Novel process for cover glass with ideal stress distribution

Abstract — This paper proposes a new process to manufacture cover glass that overcomes a strength trade‐off between the face and the edge. In the process, alkali barrier films are deposited on glass faces before an ion exchange process in order to control face stress properties without inhibiting the edge strengthening. As a demonstration of the process, alkali‐alumino‐silicate glass sheets with sputter‐deposited SiO₂ films were chemically strengthened, and then their stress properties and strengths were investigated. As a result, thicker SiO₂ films cause lower face DOL (depth of strengthened layer), and it is observed that the faces have lower DOL than the edges. In strength tests corresponding to major fracture modes of smartphone cover glass, specimens with 80–100 nm films have more balanced face performance and better edge impact strengths than the no‐film specimen.     

Mechanical properties of glass frit bonded micro packages

Frit glass bonding is a widely used technology for encapsulation of surface micro-machined structures like inertial sensors or gyroscopes on wafer level. Since for sensors in automotive applications, a lifetime of 15–20 years has to be guaranteed for, a reliable lifetime prediction is necessary. Different material parameters have to be known for a lifetime estimation based on stress corrosion cracking, which determines the long-term strength behaviour of most bonded interfaces of microsystems. Parameters needed for lifetime prediction have to describe the material’s resistance against crack propagation (fracture toughness K _IC), the stress situation in a micro package and the long-term strength behaviour. Results for fracture toughness investigations presented in this paper were determined by the micro chevron test. The stress situation in a micro package was calculated by a thermo-mechanical Finite Element Analysis. Furthermore the residual stress in the glass layer and the linear thermal expansion coefficient were determined by a crack width measurement in an environmental scanning electron microscope.     

Fabrication and mechanical testing of glass chips for high-pressure synthetic or analytical chemistry

The pressure strength of microfluidics glass chips for high-pressure chemistry has been examined. Internal pressures up to 320 bar have been measured, although variations are substantial. Long annealing steps at high temperatures did not show any improvement, but smoothening the powder blasted channel walls by isotropic etching did. Although macro cracks are observed at the bond interface it is believed that the formation of micro cracks in the channel wall and stress concentrations due to sharp corners in the channel cross-section geometry strongly limits the maximum applicable pressures.     

激光加工聚合物微流道暂态弹性热应力分析

激光直写聚合物微流道是非常有效的加工方法,但是,由于激光烧蚀聚合物的热流密度很高,在小的热影响区产生过大的集中热应力,热应力导致产生微裂纹,降低材料的强度和疲劳寿命,严重的引起材料破坏性失效.为了预测激光烧蚀时的热应力大小随时间空间的变化规律,根据烧蚀模型推出了三维弹性热应力模型.首先由烧蚀模型预测温度场分布和烧蚀流道外形,根据流道轮廓和温度场计算弹性热应力场与烧蚀过程的关系.分析结果表明弹性热应力位于低于或平行于烧蚀表面的一定厚度层内,这也许是引起表面微裂纹的原因.     

Product design for strong cover glass

Strength is one of the most important properties of cover glass. In this study, fracture analysis is used to classify the breakage mode of cover glass into four typical modes. Moreover, the mechanism and evaluation method of each mode are investigated. Consequently, a chemical strengthening design with high compressive stress (CS) and low center tension (CT) is obtained. In addition, processing design is determined to be an important factor. Fining of edge processing and surface polishing after chemical strengthening are shown to enhance the edge and surface strength, respectively.     

Glass substrate for micro display devices

Glass substrate suitable for wearable micro display devices for augmented/virtual reality was developed. This glass has highly matched thermal expansion coefficient with that of silicon, and it enables to suppress the amount of warpage caused by the boding between Si and glass wafers. The glass also shows lower thermal shrinkage than that of conventional non‐alkali glass for thin‐film transistor substrate. In this paper, the effect of the temperature dependence of the thermal expansion coefficient on warpage generation after bonding process was investigated using both numerical simulation and experiment. The newly developed glass showed remarkably low warpage after the bonding process.     

