Patent Application
20240411158
The present invention generally relates to an electrooptical component and a method for producing the layer based on a Pockels effect material forming the core of this component. The invention more specifically relates to producing optical phase modulators.
These phase modulators can advantageously be used in “optical phased array” (OPA)-type circuits, for example for designing laser remote detection systems, called LIDAR (laser imaging detection and ranging) systems.
The principle of optical phase modulators is typically based on a local modulation of the refraction index of the material, wherein the light wave propagates. This refraction index variation can be advantageously obtained by Pockels effect, for certain crystalline materials without symmetry with respect to an axis.
It is, in particular, the case of lithium niobate (LiNbO3) and of tantalum niobate (LiTaO3). Their intrinsic properties make it possible, in particular, to consider producing modulators operating at a high frequency, with low optical losses (typically around a few decibels for a TT phase shift), and with a low energy consumption. However, the thin layer synthesis of these materials is complex and the requirements linked to their crystalline quality are high. The thin LiNbO3 or LiTaO3 layer must, in particular, have a high crystalline quality and a controlled stoichiometry. It must also have a controlled crystalline orientation, according to the architecture and the position of the electrodes of the electrooptical component to be produced. Below, only LiNbO3 is mentioned, for example, for brevity. The properties of LiTaO3 and of an Li(Nb,Ta)O3 alloy are very close to the properties of LiNbO3. It is understood that LiTaO3 or Li(Nb,Ta)O3 can be directly substituted for LiNbO3.
Solutions making it possible to synthesise LiNbO3 in thin layers on a sapphire substrate have been developed. This type of substrate is expensive and the applications linked to these solutions remain limited. Producing thin LiNbO3 layers on a silicon-based substrate presents a considerable challenge. This would make it possible to consider a new generation of multifunctional systems comprising electrooptical, acoustic, microelectronic and/or quantum devices cointegrated on one same substrate. A solution consists of transferring a thin LiNbO3 layer from a donor substrate, typically from a monocrystalline LiNbO3 substrate, to a receiver substrate, typically a silicon substrate with a superficial oxide layer. This solution implementing numerous technical steps has a significant cost. The achievable range of thin layer thicknesses, typically greater than 200 nm and less than 1 μm, is also limited by the transfer and thinning technical stresses. The thickness homogeneity of a thin layer obtained by transfer is generally not sufficient for the targeted applications. The thickness standard deviation of a thin LiNbO3 layer of 200 nm obtained by transfer reaches typically around 50 nm. This type of transfer is further done “solid plate”, over the entire surface of the receiver substrate. It is thus necessary to etch the LiNbO3 layer transferred to obtain LiNbO3 layer portions locally on the receiver substrate. The etching of LiNbO3 is complex to control, in particular, to obtain roughnesses which are compatible with optical applications. Problems of contaminating production lines with lithium can occur during etching. During the solid plate transfer, it is also necessary to manage the mechanical stresses linked to the thermal dilatation difference between silicon and LiNbO3. The formation of localised LiNbO3 portions on the receiver substrate is therefore difficult and expensive.
There is therefore a need consisting of locally producing a lithium niobate-based layer on a silicon-based substrate, which has a crystalline quality which is compatible with the targeted applications, while limiting the production costs.
An aim of the present invention is to at least partially meet this need.
In particular, an aim of the present invention is to propose a method for the localised formation of an on-silicon substrate lithium niobate layer portion, which has an optimised cost and/or compatibility with respect to current methods. Such a method is advantageously implemented to produce a device comprising a lithium niobate layer portion.
Another aim of the present invention is to propose a device, typically a phase modulator-type electrooptical device, comprising a lithium niobate layer portion.
Other aims, features and advantages of the present invention will appear upon examining the description below and accompanying drawings. It is understood that other advantages can be incorporated.
To achieve this aim, according to an embodiment, a method for producing a device is provided, comprising a lithium niobate or lithium tantalate layer portion, said method comprising the following steps:
In the scope of the development of the present invention, specifications for the nucleation layer has been established. The nucleation layer must preferably have the following properties:
A technical bias of the prior art is that such a nucleation layer must necessarily be oxide-based, in order to avoid a loss of stoichiometry of the LNO portion due to a diffusion of oxygen from the LNO portion to the nucleation layer.
