Low-Temperature Monolithic Encapsulation Using Porous-Alumina Shell Anodized on Chip

A thin-film encapsulation process, featuring low-temperature steps, hermetic sealing (preliminary), and RF-compatible shell, is reported. Uniquely attractive as compared with the existing MEMS packaging approaches is its capability to monolithically package metal microstructures inside a microcavity on chip in one continuous surface-micromachining process. The key for this process is a technique to fabricate a large freestanding porous membrane on chip by postdeposition anodization of thin-film aluminum at room temperature. The porous-alumina membrane allows for the diffusion of gas or liquid etchants through the nanopores to etch away the sacrificial material underneath, freeing the movable microstructures encapsulated inside the cavity. To seal the package, a thin film is deposited over the alumina shell whose nanoscale pores of a high aspect ratio $(≫ 30)$ do not allow any detectable penetration of the sealing material. The low-temperature $(≪ 300,^{circ}hbox{C})$ encapsulation process produced a low-pressure seal (8 torr), monitored by a Pirani pressure gauge that also represents an encapsulated freestanding metal microstructure in the cavity. The thin-film package demonstrated a considerably low RF insertion loss of less than 0.1 dB up to 40 GHz.$hfill$ [2007-0267]      

A new generic surface micromachining module for MEMS hermetic packaging at temperatures below 200 °C

This paper presents the different processing steps of a new generic surface micromachining module for MEMS hermetic packaging at temperatures around 180 °C based on nickel plating and photoresist sacrificial layers. The advantages of thin film caps are the reduced thickness and area consumption and the promise of being a low-cost batch process. Moreover, sealing happens by a reflow technique, giving the freedom of choosing the pressure and atmosphere inside the cavity. Sacrificial etch holes are situated above the device allowing shorter release times compared to the state-of-the-art. With the so-called over-plating process, small etch holes can be created in the membrane without the need of expensive lithography tools. The etch holes in the membrane have been shown to be sufficiently small to block the sealing material to pass through, but still large enough to enable an efficient release.     

基于倒装技术的电容式绝对压力传感器研究

信号处理与气密封装是电容式压力传感器设计的主要难题.本文提出一种新型的绝对压力传感器结构,利用倒装技术在基片上制作电路和柱状凸点,通过回流焊接将凸点和预先加工好的膜片封接在一起,实现气密封装和电路集成,并通过金属凸点完成电极的转移.测试结果表明传感器的性能良好,在灵敏度19fF/kPa的情况下获得了良好的线性.     

Localized silicon fusion and eutectic bonding for MEMS fabricationand packaging

Silicon fusion and eutectic bonding processes based on the technique of localized heating have been successfully demonstrated. Phosphorus-doped polysilicon and gold films are applied separately in the silicon-to-glass fusion bonding and silicon-to-gold eutectic bonding experiments. These films are patterned as line-shape resistive heaters with widths of 5 or 7 μm for the purpose of heating and bonding. In the experiments, silicon-to-glass fusion bonding and silicon to gold eutectic bonding are successfully achieved at temperatures above 1000°C and 800°C, respectively, by applying 1-MPa contact pressure. Both bonding processes can achieve bonding strength comparable to the fracture toughness of bulk silicon in less than 5 min. Without using global heating furnaces, localized bonding process is conducted in the common environment of room temperature and atmospheric pressure. Although these processes are accomplished within a confined bonding region and under high temperature, the substrate temperature remains low. This new class of bonding scheme has potential applications for microelectromechanical systems fabrication and packaging that require low-temperature processing at the wafer level, excellent bonding strength, and hermetic sealing characteristics     

轴流涡轮转子下游多边界盘腔燃气入侵数值研究

研究涡轮燃气入侵对发动机性能和可靠性具有重要意义,但现有研究多集中于动叶上游盘腔,需要深化对动叶下游盘腔燃气入侵问题的研究。以某高压涡轮及其下游二次空气系统为研究对象,采用三维定常数值模拟探究了适用于转子下游多边界盘腔燃气入侵仿真方法,分析了不同压比下封严流量和燃气入侵特性,并研究了盘腔内部流动机理。结果表明：采用压力边界条件的仿真结果更符合多边界盘腔流动现实。各封严流量随盘腔进口压力增加而增大,但梯度逐渐减小,轮缘封严达到最小流量后基本呈线性变化,而盘腔内部封严流量趋于稳定。在阻止主流入侵的最小封严流量附近,轮缘封严压力分布难以判断主流是否入侵盘腔内部,而轮缘封严齿间流体温度是判断主流燃气入侵盘腔内部的重要参数。     

