Introduction to Microfabrication - McGill Physicspeter/534A/vengallatore2.pdf · Introduction to...
Transcript of Introduction to Microfabrication - McGill Physicspeter/534A/vengallatore2.pdf · Introduction to...
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PHYS 534 (Fall 2008)
Lecture 2
Introduction to Microfabrication
1Srikar Vengallatore, McGill University
How are Microsystems Designed?
Market need
C tEvaluationCreativity
Concept
Embodiment
D t il
of competitiony
& experience
Modeling andAnalysis
Manufacturingconsiderations
In house Management
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Product Specification
DetailIn-house expertise
Managementdecisions
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Structural Embodiment Phase of Design
•Selection of materials, structures, and shapes to optimize performance and reliability.
•Is electrostatic actuation optimal?
•Is a torsional hinge optimal?
•Is aluminum the best material?
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•Is sputtering the best methodto deposit aluminum?
But,….Structural Design is Severely Constrained
by Process Limitations
Micro Engine
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MACRO Engine
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5Kalpakjian and Schmid
Silicon MicroengineMetallic vs. Ceramic Materials
Patterning
Starting Material: Substrate (wafer)
What is the Origin of the Process Limitations?
PhotolithographyE-beam lithography
Processes
g
Additive Processes
SubtractiveProcesses
EvaporationSputteringCVDElectrodeposition
g p yIon beam lithographySoft lithography
Wet etchingDry etchingPlasma etching
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Package Microdevice
ElectrodepositionWafer bondingDRIE
Polishing
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Microfabrication differs from macro-machining in several ways:
•Material removal is by selective corrosion
•Structural thin films are not available from a catalog.I t d h t t th fil b f h i itInstead, we have to create the film before shaping it
•Defining a pattern follows a process that resemblesPhotography (of the old-fashioned type using photosensitivefilms)
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•Massively parallel manufacture
•Simultaneous manufacture and assembly
Catalog of Manufacturing Processes
Patterning Techniques: Photolithography, Microstamping,Electron/ion beam lithography,Soft lithography,…
Additive processes: Thin-film deposition, wafer bonding,oxidation, epitaxy, …..
Subtractive processes: Wet etching, dry etching, ion milling,
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p g, y g, g,deep reactive ion etching,…
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Starting Material: Substrate
•Typical characteristics:
•Shape: Circular plates (also called wafers)p p ( )
•Size: ~1 mm thickness~10 cm diameter
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Common Substrate Materials
•Single crystal silicon
•Single crystal Quartz (silicon oxide) •Commercially
•Amorphous silica glasses
•Pyrex
•Gallium arsenide
CommerciallyAvailable
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•SiC
•……
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How are Silicon Wafers Specified?
Chemistry: Purity; Dopant concentration
Electrical Properties: Resistivity
Geometry: Diameter;
Thickness;
Total thickness variations;
Surface finish and polish;
B d
11Virginia Semiconductor (http://www.virginiasemi.com/) and many others
Bow and warpage;
Crystallographic Orientation;
Primary & secondary flats
Specification of Chemistry
Impurities: Carbon, oxygen, heavy metals,…
Typically, O2: 5 – 25 ppmC: 1 5 ppmC: 1 – 5 ppmMetals: < 1 ppb
Some impurities are intentionally added in small,well-defined, quantities
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e de ed, qua es
Such impurities are called DOPANTS
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Crystallography of Substrates
Based on atomic arrangement, materials can be
Amorphous
Single Crystals
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Polycrystalline
Grain boundary
[Ohring]
Unit Cells of Cubic Crystals
Simple Cubic
Body Center Cubic (BCC)
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Face Center Cubic (FCC)
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Directions of a Cubic Crystals
z [0,0,1]
y
Unit Distance
[0,1,0]
[1,0,1]
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x [1,0,0]
[1,1,0]
Miller indices of Crystal Planes
[0,0,1]
[1,0,0]
[0,1,0]
Recipe: 1. Determine intercepts on each axis 1, 1, ∞
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2. Take reciprocals of these numbers 1, 1, 0
3. Reduce to smallest integers (110)
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Examples of Low Index Planes
17[Senturia]
Two Important Results for Cubic Crystals
Result 1. Plane (h k l) has unit normals [h k l]
(1 1 0)
[0,1,0]
[0,0,1] (1 1 0)
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[1,0,0] [1 1 0]
Notation: {1 0 0} indicates a family of (1 0 0) planes<1 0 0> indicates a family of [1 0 0] directions
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Two Important Results for Cubic Crystals
Result 2. The angle (γ) between two planes with indices(h1 k1 l1) and (h2 k2 l2) is given by
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212121coslkhlkh
llkkhh++++
++=γ
Example: (i) The angle between (100) and (111) is
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011 7.543
1cos31001cos ==⎥⎦
⎤⎢⎣
⎡ ++= −−γ
Crystallography of Single-Crystal Silicon Wafers
Look for the primary flat
(111) Wafers
(100) Wafers
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Representation of a Simple Process Flow
1 mm
10 cm
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Golden Rule of Process Representation:
NOTHING is EVER drawn to scale!
