U.S. patent application number 11/241965 was filed with the patent office on 2007-09-06 for bolometric detector with thermal isolation by constriction and device for detecting infrared radiation that uses such a bolometric detector.
This patent application is currently assigned to Commissariat A L'Energie Atomique. Invention is credited to Sylvette Bisotto, Jean-Louis Ouvrier-Buffet.
Application Number | 20070205364 11/241965 |
Document ID | / |
Family ID | 34952507 |
Filed Date | 2007-09-06 |
United States Patent
Application |
20070205364 |
Kind Code |
A1 |
Ouvrier-Buffet; Jean-Louis ;
et al. |
September 6, 2007 |
BOLOMETRIC DETECTOR WITH THERMAL ISOLATION BY CONSTRICTION AND
DEVICE FOR DETECTING INFRARED RADIATION THAT USES SUCH A BOLOMETRIC
DETECTOR
Abstract
A bolometric detector including an absorbing part intended to
convert incident electromagnetic radiation into calories, an active
part (2) including a sensitive area made of a bolometric material
the resistivity of which varies, in a known manner, with
temperature, and electrodes that are in contact with the bolometric
material (6) over at least part of their surface area. Support
areas or posts (3) intended to maintain the active part (2) are
suspended above a substrate (1) that accommodates, in particular,
the read-out circuit associated with said detector and to ensure
electrical conduction between the read-out circuit and the active
part, the support areas or posts (3) have regions of non-uniform
cross-sectional area between their point of contact with the
substrate and the area where they are joined to the active part or
being associated with elements that have such non uniformities.
Inventors: |
Ouvrier-Buffet; Jean-Louis;
(Sevrier, FR) ; Bisotto; Sylvette; (Grenoble,
FR) |
Correspondence
Address: |
BURR & BROWN
PO BOX 7068
SYRACUSE
NY
13261-7068
US
|
Assignee: |
Commissariat A L'Energie
Atomique
Paris
FR
|
Family ID: |
34952507 |
Appl. No.: |
11/241965 |
Filed: |
October 3, 2005 |
Current U.S.
Class: |
250/338.1 |
Current CPC
Class: |
G01J 5/04 20130101; G01J
5/023 20130101; G01J 5/046 20130101; G01J 5/20 20130101; G01J 5/02
20130101 |
Class at
Publication: |
250/338.1 |
International
Class: |
G01J 5/00 20060101
G01J005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 28, 2004 |
FR |
04.11475 |
Claims
1. A bolometric detector comprising: an absorbing part intended to
convert incident electromagnetic radiation into caloriest; an
active part consisting of: a sensitive area made of a bolometric
material the resistivity of which varies, in a known manner, with
temperature, electrodes that are in contact with the bolometric
material over at least part of their surface area; support areas or
posts maintaining said active part suspended above a substrate that
accommodates the read-out circuit associated with said detector and
ensuring electrical conduction between said read-out circuit and
said active part, wherein each of said support areas or posts has a
region of non-uniform cross-sectional area located along a
longitudinal line extending between a point of contact with the
substrate and an area where the support area or post is joined to
the active part, said region of non-uniform cross-sectional area
being geometrically solid along said longitudinal line.
2. A bolometric detector as claimed in claim 1, wherein said
regions of non-uniform cross-sectional area are in the form of
spikes or chokes creating an area where constriction of thermal
flux between said active part and the substrate is likely to
occur.
3. A bolometric detector as claimed in claim 2, wherein the spikes
or chokes are made of the same material as that of the active
part.
4. A bolometric detector as claimed in claim 2, wherein the spikes
or chokes are made of at least one material that is different from
that of the active part.
5. A bolometric detector as claimed in claim 2, wherein at least
said regions of non-uniform cross-sectional area are made of at
least one material selected from the group comprising SiO.sub.2,
Si.sub.3N.sub.4, TiN or of a material that constitutes said active
part.
6. A bolometric detector as claimed in claim 1, wherein said
regions of non-uniform cross-sectional area are made of a porous
material creating as where constriction of thermal flux between
said active part and the substrate is likely to occur.
