Measurements of True Leak Rates of MEMS Packages

Gas transport mechanisms that characterize the hermetic behavior of MEMS packages are fundamentally different depending upon which sealing materials are used in the packages. In metallic seals, gas transport occurs through a few nanoscale leak channels (gas conduction) that are produced randomly during the solder reflow process, while gas transport in polymeric seals occurs through the bulk material (gas diffusion). In this review article, the techniques to measure true leak rates of MEMS packages with the two sealing materials are described and discussed: a Helium mass spectrometer based technique for metallic sealing and a gas diffusion based model for polymeric sealing

Towards new hermeticity test methods for MEMS

Hermeticity is a measure of how well a package can maintain its intended ambient cavity environment over the device lifetime. Since many Micro-Electro-Mechanical Systems (MEMS) sensors, actuators and microelectronic devices require a known cavity environment for optimum operational performance, it is important to know the leak rate of the package for lifetime prediction purposes. In this field, limitations in the traditional leak detection methods and standards used originally for integrated circuits and semiconductors have been blindly and often incorrectly applied to MEMS and microelectronic packages. The aim of this project is to define accurately the limitations of the existing hermeticity test methods and standards when applied to low cavity volume MEMS and microelectronic packages and to demonstrate novel test methods, which are applicable to such packages. For the first time, the use of the Lambert-W function has been demonstrated to provide a closed form expression of the maximum true leak rate achievable for the most commonly used existing hermeticity test method, the helium fine leak test. This expression along with the minimum detectable leak rate expression is shown to provide practical guidelines for the accurate testing of hermeticity for ultra-low volume packages. The three leak types which MEMS and microelectronic packages are subject to: molecular leaks, permeation and outgassing, are explained in detail and it is found that the helium leak test is capable of quantifying only molecular leak in packages with cavity volumes exceeding 2.6 mm3. With many MEMS and microelectronic package containing cavities with lower volumes, new hermeticity test methods are required to fill this gap and to measure the increasingly lower leak rates which adversely affect such packages. Fourier Transform Infra-Red (FTIR) spectroscopy and Raman spectroscopy are investigated as methods of detecting gas pressure within MEMS and microelectronics packages. Measured over time, FTIR can be used to determine the molecular and permeation leak rates of packages containing infra-red transparent cap materials. Future work is required to achieve an adequate signal to noise ratio to enable Raman spectroscopy to be a quantitative method to determine molecular leaks, permeation leaks and potentially outgassing. The design, fabrication and calibration procedure for three in-situ test structures intended to monitor the hermeticity of packages electrically are also presented. The calibration results of a piezoresistive cap deflection test structure show the structure can be used to detect leak ii rates of any type down to 6.94×10-12 atm.cm3.s-1. A portfolio of hermeticity test methods is also presented outlining the limitations and advantages of each method. This portfolio is intended to be a living document and should be updated as new research is undertaken and new test methods developed

A review of the techniques to measure the hermeticity of glass frit encapsulated solar cells

Emerging 3rd generation photovoltaic technologies such as perovskite and dye-sensitized solar cells are very attractive for commercialization mainly due to their low-cost materials and fabrication processes. The main drawback of these devices is their poor long-term stability. To increase the long-term stability of these devices, a hermetic encapsulation is required. The hermeticity of encapsulated devices are measured and characterized using hermeticity tests according to standard test procedures. A review of the several techniques to measure the hermeticity is presented, addressing the test methods, limitations and applicability to perovskite and dye-sensitized solar cells glass frit encapsulated devices

