Microelectromechanical Systems Packaging Research Articles

Effect of surface morphology on the deformation mechanism of Cu nanocrystal under cyclic loading at different temperatures: Molecular dynamics simulations

Micro-electromechanical systems (MEMS) are often subject to mechanical vibration or temperature, the fatigue reliability of small metal parts is a potential problem for long-term operational stability. Since the MEMS packaging is performed at a high temperature of about 600 K. Therefore, in this paper, the deformation mechanisms of copper nano-single crystals with different surface morphologies under cyclic loading were investigated by molecular dynamics calculations at room temperature (300 K) and high temperature (600 K) to provide a theoretical basis for the design of copper nanomaterials. The results show that the plastic deformation modes of the models with different surface morphologies during cyclic loading are mainly Shockley partial dislocations slipping along {111}. In contrast, the fatigue damage modes are extrusion intrusion induced by high shear strain. Compared with the surface containing a pore, the damage caused by the circular and sharp recesses through the surface is more serious. The causes of stress concentration during deformation are mainly the entanglement of Shockley partial dislocations and the generation of Lomer-Corell lock fixed dislocations. The locations of these stress concentrations will produce pores in the subsequent deformation. In addition, since periodic interfaces do not cause particularly pronounced stress concentrations and shear strain concentrations on the surface compared to other models with defects, and the resulting shear strain concentration bands are split by shear strain bands in the other direction, extrusion intrusion is prevented. Therefore, at room temperature, the model with a one-dimensional sinusoidal surface has the strongest resistance to fatigue damage, while at high temperatures, the model with a two-dimensional sinusoidal surface has the optimal resistance to fatigue damage. This provides a basis for the design of NEMS/MEMS working at high temperatures.

Computational modelling and experimental investigation of micro-electrochemical discharge machining by controlling the electrolyte temperature

Glass vias are emerging as a favourable option for radiofrequency-based micro-electromechanical system packaging. For the micromachining of glass, electrochemical discharge machining (ECDM) could be the most suitable technique if issues pertaining to the process stability are addressed thoroughly. The electrolyte temperature has immense influence on the viscosity and conductivity of the electrolyte, which percolate the stability of the ECDM process. Therefore, this article investigates the effects of the electrolyte temperature and applied voltage on the performance characteristics of ECDM for the micromachining of borosilicate glass. The machining rate (MR) and hole overcut (HOC) of the machined microholes are considered as performance characteristics. A 3D thermal-based finite element model (FEM) was developed for the thermal analysis in the machining zone. In the thermal analysis, the heat flux by thermal discharge was assumed to have Gaussian distribution, and accordingly, temperature profiles in the thermal zone were analyzed by controlling the electrolyte temperature and voltage at various levels. Further processing of temperature profiles in the thermal zone was utilized in the estimation of MR and HOC. Electrostatic-based FEM was utilized to assess the intensity of the electric field in the proximity of the tool electrode to analyze the probable locations of thermal discharge and its impact on the geometrical characteristics of the machined microholes. The simulation outcomes were validated experimentally, and show good agreement. A field emission electron microscope with energy dispersive spectroscopy was used for the characterization of the machined surface to observe the effect of the electrolyte temperature.

A Method for Fast Au-Sn Bonding at Low Temperature Using Thermal Gradient.

