Chang-youl Moon

Application of Au-Sn eutectic bonding in hermetic radio-frequency microelectromechanical system wafer level packaging

Development of packaging is one of the critical issues toward realizing commercialization of radio-frequency-microelectromechanical system (RF-MEMS) devices. The RF-MEMS package should be designed to have small size, hermetic protection, good RF performance, and high reliability. In addition, packaging should be conducted at sufficiently low temperature. In this paper, a low-temperature hermetic wafer level packaging scheme for the RF-MEMS devices is presented. For hermetic sealing, Au-Sn eutectic bonding technology at temperatures below 300°C is used. Au-Sn multilayer metallization with a square loop of 70 µm in width is performed. The electrical feed-through is achieved by the vertical through-hole via filling with electroplated Cu. The size of the MEMS package is 1 mm × 1 mm × 700 µm. The shear strength and hermeticity of the package satisfies the requirements of MIL-STD-883F. Any organic gases or contamination are not observed inside the package. The total insertion loss for the packaging is 0.075 dB at 2 GHz. Furthermore, the robustness of the package is demonstrated by observing no performance degradation and physical damage of the package after several reliability tests.

Application of Au-Sn eutectic bonding in hermetic RF MEMS wafer level packaging

Recently the strong demands in wireless communication requires expanding development for the application of RF MEMS (Radio Frequency micro electro mechanical systems) sensing devices such as micro-switches, tunable capacitors because it offers lower power consumption, lower losses, higher linearity and higher Q factors compared with conventional communications components. To accelerate commercialization of RF MEMS products, development for packaging technologies is one of the most critical issues should be solved beforehand. Packaging for RF MEMS is more challenging compared with conventional IC (integrated Circuit) Packaging technologies because it has both electrical and mechanical component, a low temperature, and hermetic wafer level packaging technology is needed for RF MEMS device. Au-Sn metallization system has been successfully utilize for flip chip bonding in many applications such as optoelectronic packaging and microwave device because of their high strength, good wetting behaviors, and resistance for thermal fatigue compared with conventional Pb/Sn solder system. Au-Sn eutectic bonding is considered to be a promising low temperature, wafer level bonding technology. In this paper, Au-Sn eutectic bonding for RF MEMS application is presented, a closed square loop was designed for the bonding structure, test vehicle was prepared for DOE (Design of experiment) process for the optimization of bonding parameters, and bonding temperature and applied load are found to be the most critical parameters for the bonding result, bonding can be done at a relative low temperature below 300/spl deg/C. For bonded samples, shear strength, warpage, insertion loss and hermetic tests etc. are performed for the evaluation of bonding quality, AES (Auger Electron Spectrum) and SEM (Scanning Electron Microscopy) was also made to investigate the microstructure of bonded interface, and reliability test such as thermal shock and high temperature, high humidity storage test was performed for the evaluation of bonding quality.

A low temperature, hermetic wafer level packaging method for RF MEMS switch

In this paper, a low temperature hermetic wafer level packaging (WLP) scheme for RF-MEMS devices such as micro-switches is presented. The real component with size 1mm/spl times/1mm is composed of two parts: cap substrate and device substrate, cap substrate has a via-in-cavity structure with cavity depth of 20/spl mu/m. High aspect ratio via hole is fabricated by inductive coupled plasma-reactive ion etching (ICP-RIE) and electroplated with Cu for electrical feed-through. Eutectic bonding is still the most commonly used packaging technology at present. For the purpose of hermetic sealing, Au-Sn multilayer metallization with a close square loop of 100/spl mu/m width have been sputtered onto cap wafer surface as soldering system. Deposition of cap wafer metallization should be finished in one high vacuum chamber process in order to prevent oxidation of Sn layer during producing process. And Ti-Ni-Au combination structure is deposited and patterned on device wafer in accordance with the sealing and interconnection areas in cap wafer. Bonding is performed in wafer level using eutectic bonder (TPS-2000A, BNP science) at a relative low temperature of 280/spl deg/C for heating in static N/sub 2/ ambience for a period of time. As-bonded wafers are then diced into pieces and subjected to a series of performance test for evaluation. Shear strength of two bonded interfaces are measured for sample cells by shear tester ROYCE 552 100K to evaluate mechanical property. RF characteristics insertion loss at 2GHz has measured by HP 8510C network analyzer probe station, a total packaging insertion loss less than 0.05DB could be achieved. For hermeticity test, specific test vehicles which have a large cavity of 0.5/spl times/0.5/spl times/0.05cm/sup 3/ are designed for helium leak test based on M1T-STD-883F since real device cavity has a tiny volume of only 600/spl times/600/spl times/30/spl mu/m/sup 3/, test vehicles indicate a maximum equivalent leak rate in air of 1.6/spl times/10/sup -8/ mbar.l/sec. Also residual gas analysis (RGA) test is performed for bonded device sample. Reliability tests like thermal shock and high temperature, high humidity storage test are also performed according to MIL-STD-883F. For samples before and after reliability tests, measurements also have been made for comparison to evaluate the quality and reliability of packaging structure.

