Effect of Cryogenic Temperature Deposition of Various Metal Contacts on Bulk Single-Crystal <Emphasis Type="Italic">n</Emphasis>-Type ZnO

The effect of cryogenic temperatures during metal deposition on the contact properties of Pd, Pt, Ti, and Ni on bulk single-crystal n-type ZnO has been investigated. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. By sharp contrast, both Pd and Pt contacts showed rectifying characteristics after deposition with barrier heights between 0.37 eV and 0.69 eV. Changes in contact behavior were measured on Pd to anneal temperatures of ∼300 °C, showing an increase in barrier height along with a decrease in ideality factor with increasing annealing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however, low temperature contacts demonstrated some peeling/cracking for Pt and Pd.     

Improved Long-Term Thermal Stability At 350°C Of TiB2–Based Ohmic Contacts On AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/TiB₂/Ti/Au source/drain Ohmic contacts and a variety of gate metal schemes (Pt/Au, Ni/Au, Pt/TiB₂/Au or Ni/TiB₂/Au) and subjected to long-term annealing at 350°C. By comparison with companion devices with conventional Ti/Al/Pt/Au Ohmic contacts and Pt/Au gate contacts, the HEMTs with boride-based Ohmic metal and either Pt/Au, Ni/Au or Ni/TiB₂/Au gate metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C.     

Ohmic contact formation in palladium-based metallizations to n-Type InP

Two Pd-based metallizations have been systematically studied, i.e., Au/Ge/Pd and Pd/Ge contacts to n-type InP, in an attempt to better understand the role of the metallization constituents in forming ohmic contacts. Ohmic contacts were obtained with minimum specific resistances of 2.5 × 10⁻⁶ Ω-cm² and 4.2 × 10⁻⁶ Ω-cm² for the Au/Ge/Pd and the Pd/Ge contacts, respectively. The annealing regime for ohmic contact formation is 300-375°C for the Au/Ge/Pd/InP system and 350-450°C for the Pd/GelnP system. Palladium, in both cases, reacts with InP to form an amorphous layer and then an epitaxial layer at low temperatures, providing good metallization adhesion to InP substrates and improved contact morphology. Ohmic contact formation in both contacts is attributed to Ge doping, based on the solid state reaction-driven decomposition of an epitaxial layer at the metallization/InP interface, producing a very thin, heavily doped InP layer. Gold appears to be responsible for the difference in contact resistance in the two systems. It is postulated that Au reacts strongly with In to form Au-In compounds, creating additional In site vacancies in the InP surface region (relative to the Au-free metallization), thereby enhancing Ge doping of the InP surface and lowering the contact resistance. Both contacts degrade and ultimately become Schottky barriers again if over annealed, due to consumption of additional InP, which destroys the heavily doped InP layer.     

Ohmic contacts to n-type GaN using Pd/Al metallization

Ohmic contacts to n-type GaN grown by metalorganic chemical vapor deposition were formed using a Pd/Al-based metallization. Ohmic contact resistances and specific contact resistances were investigated as a function of rapid thermal annealing temperature, Pd interlayer thickness, and annealing time. As-deposited Pd/AI was found to produce rectifying contacts while the metallization exhibited ohmic characteristics after annealing at temperatures as low as 400°C. A minimum contact resistance of 0.9 ohm-mm (specific contact resistance = 1.2 × 10⁻⁵ ohm-cm²) was obtained upon annealing at 650°C for 30 s. For comparison, Al and Ti/Al contacts were also investigated. Auger electron spectroscopy, secondary ion mass spectrometry, and x-ray diffraction were used to investigate metallurgical reactions.     

Low-resistance ohmic contacts to p-type GaAs using Zn/Pd/Au metallization

We have fabricated the low resistance ohmic contacts to p-type GaAs. Specific contact resistances as low as 7 × 10^-7Ω.cm²have been obtained for contacts prepared by heat treating Zn/Pd/Au metallizations deposited on p-type epitaxial GaAs layers with an acceptor concentration of 1.5 × 10¹⁹cm^-3. These contacts are reproducible, simple to fabricate, exhibit excellent adhesion, and have a uniformly smooth surface morphology.     