Flip chip bonding technology on gold-tin (Au-Sn) microbumps for MEMS (Micro Electro Mechanical Systems) and 3D packaging is becoming increasingly important in the electronics industry. The main advantages of Au-Sn microbumps are a low electrical resistance, high electrical reliability, and fine pitch. However, the bonding temperature is relatively high, and the forming mechanism of an intermetallic compound (IMC) is complicated. In this study, Au-Sn solid-state diffusion (SSD) bonding is performed using the thermal gradient bonding (TGB) method, which lowers bonding temperature and gains high bonding strength in a short time. Firstly, Au-Sn microbumps with a low roughness are prepared by using an optimized process. Then, Au-Sn bonding parameters including bonding temperature, bonding time, and bonding pressure are optimized to obtain a higher bonding quality. The shear strength of 23.898 MPa is obtained when bonding in the HCOOH environment for 10 min at the gradient temperature of 150 °C/250 °C with a bonding pressure of more than 10 MPa. The IMC of Au-Sn is found to be Au-Sn and Au5Sn. The effect of annealing time on the IMC is also investigated. More and more Au5Sn is generated with an increase in annealing time, and Au5Sn is formed after Sn is depleted. Finally, the effect of annealing time on the IMC is verified by using finite element simulation, and the bonding strength of IMC was found to be higher when the bonding temperature is 150 °C at the cold side and 250 °C at the hot side. The temperature in the bonding area can reach 200 °C, which proves that the Au-Sn bonding process is solid-state diffusion because the temperature gradient reaches 2500 °C/cm.

Improved wear resistance of cemented carbide impact needle under high frequency micro-amplitude impact treated by high current pulsed electron beam

Assembly and packaging technology is one of the most critical factors that restrict the development of microelectromechanical systems (MEMS), including chips and other devices. Dispensing robot is a key high-end equipment in MEMS packaging. Its core component, WC-Co impact needle, is limited by fretting wear and deformation, leading to reduced service accuracy and life, which is the most important cause of failure. Impact fretting wear frequently occurs in micro-nano mechanical systems and has been widely studied. However, the combined high frequency (∼300 HZ) and micro impact amplitude (∼120 μm) wear behaviors such as WC-Co impact needle with ultrafine grain (∼100 nm) is rarely concerned. To further understand the influence of impact frequency and impact velocity on the wear rate of impact needle, dispensing experiments were designed and the wear mechanism was investigated. Results indicated that the impact velocity has more influence on the wear rate of impact needle than the impact frequency. In addition, a high-current pulsed electron beam (HCPEB) surface treatment method which can effectively increase the wear resistance of impact needle was proposed. After HCPEB treatment, the abrasion loss was reduced by 44% compared to the untreated one at the same impact frequency and different impact velocities, while the abrasion loss was reduced by 52% compared to the untreated one at the same impact velocity and different frequencies. These can be attributed to the fact that HCPEB treatment decreased the grain size of WC-Co cemented carbide impact needle and promoted WC phase transition, leading to an increase in microhardness.

Hydrogen sorption in yttrium-based getter thin films

We investigated the sorption of hydrogen by yttrium-based getters for their application to vacuum wafer-level packaging of microelectromechanical systems. Thin alloy films were co-evaporated under ultra-high vacuum on silicon wafers. Getters were activated by annealing during 1 h under inert argon atmosphere with traces of oxidizing species, at temperatures ranging from 200 °C to 400 °C. Three complementary techniques of ion beam analysis were performed on the samples: Rutherford Backscattering Spectrometry (RBS), Nuclear Reaction Analysis (NRA) and Elastic Recoil Detection Analysis (ERDA), to quantify metal, oxygen and hydrogen contents, and their in-depth distributions. The results show that oxidation occurs during annealing and prevents or not hydrogen sorption depending on the film composition. Due to its fast diffusion, hydrogen tends to accumulate near the film/substrate interface and starts to diffuse into the substrate as well. The different compositions of getter films are compared in terms of oxygen and hydrogen absorptions.

Application of DC-DC Converter in Sensor and MEMS Device Integration and Packaging

