Lecture 3-Fabrication Technologies

8/11/2019 Lecture 3-Fabrication Technologies

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Fundamentals of Microfabrication withApplications to BioMEMS

ENSC E 130


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Fundamentals of Microfabrication with

Applications to BioMEMS

Fawwaz HabbalSenior lecturer on Applied Physics

and Harvard School of Engineering and Applied SciencesExecutive Dean

Office: Pierce Hall 216

ENSC E 130

Mondays at 5:30 PM


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Teaching Assistant:Alexis Vitti

Email address: [email protected]

ENSC E 130

Mondays at 5:30 PM


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Course websitehttp://courses.fas.harvard.edu/ext/13210

Course e-mail(homework - Communications - Questions)

[email protected]

Office HoursBy Appointment only -- Write to:

[email protected]


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Course Objective

This course Teaches you microfabricationTechniques, Familiarize you with Micro-Electro-Mechanical-

Systems (MEMS). Present applicationsinseveral domains: Biologicaland medical, Electrical, Magnetic, Structural, Fluids,Thermal ..

You will not become an expert - but you will be able totake more advanced courses and complement yourworking knowledge - if any.

Discussion is important - Ask questions


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Lectures

Lectures will contain support materials andneeded background

Only general and introductory physical sciencesbackground is necessary

Some mathematics (not at a high level) will beencountered

Lectured can be viewed on the internet Questions are welcomed during class and by

e-mail We will devote a lecture or more to visit CNS labs

at Harvard


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Citation

This course is part of 4 courses that mayearn you a Citation*in

Nanotechnologies and applications in Bioscience

The courses are: ENSC E-130(BioMEMS) - offered this semester ENSC E-140 (Nanotechnology) - offered next Fall

ENSC E-150(Bio-Nano) - offered this semester ENSC E-155 (Microfluidics) - offered next semester

(*) Minimum grade B is required


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Homework and Exams

Homework: Series of questions to expand on.

Articles to read and discuss during the lectures.

40 % of your final grade Final is a combination of:

Questions (take home exam): 30% of your final grade

Term paper: 30% of your final grade

60% of final grade


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Lecture 3Fabrication Technologies


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Fabrications Technologies

Outline

! Hard Fabrication!

Lithography!Etching Methods

!Deposition of Materials

! Soft Fabrication!Micomolding

!Three Dimensional Photopolymerization

!Thick Film Technologies


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Hard Microfabrication

Will discuss mainly Silicon materials


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Device Fabrication

A materialto create the device -- Silicon A processto follow -- Micromachining Process Characteristics

Reproducible

Reliable Scalable Inexpensive Environmentally friendly

Tools to createthe device - Lithography Tools to examineand verify - Microscopy Packaging Integration methods and tools


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Micromachined Materials

Device Material -- Substrates

Silicon GaAs

Other elemental or compound semiconductors Metals (bulk and foils) Glasses Quartz

Sapphire Ceramics Plastics, polymers and other organics


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Additive Materials

Silicon (amorphous, polycrystalline, epitaxial) Silicon compounds (oxides, nitrides, carbides, )

Metals and metal compounds Glass Ceramics Polymers and other organics

Biomaterials

Micromachined Materials


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Fabrication Processes

Reference materials:

Chapter 1 in Madous BookIn Particular pages 1-31


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Process needs to be one in a Cleanroom

Small Features require cleanroomenvironmentNo particles or dust!

Different Classes for different applications


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Methods - Top down

Write the required pattern with:

1) Optical Lithography

2) Ion and Electron Beam Lithography

3) X-ray Lithography


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Bottom Up Approach

Chemical and statistical forces can create systemswith natural scale in the sub-100 nm.

Self-assembly

Energetic and statistical forces cause crystallineorder in solids, can spontaneously form ofarrays of highly ordered nanostructures.

