Very Thin Packaging of Capped MEMS Accelerometer Device (original) (raw)
Wafer Thinning Solution for Wafer-Level-Capped MEMS Devices
56th Electronic Components and Technology Conference 2006
A wafer backgrinding solution is demonstrated to successfully thin wafer-level-capped MEMS accelerometers down to 250mum thickness. Capped MEMS wafers are prepared for manufacturing by using wafer protective tapes. Holes are punched on layers of tape, matching the thickness of the caps, that serve as gap fills in-between caps and through the wafer edge. The capped wafers are fully supported while
Low stress packaging of a micromachined accelerometer
IEEE Transactions on Electronics Packaging Manufacturing, 2001
A packaging study of an acceleration microelectromechanical systems (MEMS) sensor is presented. The sensor consists of two silicon chips: a surface micromachined capacitive transducer (g-cell), which converts acceleration into signal of capacitance variation, and a microprocessor control unit (MCU) for signal conditioning. The two chips are die-bonded into a single piece of leadframe, connected via wire bonding, and finally molded with an epoxy compound. The primary goals of this paper are to provide insight and guidance for designing a package with low stress and low deformation. In particular, two die-bonding schemes: full die attach and four-dot die attach are presented in detail and their impact on performance of the transducer is discussed. Both the numerical simulation and testing data indicated that the four-dot die-attach process results in a significantly lower packaging stress to the transducer, and is appropriate for stress-sensitive MEMS devices.
Sensors, 2008
This paper discusses sputtered silicon encapsulation as a wafer level packaging approach for isolatable MEMS devices. Devices such as accelerometers, RF switches, inductors, and filters that do not require interaction with the surroundings to function, could thus be fully encapsulated at the wafer level after fabrication. A MEMSTech 50g capacitive accelerometer was used to demonstrate a sputtered encapsulation technique. Encapsulation with a very uniform surface profile was achieved using spin-on glass (SOG) as a sacrificial layer, SU-8 as base layer, RF sputtered silicon as main structural layer, eutectic gold-silicon as seal layer, and liquid crystal polymer (LCP) as outer encapsulant layer. SEM inspection and capacitance test indicated that the movable elements were released after encapsulation. Nanoindentation test confirmed that the encapsulated device is sufficiently robust to withstand a transfer molding process. Thus, an encapsulation technique that is robust, CMOS compatible, and economical has been successfully developed for packaging isolatable MEMS devices at the wafer level.
Mechanical Design and Characterization for MEMS Thin-Film Packaging
Journal of Microelectromechanical Systems, 2012
In this paper, a thin-film packaging approach is developed. It is meant to provide microelectromechanical systems (MEMS) devices with hermetic encapsulation that is sufficiently strong for transfer molding. A flat slab structure supported by columns is considered as basic geometry for the mechanical model. It takes into account both the plate deflection and the stress at the interface with the columns. To verify the model validity, thin-film packages are fabricated using silicon nitride as material for the capping layer. Both high-and low-temperature processes are used to fabricate the packages. The packages differ for the diameter of the columns (from 2 μm to 28 μm), the distances between columns (from 20 μm to 100 μm), and the capping layer thickness (from 3 μm to 7 μm). The packages are tested at different pressures up to 12.5 MPa (125 bar). Failure points agree well with the mechanical model. The largest package fabricated is a square package of 300 μm side length and with four columns (10 μm diameter) in the middle. It withstands a pressure of 10 MPa with a thin SiN capping layer with a thickness of 6 μm. Moreover, the packages are carried through grinding, dicing, and transfer molding, demonstrating that the presented thin-film encapsulation approach is robust enough for commercial first-level packaging.
A Robust Thin-Film Wafer-Level Packaging Approach for MEMS Devices
Journal of Microelectronics and Electronic Packaging, 2010
Micro-electromechanical systems (MEMS) devices are extremely sensitive to their environment, especially at the wafer level, until they are packaged in final form. The harsh back-end (BE) operations that the MEMS devices have to endure include dicing, pick-and-place, wire bonding, and molding. During these processing steps, the MEMS device is exposed to particles and contaminants. Therefore, protection at an early stage is a fundamental requirement. We describe a silicon nitride thin-film capping, which is processed using a sacrificial layer technique only with front-end technology. This approach is suitable for mass production of MEMS devices, owing to the fact that it is more cost-effective when compared to other approaches such as wafer-to-wafer bonding and die-to-wafer bonding. A bulk acoustic wave (BAW) resonator that finds application in the radio frequency (RF) front end, for example, in cell phones, is taken as a MEMS vehicle for our work. It is an example of an extremely sen...
Journal of Microelectromechanical Systems, 2007
In this paper, packaging-induced stress effects are assessed for microelectromechanical systems (MEMS) sensors. A packaged MEMS sensor may experience output signal shift (offset) due to the thermomechanical stresses induced by the plastic packaging assembly processes and external loads applied during subsequent use in the field. Modeling and simulation to minimize the stress-induced offset shift are essential for highprecision accelerometers, gyroscopes, and many other MEMS devices. Improvement of plastic package modeling accuracy is accomplished by correlating finite-element analysis package models using measured material properties and package warpage. Using a refined reduced-order MEMS sensor and package interaction model, device offset is simulated, optimized, and compared with data collected from a unique three-axis accelerometer, which uses a single mass for all three axes sensing. As a result, this accelerometer has achieved very low offset (< 1 mg/ • C) in all XY Z axes over device operation temperature range of −40 • C to +80 • C. Device offset performance was improved by at least five times after the MEMS design optimization as compared with the one prior to the optimization.
