MULTISPECTRAL INFRARED DETECTOR

Abstract

The invention relates to an infrared detector comprising an array (11) of elementary detectors (14) forming the pixels of an infrared image; – each elementary detector (14) being mounted in suspension on two anchor nails (21) with at least one anchor nail (21) shared with another elementary detector (14); – the elementary detectors (14) being arranged in columns (Co1-Co3) with, at the foot of each column (Co1-Co3), a reading circuit dedicated to said column (Co1-Co3); and – addressing means allowing successive connection of all the elementary detectors (14) of a column (Co1-Co3) to the reading circuit associated with said column (Co1-Co3); – the columns (Co1-Co3) being formed by an alignment of a single row of anchor nails (21), the pixels (17) of two consecutive columns (Co1-Co3) being placed in a staggered pattern. Figure for the abridged version: Fig 9

Description

Description Title of the invention: MULTI-INFRARED DETECTOR TISPECTRAL Technical field

[0001] The present invention relates to the field of detection of electromagnetic radiation- genetics and, more precisely, to the detection of infrared radiation.

[0002] — The invention relates more specifically to the problem of multisensor detection- pectral, that is, the detection of several distinct wavelength ranges in the thermal imaging wavelength range, i.e. the range between 8 and 14 micrometers.

[0003] — By crossing the information resulting from the detection in several ranges of wavelengths, the invention aims to obtain the absolute temperature of objects of interest of an infrared image.

[0004] — Thus, the invention can be implemented in many fields for which infrared images are currently used, such as the areas of aerospace, security, transportation, thermography, in- industrial, building inspection, leisure, health... State of the art

[0005] — In the field of detectors used for infrared imaging, it is known to use devices arranged in matrix form, capable of operating at room temperature, i.e. not requiring cooling to very low temperatures, unlike detection devices called "detectors quantum” which require operation at very low temperatures. These de- Detectors traditionally use the variation of a physical quantity of a material or assembly of appropriate materials depending on the temperature, in the vicinity of 300K.

[0006] — In the particular case of micro-bolometric detectors, the most commonly used, this physical quantity is electrical resistivity, but other quantities can be exploited, such as the dielectric constant, polarization, expansion thermal, refractive index, etc.

