nano technology- lithography

1. 1)150110105053 – Abhishek Thakkar
2)150110105054 – Yash Thakkar
G. H. PATEL COLLEGE OF ENGINEERING AND TECHNOLOGY
GUJARAT TECHNICAL UNIVERSITY
VALLABH VIDYANAGAR, ANAND - 388120
Different Techniques of Soft Lithography
Nano Technology (2170508)
Department of Chemical Engineering
Prepared By:-
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2. contents
• What is lithography
• Introduction to soft lithography
• Classification of lithography
• What is soft lithography
• Different techniques of soft lithography

3. What is Lithography
• Lithography (or patterning) refers to the series of steps that establish
the shapes, dimensions, and location of the various components of
the integrated circuit (IC). The current progress in IC design, with
the decreased dimensions (miniaturization) of the chip and increased
density of transistors, is possible only if smaller areas on the wafer
surface can be patterned. This is primarily the function of
lithography.

4. Types of Lithography
• Photolithography
• Electron Beam (e-beam) Lithography
• X-Ray Lithography
• Soft Lithography

5. Soft lithography
George M. Whitesides (Harvard)
Soft lithography:
• Low cost
• Molding, printing or transferring
• Resolution usually not very high
• Application in microfluidic, biomedical …
“Soft” means no energetic
particles (electron, ions) or
radiation (UVs, X-ray) is
involved. Instead, soft
elastomeric stamp is used.
Soft lithography opportunity assessment
“Size is not the only thing that matters, function is more important” (something like this), Whiteside 5

6. Soft Lithography
• Soft lithography represents a non photolithographic strategy
based on self assembly and replica molding for carrying out
micro and nanofabrication.
• So, soft lithography essentially was pioneered by George
Whitesides group in Harvard in mid 90s and these are rather
specific towards soft surfaces, polymers and gels as well as,
for application which do not require extremely stringent
quality control or the requirement of defect free patterns is not
as high as that is in micro electronics.

7. • Soft lithography is so called because it utilises cast moulded
stamps made from flexible materials.
• The process begins with the creation of a master. The master is
made by etching a blank – normally a silicon wafer – with a
negative photoresist. This gives a raised pattern of nanometer
sized features on the silicon wafer that corresponds with the
required channels in the polymer stamp.
• A liquid polymer is then poured on top of the silicon wafer
mould. The polymer is commonly a resin like PDMS
(poly(dimethylsiloxane)) or fluorosiliconeheat. The polymer is
heat cured and peeled off the mould.
• The mould can now be used in a number of ways. These
various alterations to the process determines the sub type of
soft lithography.

8. Applications of Soft Lithography
 Soft lithography can be used for the production
of:
• lab-on-chip systems
• biosurfaces
• biochips
• microfluidics
• microreactors
• sensors
• microelectromechanical systems (MEMS)
• microoptics

9. Different Types of Soft Lithography Techniques
1. Replica molding.
2. Micro-contact printing
3. Micro-molding in capillary.
4. Solvent assisted micro-contact molding.

10. PDMS: poly(dimethyl-siloxane)
PDMS properties:
• Silicone elastomer with a range of viscosities
• Flexible (1 MPa Young’s modulus, typical polymer 1 GPa) and easy to mold.
• Elastomer, conforms to surface over large areas.
• Chemically inert, optically transparent
• Low surface energy: bonds reversibly (or permanent).
• Seals to flat and clean surfaces for micro-fluidic channels
• Durable (reusable), low thermal expansion
• Biocompatible (even used for food additive)
• Environmentally safe
• Best Resolution: 2-10 nm (for hard PMDS)
Dow Corning brand
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11. Replica Moulding
• In replica moulding the original master is not required to be a
negative of the final piece. After producing a PDMS stamp of
the original master, the stamp then is used as a secondary
master and another stamp made from it. In this manner the
original master cannot be damaged or degraded as multiple
copies are made.

12. Replica molding (REM)
Xia & Whitesides et al, Advanced Materials, 1997, 9, 147
PU = polyurethane
It is similar to UV-curing nanoimprint lithography
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13. Replica molding (REM)
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14. Microcontact Printing

15. Microcontact Printing
• In microcontact printing the stamp is ‘inked’ with selected
chemicals, normally alkanethiols. The stamp is then pressed
onto the substrate and removed leaving a 1 molecule thick
layer with features down to 300 nm.
• Only SF that is capable of generating chemical patterns on a
surface and thus it is distict than other methods.
• Micro-contact printing (μCp) utilizes the essential physics of
silanization in conjugation with the basic concepts of micro
molding to generate ordered regions of different wettability.