微加速度计冲击可靠性及防护

通过实验研究了梳齿电容微加速度计器件在大过载冲击下的失效模式,发现了失效的主要原因是由于弹性梁的断裂,并对实验现象进行了分析和讨论,指出了微加工工艺的不确定性对器件失效的影响。检验了在结构设计中对可动质量块进行限位的作用,可以明显提高器件的抗冲击能力。提出了金属橡胶这一新材料在微加速度计冲击防护中的应用,并对其作用进行了实验验证,结果表明金属橡胶除了靠金属螺旋卷接触点之间的相互摩擦来耗散能量外,还可以明显增加冲击脉冲的宽度,有效的缓冲和吸收冲击能,提高器件的抗冲击能力和可靠性。     

Development of rollable silicon thin‐film‐transistor backplanes utilizing a roll‐to‐roll continuous lamination process

Abstract— Rollable silicon thin‐film‐transistor (TFT) backplanes utilizing a roll‐to‐roll process have been developed. The roll‐to‐roll TFT‐backplane technology is characterized by a glass‐etching TFT transfer process and a roll‐to‐roll continuous lamination process. The transfer process includes high‐rate, uniform glass‐etching to transfer TFT arrays fabricated on a glass substrate to a flexible plastic film. In the roll‐to‐roll process, thinned TFT‐glass sheets (0.1 mm) and a base‐film roll are continuously laminated using a permanent adhesive. Choosing both an appropriate elastic modulus for the adhesive and an appropriate tension strength to be used in the process is the key to suppressing deformation of the TFT‐backplane rolls caused by thermal stress. TFT backplanes that can be wound, without any major physical damage such as cracking, on a roll whose core diameter is approximately 300 mm have been sucessfully obtained. Incorporating the TFT‐backplane rolls into other roll components, such as color‐filter rolls, will make it possible to produce TFT‐LCDs in a fully roll‐to‐roll manufacturing process.     

Damage and failure in silicon-glass-metal microfluidic joints for high-pressure MEMS devices

The design, fabrication, and testing of microfluidic joints consisting of Kovar metal tubes attached to silicon using borosilicate glass for high pressure microelectromechanical systems devices are presented. The MIT microrocket, which requires microfluidic joints to sustain pressures of at least 12.7 MPa and temperatures in excess of 700 K, is used to demonstrate the feasibility of the glass sealing methodology. A key concern in such joints is the occurrence of cracks due to residual stresses during fabrication, which can affect the load-carrying capability. To obtain a better understanding of the damage and failure characteristics, a hierarchical approach was taken. First, two types of joint configurations with several glass compositions and geometries were considered at the joint-level. Axial tension and pressure tests were performed, and finite element models were used to obtain the residual stress field and to predict failure loads based on linear elastic fracture mechanics. Subsequently, tests were performed on actual and dummy microrockets to validate the methodology at the device-level. Key observations include the importance of bonding between the Kovar tube and the silicon sidewall, which can help increase joint strength, and the detrimental effects of joint proximity under differential pressure loading and manufacturing defects in multiple joint specimens. In addition to specific experimental and analyses results that allow a physical understanding of the damage and failure mechanisms, another key contribution of this work is the overall insight of the design and analysis of reliable glass-sealed microfluidic packages. This insight will help one make better design and process selections for packages in other high-pressure silicon-based MEMS applications.     

Study on demolding temperature in thermal imprint lithography via finite element analysis

Demolding in thermal imprint lithography is a process to overcome chemical and mechanical interactions of all levels between stamp and substrate formed by the process history, stamp geometries, and materials used, accounting for most of imprint failure. Undesired deformation in resist may occur depending on the stress generated in the resist with respect to the yield stress. In thermal imprint lithography, temperature is one of the most important parameters not only for the flow of resist during the molding process, but also for the demolding because the thermal stress, the friction and adhesion forces, and the mechanical strength of resist are all dependent on temperature even at below the glass transition temperature of the resist. In this paper, we have developed a method via a numerical simulation of the demolding process to determine an optimal demolding temperature that leads to the least resist deformation. Considering the viscoelasticity of the resist and the temperature dependence of friction coefficients at the resist/stamp interface, an optimal temperature was extracted via normalization of the local Von Mises stress in the resist by its yield stress. For a simple two-dimensional model system with Si stamp and PMMA as a resist layer, the optimal demolding temperature was found to be ～70°C.     