On the contrary, to meet these specifications, the nucleation layer is chosen according to the present invention made of a nitride-based refractory material. This nucleation layer made of a nitride-based refractory material is called refractory nitride-based nucleation layer below, for brevity. In the scope of the development of the present invention, it has indeed been observed that such a refractory nitride-based nucleation makes it possible, fully unexpectedly, the epitaxy of the LNO portion under good conditions. Surprisingly, it has also been observed that such a nucleation layer makes it possible to block both the diffusion of oxygen and the diffusion of lithium to the silicon-based substrate. This results that the epitaxially grown LNO portion on such a nucleation layer preserves the required stoichiometry.
Moreover, the different crystalline structures and the mesh parameters of the refractory nitrides, typically III-N nitrides, are fully compatible with those of lithium niobate and silicon. Such a nucleation layer can therefore advantageously be epitaxially grown on a silicon-based substrate, then enable the epitaxy of the LNO portion.
According to the invention, the epitaxy of LNO is done in a localised manner, in the at least one opening of the masking layer. This makes it possible to obtain only an LNO layer portion. This makes it possible to limit the mechanical stresses induced by the difference of thermal dilatation coefficients between lithium niobate and the nitride-based refractory material. The LNO portion undergoes less mechanical stresses than a lithium niobate “solid plate” layer. The crystalline quality of the LNO portion and its electrooptical properties are best preserved after the formation by epitaxy. A localised growth also makes it possible to limit the presence of lithium niobate on the entire plate. This avoids resorting to an etching of lithium niobate, which is complex to implement to obtain an LNO portion of optical quality.
The epitaxy of LNO further makes it possible to better control the thickness of the LNO portion. In particular, low thicknesses, for example, less than or equal to 200 nm, can be obtained by epitaxy with a good control of the thickness over the entire substrate. The layer transfer techniques do not make it possible to obtain such thicknesses and/or not with a sufficient precision and reproducibility over the entire substrate. The standard deviation in thickness of a thin LNO layer of 200 nm obtained by transfer can reach around 50 nm. Such a thickness variation over the entire substrate is not compatible with the optoelectronic targeted applications. On the contrary, the epitaxy of LNO makes it possible advantageously to obtain a standard deviation in thickness less than 10 nm for a layer which is 200 nm thick.
The present invention thus proposes a solution making it possible to synthesise a thin LNO layer portion directly on a silicon-based substrate—i.e. without transfer step—through the refractory nitride-based nucleation layer. Thanks to the refractory nitride-based nucleation layer, this LNO portion is stoichiometric and of high crystalline quality. Such a method can further be directly integrated in a production factory with the CMOS technology standard (based on n- and p-type complementary Metal-Oxide-Semiconductor transistors).
According to another aspect of the invention, a device is provided comprising, stacked in a vertical direction z, a silicon-based substrate, a nucleation layer on said substrate, a lithium niobate- or lithium tantalate-based layer portion, called LNO portion, made of mesa on said nucleation layer. The LNO portion is bordered by a masking layer. The device further comprises two electrodes configured to apply an electric field to the LNO portion.
Advantageously, the nucleation layer of the device is made of a nitride-based refractory material. The advantages mentioned above apply mutatis mutandis. Such a device is further directly integrable in silicon technology. It can be easily cointegrated with other microelectronic or optoelectronic or quantum devices, for example. Preferably, this device is a phase modulator-type electrooptical device. An aspect of the invention relates to a system comprising, on one same silicon-based substrate, a plurality of devices, each comprising an LNO portion according to the invention, for example, at least one electrooptical device and an electronic component such as a superconductor detector.
The aims, objectives, as well as the features and advantages of the invention will emerge best from the detailed description of embodiments of the latter, which are illustrated by the following accompanying drawings, wherein:
The drawings are given as examples and are not limiting of the invention. They constitute principle schematic representations intended to facilitate the understanding of the invention, and are not necessarily to the scale of practical applications. In particular, on the principle diagrams, the thicknesses of the different layers and portions, and the dimensions of patterns are not representative of reality.
