Method for manufacturing a mixed layer comprising a silicon waveguide and a silicon nitride waveguide

A fabricating process may include: producing a trench, in an encapsulated-silicon layer, in the location where a silicon-nitride core of the waveguide must be produced; then depositing a silicon-nitride layer on the encapsulated-silicon layer, the thickness of the deposited silicon-nitride layer being sufficient to completely fill the trench; then removing the silicon nitride situated outside of the trench to uncover an upper face with which the trench filled with silicon nitride is flush; then depositing a dielectric layer that covers the uncovered upper face in order to finalize the encapsulation of the silicon-nitride core and thus to obtain a mixed layer containing both the silicon and silicon-nitride cores encapsulated in dielectric.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is the national stage of international application PCT/EP2021/058847, filed on Apr. 6, 2021, and claims the benefit of the filing date of French Appl. No. 2 003 653, filed on Apr. 10, 2020,

The invention relates to a process for fabricating a mixed layer comprising a first waveguide the core of which is made of silicon and a second waveguide the core of which is made of silicon nitride. The invention also relates to:a process for fabricating a photonic component using this process for fabricating a mixed layer, andthe photonic component thus fabricated.

As described in U.S. Pat. No. 10,270,222B1 and in patent application WO2019002763A1, for example in the particular context of a semiconductor laser source, it is advantageous to use a waveguide made of III-V gain material and waveguides made of silicon and of silicon nitride. Specifically, this allows the performance and properties of the laser source to be improved. More broadly, it will be noted that photonic, III-V semiconductor components may benefit from improved performance when they are associated with waveguides made of silicon and of silicon nitride.

Below, the expression “silicon waveguide” or “waveguide made of silicon” designates a waveguide the core of which is made of silicon, and the expression “silicon-nitride waveguide” or “waveguide made of silicon nitride” designates a waveguide the core of which is made of silicon nitride.

In known processes for fabricating such photonic components, the three different waveguides are produced in respective layers that are stacked on top of one another. Thus, to fabricate these photonic components, it is necessary to fabricate a stack of at least three different layers.

The fabrication of these photonic components would therefore be simplified and their integration density would be improved if it were possible to produce a mixed layer containing, in the same level, silicon waveguides and silicon-nitride waveguides.

However, an effective and simple process for fabricating such a mixed layer does not exist. This is notably due to the fact that, at the present time, there is no simple process for carrying out chemical-mechanical polishing on a thick silicon-nitride layer covering most of the exterior face of a substrate and that, in addition, may be stopped as soon as a subjacent layer, for example of silicon oxide, is uncovered.

The invention therefore aims to mitigate this lack. One subject thereof is therefore a process for fabricating a mixed layer according to Claim1.

Another subject of the invention is a process for fabricating a photonic, III-V semiconductor component employing the above process for fabricating a mixed layer.

Lastly, another subject of the invention is a photonic, III-V semiconductor component fabricated using the above process.

In these figures, the same references are used to designate the same elements. In the remainder of this description, features and functions that are well known to those skilled in the art are not described in detail.

Below, the definitions of certain terms and expressions used in this patent application are given in Section I. Next, detailed examples of embodiments are described in Section II with reference to the figures. In the following section, Section III, variants of these embodiments are presented. Lastly, the advantages of the various embodiments are presented in Section IV.

Section I: Terminology and Definitions

In this description, when it is indicated that a waveguide is made of material X, this means that the core of this waveguide is made of this material X. The cladding of this waveguide is made of another material of lower refractive index.

When it is indicated that an element “is made of material X”, this means that the material X represents more than 90% or 95% or 98%, by mass, of the weight of this element.

The effective propagation index neffis also known as the “phase constant of the mode”. It is defined by the following relationship: ng=neff−Δdneff/dλ, where ngis the group index and λ is the wavelength of the optical signal guided by the guide. The effective propagation index of a waveguide depends on the dimensions of the core of this waveguide and on the materials forming this core and the cladding of this waveguide. It may be determined experimentally or by numerical simulation.

The cladding of a waveguide is generally made of dielectric. In this case, the dielectric is a dielectric the refractive index nmdof which is lower than the index nSiif the core of the waveguide is made of silicon and than the index new if the core is made of silicon nitride, where nSiand nSiNare the refractive indices of silicon and silicon nitride, respectively. Typically, the index nmdis lower than or equal to 0.85×ncor lower than or equal to 0.75×nc, where ne is the refractive index of the core of the waveguide.

By “dissolve” what is meant is the action of an operation of etching a material by wet or dry etching.