Investigating fracture strength of poly-silicon membranes using microscopic loading tests and numerical simulation

The fracture strength of poly-silicon as widely used membrane material in micro electro mechanical system applications has a critical impact in respect to design, function and reliability of e.g. pressure sensors or microphones. This circumstance necessitates the investigation of the fracture strength of these poly-silicon membranes. In this study the strength was investigated by experimental tests and numerical simulations. A new fracture test has been developed that applies a well-defined and almost constant stress within a certain region of the membrane and prevent a cracking of the membrane at the edge. The brittle behavior of poly-silicon needs a statistical evaluation of the results. To this end, a set of 45 membranes was tested at each of the three positions on the wafer in order to assure statistical accuracy and to evaluate the strength distribution across the wafer. The experimental loading tests were attended by scanning electron microscopy to examine the microstructure and the crack path. Using finite element simulation, the non-linear deformation behavior during membrane loading was analyzed and the fracture stresses were calculated. In the final step the obtained results were statistically evaluated by means of a two-parametric Weibull distribution. High values were found for the characteristic fracture stresses. They are in the range of 5400–6000 MPa.     

柔性太阳翼叠层电池弯曲适应性仿真与设计

航天器在轨进、出地影时，基于复合结构柔性基板的柔性太阳翼叠层电池在高、低温交变作用下会产生热弯曲变形。针对这一问题，采用有限元建模仿真，分析由太阳电池、盖片胶和抗辐照玻璃盖片组成的叠层电池的弯曲适应性，计算不同结构尺寸、不同叠层材料参数的叠层电池在跨中弯曲载荷下的最大应力。借助响应面技术研究设计变量与复合结构性能之间的关系，采用目标驱动优化分析，获得理想的结构尺寸参数。结果表明：叠层电池的宏观力学性能取决于微观不同叠层材料的组合方式；等效弹性模量不是定值，而是与叠层材料的参数有关；弯曲工况的失效模式取决于电池单体的最大拉应力，而不是宏观表征的整体弯曲强度。     

ZrO2氧传感器封接料开裂失效分析及改进

针对氧化锆(ZrO2)氧传感器封接料失效机理,提出了在传感器加热电路上增加预热功能,通过仿真和试验的方法验证了改进措施的有效性.仿真分析的结果表明:温度变化幅度为200℃时应力状态较温度变化400℃时的应力降低幅度约为40%.通过试验样件验证表明,传感器在进行了576次启动热应力试验后,封接料状态良好,未开裂,表明加热电路增加预热功能后降低了热应力冲击强度,增加预热电路后,传感器有效避免了使用中出现封接料开裂故障.     

Exploring the applicability of gradient elasticity to certain micro/nano reliability problems

It has long been assessed that continuum mechanics can be used successfully to address a variety of mechanical problems at both macroscopic and microscopic scales. The term “micromechanics”, in particular, has been used in considering elasticity, plasticity, damage, and fracture mechanics problems at the micron scale involving metallic, ceramic and polymeric materials, as well as their composites. Applications to automobile, aerospace, and concrete industries, as well as to chemical and microelectronic technologies have already been documented. The recent developments in the field of nanotechnology have prompted a substantial literature in nanomechanics. While this term was first introduced by the author in the early 90’s to advance a generalized continuum mechanics framework for applications at the nanoscale, it is mainly used today in considering “hybrid” ab-initio/molecular dynamics/finite element simulations, usually based on elasticity theory, to interpret the mechanical response of nano-objects (nanotubes, nanowires, nanoaggregates) and extract information on nano-configurations (dislocation cores, crack tips, interfaces). The modest goal of this article is to show that continuum elasticity can indeed be extended to describe a variety of problems at the micro/nano regime. The resultant micro/nanoelasticity theory includes long-range or nonlocal material point interactions and surface effects in the form of (phenomenological) higher-order stress/strain gradients. Coupled thermo-diffuso-chemo-mechanical processes can also be considered within such a higher-order theory. Size effects on micro/nano holes and micro/nano cracks can conveniently be modeled, and some standard strength of materials formulas routinely used for micro/nano beams can be improved, with potential applications to MEMS/NEMS devices and micro/nano reliability components.     

Micro-structures fabrication on glasses for micro-fluidics by imprinting technique

Micro-channels for microfluidics were fabricated in soda-lime glass through imprinting technique, and then joined to another soda-lime glass slab by thermal assisted direct bonding (TADB). The joined samples were characterized before and after TADB by optical and scanning electron microscopy (SEM) and surface profilometer and shear strength test. The integrity of channels is maintained also after the bonding. The bonded interface between the two glass slabs was found to be without impurities, bubbles and cracks; good bonding strength of 32 MPa between two glasses was obtained as well. These techniques are simple and low cost, suitable for mass production of glass based micro-fluidic devices.     

