WO2001027672A1

WO2001027672A1 - Adjusting assembly of a miniature optical component formed by transferring an optical integrated circuit chip onto an optical fibre connection platform

Info

Publication number: WO2001027672A1
Application number: PCT/FR2000/002825
Authority: WO
Inventors: Henri Porte; Michel De Labachelerie; Jean-Claude Jeannot; Vincent Armbruster; Neila Kaou; Pascal Mollier; Nicole Devoldere
Original assignee: Highwave Optical Technologies
Priority date: 1999-10-12
Filing date: 2000-10-11
Publication date: 2001-04-19
Also published as: FR2799553A1; FR2799553B1; AU7798000A

Abstract

The invention concerns a miniature optical component formed by transferring a chip (1) comprising axial (ZZ) optical waveguides (2), onto a platform (3) recessed with axial grooves (4, 4') for positioning optical fibres (5, 5'). The invention is characterised in that a portion of the platform surface corresponding to the site (7) for transferring the chip, is recessed with at least a series of parallel grooves alternating with interposed ridges having a reduced planar summit surface. The recessed grooves (10) at the site (7) for the chip are preferably transverse.

Description

ADJUSTING THE ASSEMBLY OF A MINIATURE OPTICAL COMPONENT

DEFERRED FORM OF AN OPTICAL INTEGRATED CIRCUTT CHIP

ON A FIBER OPTIC CONNECTION PLATFORM

The present invention relates to the field of optical integrated circuits and more particularly to miniature optical components formed by transfer of a substrate chip comprising optical waveguides on a platform hollowed out by grooves for positioning and coupling optical fibers.

It has long been known to make optical integrated circuits (in English "Integrated Optic Circuits", abbreviated IOC) by implanting a circuit of microscopic waveguides and functional optical elements on a miniature substrate plate called "chip" (in English "chip").

Recent developments in optical integrated circuits make it possible to incorporate multiple optical elements (networks, mirrors, etc.) and to associate various photonic (transmitters, detectors, diodes, lasers, etc.) or electro-optical (modulators, couplers) functions. , etc.) in a miniature chip which then constitutes almost an optical mini-bench.

The use of optical integrated circuits is of considerable interest to fiber optic telecommunications networks. It is notably envisaged to incorporate, using optical integrated circuits, signal processing functions at the level of the communication lines themselves. Optical integrated circuits are thus called upon to fulfill online processing functions, such as routing, multiplexing, demultiplexing, modulation, switching or amplification functions of optical signals. The main obstacle to the development of optical integrated circuits in fiber optic communication networks is the technical problem of coupling between optical glass fibers and optical integrated circuit waveguides. The waveguides and the optical fibers have indeed a core, that is to say an optically active axial part; whose dimensions are microscopic, and more precisely, micrometric, even submicrometric, according to implantation technologies. There are manual methods of individual fixing of a fiber in the axis of a guide after positioning, adjustment and adjustment on an optical measurement bench with a mechanical or piezoelectric micrometric displacement device. However, these precision methods implemented in the laboratory are expensive in terms of time and cost of adjustment, so that they cannot be applied industrially. In addition, these methods hardly allow to envisage the connection of a plurality of optical fibers to an optical integrated circuit.

Automatic alignment techniques are known for a series of optical fibers and optical integrated circuit waveguides which allow mass production, while offering good quality of coupling between the fibers and the guides. These self-alignment techniques use a principle of transferring an optically active chip onto another inactive optical fiber support chip, known under the English terminology of "flip-chip".

The proven flip-chip techniques thus provide:

on the one hand, to implant an optical integrated circuit, by tracing optical waveguides on a first substrate chip, the waveguides extending direc tionally up to the two opposite edges of the chip,

- on the other hand, to produce a support platform by digging on the surface of a second substrate chip, a series of directional grooves profiled in the shape of a letter V. This makes it possible to arrange the series of optical fibers in a sheet plane, each fiber resting in the hollow of the V of a respective groove, the core, that is to say the axis of the fiber, flush with the surface of the platform, - then the first integrated circuit chip optical is transferred to the second support platform chip, the face of the first chip comprising the waveguides ^~ - coming to be applied against the platform surface of the second chip, making the fibers correspond axially the guides . The substrate of the optical fiber support chip is generally silicon, a material whose crystal structure has the advantage of allowing machining or cleavage along orthogonal or oblique planes, which in particular makes it possible to engrave grooves with a V profile. On the other hand, the substrate of the optical integrated circuit chip is not necessarily silicon, or even a semiconductor, the "flip-chip" techniques having the advantage of allowing the hybridization of two chips of different substrates. According to the state of the art, the substrate of an optical chip may in particular be silicon (Si), germanium (Ge), gallium arsenide (AsGa), indium phosphide (InP), lithium niobate (Li b0 ₃ ) or glass, or even polymers. The substrate of the optical chip can thus be a semiconductor or insulating body, the electrical properties not being essential as in electronic circuits.

The techniques of "flip-chip" make it possible to automate the connection of a series of fibers to an optical integrated circuit by providing an excellent coupling between the fibers and the waveguides of the circuit, because all the axial degrees of freedom and for angular alignment are controlled, except for the transverse positioning of the chip with respect to the flat ^¬ form.

