Patent Publication Number: US-11644697-B2

Title: Phase modulator device and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of French Application No. 1909284, filed on Aug. 19, 2019, which application is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates generally to photonic integrated circuits and methods, and more particularly to phase modulator devices and methods. 
     BACKGROUND 
     Photonic integrated circuits are known comprising a phase modulator provided with a waveguide and a heating element. The heating element, typically a metallic layer portion, is configured to modify, in a controlled manner, the temperature of the material of the waveguide, and therefore the effective optical index of a signal propagating in the waveguide. When a control current circulates in the portion of the metallic layer making up the heating element, this results in a corresponding increase in its temperature by Joule effect, and therefore of the waveguide positioned near the heating element. FIG. 2 of the article by Tyler et al. titled “SiN integrated photonics for near-infrared LIDAR” and published in IEEE CPMT Symposium Japan (ICSJ) 2018, shows an example of such a modulator. 
     SUMMARY 
     There is a need to address all or some of the drawbacks of known phase modulators comprising a waveguide and a heating element configured to modify the temperature of the waveguide. 
     One embodiment addresses all or some of the drawbacks of known phase modulators comprising a waveguide and a heating element configured to modify the temperature of the waveguide, in particular when the waveguide is made of silicon nitride. 
     Another embodiment provides a method for manufacturing a device, in particular a phase modulator, comprising a waveguide and a heating element configured to modify the temperature of the waveguide, in particular when the waveguide is made of silicon nitride. 
     One embodiment provides a manufacturing method comprising the following steps: 
     a) forming a waveguide made of a first material, the waveguide being configured to guide an optical signal; 
     b) forming a layer made of a second material that is electrically conductive and transparent to a wavelength of the optical signal, 
     steps a) and b) being implemented such that the layer made of the second material is in contact with at least one of the faces of the waveguide, or is separated from the at least one of the faces by a distance of less than half, preferably less than a quarter, of the wavelength of the optical signal. 
     According to one embodiment, step a) is done before step b). 
     According to one embodiment: 
     step a) comprises the following successive steps: 
     a1) depositing a layer made of the first material on a first layer made of a third material, and 
     a2) etching the layer made of the first material to define the waveguide therein; and 
     in step b), the layer made of the second material is deposited on and in contact with exposed faces of the waveguide, or on and in contact with an intermediate layer with a thickness equal to the distance and previously deposited on and in contact with exposed faces of the waveguide. 
     According to one embodiment: 
     before step a), the method comprises a step consisting of etching a cavity in a first layer made of a third material; 
     step a) comprises the following successive steps: 
     a1) depositing a layer made of the first material so as to fill the cavity, and 
     a2) performing mechanical-chemical planarization up to the layer made of the third material, a portion of the layer made of the first material left in place in the cavity forming the waveguide; and 
     in step b), the layer made of the second material is deposited on and in contact with an exposed face of the waveguide, or on and in contact with an intermediate layer with a thickness equal to the distance and previously deposited on and in contact with an exposed face of the waveguide. 
     According to one embodiment: 
     before step a), the method comprises a step consisting of depositing another layer made of the second material; and 
     in step a1), the layer made of the first material is deposited on and in contact with the other layer made of the second material, or on and in contact with another intermediate layer with a thickness equal to the distance and previously deposited on and in contact with the other layer made of the second material. 
     According to one embodiment, the method further comprises, after step b), a step consisting of depositing a second layer made of the third material and a step consisting of forming electrically conductive vias through the second layer made of the third material up to portions of the layer made of the second material. 
     According to one embodiment, step b) is done before step a). 
     According to one embodiment: 
     in step b), the layer made of the second material is deposited on a first layer made of a third material; and 
     step a) comprises the following successive steps: 
     a1) depositing a layer made of the first material on and in contact with the layer made of the second material, or on and in contact with an intermediate layer with a thickness equal to the distance and previously deposited on and in contact with the layer made of the second material, and
 
a2) etching the layer made of the first material to define the waveguide therein.
 
     According to one embodiment: 
     before step b), the method comprises a step consisting of etching a cavity in a first layer made of a third material; 
     in step b), the layer made of the second material is deposited on the walls and the bottom of the cavity; and 
     step a) comprises the following successive steps: 
     a1) filling the cavity by depositing a layer made of the first material on and in contact with the layer made of the second material, or on and in contact with an intermediate layer with a thickness equal to the distance and previously deposited on and in contact with the layer made of the second material; and
 
a2) performing mechanical-chemical planarization at least up to the layer made of the second material, a portion of the layer made of the first material left in place in the cavity forming the waveguide.
 
     According to one embodiment, the method further comprises, after step a), a step consisting of depositing another layer made of the second material on and in contact with one or several exposed faces of the waveguide, or on and in contact with another intermediate layer with thickness equal to the distance and previously deposited on and in contact with one or several exposed faces of the waveguide. 
     According to one embodiment, the method further comprises, after step a), a step consisting of depositing a second layer made of the third material and a step consisting of forming electrically conductive vias through the second layer made of the third material up to the second material. 
     According to one embodiment, the first material is silicon nitride and the second material is indium tin oxide, or amorphous carbon, the wavelength preferably being between 450 nm and 1 μm, for example substantially equal to 905 nm, preferably equal to 905 nm. 
