Showing papers on "Contact resistance published in 1997"

Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to n-type GaN

Abstract: We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7×1017 cm−3) annealed in forming gas at 600 °C reached a minimum contact resistivity of 8×10−6 Ω cm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5×1017 cm−3) had resistivities of 7×10−6 and 5×10−6 Ω cm2 after annealing in Ar at 400 °C for 5 min and 600 °C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 °C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400–600 °C range includes Ti reducing the GaN native oxide and an Al–Ti intermetallic coming into intimate contact with the GaN.

Effects of surface treatments and metal work functions on electrical properties at p-GaN/metal interfaces

Abstract: In order to examine the possibility of preparing a nonreacted (nonalloyed) Ohmic contact to p-GaN, the effects of GaN surface treatments and work functions of the contact metals on the electrical properties between the metal contacts and p-GaN were investigated. A contamination layer consisting of GaOx and adsorbed carbons was found on the GaN substrate grown by metalorganic chemical vapor deposition. The contamination layer was not completely removed by sputtering the GaN surface with Ar and N ions where the ion densities were ∼10−2 μA/cm2. Although the contamination layer was partially removed by immersing in a buffered HF solution, little improvement of the electrical properties of the GaN/metal interfaces was obtained. Most of the contamination layer was removed by annealing the Ni and Ta contacts at temperatures close to 500 °C. These annealed contacts exhibited slightly enhanced current injection from the contact metal to the GaN. The present surface treatment study indicated that removal of the contamination layer did not significantly reduce the contact resistance. On the other hand, the resistance decreased exponentially with increasing the metal work functions, where Pt, Ni, Pd, Au, Cu, Ti, Al, Ta, and Ni/Au were deposited on the GaN. This result suggests that the Schottky barrier height at the p-GaN/metal interface might not be pinned at the GaN surface. The present study concluded that a contact metal with a large work function is desirable for nonreacted Ohmic contacts to p-GaN. However, these contacts did not provide the low contact resistance required for blue laser diodes.

The Physics of Ohmic Contacts to SiC

Abstract: The specific contact resistance of an ohmic contact will be discussed including ways to calculate and measure this parameter. Ohmic contacts to n- and p-type hexagonal SiC will then be detailed. Low resistance n-type ohmic contacts are predominately fabricated by annealing a refractory metal, thereby forming a silicide with a lowered Schottky barrier height at the metal-SiC interface. P-type contacts on the other hand generally use Al or Al alloys which upon annealing enable Al to diffuse into the SiC thus resulting in ohmic properties. Aluminium alloys however suffer from many problems which will be discussed. Other novel contacting schemes to p-type SiC will also be reviewed.

Design and understanding of anisotropic conductive films (ACF's) for LCD packaging

Abstract: Anisotropic conductive film (ACF) composed of an adhesive resin and fine conductive fillers such as metallic particles or metal-coated polymer balls are key materials for fine pitch chip-on-film (COF) and chip-on-glass (COG) LCD packaging technologies. To understand and design better quality ACF materials, the theoretical electrical conduction model with physical contact mechanism was simulated and experimentally proven. To understand the contact area changes, two pressure dependent models (1) elastic/plastic deformation; (2) finite element method (FEM) model were developed, and experimentally proven by various ACF's fabricated in our laboratory. Experimental variables were applied bonding pressure, number, size, mechanical and electrical properties of nickel powders and Au-coated polymer conductive particles. It was found that the models were in good agreement with experimental results except at higher bonding pressures. In general, as bonding pressure increases, a sharp decrease of contact resistance followed by a constant value is observed after reaching the critical bonding pressure. However, an excessive bonding pressure rather increased the connection resistance of ACF interconnection. If more conductive particles-were added, the connection resistance rapidly decreased and then became constant. This is because the counter-effect of two opposing factors, the resistance increase caused by a decrease of contact area per one particle and the resistance decrease caused by increasing number of conduction path. In addition, environmental effects on contact resistance such as thermal aging, high temperature/humidity aging, and temperature cycling were also investigated. As a whole, better design of ACF materials can be achieved by understanding the ACF conduction mechanism.