MEMS器件气密性封装的低温共晶键合工艺研究

介绍了一种采用Pb-Sn共晶合金作为中间层的键合封装技术,通过电镀的方法在芯片与基片上形成Cr/N i/Cu/Pb-Sn多金属层,在温度为190℃、压强为150 Pa的真空中进行键合,键合过程不需使用助焊剂,避免了助焊剂对微器件的污染。试验表明:这种键合工艺具有较好的气密性,键合区合金分布均匀、无缝隙、气泡等脱焊区,键合强度较高,能够满足电子元器件和微机电系统(MEMS)可动部件低温气密性封装的要求。     

A hermetic glass-silicon micropackage with high-density on-chipfeedthroughs for sensors and actuators

This paper describes the development of a hermetic micropackage with high-density on-chip feedthroughs for sensor and actuator applications. The packaging technique uses low-temperature (320°C) electrostatic bonding of a custom-made glass capsule (Corning 7740, 2×2×8 mm³) to fine grain polysilicon in order to form a hermetically sealed cavity. High-density on-chip multiple polysilicon feedthroughs (200 per millimeter) are used for connecting external sensors and actuators to the electronic circuitry inside the package. A high degree of planarity over feedthrough areas is obtained by using grid-shaped polysilicon feedthrough lines that are covered with phosphosilicate glass (PSG), which is subsequently reflown at 1100°C in steam for 2 h. Saline and DI water soak tests at elevated temperatures (85 and 95°C) were performed to determine the reliability of the package. Preliminary results have shown a mean time to failure (MTTF) of 284 days and 118 days at 85 and 95°C, respectively, in DI water. An Arrhenius diffusion model for moisture penetration yields an expected lifetime of 116 years at body temperature (37°C) for these packages. In vivo tests in guinea pigs and rats for periods ranging from one to two months have shown no sign of infection, inflammation, or tissue abnormality around the implanted package     

Fluidic packaging of microengine and microrocket devices for high-pressure and high-temperature operation

The fluidic packaging of Power MEMS devices such as the MIT microengine and microrocket requires the fabrication of hermetic seals capable of withstanding temperature in the range 20-600/spl deg/C and pressures in the range 100-300 atm. We describe an approach to such packaging by attaching Kovar metal tubes to a silicon device using glass seal technology. Failure due to fracture of the seals is a significant reliability concern in the baseline process: microscopy revealed a large number of voids in the glass, pre-cracks in the glass and silicon, and poor wetting of the glass to silicon. The effects of various processing and materials parameters on these phenomena were examined. A robust procedure, based on the use of metal-coated silicon substrates, was developed to ensure good wetting. The bending strength of single-tube specimens was determined at several temperatures. The dominant failure mode changed from fracture at room temperature to yielding of the glass and Kovar at 600/spl deg/C. The strength in tension at room temperature was analyzed using Weibull statistics; these results indicate a probability of survival of 0.99 at an operational pressure of 125 atm at room temperature for single tubes and a corresponding probability of 0.9 for a packaged device with 11 joints. The residual stresses were analyzed using the method of finite elements and recommendations for the improvement of packaging reliability are suggested.     

Effect of bonding temperature on hermetic seal and mechanical support of wafer-level Cu-to-Cu thermo-compression bonding for 3D integration

Hermetic seal and mechanical support of wafer-level Cu-to-Cu thermo-compression bonding with different bonding temperature are analyzed in this work. The investigation consists of two parts: hermetic seal study using helium bomb test and mechanical support study using four-point bending method. The wafer pairs are bonded at 250, 300 and 350 °C, respectively, under a bonding force of 5,500 N for a duration of 1 h in vacuum (~2.5 × 10^?4 mbar). The bonding medium consists of Cu (300 nm) bonding layer and Ti (50 nm) barrier layer. Excellent helium leak rate, which is smaller than the reject limit defined by MIL-STD-883E standard (method 1014.10), and outstanding interfacial adhesion energy are detected for all samples. The cavities sealed at 300 °C present an excellent reliability of temperature cycling test up to 500 cycles. Cu-to-Cu thermo-compression bonding at low temperature (≤300 °C) presents an attractive hermetic seal and a robust mechanical support for 3D integration application.     