Section II: Examples of Embodiments

FIG.1schematically shows the general architecture of a monochromatic semiconductor laser source10that emits at the wavelength λLi. Below, only the particularities of the laser source10are described in detail. For general information on the operation of a semiconductor laser source using waveguides made of silicon, silicon nitride and III-V gain material, the reader may consult U.S. Pat. No. 10,270,222B1 and patent application WO2019002763A1. As will become apparent on reading the rest of the description, the laser source10differs from known laser sources mainly in the arrangement of its waveguides made of silicon and of silicon nitride.

The laser source10comprises a rear reflector12and a front reflector14that define the ends of a Fabry-Pérot cavity inside of which the optical signal resonates. For example, the reflector12has a reflectance strictly higher than that of the reflector14. The reflectors12and14are, for example, wideband reflectors. Here, the reflectors12and14are reflectors such as Bragg gratings.

Between the reflectors12and14, the laser source comprises in succession the following photonic components from the reflector12to the reflector14:an optical waveguide15made of silicon nitride (Si3N4) in which the reflector12is produced,a bandpass filter22able to select the operating wavelength λLiof the laser source10from various wavelengths ΔRjpossible inside the Fabry-Perot cavity, this filter22being produced in the silicon-nitride waveguide15,an inversely tapered coupler24that optically connects a first region of the waveguide15to a first region of a silicon waveguide25,the silicon waveguide25,an optional tuning device16produced in the waveguide25, this tuning device16being able to move the wavelengths λRjas a function of an electrical control signal and using the properties of the silicon of the waveguide25,an adiabatic or evanescent coupler26that optically connects a second region of the waveguide25to a facing first region of a waveguide28made of III-V gain material,a semiconductor optical amplifier30(also known by the acronym SOA) produced in the waveguide28and able to generate and to amplify the optical signal resonant inside the Fabry-Pérot cavity at each wavelength ΔRj,an adiabatic or evanescent coupler32that optically connects a second region of the waveguide28to a facing third region of the waveguide25,an inversely tapered coupler34that optically connects a fourth region of the waveguide25to a first region of the waveguide36made of silicon nitride (Si3N4), andthe reflector14produced at the end of the guide36.

For a detailed description of an adiabatic coupler, the reader may refer to the following article: Amnon Yariv et al., ‘Supermode Si/III-V hybrid Lasers, optical amplifiers and modulators: proposal and analysis’, Optics Express 9147, vol. 14, No. 15, 23 Jul. 2007.

In particular, an adiabatic coupler is able to transfer almost all of the optical signal present in a first waveguide to a second waveguide situated above or below, without reflection. Such an adiabatic coupler is, for example, obtained by modifying the width of the first waveguide with respect to the width of the second waveguide. Typically, to adiabatically couple a silicon waveguide to a waveguide made of III-V material, the width of the silicon waveguide is gradually decreased as the waveguide made of III-V material is approached. In the inverse direction, to transfer, via adiabatic coupling, an optical signal from the waveguide made of III-V material to the silicon first waveguide, the width of the silicon waveguide is for example gradually increased. In addition, waveguides made of silicon and of III-V material generally have a width such that there are regions in these facing waveguides in which their respective effective propagation indices are equal.

In this embodiment, the filter22is a resonant ring filter the ring of which is produced in an Si3N4waveguide50(FIG.2). Preferably, the waveguide50in which the ring is produced is directly connected optically to two ends of the waveguide15via evanescent coupling. Evanescent coupling is obtained by bringing the two waveguides closer to each other. Here, the fitter22is identical to that described in U.S. Pat. No. 10,270,222B1 and in patent application WO2019002763A1.

To generate the electrical control signal used to control the tuning device16, the laser source10also comprises a sensor40and an electronic circuit42able to generate the electrical control signal used to control the tuning device16so as to permanently keep one wavelength ΔRjat the centre of the passband of the filter22. The sensor40and the circuit42are, for example, identical to those described in U.S. Pat. No. 10,270,222B1 and in patent application WO2019002763A1.

The light that exits via the reflectors12and14is then guided, for example, to a photodiode or an optical fibre. To this end, additional optical components are used. Given that these additional optical components are conventional, they are not described in detail here and, to simplify the figures, they are furthermore not shown.

FIG.3shows a first embodiment of the laser source10. Here, the laser source10is fabricated on a substrate60made of silicon, crystalline silicon for example, that mainly lies in a horizontal plane corresponding to the plane of the substrate.

InFIG.3, the laser source10comprises in succession stacked on the substrate60from bottom to top:a buried dielectric layer64,a mixed layer66that contains the silicon-nitride waveguides15and36and the silicon waveguide25, anda layer68made of encapsulated III-V material comprising the waveguide28inside of which is produced the amplifier30in this exemplary embodiment.

The thickness of the substrate60is large, i.e. larger than 100 μm or 300 μm.