7. A bolometric detector as claimed in claim 6, wherein the porous
material is made of an aerogel.
8. A bolometric detector as claimed in claim 7, wherein the aerogel
is made from silica.
9. A device for detecting infrared radiation comprising a plurality
of bolometric detectors as claimed in claim 1.
10. A device for detecting infrared radiation as claimed in claim
9, wherein the bolometric detectors are designed as a linear or
array configuration.
Description
TECHNICAL FIELD
[0001] The present invention relates to a bolometric detector as
well as the device for detecting infrared radiation that uses such
detectors.
[0002] The invention has application areas in the field of infrared
imaging in particular.
DESCRIPTION OF THE PRIOR ART
[0003] In infrared detectors, the use of devices configured in the
form of an array and capable of operating at ambient temperatures,
i.e. not requiring cooling to extremely low temperatures is
known,--in contrast to detecting devices called "quantum detectors"
which can only operate at extremely low temperature, typically that
of liquid nitrogen.
[0004] These uncooled detectors traditionally use the variation in
a physical unit of an appropriate material as a function of
temperature at around 300 K.
[0005] In the case of bolometric detectors, this physical unit is
electrical resistivity.
[0006] Such an uncooled detector generally consists of: [0007]
means of absorbing the infrared radiation and converting it into
heat, [0008] means of thermally isolating the detector so that its
temperature can rise due to the effect of the infrared radiation,
[0009] thermometric means which, in the context of a bolometric
detector, use a resistance element, [0010] and means of reading
electrical signals provided by the thermometric means.
[0011] Detecting devices intended for infrared imaging are produced
as a one- or two-dimensional array of elementary detectors mounted
on a substrate generally made of silicon which incorporates means
of electrically exciting said elementary detectors and means of
pre-processing the electrical signals generated by these elementary
detectors.
[0012] These means of electrical excitation and pre-processing are
thus produced on the substrate and constitute a read-out
circuit.
[0013] A device comprising such an array of elementary detectors
and an associated read-out circuit is generally placed in a package
and is connected, especially electrically, to its external
environment using classic technologies. The pressure inside such a
package is reduced in order to limit thermal losses. The package
also has a window that is transparent to the radiation to be
detected.
[0014] In order to observe a scene using this detector, the scene
is projected through suitable optics onto the array of elementary
detectors and clocked electrical stimuli are applied via the
read-out circuit (also provided for this purpose) to each of the
elementary detectors or to each row of such detectors in order to
obtain an electrical signal that constitutes an image of the
temperature reached by each elementary detector.
[0015] This signal is then processed to a greater or lesser extent
by the read-out circuit and then, if applicable, by an electronic
device outside the package in order to generate the thermal image
of the observed scene. FIGS. 1 and 2 show, respectively, a
simplified perspective and top view of a bolometric detector
according to the prior art. These Figures show the silicon
substrate, referred to as (1), which accommodates, in particular,
the read-out circuit.
[0016] This substrate accommodates an integrated electronic circuit
which comprises, on the one hand, the devices that generate the
stimuli for said detector and the devices to read out the signals
output by the latter and, on the other hand, multiplexing
components that make it possible to convert the signals output by
the various detectors into serial form and send them to a reduced
number of outputs so that they can be analysed by a processing
system, especially an imaging system of traditional style.
[0017] The detector itself consists of a suspended membrane (2),
above the substrate (1), most of which is intended to absorb
incident radiation and convert it into heat, then into an
electrical signal.
[0018] Said-membrane (2) is suspended above the read-out circuit
and therefore, in particular, substrate (1) by means of studs,
posts or anchor points (3), more generically referred to as support
areas or structures.
[0019] In this way, an empty space that extends to a height
typically of 1 to 5 .mu.m is defined between the substrate (1) and
membrane (2).
[0020] These support structures (3) are essentially vertical. They
conduct electricity, thus making it possible to apply the
excitation potentials to the conductive parts or electrodes that
are among the components of the actual bolometric detector itself
via flat stretched structures (4) that also conduct electricity but
are thermally resistant. These flat, stretched structures (4) are
conventionally referred to as arms.
