EP4111490A1

EP4111490A1 - Method for producing a layer of aluminium nitride (aln) on a structure of silicon or iii-v materials

Info

Publication number: EP4111490A1
Application number: EP21706634.9A
Authority: EP
Inventors: Maxime LEGALLAIS; Bassem Salem; Thierry Baron; Romain Gwoziecki; Marc Plissonnier
Original assignee: Centre National de la Recherche Scientifique CNRS; Commissariat a lEnergie Atomique CEA; Universite Grenoble Alpes; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Grenoble Alpes; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2020-02-27
Filing date: 2021-02-25
Publication date: 2023-01-04
Also published as: FR3107782A1; US20230111123A1; FR3107782B1; WO2021170739A1

Abstract

The invention relates to a method for producing a layer of aluminium nitride (AlN) on a structure of silicon (Si) or a III-V material, the method comprising several deposition cycles (1) carried out in a plasma reactor (200) comprising a reaction chamber (210) inside of which a substrate (70) comprising said structure is arranged, each deposition cycle (1) comprising at least the following steps: • depositing (10) aluminium species on an exposed surface (70a) of the structure, the deposition step comprising at least one injection of an aluminium (Al) precursor into the reaction chamber (210), • nitriding the exposed surface (70a) of the structure, the nitriding step comprising at least one injection of a nitrogen (N) precursor into the reaction chamber (210) and the formation (32) in the reaction chamber (210) of a nitrogen plasma. During the formation (32) of the nitrogen plasma, a non-zero bias voltage V_{bias_substrate} is applied to the substrate (70).

Description

Process for producing a layer based on aluminum nitride (AIN) on a structure based on silicon or III-V materials TECHNICAL FIELD

The present invention relates to the production of a layer based on aluminum nitride (AIN) on a structure, such as a layer or nanostructures, based on materials III-N and III-V, and preferably based on Gallium nitride (GaN). For example, it finds an advantageous application in the field of microelectronics and more particularly in the fields of power electronics, sensors and optoelectronics.

STATE OF THE ART

The multiple properties of aluminum nitride (AIN) such as its large forbidden band, its dielectric permittivity, its good temperature stability or also its piezoelectricity make this material particularly attractive for many applications in the field of sensors, optoelectronic components and also high frequency and high power components. More particularly, because of its crystalline structure close to gallium nitride (GaN), AIN is a particularly useful material for making devices based on GaN. For example, GAIN can be used as a passivation layer and thus ideally form a good quality AIN / GaN interface. For an application relating to the production of GaN-based transistors, such as high electron mobility transistors (HEMTs), when this AIN passivation layer precedes the gate dielectric of the transistor, the quality of the interface AIN / GaN and the AIN layer are then decisive in the correct operation of these transistors. Indeed, the quality of the AIN / GaN interface and of the AIN layer directly affect the electrical performance of the devices such as the mobility of the electrons, the hysteresis and also the threshold voltage. Thus, for this type of applications linked to HEMTs, but also for many other applications, there is therefore a general need to provide a solution making it possible to obtain an AIN / GaN interface and a high quality AIN layer.

Chemical vapor deposition (CVD) or physical vapor deposition (PVD) techniques are commonly used for the realization of DSA. Although the CVD technique from organometallic precursors (MOCVD) allows excellent quality layers to be achieved, it requires high deposition temperatures above 700 ° C which does not allow this technique to be considered during a process. integration with structures that have a limited thermal budget. The disadvantage of physical vapor deposition (PVD) techniques is that they generate uniformity and compliance problems.

In the literature, plasma-assisted atomic layer deposition (PEALD) appears to be a possible technique for the manufacture of AND layers. Indeed, it is based on a self-limiting growth process in which the material is deposited layer by layer. In general, the PEALD technique consists of sequentially injecting aluminum precursors, then nitride, into the reaction chamber of a plasma reactor. To achieve this sequential growth, these reactions are repeated cyclically.

Despite the existence of these known techniques of deposition by CVD, PVD, PEALD, there remains a need consisting in proposing a solution making it possible to improve the properties of an AIN / GaN interface and / or of a layer of AIN.

More generally, there remains a need to provide a solution making it possible to improve the properties of an AIN layer deposited on a layer or nanostructures based on silicon, or based on III-V materials, in particular. based on III-N materials and in particular based on GaN. An object of the present invention is to meet at least one of these needs.

ABSTRACT

To achieve this objective, according to one embodiment there is provided a method of producing a layer based on aluminum nitride (AIN) on a structure based on at least one of the following materials: silicon ( Si) or a material taken from the materials III-V. The process comprises several cycles carried out in a plasma reactor comprising a reaction chamber within which is disposed a substrate comprising the structure. Each cycle includes at least the following steps:

• deposition of aluminum-based species on an exposed surface of the structure, the deposition step comprising at least one injection into the reaction chamber of an aluminum-based precursor (Al) reacting with the exposed surface of the structure,

• nitriding of the exposed surface of the structure, the nitriding step comprising at least one injection into the reaction chamber of a nitrogen-based precursor and the formation in the reaction chamber of a based plasma. nitrogen reacting with the exposed surface of the structure.

