Soft lithography refers to a set of techniques for fabricating or replicating structures using elastomeric stamps, molds, and conformable photomasks. Some key techniques include microcontact printing, replica molding, micro-molding in capillary action, and microtransfer molding. These techniques use polydimethylsiloxane (PDMS) stamps or molds which are flexible, chemically inert, and biocompatible, allowing for low-cost and high-resolution patterning of materials for applications in microfluidics, biomedical devices, and more. Common challenges include issues with alignment and distortion due to the soft, flexible nature of PDMS.

1. “Soft” lithography
1. Soft lithography and PDMS.
2. Micro-contact printing.
3. Replica molding.
4. Micro-molding in capillary.
5. Micro-transfer molding/printing.
6. Solvent assisted microcontact molding.
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ECE 730: Fabrication in the nanoscale: principles, technology and applications
Instructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/
Textbook: Nanofabrication: principles, capabilities and limits, by Zheng Cui

2. 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 2

3. 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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4. PDMS surface treatment
Plasma oxidation
Air (10 min)
Upon treatment in oxygen plasma, PDMS seals
to itself, glass, silicon, silicon nitride, and some
plastic materials.
contact PDMS surfaces
Irreversible seal:
formation of covalent bonds
Biggest issue: it becomes hydrophobic quickly, very bad for micro-fluidic applications.
(liquid hard to get into the channels once it becomes hydrophobic)
PMDS is absolutely the most popular material for bio-medical lab-on-chip (microfluidic) applications,
but may not be suitable for commercial applications, which need chemically stable surface.
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5. PDMS problems: soft, low Young’s modulus
Shrinking makes accurate (< few
m) alignment very difficult.
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6. “Soft” lithography
1. Soft lithography and PDMS.
2. Micro-contact printing.
3. Replica molding.
4. Micro-molding in capillary.
5. Micro-transfer molding/printing.
6. Solvent assisted microcontact molding.
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7. Micro - contact printing (μCP)
100mm stamp
20m
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8. Test structures in Au, smallest dimension 400nm
Micro - contact printing (μCP)
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9. Micro - contact printing (μCP), with roller
Xia & Whitesides, Angew. Chem. Int. Ed. 1998, 37, 550-575.
A. Kumar & G. Whitesides, Applied Physics Lett. 1993
a. Printing on a planar surface with a
planar stamp.
b. Printing on a planar surface with a
rolling stamp
c. Printing on a non-planar surface
with a planar stamp
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10. Substrate Molecules
Popular “ink” molecules
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11. “Soft” lithography
1. Soft lithography and PDMS.
2. Micro-contact printing.
3. Replica molding.
4. Micro-molding in capillary.
5. Micro-transfer molding/printing.
6. Solvent assisted microcontact molding.
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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)
The patterning process has high fidelity, with little feature size loss.
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14. Replica molding (REM)
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15. “Soft” lithography
1. Soft lithography and PDMS.
2. Micro-contact printing.
3. Replica molding.
4. Micro-molding in capillary.
5. Micro-transfer molding/printing.
6. Solvent assisted microcontact molding.
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16. 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)
Line-width: 100nm
Liquid pre-
polymer
Nano-molding in
capillaries is possible 16

17. Micro-molding in capillary (MIMIC)
Kim & Whitesides et al, Nature, 1995, 376, 581
Xia, Y.; Whitesides, G. M. Ann. Rev. Mater. Sci 1998, 28, 153.
a: PU (polyurethane) on Si b: polyaniline c: ZrO2
d: polystryene colloids e+f: free standing PU
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18. “Soft” lithography
1. Soft lithography and PDMS.
2. Micro-contact printing.
3. Replica molding.
4. Micro-molding in capillary.
5. Micro-transfer molding/printing.
6. Solvent assisted microcontact molding.
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19. • Apply the liquid prepolymer
• Planarize the prepolymer
• Place the master on a planar substrate
• UV exposure or heating solidifies the
prepolymer that sticks to the substrate
Micro - transfer molding (μTM)
TM fabrication of a). one-layer microstructures;
b). three-layer polymer microstructures.
PDMS
Pre-polymer
Substrate
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20. Micro - transfer molding (μTM)
Zhao & Whitesides et al, Advanced Materials, 1996, 8, 837.
Microstructures fabricated using TM.
a) An SEM image of a fractured sample showing a
pattern of isolated stars of UV-cured polyurethane
(NOA 73) on Ag.
b) An array of parallel lines of spin-on glass on Si with an
aspect ratio (height/width) of 8.
c) A two-layer structure: isolated micro-cylinders (1.5m
in diameter) on 5m-wide lines, supported on a glass
cover slide.
d) A two-layer structure: a continuous web over a layer
of 5m-wide lines, supported on a glass cover slide.
e) A three-layer structure on a glass cover slide. The
layers of 4 m-wide lines are oriented at 60o from
each other.
Structures in c-e were made of heat-cured epoxy
(F109CLR).
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21. Metal transfer assisted nanolithography
a) SEM image of the transferred metal grating onto
PMMA layer with period of 700nm on SiO2 substrate.
b) Period 220nm on PET substrate.
c) After O2 RIE of b.
d) After metallization and lift-off process of c.
Guo, “Metal transfer assisted nanolithography on rigid and flexible substrates”, JVST B, 2008
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Glass transition temperature is 105oC

