The Interplay of Modulus,Strength, and Ductility in Adhesive Design Using Biomimetic Polymer Chemistry

High‐performance adhesives require mechanical properties tuned to demands of the surroundings. A mismatch in stiffness between substrate and adhesive leads to stress concentrations and fracture when the bonding is subjected to mechanical load. Balancing material strength versus ductility, as well as considering the relationship between adhesive modulus and substrate modulus, creates stronger joints. However, a detailed understanding of how these properties interplay is lacking. Here, a biomimetic terpolymer is altered systematically to identify regions of optimal bonding. Mechanical properties of these terpolymers are tailored by controlling the amount of a methyl methacrylate stiff monomer versus a similar monomer containing flexible poly(ethylene glycol) chains. Dopamine methacrylamide, the cross‐linking monomer, is a catechol moiety analogous to 3,4‐dihydroxyphenylalanine, a key component in the adhesive proteins of marine mussels. Bulk adhesion of this family of terpolymers is tested on metal and plastic substrates. Incorporating higher amounts of poly(ethylene glycol) into the terpolymer introduces flexibility and ductility. By taking a systematic approach to polymer design, the region in which material strength and ductility are balanced in relation to the substrate modulus is found, thereby yielding the most robust joints.     

Single-Crystalline Silicon Layer Transfer to a Flexible Substrate Using Wafer Bonding

This paper reports the successful transfer of a thin single-crystalline silicon film to a flexible, transparent polymer substrate. Thin-film silicon on polymer was realized by bonding a silicon-on-insulator (SOI) wafer to a flexible substrate using a spin-on polymer as an adhesive. The SOI wafer was thinned by a grinding operation followed by chemical mechanical polishing (CMP). The SOI was further thinned to the buried oxide using wet etchants. The residual stress in the transferred substrate was investigated by ultraviolet (UV) micro-Raman spectroscopy and numerical modeling. Both approaches showed that a low level of stress was created at the bonded interface during the layer transfer.     

Flexible Electronics: Thin Silicon Die on Flexible Substrates

Silicon thinned to 50 mum and less is flexible allowing the fabrication of flexible and conformable electronics. Two techniques have been developed to achieve this goal using thinned die: die flip chip bonded onto flexible substrates [polyimide and liquid crystal polymer (LCP)] and die flip chip laminated onto LCP films. A key to achieving each of these techniques is the thinning of die to a thickness of 50 mum or thinner. Conventional grinding and polishing can be used to thin to 50 mum. At 50 mum, the active die becomes flexible and must be handled by temporarily bonding it to a holder die for assembly. Both reflow solder and thermocompression assembly methods are used. In the case of solder assembly, underfill is used to reinforce the solder joints. With thermocompression bonding of the die to an LCP substrate, the LCP adheres to the die surface, eliminating the need for underfill.     

Flexible Electronics and Displays: High-Resolution, Roll-to-Roll, Projection Lithography and Photoablation Processing Technologies for High-Throughput Production

Flexible electronics are increasingly being used in a number of applications which benefit from their low profile, light weight, and favorable dielectric properties. However, despite these advantages, the range of practical, high-volume, applications for flexible electronics will remain limited in the future unless a number of challenges related to lithographic patterning on flexible substrates are successfully addressed. The most critical of these pertain to system parameters that affect the cost and performance of flexible circuits, including the resolution, panel size, process throughput, substrate distortion, material handling, and yield. We present a new class of roll-to-roll lithography systems, developed in recent years, that were designed to address the challenges in each of these critical areas. These systems provides high-resolution projection imaging over very large exposure areas, on flexible substrate materials. Additionally, they achieve high-precision alignment by means of image scaling to compensate for substrate distortion due to processing; and they also performs high-throughput photoablation, patterning millions of pixels simultaneously, by means of projection imaging. This technology is attractive for current and emerging applications, such as flexible circuit boards and flexible chip carriers, as well as for potential future applications such as such as flexible displays and macroelectronic systems.     

A Large-Stroke MEMS Deformable Mirror Fabricated by Low-Stress Fluoropolymer Membrane

This letter presents a deformable mirror (DM) which consists of a 25-mm square flexible membrane and an array of 67 hexagonal electrodes that can pull down a membrane by electrostatic force. The membrane is a 2.15-mum-thick fluoropolymer film and is bonded to an electrode substrate with an 80-mum-thick adhesive acrylic spacer. The fabrication process is completed by a microelectromechanical systems batch process. The surface roughness of polymer membrane is approximately 41 nm. The device demonstrated maximum 38-mum center deflection by applying 250 V because of small residual stress (2.2 MPa) and low Young's modulus (15 GPa) of fluoropolymer film. The response time of the DM is experimentally demonstrated around 10 ms in atmosphere. The proposed device is suitable for large stroke and low sampling rate applications.     

