Silicon Sensors and Actuators: The Feynman Roadmap

Foreword
Introduction
Contents
Part I Silicon as Sensor Material
1 Silicon Properties and Crystal Growth
1.1 Properties of Silicon
1.1.1 Crystal Planes and Orientation
1.1.1.1 Miller Index System
1.1.2 Electronic and Mechanical Characteristics
1.1.2.1 Elastic Properties
1.1.2.2 Strength
1.1.2.3 Fatigue
1.2 Starting Materials
1.2.1 Metallurgical-Grade Silicon
1.2.2 Electronic-Grade (EG) Polysilicon
1.3 Growth of Single Crystal Silicon
1.3.1 The Czochralski Crystal Pulling Method
1.3.2 Impurities in Czochralski Silicon
1.3.3 Defects in Silicon Crystals
1.4 New Crystal Growth Methods
1.4.1 Czochralski Growth with an Applied Magnetic Field (MCZ)
1.4.2 Continuous Czochralski Method (CCZ)
1.4.3 Neckingless Growth Method
1.5 Silicon Wafers Preparation and Properties
1.5.1 Silicon Wafers Manufacturing Process
1.5.2 Standard Measurements of Polished Wafers
Bibliography
Part II MEMS Processes
2 Epitaxy
2.1 Principle of Epitaxy
2.2 Epitaxial Reactors
2.2.1 Future Requirements for MEMS
2.3 Epitaxial Silicon Processes: Applications and Process Generalities
2.3.1 Applications
2.3.2 Process Generalities
2.3.3 High Temperature Hydrogen Annealing
2.3.4 Case Study: Single Crystal Silicon Membrane for Piezoresistive Pressure Sensors
2.4 Applications and Process Generalities of Thick Epitaxial Polycrystalline Growth
2.4.1 Introduction and Application
2.4.2 Process Generalities
2.4.3 Mechanical Properties of Thick Polycrystalline Silicon
2.4.4 Case Study: Displacement Optimization in Single Axis Accelerometer
References
3 Thin Film Deposition
3.1 Low Pressure Chemical Vapor Deposition (LPCVD)
3.1.1 Process Generalities and Equipment
3.1.2 Polysilicon
3.1.2.1 Material Properties and Applications
3.1.2.2 Case Study: Polysilicon Membrane for MEMS Microphone
3.1.2.3 Case Study: Polysilicon Interconnection for Inertial Sensors
3.1.3 Silicon Nitride
3.1.3.1 Material Properties and Applications
3.1.3.2 Case Study: Low Stress Silicon Nitride for MEMS Microphone
3.2 Plasma Enhanced Chemical Vapor Deposition (PECVD)
3.2.1 Principles and State of the Art/Methodology/Equipment
3.2.2 Silicon Oxides
3.2.2.1 Material Properties and Applications
3.2.2.2 Case Study: Warpage Compensation in MEMS Fabrication
3.2.3 Silicon Nitrides
3.2.3.1 Material Properties and Applications
3.2.3.2 Case Study: High Stress Silicon Nitride for MEMS Actuators
3.2.3.3 Others (a-Si, SiC)
References
4 Thin Films Characterization and Metrology
4.1 Films Characterization
4.1.1 Chemical Physical Characterization
4.1.1.1 Optical Properties
4.1.1.2 Thickness
4.1.1.3 Micro-structure Analysis
4.1.1.4 Contamination Control
4.1.2 Mechanical Characterizations
4.1.2.1 Film Stress and Wafer Bow/Warpage
4.1.2.2 Elastic Modulus and Hardness by Indentation
4.1.3 Dimensional Characterization
4.1.3.1 3D Measures
References
5 Deep Silicon Etch
5.1 Silicon Processing
5.1.1 Deep Silicon Etch Evolution and Bosch Process
5.1.2 Bosch Process Terminology
5.1.3 Process Parameters and Trends
5.2 DRIE Technological Challenges
5.2.1 Hardware Requirements vs. Technological Challenges
5.3 Technological Challenges in MEMS Applications
5.3.1 Multi-Features Mask Devices: Accelerometers and Gyroscopes
5.3.1.1 CD Loss and CD Control
5.3.1.2 ARDE or RIE lag
