Chau-Jie Zhan

Assembly and reliability characterization of 3D chip stacking with 30μm pitch lead-free solder micro bump interconnection

Recently, the three-dimensional chip stacking technology with fine pitch and high input/output interconnects has emerged due to the requirements of multi-function and high performance in electronic devices. When the electronic packaging technology develops toward the miniaturization trend, the reliability of interconnect with fine pitch and high density solder bump interconnections will become a critical issue in advanced 3D chip stacking technology. In this study, assembly of chip-on-chip test vehicle with a micro bump pitch of 30 µm was demonstrated and the joint reliability was also evaluated. The Si chip/carrier used in this study had more than 3000 micro bumps with Sn2.5Ag solder material. Ni/Cu under bump metallurgy layer (UBM) was selected on both the top and bottom chip. 3D chip stacking was achieved by thermo-bonding and subsequently underfill dispensing for fine gap filling was also conducted with different kinds of underfills. The temperature cycling test was performed on the chip-on-chip stacking module over 1000 cycles. In addition, with the joint size becoming small, the current for each solder bump carried continues to increase. This leads high current flowing in each individual joint. Therefore, electromigration has become a major reliability issue in microelectronic devices. In this study, electromigration in SnAg solder micro bump with a pitch of 30 μm was investigated under 0.12A at 135°C. Electromigration characteristics of fine pitch micro bump was discussed with the microstructure evolution of micro bump during current stressing. The assembly of 3D stacking chip using two layers of chip with fine pitch and lead-free interconnects was achieved in this study. The results of reliability test showed that the reliability of fine pitch solder micro bump interconnections was acceptable under mechanical and electrical evaluations.

Electromigration in Ni/Sn intermetallic micro bump joint for 3D IC chip stacking

In this study, we used a chip-on-chip test vehicle with 30μm pitch lead-free solder micro bump to study the electromigration reliability of solder micro bump interconnection used for 3D chip stacking. The structure of micro bump composed of Sn2.5Ag solder material with Cu/Ni under bump metallization (UBM) was selected. Two types of interconnection were chosen to evaluate the effect of the joint structure on electromigration behavior. The type I was the chip stacking sample with the IMC / Sn (5 um thick) / IMC joint structure, while the type II was the sample with fully transformed Ni3Sn4 intermetallic (IMC) joints made by the post-treatment of long time thermal aging. Electromigration test was performed on the four point Kelvin structure and daisy-chain structure under the current stressing of 104∼105 A/cm2 at an ambient temperature of 150°C. During the electromigration test, the resistance increase was in-situ monitored to determine the definite time to failure. The microstructure evolution was also examined at different stages of joint resistance increase. From the testing results, the rapid increase of joint resistance was found at the early stage under current stressing in the type I micro bump. After that, the joint resistance increase became slower. This mild increasing stage was much longer than the early stage. For the type II sample, however, the resistance increasing rate was quiet lower than that of the type I sample at the identical testing time. With a higher current density in the order of 105 A/cm2 in the micro joint, the effect of joule heating caused the damage happened in Al trace and Cu UBM while the residual Sn solder had been transformed to be Ni3Sn4 IMC totally and few voids were found around IMC by the microstructure observation. When applied a current density in the order of 104 A/cm2 on the micro joint, the residual Sn was also fully transformed to be IMC. However, the failure mode of the micro joint is not clear yet because the experiments are still on-going. The resistance variation is showing a steady state under such a condition of current stressing and so far no open failure happened.

Addressing bandwidth challenges in next generation high performance network systems with 3D IC integration

The bandwidth for high performance networking switches and routers increases two to ten times in every new generation. This in turn drives the bandwidth requirements for the Application Specific Integrated Circuits (ASICs) and their external memory devices designed for the high performance network systems. 3D IC integration with its low power, high density and high bandwidth advantages is proposed to address the bandwidth challenges between the ASIC and its external memory. This paper presents a novel 3D IC architecture that includes a silicon interposer with Through-Silicon-Vias (TSV) and interconnect wiring layers on both sides of the silicon interposer. An ASIC chip measured at 22 mm × 18 mm × 0.4 mm is attached on top of the silicon interposer while two smaller memory chips with a size of 10 mm × 10 mm × 0.4 mm are attached to the bottom of the silicon interposer with micro-bump interconnections. A unique, double-sided Chip to Chip (C2C) joining process is developed to enable the ASIC and memory integration in true 3D System-in-Package (SiP) format. This 3D IC architecture will help to overcome the size limitation of the current silicon interposers due to the reticle size used in the lithographic wafer processing. The 3D IC stack is assembled on an organic package substrate with conventional solder bumps. Communications between the top ASIC die and the bottom memory dice are made through the TSVs and the wiring layers of the silicon interposer. Thermal and thermo-mechanical analysis of the 3D IC stack are used to evaluate the package thermal performance and for optimizing material selection and package reliability. Both the modeling and experimental characterization results are used to gain insights into the 3D IC technology for addressing the ASIC and memory bandwidth challenges and to develop the best practice for ASIC and memory integration for next generation high performance network systems.

