Search

Controlling the oxygen content becomes a formidable challenge when using the traditional method of copper pre-oxidation to attach copper metal to AlN ceramic substrates. The issue results in copious amounts of copper oxide (CuO) formation. Excessive CuO causes voids to form at the interfacial junction, harming the long-term reliability of the copper-bonded substrate. To address these issues, this study employed the sol-gel technique, in which a copper-containing solution was applied to the AlN substrate via spin coating, followed by thermal treatment in a nitrogen-rich atmosphere. The goal was to create thin films of Cu 2-x O to improve the adhesion between copper and AlN substrates. It was observed that the surface roughness of the resulting Cu 2-x O thin film after 5 spin-coating cycles was the lowest (3.48 nm). Furthermore, at a fixed thickness of the Cu 2-x O oxide layer (approximately 36 nm), insufficient oxygen content in the thin film heated at 250 °C prevented the formation of sufficient Cu–O eutectic liquid at high temperatures, resulting in insufficient copper-bonded substrate bonding strength (0.47 MPa). When the thin film was heated to 400 °C, the proportion of the CuO phase in the film increased, effectively increasing the bonding strength of the copper-bonded substrate (0.84 MPa). [ABSTRACT FROM AUTHOR]

In this paper, an all-Si resonant pressure microsensor based on eutectic bonding was developed, which can eliminate thermal expansion coefficient mismatches and residual thermal stresses during the bonding process. More specifically, the resonant pressure microsensor included an SOI wafer with a pressure-sensitive film embedded with resonators, which was eutectically bonded with a silicon cap for vacuum encapsulation. The all-Si resonant pressure microsensor was carefully designed and simulated numerically, where the use of the silicon cap was shown to effectively address temperature disturbances of the microsensor. The microsensor was then fabricated based on MEMS processes where eutectic bonding was adopted to link the SOI wafer and the silicon cap. The characterization results showed that the temperature disturbances of the resonant pressure microsensor encapsulated with the silicon cap were quantified as −0.82 Hz/°C of the central resonator and −2.36 Hz/°C of the side resonator within a temperature range from −40 °C to 80 °C, which were at least eight times lower than that of the microsensor encapsulated with the glass cap. Compared with the microsensor using the glass cap, the all-silicon microsensor demonstrated an accuracy improvement from 0.03% FS to 0.01% FS and a reduction in short-term frequency fluctuations from 3.2 Hz to 1.5 Hz. [ABSTRACT FROM AUTHOR]

We explore compatibility of Ru with Al-Ge eutectic wafer bonding. We first present experiments to check for the presence of Ru ternary alloy poisoning inhibiting Al-Ge melting as well as evaluations of Al-Ge melt wettability on Ru and diffusion outcomes following bond-simulating anneals. Results show that Ru is stable with no observed microstructural changes or dissolution in the melt, indicating no ternary poisoning for the applied thermal budget. Ru was found to act as an effective barrier offering good melt wettability in all considered configurations with Al and Ge. From inspection of the binary constituents of Al-Ge-Ru we propose that Al-Ge eutectic melting temperature will decrease marginally for Ru contamination in a 1-2% range before a drastic increase in melting temperature (>10°C/% Ru) at higher Ru compositions. We then demonstrate wafer-level packaged 200 mm devices and MEMS with strong bond outcomes of devices bearing Ru contacts. We conclude that Ru has high compatibility with Al-Ge eutectic bonding. [2022-0037] [ABSTRACT FROM AUTHOR]