In addition, we have developed and reported systems for assembling a chip on a platform, in which it is planned to have assembly microstructures complementary, male and female, on the surface of the chip and of the platform, which makes it possible to control the transverse positioning, last degree of freedom, and which gives a complete passive self-alignment of the guides with the fibers. Reference should be made to patent document FR-A-99 05587, in the name of the applicant, for further details of description of these improved "flip-chip" techniques and of the structures and methods of assembling a chip on a fiber optic connection platform, to form a miniature optical component.

When the optical chip is transferred to the support platform, the axial optical plane of the waveguides must correspond with the axial plane of the optical fibers so that the optical transmission can be perfect. Depending on the implantation technologies used, the optical chips can include waveguides in relief or flush with the surface of the chip, or even internal waveguides "buried" in the thickness of the chip. In addition, the axial core, the optical vector, of the waveguides is covered with a layer of refractive material, of high index, to confine the light to the core of the guide.

The axial optical plane of the guides is therefore offset in height relative to the reference surface formed by the surface of the chip or by the top surface of the relief of the waveguides, a reference surface which comes into contact against the surface of the platform during carryover. In "flip-chip" techniques, it is therefore planned to adjust the depth of the grooves to offset the height of the axial plane of the optical fibers so as to correspond exactly to the axial optical plane of the raised or underlying waveguides on the surface of the chip.

In "flip-chip" techniques, it is necessary to proceed under a vacuum of extremely high dust, during the transfer of the chip to the platform, to prevent dust from interfering between the reference surface of the chip and the surface of the platform, which would level the plane of waveguides relative to the plane of the fiber web and would interfere with optical coupling.

A drawback of the optical component assembly techniques, by transferring an optical integrated circuit chip to an optical fiber connection platform, is the sensitivity of the optical coupling to the interference of dust.

The general drawback of the known "flip-chip" techniques is the need to carry out the chips under an absolute vacuum of dust. In addition, the inventors have detected that certain optical coupling problems arise from systematic defects in the flatness of the chips of optical integrated circuits. The waveguides of an optical chip are generally formed from two layers of different materials (a low index material such as silicon or light glass for the core and a high index material such as silica or glass heavy for confinement), so that the chip is likely to present a certain curvature or deformations, by effect of bimetallic strip.

It follows that the face of the chip is not perfectly flat and that all the waveguides implanted on the face of the chip cannot be perfectly aligned with the optical fibers arranged in a flat sheet on the platform.

Another drawback of known techniques for assembling an optical component by transferring an optical integrated circuit chip to a platform is therefore the impossibility of perfectly aligning the whole series of guides with the fibers because of the curvature or deformation of the chips.

The object of the present invention is to improve the transfer of an optical integrated circuit chip onto an optical fiber connection platform to form a miniature optical component while avoiding the aforementioned drawbacks.

The primary objective of the invention is to provide structures making it possible to adjust the assembly of a miniature optical component formed by transferring an optical integrated circuit chip onto a connection platform for optical fibers, the adjustment must be insensitive to dust and unevenness.

A particular objective of the invention is to implement structures or methods making it possible to flatten the face of a chip comprising waveguides against the surface of the platform where the optical fibers are arranged.

Another objective of the invention is to immobilize definitively, once adjusted, the assembly of the chip with the platform. Finally, the invention aims to achieve such structures without additional cost and without complicating the steps of manufacturing and assembling the optical component. In particular, the objective is to integrate the production of such structures during the production of the chip support platform, in a single step, without additional cost.

Briefly, these objectives are achieved, according to the invention, by providing for digging one or more series of grooves alternating with narrow ridges in the surface of the platform, at the location of the chip, to reduce the contact surface between the chip and the platform, the chip resting only on the summit area of the ridges formed in the platform. The grooves and ridges hollowed out at the location of the chip are preferably drawn transversely to the optical axis of the waveguides and axial grooves for positioning the optical fibers, so that the relief of the ridges crosses the relief of the guides waves if they are salient.

The invention is carried out with a miniature optical component formed by transferring a chip comprising axial optical waveguides, on a platform hollowed out by axial grooves for positioning optical fibers, in which a surface portion of the plate -form corresponding to the transfer location of the chip, is hollowed out by at least one series of parallel grooves alternating with intermediate ridges having a reduced flat top surface. Preferably, the series or series of parallel grooves and intermediate ridges, arranged at the location of the chip, are arranged transversely to the optical axis.

In particular, the top of the intermediate ridges is narrower than the opening of the grooves dug at the location of the chip. Furthermore, the parallel grooves and the "" intermediate ridges, formed at the location of the chip, have transverse dimensions of the order of half to twice the transverse dimension of the chip. According to the preferred embodiment of the invention, each axial end portion of the location of the chip on the surface of the platform is hollowed out by a series of parallel grooves and intermediate ridges reducing the contact surface between the respective end of the chip and the platform.

According to an alternative embodiment of the invention, the series or series of parallel grooves communicate with at least one opening passing through the platform.

