Manufacturing process of a Kerr effect electro-optical modulator

Abstract

A method of manufacturing a Kerr effect electro-optical modulator, comprising the steps of: a) using a substrate (1); b) forming a waveguide (2) on the substrate (1) so as to guide propagation of an electromagnetic wave, the waveguide (2) comprising: - a core (20), comprising a pn-type junction or a pin-type structure; - an optical cladding (21), surrounding the core (20); the core (20) and the optical cladding (21) having an interface (I); c) irradiating the interface (I) with ionizing radiation so as to trap at the interface (I) free carriers originating from the pn-type junction or the pin-type structure; d) polarizing the pn-type junction or the pin-type structure so as to apply an electric field within the core (20). Figure 3

Description

Title of the invention: Method for manufacturing a Kerr effect electro-optical modulator Technical field

[0001] The invention relates to the technical field of Kerr effect electro-optical modulators.

[0002] The invention finds its application in particular in transmitters for optical communications thanks to their wide bandwidth and their low energy consumption. State of the art

[0003] A method of manufacturing a Kerr effect electro-optical modulator known from the state of the art comprises the steps: A) use a substrate; B) forming a waveguide on the substrate so as to guide propagation of an electromagnetic wave, the waveguide comprising: - a core, comprising a pn-type junction or a pin-type structure; - an optical sheath, surrounding the heart; C) bias the pn-type junction or pin-type structure so as to apply an electric field within the core.

[0004] Such a method of the state of the art is not entirely satisfactory insofar as the modulation of the phase of the electromagnetic wave propagating within the core is not solely obtained by the modulation of the electric field applied within the core, called the Kerr effect. Indeed, the variation in the density of free carriers, originating from the pn-type junction or the pin-type structure, will also contribute to modulating the phase (and the amplitude) of the electromagnetic wave propagating within the core. This phenomenon is called the plasma dispersion effect (PDE). Such a method of the state of the art therefore leads to competition between the Kerr effect and the plasma dispersion effect.

[0005] Furthermore, the variation in the density of free carriers, originating from the pn-type junction or the pin-type structure, will also induce a change in the optical absorption of the electromagnetic wave propagating within the core, and thereby degrade the performance of the electro-optical modulator. This phenomenon is called the "chirp" effect in English, and in particular limits the maximum modulation frequency of the electro-optical modulator. Statement of the invention

[0006] The invention aims to remedy all or part of the aforementioned drawbacks. To this end, the invention relates to a method for manufacturing a Kerr effect electro-optical modulator, comprising the steps: a) use a substrate; b) forming a waveguide on the substrate so as to guide propagation of an electromagnetic wave, the waveguide comprising: - a core, comprising a pn-type junction or a pin-type structure; - an optical sheath, surrounding the heart; the core and the optical cladding having an interface; c) irradiating the interface with ionizing radiation so as to trap at the interface free carriers originating from the pn-type junction or the pin-type structure; d) bias the pn-type junction or pin-type structure so as to apply an electric field within the core.

[0007] Thus, such a method according to the invention makes it possible, thanks to step c), to limit or even eliminate the harmful influence of the free carriers in terms of the “chirp” effect and the plasma dispersion effect. Indeed, all or part of the free carriers become trapped at the interface between the core and the optical cladding of the waveguide, thanks to the ionizing radiation applied during step c). The ionizing radiation makes it possible to create defects at the interface between the core and the optical cladding, thus constituting free carrier trapping sites.

[0008] In other words, such a method according to the invention allows pure phase modulation (i.e. without amplitude modulation or with restricted amplitude modulation compared to the state of the art) of the electromagnetic wave propagating within the core of the waveguide, and allows a higher maximum modulation frequency of the electro-optical modulator compared to the state of the art.

[0009] The method according to the invention may comprise one or more of the following characteristics.

[0010] According to a characteristic of the invention: - step c) leads to a concentration of free carriers trapped at the interface; - step c) is followed by a thermal annealing step carried out according to a thermal budget adapted to reduce the concentration of free carriers trapped at the interface.

