Thermal detector comprising a thermal insulator made of expanded polymer

A thermal detector with a monolithic structure comprises a layer of material sensitive to infrared radiation and an insulating layer constituted by a thermostable polymer that can be deposited as a thin layer and has a microporous structure. This insulating layer enables the thermal decoupling of the sensitive layer from the substrate comprising reading circuits with which the detector is provided. The performance characteristics of currently used monolithic infrared detectors can thus be substantially improved through the notable reduction of the thermal losses in the sensitive layer. This is achieved through the greatly reduced thermal conductivity of the layer of dielectric polymer. Application to infrared imaging devices.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the general field of infrared detectors 
working at ambient temperature and notably to detectors comprising a 
pyroelectrical material. 
There currently exist infrared detectors that can be used to make, in 
particular, infrared imagers. These detectors work in ambient conditions 
without any cooling system. To work properly, the sensitive material in 
the detector is heated by the infrared radiation. The rise in temperature 
may lead to the appearance of charges (for the pyroelectric detector 
function), variations in dielectric constant (the dielectric bolometer 
function) and variations in resistance (the resistive bolometer function). 
The major problem with these detectors is that of confining the heat 
within the sensitive material, by the prevention, to the maximum degree, 
of losses by diffusion within the substrate comprising the circuits for 
reading the response of the sensitive material. 
2. Description of the Prior Art 
Different approaches for insulating the sensitive material have already 
been envisaged. For example, the sensitive element may be attached to the 
reading circuit by means of conductive epoxy resin pads. 
Metallized polyimide pads (U.S. Pat. No. 4,740,700 by Hughes) could also 
be used as shown in FIG. 1. The method used in both cases is a hybrid 
method that is difficult or even impossible to implement in batch 
production. This means that the matrices of detectors are made one by one. 
Another approach proposes the use, as a sensitive layer, of pyroelectrical 
conductors with low conductivity that can be adapted to a less 
sophisticated level of thermal insulation (Patent FR 89 08799). It is 
notably proposed to use, for example, a layer of a polyimide type standard 
dielectric layer, between the silicon reading circuit and the pyroelectric 
polymer. Thus it becomes possible to reduce losses by diffusion in the 
sensitive layer, with a microelectronics type of batch production mode, 
but the performance characteristics and sensitivities of these detectors 
make it necessary again to consider other approaches in order to further 
improve the thermal insulation. 
SUMMARY OF THE INVENTION 
This is why an object of the invention is a thermal detector comprising a 
layer of material sensitive to infrared radiation, contained between 
electrodes, a substrate comprising a circuit for the reading of the 
response of the sensitive material, wherein, between the sensitive 
material and the substrate, the detector comprises a layer of thermostable 
material with a microporous structure having reduced thermal conductivity. 
The thermostable polymer may advantageously be of the polyimide type. 
The sensitive material may advantageously be ferroelectric: it may be a 
polymer or a ceramic. 
The pyroelectric material may advantageously be contained between a 
continuous electrode and a set of conductive pads arranged in matrix form 
on the layer of polymer with a microporous structure, so as to define 
image elements or pixels, the layer of this polymer further comprising 
conductive vias or via holes connecting the conductive pads to the reading 
circuit integrated into the substrate. 
The sensitive material may also be a thermoresistive material such as an 
oxide, for example VO2. 
An object of the invention is also a method for the making of thermal 
detectors comprising a substrate and a layer of material sensitive to the 
infrared radiation, wherein said method comprises the following i) steps: 
the making, on the substrate, of a layer C.sub.1 of polymer dissolved in a 
solvent A, 
the separation of the layer C.sub.1 prompting the appearance of a 
heterogeneous layer with two phases; 
the elimination of the solvent A making it possible to obtain a layer 
C'.sub.1 of polymer having a microporous structure with reduced thermal 
conductivity; 
the making of the layer of sensitive material on said layer C.sub.2. 
The separation step can also be carried out by thermal treatment at a 
temperature T1 such that a phase separation appears at this temperature. 
The solvent of the polymer is then vacuum evaporated at this temperature. 
The sensitive material of the layer C2 may be of the pyroelectric polymer 
type that can easily be deposited as a thin layer. It may notably be a 
polymer of the polyvinylidene fluoride type or 
polyvinylene-trifluorethylene fluoride type. 
The sensitive material of the layer C2 may also be a ferroelectric ceramic 
which can be deposited as a thin layer, notably by sol-gel techniques. The 
value of these manufacturing methods lies in the fact that they use solely 
thin layer techniques which give small-sized infrared detectors. 