In this alternative, the transverse dimension of the grooves communicating with the opening, is substantially less than the transverse dimension of the chip.

Preferably, the opening communicating with the parallel grooves is formed at the bottom of a cavity in the form of a truncated pyramid hollowed out opposite the surface of the platform. It is expected that the cavity dug to form the opening ccmmuniculant with the parallel grooves, has a depth equal to the depth of a cavity dug to form an opening of the assembly structure.

According to another alternative embodiment, at least one groove, dug at the location of the chip, is connected with at least one bonding basin dug in the platform.

In this other alternative, the transverse dimension of the groove or grooves connected with the bonding basin is substantially greater than the transverse dimension of the chip. According to an alternative embodiment, the axial grooves for positioning the optical fibers communicate with at least one transverse groove connected with at least one bonding basin, hollowed out in the platform. According to another alternative embodiment, the axial grooves for positioning the optical fibers communicate with at least one opening passing through the thickness of the platform.

It is expected that each axial groove for positioning an optical fiber communicates with a respective opening crossing the thickness of the platform.

It is advantageously provided that the openings communicating with the axial grooves are distributed over several rows spaced axially.

Preferably, each opening communicating with an axial groove is formed at the bottom of a cavity forming a dihedral, hollowed out opposite the surface of the platform.

The invention is also obtained by implementing a method for assembling a miniature optical component formed by transferring a chip into the substrate, comprising axial waveguides on a platform made of crystalline material hollowed out with axial grooves for positioning optical fibers, comprising steps consisting in:

- dig, by anisotropic etching, in a portion of the surface of the corresponding platform, at the location of the chip, at least one series of parallel grooves, providing intermediate ridges reducing the contact surface of the platform, and

- Transfer the chip to the platform, the reference face of the chip relative to the waveguides, resting against the top surface of the ridges formed in the surface of the platform.

Preferably, the parallel grooves are hollowed out, at the location of the chip in the surface of the platform made of crystalline material, by anisotropic etching, transversely to the optical axis. According to an alternative method of procedure, other steps are provided, consisting in:

- provide, through the thickness of the platform, at least one opening communicating with a series of parallel grooves dug at the location of the chip, and

- press the chip against the surface of the platform by sucking through the opening.

Preferably, the opening communicating with the parallel grooves is formed by digging a flat-bottom cavity, by chemical attack opposite the surface of the platform through a large window mask and by stopping the chemical attack, when the bottom of the cavity reaches a thickness less than the depth of the furrows dug in the surface of the platform. According to this alternative, other steps are planned, consisting of:

- move the chip parallel to the platform, keeping the face of the chip transferred or pressed against the surface of the platform, and - optimize the position of the chip relative to the platform, by measuring the transmission optic between the axial waveguides of the chip and the optical fibers arranged in the axial grooves of the platform.

According to another alternative method, other steps are provided, consisting of:

- dig at least one bonding basin in the surface of the platform, the basin being connected with at least one groove dug at the location of the chip,

- have glue in the bonding basin, and - spread the glue in each groove connected to the basin, to fix the assembly of the chip and the platform. According to this other alternative, steps are provided, consisting of:

bonding each end of the chip to the platform by digging, respectively, in each distal portion of the location of the chip on the surface of the platform, a series of grooves connected to a bonding basin, and

- spread the glue in each series of grooves.

According to a variant of the process of the invention, other steps are planned which consist in: - digging at least one transverse groove ≈xτιτtuniculating with the axial grooves, and at least one bonding basin connected to each transverse groove,

- position the optical fibers in the axial grooves,

- have glue in the bonding basin, and - spread the glue in each transverse groove to fix the positioning of the optical fibers in the axial grooves.

According to another variant of the process according to the invention, other steps are provided, consisting in:

- provide through the thickness of the platform, at least one opening communicating with the axial grooves, and

- position the optical fibers at the bottom of the axial grooves by sucking through the opening.

Preferably, each axial groove communicates with a respective opening. According to the preferred method of the invention, each opening communicating with an axial groove is formed by digging a cavity with a pointed bottom by anisotropic chemical attack, through a mask with a narrow window, opposite the surface of the platform. According to the preferred method of the invention, the openings communicating with the grooves are formed by digging, during a single anisotropic chemical attack step, cavities with a pointed bottom and cavities with a flat bottom.

Advantageously, the pointed bottom cavities have a depth less than the depth of the flat bottom cavities. Other objectives, characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of non-limiting exemplary embodiment, with reference to the appended drawings in which: - Figure 1 shows a side view and an axial section of a structure for adjusting the assembly of a miniature optical component, according to the invention,

- Figure 2 shows a perspective view and an axial section of a second structure for adjusting the assembly of a miniature optical component, according to the invention.

- Figure 3 shows a perspective view and an axial section of a third structure for adjusting the assembly of a miniature optical component, according to the invention.

FIGS. 4A, 4B and 4C represent cross sections of the platform of FIG. 5 of a miniature optical component according to the invention, seen in section along the axes AA, BB, CC at the level of openings communicating with furrows

FIG. 5 represents a plan view of the surface of a miniature optical component platform, in which several structures are provided to adjust the assembly of the component, according to the invention, and

- Figure 6 shows a cross section of a miniature optical component platform, along a bonding groove provided in the third structure to adjust the assembly of the component, according to the invention.

