WO1998022841A1

WO1998022841A1 - Process to create the coupling in an optoelectronic device between an optical fiber and a waveguide and device obtained from this process

Info

Publication number: WO1998022841A1
Application number: PCT/EP1997/006399
Authority: WO
Inventors: Sergio Doneda
Original assignee: Italtel S.P.A.
Priority date: 1996-11-19
Filing date: 1997-11-13
Publication date: 1998-05-28
Also published as: ITMI962410A0; ITMI962410A1; IT1286385B1

Abstract

Process to create the coupling in an optoelectronic device between a waveguide defined on the surface of a sub-layer and an optical fiber in order to determine a substantially pre-formed axial alignment in relation to all six possible degrees of freedom of the coupling. The process includes the step of forming an excavation with 'V' grooves of prefixed depth in said sub-layer by means of chemical etch through an opening defined and produced through a layer of material resistant to the solution of chemical etch deposited on the surface of said sub-layer. The definition of said opening includes the further step of pre-forming a definition structure (6d) of the excavation area on the surface of said sub-layer (1), defining it photolithographically simultaneously to said waveguide (6g) by means of a same masking process. The structure of definition is obtained from the same layer (5) as the waveguide (6g).

Description

PROCESS TO CREATE THE COUPLING IN AN OPTOELECTRONIC DEVICE BETWEEN AN OPTICAL FIBER AND A WAVEGUIDE AND DEVICE OBTAINED FROM THIS PROCESS

Field of the invention.

This invention relates to a process to create seats of self-aligned coupling between an optical fiber and a waveguide defined on the surface of a sub- layer, commonly of monocrystalline silicon. The process determines an axial alignment substantially pre-formed in relation to all six possible degrees of freedom of the coupling.

The assembly and encapsulation steps of the optoelectronic devices raises very important technical problems due to the stringent alignment tolerances required to guarantee an optically efficient coupling between interconnecting waveguides defined on the face of the device and the optical fibers.

The coupling of a single mode optical fiber to a waveguide requires an extremely accurate alignment, much more than a multimode fiber. For example, a radial misalignment of one micrometer produces an excess attenuation of 0.3 dB.

Numerous techniques have been developed to guarantee the alignment in a reliable manner, but many of these techniques need laborious quality tests of the coupling to reach the required precision, with heavy severe consequence for the cost of the devices. More recently, preparation techniques of the coupling aperture have been proposed and developed which considerably reduce the number of possible degrees of freedom, thus simplifying and making the assembly and encapsulation of the devices more reliable.

Background art. US 4,639,074 discloses a technique based on the formation of apertures with "V" grooves of depth and tapering of the walls predeterminable through a choice of the parameters of aπisotropic attack of a monocrystalline silicon wafer used as substrate of the optoelectronic device. This technique is also summarized in the US patent No. 4,904,036, concerning optoelectronic devices created on monocrystalline silicon chips using waveguides of phosphor-doped silicon bioxide defined on the surface of the silicon chip to optically interconnect the devices to the optical fibers installed in seats prearranged along a corner of the silicon chip.

US 4,902,086 discloses techniques for the definition of silicon oxide waveguides supported on the surface of a monocrystalline silicon sub-layer, using chemical deposition techniques from steam step with gaseous precursors like the silanes and phosphanes to deposit amorphous strata of silicon bioxide (phosphor-doped glass) with a pre-established refractive index.

The technique of producing excavations of predetermined geometry and dimensions in a sub-layer in order to form apertures of self-aligned coupling of an equal number of optical fibers is also described in the document EP-A-0 331 332. The creation of fairly deep apertures in substantially anisotropic mode in a sub-layer, typically of monocrystalline silicon, commonly needs a wet attack of the silicon with a strong base, using a mask chemically resistant to the alkaline solutions and therefore capable of protecting the rest of the surface.

