Lecture 13 Basic Photolithographyclasses.engr.oregonstate.edu/eecs/fall2017/ece611/slides/ECE611... · 1/64 ECE611 / CHE611 –Electronic Materials Processing Fall 2017 - John Labram

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ECE611 / CHE611 – Electronic Materials Processing

Fall 2017 - John Labram

Lecture 13

Basic PhotolithographyChapter 12 Wolf and Tauber

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AnnouncementsHomework:

• Homework 3 is due today, please hand them

in at the front.

• Will be returned one week from Thursday (16th

Nov).

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Announcements

• You are expected to produce a 4-5 page term paper on a

selected topic (from a list).

• Details / regulations are on the course website.

Term Paper:

• Term paper contributes 25% of course grade.

• You should have all been assigned your first-choice topic.

• The term paper should be handed in at the start of class on

Tuesday 21st November.

• The term paper will be returned to you in class on Thursday

30th November.

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Useful LinksBerkley:

• http://www-inst.eecs.berkeley.edu/~ee143/fa10/lectures/Lec_04.pdf

University of Michigan:

• http://web.eecs.umich.edu/~peicheng/teaching/EECS598_06_Winter/Lectu

re%2016%20-%20Mar%2009.pdf

MIT:

• http://www-

mtl.mit.edu/researchgroups/hackman/6152J/SP_2004/lectures/sp_2005_Le

cture09.pdf

KTH:

• https://www.kth.se/social/upload/4f3d0e38f276545a2b000003/Lecture%20

9%20Litho.pdf

http://www-inst.eecs.berkeley.edu/~ee143/fa10/lectures/Lec_04.pdf

http://web.eecs.umich.edu/~peicheng/teaching/EECS598_06_Winter/Lecture 16 - Mar 09.pdf

http://www-mtl.mit.edu/researchgroups/hackman/6152J/SP_2004/lectures/sp_2005_Lecture09.pdf

https://www.kth.se/social/upload/4f3d0e38f276545a2b000003/Lecture 9 Litho.pdf

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Lecture 13• Overview of Photolithography Process

• Mask Fabrication.

• Photoresists.

• Photoresist Deposition.

• Exposure.

• Development.

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Overview of

Photolithography Process

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Feature Sizes

Way of

quantifying

tolerance in

distance of

mask from

wafer

Deep ultra-

violet

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Patterning Wafers• Overall the process of patterning a wafer can be

broadly divided into 3 steps:

Mask Design Wafer ExposureMask Writing

• We are interested in the process of wafer exposure.

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The Photolithography Process• Apply photoresist.

PR

Substrate

Ap

ply

DevelopEtchStrip

Expose

Mask

• Expose photoresist through a patterned mask or reticle.

• Develop PR by immersing it in a solvent which preferentially

dissolves the PR of higher solubility.

Photoresist

• Process the exposed part of the wafer.

• Strip away the remaining photoresist.

• Inspect pattern.

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Photo room

Photo Process Flowchart

Clean

Exposure

Apply HMDS

Adhesion Promoter

Develop

prebakeSpin Coat

Plasma

“descum”postbake

Process

(Etch / Implant/ Lift-off)

Strip

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Mask Fabrication

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Mask Fabrication• Starting material for reticle

manufacturing is ~80 nm thick film of

chromium covered with resist and

anti-reflective coating (ARC).

• Chromium has very good adhesion

and opaque properties.

• Substrate: quartz glass plate.

• Patterned by direct writing using e-

beam or laser usually wet etching of

Cr after exposure.

• 4 or 5× magnification is normal for projection lithography.

• Pellicle used for dust protection of reticle.

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Optical Proximity Correction• As we will see later, the wave-nature of light means that we

cannot exactly recreate the features on a wafer:

Divergent

light sourceCollimating

lens

Aperture

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Optical Proximity Correction• Optical Proximity

Correction (OPC):

Clever mask

engineering based on

software algorithms

can compensate

some of this error.