FIG. 1 shows an exploded view of a miniature optical component according to the invention, before the transfer of the chip 1 to the platform 3 for supporting optical fibers 5.5 ′.

The transfer face of the chip 1 comprises an optical integrated circuit, represented here in the very simplified form of a series of optical waveguides 2 rectilinear parallel to an axis Z'Z 'which thus constitutes the optical axis, c that is to say the axis of propagation of the optical signals, at the input and at the output of the waveguides. Of course, the optical integrated circuit implanted on the chip 1 may include waveguides of non-rectilinear layout and incorporate all kinds of functional optical elements which will not be listed herein. However, in the present it should be considered that the waveguides 2 are directed axially and reference is made to "axial waveguide", which means that the two distal parts of the guide are parallel to the optical axis Z 'Z', the middle part of the guide can follow any route. Indeed, it is provided that the two ends of each guide 2 extend to the two opposite edges of the chip, each distal part of the guide being perpendicular to the edge of the corresponding chip (the edges themselves being perpendicular to the axis (Z'Z ¹ ), each end of the guide ending in a plane cleavage parallel or preferably coincident with the plane of the edge of the chip.

Thus, the waves of the signals which propagate along the optical axis Z'Z ¹ , are normally transmitted to the input and to the output of the corresponding guide. Correlatively, axial grooves are dug in the surface of the platform 3 opposite the face of the chip 1 comprising the waveguides 2. The axial grooves are drawn in the intended extension of the guides waves, since the optical fibers positioned in the grooves must be rigorously aligned with the corresponding guides to optimize the optical transmission.

The axial grooves are therefore hollowed out in the surface of the platform, parallel to the axis ZZ which corresponds and merges with the optical axis Z'Z 'of the chip, once the component has been assembled.

The axial grooves 4,4 'are drawn on the surface of the platform, preferably outside of the location 7 of the chip, the length or axial dimension of the platform being greater than that of the chip. Generally, two series of axial grooves 4 and 4 ′ are dug respectively at each end of the platform, respectively, between the end of the platform and the axial end of the location 7 of the chip. It is expected that the platform is made up of a crystalline substrate capable of allowing anisotropic etching, that is to say a material allowing an attack, generally chemical, along preferential crystallographic axes.

The platform is notably composed of silicon, a substrate which is easily machined or cleaved along orthogonal planes as well as interesting oblique planes, by simply carrying out a chemical attack by the wet method.

The anisotropic chemical etching of the silicon is carried out for example in a sodium hydroxide solution (of formula NaOH) or in a bath consisting of a solution mixture of oxidizing agents such as hydrazine, potassium hydroxide (KOH) or ethylene diamine, and solubilizing agents, such as pyro-catechol or isopropyl alcohol (IPA).

The silicon is machined or cleaved in particular according to the planes conventionally noted [1,0,0], [1,1,1] and [1,1, -1] which correspond, for example, to the surface of the platform, on the side of an axial groove and on the other side of the axial groove.

The anisotropic etching of the platform in crystalline material has several essential advantages. The first advantage is to allow the drawing of strictly rectilinear and perfectly parallel grooves, since the etching follows a crystallographic axis despite possible localized defects in the etching mask.

The second advantage is that it makes it possible to dig grooves in the form of a perfect dihedral, that is to say with strictly flat sides and with a constant V profile.

The third consecutive advantage is that the depth of the V-shaped groove is perfectly determined by the width of the etching openings. The width of the etching mask windows fixes the opening width of the corresponding grooves. As the sides of the furrow are cut or cleaved from the edge of the opening and have an angle determined by the crystal structure, the depth of the furrow strictly corresponds to the width of the opening and to the angle between the planes of cleavage crystallographic.

It should be noted in this connection that the anisotropic chemical etching by a wet process of crystalline substrate such as silicon generally has the property of progressing very rapidly, perpendicular to the surface until the formation of a dihedron along the crystallographic planes, then of progress very slowly parallel to the surface, that is to say in the direction of widening of the dihedral. Thus, the attack on a silicon wafer covered with a mask pierced with etching windows in a soda bath, begins by forming a bowl with oblique sides and a flat bottom which is hollowed out very quickly in depth, up to that the bottom is reduced to the axis of a dihedral (point of a V); from this point, the etching no longer progresses towards the bottom, but extends very slowly in the direction of widening of the V, gradually gnawing at the sides which remain parallel to the oblique planes of the crystal structure.

To dig a groove of precise depth, it is therefore advantageously sufficient to control the width of one etching opening of the groove and to calculate the etching time so that the digging reaches this depth.

If ever by mistake, the duration of the engraving is prolonged after reaching this point of depth, the prolongation of the engraving only very slowly affects the dimensions of the groove. The error on the engraving time therefore has no consequence.