[0021] The suspended membrane (2) essentially comprises a layer (5)
that absorbs incident thermal radiation.
[0022] This absorption of radiation causes heating of this layer
which transfers the temperature thus accumulated to a layer (6)
deposited on the membrane that acts as a thermometer and is made of
a bolometric material.
[0023] This bolometric material traditionally consists of slightly
or highly resistive p or n type polycrystalline or amorphous
silicon but may also be made of vanadium oxide (V.sub.2O.sub.5 or
VO.sub.2) made in a semiconducting phase or even ferrites with a
spinel structure.
[0024] As already stated, electrodes, conventionally located in the
same plane as the detector, are used in order to define the
electrical signal in the bolometric detector and these are referred
to as coplanar electrodes or stacked electrodes or a sandwich
structure as disclosed, for example, in American Patent U.S. Pat.
No. 5,021,663.
[0025] The various parameters that affect the level of performance
of an uncooled bolometric detector include, in particular,
mastering the design and construction of the elementary detectors
and, especially, the micro bridges and posts, especially thermal
isolation between the read-out circuit (1) and the suspended
membrane (2).
[0026] As already indicated, the space between the active part of
the bolometer and the substrate comprising the read-out circuit,
possibly coated with a layer that reflects infrared radiation, is a
void apart from the posts or support areas (3) in order to prevent
thermal losses due to solid conduction.
[0027] The space is also usually filled with low-pressure gas in
order to limit convection and conduction by gases.
[0028] Generally speaking, thermal isolation of the membrane (2) is
provided by relatively narrow isolating arms (4) consisting of the
thinnest possible layers. Depending on the way they are made, these
isolating arms (4) are located in the same plane as the membrane
(2) or even produced below the latter (see for example U.S. Pat.
No. 5,367,167).
[0029] Although they fulfill their function of isolating and
supporting the membrane relatively satisfactorily, these arms
nevertheless have the drawback of penalising the performance of the
bolometric detector because of the reduction in the fill factor
they cause, especially if they are located in the same plane as
said bolometric membrane. Increasing the length of these arms
substantially or reducing their width and/or thickness
significantly compromises the rigidity of the structure. In fact,
these elements are a mechanical weak point that affects the
stability of the micro bridges which may then topple or deform and
consequently cause the membrane to come into contact with the
substrate, thereby creating thermal bridges that destroy the
thermal isolation of said membrane.
SUMMARY OF THE INVENTION
[0030] The invention aims first of all to optimise the thermal
isolation of the membrane of a bolometric detector without thereby
degrading its mechanical stability, especially that of the micro
bridges from which it is suspended. It then aims to improve the
performance of such a detector by increasing the fill factor.
[0031] To achieve this, the invention involves not only altering
the dimensional features of the arms but also working on the
structure of the thermal resistance at the level of the link
between the micro bridges and the support.
[0032] Thus, it relates to a bolometric detector comprising: [0033]
an absorbing part intended to convert incident electromagnetic
radiation into calories, [0034] an active part which itself
consists of: [0035] a sensitive area made of a bolometric material
the resistivity of which varies, in a known manner, with
temperature, [0036] electrodes that are in contact with the
bolometric material over at least part of their surface area,
[0037] support areas or posts intended to maintain said active part
suspended above a substrate that accommodates, in particular, the
read-out circuit associated with said detector and to ensure
electrical conduction between said read-out circuit and said active
part, said support areas or posts having regions of non-uniform
cross-sectional area between their point of contact with the
substrate and the area where they are joined to the active part; or
being associated with elements that have such non uniformities.
[0038] In other words, the invention involves working on the
morphology of the support areas in order to optimise the thermal
isolation of the suspended membrane. This uses a physical
phenomenon referred to as "constriction".
[0039] Producing a point-type thermal contact or a contact having
an extremely small cross-sectional area between two materials
causes constriction of the flux lines which results in a
significant increase in the thermal contact resistance.
[0040] This phenomenon can advantageously be exploited by
technologies for uncooled detectors and hence, in particular, in
the field of bolometric detectors, in order to increase the thermal
isolation of said detectors. This being so, this phenomenon applies
particularly well, but not exclusively, to obtaining vertical
contact resistance.
