Microcontact Printing (Soft Lithography) - PowerPoint PPT Presentation

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Microcontact Printing (Soft Lithography)

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Microcontact Printing (Soft Lithography) Outline Motivation History Schematic Procedure Photomask Snowflake – PowerPoint PPT presentation

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Title: 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
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