WO2006136194A1

WO2006136194A1 - Method for fabricating a turning mirror for optical devices

Info

Publication number: WO2006136194A1
Application number: PCT/EP2005/006843
Authority: WO
Inventors: Anna Merialdo; Paolo Schina
Original assignee: Telecom Italia S.P.A.
Priority date: 2005-06-24
Filing date: 2005-06-24
Publication date: 2006-12-28

Abstract

The present invention relates to a method to fabricate a turning mirror for an optical device 100. The method comprises the steps of providing a substrate, which defines a plane (X,Z) in which an optical signal propagates in a propagating direction (X). The substrate comprises a waveguide including a cladding and a core (8). The substrate is thus dry etched through a first mask aperture (15) so that a trench is obtained in the substrate, said trench comprising at least a substantially perpendicular surface with respect to the optical propagating direction (X). A protecting layer is formed on the substrate so as to cover portions of the outer surface of said substrate parallel to the (X,Z) plane and the at least a substantially perpendicular surface. The substrate is then etched by wet etching using a second masking mask with a second mask aperture (18') to protect portions of the substrate not to be etched so as to form a cavity in the substrate comprising an angled surface having an angle relative to the propagating direction (X). The protecting layer is thus removed so that the etched hole and the trench merge to form a single cavity.

Description

Method for fabricating a turning mirror for optical devices

Technical field

The present invention relates to a method to fabricate a turning mirror for optical devices, in particular for optoelectronic integrated circuits. Turning mirrors are capable of directing optical signals into and/or out planar waveguides.

Technological background A turning mirror is a structure that is capable of coupling an optical signal, for example entering to or exiting from an optical waveguide, with an optical device, typically an active device, such as a transmitter or a receiver. The turning mirror redirects the optical signal by reflecting the same at a desired angle. As an example, the mirror can make a 90° change of the direction of the light propagating in the optical waveguide so as to deflect the light toward the receiver. A 90° deflection of the incoming light beam can be typically carried out by 45°-angled mirrors.

Turning mirrors are extremely useful in integrated optical devices for directly processing optical signals; indeed such devices have become of greater importance as optical fiber communications are more and more widely used. In ^' integrated optical devices, turning mirrors can be advantageously integrated on the same substrate on which planar waveguides are fabricated. In particular, as passive optical devices, such as couplers or polarization splitters, can be made from optical waveguides, turning mirrors reflect optical signals coming from the waveguide linked to or forming at least part of a passive device to an active device and/or vice-versa. Alternatively, turning mirrors can deflect the light exiting from a waveguide semiconductor laser towards a receiver or a passive device.

Among other methods, a turning mirror may be realized by etching a cavity into a substrate. One of the sidewalls of the etched cavity has the form of an inclined surface forming an angle with respect to the plane defined by the substrate. This angled surface is the reflecting surface of the turning mirror. In other words, the turning mirror is realized by etching a wedge-shaped sloped region on a substrate. Different techniques may be employed to etch the aforementioned cavity. US patent No. 5,135,605 in the name of AT&T Bell Laboratories describes a method for making a turning mirror in an optical waveguide structure made by etching in the upper surface of the structure a cavity that intercepts the path of light propagated by the waveguide. The cavity has a side that is nearly normal to the surface on the side of the cavity adjacent the waveguide, while it has a side which is substantially at 45° to the surface on the side opposite the waveguide. Prior to etching the cavity, part of the interface of the mask layer and the body are treated such that the etchant undercuts the mask on the side of the cavity remote from the waveguide to a greater extent than it undercuts the mask on the side of the cavity adjacent the waveguide. Applicant has observed that, by using the wet etching method disclosed in the above-quoted patent, the verticality of the sidewall of the cavity adjacent the waveguide is not assured. Indeed, Applicant has observed deviations from perpendicularity of the surface on the side of the cavity adjacent the waveguide. In US patent application No. 2003/0155327 in the name of Fujitsu Limited, a manufacturing method for an optical integrated circuit including a turning mirror having a perpendicular end surface and an inclined surface formed in an optical waveguide layer is described. The manufacturing method includes the steps of applying a first photoresist to the upper surface of the optical waveguide layer, removing the first photoresist except a portion corresponding to the inclined surface, and heating the first preferred embodiment to a given temperature to melt the first photoresist at least partially and deform the first photoresist by surface tension, thereby forming a first mask having an inclined shape. The manufacturing method further includes the steps of applying a second photoresist to the upper surface of the optical waveguide layer and the first mask, removing the second photoresist at a portion ranging from a position corresponding to the perpendicular end surface to a portion corresponding to the upper end of the inclined surface to form a second mask, and etching the first mask, the second mask, and the optical waveguide layer by Reactive Ion Etching (RIE) to simultaneously form the perpendicular end surface and the inclined surface.

US patent No. 6,511,235 is relative to an integrated surface-emitting optoelectronic module and the method for making the same. A V-groove is defined for disposing an optical fiber on a silicon substrate having a crystalline surface. After dry etching a vertical groove, a dielectric layer is grown on the surface of the silicon substrate to protect the vertical wall, thereby preventing the groove from getting wider due to subsequent wet etching. A 45-degree mirror surface is formed so that an optoelectronic device can be disposed on the mirror surface in the flip chip method. US patent No. 5,116,460 pertains to a method for selectively etching materials on a semiconductor wafer that have similar etch rates. The semiconductor wafer is provided with at least a first layer. An etch mask is provided on the first layer. The layer with the etch mask is partially etched to a predetermined point. A polymer film is deposited on the partially etched layer. The polymer film is etched in an anisotropic manner creating open or clear areas in the horizontal polymer film, while leaving polymer coating on the vertical walls. The open areas are chemically etched, while the remaining polymer coating on the vertical walls protects the vertical walls from being chemically etched. Summary of the invention The present invention relates to a method for the fabrication of turning mirrors for optical devices, preferably turning mirrors for optoelectronic integrated circuits.