[0011] Thus, an advantage provided is to be able to reduce the concentration of free carriers trapped at the interface between the core and the optical cladding of the waveguide in order to reduce the optical losses of the electro-optical modulator. Such thermal annealing makes it possible to neutralize the defects obtained by the ionizing radiation applied during step c).

[0012] According to a characteristic of the invention, step c) is carried out according to an irradiation dose greater than or equal to a threshold beyond which the free carriers resulting from the pn type junction or pin type structure no longer circulate in an area of ​​the core where an electromagnetic propagation mode is guided.

[0013] Thus, an advantage provided is to annihilate the contribution of the plasma dispersion effect and the “chirp” effect for the guided propagation mode.

[0014] According to a characteristic of the invention, step c) is carried out with X-rays.

[0015] Thus, an advantage provided is the ease of obtaining such ionizing radiation. The energy of the X-rays is sufficient to obtain ionizing radiation passing through an electro-optical modulator chip, for example integrated on silicon.

[0016] According to a characteristic of the invention, step b) is carried out with a core made of a material chosen from silicon, an amorphous silicon carbide enriched in silicon, a silicon nitride enriched in silicon.

[0017] Thus, one advantage provided by such materials is to obtain a high Kerr effect due to their high second-order non-linear refractive index. Another advantage is their high breakdown voltage.

[0018] According to a characteristic of the invention, step a) is carried out with a semiconductor-on-insulator type substrate comprising successively: - a leaflet; - a dielectric layer, forming part of the optical cladding of the waveguide formed during step b); - a layer made of a semiconductor material, from which the core of the waveguide is formed during step b).

[0019] Thus, an advantage provided by such a substrate is to facilitate the manufacture of the waveguide since a part of the optical cladding is already present with the dielectric layer. When the semiconductor material is silicon, an advantage provided is to use integrated photonics on silicon, that is to say the integration of photonic functions with electronic circuits on the scale of the silicon wafer.

[0020] According to a characteristic of the invention, step b) is carried out with an optical cladding made of silicon dioxide SiO2.

[0021] Thus, the properties of this material are interesting in terms of refractive index (which must be lower than that of the core), CMOS compatibility (“Complementary Metal Oxide Semiconductor” in English), and electrical insulation.

[0022] According to a characteristic of the invention, step b) is carried out so that the formed waveguide is edge-shaped, the core comprising: - a lower, planar zone, comprising the pn-type junction or the pin-type structure; - an upper zone, forming one or two edges, surmounting the lower zone, and where a mode of propagation of the electromagnetic wave is guided.

[0023] Thus, an advantage provided by such a waveguide architecture is to facilitate the evacuation of free carriers from the lower zone to the trapping sites created at the interface between the core and the optical cladding by the ionizing radiation, in order to obtain an upper zone with the fewest possible free carriers. In particular, a double-edge architecture makes it possible to increase the proportion of the optical mode located in the intrinsic zone in order to limit losses due to doping.

[0024] According to a characteristic of the invention, the irradiation dose is greater than or equal to the threshold beyond which the free carriers from the pn type junction or the pin type structure no longer circulate in the upper zone of the core where the propagation mode of the electromagnetic wave is guided.

[0025] Thus, an advantage provided is to annihilate the contribution of the plasma dispersion effect and the "chirp" effect for the guided propagation mode in the ribbed upper zone of the waveguide. The free carriers can then no longer circulate from the lower zone to the upper zone of the core where the propagation mode of the electromagnetic wave is guided. Everything happens as if the charges trapped at the interface between the core and the optical cladding create a pinch-off closing any channel allowing the circulation of free carriers from the lower zone to the upper zone of the core where the propagation mode of the electromagnetic wave is guided.

[0026] According to a characteristic of the invention, step b) is carried out so that the formed waveguide comprises an encapsulation layer, surrounding the optical cladding.

[0027] Definitions

[0028] - By "electro-optical modulator" is meant a device comprising a medium suitable for modulating the phase of an electromagnetic wave propagating in the medium, when an external electric field is applied to this medium.

[0029] - By "Kerr effect" we mean the electro-optical Kerr effect, also called "DC Kerr Effect” in English, which differs from the optical Kerr effect (“AC Kerr Effect” in English).

