WO2018015656A1

WO2018015656A1 - Bolometric detector with guided mode filter matrix

Info

Publication number: WO2018015656A1
Application number: PCT/FR2017/051962
Authority: WO
Inventors: Giacomo BADANO; Jean-Jacques Yon
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2016-07-21
Filing date: 2017-07-19
Publication date: 2018-01-25
Also published as: FR3054318A1; FR3054318B1

Abstract

The invention relates to a matrix bolometric detector, each pixel (100) comprising a bolometric panel (110) suspended above a substrate (120). Each pixel (100) further comprises: a respective capsule (130), forming a cavity (140) inside which the bolometric panel (110) extends; and a diffractive structure (150), consisting of pads (151) distributed at a regular spacing (p) on an upper wall (132) of the capsule, on the opposite side from the bolometric panel, or in the thickness of said upper wall, in order to form, with said upper wall, a guided mode transmission filter. The invention also relates to a bolometric detector with high spectral finesse. The invention has a particularly advantageous application in the production of a multispectral bolometric detector, comprising a plurality of types of pixels each dedicated to the detection of a given wavelength.

Description

BOLOMETRIC DETECTOR WITH GUIDED MODE FILTER MATRIX

DESCRIPTION

TECHNICAL AREA

The invention relates to a bolometric detector, that is to say a detector comprising an absorber element, which absorbs incident electromagnetic radiation and whose temperature increases in response to this absorption, and a thermometric element of which an electrical or magnetic variable varies. with the temperature.

Such a detector may be in matrix form, consisting of a plurality of elementary bolometers juxtaposed each forming a pixel of the detector.

A bolometric detector is adapted in particular to detect electromagnetic radiation located in the infrared, in particular at wavelengths between 0.7 μιη and 5 mm.

STATE OF THE PRIOR ART In the prior art, bolometric detectors of the matrix type are known, in which each pixel is formed by a bolometer as described in FIG. 1A of the patent application FR-2977937.

In each pixel, the bolometer comprises a bolometric plate suspended above a reflector, at a distance λο / 4 thereof, where λο is the central wavelength of a spectral detection band.

The bolometric plate comprises an absorber element such as a thin metal layer, which absorbs incident electromagnetic radiation and whose temperature increases in response to this absorption, and a thermometric element whose resistivity varies with temperature.

The reflector and the bolometric plate together form a quarter-wave cavity allowing high absorption, typically 90%, over a spectral band ranging from 8 to 12 μιη or over a spectral band ranging from 3 μιη to 5 μιη. A disadvantage of this bolometer is that it does not offer a high spectral selectivity, all the wavelengths being bsorbed over a wide spectral band, several micrometers wide.

To overcome this drawback, patent application FR-2977937 proposes to deposit, on the bolometric plate, a metal-insulator-metal stack (M I M structure). At least one side dimension of the stack is determined to generate a plasmon resonance with incident frequency radiation occurring within said broad spectral band. A bolometer having a high spectral selectivity is thus obtained.

A disadvantage of these stacks deposited on the bolometric plate is that they increase the thermal mass of the latter, and therefore its thermal time constant.

An object of the present invention is to provide a bolometric detector of matrix type, wherein each pixel is a bolometer having a high spectral selectivity, and which does not have at least one of the drawbacks of the prior art.

In particular, an object of the present invention is to provide a bolometric detector of the matrix type, in which each pixel is a bolometer having a high spectral selectivity, and a reduced thermal mass equivalent to that of the bolometric plate considered alone.

STATEMENT OF THE INVENTION

This objective is achieved with a bolometric detector comprising a matrix of pixels, each pixel comprising a bolometric plate suspended above a substrate.

According to the invention, each pixel further comprises:

a respective capsule, forming a cavity inside which the bolometric plate extends; and

a three-dimensional structure, called a diffractive structure, consisting of studs distributed in a regular pitch on an upper wall of the capsule, on the side opposite to the bolometric board, or in the thickness of said upper wall, to form with said upper wall a mode filter guided in transmission.

It is known to implement an encapsulation of bolometric plates of a bolometer array, in order to place these bolometric plates under a controlled atmosphere, in particular under vacuum or under reduced pressure.

Individual encapsulation of each bolometric plate of a matrix of bolometers is also known per se, in particular from document EP 1 243 903. This individual encapsulation is known to those skilled in the art under the name "Pixel Level Packaging".

An idea underlying the invention is to implement an individual encapsulation of each bolometric plate of a bolometers matrix, and to exploit this individual encapsulation to refine a spectral sensitivity of each pixel of said matrix.

In particular, a capsule covering said bolometric plate is used to form a guided mode filter, by disposing on this capsule a diffractive structure.

No filter element is deposited directly on the bolometric plate, so that the filtering does not increase the thermal mass of the bolometric plate.

A bolometric detector of matrix type is thus obtained, in which each pixel is a bolometer having a high spectral selectivity, and a low thermal time constant (short response time).

The low thermal time constant is obtained without reducing the support arm section supporting the bolometric plate, therefore without degradation of the thermal insulation of the bolometric plates, and therefore without loss of sensitivity of the bolometer.

