Fabrication of vertical thin-GaN light-emitting diode by low-temperature Cu/Sn/Ag wafer bonding

Vertical thin-GaN LED was successfully fabricated on the GaN LED epi-layers grown on the patterned-sapphire substrate with the pyramidal pattern by low-temperature Cu/Sn/Ag wafer bonding at 150 °C. An inverted pyramidal pattern formed on the n-GaN surface after the GaN epi-layer was transferred onto Si wafer, which resulted from the pyramidal pattern on the patterned-sapphire substrate. The inverted pyramidal pattern has an equivalent function with roughening the n-GaN surface. With higher inverted pyramidal pattern coverage, the light extraction efficiency can be greatly enhanced. In addition, we found that the 4-fold increase (from 13.6% to 53.8%) in the pyramidal pattern coverage on patterned-sapphire substrate only gives the GaN LED epi-layer about 5.7% enhancement in the internal quantum efficiency.     

Formation of low-resistance and transparent indium tin oxide ohmic contact for high-brightness GaN-based light-emitting diodes using a Sn–Ag interlayer

We report on the formation of low-resistance and highly transparent indium tin oxide (ITO) ohmic contacts to p-GaN using a Sn–Ag alloy interlayer. Although the as-deposited Sn–Ag(6 nm)/ITO(200 nm) contacts show non-ohmic behaviors, the scheme becomes ohmic with specific contact resistance of 4.72×10⁻⁴ Ω cm² and produce transmittance of ∼91% at wavelength of 460 nm when annealed at 530 °C. Blue light-emitting diodes (LEDs) fabricated with the Sn–Ag/ITO contacts give forward-bias voltage of 3.31 V at injection current of 20 mA. LEDs with the Sn–Ag/ITO contacts show the improvement of the output power by 62% (at 20 mA) compared with LEDs with Ni/Au contacts.     

Thermal stability of back side metallization multilayer for power device application

Metallization multilayers on the back side of a power device were focused in this study. Si wafers coated with high melting point metals were exposed at 300 °C for 300 h to investigate diffusion condition of the metallization layer. We developed and examined the thermal stability of die bonding material (Au paste) including sub–micrometer–sized Au particles. Auger electron spectroscopy was applied to observe the atomic composition of the multilayers in depth direction after the high temperature aging. Surface morphology was observed using optical microscope and scanning electron microscope. While atomic composition on Ti/Au changed drastically after the high temperature aging, other multilayers maintained their metallization composition. However, the surface morphology was slightly changed on Ti/Ru/Au, W/Au, and Ta/Au. Bond strength on the Ti/Pt/Au kept over 40 MPa with unified bonding layer after exposing at 300 °C for 1000 h.     

Investigation of n-ohmic contact of vertical GaN-based light-emitting diodes on graphite substrate with Ag-In bonding

Vertical light-emitting diodes (VLEDs) were successfully transferred from a GaN-based sapphire substrate to a graphite substrate by using low-temperature and cost-effective Ag-In bonding, followed by the removal of the sapphire substrate using a laser lift-off (LLO) technique. One reason for the high thermal stability of the AgIn bonding compounds is that both the bonding metals and Cr/Au n-ohmic contact metal are capable of surviving annealing temperatures in excess of 600 °C. Therefore, the annealing of n-ohmic contact was performed at temperatures of 400 °C and 500 °C for 1 min in ambient air by using the rapid thermal annealing (RTA) process. The performance of the n-ohmic contact metal in VLEDs on a graphite substrate was investigated in this study. As a result, the final fabricated VLEDs (chip size: 1000 µm×1000 µm) demonstrated excellent performance with an average output power of 538.64 mW and a low operating voltage of 3.21 V at 350 mA, which corresponds to an enhancement of 9.3% in the light output power and a reduction of 1.8% in the forward voltage compared to that without any n-ohmic contact treatment. This points to a high level of thermal stability and cost-effective Ag-In bonding, which is promising for application to VLED fabrication.     

