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Moir fringe method for the measurement of distortions of hotembossed polymeric substrates Hayden Tay

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Title: Moir fringe method for the measurement of distortions of hotembossed polymeric substrates Hayden Tay


1
Moiré fringe method for the measurement of
distortions of hot-embossed polymeric
substratesHayden Taylor 3, Xu Zhiguang 1, 2, 4,
Li Shiguang 1, 2, Kamal Youcef-Toumi 4, Yoon
Soon Fatt 5, and Duane Boning 32 July 20081
Singapore-MIT Alliance2 Singapore Institute of
Manufacturing Technology3 Microsystems
Technology Laboratories, MIT4 Department of
Mechanical Engineering, MIT5 School of
Electrical and Electronic Engineering, NTU
2
Outline
  • Why do distortions of hot-embossed parts need to
    be measured?
  • Why use the moiré principle to do so?
  • Advantages of our method
  • Image processing procedures used
  • Outlook

3
Why measure distortions of hot-embossed parts?
  • Hot embossing of thermoplastic polymers an
    attractive manufacturing process for inexpensive
    microfluidic devices
  • Two main types of dimensional faults with the
    output
  • Out-of-plane distortions (from demolding while
    soft)
  • In-plane distortions (from post-demolding thermal
    contraction)
  • Focus here on in-plane distortions
  • Harder to eliminate from the process
  • Present challenges when registering multiple
    layers

4
Why use the moiré principle?
  • Has the potential to be effective for determining
    small distortions over large areas
  • Has already been demonstrated for quantifying
    distortions of micropatterned layers of the
    elastomer PDMS 1
  • Patterned areas 1 cm2 were measured
  • Microscopic angular positioning of samples was
    required
  • Processing of moiré patterns required extensive
    user input
  • Our aim is to demonstrate a simpler, more
    automated procedure that works over larger
    substrate areas.
  • 1 John A. Rogers, et al. Quantifying
    distortions in soft lithography, J. Vac. Sci.
    Technol. B, 1998, 16(1) 88-97
  • polydimethylsiloxane

5
Advantages of our method
  • Simplicity the only apparatus required is a
    flatbed scanner and a printed, square reference
    grid. The part is embossed using a stamp with the
    same reference pattern.
  • Samples may be positioned by hand within an
    allowable angular range of a few degrees
  • Scans are made at a variety of part-reference
    angles, and precise orientations are determined
    from the captured images

Scanners cover
Reference grid(printed acetate)
Embossed part
Scanners platen
Light from scanner
6
Information obtained from one scan
where
10 mm
Schematic of scanned image
Actual scanned image
dM, and are specific to each image
captured
7
Fitting global part shrinkage using multiple scans
One part multiple scans
dM
10 mm
  • For this part, global average post-demolding
    contraction of 0.4 is extracted
  • Consistent with known material properties
  • Could increase confidence in estimate by taking
    more scans

8
Processing of scanned images
  • Strength of the tonal contrast of moire fringes
    varies among (and within) parts depends on the
    relief and shape of the embossed topography.
  • Our image-processing strategy depends on the
    strength of contrast

Harder casesweak contrast
Easier casesstrong contrast
9
Strong fringe contrast
Steps for determining the positions of
intersections of moiré fringes for simple cases
with strong image contrast
  • Signal filtering
  • Image rotation
  • Image sharpness
  • Fine rotation, continuous region separation, and
    centroid point judgment

10
Weak fringe contrast
Steps for determining the positions of
intersections of moiré fringes for hard cases
with weak image contrast one additional step
  • Signal filtering
  • Image rotation
  • Image sharpness
  • Correlation calculation between the typical
    extracted cross and our image
  • Image sharpness, fine rotation, continuous region
    separation, and centroid point judgment

11
Processing steps in detail (weak contrast)
Original image (19442629 pixels)
12
Processing steps in detail (weak contrast)
Image after signal filtering (step 1)
13
Processing steps in detail (weak contrast)
Image after rotation (step 2)
Fringe orientations identified
14
Processing steps in detail (weak contrast)
Image after sharpness filter (step 3)
15
Processing steps in detail (weak contrast)
Cross points searching correlation calculation
between one extracted cross and our image (step 4)
16
Processing steps in detail (weak contrast)
Image sharpness, fine rotation, continuous region
separation, and centroid point judgment, to get
the positions of the fringe crosses (step 5)
17
Processing steps in detail (weak contrast)
Obtaining the best fitted grid by rotating,
translating and shrinking
Fringe cross
??
Fitted grid
extracted dM
18
Outlook
  • A promising method is described for detecting
    post-demolding distortions in hot-embossed
    substrates
  • The principle is demonstrated of using multiple
    scanned images to extract global-average part
    distortion
  • Demonstrated with a 50 mm square sample
  • A high level of automation has so far been
    achieved in image interpretation
  • Identification of fringe locations is mostly
    automatic for low-contrast images,
    identification of one fringe location is
    user-assisted
  • Identification of sample and reference grid edges
    is user-assisted
  • Have yet to confirm whether local deviations of
    the fringes from their fitted grid positions
    correspond to real part distortions.

19
Acknowledgements
  • Funding the Singapore-MIT Alliance
  • Use of the Microsystems Technology Laboratories
    at MIT
  • Matthew Dirckx
  • Brian Anthony
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