Development of fluxless chip-on-wafer bonding process for 3DIC chip stacking with 30μm pitch lead-free solder micro bumps and reliability characterization

3D IC integration can be accomplished by using the approaches such as chip-on-chip, chip-on-wafer and wafer-on-wafer integration. The scheme of chip-on-chip shows the advantages of high flexibility and yield during assembly process. However, low fabrication throughput has been a major issue. When compared to chip-on-chip integrations, wafer-on-wafer may provide the higher manufacturing throughput but its overall yield will be limited by accumulative yield. Also, good chips are forced to bond on bad chip. Therefore, the development of chip-on-wafer integration may be a better scheme for 3D chip stacking. In this study, a high-yield and fluxless chip-on-wafer bonding process with 30μm pitch lead-free solder micro bump interconnection were demonstrated and the reliability of micro joint was also evaluated. Test chip adopted in this study had more than 3000 micro bumps with a diameter of 18μm and a pitch of 30μm. Sn2.5Ag solder material was electroplated on Cu/Ni under bump metallurgy (UBM) of both the test chip and 200mm wafer. To achieve the purpose of fluxless chip-on-wafer bonding, the plasma pre-treatment was applied to both the test chips and bonded wafer. The thermo-compression bonding method with the gap control capability was also carried out. In this work, the fluxless thermo-compression bonding having more than 90% CoW yield had been accomplished. The factor of Sn thickness was found to evidently influence the CoW yield. With the optimized plasma treatment parameters and bonding conditions, a well-aligned and robust joining of micro bump without using flux could be obtained. The results of reliability test revealed that the introduction of underfill could apparently enhance the reliability performance of micro joint under mechanical evaluation. Also, the solder micro bump joint showed excellent electromigration resistance when compared to standard flip chip bump under current stress of 5×10−4 A/cm2 at an ambient temperature of 150°C.

Novel crosstalk modeling for multiple through-silicon-vias (TSV) on 3-D IC: Experimental validation and application to Faraday cage design

An equivalent circuit model to characterize the crosstalk strength in multiple TSVs is newly proposed. In this model, all the values of lumped elements in the model are given in closed-form formulas. Therefore, the computation effort for constructing the model of multiple TSVs is much lower than other previous works. The accuracy is verified by the measurement for a nine stacked silicon chips and the full-wave simulation results. The proposed model is then utilized to the design for crosstalk mitigation. With the advantages of smaller occupied area (lower cost), a rhombus-grounded Faraday cage design is recommended with lower cost and similar performance compared to conventional Faraday cage concept.

Development of Cu/Ni/SnAg Microbump Bonding Processes for Thin Chip-on-Chip Packages Via Wafer-Level Underfill Film

3-D integration provides a promising approach for the construction of complex microsystems through the bonding and interconnection of individually optimized device layers without sacrificing system performance. The use of traditional underfill processes is expected to face an arduous challenge as the filled gap of a large-scale chip is narrowed down to several micrometers. Consequently, the subsequent reliability of microbumps (μ-bumps) joints and the relative assembly compatibility of stacked chips of 3-D IC packages deteriorate. To resolve this critical issue, a novel technology for wafer-level underfill film (WLUF) is developed. This paper demonstrates the steps that the proposed technology would take. These steps include the alignment of the WLUF-coated chip to the substrate chip and the elimination of voids to make the proposed technology work. However, the coplanarity of stacked thin chips after assembling with the WLUF, is an urgent problem that needs to be understood in detail. Therefore, this paper presents a nonlinear finite element analysis (FEA) using a process-oriented simulation technique to estimate the warpage of stacked thin chips. For experimental validation, the effects of several key designed factors on the thermomechanical behavior of chip-on-chip package under various bonding forces are investigated. The analytic results indicate that a chip thickness of <; 50 μm at the outermost region of the packaging structure without μ-bumps significantly reduces approximately 2 μm of gap between chips. This phenomenon is attributed to the major structural support at the purlieus of the chip via WLUF, which is extremely weak when a uniform bonding pressure is loaded. In addition, the subsequent cooling procedure of the WLUF further aggravates the warpage magnitude of the stacked thin chips. The results of this paper could serve as a guideline for further improvement of the bonding reliability and for the design of the structural optimization of packaging assemblies via the WLUF.

Effects of UBM structure/material on the reliability performance of 3D chip stacking with 30μm-pitch solder micro bump interconnections