The dielectric of the layer64is typically an oxide. Here, the buried oxide is silicon oxide, of a thickness larger than 720 nm, and preferably larger than 1.5 μm. Here, the thickness of the layer64is equal to 2 μm.

The adiabatic couplers26and32are produced partially in the waveguide25and partially in the waveguide28.

The amplifier30is for example identical to the amplifier described in U.S. Pat. No. 10,270,222B1 and in patent application WO2019002763A1. In this case, the waveguide28and the amplifier30take the form of a stack of alternating sublayers of wells and of barriers made of ternary and/or quaternary materials. For example, in the case of epitaxy on an InP substrate, it may be a question of an alternation of quaternary InGaAsP or AlGaInAs with two different bandgaps for the wells and barriers. In the case of epitaxy on a GaAs substrate, it may be a question of an alternation of GaInNAs materials with two different bandgaps for the wells and barriers or of an alternation of GaInNAs and GaNAs. These alternations of wells and barriers are interposed between a lower sublayer70and a p-doped upper sublayer made of InP or GaAs, respectively. The sublayer70is a sublayer made of III-V material doped oppositely to the upper sublayer. For example, here, it is a question of an n-doped sublayer made of InP or GaAs, respectively. The amplifier30comprises a contact74making direct mechanical and electrical contact with the sublayer70. The p-doped (InP or GaAs) sublayer makes mechanical and electrical contact with a contact76. When a current higher than the threshold current of the laser source is applied between the contacts74and76, the amplifier30generates and amplifies the optical signal which resonates inside the Fabry-Perot cavity.

The tuning device16is here a heater able to heat the waveguide25with a view to moving the wavelengths λRj. In this embodiment, the tuning device16comprises a resistor80that is electrically connected to two electrical contacts82and84. Typically, the resistor80is separated from the waveguide25by a distance larger than 500 nm or 600 nm. These contacts82and84are electrically connected to a current or voltage source that is controlled by the electronic circuit42depending on the measurements of the sensor40.

The tuning device16, the waveguide28and the amplifier30are covered with a protective jacket90that mechanically insulates them from the exterior. Only the contacts74,76,82,84protrude beyond the jacket90. For example, the jacket90is made of silicon nitride or of silicon oxide or of benzocyclobutene (BCB).

The path of the resonant optical signal through the laser source10is represented inFIG.3by a bold double-headed arrow.

FIGS.4to10show the adiabatic coupling32and the inversely tapered coupling34. The couplings26and24are, for example, achieved in the same way as the couplings32and34, respectively.

FIG.4shows seen from above a segment of the waveguides28,25and36. In thisFIG.4, only the core of the guides25,28and36and the layer64on which these cores rest are shown. The claddings of the guides25,28and36are not shown. The limits of the core of the guide25are represented by dashed lines when they are hidden by the guide28or36.

InFIGS.5to10, the cladding of the guide28and the substrate60are not shown.

InFIG.4, dash-dotted lines A to F show the positions of cross sections A to F, respectively. The cross sections A to F are shown inFIGS.5to10, respectively. More precisely, from left to right:cross section A is situated under the guide28and between the couplers26and32,cross section B is situated substantially in the middle of the coupler32,cross section C is situated between the couplers32and34,cross section D is situated at the start of the coupler34,cross section E is situated at the end of the coupler34, andcross section F is situated between the coupler34and reflector14.

In this embodiment, the silicon core of the guide25comprises a horizontal slab100and a rib102superposed on this slab100. Such a configuration of the guide25is called a rib-waveguide configuration. This rib configuration is shown inFIGS.5to8.

The cross section of the slab100is rectangular. The slab100therefore comprises a horizontal lower face making direct contact with the upper face of the layer64and, on the opposite side, a horizontal upper face. It also comprises sidewalls100gand100d(FIG.5). The walls100gand100dextend vertically.

The cross section of the rib102is also rectangular. It comprises a horizontal lower face that is flush with the upper face of the slab100and, on the opposite side, a horizontal upper face. It also has sidewalls102gand102d(FIG.5) that extend vertically.

In this embodiment, the sidewalls100g,100d,102gand102dand the upper face of the rib102are encapsulated in a silicon-nitride block104. This block104completely covers the sidewalls100g,100d,102g,102dand the upper face of the rib102in the locations of the cross sections A to D. Thus, the cladding of the waveguide25is here made of silicon nitride at least along its sidewalls and its upper face. InFIGS.5to8, the small thickness of silicon nitride that covers the upper face of the guide25is not visible. The block104is separated from the waveguide28by a thin dielectric layer106. Thus, the waveguide28is separated from the waveguide25by the layer106and by the silicon nitride that covers the upper face of the guide25.

In the coupler32(FIG.6), the cross section of the rib102gradually flares from left to right so as to form, in the waveguides25and28, facing regions the effective propagation indices of which are equal. In this embodiment, in the coupler32, the cross section of the slab100remains constant.

