photo lithography

PhotolithographyMethod of

Microscale PatternTransfer

Matthew Jordan

Outline

Pattern Transfer

Introduction toLithography

Lithography forEngineers

Photolithography

Photoresist

Mask Aligners

Summary

PhotolithographyMethod of Microscale Pattern Transfer

Matthew Jordan

The University of Alabama

September 14, 2012

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Outline

1 Outline

2 Pattern TransferIntroduction to LithographyLithography for Engineers

3 PhotolithographyPhotoresistMask Aligners

4 Summary

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Pattern Transfer

Figure 1: Pattern Transfer is one of the oldest human compulsions[1]

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Lithography

Figure 2: A lithograph of Munich and the stone template to makeit, circa 19th century [2]

Lithography has been used by artists for centuries to transferpatterns

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Lithography

1 Coatings of hydrophilic chemicals are painted on to atemplate

2 Water based ink is applied to the template and readilyadheres to the painted pattern

3 The patterned, inked template is used to transfer theimage to the artistic medium

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Why do we do this?

Figure 3: Transistors and a memory device patterned usinglithography [3,4]

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Photolithography

Figure 4: Lithography + Light = Photolithography

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Photolithograpy Basics

A mask is made with thepattern on it

A light sensitive chemicalcalled photoresist isapplied to a substrate

Light shines through themask and exposes regionsof the photoresist

The resist is developedusing special solvents

The pattern is left in theresist and used totransfer patterns to thesubstrate

Figure 5: Basic Steps ofPhotolithography

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Photoresist

Figure 6: Photoresist being spun onto a wafer

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Photoresist Basics

Types of Photoresists

There are two types of photoresist. Positive resists willdissolve more readily after being exposed to light, andnegative resists will dissolve less readily in the developersolution after being exposed to light.

Composition of Photoresists

Photoresists are typically made of resins, photoactivecompounds, and solvents.

Photoresist Performance Measures

Photoresists are quantified by their sensitivity and theirresolution.

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Are You Awake?

Pop Quiz!

If I have a mask with circular holes to allow light through itand use it to pattern a negative resist. After developing theresist what resist shapes are left?

a) Cylindrical holes in the resist

b) Cylindrical pillars of resist

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Are You Awake?

Answer

If I have a mask with circular holes to allow light through itand use it to pattern a negative resist. After developing theresist what resist shapes are left?

a) Cylindrical holes in the resist

b) Cylindrical pillars of resist

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

How Photoresists Work: Negative

Most negative resists arelong polymers

Exposing these polymersto UV light leads tocross-linking andbranching

The cross-linked regionsare less soluble in thedeveloper solution thanthe original polymer Figure 7: Exposure to UV light

leads to cross-linking (harder todissolve)

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

How Photoresists Work: Positive

Most positive resist use adiazoquinone photoactivechemical

When exposed to UVlight a chemical changeoccurs

This leads to a chemicalthat is more soluble inthe developing solutionthat the original resist

Figure 8: Exposure to UV lightleads to chemicals that are easierto dissolve.

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Contrast

Figure 9: Contrast Curves

Contrast is a measure of photoresist performance.

γ =1

log(D100/D0)

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Applying Photoresist

Figure 10: Standard and Industrial Spin Coaters

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Mask Aligners

Figure 11: The Mask Aligner at Argonne National Lab

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Mask Aligner Basics

Figure 12: A Schematic of Mask Aligner Operation

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Photomasks

Photomasks

Sometimes called reticles

Fused silica plate

Highly transparent anddefect free

Usually Chromium isused to mask off certainregions

Depending on your resistthey are either dark-fieldor bright-field

Figure 13: Some photomasks:1× plate, a 10× plate for astepper, and a 10× plate withpellicles

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Mask Aligner Basics

Types of Mask Aligners

Mask aligners can be divided into three types. Contactaligners have the mask directly touching the photoresist,proximity aligners are similar to contact aligners but there isa small gap between the mask and substrate, and projectionprinters have the mask far away from the substrate and oftenuse optics to reduce the mask image on the substrate.

Mask Aligner performance measures

There are three primary measures of performance.Registration quantifies the ability of an aligner to align twolayers, resolution is related to the minimum feature possiblefor the aligner to resolve, and throughput relates to howmany wafers/time is possible with this system.

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Types of Aligners

Contact Printer

Simplest type of aligner

The mask comes into intimate contact with thesubstrate

Resolution is very good (g → 0), but can damage maskor substrate

Proximity Printer

Very similar to contact printing

Now there is a small gap between the mask and thesubstrate

Easier to align and the masks are less prone to receivingdamage, but there is some decrease resolution due todiffraction.

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Projection Printer

No mask damage

Image size limited byoptics and not maskfeature size.

Very low throughputgenerally (for steppersetc.)

Figure 14: A schematicprojection printer

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Minimum Feature Size

For Proximity Printers theminimum line width is givenby:

Wmin ≈√kλg

For Projection Printers theminimum line width is givenby:

Wmin ≈ kλ

NA

Where λ is the wavelength, g is the gap between the maskand the substrate, NA is called the numerical aperture, andk is a constant related to the resist’s ability to resolve smallchanges of light intensity.

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Numerical Aperture

Figure 15: Definition of Numerical Aperture [5]

NA = n sin θ

Where n is the index of refraction (air = 1, water = 1.33).

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Matthew Jordan

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Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Are you Awake?

Equations

Proximity:Wmin ≈

√kλg

Projection:Wmin ≈ k λ

NANA = n sin θ

Pop Quiz! Get out your calculators

You have access to a proximity and a projection printer andwant to know which to use to get the best resolution. Yourphotoresist has a k = 0.7. Assume that the gap in theproximity system is 15 µm and that it uses an i-line lamp(λ = 365nm). Your projection system has an objective lensthat is 20mm across, 25mm away from the substrate, anduses a g-line lamp (λ = 436nm). Assume the gap is filledwith air (n = 1). Which has the better minimum line width?

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Pop Quiz solutions

Answers

Proximity:

Wmin ≈√

0.7× .365µm × 15µm

≈ 1.96µm

= 1960nm

Projection:

sin θ =Opposite

Hypotenuse

=10mm√

102 + 252mm

= 0.37

NA = n sin θ

=⇒ NA = 0.37

=⇒ Wmin ≈0.7× 436nm

0.37= 822nm

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Registration, a.k.a. Overlay accuracy

Figure 16: Two types of registration errors: Misalignment andRunout

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Summary

Pattern Transfer

Pattern transfer is necessary to make electronic devices.

Photolithography is a technique using sets of masks andlight to transfer patterns into photoresist chemicals

Photoresist

Photoresists can be either positive or negative. Positiveresists dissolve in developer when exposed to light,negative resists are left behind in developer whenexposed to light

An important performance measure for photoresist isthe contrast.

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Summary

Mask Aligners

Three types of aligners: Contact, Proximity, andProjection.

Photomasks have the pattern you want to transfereither in bright or dark field.

Important performance measures are the resolution,registration, and throughput.

Resolution is best for contact printers.

The minimum line width is an important measure ofresolution and is related to the wavelength of light.

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Questions

Do you have any Questions?

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Matthew Jordan

Outline

Pattern Transfer



Photolithography

Photoresist

Mask Aligners

Summary

Picture References

References

[1] http://en.wikipedia.org/wiki/Cueva de las Manos[2] http://en.wikipedia.org/wiki/Lithography[3] http://spectrum.ieee.org/semiconductors/design/transistors-go-vertical[4] http://finntrack.co.uk/learners/infosys today.htm[5] http://wikis.lib.ncsu.edu/index.php/Soft Lithography

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