Wafer-level Vacuum Packaging Research Articles

A Method of Fabricating Vacuum Packages with Vertical Feedthroughs in a Wafer Level Anodic Bonding Process

Abstract This paper presents a new method for wafer level vacuum packaging of MEMS devices using anodic bonding together with vertical feedthroughs formed on an SOI cap wafer, eliminating the need for any sealing material or any complex via-refill or trench-refill vertical feedthrough steps. The packaging yield is experimentally verified to be above 95%, and the cavity pressure is characterized to be as low as 1 mTorr with the help of a thin-film getter. The shear strength of several packages is measured to be above 15 MPa.

Resonant biaxial 7-mm MEMS mirror for omnidirectional scanning

Low-cost automotive laser scanners for environmental perception are needed to enable the integration of advanced driver assistant systems into all automotive vehicle segments, which is a key to reduce the number of traffic accidents on roads. Within the scope of the European-funded project MiniFaros, partners from five different countries have been cooperating in developing a small-sized low-cost time-of-flight-based range sensor. An omnidirectional 360-deg laser scanning concept has been developed based on the combination of an omnidirectional lens and a biaxial large aperture MEMS mirror. The concept, design, fabrication, and first measurement results of a resonant biaxial 7-mm gimbal-less MEMS mirror that is electrostatically actuated by stacked vertical comb drives is described. Identical resonant frequencies of the two orthogonal axes are necessary to enable the required circle scanning capability. A tripod suspension was chosen, since it minimizes the frequency splitting of the two resonant axes. Low-mirror curvature is achieved by a thickness of the mirror of more than 500 μm. Hermetic wafer-level vacuum packaging of such large mirrors based on multiple wafer bonding has been developed to enable a large mechanical tilt angle of ±6.5 deg in each axis. Due to the large targeted tilt angle of ±15 deg and because of the MEMS mirror actuator having a diameter of 10 mm, a cavity depth of about 1.6 mm has been realized.

Wafer-Level Vacuum Packaging for Microsystems Using Glass Frit Bonding

Wafer-level glass frit bonding is an important technology for the hermetic encapsulation of microsystems. In this paper, the sintering characteristic of the glass frit material is investigated based on thermogravimetry/differential scanning calorimetry (TG/DSC) thermal analysis techniques. TG/DSC measurement results illustrate that the organic binder burnout temperature and glass transition temperature for the glass frit used in this paper are 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C and 331 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, respectively. With the TG/DSC measured results, a wafer-level vacuum packaging approach for microsystems using glass frit bonding is developed through experimental analysis. The glass frit is presintered at 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C for 30 min to completely remove the organic binder and fired at 430 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to seal the bonded wafers. The cross-sectional observation shows no void or gap is formed in the bonded layer. The average shear stress of the bonded devices is 6.2 kgf, which satisfies military standard-883E. Application of the glass frit bonding approach for a thermocouple vacuum gauge is demonstrated, which is employed to illustrate the feasibility of this process, and also to measure the vacuum degree in the packaged cavity. The measured results show that a vacuum of about 100-300 Pa can be sealed using this approach.

Vacuum sealing using atomic layer deposition of Al2O3 at 250 °C

This paper describes the use of low-temperature atomic layer deposition (ALD) of Al2O3, for vacuum seals in wafer-level vacuum packaging and other applications. The conformal coverage provided by ALD Al2O3 is shown to seal circular micromachined cavities. The cavities are 0.8 μm in height, 400 μm in diameter, and are capped by porous plasma-enhanced chemical vapor deposited dielectrics that form a membrane. The ALD Al2O3 film, of thickness ≈0.2 μm, is deposited at a temperature of 250 °C on this membrane. The retention of vacuum is indicated by the deflection of the membrane. Lifetime tests extending out to 19 months are reported.