• This requires

sophisticated

computer modeling.

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OPC Examples

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Photoresists

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Photoresists• Photoresist (PR): An organic compound (often a polymer) with

a photoactive component (PAC) whose solubility changes

upon exposure to radiation (light).

• Positive Photoresist:

• Irradiated regions become more soluble than non-

irradiated regions.

Expose

Mask

DevelopEtchStrip

More soluble

regions

hnN2

R

O

R

CO

+ N2

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Photoresists• Photoresist (PR): An organic compound (often a polymer) with

a photoactive component (PAC) whose solubility changes

upon exposure to radiation (light).

• Negative Photoresist:

• Irradiated regions become less soluble than non-irradiated

regions.

Expose

Mask

More soluble

regions

DevelopEtchStrip

hn

SU8

Cross-linked

polymer

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Comparison of Photoresists

Characteristic Positive Negative

Adhesion to Silicon Fair Excellent

Relative Cost More expensive Less expensive

Developer Base Aqueous Organic

Solubility in the

developer

Exposed region is

soluble

Exposed region is

insoluble

Minimum Feature 0.5 µm 2 µm

Step Coverage Better Lower

Wet Chemical

ResistanceFair Excellent

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Key Parameters of a Photoresist• Resolution: The smallest opening or island structure that can

be made under a given set of process conditions (related to

contrast).

• Registration: Overlay accuracy from layer to layer.

• Sensitivity: The number of photons it takes to cause the

chemical response in the PR. A resist is more sensitive if it

takes a lower dose to reproduce the mask geometry on the

wafer.

• Shelf life: The time you can reliably store a PR.

• Etch Resistance: The ability of the resist which remains on the

wafer after it is patterned to withstand the process

environment that the exposed wafer is subjected to.

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Photoresist Deposition

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HMDS: Adhesion Promoter• HMDS is the common name for

hexamethyldisilazane: ((CH3)3Si)2NH.

H H H H

Si Si O

Si Si OO

O Si

Si SiO

O Si

O O O O

Si Si O

Si Si OO

O Si

Si SiO

O Si

O O O O

SiH3C

CH

3

CH3 SiH3C

CH

3

CH3SiH3CC

H3

CH3 SiH3C

CH

3

CH3

• Common photo-resists do not wet the

surface of H-terminated Si/SiO2 very

well.

• HMDS is applied to the

surface to improve wetting

before the photoresist is

deposited.

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HMDS: Adhesion Promoter• HMDS is applied in vapor phase.

• But first hydroxyl groups must be removed

from the wafer surface.

H H H H

Si Si O

Si Si OO

O Si

Si SiO

O Si

O O O O

Si Si O

Si Si OO

O Si

Si SiO

O Si

• Cleaning or oxygen plasma can be used.

• Must be conducted in inert atmosphere.

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HMDS: Adhesion Promoter• HMDS is applied in vapor phase.

Industrially:

• HMDS is applied using a

bubbler.

In the lab:HMDS-rich

atmosphere

Cleaned

Wafer

Hotplate

(~100°C)

HMDS in

beaker

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Photoresist Deposition• Photoresist is deposited by spin-coating.

• Photoresist (in solution) is deposited onto center of wafer.

• Wafer is rotated and material is spread out by centrifugal

force.

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Photoresist Deposition1). Photoresist (in solvent) is

deposited in center of the

wafer. Wafer is held in

place with vacuum chuck.

2). Wafer is rotated slowly

(200 rpm) to distribute

material.

3). Accelerate the wafer to

final speed (~5000 rpm).

Spin the wafer at constant

speed for 30 – 60s. Forms

a uniform film and

evaporates solvent.

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Photoresist Deposition• Industrially this is done with robotic arms and automated

dispensers:

• Extremely uniform films can be deposited using spin coating

(rms roughness ~ Å’s)

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Film Thickness• It can be shown (we won’t) that the final film thickness

depends on the spinning speed via:

𝑡 ∝1

𝜔Film

thickness

Angular

velocity

• Actual thickness will also

depend on:

• Concentration.