[0041] This constriction phenomenon can be described as follows.
Let us take the fundamental case of a semi-infinite medium of
thermal conductivity k bounded by a plane that is assumed to be
isolated apart from over a surface s having a characteristic
dimension a of limited extent which is subjected to a uniform
temperature or heat flux of uniform density. The temperature field
inside the medium changes from T.varies. to T0 but this change is
mainly confined to the vicinity of s in a hemisphere having a
radius of 10a. The constriction resistance in this medium can be,
determined analytically. The condition at the boundaries of the
real contact area is either an imposed temperature condition or an
imposed flux condition. The values of the constriction resistances
(in K/W) obtained are as follows: Rc = 8 3 .times. .pi. 2 .times. 1
ka ##EQU1## for a flux condition imposed on the disc having a
radius a and Rc = 1 4 .times. ka ##EQU2## for an imposed
temperature condition.
[0042] The constriction resistance therefore appears to be
inversely proportional to the characteristic dimension a rather
than to the surface s. It also depends on the type of thermal
condition imposed on the surface s. The contact resistance is
slightly less for a temperature condition than for a uniform flux
density condition. In reality, the thermal conditions are
intermediate between these two types of conditions and the
constriction resistance values lie between these two
situations.
[0043] When the heat flux lines are channeled in a tube of finite
diameter, the constriction phenomenon is weaker than in a
semi-infinite medium.
[0044] In this case, for an imposed temperature condition and a b
.ltoreq. 0.3 ##EQU3## where b is the radius of said tube: Rc = 1 4
.times. ka .times. ( 1 - 1.41 .times. a b ) ##EQU4##
[0045] Finally, in the case of thin films, if the thickness of the
layer reduces, the constriction resistance also depends on the
condition at the boundaries on the face opposite the contact
area.
[0046] Thus, for the limiting conditions of flux imposed on the
disc of radius a, the same as on the opposite face, the thermal
contact resistance is higher than for a thick medium. The contact
resistance is then expressed as follows: Rc = 1 4 .times. ka
.times. ( 1 - 1.41 .times. a b ) .times. f .function. ( l , a , b )
.times. .times. with .times. .times. f .function. ( l , a , b )
.gtoreq. 1 ##EQU5## where 1 is the thickness of the layer and f
represents a function that modulates R.sub.c depending on the
geometric parameters.
[0047] In reality, the thermal resistance comprises two components
derived: [0048] firstly from convergence of the thermal flow lines
towards the area of lowest temperature in accordance with the
mechanisms mentioned above; the contact areas should therefore be
of reduced size so as to generate constriction regions, [0049] and
secondly from the contact resistance at the interfaces of the
thin-film layers that ensure contact. Generally speaking, contact
between two solid media is only obtained in a certain number of
areas of small surface area depending on the roughness
(.apprxeq.1%) between which there remains an interstitial medium
(in the case of microbolometers, it can be assumed that there is a
vacuum). As the conductivity of the interstitial medium is low
compared with that of the media that are in contact, this produces
convergence of the flux lines towards the contact areas which
causes a micro constriction effect similar to that described above
but on a nanostructure or microstructure scale.
[0050] Ultimately, this analysis shows that one can increase the
thermal isolation of bolometric membranes by modifying several
factors. The effect of constriction can be improved by altering the
contact shape and dimensions and hence the support areas or posts.
This is especially important for insulating media. The contact
resistance effect (micro constriction at the level of surfaces that
are in contact) depends essentially on the metallurgical structure
of the materials used. In this respect, it can be advantageous to
use materials such as aerogels and xerogels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] The manner in which the invention can be implemented and its
resulting advantages will become more apparent from the-typical
embodiments below, given merely by way of example, reference being
made to the accompanying drawings.
[0052] FIGS. 1 and 2 illustrate, as already stated, a perspective
and top view of an elementary bolometric detector according to the
prior art.
[0053] FIGS. 3 and 4 show, schematically, a first embodiment of the
invention.
