A new method for fast anodic bonding in microsystem technology

Other than temperature and voltage, load plays a key role in anodic bonding process. In this paper we present a new design of top electrode (cathode) for anodic bonding machine by which the bonding time has been reduced up to 30 % in case of bare silicon wafer at ?400 V and approximate 52 % in case of oxidized silicon wafer with Pyrex glass bonding at ?800 V. Experimentally it has been observed there was no bonding in oxidized silicon wafer with Pyrex glass up to ?600 V by using standard design while it has been successfully bonded at same voltage (?600 V) by using new design.     

Low-temperature bonding of thick-film polysilicon for microelectromechanical system (MEMS)

Polysilicon thick films have been found to be an irreplaceable option in various sensors and other microelectromechanical system (MEMS)-designs. Polysilicon is also a prospective option for replacing single-crystal silicon in customized silicon-on-insulator-substrates. Due to the nature of polysilicon, bonding for MEMS-purposes has so far concentrated on anodic bonding, which has drawbacks for instance in terms of process duration and thermal load. The objective of this work is to develop low-temperature direct bonding for various polysilicon films. Polysilicon films were grown at varying temperatures and pressures with and without boron doping. The films were polished by chemical–mechanical polishing and cleaned. Surface qualities were studied by atomic-force-microscope before bonding. Wafers were then activated with argon plasma and bonded to oxidized silicon, quartz and glass. Bonding quality was evaluated with scanning-acoustic-microscope, the crack-opening-method and HF-etching. Scanning-electron-microscopy was used to investigate film and interface quality. This development has led to a new kind of polishing process, where several microns of polysilicon are removed still leaving surface direct bondable. This is accomplished by a dedicated and effectively planarizing polishing process. Spontaneous bonding took place and good bonding quality was achieved after annealing at 200°C.     

A simple thermo-compression bonding setup for wire bonding interconnection in pressure sensor silicon chip packaging

In the process of piezo-resistive pressure sensor packaging, a simple thermo-compression bonding setup has been fabricated to achieve the wire bonding interconnection of a silicon chip with printed circuit board. An annealed gold wire is joined onto a pad surface with a needle-like chisel under a force of 0.5–1.5 N/point. The temperature of the substrate was maintained in the range of 150–200°C and the temperature of the chisel was fixed at around 150°C during wire bonding operation. The tensile strength of the wire bonding was measured with a bonding tester by the destructive-pulling experiment and was found to be at the average of 132 mN/mm². The microstructure of the bonding point was examined by scanning electron microscopy. The interface of the thermo-compression boning was shown to possess an acceptable level of reliability for a micro-electromechanical system (MEMS)-based device. The results showed that this setup can be easily operated for fabrication and is suitable for fabricating not only low-cost pressure sensors, but also other MEMS devices.

     

A simple thermo-compression bonding setup for wire bonding interconnection in pressure sensor silicon chip packaging

In the process of piezo-resistive pressure sensor packaging, a simple thermo-compression bonding setup has been fabricated to achieve the wire bonding interconnection of a silicon chip with printed circuit board. An annealed gold wire is joined onto a pad surface with a needle-like chisel under a force of 0.5?C1.5?N/point. The temperature of the substrate was maintained in the range of 150?C200°C and the temperature of the chisel was fixed at around 150°C during wire bonding operation. The tensile strength of the wire bonding was measured with a bonding tester by the destructive-pulling experiment and was found to be at the average of 132?mN/mm². The microstructure of the bonding point was examined by scanning electron microscopy. The interface of the thermo-compression boning was shown to possess an acceptable level of reliability for a micro-electromechanical system (MEMS)-based device. The results showed that this setup can be easily operated for fabrication and is suitable for fabricating not only low-cost pressure sensors, but also other MEMS devices.     

