PROCESS FOR DEPOSITING A LAYER BASED ON VANADIUM OXIDE

Abstract

The invention relates to a method for deposition by IBS of a VOx layer comprising at least chromium and / or boron. The method comprises a conditioning phase comprising a deposition of a VOx-based film on a test substrate with an initial oxygen flow rate and a determination of an intermediate oxygen flow rate comprising the following iterative process: (1) increasing the oxygen flow rate by a predetermined increment to obtain an increased oxygen flow rate, (2) deposition of a VOx-based film with the increased oxygen flow rate, (3) measuring a difference in a parameter of the film induced by the increment chosen from a deposition rate, a resistivity, a resistance of the film (4) repeating the previous steps as long as the difference is greater than a predetermined threshold. The deposition of the VOx layer is carried out with a final oxygen flow rate strictly lower than the intermediate oxygen flow rate. Figure for abstract: Figure 2

Description

Title of the invention: METHOD FOR DEPOSITING A VANADIUM OXIDE-BASED LAYER Technical field

[0001] The field of the invention is that of the deposition of layers based on vanadium oxide, more particularly that of the deposition of a layer of vanadium oxide by sputtering with an ion beam. STATE OF THE PRIOR ART

[0002] Vanadium oxide materials are used, for example, for the manufacture of microbolometers, optical switches, or optical memories. For these applications, vanadium oxide is generally deposited in the form of a layer on a substrate by various deposition techniques. These include ion beam sputtering (IBS) or cathode sputtering. It is then generally necessary to control intrinsic parameters of the layer such as its density, its electrical resistivity, its uniformity of electrical resistivity, or its atomic or stoichiometric composition.

[0003] In the field of microbolometers, a layer of vanadium oxide, for example in an amorphous form, can be used as a material sensitive to a variation in temperature, expressed by a change in its electrical resistivity. This material is appreciated in particular for its high thermal coefficient of resistance (TCR) and the low 1 / f noise that it induces.

[0004] The document "Vanadium oxide thin film with improved sheet resistance uniformity", Généreux F. et al., Proc, of SPIE Vol. 9070 90701R-1, 2014 focuses on this type of microbolometers for the production of a photodetector comprising a matrix of pixels 17 pm wide. The substrate here is a plate 150 mm in diameter. Mention is made of the importance and the difficulty of obtaining a vanadium oxide layer of uniform electrical resistivity. The sheet resistance uniformity, measured at 49 regularly spaced points on the plate, is generally between 10% and 20%, which leads to a gain variation and / or an offset between the pixels, and consequently, greatly affects the dynamics of the photodetector.A process, not disclosed in this publication, however makes it possible to obtain a resistivity uniformity of 2.5% in a layer of vanadium oxide devoid of additional chemical element, satisfactory for the production of the photodetector.

[0005] A method for obtaining a vanadium oxide layer having comparable layer resistance uniformity is described in the document "Resistance repeatability study of ion-beam deposited vanadium oxide thin films", Alvarez P. et al., Proc, of SPIE Vol. 9974 997415-1, 2016. The layer is deposited by sputtering a target containing 99.99% vanadium by an argon ion beam, on a silicon wafer 150 mm in diameter covered with a silicon nitride film 300 nm thick. A variation of the partial pressure of oxygen in the chamber causes an open hysteresis cycle to pass through the uniformity of the layer resistance, here equal to the standard deviation of the population consisting of resistance measurements at 9 determined points on the wafer. Indeed, an increase in the partial pressure of oxygen from 1.9.105 Torr to 3.2.105 Torr degrades the resistance uniformity by 1% to 3.2% through a maximum.A subsequent decrease in the oxygen partial pressure over the same value range further degrades the resistance uniformity from 3.2% to 4%, passing through a minimum at 3% and a maximum at 5%. The resistance uniformity value is then not the same at the beginning and end of the path and the hysteresis loop is said to be open. Over the same range of oxygen partial pressures, the deposition rate also follows an open hysteresis loop. On the other hand, the hysteresis loop of the average layer resistance measured at the 9 points follows a closed hysteresis loop, i.e. the average layer resistances at 1.9.105 Torr are the same at the beginning and end of the cycle.

[0006] It therefore appears that layers of pure vanadium oxide, containing for example less than 1% of additional chemical element, can be deposited on different plates, each having a layer resistance uniformity of less than 5% (lo). In addition, this document shows that with a cassette loading station and an optimized oxygen regulation system, for example implementing a residual gas analyzer (Residual Gas Analyzer, in English) and a mass flow controller (mass flow controller, in English), it is possible to achieve a pure vanadium oxide layer resistance uniformity of less than 2%, on a plate and from one plate to another.

