Byung-Gil Jeong

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.

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

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.

Warpage Modeling and Characterization to Simulate the Fabrication Process of Wafer-Level Adhesive Bonding

Since the array of wafer-level packages formed by bonding of a cap wafer to a substrate wafer, how to well encapsulate the intricate sensor devices in wafer-level is the critical issue in the development of image sensor products. This paper presents an analytical model for design and fabrication of wafer-level adhesive bonding. A novel mechanical approach is proposed, which considers each layer in bonded wafer as a beam-type plate with effective material properties. Based on this mathematical modeling, the wafer warpage under thermal loading is predicted and compared to the experimental measurements. It is shown that our newly developed wafer warpage modeling offers simple and precise evaluation of wafer-level adhesive bonding process.

Quantitative Characterization of True Leak Rate of Micro to Nanoliter Packages Using a Helium Mass Spectrometer

The helium mass spectrometer has been used extensively for qualitative analysis of fine leaks. However, qualitative analysis for the small volumes (typically < 10−3 cc) of MEMS packages can produce erroneous results. In addition, the qualitative use of the helium fine leak test does not allow comparison of two different packages. Nor can the results be correlated with data from accelerated tests such as 85°C/85%RH test. These limitations warrant the development of a method to assess hermeticity quantitatively. This paper proposes a procedure to quantitatively measure the true leak rate of micro to nanoliter packages using the helium mass spectrometer. The proposed method is based on the fact that the profile of the measured leak rate signal depends only on the true leak rate and the volume of the package. Prior knowledge of the volume of the package enables determination of the true leak rate by performing a weighted non-linear regression analysis of the data. The method was implemented to measure the true leak rate of a MEMS RF filter package. The package was tested under three different test conditions yielding three different signal profiles. The method yielded a consistent value for the true leak rate, which verified the robustness of the method. The proposed method provides a true leak rate as a quantitative measure of hermeticity. The accurate true leak rates will help evaluate new bonding materials/processes and package designs fast and effectively.