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We report the growth and characterization of ferromagnetic iron/silver (Ag/Fe) multilayer structures on gallium arsenide (GaAs) (100) substrates by molecular beam epitaxy. The samples have been characterized by x-ray diffraction and ferromagnetic resonance (FMR) techniques. X-ray read-camera studies have demonstrated the epitaxial nature of crystalline Fe film grown on GaAs substrate, while x-ray rocking curve measurements reveal a narrow linewidth of only 0.3°. On the FMR technique, we focused specifically on the information contained in FMR linewidth ΔH(f). Experimental results on ultrathin Fe/Ag films have been obtained with sufficient resolution to measure the linewidth broadening due to intrinsic damping. FMR linewidth is resolved to vary between 0.5 and 1.1 GHz in a frequency range of 20 GHz. Frequency-dependent FMR linewidth ΔH[sub pp](ω) shows a typical linear dependence with a zero frequency offset ΔH(0) of almost zero value, indicating a near-perfect crystalline quality of the deposited magnetic film. The Gilbert damping parameter G is extracted to be 1.45 x 10[sub 8] s[sup -1] from the slope of the curve. [ABSTRACT FROM AUTHOR]

Abstract The high thermal conductivity, lightweight, and low cost of aluminum (Al) make it a promising substrate material for high power electronic and photonic packages and housings. It is particularly attractive for aerospace and space applications due to its lightweight. A main challenge for these applications is poor bondability. The native aluminum oxide (Al 2 O 3) prevents aluminum from bonding by using popular die-attach materials such as solders. Zincating process is often needed to dissolve the Al 2 O 3 layer and deposit a protection zinc layer which provides a basis for subsequent metallization or soldering process. However, the zincating and metallization processes could increase the processing cost and bring more reliability issues. In this research, a novel Ag foil bonding technique has been developed to bond Ag foils directly to Al substrates to produce Ag-cladded Al substrates. Two Ag-Al bonding processes are developed: solid-state and eutectic. Subsequently, Si chips are bonded to the Ag-cladded Al substrates using solid-state process at 300 °C without any additional die-attach material. For the Ag-Al bonding processes, no surface treatment is applied to Al substrates to remove the native Al 2 O 3 layer. In the Ag-Al soli-state bonding process, Ag and Al atoms inter-diffused through the thin Al 2 O 3 to react and form Ag 2 Al and Ag 3 Al compounds. In the Ag-Al eutectic bonding process, Ag 2 Al+(Al) eutectic structure forms at the Ag/Al interface without Ag 3 Al compound formation. The native Al 2 O 3 layer, a potential fracture path, is broken into pieces during eutectic reaction and possibly dispersed into the eutectic structure. Shear test results of Si/Ag/Al joint samples far exceed the military criterion (MIL-STD-883H method 2019.8). The Si/Ag/Al structures break either along the Ag/Al interface or within the Si chip. With the advantages of high thermal conductivity, high reliability, lightweight, and process simplicity, the Ag-cladded Al structures should be highly valuable for applications in packages and housings where lightweight and high heat-conducting are necessary. Highlights • A new Ag foil bonding technique has been developed to bond Ag foil directly to Al. • Two Ag-Al bonding processes are developed: solid-state and eutectic reaction. • The breaking force of the Si/Ag/Al joint samples far exceed the military criterion. • The fracture mechanisms of the Ag-Al solid-state/eutectic bondings are studied. • The Ag-Al eutectic bonding exhibits a mixed fracture mode with ductile features. [ABSTRACT FROM AUTHOR]

Focuses on a study which proposed an image method from which the thermal profiles of the finite rectangular structure can be thermally represented exactly by the profile of the corresponding infinite plate structure. Basis of the image method; Advantages of using the method; Application of the method in integrated circuit devices.

Examines thermal properties of the multilayer infinite plate structure. Information on the temperature distribution and boundary conditions of the structure; Application of the model to integrated circuits; Temperature dependence of thermal conductivity.

The high thermal conductivity, light weight, and low cost of aluminum (Al) make it a promising substrate material for high power electronic packaging. A main challenge of using aluminum in electronic packaging is its poor bondability. The native aluminum oxide (Al 2 O 3 ) prevents aluminum from bonding to commonly used die-attach materials such as solders. Thus, zincating process is often needed to dissolve the Al 2 O 3 layer and deposit a protective zinc layer which provides a basis for subsequent metallization or soldering processes. In this research, Ag-Al solid-state bonding has been developed as a novel bonding technique to bond Ag directly to Al substrates. No surface treatment was applied on Al substrates to remove the native Al 2 O 3 layer prior to bonding. The shear strength of Ag-Al joints passes the military criterion (MIL-STD-883H method 2019.8) by a large margin. SEM and TEM imaging was utilized to study the microstructures. In the bonding processes conducted at 425 and 450 °C, Ag and Al atoms inter-diffused through the thin Al 2 O 3 to react and form Ag 2 Al and Ag 3 Al compounds. To examine the fracture modes of Ag-Al joints, the fracture surfaces after shear tests were evaluated. The effect of bonding temperatures on Ag-Al joint morphology and the fracture behaviors were investigated and discussed. An application of this new technique is to bond thin Ag foils to Al substrates and make them bondable to die-attach materials such as solders and nano-silver paste. This Ag foil bonding method provides an alternative to the zincating process. Other potential applications include making Al surfaces easier to blaze to other metals such as brass, bronze and copper. [ABSTRACT FROM AUTHOR]

A solid-state bonding technique using fine-grained silver (Ag) foils is presented. The Ag foils are manufactured using many runs of cold rolling and subsequent annealing processes to achieve the favorable microstructure. X-ray diffraction and pole figure measurement are performed to examine the crystal structure and grain orientations. Si chips are bonded to bare Cu substrates using the Ag foil as the bonding medium at 300 °C in 0.1 torr vacuum assisted by 6.9 MPa static pressure, which is much lower than that used in conventional thermal compression bonding. Cross sections prepared by focus ion beam show clear bonding interfaces with only a few voids smaller than 100 nm. The bonded structures do not crack after cooling down to room temperature, indicating that the ductile Ag layer is able to manage the strain induced by the large coefficient of thermal expansion mismatch between Si and Cu. The average shear strength of as-bonded samples is 29 MPa. High-temperature storage tests are conducted, and slight increase in strength can be observed after 300 °C aging. Fracture analyses show that the breakage occurs within the Ag foil rather than on the bonding interface. Transmission electron microscopy and energy-dispersive spectroscopy (TEM/EDX) are conducted for Ag/Cu interface after 200-h aging, and the result shows that slight diffusion proceeds during the aging. Since Ag has the highest electrical and thermal conductivities among metals, therefore the bonded structures reported in this paper probably represent the best possible design for high-temperature and high-power electronic packaging applications. [ABSTRACT FROM AUTHOR]