[0007] However, for certain applications, there is a need to introduce an additional chemical element to a layer of vanadium oxide. In the field of microbolometers, it has been taught, for example, in document FR3077879 A1, that an effective amount of boron (B) or carbon (C) in a sensitive vanadium oxide material allows the room temperature resistivity of the sensitive material to remain substantially stable after exposure to high temperature, for example above 300°C. Chromium (Cr) is also a chemical element of interest, for example example to decrease the electrical resistivity of the vanadium oxide layer, while maintaining a high TCR, as described in documents US5288380 A and US6489613B1.

[0008] However, the addition of an additional element, such as boron or chromium, degrades the resistivity uniformity of the vanadium oxide deposited by ion beam sputtering. For example, a layer of vanadium oxide comprising chromium with a resistivity of less than 20 Ω.cm can be obtained with a vanadium-chromium (VCr) target, comprising 20% ​​chromium, and an oxygen flow rate of between 4.05 sccm and 4.15 sccm (standard cubic centimeters per minute), resulting in a resistivity uniformity on a 200 mm plate greater than 30% (lo), or even equal to 50% (lo). Statement of the invention

[0009] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a method for depositing a layer of vanadium oxide comprising at least one additional chemical element chosen from chromium (Cr) and boron (B), having a resistivity uniformity lower than a resistivity uniformity obtained with a method of the prior art.

[0010] For this, the subject of the invention is a method for depositing a layer of vanadium oxide comprising at least one additional chemical element chosen from chromium and boron, in equipment for deposition by sputtering at least one target by an ion beam, the method comprising the following successive steps: • a conditioning phase comprising the following sub-steps: • deposition of a vanadium oxide-based film on a test substrate with an initial oxygen flow • determination of an intermediate oxygen flow rate comprising the following iterative process, called the first iterative process: • increasing the oxygen flow rate by a predetermined oxygen flow rate increment to achieve an increased oxygen flow rate, • deposition of a vanadium oxide-based film with increased oxygen flow, • measurement of a difference in a film parameter induced by the oxygen flow rate increment, the parameter being chosen from a deposition rate or a resistivity or resistance of the film • repetition of the previous steps as long as the difference is greater than a predetermined threshold strictly less than 0, • a deposition of the vanadium oxide layer on a substrate produced with a final oxygen flow rate strictly lower than the intermediate oxygen flow rate equal to the increased oxygen flow rate at the end of the first iterative process.

[0011] Some preferred but non-limiting aspects of this deposition method are as follows.

[0012] The deposition method may further comprise an adjustment phase subsequent to the conditioning phase, comprising the following iterative process, called the second iterative process: • reducing the oxygen flow rate by a predetermined oxygen flow rate decrement to achieve a reduced oxygen flow rate, • deposition of a vanadium oxide-based film on a control substrate with reduced oxygen flow, • measurement of resistivity of the vanadium oxide-based film, • repeating the previous steps as long as the resistivity is greater than a predetermined maximum resistivity,

[0013] The adjustment phase can be followed by the deposition of the vanadium oxide layer on the substrate produced at the final oxygen flow rate equal to the reduced oxygen flow rate at the end of the second iterative process.

[0014] The predetermined maximum resistivity can be equal to 40 Q.cm, it can preferably be equal to 20 Q.cm.

[0015] The deposition of the vanadium oxide-based film with the increased oxygen flow rate can be carried out on a different substrate at each iteration of the first iterative process.

[0016] The parameter may be a deposition rate and the measurement of the difference in deposition rates may be deduced from thickness measurements of the vanadium oxide-based films.

[0017] For each iteration of the first iterative process, the predetermined threshold may be equal to -15% of the deposition rate obtained in the previous iteration.

[0018] The deposition equipment may comprise a target comprising vanadium and the at least one additional chemical element.

[0019] The invention may also relate to a method for manufacturing a microbolometer comprising a sensitive material made of vanadium oxide comprising at least one additional chemical element chosen from chromium (Cr) and boron (B), comprising a deposition method according to any one of the preceding characteristics.

[0020] The invention may also relate to a photodetector comprising a matrix of microbolometers comprising a sensitive material common to the vanadium oxide microbolometers comprising at least one additional chemical element chosen from chromium and boron, the sensitive material possibly having a resistivity uniformity of less than or equal to 5%.

[0021] The sensitive material of the photodetector may have an average sheet resistance less than or equal to a specification of a readout circuit of the photodetector.