5.3.1.3 Profile Control and Tilt Effects
5.3.1.4 Notching on SOI
5.3.2 Complex Architectures: Micromirrors, Pressure Sensor, Microphone
5.3.2.1 Bonded Wafers
5.3.2.2 Cavity Etch
References
6 Optical Lithography
6.1 Introduction
6.2 Thick Photoresist [1, 2]
6.2.1 Introduction
6.2.2 Method and Tools
6.2.3 Case Study 1: Thick Photoresist for ECD (Electro Chemical Deposition)
6.2.4 *-1pc
6.3 Dry Film [12, 13]
6.3.1 Process Steps
6.3.2 Case Study: Lamination on Cavities
6.4 Negative Photoresist [18, 19]
6.4.1 Introduction
6.4.2 Case Study: Metal Definition Through Lift Off Process
6.5 Front to Front, Infrared (IR) and Back Side Alignment (BSA) [25, 26]
6.5.1 Introduction
6.5.2 Method and Tools
6.5.3 Case Study 1: Double Exposure
6.5.4 Case Study 2: Alignment Marks Optimization in Thick Epipoly
6.5.5 Case Study 1: Pressure Sensors' IR Alignments
6.5.6 Case Study 2: BSA for Back Side Mask
References
7 HF Release
7.1 Introduction
7.2 MEMS RELEASE: Dry or Wet?
7.3 Process Control
7.4 Vapor HF Release (vHF Release)
7.4.1 Equipment on the Market
7.4.2 Process Requirements & Characterization: Inertial Sensors
7.5 Wet HF Release
7.5.1 Hardware Requirements
7.5.2 Process Requirements & Controls
7.5.3 MEMS Applications
References
8 Galvanic Growth
8.1 Electroplating Principles
8.2 Process Integration
8.3 Electroplating Tool
8.4 Stacked Metal Plating
8.5 Alloy Plating
8.6 Electroplating Processes
8.6.1 Gold Plating
8.6.2 Copper Plating
8.6.3 Nickel Plating
8.6.4 Tin Plating
8.6.5 Ni-Fe Alloy Plating
8.6.5.1 Case Study: Ni:Fe 80:20 Through Mask Alloy Plating
8.6.6 Gold Deplating-Electrochemical Wet Etch
References
9 Wet Etching and Cleaning
9.1 Introduction
9.2 Wet Etching
9.2.1 Overview
9.2.2 Silicon Wet Etch
9.2.2.1 Isotropic Wet Etching of Silicon
9.2.3 Anisotropic Wet Etch of Silicon
9.2.4 Dielectrics Wet Etch
9.2.5 Metal Wet Etch
9.2.5.1 Gold Wet Etch
9.2.5.2 Aluminum and Aluminum Alloys Wet Etch
9.2.5.3 Ti and TiW Wet Etch
9.2.5.4 Cu Wet Etch
9.3 Wet Cleanings
9.3.1 Standard Wet Surface Preconditioning and Wet Resist Removal
9.3.2 Wet Surface Preconditioning Methods
9.3.3 Postprocessing Surface Treatments
9.3.4 MEMS Solvent-Based Cleanings
9.3.5 Case of Study: Cleaning Procedure After Deep Silicon Etch
9.3.6 Case of Study: Cleaning Post PZT Etch
9.3.7 Case of Study: Lift-off Process
References
10 Piezoelectric Materials for MEMS
10.1 Introduction
10.2 PZT
10.2.1 Material Overview
10.2.2 PZT Deposition
10.2.3 Sol-Gel Chemistry
10.2.4 Lead-Free Sol–Gel Chemistry
10.2.5 Sol–Gel Process
10.2.6 Sol–Gel Deposition Process
10.2.7 Sol–Gel Deposition Tools
10.2.8 Process Control
10.2.9 PZT Film Physical and Chemical Characterization Techniques
10.3 Study Case: First Wafer Effect
10.3.1 Introduction
10.3.2 Study of PZT 2 m—Stress and XRD
10.3.3 Titanium Stuffing
10.3.4 Actuator Deflection vs. XRD and Stress
10.4 PZT-Based Piezoelectric Stack Etch
10.4.1 Introduction to Piezoelectric Stack Etching
10.4.2 Wet Etch
10.4.3 Dry Etch
10.4.4 Process Chamber Stability and Mean Time Between Cleanings
10.4.5 Dry Etch Process Requirements for PZT-Based Piezoelectric Stack
10.4.6 TiW Etch
10.4.7 PZT Etch
10.4.8 Pt Bottom Electrode Etch
10.5 AlN
10.5.1 Material Overview
10.5.2 AlN Deposition
10.5.3 Scandium Doping
10.6 AlN Etch
