Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
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
10
Positive vs. Negative Resist
11
Positive vs. Negative Resists
12
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
15
• 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
16
• 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
18
• 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
19
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
21
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
23
So Where do we Do the Lithography?
24
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
26
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
27
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
28
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
29
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
30
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
31
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
32
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
33
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
35
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
36
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
37