These mirrors generally comprise an angled reflective surface positioned opposite to an end of an optical waveguide, wherein said angled reflective surface deflects the light either emerging from the waveguide towards an active device (such as optical transmitters or receivers) or emerging from the active device towards the waveguide in order to enable an exchange of optical signals between the active device and the waveguide. However, any other configuration can be envisaged, in all of which the turning mirror redirects optical signals which reflect on its reflective surface from a first incoming direction to a second direction.

In the present invention, the turning mirror is realized by etching a cavity into a substrate. One of the sidewalls of the etched cavity has the form of an angled surface with respect to the plane defined by the substrate. The surface of the angled surface is the reflecting surface of the turning mirror. The cavity formed by etching, in addition to the angled surface, also comprises an additional sidewall having a surface essentially opposite to the angled surface.

Preferably, but not necessarily, the substrate comprises an optical waveguide and the cavity is etched from the upper surface of an optical waveguide structure. The waveguide structure comprises a core and a cladding, where the core has a higher refractive index than that of the cladding so that the light is confined within the core and, as a consequence, the configuration of the core defines the path of the light. Applicant has noted that the surface of the sidewall near the waveguide core end, i.e., the end surface, needs to be substantially perpendicular with respect to the propagation direction of the light entering to or exiting from the waveguide. Applicant has observed that tolerances from verticality (90°) may range from about - 8° to about +0.5°. The angled surface forming the turning mirror is considered to have a negative inclination, therefore a "minus" sign in front of the tolerance angle means that the tolerance on the verticality of the end surface can be of a maximum of 8° when the end surface is inclined in the same direction as the angled surface. A tolerance angle preceded by a positive sign indicates the acceptable deviation from 90° when an inclination in the opposite direction is considered. Therefore, with substantial verticality or perpendicularity it is meant hereafter that the cavity sidewall on the side of the waveguide forms an angle with the optical propagation direction which is within the given tolerances. If the end surface's perpendicularity is not satisfactory, the emergent propagating signal from the waveguide can be refracted at the end surface itself and this causes a limit to the optical coupling tolerances between the waveguide and the active device emitting and/or receiving the light. On the other hand, a good perpendicularity allows the optical signal to enter the cavity (or to enter the waveguide) having the same propagating direction which it had in the waveguide (or in the cavity) and losses induced by scattering are minimized. Therefore, in case of end surface's perpendicularity, the optical signals can be emitted from the waveguide to the cavity (or vice versa) with minimum refraction and can be reflected with good efficiency by the angled surface opposite to the waveguide. One of the main goals of the present invention is therefore to provide a method for the realization of turning mirrors etched on a substrate, which gives a good control over the mirror angle and at the same time achieves an end surface which is substantially perpendicular with respect to the propagating direction of the optical signals (which can be either the propagating direction of a signal in the waveguide or the propagating direction of a signal in the cavity after reflection from the angled surface).

The cavity sidewall having a substantially vertical surface is then located on the side of the cavity adjacent the waveguide and substantially opposite to the angled surface. The vertical surface will be called in the following also end surface since it is the surface (which in the preferred embodiments corresponds to the end of the waveguide) to/from which the light enters/exits. Preferably, the waveguide structure is a planar waveguide where a core layer is sandwiched between a lower cladding layer and an upper cladding layer. The lower cladding layer is preferably formed on a substrate. The cavity is to be etched sufficiently deep to intercept the path of the light propagating in the optical waveguide (i.e. reaching at least the core layer level).

In planar waveguides where the core layer is substantially parallel to an underlying substrate, the perpendicularity to the direction of light propagation implies that the end surface of the sidewall near the waveguide end is substantially vertical with respect to the substrate.

In the following, therefore, the generic term "substrate" may also mean a waveguide structure.

Preferably, the etched substrate comprises silicon based material. Even more preferably, it comprises doped and/or undoped silicon oxide. The method of realization of a turning mirror according to the invention is relatively simple and inexpensive.

In order to realize a cavity in a substrate comprising an angled surface and an end surface substantially perpendicular to the light path along the waveguide, the method of the invention includes two main process steps: a dry etching step and a wet etching step.

Dry etching typically uses ionized gases as reactive agents (e.g., plasma), whereas in wet etching the material is eliminated by its dissolution in an adequate etching (aqueous or organic) solution. Dry etching allows to achieve nearly vertical sidewalls with relatively high aspect ratio (i.e., narrow and deep features can be realized) and relatively high resolution. However, this technique is expensive to implement compared to wet etching and it has a relatively low throughput.

Wet etching is a relatively simple etching technology and typically requires a container with an etching solution which dissolves the material to be etched (dip technique) or spray nozzles to spray the etching solution on the material to be etched (spray technique). To etch different materials a large variety of chemical agents can be used. However, solutions used in etching processes should etch exclusively (or chiefly) the desired film, without affecting (or affecting very little) the photolithographic masks. Thus, the solution needs to be highly selective, defining the selectivity as the ratio between the etching rates of two different films in the same etching solution. Generally, wet etching provides a higher degree of selectivity as compared to dry etching. However, etching of non-crystalline materials (e.g., amorphous materials such as silicon oxide or silicon nitride), tends to be isotropic, i.e. the etching rate is the same all directions. This implies that wet etching of a substrate through the aperture in a mask layer, which is placed on the substrate and is used to protect the underlying substrate in the region not to be etched, occurs both horizontally and vertically, thereby causing a lateral etching under the mask of the same order as the magnitude of its depth, an effect known as undercutting. The undercutting effect can provide for the formation of a cavity having an angled sidewall. However, Applicant has observed that realization by wet etching of a cavity comprising both an angled sidewall and a vertical sidewall is very complex. Accordingly, in the method of the present invention, the use of dry etching is combined with the use of wet etching, also in order to reduce implementation costs and technological complexity.

