Abstract:
A three-dimensional photonic integrated structure includes a first semiconductor substrate and a second semiconductor substrate. The first substrate incorporates a first waveguide and the second semiconductor substrate incorporates a second waveguide. An intermediate region located between the two substrates is formed by a one dielectric layer. The second substrate further includes an optical coupler configured for receiving a light signal. The first substrate and dielectric layer form a reflective element located below and opposite the grating coupler in order to reflect at least one part of the light signal.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the priority benefit of French Application for Patent No. 1654523, filed on May 20, 2016, the disclosure of which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    Embodiments relate to photonic integrated devices, and notably to the coupling of this type of device with an external optical signal, coming from, for example, but not limited to, an optical fiber. 
       BACKGROUND 
       [0003]    Conventionally, for coupling a photonic integrated circuit to an optical signal originating, for example, from an optical fiber, an optical coupler is implemented in the active layer of the integrated circuit for redirecting the light signal into a waveguide implemented in the structure. 
         [0004]    During coupling, one part of the input optical signal passes through the coupler and is not transmitted in the waveguide. Means exist for improving the efficiency of the coupling, such as optimizing the thickness of the buried insulating layer in the case of a Silicon-On-Insulator (SOI according to the abbreviation well known to the person skilled in the art) substrate. However, even with an optimum thickness, a part of the signal is lost. 
         [0005]    Another solution consists in placing a reflective layer, for example, a metal layer, under the buried layer, in order that the rays passing through the coupler are reflected and pass back again into the coupler. However, the production of such a metal layer requires specific method steps. 
       SUMMARY 
       [0006]    Thus, according to one embodiment, provision is made here to further reduce the losses of an optical signal arriving at an integrated optical coupler. 
         [0007]    In this respect, provision is advantageously made to use not a single integrated circuit but an integrated three-dimensional structure comprising multiple stacked substrates (forming a monolithic structure), and to implement a reflector in one of its substrates, under the optical coupler. 
         [0008]    This has the advantage of limiting the optical signal losses and producing the reflector with existing manufacturing methods, such as etching and deposition of dielectric material. 
         [0009]    According to one aspect, a three-dimensional photonic integrated structure is provided including a first semiconductor substrate incorporating at least one first waveguide, a second semiconductor substrate incorporating at least one second waveguide, and at least one intermediate region located between the two substrates and comprising at least one dielectric layer; the second substrate comprises at least one optical coupler configured for receiving a light signal, and the first substrate and said at least one dielectric layer comprising a reflective element located opposite said at least one grating coupler capable of reflecting at least one part of said light signal. 
         [0010]    The reflective element comprises, for example, a portion of the first semiconductor substrate and a portion of said layer of dielectric material. 
         [0011]    Thus, implementing the reflective element in a substrate comprising other photonic components avoids the need for a specific method step for obtaining the optical reflector. 
         [0012]    According to one embodiment, the intermediate region may further comprise at least one additional semiconductor layer coated in the dielectric layer and located opposite the optical coupler, the reflective element further comprising said additional layer. 
         [0013]    Preferably, the product of the thickness of the portion of the first semiconductor substrate and its refractive index and the product of the thickness of the portion of said layer of dielectric material and its refractive index are both approximately equal to a quarter of the wavelength of the light signal. 
         [0014]    The first substrate and the second substrate may be semiconductor films located on insulating layers, thus forming silicon-on-insulator substrates. In this case, the intermediate region advantageously includes the buried insulating layer on which the second substrate is located. 
         [0015]    According to one embodiment, at least one part of the reflective element, for example, said portion of the first substrate, has a thickness less than or equal to the thickness of the first substrate. Thus, said portion may be etched or left as it is. 
         [0016]    In particular, the thickness of said portion of the first semiconductor substrate may correspond to the thickness of other photonic components implemented in the first semiconductor substrate. 
         [0017]    The optical coupler may be of a single polarization type, and in this case be coupled to a single waveguide, or of a polarization splitting type and then be coupled to multiple waveguides. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Other advantages and features of the invention will appear on examination of embodiments of the invention, in no way restrictive, and the accompanying drawings in which: 
           [0019]      FIGS. 1, 3 and 4  illustrate a monolithic photonic structure; 
           [0020]      FIG. 2  shows a grating coupler as a single polarization coupler; 
           [0021]      FIG. 5  shows a grating coupler as a polarization splitting coupler. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  illustrates a monolithic photonic structure SPM. The photonic structure SPM comprises a carrier substrate  1 , on which two silicon-on-insulator substrates  2  and  3  are implemented separated by an intermediate region INT and in which multiple photonic components are implemented. 