[0022] The sensitive material of the photodetector may have a density less than or equal to 4.5. Brief description of the drawings

[0023] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

[0024] Figures 1A and 1B are schematic and partial views, respectively in perspective and in section along the plane A-A', of a microbolometer according to one embodiment, comprising a sensitive material based on vanadium oxide;

[0025] [Fig.2] is a graph illustrating a closed hysteresis cycle traversed by the deposition rate (square points) of vanadium oxide-based layers and a closed hysteresis cycle traversed by the resistivity uniformity (circular points) of the vanadium oxide-based layers, as a function of an oxygen flow rate;

[0026] [Fig.3] is a graph illustrating a closed hysteresis cycle traversed by the average resistivity of the vanadium oxide-based layers as a function of an oxygen flow rate;

[0027] [Fig.4] is a graph illustrating the density distribution of vanadium oxide-based layers deposited according to a first deposition regime (open circles) and according to a second deposition regime (solid circles) as a function of the average resistivity of the vanadium oxide-based layer.

[0028] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0029] In the figures and in the remainder of the description, the same references represent identical or similar elements. Furthermore, the different elements are not shown to scale so as to favor the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and can be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalents mean that the limits are included, unless otherwise indicated.

[0030] The invention relates to a method for deposition by ion beam sputtering (or Ion Beam Sputter Deposition, in English) of a layer of vanadium oxide comprising at least one additional chemical element chosen from chromium (Cr) and boron (B). The additional element is incorporated into a target containing vanadium or into a target independent of a target containing vanadium.

[0031] Surprisingly, the inventors have found that incorporating an amount of additional chemical element into the vanadium oxide layer greater than a minimum amount induces closed and superimposed hysteresis cycles of the resistivity uniformity and the deposition rate of the vanadium oxide layer as a function of the oxygen flow rate during deposition. More particularly, it has been found that an increase in the oxygen flow rate during a first phase results in an increase in the resistivity of the vanadium oxide layer, until an intermediate oxygen flow rate is reached for which the deposition rate drops abruptly from a high rate to a low rate, and the resistivity uniformity reaches a low plateau of the closed hysteresis cycle of the resistivity uniformity.The low plateau is such that the resistivity uniformity is substantially constant at a minimum resistivity uniformity significantly lower than a resistivity uniformity obtained during the first phase for the same oxygen flow rate. The minimum resistivity uniformity is less than 5%, or even less than 3%. The difference between the high speed and the low speed is greater than 15%, or even greater than 30%, of the high speed. The minimum quantity is equal to 0.09 atoms of boron or chromium for one atom of vanadium.

[0032] The deposition method of the invention takes advantage of this surprising observation to deposit a layer of vanadium oxide comprising boron (B) or chromium (Cr) having an average resistivity less than or equal to a predetermined maximum resistivity, for example equal to 40 Q.cm, or even equal to 20 Q.cm, and a resistivity uniformity less than or equal to 5%, or even less than or equal to 3%. The method comprises a conditioning phase during which the oxygen flow rate is gradually increased to an intermediate oxygen flow rate for which the deposition rate drops abruptly from high speed to low speed.Although the conditioning phase induces an increase in the average resistivity above the predetermined maximum resistivity, it makes it possible to deposit a layer of vanadium oxide comprising boron (B) or chromium (Cr) with an oxygen flow rate strictly lower than the intermediate oxygen flow rate, having an average resistivity lower than or equal to the predetermined maximum resistivity, this with a resistivity uniformity lower than or equal to 5%. The predetermined maximum resistivity may for example correspond to a specification for the production of a photodetector comprising a matrix of microbolometers, more particularly to the production of a sensitive material common to the microbolometers adapted to a reading circuit of the photodetector.

[0033] Throughout the description, unless otherwise stated, the “resistivity uniformity” of a layer is equal to the standard deviation of the electrical resistivities measured at several positions of uniform distribution on the layer, for example at 49 positions, given in percentage of the average electrical resistivity of the layer. Electrical resistivity can be derived, for example, from a conventional four-pin measurement of the square resistance, by multiplying the square resistance by the layer thickness, or by the average layer thickness. Layer thickness can be measured by ellipsometry or by X-ray reflection.

[0034] Figures 1A and 1B are schematic and partial views, respectively in perspective and in section along the plane AA, of a microbolometer 10 representative of the microbolometer matrix of such a photodetector 1, the microbolometer 10 comprising a sensitive material 15 based on vanadium oxide.