The underlying principle of the present invention is to form a sidewall substantially perpendicular to the direction of propagation of the light and to protect said sidewall during a subsequent wet etching step that is carried out for creating a cavity with an angled surface. More in detail, the method according to the invention comprises forming by dry etching a trench in a substrate. This substrate preferably comprises a waveguide structure in which a core is defined. A vertical sidewall of the etched trench corresponds to the end surface of the waveguide (which might be, as said above, at or close to the end surface of the core) from which optical signals will exit the waveguide and enter the cavity (or vice-versa). After the etching of the trench, a protecting layer is formed, e.g., it is deposited, so as to cover at least the sidewall close to the waveguide core end of the trench. Preferably, both opposite surfaces of the trench may be covered by the protection mask layer. According to a preferred embodiment of the invention, the trench is completely filled with the material forming the protecting layer. It is however to be understood that complete filling is not necessary, being sufficient to have a protecting layer that is thick enough to protect the end surface during the subsequent wet etching step of the method of the invention described below. The thickness of the protecting layer depends also on the etchant used and on the depth of the cavity to be etched. According to a preferred embodiment of the invention, the protecting layer is realized in polycrystalline sil icon (poly-Si). The protection mask is realized through suitable lithographic and etching techniques. According to a preferred embodiment of the invention, the protection layer is deposited on the substrate upper surface (where the trench is formed) in addition to being deposited inside the trench.

An aperture is formed by known techniques (e.g., photolithography or electron beam) in the layer deposited on the substrate upper surface. The patterned protection layer deposited on the substrate surface will then function as mask layer for the subsequent wet etching step. The aperture has dimensions preferably comprised between 4 μm and 10 μm along the propagating direction (width of the aperture) and 25 μm and 40 μm in the direction perpendicular to it (length of the aperture). A wider dimension along the propagating direction is preferably not recommended because the angled surface would be located too far from the waveguide end surface, thereby leading to an excessive dispersion. On the contrary, a shorter width may pose technical difficulties due to the close proximity of the end surface to the angled surfaces. The aperture is preferably positioned at a given transversal distance from the trench (i.e., along the propagating direction), on the opposite side of the trench with respect to the end surface of the waveguide. Preferably, the distance between the boundary of the aperture closest to the trench and the boundary of the trench itself is comprised between 1 μm and 5 μm. Deposition of a protection layer both in the trench and on the substrate surface so as to function as mask for the wet etching step can be advantageous in terms of reduction of process steps in the fabrication of the turning mirror according to the invention. It is however to be understood that a protection layer can be formed in the trench and subsequently (or before) a mask layer (which can be made of a different material than that of the protection layer) can be deposited on the substrate surface.

After a patterned mask layer is formed on the substrate surface, a wet etching step follows leading to the formation of a cavity in the substrate. The cavity in the substrate is not symmetric: due to the presence of the protecting layer covering at least part of the end surfaces of the trench, etching towards or damaging the end surface is prevented by the presence of the protecting layer. As wet etching proceeds, a surface of the cavity eventually corresponds to the sidewall (external to the waveguide) of the protection layer. On the side opposite to the waveguide, where an aperture in the substrate has been formed, wet etching can proceed and thus an undercut is formed between the mask deposited on the upper surface of the substrate and the substrate itself. The wet etching in the substrate progressively dissolves the substrate till an angled surface is produced, which is substantially opposite the sidewall of the trench covered by the protection layer. Therefore, the verticality of the end surface to the light path in the waveguide obtained by dry etching is preserved during the wet etching step.

The inclination of the angled surface with respect to the optical propagating direction depends on the substrate etching rate, the depth of the cavity, on the physical properties of the substrate, on the properties of the mask layer and on the adhesion properties of the mask layer onto the substrate. Preferably the inclination of the angled surface is comprised between 40° and 55°.

Preferably, the etching solution and the material of which the protecting layer is made are selected so that the protection mask is substantially unattacked by the chemical agents used to etch the substrate. Preferably, the substrate to be etched comprises silicon oxide based material (e.g., doped and/or undoped silicon oxide) and the protection layer comprises poly-Si. The etching solution is preferably a bath comprising hydrofluoric acid as active agents for etching the substrate.

However, also embodiments in which the protection mask is attacked at a significantly lower etching rate than the substrate by the etchant can be envisaged. In this latter case, the thickness of the protection layer covering the end surface should be selected so that it does not completely dissolve when the entire wet etching step is completed.

The protecting layer is thus removed, so that a cavity is formed in which the vertical end surface formed by dry etching faces the angled surface formed by wet etching. Refraction of optical signals in and out the waveguide at the end surface is minimized.

An additional preferred method step may follow, in which the angled surface of the cavity is metallized in order to enhance reflection. Brief description of the drawings Further features and advantages of a method to fabricate a turning mirror for optical devices according to the present invention will become more clear from the following detailed description thereof, given with reference to the accompanying drawings, where: fig 1 is a simplified schematic representation of a lateral sectioned view of a portion of an optical device prior to the fabrication of a turning mirror according to a preferred embodiment of the invention; figs. 2-4 are schematic lateral views of different steps of the method of realization of a turning mirror according to a preferred embodiment of the invention; - figs. 4a-4b are schematic top views of two different preferred embodiments of a step of the method corresponding to the lateral view of fig. 4; figs. 5-10 are schematic lateral views of different steps of the method of realization of a turning mirror according to a preferred embodiment of the invention; fig. 11 is a schematic lateral view of a turning mirror realized according to a preferred embodiment of the invention; fig. 12 is a SEM micrograph of a tapered region realized during a step of the method according to a preferred embodiment of the invention and corresponding to the step depicted in fig. 10; fig. 13 is a SEM micrograph of a turning mirror realized according to a preferred embodiment of the invention and corresponding to the mirror depicted in fig. 11; fig 14 represents a SEM micrograph of a top-plan view according to a preferred embodiment of the invention and corresponding to the step depicted in fig. 11.