22. Metal transfer assisted nanolithography: line-width reduction
Schematic of shadow (angle) evaporation of metals to
reduce pattern line-width. The line-width is reduced by
the thickness of the metal on the PDMS grating sidewall.
SEM image of the metal grating on PET substrate after
line-width reduction. The inset is a zoom-in view
showing that the line-width was reduced to 50nm.
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23. “Soft” lithography
1. Soft lithography and PDMS.
2. Micro-contact printing.
3. Replica molding.
4. Micro-molding in capillary.
5. Micro-transfer molding/printing.
6. Solvent assisted microcontact molding.
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24. Solvent assisted microcontact molding (SAMIM)
(solvent assisted imprinting)
Kim & Whitesides et al, Advanced Materials, 1997, 9,651.
Substrates
Silicon
Glass
Flexible transparency
Polymer
SU-8 (1μm)
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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25. SAMIM of SU-8 in ethanol
SU-8
2m diameter
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26. SAMIM of PMMA in acetone
Au structure fabrication
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27. 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 registration/alignment
• Compatibility with IC processes
• 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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28. PDMS elastomer for mold making
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29. PDMS elastomer for mold making
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30. PDMS elastomer for mold making
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31. PDMS elastomer for mold making
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32. PDMS elastomer for mold making
University of Washington
(coat mold release agent)
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33. 33

34. Low temperature co-fired ceramic
processing
• The very rapid evolution of semiconductor IC technology has put immense
pressure on the packaging technologies.
• The new packages must have increasingly smaller footprints, simultaneously
offering improved performance in handling high frequency clocks, very large
number of pin-outs and capability to handle and dissipate large heat densities.
• In other realms there is a need for multilayer solution for miniaturizing the HF
circuits in GHz range, while the Micro Electro Mechanical System (MEMS)
devices need an one technology solution for packaging sensors / actuators
requiring electrical, optical, fluidic and mechanical ‘interconnects’.
• The Low Temperature Co-fired Ceramic (LTCC) Technology offers and
‘integrated’ solutions to these problems.

35. LTCC (Low Temperature Co-fired Ceramics) is a multi-layer glass ceramic
substrate which is Co-fired with low resistance metal conductors at low firing
temperature (less than 1000℃). It is sometimes referred to as "Glass Ceramics"
because its composition consists of glass and alumina.

36. Low temperature co-fired ceramic
processing

37. Low temperature co-fired ceramic
processing

38. LTCC Fabrication Process
• The process starts with via punching of each layer as per individual layer design.
• If the layer requires any open or buried cavity, it is prepared using the same machine.
• The alignment holes are also punched using via puncher.
• Via filling is done subsequently, using stencil printing.
• This is followed by screen printing of conductor pattern using usual screen printing process.
• Once individual layers are ready, those layers are stacked in sequence, and aligned
mechanically.
• The stacks are then sealed in a plastic bag are and laminated under prescribed temperature-
pressure cycle using isostatic laminator.
• The individual circuits are then singulated using cutter and fired using a special programmable
batch furnace.
• All the fabrication processes described above, are carried out in Class 10000 Clean Room.
• For preparing packages, there are two main post firing processes that remain at this stage. One,
processes for preparing BGA, and second, processes for sealing. We have chosen two possible
options for sealing processes, viz. Solder sealing and Seam sealing and three BGA
formation processes, viz. sphere attachment, stencil printing and electroplating. Following
is the brief description of sealing and BGA formation processes.