It has also been proposed to create apertures with "tandem" "V" grooves, of different depths, a first and deeper excavation designed to form the supporting seat of the cladding of the optical fiber and a second excavation with "V grooves of lower depth, adjoining the first, suitable to form the supporting seat of the core of the optical fiber free of the cladding, according to a configuration like that illustrated in Fig. 10 of US 4,904,036 and the relevant manufacturing method described in US 4,810,557.

A fairly large depth of the apertures with "V grooves produced by chemical attack in conditions of anisotropy of attack and which may be between 20 and 150 mm, involves considerable difficulties in terms of dimensional control. These difficulties are mainly due to the fact that the anisotropic attack of the monocrystalline wafer of the sub-layer, from which the various devices will then be cut out, to produce the deep apertures with "V" grooves, takes place in an advanced step of the manufacturing process. This involves the definition of the excavation area by means of masking and attack (opening) of an optical stratum, for example of silicon oxide, commonly doped with phosphor in a percentage close to 1 -4% and with a refractive index similar to that of the cladding of the optical fiber, previously deposited on the surface of the wafer with cladding of the waveguides previously defined on the surface of the sublayer, which in turn may be formed by a silicon oxide with a higher phosphor content, for example around 5-10%, and with a refractive index substantially similar to that of the core of the optical fiber.

This layer of upper cladding of the waveguide is generally fairly thick (around 10-20 mm) and, being chemically deposited from steam step with a low temperature process in intrinsically conforming mode, produces an "unplanarized" surface of the wafer. The masking defining the attack area is therefore critical, as it must be carried out on a rough surface. Moreover, the definition precision is also subject to factors of dimensional error deriving from the necessity to produce an opening through an oxide layer of upper cladding of great thickness to discover the monocrystalline sub-layer and thus define the area to be attacked. These conditions of criticality of the definition process of the attack area of the sub-layer seriously prejudice the reliability of the process in terms of precision of the axial alignment between the optical fiber and the waveguide despite the use of sophisticated alignment equipment of the optical exposure mask used to define the excavation areas. Moreover, the fairly deep and extended etchings produced on a face of the sub-layer wafer, typically of monocrystalline silicon, to create the excavation to be filled with the optical material of lower cladding of the waveguide and subsequently create the excavations with "V grooves of self- aligned coupling of the optical fiber, produce macroscopic and extended lacks of homogeneity of the structural constraints in the thin monocrystalline wafer which cause an uncontrollable and often intolerable "bending" of the wafer. In practice, the need persists for a "definition" of the alignment through repeated quality tests of the coupling, before permanently fixing the optimal attitude reached, closing and encapsulating the optoelectronic device. Objects of the Invention Is one object of the present invention to identify a process to create coupling apertures in an optoelectronic device between a length of optical fiber and a waveguide conformed in such a way that the coupling is previously self aligned in relation to all the six degrees of freedom of this coupling. Summary of the Invention A manufacturing process has now been found and forms the object of this invention which is capable of practically completely eliminating these aspects of criticality in the definition of a deep excavation area with "V" grooves in a sub-layer to form a coupling seat for an optical fiber and therefore of overcoming the difficulties and limitations in terms of precision of execution of these coupling apertures of the known processes.

The method of the invention fundamentally consists in pre-forming a structure of definition of the excavation area of the sub-layer of the device through an open window of attack through a layer of material resistant to the chemical agent attacking the sub-layer, in a self-aligned manner to the same waveguide, i.e. by means of the same mask used to define the waveguide and the structure of definition of the excavation area of the sub-layer. This virtually eliminates each degree of freedom in the coupling definition, as the definition tolerances of the different parts of an optical mask of exposure for photolithography are practically negligible. In practice, in conditions of self-alignment of the window of chemical attack of the sub-layer, this structure of definition (or predefinition) is restarted from the same layer of optical glass of which the same waveguide is recovered.