Preferred embodiments of the invention

With initial reference to fig. 11, a portion of an optical device globally indicated with 100, is shown.

It is to be noted that in the following drawings (not photos) are greatly simplified and not to scale, to reveal more clearly certain features of the invention.

The device 100 preferably comprises an optical waveguide 2 suitable to transport an optical signal. Additionally, the device 100 comprises a cavity 3 realized according to the teaching of the present invention in which a turning mirror 4 is formed. The turning mirror 4 comprises an angled surface 5, which is located opposite to an end surface 6 of the waveguide 2.

The turning mirror 4 is suitable to deflect optical signals OS exiting from/entering to the waveguide 2, in particular exiting from/entering to the end surface 6, which are then reflected onto/from the reflective surface 5 (see fig.

11, the optical signal path is represented by a dotted , line). Optical signals OS are, for example, directed toward or emitted from an optolelectronic device (not shown in the figures), which can be an emitter, a receiver or a transceiver. The path of the optical signal OS is along the axis of the waveguide 2 (parallel to the X axis of fig. 11) while the signal travels therein, it is then reflected on the surface 5 of the mirror 4 and it thus follows a new axis (indicated as Y in fig. 11) so that it can exit the cavity 3. In case the optoelectronic device is an emitter, such as a laser diode, optical signals are emitted from the device (e.g., along the Y direction), impinge the angled surface 5, which deflects them and direct them into the waveguide 2. Conversely, if the optoelectronic device is a photodiode, optical signals exit waveguide 2, are deflected by the angled surface 5 and then enter the photodiode. Any turning angle of the optical signal reflecting onto the turning mirror 4 may be envisaged, i.e. any angle between the X and Y axis can be considered, depending e.g. on the turning mirror's physical characteristics and on the angle acceptance of the active devices.

In addition, even if in the depicted optical device 100 a waveguide 2 is included, any other device 100 including or not including a waveguide may be considered, the turning mirror realized by the method of the invention being suitable to redirect optical signals coming from any source. The optical waveguide 2 is realized on a first substrate 7, preferably a silicon based substrate, such as Si, SiO₂, doped-SiO₂, SiON and the like. Other conventional substrates will become apparent to those skilled in the art given the present description. The waveguide 2 comprises a core 8 and a cladding 9. In particular, the core 8 has a higher refractive index with respect to the cladding 9. Preferably, the waveguide 2 is realized in semiconductor-based materials such as doped or non-doped silicon based materials and other conventional materials used for planar waveguides. Preferably, the core 8 of the waveguide 2 may comprise a doped or un-doped silicon based material, such as Si, doped-SiO₂, SiON, and the like. Preferably, the cladding 9 comprises silicon oxide.

More in detail, with reference to fig. 1, a preferred embodiment of a waveguide structure 2, obtained before the formation of cavity 3, is shown. The substrate 7 is made of crystalline Si and is covered by a SiO₂ layer 10a, which is preferably grown by High Pressure Oxidation (HIPOX), which allows relatively fast growth of an oxide at reduced temperature. A lower cladding layer 10 is deposited on the thermally grown SiO₂ layer 10a and it is preferably realized in Borophosphosilicate Glass (BPSG). Suitable deposition techniques of the lower cladding layer 10 are for example Atmospheric Pressure Chemical Vapour Deposition (APCVD) or, alternatively, Low Chemical Vapour Deposition (LPCVD) or Plasma Enhanced Chemical Vapour Deposition (PECVD). Preferably, the lower cladding layer 10 is deposited using the so-called multilayer deposition technique, which requires a plurality of deposition steps, each followed by a reflow step at a relatively high temperature to eliminate voids and outgas. For instance, each reflow step comprises one or more hours of annealing at around 1000⁰C.

On top of the lower cladding 10, a core layer 8 is deposited, for example by PECVD or by APCVD. The core layer 8, made preferably by Ge-doped or P- doped SiO₂, is then patterned by means of optical lithography and deep oxide etching to form the core 8, e.g., of square cross section, of the waveguide 2. An end surface 8a of the core 8 is also formed (fig. 1) by dry etching. Afterwards, an additional layer, called upper cladding 11, is grown, for example by LPCVD or PECVD or APCVD, using again the multilayer technique, on top of and around the core 8. Preferably, the upper cladding 11 is formed in the same material and it has the same refractive index as the lower cladding 10. The preferred thickness of the core 8, and upper cladding 11 are preferably comprised between 2 μm and 4 μm, and between 8 μm and 10 μm, respectively. The upper surface of the upper cladding 11 defines a (X, Z) plane.

It is to be understood that the teaching of the invention is applicable to any other process for the realization of the waveguide 2, instead of the one described above. In figs. 2 - 11 a schematic representation of the fabrication process flow of the cavity 3, which includes the angled surface 5 and the end surface 6, according to a preferred embodiment of method of the invention, is illustrated. The method of the invention may be considered as comprising two main steps: a first dry etching main step in order to form an end surface 6 and assuring its substantial perpendicularity with respect to the propagating direction of the optical signal in the waveguide 2, and a second wet etching main step in which the angled surface 5 of the turning mirror 4 is realized, without modifying the verticality of the already-formed end surface 6 (fig. 11).