Thin-Film Deposition
1 mm
Thin Film
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Thin Film1 μm
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Photolithographic Patterning
•Apply thin layer of photosensitive polymer
Photoresist(~ 1 μm thick)
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•Selectively expose to light using a RETICLE
Patterning Using Photolithography
Opaque coating
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Transparent plate
•Reticles are also called Photo-masks: Commercially available
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25Exposure to Ultraviolet Radiation
Exposed photoresistis soluble
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Development of Exposed Photo-resist(Use solvent)
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Etching: Selective Corrosion to Remove Material
Chemical 1: Removes Blue only
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Chemical 2: Removes only red
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Next: A Surface-Micromachining Process..
•Addition, Patterning & Selective Removal of Thin Films
Silicon Oxide
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Silicon
Silicon oxide
Deposit polysilicon thin film
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Pattern and etchpolysilicon
Photoresist
Mask
polysilicon
polysilicon
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SiliconOxide
[Maluf]
Sacrificial Oxide
Release the structure
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2-d Representation of Process Flows
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Again, Not to Scale!
2-d Representation of Process Flow(Photolithographic details not shown)
34Substrate Silicon Oxide Polysilicon
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Interpretation of Floating Structures
Silicon Light Machines
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•Examine different cross-sections to find anchoring locations
BULK MICROMACHINING: Selective Removal Material from Substrate
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Clarifies angleof this surface
Pattern
Starting Material: Substrate (wafer)
Overview of Microdevice Manufacture
PhotolithographyE-beam lithography
Processes
Pattern Formation
Additive Processes
SubtractiveProcesses
EvaporationSputteringCVDElectrodeposition
g p yIon beam lithographySoft lithography
Wet etchingDry etchingPlasma etching
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Package Microdevice
ElectrodepositionWafer bondingDRIE
Polishing
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Lithos: Stone graphy: to write
Photolithography: Pattern transfer using photons
H d S f UV di ti ( li )Hardware: Source of UV radiation (aligner)
Reticle (master pattern)
Photoresist (polymer)
Chemical solvents
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Chemical solvents
Basic Steps in Photolithography
CoatPhotoresist
Expose toUltraviolet radiation
Develop
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mask
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Spin Coating and the Importance of Low Viscosity
Centrifugal forces vs. Viscosity
photoresist
1000 – 8000 rpm10 – 30 s
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ω
Partial evaporation of solvent during spin coating
How thin must the photoresist be?
•Depends on details of process flow
•To create small features (<1.0 μm), use thin resists (1.0 μm)
•For bulk micromachining, use thick (~10 μm) photoresist layer
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Rule of ThumbPhotoresist thickness scales with feature size
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Two Types of Photoresists
Exposed regions become soluble
Exposed regionsbecome insoluble
43POSITIVE resist NEGATIVE resist
Mechanisms linked to Bond Formation/Destruction
Before Exposure
After Exposure
44Positive Resist Negative Resist
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Examples of Positive & Negative Resists
Positive: PMMA (poly methyl methacrylate)
DQN (diaquinone ester + phenolic novolak)DQN (diaquinone ester + phenolic novolak)
Negative: bis(aryl)azide rubber resists
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•Photoresists are commercially available
[Madou]
Photolithographic Aligners
Source of Radiation
Focusing optics
Tooling for Alignment
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ALIGNER
http://www.physics.mcgill.ca/nanotools/
ALIGNER
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Pattern
Starting Material: Substrate (wafer)
Overview of Microdevice Manufacture
PhotolithographyE-beam lithography
Processes
Pattern Formation
Additive Processes
SubtractiveProcesses
EvaporationSputteringCVDElectrodeposition
g p yIon beam lithographySoft lithography
Wet etchingDry etchingPlasma etching
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Package Microdevice
ElectrodepositionWafer bondingDRIE
Polishing
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THIN FILM USUALLY IMPLIES….
•Deposited on substrate & subsequently processed
•Lateral film dimensions much larger than thickness.