Next, between the couplers32and34, the cross section of the waveguide25remains constant (FIG.7: cross section C).

In this embodiment, the waveguide28ends with an absorption region110. This region110is situated after the coupler32and is shaped to evacuate the residual fraction of the optical signal that has not been transferred, by the coupler32, to the waveguide25.

From left to right, at the start of the coupler34, the cross section of the rib102(FIG.8: section D) gradually narrows to a point. To this end, the sidewalls102gand102dgradually approach each other before meeting at an end of the rib102beyond which this rib102no longer exists. This narrowing of the cross section of the rib102forms a first tapered termination. Beyond this end of the rib102, the optical signal is therefore essentially confined to inside the slab100. Here, from the start of the coupler34to the end of the rib102, the cross section of the slab100remains constant.

Next, beyond the end of the rib102, the cross section of the slab100gradually narrows, from left to right, to a point (FIG.9: section E). To this end, as for the rib102, the sidewalls100gand100dgradually approach each other before meeting at an end of the slab100beyond which the waveguide25no longer exists and from which the guide36starts. This narrowing of the cross section of the slab100forms a second tapered termination. Thus, the guide36(FIG.10: section F) is situated in the same layer66as the waveguide25and in the extension of the waveguide25.

The cross section of the waveguide36is here rectangular. It therefore has a horizontal lower face that directly bears mechanically against the upper face of the layer64and, on the opposite side, a horizontal upper face. This waveguide36is therefore situated in the same level as the waveguide25. Specifically, the horizontal lower faces of the waveguides25and36are situated in the same horizontal plane.

Beyond the end of the slab100, most of the optical signal has been transferred from the waveguide25to the waveguide36.

A process for fabricating the laser source10will now be described with the help ofFIGS.11to26. InFIGS.20to26, the substrate60is not shown.

InFIGS.12to19, only the fabrication states of the laser source in the locations of cross sections E, F and A are shown. In these figures, the locations of these cross sections A, E and F are shown by vertical dashed lines bearing the reference letters A, E and F, respectively. InFIGS.12to19, by way of illustration, these cross sections are shown as being next to one another. However, in fact, they are situated behind one another in the direction of propagation of the optical signal along the waveguides25and36as shown, for example, inFIG.4.

The process starts with a phase118of fabricating the mixed layer66on the layer64. This phase118starts with a step120of providing a stack comprising, stacked immediately on one another, the substrate60, the buried layer64of oxide and a layer122(FIG.20) of single-crystal silicon. Such a stack is known as a silicon-on-insulator (SOI) stack. The thickness of the layer64is generally larger than 1 μm. For example, here, the thickness of the layer64is equal to 2 μm or 3 μm. The thickness of the layer122is smaller and, generally, smaller than 1 μm. For example, in this embodiment, the thickness of the layer122is equal to 500 nm.

In a step124, the layer122is structured to produce the silicon core of the waveguide25. For example, in this step, operations of photolithography, of etching and of removing the resist mask are carried out. In particular, given the configuration of the waveguide25, operations are carried out so as to thin the silicon layer122via partial etching and to leave behind only in places a thickness of 300 nm or 150 nm of single-crystal silicon. Next, generally, an operation of completely etching the layer122allows the various photonic components made of single-crystal silicon to be separated from this layer122.

More precisely, as illustrated in more detail inFIGS.21to24, typically, in step124, a sublayer126(FIG.21) of silicon oxide is deposited on top of the waveguide25. InFIGS.21to26, only one segment of the waveguide25the thickness of which is equal to 500 nm is shown. Here, the thickness of the sublayer126is for example equal to 20 nm. Next, a mask128(FIGS.22and23), which is for example made of hard silicon nitride or of resist, is deposited in regions of the layer122that must not be etched. Lastly, after the etching operations, the mask128is removed (FIG.24). The state shown inFIG.12is then reached.

In a step130, a thin etch-stop liner132(FIGS.13and25) is deposited on all the upper face. Here, the liner132is made of silicon nitride. The thickness of the liner132is typically comprised between 20 nm and 100 nm or between 20 nm and 50 nm. This liner130in particular covers the upper face of the layer64uncovered by the complete etch carried out in step124. It also covers the upper face of the core of the waveguide25. The state obtained after the deposition of the liner132is shown inFIGS.13and25. To simplifyFIGS.13to19, the residual segments of the oxide sublayer126that is situated under the liner132have not been shown in these figures.

For example, the liner132is deposited by plasma-enhanced chemical vapour deposition (PECVD).