     Another embodiment provides a phase modulator comprising: 
     a waveguide from a first material, the waveguide being configured to propagate an optical signal; and 
     a layer made of a second electrically conductive material and that is transparent to a wavelength of the optical signal, the layer made of the second material being in contact with at least one of the faces of the waveguide, or separated from the at least one of the faces by a distance of less than half, preferably less than a quarter, of the wavelength of the optical signal. 
     According to one embodiment, the first material is silicon nitride and the second material is indium tin oxide, or amorphous carbon, the wavelength preferably being between 450 nm and 1 μm, for example substantially equal to 905 nm, preferably equal to 905 nm. 
     According to one embodiment, the modulator further comprises a layer made of a thermally and electrically insulating material, preferably SiOC, the layer of the second material being inserted between the waveguide and the layer of the thermally and electrically insulating material. 
     According to one embodiment, the modulator is obtained by carrying out the method described hereinabove. 
     According to one embodiment, the layer of the second material comprises an upper portion resting on an upper face of the waveguide and/or a lower portion on which a lower face of the waveguide rests. 
     According to one embodiment, the layer of the second material further comprises, for each of the side faces of the waveguide, a side portion facing said side face. 
     According to one embodiment, the layer of the second material comprises contact portions extending laterally on either side of the waveguide and each being configured to be in contact with at least one conductive via. 
     Another embodiment provides a photonic integrated circuit comprising: a semiconductor layer of the semiconductor on insulator (SOI) type, in and/or on which components are defined; 
     an interconnect structure positioned above the semiconductor layer and configured to connect the components electrically; and 
     a modulator as described above positioned in an insulating layer of the interconnect structure, the insulating layer preferably being made of silicon oxide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features and advantages, as well as others, will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
         FIG.  1    shows schematic sectional views illustrating successive steps of one embodiment of a method for manufacturing a phase modulator; 
         FIG.  2    shows schematic sectional views illustrating successive steps of another embodiment of a method for manufacturing a phase modulator; 
         FIG.  3    shows a schematic sectional view illustrating a step of an embodiment variant of the method of  FIG.  3   ; 
         FIG.  4    shows schematic sectional views illustrating successive steps of another embodiment of a method for manufacturing a phase modulator; 
         FIG.  5    shows schematic sectional views illustrating successive steps of another embodiment of a method for manufacturing a phase modulator; 
         FIG.  6    shows schematic sectional views illustrating successive steps of another embodiment of a method for manufacturing a phase modulator; 
         FIG.  7    shows schematic sectional views illustrating successive steps of another embodiment of a method for manufacturing a phase modulator; and 
         FIG.  8    shows a schematic sectional view of an embodiment of a photonic integrated circuit comprising a phase modulator. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties. 
     For the sake of clarity, only the operations and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, the operation of the photonic integrated circuits has not been described in detail, the described embodiments being compatible with the operation of the typical photonic integrated circuits, in particular the typical photonic integrated circuits comprising a phase modulator. Furthermore, the various applications in which a photonic integrated circuit can be provided, in particular a circuit comprising a phase modulator, have not been described, the described embodiments being compatible with the typical applications in which such a photonic integrated circuit is provided. An exemplary circuit in which a modulator as described below can be provided is an optical phased array (OPA). 
     Unless indicated otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements. 
     In the following disclosure, unless indicated otherwise, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “higher”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the figures. 
     Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%. 
     The inventors here proposed to produce a device comprising an optical waveguide made of a first material and a heating element in the form of one or several portions of layers made of a second material, the heating element being directly in contact with the waveguide of the device or at a small distance from the waveguide, for example at a distance smaller than half, preferably than a quarter, of the wavelength of the optical signal that the waveguide is configured to propagate. More specifically, the heating element covers at least one face of the waveguide. 
     In the remainder of the description, as a non-limiting example, the case is considered of a phase modulator comprising a waveguide made of silicon nitride configured to guide an optical signal whose wavelength is between 450 nm and 1 μm, for example substantially equal to 905 nm, preferably equal to 905 nm. 
       FIG.  1    shows schematic sectional views A, B and C illustrating successive steps of one embodiment of a method for manufacturing a phase modulator  1  (view C of  FIG.  1   ). The sectional views A, B and C are taken in a section plane orthogonal to the longitudinal direction of the modulator, that is to say, in a section plane orthogonal to the propagation direction of an optical signal in the modulator  1 . 
     In the step illustrated by view A of  FIG.  1   , a waveguide  100  made of the first material, for example from silicon nitride, has been formed on a first layer  102  made of a third material, for example silicon oxide. In this example, the waveguide  100  is a strip waveguide, or in other words, is devoid of lateral fins as is the case for a rib or ridge waveguide. 
     In this embodiment, the waveguide  100  is formed by depositing, on the entire upper surface of the layer  102 , a layer  104  made of the first material, then the waveguide  100  is defined in the layer  104  by successive steps for masking the layer portion  104  configured to form the waveguide  100 , and removing, by etching, non-masked portions of the layer  104 . 
