvil facets, (iii) the diamond anvils can be heated while the anvil backing plates and the holders remain comparatively cool, (iv) stable hydrothermal working conditions to at least 4.5 GPa and 1000 °C can be achieved during the course of sluggish reactions typical of geoscience research thrusts, and (v) large sample volumes are possible, such as diameters of 0.5 mm and heights of 0.1 mm for a 1 mm culet.

Design of 57.5 MHz cw RFQ for medium energy heavy ion superconducting linac

The nuclear science community considers the construction of the Rare Isotope Accelerator (RIA) facility as a top priority. The RIA includes a 1.4 GV superconducting linac for production of 400 kW cw heavy ion beams. The initial acceleration of heavy ions delivered from an electron cyclotron resonance ion source can be effectively performed by a 57.5 MHz 4-m long room temperature RFQ. The principal specifications of the RFQ are (i) formation of extremely low longitudinal emittance, (ii) stable operation over a wide range of voltage for acceleration of various ion species needed for RIA operation, and (iii) simultaneous acceleration of two-charge states of uranium ions. cw operation of an accelerating structure leads to a number of requirements for the resonators such as high shunt impedance, efficient water cooling of all parts of the resonant cavity, mechanical stability together with precise alignment, reliable rf contacts, a stable operating mode, and fine tuning of the resonant frequency during operation. To satisfy these requirements a new resonant structure has been developed. This paper discusses the beam dynamics and electrodynamics design of the RFQ cavity, as well as some aspects of the mechanical design of the low-frequency cw RFQ.

Reliable Ohmic Contacts to LPE p-Type 4H-SiC for High-Power p-n Diode

Reliable Ohmic Contacts to LPE p-Type 4H-SiC for High-Power p-n Diode

Electrical properties of graphite/homoepitaxial diamond contact

We have studied the electrical properties of the graphitic electrodes formed in sulfur- and boron-doped homoepitaxial diamond films in order to make reliable ohmic contacts in diamond-based electronic devices. Sulfur-doped diamond films were achieved by sulfur ion implantation in undoped homoepitaxial diamond films. Boron-doped homoepitaxial diamond films were synthesized by gas-phase doping of boron during the chemical vapor deposition growth. The graphitization of diamond by Ar ion implantation was utilized for the graphitic electrodes. The electrical properties of graphite/diamond contacts were investigated by using the current–voltage measurement. The specific contact resistances were characterized by applying the linear transmission line model (TLM) and circular TLM extrapolation methods, and were determined to be 5.2×103 and 1.18 Ω·cm2, respectively.

Cleaning and Doping of Silicon in a BF3Plasma during Fabrication of Ohmic Contacts

The formation of reliable ohmic contacts to silicon is considered. Silicon surface cleaning, contact window doping, and molybdenum application by cathode sputtering in the BF3atmosphere are carried out in a single vacuum cycle. A model of semiconductor–plasma and dielectric–plasma contacts under stationary discharge conditions is discussed. It is shown that the rates of processes with the participation of charged particles on p-Si and n-Si surfaces substantially differ. The resistivity of Mo-n + Si and Mo-p + Si contacts is the least when the molybdenum films are applied by sputtering in the Ar + (10–15) vol % BF3atmosphere.

Through-wafer electrical interconnect for multilevel microelectromechanical system devices

This article reports the design, fabrication, and experimental demonstration of through-wafer interconnects capable of allowing direct electrical access to the interior of a multilevel microelectromechanical system device. The interconnects exploit the ability to conformally coat a high aspect ratio trench with a thick layer of tetraethylorthosilicate to isolate a through-wafer silicon plug that can provide electrical contact across two sides of a low resistivity wafer. They hold the potential of a tenfold reduction in the parasitic capacitance of previously reported through-wafer vias, and are shown to make reliable contacts to the back side of a polysilicon resistive element. The high temperature capability of the interconnects is also examined, however, their application is found to be limited to temperatures below 1000 °C due to localized degradation near the isolating trenches.

