photo lithography
DESCRIPTION
ttTRANSCRIPT
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
1/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Outline
1 Outline
2 Pattern TransferIntroduction to LithographyLithography for Engineers
3 PhotolithographyPhotoresistMask Aligners
4 Summary
2/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Pattern Transfer
Figure 1: Pattern Transfer is one of the oldest human compulsions[1]
3/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
4/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
5/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Why do we do this?
Figure 3: Transistors and a memory device patterned usinglithography [3,4]
6/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Photolithography
Figure 4: Lithography + Light = Photolithography
7/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
8/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Photoresist
Figure 6: Photoresist being spun onto a wafer
9/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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.
10/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
11/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
12/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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)
13/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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.
14/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Contrast
Figure 9: Contrast Curves
Contrast is a measure of photoresist performance.
γ =1
log(D100/D0)
15/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Applying Photoresist
Figure 10: Standard and Industrial Spin Coaters
16/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Mask Aligners
Figure 11: The Mask Aligner at Argonne National Lab
17/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Mask Aligner Basics
Figure 12: A Schematic of Mask Aligner Operation
18/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
19/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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.
20/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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.
21/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
22/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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.
23/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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).
24/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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?
25/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
26/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Registration, a.k.a. Overlay accuracy
Figure 16: Two types of registration errors: Misalignment andRunout
27/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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.
28/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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.
29/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
Photolithography
Photoresist
Mask Aligners
Summary
Questions
Do you have any Questions?
30/31
PhotolithographyMethod of
Microscale PatternTransfer
Matthew Jordan
Outline
Pattern Transfer
Introduction toLithography
Lithography forEngineers
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
31/31