Wafer-level Vacuum Research Articles

Capacitive MEMS absolute pressure sensor using a modified commercial microfabrication process

We present the design and fabrication of a capacitive absolute pressure sensor using a modified commercial microfabrication process. The pressure sensor is fabricated using MEMS Integrated Design for Inertial Sensors (MIDIS), a process recently developed by Teledyne DALSA Semiconductor Inc. (TDSI). The MIDIS process provides wafer-level vacuum encapsulation under a high vacuum pressure of 10 mtorr, which enables the absolute pressure measurement. We perform post-fabrication processing of the obtained devices from the foundry to create a thin single crystal silicon membrane and expose it to atmospheric pressure to serve as the sensing membrane for the pressure sensor. The proposed pressure device includes Through Silicon Vias (TSVs) suitable for flip-chip bonding with a signal conditioning integrated circuit. The presented sensor uses a deflectable large membrane with accurate controllable thickness and offers a high sensitivity of 16.5 fF/kPa with a good linearity over designed pressure range of 101---125 kPa. The operating pressure range can be modified by simply varying the physical dimensions of the sensing membrane. We demonstrate several prototype absolute capacitive pressure sensors with different membrane diameters ranging from 140 to 360 µm and thicknesses ranging from 1 to 10 µm with a fixed gap of 2 µm between the membrane and immovable electrode. The sensor calibration data is collected in a regulated pressure chamber using a reference commercial pressure sensor.

Design and fabrication of a high performance resonant MEMS temperature sensor

This paper presents a high performance MEMS temperature sensor comprised of a double-ended-tuning-fork (DETF) resonator and strain-amplifying beam structure. The temperature detection is based on the ‘thermal strain induced frequency variations’ of the DETF resonator. The major source of thermal strain leading to the frequency shifts is the difference in thermal expansion coefficients of the substrate and the device layers of the fabricated structures. By selecting the substrate as glass and the device layers as single crystal silicon, i.e. materials with different thermal expansion coefficients, the tines of the resonators are exposed to axial load with the changing temperature, which causes a change in the resonance frequency of the resonators. This resonance frequency shift can be related with the changing temperature by taking the thermal strain relations into consideration, which enables utilization of the resonator as a highly sensitive temperature sensor. The resonators used in this study have been fabricated by utilizing the advanced MEMS process that incorporates the simple silicon-on-glass process with the wafer level vacuum packaging Torunbalci et al (2015 J. Microelectromech. Syst. 24 556–64). The fabricated resonators have been tested in a temperature-controlled oven between −20 °C and 60 °C, and the results of two distinct designs are compared to be able to observe the effectiveness of the strain amplifying beam. Measurement results show that the design with the strain amplifying beam increases the temperature coefficient of frequency of the resonators by 33 times when compared to the one-end free DETF resonators. Minimum detectable temperature variations observed by the resonators used in this study is 0.0011 °C. This kind of very high resolution temperature sensing can be achieved by integrating this MEMS temperature sensor with any type of physical MEMS sensor where its fabrication process includes different materials for the substrate and sensor structures.

Fabrication of high-resolution strain sensors based on wafer-level vacuum packaged MEMS resonators

The paper reports on the fabrication and characterization of high-resolution strain sensors for structural materials based on Silicon On Insulator flexural resonators manufactured by polysilicon Low-Pressure Chemical Vapour Deposition vacuum packaging. The sensors present sensitivity of 164Hz/με and strain resolution limit of 150pε on steel for a measurement time of 315ms, in both tensile and compressive strain regimes. The readout of the sensor is implemented with a transimpedance oscillator circuit implemented on Printed Circuit Board, in which a microcontroller-based reciprocal frequency counter is integrated. The performance of the sensors on steel are investigated for measurement bandwidths from 1.5 to 500Hz and a comparison with conventional metal strain gauges is proposed.

Microelectromechanical Resonant Accelerometer Designed with a High Sensitivity.