DC-DC (direct current controlled direct current) converter is the core control circuit in the field of power electronics technology. Based on the theory of sensors and MEMS (microelectromechanical systems), this paper constructs a DC-DC converter device integration and packaging model and proposes an enhanced current equalization technology with offset correction function suitable for two-phase DC-DC converters. Aiming at the generation mechanism of the output ripple of the two-phase DC-DC converter, the model adopts the ripple elimination technology based on the interleaved synchronous clock and the self-calibration interleaved time generator, so that each phase of the converter is accurately staggered within the full-load range, and the problem of output ripple amplitude is solved. During the simulation process, a high-performance two-phase DC-DC converter chip is designed and implemented, which includes an adaptive on-time control logic based on ripple feedback, a self-calibrating zero-current turn-off circuit, and a robust power switch transistor drive logic. The experimental results show that the full-load current of the chip reaches 6A, the peak efficiency is 91%, the phase-to-phase current error is <0.6%, and the output ripple is <9 mV. In the 90.265 V AC input, 0-10 W load range, the output voltage error is less than 0.96%, when the load is switched between no-load and full-load, and the system response speed is less than 200 ps, which effectively improves the overall performance of the DC-DC converter.

Investigation of Adhesive's Material in Hermetic MEMS Package for Interfacial Crack between the Silver Epoxy and the Metal Lid during the Precondition Test.

A hermetic Micro-Electro-Mechanical Systems (MEMS) package with a metal lid is investigated to prevent lid-off failure and improve its reliability during the precondition test. While the MEMS package benefits from miniaturization and low cost, a hermetic version is highly sensitive to internal pressure caused by moisture penetration and the reflow process, thus affecting its reliability. In this research, the finite element method is applied to analyze the contact stress between the metal lid and the silver epoxy by applying the cohesive zone model (CZM). Moreover, the red dye penetration test is applied, revealing a microcrack at the metal lid/silver epoxy interface. Further analyses indicate that the crack is caused by internal pressure. According to the experimental testing and simulation results, the silver epoxy material, the curing process, the metal lid geometry, and the bonding layer contact area can enhance the bonding strength between the metal lid and the substrate.

Advanced utilization of 3D digital image correlation for thermal and impact reliabilities of electronics components

PurposeThis paper aims to present an adaptation of digital image correlation (DIC) to the electronics industry for reliability assessment of electronic packages. Two case studies are presented: one for warpage measurement of a micro-electro-mechanical system (MEMS) package under different temperature conditions and the other for the measurement of transient displacements on the surface of a printed circuit board (PCB) assembly under free-fall drop conditions, which is for explaining the typical camera setup requirement and comparing among different boundary conditions by fastening methods of PCB.Design/methodology/approachDIC warpage measurements on a small device, such as a MEMS package, require a special speckle pattern. A new method for the creation of speckle patterns was developed using carbon coating and aluminum evaporative deposition. To measure the transient response on the surface of a PCB during a free-fall impact event, three-dimensional (3D) DIC was integrated with synchronized stereo-high speed cameras. This approach enables the measurement of full-field displacement on the PCB surface during a free-fall impact event, contrary to the localized information that is obtained by the conventional strain gage and accelerometer method.FindingsThe authors suggest the proposed patterning method to the small-sized microelectronics packages for DIC measurements. More generally, the idea is to have a thin layer of the dark or bright color of the background and then apply the white or black colored pattern, respectively, so that the surface has high contrast. Also, to achieve a proper size of speckles, this paper does not want to expose the measuring objects to high temperatures or pressures during the sample preparation stage. Of course, it seems a complicated process to use aluminum evaporator, carbon coater and electroformed mesh. However, the authors intend to share one of the solutions to achieve a proper pattern on such small-sized electronic packages.Originality/value3D DIC technique can be successfully implemented for the measurement of micro-scale deformations in small packages (such as MEMS) and for the analysis of dynamic deformation of complex PCB.

Application of thin Au/Ti double-layered films as both low-temperature bonding layer and residual gas gettering material for MEMS encapsulation