[0030] - By "substrate" is meant a self-supporting physical support, made of a material base from which a waveguide can be formed. A substrate can be a "slice" (also called a "wafer") which is generally in the form of a disc cut from an ingot of a crystalline material.

[0031] - By "pn-type junction" is meant a junction between a p-type doped zone and an n-type doped zone. The p-type doped zone contains p-type dopants, i.e. species (e.g. impurities) which, when introduced into the matrix of the waveguide core material, accept an electron from the conduction band. The n-type doped zone contains n-type dopants, i.e. species (e.g. impurities) which, when introduced into the matrix of the waveguide core material, donate an electron to the conduction band.

[0032] - By "pin-type structure" is meant a junction between a doped zone of type p and an intrinsic zone, and a junction between the intrinsic zone and an n-type doped zone. The p-type doped zone contains p-type dopants, i.e. species (e.g. impurities) which, when introduced into the matrix of the waveguide core material, accept an electron from the conduction band. The n-type doped zone contains n-type dopants, i.e. species (e.g. impurities) which, when introduced into the matrix of the waveguide core material, donate an electron to the conduction band. The intrinsic zone is devoid of dopants.

[0033] - By "free carriers" we mean the electric charge carriers (electrons and holes) which are free to circulate within the core of the waveguide.

[0034] - By "polarize" we mean the act of applying an electrical potential difference between the p-type doped region and the n-type doped region of the pn-type junction or pin-type structure.

[0035] - By "electric field" is meant an electric field having an intensity large enough so that a third-order nonlinear susceptibility in the core of the waveguide generates a Kerr effect. The electric field strength must remain below the maximum electric field value that the electro-optic modulator can withstand before a constituent material ruptures (breaks down).

[0036] - By "ionizing radiation" is meant radiation having an energy sufficient to remove at least one electron from the atoms crossed by the ionizing radiation. In this case, the ionizing radiation is suitable for removing at least one electron from the atoms of the material in which the optical cladding is made.

[0037] - By “thermal annealing” is meant a heat treatment comprising: (i) a phase of gradual temperature increase (rise ramp) until a temperature called the annealing temperature is reached; (ii) a holding phase (plateau) at the annealing temperature, for a period called the annealing time; (iii) a cooling phase.

[0038] - By "thermal budget" we mean an energy input of a thermal nature, determined by the choice of an annealing temperature value and the choice of an annealing time value.

[0039] - By "X-rays" is meant electromagnetic radiation whose length wavelength is between 0.001 nm and 10 nm. Brief description of the drawings

[0040] Other characteristics and advantages will appear in the detailed description of different embodiments of the invention, the description being accompanied by examples and references to the attached drawings.

[0041] [Fig. 1] is a schematic sectional view of an example of an electro-optical modulator manufactured by a method according to the invention, the core of the waveguide comprising a pn-type junction.

[0042] [Fig.2] is a schematic sectional view of an example of an electro-optical modulator manufactured by a method according to the invention, the core of the waveguide comprising a pin-type structure.

[0043] [Fig.3] is a partial schematic sectional view on an enlarged scale of a waveguide (ridge structure) of an electro-optical modulator manufactured by a method according to the invention.

[0044] [Fig.4] is a partial schematic sectional view on an enlarged scale of a waveguide (double-ridge structure) of an electro-optical modulator manufactured by a method according to the invention.

[0045] [Fig.5] is a schematic sectional view of a semiconductor-on-insulator type substrate that can be used for step a) of a method according to the invention.

[0046] It should be noted that the drawings described above are schematic, and are not necessarily to scale for the sake of readability and to simplify their understanding. The sections are made along the normal to the surface of the substrate. Detailed description of the implementation methods

[0047] Elements that are identical or provide the same function will bear the same references for the different embodiments, for the sake of simplification.