Microstructural and thermal characterizations of light-emitting diode employing a low-temperature die-bonding material

A Sn/Bi bilayer was deposited on a hot air solder leveling (HASL)-treated metal-core printed circuit board (MCPCB) using electroplating as a low-temperature die-bonding material for light-emitting diode (LED). The eutectic feature of the Sn/Bi contact enabled the die-bonding process to accomplish through a liquid/solid reaction at 185 °C with a proper compression force. A high-temperature die-bonding structure composed of a Bi layer sandwiched by two intermetallic compounds (IMCs) formed after thermocompression. Employment of the Sn/Bi bilayer for low-temperature die-bonding prevented the LEDs from thermal stress problems, and the resulting high-temperature IMC/Bi/IMC die-bonding structure was capable of withstanding multiple bonding reactions and high temperature/current operation environment. Durability tests including mechanical, thermal, and optical performance were systematically performed and compared with other commercially available die-bonding materials (Ag paste and solder alloys).     

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.     

Non-conductive film with Zn-nanoparticles (Zn-NCF) for 40 μm pitch Cu-pillar/Sn–Ag bump interconnection

Non-conductive film with Zn nano-particles (Zn-NCF) is an effective solution for fine-pitch Cu-pillar/Sn–Ag bump interconnection in terms of manufacturing process and interfacial reliability. In this study, NCFs with Zn nano-particles of different acidity, viscosity, and curing speed were formulated and diffused Zn contents in the Cu pillar/Sn–Ag bumps were measured after 3D TSV chip-stack bonding. Amount of Zn diffusion into the Cu pillar/Sn–Ag bumps increased as the acidity of resin increased, as the viscosity of resin decreased, as the curing speed of resin decreased, and as the bonding temperature increased. Diffusion of Zn nano-particles into the Cu pillar/Sn–Ag bumps are maximized when the resin viscosity became lowered and the solder oxide layer was removed. To analyze the effects of Zn-NCF on IMC reduction, IMC height depending on aging time was measured and corresponding activation energies for IMC growth were calculated. For the evaluation of joint reliabilities, test vehicles were bonded using NCFs with 0 wt%, 1 wt%, 5 wt%, and 10 wt% of Zn nano-particles and aged at 150 °C up to 500 h. NCF with 10 wt% Zn nano-particle showed remarkable suppression in Cu₆Sn₅ and (Cu,Ni)₆Sn₅ IMC compared to NCFs with 0 wt%, 1 wt%, and 5 wt% of Zn nano-particles. However, in terms of Cu₃Sn IMC suppression, which is the most critical goal of this experiment NCFs with 1 wt%, 5 wt%, and 10 wt% showed an equal amount of IMC suppression. As a result, it was successfully demonstrated that the suppression of Cu–Sn IMCs was achieved by the addition of Zn nano-particles in the NCFs resulting an enhanced reliability performance in the Cu/Sn–Ag bumps bonding in 3D TSV interconnection.     

Effect of trace platinum additions on the interfacial morphology of Sn–3.8Ag–0.7Cu alloy aged for long hours

While the Sn–Ag–Cu (SAC) family of solders are considered good candidate as lead-free solder replacement materials, their relatively short processing history and application result in a host of materials as well as reliability problems. For good metallurgical bonding and electrical connection, a thin, even layer of intermetallic compound (IMC) is required but excessive growth of the IMC layer will cause various reliability problems. This is especially critical for miniaturized solder pitches in very large scale integration circuits. This work adopts the composite approach of adding 0.15 and 0.30 wt.% of Pt into Sn–3.8Ag–0.7Cu alloy to study the effect of these additions to the IMC layer thickness between the solder and substrate. Alloys were isothermally aged at 150 °C for up to 1000 h to observe contribution of Pt in suppressing excessive IMC growth. It was found that when more Pt was added to the alloy, the IMC layer became more even and continuous. Voids and IMC layer thickness were reduced. This is attributed to the role of Pt in replacing Cu in the solder and thus impeding excessive diffusion.     