Evaluation of Die Stress in MEMS Packaging: Experimental and Theoretical Approaches
IEEE Transactions on Components and Packaging Technologies, 2006
The device performance of microelectromechanical system (MEMS) inertial sensors such as accelerometers and gyroscopes is strongly influenced by the stress developed in the silicon die during packaging processes. This is due to the die warpage in the presence of the stress. It has previously been shown that most of the stress is generated during a die-attach process. In this study, we employ both experimental and theoretical approaches to gain a better understanding in a stress development induced during the packaging processes of a small silicon die (3.5 3.5 mm 2). The former approach is accompanied with an optical profilometer while the latter part by a finite element analysis and an analytical model. A specific emphasis is given to the effects of structural parameters such as the die-attach adhesive thickness and material properties on the stress development. The results from all three approaches show good agreement, in that more compliant and thicker adhesives offer great relief in the stress development, as well as bend the die convex downward from its central location. A stress model proposed from this study not only provides a diagnostic tool for very small stress-sensitive devices, but it will also present a design tool for low-stress MEMS packaging systems.
Influence of Mechanical Stress in a Packaged Frequency-Modulated MEMS Accelerometer
2020 IEEE International Symposium on Inertial Sensors and Systems (INERTIAL), 2020
Frequency modulated accelerometers composed of two double-ended tuning fork (DETF) resonators on a differential configuration were characterized for their sensitivity to force applied to their package. Commonly, differential architectures are employed to cancel commonmode errors, such as the mechanical stress or temperature dependency. The device dependence to mechanical stress was experimentally measured for forces up to 15 N and a reduction of about 5.6 times was obtained on the differential measurement. Additionally, the silicon dies were glued to chip-carriers using two different glues with distinct properties, and their sensitivity to stress was compared. The effectiveness of a viscoelastic glue over an epoxy-based glue for stress decoupling was tested. Long-term measurements under constant force were experimentally performed and for a time period of approximately 100 min, the stress relaxation and creeping of the viscoelastic glue enabled the recovery to the initial output of the sensor.
Challenges in the packaging of MEMS
Proceedings International Symposium on Advanced Packaging Materials. Processes, Properties and Interfaces (IEEE Cat. No.99TH8405)
The packaging of Micro-Electro-Mechanical Systems (MEMS) is a field of great importance to anyone using or manufacturing sensors, consumer products, or military applications. Currently much work has been done in the design and fabrication of MEMS devices but insufficient research and few publications have been completed on the packaging of these devices. This is despite the fact that packaging is a very large percentage of the total cost of MEMS devices. The main difference between IC packaging and MEMS packaging is that MEMS packaging is almost always application specific and greatly affected by its envirotient and packaging techniques such as die handling, die attach processes, and lid sealing. Many of these aspects are directly related to the materials used in the packaging processes. MEMS devices that are functional in wafer form can be rendered inoperable after packaging. MEMS dies must be handled only from the chip sides so features on the top surface are not damaged. This eliminates most current die pick-and-place fixtures. Die attach materials are key to MEMS packaging. Using hard die attach solders can create high stresses in the MEMS devices, which can affect their operation greatly. Lowstress epoxies can be high-outgassing, which can also affect device performance. Also, a low modulus die attach can allow the die to move during ultrasonic wirebonding resulting to low wirebond strength. Another source of residual stress is the lid sealing process. Most MEMS based sensors and devices require a hermetically sealed package. This can be done by pm~el seam welding the package lid, but at the cost of further induced stress on the die. Another issue of MEMS packaging is the media compatibility of the packaged device. MEMS unlike ICS often interface with their environment, which could be high pressure or corrosive. The main conclusion we can DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. , , draw about MEMS packaging is that the package affects the performance and reliability of the MEMS devices. There is a gross lack of understanding between the package materials, induced stress, and the device performance. The material properties of these packaging materials are not well defined or understood. Modeling of these materials and processes is far from maturity. Current post-package yields are too low for commercial feasibility, and consumer operating environment reliability and compatibility are often difficult to simulate. With fu~her understanding of the materials properties and behavior of the packaging materials, MEMS applications can be fully realized and integrated into countless commercial and military applications.
Cost-Efficient Wafer-Level Capping for MEMS and Imaging Sensors by Adhesive Wafer Bonding
Micromachines, 2016
Device encapsulation and packaging often constitutes a substantial part of the fabrication cost of micro electro-mechanical systems (MEMS) transducers and imaging sensor devices. In this paper, we propose a simple and cost-effective wafer-level capping method that utilizes a limited number of highly standardized process steps as well as low-cost materials. The proposed capping process is based on low-temperature adhesive wafer bonding, which ensures full complementary metal-oxide-semiconductor (CMOS) compatibility. All necessary fabrication steps for the wafer bonding, such as cavity formation and deposition of the adhesive, are performed on the capping substrate. The polymer adhesive is deposited by spray-coating on the capping wafer containing the cavities. Thus, no lithographic patterning of the polymer adhesive is needed, and material waste is minimized. Furthermore, this process does not require any additional fabrication steps on the device wafer, which lowers the process complexity and fabrication costs. We demonstrate the proposed capping method by packaging two different MEMS devices. The two MEMS devices include a vibration sensor and an acceleration switch, which employ two different electrical interconnection schemes. The experimental results show wafer-level capping with excellent bond quality due to the re-flow behavior of the polymer adhesive. No impediment to the functionality of the MEMS devices was observed, which indicates that the encapsulation does not introduce significant tensile nor compressive stresses. Thus, we present a highly versatile, robust, and cost-efficient capping method for components such as MEMS and imaging sensors.