Batch-processed vacuum-sealed capacitive pressure sensors

This paper reports two multitransducer vacuum-sealed capacitive barometric pressure sensors, one using single-lead and the other using multiple-leads to transfer the electrical signal out of the vacuum-sealed reference cavity. The first device operates with a resolution of 37 mtorr over a pressure range from 600 to 800 torr. The sensitivity is 27 fF/torr (3000 ppm/torr). The TCO at 750 torr is 3900 ppm/°C and the TCS is 1000 ppm/°C. The second device has a resolution of 25 mtorr over a range from 500 to 800 torr, with individual transducer sensitivity of 39 fF/torr. The TCO at 750 torr is 1350 ppm/°C and TCS is 1000 ppm/°C. Both devices have an on-chip compensation capacitor and are read out using an electronically-trimmed switched-capacitor charge integrator     

A hermetic glass-silicon package formed using localizedaluminum/silicon-glass bonding

A hermetic package based on localized aluminum/silicon-to-glass bonding has been successfully demonstrated. Less than 0.2 MPa contact pressure with 46 mA current input for two parallel 3.5-μm-wide polysilicon on-chip microheaters can raise the temperature of the bonding region to 700°C bonding temperature and achieve a strong and reliable bond in 7.5 min. The formation of aluminum oxide with silicon precipitate composite layer is believed to be the source of the strong bond. Accelerated testing in an autoclave shows some packages survive more than 450 h under 3 atm, 100% RH and 128°C. Premature failure has been attributed to some unbonded regions on the failed samples. The bonding yield and reliability have been improved by increasing bonding time and applied pressure     

A low-temperature thin-film electroplated metal vacuum package

This paper presents a packaging technology that employs an electroplated nickel film to vacuum seal a MEMS structure at the wafer level. The package is fabricated in a low-temperature (<250/spl deg/C) 3-mask process by electroplating a 40-/spl mu/m-thick nickel film over an 8-/spl mu/m sacrificial photoresist that is removed prior to package sealing. A large fluidic access port enables an 800/spl times/800 /spl mu/m package to be released in less than three hours. MEMS device release is performed after the formation of the first level package. The maximum fabrication temperature of 250/spl deg/C represents the lowest temperature ever reported for thin film packages (previous low /spl sim/400/spl deg/C). Implementation of electrical feedthroughs in this process requires no planarization. Several mechanisms, based upon localized melting and Pb/Sn solder bumping, for sealing low fluidic resistance feedthroughs have been investigated. This package has been fabricated with an integrated Pirani gauge to further characterize the different sealing technologies. These gauges have been used to establish the hermeticity of the different sealing technologies and have measured a sealing pressure of /spl sim/1.5 torr. Short-term (/spl sim/several weeks) reliability data is also presented.     

Device-level hermetic packaging of microresonators by RTP aluminum-to-nitride bonding

This paper presents a device-level microelectromechanical systems (MEMS) packaging process with accelerated tests and reliability analysis. Surface-micromachined microresonators are sealed inside microcavities by a rapid thermal processing (RTP) aluminum-to-silicon nitride bonding and packaging technique. Chip-to-chip bonding is used to form packages both under atmospheric pressure and in vacuum. The hermeticity of the package seals are evaluated by IPA (isopropyl alcohol) leak tests. The vacuum seal is evaluated by measuring the Q-factor (quality factor) of the packaged microresonators. The measured Q-factor of a vacuum-packaged comb-resonator is 1800/spl plusmn/200, corresponding to a 200 motor vacuum inside the micro cavity, and has not degraded over 37 weeks of shelf-life. The reliability information is evaluated by combining accelerated testing of the packages in a harsh environment (an autoclave chamber, 130/spl deg/C, 2.7 atm and 100% RH) and statistical analysis. The mean-time-to-failure (MTTF) of the packaged device is estimated as 29.7 weeks in an autoclave chamber, and tests on vacuum-packaged devices have confirmed the estimation.     

