Thi Thuy Luu - Academia.edu (original) (raw)

Papers by Thi Thuy Luu

2018 7th Electronic System-Integration Technology Conference (ESTC), 2018

Solid-liquid interdiffusion (SLID) bonding is a technique based on intermetallic compounds (IMCs)... more Solid-liquid interdiffusion (SLID) bonding is a technique based on intermetallic compounds (IMCs), enabling a thermal stability at temperatures far surpassing the bonding temperature. The technique has been developed as a die attach and interconnection technology for high-temperature applications, but is also excellent for fine-pitch bonding, and for obtaining bonds with thin layers of well-defined metallurgy. Determining the phases of IMC in a SLID bond is crucial in order to understand and predict the properties of the bond. The re-melting temperature of the bond is defined by the IMCs present, and thus directly defines the high-temperature range the SLID bond can survive. Furthermore, the phases present in a SLID bond determines whether the bond is at thermal equilibrium, or if reactions to form new IMCs are expected over the lifetime of the SLID bond (at the actual application temperature). Also, material properties such as electrical conductivity and elastic modulus will depend...

The aim of this study is to characterize a die attach method suitable for harsh environment as we... more The aim of this study is to characterize a die attach method suitable for harsh environment as well as high reliability applications. Au-Au thermosonic bonding was selected. Due to high melting temperature of gold, this technique suited well for high temperature applications. In addition, thermosonic bonding introduces ultrasonic energy to soften the material joint and enhance the bonding at the metallic interface. This allows reducing the bond temperature and force. Initial characterization focuses on the effects of bonding parameters – ultrasonic energy, bond force and bond temperature – to relative bond strength. Bonded components were subjected to die shear test to measure bond strength. Cross section and SEM were used to inspect the bond interface and Aubump deformation. Thermal shock cycling (TSC) test performed from -20 to 200C was carried out to examine bond reliability. Experiment results were compared to thermocompression bonding to evaluate the improvement of thermosonic ...

2019 IEEE 69th Electronic Components and Technology Conference (ECTC), 2019

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service... more This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Intermetallic Compounds - Formation and Applications, 2018

Solid-liquid interdiffusion (SLID) bonding for microelectronics and microsystems is a bonding tec... more Solid-liquid interdiffusion (SLID) bonding for microelectronics and microsystems is a bonding technique relying on intermetallics. The high-melting temperature of intermetallics allows for system operation at far higher temperatures than what solder-bonded systems can do, while still using similar process temperatures as in common solder processes. Additional benefits of SLID bonding are possibilities of fine-pitch bonding, as well as thin-layer metallurgical bonding. Our group has worked on a number of SLID metal systems. We have optimized wafer-level Cu-Sn SLID bonding to become an industrially feasible process, and we have verified the reliability of Au-Sn SLID bonding in a thermally mismatched system, as well as determined the actual phases present in an Au-Sn SLID bond. We have demonstrated SLID bonding for very high temperatures (Ni-Sn, having intermetallics with melting points up to 1280°C), as well as SLID with low process temperatures (Au-In, processed at 180°C, and Au-In-Bi, processed at 90-115°C). We have verified experimentally the high-temperature stability for our systems, with quantified strength at temperatures up to 300°C for three of the systems: Cu-Sn, Au-Sn and Au-In.

Lab on a chip, Jan 17, 2017

In this paper, we propose a simple method to embed transparent reactive materials in a microfluid... more In this paper, we propose a simple method to embed transparent reactive materials in a microfluidic cell, and to observe in situ the dissolution of the material. As an example, we show how to obtain the dissolution rate of a calcite window of optical quality, dissolved in water and hydrochloric acid (HCl). These fluids circulate at controlled flowrates in a channel which is obtained by xurography: double sided tape is cut out with a cutter plotter and placed between the calcite window and a non-reactive support. While the calcite window reacts in contact with the acid, its topography is measured in situ every 10 s using an interference microscope, with a pixel resolution of 4.9 μm and a vertical resolution of 50 nm. In order to avoid inlet influence on the reaction, a thin layer of photoresist is added on the calcite surface at the inlet and outlet. This layer is also used as a non reactive reference surface.