[0007] — Such an uncooled detector generally combines: — means of absorbing thermal radiation and converting it into heat ; — means of thermal insulation of the detector, so as to allow it to heat up under the action of thermal radiation; — thermometry means which, within the framework of a micro-bolometric detector, implement a resistive element whose resistance varies with temperature; — and means for reading the electrical signals provided by the thermometry means. As illustrated in [Fig. 1] of the prior art, detectors intended for thermal, or infrared, imaging are conventionally produced in the form of a matrix 110 of elementary detectors 140, forming image points or pixels 170, in one or two dimensions. To guarantee the thermal insulation of the elementary detectors 140, the latter are suspended above a substrate via support arms 19. More precisely, an elementary detector 140 is constituted by a membrane 18 incorporating a thermo-resistive material which carries out a transduction of the infrared radiations, forming the thermometry means. This membrane 18 is held in fixed suspension above the substrate by the support arms 19, a first end of which is fixed on the membrane 18 and a second end of which is fixed on anchoring nails 21. The measurement of the electrical resistance of the thermo-resistive material of the membrane 18 is ensured in particular by two electrodes, for example metallic, extending under the thermometric material and in the support arms, these electrodes being in electrical contact with the anchoring nails 21. To obtain the desired integration density of the elementary detectors 140, it is conventional to share the anchor nails 21 of two consecutive elementary detectors 140. Furthermore, each elementary detector 140 can be suspended on two or four anchor nails 21. The architecture making it possible to obtain the greatest integration density is the so-called “one nail” architecture illustrated in [Fig. 1], in which each elementary detector 140 is mounted on two shared anchor nails 21. Thus, each elementary detector 140 uses a half-nail twice, i.e. an anchor nail 21. This sharing of anchor nails 21 is only possible when the elementary detectors 140 of the same column are read consecutively, while those of the same row are read simultaneously. This makes it possible to connect the detectors of the same column together since they are not polarized and read simultaneously. To do this, the elementary detectors 140 are conventionally aligned on a horizontal axis A1 defining the base of the columns and on a vertical axis A2 defining the height of the columns. In practice, in order to be able to share the anchor nails 21, the connection of the elementary detectors 140 to its anchor nails 21 is made in two opposite corners of the elementary detector 140, that is to say along a diagonal of said detector formed between two anchor nails 21. Thus, as illustrated in the equivalent electrical diagram of [Fig.2], a column of elementary detectors 140 is formed by two rows of anchor nails 21 with elementary detectors 140 connected in a zigzag pattern on the anchor nails 21; that is to say that there is a mirror symmetry between two elementary detectors 140 placed one below the other on a column. Furthermore, these anchoring nails 21 extend from the plane of the membrane 18 to the substrate to form the poles of each elementary detector 140, that is to say the points between which the electrical resistance of each elementary detector 140 can be measured. To carry out such a reading, the substrate usually comprises sequential addressing means 150 of the elementary detectors 140. In the example of [Fig.2], the resistance Rp, of the elementary detector 140 of the first row and of the first column is measured between the nails C1 and C2 by connecting the sequential addressing means 150 to the nail C1 on the one hand and to the nail C2 on the other hand. The substrate also comprises means for electrical excitation and pre-processing of the electrical signals generated from these elementary detectors 140. These means, generally integrated into the substrate, are commonly referred to by the expression “reading circuit”. [Fig. 3] illustrates a reading circuit 16 associated with sequential addressing means 150 making it possible to consecutively select the different elementary detectors 140 of the same column. To do this, an elementary detector 140 is previously biased to a voltage Vpol via a transistor FID. The current flowing through the transistor FID and the elementary detector 140 is fixed upstream by a base branch comprising a blind elementary detector and a transistor GSK supplied by a voltage VSK. The reading circuit 16 also includes a CTIA type integrator, for the English expression "Capacitive Trans Impedance Amplifier", comprising an operational amplifier and a capacitor Cint. Thanks to the basing branch, only the current variations caused by the infrared radiation coming from the scene are integrated into the CTIA. Thus, the quiescent current of the elementary detector 140 is not integrated by the CTIA. For reasons of space occupation, the CTIA, the capacitor Cint, the base line and the transistor FID are common to all the pixels of a column and these elements are conventionally located at the bottom of each column. To obtain a scene via these detectors, it is captured through an optic adapted to the matrix of elementary detectors 140, and clocked electrical stimuli are applied via the reading circuit 16 to each of the elementary detectors 140, or to each row of such detectors, in order to obtain an electrical signal constituting the image of the temperature reached by each of said elementary detectors 140. This signal is processed in a more or less elaborate manner by the reading circuit 16, then possibly by an electronic device external to the housing, in order to generate a network of pixels 170 arranged in rows and columns Col, Co2, Co3, Co4, Co5, as illustrated in [Fig.4]. More generally, all of these pixels 170 form an infrared image 130 of the observed scene, as illustrated in [Fig.5]. In this infrared image 130, the temperature of an object of interest can be