     According to one embodiment, the layer  104  is formed directly on the layer  102 . The waveguide  100  is then in contact with the layer  102 , and more specifically, the lower face of the waveguide  100  is in contact with the upper face of the layer  102 . 
     In the step illustrated by view B of  FIG.  1   , a layer  106  made of the second material has been deposited on and in contact with the exposed surfaces of the waveguide  100 . More specifically, the layer  106  comprises a portion  106   s  covering the upper face of the waveguide  100 , the portion  106   s  being in contact with the upper face of the waveguide, and two side portions  1061  covering the two side faces of the waveguide  100 , each portion  106  being in contact with a respective side face of the waveguide  100  and with the portion  106   s.    
     In this embodiment, the layer  106  further comprises two portions  106   c  resting on the layer  102 , respectively on either side of the waveguide  100 . In section planes transverse to the waveguide  100 , that is to say, section planes orthogonal to the longitudinal direction of the waveguide  100 , each portion  106   c  extends laterally from a corresponding portion  1061 . 
     Preferably, the layer  106  is deposited consistently over the entire exposed surface of the structure illustrated by view A of  FIG.  1    (solid plate deposition) and an etching mask is formed on the portions  106   s ,  106   l  and  106   c  of the layer  106  that one wishes to leave in place. The exposed (non-masked) portions of the layer  106  are next removed by etching. 
     The material of the layer  106 , or second material, is electrically conductive. Furthermore, the second material is transparent to the wavelength of the signal configured to be propagated in a guided manner in the waveguide  100 . In the remainder of the description, it is considered for example than a material is transparent to a given wavelength if, at this wavelength, the imaginary part of the refraction index of the material is less than or equal to 0.01, preferably less than or equal to 0.005. 
     Preferably, the real part of the refraction index of the second material is less than or equal to that of the refraction index of the material of the waveguide  100 , which participates in the guided propagation of an optical signal in the waveguide  100 . 
     In this example where the waveguide  100  is made of silicon nitride and is configured to guide a signal with a wavelength between 450 nm and 1 μm, for example substantially equal to 905 nm, preferably equal to 905 nm, the second material is preferably indium tin oxide (ITO), or for example amorphous carbon. 
     In the step illustrated by view C of  FIG.  1   , a second layer  108  made of the third material has been deposited so as to cover the structure illustrated by view B in  FIG.  1   . Thus, the waveguide  100  and the layer  106  are embedded in a layer of the third material comprising, preferably constituted by, the first layer  102  and the second layer  108  made of the third material. 
     Preferably, the layer  108  is deposited over the entire structure illustrated by view B of  FIG.  1    (solid plate deposition). Preferably, the layer  108  is deposited with a thickness greater than that of the stack of the waveguide  100  and the portion  106   s  covering the upper face of the waveguide  100 . Preferably, a step for planarizing the upper face of the layer  108 , for example by chemical mechanical polishing (CMP), is provided after the deposition of the layer  108 . 
     Furthermore, in the step illustrated by view C of  FIG.  1   , the deposition of the layer  108  is followed by a step for forming electrically conductive vias  110 , for example metallic vias  110 . The vias  110  are formed on either side of the waveguide  100  considered in the direction of its length. In other words, in section planes transverse to the waveguide  100 , one via  110  is formed on one side of a first side face of the waveguide  100 , for example on the right in view C, and another via  110  is formed on the side of the other side face of the waveguide  100 , for example on the left in view C. The vias  110  are formed through the layer  108 , up to the portions  106   c  of the layer  106 . More accurately, at least one via  110  is in contact with the portion  106   c  positioned on one side of the waveguide  100 , for example on the right in view C, and at least one other via  110  is in contact with the portion  106   c  positioned on the other side of the waveguide  100 , for example on the left in view C. 
     During operation, when a voltage is applied between two vias  110  respectively positioned on either side of the waveguide  100 , a current circulates in the layer  106 . This results in heating of the layer  106 , therefore an increase in the temperature of the waveguide  100 . This causes a modification of the optical index of the waveguide  100 , and therefore a modulation of the optical signal propagating therein. Preferably, the voltage is applied between one or several first vias  110  positioned at a first end of the waveguide  100  of the modulator taken in the direction of its length, and one or several second vias  110  positioned at a second end of the waveguide  100  of the modulator taken in the direction of its length, such that the current circulates along the modulator, in a direction substantially parallel to the longitudinal direction of the waveguide  100 . 
     In this embodiment, the layer  106  thus forms the heating element of the modulator  1 . Furthermore, in this embodiment, three faces (side faces and upper face) of the waveguide  100  are covered by the layer  106  and are in contact with this layer  106 . In other words, in this embodiment, the heating element of the modulator  1  comprises an upper portion  106   s  in contact with the upper face of the waveguide  100 , two side portions  1061  in contact with the respective side faces of the waveguide  100 , and two contact portions  106   c  in contact with respective vias  110  configured to receive a control voltage of the modulator. 
     Relative to the case of a modulator where the heating element is metallic, the heating element  106  of the modulator  1  is transparent to the considered wavelengths and can therefore be positioned directly in contact with at least one face of the waveguide  100  of the modulator. As a result, for a same power supplied to the heating element, the modulator  1  allows greater modulation efficiency than a modulator having an identical waveguide and having the same length, measured in the propagation direction of an optical signal in the waveguide, but a metallic heating element necessarily positioned at a distance from the waveguide so as not to disrupt the signal propagating in the waveguide. 