During the formation of the nitrogen-based plasma, a non-zero _{bias voltage V iaS-substrate is applied to the substrate.}

Unexpectedly and particularly advantageously, the polarization of the structure during the injection of the nitride-based precursor makes it possible, on the one hand, to considerably improve the quality of the AIN layer and, on the other hand, to improve the quality. of the interface between this layer of AIN and the structure based on silicon or on a material taken from III-V materials. In particular, obtaining a crystalline layer of AIN of this quality was completely unexpected. The process thus uses the principle of PEALD-type deposits by modifying it to provide a bias voltage applied to the substrate during the nitriding step.

It will be noted that this process makes it possible, in addition to obtaining an AIN layer of particularly high quality, to promote a crystallographic orientation (002). Thanks to this control of the polarization of the substrate, the energy of the ions which arrive on the exposed surface of the substrate is perfectly controlled, which makes it possible to modify the crystalline quality of the AIN, in particular the crystalline orientation. This layer of AIN can then be used as a primer layer followed by deposition of the AIN by a relatively rapid deposition technique, such as physical vapor deposition (PVD) for example.

For the preparation of a transistor whose active layer is based on a III-V material, the method notably avoids shifting the threshold voltage towards negative voltages, and improves the slope below the threshold. The process is thus particularly advantageous for the preparation of transistors, in particular power transistors, exhibiting good electrical properties. In particular, when the structure is made of GaN and is intended to form at least part of a GaN-based device such as a HEMT, the method can significantly increase the performance of such devices.

This AIN layer can also be used as a passivation layer for LEDs based on III-N materials.

The application of a bias voltage V _{iaS-substrate} to the substrate makes it possible to increase the energy of the ions of the plasma in a controlled manner independent of the voltage V _piaSma induced by the source used to generate the nitrogen-based plasma . The efficiency of the plasma treatment can thus be modulated in a controlled manner to further improve the properties of the interface obtained. The electrical performance of the component is therefore improved.

Optionally, the method may further exhibit at least any one of the following characteristics which may be taken separately or in combination:

According to one example, the structure is based on silicon.

According to another example, the structure is based on a material taken from III-V materials. According to one example, the material is taken from III-N materials. According to one example, the material is GaN. Thus the structure is made of GaN or is based on GaN.

According to one example, the structure is a layer. It has for example a face which extends over the entire plate. It can have a flat face. Alternatively, it can follow the shape of the reliefs underlying it. According to another example, the structure may not be a layer. It can include a nanostructure or a plurality of nanostructures. A nanostructure is a structure at least one dimension of which is less than 1 millimeter and preferably less than 500 nm (10 ⁹ meters) and preferably less than 100 nm. A nanostructure can be three-dimensional (3D). It may for example be a stud or a son extending in a main direction perpendicular to a face of the support substrate and having, in a plane perpendicular to this main direction, a section of less than 1 millimeter, preferably less than 500 nm, and preferably less than 100 nm. The nanostructure can also be a trench or a rib. It can also be a structure intended to form part or to form a device such as a transistor or a micromechanical or electromechanical device (MEMS, NEMS, etc.) or else an optical or optoelectronic device (MOEMS, etc.). The nanostructure is punctual. It does not extend over the entire plate. Thus, one face of the plate extends mainly in a plane and nanostructures extend from this face and in a direction perpendicular to this plane. These nanostructures are therefore discontinuous.

According to one example, the absolute value of the bias voltage | V _{ias-substrate} | is less than or equal to 160 Volts.

According to one example, the absolute value of the bias voltage | V _{ias-substrate} | is greater than or equal to 10 Volts. According to one example, which | V _{ias-substrate} | is between 10 volts and 130 volts and preferably V _{iaS-substrate} is between -10 volts and -130 volts.

According to one example, | V _{bias-substrate} | is equal to 35 Volts.

These values of V _ias _substrate make it possible to obtain a particularly high quality of the AIN layer. In addition, unexpectedly, they induce a crystallographic orientation (002). This very high quality AIN layer can then be used as a primer layer followed by deposition of the AIN by PVD for example. This layer could also be used as a passivation layer for LEDs based on III-N materials.

According to one example, the bias voltage V _ias-SU bstrat is applied for at least 70%, and preferably 90%, of the duration T _N of the formation of the nitrogen-based plasma. According to one example, the bias voltage V _ias-SU bstrat is applied throughout the duration T _N of the formation of the nitrogen-based plasma.

According to one example, the duration T _N of the formation of the nitrogen-based plasma is sufficient to allow nitriding of the entire film formed at the surface of the structure. According to one example, the bias voltage V _ias-SU bstrat is not applied during the deposition of aluminum-based species. Preferably, the bias voltage V _ias-SU bstrat is applied only during the nitrogen-based plasma. Alternatively, V _ias-SU bstrat is applied throughout the cycle.

According to one example, the duration T _N of the formation of the nitrogen-based plasma is greater than 7 seconds, preferably T _N is greater than or equal to 10 seconds, and preferably T _N = 15 seconds. These values make it possible to obtain a particularly high quality of the AIN layer.

According to one example, each cycle comprises, at least one step of purging the reaction chamber, the purging step comprising the injection into the reaction chamber of a neutral gas, the at least one purging step being carried out. at at least one and preferably at each of the following times:

• after the injection of the aluminum-based precursor and before the formation of the nitrogen-based plasma, and

• after the formation of nitrogen-based plasma. According to one example, each cycle comprises at least one step of stabilizing the gases present in the reaction chamber, the stabilization step being carried out at least at the following instant: before the formation of the nitrogen-based plasma. Preferably after the purging step.

