CA2367708A1 - Photonic integrated circuit comprising a resonant optical component and methods for making same - Google Patents

Abstract

According to the invention, the resonant optical component (2) is placed above a photon collector (10) formed at the end of a light guide (4) of the circuit and vertically coupled to this part. To make this circuit we forms for example a substrate comprising a first layer intended for the manufacture of the guide and a second layer intended for the manufacture of a coupling layer (12) between the component and the part of the guide and report on this coupling layer, by a technique called bonding of wafer, a layer intended for the manufacture of the component or comprising this latest. The invention applies in particular to telecommunications and optical interconnections.

Description

INTB PHOTON CIRCUIT (FROM CODQPRBNANT t1N CO> üPOSANT
RESONANT OPTICS AND MANUFACTURING METHODS DS CE
CIRCUIT
DBSCRIPTION
TECÜNIQU AREA, E
The ~ present invention relates to circuits integrated photonics.
It applies in particular to systems integrated telecommunications ~ optical. . and of optical interconnection. .
PRIOR STATE OF THE ART
I1 is known to couple optically, by through a mirror for example, a guide light formed on a substrate, to a VCSEL type laser hybridized to this substrate. It is also known to emit light from a microsource based on microdisks, micro-rings or crystals photonics. It is also known to couple the ._ light vertically between two light guides.

2 0 PRESENTATION OF I ~ 'INVENTION' The subject of the present invention is a integrated photonic circuit in which a component resonant optics, for example a laser transmitter, is optically coupled to a light guide formed on a _.___ u _ - ___ ~ 25 substrate, with a coupling efficiency greater than _, US 5,513,288 A discloses urge black resonant photodiode. The principle applied in this document is as follows: an evanescent co ~ rage is formed between two guiding structures.
US 5,787,105 A discloses a device which uses a laser (resonator) and a coupling vertical optics.
- _. _ .. 'DE 3 820 171 A_describes the association of a waveguide and a detector.
US 5 838870 A discloses silicon guides formed on dis structures g ~ enrL SOI
and resonant optical components. ~ 'w'w MODIFIED SHEET

WO 00/6539

3 PCT / FR00 / 01062 that of the coupling mentioned above, between the laser type VCSEL and the light guide.
Specifically, the present invention has for object an integrated photonic circuit formed on a substrate and comprising at least one light guide integrated into this substrate, this circuit being characterized by what it further includes at least one component resonant optics which is integrated into the substrate and intended to emit or detect light or both, this resonant optical component being placed above a photon collector formed at the end of the guide light, and a means of vertical optical coupling between the resonant optical component and the collector of photons, the latter being provided to ensure the light transfer between the optical component resonant and the rest of the light guide by through the vertical optical coupling means.
It should be noted that the coupling vertical used in the present invention allows a precise adjustment of the coupling, while a coupling classic lateral is much more difficult to set up works from a technical point of view.
According to a preferred embodiment of the integrated photonic circuit object of the invention, the vertical optical coupling means comprises a layer of a material which is transparent to light and whose the optical index is lower than that of the guide light as well as that of the component material resonant optics.
Preferably, the light guide is in silicon.

The invention makes it possible to couple effectively an optical component resonating to a light guide in silicon whose small cross-section, well that it is compatible with the required dimensions by microelectronics on silicon, does not allow, when using a conventional coupling method, a sufficient light insertion efficiency.
With this silicon light guide, we preferably uses, as a substrate, a silicon on insulator type structure or SOI structure (for "Silicon On Insulator").
The possibility of this optical coupling between the resonant optical component and the light guide in silicon allows the integration of this component with other electro-optical components and with passive micro-photonic devices which are achievable on silicon type structures on insulating.
Such structures are almost the only to allow the realization of circuits passive photonics with compatible dimensions with microelectronics. It is indeed appropriate to note that a strong confinement of the light in the silicon guides due to the strong index contrast between silicon and insulator consisting of silica.
Preferably, the optical coupling means vertical includes a layer of silica. Such a layer is well suited for using a guide silicon light and a substrate having a silicon on insulator type structure.