This paper presented a temperature-insensitive resonant pressure microsensor where silicon based resonators anchored on a pressure-sensitive diaphragm were vacuum packaged by a silicon cap based on eutectic bonding. Incoming pressures deformed the pressure-sensitive diaphragm and built stresses around resonators for frequency modulation while under temperature challenges, deformations of silicon based resonators and the vacuum cap were consistent and thus no stresses were generated on resonators. The temperature-insensitive resonant pressure microsensor was analyzed in both theoretical analysis and numerical simulations with confirmed high pressure sensitivities and low temperature disturbances. The resonant pressure microsensor was then fabricated by key steps of photolithography, deep reactive ion etching, and eutectic bonding and characterized in both open-loop and close-loop testing systems. Characterization results showed that the quality factors of resonators were $\sim 10000$ with pressure sensitivity of 82.98 Hz/kPa and temperature disturbance of −0.63 Hz/° (the lowest result among previously reported resonant pressure microsensors). In summary, the temperature-insensitive resonant pressure microsensor developed in this study exhibited a fitting accuracy better than 0.02% FS within the pressure range of 10 to 120 kPa and the temperature range of −45 to $85^{\circ }\text{C}$. [ABSTRACT FROM AUTHOR]

In this paper, an all-Si resonant pressure microsensor based on eutectic bonding was developed, which can eliminate thermal expansion coefficient mismatches and residual thermal stresses during the bonding process. More specifically, the resonant pressure microsensor included an SOI wafer with a pressure-sensitive film embedded with resonators, which was eutectically bonded with a silicon cap for vacuum encapsulation. The all-Si resonant pressure microsensor was carefully designed and simulated numerically, where the use of the silicon cap was shown to effectively address temperature disturbances of the microsensor. The microsensor was then fabricated based on MEMS processes where eutectic bonding was adopted to link the SOI wafer and the silicon cap. The characterization results showed that the temperature disturbances of the resonant pressure microsensor encapsulated with the silicon cap were quantified as −0.82 Hz/°C of the central resonator and −2.36 Hz/°C of the side resonator within a temperature range from −40 °C to 80 °C, which were at least eight times lower than that of the microsensor encapsulated with the glass cap. Compared with the microsensor using the glass cap, the all-silicon microsensor demonstrated an accuracy improvement from 0.03% FS to 0.01% FS and a reduction in short-term frequency fluctuations from 3.2 Hz to 1.5 Hz.

Featured Application: A systematic method for high vacuum packaging of MEMS devices is proposed in this paper. It is applicable to many kinds of MEMS devices such as sensors, gyroscopes and actuators to reduce power consumption and improve their performance. Vacuum packaging of Micro-electro-mechanical system (MEMS) devices is a hot topic for its advantages of improving performance and reducing power consumption. In this paper, the physics package of a chip scale atomic clock (CSAC), as a typical kind of MEMS device, is performed by vacuum packaging based on a systematic method proposed by us. The whole process, including low outgassing and thermal stable materials selection, prebaking for desorption, getter firing for absorption and solder reflow for vacuum sealing is introduced thoroughly. The thermogravimetric analysis or thermal gravimetric analysis (TGA) is used to analyze the thermal stability and desorption of materials. The leak rate of physics packages is measured to be less than 4 × 10−10 Pa·m3/s by helium leak detection. The residual gas pressure and composition in physics packages are analyzed after vacuum packaging. The results show a high vacuum ~0.1 Pa in the physics package. The frequency stability is improved from 4.68 × 10−11 to 1.07 × 10−11 @40,000 s. The presented method for high vacuum packaging is also applicable to other MEMS devices. [ABSTRACT FROM AUTHOR]

In this study, we report the high‐brightness 1.53 inch 130 PPI active‐matrix light‐emitting diode display (AMLED). Dual gate coplanar amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) thin film transistor (TFT) is applied for the high brightness micro LED display. The micro LEDs optimized for the a‐IGZO TFT backplane are fabricated, and the low‐temperature eutectic bonding is applied to prevent thermal bonding damage. Finally, the glass based AMLED over ~ 10,000 cd/m2 is demonstrated, using the dual gate coplanar a‐IGZO TFT. [ABSTRACT FROM AUTHOR]