• Solvent evaporation

rate

• Viscosity.

• Local temperature.

• Local humidity.

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Exposure

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Photoresist Exposure to UV• With our photoresist deposited, we now need to develop it.

• To do this we expose it to light (typically UV).

• Our photo-resist molecules

absorb these short wavelength

photons.

• Bonds are broken in the polymer.

• Either the molecules become

more soluble in the developer

(positive photoresist).

• Or the molecules cross-link an

become insoluble (negative

photoresist).

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Exposure Techniques• Three approaches are typically taken to exposure:

Mask

Contact

Printing

• Defects

• Bowing of mask

Mask

Proximity

Printing

space

(≈25 mm)

• 2 -4 μm resolution

2-5 X

reduction

Mask

lens

Projection

Printing

1:1 Printing

Printing System Magn. Resolution (μm) Use

Contact 1 × 0.1 – 1 Research

Proximity 1 × 2 – 4 Low Cost

Projection 2-5 × 0.1 - 1 Mainstream VLSI

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Light Sources• Traditionally, mercury vapor lamps were employed for

photolithography.

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Light Sources• Mercury lamp has four

main emission lines:

• E, G, H, I:

• For projection printing,

we ideally want

monochromatic light.

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Light Sources• Nowadays for projection

printing, and excimer laser is

employed:

LaserEmission

Wavelength (nm)Resolution (μm)

Max Energy (mJ /

Pulse)

Repetition Rate

(pulses / second)

KrF 248 0.18 - 0.25 300 – 1500 150

ArF 193 0.10 – 0.13 175 – 300 400

F2 157 < 0.1 6 10

• These are pulsed lasers.

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Diffraction• Modern lithography tools are limited by the spreading of light

(and not their optical elements)

• Light passing through an aperture of similar dimensions to

the wavelength of the incident light (λ~ 100’s nm), will result

in diffraction.

Divergent

light source Collimating

lens

Aperture

Diffraction pattern

(Airy disk)

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Diffraction• The type of diffraction observed depends on the mask-wafer

separation.

• Hard-contact: (almost) no diffraction.

• Proximity: Near field (Fresnel) diffraction.

• Projection: Far field or (Fraunhofer) diffraction.

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Diffraction• The difference between Fresnel (near field) and Fraunhofer (far

field) is defined by the dimensionless Fresnel number (𝐹):

𝐹 =𝑊2

𝐿𝜆

• Where:

• 𝑊 is size of the aperture.

• 𝐿 is the distance of the screen (wafer) from the aperture.

• 𝜆 is the wavelength of the incident light.

• We define:

• 𝐹 ≫ 1 as Fresnel diffraction.

• 𝐹 ≪ 1 as Fraunhofer diffraction.

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Fresnel Diffraction• Fresnel (near field) occurs in proximity printing.

𝐿

• Minimum resolvable feature size is:

𝑊𝑚𝑖𝑛 = 𝑘𝐿𝜆

• Where:

• 𝑘 is an experimental parameter associated with the

process conditions.

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Example• Determine the minimum feature size when exposing a wafer

to i-line irradiation, using a mask 25 μm from the surface of

the wafer. Assume for this example 𝑘 = 1.

• From before we know the wavelength of the i-line is 𝜆𝑖 = 365

nm.

𝑊𝑚𝑖𝑛 = 𝑘𝐿𝜆

• Work in microns: 𝜆𝑖 = 0.365 μm.

𝑊𝑚𝑖𝑛 = 1 × 25 × 0.365

𝑊𝑚𝑖𝑛 = 3 μm

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Fraunhofer Diffraction• Fraunhofer (far field) occurs in projection printing.