Before starting a detailed review of embodiments of the invention, below, optional features are stated, which can optionally be used in association or alternatively:
According to an example, the LNO portion is lithium niobate LiNbO3-based. According to an example, the LNO portion is lithium tantalate LiTaO3-based. According to an example, the LNO portion is based on Li(Nb,Ta)O3 alloy.
According to an example, the nitride-based refractory material of the nucleation layer is chosen so as to have or to form:
This makes it possible to avoid a diffusion of oxygen and lithium from the LNO portion to the nucleation layer. The stoichiometry of the LNO portion is thus preserved.
According to an example, the nitride-based refractory material of the nucleation layer is chosen so as to have a crystallographic structure which is compatible with the substrate and the LNO portion, such as a hexagonal structure. This makes it possible to limit the appearance of structural defects during the formation of the LNO portion by epitaxy. According to an example, the mesh parameter difference between the nitride-based refractory material and the LNO material is less than 2%. This mesh parameter difference can be taken at a sub-array of the normal cell of the LNO material. For example, in the case of LiNbO3, the mesh parameter of the normal cell is 5.148 Å. In the plane (0001), it is however possible to determine a hexagonal or minicell sub-array of 3.056 Å of mesh parameter. According to an example, if the nitride-based refractory material is aluminium nitride AlN which has a mesh parameter of 3.112 Å, the mesh parameter difference between the LiNbO3 minicell and the AlN is around 1.8%. The LiNbO3 epitaxy can thus be done on AlN, surprisingly. To align the AlN hexagonal arrays and the LiNbO3 minicell, a twist of around 30° between the LiNbO3 and AlN arrays appears typically, in the plane (0001).
According to an example, the nitride-based refractory material is taken from among III-N refractory nitrides based on an element of the Ill group, such as gallium nitride GaN, aluminium nitride AlN, and AlGaN alloy.
According to an example, the substrate is silicon-based oriented in an orientation (111), the nucleation layer is aluminium nitride AlN-based, oriented in an orientation (0001), and the LNO portion is oriented in the orientation (0001).
According to an example, the LNO portion is directly in contact with the nucleation layer. In particular, there is no oxide interlayer between the nucleation layer and the LNO portion.
According to an example, the LNO portion is surmounted by an aluminium-based encapsulation portion, for example aluminium nitride AlN-based or sapphire Al2O3-based. This aluminium-based encapsulation portion typically forms, with the nucleation layer, an encapsulation of the LNO portion. This makes it possible to avoid a diffusion of oxygen and/or lithium from the LNO portion to the upper layers. The stoichiometry of the LNO portion is thus preserved. The compatibility with CMOS technologies is preserved. The electrooptical properties of the LNO portion are preserved, even improved.
According to an example, a so-called upper electrode from among the two electrodes is disposed above the LNO portion and a so-called lower electrode from among the two electrodes is disposed below the LNO portion. This device architecture is, in particular, adapted to the use of a so-called “Z-cut” oriented LNO portion (0001).
According to an example, the device is configured to form an electrooptical device, such as an optical phase modulator. In particular, the phase modulation of the light wave propagating in the device is done by Pockels effect, by applying an electric field to the LNO portion. The application of the electric field will induce a variation of the refraction index of the LNO portion, and a phase shift of the light wave.
According to an example, the LNO portion is surmounted by a silicon nitride SiN-based waveguide pattern configured to form, with the LNO portion, an edge waveguide.
According to an example, the LNO portion is surmounted by niobium nitride NbN-based structures, the device forming a superconductor detector.
According to an example, the LNO portion has a thickness e3 of between 50 nm and 500 nm, for example, of around 100 nm or 200 nm.