Ultra-low resistive ohmic contacts on n-GaN using Si implantation

Abstract: Implanted ohmic contacts were made on molecular beam epitaxy grown GaN materials. Si was implanted at a doping density of about 4×1020 cm-3 to decrease the contact resistance of the contact, followed by an activation anneal at 1150 °C for 30 s. The overlay metal Ti/Au was evaporated. Four-probe measurements were performed on transmission line model patterns. The measured maximum contact resistance was 0.097 Ω mm and the apparent specific contact resistance was 3.6×10−8 Ω cm2.

Nickel based ohmic contacts on SiC

Abstract: We have compared the chemical and structural properties of Ni/SiC and Ni2Si/SiC interfaces. In the case of Ni/SiC, the contact formation is initiated by the dissociation of SiC, due to the strong reactivity of nickel at 950 °C. Ni2Si is formed and carbon accumulates, both at the interface and throughout the metal layer. At the interface, many Kirkendall voids are observed by TEM. Despite this poor interface morphology, low contact resistances have been measured. But the presence of carbon in the contact layer and at the interface is a potential source of contact degradation at high temperature. In the case of Ni/Si multilayers evaporated on SiC instead of pure Ni, the contact formation is preceded by Ni and Si mutual diffusion in the deposited layer yielding Ni2Si. Therefore, a smaller amount of carbon is released from SiC. Low carbon segregation, abrupt interface and low contact resistance characterize this contact. The thermal stability of Ni2Si contacts is illustrated with ageing experiments carried out at 500 °C.

Semiconductor light-emitting device and manufacture thereof

Abstract: PROBLEM TO BE SOLVED: To realize a semiconductor light emitting device which is equipped with a P-type electrode which is kept high in crystallinity, low a contact resistance, and high enough in adhesive strength to an InAlGaN layer by a method wherein a first metal layer which contains an element component selected out of either group IV elements or group VI elements is deposited on a contact region. SOLUTION: A light emitting device 10 is possessed of a semiconductor multilayered structure deposited on a sapphire substrate 12. A buffer layer 14, an N-type contact layer 16, an N-type clad layer 18, an active layer 20, a P-type clad layer 22, and a P-type contact layer 24 are laminated in this sequence on the sapphire substrate 12. A P-type electrode layer 26 is deposited on the P-type contact layer 24. An N-type electrode layer 34 is deposited o the N-type contact layer 16. When an alloy of gold or the like and a group IV element or a group VI element is deposited, it is excellent in crystallinity. That is, the P-type electrode 26 is kept high in adhesive strength to the N-type contact layer 24 and can be lessened in contact resistance.

Measuring local thermal conductivity in polycrystalline diamond with a high resolution photothermal microscope

Abstract: A photothermal microscope that provides micrometer lateral and submicrometer depth resolution was designed Thermal conductivity measurements with modulation frequencies up to 12 MHz on single grains in polycrystalline diamond demonstrate its lateral resolution power even for a highly conducting material Measured conductivities strongly depend on the averaged volume and values up to 2200 W/mK are found in the high frequency limit where the properties inside a grain are sampled The capability of the instrument to measure thermal parameters on thin films is demonstrated for gold films evaporated on quartz with a thickness ranging from 20 to 1500 nm Measurements reveal a strong thickness dependence for both thin film conductivity and the contact resistance between film and substrate Thermal conductivity decreases monotonically from 230 to 30 W/mK whereas the contact resistance rises from 2×10−7 to 8×10−6 m2K/W with decreasing film thickness

