Imaging Ellipsometry

1
Imaging Ellipsometry

• Tudor Jenkins, Matt Gunn
• Institute of Mathematical and Physical Sciences,
• University of Wales,
• Aberystwyth.

2
What is ellipsometry?

• Technique that determines the change in
  polarization state of light reflected from a
  sample.
• Typically used for characterizing thin films.
• Used in a wide range of applications.
• Non contact, non destructive method.

3
(No Transcript)
4
What does an ellipsometer measure?

• The polarization change of light reflected from a
  sample in terms of two parameters ? and Y
• tanY measures the ratio of the modulus of the
  amplitude reflection ratio
• The phase difference between p- and s-polarised
  reflected light is given by D

5
What can ellipsometry tell us about a sample?

• Taken on there own values of ? and Y tell us
  little about a sample.
• We take the measured values of ? and Y, typically
  as a function of wavelength and angle of
  incidence (Variable Angle Spectroscopic
  Ellipsometry SOPRA GESP5 VASE in Aberystwyth).
• An optical model is then built using as much
  information about the sample as possible.
  Correctness of derived material parameters
  depends on model

6
Advantages of Ellipsometry

• It measures the ratio of two values so is highly
  accurate and reproducible, does not need a
  reference sample, and is not so susceptible to
  light source fluctuation
• Since it measures phase, it is highly sensitive
  to the presence of ultrathin films (down to
  submonolayer coverage).
• It provides two pieces of data at each
  wavelength. More film properties can be
  determined.

7
Sensitivity to thin film

• Bare Si

Bare Si
Si 1nm oxide
Data taken with SOPRA GESP5 spectrocopic
ellipsometer. Aber-Bangor collaboration on
CdTe/Si and CdS/CdTe/Si structures
(S.Irvine/V.Barrioz)
8
Imaging Ellipsometry

• Imaging ellipsometry combines the power of
  ellipsometry with microscopy, having a spatial
  resolution in the micron range.
• The advanced spatial resolution expands
  ellipsometry into new areas of bioanalytics and
  microelectronics.

9
Imaging Ellipsometry

• Because of the large angles of incidence
  needed in ellipsometry, images are out of focus
  and distorted. We have successfully overcome this
  using the Scheimpflug techique.

10
Imaging Ellipsometry
Conventional camera
Scheimpflug camera
11
Imaging Ellipsometry

• We employ off-null ellipsometry The
  ellipsometer is nulled on a uniform substrate and
  as the surface is modified e.g. by the deposition
  of a thin film, the ellipsometer is disturbed
  from the null condition and a signal is detected
  by the CCD camera.This allows dynamic changes to
  be observed.

12
Imaging Ellipsometry

13
Where are we?

• We have built a proof-of-principle instrument.
• Currently, single wavelength (He-Ne laser)
• Spatial resolution eight microns
• CCD camera limited to 1 frame in 4 seconds
• Set-up is manual
• Interest from several companies via Aber CCS
• Provisional US patent

14
Where are we?

15
The next phase

• The mechanical system needs upgrading
• Automatic set-up
• Better quality optical components and
  particularly a wide angle lens on the camera side
• Move to a spectroscopic capability hence
• (i) Colour CCD camera
• (ii) Laser sources
• We can achieve sub-4 micron spatial resolution.
  Full data set as fast as camera.

16
Imaging Ellipsometry