         [0023]    The structure also comprises an interconnection region (BEOL, “Back End Of Line” according to the acronym well known to the person skilled in the art) not represented here for the purposes of simplification. 
         [0024]    An optical fiber, for example, may be attached on the upper face of the structure SPM, delivering an incident optical signal L 1  whereof the wavelength is, for example, close to one thousand three hundred and ten nanometers. 
         [0025]    Here, the input optical signal arrives at the structure at a low angle θ, for example, between eight and thirteen degrees. 
         [0026]    The first SOI substrate  2  comprises a first substrate proper, or semiconductor film  22 , and a first buried insulating layer  21  (known by the person skilled in the art under the acronym “BOX”, for Buried Oxide), here a layer of silicon dioxide conventionally having a thickness of seven hundred nanometers. 
         [0027]    The first buried insulating layer  21  is located here under the first semiconductor film  22 , having, for example, here a thickness of three hundred nanometers. 
         [0028]    Multiple photonic components are implemented by etching in the first silicon film  22 , then coated in a first layer of dielectric material  23 , here of silicon dioxide, so that the assembly formed by the first silicon film  22  and the first dielectric layer  23  has a thickness of four hundred and fifty nanometers. 
         [0029]    The first substrate  22  notably comprises a first waveguide  24  and a set of active components, including, for example, a photodetector  25 . 
         [0030]    The second SOI substrate  3 , implemented directly above the layer of dielectric material  23  by molecular bonding, comprises a second buried insulating layer  31  of a thickness, for example, equal to one hundred nanometers, forming with the layer of dielectric material  23  the intermediate region INT, whereon the second substrate proper is located, or second semiconductor film  32 , made of silicon, for example. 
         [0031]    The second substrate  3  comprises photonic components etched in the second semiconductor film  32  and coated in a second layer of dielectric material  33 . Here, the components notably comprise a second waveguide  34  optically coupled to a grating coupler  36 . 
         [0032]    The structure SPM also comprises a reflective element  26 , here a Bragg mirror conventionally formed by multiple layers having different refractive indices. 
         [0033]    In this example, the Bragg mirror  26  includes two stacked layers, including a first layer formed by one portion  220  of the first silicon film  22 , and a second layer formed by the stacking of one portion  230  of the first dielectric layer  23  and one portion  310  of the second buried insulating layer  31  of the second substrate  3 . 
         [0034]    The thicknesses of the two layers of the Bragg mirror  26  are chosen here so that the product of the thickness of each layer and the refractive index of the material which composes it is as close as possible to a quarter of the wavelength of the incident signal L 1 . This feature makes it possible to further increase the efficiency of the mirror  26 . However, this value is only indicative, and the result of the product may be adapted so as to be more or less close to this value according to the thickness of the first buried insulating layer  21 . 
         [0035]    Here, the thickness of the portion  220  of the first substrate  22  is the same as the thickness of the components of the first substrate, notably of the first waveguide  24  and the photodiode  25 . Thus, the implementation of the Bragg mirror  26  does not require a specific method step. 
         [0036]    It should be noted that the drawings presented here are simplified cross-sectional views. Thus, although the second waveguide  34  and the optical coupler  36  are represented in the same cross-sectional plane, they may in reality be located in separate planes and/or be oriented in different directions. 
         [0037]    As illustrated in  FIG. 2 , the grating coupler  36  is a single polarization coupler, such that a light signal L 2  coming from the coupler and entering the second waveguide  34  is polarized according to a single polarization state, for example here a transverse electric polarization, such that a polarization in which the electric field component of the light wave is perpendicular to the plane of incidence (also known to the person skilled in the art under the term “S polarization”). 