Very large scale heterogeneous integration (VLSHI) and wafer-level vacuum packaging for infrared bolometer focal plane arrays

Imaging in the long wavelength infrared (LWIR) range from 8 to 14μm is an extremely useful tool for non-contact measurement and imaging of temperature in many industrial, automotive and security applications. However, the cost of the infrared (IR) imaging components has to be significantly reduced to make IR imaging a viable technology for many cost-sensitive applications. This paper demonstrates new and improved fabrication and packaging technologies for next-generation IR imaging detectors based on uncooled IR bolometer focal plane arrays. The proposed technologies include very large scale heterogeneous integration for combining high-performance, SiGe quantum-well bolometers with electronic integrated read-out circuits and CMOS compatible wafer-level vacuum packing. The fabrication and characterization of bolometers with a pitch of 25μm×25μm that are arranged on read-out-wafers in arrays with 320×240pixels are presented. The bolometers contain a multi-layer quantum well SiGe thermistor with a temperature coefficient of resistance of −3.0%/K. The proposed CMOS compatible wafer-level vacuum packaging technology uses Cu–Sn solid–liquid interdiffusion (SLID) bonding. The presented technologies are suitable for implementation in cost-efficient fabless business models with the potential to bring about the cost reduction needed to enable low-cost IR imaging products for industrial, security and automotive applications.

Wafer-Level Vacuum Packaging for MEMS Resonators Using Glass Frit Bonding

A wafer-level vacuum package with silicon bumps and electrical feedthroughs on the cap wafer is developed for a microelectromechanical systems (MEMS) resonator device. A MEMS resonator wafer and a cap wafer are bonded together in a vacuum chamber using glass frit bonding. The cap wafer not only provides a vacuum chamber to protect the movable resonator structure and improve the resonant performance but also realizes the redistribution of the electrical feedthroughs by using the silicon bumps. The silicon bumps provide vertical interconnections between the cap wafer and the resonator wafer, which realizes the bonding pads “transferring” from the resonator wafer to the cap wafer. A gold-aluminum eutectic is used to ensure electrical contacts between the cap wafer and the device wafer. The device fabrication and glass frit hermetic bonding process as well as the packaged MEMS resonator characterization are presented in this paper. Experimental results show that the wafer-level vacuum-packaged MEMS resonator results in over 100× higher quality factor (Q) than the resonator vibrating in atmosphere pressure, which confirms the transmission performance improvement due to vacuum packaging. Vacuum inside the package is measured indirectly by measuring the Q of the MEMS resonator inside the package. The experimental results indicate that vacuum about 1 mbar can be sealed in this approach.

Design and fabrication of high performance wafer-level vacuum packaging based on glass–silicon–glass bonding techniques

In this paper, a high performance wafer-level vacuum packaging technology based on GSG triple-layer sealing structure for encapsulating large mass inertial MEMS devices fabricated by silicon-on-glass bulk micromachining technology is presented. Roughness controlling strategy of bonding surfaces was proposed and described in detail. Silicon substrate was thinned and polished by CMP after the first bonding with the glass substrate and was then bonded with the glass micro-cap. Zr thin film was embedded into the concave of the micro-cap by a shadow-mask technique. The glass substrate was thinned to about 100 µm, wet etched through and metalized for realizing vertical feedthrough. During the fabrication, all patterning processes were operated carefully so as to reduce extrusive fragments to as little as possible. In addition, a high-performance micro-Pirani vacuum gauge was integrated into the package for monitoring the pressure and the leak rate further. The result shows that the pressure in the package is about 120 Pa and has no obvious change for more than one year indicating 10−13 stdcc s−1 leak rate.

