Determining Gold Coating Thickness in Nanolithography

1
Determining Gold Coating Thickness in
Nanolithography

• Ayesha Nzeribe1,2 with Raven Anglin2 and Weilie
  Zhou2

1 Isidore Newman High School 2 U.N.O. Advanced
Materials Research Institute
2
Abstract

• An understanding and appreciation of the
  equipment with which one works is critical
  because it allows for flexibility and scientific
  creativity. Equally important is the the
  equipment's reliability and accuracy in
  applications. This project was focused on
  determining the accuracy the Cressington 308R
  coating system as a whole and the thickness
  monitor in particular. Their use is critical in
  many applications especially FESEM viewing and
  Nanolithography. By creating cross sections of
  gold coated wafers, it was found that the
  thickness monitor gave inaccurate readings
  regarding the thickness of gold deposited by the
  coating system displaying thicknesses at less
  than half their actual value.

3
Introduction

• The process of nanolithography is one that is
  growing in importance and popularity. In this
  process, a pattern is exposed onto a
  polymer-coated silicon sample which is then
  developed in such a way that the exposed pieces
  are dissolved. A gold coating is put over the
  entire sample and the rest of the polymer is
  lifted off. It is the gold that ultimately forms
  the features of the pattern. This project dealt
  with determining optimum conditions and testing
  the limits of those conditions so that laboratory
  standards in gold thickness could be set.

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Introduction to the FESEM

• A Scanning Electron Microscope uses a beam of
  electrons generated by heating a filament and
  directing it through a series of electromagnetic
  lenses.
• A Field Emission Cathode Placed in a Scanning
  Electron Microscope will yield higher resolution
  images with fewer distortions than a regular SEM.

Beam column
Chamber
Beam Blanker
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The FESEM Beam Column
Filament Tip
Aperture Changer
Condenser Lens
Beam Booster
InLens Detector
Illuminating Lens
Scan Coils
Final Lens
Sample
http//www.nanophys.kth.se/nanophys/facilities/nfl
/manual/sem-adjust/semadj2.html
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How the FESEM Works

• The electron beam of the SEM is generated by the
  heating of a filament in the highest reach of the
  gun column. The free electrons are then drawn
  down the column by the positively charged anode.
  Electromagnetic lenses first condense then focus
  the beam as it travels down the column with the
  aperture controlling the beam width. Energized
  scanning coils draw the beam over the surface of
  the field at a set rate. The scan coils in the
  SEM are syn-

Lens
Multiple Secondary Detector
Backscatter Detector
Different detectors pick up different electrons
depending on the voltage after deflection.
Secondary detectors are used for surface features
and the backscatter detector is used to determine
sample composition.
chronized with the scan coils around the cathode
ray tube in the screen. As the electron beam is
passed over the sample, different detectors pick
up the deflected electrons and convert them into
voltage. The changes in voltage correspond to
changes in the brightness of the pixels.
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FESEM Images
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The Purpose of Coating
To be viewed by the FESEM a sample must be
conductive. It is, therefore, in most cases,
covered a layer of gold. Metallic Samples may be
simply placed in the FESEM. The LEO 1530 VP does
allows for organic samples so the sputter coater
is used primarily in connection with
nanolithography.
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Intro to Gold Sputter Coating
Sputtering consists of the coating material, in
this case gold, being literally hurled at a
sample. A stream of Argon plasma is directed at
a target of gold foil dislodging, or sputtering,
gold particles. The plasma stream continues and
the gold is distributed relatively uniformly on
the sample.
http//www.csc.uvic.ca/mserra/Fab1/html/sputterin
g.html
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Materials and Methods

• Gold Coating Thickness
• To determine the reliability of the thickness
  monitor and the consistency of the gold plasma
  stream, a Cressington 308R Coating System was
  used to coat 15 nm of gold on to five silicon
  wafer samples approximately three quarters of a
  centimeter square. A cross section of each sample
  was then viewed under the TEM to see the
  thickness of the gold on the wafer.

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Gold Sputter Set-Up
Samples
Thickness Monitor
1
2
C
4
5
Stage
4 in
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Gold Wafer 1 TEM Images
20 nm
50 nm
Difficulties with sample preparation prevented
the TEM viewing of all but one wafer.
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Use of FESEM in Nanolithography
The NPGS program is used to design a pattern
and a run file that is then sent to the FESEM,
which exposes the sample according to the
specifications set in the run file. The results
are viewed under the microscope monitors.
1. NPGS Command
3. Microscope Monitors
2. FESEM
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E-Beam Nanolithography

• Accurate measurement of gold thickness on the
  silicon wafers affects the final results after
  lift-off of e-beam written patterns.
• The upside down Chinese character happiness,
  which was designed using dots to form the image,
  means happiness comes.

1 micron
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Conclusions

• As seen in the TEM images, although the thickness
  monitor was reading fifteen nanometers of
  deposited gold the actual thickness was double
  that amount.
• From the cross section one can say that the
  actual thickness of gold on the silicon wafer is
  about 2.15 times the value read on the thickness
  monitor.

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References and Acknowledgments

• National Science Foundation (EPS-0092001)
• Louisiana Board of Regents (NSF/LEQSF(2001-04)-RII
  -03)
• Dr. Yuxi Chen and Yunhan Wang
• Trevor Kennedy
• Al Lysse