Dehui Xu

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 × 10-9 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.

Integrated micromachined thermopile IR detectors with an XeF2 dry-etching process

Based on the Seebeck effect, the CMOS compatible micromachined thermopile is widely used in infrared detection for its advantages of low-cost, high batch yield, broad spectral response and insensitivity to ambient temperature. We present two integrated thermopile IR detectors on stacked dielectric layers realized by a standard P-well CMOS technology, followed by one CMOS compatible maskless XeF2 isotropic dry-etching step. Characterizations of CMOS devices, before and after XeF2 etching, respectively, were performed to investigate the effects of XeF2 etching on the CMOS devices. With a 2.5 ?m thick stacked silicon oxide?nitride?oxide layer as an absorber, the rectangular thermopile detector and the circular thermopile detector provided responsivity of 14.14 and 10.26 V W?1, specific detectivity of 4.15 ? 107 and 4.54 ? 107 cm Hz1/2 W?1, and time constant of 23.7 and 14.6 ms, respectively. Compared with the rectangular thermopile detector, the circular thermopile detector is mechanically more stable, because its circular structure design eases the internal stress problem in the CMOS layers. After XeF2 etching, the maximum changes of threshold voltage, maximum transconductance and switching threshold voltage were 0.97%, 1.25% and 0.08%, respectively. Experimental results show that the effects of XeF2 etching on the CMOS devices are insignificant, and XeF2 etching is suitable for post-CMOS micromachining.

A high-performance bulk mode single crystal silicon microresonator based on a cavity-SOI wafer

A cavity-silicon-on-insulator (SOI)-based single crystal silicon (SCS) micromechanical resonator has been demonstrated in this paper. The most distinguishing feature of this method is that it solves the restrictions of being released from the sacrificial layer. The resonator structures can be fabricated and released in one step using dry anisotropic etching. The differential drive, single-ended sense configuration is implemented to measure the electrical characterization of the fabricated resonator. The fabricated square plate resonator has been excited in the Lame´ mode at a resonant frequency of 4.126 MHz and exhibits a quality factor (Q) as high as 5.49× 106 at a pressure of 0.05 mbar. This result corresponds to a frequency–Q product of 2.27× 1013, which is the highest value demonstrated to date for silicon-based resonators as far as we know. The dependence of Q and resonant frequency on the operating pressure is measured and characterized. The temperature stability of the device is also demonstrated, with the temperature coefficient of resonant frequency less than −20.8 ppm °C−1 in the temperature range from −10 to 60 °C. The high performance of the resonator not only benefits from the superior performance of SCS as a mechanical material, but also the merit of the cavity-SOI structure.

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.

Resonant Magnetic Field Sensor With Capacitive Driving and Electromagnetic Induction Sensing

This letter presents a novel magnetic field sensor, which consists of a square extensional mode resonator with a planar induction coil. This sensor exploits the principle of electromagnetic induction to detect external magnetic field through the electromotive force in the induction coil, which is placed on top of the resonant plate. The proposed sensor employs capacitive driving and electromagnetic induction sensing approach to detect the external magnetic field. The capacitive driving method reduces the power dissipation, and the electromagnetic induction sensing approach makes it easy to measure the output signal with high precision. The operation principle and fabrication process as well as the characterization of the magnetic field sensor are presented. Experimental result shows that the device offers a sensitivity of 3 μV/mT at its resonant frequency f0 = 4.33 MHz in air. In this letter, the vibrating characterization of the sensor is largely reduced due to air damping. Therefore, vacuum packaging is needed to enhance the performance of the magnetic field sensor.

Phononic crystal strip based anchors for reducing anchor loss of micromechanical resonators

Micromechanical resonators must be clamped to the substrate via anchors to support the suspended microstructure. However, these anchors will introduce anchor loss, and decrease quality factors (Qs) of the micromechanical resonators. To reduce the anchor loss, one dimensional phononic crystal based strips are employed as anchors of the microresonators in this paper. The dispersion relations and eigenmodes of the phononic crystal strips are presented. Flexural mode ring resonator and Lame mode square plate resonator are designed to verify the effect of phononic crystal strips. The calculated results and finite-element simulations indicate that the leaky energy could be effectively reduced by the phononic crystal strip anchor design. Resonators with different anchor designs are also fabricated and characterized. The measured Qs of the microresonators show that the phononic crystal strips could reduce the energy dissipated through anchors, and with increasing the number of phononic strip periods, Qs of the re...