Ohmic contacts to n-type GaN

Ohmic contacts to n-GaN using Ag, Au, TiN, Au/Ti, Au/Mo/Ti, and Au/Si/Ti have been studied. The Fermi level of GaN appears to be unpinned, and metals and compounds with work functions less than the electron affinity resulted in ohmic contacts. Reactively sputter deposited TiN was ohmic as deposited. However, Au/Ti, Au/Mo/Ti, and Au/Si/Ti required heat treatments to form ohmic contacts, with the best being an RTA at 900°C. Ag and Au were shown to diffuse across the GaN surface at T>500°C; therefore, they are unstable, poor ohmic contact metallizations as single metals. The other contact schemes were thermally stable up to 500°C for times of 30 min.     

Pd/Zn/Pd/Au ohmic contacts to ρ-Type InP

Low resistance ohmic contacts (ρ_c = 7 x 10^-5 Ω-cm ² ) have been fabricated to Zn-doped p-type InP using an annealed Pd/Zn/Pd/Au metallization. Palladium reacts with InP at low temperatures to form a Pd₂InP ternary phase, which is initially amorphous but crystallizes and grows epitaxially on InP. Zinc reacts with some of the overlying Pd to form PdZn (≅250° C), which decomposes at 400-425° C to form PdP₂, freeing up Zn to diffuse into Au as well as InP. The contact resistance reaches a minimum as the decomposition reaction takes place. The resultant ohmic contact is laterally uniform and consists of epitaxial Pd₂InP adjacent to InP, followed by a thin layer of PdP₂ and then the outer Au layer. Further annealing leads to a breakdown of the contact structure,i.e. decomposition of Pd₂InP, and an increase in contact resistance.     

Pd/Sb(Zn) and Pd/Ge(Zn) ohmic contacts on p-type indium gallium arsenide: The employment of the solid phase regrowth principle to achieve optimum electrical and metallurgical properties

The development of two metallizations based on the solid-phase regrowth principle is presented, namely Pd/Sb(Zn) and Pd/Ge(Zn) on moderately doped In_0.53Ga_0.47As (p=4×10¹⁸ cm⁻³). Contact resistivities of 2–3×10⁻⁷ and 6–7×10⁻⁷ Ωcm², respectively, have been achieved, where both systems exhibit an effective contact reaction depth of zero and a Zn diffusion depth below 50 nm. Exhibiting resistivities equivalent to the lowest values of Au-based systems in this doping range, especially Pd/Sb(Zn) contacts are superior to them concerning metallurgical stability and contact penetration. Both metallizations have been successfully applied for contacting the base layer of InP/In_0.53Ga_0.47As heterojunction bipolar transistors.     

Cross-Interaction Study of Cu/Sn/Pd and Ni/Sn/Pd Sandwich Solder Joint Structures

Cross-interactions between Cu/Sn/Pd and Ni/Sn/Pd sandwich structures were investigated in this work. For the Cu/Sn/Pd case, the growth behavior and morphology of the interfacial (Pd,Cu)Sn₄ compound layer was very similar to that of the single Pd/Sn interfacial reaction. This indicates that the growth of the (Pd,Cu)Sn₄ layer at the Sn/Pd interface would not be affected by the opposite Cu/Sn interfacial reaction. We can conclude that there is no cross-interaction effect between the two interfacial reactions in the Cu/Sn/Pd sandwich structure. For the Ni/Sn/Pd case, we observed that: (1) after 300 s of reflow time, the (Pd,Ni)Sn₄ compound heterogeneously nucleated on the Ni₃Sn₄ compound layer at the Sn/Ni interface; (2) the growth of the interfacial PdSn₄ compound layer was greatly suppressed by the formation of the (Pd,Ni)Sn₄ compound at the Sn/Ni interface. We believe that this suppression of PdSn₄ growth is caused by heterogeneous nucleation of the (Pd,Ni)Sn₄ compound in the Ni₃Sn₄ compound layer, which decreases the free energy of the entire sandwich reaction system. The difference in the chemical potential of Pd in the PdSn₄ phase at the Pd/Sn interface and in the (Pd,Ni)Sn₄ phase at the Sn/Ni interface is the driving force for the Pd atomic flux across the molten Sn. The diffusion of Ni into the ternary (Pd,Ni)Sn₄ compound layer controls the Pd atomic flux across the molten Sn and the growth of the ternary (Pd,Ni)Sn₄ compound at the Sn/Ni interface.     