estimated by knowing the temperature of a reference object in the image and by looking for the difference in signal measured by the detector between the reference object and that of the object of interest, making the approximation that the two objects have the same emissivity. Thus, it is possible to use an infrared image 130 to obtain a measurement of the relative temperature of an object of interest. However, it has been sought for a long time to obtain a measurement of the absolute temperature of objects in a scene captured by an infrared image, that is to say to measure the temperature of objects of interest without having to know the temperature of a reference object. To do this, it is sought to obtain several distinct responsivity measurements on the same point of an infrared image, for example with several elementary detectors 140 sensitive in different spectral bands. From several distinct responsivity measurements, it is possible to determine the absolute temperature, that is to say a measurement of the temperature of an object of interest without using a reference object. The responsiveness of an elementary detector 140 obtained by the reading circuit 16 is defined according to the following equation: [Math.1] Vpol 4 Ti Resp=Rth*"TCR*PIR*4ES at In this equation, the parameter Rth describes the thermal resistance of the elementary detector 140, the parameter Rbol describes the electrical resistance measured between the poles, and the parameter TCR relates to the material of the elementary detector 140. These parameters are therefore fixed during the production of the elementary detectors 140. To obtain elementary detectors 140 with distinct responsivities, it would therefore be possible to structurally modify the shape or the nature of the membrane of several juxtaposed elementary detectors 140. However, the different elementary detectors 140 of a matrix 110 are conventionally produced simultaneously and the differentiated production of several elementary detectors 140 would greatly complicate the production method.For example, increasing the Rth parameter or the TCR parameter on certain elementary detectors 140 to increase their responsiveness is not possible, because these parameters are already maximized as much as the . allow the available means. The infrared power PIR illustrates the infrared power received by the elementary detector 140. Another solution for obtaining distinct responsivities would therefore be to modify the infrared power transmitted to several juxtaposed elementary detectors 140. This solution would require modifying the optical window conventionally positioned above the elementary detectors 140 by customizing the optical window to each elementary detector 140. This solution is also not easily achievable, because the optical window is conventionally common to all the elementary detectors 140 and this solution would pose problems of alignment of the optical window with the elementary detectors 140. The parameters Vpol, Tint and Cint are linked to the reading circuit 16. By using the same reading circuit for a column, it seems difficult to dynamically modify the bias voltage Vpol, or the integration time Tint; and impossible to change the integration capacitor Cint to adjust the responsiveness of the different pixels of said column. In view of these constraints, the person skilled in the art could possibly obtain distinct responsivities by using distinct reading circuits between two consecutive columns and by grouping by columns the responsivities of the same type, that is to say the elementary detectors 140 whose reading circuits 16 are configured to capture distinct responsivities. To do this, the polarization voltage Vpol or, more simply, the capacitance of the capacitor Cint can vary from one column to another. This embodiment is illustrated in [Fig. 6]. It should be noted that this embodiment does not illustrate the state of the art, but only a hypothetical embodiment which would possibly be conceivable for a person skilled in the art wishing to obtain an infrared detector with pixels 171 making it possible to capture distinct responses without complicating the manufacturing process. As illustrated in [Fig.6]: — a first column Col could integrate a reading circuit 16 configured so that the pixels 171 of the first column Col illustrate a first responsiveness, — a second column Co2 could integrate a reading circuit 16 configured so that the pixels 172 of the second column Co2 illustrate a second responsiveness, and — a third column Co3 could integrate a reading circuit 16 configured so that the pixels 173 of the third column Co3 illustrate a third responsiveness. This pattern of three columns Col, Co2 and Co3 can thus be repeated to form a infrared image integrating three distinct responsivities for each point of the infrared image. This hypothetical embodiment would, however, pose a problem of spatial homogeneity of the image points. Indeed, each image point would then consist of a super-pixel 38 formed of three pixels 171-173 arranged in a line. It follows that the pitch between the super-pixels 38 would not be identical in the two dimensions x, y of the image thus formed, a first pitch would be one pixel in the y dimension, while a second pitch would be three pixels in the x dimension. This spatial inhomogeneity would pose detection problems because two-thirds of the pixels in the x dimension would be used to increase the number of responsivities, and no longer to guarantee spatial resolution. It follows that objects present on the scene would be more easily detectable along the y axis than along the x axis, thus distorting the characteristics of the resulting infrared image. To conclude, the technical problem of the invention is to obtain a multispectral infrared detector, that is to say a detector making it possible to capture distinct responsivities for each image point, with a homogeneous distribution of distinct pixels and without complicating the manufacturing process. Statement of the invention To address this technical problem, the invention proposes an infrared detector in which the columns are formed by an alignment of a single