The transverse dimension in width and depth of the grooves is chosen as a function of the size of the optical fibers, so that the core of each fiber is flush with the surface of the platform when the cylindrical fiber is arranged in the groove by pressing along the two sides. Thus, as the sectional views of FIGS. 1, 2 and 3 suggest, any optical beam coming from the axial core of a fiber shaves the plane of the surface of the platform and can be fully transmitted to the corresponding waveguide if it is correctly pressed against the surface of the platform.

Now, to achieve this objective according to the invention, it is planned, as illustrated in FIG. 1, to further dig a succession of parallel grooves 10 separated by intermediate ridges, at the location 7 of the chip in the surface of the platform. form 3.

Thus, when the chip 1 is transferred to the location 7 provided against the surface of the platform 3, the face of the chip comprising the waveguides 2 comes to bear on the top of the ridges formed in the series 10 of parallel grooves.

Thus, any dust which gets between the chip and the platform, during the transfer, is deposited at the bottom of a groove and does not interfere with the assembly of the chip on the platform.

Advantageously, the invention thus allows adjustment of the face of the chip against the surface of the platform and perfect optical alignment.

To avoid the interference of dust, it is provided according to the invention, to hollow out the maximum surface area of the platform 3 at the location 7 of the chip. Preferably, the grooves 10 are dug deeply to maximize the hollowed out area.

However, the depth of the grooves must remain significantly less than the thickness of the platform for reasons of mechanical strength. Consequently, the width of the grooves 10 dug at the location 7 of the chip is limited.

Furthermore, the sides of the grooves 10 must not meet, that is to say cut by forming edges whose level may not correspond to the level of the surface of the platform. According to the invention, as illustrated in FIG. 5, the neighboring parallel grooves 11, 12 etc. are not connected, but are separated by ridges 11 ', etc. having a flat top surface, the level of which remains at the level of the plane of the platform surface.

The plane of the summit surface of the ridges 11 ′ must remain coincident with the surface plane of the platform 3 so as to exactly transfer the face of the chip 1 against this surface and to obtain perfect optical alignment of the guides. waves 2 with the optical fibers 5 positioned on the platform.

Advantageously, however, provision is made to minimize the width of the plane top of the ridges interspersed between the grooves 11, 12 etc. , to reduce the contact surface between the chip 1 and the surface of the platform 3. In particular, the top of the intermediate ridges 12 'is significantly narrower than the opening of the grooves.

In addition, as the opening width of the grooves 11, 12 etc. is limited, it is advantageously provided to draw a maximum number of grooves parallel to the location 7 of the chip on the platform, to minimize the contact surface between the chip 1 and the platform 3.

Thus, in the preferred embodiment of the invention, as illustrated in FIG. 5, almost all of the location area 7 of the chip on the surface of the platform 3 is hollowed out with a succession of grooves 10,20,30,30 'parallels by providing ridges 12' with a tabular apex with the smallest possible surface.

Geometrically, the furrows must be strictly parallel to form intercalary ridges with minimal flat top surface.

In a particularly advantageous manner, the invention provides for proceeding by anisotropic chemical etching to dig the parallel grooves 10,20,30,30 'in the location 7 of the chip on the surface of the platform. An anisotropic wet chemical etching advantageously makes it possible to dig grooves 11, 12 etc. strictly parallel and to strictly control the width of the furrows so that the narrow flat vertices of the ridges 11 'etc. are perfectly defined.

An advantage of the anisotropic attack is to prevent the etching of a groove 11 or 12 from biting over the width of the neighboring crest 11 ′, in the event of an extension of the etching time as explained above. Another advantage of anisotropic etching is that by digging grooves 10,20,30 with a V-shaped profile, the intermediate ridges take on a trapezoidal or triangular profile which gives them a solid mechanical foundation.

According to a first embodiment not shown, the parallel grooves dug in one location 7 of the chip, can be drawn in the axial direction ZZ, therefore in the direction of the axial grooves, optical fibers and waveguides .

In an alternative embodiment not illustrated, it is planned to dig two intersecting series of grooves parallel to the location of the chip, that is to say a series of axial furrows and a series of transverse furrows, providing infill ridges reduced to truncated pyramids. This alternative offers the advantage of completely minimizing the contact surface between the chip and the platform.

These embodiments in which the location of the chip is hollowed out with a series of axial grooves are suitable insofar as the face of the chip does not include axial waveguides in relief. However, it is generally preferable that the grooves and the intermediate ridges 10 formed at the location 7 of the chip are not arranged in the axial direction ZZ to avoid entanglement of the relief of the waveguides 2 in the relief of the grooves and ridges 10, and optical alignment defects when the chip 1 is transferred to the platform. According to the preferred embodiment of the invention, as illustrated in all the figures, the succession 10, 20, 30, 30 'of grooves and ridges formed at the location 7 of the chip are drawn in the transverse direction YY to waveguides 2, therefore transverse to the optical axis ZZ and to the axial grooves 4.4 'for positioning the optical fibers 5.5'.

It is particularly advantageous to make the silicon platform and to dig the grooves 4,4 'for positioning the fibers and the grooves 10 at the location 7 of the chip by anisotropic chemical etching, because the crystalline structure of the silicon allows precisely to combine the drawing of a series of grooves 4 with a V profile strictly parallel in the axial direction and the drawing of another series of grooves 10 with a V profile strictly parallel in the transverse direction.