•Thin Films vs. Thick Films:
Thin films 0.1 μm < hfilm < 2 μm
Thick films 5 μm < hfil < 50 μm
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Thick films 5 μm < hfilm < 50 μm
THIN FILM PROCESSING TECHNIQUES
Wet (Solution) Dry (Vapor)
•Spin casting •Evaporation
•Electrodeposition
•Sol-gel & colloidaltechniques
•Sputter-deposition
•Chemical vapor deposition (CVD)
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•Pulsed-laser deposition
•Oxidation
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GENERIC VAPOR-DEPOSITION PROCESSSource of atoms (target)
Vapor of atoms
vacuum
substrate
Vapor of atoms
51Nucleation Growth Coalescence
QUALITY OF DEPOSITED FILMS
Geometry: Thickness & Thickness uniformityLateral dimensions & uniformityConformality vs. Line-of-sight coatingsConformality vs. Line of sight coatings
Kinetics: Rate of film growth
Chemistry: Fidelity of compositionCompositional uniformity
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Mechanical stress: Intrinsic (growth-related)
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GEOMETRIC PARAMETERS
Lateral uniformity &Thickness uniformity
No lateral uniformityThickness uniformity
lateral uniformityNo thickness uniformity
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y y
No lateral uniformityNo thickness uniformity
CONFORMALITY OF COATING•Ability to coat topographic featuresi.e., ability to conform to surface features
Top surface Sidewall
54CONFORMAL NON-CONFORMAL
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Material Addition Using Wafer Bonding
Wafer 1
•Direct Wafer Bonding
Wafer 2
•Intermediate Wafer Bonding
55Metal; Glass; Oxide; Polymer
High Strength Bonding is Possible(Bonded regions as strong as lattice!)
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IR Image of Bond Formation
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•Relatively simple process to implement
Pattern
Starting Material: Substrate (wafer)
Overview of Microdevice Manufacture
PhotolithographyE-beam lithography
Processes
Pattern Formation
Additive Processes
SubtractiveProcesses
EvaporationSputteringCVDElectrodeposition
g p yIon beam lithographySoft lithography
Wet etchingDry etchingPlasma etching
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Package Microdevice
ElectrodepositionWafer bondingDRIE
Polishing
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Example 1: Development of Photoresist (Wet Etching)
Key Concept: Material Removal by Chemical Corrosion
Exposed photoresist
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Exposed photoresistis soluble
How to Design a Etch Process
•What etchants (chemicals/plasma) should I use?
H l ill th t k (i KINETICS)?•How long will the process take (i.e., KINETICS)?
I th t h ISOTROPIC ANISOTROPIC?
•Is the etch SELECTIVE?(i.e., what materials can I work on?)
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•How do I detect completion of etch process?(i.e., END – POINT DETECTION)
•Is the etch ISOTROPIC or ANISOTROPIC?(i.e., what SHAPES can I make?)
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Kinetics: Diffusion vs. Reaction Control
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Diffusion-Limited vs. Reaction-Limited
Diffusion-Limited Kinetics:-Rate of arrival of reactants controls rate of reaction
-Improve by stirring, gas evolution, etc.
Reaction-Limited Kinetics:-Rate of interfacial reaction controls reaction rate
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-Control using temperature, catalyst, etc.
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Selectivity of Etch
masking layer
Substrate to be etched
SubstrateofrateEtchySelectivit
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LayerMaskingofrateEtchfySelectivit =
Sacrificial Material
Substrate
Structural Material
Selectivity is Critical for Surface Micromachining
Substrate
REQUIREMENT:
Et h S ifi i l M t i l
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Etch Sacrificial Materialwithout damaging Substrateor Structural Materials
[Maluf]
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Isotropic vs. Anisotropic Etches
Isotropic
65AnisotropicEtch front
Gases and Plasma are also used for Etching
•Vapor Phase Dry Etching (non-plasma)
•Plasma-assisted Dry Etching
-Ion Milling (Focused Ion-Beam Milling)
-Ashing
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-Reactive-Ion Etching
-Deep Reactive-Ion Etching
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Pattern
Starting Material: Substrate (wafer)
Overview of Microdevice Manufacture
PhotolithographyE-beam lithography
Processes
Pattern Formation
Additive Processes
SubtractiveProcesses
EvaporationSputteringCVDElectrodeposition
g p yIon beam lithographySoft lithography
Wet etchingDry etchingPlasma etching
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Package Microdevice
ElectrodepositionWafer bondingDRIE
Polishing
Approach to Learning Microfabrication
•This is an area of very active research
•Focus on: Fundamental principles + Established methods•Focus on: Fundamental principles + Established methods
•Learn ideas, but also a few crucial details
•Learn by assimilation: Case Studies
•Learn to find information
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Learn to find information.