Diode laser processing of PMMA and LCP materials for microsystem packaging

The thesis describes the development of laser-assisted bonding methods for assembly of microfluidic devices and MEMS packaging. A laser microwelding technique for assembly of transparent polymer substrates for fabrication of microfluidic devices was studied. The transparent PMMA substrates were bonded together using a high power diode laser system with a broad top-hat beam profile and an intermediate titanium thin film consisting of 0.7 mm diameter spots. A tensile strength of 6 MPa was achieved for this novel method which is comparable to that of the previous work in laser welding of polymers. It has been demonstrated that the method is capable of leak free encapsulation of a microfluidic channel. Furthermore, a novel laser-based method using an LCP film for packaging of MEMS, sensors and other microelectronic devices has been investigated. The results show that it is possible to use a laser based method with an LCP polymer for high quality substrate bonding applications. Glass-glass based cavities allow optical transmission and have potential applications for optical sensors and other photonic devices. For glass-glass bonding, it was shown that thin film titanium material can be used as an effective optical absorber in the laser based LCP bonding technique. Laser bonding of glass and silicon using an LCP film has also been achieved but in this case the silicon substrate acted as the absorber to capture the laser power. Laser bonding of a silicon cap to a molded LCP package has also been demonstrated successfully. The results of temperature monitoring using embedded sensors show that the temperature at the base of the LCP package (~130C) is substantially lower than the bonding temperature (> 280C). The results of shear and leak test show good reliability and hermeticity of the laser bonded microcavities. Both two-dimensional and three-dimensional models of heat transfer are developed and studied using the COMSOL Multiphysics software tool to understand the localised laser heating effects. The results are in good agreement with those of the practical work

Quantitative Hermeticity Assessment of Packages with Micro to Nano-liter Cavities

Hermeticity is a measure of the "leak-proof ness" of packages with internal cavities and is critical for ensuring proper operation of the devices/circuits enclosed in them. The most widely used hermeticity detection technique in the industry is the helium fine leak test. The exiting conduction based governing equation is examined to investigate the volume dependant limits of the test when applied to metal sealed MEMS packages. The results clearly indicate that the test has limited applicability for small internal volumes (1 nanoliter - 1 microliter). The limited applicability of the guidelines specified in Method 1014.11 of the MIL-STD-883F document for hermeticity characterization is also characterized. To cope with these limitations, a regression analysis based procedure is developed and implemented to extract the true leak rate from the apparent leak data. While the apparent leak rate obtained directly from the He mass spectrometer changes with the test parameters, the true leak rate remains constant and this can be used as a metric to evaluate a package seal. The hermeticity of polymer sealed MEMS packages is also studied. Unlike metal sealed packages, gas transport in polymer sealed packages occurs via diffusion. A gas diffusion based model is proposed to study the hermetic behavior of these packages. An effective numerical scheme is developed to implement this model and simulate the change in cavity pressure as gas flows into or out of the cavity through the polymeric seal. An optical interferometry based leak test is developed to experimentally measure this change in cavity pressure. The experimental data is used to verify the validity of the proposed numerical scheme and the assumption of adiabatic boundary conditions made in the numerical model. An inverse method is presented to determine the two diffusion properties, diffusivity and solubility, of the polymeric seal by using the experimental data iteratively with the numerical data. The proposed method offers unique advantages over the routinely practiced/existing gas diffusion property measurement techniques

Vacuum and Hermetic Packaging of MEMS using Solder

This work explores the use of solder as a material for wafer-level vacuum packaging of MicroElectroMechanicalSystems (MEMS). Two bonding techniques were developed and characterized: a standard solder bond and an advanced solder bond based on transient liquid phase (TLP) bonding. Solder was also used as a release layer as well as bond layer for forming transferred thin-film packages. Several different standard solder alloy / under bump metallization combinations were used for wafer bonding. Only the Au-Sn solder alloy, with its low tin content, proved to be compatible with the thermal limitations of commercial wafer bonders. The bond is formed at 300 °C in under an hour and has a shear strength of 28 MPa. It was used to create packages (2.3 mm X 2.3 mm X 0.5 mm) with integrated Pirani gauges. The pressures were as low as 200 mTorr and showed a worst-case leak rate of 1.5.10-15 atm.cc.s-1. TLP solder bonding was investigated because it is more compatible with the long thermal time-constant of commercial wafer bonders. Au-In and Ni-Sn TLP solder bonds were used to create vacuum packages (2.3 mm X 2.3 mm X 0.5 mm) with integrated Pirani gauges. The Ni-Sn and Au-In packages were formed at 300 °C and 200 °C, have measured shear strengths of 12.4 and 24.4 MPa, showed package pressures of 200 mTorr and 150 mTorr, and worst-case leak rates of 1.7.10-15 atm.cc.s-1 and 0.1.10-15 atm.cc.s-1. The design rules for creating bonds with these techniques are presented. Outgassing and getter activation were studied. Package pressures were reduced to 20 mTorr by outgassing for 24 hours before bonding. It was shown that titanium getters can be activated at 200 °C, enabling a MEMS vacuum packaging process with a maximum temperature of 200 °C. Solder was used to transfer thin-film electroplated nickel packages as small as 250 μm wide, 250 μm long, and 20 μm thick. A thin nickel film was electroplated over a lead-free solder transfer layer on a carrier wafer and then simultaneously bonded and transferred to a device wafer at 300 °C for 1 hour with a yield of greater than 99%.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/58520/1/welchw_1.pd