In addition, the manufacture of such a bolometric detector involves only a reduced number of steps, since the encapsulation of bolometric plates is exploited for spectral filtering.

It may be noted that it would not have been possible to deposit a MIM structure as described in the introduction, on an encapsulation element of the bolometric plate, because the MIM structure only offers effective filtering if it works in absorption. , not in transmission. Preferably, the pixel matrix is formed of several types of pixels which differ from each other by the lateral dimensions of the upper wall of the capsule, and by the distribution pitch of the pads of the diffractive structure.

In each pixel, the distribution pitch of the pads of the diffractive structure and the central wavelength of a spectral sensitivity range of the pixel are advantageously connected by the relation: p = with N _e ff the effective index of the guided mode in the upper wall of the capsule, p is the pitch of the pads of the diffractive structure, λ _£ and the central wavelength of the spectral sensitivity range of the pixel.

Preferably, in each pixel, the diffractive structure is an array of square-section studs distributed in a square or rectangular grid.

In each pixel, the diffractive structure may be metal.

In each pixel, the upper wall of the capsule preferably has a thickness of j ^ - 250 and + 250 nmj nmj with: _λ Μ the central wavelength of a spectral sensitivity range of the bolometric detector; and

N _c the average refractive index of the upper wall of the capsule, at the wavelength λ _Μ .

In each pixel, the lateral dimensions of the upper wall of the capsule can be adapted so that the mode guided in said upper wall forms a standing wave.

Preferably, in each pixel, the upper wall of the capsule has a square section, whose width is smaller than the pitch of the pixel matrix, and is equal to: m * with

m an integer greater than or equal to unity; λ _£ the central wavelength of the spectral sensitivity range of the pixel; and

N _e ff the effective index of the mode guided in the upper wall of the capsule.

Said integer may be the largest integer allowing said width to be smaller than the pitch of the pixel array.

In each pixel, the distance between the upper wall of the capsule and the bolometric plate, is advantageously between and, with λ _Μ the wavelength

10 6

center of a range of spectral sensitivity of the bolometric detector.

The different types of pixels can also be distinguished from each other by the dimensions of the bolometric plate.

Preferably, the different types of pixels are then distinguished also by the dimensions and / or the support arm structure, which extend parallel to the bolometric board to support it above the substrate, the dimensions and / or the structure of said support arms defining a thermal insulation between the bolometric plate and the substrate, and from one pixel to another of the matrix of pixels, said thermal insulation is all the higher as the surface of the bolometric plate is low.

In each pixel, the capsule advantageously has at least one through opening, said vent, each vent being closed by a stud of the diffractive structure.

In a variant, in each pixel, the capsule may have at least one through-opening, known as a vent, each vent being closed by a blocking stud that is distinct from the pads of the diffractive structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given purely by way of indication and in no way limiting, with reference to the appended drawings in which:

Figures 1A and 1B schematically illustrate, in a sectional view and in a view from above, a pixel of a bolometric detector according to the invention;

FIG. 2 schematically illustrates, in a view from above, an example of a bolometric detector according to the invention; FIG. 3 illustrates the spectral selectivity of different types of pixels of a bolometric detector according to the invention;

FIG. 4 illustrates a first embodiment of a method for manufacturing a bolometric detector according to the invention;

FIG. 5 illustrates steps of a second embodiment of a method for manufacturing a bolometric detector according to the invention; and

FIG. 6 schematically illustrates, in a sectional view, a pixel according to an advantageous variant of a bolometric detector according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The bolometric detector according to the invention is intended for the detection of an electromagnetic radiation in the infrared.

This is in particular a matrix detector, consisting of a plurality of pixels arranged for example in rows and columns, into a matrix of pixels.

For the sake of legibility of the figures, FIGS. 1A and 1B illustrate schematically a single pixel 100 of a bolometric detector according to the invention.

The width P of pixel 100 defines the pitch of the pixel array.

The pixel 100 comprises in particular a bolometric plate 110, suspended above a substrate 120, resting on support elements 111 via support arms 112.

The support elements 111 form vertical feet, which extend along an axis orthogonal to the plane of the bolometric plate 110. The support elements 111 regulate the distance between the substrate 120 and the bolometric plate 110.

The support arms 112 extend in the plane of the bolometric plate, and connect it to the support elements 111. The thermal insulation between the substrate and the bolometric plate 110 depends on the dimensions of the support arms 112.

In the bolometric matrix detector according to the invention, all the pixels are formed side by side on the same substrate. The bolometric plate 110, also called bolometric membrane, consists of an absorption membrane and a thermometric element.

The absorption membrane consists of a material adapted to absorb and convert into heat the energy of an incident electromagnetic radiation, in particular infrared radiation at wavelengths between 0.7 μιη and 3 mm, and more especially between 3 μιη and 20 μιη. The absorption membrane may be of metal, in particular of titanium nitride (TiN). The thickness of the absorption membrane is adapted to obtain a good electromagnetic coupling with the propagation medium of the incident electromagnetic radiation (impedance matching).