Both copper (Cu) and silver (Ag) have been studied extensively for electrical conductor applications. To expand the applications, two bonding designs were implemented in this paper. For the first design, a 50- \mu \textm Ag layer was annealed at 400 °C to increase Ag grain size, thereby making it more ductile. For the second design, a 5- \mu \textm Ag layer with cavities was created to release the thermal induced stress and allow easier plastic deformation for the bonding medium. For both designs, the resulting Cu substrates were bonded to Cu chips by the solid-state bonding process. The cross sections of bonded samples show no visible voids or gaps on Cu/Ag and Ag/Cu interfaces. The breaking forces of ten samples tested far exceed the requirement specified in the military standards MIL-STD-883J Method 2019.9. The successful demonstrations of solid-state bonding between Ag and Cu should open the possibility of bringing alternatives for bonding methods in the field of electronic packaging. [ABSTRACT FROM PUBLISHER]

The growth of homogeneous silver solid solution phase with zinc are conducted at two different compositions. X-ray diffraction (XRD) and Scanning electron microscope/Energy dispersive X-ray spectroscopy (SEM/EDX) are carried out for phase identification and chemical composition verification. The mechanical properties of silver solid solution phase with zinc are evaluated by tensile test. The engineering and true stress vs. strain curves are presented and analyzed, with those of pure silver in comparison. According to the experimental results, silver solid solution phase with zinc at both compositions show tempered yield strength, high tensile strength and large uniform strain compared to those of pure silver. Fractography further confirmed the superior ductility of silver solid solution phase with zinc at both compositions. Our preliminary but encouraging results may pave the way for the silver based alloys to be applied in industries such as electronic packaging and structure engineering. [ABSTRACT FROM AUTHOR]

Silver solid solution phase with indium has been discovered to have great mechanical properties and anti-tarnishing property, as shown in the results of our previous study. It is important to know the stress vs. strain curve before adopting this material in industrial applications. The growth of the homogeneous silver solid solution phase with indium is first described. The X-ray diffraction (XRD) patterns and scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDX) results are reported to verify the chemical composition of silver solid solution phase with indium samples. Based on the results, one could reversely determine the indium element composition in (Ag)–xxIn solid solution by examining the lattice constant value using XRD for unknown compositions. The preparation of ASTM tensile test samples and tensile test experimental setting are explained in details. The intrinsic mechanical material properties of silver solid solution phase with indium, i.e., characteristic stress vs. strain curves, are presented and analyzed, with pure silver stress vs. strain curve in comparison. According to the experimental results, silver solid solution phase with indium exhibits low yield strength, high ultimate tensile strength, and large elongation value before fracture, compared to pure silver. In addition, fractography of the fracture surface of the tested sample has been studied to confirm the superior ductility of silver solid solution phase with indium. These superior mechanical properties may bring silver solid solution phase with indium new applications in various industries such as electronics and brazing. [ABSTRACT FROM AUTHOR]

Soldering is a process that joins two metal parts with a low-melting-point metal or alloy, called solder. It is widely recognized that intermetallic compound (IMC) formation is essential for the solder to bond to metals. The IMC layer grows during soldering. It grows further during usage, in particular, at temperature near the solder melting temperature. Extensive IMC growth can result in reliability issues. A reason is that IMC is much less ductile than the solder and does not deform as much as the solder to manage the coefficient of thermal expansion mismatch between the parts joined. In this paper, we looked into and developed a soldering process to fabricate IMC-less solder joints. The fundamental concept is to choose a barrier metal that can bond to solder without forming IMC with the solder. With our experience and Cr–Sn phase diagram, Cr is selected as the barrier metal. Several bonding experiments have been performed and the preliminary results show that high quality Sn joints could be produced with a little IMC formation before high-temperature aging. After aging at homologous temperature of 0.9 of Sn for 100–200 h, Cu6Sn5 and Cu3 Sn IMCs are observed. They are caused by the penetration of molten Sn through microcracks or pinholes on the Cr layer to react with the underlying Cu. In production environment, the density of microcracks and pinholes can be greatly reduced to inhibit IMC growth, making the realization of solder joints without IMC a possibility. [ABSTRACT FROM PUBLISHER]

A fluxless bonding process was developed between silicon chips and aluminum substrates using Ag–In binary system. Results indicate that the Ag–In system can manage the large mismatch between the coefficient of thermal expansion (CTE) of Si chips (2.7\times 10^-6/^\circC) and Al substrates (23\times 10^-6/^\circC). The bonding structures are Si/Cr/Au/Ag and Al/Cr/Cu/Ag/In/Ag. Cross-section SEM images exhibit nearly perfect joints between the Si chips and Al substrates. Energy dispersive X-ray analysis shows that the joint consists of Ag/(Ag)/Ag2In/(Ag)/Ag, where (Ag) is a solid solution phase. The joint can achieve a melting temperature of 650^\circC at least, even though the bonding is performed at 180^\circC. Six samples are shear tested. The shear strengths obtained far exceed the requirement specified in MIL-STD-883H standards. Al is not considered as a favorable substrate material because it is not solderable and has a high CTE. The new method presented in this paper seems to have surmounted these two challenges. [ABSTRACT FROM PUBLISHER]

Abstract: We report formation of silver (Ag) micro-joints for flip-chip interconnect. For demonstration, the micro-joints were made between silicon (Si) chips and copper (Cu) substrates. Two different Ag joint sizes, 40 and 15μm, respectively, were produced. In experiment, arrays of cylindrical cavities were fabricated in thick photoresist on 2-in. Si wafers that were coated with chromium (Cr) and gold (Au) dual layer. The Si wafers are diced into 6mm×6mm chips. The cavities were filled up with pure Ag by electroplating process. The photoresist was then stripped off. The Si chip with Ag columns was bonded to Cu substrate at 260°C in 80mTorr vacuum. During bonding, Ag columns deform and their surfaces conform to and mate with the surface of Cu substrate. Solid state bonding can incur when the Ag atoms and the Cu atoms are brought within atomic distance so that they share electrons. The Ag columns were well bonded to Cu. The ductile Ag joints are able to accommodate the thermal expansion mismatch between Si and Cu. The Ag joints do not contain any intermetallic compound (IMC). Accordingly, this interconnect technology avoids all reliability issues associated with IMC growth in solder-based flip-chip joints. Compared to solders, Ag joints have superior electrical and thermal properties. Pull test results show that the strength of 40μm Ag joints passed MIL-STD-883H by wide margin. [Copyright &y& Elsevier]

