The properties and characteristics of the surface of
materials are becoming increasingly important in the development of new products and
materials. Critical dimensions on a micron scale and below can affect the performance of a
device. Properties such as surface roughness and topography can affect the appearance,
performance, functionality or manufacturability of a product. The NCLA provides a
measurement service which can quantify these properties over a wide range of sample sizes
and geometries. The NCLA currently utilises two techniques to conduct surface
analysis:
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| Scanning White-Light Interferometry
The Newview 100 surface profiler is a scanning white-light
interferometer, providing surface topography and film thickness measurements to an
accuracy of 0.1 nm. The surface profiler has a variable field of view from 6 x 4.5 mm down
to 0.18 x 0.14 mm, it has a vertical range of 100µm and it
can measure roughnesses and step -heights in the range 0.1 nm up to 10’s of microns.
One of the key advantages of optical profilometry over mechanical stylus measurements is
the fact that the instrument can make measurements on soft, easily deformed materials such
as polymers.
Among the many applications of this instrument is the measurement of roughness on
highly polished surfaces such as silicon wafers or magnetic disc reader heads. The figure
below shows the surface of a highly polished silicon wafer.
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Analysis of DLC film
click for full image (large file) |
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Analysis of Excimer machined polymer material
click for full image (large file) |
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Analysis of 5p coin
click for full image (large file) |
The profilometer can also be used in process optimisation, for example, kerf reduction in
laser hole drilling or optimisation of mechanical polishing processes.
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The diagram illustrates the principle of
operation. White light is split in a special interference microscope objective, where part
of the light travels to a spot on the sample of interest, and the remainder is directed to
a reference mirror. When the two parts recombine, bright and dark lines or, interference
fringes, appear at the point of focus. A piezoelectric stack moves the objective in the
vertical direction through a scan of between 5 and 100 m m.
Fourier transform algorithms convert the recorded data into surface topography information
which is graphically displayed. |
Click to see use of Profilometer in the
analysis of Electrode Patterning for electronic and medical
products. |
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Atomic Force Microscopy (AFM) |
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with a Digital Instruments Contact-Mode Nanoscope II Atomic Force Microscope (AFM). In the
Atomic Force Microscope the probing tip is attached to a cantilever-type spring. In
response to the force between tip and sample the cantilever is deflected. Images are taken
by scanning the sample relative to the probing tip and digitising the deflection of the
lever or the z-movement of the piezo as a function of the lateral position x,y. Contact
mode AFM operates by scanning a tip attached to the end of a cantilever across the sample
surface while monitoring the change in cantilever deflection with a split photodiode
detector. The AFM generates a real-space topographic image of a surface with both high
lateral and high vertical resolution. Operation can take place in ambient or liquid
environments. The operation of the AFM in a liquid environments is facilitated by using a
liquid cell. Imaging in liquid can be advantageous for biological samples. |
| The AFM has proven to be an extremely useful
characterisation instrument for imaging a variety of sample types. Examples of recent
experiments are provided below. |
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Gold coated calibration template |
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Thiol layer on a gold substrate |
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Etched pits in silicon |
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Quantum dot structures |
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