According to one example, the aluminum (Al) -based precursor is taken from trimethylaluminum and aluminum trichloride.

According to one example, the injection time T _Ai of the aluminum-based precursor is sufficient to saturate the exposed surface of the structure, eg, of the GaN-based layer, preferably, T _Ai is greater than or equal to 20 ms, preferably T _{A |} is of the order of 50 ms. According to one example, the nitrogen-based precursor (N) is taken from:

• a mixture of dinitrogen and dihydrogen (N ₂ -H ₂ ),

• ammonia (NH ₃ ),

• a mixture of ammonia (NH ₃ ), dinitrogen and dihydrogen (N ₂ -H ₂ ).

According to one example, during at least some of the cycles, and preferably during each cycle, the injection into the reaction chamber of an aluminum (Al) -based precursor is carried out before the injection into the. nitrogen (N) precursor reaction chamber. Thus, the deposition of the aluminum film is carried out before nitriding.

According to one example, during at least some of the cycles, and preferably during the first of said several cycles, the step of nitriding the exposed surface of the structure is carried out, before the step of injecting the precursor based on aluminum (Al).

According to one example, the method comprises at least ten cycles and preferably at least fifty cycles. Preferably it comprises at least one hundred cycles and preferably about five hundred cycles. BRIEF DESCRIPTION OF THE FIGURES

The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment of the latter which is illustrated by the following accompanying drawings in which: FIG. 1 diagrammatically represents a cycle forming an AIN film, according to one embodiment of the present invention.

Figure 2 schematically shows an example of a deposition reactor that can be used to implement the method according to the invention.

Figures 3A and 3B are graphs illustrating the effect of substrate polarization during nitrogen-based plasma on the chemical environment of nitrogen. Figure 3A illustrates X-ray photoelectron spectrometric measurements of the N1s environment for different AII deposits at different average bias voltages.

Figure 3B illustrates the ratio of the atomic percentage contribution of nitrogen bound to aluminum N1s AIN with an "a" contribution and with a "b" contribution as a function of the mean bias voltage.

Figure 4A illustrates the ratio of the atomic percentage contribution of nitrogen bound to aluminum N1s AIN with the contribution "a" as a function of the average bias voltage and the duration of the application of the bias. Figure 4B illustrates the ratio of the atomic percentage contribution of nitrogen bound to aluminum N1s AIN with the contribution "b" as a function of the average polarization voltage and the duration of the application of the polarization.

Figure 5A illustrates the ratio of the atomic percentage of nitrogen bound to aluminum N1s AIN with the contribution "a" as a function of the mean bias voltage for different chemistries and plasma durations.

Figure 5B illustrates the ratio of the atomic percentage of nitrogen bound to aluminum N1s AIN with the contribution "b" as a function of the mean bias voltage for different chemistries and plasma durations.

Figure 6 illustrates the effect of polarization on the crystallinity of AIN deposited on a GaN substrate.

FIG. 7A is a diagram of a capacitive device of the Metal-lsolant-Semiconductor type comprising metal contacts (nickel / gold), an alumina layer, an AIN layer deposited without or with a bias voltage and surmounting a layer of GaN. FIG. 7B is a graph illustrating the Capacitance - Voltage (CV) measurements for samples comprising an AIN layer deposited without polarization (0V) and with an average polarization voltage of -35V, -70V and -130V.

The drawings are given by way of example and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate understanding of the invention and are not necessarily on the scale of practical applications. In particular, in FIG. 7A, the thicknesses of the different layers are not representative of reality.

DETAILED DESCRIPTION By a substrate, a film, a layer, a gas mixture, a plasma

"Based" on a species A, a substrate, a film, a layer, a gas mixture, a plasma comprising this species A only or this species A and possibly other species. Thus, a substrate comprising a structure such as a layer or nanostructures based on Si of III-V materials can be: - either, preferably, a stack comprising the structure based on Si or on III-V material is a layer , typically a support layer on which the structure rests, or a stack comprising only the structure based on Si or on III-V material. In this case, the structure can be self-supporting, i.e. it supports its own weight.

In addition, a substrate based on Si or on a III-V material also denotes a substrate whose layer based on Si or on III-V material is surmounted by one or more layers deposited during the process described above. below. Thus, an exposed surface of the Si-based substrate or of the III-V material may be a surface formed by the structure or formed by one or more layers or films deposited on the structure.

Furthermore, a nitrogen-based plasma can be based on a chemistry comprising only nitrogen or comprising nitrogen and optionally one or more other species, for example neutral gases. In a perfectly conventional manner, a structure based on a III-V material is a structure made or comprising a material comprising at least one species from column III of the periodic table and at least one species from column V of this table. Likewise, a structure based on a III-N material is a structure made or comprising a material comprising at least one species from column III of table periodic and nitrogen (N). A III-N material can therefore for example be taken from GaN, AIGaN, AlInGaN, InN.

Several embodiments of the invention implementing successive steps of the manufacturing process are described below. Unless explicitly stated, the adjective "successive" does not necessarily imply, although this is generally preferred, that the steps follow each other immediately, intermediate steps being able to separate them.

Furthermore, the term "step" is understood to mean carrying out part of the process, and can denote a set of sub-steps. Furthermore, the term “step” does not necessarily mean that the actions carried out during a step are simultaneous or immediately successive. In particular, some actions of a first step can be followed by actions linked to a different step, and other actions of the first step can be repeated afterwards. Thus, the term step does not necessarily mean unitary and inseparable actions over time and in the sequence of the phases of the process.