A miniaturized lead zirconate titanate (PZT)-actuated Fabry-Perot interferometer (FPI) was developed for an 850 nm light beam selection by modulating the air gap between two refractive mirrors. The developed FPI consists of mirrors, a PZT actuator for air gap modulation, metal electrodes, and interface electronics capable of applying a square voltage at a frequency of 0.1 Hz. Eutectic wafer bonding method was employed to bond two mirrors through rapid thermal annealing (RTA) and applying mechanical pressure for a short time to minimize the interdiffusion between layers. The PZT actuator and metal electrodes serve as a thickness spacer to create an air gap between the two mirror plates. Alternating high and low dielectric layers (TiO 2 and SiO 2) with precise thickness and high crystallization quality were utilized for the formation of the refractive mirror because these layers offer high reflectivity and low absorption loss, enabling effective transmission of a specific wavelength in the incident light beam. By applying 0.1 Hz square pulses to the PZT actuator, the air gap was modulated, resulting in significant shifts in the center wavelength of the filtered beam at the desired wavelength. The study demonstrates the successful modulation and optimization of transmission peaks, ensuring a sharp full width at half maximum (FWHM) and high transmission rate. It also provides comprehensive experimental process factors for optimizing heat treatment conditions for PZT and refractive mirrors, minimizing metal interdiffusion between the dielectric layers, and ensuring mirror plate flatness. The device parameter effects such as cavity length orders, sequence of refractive indices for dielectric layers, and the extent of elongation of PZT with applied voltage were systematically studied to achieve optimal performance of FPI filter. The developed interface electronics allow voltage modulation up to 21.9 V at a frequency of 0.1 Hz, while preserving the elasticity of the PZT actuator. [Display omitted] • A miniaturized PZT-controlled Fabry-Perot Interferometer (FPI) designed for selecting light beam at 850 nm was developed. • Interface electronics was implemented that allow voltage modulation up to 53.8 V in DC voltage. • Transmission peaks were effectively modulated, maintaining both a sharp FWHM and a high transmission rate. • Comprehensive insights for bonding, minimal metal interdiffusion between layers, and the flatness of mirror were provided. [ABSTRACT FROM AUTHOR]

Vacuum packaging of Micro-electro-mechanical system (MEMS) devices is a hot topic for its advantages of improving performance and reducing power consumption. In this paper, the physics package of a chip scale atomic clock (CSAC), as a typical kind of MEMS device, is performed by vacuum packaging based on a systematic method proposed by us. The whole process, including low outgassing and thermal stable materials selection, prebaking for desorption, getter firing for absorption and solder reflow for vacuum sealing is introduced thoroughly. The thermogravimetric analysis or thermal gravimetric analysis (TGA) is used to analyze the thermal stability and desorption of materials. The leak rate of physics packages is measured to be less than 4 × 10−10 Pa·m3/s by helium leak detection. The residual gas pressure and composition in physics packages are analyzed after vacuum packaging. The results show a high vacuum ~0.1 Pa in the physics package. The frequency stability is improved from 4.68 × 10−11 to 1.07 × 10−11 @40,000 s. The presented method for high vacuum packaging is also applicable to other MEMS devices.

Metal wafer bonding is being investigated in industry and research fields due to the relatively low process temperatures (T < 500 °C) and, at the same time, to obtain a metallic junction able to offer greater reliability compared with other materials. In this work, electrodeposition of eutectic Sn-Cu alloy, suitable for wafer bonding, was studied using ethylene glycol (EG) as a solvent. The resulting organic electrolyte was used for its promising properties, such as a wider electrochemical window than traditional aqueous electroplating baths. Ethylene glycol solutions containing copper(II) and tin(II) chloride salts were characterised electrochemically by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV). Deposition was performed both under direct (DC) and pulsed (PC) current conditions and the advantages of the latter are discussed, also in terms of surface morphology as observed by scanning electron microscope (SEM). Wafer bonding was successfully achieved and the interdiffusion of copper and tin species in the bonding region is discussed. [ABSTRACT FROM AUTHOR]

Ultrathin buffer layers (UBLs) with varied thickness ranging from 10 to 100 nm and different materials were used in Cu/Sn eutectic bonding. A Cu/Sn film thinner than $2~\mu \text{m}$ could fully react and became stiff and rough Cu-Sn intermetallic compound layer, which leads to failure bonding. Four kinds of semiconductor compatible materials including Ti, Pd, Co, and Ni were inserted between Cu/Sn to delay interdiffusion prior to eutectic bonding. In addition to symmetric Cu/Sn bonding with UBL, asymmetric Cu/Sn-Cu bonding scheme with 50-nm Ni UBL was demonstrated. With good mechanical properties, bonding quality, and electrical characteristics, the application of submicrometer Cu/Sn wafer-level bonding by assistance of buffering layer gives a promising and flexible platform for future 3-D integration applications. [ABSTRACT FROM AUTHOR]