     Furthermore, the modulator  1  is more compact than a modulator having an identical waveguide, but a metallic heating element necessarily positioned at a distance from the waveguide so as not to disrupt the signal propagating in the waveguide. In particular, for a given modulation efficiency, and for a given power supplied to the heating element, the modulator  1  is not as long as a modulator having an identical waveguide, but a heating element positioned at a distance from the waveguide. 
     In an embodiment variant that is not illustrated, in the step of view B, a layer of a fourth thermally and electrically insulating material, for example SiOC, is deposited on the layer  106 . This makes it possible to limit, or even eliminate, a temperature increase in a waveguide positioned in the vicinity of the modulator  1 , during heating of the layer  106 . 
       FIG.  2    shows schematic sectional views A, B and C illustrating successive steps of another embodiment of the method of  FIG.  1   . The sectional views A, B and C are taken in a section plane orthogonal to the longitudinal direction of the modulator  1  (view C of  FIG.  2   ). 
     In this embodiment, rather than depositing a layer  106  made of the second material after defining the waveguide  100  in the layer  104 , a layer  200  made of the second material is deposited before the deposition of the layer  104 , the layer  104  then being formed on and in contact with this layer  200 . 
     In the step of view A of  FIG.  2   , the layer  200  has been formed on the layer  102 , then the layer  104  has been formed on and in contact with the layer  200 . As an example, the layers  200  and  104  are formed by deposition, preferably by solid plate deposition. 
     According to one embodiment, the layer  200  is formed directly on the layer  102 , the layer  200  then being in contact with the layer  102 . 
     In the step illustrated by view B of  FIG.  2   , the waveguide  100  has been defined in the layer  104 , for example by successive masking and etching steps of the layer  104  that are similar to those described in relation with view A of  FIG.  1   . The etching of the layer  104  here is stopped on the layer  200 . 
     Furthermore, in the step illustrated by view B of  FIG.  2   , portions of the layer  200  have been removed by etching. During this etching, a portion  200   i  of the layer  200  is left in place below the lower face of the waveguide  100 , and two portions  200   c  of the layer  200  are left in place on either side of the waveguide  100 . In section planes transverse to the waveguide  100 , each portion  200   c  extends laterally on a different side of the waveguide  100 , from the portion  200   i . As an example, the etching mask used for this step covers the waveguide  100  and overflows laterally, on the layer  200 , on either side of the waveguide  100 . 
     The view C of  FIG.  2    illustrates the obtained structure after the implementation of the steps described in relation with the view C of  FIG.  1   , from the structure illustrated by the view B of  FIG.  2   , the conductive vias  110  here being formed up to the portions  200   c  rather than up to the portions  106   c  as had been described in relation with  FIG.  1   . 
     In this embodiment, the heating element comprises a lower portion  200   i  in contact with the lower face of the waveguide  100 , and two contact portions  200   c  in contact with the vias  110 . In this embodiment, a single face (lower face) of the waveguide  100  is covered by the heating element. 
     The operation of the modulator of  FIG.  2    is similar to that of  FIG.  1   . Furthermore, relative to a modulator comprising a metallic heating element, the modulator  1  of the view C of  FIG.  2    benefits from the same advantages as the modulator  1  of the view C of  FIG.  1   . 
     In an embodiment variant that is not illustrated, a layer made of the fourth material is deposited on the layer  102  prior to the deposition of the layer  200 , the layer  200  being deposited on and in contact with this layer made of the fourth material, and/or a layer of the fourth material is deposited, before the steps of view C, on the portions  200   c  and on the waveguide  100 . Like before, this makes it possible to limit, or even eliminate, a temperature increase in a waveguide positioned in the vicinity of the modulator  1  when a current circulates in the heating element of the modulator. 
       FIG.  3    shows a schematic sectional view illustrating a step of an embodiment variant of the method of  FIG.  2   , and more particularly of the step described in relation with view A of  FIG.  2   . The sectional view of  FIG.  3    is taken in a section plane orthogonal to the longitudinal direction of the modulator. 
     In this variant, the layer  200  is etched prior to the deposition of the layer  104 , so as only to leave the portions  200   i  and  200   c  of the layer  200  in place. 
     The other steps of this variant are similar or identical to those described in relation with  FIG.  2    and lead to obtaining the modulator  1  as illustrated by view C of  FIG.  2   . 
       FIG.  4    shows schematic sectional views A, B and C illustrating successive steps of another embodiment of a method for manufacturing a modulator  1  (view C of  FIG.  4   ). The section views A, B and C are taken in a section plane orthogonal to the longitudinal direction of the modulator  1 . 
     This embodiment corresponds to a combination of the embodiment described in relation with  FIG.  1   , and the embodiment described in relation with  FIGS.  2  and  3   . In other words, in this embodiment, it is provided to deposit the layer  200  before forming the waveguide  100 , then to deposit the layer  106  after forming the waveguide  100 . 