Hermeticity Evaluation of Polymer-Sealed MEMS Packages by Gas Diffusion Analysis

A gas transport mechanism is studied to characterize the hermetic behavior of polymer-sealed microelectromechanical systems packages. Diffusion-based governing equations, which are fundamentally different from the conduction-based governing equations used for metallic seals, are proposed to predict a change in cavity pressure. An effective numerical scheme is developed to implement the governing equations. The validity of the governing equations is corroborated by the optical leak test. The verified gas diffusion model is utilized to investigate the effect of the diffusion properties and geometries of polymeric seals on the gas leak behavior. $hfill$[2008-0264]      

Temperature characterization of flip-chip packaged piezoresistive barometric pressure sensors

In order to miniaturize piezoresistive barometric pressure sensors, a new flip-chip packaging technology has been developed. The thermal expansions of chip and package are different. So in a standard flip-chip package the strong mechanical coupling by the solder bumps would lead to stress in the sensor chip, which is unacceptable for piezoresistive pressure sensors. To solve this problem, in the new packaging technology the chip is flip-chip bonded on compliant springs to decouple chip and package. As the first step of the packaging process an under bump metallization (UBM) is patterned on the sensor wafer. Then solder bumps are printed. After wafer-dicing the chips are flip-chip bonded on copper springs within a ceramic cavity housing. Due to the compliance of the springs, packaging stress is induced into the sensor chip. As sources of residual stress the UBM and the solder bumps on the sensor chip were identified. Different coefficients of thermal expansion of the silicon chip, the UBM and the solder lead to plastic straining of the aluminum metallization between UBM and chip. As a consequence the measurement accuracy is limited by a temperature hysteresis. The influence of the chip geometry, e.g., the thickness of the chip or the depth of the cavity, on the hysteresis was investigated by simulation and measurements. As a result of this investigation a sensor chip was designed with very low residual stress and a temperature hysteresis which is only slightly larger than the noise of the sensor.     

Long-Term and Accelerated Life Testing of a Novel Single-Wafer Vacuum Encapsulation for MEMS Resonators

We have developed a single-wafer vacuum encapsulation for microelectromechanical systems (MEMS), using a thick (20-mum) polysilicon encapsulation to package micromechanical resonators in a pressure <1 Pa. The encapsulation is robust enough to withstand standard back-end processing steps, such as wafer dicing, die handling, and injection molding of plastic. We have continuously monitored the pressure of encapsulated resonators at ambient temperature for more than 10 000 h and have seen no measurable change of pressure inside the encapsulation. We have subjected packaged resonators to >600 cycles of -50 to 80degC, and no measurable change in cavity pressure was seen. We have also performed accelerated leakage tests by driving hydrogen gas in and out of the encapsulation at elevated temperature. Two results have come from these hydrogen diffusion tests. First, hydrogen diffusion rates through the encapsulation at temperatures 300-400degC have been determined. Second, the package was shown to withstand multiple temperature cycles between room and 300-400degC without showing any adverse affects. The high robustness and stability of the encapsulation can be attributed to the clean, high-temperature environment during the sealing process     

Characterization of intermediate In/Ag layers of low temperature fluxless solder based wafer bonding for MEMS packaging

Low temperature fluxless solder for wafer bonding has received a lot of attention due to its great potential in hermetic MEMS packaging. Previous research activities mainly deploy solder alloy of eutectic composition to achieve low bonding temperature. We proposed new intermediate bonding layers (IBLs) of rich Ag composition in In–Ag materials systems. In this study, we investigated the intermetallic compounds (IMCs) at the bonding interface with respect to the bonding condition, post-bonding room temperature storage and post-bonding heat treatment. With this IBL, the IMCs of Ag₂In and Ag₉In₄ with high temperature resist to post-bonding process are derived under process condition of wafer bonding at 180 °C, 40 min and subsequent 120–130 °C annealing for 24 h. Low melting temperature IMC phase of AgIn₂ is formed in the interface after long term room temperature storage or 70 °C aging treatment. This low melting temperature IMC phase can be completely converted into high melting temperature IMCs of Ag₂In and Ag₉In₄ after 120 °C additional annealing. Based on our results, we can design the packaging process flow so as to get reliable hermetic packaged MEMS devices by using low temperature fluxless In–Ag wafer bonding.     

