Photolithography: Transferring Patterns

to 3D Topical Structures

Prof. Michael C. Murphy

Prof. Steven A. Soper

1

Overview of Photolithography• Photolithography is a fabrication process in which a geometric pattern from a mask is transferred to

a light-sensitive chemical (photoresist) using electromagnetic radiation (UV, X-ray)

2

SU-8 Negative Resist

Photolithography

• High throughput

• Small features

compared to

many other

methods

• Well established

• Multilevel

structures

Advantages Disadvantages

• Resolution limitations (~1 um

for contact lith.)

• High cost of technology

• Less accessible to chemists

and biologists

• Not easily applied to curved

surfaces

• Applicable to only a small set

of materials

• An optical lithography process used to transfer copies of a master pattern (mask) onto the surface of a

solid material (substrate, typically Si)

• Subsequent pattern transfer into the substrate material is commonly performed with etching techniques

• Resulting structure can be used as a master mold for PDMS casting

Radiation – induces photochemical reactions that modify photoresist dissolution rate

Mask – patterns the radiation to create an aerial image of mask in resist (parallel processing)

Aligner – positions mask over resist, aligns it with previous patterns

Photoresist – photoactive polymer into which pattern from the mask is copied

Substrate – serves as resist support, may be patterned in subsequent steps

Functional Components of Photolithography

Radiation

Mask + Aligner

Photoresist

Substrate

4

Process Steps to

Make a Device

5

1. Si wafer spin coated with negative SU8 resist

2. SU8 resist baked to remove solvent (not shown)

3. Exposure of SU8 with UV light through mask.

4. Post-exposure bake promotes epoxy formation of

exposed areas

5. Wafer is rinsed in developer solution to remove

unexposed areas of photoresist. Photoresist

pattern is now the negative of the mask pattern.

6. (Optional) Transfer patterned photoresist into

substrate surface with Deep Reactive Ion Etching

(DRIE) followed by removal of resist

UV light

Negative photoresist

Silicon substrate

Mask

Spin coating

Exposure

Development

Etching

Resist removal

Cross-linking

Post-exposure bake

• Typically mercury (Hg)- Xenon (Xe) vapor bulbs; light source in visible (>420 nm) and ultraviolet

(>250-300 nm and <420 nm)

• Commonly used molecular transition lines in Hg-Xe bulbs are 436 nm (g-line), 405 nm (h-line),

365 (i-line), 290, 280, 265 and 248 nm

• Excimer pulsed lasers are used to increase resolution, and decrease the optical complexity for

deep ultraviolet (DUV) lithography.

Light Sources

Hg Lamp

Spectrum

Excimer lasers

6

• Blocks (absorbs) radiation where it is not wanted

- need opaque material at the desired wavelength

• Transmits radiation where it is needed

- need material with high transmission at the desired

wavelength

Optical lithography:

opaque: thin (~100 nm) layer of Cr

transparent: glass (near UV);

quartz (deep UV)

Mask

X-ray lithography:

opaque: thick (2-50 µm) layer of Au

transparent: graphite, Kapton,

beryllium, Si, SiN

SU-8

resist

gold7

Photoresist• Viscous liquid which has a “solid” form when solvents are driven out

• Spin coat on surface to be patterned

• Exposure of resist to energy/radiation leads to (photo) chemical

reactions and changes the resist dissolution rate in the developer –

100-fold

• Remaining resist is “rugged” enough to protect underlying substrate

during subsequent processing

8

SU-8

Negative

Photoresist

Novolac Positive

Photoresist(Phenol-Formaldehyde)

• Three components: Resin, PAC and Solvent

– Inactive resin

• Novolac resin: a thermoplastic material, easily dissolvable in the developer

solution

– PAC (Photoactive compound)

