Transcript PowerPoint

Divergent Illumination Optical Testing Device
M. Fried1, Z. Horváth2, G. Juhász1, T. Mosoni1, P. Petrik1
1Research Institute for Technical Physics and Materials Science
2Research Institute for Solid State Physics and Optics,
H-1525 Budapest, POB 49, Hungary
A new technique* and device for fast testing of polarization state
changes after reflection from large surfaces is presented. Conventional
ellipsometers and other optical methods measure at one oblique angle
and a single point of the sample making these methods relatively slow
for mapping purposes. The new technique uses divergent illumination
from a point-like light-source and after reflection from the target the
image is acquired on a CCD-camera giving multiple-angle-ofincidence and mapping information.
*Patent pending
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The prototype is built in the form of a wide-angle singlewavelength ellipsometer using film polarizers. The point-like source
is an optical fiber coupled with a diode-laser and the elements are moved
by simple stepping motors. (1) point-like source (2) light-conus (3)
polarizer (4) sample on a moving stage (5) analyzer (6) CCD-camera
The axis of the light-cone is at 65o relative to the normal of the
sample surface and the opening angle of the beam is more then 14
degrees, determining the range of incidence angles that can be used for
measurement
With this arrangement, the angle of incidence is different for different
positions of the light spot on the sample surface. Using a motorised X-Y
moving stage, the sample surface can be scanned relative to the lightspot, yielding a huge amount of measured ellipsometric data
(amplitude ratio – tan y and phase shift – cos D) corresponding to the
different angles. Because of the linear translation one can measure the
polarisation state at all the points and at every angle inside the cone.
The collected data are processed by an additional computer that
provides real-time polarisation state parameter maps (and
thicknesses and/or refractive indices maps) over a large area of
the sample surface. The speed of the measuring system makes it suitable
for use even on production lines. The amount of data and the dynamics
of the CCD-detector can be optimized by grouping of the pixels.
As a demonstration, electrochemically etched porous silicon
layers were made with different porosity (refractive index) and
different thicknesses. Etched areas were limited by teflon O-ring
causing significant edge-effects.
We see now two possible enhancements to the prototype:
- A spectroscopic version where the light source (13) is a
continuos spectra white lamp (or LED) and the cone is reduced to a
line by a slit. In this case a grating (16) (or prism) deflects the light
into the perpendicular direction after the analyser (5) according to
the spectral content. In this case multiple-angle and spectral
information can be obtained simultaneously from the surface of the
sample if needed to test the complexity of the film structure.
- The other possibility is a multiwavelength version using more
than one discrete wavelength (for instance different laser-diodes) at
the entrance of the single optical fiber. After the analyser the
different colours can be split into different images detected by
discrete CCD-matrixes.
Prototype of a wide-angle multi-wavelength ellipsometer:
(1) point-like source (2) light-conus (3) polarizer (4) sample on a moving
stage (5) analyzer (6) CCD-camera (11) optical fiber (12) exit aperture
(13) multi-wavelength (white) light source (14) slit (15) optical axis (16)
grating (17) screen (18) forming optics