     In the step illustrated by view A of  FIG.  4   , the steps described in relation with views A and B of  FIG.  2    have been implemented, with the difference that only the portion  200   i  of the layer  200  has been left in place. As an example, the same etching mask is used to define the waveguide  100  in the layer  104  and to etch the layer  106 . 
     The view B of  FIG.  4    illustrates the obtained structure after the implementation of the steps described in relation with the view B of  FIG.  1   , from the structure illustrated by the view A of  FIG.  4   . 
     The view C of  FIG.  4    illustrates the obtained structure after the implementation of the steps described in relation with the view C of  FIG.  1   , from the structure illustrated by the view B of  FIG.  4   . 
     In this embodiment, the heating element of the modulator  1  comprises a lower portion  200   i  in contact with the lower face of the waveguide  100 , an upper portion  106   s  in contact with the upper face of the waveguide  100 , two side portions  1061  in contact with the respective side faces of the waveguide  100 , and two contact portions  106   c  in contact with the vias  110 . The portions  106   s  and  200   i  are electrically linked in parallel by the portions  1061 . 
     The operation of the modulator  1  of  FIG.  4    is similar to that of  FIG.  1    or  FIG.  2   . However, the waveguide  100  of the modulator  1  of  FIG.  4    comprises more faces in contact with the heating element than the modulator  1  of  FIG.  1    or  FIG.  2   . Thus, the modulator  1  of  FIG.  4    allows, for a same voltage applied between the vias  110 , a greater modulation efficiency than the modulator  1  of  FIG.  1    or of  FIG.  2   . Furthermore, the modulator  1  of  FIG.  4    allows an increase of the temperature in the waveguide  100  that is more homogeneous than in the waveguide  100  of the modulator  1  of  FIG.  1  or  2   . 
     Moreover, relative to a modulator comprising a metallic heating element, the modulator  1  of  FIG.  4    benefits from the same advantages as the modulator  1  of  FIG.  1    or  FIG.  2   . 
     In an embodiment variant that is not illustrated, the thickness of the layers  200  and  106  is configured to balance the resistance of a conductive path comprising the portions  1061 ,  106   s  of the layer  106  in contact with the waveguide  100 , with that of a conductive path comprising the portion  200   i  of the layer  200  in contact with the waveguide  100 . This makes it possible to heat the waveguide  100  more homogeneously. 
     In another embodiment variant that is not illustrated, a layer made of the fourth material is deposited on the layer  102  prior to the deposition of the layer  200 , preferably on and in contact with the layer  200 , and/or a layer made of the fourth material is deposited on the layer  106 , preferably in contact with the layer  106 . Like before, this makes it possible to limit, or even eliminate, a temperature increase in a waveguide positioned in the vicinity of the modulator  1  when a current circulates in the heating element of the modulator. 
     Embodiments and variants have been described, in relation with  FIGS.  1  to  4   , in which the waveguide  100  is defined, for example by masking and etching steps, in a layer  104  made of the material of the waveguide  100 . In these embodiments and variants, the layer  200  is deposited before the formation of the waveguide  100  and/or the layer  106  is deposited after the formation of the waveguide  100  so as to form, from layers  106  and/or  200 , a heating element, or heating layer, in contact with at least one longitudinal face of the waveguide. 
     Other embodiments and variants will now be described, in which the waveguide  100  is formed by successive steps for etching a cavity, depositing a layer made of the material of the waveguide to fill the cavity, and CMP up to the top of the cavity. Like before, a layer  200  is deposited before the formation of the waveguide  100  and/or a layer  106  is deposited after the formation of the waveguide  100  so as to form, from layers  106  and/or  200 , a heating element, or heating layer, in contact with at least one longitudinal face (side, upper or lower) of the waveguide  100 . 
       FIG.  5    shows schematic section views A, B, C and D illustrating successive steps of another embodiment of a method for manufacturing the phase modulator  1  (view D of  FIG.  5   ). The section views A, B, C and D are taken in a section plane orthogonal to the longitudinal direction of the modulator  1 . 
     In the step illustrated by view A of  FIG.  5   , a cavity  500  has been etched in the layer  102 . The dimensions of the cavity are determined by those of the waveguide  100  (absent in view A of  FIG.  5   ) that will be formed therein. Preferably, the dimensions of the cavity  500  are substantially equal to those of the waveguide  100 . 
     Preferably, the cavity  500  is etched from the upper face of the layer  102 , over only part of the thickness of the layer  102 , such that part of the thickness of the layer  102  remains below the bottom of the cavity  500 . 
     In the step illustrated by view B of  FIG.  5   , the layer  200  made of the second material has been deposited, preferably over the entire exposed surface of the layer  102  (solid plate deposition), preferably consistently, so as to cover the bottom and the walls of the cavity  500 . The layer  104  has next been deposited so as to fill the cavity  500 . The layer  104  is deposited on and in contact with the layer  200 . 