导电胶粘接可伐载板工艺的仿真与优化

导电胶是一种很有潜力的互连材料,其粘接可靠性是制约其应用的主要因素。基于对某混频模块粘接失效的分析,探索温度试验条件及载板尺寸对可伐载板粘接可靠性的影响。通过仿真和试验设计,研究不同温度试验条件下不同尺寸载板在粘接界面处的应力分布情况,并优化了可伐载板粘接工艺。结果表明,温度试验条件越严苛,载板尺寸越大,可伐载板粘接可靠性越差,可采取环氧绝缘胶加固或柔性导电胶粘接的方式对其粘接工艺进行优化。     

Correlation of silver migration to the pull out strength of silver wire embedded in an adhesive matrix

The substantial growth in the electronics industry has created a need for environmental and user-friendly alternatives to tin/lead (Sn/Pb) solders for attaching encapsulated surface mount components on rigid and flexible printed circuit boards (PCBs). Electronically conductive adhesives (ECAs) have been explored in this manner to establish mechanical as well as electrical joints between PCBs and surface mount components. Applications of conductive adhesive are limited due to serious concerns associated with the long-term reliability data of current commercial ECAs. One critical concern in wire bonding applications is the significant decrease in the bond strength and consequent loss of the conducting properties of adhesive due to silver migration. In this study, an effort is made to understand and model long-term silver migration phenomenon with respect to different parameters (duration of the migration, dry and wet conditions), and pull-out strength of silver wire embedded in an epoxy adhesive matrix. Morphology of embedded silver wire after migration and pull-out was also studied using scanning electron micrographs. Migration area increased with the duration of migration, and reduction in the pull-out strength was significant in wet condition as compared to dry condition. The increase in migration area was consistent with the reduction in pull-out strength in both wet and dry conditions.     

InSb红外探测器芯片粘接工艺研究

基于InSb红外探测器的封装特点,采用正交试验法研究了芯片粘接过程中基板平整度、粘接剂抽真空时间、配胶时间、固化条件等工艺参数对芯片性能及可靠性的影响。通过计算极差和方差分析了各因素对芯片可靠性的影响大小。结果表明,固化条件对粘接后芯片的性能影响最大,其次是配胶时间,而抽真空时间和基板平整度影响相对较小。针对极差分析得出的较优参数组合和较差参数组合,利用X射线衍射(XRD)研究了不同参数组合对晶片粘接的应力大小,所得结果与正交试验一致。     

Use of Elastic Conductive Adhesive as the Bonding Agent for the Fabrication of Vertical Structure GaN-Based LEDs on Flexible Metal Substrate

Through the use of elastic conductive adhesive (ECA) as the bonding agent and patterned laser lift-off technology, a flexible metal substrate technology for the fabrication of vertical structured GaN-based light-emitting diodes (flex-LEDs) was proposed and demonstrated. It showed that the flex-LEDs have negligible changes in dominant wavelength-current and light output intensity-current-voltage characteristics when subjected to an external bending stress, indicating that the ECA used in the present technology performed well as a buffer to external stresses. As compared with conventional sapphire substrate GaN-based LEDs, Flex-LEDs with a chip size of 600 x 600 mum² showed an increase in light output intensity (power) about 216% (80%) at 120 mA with an essential decrease in forward voltage from 3.51 to 3.3 V.     

Electrical Characterization of NCP- and NCF-Bonded Fine-Pitch Flip-Chip-on-Flexible Packages

Electronic portable devices are aimed towards higher response speed with a better viewing resolution display. Nonconductive paste (NCP) and nonconductive film (NCF) are the adhesive materials used in fine-pitch display applications. This study compares two commercially available adhesives for fine-pitch chip-on-flex (COF) applications. The electrical performance of the NCP-bonded COF was better compared to the NCF. The rheological properties of these materials in the initial stages and the mechanical properties of the adhesives after bonding are claimed to be the main factors. The semisolid form of the NCF which melts and flows slowly from the interconnection joints finally reduced the effective contact area in the joint as compared to the NCP. A low-pressure bonding caused entrapment of adhesive in the joints, induced stress accumulation in the Z-direction during high thermal loading, and a high coefficient of thermal expansion (CTE) mismatch in between bumps, adhesive, and electrode traces on the flexible substrate were the key factors for the degradation of electrical conductivity. A high load of 100 N and above was recommended since the effective contact area built into the interconnection was good and reliable after 400 cycles of a thermal shock test of -55degC-125degC. The NCP with a higher elastic modulus which ensures higher stiffness and stability towards elongation gave a better reliability in this environmental test. Cross sectioning and SEM analysis provide evidence of the effective contact area of the joint before and after the thermal cycle environmental test     