A solid state bonding technique was designed and developed to bond silicon (Si) chips to silver (Ag) buffer layer electroplated on copper (Cu) substrates at a temperature (260^\circC) far below the melting temperature of Ag (961^\circC). This is possible because Ag atoms in Ag buffer are brought in contact with the gold (Au) atoms in Au layer coated on Si chip. The close proximity in atomic scale allows the Ag and Au atoms to see each other, to share electrons and to bond to each other. The possibility of mating between Au and Ag surfaces relies on the deformation of Ag. Ag, with proper annealing, is ductile and has low yield strength. Its surface can thus deform to conform to and to follow the surface of Au layer coated on Si chip. Its ductile property can also help accommodate large mismatch in coefficient of thermal expansion between Si (2.7\times 10^-6~^\circC^-1) and Cu (17\times 10^-6~^\circC^-1). Ag also has the highest electrical and thermal conductivities among all metals. It is the dream bonding medium. The challenge is to bond Ag to other materials at a temperature compatible to electronic packaging processes, 260^\circC. In this research, Si chips are coated with thin chromium (Cr) and Au layers and then bonded to Ag buffer plated on Cu substrates. To make Ag buffer ductile, Ag buffer together with Cu substrate is annealed to increase its grain size by a factor of 25. The bonding was performed at 260^\circC with 1000 psi (6.89 MPa) static pressure. There is no molten phase involved in the bonding process. Scanning electron microscope evaluations show that on the revealed cross section no voids or gaps are observed on Si/Ag and Ag/Cu bonding interfaces. The bonded structure consists of only Si/Cr/Au/Ag/Cu and can sustain at high operating temperature due to high melting temperature of Ag. [ABSTRACT FROM PUBLISHER]

Abstract: Silver epoxy was selected to bond transducer plates on glass substrates. The properties and thickness of the bonding medium affect the electrical input impedance of the transducer. Thus, the thickness of the silver epoxy bonding layer was used as a design parameter to optimize the structure for the transducer input impedance to match the 50Ω output impedance of most radio frequency (RF) generators. Simulation and experimental results show that nearly perfect matching is achieved without using any matching circuit. At the matching condition, the transducer operates at a frequency band a little bit below the half-wavelength resonant frequency of the piezoelectric plate. In experiments, lead titanate (PT) piezoelectric plates were employed. Both full-size, 11.5mm×2mm×0.4mm, and half-size, 5.75mm×2mm×0.4mm, can be well matched using optimal silver epoxy thickness. The transducer assemblies demonstrate high efficiency. The conversion loss from electrical power to acoustic power in soda-lime glass is 4.3dB. This loss is low considering the fact that the transducers operate at off-resonance by 12%. With proper choice of silver epoxy thickness, the transducer can be matched at the fundamental, the 3rd and 5th harmonic frequencies. This leads to the possible realization of triple-band transducers. Reliability was assessed with thermal cycling test according to Telcordia GR-468-Core recommendation. Of the 30 transducer assemblies tested, none broke until 2900 cycles and 27 have sustained beyond 4050 cycles. [Copyright &y& Elsevier]

Pure silver (Ag) is used as a bonding medium to bond silicon (Si) chips to alumina substrates. The bonding process is performed at 260^\circC, which is a typical peak reflow temperature of Sn3.5Ag solders used in electronic industries. A static pressure of 1000 psi (6.9 MPa) is applied. This is a solid-state bonding process without any molten phase involved. The Ag foil sandwiched between the Si chip and the alumina substrate is ductile enough to deform and mate with the gold (Au) layers coated, respectively, on the Si chip and on the alumina substrate. Ag and Au atoms on both sides of the bonding interfaces are brought within atomic distance, and bonding is thus achieved. The ductile Ag joints can accommodate significant mismatch in coefficient of thermal expansion between Si and alumina. Scanning electron microscopy evaluations show that nearly perfect joints are achieved and no voids are observed. A standard shear test is performed to assess the bonding strength. The shear strength measured far exceeds the requirement specified in MIL-STD-883G standard. [ABSTRACT FROM PUBLISHER]

Bismuth telluride (Bi2Te3) and its alloys are the most commonly used materials for thermoelectric devices. In this paper, the fluxless bonding process was developed to bond Bi2Te3 chips to alumina substrates for high temperature applications. The silver–indium (Ag–In) system was chosen for the process development. To work with this system, the Bi2Te3 chips were coated with 100 nm titanium (Ti) and 100 nm gold (Au) as barrier layer and plated with 10 \mum Ag layer. The Bi2Te3 samples were annealed at 250^\circC for 200 h. No interdiffusion between Bi2Te3 and Ag was detected. The Ti/Au barrier layer was not affected either. It showed exceptional step coverage on the rough Bi2Te3 surface even after the annealing process. To prepare for bonding, alumina substrates with 40 nm TiW and 2.5 \mum Au were plated with 60 \mum Ag, followed by 5 \mum In and thin Ag cap layer for oxidation prevention. The Bi2Te3 chips were bonded to alumina substrates at 180^\circC. No flux was used. The resulting void-free joint consists of five regions: Ag, (Ag), Ag2In, (Ag) and Ag. (Ag) is Ag-rich solid solution. The joint has a melting temperature higher than 660^\circC. Due to the thick ductile Ag layer on alumina, the Bi2Te3 chip did not break after bonding despite its significant coefficient of thermal expansion mismatch with alumina. [ABSTRACT FROM PUBLISHER]

Acoustic wedge transducer assemblies (AWTA) for generating and detecting surface acoustic waves (SAW) on non-piezoelectric substrates were designed, fabricated and evaluated for outdoor touch panel applications. An AWTA consists of a piezoelectric acoustic transducer plate bonded to an acoustic prism that is bonded to a glass panel. The acoustic prism converts a longitudinal acoustic wave into a SAW on the glass. This conversion is based on the acoustic analogy of optics Snell's law. Thus, the prism material must have low longitudinal acoustic velocity and low acoustic attenuation. For nearly all SAW-based touch panels on the market, the acoustic prisms are made of Lucite. An issue with Lucite is high coefficient of thermal expansion (CTE), much higher than that of glass panels and piezoelectric ceramics. In outdoor environments with large temperature variations, this large CTE mismatch causes the Lucite prism to break off the glass panel and the piezoelectric transducer to break apart from the Lucite prism. After extensive search, we identified liquid crystal polymer (LCP) and bismuth (Bi) as potential acoustic prism materials having low CTE. Wedge transducer assemblies with Lucite, LCP, and Bi prisms, were fabricated and evaluated. Following Telcordia GR-468-Core recommendation, the wedge assemblies went through thermal cycling between -45^\circC and 85^\circC for 1000 cycles. The wedge assemblies with Lucite prism broke before 50 cycles. Wedge assemblies with LCP and Bi prisms survived 1000 cycles. In terms of acoustic performance, wedge assembly with LCP prisms is as good as wedge assembly with Lucite prism. Accordingly, LCP is the best prism material for SAW-based touch panels in outdoor applications. [ABSTRACT FROM PUBLISHER]