According to an example, the method further comprises, after epitaxy of the LNO portion, an at least partial removal of the masking layer by preserving the LNO portion formed locally in the at least one opening. This makes it possible, typically, to remove a deposition of LNO material on the masking layer, outside of the opening. This removes a possible source of contamination for other devices of the plate on which the device is produced, comprising a lithium niobate layer portion. This limits the impact of thermal dilatation coefficient differences with the silicon substrate during subsequent manufacturing steps.
According to an example, the removal is done by chemical-mechanical polishing.
According to an example, the method further comprises, after epitaxy of the LNO portion, a formation by epitaxy, on the LNO portion, of an aluminium-based encapsulation portion, for example, aluminium nitride AlN-based or sapphire- or alumina Al2O3-based.
According to an example, the method further comprises, after epitaxy of the LNO portion, a formation of a waveguide pattern configured to form, with the LNO portion, an edge waveguide. According to an example, the waveguide pattern is silicon nitride SiN-, or also TiO2-, ZrO2-, Ta2O5-based.
According to an example, the formation of the nucleation layer and the formation of the LNO portion are done by pulsed laser deposition, preferably without venting between the two steps. This makes it possible to avoid an exposure to air of the surface of the nucleation layer. The nucleation layer is not contaminated by water and/or hydrocarbons. The growth of the LNO portion is optimised and the overall performance is improved. According to an example, the formation of the LNO portion and the formation of the encapsulation portion are done by pulsed laser deposition successively within one same reactor without venting between said formations. This makes it possible to avoid an exposure to air of the surface of the LNO portion. The LNO portion is not contaminated by water and/or hydrocarbons. This makes it possible to avoid an intermediate surface cleaning step, to reduce the total duration of the method and to limit the costs. The overall performance is improved.
According to an example, the formation of the nucleation layer is configured such that said nucleation layer has a thickness e2 less than or equal to 200 nm, preferably less than or equal to 50 nm. This makes it possible to limit the appearance of structural defects during the formation of the nucleation layer by epitaxy.
According to an example, the formation of the LNO portion is configured such that said LNO portion has, after epitaxy, a thickness e3 of between 50 nm and 500 nm, for example, of around 200 nm. Such a thickness e3 corresponds to a thin LNO layer. Such a thickness e3 cannot be obtained with a good control of the thickness, over the entire substrate, by transfer and thinning of a layer coming from a donor substrate distinct from the receiver substrate.
According to an example, the method comprises a formation of an upper electrode on the LNO portion and a formation of a lower electrode under the LNO portion. This makes it possible, in particular, to produce “Z-cut” architecture phase modulator-type electrooptical devices.
Unless inconsistent, it is understood that all of the optional features above and/or the variants indicated can be combined so as to form an embodiment which is not necessarily illustrated or described. Such an embodiment is clearly not excluded from the invention. The features of an aspect of the invention, for example, the device or the method, can be adapted mutatis mutandis to the other aspect of the invention.
It is specified that, in the scope of the present invention, the terms “on”, “surmounts”, “covers”, “underlying”, “opposite” and their equivalents do not necessarily mean “in contact with”. Thus, for example, the deposition of a first layer on a second layer, does not compulsorily 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.
A layer can moreover be composed of several sublayers of one same material or of different materials.
By a substrate, a stack, a layer “based on” a material M, or “M-based”, this means a substrate, a stack, a layer comprising this material A only or this material A and optionally other materials, for example, alloy elements and/or doping elements. Thus, a silicon-based substrate means, for example, an Si or doped Si, or also SiGe substrate. An AlN-based layer means, for example, an AlN-, doped AlN-based layer, or AlN alloys, for example, AlGaN. A silicon nitride SiN-based waveguide can, for example, comprise non-stoichiometric silicon nitride (SixNy), or stoichiometric silicon nitride (Si3N4).
Several embodiments of the invention implementing successive steps of the manufacturing method are described below. Unless explicitly mentioned, the adjective “successive” does not necessarily imply, even if this is generally preferred, that the steps follow one another immediately, intermediate steps being able to separate them.
Moreover, the term “step” means the carrying out of a part of the method, and can mean a set of substeps.
