Book Description
So far, no book has described the step by
step fabrication process sequence along with flow chart for fabrication
of micro pressure sensors, and therefore, the book has been written
taking into account various aspects of fabrication and designing of the
pressure sensors as well as fabrication process optimization. A complete
experimental detail before and after each step of fabrication of the
sensor has also been discussed. This leads to the uniqueness of the
book.
Features
-
A complete detail of
designing and fabrication of MEMS based pressure sensor
-
Step by step
fabrication and process optimization sequence along with flow chart,
which is not discussed in other books
-
Description of novel
technique (lateral front side etching
technique) in terms of chip size reduction and fabrication cost
reduction, and comparative study on both the techniques (i.e.
Front Side Normal Etching Technology and Front Side Lateral Etching
Technology) for the fabrication of thin membrane
-
Discussion on issues
of sealing of conical tiny cavity; because the range of pressure
applied (i.e. greater or less than atmospheric pressure) can be
decided by methodology of sealing of tiny cavity
-
A complete
theoretical detail regarding aspects of designing and fabrication,
and experimental results before and after each step of fabrication
MEMS Pressure
Sensors: Fabrication and Process Optimization
will greatly benefit undergraduate and postgraduate students of MEMS and
NEMS course, process engineers and technologists
in the microelectronics industry as well as MEMS-based sensors
manufacturers.
Preface
For the great progress of MEMS
(Micro-Electro-Mechanical Systems) in recent years, there are at least
four kinds of processing methods including silicon bulk micro-machining,
silicon surface micro-machining, LIGA and CMOS (complementary
metal-oxide-semiconductor) process to fabricate the micro-sensors at
present. Among these technologies, CMOS process for micro-sensors has
the advantages of the maturity in IC (integrated circuit) foundry, the
sub-micrometer spatial resolution of device fabrication and the
functionality of on-chip circuitry. CMOS layers have been successfully
used as the mechanical structures or the sensing elements of
accelerometers, thermal sensors, magnetic sensors and pressure sensors.
A group of Scientist has also designed a magnetic Hall sensor by the
standard SPDM (single-polysilicon-double-metal) CMOS foundry service
provided by the Chip Implementation Center (CIC), Taiwan, without
post-processing. However, it caused some difficulties in achieving the
design and the fabrication of the CMOS mechanical sensors due to the
violations of some electric rules provided by the CMOS foundry line and
the uncertainty of post-processing used to form the special geometry of
sensor chip. However, recently a novel technique (Lateral front side
etching technique) has been utilized to fabricate MEMS based micro
pressure sensors. Front side etching technique is the most advanced and
recently developed MEMS technology. It takes the advantages of both the
bulk and surface micromachining technologies. Micro-electro-mechanical
systems (MEMS) are Freescale's enabling technology for acceleration and
pressure sensors. MEMS based sensor products provide an interface that
can sense, process and/or control the surrounding environment.
Freescale's MEMS-based sensors are a class of devices that builds very
small electrical and mechanical components on a single chip. MEMS-based
sensors are a crucial component in automotive electronics, medical
equipment, hard disk drives, computer peripherals, wireless devices and
smart portable electronics such as cell phones. These sensors began in
the automotive industry especially for crash detection in airbag
systems. Throughout the 1990s to today, the airbag sensor market has
proved to be a huge success using MEMS technology. MEMS-based sensors
are now becoming pervasive in everything from inkjet cartridges to cell
phones. Every major market has now embraced the technology.
Fabrication of thin membranes has been an important aspect in common
micromechanical devices owing to its numerous industrial applications.
The pressure sensing technology that provides a multiple-measurement and
multiple-range capability is also based on a thin diaphragm or membrane
fabrication process. This book describes the experimental details of the
fabrication of a thin membrane over a conical V-shaped cavity using
front side lateral etching technology and the results obtained are
discussed.
In the reported work, front side lateral etching technology has been
studied. This study proposes a novel front side lateral etching
fabrication process for silicon based piezoresistive pressure sensor. As
far as the fabrication process is concerned, this technique successfully
accomplished a front side etching process laterally to replace the
conventional back-side bulk micromachining. This novel structure
pressure sensor can achieve the distinguishing features of the chip size
reduction and fabrication costs degradation.
About
the Authors
Dr. Parvej
Ahmad Alvi
is an Assistant Professor in the Department of Physics, School of
Physical Sciences, Banasthali University, India. He has got a PhD in
Applied Physics (2009) from Department of Applied Physics, Faculty of
Engineering & Technology, Aligarh Muslim University, Aligarh, India. His
research area is MEMS, NEMS technologies, Opto-electronics, and Material
Science. He has active collaboration with CEERI (CSIR) Pilani (India),
Elettra Synchrotron (Italy), Aligarh Muslim University, Aligarh (India),
and Royal University of Phnom-Penh (Cambodia). Dr. Ahmad Alvi has
published many international research papers and articles in his field
and four books. He is an editorial board member of International Journal
of Optoelectronics Engineering (Scientific Academic Publishing, USA),
Nanoscience and Nanotechnology (Scientific Academic Publishing, USA) and
lifetime member of International Union of Crystallography.