𝑓 = Focal

length

𝑑 = Lens

diameter

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Fraunhofer Diffraction• Diffraction pattern from a single circular opening:

𝑓 = Focal

length

𝑑 = Lens

diameter

𝜆 =

Wavelength

of incident

light

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Fraunhofer Diffraction• We can think of a mask as a diffraction grating:

Divergent


lens Mask

(diffraction

grating)

• Each aperture in mask acts as a point source.

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Fraunhofer Diffraction• We can think of a mask as a diffraction grating:

Divergent


lensMask

• Each aperture in mask acts as a point source.

Focusing

lens Photoresist

on wafer

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Fraunhofer Diffraction• The resolution in Fraunhofer diffraction is defined by the

Rayleigh criterion.

• Rayleigh Criterion: when the

peak of one projection lands on

the first zero of the other.

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Fraunhofer Diffraction• The resolution in Fraunhofer diffraction is defined by the

Rayleigh criterion.

• Rayleigh Criterion: when the peak of one projection lands on

the first zero of the other:

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Fraunhofer Diffraction• The resolution in Fraunhofer diffraction is quantified by the

Rayleigh Criterion (𝑅):𝑓 = Focal

length𝑑 = Lens

diameter𝑅 = 𝑘1𝜆𝑓

𝑑• 𝑘1 is an experimental parameter associated with the system

and resist (0.6 <𝑘1 < 0.8).

• The parameter Τ𝑑 𝑓 is sometimes called 𝑁𝐴 (numerical

aperture):

𝑁𝐴 =𝑑

𝑓= 𝑛sin𝛼

• 𝑛 = index of refraction (1 in air).

• 𝛼 = maximum half angle of

incident light:

𝑅 =𝑘1𝜆

𝑁𝐴

𝛼 = Maximum half-angle

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Modulation Transfer Function• The modulation transfer function (MTF) quantifies how much

modulation of light we achieve on the wafer:

Divergent


lensMask Focusing

lens

Photoresist

on wafer

Intensity at Mask

Position

1

0

Intensity on wafer

Position

1

0

𝐼𝑀𝑎𝑥

𝐼𝑀𝑖𝑛

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Modulation Transfer FunctionIntensity at

Mask

Position

1

0

Intensity on wafer

Position

1

0

𝐼𝑀𝑎𝑥

𝐼𝑀𝑖𝑛

• We define MTF:

𝑀𝑇𝐹 =𝐼𝑚𝑎𝑥 − 𝐼𝑚𝑖𝑛

𝐼𝑚𝑎𝑥 + 𝐼𝑚𝑖𝑛

• MTF is defined between 0 (small features) and 1 (large

features).

• Generally, MTF needs to be > 0.5 for the resist to resolve

features.

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MTF vs Feature Size

𝑀𝑇𝐹 =𝐼𝑚𝑎𝑥 − 𝐼𝑚𝑖𝑛

𝐼𝑚𝑎𝑥 + 𝐼𝑚𝑖𝑛

𝑀𝑇𝐹

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Depth Focus: δ• Depth of focus defines distance along straight optical path

that wafer can be moved but keep the image in focus

Divergent


lensMask Focusing

lens

Photoresist

on wafer

𝛿

• Basically, 𝛿 defines how accurate we need to be when

positioning the lens at a distance from the wafer.

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Depth Focus: δ• Recall the resolution (Raleigh Criteria) was defined as:

𝛿

• 𝑘2 is another experimental parameter associated with the

system and resist (𝑘2~0.5).

𝑅 = 𝑘1𝜆𝑓

𝑑=𝑘1𝜆

𝑁𝐴

Focusing

lens

Photoresist

on wafer

𝑓

𝑑• The depth of focus is defined

as:

𝛿 =𝑘2𝜆

𝑁𝐴 2= 𝑘2𝜆

𝑓

𝑑

2

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Example• Consider a setup where the focal length is 5× the diameter of

the lens.

Focusing

lens

Photoresist

on wafer

𝛿

𝑓 = 5𝑑

𝑑

• Assume 𝑘1 = 𝑘2 = 0.5.