Electrical Characterization of NCP- and NCF-Bonded Fine-Pitch Flip-Chip-on-Flexible Packages

Electronic portable devices are aimed towards higher response speed with a better viewing resolution display. Nonconductive paste (NCP) and nonconductive film (NCF) are the adhesive materials used in fine-pitch display applications. This study compares two commercially available adhesives for fine-pitch chip-on-flex (COF) applications. The electrical performance of the NCP-bonded COF was better compared to the NCF. The rheological properties of these materials in the initial stages and the mechanical properties of the adhesives after bonding are claimed to be the main factors. The semisolid form of the NCF which melts and flows slowly from the interconnection joints finally reduced the effective contact area in the joint as compared to the NCP. A low-pressure bonding caused entrapment of adhesive in the joints, induced stress accumulation in the Z-direction during high thermal loading, and a high coefficient of thermal expansion mismatch in between bumps, adhesive, and electrode traces on the flexible substrate were the key factors for the degradation of electrical conductivity. A high load of 100 N and above was recommended since the effective contact area built into the interconnection was good and reliable after 400 cycles of a thermal shock test of -55degC-125degC. The NCP with a higher elastic modulus which ensures higher stiffness and stability towards elongation gave a better reliability in this environmental test. Cross sectioning and scanning electron microscopy analysis provide evidence of the effective contact area of the joint before and after the thermal cycle environmental test     

Plasma cleaning of the flex substrate for flip-chip bonding with anisotropic conductive adhesive film

Advanced integrated circuit packaging processes require good bondability and reliability between various mating surfaces. A key factor affecting this requirement is surface cleanliness. Plasma cleaning is the most suitable process for optimum surface cleanliness. An investigation of O₂, Ar, and O₂/SF₆ plasma cleaning was carried out on a flexible substrate to study the adhesion of anisotropic conductive adhesive film for flip chip bonding. Surface roughness was found to increase substantially after the plasma treatment. Adhesion strength was evaluated by 90° peeling tests both for untreated and plasma-treated flex. A higher adhesion strength of anisotropic conductive film (ACF) bond was observed after plasma cleaning. The surface morphology of plasma treated and untreated flex substrate before bonding, as well as the fracture surfaces after the peel test for both cases, was characterized by secondary electron image techniques of scanning electron microscopy (SEM). Based on the detailed SEM findings, extensive comparisons were made between the plasma treated and the untreated samples. Mechanical interlocking is found to be responsible for higher peel strength of the plasma treated flex bonding. It was also proposed to select the right flexible substrate for highly reliable, ACF bonded flip chip on flex substrate.     

Transfer Processes for Thermally Stable Large-Size TFT Flexible Substrates

We report on glass etching transfer processes to obtain thermally stable large-size TFT flexible substrates on plastic film bases. The transfer processes include high-pressure jet etching that allows us to achieve good etch-rate uniformity over the large area and the utilization of an adhesive having a low elastic modulus. From the experiments and simulations, we find that using an adhesive having a low elastic modulus is effective in reducing bend of the flexible substrate under thermal stresses. The simulations also predict that the reduction in the bend corresponds to the reduction in the principal stress of the adhesive, leading to the suppression of the film peeling off from the thinned glass. Using the transfer processes, we have successfully fabricated thermally stable TFT flexible substrates (300 mmtimes350 mm times200 mum) that have satisfactory electrical characteristics     

硅/玻璃紫外固化中间层键合

利用玻璃的透光特性和紫外固化的成熟技术,研究了一种使用紫外固化胶作为中间层的玻璃/硅室温键合工艺.通过选择一定波段的紫外固化胶,旋涂紫外胶后使用365nm光刻机作为紫外光源控制紫外固化,从而实现了硅/玻璃的中间层键合.分析测试结果表明,紫外固化辅助的中间层键合可以成功应用于硅/玻璃键合,中间层厚5～6μm,键合强度达到26MPa.该工艺只需室温条件,简单高效,成本低廉,无需额外的压力或电场,对于硅/玻璃低温键合封装具有潜在的应用价值.     