Transfer of small structures by bonding

The work presented in this paper deals with the bonding of small structures, down to 1 μm. Its aim is to evaluate the dimensional limits of anodic bonding between silicon and pyrex 7740 glass. Test structures consisting in silicon pillars with controlled radii have been developed. The silicon pillars have been fabricated by deep reactive ion etching to allow a good geometry control of the structures. A collection of matrices of 3×3 identical silicon test structures with dimensions from 200 to 1 μm has been fabricated to determine the smallest area that can bond anodically. The test results have been applied to the transfer of small structures from one wafer to another wafer by bonding, with the final objective of transferring tips for AFM probes. From the test results, a new test for bonding has been defined, based on the pull test of small structures with controlled dimensions. Preliminary simulations by FEM of the pull test of the test structures are in agreement with the experimental results. The test has been used to determine the effect of the voltage and temperature conditions during the anodic bonding on the bond strength.     

MEMS光敏BCB硅—硅键合工艺研究

光敏BCB作为粘结介质进行键合工艺实验研究。实验中选用XUS35078负性光敏BCB,提出了优化的光刻工艺参数,得到了所需要的BCB图形层,然后将两硅片在特定的温度与压力条件下完成了BCB键合。测试表明:该光敏BCB具有较小的流动性和较低的塌陷率。键合后的BCB胶厚约为11.6μm,剪切强度为18MPa,He细检漏率小于5.0×10-8atm·cm3/s。此键合工艺可应用于制作需要低温工艺且不能承受高电压的MEMS器件。     

Flexible integration of MEMS and IC for low-cost production of wireless sensor nodes

In this paper, we proposed a flexible process for size-free MEMS and IC integration with high efficiency for MEMS ubiquitous applications in wireless sensor network. In this approach, MEMS and IC can be fabricated individually by different wafers. MEMS and IC known-good-dies (KGD) are temporarily bonded onto carrier wafer with rapid and high-accurate self-alignment by using fine pattern of hydrophobic surface assembled monolayer and capillary force of H₂O; and then KGD are de-bonded from carrier wafer and transferred to target wafer by wafer level permanent bonding with plasma surface activation to reduce bonding temperature and load force. By applying above 2-step process, size of both wafer and chip could be flexible selected. Besides, CMOS processed wafer or silicon interposer can be used as the target wafer. This approach offers us excellent process flexibilities for low-cost production of wireless sensor nodes.     

A new measurement microphone based on MEMS technology

This paper presents a new type of measurement microphone that is based on MEMS technology. The silicon chip design and fabrication are discussed, as well as the specially developed packaging technology. The microphones are tested on a number of key parameters for measurement microphones: sensitivity, noise level, frequency response, and immunity to disturbing environmental parameters, such as temperature changes, humidity, static pressure variations, and vibration. A sensitivity of 22 mV/Pa (-33 dB re. 1 V/Pa), and a noise level of 23 dB(A) were measured. The noise level is 7 dB lower than state-of-the-art 1/4-inch measurement microphones. A good uniformity on sensitivity and frequency response has been measured. The sensitivity to temperature changes, humidity, static pressure variations and vibrations is fully comparable to the traditional measurement microphones. This paper shows that high-quality measurement microphones can be made using MEMS technology, with a superior noise performance.     

Modeling and characterization of an industrial inkjet head for micro-patterning on printed circuit boards

A conceptual design using computational fluid dynamics (CFD) and micro-electro-mechanical systems (MEMS) fabrication has been performed to develop an industrial inkjet head for micro-patterning on printed circuit boards. The printhead has been fabricated with silicon and silicon on insulator (SOI) wafers by MEMS process and silicon to silicon bonding method. The measured displacement waveform from a piezoelectric actuator by laser doppler vibrometer (LDV) was used as input data for the three-dimensional flow solver to simulate the droplet formation. The mechanism of droplet ejection from piezoelectric-type inkjet heads was investigated by simulating two-phase flows of the air and metal inks. As a preliminary approach, liquid metal jetting phenomena are identified by simulating droplet ejection and droplet formation in a consequent manner. Parametric studies are followed by the design optimization process to deduce key factors to inkjet head performance: nozzle geometry, droplet size, ejecting speed, pulse amplitude, and ink viscosity. The present design tool, based on a two-phase flow solver and experimental measurements, has shown its promising applicability to various concept designs of industrial inkjet system for micro-patterning on electronic chips and boards.     