A study of 3D cover glass design that improves handheld device drop reliability

Cover glass in commercial handheld devices is now evolving from flat (2D) to curved (3D) shapes. For example, some commercial devices have utilized sled‐shape cover glass, 1 which partially covers long edges of the device. According to the patents published by key handheld manufacturers, 2 , 3 we can expect more variety of 3D shaped cover glass for handheld devices in the market. In this study, we have focused on the reliability of 3D cover glass when it is dropped to a rigid surface. The key parameters under study are the corner/edge bend radius and angle of the cover glass, which determines the 3D shape of the cover glass. To achieve this goal, we developed a finite element model to simulate the drop 4 - 13 of a handheld device with 3D‐shaped glass. The model uses explicit algorithm to simulate the high speed impact on the device during the drop test. The glass performance was evaluated based on contact force between the glass and the ground and maximum principal stress in the glass. We showed that to avoid severe damage because of first impact between the glass and the ground, the bend angle of 3D glass has to be in the range between 0 and 45°. For drop angles of 45° and higher, with the proposed glass bend angle, the impact can be taken over by the edge of the back cover of the device. In addition, we showed that optimum glass bend radius is in the range of 3.8 mm and larger. This is required to reduce stress in glass because of impact. The approach and conclusions from the current study can serve as a general guideline to improve the 3D cover glass reliability of a handheld device.     

Characterizing mechanical behaviors of a flexible AMOLED during the debonding process

The stress distribution of a flexible active-matrix organic light-emitting diode (AMOLED) display during the debonding process was investigated using finite element analysis. During the fabrication of an AMOLED display, an AMOLED with a polyimide (PI) substrate is detached from a glass carrier; this is a critical process and generally results in failure of the AMOLED. To enhance the yielding rate of AMOLEDs, their stress states generated during the debonding process must be reduced. The interfacial fracture behavior between the PI substrate and glass carrier was characterized on the basis of bimaterial fracture mechanics, and the fracture toughness associated with mode mixity determined through peeling tests was considered a criterion for detaching the AMOLED from the glass carrier. The stress distribution of the AMOLED at the inception of debonding crack extension was evaluated according to fracture toughness. In addition, the parameters possibly influencing the stress states of the AMOLED in the debonding process are discussed.     

Topology optimization considering fracture mechanics behaviors at specified locations

As a typical form of material imperfection, cracks generally cannot be avoided and are critical for load bearing capability and integrity of engineering structures. This paper presents a topology optimization method for generating structural layouts that are insensitive/sensitive as required to initial cracks at specified locations. Based on the linear elastic fracture mechanics model (LEFM), the stress intensity of initial cracks in the structure is analyzed by using singularity finite elements positioned at the crack tip to describe the near-tip stress field. In the topology optimization formulation, the J integral, as a criterion for predicting crack opening under certain loading and boundary conditions, is introduced into the objective function to be minimized or maximized. In this context, the adjoint variable sensitivity analysis scheme is derived, which enables the optimization problem to be solved with a gradient-based algorithm. Numerical examples are given to demonstrate effectiveness of the proposed method on generating structures with desired overall stiffness and fracture strength property. This method provides an applicable framework incorporating linear fracture mechanics criteria into topology optimization for conceptual design of crack insensitive or easily detachable structures for particular applications.     

Calculation of film stress from pitch measurements in LCD panel manufacturing

A method based on inverting a finite element model is presented for determining film stress from pitch changes before and after a film deposition step in liquid‐crystal display panel manufacturing. It differs from the conventional methods by making use of in‐plane deformation rather than out‐of‐plane measurements to calculate film stress. The resulting film stress is confounded with glass structural relaxation. Measurements of out‐of‐plane deformation at the edge of the sheet can be used with the pitch measurements to separate the effects of glass structural relaxation and film stress.     

微加速度计在冲击环境下的可靠性研究

微加速度计用于测量载体的加速度,并提供相关的速度和位移信息.微加速度计可以和微型陀螺仪组合构成微型惯性测量单元.但是微加速度计还没有完全实现市场化,微加速度计的可靠性问题已经成为制约其广泛应用的关键因素.微加速度计在加工、封装、运输和实际使用中都可能受到冲击的作用.主要研究压阻式微加速度计在冲击环境下的可靠性问题.通过简化加速度计的结构,得出了悬臂梁上的应力分布.设计了微加速度计在冲击环境下的可靠性试验,分析了加速度计在冲击环境下的主要失效模式及失效机理.得出了压阻式加速度计在冲击环境下的主要失效模式是键合引线的脱落和悬臂梁的断裂.