Low-temperature low-pressure die attach with hybrid silver particle paste

New types of die attach pastes comprising micron-sized Ag particles hybridized with submicron-sized Ag particles were considered as lead-free die attach materials for SiC power semiconductors. Micron-sized Ag particles in alcohol solvent were prepared by mixing the die attach paste with submicron-sized Ag particles. The alcohol vaporizes completely during sintering and no residue exists in the bonding layer. The Ag layer has a uniform porous structure. The electrical resistivity of the printed tracks decreases below 1 × 10^?5 Ω cm when sintered above 200 °C. When sintered at 200 °C for 30 min, the average resistivity reaches 5 × 10^?6 Ω cm, which is slightly higher than the value obtained by using Ag nanoparticle paste. A SiC die was successfully bonded to a direct bonded copper substrate and the die-shear strength gradually increases with the increase in bonding temperature up to 300 °C. The Ag die attach bond layer was stable against thermal cycles between ?40 °C and 300 °C.     

Thermal transient characteristics of die attach in high power LED PKG

The reliability of packaged electronics strongly depends on the die attach quality because any void or a small delamination may cause instant temperature increase in the die, leading sooner or later to failure in the operation. Die attach materials have a key role in the thermal management of high power LED packages by providing the low thermal resistance between the heat generating LED chips and the heat dissipating heat slug. In this paper, thermal transient characteristics of die attach in high power LED PKG have been studied based on the thermal transient analysis using the evaluation of the structure function of the heat flow path. With high power LED packages fabricated by die attach materials such as Ag paste, solder paste and Au/Sn eutectic bonding, we have demonstrated for characteristics such as cross-section analysis, shear test and visual inspection after shear test of die attach and how to detect die attach failures and to measure thermal resistance values of die attach in high power LED PKG. From the differential structure function of the thermal transient characteristics, we could know the result that die attach quality of Au/Sn eutectic bonding with the thermal resistance of about 3.5 K/W was much better than this of Ag paste and solder paste with the thermal resistance of about 11.5–14.2 K/W and 4.4–4.6 K/W, respectively. From this results, it is possible to fabricate high power LED with a small thermal resistance and a good die attach quality by applying Au/Sn eutectic bonding die attach with a high reliability and a good repeatability.     

Electrical and thermal stability of Ag ohmic contacts for GaN-based flip-chip light-emitting diodes by using an AgAl alloy capping layer

We have investigated Ag(200 nm)/AgAl(100 nm) ohmic contacts to p-type GaN for near-UV (405 nm) flip-chip light-emitting diodes (LEDs). It is shown that the use of an AgAl alloy capping layer (with 8 at% Al) results in better electrical and optical properties as compared to single Ag contacts when annealed at 430 °C. For example, Ag/AgAl (8 at% Al) contacts give specific contact resistance of 4.6×10^–4 Ω cm² and reflectance of 90% at a wavelength of 405 nm. However, use of an AgAl (with 50 at% Al) layer is not effective. LEDs fabricated with the Ag/AgAl (8 at% Al) reflectors produce higher light output as compared with the ones with single Ag reflectors. Ohmic mechanisms of the Ag/AgAl (8 at% Al) contacts are described and discussed.     

The performance and fracture mechanism of solder joints under mechanical reliability test

The drop resistance and fracture behavior of Sn–37Pb, Sn–3.0Ag–0.5Cu (SAC305), Sn–1.0Ag–0.5Cu (SAC105), and Sn–8.5Zn–0.5Ag–0.01Al–0.1 Ga (SnZn-5e) solder ball joints under the board-level drop test (BLDT) and the ball impact test (BIT) were studied. The results show that the drop reliabilities in terms of the characteristic life ratio from the Weibul plot are SnZn-5e : Sn–37Pb:SAC105:SAC305 = 3.1:2.9:2.1:1. It was observed that failure of Sn–37Pb occurred at the eutectic tin–lead phase whereas it took place at the brittle interface between the (Cu,Ni)₆Sn₅ inter-metallic compound and Ni layer in SAC305. The failure of SAC105 was found to be located within the solder matrix as well as at the interface of the inter-metallic compound. The failure of SnZn-5e depends on the morphology of the interfacial inter-metallic compound. The failure modes of Sn–37Pb and SAC305 after the BIT were similar to those after the BLDT. The maximum impact force (F_max) and the initial fracture energy (E) from the BIT can be used to evaluate the drop reliability of solder joints.     