Electrical conduction models for isotropically conductive adhesive joints

Abstract: An electrical conduction model for silver filled isotropically conductive adhesives (ICA) was developed The model combines the microscopic resistance of the bulk silver particles and the contact between silver flakes with the macroscale resistor network calculation by percolation theory The resistivities of the composites were calculated by resistor network simulations considering both contact effects and particle size effects Three different types of film typically exist on the silver surface: residual organic films; tarnish films; and a thin epoxy layer The contact resistance between silver flakes can be due to a constriction resistance, to the tunneling resistance through insulating films, or to the resistance of more conductive layers The constriction resistance is produced by the restriction of the current flow by small contact spots and is controlled by the actual contact spot area (metallic contact), which is dependent on the contact force between flakes The tunneling resistance is caused by the very thin layer which may reside on the silver flakes between the metallic contact spots, and is dependent on a barrier film thickness and potential Oxide and sulfide tarnish films are typically degenerate semiconductors Two- and three-dimensional (2-D and 3-D) computer simulations were performed to predict the effects of particle sizes, shapes, and distribution on the percolation conduction thresholds and cluster sizes The model predicts that the percolation threshold decreases with broad particle size distributions and high aspect ratio particles The effective resistivity of the adhesive depends on the thickness dimension of the adhesive pad geometry, with very thin layers resulting in high percolation thresholds and high resistivities Resistivity does not change with the pad thicknesses greater than a certain thickness level Silver flake orientation on the surface increases the resistivity of the conductive adhesive pads, but in the same magnitude range The resistivities of the materials are controlled by silver flake sizes and the nature of the contacts

Titanium and aluminum-titanium ohmic contacts to p-type SiC

Abstract: Very low resistance ohmic contacts to p-type SiC were fabricated by depositing a 90-10 wt.% alloy of Al and Ti followed by a high temperature anneal of approximately 1000°C for 2 min. Specific contact resistances ranged from approximately 5 × 10 −6 to 3 × 10 −5 Ω cm 2 on material with a doping of 1.3 × 1019 cm−3. The initial AlTi thickness before annealing was found to be critical to controlling the AlTi sheet resistance during the anneal. In addition, chemically etching the AlTi layer after annealing revealed pitting indicative of severe reaction between the AlTi and SiC surface, as confirmed by Rutherford Backscattering. In contrast, ohmic contacts to the same SiC material were fabricated by depositing pure Ti and annealing at 800°C for 1 min. These contacts were ohmic with a specific contact resistance between 2 × 10 −5 and 4 × 10 −5 Ω cm 2 . Examination of the SiC surface after chemically etching away the annealed contact revealed a smooth surface, suggesting a much more planar Ti/SiC interface.

Latch ( latching ) type heat-driven microrelay device

Abstract: PROBLEM TO BE SOLVED: To provide an extremely small micro-relay capable of being integrated and having low contact resistance by changing the internal pressure of a storage chamber via a temperature difference, moving a conductive liquid metal in a channel, and closing or opening signal electrodes. SOLUTION: Power is applied to the heater 12 of an active storage chamber 10 or the heater 42 of a passive storage chamber 40, the internal pressure of the storage chamber is changed by temperature, a liquid metal 50 is moved in a channel 20, and signal electrodes 34, 35 or 31, 32 are closed or opened. When power is selectively applied to the heater 12 or 42, the signal can be turned on or off. This latching heat-driven micro-relay element can be integrated in a silicon wafer bulk by the technique such as macro-burnishing, electroplating, and conductor process, and an extremely small micro-relay capable of being integrated and having small contact resistance can be obtained.

Study of contact formation in AlGaN/GaN heterostructures

Abstract: The contact formation of Ti/Al and Ti metallization on AlGaN/GaN heterojunction field effect transistors (HFET) was investigated. It was found that ohmic contact formation is related to the low work function of the Ti contacting layer and the formation of a TiN phase at the Ti/nitride interface. Contact resistance as low as 1 Ω mm or less can be obtained on HFET samples with a nsμ product of ∼0.8×1016/V s and on n-GaN with a carrier concentration of 1.5×1018/cm3. Ti/Al bilayer contact scheme is superior to Ti-only contact due to a surface Al3Ti layer in the bilayer contact, which may reduce the oxidation problem when annealed in N2 at high temperatures. Preannealing the HFET samples at 850 °C for 1 h in N2 appears to improve the ohmic contact in general, but not always observed. Our results indicate that Ti/Al contact scheme yields sufficiently low contact resistance on HFET structures for microwave applications.