Examples:

Quantum dots

Langmuir-Blodgett films


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Process to Create Patterns

Pattern Generation

Design

Wafer

WRITE the Pattern


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Direct Write


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Direct Write Mask

Light Light

Ions Ions

X-ray

ElectronsElectronsWafer


Pattern Generation

Design


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Direct Write Hardware

No mask is needed

Higher end systems use Direct Write on Wafer(DWW) exposure systems

Excimer lasers: geometries down to 1 - 2 m Electron beams: geometries down to 0.1 - 0.2 m

Focused ion beams: geometries down to 0.05 - 0.1 m

But, this is a serial process

wafer cycle time is proportional to the beam writingtime, the smaller the spot, the longer it takes


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Reactive Ion Etching (RIE and DRIE)

RIE: chemical etching is accompanied by ionicbombardment

Bombardment opens areas for reactions

Ionic bombardment: No undercutting since side-walls are not

exposed Greatly increased etch rate Structural degradation Lower selectivity


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Reactive Ion Etching (RIE and DRIE)

Deep Reactive Ion Etching (DRIE)Uses electron cyclotron resonance (ECR) source to

supplement RIE

Microwave power at 245 GHz is coupled intoECR Magnetic field is used to enhance transfer of

microwave energy to resonating electrons

DRIE uses lower energy ions --> less damageand higher selectivity


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DRIE

AMMI Locus NovaBOSCH Patent


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Lithography


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Pattern Generation

Design

Direct Write Mask

Light Light

Ions Ions

X-ray

ElectronsElectronsWafer


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Photolithography

A process to transfer a pattern that is createdon a photomask onto a photoresist thinfilm

Photo masks are generated by an opticalsystem or an electron system


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Overview: Device Fabrication

Surface Preparation Coating (Spin Casting) Pre-Bake (Soft Bake) Mask Alignment Exposure Development Post-Bake (Hard Bake) Processing Using the Photoresist as a Masking Film Stripping Post Processing Cleaning (Ashing)


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Si Wafer Fabrication andCharacteristics


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Material Structure

Atoms are arranged with a certain periodicityEach side has a length (a)There are also Hexagonal structures


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Material Structure

Miller indices for a simple Cubic Crystal

[010]

[001]

[100]


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Material Structure

Miller indices for a simple Cubic Crystal

[010]

[001]

[100]

(100)(110)


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Silicon Structure


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Wafer Preparation


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Wafer Fabrication

http://www.egg.or.jp/MSIL/english/msilhist0-e.html

Czochralski Crystal Growth

Float Zone ProcessGradual pull - from a rotating

silicon seed

SEED


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Wafer Fabrication

YEAR


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Silicon Oxides


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Silicon Oxides: SiO2

Uses Diffusion masks Surface passivation Gate insulator (MOSFET)

Isolation, insulationFormation:

Grown / native Thermal: highest quality

Anodization Deposited:

CVD, evaporate, sputter


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Thermal Oxidation of Silicon

Thermal Oxidation is at high temperatures(900 - 1200 C)

Two main processes :

DryOxidationSi + O2 --> SiO2 @1 atm , 1000 C

WetOxidation

Si + 2H2O --->SiO2+ 2H2 Dry oxidation produces a better (more dense)oxide as compared to wet oxidation


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Silicon Oxide


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Other Silicon Compounds


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Doping

Doping

n-type(e.g., Sb, As, P, Bi) electron donors(5 electrons in outer shell)

p-type(e.g., B, Ga, In)acceptors(3 electrons in outer shell)


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Polysilicon

Silicon Carbide

Polycrystalline Diamond

Refractory Metals2WF6 + 3SiH4--> 2W + 3SiF4+6H2


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Silicon Nitride Si3N4

uses Diffusivity of O2, H2Ois very low in nitride

Mask against oxidation, protect against

water/corrosion Diffusivity of Nais also very low

Protect against mobile ion contamination


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Silicide Films

Silicides are metal-silicon compounds. They are used for contacts.