Moreover, the term “step” does not compulsorily mean that the actions carried out during a step are simultaneous or immediately successive. Certain actions of a first step can, in particular, be followed by actions linked to a different step, and other actions of the first step can then be resumed. Thus, the term “step” does not necessarily mean single actions and which are inseparable over time and in the sequence of phases of the method.
A preferably orthonormal system, comprising the axes x, y, z is represented in the accompanying figures. When one single system is represented on one same set of figures, this system applies to all the figures of this set.
In the present patent application, the thickness of a layer is taken in a direction which is normal to the main extension plane of the layer. Thus, a layer typically has a thickness along z. The relative terms “on”, “surmounts”, “under”, “underlying”, “inserted”, “above”, “below” refer to positions taken in the direction z. This list of terms is not exhaustive. Other relative terms can be easily specified if needed, by referring to the accompanying drawings.
The terms “vertical”, “vertically” refer to a direction along z. The terms “horizontal”, “horizontally”, “lateral”, “laterally” refer to a direction in the plane xy. Unless explicitly mentioned, the thickness, the height and the depth are measured along z.
An element located “in vertical alignment with” or “to the right of” another element means that these two elements are both located on one same line perpendicular to a plane, wherein a lower or upper face of a substrate mainly extends, i.e. on one same line oriented vertically in the figures.
In the scope of the present invention, a III-N nitride is an element nitride belonging, preferably, to the column IIIa of the periodic table (three valence electrons). This column IIIa groups together earth elements such as boron, aluminium, gallium, indium. The III-N nitrides considered in the present invention are, in particular, boron nitride BN, aluminium nitride AlN, gallium nitride GaN, indium nitride InN, and their alloys, for example, and in a non-limiting manner, AlGaN, GaInN.
In the scope of the present invention, the waveguide is intended to ensure the propagation of a light wave in a main propagation direction, taken along the axis x in the accompanying drawings. The light wave is preferably coherent, monochromatic, and of wavelength λ. It preferably propagates according to one single optical propagation mode, typically the fundamental optical mode. The waveguide can thus be configured to guide the single transverse electric (TE00) or transverse magnetic (TM00) fundamental mode of the light wave. Alternatively, the guide can locally, during propagation, be configured to enable the conversion of a TM mode into a TE mode (and reciprocally). It can have several secondary modes in addition to the fundamental mode. The guide is called monomodal, if only the fundamental mode can propagate there. It is qualified as multimodal if it accepts at least two modes.
In the examples described below, the LNO portion illustrated is lithium niobate LiNbO3-based. It is understood that LiTaO3 or Li(Nb,Ta)O3 can be directly substituted for LiNbO3 in these examples. The LNO portion projects from the underlying nucleation layer. The LNO portion is presented in the form of a mesa or of a pad with respect to the nucleation layer. The LNO portion is a finite entity. This is not an abstract and arbitrary cutting of a wider layer covering, for example, the entire plate. The LNO portion is, in particular, defined by the cross-section of the opening of the masking layer.
X-ray diffraction analyses, for example in configuration 2θ, or rotating about φ and/or Ω (phi-scan and omega-scan), can be carried out so as to determine the crystalline quality of the LNO layers and of the nucleation and/or encapsulation layers, and their epitaxial relationship.
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The nucleation layer 20 is, for example, aluminium nitride AlN-based. AlN can be deposited and/or epitaxially grown on silicon oriented along (111) and its deposition method is advantageously known and controlled. Aluminium nitride further has a mesh parameter close to that of lithium niobate. This facilitates the epitaxial growth of lithium niobate to subsequently form the LNO portion. The thermal dilatation coefficient of AlN is further located between that of LiNbO3 and that of Si. This makes it possible to limit the appearance of structural defects in the LNO portion during temperature variations linked to the different deposition steps. AlN also has a good resistance to oxidation, and a good resistance to LNO deposition methods (resistance to ionised species plasmas and to chemical precursors, making it possible to synthesise the LNO material). Boron nitride and gallium nitride have properties similar to those mentioned above for aluminium nitride. They can also be advantageously chosen for the nucleation layer 20.