High-Q MEMS Resonators for Laser Beam Scanning Displays

This paper reports on design, fabrication and characterization of high-Q MEMS resonators to be used in optical applications like laser displays and LIDAR range sensors. Stacked vertical comb drives for electrostatic actuation of single-axis scanners and biaxial MEMS mirrors were realized in a dual layer polysilicon SOI process. High Q-factors up to 145,000 have been achieved applying wafer level vacuum packaging technology including deposition of titanium thin film getters. The effective reduction of gas damping allows the MEMS actuator to achieve large amplitudes at high oscillation frequencies while driving voltage and power consumption can be minimized. Exemplarily shown is a micro scanner that achieves a total optical scan angle of 86 degrees at a resonant frequency of 30.8 kHz, which fulfills the requirements for HD720 resolution. Furthermore, results of a new wafer based glass-forming technology for fabrication of three dimensionally shaped glass lids with tilted optical windows are presented.

Small footprint wafer-level vacuum packaging using compressible gold sealing rings

A novel low-temperature wafer-level vacuum packaging process is presented. The process uses plastically deformed gold rings as sealing structures in combination with flux-free soldering to provide the bond force for a sealing wafer. This process enables the separation of the sealing and the bonding functions both spatially on the wafer and temporally in different process steps, which results in reduced areas for the sealing rings and prevents outgassing from the solder process in the cavity. This enables space savings and yields improvements. We show the experimental result of the hermetic sealing. The leak rate into the packages is determined, by measuring the package lid deformation over 10 months, to be lower than 3.5 × 10−13 mbar l s−1, which is suitable for most MEMS packages. The pressure inside the produced packages is measured to be lower than 10 mbar.

Wafer-level vacuum packaging with lateral interconnections and vertical feedthroughs for microelectromechanical system gyroscopes

A wafer-level vacuum packaging based on anodic bonded glass-silicon-glass triple stack with both lateral interconnections and vertical feedthroughs is presented. A z-axis gyroscope is packaged and tested to verify the packaging process. The packaged gyroscope achieved a Q factor of 26000, increased by a factor of 30 when compared to the same gyroscope without vacuum packaging. The pressure in the packaged cavity is ∼100 Pa, and the stability of the Q factor in three months is ∼3%. Experiment results indicate that the proposed wafer-level vacuum packaging is feasible and suitable for high-performance wafer-level packaged gyroscopes.

Wafer-Level Vacuum Packaging of Micromachined Thermoelectric IR Sensors

In the trend towards low-cost, high-performance, and miniaturization, a wafer-level vacuum package is developed for micromachined thermoelectric infrared (IR) sensor. An IR sensor wafer and a cap wafer are bonded together in a vacuum chamber using Au-Au thermocompression bonding, where the cap wafer not only protects the floating thermopile structure but also selects IR light for the sensor. The device fabrication and Au-Au thermocompression hermetic bonding process as well as the packaged IR sensor characterization is presented in this paper. Experimental results show that the wafer-level vacuum packaged IR sensor has a four times higher responsivity and detectivity than the IR sensor with atmosphere pressure package, which confirms the IR performance improvement due to vacuum packaging. IR microscope image of the packaged device proved that the Au-Au thermocompression bonding process is compatible to the handling of fragile micromachined thermopile structure. Average leak rate and shear strength are, respectively, 3.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> atm cc/s and 16.709 Kgf, which shows that the Au-Au thermocompression hermetic bonding is suitable for the wafer-level vacuum packaging of micromachined thermoelectric IR sensor.

A micro-machined Pirani gauge for vacuum measurement of ultra-small sized vacuum packaging

Quite a few MEMS devices need vacuum packaging technology to guarantee the desirable performance. Developing an absolute vacuum environment for those devices is indispensable. However, it is difficult to monitor the pressure change in vacuum chamber in on-line and real-time mode. A surface micro-machined Pirani gauge for measuring vacuum pressure inside vacuum packaging in wafer level was presented in this paper. It was designed with a simplified structure and did not need complex circuit. Only a simple Wheatstone bridge circuit is needed, which could be manufactured by conventional CMOS processes. Preliminary tests on this device were conducted. The experimental results show that the Pirani gauge is capable of measuring pressures from atmospheric value to 1 Pa and has a very good linearity in the range from1 Pa to 300 Pa. It demonstrates that the micro-machined Pirani gauge has great potential to be used in wafer level vacuum packaging. Also, the Pirani gauge is able to be the packaging hermeticity detector.

