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BACKGROUND OF THE INVENTION
This invention relates to pyroelectric devices. In particular, the present
invention relates to use of parylene C polymer film for pyroelectric
devices.
Pyroelectric detectors are a class of thermal detectors which utilize an
electrically poled pyroelectric material. When the pyroelectric material
is subjected to a change in temperature, the electrical polarization of
the material changes, thereby producing a voltage across the pyroelectric
material. In other words, the pyroelectric detector can be characterized
essentially as a capacitor upon which a time varying charge, and
consequently a voltage, appears when the temperature of the detector is
changed. Since the pyroelectric effect is a direct result of the
temperature dependence of the polarization, it can be used as a means of
detecting infrared radiation energy.
Recently, pyroelectric detectors have attracted great attention due to
their potential for low cost, medium to high performance, and room
temperature operation. Substantial effort has been spent by many
researchers to improve the performance of pyroelectric detectors utilizing
ferroelectric crystals such as SBN, TGS, LiTaO.sub.3. The state of the art
single element pyroelectric detector performance is a D* of about
1.times.10.sup.9 cm Hz.sup.1/2 /W at 10 Hz. Detectors with thickness of
about 3 microns have been fabricated from these ferroelectric crystals.
In addition to the ferroelectric crystals, certain organic polymers also
exhibit pyroelectric effects. Considerable research effort has been
expended in studying polyvinyl chloride, polyvinyl fluoride (PVF),
polyvinylidene chloride, polyvinylidene fluoride (PVF.sub.2),
polyacrylometide (PAN), polyacrylonitrile, and poly-o-fluorostyrene.
Polymer pyroelectric devices are described in the following U.S. Pat. Nos.
3,769,096 by Askin, et al.; 3,772,518 by Mureyama, et al.; 3,794,986 by
Mureyama; 3,809,920 by Coen, et al., 3,824,098 by Bergman, Jr., et al.;
3,885,301 by Mureyama; 3,912,830 by Mureyama, et al.; and British Pat.
Nos. 1,376,372 and 1,377,891. A technical article describing the
pyroelectric properties of PVF.sub.2 is Appl. Phys., 42, 5219 (1971).
Parylene C is a polymer material which has found use as a window for X-ray
radiation detectors. The properties of parylene C and related polymers
parylene N and M are discussed in a M. A. Spivack, "Mechanical Properties
of Very Thin Polymer Films," Rev. Sci., Instru., 43, 985 (1972).
Unsupported, thin parylene films have been prepared and are available in
thicknesses down to 0.025 microns. They have extremely uniform thickness,
are pin-hole free, have high thermal resistance, are highly insulating,
and are extremely mechanically rugged. The typical use of the parylene
films as windows for X-ray detectors implies that they have the mechanical
strength to support significant pressure differences. Despite the studies
of parylene polymers, there has been no report of any parylene polymer
being electrically poled, or of pyroelectric behavior in any parylene
polymer.
SUMMARY OF THE INVENTION
It has been discovered that electrically poled parylene C polymer film
exhibits pyroelectric response. Parylene C is a highly attractive new
pyroelectric detector material.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a pyroelectric detector formed by a parylene C polymer film.
FIG. 2 shows normalized resistivity as a function of frequency for parylene
C pyroelectric detectors.
FIG. 3 shows a biasing circuit used in conjunction with certain tests on
the parylene C pyroelectric detector.
FIG. 4 shows parylene C pyroelectric response as a function of bias
electric field.
FIG. 5 shows detectivity D*.sub.BB, responsivity R.sub.BB, and noise
voltage V.sub.n as a function of frequency for a parylene C pyroelectric
detector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
We have discovered that parylene C polymer film can be electrically poled
and, when electrically poled, exhibits a substantial pyroelectric
response. FIG. 1 shows a parylene C pyroelectric detector. A thin film of
parylene C polymer film 10 is stretched over the opening of a glass
capillary 12. Electrodes 14 and 16 are attached to opposite surfaces of
parylene C film.
In one successful embodiment of the invention, the parylene C film, which
was supplied by Union Carbide Corporation, had a thickness of about 20
microns. The area of the detector was 7.07.times.10.sup.-2 cm.sup.2.
An important characteristic which is required for successful pyroelectric
device application is the ability of the pyroelectric material to be
electrically poled. Prior to the present invention, there were no reports
of electrical poling of any parylene polymer.
The chemical structure of parylene C is:
##STR1##
This chemical structure, with the extra chlorine atom attached to the
carbon atoms of poly (chloro-p-xylylene) suggests that parylene C is
potentially polarizable (i.e. an ionic displacement polarization may be
imposed on this film by proper poling).
In one embodiment of the present invention, the 20 micron thick parylene C
film was poled at 120.degree. C. in a dry nitrogen atmosphere. The
parylene C film poled in this manner exhibited pyroelectric response. This
is apparently the first time either electrical poling or pyroelectric
response has been observed in parylene C.
FIGS. 2, 4 and 5 show the results of pyroelectric test measurements made on
the parylene C pyroelectric detector. FIG. 2 is a plot of normalized
responsivity R.sub.V versus frequency for parylene C detectors having long
and short leads.
It was discovered that the pyroelectric response of parylene C detectors
could be increased by applying a bias electric field. FIG. 3 shows the
circuit used to measure pyroelectric response as a function of bias
electric field, and FIG. 4 shows the resuts of these test measurements.
