Looking Inside MEMS Packages: Investigation of the Hermeticity of MEMS thin Film Packages.

MEMS (Micro Electro Mechanical Systems) are micro machines, i.e. freestanding very small devices that can move. These devices are very fragile and very sensitive to the environment. Particles might damage them, oxidation of the MEMS changes their mass and affects their properties, changes in pressure of the environment will affect their functionality, etc. For this reason, MEMS have to be encapsulated on the wafer (before dicing into chips) in a hermetic cavity. This encapsulation or package should be such that the environment inside the cavity stays stable over the lifetime of the MEMS. However, hermetical MEMS encapsulation with long-term internal environment stability is often challenging, especially for thin film packages. Due to the very small volume (normally below 100 nL) inside the thin film packages, even very small amounts of a gas will significantly increase the pressure inside the package. Thin film packages hermeticity requirements demand for an ultra-fine leak rate below 1×10-15 mbar.l/sec. The standard He storage fine leak test used in industry, with its measurement sensitivity limited to 1×10-12 mbar.l/sec, is not sensitive enough to test such an ultra-fine leak, and a more sensitive method does not exist. For this reason, a lot of research is done on new methods that allow to measure the packaging hermeticity of MEMS thin film packages and the ultra-fine level leak rate testing. The object of this PhD is to develop a method that can measure the pressure and ultra-fine leak rate of wafer level thin film MEMS packages, and to understand the mechanisms that can contribute to any change of the atmosphere inside the SiGe MEMS thin film package through experiments with some supports by models. This includes studying of the outgassing from materials located inside the package, in-diffusion of gasses from the outside world and leakage of gasses from the interface between the package and substrate. In this thesis, we demonstrate a new systematical methodology to measure the package leakage rate and internal pressure by open/sealing the package with the focused-ion-beam (FIB). We demonstrate the methodology using the capacitance between cap and bottom of the package, using the curvature of the package cap, and using the resonance frequency of the cap. Especially the latter was shown to allow measuring the internal pressure with a very high sensitivity and to enable the measurement of an ultra-fine air leak rate below the order of 1×10-15 mbar.l/sec, highly outperforming the commonly used standard fine leak method. The methodology can be applied to various MEMS packages. As a case study, we applied the measurement method on two different real MEMS packages to test their hermeticity. It is also shown that the air leakage rate measurement can be accelerated by storing the packages in a higher pressure gas (3 bar N2 for example). However, care has to be taken when testing the He leak rate, because He can leak in or out the package by both a leak path and by diffusion. We also report on a systematic study of the outgassing properties of typical materials used for imec’s polycrystalline silicon-germanium (poly-SiGe)-based MEMS platform with a thin film vacuum package. We studied the outgassing by the thermal desorption spectroscopy (TDS) method. Two different TDS techniques, atmospheric pressure ionization mass-spectrometry (APIMS) and ultra-high vacuum thermal desorption spectroscopy (UH-TDS), were used for this study. It is shown that the UH-TDS is more suitable for studying the outgassing properties of thin film materials.Table of Contents 1. Introduction and objective 1.1 Packaging of MEMS 1.1.1 General requirements for MEMS package 1.1.2 MEMS packaging classification 1.2 Why hermetic packaging is required for MEMS? 1.3 Gas sources in MEMS devices 1.3.1 Leakage 1.3.2 Outgassing 1.3.3 In-Diffusion 1.4 Review of methods to test MEMS package hermeticity 1.5 Objective and thesis organization 2. Test structures and Instrumentation 2.1 Poly-SiGe as a MEMS structural material 2.2 Poly-SiGe thin-film packaging 2.3 Test structures 2.3.1 Poly-SiGe capacitive pressure sensor (empty MEMS package) 2.3.2 Capacitive micro-machined ultrasonic transducers (CMUTs) test structures 2.3.3 MEM resonator test structure 2.4 Optical instruments 2.4.1 Light interferometry 2.4.2 Laser Doppler Vibrometer (LDV) 2.5 Electrical instruments 2.5.1 LCR meter 2.5.2 Network analyzer 2.6 Other instruments 2.6.1 Gas pressure and environmental control chamber (PAV/PMV) 2.6.2 Gas pressure and environmental control chamber (for Light interferometry) 2.6.3 The Focused Ion beam (FIB) technique 2.7 Main Conclusions 3. A FIB-assisted systematical method to determine the internal pressure and leak rate of MEMS packages 3.1 Introduction 3.2 Package hermeticity measurement by using the capacitance as a pressure sensitive factor 3.2.1 Samples and setup to measure the capacitance 3.2.2 Measurement of the package internal pressure 3.2.3 Measurement of package true leak rate 3.3 Package hermeticity measurement by using the cap resonance frequency as a pressure sensitive factor 3.3.1 Setup to measure the package cap resonance frequency 3.3.2 Measurement of the package internal pressure 3.3.3 Measurement of the package true leak rate 3.4 Package hermeticity measurement by measuring the membrane curvature as a pressure sensitive factor 3.4.1 Setup and internal pressure measurement 3.4.2 Practical issues for measuring membrane deflection 3.5 Discussion of the FIB-assisted hermeticity test methodology 3.5.1 Tightness of the FIB Sealed Region 3.5.2 How to make a good FIB drilling and sealing 3.5.3 Practical considerations of this method 3.5.4 Comparison of the discussed package hermeticity analysis methods 3.5.5 Comparison of the new FIB-assisted hermeticity test methods with commonly used hermeticity analysis methods 3.6 Acceleration test and difference gas atmosphere storage 3.6.1 Storage test and experimental results 3.6.2 Experimental results discussion 3.7 Main Conclusions 4. Case study: Application of the FIB-assisted hermeticity test methodology to MEMS Capacitive Micromachined Ultrasonic Transducers (CMUT) 4.1 Introduction to CMUT 4.2 CMUT hermeticity study by measuring capacitance 4.3 CMUT hermeticity study using the resonance frequency 4.3.1 Hermeticity study of the dual thickness structure 4.3.2 Hermeticity study of single thickness structure 4.4 Failure analysis and leak source 4.5 Main Conclusions 5. MEMS thin film outgassing study 5.1 Introduction 5.2 Motivation of outgassing study of polycrystalline silicon-germanium-based MEMS 5.3 Sample preparation and measurement procedure 5.4 TDS measurement by APIMS 5.5 TDS measurement by UHV-TDS 5.6 Results and discussion 5.7 Main Conclusions 5.8 Additional work 6. Modeling and simulation of pressure influence on MEMS thin film packaging 6.1 Thin film package membrane deflection modeling 6.1.1 Analytical calculations 6.1.2 FEM simulation 6.1.3 Further discussion considering the effect of the initially trapped gases 6.1.4 Measurement results on real fabricated devices 6.2 Modeling of squeeze-film air damping in MEMS thin film package 6.2.1 Analytical model 6.2.2 Comparison of the model with real fabricated devices 6.3 Main Conclusions 7. Conclusion and future outlook 7.1 Main conclusions 7.2 Outlook towards future research Appendix List of publications Curriculum Vitaenrpages: 191status: publishe