         [0038]    The grating coupler  36  is implemented above the Bragg mirror  26 . Accordingly, a large part of the incident rays passing through the coupler  36  arrive at the mirror  26  in order to be reflected towards the coupler  36  and coupled to the waveguide  34 . Thus, the signal losses due to the coupling are reduced. For a wavelength close to one thousand three hundred and ten (1,310) nanometers, such a mirror exhibits a reflectivity of 90%, for an incident wave L 1  in transverse electric mode (TE, according to the abbreviation well known to the person skilled in the art), arriving at an angle θ of 13°. 
         [0039]    The second waveguide  34  has a portion implemented above the first waveguide  24 , and having the same dimensions. Thus, these two parallel portions of the first and second waveguides form an adiabatic coupler for transferring light from the second waveguide to the first waveguide. 
         [0040]      FIG. 3  illustrates an embodiment in which the first portion  220  of the first semiconductor film  22  has undergone an additional etching so as to make it less thick. This thickness may, for example, and advantageously, correspond to the thickness of silicon of some semiconductor portions of photonic components etched in the first semiconductor film  22 , so that the same etching step may be used to form these photonic components and the Bragg mirror  26 . 
         [0041]    Thus, the Bragg mirror is here formed by the first portion  220  of the first silicon film  22  having in this example a thickness of one hundred and fifty nanometers, by the first portion  230  of the layer of dielectric material  23 , therefore having a thickness of three hundred nanometers, and one portion  310  of the buried insulating layer  31  of the second substrate  3 , of a thickness of one hundred nanometers. 
         [0042]    Thus, the Bragg mirror is optimized for reflecting an incident signal L having here a wavelength close to one thousand five hundred and fifty (1,550) nanometers. For this wavelength, such a mirror exhibits a reflectivity of 80%, for an incident wave L 1 ′ in transverse electric mode, arriving at an angle θ of 13°. 
         [0043]      FIG. 4  illustrates that the intermediate region INT comprises an additional silicon layer  27 , for example made of polycrystalline silicon or amorphous silicon, of a thickness of one hundred and fifty nanometers, implemented above the first silicon film  22  in order to further improve the reflectivity of the Bragg mirror  26 . 
         [0044]    Here, the portion  220  of the first silicon layer has been etched so as to have a thickness of one hundred and fifty nanometers. It has been covered with a first portion  231  of the first layer of dielectric material,  23 , which has been leveled before the deposition and etching of the additional silicon layer  27 , which has itself been covered with a second portion  232  of the first layer of dielectric material  23 . 
         [0045]    Thus, the Bragg mirror in this example comprises four layers:
       the first portion  220  of the first silicon film  22 , of a thickness of one hundred and fifty nanometers,   a first portion  231  of the first layer of dielectric material  23 , here of a thickness of one hundred and fifty nanometers,   the additional silicon layer  27 , and   the stack of a second portion  232  of the first layer of dielectric material  23 , of a thickness of fifty nanometers and a portion  310  of the buried insulating layer  31  of the second substrate  3 , of a thickness of one hundred nanometers.       
 
         [0050]    Thus, it is particularly advantageous that the two thicknesses of the pairs of silicon and silicon dioxide layers are identical, which provides improved reflectivity. However, it would be conceivable to have a mirror with different thicknesses of layers. 
         [0051]    As illustrated in  FIG. 5 , the grating coupler is in this example a polarization splitting coupler (PSGC, for “Polarization Splitting Grating Coupler” according to the abbreviation well known to the person skilled in the art). Nevertheless, this embodiment is compatible with a single polarization coupler. 
         [0052]    Thus, a light signal L 1  passing into the coupler  36  will be split into two separate polarization subsignals. For example, a first subsignal L 3  will here be transverse electrically polarized and directed into the second waveguide  34 , and a second subsignal L 4  will be directed into a third waveguide  37  and transverse magnetically polarized (or P polarization), meaning a polarization in which the magnetic field component of the light wave is perpendicular to the plane of incidence. 
         [0053]    It should be noted that the embodiments described here are in no way restrictive. Notably, although a Bragg reflector with a thickness of five hundred and fifty nanometers has been described, it is quite possible to envisage a reflector having a different thickness, preferably but not restrictively with layers whereof the product of the thickness and the refractive index is close to a quarter of the wavelength of the incident signal. The same applies to the number of layers of the mirror, which may vary with respect to the examples illustrated in  FIGS. 1, 3 and 4 .