2016 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2016

Solid-Liquid Interdiffusion (SLID) bonding has received substantial interest, primarily due to it... more Solid-Liquid Interdiffusion (SLID) bonding has received substantial interest, primarily due to its ability to withstand higher temperatures than the bonding temperature. Whereas SLID bonding typically is performed in the range of normal solder temperatures, the remelting temperature is far higher. This makes SLID very promising for high-temperature applications, but it is also attractive from the viewpoint of creating well-defined, thin-layer metallic joints. This paper presents our work in the Cu-Sn, Au-Sn and Au-In SLID systems. For all these three systems we obtain well-defined layered bond structures with high strength. We verify experimentally that they are all solid to temperatures higher than 300 °C, being substantially higher than the respective melting temperatures of the initial low-temperature metal. For Au -In, a substantial increase in the bond strength at a die shear temperature of 300 °C is explained by an occurring phase transition. We show that a Au-Sn SLID assembly of materials with different Coefficients of Thermal Expansion can withstand thermal storage followed by thermal cycling, and that Au-Sn SLID also is compatible with bonding of rough substrates.

Journal of Micromechanics and Microengineering, 2015

In this paper, we report wafer-level bonding using solid-liquid inter-diffusion (SLID) processes ... more In this paper, we report wafer-level bonding using solid-liquid inter-diffusion (SLID) processes for fabricating micro-joints Cu–Sn at low temperature (270 °C). The evolution of multilayer Cu/Sn to micro-joint alloys has been characterized by optical microscopy and mechanical die-shear testing. The Cu–Sn joints with line width from 80 to 200 μm prove to be reliable packaging materials for bonding vacuum micro-cavities with controllable Sn overflow, as well as high mechanical strength (>70 MPa). A thermodynamic model has been performed to further understand the formation of Cu–Sn intermetallic alloys. There are two important findings for this work: 1) Using a two-step temperature profile may significantly reduce the amount of Sn overflow; 2) for packaging, a bond frame width greater than 80 μm will result in high yield.

Metallurgical and Materials Transactions A, 2015

Wafer-level Cu-Sn SLID (Solid-Liquid Interdiffusion)-bonded devices have been evaluated at high t... more Wafer-level Cu-Sn SLID (Solid-Liquid Interdiffusion)-bonded devices have been evaluated at high temperature. The bonding process was performed at 553 K (280°C) and the mechanical integrity of the bonded samples was investigated at elevated temperatures. The die shear strength of Cu-Sn systems shows a constant behavior (42 MPa) for shear tests performed from room temperature [RT-298 K (25°C)] to 573 K (to 300°C). This confirms experimentally the high-temperature stability of Cu-Sn SLID bonding predicted from phase diagrams. The fractography of sheared samples indicates brittle-fracture mode for all samples shear tested from RT to 573 K (300°C). The two dominating failure modes are Adhesive fracture between the Ti-W adhesion layer and the Si, and interface fracture at the original bond interface. This indicates that the bonding material itself is stronger than the observed shear strength values, and since these interfaces can be improved with process optimization even stronger bonds can be achieved. The presented work offers fundamental evidence of the Cu-Sn SLID bonding process for operating microelectronics and MEMS at high temperature.

2014 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2014

Solid-Liquid Interdiffusion (SLID) bonding has emerged as a promising bonding technique, particul... more Solid-Liquid Interdiffusion (SLID) bonding has emerged as a promising bonding technique, particularly for high-temperature applications. It uses well-establish processing steps such as electroplating, and it is well suited for wafer-level processing. SLID is based on intermetallic compounds (IMCs) as the bonding medium, enabling a thermal stability at temperatures well above the bonding temperature. In this paper, we present our work on Cu-Sn and Au-Sn SLID bonding. For Cu-Sn SLID bonding, our focus has been on wafer-level processing and optimization of manufacturing parameters. We present successful flux-free Cu-Sn SLID bonding for hermetic sealing, and an experimentally obtained kinetics model for IMC formation as function of time, showing different mathematical relations above and below the melting point of Sn. The model is used for predicting the effect of a given bonding temperature profile. For Au-Sn SLID bonding, our focus has been on phase identification and verification of ...