According to a first step, a masking layer 12 is deposited over the upper cladding 11 of the waveguide 2 (see fig. 2 in which the resulting configuration after this deposition step is depicted). Layer 12 can be patterned by using suitable photolithographic techniques by means e.g. of a photoresist 14 in order to form a hard mask 13 (figs. 3 and 4), which is used to protect portions of the underlying substrate which are not going to be etched in a subsequent dry etching method step. In particular, for example, the masking layer 12 can be covered by a photoresist, e.g., a positive photoresist, which is exposed to UV radiation according to a selected pattern which is to be transferred to the masking layer 12. The exposed portions of the photoresist are thus developed to resolve the pattern using a suitable developer. The photoresist 14 remaining after development is used as a mask in the subsequent etching process of the first masking layer 12 (see fig. 3 which shows the photoresist layer 14 after development). The masking layer 12, covered by mask 14, is thus etched using any suitable etching technique, such as Reactive Ion Etching (RIE), to obtain the first hard mask 13 (see fig. 4 which shows the configuration in which the hard mask 13 is formed). The material of masking layer 12 and the etching parameters are selected so as to ensure a relatively high selectivity. Preferably, the selectivity between the masking layer and the substrate is not less than 3: 1.

The masking layer 12 is preferably a hard mask (such as silicon nitride), more preferably made of polycrystalline silicon (poly-Si), which can be deposited by a suitable technique, such as LPCVD, even if alternative deposition techniques might be used. The thickness of the layer 12 depends on the depth which has to be etched in the substrate in order to realize a substantially vertical end surface, on the selectivity between the mask layer 12 and the substrate material to be etched. Preferably, the thickness of the masking layer 12 deposited over the upper cladding 11 is comprised between 1.2 and 5 μm. The thickness to be etched in the waveguide 2 is preferably between 10 and 25 μm, more preferably between 13 and 20 μm.

Two alternative preferred embodiments of mask 13 layouts are depicted in figs.

4a and in fig. 4b (the configurations shown in figs. 4a and 4b are such that the mask layer 13 is still covered by the remaining of the developed photoresist

14), which are alternative top views of the configuration depicted in fig. 4.

The mask 13 of fig. 4a presents and U-shaped aperture 15, which comprises three rectangular branches, called first, second and third arm 16a, 16b, 16c.

From the two opposite ends of the third arm 16c, the first and the second arms 16a, 16b depart, in such a way that they are parallel among them and perpendicular to the third arm 16c. Said third arm aperture 16c uncovers a portion of the upper surface on the (X,Z) plane of the cladding 9 located at or adjacent to the end 8a of the core 8 along the Y direction.

A distance of few microns may be present between the end of the core 8a and the end surface of the waveguide, as described more in detail below. In other words, in the embodiment of fig. 4a, the boundary of the aperture 16c is located at a distance of few microns from the end surface 8a.

Preferably, the aperture 16c spans a given width w_α along the X direction from the desired location of the end surface 6, and its length Iu is longer than the maximum dimension of the turning mirror 4 in the direction Z perpendicular to the X propagating direction. Preferably, the width Wu and the length Iy of the third arm 16c are comprised between 1.5 μm and 4 μm, and between 40 μm and 60 μm, respectively.

Regarding the two parallel arms 16a, 16b, which are preferably symmetric with respect to the axis of the U-aperture 15, they have a width W₂ which is substantially identical to the width Wu of the third arm 16c, and a length I₂ which is longer than the maximum extension in the X direction of an upper surface cladding portion on which a subsequent wet etching phase will be performed in order to form the angled surface 5, as it will described in detail in the following. Preferably, the length I₂ is comprised between 25 μm and 42 μm. In figs. 4a and 4b, the aperture corresponding to the area of the upper surface in which the wet etching phase will take place in a subsequent step is represented by a rectangular area 18'. The U-shaped aperture 15 uncovers the area to be dry etched in a subsequent step of the method of the invention. A U-shaped aperture can be advantageous if verticality of the walls of the turning mirror lateral to the inclined surface is desired.

According to a different embodiment of the method of the invention, represented in fig. 4b, instead of the U-shaped aperture 15, the first hard mask 13 comprises a strip-like aperture 15', which includes a rectangular aperture analogous to the third arm 16c of the U-shaped aperture 15. The dimensions of the strip 15' in the X and Z directions are respectively preferably comprised between 1.5 μm < w_s ≤ 4 μm and between 40 < I₉ < 60. Although in fig. 4 only the number 15 is written for conciseness in notations, it is to be understood that an hard mask 13 having either the U-shaped aperture 15 or the strip aperture 15' can be alternatively employed in the method of the invention. Other apertures having different shapes can be patterned as well, as long as a substantially perpendicular end surface 6 is formed in the subsequent dry etching step of the method of the invention. The following process steps according to the method of the invention are realized in order the obtain a trench 19 in the waveguide 2, in particular a trench having at least a perpendicular wall with respect to the propagating X- direction, said perpendicular wall forming the end surface 6 (fig. 5). The patterned masking layer 13 above described, on which the remaining portions of the photoresist 14 after development can be optionally left, is used to mask the underlying cladding 9 during an etching step to form trench 19. In detail, the trench 19 is realized in the waveguide 2, i.e. in the cladding 9, using a dry etching technique (see fig. 5). Preferably, a deep oxide etching technique is applied. Even more preferably, the etching is performed using an inductively coupled plasma (ICP) system, the Advanced Oxide Etching (AOE) system being a preferred example. However any other dry etching technique achieving the desired trench depth and verticality of the walls can be used in the present invention. If the mask 13 comprises the strip aperture 15', also the trench 19 has a linear shape, otherwise, in case of U-shaped aperture, also the configuration of the trench has the form of a U. In the following, only the portion of the trench 19 located in front of the core end 8a of the waveguide 8 (which is the whole trench in case of the 15' aperture) is described. Thanks to the accuracy achieved through the use of a dry etching technique, a lateral surface, in particular the lateral surface of the trench 19 closest to the end 8a of the core 8 - which corresponds to the end surface 6 - is substantially vertical, the angle formed between the (X,Z) plane and this surface 6 being is substantially of 90°. Preferably, the angle of the substantially vertical surface is comprised between 82° and about 90.5°, more preferably between 89° and 90.5°. In the preferred embodiment depicted in fig. 6, both lateral facing surfaces 6, 6a of the trench 19 are substantially vertical with respect to the

(X,Z) plane.