According to an essential aspect of the process of the invention, this structure of predefinition of the excavation area of the coupling seat of the optical fiber is formed above a layer of lower cladding material of the waveguide and the subsequent stage of attack of opening of the underlying layer of material resistant to the chemical attack of excavation of the coupling seat, previously deposited on the face of the monocrystalline sub-layer, is carried out by means of R.I.E. (Reactive Ion Etching) attack of the layer of lower cladding and the underlying layer of material resistant to the chemical agent attacking the monocrystalline sub-layer, in conditions of accentuated anisotropy. These conditions of anisotropy of attack make it possible to maintain a high precision in the definition of the excavation area despite the fact that said structure of predefinition is formed on the surface of a stack formed by the chemically resistant layer of material and the layer of lower optical cladding material of the waveguide.

During this step of anisotropic attack in R.I.E. plasma, the waveguide predefined above the layer of lower cladding material is protected by a specially deposited and defined layer of a polymer sufficiently resistant to the R.I.E. attack, for example a polyacrylate. According to another aspect of the invention, the waveguide is defined on said layer of lower cladding material at a higher level, defining in the formation area of the waveguide a base composed of a buffer layer of oxide, deposited on top of the chemically resistant layer which covers the surface of the monocrystalline sub-layer and a first layer of lower cladding material. In this way, the monocrystalline sub-layer is etched in fairly deep manner exclusively in the areas of the coupling seats of the optical fibers so as not to develop internal stress or limit its extent, thus effectively preventing bending problems of the wafer during the manufacturing process of the devices. The formation of the waveguides at a fairly high level to the plane of the monocrystalline sub-layer also contributes to this purpose, thus reducing the required depth of the "V" grooves to be produced in the coupling seats of the fibers.

In the case of a sub-layer of monocrystalline silicon, which may therefore be attacked in anisotropic manner through contact with a strongly basic solution, the "masking" of attack, or the layer deposited directly on the surface of the sub-layer, may be of a material sufficiently resistant to the chemical attack from the strongly basic solution used. Silicon nitride possesses these requisites and is therefore particularly suitable for the purpose.

The object of this invention therefore consists of a process to create the coupling in an optoelectronic device between a waveguide defined on the surface of a sub-layer and an optical fiber, axially aligned to each other, which includes the step of forming an aperture with "V" grooves of prefixed depth in said sub-layer by means of chemical attack, characterised by the fact that it includes the further step of pre-forming a structure of definition of the excavation area on the surface of said layer defining it photolithographically simultaneously to said waveguide by means of a same masking process, said structure of definition being obtained from the same layer of a material of said waveguide deposited above a lower layer of cladding material of the waveguide.

The object of this invention also includes a process to create at least one coupling seat of a length of optical fiber on an optoelectronic device in axial alignment with a waveguide defined above a sub-layer of the device, characterised by the fact that it includes the steps of: a) depositing on the surface of said sub-layer a first protective layer of a material resistant to the attack by chemical weapons of wet attack of said sub- layer; b) depositing a second layer of a material with different characteristics of attackability in plasma from those of the material of said first protective layer and similar to those of an optical glass; c) chemically depositing from steam step a third layer of optical glass with suitable refractive index to form a lower cladding of the waveguide; d) defining a formation area of said waveguide by means of a first masking and attack operation to remove said second and third layers from the surface external to said area; e) chemically depositing from steam step a fourth layer of optical glass with suitable refractive index to form a lower cladding of the waveguide; f) depositing a fifth layer of optical glass with a suitable refractive index to form said waveguide; g) defining by means of a second stage of masking and attack in plasma said fifth layer of optical glass, defining said waveguide inside said area and at least one frame of definition of the excavation area of at least one coupling seat external to and adjoining said area; h) depositing and defining a sixth layer of polymeric material to protect the waveguide in said area; i) attacking in conditions of high anisotropy said fourth layer of optical glass in the zones not masked by said sixth protective layer and by said frame of definition of said third layer of optical glass and continuing the attack through the underlying first layer of protective material until the surface of the sub-layer in the area thus defined by said frame is uncovered;

I) depositing a seventh layer of optical glass with a suitable refractive index to form an upper layer of cladding of said waveguide; m) masking said area and wet attacking said seventh layer of optical glass removing it from the surface extant to said area; n) wet etching said sub-layer in isotropic manner through the aperture defined in said first protective layer to form a "V" shaped aperture adapted to support said length of optical fiber.