Estado del arte en detección de fugas y propuesta de máquina de soporte vectorial para el análisis de estanqueidad en envases

El presente artículo es una recopilación de los métodos más utilizados en análisis de estanqueidad o detección de fugas, como antecedentes de la realización de un sistema que aplica la inteligencia artificial para este tipo de análisis. Se presenta también la propuesta de usar una máquina de soporte vectorial en este sistema

Packaging Technologies for Millimeter Scale Microsystems in Harsh Environment Applications

Microsystems capable of sensing temperature, pressure and other parameters are needed for many applications, for example, gathering information in downhole environments for oil and gas exploration. Certain target locations limit the size of the microsystems to millimeter or even sub-millimeter scale. In addition, the high temperature, high pressure, and corrosive ambient environments are challenging for microsystems. Target environments include 125°C temperature, 50 MPa pressure, and salinity standards consistent with American Petroleum Institute (API) brine (8% NaCl + 2% CaCl2). Other chemicals including hydrocarbons and cement slurry are also found in these environments. The system package plays a critical role as it protects the system components against environment, while also providing the physical coupling to the environment, e.g., for communication modules and pressure sensors. The package must be made of mechanically and chemically robust materials. High temperature assembly steps must be avoided in the packaging process (such as bonding above 200°C), because these steps are generally incompatible with embedded batteries and polymer-based sensors. The development of system package and relevant technologies is the focus of this dissertation. This dissertation first describes the design and fabrication of sapphire-on-steel packages in two sizes (0.8 mm and 8 mm), which are capable of isolating high pressure while allowing optical communication. These packages have been operated with embedded electronics at 125ºC and ≈70 MPa in API brine, hydrocarbons, and cement slurry. Additionally, polymer-in-tube packages are reported, which allow the embedded pressure sensors to couple with the environment. These packages have been successfully operated with embedded electronics and sensors at 125ºC and 50 MPa in API brine. A third approach of encapsulation that is reported involves polymer film encapsulation, which has the potential to significantly improve the chemical resistance of microsystems. Finally a batch-mode packaging process is presented based on micro-crimping, enabling room temperature assembly for sub-millimeter scale packages made by metal alloys. This packaging process has been demonstrated by a 5×5 array of 0.5 mm packages. These packages have survived at least 200 MPa pressure and at least 72 h in API brine.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135761/1/yushuma_1.pd

Sistema para pruebas de estanqueidad en envases de tereftalato de polietileno basado en máquina de soporte vectorial

Este proyecto consiste en el desarrollo e implementación de un algoritmo basado en inteligencia artificial, que permita realizar las pruebas de estanqueidad y con esto identificar envases de Tereftalato de Polietileno (PET) que presentan fugas. Se realiza el desarrollo del sistema a partir de mediciones de presión. Para esto se propone elaborar un mecanismo prototipo, mediante el cual se realice la adquisición de la señal de presión, este mismo mecanismo permite la inyección de aire a presión en el envase y el sellado del mismo. La señal adquirida es procesada digitalmente y mediante una máquina de soporte vectorial (SVM por sus siglas en inglés) se clasifica en una de dos categorías. Existen muchas herramientas de procesamiento digital de señales, sin embargo en aplicaciones de clasificación las SVM, han demostrado ser una muy buena alternativa