Contents:
Preface
About the Autor
1. Introduction
1.1. MEMS Technology
1.1.1. Bulk Micro-machining
1.1.2. Surface Micro-machining
1.1.3. High Aspect Ratio Micro-machining
1.1.3.1. LIGA
1.1.3.2. Other Techniques
1.2. Transduction Mechanism
1.2.1. Piezoresistance
1.2.2. Piezoelectricity
1.2.3. Bimorphs
1.2.4. Capacitance
1.2.5. Electrostatics
1.3. Micro-machined Pressure Sensors
1.3.1. Macro- and Micro-scale Devices
1.3.2. Piezoresistive Pressure Sensors
1.3.3. Capacitive Pressure Sensors
1.3.4. Piezoelectric Pressure Sensors
1.3.5. Optical Sensors
References
2. Micro-fabrication Technologies
2.1. Micro-fabrication Techniques
2.2. Silicon Wafers
2.3. Wafers Cleaning
2.3.1. Degreasing
2.3.2. RCA 1
2.3.3. RCA 2
2.3.4. Piranha Treatment
2.4 Thermal Oxidation
2.5. CVD Techniques
2.5.1. LPCVD (Low pressure Chemical Vapor
Deposition)
2.5.2. PECVD (Plasma Enhanced Chemical Vapor
Deposition)
2.5.3. APCVD (Atmospheric Pressure Chemical Vapor
Deposition)
2.6. Photolithography
2.6.1. Wafer Alignment and Wafer Exposure
2.6.2. Photoresist
2.6.3. Etching
2.6.3.1. Dry Etching
2.6.3.2. Wet Etching
2.7. Metallization
2.8. Device Processing
References
3. Thin Film Materials
3.1. Silicon Dioxide (SiO2)
3.2. Silicon Nitride (Si3N4)
3.3. Polycrystalline Silicon (Polysilicon)
3.3.1. Properties of Polysilicon Film
3.3.1.1. Residual Stress
3.3.1.2. Young’s Modulus
3.3.1.3. Roughness
3.3.1.4. Electrical Properties
3.3.2. Polysilicon as a Sacrificial Layer
3.3.3. Effect of Doping Temperature on
Polysilicon Grains
3.3.4. Advantages and Disadvantages of
Polysilicon Film
3.3.5. Applications of Polysilicon
References
4. Anisotrpic Etching
4.1. Introduction
4.2. Wet Etching Fundamentals
4.2.1. Isotropic Etching
4.2.2. Anisotropic Etching
4.2.3. Wet Anisotropic Etching Using Aqueous KOH
Solution
4.3. Experimental Methodology
4.4. Surface Roughness due to KOH Etching
4.5. Physical Models for KOH Etching
4.6. Results and Inferences
References
5. Fundamental Theory and Design of Pressure Sensor
5.1. Stress Analysis for Thin Diaphragm
5.2. Wheatstone Bridge
5.3. Piezoresistors
References
6. Fabrication of Micro Pressure Sensor (using Fron-side
Lateral Etching Technology)
6.1. Front-Side-Etching Technology
6.1.1. Front Side Normal Etching Technology
6.1.2. Front Side Lateral Etching Technology
6.2. Fabrication Process Sequence Optimization
6.2.1. Fabrication Process Sequence (I)
6.2.2. Fabrication Process Sequence (II)
6.3. Fabrication Process Detail
6.3.1. Detail of Process Sequence (I)
6.3.1.1. Thermal Growth of SiO2
6.3.1.2. LPCVD of Si3N4
6.3.1.3. Photolithography-1 (PLG-1)
6.3.1.4. RIE of Si3N4 (0.15 Microns)
6.3.1.5. Wet etching of SiO2 (0.5 Microns)
6.3.1.6. Stripping Off of Photoresist
6.3.1.7. LPCVD of Polysilicon (Thickness 1.0 μm)
6.3.1.8. Etching of Polysilicon
6.3.1.9. PECVD of (Si3N4 + SiO2 + Si3N4)
6.3.1.10. PECVD of Si3N4 for Thickness 0.2
Microns
6.3.1.11. PECVD of SiO2 for Thickness 0.5 Microns
6.3.1.12. Boron Doping of Polysilicon for
Piezoresistors
6.3.1.13. Boron Diffusion Conditions
6.3.1.14. Metallization for Contact Lines and
Contact Pads
6.3.1.15. Photolithography (PLG-4) for Defining
Metal Contact Lines and Contact Pads
6.3.1.16. Aluminium Etching
6.3.1.17. Sintering
6.3.1.18. Details of the PLG-5
6.3.1.19. KOH Etching
6.3.2. Detail of Process Sequence (II)
6.3.2.1. Membrane and Cavity Formation
6.3.2.2. Sealing of the Cavity and Deposition of
Resistors
References
7. In-Process Observations
7.1. Photographs for Process Sequence (I)
7.1.1. Photograph after PLG-1, SiO2 and Si3N4
Etching
7.1.2. Photograph after PLG-2 and Poly Etch (with
Photoresist)
7.1.3. Photograph after PLG-2 and Poly Etch
(without Photoresist)
7.1.4. Photograph after PECVD (Si3N4 + SiO2 +
Si3N4)
7.1.5. Photograph after (LPCVD of Polysilicon +
Boron Doping + + PLG-3 + RIE of Polysilicon) Defining of Boron Doped
Polysilicon Resistors
7.1.6. Photograph after (Al metallization + PLG-4
+ Wet Etching of Al) Defining Metal Contact Pads and Contact Lines
7.1.7. Photographs after KOH Etching for Cavity
Formation
7.2. Photographs for Process Sequence (II)
7.2.1. Photograph after LPCVD (Si3N4 + SiO2 +
Si3N4)
7.2.2. Photographs after PLG-3 and RIE of LPCVD
(Si3N4 + SiO2 + Si3N4)
7.2.3. Photographs after KOH Operation
7.2.4. Photographs after Defining Resistors and
Metallic Lines
References
8. Summary
Future Scope of the Work
Index
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