NASA Technical Reports Server (NTRS) 20110014684: Fiber-Optic Sensor Would Monitor Growth of Polymer Film

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by  NASA Technical Reports Server (NTRS)

Technology Focus: Sensors
Fiber-Optic Sensor Would Monitor Growth of Polymer Film
Changes in thickness would be inferred from interference fringes.
Goddard Space Flight Center, Greenbelt, Maryland
A proposed optoelectronic sensor sys- control box to a fused-silica faceplate in light beam would vary in proportion to
tem would measure the increase in a sensor head. The sensory tips of the the increasing thickness of the film and
thickness of a film of parylene (a ther- optical fibers would be polished flush the known index of refraction of the
moplastic polymer made from para-xy- with the free surface of the faceplate. In film, causing the photodetector reading
lene) during growth of the film in a preparation for use, the sensor head to vary in proportion to a known sinu-
vapor deposition process. By enabling would be mounted with a hermetic seal soidal function of film thickness. Elec-
real-time monitoring of film thickness, in a feed-through port in the deposition tronic means of monitoring this varia-
the system would make it possible to chamber, such that free face of the face- tion and the corresponding variation in
identify process conditions favorable for plate and the sensory tips of the optical phase and thickness are well established
growth and to tailor the final thickness fibers would be exposed to the deposi- in the art of interferometry. Hence, by
of the film with greater precision than is tion environment. tracking the cumulative change in phase
now possible. During operation, light would travel difference from the beginning of depo-
The heart of the sensor would be a along each optical fiber from the con- sition, one could track the growing
pair of fiber-optic Fabry-Perot interfer- trol box to the sensor head. A small por- thickness of the film to within a small
ometers, depicted schematically in the tion of the light would be reflected to- fraction of a wavelength of light.
figure. (In principle, a single such inter- ward the control box from the end face This work was done by Michael Beames-
ferometer would suffice. The proposal of each fiber. Once growth of the pary- derfer of Goddard Space Flight Center.
calls for the use of two interferometers lene film started, a small portion of the Further information is contained in a TSP
for protective redundancy and increased light would also be reflected toward the (see page 1).
accuracy.) Each interferometer would control box from the outer surface of This invention is owned by NASA, and a
include a light source, a fiber-optic cou- the film. In the control box, the two re- patent application has been filed. Inquiries con-
pler, and photodetectors in a control flected portions of the light beam would cerning nonexclusive or exclusive license for its
box outside the deposition chamber. A interfere in one of the photodetectors. commercial development should be addressed to
single-mode optical fiber for each inter- The difference between the phases of the Patent Counsel, Goddard Space Flight Cen-
ferometer would run from inside the the interfering reflected portions of the ter, [phone]. Refer to GSC-14757-1.
Fiber-Optic
Coupler
Fused-Silica
Potting
Single-Mode Material Faceplate
Optical Fibers
Parylene
Film
Electrical
Outputs Photodetectors
Sensor
Head
Reflection From
Outside Inside End Face of
Optical Fiber
Control Unit
Wall of
Deposition
Chamber
Reflection From
Optical Outer Surface of
Fiber Parylene Film
Fused-Silica Parylene
Faceplate Film
Portions of the Light Beam in each optical fiber would be reflected from opposite faces of the parylene and would interfere in one of the photodetectors.
Variations in the photodetector output would be interpreted in terms of increases in the thickness of the parylene film according to well-known principles
of interferometry.
NASA Tech Briefs, January 2005 5