In this paper, silicon (Si) chips were bonded to 304 stainless steel (304 SS) substrates using silver–indium (Ag–In) binary system without any use of flux. 304 SS substrates were also bonded to 304 SS substrates to develop a low-temperature fluxless process to seal two 304 SS parts together. In the bonding design, Ag and In were deposited separately in a layered structure. Various processes and solutions were experimented to plate Ag on 304 SS. So far, we have not found a process that could plate Ag directly on 304 SS without an intermediate layer. At present, the most successful intermediate layer seems to be nickel (Ni). Thus, Ni was plated on 304 SS, followed by Ag. The resulting 304 304 SS substrates were annealed to increase Ag grain size if grain growth is needed for successful bonding. Nearly joints were produced on Si to 304 SS bonding and 304 SS to 304 SS bonding. The resulting joints consist of Ag, Ag-rich solid solution (Ag), Ag3In, and Ag2In. The joints were fabricated at only 190^\circC. Yet, the resulting joints exceed 660^\circC in melting temperature. This new bonding technique should be valuable for packaging electronic devices that need a high operating temperature. It is also useful for bonding 340 SS parts together at low temperature. [ABSTRACT FROM PUBLISHER]

In this paper, a silicon photomultiplier (SiPM) array module is evaluated and implemented as the receiver of laser rangefinding experiments to simultaneously measure the range of multiple targets. The array consists of 4 \times 4 SiPMs on a single chip. Each SiPM is followed by a transimpedance amplifier to have an adequate output signal voltage. The 4 \times 4 array is capable of measuring the range of 16 targets. A SiPM is an avalanche photodiode that operates in Geiger mode, where a huge multiplication factor, as high as 1 000 000, is obtainable. Thus, it can achieve high sensitivity. Our performance evaluation results show that the sensitivity of the measured SiPMs actually approaches the quantum limit. The SiPM can detect one single photoelectron in the depletion region. Each SiPM has a large active area, i.e., 8 \mm^2, and can collect much more returned light compared with typical silicon avalanche photodiodes (Si-APDs). The rise time of output voltage waveforms from SiPMs varies from 6 to 300 ns, depending on incident light power. The rising edges of output waveforms are compared for time measurement. Using a digital oscilloscope, the time of flight can be measured with 1-ns accuracy. This corresponds to a range resolution of 15 cm. [ABSTRACT FROM PUBLISHER]

In this paper, pure silver (Ag) joints between silicon (Si) chips and copper (Cu) substrates are produced successfully at a temperature much lower than its melting point. Silver is ductile and has low-yield strength. It can deform to release the shear stress caused by the large mismatch in coefficient of thermal expansion between Si and Cu. Silver also has the highest electrical conductivity and thermal conductivity among metals. As a bonding medium and interconnect material, it can provide the best electrical and thermal performances. In experiments, Ag in the form of foil is chosen as the bonding medium. Prior to bonding, the Si chips are coated with thin Cr and Au layers. The Si chip, Ag foil, and Cu substrate are bonded together in one step. The bonding process is conducted at 250 °C in 50-mtorr vacuum environment. There is no molten phase involved during the bonding process. The resulting joints exhibit nearly perfect quality. No voids are observed at the Si/Ag and Ag/Cu bonding interfaces. The bonding strengths at these two bonding interfaces pass MIL-STD-883G standards. We believe that the bonds at the Au/Ag and Ag/Cu interfaces are formed by shortrange interdiffusion. Since the melting point of Ag is 961 °C, the Ag joints are expected to sustain high-temperature. The 250 °C bonding temperature is the typical reflow temperature of Sn3.5Ag solders used in electronic industries. This novel bonding process can be applied to various electronic devices that require high-thermal performance or high-operation temperature. It is particularly valuable to packaging high-temperature devices. [ABSTRACT FROM AUTHOR]

This article extends the findings from the Resources for Enhancing Alzheimer’s Caregiver Health (REACH II) program, a multisite randomized clinical trial of a multicomponent psychosocial intervention, to improve the well-being of informal caregivers (CGs) of persons with dementia. We used residual change scores and stepwise hierarchical regression analyses to explore separately in 3 racial ethnic groups (Hispanic or Latino, Black or African American, and White or Caucasian) how the effects of the intervention were moderated by CG characteristics (sex, age, education, and relationship), CG resources (social support), and religious coping. The results indicated that CG’s age and religious coping moderated the effects of the intervention for Hispanics and Blacks. The older Hispanic and Black CGs who received the intervention reported a decrease in CG burden from baseline to follow-up. Black CGs with less religious coping who received the intervention also reported a decrease in depressive symptoms from baseline to follow-up. [ABSTRACT FROM PUBLISHER]

We report a new bandpass filter on coplanar strip (CPS) lines for RF and microwave applications. The filter is implemented as electrode topology on the CPS electrodes. The topology consists of basic electrode patterns that emulate inductors and capacitors in a two-port filter section. In the design process, equivalent circuit model is used to identify the initial topology, which is then fine tuned using full wave analysis. The bandpass filter is very compact in size and planar in structure. It does not use quarter-wavelength sections, vias, or bonding wires. It can be fabricated without incurring additional cost. Bandpass filters for the 2.4 GHz ISM (Industrial, Scientific, and Medical) band were designed, fabricated, and measured. The measurement responses agree well with the full-wave simulation results. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2008. [ABSTRACT FROM AUTHOR]

We report the characterization of coplanar strip (CPS) transmission lines on dielectric boards at millimeter-wave frequency up to 40 GHz. The CPS line was analyzed and simulated using computer-aided design tools. Many CPS lines were built on RT/Duroid board and the measured attenuation is as low as 0.8 dB/cm at 30 GHz. CPS lines of various electrode width and width-to-gap ratios were measured to obtain key design guidelines. The CPS line has both the signal and ground electrodes on the front surface of the board. In contrast to the microstrip line, the CSP line does not include the backside of the board as a part of the transmission line. Thus, the backside can be left open as dielectric or metallized. The backside condition can affect the wave propagation if it is too close to the electrodes. We simulated and measured the attenuation coefficient, line impedance, and electrical field distributions for different backside conditions. Our results clearly indicate that the CPS line is an attractive transmission line on the board level in millimeter electromagnetic waves applications. Its applications can be extended to microwave and millimeter-wave integrated circuits. © 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2008. [ABSTRACT FROM AUTHOR]