Next, a step136of encapsulating the silicon core then of planarizing is carried out. In step136, a dielectric138(FIG.14) is deposited to encapsulate the silicon core. Here, this dielectric is silicon oxide. By way of illustration, it is a question of a high-density plasma (HDP) silicon oxide, or indeed of a silicon oxide based on tetraethylorthosilicate (TEOS). The deposited thickness of this dielectric is larger than the maximum thickness of the silicon core structured in the preceding steps. Typically, this thickness is more than 1.5 times larger than the maximum thickness of the silicon core. In addition, this thickness may be increased depending on the thickness of dielectric that it is desired to obtain above the silicon core.

Next, the upper face is planarized by carrying out chemical-mechanical polishing (CMP). This planarizing operation is stopped when the liner132situated above the segments of the waveguide25the thickness of which is equal to 500 nm is uncovered. To do this, typically, the etchant used in this operation of chemical-mechanical polishing is a selective etchant that dissolves the dielectric138at least two or four times more rapidly than silicon nitride.

After this operation of chemical-mechanical polishing, the upper face is planar. Here, at this stage, a new sublayer140(FIGS.14and26) of silicon oxide is deposited in order to obtain an upper face made entirely of silicon oxide. The thickness of the sublayer140is chosen so as to adjust the thickness of the silicon-nitride core of the waveguide36and the thickness of the block104. For example, here, the thickness of the sublayer140is chosen to be equal to 40 nm. At the end of step136, the encapsulated-silicon layer shown inFIG.14is obtained.

Next, in a step144, a trench146(FIG.15) is produced in the location where the silicon-nitride core of the guide36must be produced. In addition, in this embodiment, this trench comprises an extension148(FIG.15) along the core of the waveguide25in the location where the silicon-nitride block104must be formed. To this end, this extension148uncovers the sidewalls of the core of the waveguide25.

For example, the trench146and its extension148are produced by dry etching, through a mask, the underlayer140and the material138. The dry etching is stopped when the liner132situated in the location of the trench146is uncovered. To do this, typically, the etchant used is a selective etchant that dissolves silicon oxide two or four times more rapidly than silicon nitride.

In a step150, a layer152(FIG.16) of silicon nitride is deposited on all the exterior face. The thickness of the layer152is sufficient to completely fill the trench146and its extension148. For example, here, the thickness of the layer152is larger than 600 nm. For example, the layer152is deposited by PECVD.

In a step160, the segments of the layer152deposited outside of the trench146and its extension148are removed. To do this, in this embodiment, in an operation162, a layer164(FIG.17) of silicon oxide is deposited on all the upper face of the layer152. The thickness of the layer164is sufficient to completely fill the recesses formed in the layer152plumb with the trench146and its extension148. For example, the thickness of the layer164is larger than 100 nm or 150 nm.

Next, in an operation166, the layer164is planarized by chemical-mechanical polishing. Here, the polishing is stopped when the upper face of the layer152is reached. For example, the operation166is carried out just like the planarizing operation described with reference to step136. At the end of this operation, an exterior face essentially consisting of silicon nitride and of a little silicon oxide plumb with the trench146and with the extension148is obtained. The state shown inFIG.18is then reached. InFIG.18, the black spots plumb with the trench146and with its extension148represent the residues of silicon oxide plumb with these elements.

Next, an operation168of non-selective etching is executed. In the operation168, the etchant used dissolves silicon oxide as rapidly as silicon nitride. In this text by “dissolves element A as rapidly as element B”, what is meant is the fact that the etch rate of element B is comprised between 0.8 vAand 1.2 vAand, preferably, between 0.9 vAand 1.1 vA, where vAis the etch rate of element A. The operation168is stopped when the silicon oxide situated under the layer152is reached. To do this, the end of this non-selective etching is detected by optical emission spectroscopy. This process allows the material present on the exterior face during etching to be identified. As soon as the identified material is the dielectric under the layer152, the non-selective etching is stopped. However, in practice, a small thickness, of about 20 nm, of dielectric situated under the layer152is nonetheless consumed. The non-selective etching applied to the planarized face of the layer164allows the initial planarity to be preserved. Thus, at the end of the operation168, a planar exterior face with which the upper faces of the block104and of the waveguide core36are flush is obtained. The step160of removing the silicon-nitride layer152is then finished.

Optionally, after step160and in a step169, operations of locally etching the upper face of the waveguide36are carried out. These operations are, for example, performed to structure the reflector14in the waveguide36.

To compensate for the thickness of dielectric consumed in the operation168and to cover the tops of the block104and of the silicon-nitride core, in a step170, the dielectric sublayer106is deposited on the exterior face. Here, the dielectric deposited in step170is silicon oxide. The state shown inFIG.19is then reached.

For example, at the end of step170, the thickness of the sublayer106is equal to 20 nm.

The phase118of fabricating the mixed layer66is then finished.