The word "dielectric" denotes a material whose electrical conductivity is low enough in the given application to serve as an insulator. In the present invention, a dielectric material preferably has a dielectric constant greater than 4. The spacers are typically formed from a dielectric material. In the present patent application, when a gas mixture is expressed with percentages, these percentages correspond to fractions of the total flow rate of the gases injected into the reactor. Thus, if a gas mixture, for example intended to form a plasma, comprises x% of gas A, this means that the injection flow rate of gas A corresponds to x% of the total flow rate of the gases injected into the reactor to form the plasma .

In the present invention, the term “HEMT type transistors” (acronym for “High Electron Mobility Transistor”) means field effect transistors with high electron mobility, sometimes also designated by the term of field effect transistor. with heterostructure. Such a transistor includes the superposition of two semiconductor layers having different forbidden bands which form a quantum well at their interface. Electrons are confined in this quantum well to form a two-dimensional gas of electrons. For reasons of high voltage and temperature resistance, the materials of these transistors are chosen so as to present a wide band gap of forbidden energy. By microelectronic device is meant any type of device produced with microelectronic means. These devices include in particular, in addition to purely electronic devices, micromechanical or electromechanical devices (MEMS, NEMS, etc.) as well as optical or optoelectronic devices (MOEMS, etc.).

It may be a device intended to perform an electronic, optical, mechanical etc. function. It may also be an intermediate product intended solely for the production of another microelectronic device.

It is specified that in the context of the present invention, the thickness of a layer or of the substrate is measured in a direction perpendicular to the surface along which this layer or this substrate has its maximum extension. The thickness is thus taken in a direction perpendicular to the main faces of the substrate on which the different layers rest.

It is specified that, within the framework of the present invention, the terms “on”, “overcomes”, “covers”, “underlying”, in “vis-à-vis” and their equivalents do not necessarily mean “in the face”. contact of ”. Thus, for example, the deposition, the transfer, the bonding, the assembly or the application of a first layer on a second layer, does not necessarily mean that the two layers are directly in contact with one another, but means that the first layer at least partially covers the second layer by being either directly in contact with it, or by being separated from it by at least one other layer or at least one other element.

The terms “substantially”, “approximately”, “of the order of” mean “to within 10%”.

The general principle of the present invention will now be described with reference to FIG. 1.

FIG. 1 illustrates the different steps which take place in each production cycle of an AIN film on a structure based on silicon or based on a III-V material. This cycle of steps is repeated several times until the stack of these films forms an AlN layer having the desired thickness. Each cycle comprises a sequential injection of precursors into a plasma deposition reactor such as that illustrated in FIG. 2 and which will be described below. Preferably it is a reactor configured to perform atomic layer deposition (PEALD). In order to carry out these cycles, a substrate 70 comprising a structure based on silicon or based on a III-V material is placed in a reaction chamber of the reactor. Such a structure is for example based on a III-N material. In the nonlimiting example which will be described in detail, this structure is based on GaN. More precisely, this structure will be described as being a layer of GaN. All the characteristics, stages and technical effects which will be described below are perfectly applicable to a structure based on silicon or on a III-V material other than GaN. Moreover, all the characteristics, steps and technical effects which will be described below are perfectly applicable to a structure, possibly other than a layer, such as a nanostructure, for example in three dimensions, or a plurality of such structures. The substrate 70 can be formed only of this structure in Si or in material III-V. Alternatively, this substrate 70 can comprise a support layer surmounted at least by such a structure. The structure has a free surface, exposed to species present in the reaction chamber.

Each cycle preferably comprises five main steps: A first step consists in depositing 10 aluminum-based species on the exposed surface 70a of the structure forming or resting on the substrate 70. This step is referenced 10 in FIG. 1. It step comprises the injection into the reaction chamber of the reactor of an aluminum-based precursor. The aluminum reagent reacts with the exposed surface 70a by chemisorption. If this injection is carried out during the first production cycle of the ANI layer, the exposed surface 70a is the upper face of an Si structure or of III-V material, in this non-limiting example a GaN layer. If this injection is performed in a subsequent cycle, the exposed surface 70a corresponds to the upper face of the AIN film formed during the previous cycle or being formed during the current cycle.

A second step, referenced 20, usually referred to as a purge, has the function of removing the reactants from the aluminum-based precursor which has not reacted as well as the reaction products. This purge generally consists of injecting an inert gas such as argon (Ar) into the reaction chamber. A third step, referenced 30, consists in carrying out a nitriding of the exposed surface 70a of the substrate 70. This step comprises the injection into the reaction chamber of a nitrogen-based precursor (N) then the formation of a plasma. 32 whose chemistry includes nitrogen-based species. This plasma is configured to allow nitriding of the exposed surface 70a. This third step 30 may include a phase 31 for stabilizing the gases used for the nitrogen-based plasma. This stabilization phase 31 is preferably carried out before the formation 32 of the nitrogen-based plasma.

During the formation of the nitrogen-based plasma, a polarization is applied to the structure based on Si or on III-V material, in this example a layer based on GaN. The voltage of this polarization can be qualified as V _{iaS-substrate} , by differentiation with the voltage V _piaSma which is induced, in a perfectly conventional manner by the source of the plasma in order to generate the ions and radicals and therefore initiate the deposition of dielectric. The bias voltage V _{biaS-substrate} is controlled independently of the voltage V _piaSma induced by the source.