4 Preferably, the optical component resonant includes a microresonator.
It should be noted that the use of a laser transmitter comprising a microresonator or resonant microcavity (resonator or resonant cavity whose dimensions are of the order of a few micrometers) allows, by confining the photons produced, to favor the rate of spontaneous emission in the mode desired and therefore lower the stimulated emission threshold of the laser transmitter.
It should also be noted that such laser transmitter is compatible with the requirements of size of photonic integration on silicon.
Preferably, we make sure that the density of photonic states of the laser mode enhanced so that the photons are constrained to occupy this mode.
We recall that this selective reinforcement of the spontaneous emission rate depends on the Q / V ratio where Q
is the quality factor of the cavity and V the volume associated mode. If the quality factor of the microcavity is sufficient, this then allows transfer a large percentage (a few tens of ~) of spontaneous emission in laser mode.
Preferably, the microresonator is a microdisc microresonator ("microdisk") or micro-ring or a microstructure based on a two photonic crystal dimensions.
A microdisc or resonator micro-ring, exploiting gallery modes (called "Whispering gallery modes" in articles in English language) provides strong confinement photons. It is the same for a microresonator consisting of a crystal-based microstructure

5 two-dimensional photonics.
Regarding the latter, consult the document [1J which, like the other documents cited by the following is mentioned at the end of this description.
Preferably, the material of the component resonant optics is selected from the compounds III-V semiconductors which are among the materials most efficient transmitters and / or detectors.
This material can be a heterostructure III-V semiconductor with quantum wells ("quantum wells ”) or quantum dots.
This allows, in case the optical component resonant is a laser transmitter, to lower the threshold of this laser transmitter by increasing the confinement of excite in the latter.
The stimulated emission is all the more favored that the density of convolved states exciton photon is important in the desired laser mode. A
one- or two-dimensional laser transmitter thus has a very low threshold.
It should be noted that the wells are easier to form than boxes quantum but that these allow a optimal confinement of excitons.
It should also be noted that well-based semiconductor III-V heterostructure

6 quantum or quantum dots can emit to wavelengths that are within the range ranging from 1.3 µm to 1.5 µm, wavelengths at which the silicon is transparent.
Note also that, to achieve integrated photonic circuits, you should have light sources and passive optical components of micron dimensions. Concerning the passive optical components, silicon technology on insulator "allows, thanks to the high index contrast optics between, on the one hand, silicon and, on the other hand, silica and air, to ensure strong containment of the light that is needed for miniaturization of these components.
In addition, the realization of sources of microcavity light allows to satisfy miniaturization and low consumption requirements while being compatible with the "silicon on insulator" technology.
The present invention also relates to a manufacturing process of the integrated photonic circuit object of the invention, in which a substrate is formed comprising a first layer intended for the manufacture of the light guide and a second layer intended for the manufacture of the optical coupling means and a third layer or portion of layer on this second layer by a technique called wafer bonding, this third layer or portion of layer being intended for the manufacture of the resonant optical component or including the latter.

7 The present invention further relates to a other method of manufacturing the photonic circuit integrated object of the invention, in which a substrate comprising a first layer intended for the manufacture of the light guide and a second layer intended for the manufacture of the optical coupling means and we form, on this second layer, a heterostructure based on an active material comprising nanocrystals of InAs (forming boxes quantum) in a matrix of Si or Si3N4, this heterostructure being intended for the manufacture of resonant optical component.
BR ~ VE DESCRIPTION OF THE DRAWINGS
The present invention will be better understood from reading the description of exemplary embodiments given below, for information only and in no way limiting, with reference to the drawings annexed on which.
~ Figure 1 is a longitudinal sectional view schematic of an integrated photonic circuit according to the invention, ~ Figure 2 is a schematic top view of a first particular embodiment of the integrated photonic circuit object of the invention, comprising a microresonator of the type microdisc, ~ Figure 3 is a longitudinal sectional view schematic ~ of the circuit of FIG. 2, ~ Figure 4 is a schematic top view of a second particular embodiment of the