0.12-μm gate III-V nitride HFET's with high contact resistances

Abstract: HFET's with 0.12-/spl mu/m gate length were fabricated on a III-V nitride wafer. The contact resistance from unannealed Ti/Au ohmic contact was 10 /spl Omega//spl middot/mm. Even with this relatively high contact resistance, f/sub T/ of 46.9 GHz and f/sub max/ of 103 GHz were measured with the Ti/Au contacts, the highest yet achieved on III-V nitride FETs. The improvement in the frequency response was mainly due to the decrease in the gate length (0.12 /spl mu/m). In addition, the effects of high contact resistances at high frequency are discussed.

Temperature Behavior of Pt/Au Ohmic Contacts to p-GaN

Abstract: Ohmic contacts to Mg-doped p-GaN grown by MOCVD {xc[1]} are studied using a circular transmission line model (TLM) to avoid the need for isolation. For samples which use a p-dopant activation anneal before metallization, no appreciable difference in the specific contact resistance, rc, as a function of different capping options is observed. However, a lower rc is obtained when no pre-metallization anneal is employed, and the post-metallization anneal simultaneously activates the p-dopant and anneals the contact. This trend is shown for Pt/Au, Pt, Pd/Pt/Au, and Ni/Au contacts to p-GaN. The rc ‘s for these metal contacts are in the range of 1.4‐7.6 x 10-3 ohm-cm2 at room temperature at a bias of 10mA. No particular metallization formula clearly yields a consistently superior contact. Instead, the temperature of the contact has the strongest influence. Detailed studies of the electrical properties of the Pt/Au contacts reveal that the I-V linearity improves significantly with increasing temperature. At room temperature, a slightly rectified I-V characteristic curve is obtained, while at 200°C and above, the I-V curve is linear. For all the p-GaN samples, it is also found that the sheet resistance decreases by an order of magnitude with increasing temperature from 25°C to 350°C. The specific contact resistance is also found to decrease by nearly an order of magnitude for a temperature increase of the same range. A minimum rc of 4.2 x 10-4 ohm-cm2 was obtained at a temperature of 350°C for a Pt/Au contact. This result is the lowest reported rc for ohmic contacts to p-GaN.

A micro variable inductor chip using MEMS relays

Abstract: An adjustable inductor which is digitally controlled by microrelays has been made using combined surface and bulk micromachining technology. The microrelays were fabricated using a TaSi/sub 2//SiO/sub 2/ bimorph cantilever beam, a gold-to-gold electrical contact, aluminum as sacrificial layer, and a combined electrostatic and thermal actuation mechanism. The silicon substrate underneath the inductor region was etched out to reduce the substrate eddy current loss. Sixteen different inductance values ranging from 2.5 nH to 324.8 nH were obtained using four microrelays. The minimum self-resonant frequency is 1.9 GHz. The lowest measured thermal power and electrostatic voltage for the combined actuation of microrelays are 8.0 mW and 20 V, respectively. The measured contact resistance is typically 0.6 to 0.8 ohms.

Carrier transport mechanism of ohmic contact to p-type diamond

Abstract: The carrier transport mechanism through the p-diamond/metal interface was studied by measuring specific contact resistances (ρc) using a transmission line method for Ti, Mo, and Cr (carbide forming metals) and Pd and Co (carbon soluble metals) metals contacting to the boron-doped polycrystalline diamond films. Schottky barrier heights (φB) of around 0.5 eV were measured for the annealed contacts. The present result indicates that formation of thermally stable graphite layers at the diamond/metal interfaces upon annealing would pin the Fermi level of the p-diamond. This model led to the preparation of in situ Ohmic contacts by depositing a thin diamondlike carbon on the p-diamond surface prior to metal deposition, and also to excellent Schottky contacts with breakdown voltages higher than 900 V. The present experiment concluded that the existence of a graphite layer at the diamond/metal interface controlled the electrical properties through the p-diamond/metal interface.