In practice, the reaction chamber comprises a plate for receiving the substrate 70. The plate is electrically conductive and the bias voltage is applied to this plate, also referred to as the sample holder, supporting the substrate 70. It can thus be said that this voltage is transmitted. or applied to the substrate 70 and the structure. It will be noted that the expression “applied to the substrate” means that the bias voltage V _ias _substrate is applied to the plate on which the substrate 70 rests, whether the substrate 70 is conductive or not.

Conventionally, in a remote plasma configuration, the plasma, generated by a main source (ICP or CCP), is located away from the substrate 70. A zone of positive space charge called the cladding is formed between the plasma and the. substrate due to the difference in mobility between heavy ions and electrons. This cladding simply corresponds to the difference between the potential of the plasma Vpiasma and the potential of the substrate V _ia s_substrat · Thus, when no bias voltage on the substrate V _ias _substrat (to ground), is applied, which is the standard case during deposits by PEALD, V _ia s_substrat = 0. Within the framework of the proposed method, a _{non-zero bias is applied to V ias} _substrate, typically strictly less than 0 (<0). We can therefore increase / adjust the energy of the ions independently of V _piaSma since the energy of the ions indeed depends on the voltage of the plasma and on the bias voltage of the substrate V _ias _substrate, according to the following relation, with q the charge of the ion:

The application of this bias voltage V _ias _substrate provides considerable advantages. In particular, this polarization makes it possible to improve the quality of the AIN layer formed. On the other hand, it makes it possible to improve the quality of the interface between the structure based on Si or on III-V material and the AlN layer. Figures 3A to 7B, which will be described in detail below, explain the advantages conferred by the application of this bias voltage during the formation step 32 of the nitrogen-based plasma.

By applying a _{non-zero V iaS-substrate} bias voltage, the efficiency of ion bombardment on the surface can be increased and adjusted, while preserving the exposed surface 70a. The quality of the AIN layer and the quality of the interface between this AIN layer and the substrate 70 are considerably improved. The repeatability of this process is also improved compared to existing solutions, in particular those using a single plasma source which makes it possible to control only the flow of ions reaching the substrate and therefore to play on V _piasma .

The applied bias voltage is less than 160 volts. It will be noted that this bias voltage is much lower than the bias voltages usually used for performing plasma etchings or implantations. In addition, this method is preferably carried out in a plasma deposition reactor. Plasma etching reactors are not configured to apply such low bias voltages to the substrate.

Preferably, the bias voltage V _{biaS-substrate} is applied only during the nitrogen-based plasma and not during the deposition of aluminum-based species. The precursor of alumina (TMA for example) decomposes thermally. Nitrogen, on the other hand, requires a lot more energy and therefore requires a plasma to break it down. Therefore, it is possible to apply V _ias _substrate only during nitrogen-based plasma. Alternatively, V _ias-SU bstrat is applied throughout the cycle.

A fourth step, referenced 40, consists in carrying out a purge so as to evacuate the reactants of the nitrogen-based precursor which have not reacted as well as the reaction products. During this purge, neutral gas such as argon (Ar) is injected into the reaction chamber. This step, although advantageous, is only optional.

It will be noted that in the process illustrated in FIG. 1, the injection of the aluminum-based precursor is carried out before the injection of the nitrogen-based precursor and the formation of the nitrogen-based plasma. However, provision can be made for the order of these groups of steps to be reversed. Thus, the sequence comprising the injection of the aluminum-based precursor can be carried out after the sequence comprising the injection and formation of the nitrogen-based plasma 32. Typically, at the end of a cycle the thickness of AIN formed is less than Angstrom (10 ¹ ° meters). Preferably this thickness is less than 0.7 Angstrom. Preferably, this thickness is between 0.4 Angstrom and 0.7 Angstrom.

Figure 2 illustrates a diagram of a plasma reactor that can be used to implement the proposed method.

Preferably, the proposed method is implemented in a plasma deposition reactor. More particularly in an inductively coupled plasma reactor, usually qualified by its acronym ICP from the English term Inductively Coupled Plasma. The reactor 200 comprises a reaction chamber 210 inside which is disposed a plate 220. This plate 220 is configured to receive the substrate 70 comprising GaN. The substrate 70 rests on the tray 220 by a rear surface. The front surface of the substrate 70, opposite its rear surface, is exposed to the species present in the reaction chamber 210. In this non-limiting example, the substrate 70 forms the structure on which it is desired to deposit the AlN-based layer. This front surface of the substrate 70 therefore constitutes the surface 70a of the structure. The plate 220 is electrically conductive. In a relatively conventional manner, the reactor comprises a gas inlet 230 making it possible to inject into the interior of the chamber 210 the gases intended to form the chemistry of the plasma as well as the gases intended for the purge phases. It also comprises an induction coupling device 260, a coil of which is illustrated in FIG. 2, and which allows the formation of the plasma. A wall of the reaction chamber 210 is electrically connected to the earth 270. Conventionally, as clearly shown in this FIG. 2, the plasma source 260 is offset with respect to the reaction chamber 210. Thus, the voltage V _piasma is offset from the substrate 70. This bias voltage V _piaSma is not applied to the substrate 70. The reactor 200 also comprises a valve 240 for isolating the reaction chamber 210. The reactor 200 also comprises a pump 250 for extracting the particles. species present in the reaction chamber 210.