     In the step illustrated by view C of  FIG.  5   , the portions of the layer  104  positioned above the upper level of the cavity  500 , that is to say, here, the upper level of the layer  200 , have been removed by CMP up to the layer  200 . The portion of the layer  104  left in place in the cavity  500  then forms the waveguide  100  of the modulator. The lower face and the side faces of the waveguide  100  are then covered by the portions of the layer  200  left in place in the cavity  500 . More particularly, the lower face of the waveguide  100  is in contact with a portion  200   i  of the layer  200 , the side faces of the waveguide  100  being in contact with respective portions  200   l  of the layer  200 . The portions  200   l  are in contact with the portion  200   i.    
     Furthermore, in the step illustrated by view C of  FIG.  5   , portions of the layer  200  resting on the layer  102 , that is to say, portions of the layer  200  positioned outside the cavity  500 , have been removed by etching, leaving in place, on the layer  102 , portions  200   c  of the layer  200  positioned, in planes transverse to the waveguide  100 , respectively on either side of the waveguide  100 . Each portion  200   c  left in place is in contact with a corresponding portion  200   l  or, in other words, each portion  200   c  extends laterally from a corresponding portion  200   l.    
     As an example, this step for definition by etching of the portions  200   c  in the layer  200  is done using an etching mask covering the upper face of the waveguide  100  and laterally overflowing on the layer  200 , and more specifically on the portions  200   c  of the layer  200 . 
     View D of  FIG.  5    illustrates the structure obtained after the implementation of the steps described in relation with the view C of  FIG.  1   , from the structure illustrated by the view C of  FIG.  5   , with the difference that the vias  110  are formed up to the portions  200   c  rather than up to portions  106   c.    
     In this embodiment, the heating element of the modulator  1  comprises a lower portion  200   i  in contact with the lower face of the waveguide  100 , two side portions  200   l  in contact with the respective side faces of the waveguide  100 , and two contact portions  200   c  in contact with the vias  110 . A first portion  200   c , a first portion  200   l , the portion  200   i , a second portion  200   l  and a second portion  200   c  are linked in series between the vias  110 . 
     The operation of the modulator of  FIG.  5    is similar to that of  FIG.  1   . Furthermore, relative to a modulator comprising a metallic heating element, the modulator  1  of  FIG.  5    benefits from the same advantages as the modulator  1  of  FIG.  1   . 
     In an embodiment variant that is not illustrated, a layer made of the fourth material is deposited on the layer  102  prior to the deposition of the layer  200 , the layer  200  being deposited on and in contact with this layer made of the fourth material, and/or a layer of the fourth material is deposited, between the steps of view C and those of view D, on the portions  200   c  and on the waveguide  100 . Like before, this makes it possible to limit, or even eliminate, a temperature increase in a waveguide positioned in the vicinity of the modulator  1  when a current circulates in the heating element of the modulator. 
       FIG.  6    shows schematic section views A and B illustrating successive steps of another embodiment of a method for manufacturing the modulator  1  (view B of  FIG.  6   ). The section views A and B are taken in a section plane orthogonal to the longitudinal direction of the modulator  1 . 
     In this embodiment, rather than depositing a layer  200  before the formation of the waveguide  100  in the cavity  500 , a layer  106  made of the third material is deposited after the formation of the waveguide  100  in the cavity  500 . 
     In the step illustrated by view A of  FIG.  6   , the cavity  500  (not referenced in  FIG.  6   ) has been etched in the layer  102  similarly to what was described in relation with view A of  FIG.  5   , then the layer  104  has been deposited to fill the cavity  500  similarly to what was described in relation with view B of  FIG.  5   . The portions of the layer  104  positioned above the upper level of the cavity  500 , that is to say, here, the upper level of the layer  102 , have next been removed by CMP up to the layer  102 . The portion of the layer  104  left in place in the cavity  500  then forms the waveguide  100 . 
     Furthermore, in the step illustrated by view A of  FIG.  6   , the layer  106  has been deposited on the exposed face of the structure, that is to say, here, the upper face of the waveguide  100  and the exposed face of the layer  102 . 
     Portions of the layer  106  have next been removed by etching. The etching is done so as to leave in place a portion  106   s  of the layer  106  covering the upper face of the waveguide  100  and, on either side of the waveguide  100 , portions  106   c  of the layer  106 . The portion  106   s  is in contact with the upper face of the waveguide. Each portion  106   c  extends laterally from the portion  106   s.    
     The view B of  FIG.  6    illustrates the obtained structure after the implementation of the steps described in relation with the view C of  FIG.  1   , from the structure illustrated by the view B of  FIG.  6   . 
     In this embodiment, the heating element of the modulator  1  comprises contact portions  106   c  and an upper portion  106   s . A first portion  160   c , the portion  106   s , and a second portion  106   c  are linked in series between the vias  110 . 
     The operation of the modulator of  FIG.  6    is similar to that of  FIG.  1   . Furthermore, relative to a modulator comprising a metallic heating element, the modulator  1  of  FIG.  6    benefits from the same advantages as the modulator  1  of  FIG.  1   . 
     In an embodiment variant that is not illustrated, a layer made of the fourth material is deposited on the layer  106  prior to the deposition of the layer  108  and/or a layer made of the fourth material is deposited (solid plate) on the layer  102  between the etching of the cavity  500  and the deposition of the layer  104 . In this last case, after the deposition of the layer  104 , the CMP step can be stopped on the layer of the fourth material deposited on the layer  102 , or on the layer  102 . Like before, the deposition of one or two layers of the fourth material makes it possible to limit, or even eliminate, a temperature increase in a waveguide positioned in the vicinity of the modulator  1  when a current circulates in the heating element of the modulator. 
