Hanyang University
Soft Lithography
Jin-Goo Park
Materials and Chemical Engineering Hanyang University, Ansan
Electronic Materials and Processing Lab.
Introduction to Soft Lithography
Micro-
Electronics Microelectronics
(low cost, < 100 nm) Research
Micro- Electro- Mechanical
System
Micro- Reactors
Micro- Analysis
Photolithography vs. Imprinting
Photo Lithography Imprinting
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Rigid photomask
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High cost
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Optical diffraction
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Not surmount 100nm barrierz
Not apply for nonplanr surface
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No control over chemistry
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Chemical functionalities on surfacez
2-D structure
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Limited by photosensitive material
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Elastomeric stamp or mold
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Non-photolithography
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Low cost, easy to use,
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30 nm ~ 500 um
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Apply nonplaner surface
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2-D, 3-D structure
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Use variety of materials
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Surface chemistry
Photo Litho. vs. Imprinting
Wafer
PR Spin Coat Photolithography
To make metal lines
Wafer
Soft Bake Mask
Alignment
Exposure Wafer
Wafer
Develop
Wafer Deposition
Wafer Lift Off
Wafer
Resist Spin Coat Imprinting
Bake
Hard Bake
Wafer Alignment
Wafer
Heating and Pressing
Wafer
RIE (Reactive Ion Etching)
Wafer Deposition
Wafer Lift Off
Embossing
Embossing Embossing
Nano Imprint Lithography (NIL)
Imprint Lithography
Soft Lithography
Soft Embossing Cold Embossing
Step and Flash Imprint Lithography (SFIL)
Hot Embossing
Two Types of Embossing
UV Light
• Cold Embossing
Heat and Pressure
• Nano Imprint
Many different names for the same process
Approaches to Nano Imprinting
•Temperature > Tg
•Contact Force ~2-40kN
•Vacuum
•Achieved Resolution :
< 100 nm
•Room Temperature
•Contact Force ~ 1-100N
•UV Light (350-450nm)
•Achieved Resolution :
< 15 nm
•Room Temperature
•Contact Force ~1-40N
•“Inked” stamp
•Achieved Resolution :
< 50 nm
Typical Equipment set
EV520 Semi-Automated Wafer Bonder and Hot Embosser
Automated Bonding process
Hot Embossing and Nanoimprinting
Support for All Wafer Bonding Processes
Temp. 550C max.
Voltage 2kV max.
Pressure 8,000 lbf max.
Up to 8” Wafers & Substrates
Hot Embossing Chamber Cross Section
Contact Force generated by external pneumatic cylinder
Top and bottom side heater with independent
temperature control Small Chamber
Uniforce Compliant Membrane
Center Contact Pin (Ceramic or Teflon)
Mold Align Heating Plate Molding Demolding
Hot Embossing Procedure
Silicon Master
Wafer after demolding step Embossing tool: Electroplated nickel
from a silicon master
Nickel Master
Embossing from Material : Ni
500 μm
Embossed Microstructure Material: Polycarbonate Height : 50 μm
Embossing Technique
Micro-Embossing Solutions for Polymer-Bio-Chips
Hanyang University
PDMS Based Technique
Electronic Materials and Processing Lab.
Introduction to PDMS
Siloxane oligomer Siloxane cross-linker
•PDMS (Polydimethylsiloxane) has many unique properties and is therefore used in many various applications.
•PDMS stem from the nature of the siloxane bond
• Curing
Product specification Dow Corning Sylgard 184 CH3 CH3
| | ..-(-Si-O-Si-O-)n-..
| | CH3 CH3
The siloxane bond
Recommend curing conditions
• 24 hr at 23°C or
• 4 hr at 65°C or
• 1 hr at 100°C or
• 15 min at 150°C
24 months Shelf life
0.18 WmºK Thermal conductivity
150 K Glass transition temperature
1.08 Specific gravity
3900 mPa⋅s Viscosity
Clear Color
Value Property
Characteristics of PDMS
Elastic characteristic
Low interfacial free energy
(~ 21.6 dynes/cm)
Chemically inert
- do not adhere to, react with
Not hydroscopic - not swell with humidity
Easy to pass gases
Good thermal stability (~ 186℃ in air)
Transparent down to ~300 nm
Durable elastomer ( over 50 time)
Thermal expansion - difficult to get high accuracy
Sagging / pairing
- deform or distort and generate defects in the pattern - relief structure can’t withstand - aspect ratio must be 0.2~2
Shrinking
- shrinks by ~1% upon curing - readily swelled by
non-polar organic solvents
Properties Technical problems
Techniques of Soft Lithography
μCP (Microcontact Printing) REM (Replica Molding)
μTM (MicroTransfer Molding)
Transfer of SAM precursor with elastomeric stamp onto substrat
⇒ master generation by photolithography and similar techniques
⇒ stamp is obtained by casting of elastomer (PDMS, e.g.) over master
Pattern generation by stamping of SAM precursor onto substrate
Microcontact Printing
Stamped SAM patterncan be further processed by etching or deposition:
⇒µCPtechnique can also be applied to curved surfacesof stamp or substrate
Quality of µCP SAMs is comparableto films obtained byadsorption from solution
Microcontact Printing
Replica Molding
Use elastic polymer as master for molding of prepolymer
Elasticity and low surface energy of stamp make release of mold easy
Allows duplication of three-dimensional topologis in a single step
Faithful duplication of complex structure in the master
Nanometer resolution (~10nm)
UV curable prepolymers : shrinkage of less than 3% on curing (no solvent)
- Mold prepolymer
- Cure - Peel off
Replica Molding
(a) (b) (a)Cr nano-structures on a master (b)Polyurethanenano-structures
(C) (d) (c)Au structures on a master (d) Polyurethane nano-structures
Microtransfer Molding
Convenient method for fabrication of microstructures
Nonplanr substrates and 3D structures layer by layer
Generating both interconnected and isolated microstructures
Variety materials other than organic polymers: glassy carbon, sol-gels, ceramics Procedures
1. A drop of liquid prepolymer is applied to patterned surface of a PDMS mold
2. The excess liquid removed by scraping with PDMS block or by blowing off with N2 3. The filled mold is placed in contact with
a substrate and heated
4. After curing the mold is peeled away 5. Thin films must be removed using O2RIE
Microtransfer Molding
Polymeric microstructures fabricated using microtransfer molding
(a) Arrays of 3-cm long wave guides of PU fabricated on Si/SiO2.- different lateral dimensions and are separated by different spacing (b) An SEM image of the ends of the wave guides ( ~3um2 )
(c) An SEM image of an array of isolated micro-cylinders of epoxy on 5-um lines of epoxy, supported on a glass slide.