The object of this invention also includes an optoelectronic device composed of a silicon sub-layer on a portion of which waveguides are created and on at least one further portion of which "V" grooves are created by means of chemical and wet isotropic etch, characterised by the fact that it includes a layer of masking with a window coinciding with the excavation area on the surface of said sub-layer photolitographically defined in self-alignment to said waveguide by means of a single masking process.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. Brief description of the drawings The invention, together with further objects and advantages thereof, may be understood with reference to the following description, taken in conjuction with the accompanying drawings, and in which:

Figure 1 shows a view of a monocrystalline silicon sub-layer on which the coupling seats between an optical fiber and a waveguide are created;

Figures 2A to 11A illustrate the significant steps of the manufacturing process according to the invention, and, in particular, illustrate a section of the sub-layer made according to plan A-A of figure 1 ;

Figures 2B to 11 B illustrate corresponding steps of the manufacturing process according to the invention, and, in particular, illustrate a section of the sub-layer made according to plan B-B of figure 1 ;

Figure 12 illustrates a section of the silicon sub-layer made according to plan C-C of figure 1 at the end of the manufacturing process;

Figure 13 shows a part of the same section illustrated in section 12 with the piece of the optical fiber 9 placed in contact with the waveguide comprising the core 6g. Detailed description of a preferred embodiment of the Invention

The process of the invention is exemplified for the case in which the sublayer of the device with waveguide is composed of monocrystalline silicon according to the commonest manufacturing techniques of optoelectronic devices, but it is understood that the invention is equally applicable to manufacturing processes which use a different sub-layer material, possibly adapting the type of material used to pre-form the mask of wet isotropic etch to create the excavation of the sub-layer in order to guarantee the necessary selectivity of the etches or the resistance of the material of this mask of etch of the sub-layer to the chemical substance used to etch it and to that used to "open" the lower optical layer of cladding of the device with waveguide.

Figure 1 shows the view of a monocrystalline silicon sub-layer which is functionally subdivided into three portions, schematically represented and indicated with the letters a, b and c. The portion a is functionally reserved to the manufacture of waveguides, while the portions b and c are functionally reserved to the manufacture of V grooves, in each of which a length of optical fiber is destined to be allocated.

This last element must be positioned in such a way as to be optically coupled to a respective waveguide, and, according to the invention, this scope is achieved without the need to actively search for the correct position of the optical fiber - with respect to the waveguide - in order to improve the transfer of luminous energy between the above mentioned optoelectronic devices.

In other words, the process according to the invention guarantees the correct positioning of the length of optical fiber with respect to the waveguide without the need to search for the optimal value of one or more of the six degrees of freedom of this piece of optical fiber with respect to the waveguide.

The definition of the various layers deposited or grown on the face of the substratum may take place using the common photolithographic techniques which provide the application of a layer of resist (positive or negative), its exposure through a special optical mask (master), followed by the development treatments, used in the manufacturing processes of the integrated circuits. These techniques, and the application equipment of the resist, of alignment of the optical masks and of exposure and development used are well known to a technician and do not need specific description here.