The absorption membrane is in thermal contact with the thermometric element. The thermometric element is for example a layer of a material having a high variation of resistivity as a function of temperature, deposited directly on the absorption membrane. The thermometric element is, for example, vanadium oxide or amorphous silicon.

In a manner known per se, the thermometric element is connected to means for measuring an electrical resistance integrated in the substrate 120 and not shown in the figures.

The thermometric element thus makes it possible to measure heating of the absorption membrane, and therefore the energy of the electromagnetic radiation incident thereon.

The electrical connection between the bolometric plate and reading circuits located in the substrate is preferably provided by the support elements 111.

The substrate is here in silicon.

The upper surface of the substrate is covered with a thin metal layer (not shown), for example aluminum or copper or titanium, with a thickness of between 50 nm and 200 nm.

This thin metal layer forms a reflective surface 121, which extends below the bolometric plate 110 and reflects the wavelengths to which the bolometric board is absorbent.

The reflective surface 121 and the bolometric plate 110 together form a quarter-wave cavity, making it possible to improve the absorption by the bolometric plate. In the following, we will give more details on the dimensioning of the quarter-wave cavity. According to the invention, each pixel further comprises a capsule 130, also called hood.

The inside of the capsule 130 defines a cavity 140 within which the bolometric plate 110 extends.

In other words, each capsule 130 covers a single bolometric plate

110.

The capsule 130 designates a three-dimensional structure comprising side walls 131, and an upper wall 132.

The lateral walls 131 of the capsule laterally surround the bolometric plate 110.

The side walls 131 here extend vertically, that is, parallel to the normal to the substrate.

The smallest edge-to-edge distance between the bolometric plate 110 and the side walls 131 is generally of the order of one micrometer, preferably greater than 100 nm.

In order to ensure the thermal insulation of the bolometric plate 110, it is not in contact with the side walls 131 of the capsule 130.

The upper wall 132 of the capsule extends above the bolometric plate 110, so that the bolometric plate 110 is between said upper wall 132 and the substrate 120.

The upper wall 132 of the capsule extends horizontally, that is to say parallel to the substrate.

The upper wall 132 of the capsule is in direct physical contact with the side walls 131, throughout its perimeter.

The upper wall 132 of the capsule preferably has a square section, in a plane parallel to the substrate (see Figure 1B).

However, the invention is not limited to this example, and we can realize many variants in which this section is not square, for example circular. The upper wall 132 of the capsule extends away from the bolometric plate. We will give in the following more details on this distance.

In the bolometric matrix detector according to the invention, the respective upper walls of the different capsules are spaced from each other, without direct physical contact between them.

The capsule 130 is made of transparent material in the range of wavelengths that the bolometric detector is intended to detect, here infrared.

Preferably, this material is a dielectric material.

The capsule 130 is for example amorphous silicon.

The cavity 140 formed by the capsule is closed by the substrate 120 so as to define inside each capsule 130 a closed volume receiving a bolometric plate.

Alternatively, the cavity 140 formed by the capsule is closed by a layer covering the substrate 120.

In any case, the bolometric plate 110 extends inside a closed volume, preferably hermetically, within which there is a vacuum or a reduced pressure, for example a pressure of less than 10 ^{~ 3} mbar .

It is known per se to encapsulate the bolometric plates, each inside a respective capsule.

In the prior art, each bolometric plate thus encapsulated defines a bolometer with low spectral selectivity, absorbing all wavelengths over a broad spectral band, the so-called initial spectral band.

The initial spectral band has a spectral width of several micrometers. The idea underlying the invention is to use the top wall 132 of the capsule, to form a guided mode filter that transmits only a portion of said initial spectral band.

Guided mode filters are known to those skilled in the art. For example, they are studied by ZS Liu et al. in the article "High-efficiency guided-mode resonance filter", Optics Letters, Vol. 23, No. 19, October ^1, 1998.

They are also described in US Pat. No. 8,698,207, which is particularly concerned with transmission-guided mode filters. A guided mode filter exploits a guided mode resonance phenomenon that results from the coupling between a diffraction order of a diffractive structure and a guided mode of a waveguide.

According to the invention, a transmission-guided mode filter is formed in the pixel 100, using the upper wall 132 of the capsule as a waveguide, and forming a diffractive structure 150 thereon.

The upper wall 132 of the capsule therefore has both a guide function for the guided mode filter, and a sealing function for enclosing the bolometric plate in a vacuum or under reduced pressure.

The guided mode filter transmits to the bolometric board 110 only a small range of wavelengths.

This small wavelength range thus forms the range of wavelengths that the pixel 100 is adapted to detect, called the spectral sensitivity range of the pixel 100.

In other words, the spectral sensitivity range of the pixel 100 corresponds to the transmission range of the guided mode filter according to the invention.

The central wavelength of the spectral sensitivity range of the pixel 100 is noted -

The spectral sensitivity range of the pixel is located in the initial spectral band mentioned above, and has a lower spectral width, for example less than one micrometer.