(d ) An SEM image of a three-layer structure on a glass slide made from a thermally curable epoxy.
Micromolding in Capillaries
Procedures
1. PDMS is placed on a substrate ( network of empty channels)
2. Low-viscosity liquid prepolymer is placed at the end of channels
3. Spontaneous filling by capillary action into the network of channels
4. After curing, mold is removed and network of Material remains
3D microstructureformation by filling of micro-capillarieswith liquid precursor
Low viscosityprepolymer
Capillary filling is rapid and complete over short distance( ~ 1cm)
Rateof filling decreasesas the cross-sectional dimensions of the capillary decrease
Micromolding in Capillaries
System without solvents Systems with solvents
Solvent-Assisted Micromolding
procedure
1. Wet a PDMS mold with the solvent
2. Bring it into contact with the surface of the substrate 3. Solvent dissolves (or swells) a thin layer of the substrate, 4. Fluid or gel is molded against the relief structures
in the mold.
5. The solvent dissipates and evaporates, 6. Fluid solidifies and forms a patterned
SAMIM uses a solvent instead of temperature to soften the material
Solvent have high vapor pressure and a moderately high surface tension - rapid evaporation of the excess solvent and minimal swelling of the PDMS
Hydrophilic elastomers or surface modification of PDMS is required - partially wet
Solvent-Assisted Micromolding
(a) SEM images of structures in photoresist (1.6um) spin-coated on Si/SiO2, (b) Polystyrene ( 2.0 um thick)
(c) ABS ( 0.85 um thick)
(d )AFM image of nanostructures in a thin (0.4 mm thick) film of Microposit
Common characteristic of structure are joined
by a thin, underlying film of the polymer
Film can be removed by O2RIE
Polymeric structures can be used as masks in the etching of underying substrates
Summary in PDMS Based Pattern Generation
Advantage Disadvantage
Non-Photolithographic technics
Patterning on scales < 100nm
Patterning : solid materials liquid materials surface functionalities large areas
Three dimensional microstructures
No diffraction limit (30nm)
Optical transparency of the mask
Good control over surface chemistry
Convenient, inexpensive
Minimize waste of materials
Patterns, mold may distorted , deformed ( pairing, sagging,swelling, shrinking )
Difficult to achieve accurate registration with elastomers (<1um)
Defects higher than photolithography
Micro contact printing works well only a limited range of surfaces
Micro molding in capillaries is slow
REM, uTM, SAMIM leave a thin film -- must be removed by O2RIE
The soft-lithography model system
Master Elastomer
Stamp
Based on contact and pattern replication Silicon, SOI : Photolithography, e-beam
PDMS(dimenthylsiloxane) : curing 20-80℃ 48 hours
Thermal and chemical shrinking : To consider the design of the master
Microcontact printing of alkanethiols on gold was the first representative
of soft-lithography processes
Micro-Contact Printing
Micro-Contact Printed of Thiols
SEM image / Scheme SEM image / Scheme
•Feature of the stamps
; 0.6 ㎛ Ⅹ 3.0 ㎛
•Scheme showing diffusion paths of molecular ink during printing
; The diffusion of ink molecules
; Zone of contact is dominant
; Printing & reaction time
; reactant contcentration
; Pattern width
The Factor of Defect
Wet inking-print-etch Wet inking-print-etch Contact inking-print-etch Contact inking-print-etch
DDT : Dodecanethiol - No contrast
HDT : Hexadecanethiol - >500nm ink diffusion ECT : Eicosanethiol
- ∼100nm ink diffusion
SEM Image of Gold Patterns
Contact inking – printing - etching
The formation of this pattern is difficult using immersion inking
The Effect of Stamp Hardness
The SEM images were acquired after coating molded PDMS stamps with a thin layer of gold
Sylgard 184 with a Young’s modulus of 3MPa Material with a Young’s modulus of 9.7MPa