Proceedings of the 5th Electronics System-integration Technology Conference (ESTC), 2014

Solid-Liquid Interdiffusion (SLID) bonding is a promising bonding technique, particularly for hig... more Solid-Liquid Interdiffusion (SLID) bonding is a promising bonding technique, particularly for high-temperature applications. Based on intermetallics as the bonding medium, the bonds are stable at temperatures far above the processing temperature which is in the range of normal solder temperatures. This work confirms experimentally this high-temperature stability through shear strength testing as function of temperature (room temperature to 300 °C) for three different SLID systems: Cu-Sn, Au-Sn and Au-In. All three systems remain solid within the tested temperature range, as expected, but they show remarkably different temperature dependence of mechanical strength: Au-Sn SLID bonds show strongly decreasing shear strength with temperature (but at 300 °C it is still well above the MIL-STD requirement); Cu-Sn SLID bonds show only small changes; whereas Au-In SLID bonds show increased shear strength at 300 °C, accompanied with a change in fracture mode from brittle to ductile. All three behaviours can be explained from the phase diagrams with the actual phases in use.

2012 4th Electronic System-Integration Technology Conference, 2012

ABSTRACT The aim of this study is to optimize the bond process for Cu/Sn wafer-level bonding, a c... more ABSTRACT The aim of this study is to optimize the bond process for Cu/Sn wafer-level bonding, a competitive material for modern MEMS encapsulation due to its low cost and high performance. For this Solid-Liquid Interdiffusion (SLID) bonding technique, it is important to understand the formation of the intermetallic compounds (IMCs), which takes place during the bond process. In order to estimate thermodynamic kinetics coefficients, electroplated Cu/Sn multilayer stacks were annealed at temperatures in the range 150–300°C for 0–320 min. The formation process of intermetallic compounds (IMC) were investigated by cross-section microscopy of annealed samples at different time and temperature. The kinetics constants of Cu3Sn growth, as well as decreasing Sn thickness, are derived from measured IMC thicknesses. Based upon these extracted kinetics constants, a simulation model for IMC growth and remaining Sn thickness profile during wafer bonding process has been implemented by MATLAB. This model is used to predict an optimized wafer-level Cu/Sn bonding process.

2013 IEEE 63rd Electronic Components and Technology Conference, 2013

ABSTRACT The objective of this study is to optimize the Cu/Sn solid liquid bonding process, which... more ABSTRACT The objective of this study is to optimize the Cu/Sn solid liquid bonding process, which is an attractive technique for wafer-level MEMS packaging and encapsulation. In order to optimize the bonding process, the effect of bonding temperature profile, initial Sn layer thickness and bond pressure are investigated and discussed. Bond performance is characterized by sealing yield, dicing yield and cross section analysis of the bond interface. With correct design of Cu/Sn layer thickness and temperature profile, high bond yield at bond temperature 270°C and 250°C was obtained.

2012 4th Electronic System-Integration Technology Conference, 2012

ABSTRACT A low-temperature bonding process for ultrasound transducers is presented: compatible wi... more ABSTRACT A low-temperature bonding process for ultrasound transducers is presented: compatible with poling requirements, manufacturability and reliability. In this work, we demonstrate that a thermosonic bonding process can provide a reliable, metallurgical bond at moderate temperatures, even down to room temperature, with bonding times in the order of seconds. Bonding parameters (temperature, compression force, ultrasonic energy) were optimized by evaluating shear strength on Au stud bump bonded Si chips. Model systems have been bonded, mimicking a complete Electro-Acoustic Module (EAM), including a stack of IC emulator / flex interconnection / interface part of the ultrasound transducer.