基于吹塑成型方法的微玻璃空腔的设计与制备

根据硼硅酸盐玻璃的内部结构特殊性和热学性质,设计并制备出两种3D微玻璃空腔,主要讲述了3D微玻璃空腔的设计过程和吹塑成型的制备方法。 CORNING Pyrex 7740玻璃是硼硅酸盐玻璃的代表。将硅片进行深硅刻蚀形成深槽,并与7740玻璃进行常压下的阳极键合,形成微空腔;将得到的微空腔放入真空退火炉中进行退火,使玻璃空腔内部空气膨胀,最终形成3D微玻璃空腔。经过实验得到的两种3D微玻璃空腔表明其制备工艺的可行性,将制备出的3D微玻璃空腔运用到导航器件的设计和微结构的封装等方面,具有比较好的发展前景。     

瓦斯抽采钻孔漏气通道检测装置研制及应用

瓦斯抽采钻孔孔周裂隙和封孔段空隙通道是造成钻孔漏气失效的主要原因。为有效检测钻孔漏气通道,基于管流流体力学理论和漏气检测判别方法,研制了瓦斯抽采钻孔漏气通道检测装置。通过检测不同钻孔深度气样参数并分析其分布规律和突变情况,确定抽采钻孔失效原因和漏气通道位置;检测装置采用高稳压阻式压力传感器、激光甲烷传感器和荧光氧气传感器实现抽采负压、瓦斯浓度和氧气浓度检测,并采用1.5 m/节快接式25 mm薄壁不锈钢管作为取气管件,钻孔检测深度达30 m。现场应用结果表明,抽采管段检测参数变化稳定,说明抽采管未发生破损或接口漏气等,抽采管密封效果较好;在封孔段,距孔口9~18 m范围内存在多处不同程度的突变点,最大漏气通道在距孔口9~12 m范围内,说明原封孔深度不足,原封孔工艺无法有效密封漏气通道。将封孔深度增加至12 m,并采用“两堵一注”带压注浆封孔工艺,进行对比试验,结果表明,改进后试验钻孔整体抽采效果大幅改善,孔口瓦斯体积分数提升至55%以上,在距孔口12 m以深范围内瓦斯体积分数变化稳定,氧气体积分数近乎为0,漏气通道减少。试验结果验证了瓦斯抽采钻孔漏气通道检测装置能够有效检测漏气通道,为有针对性地调整封孔方式和相关参数及后续改进工作提供依据。     

Dispensing and hermetic sealing Rb in a miniature reference cell for integrated atomic clocks

An innovative method for fabricating the reference cell for a Rubidium (Rb) integrated atomic clock is presented. This method uses low-temperature solder sealing technique for producing mini-cells of the size of 14 mm × 10 mm × 3 mm, suitable for Rb miniature atomic clocks. Top and bottom of the cell consists of two glass slides. An LTCC (low-temperature cofired ceramic) module of 2 mm thickness, equipped with a small reservoir for confining the Rb is placed in between the two walls, acting as a spacer, increasing the total volume of the cell. A solder ring joins together the LTCC and the top of the cell. This paper also presents a new technique for handling the Rb, which allows its safe handling and long storage. The alkali metal is stored inside a pool of dodecane, which protects it from oxidation. Pure liquid Rb is then dispensed inside the adjacent reservoir using a glass micropipette; finally, the cell is heated in vacuum, in order to carry out the sealing. The achieved sealing hermeticity was tested, without Rb, by sealing N₂O gas and monitoring its pressure through absorbance measurements using FTIR spectroscopy. Hermeticity was also tested with Rb by integrating a pressure sensor in the LTCC module.