       FIG.  7    shows schematic section views A, B, C and D illustrating successive steps of another embodiment of a method for manufacturing a modulator  1  (view D of  FIG.  7   ). The section views A, B, C and D are taken in a section plane orthogonal to the longitudinal direction of the modulator  1 . 
     This embodiment corresponds to a combination of the embodiment described in relation with  FIG.  5   , and the embodiment described in relation with  FIG.  6   . In other words, in this embodiment, it is provided to deposit the layer  200  before forming the waveguide  100 , then to deposit the layer  106  after forming the waveguide  100 . 
     Views A and B of  FIG.  7    are identical to respective views A and B of  FIG.  5    and illustrate the same steps as those described in relation with respective views A and B of  FIG.  5   . 
     In the step illustrated by view C of  FIG.  7   , the portions of layers  200  and  104  positioned above the upper level of the cavity  500 , that is to say, here, above the upper level of the layer  102 , have been removed by CMP up to the layer  102 . The portion of the layer  104  left in place in the cavity  500  then forms the waveguide  100  of the modulator. The lower face and the side faces of the waveguide  100  are then covered by the portions of the layer  200  left in place in the cavity  500 . More particularly, the lower face of the waveguide  100  is in contact with a portion  200   i  of the layer  200 , the side faces of the waveguide  100  being in contact with respective portions  200   l  of the layer  200 . The portions  200   l  are in contact with the portion  200   i.    
     It will be noted that relative to the embodiment described in relation with  FIG.  5   , the portions  200   c  of the layer  200  are removed during the CMP step. 
     Still in the step illustrated by view C of  FIG.  7   , the layer  106  has next been formed over the entire exposed surface of the structure, that is to say, here, the upper face of the waveguide  100  and the exposed face of the layer  102 . 
     Portions of the layer  106  have next been removed by etching. The etching is done so as to leave in place a portion  106   s  of the layer  106  covering the upper face of the waveguide  100  and, on either side of the waveguide  100 , portions  106   c  of the layer  106 . The portion  106   s  is in contact with the upper face of the waveguide. Each portion  106   c  extends laterally from the portion  106   s . Furthermore, the apex of each portion  200   l  of the layer  200  is in contact with a corresponding portion  106   c.    
     View D of  FIG.  7    illustrates the obtained structure after the implementation of the steps described in relation with the view C of  FIG.  1   , from the structure illustrated by the view C of  FIG.  7   . 
     In this embodiment, the heating element of the modulator  1  comprises two side portions  200   l  covering the respective side faces of the waveguide  100 , a lower portion  200   i  covering the lower face of the waveguide  100 , an upper portion  106   s  covering the upper face of the waveguide  100  and two contact portions  106   c  in contact with the vias  110 . The portion  200   i  electrically links the portions  200   l  to one another, the portions  200   l  and  200   i  being connected in series. Furthermore, between the portions  106   c , the set of portions  200   i  and  200   l  is connected in parallel with the portion  106   s , the portions  106   c  each being connected to a different via  110 . 
     The operation of the modulator of  FIG.  7    is similar to that of  FIG.  1   . Furthermore, relative to a modulator comprising a metallic heating element, the modulator  1  of  FIG.  7    benefits from the same advantages as the modulator  1  of  FIG.  1   . 
     In an embodiment variant that is not illustrated, a layer made of the fourth material is deposited on the layer  102 , and in particular in the cavity  500 , prior to the deposition of the layer  200  and/or a layer made of the fourth material is deposited on the layer  106 , prior to the deposition of the layer  108 . Like before, this makes it possible to limit, or even eliminate, a temperature increase in a waveguide positioned in the vicinity of the modulator  1  when a current circulates in the heating element of the modulator. 
       FIG.  8    shows a section view illustrating an embodiment of a photonic integrated circuit  8  comprising a modulator  1  as previously described. More specifically, in this example, the modulator  1  is of the type of that illustrated by view C of  FIG.  4   . 
     The circuit  8  comprises a substrate  800  covered by an insulating layer  802 , in turn covered by a semiconductor layer  804 , for example monocrystalline silicon. The layer  804  and the layer  802  form a structure of the semiconductor on insulator (SOI) type. 
     Various components  806  have been formed from the layer  804 , for example by conventional masking, etching, epitaxy and/or doping steps. In other words, the components are formed or positioned in and/or on the layer  804 . In the illustrated example, the circuit comprises a photodiode  806 , for example a photodiode made of germanium epitaxied from the layer  804 , or a silicon photodiode formed in a silicon layer  804 . 
     Waveguides  808  have also been defined in the layer  804 , for example by conventional masking and etching steps of the layer  804 . 
     In the illustrated example, the circuit  8  comprises, from left to right in  FIG.  8   , a strip waveguide  808 , the photodiode  806  and a rib or ridge waveguide  808 . 
     The components  806  and the waveguides  808  are covered by an insulating layer  810 , for example of silicon oxide. 