8 integrated photonic circuit object of the invention, including a crystal-based microresonator two-dimensional photonics, ~ Figure 5 is a longitudinal sectional view schematic of the circuit of FIG. 4, and ~ Figure 6 is a longitudinal sectional view schematic of a structure usable for manufacture of a circuit according to the invention.
DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS
In the compliant integrated photonic circuit to the invention, which is schematically represented on FIG. 1, a laser microsource 2, with microcavity resonant, is associated with a light guide 4 of so as to allow the transfer of the generated light by the microsource in this light guide.
The latter can be optically connected to other passive optical components (not shown) of the photonic circuit, which then receive the light generated by the microsource and transmitted by the guide from light.
The light guide 4 is formed, in the example shown, from a type structure silicon on insulator. This structure includes a silicon substrate 6, on which a layer of silica 8, as well as a layer of silicon formed on this layer of silica 8 and treated to form the light guide 4.
Laser microsource 2 is formed above of one end 10 of this light guide and a layer

9 intermediate in silica 12 is interposed between the laser microsource and this end of the guide light.
This achieves vertical integration an active part (laser microsource) of the circuit photonics and a passive part (light guide) of this circuit, the active part being positioned above above this passive part.
The intermediate layer 12 has the function to transfer the light emitted by the laser microsource towards end 10 of the light guide by coupling evanescent vertical, which is symbolized by the arrow F1 in Figure 1.
The end 10 of the waveguide, which is found below this middle layer, is intended to recover the light thus transferred.
The latter then spreads to the rest of the 14 light guide (which is symbolized by the arrow horizontal F2) to be optionally sent to other passive components (not shown) of the integrated photonic circuit.
The use of the laser microsource resonant microcavity 2 and type structure silicon on insulator makes it particularly efficient the transfer of light from the microsource to the guide light due to compatibility, in terms of optical confinement, of such a laser source and of a such structure.
As will be seen below, the end of the waveguide intended to recover the light emitted by the laser microsource and redirect it to the light should be what is called a "collector of photons "and then the rest 14 of the waveguide is sufficiently coupled to this collector of photons to inhibit the resonant nature of this 5 last which would otherwise cause filtering additional light (not desired).
In the example of Figures 2 and 3, the laser microsource has a type configuration microdisc 2a. We see in these figures the structure of

10 silicon on insulator type including the substrate of silicon 6, the layer of silica 8 formed on this last and, on this layer of silica, the guide silicon light 4 whose end 10 constitutes then a photon collector (made of silicon).
On the latter is the layer silica intermediate 12. The microsource type microdisc 2a is formed on this layer intermediate.
For information only and not at all limiting the width L of the light guide is 0.3 llm, the diameter D of the microdisk microsource is worth about 5 ~ .un, the thickness E1 of the silica layer 8 is 0.5 llm, the thickness E2 of the silicon layer, at from which the collector 10 and the rest 14 of the light guide, worth 0.2 llm, the thickness E3 of the intermediate silica layer 12 is 0.2 Dun and the thickness E4 of the microdisc laser microsource is 0.2 µm.
In the example of Figures 4 and 5, the laser microsource includes a 2b resonator based on a two-dimensional photonic crystal. We still see

11 in these figures 4 and 5 the silicon substrate 6 covered by the silica layer 8, itself surmounted by the silicon light guide 4.
In the case of Figures 4 and 5, the end 10 of this light guide is a photon collector which is formed as a microcavity based on a crystal two-dimensional photonics and extends the rest 14 (itself rectilinear) of this light guide. This end is covered by the intermediate layer in silica 12, itself covered by the microsource laser 2 based on two-dimensional photonic crystal.
For information only and not at all limitative, the area S1 of this microsource based on photonic crystals is worth about 10 ~ un2 while the surface S2 of the photon collector is of the order of um2, the width L of the light guide is 0.3 µm, the thickness E1 of the silica layer 8 is 0.5 μm, the thickness E2 of the silicon layer, starting from which are formed the end 10 and the rest 14 of the 20 light guide, is 0.2 ~ .zm, the thickness E3 of the intermediate layer 12 is 0.2 μm and the thickness E4 of photonic crystal microsource is worth 0.2 µm.
In the particular configuration of Figures 2 and 3 and in the other configuration particular of Figures 4 and 5 we find the important elements of the diagram in Figure 1 that are the resonant microcavity source, the layer evanescent coupling intermediary and guide light (in silicon in the examples considered).

