Common test standards for evaluating the hermeticity of microsystem
packages are unsuitable for small MEMS-devices.
It is the task of this Master thesis to create a universal test device to
measure and to compare the hermeticities of different wafer-level
packaging concepts, especially for RF MEMS devices. Resonator structures
were found to be most suitable to measure low pressures and low
pressure changes over time, due to the high sensitivity of their Q-value to
the pressure in the cavity. The resonators are electrostatically actuated by
using a novel coupling concept of the excitation voltage. The detection of
the resonator movement is done by laser-interferometry. Sensors fulfilling
the specific demands were designed, simulated and fabricated in the
cleanroom. The fabrication process is based on SOI (Silicon On Insulator)
wafers. Finally, the sensors were evaluated and characterized.
A suitable resonator with a length of 500 µm reaches a Q-factor of 8070,
at an ambient pressure of 0,02 mbar, and a resonance frequency of
36329 Hz. The sensitivity of the Q-value to pressure change is
4000 %/mbar at 0,02 mbar.
This work was carried out within the Summit RF MEMS project,
a collaborative project involving Ericsson, the Royal Institute of
Technology-S3, Acreo and Saab Ericsson Space.
Table of Contents
1. Introduction
1.1 What is MEMS ?
1.2 Hermeticity
1.3 Wafer-level sealing technologies
1.3.1 Anodic bonding
1.3.2 Silicon direct bonding
1.3.3 Eutectic bonding
1.3.4 Adhesive bonding
1.4 How to measure hermeticity
2. Theory
2.1 Bending of a beam
2.2 Mechanical vibrations
2.2.1 Free, undamped vibration
2.2.1.1 Flexural mode vibration
2.2.1.2 Shear deformation
2.2.1.3 Torsional vibration
2.2.1.4 Longitudinal vibration
2.2.2 Damping
2.2.3 Quality factor
2.2.4 Pressure dependency of the quality factor
2.3 Electrostatic excitation
2.4 Optical detection
2.4.1 Fabry – Perot – Interferometer
2.4.2 Optical properties of silicon
3. Project management
4. Design
4.1 System requirements
4.2 Design considerations
4.3 Design Process
4.4 Stiction
4.5 Mode coupling
4.6 Material
4.7 Excitation and detection
4.7.1 Electrostatic excitation of the beam
4.7.2 Mechanical stability of the bottom-bottom electrode configuration
4.7.3 Electrical connections
4.7.4 Detection technique
4.8 Designing a basic beam structure
4.9 Design variations and other resonator shapes
4.10 Mask drawing
5. Fabrication
5.1 Flowchart
6. Measurement setup
6.1 Measurement Results and Discussion
7. Final Discussion and Conclusions
Objectives and Topics
The primary goal of this master's thesis is the development of a universal resonant sensor device designed to measure and evaluate the hermeticity of various wafer-level packaging concepts for RF MEMS. By utilizing the high sensitivity of a resonator's Q-value to internal cavity pressure, the research explores novel excitation and optical detection methods to overcome the limitations of standard hermeticity test methods when applied to very small MEMS volumes.
- Design and optimization of silicon-based micro-resonator structures for pressure sensing.
- Development of fabrication processes using SOI (Silicon On Insulator) technology in cleanroom environments.
- Implementation of electrostatic excitation and laser-interferometric detection techniques.
- Characterization of sensor performance, including resonance frequency and pressure-dependent Q-factor analysis.
Excerpt from the Book
4.4 Stiction
Stiction is one potentially dominant problem during fabrication or operation. Stiction means that a structure intended to be moving sticks to a surface. Thus the device may not function.
Stiction can occur during fabrication or during operation. Electrostatic charges on the surface may cause stiction during operation. Stiction during fabrication can occur after a free hanging structure is wet chemically released. During drying, after the final cleaning step, capillary forces between both surfaces may be high enough to pull the surfaces together, thus the released structure will stick to the surface fig. 4.3, fig. 4.4.
Summary of Chapters
1. Introduction: Provides background on MicroElectroMechanical Systems (MEMS), the importance of hermetic packaging for small volumes, and an overview of existing sealing and measurement technologies.
2. Theory: Establishes the theoretical foundation regarding beam deflection, mechanical vibration modes, damping mechanisms, and the principles of electrostatic excitation and optical interference.
3. Project management: Outlines the project structure, timelines, and the specific equipment utilized for literature research, simulation, and fabrication.
4. Design: Details the systematic design process, addressing requirements like stiction prevention, mode coupling minimization, material selection, and electrode configuration.
5. Fabrication: Describes the cleanroom processes employed to manufacture the sensor, including DRIE etching, wafer bonding, and the specific fabrication flowcharts.
6. Measurement setup: Explains the experimental configuration used for evaluating the sensors, specifically describing the laser-interferometric detection and spectrum analysis techniques.
7. Final Discussion and Conclusions: Summarizes the key findings regarding the sensor's performance, discusses encountered challenges, and provides recommendations for future improvements in sensor handling and alignment.
Keywords
MEMS, RF MEMS, Hermeticity, Wafer-level packaging, Micro-resonator, Q-factor, Electrostatic excitation, Laser interferometry, SOI wafers, Stiction, Pressure sensor, Fabrication, Silicon, Vacuum sensor, Resonant sensor.
Frequently Asked Questions
What is the core purpose of this research?
The research focuses on designing and fabricating a universal, resonant-based sensor device capable of measuring the hermeticity of micro-scale cavities, which is a critical challenge for small RF MEMS devices.
What are the primary technical fields covered?
The thesis integrates fields including microsystems technology (MEMS), mechanical engineering, vacuum physics, semiconductor fabrication, and optical measurement techniques.
What is the central research challenge addressed?
The primary challenge is that traditional test standards for hermeticity are unsuitable for the extremely small cavity volumes found in modern MEMS devices, requiring a new approach based on resonant pressure-sensitive structures.
Which scientific methods are employed?
The study utilizes analytical modeling (beam mechanics), numerical simulations (ANSYS), cleanroom fabrication (SOI processing, DRIE etching), and experimental characterization using laser interferometry and spectrum analysis.
What is covered in the main body of the work?
The main body systematically explores the theoretical background of mechanical vibrations, the design considerations for optimizing resonator geometry, the detailed fabrication process flow, and the experimental results regarding pressure-dependent Q-factors.
Which keywords characterize this work?
Key terms include MEMS, hermeticity, micro-resonators, Q-factor, electrostatic excitation, laser interferometry, and SOI technology.
Why is SOI technology specifically used for this project?
SOI wafers are chosen because they facilitate the easy creation of free-hanging monocrystalline silicon structures, which are essential for sensitive resonator designs, and they offer superior mechanical properties compared to deposited thin films.
How does the proposed optical detection method function?
The method uses the silicon substrate's transparency in the infrared region to create a Fabry-Perot interferometer between the non-moving bottom surface and the vibrating beam, allowing for the precise measurement of vibration frequency and amplitude via reflected light.
- Quote paper
- Dr. Sebastian Fischer (Author), 2002, Mechanical resonator for hermeticity evaluation of RF MEMS wafer–level packages, Munich, GRIN Verlag, https://www.grin.com/document/21041