Typical thickness 0.1 to 0.2 m


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Silicide Films


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Silicide Films

Ion Implant for mixing


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Method for Creating Features


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Si Etching and Characteristics


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Etchant Properties

Selectivity to masking layer(s)

Selectivity to metals (e.g., Al)

Etch rateAnisotropy(crystal plane selectivity)

Surface roughness

Control of etch parameters


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Etching Plans


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Etching

Silicon etching: different rates

Anisotropic

Isotropic


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KOH Etching

Etching Rate: Varies with Temperature andConcentration

(110) > (100) > (111)

(100) > (110) > (111)


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Anisotropic Etching

(100 Surface)

(110 Surface)

Petersen

Anisotropic = direction dependent


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Anisotropic Etching


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Creating Patterns with Lithography


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An Overview


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Major Pieces of Equipment

Stepper positionAccuracy can beas good as 50 nm


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Masking with Photoresist


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Creating a Mask

The mask is the stencil of the requiredpattern

CAD systems are used to create the patterns

Pattern is created by photo projectionexposure


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Masks

Create master patterns are transferred to wafers Both glass and quartz are used

Photographic emulsion on soda-lime glass (cheap)

Fe2O3on soda-lime glass Cr on soda-lime glass

Cr on quartz glass (expensive, used with deep UV)

Polarity

light-field: mostly clear, drawn feature are opaque dark-field: mostly opaque, drawn feature are clear


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Masking

Shadow masking 15nmdiameter had been prepared

Exposure1:1 to 10:1

Lateral resolution (b)is

b = k (

/ NA)

NA is the numerical aperture;k = 0.5 theoretically

bis affected by depth offocus


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Mask Alignment


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Mask Alignment

Create marks on wafer to consecutivelyalign several masks


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Alignment

3 degrees of freedom between mask and wafer Modern process lines use automatic pattern

recognition and alignment systems Usually takes 1-5 seconds to align and expose on a

modern stepper Human operators usually take 30-45 seconds with

well-designed alignment marks Normally requires at least two alignment mark sets

on opposite sides of wafer or stepped region Use a split-field microscope to make alignment

easier


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Alignment - Exposure


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Alignment and Exposure Hardware

Projection systems give the ability to change thereproduction ratio 10:1 reduction allows larger size patterns on the

mask - more robust to mask defects

Most wafers contain an array of the same pattern,so one cell of the array is needed on the mask These machines are also called Steppers

Example: GCA-4800

Disadvantage of steppers: absolutely nodefects, since it will be reproduced all over thewafer


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An Alignment Machine (Karl-Suss)


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PhotoresistMaterials and Application


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Photoresist

PR: Radiation-sensitive compound Requirements

Etch resistance

Thermal stability Ease of development

Good adhesion

Difficult to achieve in the UV region


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Photoresist -Types

Positive resists Exposed region becomes more soluble Patterns are the same as those on the mask

Negative resists Exposed regions become less soluble Patterns are the reverse of the mask patterns


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Components of Photoresist

Conventional optical photoresist has three components 1) Matrix material 2) Sensitizer 3) Solvent

Sensitizer (also called inhibitor) Photoactive compound (PAC) - Insoluble without

radiation - preventing resist to be dissolved Take photochemical reaction upon exposing to light,

transferring from dissolution inhibitor to dissolutionenhancer


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Photoresist - Matrix and Solvent

Solvent Keep photoresist in liquid state Allows spin coating of the resist Solvent content determines resists viscosity and hence the

its thicknessMatrix Material (resin) Serves a binder Inert to radiation Dissolves fast in developer (~ 150 A/s)

Provides resistant to etchers Provides adhesion to the substrate Contributes to the mechanical properties of the resist


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Photoresist - Photo active Compound

Function of PAC Dissolution Enhancer 1000 2000 /s Matrix +

Sensitizer with Radiation

Dissolution Inhibitor 10 20 /s Matrix + Sensitizer without Radiation NA 150 /s Matrix Differential solubility before and after exposure

100 : 1


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Photoresist - Types

Positive photoresist Three constituents: a photosensitive compound, a base resin,

and an organic solvent. After irradiation, the photosensitive compound changes its

chemical structure, and transforms into a more soluble species.Upon developing, the exposed areas are expunged.

Negative photoresist Polymers combined with a photosensitive compound. Photosensitive compound absorbs the radiation energy - initiate a

chain reaction that causes crosslinking of the polymer molecules.The cross-linked polymer has a higher molecular weight andbecomes insoluble in the developer solution.

After development, the unexposed portions are removed.