Low resistance Pd/Zn/Pd Au ohmic contacts to P-type gaas

Many optoelectronic devices require contacts top-doped epitaxial layers. To achieve low contact resistance, the semiconductor has to be doped to high levels. Thep-dopants most commonly used are Be, Mg, and Zn. The contacts were formed by the sequential e-beam evaporation of 10 nm Pd, ≤5 nm Zn, 20 nm Pd and 40 nm Au layers onto a 0.2 μm thick Be-doped (5 × 10¹⁸ cm) GaAs layer grown by MBE. The minimum contact resistance of 0.04Ω-mm (≤1 × 10⁻⁷ Ω-cm²), as measured using the transmission line method, was obtained for contacts annealed at 500° C for 30s. These are the lowest contact resistance values reported to date for alloyed contacts top-GaAs.     

Ohmic contact behaviour of Co/C/4H-SiC structures

The electrical contact properties of Co/4H-SiC structures are investigated.A carbon interfacial layer between a Co film and SiC is used to improve the Ohmic contact properties significantly.The C film is deposited prior to Co film deposition on SiC using DC sputtering.The high quality Ohmic contact and specific contact resistivity of 2.30×10^-6Ω·cm² are obtained for Co/C/SiC structures after two-step annealing at 500℃for 10 min and 1050℃for 3 min.The physical properties of the contacts are examined by using XRD.The results indicate that the Co-based metal contacts have better structural stability of silicide phases formed after the high temperature annealing and carbon-enriched layer is produced below the contact,playing a key role in forming an Ohmic contact through the reduction of effective Schottky barrier height for the transport of electrons.The thermal stability of Au/Co/C/SiC Ohmic contacts is investigated.The contacts remain Ohmic on doped n-type(2.8×10¹⁸ cm^-3) 4H-SiC after thermal aging treatment at 500℃for 20 h.     

Solid-state interfacial reactions at the solder joints employing Au/Pd/Ni and Au/Ni as the surface finish metallizations

A tri-layer of nickel/palladium/gold (Au/Pd/Ni) is a promising candidate to replace the conventional Au/Ni bi-layer as the surface finish metallization for lead-free packaging. A surface finish metallization (Au/Pd/Ni or Au/Ni) and a Sn layer are sequentially deposited on a Cu substrate and then are subjected to thermal aging at 150 and 200 °C to investigate the interfacial reactions in the stacking multilayer structure made by low-temperature solid-state bonding. Because of the absence of the reflow process, the Pd and Au layers do not dissolve in the Sn matrix but remain at the interface and participate in the interfacial reaction to form the (Pd,Ni,Au)Sn₄ and (Au,Ni)Sn₄ phases at the Au/Pd/Ni- and Au/Ni-based interfaces, respectively. Though the Pd layer was only 0.4 μm, its resulting (Pd,Ni,Au)Sn₄ phase is much thicker than the (Au,Ni)Sn₄ phase. These two intermetallic compounds exhibit very different microstructural evolution which significantly affects the interfacial microstructures and growth rate of other intermetallic compound formed at the same interfaces.     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

All-refractory GaAs FET using amorphous TiWSixsource/drain metallization and graded-InxGa1-xAslayers

We report on the fabrication of an all-refractory GaAs field-effect transistor having non-alloyed source and drain ohmic contacts and a TiW/Au refractory gate metallization. The ohmic contacts consist of amorphous TiWSi_x metallization and intervening graded InGaAs layers grown by low pressure organometallic vapor phase epitaxy (LPOMVPE). The amorphous TiWSi_x, is formed using alternating layers of TiW(10 Å) and Si(1.5 Å) deposited by an RF magnetron sputtering technique. The resulting all-refractory FET devices exhibited excellent dc transistor characteristics with measured transconductance of 140 mS/mm. The dc performance of these devices was comparable to conventional devices with AuGe/Ni/Au contacts fabricated using similar material structures     