More particularly, the method is implemented in an inductively coupled plasma reactor, usually qualified by its acronym ICP from the English term Inductively Coupled Plasma. Preferably, the source is an inductive radiofrequency source, which makes it possible to have a stable plasma at a power P _piaS ma much lower compared to other sources, for example a microwave source, typically 1500 W to 2000 W According to one example, the power of the inductive radiofrequency source is between 100 and 300 W, preferably 300 W. Advantageously, this reactor 200 comprises a bias device 270 configured to allow the application of the bias voltage V _{iaS-substrate} to the plate 220. According to one example, this voltage can ultimately be applied to the substrate 70, at least. on its face facing the plate 220, whether this face is electrically conductive or not. This polarization device 270 comprises a control device 281 and makes it possible to apply an alternating voltage to the plate 220. This control device 281 preferably comprises an automatic adaptation unit (referred to by its English term as auto match unit) which matches the impedance in the chamber and of the ion source to that of the radiofrequency generator.

This biasing device 270 is configured to allow the application to the plate 220 of the bias voltage V _{biaS-substrate} , the amplitude of which is low, typically less than 160 volts, preferably less than 130 volts.

The biasing device 270 and the plasma source 260 are configured so as to be able to adjust the bias voltage V _ias _substrat applied to the plate 220 independently of the plasma voltage Vpi _aS ma · V _ias-SU bstrat and V _piasma are independent. V _bias-SU bstrat and V _piasma are independently controlled.

With reference to Figures 3A to 7B, the effects and advantages of applying a bias voltage during the formation of the nitrogen-based plasma will now be described in detail.

Figures 3A and 3B are graphs illustrating the effect of substrate polarization, during nitrogen-based plasma, on the chemical environment of nitrogen. More precisely, in this example, the plasma is an N ₂ -H ₂ plasma with 33% hydrogen during the deposition of the AIN on an orientation silicon layer (100). FIG. 3A illustrates measurements by X-ray photoelectron spectrometry of the N1s environment for different deposits of AIN, produced on the orientation silicon layer (100), this at different substrate bias voltages (V). Three contributions are observed. A first contribution corresponds to the N-AI bonds at approximately 396.7 eV. Two other contributions 'a' and 'b', unwanted and corresponding to impurities, are observed. These contributions 'a' and 'b' are referenced 310 and 320.

FIG. 3B illustrates, on curve 330, the ratio of the atomic percentage of N 1s bonds with Al to the atomic percentage of N 1s bonds with the 'a' contribution. Curve 340 of this figure 3B illustrates the ratio of the atomic percentage of N1s bonds with AI to the atomic percentage of N1s bonds with the contribution 'b'. The angle of analysis by XPS (X-ray photoelectron spectrometry) is 23.75 °. According to these measurements by XPS, the ratio of the atomic percentage of the N1s N-Al bonds to the other 'a' and 'b' contributions increases and then decreases. This thus shows that the quality of the layer can be improved with the polarization of the substrate in a range of about 0 to -70V with an optimum emerging at about -35V. Similar results can be obtained with layers thicker by around 25 nm and also on a GaN layer instead of a silicon (Si) layer.

The duration of the nitrogen-based plasma must be long enough to allow nitriding, preferably complete nitriding, of the surface exposed to the plasma and thus benefit from the effects of the polarization of the substrate. Preferably, this duration of the plasma, denoted T _P, is greater than or equal to 70% and preferably greater than equal to 90% of the duration T _N of formation 32 of the nitrogen-based plasma. Preferably TP = TN. Figures 4A and 4B illustrate the ratio of the atomic percentage of the contribution of nitrogen bound to aluminum N1s AIN with the contribution 'a' (Figure 4A) and with the contribution 'b' (Figure 4B) as a function of the average bias voltage for a duration T _P of 5s (seconds) (curves 41a and 41b) and for a duration T _P of 15s (curves 42a and 42b), T _P being the duration of the plasma, here a plasma based on a chemistry of N ₂ -H ₂ with 33% hydrogen.

It emerges from these figures that for an N ₂ -H ₂ plasma with 33% hydrogen, a _{plasma duration T P} of 5 s is too short since it only forms very few AIN bonds. Thus, the polarization of the substrate then has little effect on the quality of the deposit if T _P is not correctly adjusted. Similar results, illustrated in FIGS. 5A and 5B, were obtained for deposits carried out with three different chemistries, more precisely with respectively an NH ₃ plasma, a N ₂ -H ₂ -Ar plasma, and also with a plasma of N ₂ H ₂ .

Curves 51a and 51b (legend “5s, NH ₃ ”) each correspond to an ammonia plasma only with a duration of 5s. Curves 52a and 52b (legend “15s, N ₂ -H ₂ -Ar”) each correspond to a plasma composed of 86% H ₂ , 7% N ₂ and 7% Ar and a duration from 15s.

The curves 53a, 53b (legend “15s, N ₂ -H ₂ ”) each correspond to a plasma composed of 33% hydrogen and with a duration of 15s. FIG. 6 illustrates the effect of polarization on the crystallinity of the AIN deposited on the structure, here a GaN substrate. Deposits of AIN at different bias voltages were carried out on a GaN substrate. More precisely, in this example, the GaN substrates were obtained by MOCVD growth on silicon.

