Bolometric detector with antenna comprising a switch and imagery device comprising same

The bolometric detector comprises at least one receiver antenna designed to collect electromagnetic waves. The antenna is extended by a resistive load converting the power from the electromagnetic waves into calorific power. Said resistive load is in contact with a thermometric element detecting the temperature rise relating to the calorific power generated. A switch arranged between the antenna and the resistive load enables the current flow between the antenna and the resistive load to be modulated by means of control means. The switch can for example be a micro-device with a bimetal effect thermal actuator.

BACKGROUND OF THE INVENTION

The invention relates to a bolometric detector comprising at least one receiver antenna designed to collect electromagnetic waves and extended by a resistive load converting the power of the electromagnetic waves into calorific power, said resistive load being in contact with a thermometric element detecting the temperature rise relating to the calorific power generated, said detector also comprising modulating means.

STATE OF THE PRIOR ART

Antenna-based bolometric detectors are generally used to achieve cameras or imagery systems able to passively detect sub-millimeter and millimeter wavelengths. This range of wavelengths enables objects to be detected under difficult conditions such as in the dark, fog, rain, or smoke. Thus the document FR-A1-2,788,129 corresponding to WO-A1-0040937 describes a bolometric detector with antenna comprising a resistive load that creates a calorific power when the antenna detects a millimeter electromagnetic wave, the resistive load being the load resistor of the antenna.

This type of detector generally generates a large low frequency noise at the output of a thermometric element designed to measure the temperature increase. The low frequency noise generated can be limited by applying absorption modulation at the incident wave. The electrical signal detected is then translated to higher frequencies, which has the consequence of making the thermometric element and the system located down-line from the detector, such as an electronic processing circuit, work in a frequency range where they are less noisy.

To achieve this modulation, it is known to use a chopper enabling the incident wave absorbed by the detectors of an imagery system to be modulated. The chopper is formed by a wheel comprising periodic openings, the wheel being driven by a motor the frequency of rotation whereof is the frequency at which the response of the detectors is translated, thus limiting the low frequency noise. It does however present numerous shortcomings, such as that of generating mechanical vibrations, of being bulky and of presenting a fluctuating speed of rotation of the motor.

OBJECT OF THE INVENTION

The object of the invention is to provide an antenna-based bolometric detector enabling modulation so as to limit low frequency noise, the modulating means having to be of small size, dependable and vibration-free.

According to the invention, this object is achieved by the fact that the modulating means comprise a switch located between the antenna and the resistive load, control means periodically causing opening and closing of the switch at a preset switching frequency.

According to a development of the invention, the control means comprise thermal switching means.

According to another feature of the invention, the thermal switching means comprise a bimetal effect thermal actuator.

According to a preferred embodiment, the switch achieved on a substrate comprises a deformable element, attached via its two ends to the substrate and naturally convex with respect to the latter, a conducting stud, located on the deformable element, being designed to make the electrical contact between the antenna and the resistive load when the switch is closed, the antenna and resistive load being achieved on said substrate.

A further object of the invention is to provide an imagery device comprising at least one such bolometric detector.

DESCRIPTION OF PARTICULAR EMBODIMENTS

The bolometric detector is achieved by means of technologies used in the semiconductor field. It is achieved on a substrate, preferably made of silicon.

InFIG. 1, a bolometric detector with antenna1, represented by a broken line inFIG. 1, comprises at least one flat metallic receiver antenna2designed to collect electromagnetic waves, thus creating an electrical signal corresponding to the electromagnetic power detected. The antenna2is connected to a resistive load3converting the electromagnetic power into calorific power. The current flowing from the antenna2to the resistive load3creates a temperature rise of the latter by Joule effect corresponding to a calorific power given off by the resistive load.

The resistive load2is in contact with a thermometric element4, enabling the calorific power to also dissipate in the thermometric element4. The thermometric element4, preferably formed by a diode, is designed to convert the temperature increase due to its heat rise into a current or voltage variation. Thus, if no electromagnetic waves are detected, a reference current flows through the thermometric element4, the reference current corresponding to the temperature of the thermometric element4in the absence of heating.

When an electromagnetic wave is detected, the thermometric element4heats and the current flowing therethrough differs from the reference current. An electronic processing circuit5connected to the thermometric element4detects and analyses the variation of the current flowing through the thermometric element4and therefore the temperature variation due to the heat rise of the thermometric element4.

The bolometric detector1comprises a switch6arranged between the antenna2and the resistive load3. Control means periodically cause opening and closing of the switch6at a preset switching frequency. When the switch is closed, the electric current generated by the detected electromagnetic waves flows from the antenna to the resistive load. The control means can be of any known type and comprise switching means which can be for example electrostatic, thermal or electromagnetic, and which are controlled by the electronic processing circuit5(FIG.1).