Thus, the central wavelength of the sensitivity range of the pixel 100 can only take values located in the initial spectral band mentioned above.

The diffractive structure 150 is a three-dimensional structure consisting of a plurality of studs 151 distributed in a regular pitch, on the upper wall 132 of the capsule 130, on the opposite side to the bolometric plate 110.

The diffractive structure 150 here extends directly on the upper wall 132 of the capsule.

Alternatively, a thin layer of hooked can be interposed between the upper wall 132 of the capsule and the diffractive structure 150. The diffractive structure 150 is preferably metal, for example gold, copper, or aluminum.

The diffractive structure 150 can thus have a high refractive index, and therefore a reduced thickness.

The thickness h of the diffractive structure 150 is advantageously

30 nm and 250 nm, and even between 30 nm and 200 nm, for example equal to 150 nm.

According to the advantageous embodiment illustrated in FIGS. 1A and 1B, the diffractive structure 150 consists of an array of studs 151, distributed in a square mesh, and each having a square section.

The diffractive wave structure itself has a square shape, with as many pins in width as in length.

The pitch p of the array of pads is less than the central wavelength of the spectral sensitivity range of the pixel 100, λ _£ .

In particular, the pitch p of the array of pads and the central wavelength of the spectral sensitivity range of the pixel 100, λ _£ , are connected by the following formula:

Neff with N _e ff the effective mode index guided in the upper wall 132 of the capsule.

The guided mode is preferably the TMo mode, that is to say the zero-order magnetic transverse mode of the dielectric waveguide, here the upper wall 132 of the capsule.

The effective index N _e ff depends on the wavelength Λ _£ and the mode considered, as well as the properties of the medium (in particular the refractive index and geometry).

For the same value of the pitch p, the width of the pad 151 modifies the width of the spectral sensitivity range of the pixel. The larger the studs, the closer this range is.

The lateral dimensions of the upper wall 132 of the capsule also depend on the value of the central wavelength of the spectral sensitivity range of the pixel 100, Λ _..

In particular, the lateral dimensions of the upper wall 132 of the capsule are adapted so that the mode guided in the upper wall of the capsule, forms a wave stationary in said upper wall 132, with nodes at the edges of the upper wall 132.

With a diffractive structure 150 as described with reference to FIGS. 1A and 1B, this condition is verified with an upper wall 132 with a square section of side L ₀ , where L ₀ is a length equal to an integer number of wavelengths of said mode. guided.

This condition is written:

m being a strictly positive integer;

λ _£ being the central wavelength of the spectral sensitivity range of the pixel 100; and

N _e ff being the effective index of the mode guided in the upper wall 132 of the capsule. The ratio-- is equal to the pitch p of the network of studs forming the diffractive structure

Neff

150.

In addition, and obviously, the length L ₀ must be strictly less than the width P of the pixel 100, or in other words, the pitch of the bolometer array forming the bolometric detector according to the invention.

This condition is written:

0 <L ₀ <P (4)

Considered otherwise, by fixing the pitch P of the bolometer matrix, the effective index N _e ff and a length L _gap equal to the difference between the pitch P of the bolometer matrix and the length L ₀ of the upper wall side. 132 of the capsule, the central wavelength of the spectral sensitivity range of the pixel 100 is given by:

λ _{ι = Ν} (5)

Preferably, the largest value of L ₀ is chosen which satisfies both conditions (3) and (4). In other words, we then have:

L ₀ = P-mod (P, p) (6) where mod (P, p) is the remainder of the division of P and p. mod (P, p) defines the length L _gap equal to the difference between the pitch P of the pixel matrix and the length L ₀ of the side of the upper wall 132 of the capsule.

Preferably, the thickness e of the top wall 132 of the capsule is adapted to the central wavelength Λ _£ of the spectral sensitivity range of the pixel 100.

In particular, this thickness e is substantially equal to - with N _c the refractive index of the material of the upper wall 132 of the capsule, at the wavelength λ _£ .

We can have in particular:

In addition, this thickness e is preferably greater than or equal to 800 nm, which ensures a thickness of at least 600 nm on the side walls 131 of the capsule, and thus a good mechanical strength of the capsule.

In practice, the thickness e is advantageously between 1.0 μιη and 1.5 μιη, for example 1.3 μιη for detector wavelengths between 8 μιη and 12 μιη.

In any case, this thickness e is advantageously less than or equal to 2 μιη, especially less than or equal to 1.8 μιη, to avoid the appearance of a multimode guidance in the upper wall of the capsule.

The value of the center wavelength Λ _£ of the spectral sensitivity range of the pixel 100 also sets an optimum height of the board bolometric 110, inside the capsule 130.

In known manner, the distance D ₁ between the reflecting surface 121, and the underside of the bolometric plate 110 located on the side of the reflecting surface 121, is substantially equal to, to form a resonant cavity quarter wave length

4

wave Λ _£ . We can provide a certain tolerance on D _lr for example

In addition, the distance D ₂ between an upper face of the bolometric plate 110, on the side of the upper wall 132 of the capsule, and a lower face of said wall 132, on the side of the bolometric plate, is adapted to form a cavity non-resonant at the wavelength Λ _£ . In other words, it is to avoid a coupling of the Fabry-Perrot type between the upper wall of the capsule and the bolometric plate.