Novel SU-8 based vacuum wafer-level packaging for MEMS devices

This work presents a simple and low-cost SU-8 based wafer-level vacuum packaging method which is CMOS and MEMS compatible. Different approaches have been investigated by taking advantage of the properties of SU-8, such as chemical resistance, optical transparence, mechanical reliability and versatility. A novel technique of wafer level adhesive bonding, which uses SU-8 as structural and adhesive material, has been developed and successfully demonstrated. Optical inspection and SEM images were used in order to measure the package lid bending and probe the encapsulation sealing. In addition, an indirect vacuum level measurement has been carried out by comparing the different quality factors of a test cantilever resonator when this element is packed or unpacked.

Fabrication and Testing of a Wafer-Level Vacuum Package for MEMS Device

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A wafer-level vacuum package with getters deposited on the cap wafer is developed for an accelerometer device. An accelerometer wafer and cap wafer is bonded together in a vacuum of 1 mtorr and is characterized using a micro-electro-mechanical systems (MEMS) motion analyzer (MMA). Vacuum inside the package is measured indirectly by measuring the <emphasis emphasistype="boldital">Q</emphasis>-factor response of the accelerometer structure inside the package. The obtained results indicated that there is variation from the center to the edge of the wafer. This may be due to difference in the outgassing of the package. Different reliability tests on the wafer-level package showed the package is robust to the reliability conditions. A progressive test on the <emphasis emphasistype="boldital">Q</emphasis>-factor for different cycles of reliability test proved that there is no shift in the measurement value. A 3-D wafer-level package for accelerometer device is also developed to meet the requirements of vacuum packaging. Hermeticity and CV test showed no degradation in the device performance when subjected to reliability tests. </para>

Pirani Vacuum Gauges Using Silicon-on-Glass and Dissolved-Wafer Processes for the Characterization of MEMS Vacuum Packaging

This paper presents the design and implementation of Pirani vacuum gauges for the characterization of vacuum packaging of microelectromechanical systems (MEMS). Various Pirani vacuum gauges are fabricated with two different standard in-house fabrication processes, namely the silicon-on-glass (SOG) process and dissolved-wafer process (DWP). The Pirani gauges utilize meander-shaped suspended silicon coils as the heaters and two isolated silicon islands in the close proximity of the heater that function as dual-heat sinks to enhance the sensitivity and dynamic range as compared to a microbridge with a single heat sink. The gauges are designed to occupy an area of 4 mm × 1.5 mm. The DWP Pirani gauge fabricated with a structural thickness of 14 mum and a gap of 2 mum shows a measured sensitivity of 4.2×10⁴ (K/W)/Torr in a dynamic range of 10-2000 mTorr. The SOG Pirani gauge fabricated with a structural thickness of 100 mum and a gap of 3 mum shows a lower measured sensitivity of 3.8×10³ (K/W)/Torr in a dynamic range of 50-5000 mTorr; however, the 100 mum-thick structural layer results in a much more robust process against stress-based deformations in suspended silicon compared to the DWP Pirani gauges. Each gauge is used to monitor the pressure of a different packaging approach. The DWP Pirani gauge is used to detect the pressure of a wafer-level vacuum package, where the pressure inside the cavity is measured to be about 2.4 mTorr. The SOG Pirani gauge is used the monitor the pressure inside a hybrid platform package which is vacuum-sealed using a projection welder, where the pressure is measured to be about 1400 mTorr. These measurements verify that the DWP and SOG Pirani gauges can be used for the characterization of wafer-level or hybrid platform vacuum packages for MEMS devices.