[0035] Throughout the description, a layer or material or film is said to be "vanadium oxide-based" when the material or layer or film comprises mainly vanadium oxide, and at least one additional chemical element chosen from chromium (Cr) and boron (B). The vanadium oxide is in any of its phases, the value of the stoichiometric ratio of the number of oxygen atoms to the number of vanadium atoms is arbitrary, that is to say that the atomic composition of the layer or material is arbitrary provided that it comprises mainly oxygen and vanadium atoms. It may be amorphous, possibly at least partly polycrystalline. The quantity of additional chemical element is greater than the minimum quantity.

[0036] By material predominantly comprising a compound, we mean a material of which at least 50% of its volume is formed or comprises the compound.

[0037] The indications relating to the atomic composition of a compound are expressed by its gross chemical formula, conventionally expressed relative to one (1) vanadium atom. Thus, for a VOxBy compound, mentioned here for purely illustrative purposes, the x value of the quantity of oxygen is the number of oxygen atoms per 1 vanadium atom, and the y value of the quantity of boron is the number of boron atoms per 1 vanadium atom. The value of the quantity of the chemical element is given to within 10%. Furthermore, the atomic proportion of each chemical element in the VOxBy compound is l / (1+x+y) for vanadium, x / (l+x+y) for oxygen, and y / (l+x +y) for boron.

[0038] The atomic composition of a layer based on vanadium oxide can be determined in particular by NRA (Nuclear Reaction Analysis), by RBS (Rutherford Backscattering Spectroscopy), by SIMS (Secondary Ion Mass Spectrometry), by XPS (X-ray Photoelectron Spectroscopy), using suitable standards.

[0039] The microbolometer 10 comprises an absorbent membrane 11 comprising the sensitive material 15 based on vanadium oxide, suspended above a substrate 2 by anchoring pillars 12 and thermal insulation arms 13, as well as an electronic control and reading circuit (not shown) located in the substrate 2. The absorbing membrane 11 is spaced from the substrate 2, and in particular from a reflective layer 14 resting on the substrate 2, by a non-zero distance. This distance is preferably adjusted so as to form a quarter-wave cavity optimizing the absorption of the electromagnetic radiation to be detected by the suspended membrane 11. The microbolometer 10 is here adapted to absorb infrared radiation included in the long infrared wavelength band (called LWIR), ranging from approximately 8 pm to 14 pm.

[0040] Here and for the remainder of the description, we define a direct three-dimensional orthogonal reference frame (X, Y, Z), where the XY plane is substantially parallel to the plane of the substrate 2, the Z axis being oriented in a direction substantially orthogonal to the plane of the substrate 2. Furthermore, the terms “lower” and “upper” are understood as relating to an increasing positioning when moving away from the substrate 2 in the +Z direction.

[0041] As illustrated in [Fig.lB], the absorbent membrane 11 may comprise a lower support layer 20 made of an electrically insulating material on which rest two electrodes 21.1, 21.2 distinct from one another, made for example of TiN which has a strong absorption of infrared radiation. A thin layer of the sensitive material 15 rests on the support layer 20 and comes into contact with each of the two electrodes 21.1, 21.2. The sensitive material 15 is here covered with a protective layer 22, made for example of a silicon nitride SiN or a silicon oxide SiO, which makes it possible to avoid any subsequent contamination of the sensitive material 15. This example is given purely for illustrative purposes and other arrangements of the electrodes and the sensitive material are possible.

[0042] Furthermore, the microbolometer 10 may be located in a vacuum-sealed cavity defined by an encapsulation structure (not shown), such as that described in patent application EP3067675. Alternatively, the microbolometer array 10 may be placed in a vacuum-sealed cavity where the entire photodetector 1 may be placed in a vacuum-sealed enclosure.

[0043] In connection with figures 2, 3 and 4, a cycle of deposition of a layer based on vanadium oxide comprising chromium (Cr) will be described.

[0044] A target comprising vanadium (V) and chromium (Cr) is introduced into a vacuum chamber of an ion beam sputtering deposition equipment. Here, the target comprises 20% of chromium (Cr) atoms relative to the number of vanadium (V) atoms, i.e. the target comprises 4 vanadium (V) atoms for one chromium (Cr) atom.

[0045] The pressure in the chamber is between 1.105 and 3.10 4 Torr, for example equal to 10 4 Torr. The equipment comprises a source of krypton (Kr) ions. The krypton (Kr) ions are accelerated by an electric field to reach an energy, here equal to 1000 eV, then neutralized at the output of the source. The krypton (Kr) ion current here is equal to 150 mA. The krypton atoms form a beam directed towards the target, thus bombarding it with sufficient energy to tear off vanadium (V) and chromium (Cr) atoms from the target.