• Positive resists: DNQ (Diazonaphtoquinone) dissolution inhibitor

– Retardation in the dissolution rate

– Upon exposure to the light, PAC is destroyed and resists becomes soluble

• Negative resists

– The PAC releases nitrogen gas upon exposure to light, and the radicals

generated react with the double bonds to form cross-links between resin

molecules

– Organic solvent

• Provide the ability to spin coat the resists

Photoresist Composition

9

Types of Photoresists

Positive: PR pattern is the same as mask. During exposure to light, light degrades polymers

resulting in the photoresist being more soluble in developers (usually organic solvents)

Negative: PR pattern is the inverse of the mask. Light induces 3D crosslinking of the polymer

structure, which strengthens it’s resistance to dissolution in developer

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Positive vs. Negative Resist

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Positive vs. Negative Resists

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Characteristics Positive Negative

Adhesion to Si Fair Excellent

Adhesion to SiO2 Poor (requires adhesion

promoter)

Poor (requires adhesion promoter)

Developer Aqueous base Organic solvent

Developer process window Small Very wide, insensitive to

overdeveloping

Residue after developing Mostly for < 1 um, good for high

aspect ratio

Often a problem

Lift-off Yes Yes (with newer types)

Pinholes Higher Lower

Plasma etch resistance Very good Not very good

Thermal stability Good Fair

Wet chemical resistance Fair Excellent

Resist strippers Acids, simple solvents Acids (piranha)

1) Dehydration in an oven at ~120°C for ~30 min

2) Spin coat adhesion promoter such as hexamethyldisilane

(HMDS)

3) Spin coat resist

4) Soft bake to partially solidify PR (85-95°C for 1 - 30 min

depending on resist)

5) Expose with few hundred mJ/cm2 of light

6) Hard bake (Optional) , removes more solvent (110-150°C)

7) Develop, weak regions of PR dissolved

8) Additional Hard bake or chemical treatment to harden PR

for aggressive processes

such as ion implantation or plasma etching

Lithography Process Flow

ω – spin speed

Spin Coating Resists onto Substrates

14

• Dispense resist onto substrate

• Ramp up spin coater to preprogrammed spin speed

• Achieve desired resist thickness by choosing spin speed and resist viscosity

• Many more details here: http://www.slideshare.net/ferdoussarwar/spin-

coating

Spin curve for SU8 negative resist

Masks

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• Mask is a stencil used to repeatedly generate a desired pattern

• Consists of flat transparent plate with an absorber layer (opaque to UV)

• Usually mask is kept in direct contact with photoresist while exposing to UV. This results in a 1:1 image

transfer on the wafer (contact lithography)

Transparent plate:

Absorber layer:

Quartz or soda-lime PET or maylar

Chromium with anti-

reflective layerEmulsion

Minimum feature size: ~ 1 um ~ 10 um

Cost: $100s to $1,000s $10s to $100s

Making Masks

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• Use CAD program to draw pattern

– AutoCAD

– TurboCAD

– Layout Editor

• Laser writer directly writes patterns onto

photoresist

– Direct-write procedure similar to ion-

beam milling, electron beam

lithography

• Develop, etch and strip

• More details about mask fabrication

process:

http://www.slideshare.net/gkdelhi8/slide-

26-36278966

Optical Lithography – Exposure Concepts

17

Contact Lithography Resolution

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• Diffraction of light at the edge of mask

• Non-uniformities in wafer flatness

• Debris between wafer and mask

• Resist thickness

• Wavelength

R: resolution

λ : wavelength

s: gap between mask and substrate

z: thickness of resist

Improve resolution by reducing:

• wavelength

• mask/substrate gap

• resist thickness

Resolution for 400 nm light, 1 um thick resist, and no gap: ~1 um

Factors that affect resolution:

Mask Alignment

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Exposure is performed with a mask aligner:

• UV light source

• Proper intensity

• Directionality

• Spectral characteristics (wavelengths)

• Uniformity across wafer

• Typically a mercury arc lamp (365, 405, 436 nm)

• Mask/substrate alignment and parallelizationUV light with mercury arc lamp

Mask aligner instrument

Mask 1

Mask 2

Align Mask 2 to features made using

Mask 1

Mask Alignment

Exposure Times

20

Calculating Dose:

Exposure time (s) = Dose (mJ/cm2)

Power (mW/cm2)

Power: intensity of the UV light source, which varies with wavelength

Dose: amount of energy needed for complete exposure

Lamp intensity in our

cleanroom:

• 11 mW/cm2 (365 nm)

• ~20 mW/cm2 (405, 436 nm)

Photoresist Profiles

overcut

undercut

vertical walls

Control radiation dose and development to achieve desired profile

DoseDeveloper

influenceUses

High

Medium

Low

Low

Moderate

Dominant

Lift-off

Ion implant

Dry etch

Lift-off

Wet etch

Wet etch

Negative resist image

Development stepExterior

scatter

zone

Positive resist image

Exterior

scatter

zone

Development step

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Photolithography - Baking

• Only needed for negative resists

• Heating of exposed resist completes

the formation of insoluble layer. For

SU8, this is an epoxy based reaction

promoted by formation of acid groups

generated during exposure step

• Gently raise temperature of thick

resist to avoid cracks and

delamination

Soft baking

• Performed after spin coating to drive

off solvent

• Bake time is determined by resist

thickness

Post exposure baking

Typical temperature profile for SU8 post-exposure bake

SU8 soft bake times

SU8 post-exposure bake times

Hard baking

• Performed after all development steps

• Further cross-links resist to improve

structural stability and etch resistance.

• SU8: 140C-200C for 20-30 mins.

• Generally not necessary

Uses of Photolithography

1. Etching processes:

Spin photoresist (PR) PhotolithographyEtch using PR

as maskRemove PR

2. Lift off processes

Evaporate metalSpin (PR)Lift Off excess

metal with PRPhotolithography

patterning of difficult to etch metals (Pt)

preparation of optical masks, patterning metals, oxides, etc…,

patterning microfluidic channels in glass, silicon

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So Where do we Do the Lithography?

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Properties

1. Environment free of contaminants

and particulates

2. Classified based on number of

particles per m3

3. Input air filtered with HEPA to

remove particles and biological

contaminates

4. For resist processing – yellow

lights so as not to interfere with

light sensitive resists (exposure)

UNIST Clean Room

Class ≥0.1 µm ≥0.2 µm ≥0.3 µm ≥0.5 µm ≥5 µm

10 350 75 30 10 0.07

100 3,500 750 300 100 0.7

1,000 35,000 7,500 3,000 1,000 7.0

10,000 350,000 75,000 30,000 10,000 70

Cleanroom Classification (particles per ft3)

LaminarTurbulent

Direct Fabrication

Absorber

MaskResist

Base Plate

Resist

Microstructure

Absorber

Optical maskResist

Base Plate

Resist

Microstructure

Electroplating

MetalMicrostructure

Conductive

Base Plate

Electroplated

part

Photolithography

Resist removal

Lithography

Part release

preparation of metal parts

(e.g. mold masters)

fabrication of X-ray masks

preparation of polymer parts,

fabrication of polymer microfluidics

fabrication of mold masters for casting25

Micropatterns for Metal

Electroplating

Tilted and Rotated Exposures

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SU8 Process Flow - Example

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

Develop

SU8: negative resist commonly used to make microfluidic molds

SU-8

Negative

Photoresist

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Substrate PretreatmentPurpose:

• Clean substrate

• Promote adhesion

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

Develop

Si wafer after HF dip

A common Si wafer cleaning procedure:

1. Solvent Clean – cleans oils and organics

2. RCA-1 clean – removed additional organics and

oxidizes Si surface

3. HF dip – remove native oxide and creates

hydrophobic surface (good contact with resist)

4. Dehydration bake – drives off water vapor and

promotes adhesion

*Details of process can be found in “Si wafer

cleaning procedure.pdf”

Prime Si wafers straight out of box can skip steps 3 and 4

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Spin Coating• Achieve desired resist thickness by choosing spin speed

and resist viscosity

• Dispense resist onto substrate

– Pour straight from bottle of highly viscous resists (>

~50um)

• Ramp up spin coater to preprogrammed spin speed

• Always clean the spin coater afterwards!