Abstract: A fluxless bonding process in vacuum environment using newly developed electroplated Sn–Ag multilayer structure at eutectic composition is presented. The new bonding process is entirely fluxless, or flux-free. It is performed in vacuum (100mTorr), in which the oxygen content is reduced by a factor of 7600 comparing to air, to inhibit solder oxidation. In the design, Cr/Au dual layer is employed as the UBM as well as the plating seed layer. This UBM design, seldom used in the electronic industry, is explained in some details. To realize the fluxless possibility, a proper layer design of the solder structure is needed. In this connection, we wish to point out that it is hard to achieve fluxless bonding using Sn-rich alloys because these alloys have numerous Sn atoms on the surface that are easily oxidized. To prevent Sn oxidation, a thin Ag layer is plated immediately over Sn layer. XRD results confirm that this thin Ag layer does act as a barrier to prevent oxidation of the inner Sn layer. The resulting solder joints are void free as examined by a scanning acoustic microscope (SAM). SEM and EDX studies on the cross section of the joint indicate a homogeneous Sn-rich phase. The melting temperature is measured to be between 219 and 226°C. This new fluxless bonding process is valuable in many applications where the use of flux is prohibited. [Copyright &y& Elsevier]

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 75×75×25 mm3, only 270 W of power is needed to drive the platform to 400 °C. At this temperature, the temperature of the furnace enclosure is below 55 °C, allowing O-rings to be used to seal the vacuum chamber. Using a mechanical pump, the furnace can be pumped down to 40 mTorr, which is low enough for our fluxless bonding processes. With a temperature controller, the platform temperature can be controlled within 1%. The heat-up time to 400 °C is only 7 min. [ABSTRACT FROM AUTHOR]

Abstract: This letter reports a new current versus voltage model for light-emitting devices with a quantum well where electrons and holes are injected and recombine. The current is entirely caused by the recombination of electrons and holes. Historically, the equation used for light-emitting diodes (LEDs) and laser diodes (LDs) has been the renowned Sah–Noyce–Shockley (SNS) diode equation. In this equation at typical forward bias condition, most of the current is caused by the diffusion of carriers over the depletion region. It is clear that this condition is different from what actually happen in LEDs and LDs. We thus looked into the fundamental of carrier transport and developed a new model for devices with a quantum well. Based on the new model, calculated I–V curves agree well with measurement results of GaN/sapphire LEDs with GaInN quantum wells. In calculation, junction temperature T j rather than case temperature T c is used to achieve better agreement. [Copyright &y& Elsevier]

Abstract: We report our initial result of bonding two 2in. silicon wafers using Sn-rich Sn–Au dual-layer structure that is produced by electroplating process. No flux is used in the bonding process. Comparing to Au-rich Au80Sn20 eutectic alloy, it is more difficult to achieve fluxless feature using Sn-rich Sn–Au alloys due to tin oxidation. In this initial effort, two samples are produced. The resulting Sn-rich solder joint layer, about 30μm in thickness, is very uniform over the entire 2-in. sample. The quality of the joint is examined using scanning acoustic microscope (C-SAM) and X-ray micro-imaging technique. Images of these two techniques indicate that the joints are of high quality. Of the two samples, the better one shows nearly perfect joint with only 2% of possible void area. This initial success shows that it is indeed possible to bond entire wafers together with a thin metallic joint of high quality. This fluxless technique can be extended to bonding wafers of different materials for new device and packaging applications. The use of metallic alloy layers of high melting temperature is also possible and is being considered. [Copyright &y& Elsevier]

In this paper, a new configuration of nematic liquid crystal thermography (NLCT) that uses laser illumination to im- prove the contrast of thermal images on device chips is reported. The wavelength is selected so that the laser light is not absorbed by the device under measurement. The most important application of this new configuration is to measure the junction temperature of light-emitting devices (LEDs). Traditional NLCT cannot be used on LEDs, because the light emitted by the device overwhelms the light reflected from the nematic liquid crystal (NLC) layer coated on the device surface. Thus, the temperature information carried by the reflected light is lost. Using laser illumination and a color filter, this difficult measurement has been made a reality. This new configuration is extended to studying the thermal performance of heterojunction field effect transistors (HFETs) built on AIGaN/GaN structure grown on sapphire and SiC substrates, respectively. This technique is nondestructive and can be performed in real time during device operation. It has a submicrometer spatial resolution and a ±1 °C temperature accuracy. The thermal resistance of HFETs and LEDs has been measured with this method. The new measurement configuration is a valuable tool for studying the thermal performance of GaN-based as well as other semiconductor devices. It provides the device engineers and physicists additional information to improve device structures and performance. [ABSTRACT FROM AUTHOR]

Abstract: Initial success of new fluxless flip–chip interconnect technique is reported. Non-eutectic Sn-rich Sn bumps are used to connect thin silicon chip to glass substrate and Cu laminated polyimide film, respectively. Fluxless process development on tin-rich alloys is challenging because tin atoms get oxidized easily. The solder bumps of Snayer composite with overall composition of 95at.% Sn and 5at.% Au are fabricated by vacuum deposition on thin silicon wafers to prevent solder oxidation. The bonding is performed at 250°C in hydrogen environment without using any flux. The diameter of the bump is 200μm and height is 20μm. Nearly void-free solder joints with randomly distributed AuSn4 intermetallic compound (IMC) are produced using this fluxless bonding process. Continuous layers of Cu3Sn and Cu6Sn5 instead of commonly known scallop-type Cu6Sn5 are formed between laminated Cu layer and the Sn-rich non-eutectic Au. The re-melting temperature of solder joints is measured to be 220°C. The bonding process is entirely flux-free and thus is particularly valuable for many devices that cannot take any flux, such as biomedical, photonic, MEMS, and sensor devices. [Copyright &y& Elsevier]

A dry electromigration technique was developed to fabricate deep multimode channel and planar waveguides on BK7 optical glass. In contrast to earlier ion exchange processes based on thermal diffusion, a relatively high electric field (∼440 V/mm) was applied on the glass to accelerate the field-driven ion exchange process by expeditiously replacing host sodium ions in the glass with silver ions. As a result, the process temperature is significantly reduced, leading to lower optical attenuation on the waveguides. Lowest optical loss was achieved on waveguides fabricated at 320 °C. The increase on the index of refraction for the planar waveguide was determined to be 0.076 using the prism coupling method. The optical loss for channel waveguides was measured using the edge coupling technique with a 0.6328 μm He–Ne laser. Loss of 2 dB/cm was obtained for channel waveguides of 25 μm in depth, relatively low for waveguides of such depth at red wavelength. The scanning electron microscope (SEM) with energy-dispersive X-ray (EDX) spectroscope was utilized to obtain the concentration profile of silver ions in the waveguide region. A modified Huggins–Sun model with Gladstone–Dale relation was employed to deduce the refractive index profile from the silver ion concentration profile. A nearly step-like profile was observed from every deep multimode waveguide fabricated. [Copyright &y& Elsevier]