The series of figures illustrated below schematically and partially show the details gradually defined on the face of the sub-layer at different stages of progress of the manufacturing process, showing:

- in figures 2A to 11 A an elevated section (left) made according to plan A- A of figure 1 is illustrated to show the cross-section of the formation process of a waveguide;

- in figures 2B to 1 1 B an elevated structure (right) made according to plan B-B is illustrated to show the cross-section of the formation process of a "V" groove. With reference to the series of illustrations, a manufacturing process according to the invention may be divided as follows. With reference to figures 2A and 2B, (Si₃N₄ and buffer layer deposition) on a wafer of monocrystalline silicon 1 , a first layer 2 of a material resistant to the chemical etchart of the silicon, for example of silicon nitride (Si₃N₄), is deposited from vapour phase. The thickness of the layer 2 of nitride may be between 0.1 and 0.4 μm.

Above this first layer of nitride a second buffer layer 3 of a material like silicon oxide with different characteristics of etchability from those of the nitride and similar to those of the optical glass may be placed.

Above this bottom buffer layer of silicon oxide (Si02), is chemically deposited, according to a low temperature deposition process (LPCVD), a third layer of optical glass of lower cladding 4, with for example a phosphor content of around 2-4% in weight (see figures 3A and 3B, "I lower cladding deposition"). The phosphor content preferably produces an optical layer of lower cladding of the waveguide with a similar refractive index to that of the cladding of the optical fiber to be coupled to the waveguide. The thickness of this third layer of lower cladding 4 may be between approximately 4 and approximately 8 μm.

With reference to Figures 4A and 4B (Si₃N₄ opening in the "V" grooves zone), at this point of the manufacturing process a first masking stage is carried out to define the area on which the waveguide will be formed and, according to an aspect of the manufacturing process of the invention, the third layer 4 of optical glass of lower cladding and the possible second buffer layer 3 of silicon oxide 3 are chemically and wet etched, in substantially isotropic manner, until they are completely eliminated from the surface external to the defined (masked) area of formation of the waveguide. The wet isotropic etch produces a marked tapering of the step of the formation with ".mesa" or with pedestal thus produced above the layer 2 of silicon nitride, in correspondence with the formation area of the waveguide.

With reference to Figures 5A and 5B (the lower cladding and core deposition), the manufacturing process continues with the chemical deposition from vapour phase at low temperature of a further layer 5 of optical glass of lower cladding, which, thanks to the tapering of the pre-formed mesa formation, will reproduce a profile with the stepped walls tapered in substantially conforming manner to the underlying mesa structure.

The thickness of this further layer 5 of optical glass of lower cladding is added to the thickness of the layer 4 previously deposited and defined in the formation area of the waveguide and is sufficient (from 1 to 4 μm) to obtain an adequate total thickness, of at least approximately 9 μm, below the waveguide.

Continuing with reference to Figures 5A and 5B, above the layer of optical glass of lower cladding 5 a layer of phosphor glass (of phosphor-doped silicon oxide) is deposited, also in this case chemically fromvapour phase, with a process carried out at low temperature, in order to produce a substantially conforming deposit with a thickness that may be between 1 and 4 μm. The phosphor content may be between 5 and 10% in weight and a refractive index substantially similar to that of the core of the optical fiber to be coupled to the waveguide should preferably be produced in the material of layer 6. The accentuated tapering of the stepped walls helps to prevent the start of cracks in the layers of optical glass 6 which might be induced during the conforming deposit by the presence of sharp corners or of steps with substantially vertical walls.

At this point a definition mask of the waveguide and of the structure or structures of definition of the excavation areas of the sub-layer is created to form the coupling seats of an equal number of optical fibers, from the same layer of optical glass 6, which, after being attacked in plasma and having removed the resist mask of definition, appears as shown in Figures 6A and 6B (waveguide and "V groove definition). As you may observe, the waveguide 6g is defined above the pre-formed "mesa", at a higher level than that of the structures of definition 6d.

A layer of a protective material 7, for example a particularly resistant polymer such as a polyacrylate is then deposited and defined (using the same master mask used to define the formation area of the waveguide), to protect the waveguide 6g, as illustrated in Figure 7A (waveguide protection). An attack in R.I.E. plasma as shown in Figures 8A and 8B (Si₃N₄ opening) is then carried out, etching in plasma in conditions of high anisotropy the layer 5 of optical glass of lower cladding and subsequently the layer 3 of silicon nitride through the structure of definition 6d, substantially composed of a frame of optical glass with phosphor core, previously defined in self-alignment to the waveguide 6g.

