OPTICAL MONITORS
Our optical thickness monitors operate on substrates
on rotating workholders, ensuring accurate and repeatable measurements of thin films
during deposition
Our optical thickness monitors use a
single wavelength transmission or reflection method as well as sophisticated data modeling
and filter techniques to determine the thickness of a deposited film.
The IL540 can be equipped with either a white
light source and an automated monochromator, or with a narrow band IR laser for DWDM
applications. All DWDM optical monitors come with a detector module, industrial computer,
color monitor, keyboard, mouse, and control software. The IL540 also provides for data
capture on rapidly rotating substrates.
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Specifications
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Layout
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Design Features
With a white light source, the IL540 is suitable for a variety of box
coaters and smaller vacuum chambers. Layer thicknesses are automatically terminated and
the instrument can deal with complex multilayer structures. The standard wavelength range
is 350 to 800 nm. The typical mechanical arrangement is for reflectance, but the source
can be moved to a third port to provide transmitted light. Layer thickness termination
accuracy is under 1 percent of the wavelength in use.
With an IR laser source centered on 1550 nm, the IL540 is
the unsurpassed solution for accurate, in-process measurement of film thickness during the
production of DWDM optical filters. It measures filters with passbands as narrow as 25
GHz. The following design features make this possible:
How It Works
The IL540 operates on the principle of interference between light that is
reflected/transmitted at multiple interfaces in a thin film multilayer. If light of a
single wavelength is reflected off (or transmitted through) a substrate with an overlying
coating of thin transparent film that has a thickness of the same order of magnitude as
the wavelength of light, then the detected signal shows an oscillatory behavior, which is
periodic with the thickness of the thin film. The periodic nature of this signal is due to
the phenomenon of interference. At one point, the waves reflected from the film/substrate
and ambient/film interface will be in step with one another and interfere constructively.
At a different thickness, they will be completely out of step and a null signal will
result.
The difference in distance that the light has to travel between the constructive and
destructive case is half a wavelength. Since the optical beam passes through the coating
twice, the distance on the trace from maximum to minimum is traditionally referred to as a
quarter
wave thickness.
The performance of an optical coating depends on the
phenomenon of interference within a simple single layer or a complex multiple component
system of thin films. The crucial parameter for each film is how long the light will take
to go through it, given by nx/c where n=refractive index, x=physical film thickness, and
c=speed of light within a vaccuum. The form of this equation prompted the term optical
thickness for its numerator, nx, and it is the measurement and control over this
parameter that is crucial to a coating's performance.
The crucial measurement of optical thickness depends on
refractive index as much as it does on physical thickness. Experienced coating engineers
will recognize that while small changes in deposition conditions can have a significant
effect on the film's physical properties, such as density, they can have quite an enormous
effect on refractive index. If you are aiming for high specification films, it is simply
not enough to assume that "n" is constant — you have to monitor optical
thickness with an optical technique.
Benefits
The
key to successful thin film optical monitoring is
to design hardware that is immune to noise and
interference, and to couple it with software that
can handle complicated multilayer designs and
accurately predict end points in accordance with
the goals of the filter design. The IL540 is the
most refined, robust and successful solution
available to the problem of accurate endpointing
in complex multilayers.
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