16. • In silanization, the surface active silane molecules self
assemble on a substrate, forming a self assembled monolayer
(SAM).
• The presence of a ligand (Y(CH2)nX), which is reactive
toward the surface ensures the attachment of the silane
molecules to the substrate.
• The success of μCp, particularly the dimension control and
sharpness of the features depends on the rapid reaction and
attachment of the surface active molecules to the substrate.
• The autophobicity of the resulting SAM layer is also
important as it prevents the spreading of the silane molecules
on the substrate, preventing lateral distortion

17. • The intrinsic viscosity of the ink solution is also important, as
a low solution viscosity leads to flooding of the ink on the
surface.
• On the other hand, a very high viscosity hinders the transfer of
the molecules from the stamp to the substrate within a finite
contact time.
• Solvent evaporation rate is also a key parameter, as a very
rapidly evaporating solvent would make the ink too dry even
before evaporation and hinder the transfer of the molecules.
• In contrast, a very slow evaporating solvent may result in
smearing of the patterns and possible joining of the near by
structures.
• The typical contact time varies depending on the nature of the
substrate. For example, it is ~ 10 – 20 seconds when a gold
coated surface is imprinted with alkanethiol

18. Micro - contact printing (μCP)
100mm stamp20m
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19. Applying alkanethiols on stamp to form SAM
• Linear alkanethiols with various molecular weights
o 158 g/mol (dodecanethiol, DDT)
o 258 g/mol (hexadecanethiol, HDT)
o 314 g/mol (eicosanethiol, ECT)
in ethanol
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20. Substrate Molecules
Popular “ink” molecules
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21. Micromoulding in Capillaries (MIMIC)
• The stamp can be brought into contact with a solid substrate
and capillary action used to add a polymer material to the
channels. The polymer is cured and the stamp removed leaving
a pattern with features as small as 1 µm.

23. Uses capillary forces to fill the gaps between substrate and PDMS master.
1. The PDMS master is pressed tightly on a planar substrate.
2. Elastic PDMS seals off walls and creates capillary channels.
3. A drop of liquid prepolymer is placed at the ends of these channels and
fills them automatically due to capillary force.
4. PDMS can absorb the solvent, which creates a partial vacuum inside the
PDMS cavity and helps to draw in liquid polymer.
5. Cure and peel of the PDMS master.
Micro-molding in capillary (MIMIC)
Liquid pre-
polymer
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24. Solvent-Assisted Microcontact
Moulding (SAMIM)

25. Solvent-Assisted Microcontact Moulding
(SAMIM)
• In Solvent-Assisted Microcontact Moulding (SAMIM) the
stamp is coated with a solvent and then placed on a polymer
photoresist matched to the solvent. The solvent causes the
polymer to swell and spread into the features of the stamp. The
resulting photoresist.
• SAMIM is a Soft Lithography technique that combined the
essential components of both CFL and MIMIC.
• It requires the existence of a film coated on the substrate as
well as the capillarity driven flow is in the vertical direction
along the side walls of the stamp patterns.

26. • the stamp is first pre-soaked in a solvent of the polymer to be
patterned.
• Then the wet stamp is then brought in conformal contact with
a thin film of the polymer which is desired to be patterned
• As the stamp is wet, the solvent molecules carried along with
stamp come in contact with the polymer molecules of the film
and locally dissolve them. This results in significant reduction
in viscosity of the polymer.
• Beyond this stage, the polymer solution climbs up along the
walls of the stamp features due to capillary action, resulting in
mold filling and pattern replication. The schematic of the
process is shown in figure.

27. Solvent assisted microcontact molding (SAMIM)
(solvent assisted imprinting)
Kim & Whitesides et al, Advanced Materials, 1997, 9,651.
Substrates
Silicon
Glass
Polymer
Shipley 1805 Photoresist (500nm)
3% PMMA (70nm)
Solvent
SU-8 (ethanol)
Shipley 1805 photoresist (ethanol)
3% PMMA (acetone)
Uses a solvent to wet the PDMS stamp and soften the structure polymer.
Dissipate and evaporate the solvent through PDMS.
(PDMS stamp can absorb the solvent because of the solvent permeability of PDMS.)
The molded polymer structure becomes solidified in a
few minutes after evaporation of solvent ,while the
stamp is still in conformable contact with the substrate.
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28. SAMIM of PMMA in acetone
Au structure fabrication
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29. Advantages:
• Convenient and low cost
• Rapid prototyping
• Deformation of PDMS provides route to
complex patterns
• No optical diffraction limit
• Non-planar or curved surfaces
• Generation of 3D -structures
• Control over surface chemistry
• A broad range of materials
• Applicable to manufacturing
• Patterning over large areas
Disadvantages:
• Distortion of patterns
• Poor alignment
• Defects and their densities
• μCP can only be applied to a number
of surfaces
• MIMIC is a relatively slow process
Soft lithography: advantages and disadvantages
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30. References
• Younan Xia and George M. Whitesides, Annu. Rev. Mater.
Sci. 1998. 28:153–84
• nptel.ac.in/courses/103105065/M13l18.pdf
• https://ece.uwaterloo.ca/~bcui/content/NE%20353/17%20Soft
%20lithography.pptx

31. THANK YOU