Effects of Au and Pd Additions on Joint Strength, Electrical Resistivity, and Ion-Migration Tolerance in Low-Temperature Sintering Bonding Using Ag2O Paste

A new bonding process using an Ag₂O paste consisting of Ag₂O particles mixed with a triethylene glycol reducing agent has been proposed as an alternative joining approach for microsoldering in electronics assembly, which currently uses Pb-rich, high-temperature solders. Ag nanoparticles were formed at approximately 130°C to 160°C through a reduction process, sintered to one another immediately, and bonded to a metal substrate. An Au-coated Cu specimen was successfully bonded using the Ag₂O paste. The resulting joint exhibited superior strength compared with joints fabricated using conventional Pb-rich solders. To improve ion-migration tolerance, the Ag₂O paste was mixed with Au and Pd microparticles to form sintered Ag-Au and Ag-Pd layers, respectively. The additions of Au and Pd improved the ion-migration tolerance of the joint. Regarding the mechanical properties of the joints, addition of secondary Au and Pd both resulted in decreased joint strength. To match the joint strength of conventional Pb-10Sn solder, the mixing ratios of Au and Pd were estimated to be limited to 16?vol.% and 7?vol.%, respectively. The electrical resistivities of the sintered layers consisting of 16?vol.% Au and 7?vol.% Pd were lower than that of Pb-10Sn solder. Thus, the additive fractions of Au and Pd to the Ag₂O paste should be less than 16?vol.% and 7?vol.%, respectively, to avoid compromising the mechanical and electrical properties of the sintered layer relative to those of contemporary Pb-10Sn solder. Following the addition of Au and Pd to the paste, the ion-migration tolerances of the sintered layers were approximately 3 and 2 times higher than that of pure Ag, respectively. Thus, the addition of Au was found to improve the ion-migration tolerance of the sintered Ag layer more effectively and with less sacrifice of the mechanical and electrical properties of the sintered layer than the addition of Pd.     

Thermal Stability of Nitride-Based Diffusion Barriers for Ohmic Contacts to <Emphasis Type="Italic">n</Emphasis>-GaN

The use of TaN, TiN, and ZrN diffusion barriers for Ti/Al-based contacts on n-GaN (n ∼ 3 × 10¹⁷ cm⁻³) is reported. The annealing temperature (600–1,000°C) dependence of the Ohmic contact characteristics using a Ti/Al/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, deposited by sputtering was investigated by contact resistance measurements and Auger electron spectroscopy (AES). The as-deposited contacts were rectifying and transitioned to Ohmic behavior for annealing at ≥600°C. A minimum specific contact resistivity of ∼6 × 10⁻⁵ Ω-cm⁻² was obtained after annealing over a broad range of temperatures (600–900°C for 60 s), comparable to that achieved using a conventional Ti/Al/Pt/Au scheme on the same samples. The contact morphology became considerably rougher at the high end of the annealing range. The long-term reliability of the contacts at 350°C was examined; each contact structure showed an increase in contact resistance by a factor of three to four over 24 days at 350°C in air. AES profiling showed that the aging had little effect on the contact structure of the nitride stacks.     

Microstructure and Electrical Properties of Low Temperature Processed Ohmic Contacts to p‐Type GaN

With Ni/Au and Pd/Au metal schemes and low temperature processing, we formed low resistance stable Ohmic contacts to p‐type GaN. Our investigation was preceded by conventional cleaning, followed by treatment in boiling HNO₃:HCl (1:3). Metallization was by thermally evaporating 30 nm Ni/15 nm Au or 25 nm Pd/15 nm Au. After heat treatment in O₂ + N₂ at various temperatures, the contacts were subsequently cooled in liquid nitrogen. Cryogenic cooling following heat treatment at 600 ·C decreased the specific contact resistance from 9.84·10^?4 Ωcm² to 2.65·10^?4 Ωcm² for the Ni/Au contacts, while this increased it from 1.80·10^?4 Ωcm² to 3.34·10^?4 Ωcm² for the Pd/Au contacts. The Ni/Au contacts showed slightly higher specific contact resistance than the Pd/Au contacts, although they were more stable than the Pd contacts. X‐ray photoelectron spectroscopy depth profiling showed the Ni contacts to be NiO followed by Au at the interface for the Ni/Au contacts, whereas the Pd/Au contacts exhibited a Pd:Au solid solution. The contacts quenched in liquid nitrogen following sintering were much more uniform under atomic force microscopy examination and gave a 3 times lower contact resistance with the Ni/Au design. Current‐voltage‐temperature analysis revealed that conduction was predominantly by thermionic field emission.     