Finally, it will be noted that, according to one improvement of the 
invention, the micropores of the polymer layer are filled not with air but 
with a substance having better properties of thermal insulation than air, 
to obtain a layer that is an even greater thermal insulator. Xenon is 
particularly appropriate: it can easily be incorporated into the layer and 
remain trapped therein.

MORE DETAILED DESCRIPTION 
FIG. 1 shows an infrared detector based on pyroelectrical material 
according to the above-mentioned Hughes patent application. This detector 
comprises elements made of pyroelectric material, at a rate of one element 
per pixel, between a continuous electrode and elementary electrodes. 
Conductive pads electrically connect the elementary electrodes to the 
inputs of processing circuits prepared on a semiconductor substrate. This 
type of hybrid structure remains far more complex than the structures that 
are made solely by using thin-layer batch processing techniques such as 
those used in microelectronics. This is why the invention proposes a 
thermal detector in which the use of a thermostable material that can be 
deposited in a thin layer and that has a notably reduced level of thermal 
conductivity, enables the making of a structure in which a thin layer of 
sensitive material is made on a substrate comprising a reading circuit, 
said layer being thermally decoupled from the substrate by a layer of 
thermostable material. 
The substrate used may advantageously be a silicon type semiconductor 
substrate comprising reading circuits. In the case of matrix detectors 
that are useful in infrared imaging, the substrate may comprise processing 
circuits and a 2D matrix of reading switches. 
The thermal detector according to the invention comprises a layer 
constituting the thermal barrier, this layer being provided with 
conductive vias that enable the electrical contacts to be set up between 
the elements of the reading circuit and the lower electrodes of the layer 
of sensitive material whose physical variations in the course of a 
temperature variation have to be recorded. 
Preferably, the layer constituting the thermal barrier is a layer of 
polyimide type thermostable polymer. This layer may be got from a solution 
by a standard coating method (by spraying, centrifugation, film-casting, 
etc.). It is advantageously possible to use a solution of polyimide type 
precursor of polymer (polyimide being difficult to dissolve) that is 
conventionally used to make insulating layers in microelectronics. 
Before this solution is dried, the so-called phase-separation or 
phase-inversion technique is used. The aim of this technique is the 
conversion, in this layer, of the solution, which is initially 
homogeneous, into a two-phase system that can be used to obtain a 
polymer-rich phase that, after drying, will constitute a solid structure 
and a polymer-depleted phase that will constitute a porous structure. This 
complex structure thus leads to the preparation of a polymer with a 
microporous structure whose thermal conductivity is notably lowered, after 
evaporation of a solvent of the initial solution containing the 
thermostable polymer. 
Several methods may be implemented to carry out this phase-inversion 
technique. Preferably, the phase-inversion operation may be done by 
coagulation of the solution by immersing it a bath that is not a solvent 
of polymer but is miscible with the solvent of said solution. 
After the elimination of the solvent and of the non-solvent by drying and 
after thermal treatment to stabilize the structure notably in the case of 
a precursor of polyimide to be "imidized", there is obtained an expanded 
polymer whose microporous structure gives it low density and hence reduced 
thermal conductivity. The layer thus obtained may typically have a 
thickness ranging from one micron to some hundreds of microns. 
To obtain proper adhesion of the porous layer to the substrate, especially 
during the coagulation phase, it is possible to use a clinging layer C0 or 
a promoter of adhesion. In this particular case, it may be polyimides, 
aminosilanes or aluminium chelate. It is also possible, prior to the 
deposition of the polymer layer which will be made porous, to make a fine 
polymer layer (preferably the same one) that is dried but is not totally 
annealed so that it can be allied with this layer during the final thermal 
treatment. 
In order to ensure the imperviousness and the proper surface condition of 
the porous layer, it is possible to densify a surface zone C'1a by 
subjecting the upper face of the deposit to a summary drying before the 
phase-inversion step. This summary drying is done by setting up a 
temperature gradient within the layer. To this end, it is possible to send 
a gas flow superficially, or to deposit the substrate covered with the 
layer within a stove, the substrate/layer unit lying on a cooled plate, to 
ensure the necessary temperature gradient inside the layer. The etching of 
the connection vias can be done similarly to the etching of the standard 
polymers used in microelectronics. The technique used may be plasma 
etching or photo ablation by excimer laser. It suffices thereafter to 
deposit metallizations locally in order to set up contacts between the 
reading circuits and the lower electrodes of the sensitive layer. It may 
be noted that the cellular structure of the thermally insulating layer 
enables it to be etched at a speed which is higher than that of the same 
polymer in a nonexpanded state. This thus enables the making of layers 
with a greater thickness while at the same time keeping the same etching 
method. 