3rd Electronics System Integration Technology Conference ESTC, 2010

Fluxless SLID (Solid-Liquid InterDiffusion) bonding based on Au and Sn is presented, using two di... more Fluxless SLID (Solid-Liquid InterDiffusion) bonding based on Au and Sn is presented, using two different processes, and bonding temperatures in the range 300-350°C. The decomposition of the bond was tested by applying shear force while heating the samples. No bond delamination was observed for temperatures up to 350-400°C, with 95% of the tested samples surviving 400°C without bond delamination. This

Journal of Electronic Materials, 2013

The objective of this study is to optimize the Cu/Sn solid-liquid interdiffusion process for wafe... more The objective of this study is to optimize the Cu/Sn solid-liquid interdiffusion process for wafer-level bonding applications. To optimize the temperature profile of the bonding process, the formation of intermetallic compounds (IMCs) which takes place during the bonding process needs to be well understood and characterized. In this study, a simulation model for the development of IMCs and the unreacted remaining Sn thickness as a function of the bonding temperature profile was developed. With this accurate simulation model, we are able to predict the parameters which are critical for bonding process optimization. The initial characterization focuses on a kinetics model of the Cu 3 Sn thickness growth and the amount of Sn thickness that reacts with Cu to form IMCs. As-plated Cu/Sn samples were annealed using different temperatures (150°C to 300°C) and durations (0 min to 320 min). The kinetics model is then extracted from the measured thickness of IMCs of the annealed samples.

2015 European Microelectronics Packaging Conference (EMPC), 2015

Solid-Liquid Interdiffusion (SLID) bonding is based on intermetallics, enabling a thermal stabili... more Solid-Liquid Interdiffusion (SLID) bonding is based on intermetallics, enabling a thermal stability at temperatures far surpassing the bonding temperature. The Cu-Sn SLID process is performed at a temperature above the melting point of Sn, creating Cu-Sn intermetallic compounds (IMCs) with much higher melting points. A final bondline structure of Cu / Cu3Sn / Cu ensures high-temperature stability. Voids can appear at different locations in aIMCs SLID bond: Along the original bond interface, in the Cu / Cu3Sn interface, or scattered within the Cu3Sn layer. Whereas a certain amount of voids is tolerated, an excessive amount challenges mechanical strength, hermeticity and reliability. In this work we perform wafer-level Cu-Sn bonding, patterned as bond frames on a large number of chips, with equal processing conditions for the whole wafer pair. The bondline thickness is of the order of 10 μm. The bonded wafers are singulated to chips, and selected chip pairs are cross-sectioned by Ar i...

Metallurgical and Materials Transactions A, 2015

ABSTRACT Wafer-level bonding using Au-In solid liquid interdiffusion (SLID) bonding is a promisin... more ABSTRACT Wafer-level bonding using Au-In solid liquid interdiffusion (SLID) bonding is a promising approach to enable low-temperature assembly and MEMS packaging/encapsulation. Due to the low-melting point of In, wafer-level bonding can be performed at considerably lower temperatures than Sn-based bonding; this work treats bonds performed at 453 K (180 °C). Following bonding, the die shear strength at elevated temperatures was investigated from room temperature to 573 K (300 °C), revealing excellent mechanical integrity at these temperatures well above the bonding temperature. For shear test temperatures from room temperature to 473 K (200 °C), the measured shear strength was stable at 30 MPa, whereas it increased to 40 MPa at shear test temperature of 573 K (300 °C). The fracture surfaces of Au-In-bonded samples revealed brittle fracture modes (at the original bond interface and at the adhesion layers) for shear test temperatures up to 473 K (200 °C), but ductile fracture mode for shear test temperature of 573 K (300 °C). The as-bonded samples have a layered structure consisting of the two intermetallic phases AuIn and γ′, as shown by cross section microscopy and predicted from the phase diagram. The change in behavior for the tests at 573 K (300 °C) is attributed to a solid-state phase transition occurring at 497 K (224 °C), where the phase diagram predicts a AuIn/ψ structure and a phase boundary moving across the initial bond interface. The associated interdiffusion of Au and In will strengthen the initial bond interface and, as a consequence, the measured shear strength. This work provides experimental evidence for the high-temperature stability of wafer-level, low-temperature bonded, Au-In SLID bonds. The high bond strength obtained is limited by the strength at the initial bond interface and at the adhesion layers, showing that the Au-In SLID system itself is capable of even higher bond strength.

