Vivek Chidambaram

Titanium-Based Getter Solution for Wafer-Level MEMS Vacuum Packaging

Ultrahigh-vacuum conditions can be achieved by employing porous absorbent materials such as Ti, Zr, Ta, and Yt. Commercial getters are primarily Zr-based, since Zr possesses the best adsorption characteristics. Titanium is not considered as a candidate, since adsorption of gases by Ti is significantly reduced due to oxidation and other contamination. In the present work, it is demonstrated that the adsorption property of Ti can be substantially enhanced and benchmarked against other Zr-based commercial getters by employing a sacrificial layer such as Ni over Ti, and also by using other surface engineering techniques. It has been confirmed that, in addition to the activation temperature, the vacuum level during getter activation also plays a pivotal role in influencing the adsorption characteristics of Ti. It has been determined that the getter life could be significantly improved by the reversible adsorption characteristic of H2 gas, facilitating regeneration cycles.

Cyanate Ester-Based Encapsulation Material for High-Temperature Applications

Cyanate ester resin-based composite materials have been proposed as potential encapsulants for high-temperature applications. The objective of this study is to develop a cyanate ester-based encapsulant, which can also serve as a flip-chip underfill as well as for traditional encapsulation. Two different materials, quartz and alumina fillers, have been studied. The impact of shapes and sizes of the fillers on the overall thermomechanical properties has been investigated. The adhesion strengths of the materials to the ceramic substrate, Kovar lid, and silicon die have also been characterized. The modulus of the resin and the shape of the fillers play a pivotal role in minimizing thermal stress, generated by coefficient of thermal expansion mismatches. Smaller filler particles were found to have better adhesion to the cyanate ester resin. The high-temperature performance of the cyanate ester-based encapsulants was evaluated by thermal aging at 300°C for up to 500 h.

High reliability gold based solder alloys for micro-electronics packaging for high temperature applications

The performance of the Au-Ge eutectic solder alloy and the Au-Si eutectic solder alloy at 300°C up to 500 h has been extensively reported. Coarsening of the dispersed (Ge) phase as well as the dissolution of the hard (Ge) phase into the soft (Au) matrix is observed during thermal aging. Shear testing and nano-indentation confirmed the loss of strength of the Au-Ge bulk solder during thermal aging at 300°C. However, a fraction of the lost strength was recovered during the final stages of thermal aging at 300°C for 500 h. The coarsening effect was more pre-dominant in the Au-Si eutectic alloy. The pace at which the Au-Si eutectic alloy loses its strength during aging at 300°C is significantly higher, when compared to Au-Ge eutectic alloy. The possibility of averting the grain coarsening in Au-Ge eutectic solder alloy by micro-alloying has also been explored. Among the low melting point metals, Sn has been identified as a potential candidate, since it can dissolve in the (Ge) phase. The reliability of the solder joint is also influenced by the intermetallic compounds (IMC) formed between the bulk solder and the solder wettable layer of the under-bump metallization (UBM). Hence, the interfacial reactions between the Au-Ge and Au-Si eutectic solder alloys and the electroless nickel immersion gold (ENIG) and Cu/Au UBMs have also been extensively studied. ENIG has been identified as a prospective UBM candidate for these eutectic alloys as their consumption during aging and wetting is minimal. Based on the present works findings, it can be concluded that among the binary eutectic alloys, Au-Ge eutectic alloy is better suited for high-temperature applications.

High-temperature endurable encapsulation material

The accomplishment of fully functional high-pressure high-temperature (HPHT) well is possible only, when the packaging and interconnections in the well logging equipments can survive at higher temperatures. Currently, there are numerous choices for substrate materials and interconnection materials. However, there are hardly any encapsulation materials that can endure at 300°C. Thus, the limiting factor for the evaluation and monitoring of HPHT wells is; the availability of high-temperature endurable encapsulation material. In this paper, the endurability of three prospective candidates for high-temperature encapsulation have been characterized and reported. The three prospective candidates are benzocyclobutene (BCB), ceramic filled cyanate ester and quartz filled cyanate ester. The high-temperature endurability has been evaluated in this work by high-temperature storage at 300°C up to 500 hours. Adhesion strength of these prospective candidates with the alumina ceramic substrate and the Si die was verified by room shear testing and hot shear testing. It has been determined that the quartz filled cyanate ester could comply with the minimum indispensable requirement for this application, when sandwiched between alumina ceramic substrates, despite the loss of strength during long-term thermal aging at 300°C. The material degradation has been studied in this work, using thermo-gravimetric analysis.