row of anchor nails, each elementary detector being produced between two superimposed anchor nails of the same column. Unlike the prior art in which a column of elementary detectors is formed by two rows of anchor nails, the formation of a column of elementary detectors by an alignment of a single row of anchor nails makes it possible to nest the elementary detectors in a staggered manner and, thus, to improve the homogeneity of the distribution of the distinct pixels. The invention therefore relates to an infrared detector comprising a matrix of elementary detectors forming the pixels of an infrared image; — each elementary detector being suspended on two anchor nails with at least one anchor nail shared with another elementary detector; — the elementary detectors being arranged in columns with, at the foot of each column, a reading circuit dedicated to said column: and — addressing means for successively connecting all of the elementary detectors of a column to the reading circuit associated with said column. The invention is characterized in that the columns are formed by an alignment of a single row of anchor nails, the pixels of two consecutive columns —or of two consecutive lines - being placed in a staggered fashion. For the purposes of the invention, pixels placed in a “staggered” pattern means that the center of the pixels in a column is not aligned horizontally with the center of the pixels in its neighboring columns, in a pixel matrix having a constant pitch. The term “horizontally” refers to an alignment parallel to an axis defining the base of the columns, and correspondingly, the term “vertically” refers to an alignment perpendicular to the axis defining the base of the columns. To ensure optimal nesting of pixels in neighboring columns, the anchor nails of two adjacent columns are preferably offset horizontally and vertically by half a step so that all pixels have edges aligned along axes forming an angle of 45° with the axis defining the base of the columns. Thus, compared to the pixels of the prior art, each pixel has undergone a rotation of 45° on itself. Preferably, this specific positioning of the pixels makes it possible to form a multispectral infrared detector, that is to say a detector making it possible to capture distinct responsivities. To do this, the readout circuits of two adjacent columns have separate bias voltages or integration capacitors. More precisely, it is advantageous to capture three distinct responsivities according to a homogeneous spatial distribution in each direction to form each image point. In this embodiment, three consecutive columns are configured to measure three distinct responsivities: — a first responsiveness in a range of wavelengths; — a second responsivity in a first sub-part of said wavelength range; and — a third responsivity in a second sub-part of said wavelength range distinct from the first sub-part. Typically, the wavelength range of said first responsivity corresponds to the range of 8 to 14 micrometers. Preferably, the first sub-portion of said second responsivity corresponds to the wavelength range between 8 and 11 micrometers and the second sub-portion of said third responsivity corresponds to the wavelength range between 11 and 14 micrometers. Brief description of the figures The manner of carrying out the invention as well as the advantages which result therefrom will emerge clearly from the following embodiments, given for informational but non-limiting purposes, supported by figures 1 to 15 in which: [Fig.1] is a schematic top view of an infrared detector comprising elementary “one nail” detectors according to the state of the art; [Fig.2] is a schematic representation from above of the equivalent electrical diagram of the detector of [Fig.1]; [Fig.3] is an equivalent electrical diagram of the addressing means and the reading circuit of the detector of [Fig.1]; [Fig.4] is a schematic representation of several pixels of the detector of [Fig.1]; [Fig.5] is an illustration of an infrared image captured by the detector of [Fig.1]; [Fig.6] is a schematic representation of a hypothetical embodiment incorporating responsivities associated with distinct columns; [Fig.7] is a graphical representation of several distinct responsiveness measurements according to a first embodiment; [Fig.8] is a graphical representation of several distinct responsiveness measurements according to a second embodiment; [Fig.9] is a schematic representation from above of an infrared detector comprising elementary detectors according to an embodiment of the invention; [Fig.10] is a schematic representation from above of the equivalent electrical diagram of the detector of [Fig.9]; [Fig.11] is a schematic representation from above of the equivalent electrical diagram of a hypothetical detector integrating a displacement of the elementary detectors of the state of the art; [Fig.12] is an equivalent electrical diagram of the addressing means and the reading circuit of the detector of [Fig.9]; [Fig.13] is a schematic representation of several pixels of the detector of [Fig.9]; [Fig.14] is an illustration of an infrared image captured by the detector of [Fig.9]; and [Fig.15] is a schematic representation of several pixels of the detector of [Fig.9] forming different points of the infrared image of [Fig.14]. Detailed description of the invention The invention proposes a new solution for forming the elementary detectors 14 of an infrared detector in order to guarantee the nesting of the pixels 17 of several distinct columns Co1, Co2, Co3. This solution is particularly effective for obtaining a multispectral infrared detector, that is to say an infrared detector making it possible to capture image points formed from several pixels obtained with distinct responsivities. As illustrated in [Fig.7], a multispectral detector can be used to capture a first responsivity Resp1 in the wavelength range of 8 to 14 micrometers; as well as two responsivities Resp2, Resp3 associated with sub-parts of this wavelength range, for example a responsivity Resp2 in the range of 8 to 11 micrometers and a responsivity Resp3 in the range of 11 to 14 micrometers. An