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Positive and Negative Photoresist

Positive ResistThe solubility of exposedregions is much higher thanthe unexposed region in asolvent (developer) produces a

positive image of the maskNegative Resist

The solubility of exposedregions is much lower than the

unexposed region in developerproduces a negative image ofthe mask


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Commercial Photoresist


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Absorption of x-rays in some materials


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Applying the PhotoresistSpin Coating


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Photoresist Spin Coating

Wafer is held on a spinner chuck by vacuum Resist is coated to uniform thickness by spin coating Typically 3000-6000 rpm for 15-30 seconds Resist thickness is set by

Resist viscosity Spinner rotational speed

Resist thickness is given by

t = kp2/ " wk = spinner constant, typically 80-100p = resist solids content in percentw = spinner rotational speed in rpm/1000


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Spin Coating

Use a centrally rotatingsubstrate. Fast rotationcreates centrifugal force withsolvent evaporation create aconstant thickness.

Thickness can be 100nm Organic polymers and

biopolymers can be deposited

Stretching and orienting ofmolecules


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Spin Coating


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Spin Coating Machine

PR applicator

Wafer


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Stages of Coating


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Spin Coating - Defects

Striations 30 nm variations in resist thickness due to nonuniform dryingof solvent during spin coating

80-100 mm periodicity, radially out from center of wafer Edge Bead

residual ridge in resist at edge of wafer; 20-30 times thenominal thickness of the resist

radius on wafer edge greatly reduces the edge bead height Solvents are spun on after resist coating - partially dissolve

away the edge bead

Streaks radial patterns caused by hard particles of diameter greater

than the resist thickness


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Pre-Bake

Pre-bake evaporate coating solvent

Increase the density of the resist after spin coating.

Typical thermal cycles

90-100C for 20 min. in a convection oven75-85C for 45 seconds on a hot plate

Microwave heating and IR lamps are also used inproduction lines

P B k


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Pre-Bake

A narrow time-temperature window isneeded to achieve best linewidth control.

The thickness of the resist is usually

decreased by 25 % during prebake for bothpositive and negative resists.

P B k


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Pre-Bake

Convection ovensSolvent at surface of resist is evaporated first candevelop impermeable skin, trapping the remainingsolvent inside

Heating must go slow to avoid solvent burst effects Conduction (hot plate)Need an extremely smooth surface for good thermal

contact and heating uniformity

Temperature rise starts at bottom of wafer -- morethorough evaporationFaster and more suitable for automation

H d B k


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Hard Bake

Removes all traces of the coating solvent ordeveloper. Harden the developed photoresist prior to the

processing steps - e.g. metal deposition, acid

etching Main parameter is the plastic flow or glasstransition temperature

Some shrinkage of the photoresist may occur;

introduces some stress into the photoresist


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Photoresist Removal


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Creating PatternsUsing Masks and Photoresist

P tt T f


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Pattern Transfer

Now we have a substrate coated with a resist stencil. The stencil can be used to protect parts of the

substrate during an additive step like a metaldeposition.

Or the stencil can allow some etching procedure toreach the substrate in well-defined locations in asubtractive step.

Deposition is usually through evaporation orsputtering.

Ph t k d P tt C ti


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Photomask and Pattern Creation

Mask Silicon wafer covered with Photoresist

Expose

Remove the exposed (orthe unexposed) areas

Z

A th P tt i M th d


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Another Patterning Method

mask

Subtractive Additive

Function layer

mask

Remove mask


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Exposure

E M th d


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Exposure Methods

Projection


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Projection

Wafer

Dioptric

Reticle

Projection Lithography


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Projection Lithography

Characteristics of a Microlithography System


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ResolutionThe resolution of an optical system is its capability todistinguish closely spaced objects. For amicrolithography system, resolution defines the

minimum linewidth or space that the system can print.



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Registration CapabilityA measure of degree to which the pattern being printed can be fit(aligned) to previously printed patterns.

Dimensional Control

Ability to produce the same feature size with the same toleranceand position accuracy across an entire wafer and wafer-to-wafer

Throughput

The time to complete a print

Resolution


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Resolution

Airy Disk: the smallest distance, Lmin, an optical system canresolveRayleigh Criterion: The central maximum of each point sourceslie at the first minimum of the Airy disk

Lmin= 0.61 !/ NA

Numerical Aperture:NA = sin

For small sin =

Numerical Aperture NA = ,

Lmin

Depth of Focus (DoF) Requirement


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Depth of Focus (DoF) Requirement

Why do we need to meet DoF Requirement?