Waferlevel Vacuum Packaging: Essential for Microscanners in Mobile Applications

MEMS scanning mirrors are a key part of miniaturized laser based display systems. Due to their small outline and high performance these systems are supposed to be integrated into next generation mobile devices such as cellular phones or PDAs. But mobile applications demand for ruggedness against harsh environmental conditions, which can only be guaranteed by a hermetic package. Packaging of MEMS is an important expense factor within the production costs and to ensure mass producibility the packaging has to be done on a wafer level. While for inertial MEMS this is state of the art, it has not yet been reported for MOEMS. Therefore, Fraunhofer ISIT has developed a complete process technology for the manufacturing of wafer level vacuum packaged MEMS scanning mirrors. It allows the fabrication of vertical stacked combdrives for quasistatic mirror operation and a low damping environment for the reduction of needed driving signals. It has been achieved that scan angles of above 50 deg can be reached even at a low driving voltage of below 6 V.

Latching ultra-low power MEMS shock sensors for acceleration monitoring

A microelectromechanical shock sensor, which uses a latching mechanism to record a shock event above a specified threshold level, is discussed in this article. The fabrication process for the shock sensor, which includes wafer-level vacuum packaging, is detailed along with the design features. These features include a reset actuator for reuse of the sensor, a no-power operation scheme when the reset actuator is not activated, and a control circuit to minimize power used to unlatch the sensor. In order to describe the shock-sensor dynamics and interaction with the latch mechanism, a preliminary non-linear model has been developed. Experimental results are presented and compared with model predictions.

Fabrication and Characterization of a Wafer-Level MEMS Vacuum Package With Vertical Feedthroughs

This paper reports a MEMS vacuum package with vertical feedthroughs formed in a glass substrate all at the wafer level. This approach satisfies requirements for MEMS vacuum packages, including small size, vacuum/hermetic capability, sealed and low parasitic feedthroughs, wafer-level processing, compatibility with most MEMS processes, and low cost. It also enables flip-chip solder attachment to a PC board. The package has an integrated micro-Pirani gauge on a glass substrate for in situ monitoring, a silicon cap, and vertical feedthroughs through the glass. The integrated Pirani gauge has 0.6 milli-torr resolution at 0.1 torr and 0.2 torr resolution at 100 torr. Using the Pirani gauge, the fabricated vacuum package is characterized. The package has maintained ~33 torr base pressure with plusmn1.5 torr uncertainty for more than four months without a getter. The long-term measured pressure uncertainty is from the measurement setup and environment, and can be improved using a getter inside the package. [2007-0160]

Gold metallizations for eutectic bonding of silicon wafers

Gold eutectic bonding of silicon wafers is a good candidate for wafer level vacuum packaging of vibrating MEMS: in this paper we investigated several e-beam evaporated metallizations stacks including a titanium adhesion layer, an optional diffusion barrier (Ni or Pt) and a gold film for eutectic bonding on Si and SiO2/Si wafers. Interdiffusion in the multilayers for annealing temperatures (380–430°C) larger than the Au–Si eutectic temperature (363°C) and times corresponding to a bonding process was characterized by RBS, roughness and resistivity measurements. Au/Pt/Ti and Au/Ti/SiO2 were found to have the best characteristics for bonding. This was confirmed by bonding experiments.

Experimental Reliability Estimation and Improvement for a Wafer Level Vacuum Packaged MEMS Device

We carry out reliablity tests and investigate the failure mechanisms for wafer level vacuum packaged (WLVP) MEMS resonator using an accelerated degradation test (ADT). ADT is also used to estimate the mean life-time (or failure) of WLVP. The main failure mechanism of WLVP is found to be outgassing inside the package. Failure distribution of log-logistic is well fitted with the failure data from ADT, and Arrhenius model is used as a stress-life model because the main stress parameter is temperature. Acceleration factor (AF) for temperature levels is calculated and used to evaluate mean life-time of the WLVP after redesign. Finally the accuracy of ADT model is examined by verification test. The successful WLVP is achieved by reducing outgassing through the deposition of titanium coating inside the package as a getter material.