In this example, the AIN layer is obtained by a succession of 500 cycles. Each cycle comprises a plasma of N ₂ -H ₂ (33% hydrogen) maintained for a period of 15 seconds. X-ray diffraction measurements were carried out at grazing incidence. The diffractogram was indexed according to the ICDD 00-025-1133 sheet of the hexagonal AIN.

The grazing incidence X-ray diffraction measurements show that the layers are poly-crystalline and have a hexagonal structure. Note that a preferential growth orientation along the (002) axis seems to be favored during deposition at low bias voltages. Typically, these bias voltages are between -40V and -80V.

Deposits of AII at different bias voltages were also performed on GaN substrate to study the effect induced by the application of a bias voltage on the electrical characteristics of capacitors.

In this example, the doping of the GaN is of type N. The doping is carried out so that the concentration of doping species is 5 × 10 ¹⁸ cm ³ .

Before their introduction into the PEALD reactor, the samples were immersed in a 12% hydrochloric acid solution for 2 minutes then rinsed for 1 minute with deionized water and finally dried under a stream of nitrogen. Then, an AIN layer is formed by 40 cycles. For certain substrates, these cycles are carried out without polarization during the formation 32 of the nitrogen-based plasma (curves 701). For other substrates, these cycles are performed with an average bias voltage of -35V (curve 702), -70V (curve 703) and -130V (curve 704), respectively.

After this deposition, 125 cycles of alumina (Al ₂ O ₃ ) are deposited on the layer of AIN. Preferably, this Al ₂ O ₃ deposition is carried out in the same reactor. Finally, circular spots are defined in a positive photosensitive resin by lithography. Then, 30 nm of nickel and then 100 nm of gold are evaporated by electron beam. Finally, the excess metal is removed by lift-off in acetone. FIG. 7A schematically illustrates the device obtained. This device comprises a GaN layer, for example forming a substrate 70, surmounted by an AlN layer 71 obtained by applying or not a bias voltage during successive deposition cycles. On and preferably directly in contact with the AlN layer 71 include Al ₂ 0 layer _3. The pins 73 of nickel and gold overcome the AI ₂ 0 _{3 layer.}

The various devices are characterized by electrical measurements of the “Capacitance-Voltage” (C-V) type at a frequency of 10 kHz and on a pad size of 600 μm. FIG. 7B illustrates the results of these electrical measurements. The curves 701 correspond to the substrates obtained without applying a bias voltage during the cycles of formation of the nitrogen-based plasma. The curves 702, 703 and 704 correspond respectively to the results obtained with substrates produced by applying, during the formation of the nitrogen-based plasma, an average polarization voltage of -35V, -70V and -130V.

Applying the bias voltage to the substrate has been observed to change the charge plane at the interface between the GaN and the gate stack and to shift the threshold voltage to positive voltages.

It emerges from this FIG. 7B that the polarization of the substrate during the deposition of the AIN strongly affects the electrical characteristics. Thanks to the substrate polarization, the CV characteristics are shifted towards positive voltages, which is the desired effect in the context of improving many devices and in particular HEMTs. However, if the bias voltage is too high in absolute value, the switching between the depletion and accumulation regime is less abrupt and the hysteresis also increases (curves 704). An optimum then emerges at an average bias voltage of -35V which corresponds to the optimum observed previously for an N ₂ -H ₂ plasma with 33% hydrogen on the XPS measurements as illustrated in FIGS. 3A and 3B. Specific examples and variant embodiments

The following paragraphs aim to describe specific embodiments of the present invention and to propose certain variants. The characteristics and the examples and variants proposed below are applicable and can be combined with each of the examples mentioned above. According to one embodiment, at least for certain cycles, the formation of the nitrogen-based plasma is carried out before the injection of the aluminum-based precursor. Thus, steps 10 and 30 of Figure 1 can be reversed. According to one embodiment, this order of the sequences is identical for all the cycles of the formation of the AIN layer.

According to one example, the purge phase uses a neutral gas such as dinitrogen (N ₂ ) OR argon (Ar). Advantageously, the purge time is long enough to remove the excess reagent and / or the reaction by-products. Typically, the purge face lasts several seconds. It lasts for example about 3s. According to one example, the aluminum-based precursors can be trimethylaluminum or aluminum trichloride. The duration of injection of the precursor must be sufficient to saturate the surface of the GaN-based layer or the surface already deposited with AIN. This injection duration is typically about 50 ms (10 ⁶ seconds). According to one example, the pressure of the reaction chamber of the reactor during the plasma must be adjusted so as to have a non-collisional cladding. Typically the pressure is less than 50 mTorr (ie 6.67 Pa) and preferably equal to 10 mTorr.

According to one example, the RF-ICP power must be large enough to have a stable plasma. This power is greater than 100W. Preferably this power is between 100-300W.

According to one example, the nitrogen-based precursor can be a mixture of dinitrogen and dihydrogen (N ₂ -H ₂ ), ammonia (NH ₃ ), or a mixture of these gases. Argon can be added with all of these gases. According to one example, the duration of the nitrogen-based plasma must be long enough to allow nitriding of the film or of the layer previously formed.

According to one example, the bias voltage of the substrate during the nitrogen-based plasma is between -10 V and -130 V.

It is clear from the above description that the proposed process considerably improves the quality of the deposited ANI layer as well as the quality of the interface between the ANI and GaN.

The proposed process thus confers considerable advantages, in particular for the manufacture of HEMTs based on GaN and AIGaN.