The width w of the trench 19 corresponds to the width of the aperture 15' or of the third arm 16c of aperture 15 in the mask 13, and therefore w is substantially equal to either Wu or w_s. The depth d of the trench 19 is preferably comprised between 12 μm and 16 μm.

Preferably, a distance between the end surface 8a of the core 8 and the end surface 6 of the trench 19 is present. This distance should preferably be small (i.e., not larger than few microns) so that scattering and dispersion of the light exiting the core end surface is acceptable for the applications of the optical device and is in general negligible. More preferably, the distance is comprised between 1 and 5 μm. The presence of a small distance between the end surface 8a of the core and the end surface 6 of the waveguide reduces the risk of tampering with the core end surface during the dry etching phase. After the dry etching phase, removal of photoresist 14 and mask 13 is carried out, through an additional etching phase, which might be either a wet or a dry etching step, or a sequence of the two in any order, according to the mask 13 and photoresist 14 characteristics. The configuration after this removal step is shown in fig. 6. After the above mentioned steps, a substantially perpendicular end surface 6 is formed. In the following, the formation of the angled surface according to a preferred embodiment of the present invention is described. A protecting layer 20 is deposited in the trench and in particular covering at least a sidewall of trench 19 that will define the end surface 6 adjacent to the waveguide core end. Preferably, both facing surfaces 6 and 6a of the trench 19 are covered by the second masking layer 20, thus the masking layer 20 in the trench 19 follows the trench's walls forming an U contour in lateral section. According to an additional preferred embodiment of the invention, such as the one illustrated in fig. 7, the entire trench 19 is filled by the layer 20; this can easily obtained if the masking layer 20 thickness is more or equal than half the trench 19 width.

Preferably, protecting layer 20 covers also at least a portion of the upper surface of the waveguide structure 2, as illustrated in fig. 7. In this way, protecting layer 20, if suitably patterned and if made of a suitable material, can also function as mask for the subsequent wet etching phase of the cladding 9 of the waveguide 2.

It is however to be understood that, instead of a single masking layer covering the upper surface of the waveguide structure 2 and filling the trench 19, two different masking layers deposited in two different deposition steps, may be used to realize the turning mirror according to the method of the present invention.

According to a preferred' embodiment of the present invention, the protecting layer 20 is realized in poly-Si, deposited e.g. by LPCVD. Alternative materials are Silicon Carbide or Silicon Nitride. The preferred thickness of the layer ranges between 0.5 μm and 1.5 μm when measured on top of the upper cladding 11. Preferably the walls of the trench 19 are covered by the same thickness (0.5-1.5 μm) of masking layer 20.

The masking layer 20 is thus patterned to form an aperture 18 (fig. 8), preferably a rectangular aperture, which corresponds to the "wet etching area" already indicated with 18' in figs. 4a and 4b. The patterning is made Using suitable photolithographic techniques, for example the same technique used to pattern the masking layer 12: a photoresist 22 is deposited and then patterned to form a first aperture. Then, an etching phase of the layer 20 to transfer in the latter the aperture formed in the photoresist 22 so as to form a patterned masking layer 24 (see fig. 8) is realized. The masking layer 24 is preferably a hard mask made e.g. of poly-Si, silicon nitride or silicon carbide. The aperture 18 is located adjacent to the trench 19, in particular to the side of the trench opposite to the core 8, and its dimensions are preferably comprised between 4 μm and 10 μm along the X axis and between 25 μm and 40 μm along the Z axis. The distance between the boundary of the aperture 18 closest to the trench 19 and the boundary of the trench 19 itself is preferably comprised between 1 μm and 5 μm.

At the end of the patterning of the aperture 18, the photoresist 22 is removed (this configuration is shown in fig. 9). The portion of the cladding 9 of the waveguide 2 exposed by the aperture 18 is then subjected to wet etching using an etching solution that attacks the material in which the waveguide 2, in particular cladding 9, is made, thereby forming a cavity 3' (fig. 10). The etching solution, however, does not appreciably attack the mask 24 and thus the end surface 6, covered by the hard mask 24, is protected.

The material for the hard mask 24 and the etchant are chosen such that the etching rate for the hard mask 24 is much less than the etching rate for the material forming the cladding 9 of the waveguide 2. Preferably, the etching solution and the mask are selected so that the etching rate of the etching solution for the hard mask 24 can be considered to be negligible compared to that of the cladding.

In particular, during wet etching, an undercut 23 is formed between the hard mask 25 and the cladding 9 (see fig. 10) so as to form an etched hole 3'. Etching of the wall 6 is essentially prevented by the presence of the protecting layer 20. In the example shown in figs. 8-10, being the aperture 18 slightly shifted from the location of the surface 6a of the trench 19, a small undercut may be formed below the hard mask 24 toward the wall 6a. However, due to the small distance between aperture 18 and wall 6a, the profile of the surface of the hole 3' substantially follows the whole profile of the hard mask 24 deposited over the wall 6a.