[0048] An object of the invention is a method of manufacturing a Kerr effect electro-optical modulator, comprising the steps: a) use a substrate 1; b) forming a waveguide 2 on the substrate 1 so as to guide propagation of an electromagnetic wave, the waveguide 2 comprising: - a core 20, comprising a pn-type junction 200 or a pin-type structure 201; - an optical sheath 21, surrounding the core 20; the core 20 and the optical cladding 21 having an interface I; c) irradiating the interface I with ionizing radiation so as to trap at the interface I free carriers originating from the pn-type junction 200 or the pin-type structure 201; d) polarizing the pn-type junction 200 or the pin-type structure 201 so as to apply an electric field within the core 20.

[0049] Step a)

[0050] Step a) consists of using a substrate 1 from which the waveguide will be formed. 2.

[0051] Step a) is advantageously carried out with a substrate 1 of the semiconductor-on-insulator type comprising successively: - a 10 plate; - a dielectric layer 11, forming part of the optical cladding 21 of the waveguide 2 formed during step b); - a layer 12 (called useful layer) made of a semiconductor material, from which the core 20 of the waveguide 2 is formed during step b).

[0052] The wafer 10 can be made of silicon. The dielectric layer 11 forms the lower part of the optical cladding 21 of the waveguide 2 formed during step b). The dielectric layer 11 is therefore advantageously made of the same material as the upper part of the optical cladding 21 of the waveguide formed during step b). The useful layer 12 is advantageously made of a semiconductor material which is the material of the core 20 of the waveguide 2 formed during step b).

[0053] By way of non-limiting example, a substrate 1 of the semiconductor-on-insulator type can be obtained by the Smart-Cut™ technology known to those skilled in the art.

[0054] Step b)

[0055] Step b) consists of forming a waveguide 2 on the substrate 1. The waveguide 2 is arranged to guide an electromagnetic wave within it.

[0056] The waveguide 2 comprises a core 20. Step b) is advantageously carried out with a core 20 made of a material chosen from silicon, a silicon-enriched amorphous silicon carbide, and a silicon-enriched silicon nitride.

[0057] In particular, silicon-enriched amorphous silicon carbide has a second-order nonlinear refractive index of 2.5 10 17 m2 / W, approximately four times higher than that of silicon. In addition, silicon-enriched amorphous silicon carbide has a band gap of 2.1 eV, higher than silicon, suppressing two-photon absorption at telecommunications wavelengths. Silicon-enriched amorphous silicon carbide has a refractive index of 2.6, which reduces the confinement of the electromagnetic wave compared to silicon. Finally, silicon-enriched amorphous silicon carbide has a higher breakdown voltage than silicon.

[0058] Silicon-enriched silicon nitride has a second-order nonlinear refractive index of 2 10 17 m2 / W, a refractive index of 3, and a higher breakdown voltage than silicon.

[0059] The core 20 of the waveguide 2 is advantageously obtained from the useful layer 12 of the substrate 1.

[0060] The core 20 comprises a pn-type junction 200. According to an alternative, the core 20 comprises a pin-type structure 201. The pn-type junction 200 and the pin-type structure 201 can be obtained by doping techniques known from the person skilled in the art, for example by implantation or by diffusion. As a non-limiting example, the p-type doping can be obtained by introducing boron. The n-type doping can be obtained by introducing phosphorus. The p-type dopants of the pn-type junction 200 or of the pin-type structure 201 can have a concentration of the order of 1017 cm 3. The n-type dopants of the pn-type junction 200 or of the pin-type structure 201 can have a concentration of the order of 1018 cm 3. The waveguide 2 advantageously comprises a heavily doped p++ p-type zone, arranged to connect the p-type doped zone of the pn-type junction 200 or of the pin-type structure 201 with a metal contact zone E. The concentration of the p-type dopants of the p++ zone can reach 1020 cm 3.The waveguide 2 advantageously comprises a heavily doped n++ zone of n-type, arranged to connect the n-type doped zone of the pn-type junction 200 or of the pin-type structure 201 with a metal contact zone E. The concentration of the n-type dopants of the n++ zone can reach 1020 cm 3.

[0061] The waveguide 2 comprises an optical cladding 21, extending around the core 20 along an interface I. The optical cladding 21 has a refractive index strictly lower than that of the core 20. The upper part of the optical cladding 21 can be obtained by a deposition technique known to those skilled in the art. The lower part of the optical cladding 21 can be the dielectric layer 11 of the substrate 1. Step b) is advantageously carried out with an optical cladding 21 made of silicon dioxide SiO2.