Phase formation and diffusion soldering in Pt/In,Pd/In,and Zr/Sn thin-film systems

Potential candidates for thin-film diffusion soldering were investigated by analysis of phase formation and measurements of mechanical and thermal stability of thin-film bonds. Bilayers of Pt/In, Pd/In, and Zr/Sn of 500 nm/500 nm thickness were prepared by direct current magnetron sputtering followed by a 5 nm, thin protective Au layer. Single bilayer samples were heat-treated between 200°C and 500°C for 3–30 min and studied by x-ray diffraction (XRD) analysis. Some bilayers were bonded face-to-face between 300°C and 500°C for 3–30 min and sheared-off either in shear-strength measurements at room temperature or in remelting experiments up to 1,100°C. Phase formation in Pd/In and Pt/In thin films is much faster than in Zr/Sn thin films. An interaction of Au in addition to a questionable thermal stability of PtIn₂ complicated the reaction in Pt/In samples. A revised partial Pd-In phase diagram was constructed, correcting the compound ‘PdIn₃’ to Pd₃In₇. The Pt/In and Pd/In thin-film systems are very promising candidates for thin-film diffusion soldering.     

Mounting of high power laser diodes on boron nitride heat sinksusing an optimized Au/Sn metallurgy

High power diode lasers have become more and more important to industrial and medical applications. In contrast to low power applications, long cavity lasers or laser bars are used in this field and mounting quality influences considerably laser performance and life time. In this paper we focus on the solder metallurgy and stress-induced laser behavior after mounting. The laser chips have been bonded fluxless epi-side down on translucent cubic boron nitride (T-cBN) using Au/Sn solder. The laser behavior has been tested with different chip metallizations preserving the eutectic solder composition or forming the Au rich ζ-phase during reflow. The resulting additional stress in the lasing region has been independently indicated by polarization measurements of the emitted light. A metallization scheme has been developed which forms the highly melting ζ-phase during soldering within a wide process window. This procedure yields better results then using eutectic Au/Sn which has a higher hardness than the ζ-phase. Laser diodes up to a cavity length of 2000 μm and an aperture of 200 μm have successfully been mounted on T-cBN. State of the art laser data, excellent thermal stability, high yield and reliability have been obtained     

Comparison of electroplated eutectic Bi/Sn and Pb/Sn solder bumpson various UBM systems

The effect of a reflow process and under bump metallurgy (UBM) systems on the growth of intermetallic compounds (IMC) of the 57Bi/43Sn and 37Pb/63Sn solder bump/UBM interfaces was investigated. The selected UBM systems were sputtered Al/Ti/Cu, sputtered Al/NiV/Cu, Al/electroless Ni/immersion Au, and Al/Ti/electroless Cu. An alloy electroplating method was used for the solder bumping process. The microstructure and composition of intermetallic compound (IMC) phases and their morphologies were examined using scanning electron microscopy and X-ray diffraction. The Cu₆Sn₅ η'-phase IMC appeared on all Cu containing UBM cases with Pb/Sn and Bi/Sn solders and the Cu ₃Sn ϵ-phase was detected only with Pb/Sn solder bumps. The Ni₃Sn₄ IMC was found to be the main IMC phase between Ni and solder. The Ni₃Sn secondary IMC was also detected on the electroless Ni UBM with PbSn solder after ten times reflow. Through the bump shear test, Al/NiV/Cu, Al/elNi/Au, and Al/Ti/elCu UBMs showed good stability with Bi/Sn and Pb/Sn solder in terms of metallurgical aspects