2018 7th Electronic System-Integration Technology Conference (ESTC), 2018

Solid-liquid interdiffusion (SLID) bonding is a technique based on intermetallic compounds (IMCs)... more Solid-liquid interdiffusion (SLID) bonding is a technique based on intermetallic compounds (IMCs), enabling a thermal stability at temperatures far surpassing the bonding temperature. The technique has been developed as a die attach and interconnection technology for high-temperature applications, but is also excellent for fine-pitch bonding, and for obtaining bonds with thin layers of well-defined metallurgy. Determining the phases of IMC in a SLID bond is crucial in order to understand and predict the properties of the bond. The re-melting temperature of the bond is defined by the IMCs present, and thus directly defines the high-temperature range the SLID bond can survive. Furthermore, the phases present in a SLID bond determines whether the bond is at thermal equilibrium, or if reactions to form new IMCs are expected over the lifetime of the SLID bond (at the actual application temperature). Also, material properties such as electrical conductivity and elastic modulus will depend...

The aim of this study is to characterize a die attach method suitable for harsh environment as we... more The aim of this study is to characterize a die attach method suitable for harsh environment as well as high reliability applications. Au-Au thermosonic bonding was selected. Due to high melting temperature of gold, this technique suited well for high temperature applications. In addition, thermosonic bonding introduces ultrasonic energy to soften the material joint and enhance the bonding at the metallic interface. This allows reducing the bond temperature and force. Initial characterization focuses on the effects of bonding parameters – ultrasonic energy, bond force and bond temperature – to relative bond strength. Bonded components were subjected to die shear test to measure bond strength. Cross section and SEM were used to inspect the bond interface and Aubump deformation. Thermal shock cycling (TSC) test performed from -20 to 200C was carried out to examine bond reliability. Experiment results were compared to thermocompression bonding to evaluate the improvement of thermosonic ...

2019 IEEE 69th Electronic Components and Technology Conference (ECTC), 2019

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service... more This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Intermetallic Compounds - Formation and Applications, 2018

Solid-liquid interdiffusion (SLID) bonding for microelectronics and microsystems is a bonding tec... more Solid-liquid interdiffusion (SLID) bonding for microelectronics and microsystems is a bonding technique relying on intermetallics. The high-melting temperature of intermetallics allows for system operation at far higher temperatures than what solder-bonded systems can do, while still using similar process temperatures as in common solder processes. Additional benefits of SLID bonding are possibilities of fine-pitch bonding, as well as thin-layer metallurgical bonding. Our group has worked on a number of SLID metal systems. We have optimized wafer-level Cu-Sn SLID bonding to become an industrially feasible process, and we have verified the reliability of Au-Sn SLID bonding in a thermally mismatched system, as well as determined the actual phases present in an Au-Sn SLID bond. We have demonstrated SLID bonding for very high temperatures (Ni-Sn, having intermetallics with melting points up to 1280°C), as well as SLID with low process temperatures (Au-In, processed at 180°C, and Au-In-Bi, processed at 90-115°C). We have verified experimentally the high-temperature stability for our systems, with quantified strength at temperatures up to 300°C for three of the systems: Cu-Sn, Au-Sn and Au-In.

Lab on a chip, Jan 17, 2017

In this paper, we propose a simple method to embed transparent reactive materials in a microfluid... more In this paper, we propose a simple method to embed transparent reactive materials in a microfluidic cell, and to observe in situ the dissolution of the material. As an example, we show how to obtain the dissolution rate of a calcite window of optical quality, dissolved in water and hydrochloric acid (HCl). These fluids circulate at controlled flowrates in a channel which is obtained by xurography: double sided tape is cut out with a cutter plotter and placed between the calcite window and a non-reactive support. While the calcite window reacts in contact with the acid, its topography is measured in situ every 10 s using an interference microscope, with a pixel resolution of 4.9 μm and a vertical resolution of 50 nm. In order to avoid inlet influence on the reaction, a thin layer of photoresist is added on the calcite surface at the inlet and outlet. This layer is also used as a non reactive reference surface.