The periodical opening and closing of the switch6enables the electrical signal transmitted to the resistive load3to be modulated in amplitude so as to limit the low frequency noise received by a device located down-line from the detector, such as the electronic processing circuit5. The switching frequency of the switch is lower than the thermal cut-off frequency of the detector. A thermal variation occurring at a frequency higher than the thermal cut-off frequency of the detector can not in fact be detected. The thermal cut-off frequency is equal to the ratio C/R, C being the heating capacity of the thermometric element and R the thermal resistance of the resistive load. The switching frequency of the switch6is preferably about a few tens to a few hundred Hertz.

According to a particular embodiment represented inFIG. 2, the bolometric detector1, achieved on a silicon substrate (not shown in FIG.2), comprises two opposite flat metallic elements corresponding to a first and a second receiver antenna2aand2b, triangular in FIG.2. They are arranged on each side of a hollow central part8, along an axis of symmetry s1.

An arm10, preferably made of metal, in the form of a beam constituting the resistive load of the detector, is suspended above the hollow central part8along the axis s1. It comprises a first end directly connected to the first antenna2aand a second end connected to the second antenna2bby means of a switch6. Electrodes7connect the switch6to the electronic processing circuit5which periodically causes opening and closing of the switch6so as to modulate the electrical signal generated by the antennas before the latter is input to the resistive load.

A diode9is fitted on the arm10, above the hollow central part8, in thermal contact with the resistive load constituted by the arm10. Thus, when the arm10receives the electrical signal corresponding to the detected wave, it heats and this heat rise is transmitted to the diode9, causing a current or voltage variation in the diode9. The diode9is connected to terminals11connecting it to the electronic processing circuit5, which measures and analyses the current or voltage variation generated by the heat rise.

InFIG. 3, the bolometric detector1is achieved on a silicon substrate12divided into two parts12aand12bboth designed to support a receiver antenna. The two parts12aand12bare separated by the hollow central part8. The first part12ais totally covered by a first layer13of silicon oxide itself covered by a second layer14of epitaxial silicon, the first antenna2abeing arranged on the second layer14. The second part12bof the substrate is covered by the first and second layers13and14, wherein a cavity15is formed whereon the switch6is arranged. The second antenna2b, arranged on the second layer14, is adjacent to the switch6. The latter is thus arranged between the second antenna2band the hollow central part8.

The two parts12aand12bof the substrate are connected by the arm10forming a beam and supporting the diode9above the hollow central8. A first free end of the arm10is arranged between the second layer14of the first part12aof the substrate and the first antenna2a, whereas the second free end of the arm10is connected to a first connection terminal18aof the switch6, located at the bottom of the cavity15. The second antenna2bis also connected to a second connection terminal18bof the switch6, located at the bottom of the cavity15, so that the first and second connection terminals18aand18bare separated by a space of preset width.

The switch6preferably comprises a deformable element16, attached via its two ends to the first layer13. The deformable element is naturally convex with respect to the plane of the substrate12. A conducting stud17is provided in the convex part of the deformable element16, on the same side as the cavity15. The switching means enable the deformable element16to periodically switch from a first state (FIG.3), corresponding to its naturally convex shape, to a second state (FIG.4). The second state of the deformable element16enables the conducting stud17to make the electrical contact between the connection terminals18aand18b, and consequently between the second antenna2band the metal arm10. The space comprised between the two connection terminals18aand18bis smaller than or equal to the width of the stud17.

Switching of the switch can be achieved by any known type of means. A bimetal effect thermal actuator, such as the micro-devices with bimetal effect thermal actuator described in the documents FR-A1-2,818,795 corresponding to US-A1-2002097133 and FR-A1-2,772,512, can for example be used to cause periodic opening and closing of the switch. InFIG. 2, the electrodes7are formed by coils constituting heating elements.

According to another embodiment, the deformable element16is made of magnetic material and a coil is arranged on the substrate12so as to cause periodic opening and closing of the switch by means of electromagnetic switching means.

In another alternative embodiment, the stud17can be electrostatically attracted by a part of the second antenna2b.

The use of a switch of the type of those described above enables the electrical signal sent to the resistive load to be modulated, while at the same time presenting the advantage of being integrated in the bolometric detector, thus making the volume occupied by the latter small. As this type of switch is not mechanical, it does not create any vibration troubling operation of the detector and is very dependable.

The switch also enables the very low frequency noise inherent to a pixel to be limited and enables adjustment to be made on the offset voltage fluctuation. It is then not necessary to use an optically shielding screen to eliminate this fluctuation, which presents the advantage of eliminating an embedded mechanical system.

An imagery device comprising at least one bolometric detector such as those described above presents the advantage of detecting objects under difficult conditions, while limiting the low frequency noise generated by the detector. The imagery device can also comprise a matrix of bolometric detectors.

The invention is not limited to the embodiments described above. Several bolometric detectors can be arranged in such a way that their respective diodes are connected in parallel, thus forming a matrix of detectors used in an imagery system enabling the range of wavelengths detected to be broadened.