In this study, we evaluated the applicability of thin Au/Ti double-layered films as both bonding layers and gettering material for residual gases in micro-electromechanical system (MEMS) packaging. The thin Au/Ti films on Si substrates with thermal SiO2 films were annealed at a maximum temperature of 400 °C for 1 h in atmospheric air to examine getter activation by the diffusion of Ti atoms to the surface through the Au film. X-ray photoelectron spectroscopy measurements revealed that the Ti atoms under the Au film diffused to the surface by the getter activation annealing at temperatures above 200 °C. The bonding characteristics of the thin Au/Ti films on the Si substrates were inspected through tensile tests, scanning acoustic microscopy (SAM), and transmission electron microscopy (TEM). With pre-bond annealing at or below 200 °C, no significant voids were observed at the bonding interfaces using SAM, and peeling was not observed at the bonding interface after the tensile tests. By contrast, the films could not bond with pre-bond annealing at 300 °C. At the bonding interface prepared after pre-bond annealing at or below 200 °C, no significant void was observed using cross-sectional TEM even after post-bond annealing at 300 °C, which is the required temperature for activation as a gettering layer. The results confirm that the thin Au/Ti films are applicable as both bonding layers and getter materials for MEMS packaging.

Gas Absorption in Package Using Au/Pt/Ti Bonding Layer

Vacuum packaging is required for the successful long-term operation of various kinds of micro-electromechanical system (MEMS) devices. The following points are important to complete and maintain encapsulated internal vacuum conditions: Degassing MEMS assemblies before packagingPackaging MEMS devices without leakageAbsorbing residual gases after packaging Our research group has developed a simplified MEMS packaging process using a Au/Ti layer (from top to bottom) as a bonding and gettering layer[1]. The assemblies were firstly metalized with thin Au/Ti films. The Au surfaces formed direct bonding without leakage even at room temperature. Moreover, when the packaged structure was annealed at 200 °C, the Ti atoms diffused to the inner surface of the package and absorbed residual gas molecules. This approach allows the simplification of the MEMS packaging process as the bonding and gettering layers can be simultaneously fabricated.However, we found that the Au/Ti film cannot form bonding after degas annealing at 200 °C because Ti oxides were formed on the surface. The annealing at 200 °C effectively removes the water molecules physi and chemi-sorbed on the MEMS assemblies, which deteriorate the packaged environment. The purpose of this study is to develop a metal multilayer that enables both the packaging after degassing at 200 °C and the absorption of residual gas at higher temperatures. The diffusion of Ti atoms can be controlled by fabricating a Pt barrier layer between the Au and Ti layers. A thick Pt layer enables bonding after degassing by preventing the diffusion of Ti during degassing. However, the gas absorption process requires a moderate thickness of the Pt layer because the Ti atoms should reach the surface at high temperatures. In this study, the packaging using thin Au/Pt/Ti multilayers were performed for the advanced MEMS packaging process.For the fabrication of packaged environments, a Si substrate with cavities was bonded with a flat Si substrate using the Au/Pt/Ti layers, as illustrated in Fig. 1. They were thermally oxidized before the bonding procedures. Firstly, the substrates were metalized with Au/Pt/Ti layers of 3/5/40 nm in thickness. The substrates were annealed at 200 °C under vacuum conditions for degassing. Immediately after degassing, the Au surfaces were brought in contact and pressed for the bond formation. Figure 2 shows the mapping of the interfacial gaps observed using a scanning acoustic microscope. Ultrasonic waves reflected in the bright areas, indicating the presence of unbonded areas at the interface. The bright square-shaped regions were due to the packaged areas. While particles on the surface were observed as tiny round voids, there was no leak path for the packages.For the absorption of residual gas molecules, the bonded specimens were annealed at 450 °C for 180 min. Figure 3 depicts the X-ray photoelectron spectra obtained from the inner surface of the annealed package. While the Ti atoms were absent on the surface before the high-temperature annealing step, Ti compounds were present after the high-temperature annealing. This is indicated that the Ti diffused through the Au and Pt layers and reacted with the residual gas molecules on the surface.These results suggested that the 3/5/40-nm-thick Au/Pt/Ti layers can form direct bonding after degassing and also absorb residual gas molecules in the package. We expect that this packaging technique would contribute to future MEMS manufacturing because high-vacuum sealing can be obtained by the simple metal multilayer structure.[1]. Y. Kurashima et al., 45th International Conference on Micro and Nano Engineering, 2019. Figure 1

Effect of Au Film Thickness and Surface Roughness on Room-Temperature Wafer Bonding and Wafer-Scale Vacuum Sealing by Au-Au Surface Activated Bonding.