2016 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2016

Solid-Liquid Interdiffusion (SLID) bonding has received substantial interest, primarily due to it... more Solid-Liquid Interdiffusion (SLID) bonding has received substantial interest, primarily due to its ability to withstand higher temperatures than the bonding temperature. Whereas SLID bonding typically is performed in the range of normal solder temperatures, the remelting temperature is far higher. This makes SLID very promising for high-temperature applications, but it is also attractive from the viewpoint of creating well-defined, thin-layer metallic joints. This paper presents our work in the Cu-Sn, Au-Sn and Au-In SLID systems. For all these three systems we obtain well-defined layered bond structures with high strength. We verify experimentally that they are all solid to temperatures higher than 300 °C, being substantially higher than the respective melting temperatures of the initial low-temperature metal. For Au -In, a substantial increase in the bond strength at a die shear temperature of 300 °C is explained by an occurring phase transition. We show that a Au-Sn SLID assembly of materials with different Coefficients of Thermal Expansion can withstand thermal storage followed by thermal cycling, and that Au-Sn SLID also is compatible with bonding of rough substrates.

Journal of Micromechanics and Microengineering, 2015

In this paper, we report wafer-level bonding using solid-liquid inter-diffusion (SLID) processes ... more In this paper, we report wafer-level bonding using solid-liquid inter-diffusion (SLID) processes for fabricating micro-joints Cu–Sn at low temperature (270 °C). The evolution of multilayer Cu/Sn to micro-joint alloys has been characterized by optical microscopy and mechanical die-shear testing. The Cu–Sn joints with line width from 80 to 200 μm prove to be reliable packaging materials for bonding vacuum micro-cavities with controllable Sn overflow, as well as high mechanical strength (>70 MPa). A thermodynamic model has been performed to further understand the formation of Cu–Sn intermetallic alloys. There are two important findings for this work: 1) Using a two-step temperature profile may significantly reduce the amount of Sn overflow; 2) for packaging, a bond frame width greater than 80 μm will result in high yield.

Metallurgical and Materials Transactions A, 2015

Wafer-level Cu-Sn SLID (Solid-Liquid Interdiffusion)-bonded devices have been evaluated at high t... more Wafer-level Cu-Sn SLID (Solid-Liquid Interdiffusion)-bonded devices have been evaluated at high temperature. The bonding process was performed at 553 K (280°C) and the mechanical integrity of the bonded samples was investigated at elevated temperatures. The die shear strength of Cu-Sn systems shows a constant behavior (42 MPa) for shear tests performed from room temperature [RT-298 K (25°C)] to 573 K (to 300°C). This confirms experimentally the high-temperature stability of Cu-Sn SLID bonding predicted from phase diagrams. The fractography of sheared samples indicates brittle-fracture mode for all samples shear tested from RT to 573 K (300°C). The two dominating failure modes are Adhesive fracture between the Ti-W adhesion layer and the Si, and interface fracture at the original bond interface. This indicates that the bonding material itself is stronger than the observed shear strength values, and since these interfaces can be improved with process optimization even stronger bonds can be achieved. The presented work offers fundamental evidence of the Cu-Sn SLID bonding process for operating microelectronics and MEMS at high temperature.

2014 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2014

Solid-Liquid Interdiffusion (SLID) bonding has emerged as a promising bonding technique, particul... more Solid-Liquid Interdiffusion (SLID) bonding has emerged as a promising bonding technique, particularly for high-temperature applications. It uses well-establish processing steps such as electroplating, and it is well suited for wafer-level processing. SLID is based on intermetallic compounds (IMCs) as the bonding medium, enabling a thermal stability at temperatures well above the bonding temperature. In this paper, we present our work on Cu-Sn and Au-Sn SLID bonding. For Cu-Sn SLID bonding, our focus has been on wafer-level processing and optimization of manufacturing parameters. We present successful flux-free Cu-Sn SLID bonding for hermetic sealing, and an experimentally obtained kinetics model for IMC formation as function of time, showing different mathematical relations above and below the melting point of Sn. The model is used for predicting the effect of a given bonding temperature profile. For Au-Sn SLID bonding, our focus has been on phase identification and verification of ...