[0054] FIGS. 5 and 6 illustrate a second embodiment of the
invention.
[0055] FIG. 7 is a schematic cross-sectional view of a detail of a
bolometric membrane in accordance with the invention.
[0056] FIGS. 8 to 10 are views similar to FIG. 7 of other membranes
in accordance with the invention.
[0057] FIG. 11 aims to illustrate the contact between a porous
material and a smooth surface.
[0058] FIGS. 3 and 4 illustrate a first way of implementing the
invention in principle. According to this, the studs or posts (3)
are of traditional style but their upper end is not fixed directly
to the bolometric membrane (2) but to an intermediate frame (9)
which makes it, possible to separate the fabrication of the
isolating arms, that of the spikes or micro spikes (7) described
below and that of the membrane (2). This intermediate frame
corresponds to the associated element mentioned earlier in the
general definition of the invention.
[0059] Frame (9) can be made of any material. However, it is
preferable to use a material selected from the group comprising
SiO, SiN, TiN or a material from which the active suspended part is
made.
[0060] This frame (9) has micro spikes (7), the pointed tips of
which are in contact with the bolometric membrane (2). These micro
spikes are an integral part of the frame and are therefore made of
the same material as the latter. Nevertheless, it is perfectly
possible to envisage making said micro spikes from a different
material.
[0061] In addition, the frame and the micro spikes conduct
electricity. However, this conduction may be only partial to the
extent that both the frame and the micro spikes may have a
conducting core covered by an insulating material or vice versa,
i.e. a core made of an insulating material covered in a conductive
layer. In addition, the materials used to make the micro spikes are
not necessarily identical to those used to make the frame.
[0062] In the configuration in FIG. 3, the areas where the thermal
flux lines are constricted are mainly located on the membrane (2).
FIG. 4 is similar to FIG. 3 apart from the fact that the micro
spikes (8) have a substantially inverted configuration because
their tip points down towards the frame. In this configuration, the
micro spikes may be part of said membrane or be made of a different
material. In addition, the areas where the thermal flux lines are
constricted are chiefly located in the frame.
[0063] The studs or posts (3) rest on the read-out circuit, the
frame (9) and the bolometric membrane (2) are produced using
conventional microelectronics and microtechnology processes.
[0064] Micro spikes that point towards the membrane (2) can be
made, for example, as follows. A layer of photosensitive resin is
spread out on the structure intended to form said micro spike. A
disc, the diameter of which is substantially equivalent to twice
the thickness of the layer that constitutes the structure is
lithographed. This assembly is then immersed in an appropriate
etching solution depending on the nature of the material. The rate
of horizontal etching that results in over-etching under the resin
mask equals the vertical etching rate. The structure changes over
time into a micro spike shape.
[0065] Obviously, this method must be adapted to the underlying
sandwich structure that is already present. In particular the
material or materials of which the spikes are made may be an
electrical insulator or an electrical conductor such as silica or a
nitride.
[0066] In a second configuration of the invention as shown in FIGS.
5 and 6, the bolometric membrane rests directly on micro spikes
(10, 11) that are an extension of the posts or support areas (3),
hence without any intermediate frame.
[0067] In FIG. 5, the micro spikes are not made of the same
material as the membrane and the support areas. In contrast, in the
configuration described in FIG. 6 where the micro spikes are
mounted tip to tip, the material of said spikes and membrane (2)
are identical.
[0068] According to one advantageous embodiment, the membrane also
has isolating arms making it possible to increase the thermal
resistance.
[0069] Said membrane is made using conventional techniques.
[0070] Thus the structure (5) that supports the thermometer
material (6) consists of two insulating layers that enclose metal
electrodes. The insulating layer deposited on the metal layer that
constitutes the electrodes comprises contact openings in order to
connect the thermistor. Etching of the thermometer material makes
it possible to expose the materials in the regions that separate
the detectors (reticulation).
[0071] In another embodiment that is also known, the structure that
supports the bolometric material consists of an insulating layer on
which metal electrodes that are totally in contact with the
thermistor rest. Etching of the thermometer material makes it
possible to expose the materials in the regions that separate the
detectors.