Distinct from conventional diaphragm-type pressure sensors, a silicon-based bulk-type high pressure sensor has been analyzed, realized, and characterized. External hydrostatic pressure acting on the sensor is converted to a biaxial compression inside an all-silicon encapsulated vacuum cavity. The stress anisotropy is analytically modeled and numerically simulated. The biaxial compression is measured using two pairs of piezoresistors oriented to optimally utilize the anisotropy of silicon piezoresistance. An improved pressure seal in the test-package allowed extended testing of the sensor up to a pressure of 200 MPa and a temperature of 175 °C. Reported also is a zero-offset of the sensor, largely attributed to the tensile stress induced by the insulating cover oxide after cooling from the high-temperature dopant activation anneal. [2017-0253] [ABSTRACT FROM PUBLISHER]

A Ge thin film deposited by chemical vapor deposition (CVD) was used to obtain a uniform bonding between Au and Ge films for applications of wafer level packages (WLPs). This Ge CVD thin film showed selective growth on Au and Cu metals when the substrate has both metal and oxide. A one-step and two-step Ge deposition followed by eutectic bonding method was employed to bond the wafers. The samples were characterized by X-ray diffraction, field emission scanning electron microscopy equipped with an energy dispersive spectroscopy (FESEM-EDS), atomic force microscopy, high resolution Field emission transmission electron microscopy, IR inspection tool and secondary ion mass spectroscopy (SIMS). According to the IR inspection results, the two-step Ge deposited sample showed more uniform film compared to one-step deposition after eutectic bonding. Moreover, an improved bonding quality was obtained from the two-step process. Based on FESEM observations, a uniform and crater-free interface was detected between the bonded 4-inch wafers, in which the presence of Ge beside Au and Si was confirmed by EDS. SIMS profiles proved the formation of a thin Au-Ge interlayer at the bonded interface, which enhanced the bonding conditions. [ABSTRACT FROM AUTHOR]

The reach and impact of the Internet of Things will depend on the availability of low-cost, smart sensors--"low cost" for ubiquitous presence, and "smart" for connectivity and autonomy. By using wafer-level processes not only for the smart sensor fabrication and integration, but also for packaging, we can further greatly reduce the cost of sensor components and systems as well as further decrease their size and weight. This paper reviews the state-of-the-art in the wafer-level vacuum packaging technology of smart sensors. We describe the processes needed to create the wafer-scale vacuum microchambers, focusing on approaches that involve metal seals and that are compatible with the thermal budget of complementary metal-oxide semiconductor (CMOS) integrated circuits. We review choices of seal materials and structures that are available to a device designer, and present techniques used for the fabrication of metal seals on device and window wafers. We also analyze the deposition and activation of thin film getters needed to maintain vacuum in the ultra-small chambers, and the wafer-to-wafer bonding processes that form the hermetic seal. We discuss inherent trade-offs and challenges of each seal material set and the corresponding bonding processes. Finally, we identify areas for further research that could help broaden implementations of the wafer-level vacuum packaging technology. [ABSTRACT FROM AUTHOR]

The high-temperature requirement of Al-Ge eutectic bonding stands as a major obstacle to its wider acceptance for hermetic sealing application in the microelectromechanical systems packaging industry, in particular for temperature-sensitive devices. It has been demonstrated that a reduction in bonding temperature is feasible without compromising the hermeticity. The change in the mode of bonding from eutectic to solid-state diffusion did not have a dramatic impact on the bonding quality. However, this resulted in a substantial increase in bonding time. The shear strength also deteriorated as a result of the decrease in thickness of the reaction interface. However, the shear strength still complied with military standards. It has been confirmed that a hermetic seal could still be achieved without any solidification occurring at the interface. This is feasible since the interdiffusion coefficients of Al in (Ge) phase and Ge in (Al) phase are closer and are comparable to diffusion between solid-solution phases of identical metals such as in Au-Au, Cu-Cu, and Si-Si bonding, which are generally used for such hermetic sealing application. An appropriate stacking mechanism for Al-Ge diffusion bonding is identified to overcome the limitations with respect to surface topography. [ABSTRACT FROM AUTHOR]

