Chin C. Lee

A new bonding technology using gold and tin multilayer composite structures

A bonding technology which utilizes chromium, gold, and tin and gold deposited directly on the backside of a device die to form a multilayer composite is reported. The substrate accepting the die is coated with chromium and gold layers. The die and the substrate are brought into contact and heated to 310-320 degrees C. Due to the unique feature of the gold-tin alloy system, the tin layer melts first and dissolves the gold layers of the composite to produce a solution mixed with solid, which in turn would dissolve a portion of the gold layer on the substrate to develop a near eutectic bonding. In the composite, since the tin layer is protected by an outer gold layer in the same vacuum cycle, tin oxidation, which is a major cause of difficulty in achieving quality bondings, is reduced. This technology thus eliminates the requirement of preforms, prevents tin oxidation, and provides precise control of the bonding thickness. Results of bonding 4-mm by 4-mm GaAs dice on alumina substrates show that high-quality bondings are obtained as determined by a scanning acoustic microscope (SAM). >

AuSn alloy phase diagram and properties related to its use as a bonding medium

Abstract AuSn eutectic alloy has been successfully used in microelectronic packaging for high reliability applications where a hard solder as well as a low processing temperature are required. A new multilayer bonding technology not only has produced nearly perfect bonding but also has reduced the processing temperature even below the eutectic melting point. Knowledge of the different phases of the alloy and their formation, as well as the interdiffusion that occurs, thus becomes important in studying the bonding principle and the long-term reliability. In this paper, we review a large number of publications on the AuSn system and summarize the important properties. We hope that this summary would further enhance the development of new AuSn bonding methods as a result of an overall understanding of oxidation and diffusion properties.

Au-In bonding below the eutectic temperature

A bonding method using Au-In alloy which requires a low process temperature of 200 degrees C to produce high temperature (454 degrees C) bonds is reported. Multiple layers of Au and In are deposited on semiconductor wafers in one vacuum cycle to reduce In oxidation. The semiconductor dice are then bonded to substrates coated with Au. Above 157 degrees C, the indium layer melts and dissolves the Au layers to form a mixture of liquid and solid. The solid-liquid interdiffusion process continues until the mixture solidifies to form the Au-In bond. A scanning acoustic microscope (SAM) was used to determine the excellent bonding quality before and after thermal shock tests while an energy dispersive X-ray (EDX) was employed to determine the composition of the resulting bonds. The resulting bond has an unbonding temperature greater than 545 degrees C. Due to the low process temperature, the stress on the bonded structure caused by thermal expansion mismatch is reduced. This type of bonding is useful when bonding at a low temperature is followed by a subsequent higher temperature process. >

Silver-indium joints produced at low temperature for high temperature devices

A two-step fluxless bonding process adopted to produce high temperature silver-indium joints (80 wt% silver and 20 wt% indium) at relatively low process temperature of 206/spl deg/C has been developed. After annealing the joint continuously for 26 h at 145/spl deg/C, its melting temperature increases to 765-780/spl deg/C, as confirmed by a de-bonding test. The technique thus developed provides a viable alternative to packaging many high temperature devices running at 350/spl deg/C and above. The bonding quality of the Ag-In joints produced was examined using scanning acoustic microscopy. The joint cross-section was also studied using a scanning electron microscope equipped with an energy dispersive X-ray (EDX) spectroscope to find the local microstructure and composition. The results have shown that the joint is nearly void-free and uniform in thickness ranging from 7.2 to 7.8 /spl mu/m. The annealed sample joint, as determined by EDX, is mainly composed of AgIn/sub 2/, Ag/sub 2/In, and AuIn/sub 2/ grains embedded in an Ag-rich Ag-In alloy matrix. During joint formation, the intermetallic compound AgIn/sub 2/, in particular, prevents the indium layer from oxidation, and therefore, no flux is needed. In addition, low process temperatures help to reduce the thermal stresses developed in the bonded structure due to thermal expansion mismatch. Finally, reliability tests were conducted on three sets of annealed joints using a high temperature oven running continuously at 500/spl deg/C for 10, 100, and 1000 h each. Scanning acoustic microscopy (SAM) images on these samples confirmed that the joints have an excellent survivability in a high temperature environment.

Measurement of temperature profiles on visible light-emitting diodes by use of a nematic liquid crystal and an infrared laser

We present a new technique for measuring the temperature profiles of visible LED chips by use of a nematic liquid crystal with IR laser illumination. The LEDs studied have a multi-quantum-well InGaN/GaN/sapphire structure. New features in this technique are the use of a high-power IR laser beam as the sensing light and the insertion of a color filter in the optical path to block the high-intensity LED light. For the LEDs measured, the conversion efficiency decreases by 70% when the junction temperature rises from 25 to 107 degrees C. This technique is a valuable tool for studying the performance of LEDs as a function of junction temperature.