Next, in a step176, a transfer or a substrate made of III-V gain material is direct bonded to the mixed layer66. This transfer or this substrate for example comprises:the lower sublayer70,the stack of alternating sublayers of wells and of barriers made of ternary and/or quaternary materials, andthe doped upper sublayer.

In a step178, the transfer or the substrate is structured by etching to form the core of the waveguide28. Typically, in a first etch, the upper sublayers of the transfer are etched to structure the amplifier30. Next, in a second etch, the sublayer70is etched to complete the structuring of the amplifier30. In step178, the resistor80is also fabricated.

Lastly, in a step180, the jacket90and the contacts74,76,82and84are produced. The layer68made of encapsulated III-V material is then obtained and the fabrication of the laser source10is finished.

FIG.27shows a second process for fabricating the mixed layer66capable of being implemented instead of phase118. This second process is identical to phase118ofFIG.11, except that:step124is replaced by a step200, andsteps160,169,170and172are replaced by steps202,215,216and218, respectively.

Step200is identical to step124, except that the thickness of the sublayer126is larger than 20 nm so as to obtain a larger margin of security during the removal of the liner132. For example, in step200, the thickness of the sublayer126is larger than 30 nm or 40 nm. Here, the thickness of the sublayer126is equal to 40 nm.

Step202starts with an operation204of forming a protective mask206(FIG.28) plumb with the trench146and its extension148. This mask206is designed to prevent the silicon nitride situated under this mask from being etched. For example, the mask206is made of resist. The state shown inFIG.28is then reached.

Next, an operation208of non-selectively etching the silicon nitride is executed. This operation280is stopped when the silicon oxide situated under the layer152is uncovered. For example, this operation is carried out as was described in the context of operation168.

Next, in an operation210, the mask206is removed. The state shown inFIG.29is then reached. At this stage, there is a silicon-nitride protuberance212above the trench146and its extension148. This protuberance212protrudes beyond the silicon-oxide exterior face. However, the exterior face is essentially made of silicon oxide, i.e. the protuberance212covers less than 75% and, typically, less than 50% or less than 25% of the exterior face. The rest of the exterior face is made of silicon oxide.

In an operation214, the exterior face is planarized by carrying out chemical-mechanical polishing in order to remove the protuberance212. For example, in this embodiment, the operation214is stopped when the sublayer126is reached. The operation214decreases nonetheless by 20 nm the thickness of the sublayer126. Step202of removing the layer152is then finished.

Optionally, after the operation214, an operation215identical to the operation169is executed.

Next, in an operation216, the sublayer106is deposited. This operation is for example identical to the operation170.

Lastly, an optional operation218of planarizing the sublayer106is carried out. The operation218is identical to the operation172.

Here, at the end of the operation218, the thickness of the deposited sublayer106is larger than 50 nm or 80 nm. For example, the thickness of the deposited sublayer106is equal to 80 nm.

The process for producing the mixed layer66is then finished.

FIG.30shows a third process for fabricating the mixed layer66capable of being implemented instead of phase118in the process ofFIG.11. This third process is identical to phase118, except that it comprises, between steps130and136, an additional step230of producing a cavity under the waveguide36.

More precisely, in an operation232, the liner132is etched to form apertures234(FIG.31) in this liner132. These apertures234extend along the location where the guide36must be produced. Here, these apertures234also extend along the location where the block104must be produced.

Next, in an operation236, trenches238that pass right through the thickness of the layer64are produced using the liner132as etch mask. These trenches238each emerge onto the substrate60. The state shown inFIG.31is then reached.

In an operation240, the substrate60is etched to form a cavity242inside the substrate60. To do this, a selective etchant is introduced into the trenches238. This etchant dissolves silicon two or four times more rapidly than silicon oxide. In addition, it is here a question of an isotropic etchant, i.e. an etchant that dissolves silicon at the same rate in the vertical direction and in the horizontal direction. Thus, at the end of the operation240, the cavity242extends under the location where the guide36and the block104must be produced. Here, the cavity242also extends under the location where the coupler34must be produced.

Next, the trenches238are filled. For example, here, this is carried out in step136. Specifically, the deposition of the dielectric138in step136fills the trenches238. The state shown inFIG.33is then reached, at the end of step136.

From step136, the process for fabricating the mixed layer66is for example identical to that already described with reference toFIG.11or27. The dots inFIG.30indicate that the steps that follow step136have not been shown.

The cavity242allows the guide36to be distanced from the substrate60and this guide36to be better isolated from the substrate60. This improvement in the isolation between the waveguide36and the substrate60decreases optical losses, notably when the transverse dimensions of the guide36are small.