A fluxless bonding process to produce tin-rich bismuth–tin solder joints is presented. The process was designed using Sn–Bi multilayer structure deposited in high vacuum to inhibit oxidation and to achieve the fluxless feature. The composition was chosen to be tin-rich, 95.0 wt.% Sn and 5.0 wt.% Bi, to achieve a solidus temperature significantly higher the 139 °C eutectic temperature of the 57Bi–43Sn eutectuic alloy. The resulting joints were imaged with a scanning acoustic microscope (SAM) and shown to be nearly void-free. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy were used to study the microstructure and compositions of the joints. The measured joint thickness was 4.1 μm. SAM and EDX results indicate that the solder joint consists of (β-Sn) matrix embedded with small grains of (Bi) solid solution phase. The melting temperature of the joint was measured to be 210–215 °C, as expected from the designed composition. The fluxless feature of this process gives the packaging community another process to consider for use in devices and packages where the use of flux is prohibited, such as some biomedical, MEMs, fiber optical and photonic devices. [Copyright &y& Elsevier]

A new fluxless bonding technique using Sn–Bi–Au design is presented. Microstructure and phase growth mechanism of as deposited Au/Sn/Bi films were first studied by scanning electron microscopy (SEM), X-ray diffraction method (XRD) and X-ray photoelectron spectroscopy (XPS). It is found that the crystallites of Au–Sn intermetallics are randomly distributed in Bi-rich matrix that forms a top-capping layer. The average size of AuSn2 and AuSn4 crystallite are 756 and 667 A˚, respectively. The bonding process was performed at 250 °C in air without the use of flux. The resulting joints exhibit high shear strength of 4.9 kg and show only a few voids. Melting temperature of 225 °C was measured. The joint is composed of near eutectic Sn–Bi alloy with ribbon shaped AuSn2+AuSn4 intermetallic compound layer embedded inside the solder joint. The fracture of the Sn–Bi–Au solder joint mainly occurs along the interface between the Au–Sn intermetallic layer and the Sn-rich regions. This facture mode is ductile. The flux-free process is compatible with many pick-and-place machines that operate in ambient air. [Copyright &y& Elsevier]

Structure and phase growth mechanism of vacuum deposited Au/Sn/Bi thin films have been studied by X-ray diffraction (XRD) method, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). It is found that the crystallites of Au–Sn intermetallic compounds are randomly distributed in Bi-rich matrix that forms a top-capping layer. The average sizes of AuSn2 and AuSn4 crystallites are 756 and 667 A˚, respectively. The lattice constants of AuSn4 in the capping layer are determined to be a=6.3694, b=6.4092 and c=11.4785 A˚ by Cohen's analytical extrapolation method. The capping layer of Au–Sn intermetallic grains in Bi-rich matrix covers most surface area, thus minimizing the exposure of inner Sn atoms to air and effectively prevents oxidation of the resulting thin film structure. [Copyright &y& Elsevier]

Deals with fluxless bonding processes using two different noneutectic gold-tin multilayer composites to fabricate high quality solder joints. Gold-tin binary system; Fluxless bonding technology; Experimental results and discussion; Conclusion.

Device packages usually need more than one soldering operation to complete. For photonic and fiber optic device packaging, indium solder is often used due to its ductility. Since indium has a relatively low melting temperature of 156.6 °C, the subsequent bonding operation requires a process temperature that is lower than 156 °C. In this paper, we report a new bonding process at 140 °C based on an indium–tin multilayer composite. This is a fluxless bonding technique. In fabrication, a chromiurn–tin–indium–gold multilayer composite is deposited on silicon die in one high vacuum cycle to prevent oxidation. Immediately upon deposition, gold and indium react to form a stable AuIn2 protective outer layer against oxidation. Silicon substrates are deposited with thin chromium–gold layers. The silicon die and substrate are bonded in a hydrogen environment at 140 °C. Scanning acoustic microscopy (SAM) analysis is used to evaluate the joint quality. This bonding technique consistently achieves uniform and void-free joints. Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analyses also are performed on the joint cross sections. The SEM image shows a uniform joint thickness of 5 μm and the joint microstructure. SEM and EDX results indicate the joint consists of In–Sn alloy with embedded AuIn2 grains. The re-melting temperature of the joint is found to be 125–150 °C, which shows that the joint composition is not exactly eutectic that has a melting temperature of 118 °C, but rather is Sn-rich. The success of this design illustrates the solid-state interdiffusion between Sn and In to form a thin layer of Sn–In eutectic alloy. The fluxless feature of this technology is valuable for bonding and assembling many emerging photonic devices that simply cannot tolerate fluxes. [Copyright &y& Elsevier]

A new alternative solder joint made of a non-eutectic indium–tin (In–Sn) multilayer composite deposited in high vacuum is reported. The unique features of this design are that it is fluxless, oxidation-free, and more importantly the fabricated joint achieves a re-melting temperature significantly higher than the bonding temperature. The In–Sn non-eutectic multilayer structure with a thin gold film evaporated as a cap layer has a predominantly Sn-rich matrix with a composition of 6 wt.% Au, 14 wt.% In and 80 wt.% Sn. The quality of the joints was first evaluated using a combination of X-ray microfocus and scanning acoustic microscopy techniques, and results have shown that the joints are nearly void-free. In addition, analysis by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray (EDX) spectroscope performed on the joint cross-section clearly indicated a uniform joint thickness of 7.6 μm, while AuIn2 grains embedded in a heavily Sn-rich matrix were clearly detected. Lastly, we set out to determine the re-melting temperatures of these fabricated joints. Temperature values ranging from 175 to 190 °C were found, higher than the bonding temperature of 150 °C. The results clearly show that the joint composition is heavily Sn-rich, leading to the high re-melting temperature The increase in re-melting temperature opens up the post-processing temperature window of devices in manufacturing processes. No flux is needed during the bonding process, making it particularly useful to packaging devices for which the use of flux is strictly prohibited. [Copyright &y& Elsevier]

Presents information on a study which focused on the application of oxidation-free fluxless technology in developing indium-gold bonding process on copper substrates. Principle of flux action; Design principle and fabrication of joints; Experimental results and discussion; Conclusions.

Formulates the boundary element method to carry out a full wave analysis of microstrip lines. Effective dielectric constant; Problem formulation; Numerical procedure; Absolute value of determinant versus effective dielectric constant.

Presents information on a study which derived the analytical three-dimensional temperature solution of a five-layer anisotropic plate structure with infinite lateral boundaries and with embedded heat sources. Application of the double Fourier integration; General approaches to thermal analysis methods; Derivation of the temperature solutions; Boundary conditions of the structures.