Metals on 6H-SiC: contact formation from the materials science point of view

Abstract: Metal-6H-SiC interface reactions were studied under three different aspects; phase relations, diffusion reaction, and electronic properties. For the first, powder pellets were annealed at different temperatures to achieve thermodynamic equilibrium. The resulting phases were evaluated using XRD. For the second, bulk diffusion couples were prepared and annealed. Cross sections of the reaction layers were investigated in a SEM using secondary and backscattered electrons as well as energy dispersive X-ray analysis. For the third, the electronic properties of thin film metal contacts were evaluated as a function of annealing treatment. The results of the three aspects were combined. The four metals investigated (Ni, Cr, W, Ti) are unstable on 6H-SiC. They react at elevated temperatures in accordance with the phase diagrams but in different morphologies. Ni is the most, W the least reactive metal. All metals form ohmic contacts on n-type 6H-SiC after an appropriate annealing procedure. Ti gives the lowest, Ni the highest contact resistance. Ti forms either ohmic or Schottky contacts depending on the annealing temperature which correlates with a change in the diffusion path.

Semiconductor device having contact resistance reducing layer

Abstract: In a wide band cap semiconductor, a GaPx N1-x (01≦x≦09) layer is inserted between a layer comprising AlGaInN and an electrode, The potential barrier between the electrode and the surface layer can be reduced Contact resistance can be decreased, and ohmic contact can be easily taken up

DC measurement of electrical contacts between strands in superconducting cables for the LHC main magnets

Abstract: In the LHC main magnets, using Rutherford type cable, the eddy current loss and dynamic magnetic field error depend largely on the electrical resistance between crossing (Rc) and adjacent (Ra) strands. Cables made of strands with pre-selected coatings have been studied at low temperature using a DC electrical method. The significance of the inter-strand contact is explained. The properties of resistive barriers, the DC method used for the resistance measurement on the cable, and sample preparation are described. Finally the resistances are presented under various conditions, and the effect is discussed that the cable treatment has on the contact resistance.

Low resistance ohmic contact to n-GaN with a separate layer method

Abstract: We have developed an ohmic metalization method to n-GaN through which low contact resistances down to 3 × 10−6 Ω cm2 have been achieved. The process consists of: depositing a 200 A thin layer Ti; annealing at 975°C for 30 s, realigning and depositing another 200 A thin layer of Ti, followed by a 2000 A overlayer of Au. The high temperature annealing effect on bulk GaN has been investigated. No degradation of semiconductor bulk properties was found up to 1050°C. TiN formation is believed to be the key to the low contact resistance.

Conduction mechanisms in anisotropic conducting adhesive assembly

Abstract: This paper explores experimentally and through analytical and computational models, the mechanisms of conduction in flip chip interconnects using anisotropic conducting adhesives. A large number of assemblies were constructed with geometries in the 200-500 /spl mu/m range, and wide variations in joint resistance were found to occur both within the same assembly and between assemblies under the same experimental conditions. In order to explain the origin of these unsatisfactory connections, a series of tests to measure the contact resistance linearity of both high and low resistance joints were made. The results from these measurements show that a large number of low resistance joints are ohmic, while most joints of relatively high resistance show resistive heating. However, in some of the initially high resistance joints there is an initial ohmic behaviour which is followed by a breakdown of a dielectric or insulating film, resulting in lower resistance. In addition to linearity measurements, computational models of metallic conduction in solid and polymer core particles were constructed to help understand the conduction mechanism. These models, which are based on the finite element method, represent typical conductor particles trapped between appropriate substrate and component metallisation. The model results show that the contact area required to explain high resistances is small and that the likelihood of obtaining a high resistance through such a small area of metal-to-metal contact is small, thus giving a strong indication of the presence of high resistivity films at the joint contact surfaces.