For example, the surface of the platform which forms the crystallographic plane [1,0,0] is dug along the crystallographic planes [1,1,1] and [1, -1, -1] to form the two opposite flanks of the axial grooves 4, then dug along the crystallographic planes [1, -1,1] and [1,1, -1] to form the two opposite flanks of the transverse grooves 10.

From these indications and his crystallographic knowledge, a person skilled in the art can, without departing from the scope of the present invention, determine other cleavage planes and other directions of digging the grooves for positioning the optical fibers. and recess grooves at the location of the chip, in a silicon substrate as well as in other crystalline substrates.

Thus, according to an alternative embodiment, the series of grooves and intermediate ridges formed in the location of the chip on the surface of a silicon platform can be traced in an oblique direction intermediate between the transverse direction YY and the axial direction ZZ of the 4.4 'grooves for positioning the optical fibers. Indeed, the plane [1,0,0] of the silicon surface can still be cut or cleaved according to crystallographic planes noted [2,1,1], [2, - 1,1], ... to cut the opposite flanks of a series of oblique grooves compared to the axial grooves.

The primordial advantage of the invention is therefore to integrate the production of a structure of transverse grooves making it possible to adjust the assembly of the optical component, during the single stage of production of the platform which consists to etch the structure of axial grooves allowing precise positioning of the optical fibers. The structure of transverse grooves is advantageously obtained by simply modifying the etching mask of the platform. A single process step therefore makes it possible to carry out the various functional structures provided, without additional cost or complication and with optimal precision (a single mask). Now, according to another embodiment of the invention illustrated in FIG. 2, it is planned to pierce one or more openings 29 in the middle of a series of parallel grooves 20, dug at the location 7 of the chip, in the surface of the platform 3. The opening 29 crosses the thickness of the platform 3 and opens into the series of grooves 20.

The opening 29 has the function of allowing the suction of the chip 1 against the surface of the platform 3, by carrying out the vacuum opposite the surface of the platform. The invention thus provides a method consisting in:

- provide, through the thickness of the platform 3, at least one opening 29 communicating with a series of parallel grooves 20 hollowed out at the location 7 of the chip, and

- press the chip 1 against the surface of the platform 3 by sucking through the opening 29. to allow the assembly of the chip 1 of the optical integrated circuit on the connection platform 3 to be perfectly adjusted 4.4 'optical fibers forming the miniature optical component.

The dimensions of the suction opening 29 are therefore less than the width of the chip 1, the section of the opening 29 preferably being significantly smaller than the surface of the chip to avoid mechanically weakening the platform 3.

To make the opening 29, provision is preferably made for wet chemical etching opposite the surface of the platform 3.

Advantageously, provision is made to dig the grooves on the surface of the platform and to pierce the openings in the opposite face, during a single step of chemical etching consisting in immersing the platform in a bath of chemical attack by protecting each of these faces with an etching mask.

The advantage of the method according to the invention is to integrate the production of the various structures for adjusting the optical component and positioning the fibers in a single manufacturing step, without any additional cost.

According to the preferred method, a wide cavity 29 with a flat bottom is hollowed out in the opposite face of the platform 3 which is covered with a mask with wide windows and immersed in a chemical attack bath. The duration of chemical etching is calculated to stop when the thickness of the bottom of the cavity 29 becomes less than the depth of the grooves 20, that is to say when the flat bottom of the cavity 29 reaches the point in V grooves 20, as illustrated in FIG. 4C. It is not necessary for the etching to extend beyond it since the cavity 29 therefore communicates with the corresponding grooves 20 and forms an opening.

It is provided, in fact, that the opening 29 communicates with a series 20 of extended grooves. The grooves 20 communicating with the opening, however, have transverse dimensions smaller than the chip 1 to avoid leakage during suction.

Advantageously, such a structure allows during suction, that the suction effect extends over the entire opening surface of the grooves 20 communicating with the opening suction 29, while avoiding deformation of the chip 1 of the optical integrated circuit insofar as the intermediate ridges support the chip.

Such a structure, analogous to the arrangement of a grid on a suction mouth, advantageously allows the chip to be pressed perfectly against the ridges formed on the surface of the platform without risk of deformation of the waveguides, nor of optical aberration.

Another advantage of vacuuming is to remove any dust that could get between the chip and the platform during assembly.

A particular advantage of such a suction system is that it makes it possible to straighten a non-planar chip by pressing it against the plane of the ridges formed on the surface of the platform during assembly.

Consequently, in the preferred embodiment of the invention, it is planned to pierce several suction openings at the location 7 of the chip in the platform 3, each distal part of the location 7 of the chip comprising preferably a respective suction opening.

The suction openings drilled at the axial ends of the location 7 of the chip advantageously make it possible to press each end of the chip 1 against the surface of the platform 3, which eliminates the curvature and any defect in the flatness of the chip . The ends of the waveguides 2 are thus brought exactly into the optical reference plane formed by the surface of the platform 3 which corresponds to the plane grazing the core of the optical fibers 5.5 ′ arranged in the axial grooves 4.4 . According to another arrangement of the invention, indicated in FIG. 2, provision is also made to pierce suction openings 50,59 communicating with the axial grooves 4,4 ′ for positioning the optical fibers 5,5 ′.