In other words, the distance D ₂ is substantially equal to.

8

We can provide a certain tolerance on D ₂ , for example:

In addition, D ₂ is preferably greater than 1 μιη, to avoid any risk of contact between the bolometric plate 110 and the capsule 130, and any evanescent coupling between the bolometric plate 110 and the capsule 130

For example, D ₂ is a distance between 1 μιη and 1.7 μιη, in a bolometric detector adapted to the detection of electromagnetic radiation at wavelengths between 8 μιη and 12 μιη.

It is noted that the values that can take _£ Λ are limited, and located within an initial spectral band mentioned above, corresponding to the spectral band absorbed by the bolometric plate in the absence of filter.

Therefore, the distribution pitch of the pads of the diffractive structure only takes values within a predetermined range.

In the same way, the length L ₀ and / or the thickness e and / or the distance D ₁ and / or the distance D ₂ , takes only values situated within a respective predetermined interval.

All pixels of the bolometric detector according to the invention may be the same, and associated with the same value of the center wavelength Λ _£ of the spectral sensitivity range of the pixel. In this case, the spectral sensitivity range of the bolometric detector according to the invention is defined as being the spectral sensitivity range of a pixel of the bolometric detector.

The spectral sensitivity range of the bolometric detector is then centered on a wavelength λ _Μ equal to the wavelength λ _£ .

According to an advantageous variant, the bolometric detector according to the invention has several types of pixels, each type of pixel being associated with a value that is distinct from the central wavelength of its sensitivity range, λ _£ .

The bolometric detector according to the invention is then a multi-spectral detector. In this case, the spectral sensitivity range of the bolometric detector according to the invention is defined as being the set of spectral sensitivity ranges of the different types of pixels of the bolometric detector.

The spectral sensitivity range of the bolometric detector is then centered on a wavelength λ _Μ , equal to the median value of the central wavelengths of the spectral sensitivity ranges of the different types of pixels.

In such a multi-spectral bolometric detector, the different types of pixels differ from one another by the lateral dimensions of the upper wall of their respective capsules, here the value of the length L ₀ , and by the dimensions of their respective diffractive structures. , here the value of the step p.

The highest value of the central wavelength values of the spectral sensitivity ranges associated with the different types of pixels of the multi-spectral bolometric detector is _designated λ _Μαχ .

At _Min is called the smallest of these values.

Preferably, the difference between λ _Μαχ and λ _min is less than or equal to 2.5 μιη, and even less than or equal to 2 μιη.

Those skilled in the art will be able to produce a bolometric detector, in which the upper walls of the capsules have different thicknesses depending on the type of pixel considered. However, in order to simplify the manufacture of a multi-spectral detector according to the invention, the upper wall 132 of the capsule advantageously has the same thickness e on all the pixels.

In particular, this thickness e is substantially equal to with N _c the refractive index of the material of the upper wall 132 of the capsule at the wavelength λ _Μ , and λ _Μ the central wavelength of the range of spectral sensitivity of the bolometric detector.

AT. -AT.

In other words, this thickness e is substantially equal to -, with λ _Μ =

^ ~ ^ -Max -Min

We can have in particular:

(10)

In the same way, in order to simplify the manufacture of a multi-spectral detector according to the invention, the bolometric plate is advantageously located at the same distance from the reflecting surface on all the pixels.

In other words, the distance D ₁ is the same on all the pixels, and substantially equal to.

With a certain tolerance on D _lt we obtain:

In the same way, the bolometric plate is advantageously located at the same distance from the upper wall of the capsule on all the pixels.

In other words, the distance D ₂ is the same on all the pixels, and substantially equal to.

^& 8

With a certain tolerance on D ₂ , we obtain: ≤ D ₇ ≤ (12)

10

By way of example, the following table gives the values of the pitch p of the array of pads and of the length L _gap = P-L ₀ , for the different types of pixels of a multi-spectral bolometric detector according to the invention.

It is desired that the different types of pixels respectively detect at central wavelengths at 10 μιη, 11 μιη, 12 μιη, 13 μιη, 14 μιη, 15 μιη and 16 μιη.

The median value of these different wavelengths is λ _Μ = 13 μιη.

An optimum thickness of the upper wall 132 of the capsule is deduced: e = - ^ -, with N _c the refractive index of the amorphous silicon at 13 μιη.

From this optimal thickness, a value of N _e // is obtained, using a modal calculation known to those skilled in the art. Here, N _eff = 3.10.

Given a pixel pitch P = 34.0 μπι, the following values are deduced:

Table 1

As illustrated in particular by Table 1, in a multi-spectral bolometric detector according to the invention, the different pixel types differ in particular by the lateral dimensions of the upper wall of the capsule. According to a first embodiment of the invention, the lateral dimensions of the bolometric plate are the same for all types of pixels, and adapted to the dimensions of the smallest capsule size.