Eutectic compounds play an important role in the fabrication and packaging of devices with low thermal budgets. Taking low-cost polymers into account, eutectic soldering and eutectic bonding temperatures below the glass transition temperature of the aforementioned polymers, 150 °C and below, need to be met. In this regard, thermocompressive eutectic bonding processes of silicon integrated circuits by means of the binary eutectic compounds indium–tin and indium–bismuth were optimized with regard to least necessary thermal energy, and the feasibility thereof onto polycarbonate and poly(methyl methacrylate) substrates was investigated. [ABSTRACT FROM PUBLISHER]

This paper describes the investigation of a microdischarge-based approach for sensing the diaphragm deflection in a monolithically fabricated pressure sensor. This transduction approach is appealing from the viewpoint of miniaturization. The device consists of a deflecting Si diaphragm with a sensing cathode, and a glass substrate with an anode and a reference cathode. The total exterior volume of the device is 0.05 mm3; typical electrode size and separations are 35 and 10 mm3. Pulsed microdischarges are initiated in a sealed chamber formed between Si and glass chips, and are filled with Ar gas. External pressure deflects the Si diaphragm and changes the interelectrode spacing, thereby redistributing the current between the anode and two competing cathodes. The differential current is indicative of the diaphragm deflection which is determined by the external pressure. A 6-mask microfabrication process is investigated for device fabrication. Electrode connections to the interior of the chamber are provided by laser drilling and copper electroplating through high aspect ratio glass vias. The Si and glass substrates are bonded by Au-In eutectic. The redistribution of plasma current between competing cathodes, as a consequence of diaphragm deflection over a range of pressure, was experimentally demonstrated. First principles modeling of transient microdischarges have provided insights to the fundamental processes responsible for the differential current and guidance for scaling the device to smaller dimension. [ABSTRACT FROM AUTHOR]

The reach and impact of the Internet of Things will depend on the availability of low-cost, smart sensors—“low cost” for ubiquitous presence, and “smart” for connectivity and autonomy. By using wafer-level processes not only for the smart sensor fabrication and integration, but also for packaging, we can further greatly reduce the cost of sensor components and systems as well as further decrease their size and weight. This paper reviews the state-of-the-art in the wafer-level vacuum packaging technology of smart sensors. We describe the processes needed to create the wafer-scale vacuum microchambers, focusing on approaches that involve metal seals and that are compatible with the thermal budget of complementary metal-oxide semiconductor (CMOS) integrated circuits. We review choices of seal materials and structures that are available to a device designer, and present techniques used for the fabrication of metal seals on device and window wafers. We also analyze the deposition and activation of thin film getters needed to maintain vacuum in the ultra-small chambers, and the wafer-to-wafer bonding processes that form the hermetic seal. We discuss inherent trade-offs and challenges of each seal material set and the corresponding bonding processes. Finally, we identify areas for further research that could help broaden implementations of the wafer-level vacuum packaging technology.

As a major step in the conduction of heat dissipation, wafer bonding significantly contributes to device reliability. This work presents the Cu–Sn eutectic bonding with a Mo buffer layer for light emitting diodes to increase its reliability. The conventional Ag-paste bonding and Cu–Sn bonding (without Mo top layer) are also studied for comparison. Their reliability is measured using thermal shock treatments ranging from − 40 to 120 °C (200 cycles). Experimental results indicate that the eutectic bonding with a Mo buffer layer demonstrates a better device performance than the other two counterparts. Following thermal shock treatment, the light droops for the Ag-paste, Cu–Sn, and Mo/Cu–Sn bonding samples are 15, 11 and 7%, respectively. Additionally, the increasing ratios of thermal resistance are 26, 33 and 19%, respectively. Moreover, adding the Mo buffer layer can relieve the thermal stress problem, owing to the thermal expansion mismatch in Gallium–Nitride/Cu–Sn/submount wafer bonding structures. [ABSTRACT FROM AUTHOR]

