Wafer-level Vacuum Packaging Research Articles

Reliability of MEMS packaging: vacuum maintenance and packaging induced stress

In this study, the dominant reliability issues of MEMS packaging that include vacuum maintenance and packaging induced stress, are discussed, and design considerations to improve the reliability are presented. The MEMS vibratory gyroscope sensor is fabricated with anodically bonded wafer level vacuum packaging followed by die-bonding and wire-bonding processes. The epoxy-molding compound (EMC) is applied to encapsulate the gyroscope sensor. Several methods to improve reliability of the vacuum packaging are suggested based on extensive work. The two major failure mechanisms of anodic vacuum packaging, namely leakage and outgassing are investigated. Leakage is effectively reduced by optimization of the bonding process. Outgassing inside the cavity can be minimized by application of Ti coating. Packaging induced stress is mainly caused by thermal expansion mismatch among the materials used to fabricate the package. During anodic bonding, thermo-mechanical stress is generated to the bonded wafer, which increases with temperature. The EMC encapsulation also produces package-induced stress. In order to increase robustness of the structure against deformation, a “crab-leg” type spring is replaced with a semi-folded spring. The results show that the frequency shift is greatly reduced after applying the semi-folded spring.

Selective bonding and encapsulation for wafer-level vacuum packaging of MEMS and related micro systems

A laser-assisted bonding technique is demonstrated for low temperature region selective processing. A continuous wave carbon dioxide (CO 2) laser ( λ=10.6 μm) is used for solder (Pb37/Sn63) bonding of metallized silicon substrates (chips or wafers) for MEMS applications. Laser-assisted selective heating of silicon led to the reflow of an electroplated, or screen-printed, intermediate solder layer which produced silicon–solder–silicon joints. The bonding process was performed on fixtures in a vacuum chamber at an air pressure of 10 −3 Torr to achieve fluxless soldering and vacuum encapsulation. The bonding temperature at the sealing ring was controlled to be close to the reflow temperature of the solder. Pull test results showed that the joint was sufficiently strong. Helium leak testing showed that the leak rate of the package met the requirements of MIL-STD-883E under optimized bonding conditions and bonded packages survived thermal shock testing. The testing, based on a design of experiments method, indicated that both laser incident power and scribe velocity significantly influenced bonding results. This novel method is especially suitable for encapsulation and vacuum packaging of chips or wafers containing MEMS and other micro devices with low temperature budgets, where managing stress distribution is important. Further, released and encapsulated devices on the sealed wafers can be diced without damaging the MEMS devices at wafer level.

A study on wafer level vacuum packaging for MEMS devices

A new vacuum packaging process at the wafer level is developed for the surface micromachining devices using glass–silicon anodic bonding technology. The rim for the glass–silicon bonding process which is needed to prevent vacuum leakage is built up simultaneously as the structure is being etched. The mechanical resonator is used as a tool for evaluating the vacuum level of the packaging. The inside pressure of the packaged device was measured indirectly by measuring the quality factor of the mechanical resonator. The measured Q factor was about 5 × 104 and the estimated inner pressure was about 1 mTorr. It is also possible to change the inside pressure of the packaged devices from 2 Torr to 1 mTorr by varying the amount of Ti getter material. The yield of the vacuum packaging process is about 80% and vacuum degradation was not observed after 1000 h had passed. The developed vacuum packaging process is also applied to resonant accelerometers which need a high vacuum environment to implement higher performance.

Low-Cost Wafer-Level Vacuum Packaging for MEMS

AbstractVacuum packaging of high-performance surface-micromachined uncooled microbolometer detectors and focal-plane arrays (FPAs) for infrared imaging and nonimaging applications, inertial MEMS (microelectromechanical systems) accelerometers and gyroscopes, and rf MEMS resonators is a key issue in the technology development path to low-cost, high-volume MEMS production. In this article, two approaches to vacuum packaging for MEMS will be discussed. The first is component-level vacuum packaging, a die-level approach that involves packaging individual die in a ceramic package using either a silicon or germanium lid. The second approach is wafer-level vacuum packaging, in which the vacuum-packaging process is carried out at the wafer level prior to dicing the wafer into individual die. We focus the discussion of MEMS vacuum packaging on surface-micromachined uncooled amorphous silicon infrared microbolometer detectors and FPAs for which both component-level and wafer-level vacuum packaging have found widespread application and system insertion. We first discuss the requirement for vacuum packaging of uncooled a-Si microbolometers and FPAs. Second, we discuss the details of the component-level and wafer-level vacuum-packaging approaches. Finally, we discuss the system insertion of wafer-level vacuum packaging into the Raytheon 2000AS uncooled infrared imaging camera product line that employs a wafer-level-packaged 160 × 120 pixel a-Si infrared FPA.

Amorphous Silicon Microbolometer Technology

ABSTRACTHighly sensitive hydrogenated amorphous silicon (a-Si:H) microbolometer arrays have been developed that take advantage of the high temperature coefficient of resistance (TCR) of aSi:H and its relatively high optical absorption coefficient. TCR is an important design parameter and depends on material properties such as doping concentration. Ultra-thin (∼2000 Å) aSiNx:H/a-Si:H/ a-SiNx:H membranes with low thermal mass suspended over silicon readout integrated circuits are built using RF plasma enhanced chemical vapor deposition (PECVD) and surface micromachining techniques. The IR absorptance of the bolometer detectors is enhanced by using quarter-wave resonant cavity structures and thin-film metal absorber layers. To ensure high thermal isolation the microbolometer arrays are vacuum packaged using wafer level vacuum packaging. Imaging applications include a 120×160 a-Si:H bolometer pixel array IR camera operating at ambient temperature. Non-imaging applications are multi-channel detectors for gas sensing systems.

Wafer-level vacuum packaging for MEMS

Vacuum packaging of high performance infrared (IR) MEMS uncooled detectors and arrays, inertial MEMS accelerometers and gyros, and radio frequency (rf) MEMS resonators is a key issue in the technology development path to low cost, high volume MEMS production. Wafer-level vacuum packaging transfers the packaging operation into the wafer fab. It is a product neutral enabling technology for commercialization of MEMS for home, industry, automotive, and environmental monitoring applications. 4 in. wafer-level vacuum packaging has been demonstrated using IR MEMS bolometers and results will be presented in this article. In addition to the wafer-level packaging results, vacuum package reliability results obtained on component-level ceramic vacuum packages will also be presented.