Characterization and Reliability Verification of Wafer-Level Hermetic Package with Nano-Liter Cavity for RF-MEMS Applications

Wafer-level packaging (WLP) is a very promising candidate for RF-MEMS packaging, especially in the mobile applications, due to the lower cost and higher volume throughput relative to the component level packaging. However, the long-term reliability of WLP is still one of the critical concerns for the commercialization of RF-MEMS devices. In this paper, a wafer-level hermetic packaging scheme based on through-wafer interconnects and wafer-to-wafer bonding will be reviewed in terms of their construction, fabrication process, and electrical/mechanical performance. The film bulk acoustic resonators (FBARs) sealed with the wafer-level packaging scheme were also undergone through harsh environment tests, such as the pressure cooker test for 300 hours, the high humidity storage test at 85degC/85%RH for 1000 hours, the high temp storage test at 125degC for 1000 hours and the temperature cycling test (-55~125degC) for 1000 cycles, to investigate the long-term reliability of the packages. The performance evaluation and reliability results of the package will also be presented.

Wafer-level low temperature bonding with Au-In system

Wafer bonding at low temperature is an essential process for next generation MEMS & Sensor packaging. Optoelectronic devices, such as image sensor module and laser diode integrated circuit, need low bonding temperature, high re-melting temperature, high thermal conductivity, and stress-relaxed structure in many cases. Eutectic Au-In system was developed as a replacement of previous Au-Sn system for specific systems require bonding temperature lower than 200degC. Bonding temperature of developed Au-In system was set at 180degC, which was 100degC lower than that of Au-Sn system. Though polymer materials has been used for low temperature bonding, out-gassing and volume shrinkage during the bonding process often degraded bonding quality and accurate alignment between the wafers. Clean packaging with accurate alignment was achieved with eutectic Au-In bonding which also possessed high re-melting temperature over 450degC. Majority of the deposited metallizations to construct the system was converted to intermetallic compounds (AuIn & AuIn2) after bonding reaction. Peak temperature and duration time were varied to investigate optimum condition of wafer-level bonding and diced separate dies are used for X-ray inspection, microstructural observation of the cross-section, and shear test. The results showed that bonding parameters critically affected mechanical reliability of the bonded joint. Failure through the solder layer (unreacted pure In) resulted in higher shear strength, while clear separation between the wafer and under bump metallization (UBM) revealed low bond strength. Re-melting temperature of Au-In system was measured using TMA and the result showed that it was closely related with melting phenomena of pre-formed intermetallic compounds such as AuIn and gamma phases. The wafer-level bonding with Au-In system showed good feasibility for MEMS & sensor packagings that require low temperature bonding with high quality.

Implantable flexible wireless pressure sensor module

A chip embedded flexible packaging scheme has been developed using a thinned silicon chip. Mechanical characteristics of thinned silicon chips are examined by bending test and finite element analysis. Thinned silicon chips (t<50 /spl mu/m) are fabricated by a chemical etching process to avoid possible surface damage. These technologies can be used for an implantable real-time monitoring of blood pressure. Our research targets are developing an implantable blood pressure sensor module and its telemetric measurement. By winding around the coronary arteries, we can measure the blood pressure by capacitance variation of blood vessels.