10.6.1 TE Etch
10.6.2 AlN Bulk Etch
10.6.3 BE Etch
10.6.4 MTBC
10.6.5 Endpoint Detection
10.7 Piezoelectric Materials: Figure of Merit
References
11 Wafer-to-Wafer Bonding
11.1 Introduction
11.2 Bonding Classification
11.2.1 Direct Wafer Bonding
11.2.2 Intermediate Layer Wafer Bonding
11.3 Temporary Bonding and Debonding Overview
11.4 Bonding Characterization
11.4.1 SAM and IR Inspection
11.4.2 SEM and TEM Inspections
11.4.3 Pull Test
11.4.4 Shear Test
11.4.5 Micro-Chevron Test
11.4.6 Mazdara's Method
11.5 Hardware Description
11.6 Permanent Bonding Techniques
11.6.1 Glassfrit Bonding
11.6.1.1 Screen Printing Technique
11.6.2 Au–Au Thermocompression Bonding
11.6.3 AlGe Eutectic Bonding
11.6.4 Adhesive Bonding: BCB
11.6.4.1 Layer Transfer
11.7 Temporary Bonding
11.7.1 Introduction to Temporary Bonding Approach
11.7.2 Temporary Bonding Study Case—Microcracks
11.7.2.1 Simulation
11.7.2.2 Thermal Cycle Measurements
11.8 Final Remarks
References
Part III MEMS Sensors
12 Linear and Nonlinear Mechanics in MEMS
12.1 Introduction
12.2 Mechanical and coupled problems in MEMS
12.2.1 One degree-of-freedom oscillator
12.2.2 Mechanical forces and nonlinearities
12.2.3 Electrostatic forces and nonlinearities
12.2.4 Fluid-structure interaction
12.2.5 Thermal effects
12.3 Analytical methods for linear and nonlinear problems in MEMS
12.3.1 Hamilton's principle for a discrete formulation
12.3.1.1 Duffing oscillator
12.3.2 Application to MEMS Devices
12.3.2.1 Oscillator for a resonant accelerometer showing hardening and softening behaviour
12.3.2.2 Double Ended Tuning Fork for a resonant accelerometer showing hardening behaviour under varying temperature conditions
12.3.2.3 Torsional resonator for a resonant accelerometer showing softening behaviour
12.3.2.4 Micromirror showing nonlinearity
12.4 Numerical methods for linear and nonlinear problems in MEMS
12.4.1 Methodology
12.4.1.1 Geometric nonlinearities
12.4.1.2 Electrostatic Nonlinearities
12.4.1.3 Thermoelastic and fluid dissipation
12.4.2 Reduced order model and validation
12.5 Other mechanical and coupled problems in MEMS
12.5.1 PiezoMEMS and material nonlinearities
12.5.2 Material nonlinearities induced by high doping levels
12.5.3 Low distance and contact forces
12.5.4 Internal resonance in MEMS
12.5.5 Parametric resonance in MEMS
12.5.5.1 Disk Resonating Gyroscope (DRG)
12.5.5.2 Electrostatic Micromirrors
12.6 Closing Remarks
References
13 Inertial Sensors
13.1 Inertial Sensors: An Historical Background
13.2 Capacitive MEMS Accelerometers
13.2.1 Accelerometer Working Principles
13.2.2 Accelerometer Specifications and Requirements
13.2.3 MEMS Accelerometers Design Principles
13.3 Capacitive MEMS Gyroscopes
13.3.1 Gyroscope Working Principles
13.3.2 Gyroscope Specifications and Requirements
13.3.3 MEMS Gyroscopes Design Principles
13.4 THELMA Technology Introduction
13.4.1 Process Specificities: Wafer-Level Package (WLP): Vacuum Level through Getter Technology
13.4.2 THELMA-60 Technology Platform
13.4.3 THELMA Technology Solutions for MEMS Area Shrinkage
13.4.3.1 SMERALDO Technology
13.4.3.2 VIA FIRST Option
13.4.3.3 THELMA-PRO: THELMA with PROtective Permanent Oxide Coating
13.5 Conclusions
References
14 Magnetometers
14.1 *-16pt
14.1.1 Hall Sensors