The major adoptions of silver-based bonding wires and silver-sintering methods in the electronic packaging industry have incited the fundamental material properties research on the silver-based alloys. Recently, an abnormal phenomenon, namely, solid solution softening, was observed in stress vs. strain characterization of Ag-In solid solution. In this paper, the mechanical properties of additional concentrated silver solid solution phases with other solute elements, Al, Ga and Sn, have been experimentally determined, with their work hardening behaviors and the corresponding fractography further analyzed. Particularly, the concentrated Ag-Ga solid solution has been discovered to possess the best combination of mechanical properties, namely, lowest yield strength, highest ductility and highest strength, among the concentrated solid solutions of the current study. Microscopically, a clear evidence of narrow twin lamella feature has been observed in the Ag-Ga solid solution, and further identified as the deformation twinning, using high resolution transmission electron microscopy (HRTEM). To explain solid solution softening mechanism in low Peierls potential FCC metals, the authors have proposed a self-contained theoretical interpretation associated with fractional kink-pairs formation, and originally introduced the concept of the short range order (SRO) induced localized homologous temperature (LHT). Furthermore, the mechanism of twinning-induced plasticity (TWIP) can be referred as the underlying reason of the enhanced ductility in the Ag-Ga solid solution alloy. With such excellent mechanical properties, the Ag-Ga solid solution alloy is expected to have a great potential in the development of advanced joining materials for the various applications in electronics industries. [ABSTRACT FROM AUTHOR]

Microwave microstrip line technique is utilized to determine the magnetic properties of ultrathin iron (Fe) films. Wideband tunable microwave filters are fabricated, either in integrated or flip-chip type, on ultrathin Fe films grown on GaAs substrates using molecular beam epitaxy. Taking advantage of the ferromagnetic resonance effects of the Fe films, four-fold variation of the resonance frequency is found to relate directly to the tetragonal symmetry of in-plane anisotropy. As a result, effective magnetization 4πM[sub eff], four-fold in-plane anisotropy K[sub 1], second order term of perpendicular uniaxial anisotropy K[sub u⊥], and the Gilbert damping factor of the ultrathin films are derived from Landau–Lifshitz equation and transmission characteristics of the microwave filters involved. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper, aluminum circuit boards (ACBs) were designed, fabricated, and tested to demonstrate the possibility and advantages of the ACB technology. Processes were developed to grow high quality alumina (Al2O3) on Al boards and coat thick copper (Cu) layer over the alumina to produce an Al/alumina/Cu structure. The measured resistance and breakdown voltage of the as-formed 50 \mum alumina layer is >40~M\Omega and 600 VDC respectively. In this design, heat generated by a high power circuit component attached to the Cu layer can conduct through the alumina layer and reach the Al base. Alumina has much higher thermal conductivity than epoxy-glass insulating layer of the popular FR-4 printed circuit boards. The quality of the boards produced in this paper was evaluated rigorously using scanning electron microscope. To test the reliability of the boards, they were put through 500 cycles of thermal cycling test between -40^\circC to +85^\circC and 100 h of high temperature storage test at 250^\circC. To ensure its compatibility with soldering operations, 10 mm \times\,12 mm Cu substrates were bonded to the Al boards using a fluxless tin process. The thickness of the joint is 9.4 \mum including the intermetallic layers. Despite significant coefficient of thermal expansion mismatch of the structure and large Cu size, the bonded samples show no sign of cracks, breakage, or degradation. [ABSTRACT FROM AUTHOR]

Transient liquid phase (TLP) bonding technique is one of the promising solutions to third-generation wide-bandgap semiconductor power packaging. However, its prolonged heating process of the components restricts its application to some extent. To address this issue, the singular impact of acoustic cavitation induced by ultrasound emerges as a promising solution. For the first time, this study proposes the incorporation of ultrasound into the silver‑indium (Ag-In) TLP bonding process, enabling rapid and efficient bonding with 50 μm interlayers within seconds, while achieving a maximum shear strength of 60 MPa. A comprehensive comparative and analytical investigation of the interfacial microstructure, mechanical properties, and grain distribution has been carried out, in comparison to that of the conventional Ag-In TLP joint. The overall temperature profile was monitored utilizing a non-contact infrared camera and subsequently corroborated with finite element simulation outcomes. The phase analysis unveiled a predominant composition of the solid solution phase (Ag)-In within the bonding region, which contributes to a pronounced enhancement of the mechanical properties of the joints. Transmission electron microscopy (TEM) confirmed the complete degradation of the initial bonding interface, resulting in a bond with a bulk-like appearance. Finally, this work has elucidated the underlying mechanisms for the ultrasonic-enhanced transient liquid phase bonding (U-TLP) joint formation, where the influence of acoustic cavitation on element diffusion and the metallurgical bonding process has been discussed. [Display omitted] • The inaugural implementation of ultrasonic-enhanced bonding for silver‑indium. • With merely 2 s of ultrasonic enhancement, a compact Ag-In joint is achieved. • The impressive maximum strength of the nearly flawless joint reaches 60 MPa. • Ultrasonically-enhanced joints feature a dominant solid solution phase, not IMCs. [ABSTRACT FROM AUTHOR]

Flip-chip interconnect joints of copper/gold (Cu/Au) with 40-\mum diameter and 100-\mum pitch were made between silicon (Si) chips and Cu substrates using solid-state bonding at 200^\circC with a static pressure of 250–400 psi (1.7–2.7 MPa). The array of 50\,\times\,50 Cu/Au columns was first created. In fabrication, photoresist with 50\,\times\,50 cavities of 40-\mum diameter and 45-\mum depth were produced on Si wafers, which were first coated with 30 nm chromium and 100 nm Au films. Cu of 25-\mum thickness was electroplated in the cavities, followed by 10 \mum of Au. After stripping the photoresist, the array of 50\,\times\,50 Cu/Au columns was obtained on a chip region of the wafer. The 50\,\times\,50 Cu/Au columns on the chip were bonded to a Cu substrate by solid-state bonding. No molten phase was involved and no flux was used. No underfill was applied. The corresponding load for each column was only 0.22–0.35 g. Cross-section scanning electron microscopy images show that Cu/Au columns were well bonded to the Cu substrate. Despite the large mismatch in the coefficient of thermal expansion between Si and Cu, no joint breakage was observed. The pull test was performed and the fracture modes were evaluated. The fracture force and fracture strength obtained were 11.2–14.2 kg and 35–44 MPa (5000–6400 psi), respectively. The measured fracture force is four times larger than the criterion of the pull-off test in MIL-STD-883E. [ABSTRACT FROM PUBLISHER]