[0046] The chamber further comprises an oxygen inlet associated with a means for regulating the oxygen flow rate and a plate support intended to receive a plate, here a silicon plate having a diameter of 200 mm. The vanadium (V) and chromium (Cr) atoms torn from the target combine with the oxygen atoms and are deposited on a plate deposited on the plate support to form a layer based on vanadium oxide. The plate support is maintained at room temperature.

[0047] In Figures 2 and 3, the oxygen flow rate arriving in the chamber is given on the x-axis in sccm. In [Fig.2], the left y-axis gives the deposition rate in nm / min and the right y-axis, the resistivity uniformity in percentage. In [Fig.3], the y-axis gives the average resistivity in Q.cm.

[0048] In Figures 2 to 4, each experimental point corresponds to a deposition of a layer based on vanadium oxide on a separate plate. The duration of the deposition is the same for all the experimental points, here equal to 30 minutes. Before each deposition, a plate is introduced into the chamber from a vacuum airlock, the vacuum airlock being able to be a loading station for a cassette containing several plates. Between each deposition of a separate plate, the pressure in the chamber then remains substantially constant, for example between 105 Torr and 3.10 4 Torr. The target is notably the same for all the experimental points of the cycle.

[0049] Electrical resistivity is measured by placing the plate on a temperature-controlled support, here at room temperature. A conventional four-point measurement of the square resistance is carried out at 49 positions regularly distributed on the plate, the vanadium oxide layer being deposited on a dielectric layer, here a silicon nitride layer. Electrical resistivity is obtained by multiplying the square resistance by the thickness of the layer, measured for example by ellipsometry or X-ray reflectometry (or XRR, for "X-Ray Reflectometry" in English). The average resistivity is then equal to the average of the electrical resistivities measured at the 49 positions. The resistivity uniformity is here equal to the standard deviation of the electrical resistivities measured at the 49 positions, as a percentage of the average resistivity.

[0050] A first plate is introduced into the chamber and placed on the plate support. A first deposition of a vanadium oxide-based layer is carried out for a first duration with an oxygen flow rate equal to an initial oxygen flow rate of 4.00 sccm. The first plate is removed from the equipment. A measurement of a thickness of the vanadium oxide-based layer is carried out, for example by ellipsometry or by X-ray reflectometry (or XRR, for "X-Ray Reflectometry" in English). A deposition rate is determined by dividing the thickness by the first time, here equal to 6 nm / min (empty square superimposed on a solid square in [Fig.2]). A resistivity measurement in 49 positions is also carried out to determine an average resistivity, here equal to 20 Q.cm (empty circle superimposed on a solid circle in [Fig.3]), and a resistivity uniformity, here equal to 22% (lo -empty circle superimposed on a solid circle in [Fig.2]).

[0051] The same procedure is then repeated for 9 additional plates to obtain a deposition of a layer of vanadium oxide on 9 separate plates. Before each deposition on a separate plate, the oxygen flow rate is increased by an increment of 0.03 sccm. From one additional plate to the next, the resistivity increases progressively to reach a maximum resistivity equal to 150 Q.cm for an oxygen flow rate of 4.27 sccm. At the same time, the resistivity uniformity remains above 10% (lo). For these additional plates, the deposition rate is equal to a high rate of 6.1 nm / min.

[0052] The oxygen flow rate is again increased by an increment of 0.03 sccm to reach an intermediate oxygen flow rate equal to 4.30 sccm. A tenth layer of vanadium oxide is then deposited on a tenth plate with this intermediate oxygen flow rate. When changing from an oxygen flow rate of 4.27 sccm to the intermediate oxygen flow rate, a sudden drop in the deposition rate and the average resistivity was observed. At the intermediate oxygen flow rate, the deposition rate is equal to a low rate of 3.7 nm / min, and the average resistivity is equal to 60 Ω.cm. The sudden drop in the deposition rate reveals a switch between a first deposition regime at a high rate and a second deposition regime at a low rate.

[0053] The oxygen flow rate is then decreased in successive increments and a layer of vanadium oxide is deposited for each incremental value of the oxygen flow rate. Up to an oxygen flow rate of 4.03 sccm, the resistivity uniformity remains substantially constant, equal to a low plateau of 3% (lo), characteristic of the second deposition regime. During this second deposition regime, the average resistivity decreases until it reaches a value of 10 Q.cm for an oxygen flow rate of 4.03 sccm. Thus it is possible to adjust the average resistivity of a layer based on vanadium oxide without undergoing a variation in the resistivity uniformity.