In a variant, the lateral dimensions of the bolometric plate are adapted to the capsule size of each pixel type, and differ from one type of pixel to another on the matrix of pixels. The larger the capsule, the wider the bolometer plate.

However, the dimensions of the bolometric plate impact the sensitivity of the corresponding pixel. The larger the surface of the bolometric plate, the better the sensitivity of the pixel.

In order to avoid that in the same multi-spectral bolometric detector, certain wavelengths are detected with a better sensitivity than others, a lower surface area of the bolometric plate is compensated for by better thermal insulation thereof.

The thermal insulation of the bolometric board can be adjusted by modifying the dimensions and / or the structure of the support arms 112 (see FIG. 1A) which support the board.

In particular, the thermal insulation of the bolometric board can be increased by refining the diameter of the support arms.

Alternatively, the length of these support arms can be increased to increase this thermal insulation.

According to another variant, a layer of thermal insulation can be included in the support arms.

Thus, on a multi-spectral bolometric detector according to the invention, the different types of pixels are also distinguished by the dimensions and / or the structure of the support arms.

FIG. 2 schematically illustrates, in a view from above, an example of a bolometric detector 1000 according to the invention, in particular a multi-spectral bolometric detector. The detector 1000 consists of a matrix of pixels, and has four types of pixels 100A, 100B, 100C, IOOD which differ by the value of the central wavelength of their spectral sensitivity range.

Each pixel type 100A, 100B, 100C, IOOD has a capsule of width L _QA , L _QB , L _QC , respectively L _QD , and a diffractive structure with a network pitch p _A , p _B , p _c , p _D ( not shown in Figure 2).

The different types of pixels are arranged in a periodic pattern characterized by a square macro-pixel, comprising a 100A pixel, a 100B pixel, a 100c pixel, and an IOOD pixel.

The invention offers a great flexibility in the arrangement of the different types of pixels, each pixel being independent of the neighboring pixel (s).

According to a variant not shown, the detector 1000 has three types of pixels 100A, 100B, 100C, arranged in a periodic pattern characterized by a square macro-pixel, comprising a pixel 100A, a pixel 100B, and two pixels 100c arranged according to a diagonal macro-pixel (Bayer matrix).

FIG. 3 illustrates the performances of a multi-spectral bolometric detector according to the invention. The x-axis is a wavelength, in meters. The y-axis is an absorption rate by the bolometric plate.

Each absorption peak 31, 32, 33, 34, 35, 36 corresponds to a pixel type and a corresponding value of the central wavelength of the pixel sensitivity range.

FIG. 4 schematically illustrates a first embodiment of a method for manufacturing a bolometric detector according to the invention.

In a first step 401, the bolometric plates 110, the support arms (not shown) and the support elements 111 are formed by depositing and etching on a first sacrificial layer 41, itself deposited on the substrate 120.

The whole is then covered with a second sacrificial layer 42, preferably of the same kind as the first sacrificial layer 41, and trenches 43 are etched between the bolometric plates 110 (step 402).

The sacrificial layers may be organic or inorganic, for example polyimide or silicon dioxide. The trenches 43 are etched by photolithography, and the position of the trenches is defined by the desired dimensions for the upper wall of the capsule.

A layer 44 of an infrared-transparent material, for example amorphous silicon, is then deposited on and around the sacrificial layer pads formed by the etching of the trenches (step 403).

In particular, the infrared-transparent material 44 lines the top of the second sacrificial layer 42, as well as the bottom and the sides of the trenches 43. The capsules according to the invention are thus formed.

Openings 45, or vents, are then opened in the layer 44, through which the material of the first and second sacrificial layers is removed by chemical etching (step 404).

The vents 45 may each have a diameter between 0.1 and 1 μιη.

Each bolometric plate, or capsule, corresponds to at least one vent 45.

A metal film 46 is then deposited on each capsule (step 405).

On each capsule, the metal film 46 closes the vent 45, so as to seal the capsule.

Each metal film 46 is constituted for example by an aluminum layer with a thickness of 200 nm.

The deposition of metal film is carried out for example under vacuum, by physical vapor deposition, preferably by evaporation of a source of aluminum heated by electron beam or Joule effect.

In a last step 406, the metal films 46 are etched to form the diffractive structures according to the invention. During this etching, portions 46A are spared from the metal films 46 which seal the vents 45.

The dimensions of the portions 46A may differ slightly from the dimensions of the other pads of the network, without this adversely affecting the quality of the diffractive structure thus produced.

For the sake of brevity, the standard preliminary steps for producing read circuits in the substrate are not described here. FIG. 5 illustrates steps of a second embodiment of a method for manufacturing a bolometric detector according to the invention.

This method firstly comprises steps 401 to 403 as described with reference to FIG. 4.

In a next step 504, a metal film 56 is deposited on each capsule, for example by physical vapor deposition or by chemical vapor deposition.

Each metal film 56 is then etched to form the diffractive structures according to the invention, then the diffractive structures are each covered with a thin layer 57 for stopping the etching (step 505).