A flip-chip interconnect process at 180^\circC using the silver–indium (Ag–In) binary system is reported. An array of 50\,\times\,50 flip-chip joints with 100 \mum pitch and 40 \mum joint diameter was fabricated. Each joint has the column structure of Ag/(Ag)/Ag2In/(Ag) that connects the silicon (Si) chip to the copper (Cu) substrate. The joint height is approximately 50 \mum. In this structure, Ag2In is a dominating intermetallic compound in the Ag–In system with melting temperature of 660^\circC. (Ag) is a solid solution phase of Ag with In composition up to 20 at.%. It has a solidus temperature range of 695 to 962^\circC depending on In composition. In long-term operation, (Ag)/Ag2In/(Ag) is expected to gradually convert to a single (Ag) phase, which is more reliable. Thus, the flip-chip joints will get better in use. In fabrication, 50\,\times\,50 Ag columns were made on Si wafer coated with chromium (Cr) and gold (Au). The Cu substrate was electroplated with Ag(10~\mum)/In(5~\mum)/Ag(thin). Si chips with Ag columns were bonded to Cu substrates at 180^\circC for 5 min. No flux was used. Cross-sectional scanning electron microscopy images show that all 50 Ag columns in one row are well bonded to the Cu substrate without visible voids or cracks. Energy-dispersive X-ray spectroscopy data indicate that the resulting column structure is Ag/(Ag)/Ag2In/(Ag). The process temperature of this new interconnect method is 80^\circC below the typical reflow temperature of tin-based lead-free solders. [ABSTRACT FROM AUTHOR]

A fluxless bonding process is successfully developed between silicon (Si) chips and copper (Cu) substrates using the silver-indium (Ag-n) binary system. This is a new design concept that utilizes thick Ag plated over the Cu substrate to deal with the large mismatch in coefficient of thermal expansion between semiconductors, such as Si (3 ppm/°Cu (17 ppm/°C). The Ag layer actually becomes a part of the Ag-Cu substrate. Ag is chosen for the cladding because of its superior physical properties of ductility, high electrical conductivity, and high thermal conductivity. Following the thick Ag layer, 5µm In and 0.1 µm Ag layers are plated. The thin outer Ag layer inhibits oxidation of inner In. After many bonding experiments, we realize that the success of producing a joint relates to the microstructure of the Ag layer. Ag with small grains results in rapid growth of solid Ag2In intermetallic compounds through grain boundary diffusion. Thus, a joint is not obtained because of lack of molten phase (L). To coarsen Ag grains, an annealing step is added to the Ag-plated Cu substrate. This step makes Ag grains 200 times coarser compared to the as-plated Ag. The coarsened microstructure slows down the Ag2In growth. Consequently, the (L) phase stays at the molten state with sufficient time to react with the Ag layer on the Si chip to produce a joint. Nearly perfect joints are produced on Ag-plated Cu substrates. The resulting joints consist of pure Ag, Ag-rich solid solution, Ag2In, and Ag3In. The melting temperature exceeds 650 °C. Using the present process, high temperature joints of high thermal conductivity are made between Si chips and Cu substrates at low bonding temperature (200 °C). We foresee the Ag-In system as an important system to explore for various fluxless bonding applications in electronic packaging. This system provides the possibilities of producing joints of wide composition choices and wide melting temperature range. This paper provides preliminary but useful information on how the microstructure of Ag affects the bonding results. [ABSTRACT FROM AUTHOR]

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/° C) and Cu (17 ppm/° 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 15200. The resulting joints consist of three distinct layers, i.e., Sn-rich layer, Ni3 Sn4 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. [ABSTRACT FROM AUTHOR]

We present a fluxless bonding process between silicon and Ag-copper dual-layer substrate using electroplated indium/silver solder. The nucleation mechanism of In plated over Ag layer is first investigated. It is interesting to discover that In atoms react with underlying Ag to from AgIn2 compound layer during electroplating. A novel Ag laminating technique on Cu substrates is developed. The Ag cladding functions as a strain buffer to manage the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/°C) and Cu (17 ppm/°C) substrates. To bond Si chips to the Ag layer on copper substrates, In-based alloy (InAg) is used. A fluxless bonding process is developed between Si/Cr/Au/Ag and Cu/Ag/ln/Ag. The process is performed in 50-militorr vacuum to suppress solder oxidation. No flux is used. The resulting joints consist of three distinct layers of Ag, Ag2In and Ag. Microstructure and composition of the joints are examined using scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). Bonded samples are further annealed to convert the Ag2In phase into solid solution phase (Ag). The joint has a melting temperature above 850 °C. This technique presents our success in overcoming the very large CTE mismatch between silicon and copper. It can be applied to mounting numerous high-power silicon devices to Cu substrates for various industrial applications. [ABSTRACT FROM AUTHOR]

A fluxless process of bonding silicon to Ag-cladded copper using electroplated In-Ag multilayer structure is developed. The Ag cladding on the copper substrate is a stress buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/°C) and Cu (17 ppm/°C). To manufacture Ag on copper substrate, two techniques are developed. The first is an electroplating process to fabricate a thick Ag layer. The second technique is a novel laminating process that bonds Ag foil directly on Cu substrate. On Si chips, two underbump metallurgy (UBM) structures are designed, Si/Cr/Au and Si/Cr/Ni/Au. To produce a solder layer, Si chips are electroplated with In followed by thin Ag. The thin Ag cap layer prevents oxidation of the inner In region. To achieve a fluxless feature, the bonding process is performed in a vacuum environment (50 mtorrs) to suppress indium oxidation. Compared to bonding in air, the oxygen content is reduced by a factor of 15200. Using Cr/Au UBM structure, the silicon chip was detached from Cu substrate. The broken interface lies between Si/Cr and Ag2InIMC on Cu substrate. Using a new UBM design of Si/Cr/Ni/Au, high-quality joints are produced that comprise of three distinct layers of In7Ni3, Ag2In, and Ag. Microstructure and composition of the joints are studied using a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). [ABSTRACT FROM AUTHOR]

A new fluxless hermetic sealing technique using electroplated Sn-rich soft solder is reported. Specific glass (SCG72) is chosen as lid material to seal alumina packages. It has nominal coefficient of thermal expansion of 7 ppm/° C, close to that of alumina. Alternatively, sapphire can also be used as the lid material. A thick Sn layer is plated over the Cr/Au patterned glass wafer, followed immediately by thin Au layer. This outer Au layer prevents the inner Sn from oxidation. In bonding, the glass lid is placed over the package rim which has a metallization structure of W/Ni/Au. The fluxless sealing process is performed in vacuum (50 millitorrs) to suppress tin oxidation. Compared to bonding in air, the oxygen content is reduced by a factor of 15200. Fluxless bonding is valuable in many hermetic sealing applications such as microelectromechanical systems (MEMS), sensors, photonic devices, and imaging devices. Nearly void-free sealed joints are achieved with Sn-rich composition. Helium leakage tests are performed to evaluate hermetic quality. Scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) is used to evaluate the composition and microstructure of the sealing joints. The results show how the Sn-rich solder reacts with the W/Ni/Au on the package. This new sealing process can be applied to nearly all devices and packages that require hermetic sealing. [ABSTRACT FROM AUTHOR]