In an exemplary embodiment not illustrated, a single suction opening can lead to all of the grooves axial hollowed out at one end of the platform. In this case, it suffices to dig a large flat-bottom cavity covering the extent of the entire sheet of axial furrows and connecting with each of them. So that the fiber does not engage in the opening of the bottom of the groove, the tubular periphery of the fiber must not be in contact with the edges of the opening, but only come into contact with the oblique sides of the groove, which gives the following relation: 1 <D.sin (α) in which:

- 1 is the width of the opening,

- D is the diameter of the fiber, typically 125 μm,

- α is the crystallographic angle between the normal to the surface and the normal to the flanks of the groove, that is to say, according to the previous example, the crystallographic angle between the plane

[1,0,0] of the surface of the platform and the plane [1,1,1] or symmetrically [1, -1, -1] of a groove flank, which is approximately 55 ° for silicon. In other words, as the width 1 of the opening is limited, the depth of the cavity opening into an axial groove must be limited so that the thickness of the bottom of the cavity respects the following formula:

2 (e + Δ)> D.cos (α) in which:

- e is the thickness of the bottom of the cavity, that is to say the thickness of the membrane of crystalline material lying between the plane of the surface of the platform and the plane of the bottom of the cavity, - Δ is the height of the axis of the fiber core relative to the surface of the platform,

- D is the diameter of the fiber core,

- α is the angle between the normal to the surface and the normal to the side of the groove. However, it is difficult to precisely control the depth of the flat bottom, of this large cavity, and therefore the size of the opening opening into the groove.

However, it is preferable that the opening communicating with a groove is narrow to avoid that the fiber does not engage in the opening and does not undergo deformations and optically harmful stresses.

According to the preferred embodiment of the invention, it is provided that each axial groove communicates with a suction opening having a transverse profile in the form of an inverted letter V or of a Greek letter Λ whose engraving depth is perfectly controlled, as previously exposed.

In fact, the engraving depth is calculated so that the tip of the top of the Λ of the cavity just reaches the tip of the V of the bottom of the corresponding groove, like the junction of the letters V and in the well-known acronym of the Volkswagen ^® brand and as illustrated in section views 4A and 4B.

In the preferred embodiment of the invention, it is therefore provided that each axial groove 4 in V, common with a respective suction opening formed by digging a cavity 51,52,53,54,55 with inverted V profile or in Λ.

According to the invention, it is advantageously provided to carry out an anisotropic chemical attack through a mask pierced with narrow etching windows, arranged on the rear face of the platform, in order to dig the cavities communicating respectively with the grooves 4.

As explained above, the width of the narrow windows 50, 51, 52, 53, 54 of the etching mask is calculated precisely, since it determines the depth of the cavity with a pointed bottom in Λ hollowed out by anisotropic etching.

Advantageously, the etching time is calculated so that the tip of the Λ of the cavity reaches the tip of the V of the grooves, by planning to slightly extend the etching time by controlling the etching extension to adjust the width of the opening connecting the cavity and the groove. Advantageously, it is planned to dig the cavities with a pointed bottom by Λ providing the openings communicating with the axial grooves, at the same time as other flat-bottom cavities of different depth dug at the location of the chip and used to other functions (in particular for piercing assembly microstructure openings or openings communicating with transverse grooves).

So that each axial groove 4 communicates with a respective opening formed by digging an independent cavity with transverse profile in Λ, the windows should

50,51,52,53,54 of the etching mask are separated from each other.

However, the depth of the axial grooves 4 and 4 ′ is generally much less than the depth of the cavities 50-59 leading to the axial grooves, since the radius of the optical fibers 5 and 5 ′, typically of the order of several tens of micrometers, is significantly less than the standard thickness of the substrate platforms, typically several hundred micrometers. Consequently, the width of the cavities 50-59 is considerably greater than the width of the corresponding axial grooves 4 and 4 ′.

Also, as illustrated in FIG. 5, according to a preferred method, the cavities 50 to 55 communicating with the axial grooves 4 are distributed axially in staggered rows on several transverse rows, for example, AA and BB, so that the etching windows do not not overlap and that the profil profile cavities are hollowed out independently of each other.

Such openings communicating with the axial grooves make it possible to aspirate and press the fibers one by one in the corresponding grooves.

The essential advantage of the method according to the invention is to allow an excellent adjustment of the position of each fiber and to transfer the engraving precision of the axial grooves 4 and 4 'to the positioning accuracy of the fibers 5 and 5'. In practice, it has been measured, for example, that the coupling between optical fibers maintained in their axial groove by suction, provides a transmission greater than 95%, the attenuation being almost negligible optically. The anisotropic chemical attack makes it possible to dig, through an etching mask, narrow cavities 50-55 with a pointed bottom and wide cavities 29 with a flat bottom, the depths of which are of the same order. This property is advantageous insofar as the axial grooves 4 and the corresponding transverse grooves 20 generally have the same depth.

