Photo Lithography

Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

Summary

Photolithography Method of Microscale Pattern Transfer Matthew Jordan The University of Alabama

September 14, 2012

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Outline Photolithography Method of Microscale Pattern Transfer

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Outline

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Pattern Transfer Introduction to Lithography Lithography for Engineers

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Photolithography Photoresist Mask Aligners

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Summary

Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Pattern Transfer Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Figure 1: Pattern Transfer is one of the oldest human compulsions [1] 3/31

Lithography Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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

Lithography has been used by artists for centuries to transfer patterns 4/31

Lithography Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Coatings of hydrophilic chemicals are painted on to a template

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Water based ink is applied to the template and readily adheres to the painted pattern

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

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Why do we do this? Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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

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Photolithography Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Figure 4: Lithography + Light = Photolithography

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Photolithograpy Basics Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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A mask is made with the pattern on it A light sensitive chemical called photoresist is applied to a substrate Light shines through the mask and exposes regions of the photoresist The resist is developed using special solvents The pattern is left in the resist and used to transfer patterns to the substrate

Figure 5: Basic Steps of Photolithography 8/31

Photoresist Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

Summary

Figure 6: Photoresist being spun onto a wafer 9/31

Photoresist Basics Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Types of Photoresists There are two types of photoresist. Positive resists will dissolve more readily after being exposed to light, and negative resists will dissolve less readily in the developer solution after being exposed to light. Composition of Photoresists Photoresists are typically made of resins, photoactive compounds, and solvents. Photoresist Performance Measures Photoresists are quantified by their sensitivity and their resolution. 10/31

Are You Awake? Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

Summary

Pop Quiz! If I have a mask with circular holes to allow light through it and use it to pattern a negative resist. After developing the resist what resist shapes are left? a) Cylindrical holes in the resist b) Cylindrical pillars of resist

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Are You Awake? Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

Summary

Answer If I have a mask with circular holes to allow light through it and use it to pattern a negative resist. After developing the resist what resist shapes are left? a) Cylindrical holes in the resist b)

Cylindrical pillars of resist

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How Photoresists Work: Negative Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline

Most negative resists are long polymers

Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Exposing these polymers to UV light leads to cross-linking and branching The cross-linked regions are less soluble in the developer solution than the original polymer

Figure 7: Exposure to UV light leads to cross-linking (harder to dissolve) 13/31

How Photoresists Work: Positive Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Most positive resist use a diazoquinone photoactive chemical When exposed to UV light a chemical change occurs This leads to a chemical that is more soluble in the developing solution that the original resist

Figure 8: Exposure to UV light leads to chemicals that are easier to dissolve.

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Contrast Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Figure 9: Contrast Curves

Contrast is a measure of photoresist performance. 1 γ= log (D100 /D0 ) 15/31

Applying Photoresist Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Figure 10: Standard and Industrial Spin Coaters

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Mask Aligners Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Figure 11: The Mask Aligner at Argonne National Lab 17/31

Mask Aligner Basics Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Figure 12: A Schematic of Mask Aligner Operation

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Photomasks Photolithography Method of Microscale Pattern Transfer Matthew Jordan

Photomasks Sometimes called reticles

Outline

Fused silica plate

Pattern Transfer

Highly transparent and defect free

Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Usually Chromium is used to mask off certain regions Depending on your resist they are either dark-field or bright-field

Figure 13: Some photomasks: 1× plate, a 10× plate for a stepper, and a 10× plate with pellicles

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Mask Aligner Basics Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Types of Mask Aligners Mask aligners can be divided into three types. Contact aligners have the mask directly touching the photoresist, proximity aligners are similar to contact aligners but there is a small gap between the mask and substrate, and projection printers have the mask far away from the substrate and often use 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 two layers, resolution is related to the minimum feature possible for the aligner to resolve, and throughput relates to how many wafers/time is possible with this system. 20/31

Types of Aligners Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

Summary

Contact Printer Simplest type of aligner The mask comes into intimate contact with the substrate Resolution is very good (g → 0), but can damage mask or substrate Proximity Printer Very similar to contact printing Now there is a small gap between the mask and the substrate Easier to align and the masks are less prone to receiving damage, but there is some decrease resolution due to diffraction. 21/31

Projection Printer Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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No mask damage Image size limited by optics and not mask feature size. Very low throughput generally (for steppers etc.) Figure 14: A schematic projection printer

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Minimum Feature Size Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline

For Proximity Printers the minimum line width is given by:

For Projection Printers the minimum line width is given by:

Pattern Transfer Introduction to Lithography Lithography for Engineers

Wmin ≈

p kλg

Wmin ≈ k

λ NA

Photolithography Photoresist Mask Aligners

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Where λ is the wavelength, g is the gap between the mask and the substrate, NA is called the numerical aperture, and k is a constant related to the resist’s ability to resolve small changes of light intensity.

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Numerical Aperture Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Figure 15: Definition of Numerical Aperture [5]

NA = n sin θ Where n is the index of refraction (air = 1, water = 1.33). 24/31

Are you Awake? Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline

Equations Proximity: √ Wmin ≈ kλg

Projection: λ Wmin ≈ k NA NA = n sin θ

Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

Summary

Pop Quiz! Get out your calculators You have access to a proximity and a projection printer and want to know which to use to get the best resolution. Your photoresist has a k = 0.7. Assume that the gap in the proximity system is 15 µm and that it uses an i-line lamp (λ = 365nm). Your projection system has an objective lens that is 20mm across, 25mm away from the substrate, and uses a g-line lamp (λ = 436nm). Assume the gap is filled with air (n = 1). Which has the better minimum line width? 25/31

Pop Quiz solutions Photolithography Method of Microscale Pattern Transfer Matthew Jordan

Answers Proximity:

Projection: Opposite Hypotenuse 10mm =√ 2 10 + 252 mm = 0.37

Outline

sin θ =

Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

NA = n sin θ

Summary

Wmin ≈

p 0.7 × .365µm × 15µm

≈ 1.96µm = 1960nm

=⇒ NA = 0.37 0.7 × 436nm =⇒ Wmin ≈ 0.37 = 822nm 26/31

Registration, a.k.a. Overlay accuracy Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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Figure 16: Two types of registration errors: Misalignment and Runout

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Summary Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

Summary

Pattern Transfer Pattern transfer is necessary to make electronic devices. Photolithography is a technique using sets of masks and light to transfer patterns into photoresist chemicals Photoresist Photoresists can be either positive or negative. Positive resists dissolve in developer when exposed to light, negative resists are left behind in developer when exposed to light An important performance measure for photoresist is the contrast. 28/31

Summary Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

Mask Aligners Three types of aligners: Contact, Proximity, and Projection. Photomasks have the pattern you want to transfer either in bright or dark field. Important performance measures are the resolution, registration, and throughput.

Summary

Resolution is best for contact printers. The minimum line width is an important measure of resolution and is related to the wavelength of light.

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Questions Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Do you have any Questions?

Photolithography Photoresist Mask Aligners

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Picture References Photolithography Method of Microscale Pattern Transfer Matthew Jordan Outline Pattern Transfer Introduction to Lithography Lithography for Engineers

Photolithography Photoresist Mask Aligners

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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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