Substrate is not flat - can varies as much as 10 m across a wafer

There are previously fabricated patterns on the wafer

DOF - The range over which there are clear optical images

Depth of focus,DoF, can be expressed as:

DoF = n l / [2(NA)2]

DOF decreases fast when NA increase!

Homework


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Homework

Due next week. Send by e-mail to

[email protected]

Also copy to:[email protected]

Homework

What is the depth of focus for a situation with!= 435 nm, NA = 0.6, n =1.47 (DI water)

Depth of Focus


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Depth of Focus

Photoresist Contrast


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Photoresist - Contrast

Contrast is determined by the Gamma (slope) of theresponse curve

GammaRepresents the

ability of resist to distinguish

between light and darkregions

Resist UV DUV

+ ive 2 - 3 1 - 2

- ive 5 - 10 3 - 6

Sensitivity(mJ/Cm2)

Resist UV DUV

100 20 - 40

Photoresist - Fabrication Issues


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Photoresist - Fabrication Issues

Surface Reflection Standing Wave

Anti-reflection coatingAdd unbleachable dyes to resistPost baking after exposure (before development)Multi-wavelength exposure

Light Transmission Near the Edges


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Light Transmission Near the Edges

Z

D = the thickness of the photoresist

2b = the minimum pitch of line spacing

Z = the spacing

For Contact Imaging: 2 b = 3 " (0.5 d !)

For Proximity Imaging: 2 b = 3 " !(Z + 0.5 d)

Homework


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Homework

Due next week. Send by e-mail to

[email protected]

Also copy to: [email protected] Homework

For the case of

!= 400 nm, d = 1m , Z = 10 m

What is the minimum resolution for contact andproximity imaging?

Edges and Profiles


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Edges and Profiles

Feature edge profile is affected by

The distance between the mask and the photoresist(Reduce the diffraction)

The thickness of the photoresist

The Exposure time

The development

Modulation Transfer Function


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Modulation Transfer Function

How to Create Contact


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How to Create Contact

Problems: Optically flat photoresist

Dust

Stiff masks

Smallest Features


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Smallest Features

In the far field approximation (like inmicroscopy), resolution limit is determinedby diffraction

Lmin = !/ (2#$)

In contact printing, the exposure takes placevia near field.

So, diffraction is not a limiting factor

Near Field Diffraction


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Near Field Diffraction

Z

Proximity Exposure


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Proximity Exposure

Phase Shifting Masks


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Phase Shifting Masks

Conformable Contact Lithography


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Conformable Contact Lithography

Wave Length = 220 nm Pattern Resolution = 100 nm

L min= 50 nm

200 nm Grating


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200 nm Grating

Scanning electron micrograph of a 200-nm-pitch gratingembedded in a deep-ultraviolet transparent SiO2 substratedepicts the structure of the embedded-amplitude mask

SEM image of a pattern replicated byD UV (100 )


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Deep UV (100nm)

500 nm

100 nm

45 nm


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45 nm


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Other Exposing Beams

Electron and X-rays Beams

Other Methods

E-Beams and X-ray Lithography


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y g p y

DoF and resolution are improved with shortwave illumination

Throughput is an issue. So, these are used to

create the masks Early 80s, deep UV (248 nm and 193 nm) wasused with ArF and KrF excimer lasers

X-ray required using synchrotron generators

Advanced Lithography Technology


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g p y gy

E-Beam Lithography X-Ray Lithography

Focused Ion Beam Lithography

Imprinting Lithography

Electron-beam lithography


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g p y

The most common method to create verysmall features

Electron beam exposure alters the chemistry

of the resist instead of light exposure.

Electron Beam Lithography


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g p y

Electron beam lithography is one of the mostpromising of nanolithography. Similar to a Scanning Electron Microscope and

often a scanning electron microscope is used. An electron beam is formed and scanned at a

controlled rate over the surface of a photoresist. Scan rate is adjusted to deliver a "critical" dose of

electrons to a selected area of the resist. The resist is either developed in a chemical bath

similar to photolithography, or the electron beaminteracts with the material to remove the resistmaterial.