MEMS悬浮结构深反应离子刻蚀保护方法对比研究

分别对基于硅玻键合与硅硅键合的MEMS加工工艺中悬浮结构深反应离子刻蚀保护方法进行对比研究,获得最佳结构刻蚀保护方法。基于硅玻键合工艺的最佳刻蚀保护方法：在结构层背面溅射金属作为保护层;基于硅硅键合工艺的最佳刻蚀保护方法：将结构刻通区域衬底硅暴露及结构层背面制作图形化的SiO2保护层相结合的方式保护。结构保护后,经长时间过刻蚀,结构依然完整无损。     

A Microfabricated Planar Electrospray Array Ionic Liquid Ion Source With Integrated Extractor

This paper reports the design, fabrication, and experimental characterization of a fully microfabricated planar array of externally fed electrospray emitters that produces heavy molecular ions from the ionic liquids $hbox{EMI-BF}_{4}$ and EMI-Im. The microelectromechanical systems (MEMS) electrospray array is composed of the following two microfabricated parts: 1) an emitter die with as many as 502 emitters in 1.13 $hbox{cm}^{2}$ and 2) an extractor component that provides assembly alignment, electrical insulation, and a common bias voltage to the emitter array. The devices were created using Pyrex and silicon substrates, as well as microfabrication techniques such as deep reactive ion etching, low-temperature fusion bonding, and anodic bonding. The emitters are coated with black silicon, which acts as a wicking material for transporting the liquid to the emitter tips. The extractor electrode uses a 3-D MEMS packaging technology that allows hand assembly of the two components with micrometer-level precision. Experimental characterization of the MEMS electrospray array includes current–voltage characteristics, time-of-flight mass spectrometry, beam divergence, and imprints on a collector. The data show that with both ionic liquids and in both polarities, the electrospray array works in the pure ionic regime, emitting ions with as little as 500 V of bias voltage. The data suggest that the MEMS electrospray array ion source could be used in applications such as coating, printing, etching, and nanosatellite propulsion. $hfill$[2008-0270]      

Microrivets for MEMS packaging: concept, fabrication, and strengthtesting

Microriveting is introduced as a novel and alternative joining technique to package MEMS devices. In contrast to the existing methods, mostly surface bonding, the reported technique joins two wafer pieces together by riveting, a mechanical joining means. Advantages include wafer joining at room temperature and low voltage, and relaxed requirements for surface preparation. The microrivets, which hold a cap-base wafer pair together, are formed by filling rivet holes through electroplating. The cap wafer has a recess to house the MEMS devices and also has through-holes to serve as rivet molds. The seed layer on the base wafer becomes the base of the rivet. The process requires only simple mechanical clamping of the wafer pair during riveting, compared with the more involved procedures needed for wafer bonding. Directionality of electroplating in an electric field is what makes this process simple and robust. Strength testing is carried out to evaluate the joining with microrivets. Different modes of rivet failure under different loading conditions are identified and investigated. Effective strength between 7 and 11 MPa was measured under normal loading with nickel microrivets. Joining strengths comparable to conventional wafer bonding processes, ease of fabrication with repeatability, and compatibility with batch fabrication show that microriveting is a feasible technique to join wafers for MEMS packaging, especially when hermetic sealing is not essential     

碳化硅直接键合机理及其力学性能研究

随着碳化硅(SiC)材料的MEMS器件在恶劣环境测量中的应用前景和迫切需求,进行了碳化硅的直接键合实验.研究了工艺条件对键合样品力学性能的影响,同时借助激光共聚焦扫描显微镜(CLSM)、扫描电子显微镜(SEM)、能谱仪(EDS)和拉曼光谱仪等对碳化硅键合样品界面的微观结构进行了分析.结果表明:退火温度和加载压力是影响键合效果的关键性因素.当退火温度为1 300℃,加载压力为3 MPa和退火时间为3 h时,此时键合样品的气密性非常好,力学性能达到最佳,键合强度2 MPa.最后通过样品微观界面分析表明碳化硅直接键合的机理为界面氧化硅过渡层的形成及粘性流动与碳化硅和碳化硅的熔融直接键合.     