Naturally, also the structure 6d of definition of the excavation area of optical glass used for the core formation is etched in this step, but its thickness

(of approximately 2-6 μm) is sufficient to cause the opening of the etch area of the sub-layer 1 before being completely destroyed as illustrated in Figures 8A and 8B (SI₃N₄ opening).

The attack in R.I.E. plasma conducted in conditions of accentuated anisotropy, guarantees a progression of etch with substantially vertical excavation walls so as to maintain a high precision in the opening of the window of etch of the area of the monocrystalline sub-layer 1. This is guaranteed by the fact that the thickness of the layer 5 of optical glass of lower cladding which is opened in this step of R.I.E. attack is fairly small (essentially smaller than the minimum thickness of the layer of lower cladding of the waveguide which is commonly at least approximately 9 μm) to satisfy the functional requisites of optical nature.

This fundamental condition is guaranteed by the fact that the entire thickness of the layer of lower cladding of the waveguide is formed in two distinct steps of deposition or from the sum of the thicknesses of the layers 4 and 5. Therefore the thickness to be opened in this step of anisotropic attack in R.I.E. plasma may be fairly small, for example from 1 to 4 μm.

On completion of the R.I.E. attack step, the layer of polymer 7 protecting the waveguide 6g is removed by means of oxygen plasma attack.

The process may then proceed with the step of chemical deposition from vapour phase at low temperature of a conforming layer 8 of optical glass, essentially similar to the optical glass of the layers 4 and 5 to form an adequate upper cladding 8 of the waveguide 6g as illustrated in Figures 9A and 9B (upper cladding deposition).

After re-forming a definition mask of the waveguide area, the layers of optical glass of upper and lower cladding are chemically etched through contact with a solution containing hydrofluoric acid for example, preferably plugged with fluoride ammonium, to remove the glass material from the entire surface of the wafer unmasked. The intrinsically isotropic chemical etch produces an accentuated tapering of the cladding structure of the waveguide, as shown in Figure 10A ("V" grooves opening). At the end of this step of the process, the excavation area of the sub-layer of monocrystalline silicon 1 to create coupling seats of the optical fibers is defined by the inside edge of the window opened previously in the masking layer 1 of silicon nitride, defined in conditions of self-alignment to the waveguide 6g, according to the process illustrated above. The subsequent attack of the sub-layer 1 in controlled conditions of anisotropy of attack, using a KOH (potassium hydroxide) or EDP (ethylene diamine pyrocatacol) solution, produces an excavation of the sub-layer 1 with tapered walls of a pre-established angle, typically for the monocrystalline silicon around 54°, and the etch continues to produce a tapered excavation until a pre-established depth is reached, as shown schematically in figure 11 B (SI etching).

Since the monocrystalline structure of the silicon sub-layer 1 guarantees the perfect execution of the tapered surfaces of the "V" grooves at an angle of 54°, it is evident that, adjusting the amplitude of the window of attack to the outside diameter of the cladding of the optical fiber destined to be positioned in the "V" groove obtained as specified above (in figure 11 B this optical fiber is represented with broken line and marked by number 9 with the support of the cladding of the fiber on the sides of the excavation made in the silicon sublayer), the core 10 of the optical fiber is perfectly axially aligned with the waveguide 6g, defined above the sub-layer of the device as illustrated below with reference to figure 12. In other words, if the "V" groove must be dimensioned in such a way that the piece of optical fiber rests only on the sides of the groove, the above dimensioning involves only the definition of the amplitude of the window of etch. On the contrary, if the "V" groove is dimensioned in such a way that the piece of optical fiber rests on the bottom and sides of the "V" groove, the above dimensioning must also involve the depth of etch.