14.1.2 Lorentz Force Magnetometer
14.1.3 Fluxgate and Magnetoinductive
14.1.4 Magnetoresistive
14.2 A Typical AMR Sensor Structure
14.2.1 Barber Poles to Linearize R(B)
14.2.2 Set and Reset Coils
14.3 ST AMR Technology Overview
14.4 Device Performances Versus Device Parameters
14.4.1 Sensitivity and Linearity Range
14.4.2 Set and Reset Efficiency
14.4.3 Cross-Axis (2-Axis Magnetometers)
14.5 Conclusions
References
15 Microphones
15.1 Microphone Overview
15.2 MEMS Microphone Description and Specification
15.2.1 MEMS Microphone Architecture
15.2.2 Acoustic Specification
15.2.3 Reliability Requirements
15.3 Capacitive Microphones
15.4 Capacitive Microphone Technology
15.5 Piezoelectric Microphones
15.6 Piezoelectric Microphone Technology
References
16 Pressure Sensors
16.1 History of MEMS Pressure Sensors
16.2 Conventional Piezoresistive and Capacitive Pressure Sensors: Description and Specification
16.2.1 Piezoresistive Effect and Conventional Piezoresistive Pressure Sensors
16.2.2 Capacitive Pressure Sensors
16.3 VENSEN Technology: Architecture and Schematic Process Flow
16.3.1 VENSEN Schematic Process Flow
16.3.2 An Industrialization Challenge: Piezoresistive Pressure Sensor in a Full Molded Package, “the Bastille Architecture”
References
17 Environmental Sensors
17.1 Humidity Sensors
17.1.1 Introduction
17.1.2 Relative Humidity Definition
17.1.3 Principle of Operation
17.1.4 Capacitive Humidity Sensor with Planar Electrodes
17.1.5 Interdigitated Capacitive Humidity Sensor
17.2 VOC GAS Sensor
17.2.1 Air Pollution
17.2.2 Sensors for VOC Detection
17.2.3 Metal Oxide Semiconductor Sensors
17.3 Temperature Sensors
17.3.1 Introduction
17.3.2 Temperature Compensation
17.3.3 Temperature Monitoring
17.3.4 Resistive Temperature Sensor
References
Part IV MEMS Actuators
18 Micromirrors
18.1 Application Fields
18.2 Types of Micromirrors
18.3 Actuation Principles
18.3.1 Capacitive Actuation and Sensing
18.3.2 Magnetic Actuation
18.3.3 Thin-Film Piezoelectric Actuation
18.3.4 Comparison Between Actuation Principles
18.4 Technology Platform
18.4.1 Comb Finger Structure Realization for Capacitive micromirror
18.4.2 Electromagnetic micromirror Technology
18.4.3 Thin-Film Piezo Technology
18.4.4 Reflective Surface: Requirements and Realization
18.4.5 Process Specificities: Vacuum Usage for Improved Performance and Reliability
References
19 Inkjet Printhead
19.1 Continuous Jet
19.2 Drop on Demand (DOD)
19.2.1 Head Droplet Formation
19.2.2 Pinch-off, Breakup, and Satellites
19.3 Thermal Jetting Actuation
19.3.1 Thermal Inkjet Printing
19.3.2 Bubble Growth
19.3.3 Main Jetting Issues
19.4 Piezoelectric Actuation
19.4.1 Piezoelectric Inkjet Printing
19.4.2 Working Principle
19.4.3 Acoustics
19.4.4 Frequency Response
19.4.5 Lumped Elements Modelling
19.4.6 Nozzle Pressure
19.4.7 Efficiency
19.4.8 Pulse Design
19.4.9 Jetting Issues
19.5 Thin-Film Piezo Printhead: Technology Description
19.6 Inkjet Applications
19.6.1 Graphics
19.6.2 Electronics
19.6.3 Displays
Bibliography
20 Micro Speakers
20.1 Micro Loudspeakers Applications and Requirements
20.1.1 Voice Coil Transducers for In-ear Applications
20.2 “More than Moore” Technologies: Background for Audio MEMS
20.3 MEMS Speaker Architecture and Performances
20.4 PZT Thin-Film MEMS Speaker Modeling and Design
