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In recent years,
interferometers and confocal imaging profilers have been competing fiercely to
conquer the non-contact surface metrology market. They are both capable of
accurately and reliably measuring surface topographies on the scale of
millimeters to nanometers. Today, ST Instruments is offering a breakthrough in
non-contact optical profiling: a new dual-technology sensor head combining both
confocal and interferometry techniques, which outperforms all existing systems
due to its unique combination of techniques. In addition to its compact and
robust design, the multi-head technology is a complete tool that is ideal for
obtaining a fast, non-invasive assessment of the micro- and nano-geometry of
technical surfaces, in multiple configurations: from the standard set-up for
R&D and quality inspection laboratories to the manipulator or robot driven
system for online process controls. |
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| The
basic set up of the illumination hardware is shown in the figure above.
The light source is a high power LED, which is emitting at a peak
wavelength of 480 nm with a lambertian emission pattern. The light beam
coming from the LED is collimated before reaching a polarizing beam
splitter cube (PBS). The resulting polarized beam strikes the
ferroelectric liquid-crystal-on silicon (F-LCOS) microdisplay, which is
placed on the field diaphragm position of the optical epi-illuminator set
up. The F-LCOS is the key active device of the sensor head and the
information transferred to the microdisplay will be imaged on the surface
of the sample by the f/100mm field lens and a CFI60 infinity-corrected
microscope objective. In this standard episcopic arrangement the surface
of the illuminated sample will be also imaged on the CCD array. When a
white image (i.e. full-on state for all pixels of the LCOS) is displayed,
the sample is illuminated in the whole field of view as in a standard
microscope. Conventional microscope objectives will provide standard
microscope imaging, whereas interference objectives will provide
interference imaging. In order to obtain confocal images a binary pattern
is displayed in the LCOS and imaged onto the surface of the sample. The
optical set up provides the required matching between pixels of the LCOS
and pixels of the CCD array, which in turn behave as confocal apertures.
The axial response at each pixel position is calculated from the CCD
frames using the appropriate algorithms. Because only one or a few points
of the surface are illuminated at the same time, in-plane raster scanning
is necessary to build the axial response (i.e. the confocal image) at all
pixel locations. In order to carry out the raster scan, a sequence of
binary patterns is displayed in the LCOS until the complete field of view
has been completely filled. As a result, confocal imaging is slower in
comparison with standard or interference imaging because a sequence of CCD
frames must be acquired to build one single confocal image.
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| The complete epi-illuminator
and the revolving nosepiece with the objectives are mounted on a vertical
motor-driven linear stage. This allows the sensor head to scan the sample
vertically in steps so that every point on the surface passes through the
focus. The height of the surface at each pixel location is found by using
the profiling technologies. The user simply has to choose the right
objective and select the appropriate acquisition and data processing
algorithms (confocal, PSI and VSI).
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The
Multi-head is able to work in the following modes:
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| The compact design
of the sensor head breaks the trend in traditional microscope appearance
for most optical profilers and makes many different configurations
possible. Anything from the simple stand set-up for R&D and quality
inspection laboratories to the most complete and sophisticated
arrangements for on-line process control, such as the robot-driven set up
are now possible. |
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