[0062] According to one embodiment, step b) is executed so that the waveguide 2 formed is edged (“rib” in English, also referred to as a rib), the core 20 comprising: - a lower zone 20a, planar, comprising the pn-type junction 200 or the pin-type structure 201; - an upper zone 20b, forming an edge, surmounting the lower zone 20a, and where a mode of propagation of the electromagnetic wave is guided.

[0063] Other waveguide 2 architectures are conceivable, for example a rectangular waveguide 2 (“strip” in English). However, an advantage provided by a ridge architecture is to facilitate the evacuation of free carriers from the lower zone 20a to the trapping sites created at the interface I between the core 20 and the optical cladding 21 by the ionizing radiation, in order to obtain an upper zone 20b with as few free carriers as possible. As a non-limiting example, the upper zone 20b of the ridge waveguide 2 may have a width L of 400 nm, and a height H of 220 nm.

[0064] According to one embodiment, step b) is executed so that the waveguide 2 formed is edge-shaped, the core 20 comprising: - a lower zone 20a, planar, comprising the pn-type junction 200 or the pin-type structure 201; - an upper zone 20b, forming two edges Ainf, Asup, surmounting the lower zone 20a, and where a mode of propagation of the electromagnetic wave is guided.

[0065] The upper edge Asup is arranged to laterally confine the optical mode. The lower edge Ainf extends under the upper edge Asup, on either side of the upper edge Asup. The lower edge Ainf delimits two underlying zones, a first underlying zone comprising n-type dopants, a second underlying zone comprising p-type dopants. Such a double-edge architecture makes it possible to apply the electric field during step d) as close as possible to the optical mode.

[0066] Step b) is advantageously carried out so that the waveguide 2 formed comprises an encapsulation layer (not illustrated), surrounding the optical cladding 21. In other words, the encapsulation layer extends around the optical cladding 21. The encapsulation layer, which may be made of a polymer, may be deposited on the upper part of the optical cladding 21 by a deposition technique known to those skilled in the art.

[0067] Step c)

[0068] Step c) consists of applying ionizing radiation to the interface I between the core 20 and the optical cladding 21 of the waveguide 2. For practical reasons, the irradiation of step c) is advantageously global in the sense that the entire waveguide 2 formed during step b) is irradiated. The ionizing radiation must at least reach the interface I between the core 20 and the optical cladding 21. More precisely, the ionizing radiation must at least reach the interface I between the core 20 and the upper part of the optical cladding 21. In practice, the irradiation of step c) is global and the entire interface I between the core 20 and the optical cladding 21 is reached by the ionizing radiation.

[0069] Step c) is executed so that free carriers, originating from the pn-type junction 200 or originating from the pin-type structure 201, are trapped at the interface I between the core 20 and the optical cladding 21 of the waveguide 2. As a non-limiting example, for a dose of ionizing radiation of the order of 106 Gy (Gray), the concentration of free carriers trapped at the interface I is of the order of 1020 cm 3.

[0070] Step c) is advantageously carried out with X-rays. The energy of the X-rays is sufficient to obtain ionizing radiation passing through an electro-optical modulator chip, for example integrated on silicon. Gamma radiation can also be considered as ionizing radiation for certain applications.

[0071] Step c) is advantageously carried out according to an irradiation dose greater than or equal to a threshold beyond which the free carriers (from the pn-type junction 200 or from the pin-type structure 201) no longer circulate in a zone of the core 20 where a propagation mode of the electromagnetic wave is guided. In the case of an edge waveguide 2, the irradiation dose is greater than or equal to the threshold beyond which the free carriers (from the pn-type junction 200 or from the pin-type structure 201) no longer circulate in the upper zone 20b of the core 20 where the propagation mode of the electromagnetic wave is guided. As a non-limiting example, in the case of X-rays with a silicon core 20 and an optical cladding 21 made of silicon dioxide, the threshold is of the order of 106 Gy.