20.4.1 PZT Nonlinearities Effects on MEMS Speakers
20.5 MEMS Speakers Compared to Electrodynamic Micro Speakers
20.6 MEMS Speaker Applications
20.6.1 In-ear Headphones: USB-C and Audio Module for TWS
20.6.2 Virtual Reality and Gaming
20.6.3 Smart Glasses and Augmented Reality
20.7 MEMS Speaker Manufacturing Process
References
21 Tunable Lenses for Autofocus
21.1 Tunable Lenses and Voice Coil Motor
21.2 TLens® Concept
21.3 Focusing Tunable Lens and Optical Components
21.3.1 Introduction
21.3.2 The Basic Optical Performances
21.3.3 Main Mechanical Features for an Autofocus System
21.4 TLens® Product Architecture
21.4.1 TLens® Components
21.4.2 TLens® Target Specifications
21.5 Thin-Film PZT MEMS Technology for Tunable Lens
21.5.1 The MEMS Actuator: General Considerations
21.5.2 MEMS Actuator Architecture
21.5.3 Thin-Film PZT MEMS: The Front-End Processing
21.5.3.1 Thin-Glass Membrane Deposition
21.5.3.2 PZT Stack Fabrication
21.5.3.3 Passivation and Antireflective Coating of the Lens
21.5.3.4 Cavity Patterning and Membrane Release
21.6 Design and Simulations of the Actuator
21.7 TLens® Real Performances Versus Simulations: Tolerance Analysis
21.8 Conclusions
21.9 Potential Future and Improvement
References
Part V Electronic Interfaces
22 Electronic Sensors Front-End
22.1 Integrated Interfaces for MEMS Microphones
22.1.1 Introduction
22.1.2 Interface Circuits for Capacitive MEMS Microphones
22.1.3 Preamplifier
22.1.4 A/D Converter
22.1.5 Digital Microphones with Extended Dynamic Range
22.1.6 Analog and Digital Microphones ASICs
22.2 Interfaces for Capacitive Accelerometer
22.2.1 Introduction
22.2.2 MEMS Basic Concepts
22.2.3 Reading Interfaces Overview
22.2.4 Open-Loop Architectures
22.2.4.1 DT Approach
22.2.4.2 CT Approach
22.2.4.3 DC Level Control of High Impedance Node
22.2.4.4 Circuital Implementations
22.2.5 Closed-Loop Architectures
22.2.5.1 Circuital Implementations
22.2.6 Charge Balanced Architectures
22.2.7 Interfaces for High-End Applications
22.2.8 Conclusions
22.3 Electronic Interfaces for Coriolis Gyroscope
22.3.1 Introduction
22.3.2 Functionalities of Gyroscope Interface Circuit
22.3.3 Main Circuit Blocks and Requirements
22.3.3.1 Capacitance-to-Voltage Converters (C2V)
22.3.3.2 Precise-Amplifier (PreAmp)
22.3.3.3 High-Voltage Charge Pump (CP)
22.3.3.4 Analog-to-Digital Converter (ADC)
22.3.3.5 Phase-Locked Loop (PLL)
22.3.4 System and Architecture Design
22.3.5 IC Technology Selection for Gyro Interface Circuits
22.4 Interfaces for Pressure Sensors
22.4.1 Introduction
22.4.2 Pressure Sensor Design Consideration
22.4.3 Switched-Capacitor Amplifier Front-End
22.4.4 Continuous Time Amplifier Front-End
22.5 Interface for PTAT Temperature Sensors
22.5.1 Introduction
22.5.2 Principle of Operation and Accuracy Requirements for the Interface Electronics
22.5.3 Analog-to-Digital Conversion
22.5.4 Calibration Techniques
References
23 Electronic Interfaces for Actuators
23.1 MEMS Mirror Driving
23.1.1 Linear Driver Architectures
23.1.2 Resonant Driver Architectures
23.2 MEMS Mirror Sensing
23.2.1 Capacitive Sensing
23.2.2 Resistive Sensing
23.2.3 Piezoelectric Sensing
23.2.4 ASIC Implementation Examples (Figs. 23.31 and 23.32)
23.3 MEMS Mirror Control Loop
23.3.1 Resonant Control Loop