[0054] The oxygen flow rate is decreased one last time and fixed at the initial oxygen flow rate. A final layer of vanadium oxide is deposited on a last plate at the initial oxygen flow rate. The average resistivity, the resistivity uniformity and the deposition rate measured on this last plate are respectively identical to the average resistivity, the resistivity uniformity and the deposition rate measured on the first plate. Thus, the average resistivity, resistivity uniformity and deposition rate travel through closed hysteresis cycles.

[0055] [Fig.4] shows the value of the density and the average resistivity of each vanadium oxide-based layer obtained during the deposition cycle of Figures 2 and 3. The ordinate axis gives the density and the abscissa axis the average resistivity in Q.cm. The open circles correspond to the vanadium oxide-based layers obtained with the first deposition regime. The solid circles correspond to the vanadium oxide-based layers obtained with the second deposition regime. With the first regime, the densities are between 5.15 and 5.45. With the second regime, the densities are between 4.2 and 4.5. Thus, the density of the vanadium oxide-based layer is an intrinsic characteristic of the deposition regime used to obtain the vanadium oxide-based layer. A vanadium oxide layer with a density lower than 4.5 was obtained with the second deposition regime.

[0056] The atomic compositions of a first layer and a second layer based on vanadium oxide were determined. The first and second layers were deposited respectively according to the first and second deposition regimes with oxygen flow rates leading to substantially equal average resistivities. It was then found that the first layer comprises a number of oxygen atoms (O) per vanadium atom (V) lower than the number of oxygen atoms (O) per vanadium atom (V) in the second layer, while the numbers of chromium atoms (Cr) per vanadium atom are substantially equal. Thus, the number of oxygen atoms per vanadium atom and the resistivity together define a pair of intrinsic characteristics of the deposition regime used to obtain the layer based on vanadium oxide. Table 1 gives the characteristics of the first layer (first row) and the second layer (second row).O / V is the number of oxygen (O) atoms per vanadium atom. Cr / V is the number of chromium (Cr) atoms per vanadium atom. [Tables 1] Average resistivity (Q.cm) Deposition rate (nm / min) Resistivity uniformity (% lo) O / V Cr / V 8.96 6.1 33 1.74 0.263 12.9 3.9 3 2.19 0.254

[0057] A similar deposition cycle with a vanadium-boron (VB) target instead of the vanadium-chromium target made it possible to verify that the average resistivity, resistivity uniformity and deposition rate of a vanadium oxide-based layer comprising boron as an additional element also go through hysteresis cycles. The vanadium-boron target comprises 10% boron (B) atoms relative to the number of vanadium (V) atoms.

[0058] For the first deposition regime, a high speed of between 5 and 6 nm / min, a density of the vanadium oxide-based layer of between 5.1 and 5.3, and a resistivity uniformity of the vanadium oxide-based layer of between 30% and 40% (lo) were measured. For the second deposition regime, a low speed of between 4 and 4.3 nm / min, a density of the vanadium oxide-based layer of between 4.3 and 4.4, and a resistivity uniformity of the vanadium oxide-based layer of less than 5% (lo) were measured.

[0059] Now, an example of a method for depositing a vanadium oxide-based layer according to the invention will be described. The deposition method exploits the closed hysteresis cycles of Figures 2 and 3 to obtain a vanadium oxide-based layer on a substrate.

[0060] The deposition process takes place in ion beam sputtering deposition equipment comprising a vacuum pump system, a heavy ion source, an airlock and a sealed chamber. The vacuum pump system may consist of a single vacuum pump. It maintains the chamber at an internal pressure corresponding to at least a secondary vacuum. It also maintains the airlock under at least a primary vacuum. The internal pressure is for example between 105 and 3.10 4 Torr. The heavy ions may be for example argon, krypton or xenon ions. The airlock may accommodate a plate or a cassette containing several plates.

[0061] The chamber comprises at least one target, an oxygen inlet associated with a means for regulating the oxygen flow rate, a communication path with the ion source, a communication path with the airlock, a plate support and a means for transferring a plate from the airlock via the communication path with the airlock to the plate support and vice versa. The means for transferring a plate is typically a robot.

[0062] The target comprises vanadium (V) and an additional chemical element selected from chromium (Cr) and boron (B). The target comprises, for example, 20% of additional chemical element relative to the number of vanadium (V) atoms. Alternatively, the chamber may comprise two targets, each comprising vanadium and / or an additional chemical element selected from chromium (Cr) and boron (B).

[0063] In operation, the ion source generates a beam of atoms and / or heavy ions through the communication path with the ion source, towards the target(s). The beam has sufficient energy to tear off the additional atomic element and vanadium from the target(s). The vanadium (V) and the additional chemical element combine with oxygen and are deposited on a plate placed on the plate holder by the means for transferring a plate. The plate holder can maintain the plate at a constant deposition temperature. The deposition temperature can be room temperature.