This paper describes the design and experimental evaluation of a silicon micro-machined resonant accelerometer (SMRA). This type of accelerometer works on the principle that a proof mass under acceleration applies force to two double-ended tuning fork (DETF) resonators, and the frequency output of two DETFs exhibits a differential shift. The dies of an SMRA are fabricated using silicon-on-insulator (SOI) processing and wafer-level vacuum packaging. This research aims to design a high-sensitivity SMRA because a high sensitivity allows for the acceleration signal to be easily demodulated by frequency counting techniques and decreases the noise level. This study applies the energy-consumed concept and the Nelder-Mead algorithm in the SMRA to address the design issues and further increase its sensitivity. Using this novel method, the sensitivity of the SMRA has been increased by 66.1%, which attributes to both the re-designed DETF and the reduced energy loss on the micro-lever. The results of both the closed-form and finite-element analyses are described and are in agreement with one another. A resonant frequency of approximately 22 kHz, a frequency sensitivity of over 250 Hz per g, a one-hour bias stability of 55 μg, a bias repeatability (1σ) of 48 μg and the bias-instability of 4.8 μg have been achieved.

Wafer level vacuum encapsulated tri-axial accelerometer with low cross-axis sensitivity in a commercial MEMS Process

We present the design, fabrication and testing of a novel integrated tri-axial capacitive accelerometer with low cross-axis sensitivity. The accelerometer is fabricated in MEMS Integrated Design for Inertial Sensors (MIDIS), a process recently introduced by Teledyne DALSA Semiconductor Inc. (TDSI). The MIDIS process is based on high aspect ratio bulk micromachining of single-crystal silicon wafer that is vacuum encapsulated between two other silicon wafers. The fabrication process includes Through Silicon Vias (TSVs) with sealed anchors for compact flip–chip integration with microelectronic signal processing circuitry. The proposed accelerometer is designed to provide simultaneous detection of the input accelerations along the 3 principle axes enabling high-g measurement up to ±50g. The integrated structure design uses decoupled frames supported by strategically designed springs and employs capacitive compensators for the purpose of achieving low cross-axis sensitivity. The capacitive transduction is based on differential measurement along X- and Y- axis and absolute measurement along the Z-axis. The initial measured capacitances are 2.18pF, 2.4pF and 1.14pF along X, Y and Z-axis, respectively. The dynamic specifications of the accelerometer are characterized by its lowest-order operating mode at 4kHz.

A Lateral Differential Resonant Pressure Microsensor Based on SOI-Glass Wafer-Level Vacuum Packaging.

This paper presents the fabrication and characterization of a resonant pressure microsensor based on SOI-glass wafer-level vacuum packaging. The SOI-based pressure microsensor consists of a pressure-sensitive diaphragm at the handle layer and two lateral resonators (electrostatic excitation and capacitive detection) on the device layer as a differential setup. The resonators were vacuum packaged with a glass cap using anodic bonding and the wire interconnection was realized using a mask-free electrochemical etching approach by selectively patterning an Au film on highly topographic surfaces. The fabricated resonant pressure microsensor with dual resonators was characterized in a systematic manner, producing a quality factor higher than 10,000 (~6 months), a sensitivity of about 166 Hz/kPa and a reduced nonlinear error of 0.033% F.S. Based on the differential output, the sensitivity was increased to two times and the temperature-caused frequency drift was decreased to 25%.

A wafer level vacuum encapsulated capacitive accelerometer fabricated in an unmodified commercial MEMS process.

We present the design and fabrication of a single axis low noise accelerometer in an unmodified commercial MicroElectroMechanical Systems (MEMS) process. The new microfabrication process, MEMS Integrated Design for Inertial Sensors (MIDIS), introduced by Teledyne DALSA Inc. allows wafer level vacuum encapsulation at 10 milliTorr which provides a high Quality factor and reduces noise interference on the MEMS sensor devices. The MIDIS process is based on high aspect ratio bulk micromachining of single-crystal silicon layer that is vacuum encapsulated between two other silicon handle wafers. The process includes sealed Through Silicon Vias (TSVs) for compact design and flip-chip integration with signal processing circuits. The proposed accelerometer design is sensitive to single-axis in-plane acceleration and uses a differential capacitance measurement. Over ±1 g measurement range, the measured sensitivity was 1fF/g. The accelerometer system was designed to provide a detection resolution of 33 milli-g over the operational range of ±100 g.