Au-Au surface activated bonding (SAB) using ultrathin Au films is effective for room-temperature pressureless wafer bonding. This paper reports the effect of the film thickness (15–500 nm) and surface roughness (0.3–1.6 nm) on room-temperature pressureless wafer bonding and sealing. The root-mean-square surface roughness and grain size of sputtered Au thin films on Si and glass wafers increased with the film thickness. The bonded area was more than 85% of the total wafer area when the film thickness was 100 nm or less and decreased as the thickness increased. Room-temperature wafer-scale vacuum sealing was achieved when Au thin films with a thickness of 50 nm or less were used. These results suggest that Au-Au SAB using ultrathin Au films is useful in achieving room-temperature wafer-level hermetic and vacuum packaging of microelectromechanical systems and optoelectronic devices.

Fabrication and characterization of polyimide-based ‘smooth’ titanium nitride microelectrode arrays for neural stimulation and recording

Objective. As electrodes are required to interact with sub-millimeter neural structures, innovative microfabrication processes are required to enable fabrication of microdevices involved in such stimulation and/or recording. This requires the development of highly integrated and miniaturized systems, comprising die-integration-compatible technology and flexible microelectrodes. To elicit selective stimulation and recordings of sub-neural structures, such microfabrication process flow can beneficiate from the integration of titanium nitride (TiN) microelectrodes onto a polyimide substrate. Finally, assembling onto cuffs is required, as well as electrode characterization. Approach. Flexible TiN microelectrode array integration and miniaturization was achieved through microfabrication technology based on microelectromechanical systems (MEMS) and complementary metal-oxide semiconductor processing techniques and materials. They are highly reproducible processes, granting extreme control over the feature size and shape, as well as enabling the integration of on-chip electronics. This design is intended to enhance the integration of future electronic modules, with high gains on device miniaturization. Main results. (a) Fabrication of two electrode designs, (1) 2 mm long array with 14 TiN square-shaped microelectrodes (80 × 80 µm2), and (2) an electrode array with 2 mm × 80 µm contacts. The average impedances at 1 kHz were 59 and 5.5 kΩ, respectively, for the smaller and larger contacts. Both designs were patterned on a flexible substrate and directly interconnected with a silicon chip. (b) Integration of flexible microelectrode array onto a cuff electrode designed for acute stimulation of the sub-millimeter nerves. (c) The TiN electrodes exhibited capacitive charge transfer, a water window of −0.6 V to 0.8 V, and a maximum charge injection capacity of 154 ± 16 µC cm−2. Significance. We present the concept, fabrication and characterization of composite and flexible cuff electrodes, compatible with post-processing and MEMS packaging technologies, which allow for compact integration with control, readout and RF electronics. The fabricated TiN microelectrodes were electrochemically characterized and exhibited a comparable performance to other state-of-the-art electrodes for neural stimulation and recording. Therefore, the presented TiN-on-polyimide microelectrodes, released from silicon wafers, are a promising solution for neural interfaces targeted at sub-millimeter nerves, which may benefit from future upgrades with die-electronic modules.

Ultra-low thermal expansion coefficient of PZB/β-eucryptite composite glass for MEMS packaging