Naturally with reference to the figures, the devices will be obtained from the wafer, at the end of the manufacturing process, by cutting the wafer illustrated in figure 6 at the end of the excavation (or of the excavations) produced, or at plans 1 1 and 12 of figure 1.

When the various devices have been cut, the excavations produced will from an equal number of "V" grooves in which the ends of an equal number of optical fibers will be supported, as illustrated in figure 12 which shows a section of the silicon sub-layer 1 made according to plane A-A of figure 1. According to one of the commonly used techniques, the encapsulation may be contemplated through the use of "covers" (illustrated in figure and marked by the number 13), also preferentially composed of monocrystalline silicon dices, which may be manufactured similarly to that described above, thus forming also in the covers simiiar "complementary" "V" notches to those formed on portions b and c of the silicon sub-layer, to close and block the optical fiber in a "rhomb-shaped" coupling seat.

Figure 12 shows a section of the silicon sub-layer made according to plane C-C of figure 1 to better show the device obtained from this process and in particular the coupling of the core 6g in the waveguide to the core 10 of the piece of optical fiber 9 which is positioned in the hollow with 54° walls obtained as illustrated in figure 1 1 B.

The glass material which separates the end of the waveguide 6 from the core 10 of the piece of optical fiber is suitably sized to prevent the introduction of notable attenuations of the luminous radiations which cross these two elements. Figure 12 also illustrates the covering element 13 mentioned previously with reference to figure 11 B which makes it possible to block the piece of optical fiber 9.

To facilitate the transfer of luminous energy from core 6g of the waveguide to core 10 of the optical fiber, the edge of the silicon wafer is preferentially removed by saw dicing or other mechanical process suitable to remove a portion A of silicon sub-layer indicated with broken line in figure 12 n such a way as to put the two above mentioned cores 6g and 10 practically in physical contact, as illustrated in detail in figure 13. Therefore, while a particular embodiment of the present invention has been shown and described, it should be understood that the present invention is not limited thereto since other embodiments may be made by those skilled in the art without departing from the scope thereof. It is thus contemplated that the present invention encompasses any and all such embodiments covered by the following claims.

Claims

1. Process to create the coupling in an optoelectronic device between a waveguide defined on the surface of a sub-layer and an optical fiber axially aligned to each other which includes the step of forming an excavation with "V grooves of pre-fixed depth in said sub-layer by means of chemical etch through an opening defined and produced through a layer of material resistant to the solution of chemical attack deposited on the surface of said sub-layer, characterised by the fact that the definition of said opening includes the further step of pre-forming a structure of definition (6d) of the excavation area on the surface of said sub-layer (1 ) defining it photolithographically simultaneously to said waveguide (6g) by means of a same masking process, said structure of definition being obtained from the same layer (5) as the waveguide (6g).

2. Process according to claim 1 , characterised by the fact that said sublayer (1) is a silicon monocrystal, said layer of resistant material (2) is of silicon nitride and said structure of definition (6d) is defined above a layer (5) of optical glass of lower cladding deposited above said layer of nitride (2) outside the formation area of said waveguide and above at least a first defined layer (4) of optical glass of lower cladding inside said area.

3. Process according to claim 1 , characterised by the fact that said waveguide (6g) is defined above an area of said sub-layer (1 ) above which is defined a mesa composed of a buffer layer (3) of silicon oxide, of said first layer (4) of optical glass of lower cladding and of said further layer (5) of optical glass of lower cladding.