23.3.2 Linear Control Loop
23.4 Piezo Inkjet Printers
23.4.1 Nozzle Drive Waveform
23.4.2 Driver Architectures
23.4.2.1 Switch Connected to Ground
23.4.2.2 Switch Floating
23.4.2.3 Multiswitch Floating
23.4.2.4 Print Head Assembly
23.5 PMUT Sensing
23.5.1 Sensor Modeling
23.5.2 Charge Reading Mode
23.5.3 Voltage Reading Mode
23.5.4 Linear Array Sense Architecture
23.5.4.1 Parallel Membranes Connection
23.5.4.2 Series Membranes Connection
References
Part VI MEMS Back-end
24 MEMS Package Design and Technology
24.1 Introduction
24.2 Sensor Package Design Structures
24.3 Basic Technology and Processes for Sensor Package
24.3.1 Fan-out Technology
24.3.2 Die Attach Processes and Materials
24.3.3 Electrical Interconnection Methods
24.3.4 Molding Processes
24.3.5 Cap/Cans Material and Attach Processes
24.4 Computed-Aided Engineering for MEMS Sensor
24.5 Advanced Design and Processes
24.6 Conclusion
References
Links
25 MEMS Testing: Sensors Testing and Calibration Impact on Product Performances
25.1 Introduction
25.1.1 Testing Purposes: Screening, Calibration, Compensation
25.2 Wafer Level Testing
25.2.1 Calibration of the Measurement Setup
25.2.2 Specific Electromechanical Screening (Capacitive, Magnetic): Offset, Sensitivity, Quadrature, Freq Response
25.2.2.1 Capacitance Measure
25.2.2.2 Sensitivity Measure
25.2.2.3 Offset Measure
25.2.2.4 Resonance Frequency and Q Factor
25.2.2.5 Exponential Decay Method
25.2.2.6 Bandwidth Amplitude at Half Power Method
25.2.2.7 Quadrature Error Measurement
25.2.3 Physical Stimuli Applied at Wafer Level for Screening and Calibration
25.2.3.1 Pressure-Temperature Sensor
25.2.3.2 Humidity Sensor
25.2.3.3 Magnetometer
25.2.4 MEMS Actuators: Testing Characteristics
25.2.4.1 Micromirrors
25.2.4.2 Printheads
25.2.4.3 Loudspeakers
25.3 System in Package Calibration
25.3.1 Physical Stimulus: Characteristics, Main Requirements, Limits
25.3.1.1 Gravity Stimulus Used for Low-G Accelerometer
25.3.1.2 Rotational Stimulus for Gyroscope Calibration
25.3.2 Temperature and Humidity Sensors
25.3.2.1 Temperature Sensors
25.3.2.2 Humidity Sensors
25.3.3 Acoustic Waves Sensors: Microphone
References
Part VII Reliability
26 Reliability
26.1 Introduction and Historical Evolution
26.2 Material Characterization and Basic Failure Modes
26.3 MEMS Device General Reliability Approach
26.4 Sensors Reliability Overview
26.4.1 Inertial Sensors
26.4.2 Magnetometer
26.4.3 Microphone
26.4.4 Pressure Sensor
26.5 Actuators Reliability Overview
26.5.1 Case Study: Electromagnetic Mirror
References
Part VIII The Future of Sensor and Actuators
27 MEMS: From a Bright Past Towards a Shining Future
27.1 Introduction
27.2 The Quantum Leaps of Offline Era of MEMS
27.3 MEMS Moving from the Offline Era to the Online Era
27.3.1 Consumer and IoT
27.3.1.1 Smartphones Boom and Feature Increase
27.3.1.2 Fitness Bands and Entertainment Devices
27.3.1.3 TWS and Hearables
27.3.2 Fourth Industrial Revolution
27.3.2.1 Industrial IoT
27.3.2.2 Medical Devices
27.3.3 Automotive
27.4 MEMS: The Journey Has Just Begun – The Online Era
27.4.1 Personal Electronics Trends and Future Applications
27.4.1.1 Augmented Reality and Virtual Reality Devices
27.4.1.2 From an Idea to a Vision
27.4.2 The Automotive Future
27.5 Conclusion
Index