With the increased demand of functionality in electronic device, three dimensional integration circuits technology together with downscaling of interconnection pitch present an important role for the development of next generation electronics. In the current types of interconnects, solder micro bumps have received much attention due to its low cost in material and process. For fine pitch solder micro bump interconnections, selection of under bump metallurgical material is a crucial issue because the UBM structure/material will show a significant influence on the reliability performances of the solder micro bump joints. However, which UBM structure/material for fine pitch solder micro bump joint presents the better reliability properties is not concluded yet until now. In this study, the effect of UBM structural/material on the reliability properties of lead-free solder micro interconnections with a pitch of 30μm was discussed. The chip-on-chip test vehicle with solder micro bump interconnections having a diameter of 18 μm was adopted to evaluate the effects of UBM structure/material. There were more than 3000 micro bumps with Sn2.5Ag solder material on both the silicon chip and carrier. In this study, three types of UBM were selected on the silicon carrier: they were single copper layer with a thickness of 8μm; the Cu/Ni layer with the thickness of 5μm/3μm and the Cu/Ni/Au layer with the thickness of 5μm/3μm/0.5μm. The UBM was electro-plated on the Al trace and then the Sn2.5Ag solder with a thickness of 5 μm was deposited. For the silicon carrier with the bump structure of Cu/Sn and Cu/Ni/Sn, the silicon test chip with solder micro bump of Cu/Sn was used for chip bonding. The test chip having the solder micro bump of Cu/Sn and Cu//Ni/Sn was used for bonding with the silicon carrier having the Cu/Ni/Au UBM. In chip stacking process, we adopted the fluxless thermocompression process for each type of micro joints. After chip bonding process, the fine gap between bonded chips was filled by capillary type of underfill. The influence of underfill on the reliability of solder micro bump interconnects with various combinations of UBM structures were estimated also. After the assembly process, temperature cycling test (TCT), high temperature storage (HTS) and electromigration test (EM) were performed on the chip-stacking module to assess the effect of UBM structure/material on the reliability of solder micro bump interconnections. The results of reliability test revealed that only Cu/Sn/Au/Ni/Cu micro joint could not pass the TCT and HTS of 1000 cycles. After reliability test, the Cu/Sn/Cu joints showed the evidently microstructural evolution while the Cu/Ni/Sn/Cu joints did not show apparent microstructure change among all the types of micro joints and still revealed the most contents of residual solder within the joint. On the other hand, the existence of Au layer upon the UBM caused the complicated interface reaction between solder and UBM, which presented a negative effect during long-term reliability performance. The reliability results also displayed that the introduction of underfill could apparently enhance the reliability of micro joint under mechanical evaluation. From the results of EM reliability test, all the types of the micro joints showed excellent electromigration resistance under current stress of 0.08A at an ambient temperature of 150°C irrespective of the types of UBM. This superior property was attributed to the microstructure change transformed from the solder joint to the IMC joint within the solder micro bump interconnections during electromigration test. All the results of reliability test illustrated that the selection of UBM structure/material well influenced the reliability performance of fine-pitch solder micro bump interconnections in 3D chip stacking.

Process, assembly and electromigration characteristics of glass interposer for 3D integration

Glass interposer is proposed as a superior alternative to organic and silicon-based interposers for 3DIC packaging in the near future. Because glass is an excellent dielectric material and could be fabricated with large size, it provides several attractive advantages such as excellent electrical isolation, better RF performance, better feasibility with CTE and most importantly low cost solution. In this paper, we investigated the EM performance of Cu RDL line with glass substrate. Three different physical properties of glass materials were used for studying the EM performance of Cu RDL line. The used testing conditions are under 150~170 °C and 300~500mA. The glass type material with best performance was applied for glass interposer process integration and assembly investigation. Therefore, a wafer-level 300mm glass interposer scheme with topside RDLs, Cu TGVs, bottom side RDLs, Cu/Sn micro-bump and PBO passivation has been successfully developed and demonstrated in the study. The chip stack modules with glass interposer were assembled to evaluate their electrical characteristics. Pre-conditioning test was performed on the chip stacking module with the glass interposer to assess the reliability of the heterogeneous 3D integration scheme. All the results indicate that the glass interposer with polymer passivation can be successfully integrated with lower cost processes and assembly has been successfully developed and demonstrated in the study.

Assembly process and reliability assessment of TSV/RDL/IPD interposer with multi-chip-stacking for 3D IC integration SiP

In this study, a 3D IC integration system-in-package (SiP) with TSV/RDL/IPD interposer is designed and developed. Emphasis is placed on the Cu revealing, embedded stress sensors, non-destructive inspection, thermal modeling and measurement, and final assembly and reliability assessments.

Performance and process characteristic of glass interposer with through-glass-via(TGV)

Primary approach of 3DIC packaging usually adopts organic substrates or silicon interposer as the intermedium between multi-integrated circuits (ICs) and printed circuit board. Current organic substrates face the limitations in poor dimensional stability, trace density and CTE mismatch to silicon. Silicon interposer is a good solution for high-pin-count ICs and high performance applications based on the mature Si technology of advance via formation and fine line Cu damascene multilevel interconnection process, but silicon interposer is limited by high cost. Glass is proposed as ideal interposer material due to high resistivity, low dielectric constant, low insertion loss and adjustable coefficient of thermal expansion (CTE) for the 3DIC assembly integration and most importantly low cost solution, [1-4]. The main focus of this paper is on (a) TGV electrical design, simulation and characterization, (b) wafer level integration in TGV formation, two RDL on the front-side, one RDL on the backside and polymer-based PBO for the passivation, (c) assembly process of silicon chip stack on the glass interposer with Kelvin resistance measurement. The glass interposer was assessed to have excellent electrical characteristics and is potentially to be applied for 3D product applications.