[0064] The method begins with a conditioning phase comprising a first iterative process during which N vanadium oxide-based films are successively deposited on N separate plates, the chamber pressure being maintained at the internal pressure. The N films and the N plates are numbered from 0 to N1, in the order of successive depositions. The integer N may be unknown at the start of the conditioning phase.

[0065] The vanadium oxide-based film 0 is deposited with an oxygen flow rate equal to an initial oxygen flow rate, for example between 4.00 sccm and 4.27 sccm, for a duration t0. A measurement of a thickness e0 of the film 0 is carried out, for example by ellipsometry, or by X-ray reflectometry (or XRR, for “X-Ray Reflectometry” in English). The measurement can be carried out in-situ, that is to say inside the chamber, for example with the plate 0 on the plate support. The measurement can also be carried out ex-situ, that is to say once the plate 0 has been removed from the equipment. A deposition rate v0 of the film 0 is then determined by dividing the thickness e0 by the duration t0, that is to say that v0 = eo / to.

[0066] The deposition step and the determination of a deposition rate v; from a thickness measurement are repeated for each iteration i of the first iterative process, while increasing the oxygen flow rate from one iteration to the next. That is to say that each film i is deposited on the plate i with an oxygen flow rate increased by an increment d; relative to the oxygen flow rate used during the deposition of the film (i-1), i being an integer between 1 and N. The increments d; are advantageously all equal, for example equal to 0.3 sccm. If the thickness measurement is carried out in situ, several films i can be deposited on the same plate. The deposition rate v; at iteration i is then equal to the difference in thicknesses measured at iterations i and i-1 divided by the duration h of the deposition, that is to say that v, = (e; - 6^) / ^.

[0067] The first iterative process ends when the difference in the deposition rate induced by the oxygen flow rate increment is less than a predetermined threshold Sv strictly less than 0. That is to say that (VN - VN.i) is strictly less than the predetermined threshold, and (V; - Vm) is strictly greater than the predetermined threshold for any integer i between 1 and NL. The predetermined threshold Sv can also be defined as a percentage of the deposition rate at the previous iteration. In this case, 100(VN - VN_i ) / VN-i is strictly less than the predetermined threshold, and 100(V; - Vi-O / Vu is strictly greater than the predetermined threshold for any integer i between 1 and NL. The predetermined threshold is for example equal to -15% of the deposition rate obtained at the previous iteration.

[0068] Referring to the deposition cycle described in connection with Figures 2 to 4, the oxygen flow rate of iteration N is equal to an intermediate oxygen flow rate for which the deposition passes from a first deposition regime to a second deposition regime.

[0069] Alternatively, the transition from the first deposition regime to the second deposition regime may be determined by a comparison of a difference in resistivity p or layer resistance R to a predetermined threshold. That is, (pN - pN4) or (RN - RN i) is strictly less than the predetermined threshold, and (p; - p, J or (R; - R, J is strictly greater than the predetermined threshold for any integer i between 1 and N1. In this case, the deposition rate and thickness measurements are optional.

[0070] Subsequent to the conditioning phase, the deposition method comprises an adjustment phase comprising a reduction in the oxygen flow rate to obtain a final oxygen flow rate strictly lower than the intermediate oxygen flow rate. At the end of the conditioning phase, the vanadium oxide-based layer is deposited with the final oxygen flow rate. Thus, the vanadium oxide-based layer is deposited in the second deposition regime and consequently has a low resistivity uniformity, for example less than 5% and a low density, for example less than 4.5.

[0071] Reducing the oxygen flow rate to obtain the final oxygen flow rate may comprise a second iterative process.

[0072] At each iteration j of the second iterative process, a film j based on vanadium oxide is deposited on a separate plate j, with an oxygen flow rate reduced by a decrement d'j relative to the oxygen flow rate of iteration j-1. A measurement of the resistivity or layer resistance of the film j is carried out. Advantageously, all the decrements d'; are equal, here at -0.03 sccm.

[0073] The second iterative process ends when the oxygen flow rate reaches a final oxygen flow rate for which the resistivity or sheet resistance of the film j is lower than a predetermined maximum resistivity or maximum sheet resistance, respectively. With reference to Figures 1 and 2, the resistivity or sheet resistance is adjusted during the second iterative process without impacting the resistivity uniformity. It is for example possible to measure the resistivity at a reduced number of points at each iteration, for example 1 to 3 points.

[0074] At the end of the second iterative process, the vanadium oxide layer is deposited at the final oxygen flow rate.