The invention is not limited to the embodiments described above and extends to all the embodiments covered by the claims. For example, in the preceding examples, the GaN-based layer on which the AlN-based layer is formed consists of GaN. However, the present invention also extends to embodiments in which the GaN-based layer on which the AlN-based layer is formed is a layer made of a gallium nitride and at least one among indium and aluminum. Thus, this GaN-based layer can be GaN, AIGaN, InGaN or AlinGaN.

The invention also extends to embodiments in which the structure on which the AIN layer is deposited is silicon-based.

The invention also extends to embodiments in which the structure on which the ANI layer is deposited is based on a material taken from the III-V materials. Preferably it is an III-N material.

Thus, all the examples, characteristics, steps and technical advantages mentioned above with reference to a GaN-based structure are applicable to a silicon-based structure or a material taken from III-V materials. Moreover, in the examples described above, the structure is a layer.

Nevertheless, all the examples, characteristics, steps and technical advantages mentioned above with reference to a structure forming a layer are applicable to a structure not forming a layer but forming a point structure, for example a three-dimensional relief. The structure can be a nanostructure or include a plurality of nanostructures.

Claims

1. Method for producing a layer based on aluminum nitride (AIN) on a structure based on silicon (Si) or based on a III-V material, the method comprising several deposition cycles (1) carried out in a plasma reactor (200) comprising a reaction chamber (210) inside which is disposed a substrate (70) comprising said structure, each deposition cycle (1) comprising at least the following steps:

• deposition (10) of aluminum-based species on an exposed surface (70a) of the structure, the deposition step comprising at least one injection into the reaction chamber (210) of a precursor based on aluminum (Al),

• nitriding of the exposed surface (70a) of the structure, the nitriding step comprising at least one injection into the reaction chamber (210) of a nitrogen-based precursor (N) and the formation

(32) in the reaction chamber (21 0) of a nitrogen-based plasma, characterized in that during the formation (32) of the nitrogen-based plasma, a voltage is applied to the substrate (70) of non-zero V ias_substrat polarization.

2. Method according to the preceding claim wherein the absolute value of the bias voltage | V _b ias_substrat | is less than or equal to 160

Volts.

3. Method according to any one of the preceding claims the absolute value of the bias voltage | V _i a _S _substrate | is greater than or equal to 10 Volts. 4. A method according to any preceding claim wherein | V _i as_substrat | is between 10 volts and 130 volts and preferably V _{bias-substrate} is between -10 volts and -130 volts.

5. A method according to any preceding claim wherein | V _bias _substrat | is equal to 35 Volts. 6. Method according to any one of the preceding claims wherein the bias voltage V _i a _S _substrate is applied for at least 70%, preferably 90%, and even more preferably throughout the duration T _N of the formation (32) of the nitrogen-based plasma.

7. A method according to any preceding claim wherein the duration T _N of the formation (32) of the nitrogen-based plasma is sufficient to allow nitriding of the entire exposed surface (70a) of the structure.

8. A method according to any preceding claim wherein the bias voltage V ias_substrat is controlled independently of a voltage V _piaS ma induced by a source of said nitrogen-based plasma. 9. A method according to any preceding claim wherein each cycle (1) comprises, at least one step of purging (20, 40) of the reaction chamber (210), the step of purging (20, 40) comprising the injection into the reaction chamber (210) of a neutral gas, the at least one purging step (20, 40) being carried out at at least one and preferably at each of the following times:

• after the injection of the aluminum-based precursor and before the formation (32) of the nitrogen-based plasma, and

• after the formation (32) of nitrogen-based plasma.

10. A method according to any preceding claim wherein each cycle (1) comprises at least one stabilization step (31) of the gases present in the reaction chamber (210), the stabilization step (31) being carried out at least at the following instant:

• before the formation (32) of nitrogen-based plasma.

11. Process according to any one of the preceding claims, in which the aluminum-based precursor (Al) is taken from trimethylaluminum and aluminum trichloride.

12. Method according to any one of the preceding claims, in which the duration of injection T _Ai of the aluminum-based precursor is sufficient to saturate the exposed surface (70a) of the structure, preferably T _Ai is greater than or equal. at 20 ms, preferably T _A! is of the order of 50 ms.

13. Process according to any one of the preceding claims, in which the nitrogen-based precursor (N) is taken from: • a mixture of nitrogen and hydrogen (N2-H2),

• ammonia (NH3),

• a mixture of ammonia (NH3), nitrogen and hydrogen (N2-H2).

14. Method according to any one of the preceding claims wherein during at least some of the cycles (1), and preferably during each cycle (1), the injection of the aluminum-based precursor is carried out before nitriding comprising forming (32) the nitrogen-based plasma.

15. Method according to any one of the preceding claims, in which during at least some of the cycles (1), and preferably during the first of said several cycles (1), the step of nitriding the exposed surface is carried out. (70a) of the structure, before the injection of the aluminum-based precursor.

16. A method according to any preceding claim wherein the structure is one of: a layer, a three-dimensional structure, a plurality of three-dimensional structures.

17. A method according to any preceding claim wherein the structure is based on silicon.

18. A method according to any one of claims 1 to 16 wherein the structure is based on a III-V material. 19. Method according to the preceding claim, in which the structure is based on a III-N material.

20. Process according to the preceding claim, in which the structure is based on gallium nitride (GaN).