[0072] This type of sandwich structure produces a component that is
optimised in terms of signal-to-noise ratio.
[0073] These structures can be produced using various processes
that result in the fabrication of micro bridges.
[0074] Microbolometer technology is implemented on a sacrificial
layer made of polyimide having a thickness of 1 to 5 .mu.m,
preferably equal to a quarter of the wavelength to be detected so
as to produce a quarter-wavelength cavity between the electrodes
and the reflector (metallic material deposited on the multiplex or
read-out circuit) that ensures maximum absorption.
[0075] The thin-film layers (having a thickness from 0.005 .mu.m to
0.1 .mu.m for example) of insulating elements (SiN, SiO, ZnS, etc.)
are obtained using low-temperature deposition techniques that are
customarily used for these materials: cathode spluttering, plasma
decomposition (PECVD). These materials are generally etched by
using plasma-assisted chemical etching processes.
[0076] The metal materials (Ti, TiN, Pt, etc.) that constitute the
electrodes are preferably deposited by cathode spluttering. These
metallised areas are defined by chemical or plasma etching
processes. The thickness of these layers is from 0.005 .mu.m to 0.1
.mu.m. The sheet resistance of the layer that constitutes the
electrodes will be adjusted so as to encourage the absorption of IR
radiation.
[0077] The thermometer material can be an amorphous or
polycrystalline semiconductor (Si, Ge, SiC, a-Si:H, a-SiC:H, etc.)
obtained using low-temperature deposition techniques that are
customarily used for these materials: cathode spluttering, thermal
decomposition (LPCVD) or plasma decomposition (PECVD).
[0078] Any doping of these layers is produced by introducing a
doping gas (BF3, PH3, etc.) in the reactor or by ion implantation.
These materials are generally etched using plasma-assisted chemical
etching processes. This may involve a metal material or even a
vanadium or ferrite oxide.
[0079] The process to etch the sacrificial layer is adapted to suit
the nature of the layer. It is preferably a plasma etching
process.
[0080] A first embodiment of a constriction area in the vicinity of
the bolometric membrane in accordance with the invention is
described in relation to FIG. 7.
[0081] In this example, the opening of the hole (12) in layers (5,
6) constituting the bolometric structure and the sacrificial layer
(not shown) is produced firstly by using high-resolution electronic
or optical photolithographic means and secondly with the aid of dry
etching equipment. 50 nm holes are currently feasible in
association with classic technologies (spacer techniques, etc.),
many publications report this, such as for example, "Fabrication of
thin-film metal nanobridges" Ralls et al.--Appl. Phys. Letter
5(23), 4 Dec. 1989).
[0082] Also, the dimensions of the pivot (13) located between the
lower surface of the membrane and the support (frame (9) or support
area (3)) can always be reduced by using an isotropic plasma
etching process that is suitable for the thermometer material,
thereby favouring thermal isolation of said membrane.
[0083] In order to increase the thermal resistance even more,
roughness of the interface with the support (frame (9) or support
area (3)) can be obtained during deposition of the thin-film
layers. Nevertheless, such roughness can be obtained more reliably
by appropriate heat treatments that leave voids resulting from
interdiffusion of materials. An appropriate chemical treatment can
also be used to etch the interdiffusion regions or even simply
expose the grain boundary. Although the surface area of the
materials that are in contact is relatively large, the effective
thermal contact surface is around 1% of that.
[0084] In the configuration described in relation to FIG. 8,
constriction of the thermal flux lines is obtained by using porous
materials, for example silica or carbon polymer based aerogel type
materials (as described in the document entitled "Ordered porous
materials for emerging applications"--NATURE--Vol. 417--20 Jun.
2002).
[0085] FIG. 11 schematically shows the contact between a porous
material and a smooth surface. In it one can see, in particular,
that this contact actually consists of a plurality of areas
configured as spike shapes that are the result of the physics of
the porous material used and are capable of constituting the
constriction areas sought-after within the meaning of the present
invention.