• Many more details here:

http://www.slideshare.net/ferdoussarwar/spin-coating

Spin curve for SU8 negative resist

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

Develop

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Soft BakingSoft baking process

• Performed after spin coating to drive off

solvent

• Bake time is determined by resist thickness

• First step at 65C allows resist to flow and level

out

• Baking can be done on both hot plate or oven,

but oven needs slightly more time

SU8 soft bake times

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

Develop

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Laminating SU8 sheets

https://djmicrolaminates.com

• Thick dry film sheets of epoxy resists

• Range of thickness 100 um – 1 mm

• Variety of shapes and sizes to match common

substrate geometries

• Apply sheets to substrate using an office

laminator

Alternative to spin coating and soft baking Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

Develop

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ExposureExposure is performed with a mask aligner,

which offers:

• UV light source that offers

• Proper intensity

• Directionality

• Spectral characteristics

(wavelengths)

• Uniformity across wafer

• Typically a mercury arc lamp (365,

405, 436 nm)

• Mask/substrate alignment and

parallelization

UV light with mercury arc lamp

Mask 1

Mask 2

Align Mask 2 to features

made using Mask 1

Mask Alignment

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

Develop

Karl Suss MA6 mask aligner

Transparency mask

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ExposureCalculating Dose:

Exposure time (s) = Dose (mJ/cm2)

Power (mW/cm2)

Power: intensity of the UV light source at some wavelength

Dose: amount of energy needed for complete exposure

Lamp intensity in cleanroom: 11 mW/cm2 (365 nm)

25 mW/cm2 (405, 436 nm)

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

DevelopRecommended exposure energies (dose)

Exposure energy should be increased

for less reflective substrate surfaces

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SU8 Exposure TipsOverexposure:

• Loss of resolution

• But, improved adhesion (negative resist)

• Excessive overexposure: stress and

delamination

Underexposure:

• Delamination of resist

• Cracking

• Soft features

If can sacrifice resolution,

overexposure is recommended

SU resist profile

Without UV filter – T-topping

With UV filter

T-topping with SU8 resists:

• SU8 absorbs heavily at λ < 350 nm,

resulting in increased exposure near top

• Use a UV filter to eliminate short

wavelengths

• Increase dose by > 40%

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure

Bake

Develop

34

Contact lithography resolution limits

• Debris between wafer and mask

• Non-uniformities in wafer

flatness

• Variation in resist thickness

(edge bead)

• Diffraction

• Wavelength

R: resolution

λ : wavelength

s: gap between mask and

substrate

z: thickness of resist

Resolution for 400 nm light, 1 um thick resist, and no gap: ~ 1 um

Factors that affect resolution:

s: substrate/mask gap

z: resist thickness

λ : wavelength

Substrate

Pretreat

Spin coat

Soft Bake

Expose

Post Exposure

Bake

Develop

Diffraction profiles obtained from opaque square

patterns on the mask: 4um down to 1.6 um. Resist: 3 um

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Post Exposure Baking• Only needed for negative resists

• Heating of exposed resist completes the formation of insoluble layer.

• Epoxy based reaction promoted by formation of acid groups that were

generated during exposure step

• Gently raise temperature of thick resist to avoid cracks and delamination

Temperature profile for SU8 post-exposure bake

SU8 post-exposure bake times

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

Develop

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Development

Orbital shaker

1. Place substrate in bath of develop

2. Agitation can speed development. Options:

a) Swirl by hand – single substrates

b) Orbital shaker – multiple substrates

c) Megasonic agitation (not ultrasonic) – hard to

develop features

3. Blow dry with N2

4. Rinse with isopropyl alcohol and blow dry with N2

- presence of white residue indicates incomplete

development: squirt with fresh developer

Developers:

• Propylene glycol monomethyl ether acetate (PGMEA)

• Acetone Process:

Agitate, but don’t splash!

Typical development times

Substrate Pretreat

Spin coat

Soft Bake

Expose

Post Exposure Bake

Develop

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