In FIG. 3, a bias voltage was applied across pyroelectric detector DET1 by
voltage source V.sub.S and resistor R.sub.B. The voltage from V.sub.S was
varied between 0 and 1 KV. The drain of FET1 was connected to a positive
voltage, and the source was connected through resistor R.sub.S to a
negative voltage. The output signal was derived from the source of FET1.
During the tests, the incident irradiance on the parylene C pyroelectric
detector DET1 was 6.75.times.10.sup.-5 W/cm.sup.2. The chopping frequency
was 24 Hz.
As shown in FIG. 4, the signal voltage, V.sub.S, from the parylene C
detector increased nearly linearly with increasing bias electric field. An
increase in signal voltage by a factor of about 2.5 was achieved with an
increase in bias electric field from 0 V/cm to 45.times.10.sup.+4 V/cm
bias field.
FIG. 5 shows detectivity D*.sub.BB, responsivity R.sub.BB and noise voltage
V.sub.n as a function of frequency for a parylene C pyroelectric detector.
These measurements were made about 4 weeks after the detector had been
electrically poled. The detector area was about 0.07 cm.sup.2 and the
thickness was 20 microns. As shown in FIG. 5, the detectivity exceeded
10.sup.7 cm Hz.sup.1/2 /W at low frequencies (less than about 3 Hz). These
measurements are very encouraging, since no attempt had been made to
optimize the electrical poling of the parylene C itself. With proper
poling and reduced detector thickness (preferably less than 1 micron),
10.sup.9 detectivity at a few hundred Hz chopping frequency appears
possible.
Parylene C polymer films have considerable potential for pyroelectric
devices, particularly to detector arrays which are direct coupled to
charged coupled devices (CCD's). Parylene C offers the following
advantages for large area pyroelectric mosaic applications: (1) ease of
fabrication of large area thin samples, (2) low dielectric constant
required for direct coupling to CCD's, (3) low cost, and (4) low thermal
diffusivity.
The ability to prepare very thin parylene C film is a substantial advantage
in obtaining detector performance goals. For a well designed, thermally
isolated pyroelectric detector, the D* rolls off as a function of
1/(f).sup.1/2 in the region f>f.sub.o, where f.sub.o =1/2.pi..tau..sub.th,
and .tau..sub.th is the detector thermal time constant. Using a lumped
thermal loss model, .tau..sub.th can be simply repressed as:
.tau..sub.th =c .rho.d/g (1)
where c.rho. is the detector volume specific heat, d is the detector
thickness, and g is the lumped thermal conductance per unit area. Equation
(1) clearly demonstrates that the thinner the detector thickness, the
faster is the detector thermal response time. Thus, one can expect better
D* at higher frequencies without too stringent a requirement on the
pyroelectric material figure of merit, p/(.epsilon.).sup.1/2 where p and
.epsilon., are the material pyroelectric coefficient and dielectric
constant, respectively.
For example, for a pyroelectric detector with a thickness of 2000A, c.rho.
of 1.9 Joule/.degree. K. cm.sup.3 and g of 2.4.times.10.sup.-2 W/.degree.
K. cm.sup.2, a thermal time constant of 1.5 ms is obtainable. The detector
noise limited D* of 1.times.10.sup.9 cm Hz.sup.1/2 /W at 100 Hz can be
achieved with a pyroelectric material having a p(.epsilon.).sup.1/2 ratio
of 8.times.10.sup.-10 coul/.degree. K. cm.sup.2 and dissipation factor
0.005. Among the commonly known pyroelectric polymers, both PVF and
PVF.sub.2 have p/(.epsilon.).sup.1/2 ration larger than
8.times.10.sup.-10.
Presently, PVF.sub.2 is the most commonly used polymeric pyroelectric
material. PVF.sub.2 detectors with 1.times.10.sup.9 detectivity at 4 Hz
have been reported. Detector grade PVF.sub.2 is typically prepared by a
technique of extrusion and then stretching. A 4 .mu.m thickness is
probably the limit of the thinnest sample which can be prepared by this
technique.
In order to reach 1.times.10.sup.9 cm Hz.sup.1/2 /W at 100 Hz, therefore, a
new material is needed. Parylene c appears to fill this need. Unsupported,
thin parylene films have been prepared and are available in thicknesses
down to 0.025 .mu.m. These parylene polymers are extremely uniform in
thickness, pin-hole free, high thermal resistance, highly insulating and
extremely mechanically rugged. The thickness requirements, therefore are
met by parylene C.
Another advantage of parylene C is that the dielectric constant of the film
is about 3.15. One criteria for directly introducing a pyroelectric signal
into the MOSFET input of a CCD requires that the dielectric constant of
the detector material is the same, or preferably less than that of
SiO.sub.2 (.epsilon.=3.9)
Parylene C polymer film, therefore, possesses all the mechanical and
dielectric properties of a pyroelectric material which is needed for a
large element pyro/CCD focal plane. These factors, combined with the
demonstrated pyroelectric response, make parylene C an attractive
pyroelectric material.
In conclusion, we have demonstrated that parylene C polymer film is capable
of being electrically poled and has a substantial pyroelectric response.
The pyroelectric, mechanical and dielectric properties of parylene C make
it an attractive new pyroelectric detector material. While the invention
has been described with reference to preferred embodiment, workers skilled
in the art will recognize the changes made in form and detail without
departing from the spirit and scope of the present invention. For example,
although certain applications of parylene C pyroelectric material have
been discussed, other applications and configurations for pyroelectric
devices are also possible and contemplated as a part of the present
invention.
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