[0072] Step c) leads to a concentration of the free carriers trapped at the interface I between the core 20 and the optical cladding 21 of the waveguide 2. Step c) is advantageously followed by a thermal annealing step, executed according to a thermal budget adapted to reduce the concentration of the free carriers trapped at the interface I between the core 20 and the optical cladding 21 of the waveguide 2. The thermal annealing step is executed before step d). As a non-limiting example, in the case of X-rays with a core 20 made of silicon and an optical cladding 21 made of silicon dioxide, the annealing temperature may be of the order of 200°C.

[0073] Step d)

[0074] Step d) consists of polarizing the pn-type junction 200 or the pin-type structure 201. Step d) is executed so as to apply an electric field within the core 20 of the waveguide 2.

[0075] The intensity of the electric field must be sufficiently large so that a third-order non-linear susceptibility in the core 20 of the waveguide 2 generates a Kerr effect. The intensity of the electric field must remain lower than the maximum value of the electric field that the electro-optical modulator can withstand before a rupture (breakdown) of a constituent material. For example, for a core 20 made of silicon, the breakdown field is of the order of 2.5 105 V / cm. The voltage applied between the metal contact zones E can then be of the order of ten volts to a few tens of volts.

[0076] The invention is not limited to the embodiments disclosed. Those skilled in the art are able to consider their technically effective combinations and to substitute equivalents for them.

Claims

Claims

1. A method of manufacturing a Kerr effect electro-optical modulator, comprising the steps of: a) using a substrate (1); b) forming a waveguide (2) on the substrate (1) so as to guide propagation of an electromagnetic wave, the waveguide (2) comprising: - a core (20), comprising a pn-type junction (200) or a pin-type structure (201); - an optical cladding (21), surrounding the core (20); the core (20) and the optical cladding (21) having an interface (I); c) irradiating the interface (I) with ionizing radiation so as to trap at the interface (I) free carriers originating from the pn-type junction (200) or the pin-type structure (201); d) biasing the pn-type junction (200) or the pin-type structure (201) so as to apply an electric field within the core (20).

2. Method according to claim 1, in which: - step c) leads to a concentration of the free carriers trapped at the interface (I); - step c) is followed by a thermal annealing step carried out according to a thermal budget adapted to reduce the concentration of the free carriers trapped at the interface (I).

3. Method according to claim 1 or 2, in which step c) is carried out according to an irradiation dose greater than or equal to a threshold beyond which the free carriers coming from the pn-type junction (200) or the pin-type structure (201) no longer circulate in a zone of the core (20) where a propagation mode of the electromagnetic wave is guided.

4. Method according to one of claims 1 to 3, in which step c) is carried out with X-rays.

5. Method according to one of claims 1 to 4, in which step b) is carried out with a core (20) made of a material chosen from silicon, a silicon-enriched amorphous silicon carbide, and a silicon-enriched silicon nitride.

6. Method according to one of claims 1 to 5, in which step a) is carried out with a substrate (1) of the semiconductor on insulator type comprising successively: - a wafer (10); - a dielectric layer (11), forming part of the optical cladding (21) of the waveguide (2) formed during step b); - a layer (12) made of a semiconductor material, from which the core (20) of the waveguide (2) is formed during step b).

7. Method according to one of claims 1 to 6, in which step b) is carried out with an optical cladding (21) made of silicon dioxide SiO2.

8. Method according to one of claims 1 to 7, in which step b) is carried out so that the waveguide (2) formed is ridged, the core (20) comprising: - a lower zone (20a), planar, comprising the pn-type junction (200) or the pin-type structure (201); - an upper zone (20b), forming one or two edges (Ainf, Asup), surmounting the lower zone (20a), and where a propagation mode of the electromagnetic wave is guided.

9. Method according to claim 8 in combination with claim 3, in which the irradiation dose is greater than or equal to the threshold beyond which the free carriers from the pn-type junction (200) or the pin-type structure (201) no longer circulate in the upper zone (20b) of the core (20) where the propagation mode of the electromagnetic wave is guided.

10. Method according to one of claims 1 to 9, in which step b) is carried out so that the waveguide (2) formed comprises an encapsulation layer, surrounding the optical cladding (21).

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