Proceedings of the 5th Electronics System-integration Technology Conference (ESTC), 2014

Solid-Liquid Interdiffusion (SLID) bonding is a promising bonding technique, particularly for hig... more Solid-Liquid Interdiffusion (SLID) bonding is a promising bonding technique, particularly for high-temperature applications. Based on intermetallics as the bonding medium, the bonds are stable at temperatures far above the processing temperature which is in the range of normal solder temperatures. This work confirms experimentally this high-temperature stability through shear strength testing as function of temperature (room temperature to 300 °C) for three different SLID systems: Cu-Sn, Au-Sn and Au-In. All three systems remain solid within the tested temperature range, as expected, but they show remarkably different temperature dependence of mechanical strength: Au-Sn SLID bonds show strongly decreasing shear strength with temperature (but at 300 °C it is still well above the MIL-STD requirement); Cu-Sn SLID bonds show only small changes; whereas Au-In SLID bonds show increased shear strength at 300 °C, accompanied with a change in fracture mode from brittle to ductile. All three behaviours can be explained from the phase diagrams with the actual phases in use.

2012 4th Electronic System-Integration Technology Conference, 2012

ABSTRACT The aim of this study is to optimize the bond process for Cu/Sn wafer-level bonding, a c... more ABSTRACT The aim of this study is to optimize the bond process for Cu/Sn wafer-level bonding, a competitive material for modern MEMS encapsulation due to its low cost and high performance. For this Solid-Liquid Interdiffusion (SLID) bonding technique, it is important to understand the formation of the intermetallic compounds (IMCs), which takes place during the bond process. In order to estimate thermodynamic kinetics coefficients, electroplated Cu/Sn multilayer stacks were annealed at temperatures in the range 150–300°C for 0–320 min. The formation process of intermetallic compounds (IMC) were investigated by cross-section microscopy of annealed samples at different time and temperature. The kinetics constants of Cu3Sn growth, as well as decreasing Sn thickness, are derived from measured IMC thicknesses. Based upon these extracted kinetics constants, a simulation model for IMC growth and remaining Sn thickness profile during wafer bonding process has been implemented by MATLAB. This model is used to predict an optimized wafer-level Cu/Sn bonding process.

2013 IEEE 63rd Electronic Components and Technology Conference, 2013

ABSTRACT The objective of this study is to optimize the Cu/Sn solid liquid bonding process, which... more ABSTRACT The objective of this study is to optimize the Cu/Sn solid liquid bonding process, which is an attractive technique for wafer-level MEMS packaging and encapsulation. In order to optimize the bonding process, the effect of bonding temperature profile, initial Sn layer thickness and bond pressure are investigated and discussed. Bond performance is characterized by sealing yield, dicing yield and cross section analysis of the bond interface. With correct design of Cu/Sn layer thickness and temperature profile, high bond yield at bond temperature 270°C and 250°C was obtained.

2012 4th Electronic System-Integration Technology Conference, 2012

ABSTRACT A low-temperature bonding process for ultrasound transducers is presented: compatible wi... more ABSTRACT A low-temperature bonding process for ultrasound transducers is presented: compatible with poling requirements, manufacturability and reliability. In this work, we demonstrate that a thermosonic bonding process can provide a reliable, metallurgical bond at moderate temperatures, even down to room temperature, with bonding times in the order of seconds. Bonding parameters (temperature, compression force, ultrasonic energy) were optimized by evaluating shear strength on Au stud bump bonded Si chips. Model systems have been bonded, mimicking a complete Electro-Acoustic Module (EAM), including a stack of IC emulator / flex interconnection / interface part of the ultrasound transducer.