The etched hole 3' thus comprises an angled surface 5 formed during the wet etching below the undercut 23 and on the opposite side, it substantially follows the profile of the protection layer 20 covering the trench 19. Therefore the etched hole 3' is substantially delimited by the vertical wall of the protection layer 20.

The depth of the hole 3' reached in this wet etching phase is preferably less than the depth of the trench 19 to avoid that the etchant may etch the cladding 9 deeper than the bottom wall of the trench. Preferably this depth difference is of the order of 1 μm. Therefore, the cavity 3' has preferably a depth comprised between 11 μm and 15 μm.

Although in the figures it is schematically illustrated that the trench 19 extends vertically down to the thermal oxide layer 10a, it is to be understood that the trench can also extends (also partially) into the thermal oxide layer. By properly selecting the parameters of wet etching and the masking layer, it is possible to obtain angles of the angled surface 5 forming the mirror 4 in a wide range of values, even if obviously a narrower range is generally preferable, i.e. the preferred range is comprised between 40 and 55 degrees, more preferably

50° ± 2.5°.

Fig. 12 represents a Scanning Electron Microscope (SEM) micrograph of the configuration of the optical device 100 after this wet etching step. The hole 3' and aperture 18 are visible. The mask 24 is still present.

After the cavity 3' has been defined, an etching step is carried out to remove the mask 24. After removal of mask 24, the cavity 3' and the trench 19 merge to form a single cavity 3, said cavity comprising on one side the perpendicular end surface 6, formed by the dry etching step, and opposite to it, the angled surface 5, realized by the above described wet etching step (fig. 11).

A SEM micrograph of the so formed cavity 3 including the angled surface 5, obtained using the process described, is shown in Fig. 13. In the example of fig.

13, the mirror is cut with a dicer along a direction parallel to the optical axis. In fig. 14, a cross section Scanning Electron Microscope (SEM) micrograph of the same configuration schematically depicted in fig.11 and obtained using the process described above is shown. In this photograph, the cavity is seen from above and the poly-Si layer has been removed.

Preferably, an additional metallization phase, in which a metallic layer is deposited on top of the angled surface 5 (not shown in fig. 11), is realized according to the method of the invention.

Example 1

The turning mirror of fig. 13 is obtained according to the following example. A 10 μm thick silicon dioxide (SiO₂) layer (the first sub layer 10a) is grown by thermal oxidation on a monocrystalline silicon substrate 7, in particular using

HIPOX. Then, a 4 μm thick borophosphosilicate glass (BPSG) layer 10b is grown on top of the first layer 10a by APCVD using a multilayer deposition technique, which requires sub-steps of reflow at around 1,000⁰C in an N-rich atmosphere. Alternatively, this layer 10b can be deposited using either LPCVD or APCVD. The layer 10b is doped with Boron and Phosphorus concentrations equal to 3.4 % and 2.4 % by weight, respectively, so that a refractive index of 1.4594 + 0.0003 at 633 nm is obtained. First and second sub-layers 10a, 10b form the lower cladding 10. Then, a Ge-doped (Germanium concentration equal to 2.5% by weight) silicon oxide core layer 8 having refractive index of 1.4821 ± 0.0002 at 1554 nm is deposited by PECVD (or alternatively by APCVD) on top of the second sub-layer 10b. The core layer 8 is then patterned by means of optical lithography and Deep Oxide Etching, forming, among others, the end surface 8a. The thickness of the core is equal to 2.6 μm.

Afterwards, an additional 8 μm thick BPSG layer 11 (upper cladding) is deposited by APCVD on top of and around the patterned core 8. The refractive index and the physical characteristics of this BPSG upper cladding layer 11 are as those of the second sub-layer 10b. The waveguide structure 2 is thus obtained.

On top of the upper cladding 11, a poly-Si layer 12 having a thickness of 3 μm is deposited by LPCVD performed in a reactor from SiH₄ at a deposition temperature of 600⁰C and at a pressure of 250 mTorr for 460 min. A 2 μm-thick positive photoresist HPR 504™ layer 14, made by Fujifilm Arch. Co., is deposited on the poly-Si layer 12 and it is patterned (for example using UV radiation and a developer) so as to form a hard mask. Patterning of poly-Si

12 is made by Reactive Ion Etching (RIE) in order to form the aperture 15' with w_s= 2 μm (fig. 4b) on first masking layer 12. In the RIE step, one of the following gases/ gas mixtures can be used: SF₆, CF₄ , CF₄+O₂ or CI₂+Ar. By using the so-formed hard mask 13, the trench 19 is formed by deep oxide etching into the cladding 9. Deep etching is carried out by means of an Advanced Oxide Etching (AOE) System made by STS (Surface Technology Systems) including a C₄F₈ plasma. The width of the trench is equal to 2 μm and its depth to 12 μm. Two substantially parallel facing surfaces 6 and 6a are obtained.

The photoresist 12 is thus removed using oxygen as an etching gas in a plasma reactor. The first poly-Si hard mask 13 is removed either by using a RIE step or by a wet etching step in a basic solution (a 25% TMAH™ solution made by Fujifilm Arch. Co. at 90⁰C has been employed). A second poly-Si layer 20 having a thickness of 0.7 μm is deposited by LPCVD performed in a reactor in a SiH₄ atmosphere at a deposition temperature of 600⁰C and pressure of 250 mTorr covering the upper surface of the cladding 9 and the trench surfaces 6,6a. A HPR 504™ photoresist 22 is deposited on top of the poly-Si layer 20 with thickness of 1.4 micron it is patterned, and used as a mask in a subsequent RIE etching phase to realize the aperture 18 on the layer 20 forming the second hard mask 24. The aperture 18 has a rectangular section of 30 X 5 μm² (along the Z and X axis, respectively) section, with a distance of 3 μm from the closest boundary (the surface 6a) of the trench 19. The photoresist 22 is thus removed by oxygen plasma. A wet etching phase thus follows using the hard mask 24 to protect the underlying cladding 9. The used solution is a 7: 1 by volume of Buffered Oxide Etchant OHS CPG with surfactant made by Fujifilm Arch. Co., containing HF and NH₄F, at 20⁰C. The etching rate of the cladding 9 is equal to about 234 λ/min. The hard mask 24 (poly-Si) results substantially unaffected by this etchant.