Low Temperature, Wafer Level Au-In Bonding for ISM Packaging

A low process temperature, hermetic and reliable wafer level packaging (WLP) technology is required for image sensor module (ISM) packaging. Eutectic bonding is regarded as one of the most common used methods for WLP. Au-Sn metallization system has been applied as a wafer level bonding technology in many applications, but it still has process temperature around 300degC which is not applicable for temperature sensitive materials contained device wafer like ISM. In this paper, a fluxless Au-In solder system with Au, In multilayer metallizations has been developed and fabrication process is also presented, the metallization is achieved using e-beam evaporation, test vehicle was then prepared for bonding quality evaluation. Bonding process is performed at 180degC with static force for a relatively long dwelling time of 30minutes in N2 ambience, finally a void free joint is formed. Microstructure observation reveals a combination of different Au-In intermetallic compounds AuIn2 and AuIn at the interface. Shear strength around 20MPa could be obtained for as-bonded samples, and a remelting temperature over 300degC is confirmed using thermomechanical analysis (TMA) test. Real time helium leak rate test are performed to check hermeticity of the package, samples are also subjected to pressure cooker test (PCT) for evaluation of bonding performance after reliability test

Microstructure of AuSn Wafer Bonding for RF-MEMS Packaging

RF-MEMS is one of the most potential applications for MEMS products. Eutectic solder wafer bonding is one of the attractive methods for RF-MEMS wafer level packaging. A process of gold-tin hermetical wafer bonding was developed in SAIT, Korean. Different UBM systems and thin films of gold-tin were deposited on cap wafer, RF-MEMS device wafer and substrate wafer (if needed). The bonding was performed in N2 ambience with pressure. The cross section of bonding layer had been studied using SEM/EDAX. The thickness of bonding layer is uniform, ranging from 5mum to 7mum. Pretreatment is important to obtain good adhesion and successful microstructure. Voids could be detected without ashing. Optimal process, such as plasma cleaning, would eliminate these voids. There existed different regions in the bonding layer due to inter-diffusion between Au-Sn and other elements. Different intermetallic formed at the bonding layer. The compositions of the intermetallic was identified by EDAX and analyzed according to the Au-Sn phase diagram. The microstructures of the bonding layer are similar for different bonding temperatures in experiments, which indicates lower bonding temperature can get the same hermetical sealing. Typically, there are Au rich layer, AuSn IMC layer and Sn/Au-Ni-Ti layer in the bonding layer between cap wafer and substrate wafer, while there are Au layer, Au-Sn-Ni compound layer in the bonding layer between device wafer and substrate wafer. From the EDAX analysis, different intermetallic compound (IMC) can be identified as AuSn2, AuSn and other composition. Hermetical and shear strength test were performed for as-bonded dice. The test results indicated there is little difference among different bonding process. Fracture surfaces after shear test were investigated as well. The fracture was inside Au-Sn IMC. It indicates the UBM selected is suitable for application. Reliability test was also performed

Reliability Verification of Hermetic Package With Nanoliter Cavity for RF-Micro Device

With the advance of high-performance and small-size microelectromechanical systems (MEMS) devices, wafer-level packaging has gained increased attention over the past few years. Most MEMS packages must protect the often-fragile mechanical structures against the environment and provide the interface for the interaction with the next level in the packaging hierarchy. It is obvious that stable performance and high reliability are essential requirements of a packaged device. In this paper, a novel hermetic package, called the WL-¿P, recently developed for radio-frequency (RF)-filter and RF-duplexer, will be reviewed in terms of its construction, fabrication process, and electrical/mechanical performance. The package consists of a device wafer for a MEMS device and a cap wafer that has a micromachined cavity and through-wafer vias for electrical connections. The cap and device wafers are bonded to each other through a closed square loop of gold/tin eutectic solder at the peripheral edge. The via-in-cavity structure is designed in the cap substrate, with vertical via holes fabricated and fully electroplated with copper. The detailed design and fabrication technology of this new type of hermetically sealed package are presented with process flow. The performance evaluation and reliability results of a hermetic package will also be presented. The developed wafer-level hermetic package technology is able to fulfill todays requirements for hermetic and cost-effective packaging of high-speed RF-MEMS applications.

Polymeric light delivery via a C-shaped metallic aperture

A polymeric light delivery system with a C-shaped metallic nanoaperture is proposed for the heat-assisted magnetic recording. This light delivery system has high optical efficiency and easy fabricability in the low temperature process that is compatible with the conventional magnetic head. The light delivery characteristics are demonstrated analytically and experimentally. In particular, the near-field spot size of the light delivery system was measured using the virtual scanning near-field optical microscopy (VSNOM) method, in which the probe tip geometry is not reflected. The probable spot size of the developed light delivery is under 100 nm at a wavelength of 780 nm from a polymeric light delivery with the C-shaped metallic nanoaperture.