Wafer bonding using Cu–Sn intermetallic bonding layers

Wafer-level Cu–Sn intermetallic bonding is an interesting process for advanced applications in the area of MEMS and 3D interconnects. The existence of two intermetallic phases for Cu–Sn system makes the wafer bonding process challenging. The impact of process parameters on final bonding layer quality have been investigated for transient liquid phase wafer-level bonding based on the Cu–Sn system. Subjects of this investigation were bonding temperature profile, bonding time and contact pressure as well as the choice of metal deposition method and the ratio of deposited metal layer thicknesses. Typical failure modes in intermetallic compound growth for the mentioned process and design parameters have been identified and were subjected to qualitative and quantitative analysis. The possibilities to avoid abovementioned failures are indicated based on experimental results.     

Direct bonding with on-wafer metal interconnections

A low temperature direct bonding process with encapsulated metal interconnections was proposed. The process can be realized between silicon wafers or silicon and glass wafers. To establish well-insulated electric connection, sputtered aluminum film was patterned between a bottom thermal SiO₂ and a top PE-SiO₂; the consequential uneven wafer surface was planarized through a chemical mechanical polishing (CMP) step. Benefit from this smooth surface finish, direct bonding is achieved at room temperature, and a general yielding rate of more than 95% is obtained. Test results confirmed the reliability of the bonding. The main advantages of this new technology are its electric connectivity, low thermal stress and hermeticity. This process can be utilized for the packaging of micro electro mechanical system (MEMS) devices or the production of SOI wafers with pre-fabricated electrodes and wires.     

RF MEMS电容式开关结构层释放技术

研究了RF MEMS开关的制造工艺流程和聚酰亚胺牺牲层的去除工艺。在开关的设计和加工中采用在信号线两侧的地线上生长一层绝缘介质层,直流偏置线生成在绝缘介质层之上,与桥的锚点相连接,实现了交直流隔离。讨论了干法刻蚀和湿法刻蚀牺牲层技术。干法刻蚀容易造成绝缘介质层的刻蚀和损伤。采用湿法刻蚀结合临界点干燥技术,可以获得理想的微梁结构。通过测试,开关样品的下拉电压为34 V～40 V,下拉距离为(1.7±0.2)μm,满足设计要求。     

基于微模塑复制技术的平面线圈研制

作为硅微机械技术的替代,低成本的聚合物微制作变得越来越重要.在本研究中,开发了一种微模塑工艺,用以制作以聚合物为基底的平面线圈,将提高其可靠性,降低制造成本.双面密集微沟槽(100μm×70 μm)阵列是线圈基底的一大特点,其初始图形用MEMS技术刻制在硅片上;利用PDMS良好的柔性、弹性和复型分辨率的特性制作了过渡模版,通过图形转移技术成功解决了刚性脱模难的问题.探索出了中温注射、低温固化、高温后固化的最佳模塑成型方法.通过电铸,制得了厚度为0.7 mm、直径φ20 mm的平面微电机定子线圈.     

Effects of dielectric charging on MEMS-based grating light modulator

Silicon dioxide and silicon nitride coatings are preferably used as dielectric layers for short-circuit protection in capacitive silicon MEMS based Grating Light Modulator (GLM). However, their tendency to electrostatic charging can diminish the device reliability. The influence of dielectric charges on GLM was discussed. Gas discharges and charges in the air gap of silicon cantilever GLM have been modeled and observed, resulting in surface charge accumulation on the electrode passivation of the devices. The impact of parasitic surface charges on the deflection versus voltage characteristics of bulk micromachined silicon cantilever GLM is investigated and measurements of the charge decay are shown in experimental results. Experimental results agree with the theoretical calculation well, which illustrates the influence of charging and discharging processing. One method is proposed to reduce the influence of dielectric charges.     

Low temperature wafer bonding for thin silicon film transfer

In this work, we investigate the low temperature (<200 °C) wafer bonding using wet chemical surface activation and we demonstrate high bonding strength sufficient to achieve the transfer of a thin silicon film of thickness less than 400 nm on top of another silicon wafer using spin-on-glass (SOG) film as an intermediate layer. The process developed is the first critical step that can enable three-dimensional (3D) integration and wafer level packaging of MEMS with electronic circuits.     

低温阳极键合技术研究

通过键合温度220~250℃、键合电压400~600 V的硅玻璃低温阳极键合实验,分析了温度和电场分别对键合强度和键合效率的影响,并讨论了键合机理。