• Use F2 laser: 𝜆 = 157 nm,

𝑁𝐴 =𝑑

5𝑑= 0.2

𝑅 =0.5 × 157

0.2= 392.5 nm

𝛿 =0.5 × 157

0.2 2= 1.96 μm

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Standing Waves• Standing waves a problem, in particular when exposing on

reflective layers such as metals.

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Standing Waves

• Suppressed by antireflective coating (ARC) prior to resist

spinning

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Development

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Development• We now wish to dissolve the regions of the photoresist which

have higher solubility.

• Exposed regions (positive photoresist).

• Masked regions (negative photoresist).

• Three strategies:

Immersion

Developing

develop solution

Wafers

pH and # of lots processed

are monitored

Spray Developing

Developer

fresh developer with each batch

Puddle Technique

fixed amount of developer

dispensed and rinsed

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Development• We now wish to dissolve the regions of the photoresist which

have higher solubility.

• Exposed regions (positive photoresist).

• Masked regions (negative photoresist).

• We need the following:

• Original thickness of positive resist should not be

measurably reduced.

• Development time should be short.

• Minimum pattern distortion (negative resists tend to

swell).

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Prebake & Postbake• Evaporates resist solvent.

Clean

Exposure Develop

Apply HMDS

Plasma

“descum”

StripProcess

Adhesion Promoter


Photo room

Spin Coat prebake

postbake

• Improves adhesion.

• Anneals out stress in PR.

• Increases etch resistance

(postbake).

• Bake ovens:

• Convection/ hot air.

• Infrared (IR).

• Hot-plate.

• Temperature:

• Prebake: 90 - 100°C.

• Postbake: 120°C.

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Plasma “descum” and Strip• Not all the resist is removed

during development. Clean

Exposure Develop

Apply HMDS

Plasma

“descum”

StripProcess

Adhesion Promoter


Photo room

Spin Coat prebake

postbake

• Plasma “descum” or ashing is

used to remove the residue.

• Plasmas are also used to strip

the PR.

• Barrel or downstream etchers are used.

• O2 gas is used.

• Since the photoresist is organic we make use of elemental

oxygen free radicals in the plasma.

C-H Photoresist(s) + O(g) → CO2(g) + H2O(g)

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Downstream Plasma• Plasma Ashing: Downstream Etcher

• Charged particles (ions & electrons) stay in plasma.

Electronics &

Power supply

(RF Microwave)

R

E

A

C

T

A

N

T

Plasma

Free

radicals

Wafer

Wafer is downstream of the plasma

G

A

S

• Free radicals flow

to wafer.

• Free radicals etch

the wafer

isotropically

• No surface

damage and

heating due to ion

bombardment.

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Resist Contrast• Resist Contrast quantifies the ability of the resist to distinguish

light/dark in the aerial image.

• To evaluate it we plot the developed thickness (i.e. thickness

remaining after development) as a function of dose (𝑄).

Dose (mJcm-2) = Intensity (mW/cm-2) × time (s)

𝑄 = 𝐼𝑡

𝑄0𝑄0

𝑄𝑓𝑄𝑓

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Resist Contrast• Quantitatively, the contrast is defined by 𝛾:

𝛾 =1

log10𝑄𝑓𝑄0

• Where:

• 𝑄0 is the onset of

exposure effect.

• 𝑄𝑓 is the dose at

which the exposure

is completeD

evelo

ped

Th

ickn

ess

Log10(Q)

𝑄0𝑄0

𝑄𝑓

• 𝛾 is typically 2 – 10.

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Lift Off• Not used in VLSI, but can be used in research.

• Avoid etching of

difficult materials

• Can produce a wider

range of structures.

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Next Time…• We will be talking about advanced techniques.

• Techniques for going down to sub-100nm resolution.

Documents

Lecture 13 Basic Photolithographyclasses.engr.oregonstate.edu/eecs/fall2017/ece611/slides/ECE611... · 1/64 ECE611 / CHE611 –Electronic Materials Processing Fall 2017 - John Labram