12 Given the small dimensions given above as an example for circuits of Figures 2 to 5, the free spectral interval between the modes of cavity is appreciable. in the case of Figures 2 and 3, the free spectral interval between two modes is of the order of a few tens of nanometers for a microdisc whose radius is worth a few micrometers (see document [2] on this subject) and, in the case of FIGS. 4 and 5, where a crystal is used two-dimensional photonics, this interval can exceed one hundred nanometers (see on this subject the document [3]).
Such characteristics are in agreement with the performance that is generally required for optical telecommunications for example (currently 30 nm of useful spectral range).
For the manufacture of the structures of figures 2 to 5 we can start by developing the active material of the laser microsource (we will come back on that later) and then define this microsource laser, for example by electronic lithography then reactive ion etching (etching) and then define the passive components (likely to be on the substrate 6) and the light guide 4 with its end 10 (photon collector) as well as any other light guides for example by optical or electronic lithography or by etching dry and / or wet.
The laser microsource can be pumped either optically or electrically but the pumping optically is easier to set up

13 works that pumping by electric means because the latter requires the deposition of metallic layers (to form electrical contacts) as well as the doping of semiconductor materials.
The integrated photonic circuits of Figures 2 to 5 have the advantage of allowing the exploitation of electro-optical materials, such as example InP, which have strong refractive indices and are in contact with air and with materials, such as for example silica, which have low indices of refraction, which ensures excellent confinement optical.
Therefore it is not necessary to make very thick components (several micrometers thick) or to suspend these components above an air gap by means of micro-machining.
We thus solve in particular the problems of manufacturing and mechanical reliability which are generally observed in the case of microlasers resonant microdisc type suspension (see on this subject document [4]).
In addition, the manufacturing processes for active and passive parts of photonic circuits integrated figures 2 to 5 are sufficient independent to keep all the skills of techniques for manufacturing photonic circuits on silicon on insulator type structures.
In what follows, we return to the fabrication of the laser microsource. More precisely, we now consider integration, to the rest of the

14 circuit, of the active material from which is formed this laser microsource.
As active material, we use preferably a III-V semiconductor compound with a gap direct, material that is perfectly mastered for manufacture the lasers conventionally used in the optical telecommunications domain at 1.3 ~ .un and 1.55 µm.
Two ways are possible and indicated in what follows.
According to a first mode of implementation particular we make a silicon type structure on insulator comprising a silicon substrate 6, on which is formed a layer of silica 8, as well as a layer of silicon 16 formed on this layer of silica 8, where a structure is used commercially available like this.
A layer of silica 18 is then formed, intended for the formation of the intermediate layer 12, on the silicon layer 16 then a complete layer 20 or a layer portion of a semiconductor compound III-V, for example InP, on this layer of silica 18.
This layer 20 or this portion of layer of InP is added to this layer of silica 18 by the technique called wafer bonding ("wafer bonding ”).
When using a diaper portion of InP, this portion therefore only extends over a part of layer 18.
We then manufacture the laser microsource 2 from this layer 20 in InP, by epitaxy of InP / GaInAsP and known manufacturing techniques laser microsources, so as to obtain a GaInAsP / InP heterostructure (quantum well or quantum dots).
5 The intermediate layer is then made vertical coupling 12 from the silica layer 18 then we make the light microguide 4 (y included the end 10 of the latter which is under layer 12) from the silicon layer 16 of 10 the silicon on insulator structure.
Instead of forming the laser microsource by epitaxy from layer 20 or the portion of layer in InP, we can also transfer, on the silicon on insulator type structure provided with the

15 layer of silica 18, a complete laser structure formed from InP, this transfer still taking place by the plate bonding technique.
The second mode of implementation particular also uses a silicon type substrate on insulator.
On this substrate, a layer of silica for the formation of the coupling layer intermediate and, on this layer of silica, we manufactures by chemical vapor deposition ("chemical vapor deposition ”) and epitaxy a heterostructure to base of an active material, called SIA material and consisting of nanocrystals of InAs distributed in a matrix of Si or Si3N4. This SIA material is manufactured by a combination of chemical deposition techniques in vapor phase, to obtain the Si or Si3N4 matrix,