Wafer level vacuum packaging of micro-mirrors with buried signal lines

This paper presents the vacuum wafer level packaging of in-house manufactured two-dimensional micro- mirrors based on the electrostatic driving principle, fabricated using a bulk technology on bonded-silicon-on-insulator wafers. The so called buried signal line concept is used in order to obtain a wafer bond friendly bonding frame. Two versions of the buried signal line concept are described in the paper: crystalline Si and poly-Si based. Anodic and glass frit bonding methods were used to bond a stack of four wafers, resulting in a vacuum-sealed package. The fabrication technology, alignment, bonding, dicing and electrical characterization are discussed. A vacuum level of 120 Pa with no getter materials was reached in the package. Due to vacuum packaging the driving voltage of the micro-mirrors after dicing could be significantly decreased from approximately 170 V to less than 35 V.

Low temperature activation of Au/Ti getter film for application to wafer-level vacuum packaging

Non-evaporable getter (NEG) thin films based on alloys of transition metals have been studied by various authors for vacuum control in wafer-level packages of micro electro mechanical systems (MEMS). These materials have typically a relatively high activation temperature (300–450 °C) which is incompatible with some temperature sensitive MEMS devices. In this work we investigate the potential of Au/Ti system with a thin or ultrathin non oxidizable Au layer as a low activation temperature getter material. In this bilayer system, gettering activation is produced by thermal outdiffusion of titanium atoms through the gold film. The outdiffusion kinetics of titanium was modelled and characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS) at various temperatures. Results confirm that Au/Ti bilayer is a promising getter material for wafer-level packaging with an activation temperature below 300 °C for 1 h annealing time.

Vacuum-packaged Resonant Pressure Sensor with Dual Resonators for High Sensitivity and Linearity

This paper presents a vacuum-packaged resonant pressure sensor featured with high sensitivity and linearity using dual resonators. The frequency difference between these two resonators is used to represent pressure, so that the nonlinear responds of each resonator is eliminated and the sensitivity of the sensor is also improved at the same time. The sensitivities of the dual resonators were matched through improved diaphragm uniformity and controlled anchor undercut, which leaded to a better linear performance. With this method, the nonlinear error is reduced from 1.3% F.S. to 0.2% F.S. over the pressure range from 0 to 250kPa. Furthermore, to achieve better performance, the micro resonators were wafer-level vacuum packaged with anodic bonding to provide the needed low damping environment and the stable reference pressure. Test results show that the pressure in the vacuum-packaged micro chamber of the sensor is less than 5Pa and kept for 6 months with no drop.

A high-Q resonant pressure microsensor with through-glass electrical interconnections based on wafer-level MEMS vacuum packaging.

This paper presents a high-Q resonant pressure microsensor with through-glass electrical interconnections based on wafer-level MEMS vacuum packaging. An approach to maintaining high-vacuum conditions by integrating the MEMS fabrication process with getter material preparation is presented in this paper. In this device, the pressure under measurement causes a deflection of a pressure-sensitive silicon square diaphragm, which is further translated to stress build up in “H” type doubly-clamped micro resonant beams, leading to a resonance frequency shift. The device geometries were optimized using FEM simulation and a 4-inch SOI wafer was used for device fabrication, which required only three photolithographic steps. In the device fabrication, a non-evaporable metal thin film as the getter material was sputtered on a Pyrex 7740 glass wafer, which was then anodically bonded to the patterned SOI wafer for vacuum packaging. Through-glass via holes predefined in the glass wafer functioned as the electrical interconnections between the patterned SOI wafer and the surrounding electrical components. Experimental results recorded that the Q-factor of the resonant beam was beyond 22,000, with a differential sensitivity of 89.86 Hz/kPa, a device resolution of 10 Pa and a nonlinearity of 0.02% F.S with the pressure varying from 50 kPa to 100 kPa. In addition, the temperature drift coefficient was less than −0.01% F.S/°C in the range of −40 °C to 70 °C, the long-term stability error was quantified as 0.01% F.S over a 5-month period and the accuracy of the microsensor was better than 0.01% F.S.

Silicon MEMS Disk Resonator Gyroscope With an Integrated CMOS Analog Front-End

We present a 2-mm diameter, 35-μm-thick disk resonator gyro (DRG) fabricated in silicon with integrated 0.35-μm CMOS analog front-end circuits. The device is fabricated in the commercial InvenSense Fabrication MEMSCMOS integrated platform, which incorporates a wafer-level vacuum seal, yielding a quality factor (Q) of 2800 at the DRGs 78-kHz resonant frequency. After performing electrostatic tuning to enable mode-matched operation, this DRG achieves a 55 μV/°/s sensitivity. Resonator vibration in the sense and drive axes is sensed using capacitive transduction, and amplified using a lownoise, on-chip integrated circuit. This allows the DRG to achieve Brownian noise-limited performance. The angle random walk is measured to be 0.008°/s/√(Hz) and the bias instability is 20°/h.