Telematics in geriatrics--potentials, problems and application experiences

Modern telecommunication technology (telematics) has the potential to improve the quality of life for elders with physical and mental impairments as well as for their care giving relatives. Videophones, internet resources, and multimedia computers can be used for networking them together with social workers, nurse practitioners, physicians and therapeutic staff in service-centers. This can be viewed as a unique opportunity to establish and maintain instant and personalized access to various medical services in a situation where increasing needs are opposed to decreasing resources. However, it is not yet clear whether telematics is adequate, efficient, and effective in supporting care for geriatric patients. Some studies already showed its applicability and feasibility, but there are still no larger trials showing that maintenance or enhancement of autonomy can be achieved effectively by using new technologies. This article reviews the literature on telematics in geriatrics and presents data of a tele-rehabilitation project ("TeleReha", conducted at the Berlin Geriatric Center) which comprised mobility-impaired patients (N = 13, mean age 72 yrs), care giving relatives (N = 8), and geriatric professionals. Networking was established using ISDN technology with videophones or PC-based videoconferencing systems. Results showed that participants regard telecommunication devices as a valuable resource for their informational and communicational needs. Use of telecommunication systems was inversely related to physical mobility. Having access to professional service and counselling was rated highly important but also the opportunity to establish reliable contacts with non-professionals (relatives, other participants). Despite experienced technical problems, use of telecommunication systems was evaluated more positively in the post-test as compared to the pre-test. In summary, current experience suggests that telematics can be used efficiently by geriatric patients and by relatives and professionals caring for them. However, evidence for a medically and economically effective use is still scarce. A lack of structural and organizational concepts for geriatric telematics initiatives can be identified which in part may be due to the fact that the considerable potentials of telematics applications are still largely unrecognized by geriatricians.

Reliable contacts to two-dimensional conduction layers

For many experiments and device applications, electrical contacts to a two-dimensional conduction layer must remain reliable under repeated temperature cycling between 300 and 77 K or lower. This work introduces the use of a silicon-doped InAs contact to the AlGaAs/GaAs two-dimensional electron gas which demonstrates exceptional reliability under such temperature cycling. The noise spectrum of AlGaAs/GaAs contacted with silicon-doped InAs shows almost no dependence on bias current; this fact can be used to improve the performance of device applications such as Hall sensors. In addition, this work introduces an alternative two-dimensional conduction structure, highly mismatched InAs/GaP. InAs/GaP contacted with Ti/Au shows reliability equal to AlGaAs/GaAs contacted with silicon-doped InAs. The InAs/GaP material may be more desirable for some applications because of the lower temperature dependence of its electronic properties and potentially easier integration with silicon-based microelectronics.

Optimization of The Processing Parameters for Pulsed Laser Deposition of Nickel Silicide Ohmic Contacts On SiC

AbstractThis research investigates the potential of pulsed laser deposition to create reliable high current ohmic contacts of Ni2Si on single crystal 4H-SiC. Since this stoichiometry is the stable interphase in the nickel-silicon carbide diffusion couple, direct deposition eliminates the detrimental excess carbon normally formed by direct sintering Ni on SiC, the surface roughening that results from this sintering as well as the need for post-deposition high-temperature (900°C) anneals that are required in complex multi-component contacts. This study examines the processing parameters that must be used during deposition to obtain the desired microstructural characteristics for the contact. Pulsed laser deposition of nickel silicide produces smooth films with an amorphous or nanocrystalline structure interspersed with macroparticles. Macroparticle formation on the resulting films appear in the form of solidified droplets of the eutectic composition nickel silicide (3:1) that form during the long term target processing. The dependence of the number and size distributions of these droplets on laser fluence sample temperature is examined.

Fabrication of high-quality one- and two-dimensional electron gases in undoped GaAs/AlGaAs heterostructures

We have developed a technique for the fabrication of high-mobility electron gases formed in undoped GaAs/AlGaAs heterostructures. The use of an insulated gate allows independent control over the carrier density in the Hall bar and ohmic contact regions of the device. This unique design eliminates difficulties in obtaining reliable ohmic contacts, particularly in the low carrier density regime. In the absence of remote ionized impurity scattering, extremely high transport mobilities are obtained at low carrier densities (1×106 cm2 V−1 s−1 at 1×1010 cm−2). This design has been adapted to the formation of undoped one-dimensional electron gases that show clean and reproducible conductance plateau at 1.5 K.

An ultrahigh vacuum system for the fabrication and characterization of ultrathin metal–semiconductor films and sensors

An ultrahigh vacuum system has been designed and built to study the magnetic and electrical behavior of ultrathin metal films deposited on semiconductors. The system allows variable temperature metal film deposition by electron beam evaporation onto an electrically active, low noise device structure. Significant features include, the use of microfabricated substrates to create reliable zero-force electrical contacts to ultrathin metal–semiconductor devices, a dark atomic beam source, and a compact magneto-optic Kerr effect (MOKE) magnetometer with an external electromagnet. A temperature controlled rotating sample manipulator allows the active metal surface to be deposited in one position and subsequently rotated between the poles of the electromagnet for simultaneous MOKE and electrical measurements while the surface undergoes controlled dosing from a molecular or atomic beam. Low-energy electron diffraction is available for sample characterization and a quadrupole mass spectrometer is used to monitor the beam. Results of iron on Si(111) show magnetic coercivity increasing approximately linearly with increasing film thickness to 6.4 kA/m at 100 Å. Current–voltage measurements of 50 Å iron and copper on Si(111) when fit to a thermionic emission model showed, respectively, ideality factors of approximately 4 and 1, and barrier heights of 0.45 and 0.65 eV after deposition at 160 K and annealing to room temperature. The use of the thin Cu film Schottky diode for atomic hydrogen detection is demonstrated.