The etching stop layer 57 is, for example, silicon dioxide having a thickness of, for example, between 20 nm and 100 nm.

Then vents 55 are opened in the capsules and the sacrificial layers are evacuated as described with reference to Fig. 4 (step 506).

A closure layer 58 is then deposited on each capsule, which covers the diffractive structures covered by the stop layers of the etching, and blocks the vents 55 (step 507).

On each capsule, the capping layer 58 closes the vent 55, so as to seal the capsule.

The capping layer is for example germanium or silicon, deposited by physical vapor deposition electron beam.

The thickness of the capping layer is for example between 1 μιη and 1.7 μιη, to ensure hermetic sealing of the vents.

In a last step 508, the capping layer 58 is etched by sparing the portions 58A of the capping layer which seal the vents.

The control of this etching is facilitated by the stop layer of the etching 57 which stops the etching before it starts the metal of the diffractive structure.

The dimensions of the portions 58A may differ slightly from the dimensions of the pads of the network, without this adversely affecting the quality of the diffractive structure thus produced.

An etching with hydrofluoric acid then makes it possible to eliminate the stop layer of the etching 57. The stop layer of the etching 57, of fine thickness, can be omitted without starting a possible portion of said layer incorporated in the portions 58A for closing the vents.

According to a variant not shown, the stop layers of the etching are deposited on the metal films 56, before etching a diffractive structure. Each diffractive structure is then etched by etching the stack of a metal film and a stop layer of the etching.

This second embodiment of a method of manufacturing a bolometric detector according to the invention allows the etching of the diffractive structure to be made on a flat surface, the vents not yet open during this etching.

The upper wall 132 of each capsule therefore has at least one vent, hermetically sealed by a plug inserted in the vent.

This vent, not shown in the schematic representation of Figure 1A, allows the evacuation of a sacrificial layer used to manufacture the capsule, as described with reference to Figures 4 and 5.

The plug inserted into the vent may be constituted by a pad of the diffractive structure, or by a blocking stud made of a material other than the diffractive structure.

Many variants of a bolometric detector can be implemented without departing from the scope of the invention.

More particularly, the case of a diffractive structure formed of a network of square studs on a top wall of square section capsule has been described.

The bolometric detector according to the invention may have many other forms of diffractive structures.

For example, each diffractive structure may consist of concentric rings spaced from each other in a regular step p = -.

Neff

As a variant, each diffractive structure may consist of two series of concentric semicircles, the semicircles of each series being spaced apart from each other in a regular step p =

According to another variant, each diffractive structure may consist of two nested networks, each in the shape of a rake, etc.

Such examples of diffractive structure are illustrated in particular in US Pat. No. 8,698,207.

In each case, the diffractive structure consists of studs distributed in a large pitch p = -.

Neff

In any case, in each case, the pitch p of the diffractive structure sets the value of the central wavelength λ _£ of the spectral sensitivity range of the pixel, and the width of the pads of the diffractive structure sets the width this range of spectral sensitivity (the larger the pads, the higher the transmission peak of the guided mode filter is narrow and therefore the range of spectral sensitivity of the pixel is narrow).

The lateral dimensions of the upper wall of the capsule, and the shape of the diffractive structure, are adapted to each other so that the wave guided in said upper wall is a standing wave.

For example, for a diffractive structure formed by concentric rings, the upper wall of the capsule has a circular section. The radius for obtaining said stationary wave can be calculated using a Bessel function.

The section of the top wall of the capsules may also have other shapes, for example a hexagonal shape.

According to another variant, the diffractive structure is embedded in the thickness of the upper wall 632 of the capsule.

FIG. 6 illustrates, in a sectional view, a pixel 600 of a bolometric detector according to this variant.

The diffractive structure 650, consisting here of a network of metal studs 651, is embedded in the material forming the capsule.

Here, the array of pads 651 is embedded in amorphous silicon. For example, it is located between a first amorphous silicon elementary layer 650 nm thick, and a second amorphous silicon elementary layer 650 nm thick.

The thickness of the upper wall of the capsule is then measured without taking into account the network of pads 651. Above this array of pads 651, the material forming the capsule reproduces the identical topography of the network of pads 651.

Formally, it can therefore also be considered that there is a diffractive structure in amorphous silicon, resting on the upper wall of the capsule. The upper wall of the capsule then integrates the metal network.

This variant makes it easier to lithograph the metal network. In particular, this lithography is on a flat surface.

This variant is particularly suitable when the metal pads have a thickness of the order of 30 nm.

The invention is particularly advantageous in the context of a multi-spectral bolometric detector, for applications in imaging, thermography, gas detection, etc.

The multi-spectral bolometric detector according to the invention can form in particular a hyper-spectral detector, making it possible to distinguish many spectral bands beyond the visible.