Low Activation Temperature Au/Ti Getter Films for Wafer-Level Vacuum Packaging

In this work thermal diffusion of titanium through an ultrathin capping gold layer has been investigated as an alternative way to produce a getter film activated at low temperature. Ti out-diffusion from Au/Ti thin films was studied using four probe resistivity measurements, XPS analysis, residual stress measurements, SEM cross section observation and Ion Beam Etching coupled with mass spectrometry. The results show that a titanium oxide film is growing as the titanium out diffuses and comes in contact with traces of oxidizing species in N2 annealing gaz. The pumping capacity is large enough to integrate Au/Ti film as a low temperature getter film for wafer-level vacuum packaging.

Low Activation Temperature Au/Ti Getter Films for Wafer-Level Vacuum Packaging

Vacuum packaging is required for mechanical microresonators and microbolometers to obtain a high quality factor and a low heat loss respectively. Because the internal volume of wafer-level packages is small, integration of getter films in the cavity is needed to maintain a low internal pressure (<10-2 mbar) for a long period of time (several years or more) even if a highly hermetic wafer bonding process is used [1,2]. Many Non-Evaporable Getter (NEG) films based on sputtered highly reactive transition metal have been investigated for this purpose. In this work we investigate thermal diffusion of titanium through an ultrathin capping Au layer as an alternative way to produce a getter film activated at low temperature. Both films were evaporated to avoid rare gas contamination found in sputtered films which can outgas afterwards. [3]The films have been annealed in N2 and Ti out-diffusion from Au/Ti thin films was studied as a function of the annealing temperature using four probe resistivity measurements, spectroscopic ellipsometry, XPS analysis, residual stress variations, SEM cross section observations and Energy Dispersive X-Ray Spectrometry (EDX) measurements. These measurements were completed by AFM and SEM surface analyses. All results are consistent and support the occurrence of significant Ti out-diffusion followed by oxidation, even at low temperature (<300°C). The electrical resistivity already increases by 10% after 1 hour annealing at 200°C. XPS measurements confirm the presence of a TiOx layer after annealing which contribution increases with the temperature and with a composition close to x » 1.8. SEM and AFM images of the as-deposited films surface show a smooth surface of gold, with RMS roughness below 1.5 nm. After annealing at 300°C, the titanium oxide is visible at the surface, in a shape of flakes and after annealing at 350°C, this layer covers completely the gold. Analysis of the sample cross-section shows that after annealing the gold layer is buried under the TiOx layer, that this layer is continuous and that its depth, corresponding to the TiOx thickness, increases with the annealing temperature (see fig., left-SE and right-BSE). Finally, TiOx films of several nm can be obtained by diffusion of the Ti through the Au at low temperature, showing that Au/Ti film can be used as a low temperature getter film for wafer-level vacuum packaging.[1]. M. Deniel, A. Naumann, D. Sorger, S. Tenholte, J. Voigt, O. Mehner, The influence of vacuum on the design and characterization of MEMS, Vacuum, 2012, 86 pp. 536-546.[2]. S. García-Blanco, P. Topart, Y. Desroches, J. Caron, F. Williamson, C. Alain and H. Jerominek, Low temperature hermetic wafer-level package for uncooled microbolometer FPAs, Proceedings of SPIE, 2008, 68840P-1.[3]. L. Tenchine, X. Baillin, C. Faure, P. Nicolas, E. Martinez, NEG thin films for under controlled atmosphere MEMS packaging, Sensors and Actuators A, 2011, 172 pp. 233-239.

The Design and Simulation of Vibratory Ring Gyroscope

This paper deals with design and simulation of vibratory ring gyroscope which has good performance as resisting vibration, resisting impact, small temperature drift because of the inherent symmetry of structure. The good performance of gyroscope has verified by detailed modeling and simulation. The manufacture of gyroscope using wafer level vacuum packaging technology making the gyroscope has high quality factor .At last we test the gyroscope and the Q factor is 20300 by detecting the resonant state of the gyroscope.

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.