image point is then formed from these three responsivity information captured by several separate elementary detectors 14. Furthermore, as illustrated in [Fig.8], it is possible to amplify the responsivities captured by certain elementary detectors 14, for example those used to capture the sub-parts of the wavelength range from 8 to 14 micrometers. To ensure homogeneity in the arrangement of the elementary detectors 14 associated with distinct responsivities, the invention proposes to form columns Co1 -Co3 by aligning a single row of anchoring nails 21, unlike the state of the art illustrated in [Fig.1] which uses two rows of anchoring nails 21 per column. Thus, the embodiment of the invention illustrated in [Fig.9] has juxtaposed columns with a single row of anchor nails 21 per column. The structure of each elementary detector 14 is not modified, so that each elementary detector still comprises a membrane 18, support arms 19 and anchor nails 21. The membrane 18 and the support arms 19 can be produced using traditional techniques from the microelectronics and MEMS (Micro Electron Mechanical Systems) industries using a sacrificial layer on which the electrodes and the membrane layers are deposited. More specifically, a support layer is conventionally deposited on a sacrificial layer at which the anchoring nails 21 are flush. The metal electrodes are deposited on this support layer. The thermometric material, for example a layer of vanadium oxide, is then deposited in contact with the two electrodes connected to the two anchoring nails of the elementary detector 14. An encapsulation layer is then conventionally deposited on the electrodes and the thermometric layer. The support arms 19 are structured to ensure thermal insulation of the membrane 18 from the substrate. It should be noted that the shape of the membrane 18 and the support arms 19 may be similar to that of the elementary detectors 140 of the prior art, as illustrated in [Fig.1]. Unlike the state of the art, although the elementary detectors are formed in the diagonal of a pixel 17 between two anchor nails 21, this diagonal is now arranged perpendicular to the axis A1 of the base of the columns. The rectangular shape of the 17 pixels now extends along two axes A3 and A4 oriented respectively at an angle of 45° to the A1 axis. It follows that the columns are formed by squares or rectangles aligned by the vertices of the pixels 17 instead of being aligned by their base. In the equivalent electrical diagram illustrated in [Fig. 10], the resistors are therefore aligned in the height of the columns Co1-Co3 instead of being arranged in a zigzag pattern. The sequential addressing means 15 are simple to implement. For example, to capture the resistance value of the elementary detector 14 highlighted in [Fig. 10], it is sufficient to connect the anchor nails C1 and C2 to measure the resistance Rpol- It should be noted that the equivalent electrical diagram does not result from a simple 45° turn of the equivalent diagram of [Fig.2], since this simple 45° turn is illustrated in [Fig.11] and it would always present bolometric resistances arranged in a zigzag pattern. Thus, to arrive at the structure of the invention, it is necessary to modify the topology of the anchor nails 21 and also to modify the construction diagonal of each resistance of the elementary detectors 14. Regarding the reading circuit 16 of the invention, it can be structurally identical to that of the prior art by using an operational amplifier and a capacitor Cint. The capacitor Cint is connected between the inverting input and the output of the amplifier, while the non-inverting input is supplied by a constant voltage VBUS. Conventionally, the reading of a resistor Ro of an elementary detector 14 can be carried out by polarizing the resistor Ra at a voltage Vpol which is applied via the transistor FID. Thus, the current variations passing through this resistor Rya, which are caused by the incident infrared radiation, are integrated on the capacitor Cint to obtain the responsivity Resp. This strategy for forming the elementary detectors 14 makes it possible to obtain pixels 17 placed in a staggered pattern, as illustrated in [Fig.13]. Although these pixels 17 are oriented angularly relative to the axis A1 of the base of the columns, when the number of pixels 17 thus captured is large, the orientation of the pixels 17 is no longer visible because a pixel provides information on a point in space and its arrangement has no effect on the format of the image. It is therefore possible to obtain a rectangular infrared image 13 with the invention. Unlike the infrared image 130 of the state of the art, the measurement of several responsivities also makes it possible to obtain more precise information 40 on the absolute temperature of the objects in the observed scene. Thus, the infrared image 13 of [Fig.14] illustrates, for example, the capture of the precise temperature of the individuals moving around the scene. To do this, the reading circuits 16 associated with several juxtaposed columns preferably have polarization voltages Vpol or separate integration capacitors Cint. As illustrated in [Fig.15], the first column Co1 allows for example to capture a first responsivity Respl, typically a responsivity in the wavelength range of 8 to 14 micrometers, while a second column Co2 allows to detect the second responsivity Resp2 and the third column Co3 allows to capture the third responsivity Resp3. This alternation of responsivities is then reproduced to form all the columns Co1-Co3 of the infrared image 13. Due to this staggered placement of the Col-Co3 columns, the super pixel 37 obtained by integrating the different responsivities forms a square of two pixels by two pixels 17. Thus, the pitch between two super pixels 37 is homogeneous in both directions, respectively horizontal and vertical of the image. It follows that the invention makes it possible to obtain an infrared image 13 with image points having several distinct responsivities, making it possible to measure an absolute temperature, for example to obtain additional information 40 on the infrared image 13.