FIGS.34and35show an inversely tapered coupler250capable of being used instead of the coupler34. The dashed line G inFIG.34shows the location of cross section G of the coupler250.FIG.35shows cross section G. The coupler250is identical to the coupler34, except that the tapered termination of the slab100of the waveguide25is replaced by a tapered termination comprising subwavelength features. In this embodiment, beyond the tapered termination of the slab100, the coupler250comprises features252that are aligned one after the other in the direction260of propagation of the optical signal at the wavelength λLi. Each feature252is made of silicon and entirely encapsulated in the silicon-nitride block104. The horizontal dimensions of these features are smaller than the wavelength ΔLi. In addition, the length of the features252in the direction260decreases with distance from the tapered termination. The cross section of each feature252is for example rectangular. In this embodiment, the coupler250comprises three features252.

Section III: Variants

Variants of the Laser Source:

The waveguide15may be made of silicon.

Other embodiments of the waveguide25are possible. For example, as a variant, the rib102is omitted. In this case, the waveguide25is a slab waveguide. In this case, the coupler34comprises a single tapered termination, i.e. that produced by narrowing the cross section of the slab100.

In another variant, the upper face of the rib102is flush with the upper face of the block104.

The cladding of the waveguide25is not necessarily made of silicon nitride. It may also be made of silicon oxide for example. In this case, the silicon-nitride block104is omitted from the segment of the guide25situated before the coupler34. The dielectric situated between the waveguides28and25then comprises no thickness of silicon nitride.

Many variants of the laser source are possible. For example, the couplers26and32may be produced differently. Thus, by way of example, the tapered termination may be produced in the guide25and/or in the guide28.

Other embodiments of the couplers24and34are possible. In particular, these couplers are not necessarily produced using an inversely tapered transition. For example, as a variant, if the cross-sectional area of the waveguide25is small, i.e. typically smaller than 100 nm×100 nm, then butt coupling may be employed. In this case, the cladding of the waveguide25is generally made entirely from a dielectric different from silicon nitride, such as silicon oxide.

The various variants of a semiconductor laser source described in U.S. Pat. No. 10,270,222B1 and in patent application WO20190027631 are applicable to a laser source comprising a mixed layer as described in this patent application. In particular, there are many other possible embodiments for the filter22and the reflectors12and14.

As a variant, the filter22is produced in the silicon waveguide25.

Up to now, the laser source has been described in the particular case where it is a question of a DBR laser source (DBR being the acronym of distributed Bragg reflector). However, a mixed layer such as the mixed layer66may also be used to produce a DFB laser source (DFB being the acronym of distributed feedback). In the latter case, the silicon-nitride waveguide is also situated in the extension of the silicon waveguide produced under the waveguide made of III-V material. In contrast, the Bragg grating is produced in the silicon waveguide or in a silicon-nitride sublayer interposed between the silicon waveguide and the waveguide made of III-V material.

Variants of the Processes for Fabricating the Mixed Layer:

In step120, the layer122may also be made of amorphous silicon.

Other embodiments of the step124of structuring the silicon core are possible. For example, the silicon-oxide sublayer126may be omitted.

In step124, the structuring of the silicon core may also comprise local thickness-increasing operations (either growth of single-crystal silicon or deposition of amorphous silicon, or deposition of amorphous silicon followed by a recrystallisation by heat treatment) to locally increase the thickness of the silicon core. In this case, the initial thickness of the single-crystal silicon layer122may be small. For example, the initial thickness of the layer122is then 300 nm.

In step130, the etch-stop liner132may be made from materials other than silicon nitride. For example, it may be made of Al2O3or HfO2. In this case, the lower face of the guide36is situated at a level slightly above the lower face of the guide25. Typically, the height difference between these two lower faces however remains smaller than 100 nm and, generally, smaller than 50 nm.

In operation162, a material other than silicon oxide may be used provided that this material may be planarized then etched as described in operations166and168.

In operation166, the chemical-mechanical polishing of the silicon-oxide layer may also be stopped before the silicon-nitride layer152is reached. In this case, there is, at the end of this polishing operation, a residual silicon-oxide layer that completely covers the silicon-nitride layer152. Next, this residual layer is completely removed in the operation168of non-selective etching.

In another variant, the operation168of non-selective etching is continued until the liner132or the sublayer126is reached. In this case, at least some or all of the silicon-nitride liner132is also removed.

In the operation170, a dielectric other than silicon oxide may be deposited to form the sublayer106. Typically, this other dielectric is then chosen to allow a good direct bonding of the transfer or substrate made of III-V material to be achieved.

In step176, instead of bonding a transfer or substrate made of III-V gain material, it is possible to deposit this material on the mixed layer66.

As a variant, the operation208of non-selectively etching the silicon nitride is stopped before the sublayer126is reached. For example, it is stopped when the sublayer140is reached.