16 and molecular beam epitaxy ("molecular beam"
epitaxy ”), to obtain the nanocrystals of InAs.
Regarding such SIA material we can see document [5].
After manufacturing the heterostructure, as before, we make the intermediate layer vertical coupling 12 and light guide 4 (y including end 10 of the latter).
Details are given below additional information on circuit manufacturing integrated photonics according to the invention.
SOI substrates (Si / Si02 / Si) called "Unibond" are commercially available from SOITEC.
About laser microsources microdisc or micro-ring we will also consult the documents [6], [7] and [8].
About photonic crystals, we see also document [9].
In addition, the invention is not limited to integration of a laser microsource. a integrated microresonator according to the invention can be used not only as a laser transmitter but also as a light amplifier or as a resonant photodetector or even as as a laser emitter and a resonant photodetector, alternating these two uses.
The documents cited herein description are as follows.
[1] T. Baba, IEEE J. Select. Topics. In Quantum Electron. 3 n ° 3 (1997) p.808

17 [2] BE Little et al., J. of Lightwave Technol. 15 # 6 (1997) p.998 [3] OJ Painter et al., J. Lightwave Technol. 17 (1999) [4] DY Chu et al., Appl. Phys. Lett. 65 no.25 (1994) [5] J. Shi et al., Appl. Phys. Lett. 70 (1997) 2586 [6] SI McCall et al., Appl. Phys. Lett. 60 (1992) 289 [7] JP Zhang et al., Phys. Rev. Lett. 75 (1995) 2678 [8] T. Baba et al., IEEE Photon. Technol. Lett. 9 (1997) 878 [9] E. Yablonovitch, Phys. Lett. 58, 2059 (1987).

Claims (12)

Claims 1. Integrated photonic circuit formed on a substrate and comprising at least one light guide (4) integrated into this substrate, this circuit being characterized in that it further comprises at least one component resonant optics (2) which is integrated into the substrate and intended to emit or detect light or two, this resonant optical component being placed at the above a photon collector (10) formed from the end of the light guide (4), and a means (12) of vertical optical coupling between the optical component resonant and the photon collector, the latter being intended to ensure the transfer of light between the resonant optical component and the rest (14) of the guide light via the coupling means vertical optics. 2. Integrated photonic circuit according to claim 1, wherein the coupling means vertical optic includes a layer (12) of a material which is transparent to light and whose index optics is lower than that of the light guide as well than that of the material of the resonant optical component. 3. Integrated photonic circuit according to one any of claims 1 and 2, wherein the light guide (4) is made of silicon. 4. Integrated photonic circuit according to claim 3, wherein the substrate is 5. Integrated photonic circuit according to one any of claims 1 to 4, wherein the made of silicon_ a single insulating layer is formed on this substrate and the waveguide light is formed on this insulating layer_ vertical optical coupling means comprises a layer silica (12). 6. Integrated photonic circuit according to one any of claims 1 to 5, wherein the resonant optical component (2) comprises a microresonator. 7. Integrated photonic circuit according to claim 6, wherein the microresonator is a microdisk or microring microresonator (2a). 8. Integrated photonic circuit according to claim 6, wherein the microresonator is a microstructure based on a photonic crystal (2b) with two dimensions. 9. Integrated photonic circuit according to one any of claims 1 to 8, wherein the material of the resonant optical component (2) is chosen among III-V semiconductor compounds. 10. Integrated photonic circuit according to claim 9, wherein the component material resonant optic (2) is a heterostructure III-V semiconductor with quantum wells or boxes quantum. 11. Circuit fabrication process integrated photonics according to any of claims 1 to 10, wherein a substrate is formed comprising a first layer (16) intended for the manufacture of the light guide and a second layer (18) intended for the manufacture of the coupling means optics and a third layer or layer portion (20) on this second layer by a technique called wafer bonding, this third layer or layer portion being intended to the manufacture of the resonant optical component (2) or including the latter. 12. Circuit fabrication process integrated photonics according to any of claims 1 to 10, wherein a substrate is formed comprising a first layer (16) intended for the manufacture of the light guide and a second layer (18) intended for the manufacture of the coupling means optics and one forms, on this second layer, a heterostructure based on an active material comprising InAs nanocrystals in a matrix of Si or Si3N4, this heterostructure being intended for the manufacture of the resonant optical component (2).