39. Solder sealing
Solder sealing is done using solder performs. This needs conducting and
solderable surfaces on either side. Further, interconnection from lid to base would
also require similar pads. As per present plan, these conducting surfaces would
be made using a co-fired Ag-Pd paste. Finally, solder sealing is done by reflow
after components are placed.
Seam Sealing
Seam sealing requires brazing of metallic parts to the ceramic substrate. Brazing
is a three step process that requires post-fire adhesion layer, post fire barrier layer,
brazing layer, each being individually printed and fired. The brazing layer is a low
temperature alloy, and is required to be fired with the metallic part. The BGA could
then be formed by one of the processes described below. It may be noted here
that final sealing process by seam sealing can be done only after components are
placed inside the base.
BGA Formation Processes
The three process options selected here have their own advantages and disadvantages. Sphere
attachment is done using pre-formed solder spheres which could be attached using another
solder paste or through reflow of the same BGA spheres. This process is simple, and cheap, but
has limitations in terms of minimum BGA size. The stencil printing process is also cheap, slightly
more process dependent but can make BGAs with sizes up to even 100µm but with relaxed
pitch. The electroplating process is having more process steps making it costlier, but is capable
of even sub-100µm BGA with equal pitch. Through the present project we are attempting an
exploration of each one of this process. The materials selection for each process has to be done
very carefully so as to ensure subsequently lower temperature steps.

40. Sphere Attachment
BGA by solder sphere attachment is done by directly placing micro-spheres of high-Pb
solder on conducting pads with high alignment accuracy. The solder pads are
dispensed with a viscous flux or attachment solder paste prior to this placement. Reflow
of these solder spheres or attachment solder paste would produce BGAs.
Stencil printing
For good adhesion, one requires Under Bump Metallurgy (UBM) or adhesion
/ barrier layers before solder deposition. High lead Sn-Pb Solder is deposited
by printing solder paste onto the Under Bump Metallurgy (UBM) by stencil
printing process and reflowing the solder paste to form a spherical solder
bump.
Electroplating
In this case the UBM consists typically of an evaporated adhesion layer that provides
conducting path for electroplating, followed by photolithography to open areas of BGA
pads. The BGA pads are then electroplated with adhesion enhancing and solder-barrier
layers. Plating of the high lead Sn-Pb solder alloy is then done over the UBM followed by
removal of resist and underlying evaporated seed layer. The final step is of reflow that
forms the solder bump.

41. LTCC Advantages
•Multilayer capability- means interconnect density limited only by the installed
process capabilities
•Compatibility with flip chip process
means capability to handle and redistribute large number of I/Os
•Capability to integrate passives
means appropriation of additional components within the package which otherwise are
accommodated out of the package
•HF capability up to millimeter wave…
means integration of front-end modules in communication devices
•Integration of Si and other technology devices simultaneously
Flexibility to choose integration scheme
•but the system designer has to work in tandem with chip manufacturers
Ceramic process brings high reliability
•Capability of handling electrical, optical, fluidic and mechanical interconnects in
and out of the package
Amenable to automated wafer assembly; useful tool for fast prototyping
•TCE matching with semiconductor device materials
High working temperature

42. Application examples
- Nozzles that require conductors.
• Nozzle for atmospheric pressure plasma cleaning equipment.
It is possible to appropriately design the internal conductor pattern shape and the
distance between conductors depending on the type of gas used. The shape for
rectification at the nozzle tip is also available.
- Substrate utilizing the hollow as a flow path.
• Sensor unit that analyzes gas and liquid components.
• Substrate that resists high temperatures by the flow of cooling gas and liquid through
it.
- Substrate with special characteristics, created by filling the hollow part with a
different material.
• Substrate with excellent high frequency characteristics.
• Substrate filled with high shielding material to reduce signal interference.
• High heat dissipation substrate filled with high heat dissipation material.
- Built-in integrated component substrate for miniaturization and space saving.
(Note: The mounting part requires an opening)
• Component mounting, or built-in cavity substrate.
- Substrate that requires partial thermal insulation due to product configuration.
• Substrate where the effective temperature range differs between the upper and lower