High temperature endurable hermetic sealing material selection and reliability comparison for IR gas sensor module packaging

Infrared (IR) sensor module deploy for hazardous gas leakage detection is crucial to provide and maintain offshore Oil & Gas platform asset integrity and improve operational risk management by avoiding accidental disaster and mitigating risk associated with danger involve in oil production. These sensor modules must remain robust under harsh ambient environment. Hence, designing novel high temperature interconnects material bill of materials (BOM) with compatible barrier metallization [1-2] and robust hermetic sealing material further enhanced sensor reliability. By reducing BOM oxidation degradation at high ambient temperature [3-4], reliability of the sensor is maintained. For this study, the TV (test vehicle, Figure 1) consist of an Alumina (Al2O3) substrate casing which houses all the active components and is hermetically sealed with a Silicon (Si) die that acts as an IR filter. Laser and seam welding are common method of performing hermetic sealing but they suffer from low throughput issues. An investigative benchmark of various hermetic sealing methods and materials will be discussed in great details, targeting Alumina (Al2O3) to Silicon (Si) interfaces sealing. Both sealing surfaces have no metallization. Materials such as high temperature low outgassing adhesive, glass frit paste and ceramic paste will be applied on the TV via dispensing or screen printing method and their corresponding hermeticity performance of the sealing interface will be investigated. The silicon filter is 3.7 × 3.7 mm in size and will be mounted on a 3.8 × 3.8mm ceramic substrate casing. TV hermeticity degradation response (base on MIL-STD-883J Method 1014.14 which requires 10−9 cc/sec leak rate) is examined after reliability evaluation is conducted, which include Thermal Cycling (TC, −55°C to 250°C, 500 cycles) and High Temperature Storage (HTS, 250°C, 500 hours). The objective of this paper is to report the high temperature reliability performance of various benchmarked sealing materials and understand and document the degradation mechanism and hermeticity response (via MIL-STD leak test) after HTS and TC test at 250°C.

Al-Ge Diffusion Bonding for Hermetic Sealing Application

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.

Performance enhancement of Au-Ge eutectic alloys for high-temperature electronics

Au-Ge eutectic has been used as a high temperature electronics interconnection material up to 250°C due to its favorable characteristics. However, both nano-indentation and shear testing, confirmed the loss of strength of the Au-Ge eutectic at a high temperature of 300°C due to the growth of the (Ge) phase. This coarsening has also resulted in the weakening of the (Au) phase due to the deterioration of the precipitation hardening of the (Au) matrix by the (Ge) dispersed phase. In this paper, various techniques for averting the coarsening of the (Ge) phase have been explored. It has been determined that Sn can dissolve in the Au-Ge and segregate in the (Ge) phase, resulting in restraining the coarsening of the (Ge) phase. The composition of the ternary Au-Ge-Sn alloy has been designed by taking into account; the compliance with the solidification criterion and precipitation of phases in the bulk solder. It has been ensured that no brittle intermetallic compounds (IMCs) precipitate in the matrix of the Au-Ge eutectic bulk solder, as a result of micro-alloying with Sn.

Extreme high pressure and high temperature package development

As oil and gas industries ventured further and deeper into the earth or ocean in search for new reservoirs, the requirements of depth, pressure and temperature are ever expanding. Conventionally, ceramic based hermetic sealed packaging is used for high temperature endurable package. However, for the case of highly pressurized application, the stress on the package is substantial and the hermetically sealed ceramic package cannot survive under a high pressure up to 30kpsi. To overcome this limitation, the authors are proposing to fill high temperature and high pressure endurable protective materials inside of ceramic substrate cavity to absorb the package internal stress caused by the external high pressure loading. The reliability of the package has been successfully demonstrated under combined 30kpsi isostatic pressure and 300°C temperature (HPHT) aging condition for 500 hours as well as thermal cycling condition for 500 cycles.

High Vacuum and High Robustness Al-Ge Bonding for Wafer Level Chip Scale Packaging of MEMS Sensors

This paper presents the development of Al-Ge eutectic bonding for wafer level chip scale packaging of MEMS sensors. Al is sputtered on the MEMS wafer while an Al/Ti/Ge stack is sputtered on the cap wafer. The bonding temperature and bonding time are 430°C and 30min, respectively. A CMOS compatible Ti/Ni stack was deposited as getter on the cap wafer to maintain the vacuum level inside the cavity. A silicon pirani gauge was used as hermeticity indicator to evaluate the vacuum level of the bonding. The operational dynamic range of the pirani gauge is 100mTorr-76Torr. The experimental results over three wafers show that the achieved vacuum level of the bonding is less than 200mTorr with yield more than 90%, 1000 rapid thermal cycles from -50°C to 150°C with 1 hour for each cycle were conducted, and the related vacuum level were recorded at 150th, 400th, 650th, 800th, 1000th cycle. The normalized vacuum degradation percentage are about 1.6%, 2.05%, 2.25%, 2.54%, 2.78%, 2.82%, respectively. The results also show the vacuum level trends to be stable from the 650th cycle.