Uncooled Thermoelectric Infrared Sensor With Advanced Micromachining

A simple mass producible uncooled thermoelectric infrared microsensor has been designed and fabricated. To improve the cost-efficiency, an advanced micromachining process, which combines wet anisotropic pre-etching and XeF2 dry isotropic post-etching, is adopted for the sensor fabrication. The wet anisotropic pre-etching removes bulk silicon from back-side and forms a thin silicon membrane for device fabrication, the XeF2 dry isotropic post-etching undercuts silicon membrane and releases the microstructure. Experimental results show that the sensor with advanced micromachining exhibits a two times higher responsivity and detectivity than the sensor with only XeF2 front-side etching. In air at room temperature, the sensor with advanced micromachining has a responsivity of 71.57 V W-1, noise equivalent power of 0.64 nW Hz-1/2, detectivity of 6.21×107 cm Hz1/2 W-1 and a time constant of 13.2 ms. The effect of back-side etch window size on sensor performance is also characterized by finite-element method simulation.

Isotropic Silicon Etching With

Wafer-level isotropic etching of silicon with XeF2 gas has been investigated for microelectromechanical-system (MEMS) fabrication. Because of the large exposed silicon area in the wafer-level process, XeF2 gas diffusion in the wafer-level process is different from the chip-level process. The silicon etch rate for the wafer-level XeF2 process is much smaller than chip-level XeF2 etching. Additionally, the silicon etch rate drops off as the etching time increased. The aperture size effect is apparent in the wafer-level XeF2 processing. However, for etching windows with a large size, the aperture size effect will be minimized. Both vertical and lateral aperture size effects depend on the number of etch cycle. Although slight anisotropy is also observed, wafer-level XeF2 etching shows a better isotropy than the chip-level process. Compared with the chip-level process, wafer-level XeF2 etching shows a large etch rate for SiO2. The etch selectivity between silicon and SiO2 is lower than 1000:1. Based on the characteristics of XeF2 etching, the layout design rule for the MEMS device with XeF2 releasing is developed and demonstrated.

\hbox{XeF}_{2}

This paper presents the design, fabrication and experimental results of a front-etched CMOS compatible micromachined thermopile IR detector. The N-polysilicon/Al thermocouples were embedded in a 2.5 ?m thick SiO2?Si3N4?SiO2 sandwich membrane, and XeF2 front-side isotropic post-etching was adopted to release and thermally isolate the thermopile structure. Due to the isotropy of XeF2 etching, a lot of flexibility was allowed in etching window layout and thermopile structure design. Etching windows in the dielectric absorber area were designed to avoid cutting off the heat transfer path from the absorber to hot junctions, and aluminum strips were patterned in the absorber to ensure the temperature was uniform across the absorber area. Two different thermopile structures, circle and rectangle, were designed and fabricated to investigate detector performance with respect to the thermopile structure. The steady-state behavior of fabricated detectors was anticipated by thermal ANSYS simulation. Due to the fact that the circular structure can get a higher temperature difference between hot and cold junctions, the circular thermopile detector has a quicker response, two times higher responsivity and detectivity than the rectangular thermopile detector. The circular thermopile detector exhibits a responsivity of 102.0 V W?1, a detectivity of 9.2 ? 107 cm Hz1/2 W?1 and a time constant of 16.8 ms, in air at room temperature.

Gas for Wafer-Level Micromachining Applications

CMOS-compatible thermopile detectors are widely used for IR detection. In this paper, an analytical model is developed for front-etched thermopile IR detectors using CMOS technology. By dividing the front-etched microbridge thermopile detector into cantilever thermocouple detectors with separate absorber areas, the thermal gradient in each zone of the single thermocouple IR detector (absorber and thermocouple transducer) is analyzed using 1-D method. During thermal gradient modeling, the IR absorption of dielectric layers in the thermocouple area is also considered. The thermopile IR detector performance is then calculated by adding the temperature differences of each single thermocouple IR detector. The developed analytical model has been verified by comparing simulations with experiments. The simulation results closely agree with the measured results. For optimizing the geometry of the front-etched thermopile IR detector, a quantitative study of the detector performance is conducted with respect to the absorber width, polysilicon length, polysilicon width, and etching window width.