The nucleation layer 20 can be formed by a physical or chemical deposition technique, for example, by pulsed laser deposition (PLD). It can be alternatively formed by one of the following techniques: chemical vapour deposition (CVD), preferably metal organic chemical vapour deposition (MOCVD), PVD sputtering, plasma-enhanced atomic layer deposition (PEALD).
The nucleation layer 20 is preferably monocrystalline. It has a preferable orientation. An orientation along a growth plane (0001) can be typically chosen for an AlN nucleation layer 20 on a silicon topSi layer 13 (111). The nucleation layer 20 is preferably stoichiometric. It has a thickness e2 typically of between 5 and 20 nanometres, for example, around 10 nm.
Using a refractory nitride, in particular AlN nucleation layer 20, advantageously makes it possible to carry out a heteroepitaxy of LiNbO3 on a silicon-based substrate 10. Epitaxy in particular makes it possible to obtain a stoichiometric LiNbO3 layer, preferably oriented and of high crystalline quality. The thickness of this LiNbO3 layer obtained by epitaxy is further fully controlled. The LiNbO3 layer can be formed directly on substrates 10 of different sizes, without an intermediate transfer step. This advantageously makes it possible to decrease the manufacturing costs of an LNO/AlN/Si layer stack.
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This masking layer 300 is structured, for example standardly by lithography and etching, so as to form one or more openings 3a opening onto the nucleation layer 20. The opening(s) 3a typically has/have lateral dimensions l3 of several microns, for example, between 1 μm and 100 μm.
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The LNO portion 30 is advantageously epitaxially grown on the nucleation layer 20. It is preferably monocrystalline. It has a preferable orientation. An orientation along a growth plane (0001) can be typically chosen. This makes it possible to maximise the Pockels effect within the LNO portion 30 with electrodes placed respectively below and above the LNO portion 30, along z. Other orientations can be chosen according to the desired device architecture and/or of the desired application.
The LNO portion 30 formed on the nucleation layer 20 is preferably stoichiometric, for example Li1Nb1O3. The atomic percentage of lithium is ideally close to that of niobium. According to an example, the atomic percentages of lithium and of niobium are around 20%. The LNO portion 30 has a thickness e3 of between 50 and 500 nanometres, for example, around 200 nm. The thickness e3 of the LNO portion 30 can be chosen according to the desired application.
After formation of the LNO portion 30 in the opening 3a, an encapsulation portion 31, typically AlN-based, is preferably formed by epitaxy on the LNO portion 30. The portion 31 has a thickness e31 typically of between 5 and 20 nanometres, for example, around 10 nm. The sum of the thicknesses of the portions 30, 31 is typically less than the thickness e301 of the layer 301 underlying the layer 302. The layer 302 thus remains above the portions 30, 31 along z. This makes it possible to use the layer 302 as a layer for detecting the end of polishing during the subsequent CMP polishing.
According to an option, the encapsulation portion 31 and the LNO portion 31 are both produced in situ by PLD in one same growth reactor. The growth of the encapsulation portion 31 can thus be produced directly after the end of growth of the LNO portion 30. This makes it possible to avoid a venting of the LNO portion 30 before epitaxy of the encapsulation portion 31. The surface of the LNO portion 30 therefore remains clean. This avoids an intermediate cleaning step. The duration of the method is thus decreased. This also makes it possible to limit the appearance of roughness during the formation of the encapsulation portion 31. The surface state of the latter is thus optimised.
Growth residues 30r, 31r can be deposited at the periphery of the opening 3, on the masking layer 300, during the epitaxy of the portions 30, 31 in the opening 3a.
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Other applications can also be considered. The localised on-silicon LNO epitaxy can be advantageously implemented to cointegrate different devices having different functions.
From the above, it clearly appears that the present invention advantageously makes it possible to form localised portions of thin LNO layers of good crystalline quality on silicon-based substrates comprising a nucleation layer made of a nitride-based refractory material, typically III-N-material-based. These localised LNO portions are advantageously directly integrable in electrooptical or quantum devices. Other applications can be considered. The invention is not limited to the embodiments described above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2305745 | Jun 2023 | FR | national |