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.     

Reliability modeling of Sn–Ag transient liquid phase die-bonds for high-power SiC devices

In this work, a novel foil-based transient liquid phase bonding process has been used to mount the SiC Schottky diodes. The Sn–Ag TLP interlayer material was produced in the form of preforms of multilayer foils, using electrochemical deposition. The foils were designed to keep the overall composition of Ag and Sn about 80% and 20% respectively. The optimized TLP bonding process parameters were used during the assembly process. The die-attachment characterizations revealed that resulting intermetallic compounds (Ag₃Sn and ζ) have melting point beyond 480 °C. The die-attachment produced low bending stresses, while heated from 30 °C to 400 °C. The reliability of Sn–Ag TLP bonded samples was studied during passive temperature cycling and during active power cycling. During power cycling, the crack rates were determined by measuring the crack lengths of the TLP bonded joints after failure. The failure criteria were set to be an increase of diode's forward voltage by 10% since the start of the power cycling tests. The thermo-mechanical simulations were performed to determine the damage parameter i.e. strain range amplitude ∆ ε_p. Based on mechanical characterization of the TLP bonded layers, a plastic material model was used. The crack propagation rates were modeled using Paris' Law. Based on comparisons with state-of-the-art silver sintering technique, it can be stated that the TLP bonding is a promising die-attachment technique and its power cycling reliability is similar to silver sintering.     

A study of large area die bonding materials and their corresponding mechanical and thermal properties

We tested the ability of Au/Sn eutectic, silver paste, and solder paste to bond to a large area as well as the bonding of a high power LED die to a highly conductive submount. The samples ran through several tests including ultrasound image, shear force, and thermal resistance measurement. Finite element analysis (FEA) models were built for comparison and analysis. Au/Sn bonding shows the best thermal and mechanical properties. Silver paste shows lower contact thermal resistance compared with solder paste. Although the thickness of the silver paste bonding layer is greater than the solder paste bonding layer, the average total thermal resistance is noticeably lower than the solder paste bonded samples.     

Development of chip-on-flex bonding using Sn-based bumps and non-conductive adhesive

We developed a reliable and low cost chip-on-flex (COF) bonding technique using Sn-based bumps and a non-conductive adhesive (NCA). Two types of bump materials were used for the bonding process: Sn bumps and Sn–Ag bumps. The bonding process was performed at 180 °C for 10 s using a thermo-compression bonder after dispensing the NCA. Sn-based bumps were easily deformed to contact Cu pads during the bonding process. A thin layer of Cu₆Sn₅ intermetallic compound was observed at the interface between Sn-based bumps and Cu pads. After bonding, electrical measurements showed that all COF joints had very low contact resistance, and there were no failed joints. To evaluate the reliability of COF joints, high temperature storage tests (150 °C, 1000 h), thermal cycling tests (−25 °C/+125 °C, 1000 cycles) and temperature and humidity tests (85 °C/85% RH, 1000 h) were performed. Although contact resistance was slightly increased after the reliability test, all COF joints passed failure criteria. Therefore, the metallurgical bond resulted in good contact and improved the reliability of the joints.     

Study of free air ball formation in Ag–8Au–3Pd alloy wire bonding

An innovative Ag–8Au–3Pd alloy wire has been developed as an alternative to the traditional gold wire bonding. This paper focused on the free air ball (FAB) formation of 0.7 mil Ag–8Au–3Pd alloy wire, which was vital for the yield of the subsequent bonding process. During electric flame-off (EFO) process, the wire tail was melted by a high voltage spark, and then the FAB was shaped by the effects of surface tension and gravity. The EFO current was the key factor to influence the Ag–8Au–3Pd alloy FAB size and morphology due to the energy input via arc discharging. The defects including off-center and ripple appeared on the Ag–8Au–3Pd alloy FABs were discussed by cooling and solidification. It is suggested that low EFO current will effectively avoid FAB defects. The contaminants on the Ag–8Au–3Pd alloy FAB surface were analyzed by Auger electron spectroscopy (AES). Under the protection of the shielding gas, oxidation and sulfuration have been effectively prevented.     