A cavity 3' is formed in the cladding 9, which includes an angled surface 5 having an angle with respect to the (X,Z) plane of 50° ± 2.5°. The poly-Si layer 24 is thus removed in a wet etching phase employing a basic Silicon Etch TMAH™ 25% solution made by Fujifilm Arch. Co. at 90⁰C, which does not substantially etch the cladding material 9.

The resulting cavity 3 including the vertical end surface 6 and inclined surface 5 according to the described example is illustrated in fig. 13.

Claims

1. Method for fabricating an optical device (100) comprising a turning mirror (4), said method comprising the steps of: - providing a substrate (9) having a substrate surface and being apt to propagate an optical signal along a propagating direction (X); dry etching a trench (19) in the substrate surface down to a predetermined depth into said substrate (9), said trench (19) comprising at least a sidewall defining a surface (6) substantially perpendicular with respect to the propagating direction (X); forming a protecting layer (20) so as to cover at least part of the substantially perpendicular surface (6); forming a first mask (24) on said substrate (9), said first mask (24) defining a first aperture (18) exposing a portion of said substrate, said first aperture (18) being adjacent to the trench (19); etching said substrate (9) through said first aperture (18) by using a wet etchant so as to form a hole (3') in said substrate (9), said hole defining an angled surface (5) forming an angle relative to the propagating direction (X), and - removing said protecting layer (20) so that said hole (3') and said trench (19) merge to form a single cavity (3).

2. A method according to claim 1, wherein said angled surface (5) is opposite to said substantially perpendicular surface (6).

3. A method according to claim 1, wherein said protecting layer (20) is substantially unattacked by said wet etchant.

4. A method according to claim 1, wherein said dry etching is a deep oxide etching phase.

5. A method according to claim 1, wherein the step of forming said trench (19) by dry etching comprises the sub-steps of: - forming a first layer (12) on said substrate surface; patterning said first layer (12) to define a second aperture (15, 15') so as to form a second mask (13), and dry etching the trench (19) using said second mask (13), wherein said second aperture (15,15') is adjacent to said first aperture (18).

6. A method according to claim 5, wherein the first layer (12) is a hard masking layer.

7. A method according to claim 5, wherein said second aperture (15') is rectangular.

8. A method according to claim 4, wherein said second aperture (15) is U- shaped and comprises a first, a second and a third arm (16a, 16b, 16c), the first and the second arm (16a, 16b) extending from two opposite ends of said third arm (16c), wherein said first and second arm (16a, 16b) are parallel among them and perpendicular to said third arm (16c).

9. A method according to claim 6, wherein said second aperture (15') has a width (Ws) along the propagating direction (X) comprised between 1.5 μm and

4 μm.

10. A method according to claim 7, wherein the width (wy) of said third arm (16c) along the propagating direction (X) is comprised between 1.5 μm and 4 μm.

11. A method according to claim 1, wherein said depth of said trench (19) is comprised between 12 μm and 16 μm.

12. A method according to claim 1, wherein the step of forming said first mask (24) comprises the sub-steps of depositing a second layer (20) on said substrate (9) and patterning a first aperture (18) on said layer (20).

13. A method according to claim 12, wherein said step of forming said second layer (20) on said substrate (9) and of forming a protective layer (20) covering at least part of the substantially perpendicular surface (6) are a single deposition step.

14. A method according to claim 11 or 12, wherein the distance between the boundary of said aperture (18) and the boundary of said trench (19) is comprised between 1 μm and 5 μm.

15. A method according to any of claims from 11 to 13, wherein the width of said aperture (18) along said propagating direction (X) is preferably comprised between 4 μm and 10 μm.

16. A method according to any of the preceding claims, wherein the material in which said protective layer (20) is formed comprises polycrystalline silicon.

17. A method according any of the preceding claims, wherein said protective layer (20) has a thickness comprised between 0.5 μm and 1.5 μm.

18. A method according to any of the preceding claims, wherein said first mask (24) is substantially unattacked by said wet etchant.

19. A method according to any of the preceding claims, wherein said wet etching phase comprises the steps of a. forming an undercut (23) between said first mask (24) and said substrate (9) and b. forming said angled surface (5) in correspondence of said undercut

(23).

20. A method according to any of the preceding claims, wherein said substrate (9) comprises silicon based material.

21. A method according to claim 1, wherein said substrate (9) comprises silicon oxide material.

22. A method according to claim 1, wherein the step of providing a substrate (9) comprises the steps of forming a waveguide (2) on a first supporting substrate (7), said waveguide including a core (8) and a cladding (9).

23. A method according to claim 22, wherein said core (8) comprises an end (8a) and wherein said substantially perpendicular surface (6) is located adjacent to said end (8a) of said waveguide (2), so that an optical signal (OS) propagating in said waveguide (2) exits the cavity through said perpendicular surface (6) or said optical signal propagating in said cavity (3) enters said waveguide through said perpendicular surface (6).

24. A method according to claim 23, wherein the distance along the propagating direction (X) between said core end (8a) and said perpendicular surface (6) is comprised between 1 μm and 4 μm.

25. A method according to claim 1, wherein the angle comprised between said propagating direction (X) and said angled surface (5) is comprised between 40° and 55°.

26. A method according to claim 1, comprising the step of forming a metallic layer on said angled surface (5).