PbO–ZnO–B2O3 (PZB) glass with low-melting temperature has been widely applied in microelectromechanical systems (MEMS) packaging. However, the high thermal expansion coefficient (TEC) of PZB glass usually leads to strict packaging requirements. Here, in order to achieve ideal TEC, we have introduced β-eucryptite into PZB/β-eucryptite composite glass and optimized the composite fabrication process. Firstly, PbO:ZnO:B2O3 in mol ratio of 65:15:20 was selected as glass matrix owing to its low-melting temperature and good wettability. Then, β-eucryptite ceramics with ultra-low TEC were synthesized via a facile solid-state reaction method to optimize subsequent composite TEC value. The combination of corresponding material and device characterizations suggest that the pure phase and regular columnar grains of β-eucryptite ceramic can be obtained by sintering at 1450 °C for three hours, which can result in the lowest negative TEC of α30-300°C = −10.74 × 10−6 K−1. Furthermore, regarding the PZB/β-eucryptite composite glasses, systematic experiments demonstrated that the β-eucryptite introduction can greatly impact the TEC of matrix glass with only negligible effect on the softening point (Ts). Importantly, composite glasses with TEC of α30-300°C = 3.26 × 10−6 K−1, Ts = 380 °C and wetting angle < 30° are ideal for MEMS packaging. This work implements optimized fabrication process to obtain low-melting PZB/β-eucryptite composite glasses with ideal TEC values, which can be envisaged to be applied in advanced MEMS packaging applications.

Integrated piezoelectric micromechanical vibration platform for six degree of freedom motion

Piezoelectric micromechanical actuators are featured with a low driving voltage, fast response and large-range output, which make them desirable for applications of long-term drift error self-calibration in microelectromechanical systems (MEMS) inertial measurement units. A micromechanical vibration platform supported by four folded beams and excited by 32 partitioned electrodes based on PZT film is presented in this paper. PZT-silicon on insulator based wafer-level MEMS process and packaging technique are conducted in the manufacture and integration for the micro vibration platform. The six-degree-of-freedom (6 DOF) motion of the micro vibration platform is experimentally verified. In addition, the motion behavior control, damping characteristics and fatigue characteristics are analyzed. The 6 DOF micro vibration platform has a stable capability of large-range dynamic output.

A Novel Seedless TSV Process Based on Room Temperature Curing Silver Nanowires ECAs for MEMS Packaging.

The through-silicon-vias (TSVs) process is a vital technology in microelectromechanical systems (MEMS) packaging. The current via filling technique based on copper electroplating has many shortcomings, such as involving multi-step processes, requiring sophisticated equipment, low through-put and probably damaging the MEMS devices susceptible to mechanical polishing. Herein, a room temperature treatable, high-efficient and low-cost seedless TSV process was developed with a one-step filling process by using novel electrically conductive adhesives (ECAs) filled with silver nanowires. The as-prepared ECAs could be fully cured at room temperature and exhibited excellent conductivity due to combining the benefits of both polymethyl methacrylate (PMMA) and silver nanowires. Complete filling of TSVs with the as-prepared 30 wt% silver nanowires ECAs was realized, and the resistivity of a fully filled TSV was as low as 10−3 Ω·cm. Furthermore, the application of such novel TSV filling process could also be extended to a wide range of different substrates, showing great potential in MEMS packaging, flexible microsystems and many other applications.

Electroplating of 3D Sn-rich solder for MEMS packaging applications

Electroplating is a popular method to produce solders in microelectromechanical systems (MEMS) encapsulation and interconnection applications. In this work, a fabrication process is introduced for the generation of three-dimensional (3D) Sn solder microstructures. An aluminum membrane was employed as the conducting layer while electroplating. Based on the proposed method, Sn solders with different heights can be easily obtained after reflow. Taking advantage of these 3D structures, MEMS packaging, electrical interconnections, hermetic seals and metal-armoring can be formed simultaneously in one step. Metal-armoring provided by the solder–solder contact can be used at the extremes of the suspension travel, greatly increasing the capability of shock resistance. The helium leak rates of the packaged samples are lower than the reject limit (5 × 10−8 atm cc s−1 based on MIL-STD-883E standard). The bonding strength of samples is measured by a die shear strength tests with an average value of 7.4 MPa. In addition, the ohmic behavior of the Sn solder bonding is verified as well. These excellent results show an outstanding packaging quality in MEMS encapsulation applications.