Claims

Claims

1. Infrared detector comprising an array (11) of ele- elements (14) forming the pixels (17) of an infrared image (13); — each elementary detector (14) being mounted in suspension on two anchor nails (21) with at least one anchor nail (21) shared with another elementary detector (14); — the elementary detectors (14) being arranged in columns (Co1-Co3) with, at the foot of each column (Col-Co3), a reading circuit (16) dedicated to said column (Co1l-Co3); and — addressing means (15) allowing successive connection the set of elementary detectors (14) of a column (Co1l-Co3) to the reading circuit (16) associated with said column (Co1-Co3); characterized in that the columns (Co1-Co3) are formed by a alignment of a single row of anchor nails (21), pixels (17) of two consecutive columns (Co1-Co3) being placed in a staggered manner.

2. Infrared detector according to claim 1, in which the nails anchoring (21) of two consecutive columns (Col-Co3) are offset horizontally and vertically by half a step so that all pixels (17) have edges aligned along axes (A3, A4) forming an angle (a) of 45° with an axis (A1) defining the base of the columns (Col-Co3).

3. An infrared detector according to claim 1 or 2, wherein the reading circuits (16) of two consecutive columns (Co1-Co3) have bias voltages (Vpol) or capacitors distinct integration (Cint).

4. An infrared detector according to claim 3, wherein three consecutive columns (Co1-Co3) are configured to measure three distinct responsivities (Resp1, Resp2, Resp3): — a first responsivity (Resp1) in a range of lengths wave; — a second responsivity (Resp2) in a first subsection of said wavelength range; and — a third responsivity (Resp3) in a second subsection of said wavelength range distinct from the first sub-part.

5. An infrared detector according to claim 4, wherein the wavelength range wavelength of said first responsivity (Respl) corresponds to the range of 8 to 14 micrometers.

6. An infrared detector according to claim 5, wherein the first subsection of said second responsivity (Resp2) corresponds to the wavelength range between 8 and 11 micrometers.

7. An infrared detector according to claim 5 or 6, wherein the second sub-part of said third responsivity (Resp3) corresponds to the wavelength range between 11 and 14 micrometers.