4. Process to create at least one coupling seat of a length of optical fiber (9, 10) on an optoelectronic device, in axial alignment with a waveguide

(6g) defined above a sub-layer (1 ) of the device, characterised by the fact that it includes the steps of: a) depositing on the surface of said sub-layer (1) a first layer (2) of a material resistant to the etch from chemical etcharts of said sub-layer; b) depositing a second layer (3) of a material with characteristics of wet etchability different from those of the material of said first layer (2) and similar to those of an optical glass; c) chemically depositing from vapour phase a third layer (4) of optical glass with suitable refractive index to form a lower cladding of the waveguide; d) defining a formation area of said waveguide by means of a first operation of masking and wet isotropic etch to remove said second (3) and third (4) layer from the surface external to said area; e) chemically depositing from vapour phase a fourth conforming layer (5) of optical glass with suitable refractive index to form a lower cladding of the waveguide; f) depositing a fifth conforming layer (6) of optical glass with a suitable refractive index to form said waveguide; g) defining by means of a second stage of masking and attack in plasma said fifth layer of optical glass, defining said waveguide (6g) inside said area and at least one structure of definition (6d) of the window of excavation of at lest one coupling seat externally and adjacently to said area containing the waveguide (6g); h) depositing and defining a sixth layer (7) of polymeric material to protect the waveguide (6g) in said area; i) attacking in conditions of high anisotropy said fourth layer (5) of optical glass in the zones not masked by said sixth protective layer (7) and by said structure of definition (6d) and continue the etch across the underlying first layer (2) of resistant material until the surface of the sub-layer (1) in the area thus defined by said window is uncovered;

I) depositing a seventh layer (8) of optical glass with a suitable refractive index to form a layer of upper cladding of said waveguide; m) masking said area containing said waveguide (6g) and etching said seventh layer (8) of optical glass removing it from the surface external to this area; n) chemically and wet etching, in anisotropic manner, said sub-layer (1 ) through the window opened in said first resistant layer (2) and forming a "V" groove to support said length of optical fiber.

5. Process according to claim 4, characterised by the fact that it provides the further steps of: o) removing, by means of mechanical processing, the edge (A) of the portion of monocrystalline silicon placed at the end of the above mentioned excavation with "V" grooves; p) positioning inside said excavation with "V grooves a length of waveguide (9) in such a way that its end is opposite and in contact with the end of the waveguide obtained as specified above.

6. Process according to claim 4, characterised by the fact that it provides the further step of blocking the above mentioned length of waveguide (9) by means of a covering element obtained by making an excavation with "V" grooves on a monocrystalline silicon dice.

7. Process according to claim 4, characterised by the fact that said sublayer (1) is a silicon monocrystal and said first resistant layer (2) is of silicon nitride.

8. Process according to claim 4, characterised by the fact that in said step d) and in said step n) said attack of the sub-layer is conducted using a strongly basic solution.

9. Process according to claim 8, characterised by the fact that strongly basic solution is a potassium hydroxide solution.

10. Process according to claim 8, characterised by the fact that said strongly basic solution is an ethylen-diamine pyracatacol solution.

11. Process according to claim 4, characterised by the fact that in said step g) the attack is conducted in a fluoromethane plasma.

12. Process according to claim 4, characterised by the fact that in said step i) the dry etch in R.I.E. plasma is conducted in presence of CxFy.

13. Process according to claim 4, characterised by the fact that in said step m) the etch is conducted through contact with a solution containing hydrofluoric acid.

14. Process according to claim 13, characterised by the fact that said solution is a solution of HF plugged with ammonium fluoride.

13. Optoelectronic device composed of a sub-layer of silicon (1) on a portion of which waveguides (6g) are created and on at least a further portion of which "V" grooves are created by means of chemical attack, characterised by the fact that it includes a masking layer with a window coinciding with the excavation area on the surface of said sub-layer photolithographically defined in self-alignment to said waveguide by means of a single masking process.

14. Device according to claim 15, characterised by the fact that said sub-layer (1 ) is of silicon monocrystal and said masking layer (2) of silicon nitride. Device according to claim 13, characterised by the fact that said waveguide (6g) is defined above an area of said sub-layer (1 ) on which a mesa composed of at least one pre-layer (4) of optical glass of lower cladding defined above said area is previously created.