Claims

1. A bolometric detector (1000) comprising an array of pixels (100; 100A; 100B; 100C; _100D; 600), each pixel (100; _100a; _100b; 100 _c; 100 _D; 600) comprising a bolometric plate (110) suspended above a substrate (120), characterized in that each pixel further comprises:

a respective capsule (130) forming a cavity (140) within which the bolometric plate (110) extends; and

a three-dimensional structure (150; 650), said diffractive structure, consisting of studs (151; 651) distributed in a regular pitch (p) on an upper wall (132) of the capsule, on the opposite side to the bolometric plate, or in the thickness of said upper wall (632), to form with said upper wall (132; 632) a mode filter guided in transmission.

2. Detector (1000) according to claim 1, characterized in that the pixel matrix is formed of several types of pixels (100, 100 _A , 100 _B , 100 _c , 100 _D , 600) which differ from one another by the dimensions side members (L ₀ ; L _QA ; L _QB ; L _QC ; L _0D ) of the top wall (132; 632) of the capsule; and by the step (p) of distribution of the studs (151; 651) of the diffractive structure .

3. detector (1000) according to claim 1 or 2, ac terized in that da ns each pixel (100; _100a; _100b; 100 _c; 100 _D; 600), the pitch (p) of the distribution of pads the diffractive structure and the central wavelength (Α _έ ) of a spectral sensitivity range of the pixel are connected by the relation:

^VN eff

with N _e ff the effective index of the guided mode in the top wall (132; 632) of the capsule, p the distribution pitch of the diffractive structure, and Α _έ the central wavelength of the sensitivity range spectral pixel.

4. Detector (1000) according to any one of claims 1 to 3, characterized in that in each pixel (100; 100 _A ; 100 _B ; 100 _c ; 100 _D ; 600), the diffractive structure (150; 650) is a network of studs (151; 651) of square section distributed in a square grid.

5. detector (1000) according to any of claims 1 to 4, characterized in that in each pixel (100; _100a; _100b; 100 _c; 100 _D; 650), the diffractive structure (150; 650) is metal.

6. Detector (1000) according to any one of claims 1 to 5, characterized in that in each pixel (100; 100 _A ; 100 _B ; 100 _c ; 100 _D ; 600), the upper wall (132; of the capsule has a thickness of between 250 nm and 250 nm, with:

λ _Μ the central wavelength of a range of spectral sensitivity of the bolometric detector (1000); and

N _c the average refractive index of the top wall (132; 632) of the capsule, at the wavelength λ _Μ .

7. detector (1000) according to any one of claims 1 to 6, characterized in that in each pixel (100; _100a; _100b; 100 _c; 100 _D; 600), the lateral dimensions of the top wall ( 132; 632) of the capsule are adapted so that the mode guided in said upper wall (132; 632) forms a standing wave.

8. Detector (1000) according to any one of claims 1 to 7, characterized in that in each pixel (100; 100 _A ; 100 _B ; 100 _c ; 100 _D ; 600), the upper wall (132; of the capsule has a square section, whose width (L ₀ ; L _QA ; L _QB ; L _QC ; L _QD ) is smaller than the pitch (P) of the pixel matrix, and is equal to: m * with m an integer greater than or equal to unity;

λ _£ the central wavelength of the spectral sensitivity range of the pixel; and

N _e ff the effective index of the mode guided in the upper wall (132; 632) of the capsule.

9. Detector (1000) according to claim 8, characterized in that said integer is the largest integer allowing said width (L ₀ ; L _QA ; L _QB ; L _QC ; L _0D ) to be smaller than the pitch (P ) of the pixel matrix.

10. detector (1000) according to any one of claims 1 to 9, characterized in that in each pixel (100; _100a; _100b; 100 _c; 100 _D; 600), the distance between the top wall (132 632) of the capsule and the bolometric plate (110) is between

And, with λ _Μ the central wavelength of a spectral sensitivity range of

10 6

bolometric detector (1000).

11. Detector (1000) according to any one of claims 2 to 10, characterized in that the different types of pixels (100, 100 _A , 100 _B , 100 _c , 100 _D , 600) are also distinguishable from each other by the dimensions of the bolometric board (110).

12. Detector (1000) according to claim 11, characterized in that the different types of pixels (100, 100 _A , 100 _B , 100 _c , 100 _D , 600) are also distinguished by the dimensions and / or the arm structure. support members (112), which extend parallel to the bolometric board (110) to support it above the substrate (120), the dimensions and / or the structure of said support arms defining thermal insulation between the board bolometric (110) and the substrate (120), and in that from one pixel to the other of the pixel matrix, said thermal insulation is even higher than the surface of the bolometric plate (110) is weak .

13. Detector (1000) according to any one of claims 1 to 12, characterized in that in each pixel (100; 100 _A ; 100 _B ; 100 _c ; 100 _D ; 600), the capsule (130) has at least a through opening (45), said vent, each vent (45) being closed by a stud (46A) of the diffractive structure.

14. Detector (1000) according to any one of claims 1 to 12, characterized in that in each pixel (100; 100 _A ; 100 _B ; 100 _c ; 100 _D ; 600), the capsule (130) has at least a through opening (55), said vent, each vent (55) being closed by a sealing stud (58A) separate from the pads of the diffractive structure.