Void–free Au–Sn eutectic bonding of GaAs dice and its characterization using scanning acoustic microscopy

A new technique to produce perfect bonding between GaAs dice and alumina substrates is reported. Utilizing this technique, void-free bondings have been achieved consistently. The quality of the bonded devices is confirmed by a Scanning Acoustic Microscope (SAM) having a spatial resolution of 25 µm. Thermal cycling between -25° C and 125° C, and thermal shock between -196° C and 135° C, have been used to assess the reliability of the specimens. The SAM was used to study the variation of the bonds in the tests. After the tests, the bonds show no sign of degradation and the GaAs dice did not crack. Shear test has also been performed. All the well bonded specimens passed the shear test. The shear strength correlated very well with the SAM images of the specimens taken before the test.

A fluxless bonding technology using indium-silver multilayer composites

A fluxless bonding process has been developed using indium-silver multilayer composites deposited on silicon and GaAs wafers in one high vacuum cycle to inhibit the oxidation of the bonding media. The in situ formation of AgIn/sub 2/ intermetallic outer layer protects the inner media from oxidation when exposed to atmosphere. The bonding process is performed at 180/spl deg/C temperature in inert environment to prevent oxygen from getting into the specimens. High quality joints are produced as confirmed by a scanning acoustic microscope. The joints are very uniform with a thickness of 4 /spl mu/m. Scanning electron microscopy (SEM) evaluations reveal that the joint is composed of indium matrix with embedded intermetallic grains. Neither flux nor scrubbing motion is used in the bonding process. The process should be valuable in manufacturing applications where the use of flux cannot be tolerated.

Noncollinear coplanar magneto-optic interaction of guided optical wave and magnetostatic surface waves in yttrium iron garnet-gadolinium gallium garnet waveguides

Wideband noncollinear coplanar guided‐wave magneto‐optic diffraction and mode conversion by magnetostatic surface waves at multigigahertz (3–7 GHz) carrier frequencies in yttrium iron garnet‐gadolinium gallium garnet waveguides have been observed and measured in detail for the first time. Interaction configuration, physical mechanisms, summary of a theoretical treatment, and some experimental results obtained at 1.152 μm optical wavelength are presented. This noncollinear coplanar magneto‐optic interaction configuration should result in a number of integrated optic devices for wideband communications and signal processing applications at electronically tunable microwave carrier frequencies. The potential advantages of the resulting magneto‐optic devices over the existing acousto‐optic devices are also discussed.

Design and construction of a compact vacuum furnace for scientific research

The design, construction, and measurement of a compact vacuum furnace are reported. This type of furnace has many scientific applications in material processing and growth research. One example is the fluxless bonding process, where elevated temperature is needed to melt the solder and vacuum environment is required to inhibit solder oxidation. The primary objective of the furnace design is to keep the vacuum enclosure cool using only natural convection while allowing the heating platform to reach high temperature. This characteristic is necessary to enable us to seal the vacuum chamber using O-rings. To achieve this, the platform was designed to be thermally isolated from the chamber enclosure. Heat losses from the platform by conduction, convection, and radiation were analyzed. The dominating loss was found to be caused by the blackbody radiation, which can thus be used to estimate the relationship between platform temperature and the drive power needed. With a graphite platform of 75×75×25mm3, only 270...

Temperature measurement of visible light-emitting diodes using nematic liquid crystal thermography with laser illumination

In this letter, we present a new configuration with laser illumination to measure the temperature of visible light-emitting diode (LED) chips using nematic liquid crystals. This method is applied to measuring the junction temperature of multiquantum well (MQW) LEDs in InGaN-GaN-sapphire structure. A color filter is inserted in the optical path to attenuate the overwhelming LED light. A high-power laser beam is used as the sensing light to enhance the contrast of the thermal image on LED chips. This technique is nondestructive and can be performed in real-time during device operation. One objective is to investigate the effect of the junction temperature on the electrical and optical performance of the LED devices. For the LEDs measured, the conversion efficiency decreases by 67% when the junction temperature rises from 22/spl deg/C to 107/spl deg/C. The new measurement configuration is a valuable tool to study the thermal performance of GaN-based LED devices and subsequently to investigate the degradation on electrical and optical performance due to junction temperature increase.