Platinum Metals in Ohmic Contacts to III-V Semiconductors

Platinum metals, particularly platinum and palladium, are used extensively in low resistance, relatively stable and laterally uniform ohmic contacts made to semiconductor devices formed from Group III and V elements. Palladium is primarily utilised as one component of a multilayer metallisation structure, improving adhesion to the semiconductor, while platinum is commonly used as a diffusion barrier to minimise interdiffusion between metal and semiconductor components. This paper describes the roles of platinum and palladium in ohmic contacts, gives examples of their typical utilisation and outlines the design considerations associated with the formation of reliable ohmic contacts.

Contact Issues of GaN Technology

In this paper, we discuss the issue of fabricating reliable and reproducible ohmic contacts on AlGaN HFET structures. During the course of our investigation of fabricating contacts to HFETs, we found that the contact properties could vary significantly from one sample to another, even though they were nominally the same. This problem was prominently manifested in the ohmic contact behavior. The origin of this problem was traced back to the variation of the HFET structure during growth. In this paper, we report an attempt to fabricate reproducible ohmic contacts of these structures.

Low Resistance Gold Contacts for Gallium Nitride

Gold is playing an important role in the development of reliable contacts with a very low contact resistance to gallium nitride.

Reliable electrical contacts on single crystals

We investigate the making of reliable electrical contacts on single crystals. Numerous silver pastes and the corresponding application procedures do not yield satisfactory results. Although the electrical contacts are initially of low resistance, they deteriorate in time. We present a procedure which overcomes this degradation of the contacts and ensures low-resistance contacts for at least a year, with no sign of possible future departure from the initial quality.

Characterization of As Implanted Silicides by Frequency Noise Level Measurements

Silicides belong to a very promising group of materials which are of great interest both in the physics of thin films and in microelectronics. Their low resistivity and expected good temperature stability make them desirable for fabrication of reliable and reproducible electrical contacts. Investigations of this type of contacts include both their experimental development and the development of methods for their characterization such as noise level measurement and RBS analysis. The noise level measurements enable the control of the noise, which is important characteristic of metal-semiconductor (M/S) electrical contacts (especially l/f noise). In our work we compare the RBS spectra and low frequency noise spectra of As + implanted layers of TiN-Ti-Si structures, obtained in different conditions. Low frequency noise spectra are also obtained at different sample temperatures. Results of our measurements and analysis are of interest for solving the problems related to the application of ion implantation of As + ions for the formation of silicides on p-Si, that are discussed by many authors in their investigations recently.

Ohmic Contacts for Compound Semiconductors

Although there has been strong demand for low -resistance, highly reliable Ohmic contacts for compound semiconductors to realize high-frequency transistors, high-power devices, and light-emitting (LED) and laser diodes (LD), development of the Ohmic contact technologies has been made on a trial-and-error basis. The primary reason is lack of fundamental data to design ideal metal/semiconductor interfaces due to complexity of elements involved at the interfacial reaction. In this article, we review recent systematic studies carried out for Ohmic contact materials to n-GaAs and then address critical issues to apply the methodology established in n-GaAs to develop low-resistance Ohmic contacts for p-ZnSe and p-GaN, which are desperate for blue-green LED and LD.

Fabrication of independently contacted and tuneable 2D-electron-hole systems in GaAs/AlGaAs double quantum wells

We have investigated the properties of closely spaced, but electrically separated two-dimensional electron-hole systems formed in GaAs/AlGaAs double quantum wells (QWs). These novel structures were fabricated using MBE by applying a special Si and C modulation doping technique in order to form intrinsically both a high-quality two-dimensional electron (2DEG) and hole gas (2DHG) with a separation of less than 1500 Å. The devices are additionally equipped with buried p +-GaAs back-gates, which have been fabricated by regrowth over a patterned substrate. Reliable independent contacts to each of the 2D-systems have been achieved by a stepwise annealing technique for the p- and n-type metallisation. We have done so-called `drag' measurements to study the interaction effects between the two systems, from which we could conclude on the non-parabolic subband dispersion of the 2DHGs.

Contact Issues of GaN Technology

AbstractIn this paper, we discuss the issue of fabricating reliable and reproducible ohmic contacts on AlGaN HFET structures. During the course of our investigation of fabricating contacts to HFETs, we found that the contact properties could vary significantly from one sample to another, even though they were nominally the same. This problem was prominently manifested in the ohmic contact behavior. The origin of this problem was traced back to the variation of the HFET structure during growth. In this paper, we report an attempt to fabricate reproducible ohmic contacts of these structures.