Fluxless GaAs-on-Si wafer bonding using Ag/Sn solder was investigated to realize uniform and void-free heterogeneous material integration. The effects of the diffusion barrier, Ag/Sn thickness, and Ar plasma treatment were studied to achieve the optimal fluxless bonding process. Pt on a GaAs wafer and Mo on a Si wafer act as diffusion barriers by preventing the flow of Ag/Sn solder into both the wafers. The bonding strength is closely related to the Ag/Sn thickness and Ar plasma treatment. A shear strength test was carried out to investigate the bonding strength. Under identical bonding conditions, the Ag/Sn thickness was optimized to achieve higher bonding strength and to avoid the formation of voids due to thermal stress. An Ar plasma pretreatment process improved the bonding strength because the Ar plasma removed carbon contaminants and metal-oxide bonds from the metal surface. [ABSTRACT FROM AUTHOR]

Highlights: [•] We develop a method to enhance the vacuum package of the capacitive pressure sensor in the process of anodic bonding. [•] The method is realized by using localized Si–Au eutectic bonding with a new structure with sunken pad. [•] The sunken pad is with a larger size than the opened window in silicon wafer. [•] Eutectic bonding occurred around the sunken pad. [Copyright &y& Elsevier]

Abstract: In this paper, an nickel-chromium (NiCr) alloy piezoresistive atmosphere pressure sensor, based on eutectic bonding is presented. Compared with conventionally silicon piezoresistive pressure sensor, it has the advantages with low cost and easy fabrication processes. First, NiCr alloy (80:20 wt%) is used as the strain detecting material, with a smaller but acceptable sensitivity and simpler processes. Second, eutectic bonding technology based on silver-tin (Ag-Sn) alloy, which is 3.5%wt Ag and 20um thickness, is used as an alternative and easy bonding choice to complete the vacuum package. Bonding quality of eutectic bonding is evaluated by inspection through the deflection of diaphragm of silicon with more than 95% of the area successfully bonded. The Pressure-Voltage characteristic test results suggest a precision within 0.3% in square fitting. The temperature characteristics is also tested and theoretically analyzed. The temperature coefficient offset (TCO) is 620ppm/(°C FSO). [Copyright &y& Elsevier]

Abstract: The simple Auic, which melts at 363°C, can be used to bond Si wafers. However, faceted craters can form at the Au/Si interface as a result of anisotropic and non-uniform reaction between Au and crystalline silicon (c-Si). These craters may adversely affect active devices on the wafers. Two possible solutions to this problem were investigated in this study. One solution was to use an amorphous silicon layer (a-Si) that was deposited on the c-Si substrate to bond with the Au. The other solution was to use solid-state bonding instead of eutectic bonding, and the wafers were bonded at a temperature (350°C) below the Auic temperature. The results showed that the a-Si layer prevented the formation of craters and solid-state bonding not only required a lower bonding temperature than eutectic bonding, but also prevented spill out of the solder resulting in strong bonds with high shear strength in comparison with eutectic bonding. Using amorphous silicon, the maximum shear strength for the solid-state Aueached 15.2MPa, whereas for the eutectic Aut was 13.2MPa. [Copyright &y& Elsevier]

This paper presents a novel gas microvalve design concept, in which a flow control gate is opened by a pneumatic pressure and closed by a shape memory alloy actuator, allowing large flow control. Two different design variations were fabricated using a novel wafer-level Au–Si eutectic bonding process for TiNi to silicon integration. The resulting microvalves demonstrate a record pneumatic performance per footprint area; a microvalve with a footprint of only 1 \times 3.3\ \mm^2 successfully controls a flow difference of 3100 sccm at a pressure drop of 70 kPa using a power of 0.35 W. [ABSTRACT FROM PUBLISHER]