Example of the manufacture of a thermal detector according to the 
invention, based on ferroelectric material 
To a silicon substrate comprising reading circuits, there is applied a 
first thin layer of polyamic acid solution (for example the material 
referenced PIQ 13 HITACHI) which is a polyimide precursor dissolved in the 
solvent A, namely N Methyl-Pyrrolidone by spin-coating centrifugation. The 
dilution of the solution and the speed of centrifugation are adjusted in 
order to obtain a final thickness of the order of 0.5 .mu.m. This layer is 
then dried for 10 minutes at 200.degree. C. and constitutes a catching 
layer C0 on the substrate (S). On this layer, the layer designed to be 
expanded is deposited. The solution is then more concentrated and the 
centrifugation speed is reduced to obtain a greater thickness (of the 
order of 10 .mu.m). 
In order to densify the surface zone C'1a of this layer, the solvent A is 
evaporated summarily on the surface of the deposit in a stove ventilated 
at 80.degree. C. for two minutes. 
The phase inversion is then obtained by plunging the substrate covered with 
the capturing layer and the partially densified layer into a quantity of 
non-solvent B while stirring. This non-solvent may be methanol or a 
chlorinated solvent that is miscible with N-Methyl Pyrrolidone. At the end 
of some minutes, the originally transparent layer becomes totally 
diffusive during the separation of the medium into two phases. 
To eliminate the solvent A and the non-solvent B, the layer is vacuum-dried 
and then processed thermally at 300.degree. C. to convert the polyamic 
acid into polyimide. The layer C'1 and a structure illustrated in FIG. 2 
are then obtained. 
The connection vias (VC) are then etched into this layer of expanded 
polymer to gain access to the reading elements as illustrated in FIG. 3, 
by reactive ion etching. 
Then the metallization of the vias is carried out, followed by the etching, 
by photolithography, of the zones in which it is sought to eliminate the 
metallizations so as to define a matrix arrangement of the lower 
electrodes (Eij) of the thermal detector, said electrodes being thus 
disconnected from one another and being connected to the reading circuit 
of the substrate (S) as can be seen in FIG. 4. On these electrodes (Eij), 
there is then deposited the layer of sensitive material, for example a 
layer of pyroelectrical polymer with reduced thermal conductivity: for 
example, a copolymer PVDF-TrFE 75-25 (75% in moles of PVDF for 25% in 
moles of TrFE) in dimethylformamide (DMF). Several layers of 
pyroelectrical polymer can thus be deposited by centrifugation to obtain a 
thickness of five to several tens of microns. 
On this layer, there is then deposited a continuous electrode (E) to 
constitute a counter-electrode, for example in the form of an aluminium 
layer with a thickness of 1000 Angstroms, the deposition being done by 
vacuum evaporation. 
A final infrared absorbent layer (A) may then be deposited on the 
continuous electrode (E): this layer may notably be aluminium black 
obtained by evaporation of aluminium under nitrogen atmosphere according 
to a known technique. 
The continuous layer (E) may also fulfil the function (A) by being made of 
metal of the titanium or chromium type. 
After the pyroelectric polymer material has been polarized, typically with 
a DC electrical field in the region of 100 V/.mu.m, the thermal detector 
thus obtained is capable of working. 
The performance characteristics of a detector such as this have been 
compared with those of a detector made of a pyroelectric polymer film 
deposited on an insulating layer of dense polyimide. 
With a self-supported film of ferroelectric polymer having a thickness of 
10 .mu.m, a measurement is made of a pyroelectric current assigned a 
standardized value of 1 under the effect of a 10. .mu.m CO2 laser 
radiation at a frequency of 50 Hz. 
For a 10 .mu.m film of ferroelectric polymer deposited on a dense polyimide 
film with a thickness of 20 .mu.m, the measured pyroelectric current drops 
to 0.4 in terms of the standardized value. 
For a 7 .mu.m film of ferroelectric polymer (hence one that is less thick 
and generates fewer pyroelectric charges) deposited on a 
porous-structured, expanded polyimide film with a thickness of 14 .mu.m, 
the pyroelectric current measured is in the region of 0.95 in terms of the 
standardized value. This is a very conclusive result with respect to the 
thermal insulation conferred by the porous-structured dielectric layer. 
To further improve the properties of thermal insulation of the microporous 
layer according to the invention, it may be planned to fill the micropores 
of the layer not with air but with a substance that has a higher heat 
insulation coefficient than that of air. The substance may be liquid or 
gaseous. For example, the pores may be filled with xenon which is a better 
heat insulator than air. Xenon gets placed in the pores, for example, if 
solvent-elimination annealing operations are carried out in a xenon 
atmosphere rather than in under ambient atmospheric conditions. After the 
formation of the microporous layer, the xenon remains trapped in the 
pores.