CMOS semiconductor device having dual-gate electrode construction and method of production of the same

Abstract: A semiconductor device in which mutual diffusion of doped impurities occurring through an upper silicide electrode layer is prevented. A silicide electrode layer is doped with both the same degree of p-type impurities as the concentration of p-type impurities of the lower gate electrode layer and the same degree of n-type impurities as the concentration of n-type impurities. As a result, the concentration of doped impurities of the gate electrode layer is balanced at the two sides of the interface of the pMOS side and nMOS side. Therefore, heat diffusion caused by subsequent heat treatment is prevented and the problem of mutual diffusion can be solved. The present invention is also suitable for the SALICIDE process. Even when the silicide electrode layer is formed simultaneously on an extremely shallow source or drain region, since the concentration of the impurities of the silicide electrode layer was initially high, the lower impurities will not be drained so the contact resistance will not be made to deteriorate. As a result, it becomes easy for the SALICIDE process to be applied to submicron devices. In the method of production of the present invention, the silicide electrode layer is formed by the CVD method or the sputtering method and the impurities doped during this process, so no special step has to be provided for introducing the impurities.

Effect of contact resistance between particles on the current distribution in a packed bed electrode

Abstract: A microscopic, modelistic approach was carried out to elucidate the electrochemical reduction of a nuclear waste solution in a packed bed electrode. The interfacial surface reactions within the packed bed were taken into account and the particle–particle contact resistance through oxide films was found to be big enough to effect the potential distribution throughout the bed. On the basis of equations developed here, the contribution of the resistance of the oxide film to the potential and current distribution throughout the bed was compared with the macroscopic homogeneous approach.

Semiconductor device having improved lamination-structure reliability for buried layers, silicide films and metal films, and a method for forming the same

Abstract: After formation of a connection hole or before deposition of an insulator film, a semiconductor device is placed onto a cathode of a plasma generator. A surface of a metal silicide film such as a silicide of titanium is exposed to a plasma of a nitrogen-containing gas at 550 degrees centigrade or less. As a result of such processing, a barrier compound layer, composed of a compound of nitrogen, oxygen, metal and silicon, is formed at a near-surface region of the metal silicide film of the titanium silicide film. Thereafter, while forming a buried layer from material superior in step coverage such as an Al--Ti compound and an aluminum alloy, reaction between the metal silicide film and the buried layer in a later annealing treatment can be avoided without depositing a barrier metal such as a titanium nitride/nitride film in the connection hole. Accordingly, contact resistance, sheet resistance and junction leakage can be reduced and reliability can be improved.

The effect of mechanical loading on the contact resistance of coated aluminium

Abstract: Measurements of contact resistance, related to resistance spot welding, were made using pre-treated (coated) and abraded aluminium alloy strip. With conventional domed electrodes, the contact resistance was much larger at the sheet-to-sheet (faying) contact than at the electrode-sheet contact. The effect is believed to be associated with the role of sliding in breaking down contact resistance of sheet with an insulating surface film. When the coating was removed the difference between faying surface and electrode-sheet contact was much smaller. Macroscopic shear stresses are developed by electrode-sheet contact whereas no shear stresses are present at the faying surface. The hypothesis is supported by experiments made with asymmetrical electrode pairs which give rise to varying shear stresses in faying surface contact. Some implications for the control of spot welding of different aluminium surfaces are discussed.

Intermittence detection in fretting corrosion studies of electrical contacts

Abstract: The occurrence of intermittences in electrical circuits can result in serious reliability problems particularly where low signal voltages and currents are used. It is well known that differential thermal expansion or vibration in electrical connectors can result in micromotion resulting in fretting corrosion. This can lead to an increase in contact resistance and eventual loss of electrical contact. The occurrence of electrical intermittences has been considered a precursor to contact failure associated with fretting corrosion and can cause disruption in digital circuit signals. We report the development of instrumentation that simultaneously measures the occurrence of electrical intermittences along with contact resistance during the fretting of electrical contacts. In addition, the measurement system records contact friction and normal force dynamically. Intermittences can be counted and timed with durations from 20 ns to milliseconds as a function of fretting cycles and correlated with the increase in contact resistance. All systems are integrated under LABVIEW computer control software. Measurements were made on Cu-Cu and Sn-Sn rider/flat combinations. Results will be interpreted in terms of the influence of wear debris on the electrical properties of the contacts.