Yanmei Kong

Elimination of initial stress-induced curvature in a micromachined bi-material composite-layered cantilever

Micro-devices with a bi-material-cantilever (BMC) commonly suffer initial curvature due to the mismatch of residual stress. Traditional corrective methods to reduce the residual stress mismatch generally involve the development of different material deposition recipes. In this paper, a new method for reducing residual stress mismatch in a BMC is proposed based on various previously developed deposition recipes. An initial material film is deposited using two or more developed deposition recipes. This first film is designed to introduce a stepped stress gradient, which is then balanced by overlapping a second material film on the first and using appropriate deposition recipes to form a nearly stress-balanced structure. A theoretical model is proposed based on both the moment balance principle and total equal strain at the interface of two adjacent layers. Experimental results and analytical models suggest that the proposed method is effective in producing multi-layer micro cantilevers that display balanced residual stresses. The method provides a generic solution to the problem of mismatched initial stresses which universally exists in micro-electro-mechanical systems (MEMS) devices based on a BMC. Moreover, the method can be incorporated into a MEMS design automation package for efficient design of various multiple material layer devices from MEMS material library and developed deposition recipes.

Infrared camera based on optical-readout bi-material FPA

With the trend of developments of infrared-focal plane array (FPA) which is particularly pronounced in many applications, in this paper, we demonstrated an uncooled infrared (8~14μm) camera based on the optically read-out Bi-material Infrared FPA. We reported on the fabrication and characterization of arrays of bimaterial microcantilevers. On the basis of opto-mechanical effect and micro electromechanical system (MEMS) technology, a substrate-free FPA with the thermal isolated structure SiNx and Au for uncooled infrared imaging is developed, with 49.5μm×49.5μm of the pixel and 240×240 array, moreover, the camera had an average noise equivalent temperature difference (NETD) and a response time of 100mK and 100ms at 7Pa atmospheric pressure, respectively.

An electrical test method for quality detecting of wafer level eutectic bonding

As the costs of packaging and testing account for a substantial portion of microelectromechanical system (MEMS) devices, an effective and convenient characterization method is urgent to be investigated to lower the cost. In this paper, an electrical test method was utilized, and the test key used for a four-probe current-voltage test was designed to monitor the quality of the AuSn eutectic bonding. The electrical test can directly detect whether or not voids existed in the bonding layer. The difference in alloy state, for example, the existence of the (Au, Ni) 3Sn2 phase confirmed by the scanning electron microscope and energy dispersive x-ray spectroscopy test, can also be reflected by resistivity variation. The electrical test can be implemented automatically and conveniently unlike other characterization methods. Therefore, it is suitable to be applied in quality inspection in industrial production.

Study of cavity effect in micro-Pirani gauge chamber with improved sensitivity for high vacuum regime

Ultra-low pressure application of Pirani gauge needs significant improvement of sensitivity and expansion of measureable low pressure limit. However, the performance of Pirani gauge in high vacuum regime remains critical concerns since gaseous thermal conduction with high percentage is essential requirement. In this work, the heat transfer mechanism of micro-Pirani gauge packaged in a non-hermetic chamber was investigated and analyzed compared with the one before wafer-level packaging. The cavity effect, extremely important for the efficient detection of low pressure, was numerically and experimentally analyzed considering the influence of the pressure, the temperature and the effective heat transfer area in micro-Pirani gauge chamber. The thermal conduction model is validated by experiment data of MEMS Pirani gauges with and without capping. It is found that nature gaseous convection in chamber, determined by the Rayleigh number, should be taken into consideration. The experiment and model calculated results show that thermal resistance increases in the molecule regime, and further increases after capping due to the suppression of gaseous convection. The gaseous thermal conduction accounts for an increasing percentage of thermal conduction at low pressure while little changes at high pressure after capping because of the existence of cavity effect improving the sensitivity of cavity-effect-influenced Pirani gauge for high vacuum regime.Ultra-low pressure application of Pirani gauge needs significant improvement of sensitivity and expansion of measureable low pressure limit. However, the performance of Pirani gauge in high vacuum regime remains critical concerns since gaseous thermal conduction with high percentage is essential requirement. In this work, the heat transfer mechanism of micro-Pirani gauge packaged in a non-hermetic chamber was investigated and analyzed compared with the one before wafer-level packaging. The cavity effect, extremely important for the efficient detection of low pressure, was numerically and experimentally analyzed considering the influence of the pressure, the temperature and the effective heat transfer area in micro-Pirani gauge chamber. The thermal conduction model is validated by experiment data of MEMS Pirani gauges with and without capping. It is found that nature gaseous convection in chamber, determined by the Rayleigh number, should be taken into consideration. The experiment and model calculated res...

Design and fabrication of wafer-level packaged MEMS Pirani gauge with sorrounded heat sinks

This paper presents a wafer-level packaged MEMS (Micro-electromechanical Systems) Pirani gauge with surrounded heat sinks with improved characteristics during the low limit of the cavity vacuum. Different to the reported gauges in other papers, it has four heat sinks, and one of these is the cap of the wafer-level package. And it utilizes two heaters and two constant resistors to construct an Whetstone Bridge, two meander-shaped suspended silicon coils is set as the heaters and two isolated comb-shaped silicon structures as the dual-lateral heat sinks. In addition, the substrate and capped wafers are used as dual-vertical heat sinks. The gauge is finally realized with the volume 1.4mm×1.2mm×0.5mm using the CMOS (Complementary Metal Oxide Semiconductor)-MEMS compatible process. The gauge was measured twice before and after wafer-level packaging. It denotes that with the introduction of the capped heat sink, the device bears a better sensitivity compared to the intermediate device before packaging. With a hole on the edge of the device, the gauge could be widely and flexibly used for the characterization of vacuum packaged MEMS devices.

Investigation and Optimization of Pirani Vacuum Gauges With Monocrystal Silicon Heaters and Heat Sinks

This paper presents micro-Pirani vacuum gauges using low-resistivity monocrystal silicon for the heaters and the heat sinks. We designed a new Pirani gauge with an embedded Wheatstone bridge made by thick-silicon resistors that allows an improvement in resolution and sensitivity. With the optimized Wheatstone bridge structure, the temperature coefficient of the gauges pressure measure decreases from 2.5%/°C to 0.74%/°C, and the resolution was optimized from 0.071 to 0.008 Torr at 1 Torr. In addition, the gauges were packaged into non-hermetic cavities by wafer-level AlGe eutectic bonding to undergo the wafer-dicing process, with sensitivity improved from 1.65 to 17.1 V/Torr at 1 Torr. [2016-0320]