Performances and procedures modules in micro electro mechanical system packaging technologies

Abstract This paper presents the challenging issues in micro electro mechanical system (MEMS) packing technologies. The MEMS package includes a MEMS device and a signal conditioning electronic circuit. In one viewpoint, MEMS application is categorized by the type of sensor, actuator, and structure. MEMS technology applications in general domains are automotive, consumer, industrial, biotechnology and commercial applications. Few methods such as the low power oven, vacuum, and silicon glass packages were discussed. The major technique as die level and wafer level of packing including thin-film packing were reviewed in this paper. The problem of several failure mechanisms and challenges and their solutions were discussed in this review.

Low-temperature hermetic thermo-compression bonding using electroplated copper sealing frame planarized by fly-cutting for wafer-level MEMS packaging

Hermetic packaging plays an important role for optimizing the functionality and reliability of a wide variety of micro-electro-mechanical systems (MEMS). In this paper, we propose a low-temperature wafer-level hermetic packaging method based on the thermo-compression bonding process using an electroplated Cu sealing frame planarized by a single-point diamond mechanical fly-cutting. This technology has an inherent possibility of hermetic sealing and electrical contact as well as a capability of integration of micro-structured wafers. Hermetic sealing can be realized with the sealing frame as narrow as 30 μm at a temperature as low as 250 °C. At such a low bonding temperature, a less amount of gases is desorbed, resulting in a sealed cavity pressure lower than 100 Pa. The leak rate into the packages is estimated by a long-term sealed cavity pressure measurement for 7 months to be less than 1.67 × 10−15 Pa m3 s−1. In addition, the bonding shear strength is also evaluated to be higher than 100 MPa.

Surface activated bonding of Ti/Au and Ti/Pt/Au films after vacuum annealing for MEMS packaging

For micro electro mechanical system (MEMS) packaging that can control the internal atmosphere for a long period, we demonstrated a low-temperature bonding process for Au films after vacuum annealing for degassing. Ti (5 nm)/Au (12 nm) and Ti (5 nm)/Pt (10 nm)/Au (12 nm) (from bottom to top) films on Si substrates were bonded by a sequence consisting of surface activation by Ar plasma, vacuum annealing at 200 °C, and bonding at 200 °C or 40 °C. The Ti/Au films failed to form bonding at low temperatures in the case of bonding after the vacuum annealing. However, the Ti/Pt/Au films were successfully bonded even after the vacuum annealing. After the vacuum annealing step, TiO2 formed on the Ti/Au film surface due to thermal diffusion of Ti atoms through the thin Au film. Neither Pt nor Ti atoms were detected on the Ti/Pt/Au film surface after the vacuum annealing step. It is assumed that the thin Pt layer blocked the thermal diffusion of Ti atoms.

Comprehensive Die Shear Test of Silicon Packages Bonded by Thermocompression of Al Layers with Thin Sn Capping or Insertions.

Thermocompression bonding for wafer-level hermetic packaging was demonstrated at the lowest temperature of 370 to 390 °C ever reported using Al films with thin Sn capping or insertions as bonding layer. For shrinking the chip size of MEMS (micro electro mechanical systems), a smaller size of wafer-level packaging and MEMS–ASIC (application specific integrated circuit) integration are of great importance. Metal-based bonding under the temperature of CMOS (complementary metal-oxide-semiconductor) backend process is a key technology, and Al is one of the best candidates for bonding metal in terms of CMOS compatibility. In this study, after the thermocompression bonding of two substrates, the shear fracture strength of dies was measured by a bonding tester, and the shear-fractured surfaces were observed by SEM (scanning electron microscope), EDX (energy dispersive X-ray spectrometry), and a surface profiler to clarify where the shear fracture took place. We confirmed two kinds of fracture mode. One mode is Si bulk fracture mode, where the die shear strength is 41.6 to 209 MPa, proportionally depending on the area of Si fracture. The other mode is bonding interface fracture mode, where the die shear strength is 32.8 to 97.4 MPa. Regardless of the fracture modes, the minimum die shear strength is practical for wafer-level MEMS packaging.