晶圆键合技术在LED应用中的研究进展

简要介绍了晶圆键合技术在发光二极管(LED)应用中的研究背景,分别论述了常用的黏合剂键合技术、金属键合技术和直接键合技术在高亮度垂直LED制备中的研究现状,包括它们的材料组成和作用、工艺步骤和参数以及优缺点.其中,黏合剂键合是一种低温键合技术,且易于应用、成本低、引入应力小,但可靠性较差;金属键合技术能提供高热导、高电导的稳定键合界面,与后续工艺兼容性好,但键合温度高,引入应力大,易造成晶圆损伤;表面活化直接键合技术能实现室温键合,降低由于不同材料间热失配带来的负面影响,但键合良率有待提高.     

Sensitive strain measurements of bonded SOI films using Moire/spl acute/

The authors have developed a simple technique to quantify strain in bonded Si films and used it to compare the strain induced by two distinct wafer bonding methods. This method consists of patterning sets of moire/spl acute/ gratings on silicon-on-insulator (SOI) substrates prior to bonding using a G-line stepper. After planarization, bonding, and etch-back, the same lithography step is performed on the flipped patterns. The resultant interference between upper and lower gratings produces moire/spl acute/ fringes which is a measure of the strain. In the experiments, the sensitivity of the measurement is approximately 20 nm. This approach has been used to compare two methods of wafer bonding. The first method, a manual bonding technique, yielded strain of up to 100 nm/mm. The second method employed a commercial-grade bonder and resulted in film strains below 40 nm/mm. In the bonding schemes the authors have studied, they believe strain results mainly from induced wafer bow during bonding and stress contributions of deposited films. This scheme was developed to address wafer strain that arises from a direct-alignment double-gate MOSFET fabrication scheme (Meinhold, 1994).     

Investigation and Effects of Wafer Bow in 3D Integration Bonding Schemes

This paper investigates and reviews the effects of wafer bow in three- dimensional (3D) integration bonding schemes, including copper wafer bonding and oxide fusion wafer bonding with silicon on insulator (SOI)-based layer transfer technology. Wafer bow criteria for good bonding quality and fabrication techniques to minimize wafer bow are introduced for 3D integration technology and applications.     

Polysilicon thin-film transistors on polymer substrates

Different approaches to fabricate low-temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) on polymer substrates are reviewed and the two main routes are discussed: (1) standard fabrication of LTPS TFTs on glass substrates followed by a transfer process of the devices on the polymeric substrate; (2) direct fabrication of the devices on the polymeric substrate. Among the different techniques we have described in more detail the process we have recently developed for the fabrication of LTPS TFTs directly on ultra-thin polyimide (PI) substrate. LTPS TFT technology is particularly suited for high performance flexible electronics applications, due to the excellent device characteristics, good electrical stability and CMOS technology. Flexible display application remains the most attractive application for LTPS technology, especially for AMOLED displays, where device stability and the possibility to integrate the driving circuits make LTPS technology superior to all the other competitive TFT technologies. Among the other applications, particularly promising is also the application to flexible smart sensors, where integration of a front-end electronics is essential. Some examples of flexible gas sensors and pressure sensors, integrated with simple readout electronics based on LTPS TFTs and fabricated on ultra-thin PI substrate, are presented.     

Bonding stress and reliability of high power GaAs-based lasers

GaAs-based lasers were bonded to oxygen-free high-conductivity (OFHC) copper heat sinks using a eutectic PbSn solder or a silver-filled conductive epoxy, and life tested. Epoxy-bonded devices were observed to have a larger failure rate on life test than solder-bonded devices. Bonding stress, as measured by the degree of polarization (DOP) of photoluminescence, was found to be the largest in epoxy-bonded devices. As well, the type of heat sink and bonding adhesive affected the stress in the laser material, with bonding stress increasing when there was a larger mismatch of coefficients of thermal expansion between the laser material, adhesive, and heat sink. Results suggested that heat sink material and bonding adhesive contribute to stress within the laser material and the resulting performance of the device     

基于临时键合与解键合技术的THz集成电路研究

太赫兹频段部件的工艺实现是决定器件性能的关键环节。从太赫兹频段部件用50mm 超薄石英基片薄膜电路的集成工艺技术难点和可实现性出发,重点介绍了一种灵活快速的解决方案———临时键合与解键合技术。结果表明,临时键合与解键合技术克服了超薄石英基片脆性大、易破碎的弊端,在50mm 及以下厚度超薄石英基片薄膜电路集成中具有很好的应用前景。