[0075] The method described above can be implemented for the production of the sensitive material 15 of the microbolometers 10 of the photodetector 1 of FIGS. 1A and 1B. In this case, the first and second iterative processes can be carried out on blank test plates, i.e. plates devoid of structured layers. The plates test plates can be silicon plates, possibly covered with a dielectric layer.

[0076] Next, a vanadium oxide-based layer is deposited with the final oxygen flow on a stack of intermediate layers resting on the substrate 2 which may, at this stage, comprise the electronic control and reading circuit. The vanadium oxide-based layer is then etched to produce the sensitive material 15.

[0077] Thus, since the vanadium oxide-based layer comes from the second deposition regime, the sensitive material 15 has substantially the same resistivity at all the microbolometers of the microbolometer array of the photodetector 1, which makes it possible to increase the dynamic range of the photodetector 1. In addition, if several photodetectors 1 are manufactured in parallel on the same substrate 2, the sensitive material has substantially the same resistivity at all the microbolometers of all the detectors, suitable for a readout circuit design common to all the photodetectors. The low density of the vanadium oxide-based layer from which the sensitive material 15 comes, for example less than 4.5, makes it possible to reduce the heat capacity of the microbolometer board and, consequently, to improve the sensitivity of the photodetector 1.

[0078] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims

Claims

1. Method for depositing a layer of vanadium oxide comprising at least one additional chemical element chosen from chromium (Cr) and boron (B), in equipment for deposition by sputtering at least one target by an ion beam, the method comprising the following successive steps: • a conditioning phase comprising the following sub-steps: • deposition of a film based on vanadium oxide on a test substrate with an initial oxygen flow rate • determination of an intermediate oxygen flow rate comprising the following iterative process, called the first iterative process: • increasing the oxygen flow rate by a predetermined oxygen flow rate increment to obtain an increased oxygen flow rate, • deposition of a film based on vanadium oxide with the increased oxygen flow rate, • measurement of a difference in a parameter of the film induced by the oxygen flow rate increment,the parameter being chosen from a deposition rate or a resistivity or a resistance of the film • repetition of the previous steps as long as the difference is greater than a predetermined threshold strictly less than 0, • a deposition of the vanadium oxide layer on a substrate produced with a final oxygen flow rate strictly lower than the intermediate oxygen flow rate equal to the increased oxygen flow rate at the end of the first iterative process.,

2. A deposition method according to claim 1, further comprising an adjustment phase subsequent to the conditioning phase, comprising the following iterative process, called the second iterative process: • reducing the oxygen flow rate by a predetermined oxygen flow rate decrement to obtain a reduced oxygen flow rate, • depositing a vanadium oxide-based film on a control substrate with the reduced oxygen flow rate, • measuring a resistivity of the vanadium oxide-based film, • repeating the previous steps as long as the resistivity is greater than a predetermined maximum resistivity, the adjustment phase being followed by the deposition of the vanadium oxide layer on the substrate produced at the final oxygen flow rate equal to the reduced oxygen flow rate at the end of the second iterative process.

3. Deposition method according to claim 2, in which the predetermined maximum resistivity is equal to 40 Q.cm, preferably equal to 20 Q.cm.

4. A deposition method according to any one of claims 1 to 3, wherein the deposition of the vanadium oxide-based film with the increased oxygen flow rate is carried out on a different substrate at each iteration of the first iterative process.

5. A deposition method according to any one of claims 1 to 4, wherein the parameter is a deposition rate and the measurement of the difference in deposition rates is deduced from thickness measurements of the vanadium oxide-based films.

6. A deposition method according to claim 5, wherein for each iteration of the first iterative process, the predetermined threshold is equal to -15% of the deposition rate obtained in the previous iteration.

7. A deposition method according to any preceding claim, wherein the deposition equipment comprises a target comprising vanadium and the at least one additional chemical element.

8. A method of manufacturing a microbolometer comprising a sensitive material made of vanadium oxide comprising at least one additional chemical element chosen from chromium (Cr) and boron (B), comprising a deposition method according to any one of claims 1 to 7 for producing the sensitive material.

9. Photodetector (1) comprising a matrix of microbolometers (10) comprising a sensitive material (15) common to the microbolometers made of vanadium oxide comprising at least one additional chemical element chosen from chromium (Cr) and boron (B), the sensitive material having a resistivity uniformity of less than or equal to 5%.

10. Photodetector (1) according to claim 10, wherein the material sensitive to an average sheet resistance less than or equal to a specification of a photodetector readout circuit (1).

11. Photodetector (1) according to claims 10 or 11, wherein the sensitive material (15) has a density less than or equal to 4.5.

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