3rd Electronics System Integration Technology Conference ESTC, 2010

Fluxless SLID (Solid-Liquid InterDiffusion) bonding based on Au and Sn is presented, using two di... more Fluxless SLID (Solid-Liquid InterDiffusion) bonding based on Au and Sn is presented, using two different processes, and bonding temperatures in the range 300-350°C. The decomposition of the bond was tested by applying shear force while heating the samples. No bond delamination was observed for temperatures up to 350-400°C, with 95% of the tested samples surviving 400°C without bond delamination. This

Journal of Electronic Materials, 2013

The objective of this study is to optimize the Cu/Sn solid-liquid interdiffusion process for wafe... more The objective of this study is to optimize the Cu/Sn solid-liquid interdiffusion process for wafer-level bonding applications. To optimize the temperature profile of the bonding process, the formation of intermetallic compounds (IMCs) which takes place during the bonding process needs to be well understood and characterized. In this study, a simulation model for the development of IMCs and the unreacted remaining Sn thickness as a function of the bonding temperature profile was developed. With this accurate simulation model, we are able to predict the parameters which are critical for bonding process optimization. The initial characterization focuses on a kinetics model of the Cu 3 Sn thickness growth and the amount of Sn thickness that reacts with Cu to form IMCs. As-plated Cu/Sn samples were annealed using different temperatures (150°C to 300°C) and durations (0 min to 320 min). The kinetics model is then extracted from the measured thickness of IMCs of the annealed samples.

2015 European Microelectronics Packaging Conference (EMPC), 2015

Solid-Liquid Interdiffusion (SLID) bonding is based on intermetallics, enabling a thermal stabili... more Solid-Liquid Interdiffusion (SLID) bonding is based on intermetallics, enabling a thermal stability at temperatures far surpassing the bonding temperature. The Cu-Sn SLID process is performed at a temperature above the melting point of Sn, creating Cu-Sn intermetallic compounds (IMCs) with much higher melting points. A final bondline structure of Cu / Cu3Sn / Cu ensures high-temperature stability. Voids can appear at different locations in aIMCs SLID bond: Along the original bond interface, in the Cu / Cu3Sn interface, or scattered within the Cu3Sn layer. Whereas a certain amount of voids is tolerated, an excessive amount challenges mechanical strength, hermeticity and reliability. In this work we perform wafer-level Cu-Sn bonding, patterned as bond frames on a large number of chips, with equal processing conditions for the whole wafer pair. The bondline thickness is of the order of 10 μm. The bonded wafers are singulated to chips, and selected chip pairs are cross-sectioned by Ar i...

Metallurgical and Materials Transactions A, 2015

ABSTRACT Wafer-level bonding using Au-In solid liquid interdiffusion (SLID) bonding is a promisin... more ABSTRACT Wafer-level bonding using Au-In solid liquid interdiffusion (SLID) bonding is a promising approach to enable low-temperature assembly and MEMS packaging/encapsulation. Due to the low-melting point of In, wafer-level bonding can be performed at considerably lower temperatures than Sn-based bonding; this work treats bonds performed at 453 K (180 °C). Following bonding, the die shear strength at elevated temperatures was investigated from room temperature to 573 K (300 °C), revealing excellent mechanical integrity at these temperatures well above the bonding temperature. For shear test temperatures from room temperature to 473 K (200 °C), the measured shear strength was stable at 30 MPa, whereas it increased to 40 MPa at shear test temperature of 573 K (300 °C). The fracture surfaces of Au-In-bonded samples revealed brittle fracture modes (at the original bond interface and at the adhesion layers) for shear test temperatures up to 473 K (200 °C), but ductile fracture mode for shear test temperature of 573 K (300 °C). The as-bonded samples have a layered structure consisting of the two intermetallic phases AuIn and γ′, as shown by cross section microscopy and predicted from the phase diagram. The change in behavior for the tests at 573 K (300 °C) is attributed to a solid-state phase transition occurring at 497 K (224 °C), where the phase diagram predicts a AuIn/ψ structure and a phase boundary moving across the initial bond interface. The associated interdiffusion of Au and In will strengthen the initial bond interface and, as a consequence, the measured shear strength. This work provides experimental evidence for the high-temperature stability of wafer-level, low-temperature bonded, Au-In SLID bonds. The high bond strength obtained is limited by the strength at the initial bond interface and at the adhesion layers, showing that the Au-In SLID system itself is capable of even higher bond strength.