Electron-Beam Lithography (EBL)


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g p y ( )

Diffraction is not a limitation on resolution Resolution depends on beam size, can reach ~ 5 nm Two applications:

Direct Writing

Projection (step and repeat)Issues: Throughput of direct writing is very low research

tool or low pattern density production

Projection stepper is in development stage. Maskmaking is the biggest challenge. Back-scattering and second electron reduce

resolution with dense patterns

Schematic of E-Beam System


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y

E-Beam Issues


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Electron Scattering in Resist and SubstrateThe scattered electrons also expose the resist!

E-Beam Issues - Proximity Effect


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y

MTF is greatly reduced at high pattern density- requires Use thin resist and thin substrate Adjust acceleration voltage Split pattern into several writings using different doses

Adjust pattern size and shapes Adjust dose level to compensate scattering

Raith-150 EBL System at CNS


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y

Direct Writing and SEM system - Thermalassisted field emission

Resolution: 2 nm @ 1.0 KeV

Column voltage range: 200 30VResolution of laser interferometer register: 2 nmMaximum wafer size: 6Writing speed: 10MHz


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X-ray Lithography

X-ray Lithography


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y g p y

X-ray lithography is one of the most promisingtechnologies for nanolithography. Mask is made of an X-ray transparent material with

a pattern of high Z material either etched ordeposited on it.

The mask is the limiting factor in X-raylithography.

Resolution of the pattern is dependant on thevariations in the mask.

Scalability to manufacturing would be relativelyeasy compared to some other techniques such asSPM lithography.

X-Ray Lithography (XRL)


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Very short wavelength (1.0 0.01 nm), Very high resolution Area exposure:higher throughput than e-beam system X-ray is transparent: Low level of dust/contamination impactIssues:

Optics is extremely difficult no lens available for focused of

defocused Point source and shadow exposure Geometric error Expensive and complicated X-ray sources

Very complicated mask (Boron nitride) and fabrication Heavy metal (Au) as opaque material Low mass membrane(1 ~ 2 micrometer Si3N4) as substrate

Minimum Feature Size in a Proximity Exposure withsoft x-rays


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soft x-rays

S

nm

Resist

Mask


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Focus Ion Beam Lithography

Focused Ion-Beam (FIB) Lithography


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Like EBL, FIB is used as direct writing exposurePotential: Less backscattering (larger mass than electron) Resist for FIB lithography is more sensitive Energy higher than electrons

Better resolution and faster exposure speed than E-beamIssues:

Lack of reliable ion sources Harder to be focused

Shorter penetrate (absorption) depth in resist (~ 30 500 nm) -multilayer resist process Unexpected ion implantation on substrate beneath resist



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Conventional Photoresist Resist Implantation



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Inorganic Resist Ion Induced etching andDevelopment



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Ion Beam Etching and Ion Implantation


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Scanned Probe Lithography

Scanned probe lithography (SPL)


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Plowing: use an AFM tip to literally plow a groovethrough either a very thin resist layer or a self-assembledmonolayer (SAM) on the substrate.

Can produce lines in these layers as narrow as 20-30 nm

Local oxidation: use either a conductive AFM tip or anSTM tip to do local electrochemistry on the substrate

Dip-pen lithography: use the tip of a AFM to transfer

SAMs from reservoirs down the tip to the substrateSTM lithography: position individual atoms one at atime to build up structures

Scanning Probe Microscope Lithography


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Performed by oxidizing a material with theelectric field created at the tip of a scanning probemicroscope.

Oxidized material can be removed by preferentialetching.

Resist materials can include Si and Ti (easilyoxidized).

25 nm and 35 nm lines were formed by oxidizingSi with the AFM and then dry etched to a depth of

30 nm.


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References

References


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B. J. LinContact and proximity Printingin Fine Line Lithography, Elsevier

J. Goodberlet

Applied Physics Letters Volume 76, 2000 Introduction to Microelectronic Fabrication

Richard C. Jaeger, Addison-Wesley, 1993

S.K. Ghandi,VLSI Fabrication Principles

John Wiley and Sons, New York, 1983 - Chapter 4

Lecture 3-Fabrication Technologies

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Transcript of Lecture 3-Fabrication Technologies