Microcontact Printing (Soft Lithography)

1
Microcontact Printing (Soft Lithography)
2
Outline

• Motivation
• History
• Schematic
• Procedure
• Photomask
• Snowflake
• "I Love NanoLab"
• OU interlock logo
• Stamp Master
• The stamp
• Chemistry of the Process
• Transferring the Pattern
• Etching the Substrate
• Examples

3
Motivation

• Production of silicon integrated circuits
• Patterning biological macromolecules, organic and
  inorganic salts, colloidal materials, conducting
  polymers, polymer beads
• The figure to the right is a patterned rod

Image courtesy of Jackman et al, Science 1998.
vol. 280(5372) pp 2089-2091
4
Motivation Novel Example

• Transferring micrometer size pattern onto various
  surfaces
• Surface does not have to be flat (as opposed to
  photolithography)
• The figure to the right is an example of how one
  would transfer patterns to a cylindrical rod

Image courtesy of Jackman et al, Science 1998.
vol. 280(5372) pp 2089-2091
5
History

• Both lithography and stamp printing have been
  around for centuries
• It was the combination of the two that gave rise
  to microcontact printing
• In 1993, Amit Kumar and George M. Whitesides
  developed microcontact printing at Harvard
  University
• Subsequent methods of soft lithography have since
  been explored
• In 1996 IBM began research on improving optical
  lithography in order to enhance the precision of
  the printing process

6
Process Schematic
A prepolymer (2) covering the master (1) is cured
by heat or light and demolded to form an
elastomeric stamp (3), which is inked by
immersion (4) or with an ink pad (5) and printed
onto the substrate (6), forming a self-assembled
monolayer, which is transferred into the
substrate by a selective etch. Scanning electron
micrographs show the master, image of the stamp,
and the printed and etched pattern.
Image taken from http//www.aip.org/tip/INPHFA/vol
-8/iss-4/p16.pdf
7
Photomask
opaque

• Create desired pattern on computer (AutoCAD,
  Adobe Illustrator, or your favorite graphics
  program)
• Print onto transparency
• Further reduction of size can done
  photographically
• Pattern must be in solid black and white and
  printed using an opaque ink, which is determined
  by the photoresist

8
About the Photomask Snowflake

• The snowflakes in the photomask are made such
  that each subsequent one was half the size of the
  previous one
• Measurements were made regarding snowflake size
  as well as distance between the snowflakes
• These measurements were used to evaluate how well
  the pattern transfer occurred
• The depth of the stamp was determined (determined
  by the master)
• If the stamp is too shallow, there can be contact
  in between the features
• If the stamp is too deep, the features can be
  distorted

0.629 mm
11.3 mm
9.175 mm
From Whitesides et. al paper
9
About the Photomask "I Love Nanolab"

• In this photomask, I Love Nanolab the largest
  text (1) are 3mm by 21.3 mm. Each subsequent
  set is half the size of the previous.
• The smallest size (8) is 128 times smaller than
  the biggest size, and is 23.4µm X 166.4µm

21.3 mm
3 mm
Dont YOU love NanoLab?
10
About the Photomask OU Logo

• In this photomask, the largest OU logo is 10 mm
  wide and 13.7 mm long.

10 mm
13.7 mm
11
Stamp Master (mold)

• Done on glass slide using the mask as the source
  of the pattern
• Apply photoresist (10-20 µm)
• (MicroChem SU-8 2010) Spin coat for even
  distribution
• Cover Sample with Mask and Expose to UV light
• The resist will harden upon exposure (negative
  resist)
• Soft-Baking
• Put on hotplate, or oven for 3 minutes
• Develop
• Unexposed resist will be dissolved, while the
  exposed resist remains
• Wash Master and clean up

12
The Stamp

• Clean master sample and place in walled container
  (disposable petri dish).
• Pour silicone resin evenly over master
• The silicone resin is liquid polydimethylsiloxane
  PDMS
• Bake the PDMS to solidify (65C for 15-20h).
• Remove cured stamp from master and wash both in
  EtOH

13
Chemistry of the Process

• Stamp is inked with a thiol (R-S-H)
• The thiol ink pattern is stamped onto the metal
  surface
• The thiol ink works as an etch resist
• Etching removes metal on uninked areas
• A thiol ink is 2mM solution of hexadecanethiol
• The substrate is a Pd or Au thin film evaporated
  on polished Si wafer

14
Transferring the Pattern to a Thin Metal Film

• Wash both substrate (the metal) and the stamp in
  EtOH
• Using Q-Tip, coat the stamp with the thiol
  solution and dry it
• dry under N2 stream for 30sec
• Bring stamp into contact with substrate for 10
  sec., remove stamp and dry substrate under N2

15
Useful Chemistry

• The sulfur atoms bonds to the metal (Pd or Au)
• The carbon chains of the thiol will then align
  with each other to create a hydrophobic SAM
  (self-assembling monolayer)
• The monolayer acts as a protective coating
  against the etchant
• The best results are often obtained by the
  longest chains because they create a larger and
  therefore better barrier

16
Etching the Substrate

• Immerse substrate in diluted (13) etchant for
  lt30 sec (Pd Etchant, type TFP in CYANIDE!!! be
  careful, its highly toxic!)
• The time here is very important!!! If you leave
  it too long in the etch, you will most likely
  remove ALL the Pd, if its not long enough, then
  your pattern will not be fully developed.
• Remove from etchant and immediately quench it in
  deionized water, rinse and dry.

17
Good and Bad Examples
Black lines drawn to mark the edge of the
snowflakes