[0086] According to this configuration (FIG. 8), the opening of
hole (12) through the layers (5, 6) of the bolometric structure is
obtained in the same way as previously. The material that
constitutes the spike is deposited over the entire surface of the
device preferably using the sol-gel method. This material is then
etched (self-aligned etching process) in order to localise the
spikes near the holes in order to form supports for the bolometric
membrane.
[0087] Depending on the nature of the material used to produce the
contact spike, the latter's contribution to the thermal resistance
is not negligible in terms of pure constriction. This is
particularly true if the spike is made of an aerogel type material
that has a very low thermal conductivity such as a silica-based
gel. In fact, once they are in a vacuum, these materials have a
thermal conductivity of the order of 0.02 Wm.sup.-1K.sup.-1 M, i.e.
two orders of magnitude below that of silicon oxide or nitride. The
thermal constriction resistance and the thermal resistance of the
spike are then in series and are therefore cumulative and this
improves the performance of the device.
[0088] Aerogels are extremely porous materials (porosity from 84 %
to 99.5%). They are produced by using a sol-gel process. Preparing
an aerogel therefore involves obtaining a gel. This gel can be
obtained simply by destabilising a sol consisting of a suspension
of silicon particles in water. Aggregation of the particles results
in the formation of a gel. One can also hydrolyse an organosilicate
in solution in an alcohol in order to cause polycondensation of the
radicals. Under certain chemical conditions (alkaline pH), this
forms particles that then assemble in the same way as in the
previous case. In a neutral or acid solution, polycondensation
results in the formation of a polymer and also gives rise to a
gel.
[0089] The solid porous material referred to as an aerogel is
obtained by drying the gel. Drying in air causes densification and
often splitting of the solid backbone: the air-liquid separation
surface propagates into the structure and capillary forces heavily
disrupt the lattice. In order to obtain an aerogel, the solvent is
brought to temperature and pressure conditions that are above the
critical point. The solvent can then be removed without damaging
the solid part. The macroscopic density obtained depends on the
initial organosilicate concentration of the solution. It may be
very low: aerogels having a density of 4 kg/M.sup.3 can be
produced. It is also possible to use aerogels that conduct
electricity in order to ensure electrical bonding between the
bolometric membrane and the support/frame or the read-out circuit.
Carbonaceous or organic aerogels have electrical resistivities
which are perfectly suitable in this respect.
[0090] The view shown in FIG. 9 is similar to that in FIG. 8 in the
sense that it also uses a spike made of an aerogel material.
However, it is intended to illustrate the constriction area at the
interfaces between the spike and frame or spike and substrate or
read-out circuit.
[0091] Finally, FIG. 10 shows the principle whereby the thermal
flux lines are constricted using nanotubes or nanowires. In this
configuration the spike is made as a carbon nanotube or
nanowire.
[0092] A carbon nanotube is grown catalytically, i.e. growth
requires the formation of catalysing clusters. Growth can be
obtained in a Plasma Enhanced Chemical Vapour Deposition setup at a
temperature that is compatible with the read-out circuit. A flow of
acetylene and ammonia is introduced into the growth chamber and,
through thermal decomposition, makes it possible to form the walls
of the nanotube. The ammonia makes it possible to etch the graphite
carbon formed on the top of the tube. Besides its function of
generating carbon particles, the plasma orients the nanotubes
vertically thanks to the voltage applied. The size of the
catalysing stud determines the diameter of the nanotube.
[0093] The following advantage is apparent from the present
invention. Using conventional posts according to the prior art
provides a minimum cross-section or diameter of 1 .mu.m. In
contrast, the spikes produced within the framework of the present
invention can have a diameter of the order of 100 .ANG. or even of
the order of 10 .ANG. for nanotubes. Given this fact and as it has
been shown that the constriction resistance is inversely
proportional to the diameter of said constriction areas, there is a
gain of the order of a factor of 100 or even 1,000 for nanotubes
compared with conventional posts and, consequently, this improves
the thermal resistance and hence the thermal isolation of the
suspended membrane extremely significantly. Ultimately, this
improves the performance of detecting devices that use such
bolometric detectors considerably. In a known manner, these
bolometric detectors are designed as a linear or array
configuration.
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