     The circuit  8  further comprises an interconnect structure  812  resting on the layer  810 . The interconnect structure  812  comprises portions of metallic layers  814  separated from one another by insulating layers  816 . The interconnect structure  812  further comprises electrically conductive vias  818  passing through layers  816  in order to connect portions of metallic layers  814  to one another, to contact pads  820  positioned at the upper face of the interconnect structure  812  and/or to components  806 . 
     According to one embodiment, the modulator  1  is positioned in an insulating layer  816  of the interconnect structure  812 , preferably a layer  816  positioned between the layer  810  and the lower metal level of the interconnect structure  812 . As an example, the lower metal level comprises the metallic layer portions  814  positioned at a same level and which are closest to the layer  804 . 
     The layer  816  in which the modulator  1  is positioned comprises, preferably is made up of, the layers  102  and  108  previously described (not shown in  FIG.  8   ). 
     The interconnect structure  812  comprises the vias  110  of the modulator  1 . The vias  110  are electrically linked to metallic layer portions  814  of the interconnect structure  812 , for example portions  814  of the lower metal level. 
     As an example, the circuit  8  is obtained by carrying out the following steps: 
     forming components  806  and/or waveguides  808  from a layer  804  of the SOI type; and 
     forming an interconnect structure  812  above the layer  804 , forming the interconnect structure  812  comprising the step consisting of forming a layer  102  (not shown in  FIG.  8   ) and carrying out one of the methods described hereinabove in relation with  FIGS.  1  to  7    such that the layers  102  and  108  (not shown in  FIG.  8   ) correspond to an insulating layer  816  of the interconnect structure  812 , that is to say, form an insulating layer  816  of this interconnect structure. 
     According to one embodiment, the circuit  8  is configured to carry out a light detection and ranging (LIDAR) function. 
     Described hereinabove, in relation with  FIGS.  1  to  8   , are embodiments and variants in which the modulator  1  comprises a waveguide  100  made of silicon nitride embedded in insulating layers  102  and  108 , for example made of silicon oxide, and a heating element, for example made of ITO, in contact with at least one face of the waveguide. These embodiments and variants can be adapted to the case of a modulator  1  comprising a waveguide made of another material, by modifying the material of the heating element, as long as the material of the heating element remains electrically conductive and transparent to the wavelengths of a signal configured to be propagated in the waveguide. 
     As an example, the waveguide  100  can be made of monocrystalline silicon in a layer of the SOI type, preferably by carrying out the method described in relation with  FIG.  1   , the layer  102  for example corresponding to a layer of silicon oxide on which the SOI-type layer rests and/or the material of the layer  106  for example being ZnO, Cd 2 SnO 4  or amorphous carbon. 
     Furthermore, although examples of embodiments and variants have been described in which the waveguide  100  is a strip waveguide, one skilled in the art is able to adapt these embodiments and variants to the case of a rib or ridge waveguide  100 . 
     Moreover, embodiments and variants have been described in which the heating element is in direct contact with one or several faces of the waveguide. These embodiments and variants can be adapted to the case where the heating element is not in direct contact with these faces of the waveguide, but remains positioned at a distance from the waveguide smaller than half, preferably than a quarter, of the wavelength of the signal that the waveguide propagates. 
     For example, it is possible to provide that the heating element is separated from one or several faces of the waveguide by an intermediate layer with a thickness smaller than half, preferably than a quarter, of the wavelength of the optical signal that the waveguide propagates. In other words, the intermediate layer comprises a first face in contact with the waveguide and a second face, opposite the first, in contact with the heating element, the thickness of the intermediate layer being measured between its first and second faces. Providing such an intermediate layer can make it possible for the heating element to disrupt the optical signal propagated in the waveguide less relative to the case where the heating element is in contact with the waveguide. The intermediate layer is preferably made up of a single layer. The intermediate layer is preferably made of a material with an optical index lower than that of the first material, for example from the third material. 
     One skilled in the art is able to adapt the described embodiments and variants to the case where such an intermediate layer is provided, in particular to provide the step for depositing this intermediate layer. In particular, an intermediate layer can be deposited on and in contact with the waveguide before depositing a layer of the second material on and in contact with the intermediate layer, and/or an intermediate layer can be deposited on and in contact with a layer made of the second material, before forming the waveguide on and in contact with the intermediate layer. 
     Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art. 
     Finally, the practical implementation of the embodiments and variants described herein is within the capabilities of those skilled in the art based on the functional description provided hereinabove. In particular, one skilled in the art is able to determine the length of each portion  106   c  and  200   c , for example measured from side faces of the waveguide  100 , for example in a direction orthogonal to these faces, and the distance between the side faces of the waveguide  100  and the conductive vias  110 , for example measured in a direction orthogonal to these faces, so that the metallic vias  110  do not disrupt the propagation of an optical signal in the waveguide  100 . More generally, one skilled in the art is able to determine the various dimensions of the modulator  1 , for example the dimensions of the cross-section of the waveguide  100  as a function of the signal to be guided or the thickness of the layers  106  and  200 , from the functional description given above. To that end, one skilled in the art can use computer-assisted simulation software such as the software designated by the commercial name Lumerical.