Ag–Copper Structure for Bonding Large Semiconductor Chips

Ag–copper dual-layer substrate design is presented. The Ag cladding on the copper substrate is a buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/$^{circ}$C) and Cu (17 ppm/$^{circ}$ C). Ag is chosen because of its low yield strength, only one-tenth of that of Cu and one-third of the popular Sn3.5Ag solder. Other advantages are high electrical conductivity and high thermal conductivity. To bond Si chips to the Ag layer on copper substrates, Sn-rich solder is used. A fluxless bonding process is designed and developed. The bonding media are Ni/Sn/Au multilayer solder structure plated over Ag. In this design, Ni is a diffusion barrier between Sn and Ag. The thin (100 nm) outer Au layer prevents inner Sn from oxidation. The Si chip is deposited with Cr/Au under bump metallurgy (UBM). The bonding process is performed in 50-mtorr vacuum atmosphere without any flux. Comparing to bonding in air, the oxygen content is reduced by a factor of 15 200. The resulting joints consist of three distinct layers, i.e., Sn-rich layer, Ni$_{3}$ Sn$_{4}$ intermetallic compound, and Ni. Scanning acoustic microscopy (SAM) is used to verify the quality of the joint. Microstructure and composition of the joints are studied using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). This technique presents an initial success in overcoming the very large mismatch in thermal expansion between silicon and copper. It can be applied to mounting numerous high-power silicon devices to Cu substrate for various applications such as hybrid automotive and high-voltage power networks.      

High-lead flip chip bump cracking on the thin organic substrate in a module package

Flip chip bump cracking was observed after Si die attach reflow on the organic substrate of a module package. High-lead bump and eutectic SnPb cladding were used on Si die and the substrate sides, respectively. The reflow peak temperature was 260 °C for compatibility with passive components attach using lead-free solder. Flip chip bump cracking occurred at high-lead solder close to the die side. The cracking was eliminated by lowering the reflow peak temperature down to 225 °C. Main cause of the cracking at 260 °C reflow was attributed to the extensive Sn diffusion into high lead bump. This decreased the melting point of the high-lead solder around the die side, which in turn worsened the adhesion between solder and die due to the coexistence of solid and liquid. Diffusion length estimation showed both of the liquid- and solid-state diffusions of Sn. Crack gap in the solder bump was consistent with thermal expansion mismatch between Si die and organic substrate. The bump cracking was mitigated by use of 225 °C reflow, limiting Sn diffusion and providing a good integrity of high lead bumps on die side.     

Influence of trace alloying elements on the ball impact test reliability of SnAgCu solder joints

In this work, we present ball impact test (BIT) responses and fracture modes obtained at an impact velocity of 0.8 m/s on SAC (Sn–Ag–Cu) package-level solder joints with a trace amount of Mn or RE (rare earth) additions, which were bonded with substrates of OSP Cu and electroplated Ni/Au surface finishes respectively. With respect to the as-mounted conditions, the Ni/Au joints possessed better impact fracture resistance than those with Cu substrate. Subsequent to aging at 150 °C for 800 h, multi-layered intermetallic compounds emerged at the interface of the Ni/Au joints and gave rise to degradation of the BIT properties. This can be prevented by RE doping, which is able to inhibit the growth of interfacial IMCs during aging. As for aged Cu joints, the Mn-doped samples showed the best performance in impact force and toughness. This was related to the hardened Sn matrix, and most importantly, a greater Cu₃Sn/Cu₆Sn₅ thickness ratio at the interface. Compared to Cu₆Sn₅, Cu₃Sn with a similar hardness but greater elastic modulus possessed better plastic ability, which was beneficial to the reliability of solder joints suffering high strain rate deformation if no excess Kirkendall voids formed.