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

2

Product Specification

DetailIn-house expertise

Managementdecisions

2

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?

3

•Is sputtering the best methodto deposit aluminum?

But,….Structural Design is Severely Constrained

by Process Limitations

Micro Engine

4

MACRO Engine

3

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

4

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

7

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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22

22

21

21

21

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

20[Maluf]

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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

29[Maluf]

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

32[Maluf]

17

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

36(Maluf)

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Clarifies angleof this surface

Pattern


Overview of Microdevice Manufacture


Processes

Pattern Formation

Additive Processes





38

Package Microdevice


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

46[Senturia]

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ALIGNER

http://www.physics.mcgill.ca/nanotools/

ALIGNER

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Pattern




Processes

Pattern Formation

Additive Processes





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Package Microdevice


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




Processes

Pattern Formation

Additive Processes





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Package Microdevice


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

61(Madou)

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




Processes

Pattern Formation

Additive Processes





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Package Microdevice


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.

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