The phase of fabricating a mixed layer may be implemented in processes other than a process for fabricating a laser source. In particular, the described process is also suitable for fabricating a mixed layer in which the waveguides made of silicon and of silicon nitride contained in this mixed layer are not optically coupled to each other by an optical coupler such as the couplers24and34. For example, the optical coupling between these two waveguides may be achieved using an optical coupler at least one portion of which is produced in a layer situated above or below the mixed layer. The processes described here also allow a mixed layer in which the waveguides made of silicon and of silicon nitride are not optically coupled to each other to be produced. In this case, the production of an optical coupler such as the couplers24and34is omitted.

For example, the process for fabricating a mixed layer described here may be used to fabricate a mixed layer of photonic components other than a laser source. For example, this process may be used to fabricate a mixed layer comprising the electrodes of an optical modulator. It may also be used to fabricate the mixed layer of any photonic component comprising a layer made of Ill-V material stacked on the mixed layer. For example, the processes described here may be implemented during the fabrication of the following photonic components: a semiconductor optical amplifier (SOA), an electro-absorption modulator (EAM), a photodiode. For example, an SOA may be obtained from the photonic component ofFIG.1by omitting the reflectors12and14and the filter22. An EAM may be obtained from the component ofFIG.1by omitting the reflectors12and14, the filter22and by shortening the length of the component30. The processes described here for fabricating the mixed layer may also be implemented to fabricate the following photonic components:a transmitter comprising a plurality of laser sources that emit optical signals at various wavelengths and, produced in the silicon-nitride waveguide, a multiplexer of the type known as an arrayed waveguide grating (AWG),a receiver comprising a plurality of photodetectors and, produced in the silicon-nitride waveguide, an AWG demultiplexer,a set of a plurality of wideband semiconductor optical amplifiers connected to the entrance and/or exit of an AWG multiplexer/demultiplexer that splits the optical signals amplified at various wavelengths, the AWG multiplexer/demultiplexer being produced in the silicon-nitride waveguide,wavemeters using the different thermo-optic coefficient of silicon nitride,optical combs generated by solid bodies pumped by laser diode.

The processes described here for fabricating a mixed layer may also be implemented in processes for fabricating photonic components that comprise no layer made of encapsulated III-V material.

Other Variants:

The mixed layer may comprise additional waveguides made of silicon or of silicon nitride. For example, one of these additional waveguides is optically connected to the exit of the reflector12or14.

Alternatively, by inverting the polarity of the supply signal between the contacts74,76of the amplifier30, the input optical signal is absorbed instead of being amplified. The amplifier30is then used to carry out photo-detection or signal modulation.

As a variant, these photonic components, and in particular the photonic components, such as the laser source10, comprising a layer made of Ill-V material stacked on the mixed layer, may be fabricated using a process for fabricating the mixed layer66other than those described here. Thus, the embodiments of the laser source described here may be implemented independently of the processes that have been described for fabricating this mixed layer.

Section IV: Advantages of the Described Embodiments

The processes for fabricating a mixed layer that have been described here are simple. Specifically, they do not require the substrate to be flipped. Thus, all the etching, planarizing and depositing steps required to produce the mixed layer are carried out on the same side of this substrate60.

The processes for fabricating a mixed layer that have been described here thus allow a higher integration density to be obtained than the known processes. Specifically, known processes transfer a silicon-nitride transfer and/or a single-crystal-silicon transfer to the layer64. In the former case, a space of several hundred microns must necessarily exist between these silicon and silicon-nitride transfers. Thus, the cores made of silicon and of silicon nitride that are produced by structuring these transfers are necessarily spaced apart from each other by several hundred microns.

The use of a stop liner allows the bottom of the trench146to be precisely positioned at less than 100 nm from the upper face of the layer64. Thus, this in the end allows a silicon-nitride core and a silicon core the lower faces of which are situated in levels that are spaced apart from each other by a distance smaller than 100 nm and that are both situated at less than 100 nm from the layer64to be obtained.

When the liner132is made of silicon nitride, the lower faces of the cores made of silicon and of silicon nitride are situated in the same horizontal plane.

The production of an extension of the trench146above the one or more tapered terminations of the silicon core allows an optical coupling to be obtained, between the waveguides made of silicon and of silicon nitride, that occurs entirely inside the mixed layer. It is therefore not necessary, to achieve this coupling, to use an additional layer situated above or below this mixed layer.

The fact of forming the cladding of the guide25from silicon nitride allows the optical properties of the silicon waveguide to be improved.

The use of a mixed layer to fabricate a photonic component allows the integration density of this photonic component to be increased. Specifically, it is no longer necessary to have silicon waveguides in a first layer and silicon-nitride waveguides in a second layer situated above or below this first layer.