In the preferred embodiment of the invention, it is however provided that the depth of the pointed bottom cavities and the depth of the flat bottom cavities are substantially different, although they remain of the same order. Indeed, the arrangement of grooves and ridges made in the location 7 of the chip on the surface of the platform according to the invention is particularly advantageously associated with the arrangement of male and female assembly microstructures provided according to the Another invention described in the aforementioned patent document FR-99-05587, which will be referred to for further description details.

As illustrated in FIGS. 4C and 5, said female microstructures 8 and 8 ′ consist of openings in the form of axial buttonholes formed in the surface of the platform 3, at the location 7 of the chip, also.

It is also provided according to said document that the openings 8 and 8 'are pierced through thin membranes formed by digging large cavities 9 and 9' opposite the locations 8 and 8 'of said female microstructures.

The membranes provided for piercing the assembly openings 8 and 8 ′ have a thickness of the order of one to a few tens of micrometers, while the axial grooves 4 communicating with the suction openings 51 and 52 have a depth in the range of several tens to a hundred micrometers and are dug into a platform thickness of the order of a hundred micrometers.

Advantageously, the method of method according to the invention makes it possible to dig cavities of slightly different depths, during a single and same step of anisotropic chemical etching, by providing for digging large cavities 9, 9 ′ and 29, with flat bottom at location 7 of the chip and narrow cavities 50-55 with pointed bottom at the location of axial grooves 4.

Indeed, the depth of the wide cavities 9,9 ′ and 29 with a wide bottom is determined by the etching time, while the depth of the narrow cavities 50-54 with a pointed bottom is determined by the etching width of the cavity, a prolongation of the etching time hardly affecting the depth of a cavity with a pointed bottom, as explained above. During the wet chemical etching step, the depth of the large cavities 9, 9 ', 29 and the narrow cavities 50 to 54 increases first at the same speed until the etching of the narrow cavities 50-54 stops when reaching the tip of Λ, while the engraving of the large cavities continues to progress, in depth.

It is therefore sufficient to extend the duration of anisotropic wet chemical attack beyond the etching time corresponding to the depth of the narrow cavities with a pointed bottom in Λ, to dig cavities 9, 9 ', 29 with a flat bottom deep significantly greater without affecting the depth of the narrow cavities 50 to 54.

The etching method according to the invention therefore advantageously makes it possible to modulate the depth of the cavities and the corresponding dimensions of the assembly and suction openings formed in the surface of the platform. Consequently, the assembly and adjustment of the miniature optical component according to the invention consists in:

- apply a vacuum pump opposite the surface of the platform 3, - place the chip 1 at the location 7 provided on the surface of the platform and,

- arrange the optical fibers 5-5 'in their respective grooves 4-4', the suction of the vacuum pump making it possible both to press the chip 1 and the fibers 5-5 'in their respective locations.

According to an assembly method, provision is made to move the chip 1 transversely while the suction keeps the chip 1 pressed against the surface of the platform 3 by seeking the transverse position of coincidence of the guides on an optical measurement bench of waves 2 with optical fibers 5 and 5 ^" .

According to a variant of the assembly method, the platform expressly comprises female microstructures in the form of an axial buttonhole with a pointed V-shaped end, making it possible to position exactly male microstructures in the form of a stud or mushroom arranged on the face of the chip .

The assembly then consists in fitting the chip onto the platform and sliding it axially to position it precisely transversely (and axially), the arrangement according to the invention of grooves alternating with ridges and the aspiration to the 'location of the chip, ensuring the plating and adjustment of the chip, according to the last degree of freedom "vertical".

Now, if it is necessary to immobilize the assembly of the optical component according to the invention, there is provided a third structure finally allowing fixing by gluing. According to this alternative embodiment, a transverse groove or a series of transverse grooves 30.30 ′ hollowed out at the location 7 of the chip, are connected to a bonding basin 39.39 ′ also hollowed out on the surface of the platform. form 3. Such an arrangement makes it possible to proceed with the final fixing of the assembly of the chip 1 on the platform 3, as illustrated in FIG. 6, by disposing of the liquid glue in the bonding basin 39 ′ and in letting it flow or diffuse, in particular by gravity or by capillarity, possibly joined to the action of heat, in the groove (s) 30 'connected to the basin 39'.

The glue thus coats all the hollow of the groove 30 ′ as well as the face of the chip 1 comprising the waveguides 2.

Taking the glue then locally secures the chip with the platform 3.

The grooves 30.30 ′ connected to a bonding basin 39.39 ′ preferably have a transverse dimension, that is to say a length of the chip greater than the transverse dimension, that is to say the width , so that the end of each groove 30 'of bonding opens freely out of the location 7 of the chip 1, as illustrated in FIG. 6.

The glue can then flow freely over the entire length of the groove 30 '.

Preferably, a bonding basin 39 is connected to a series 30 of bonding grooves 32 alternating with blind grooves 31 not connected to the basin. Such blind grooves are used to collect any dust.

According to the preferred embodiment of the invention, as illustrated in FIGS. 3 and 5, two series of gluing grooves 30 and 30 ′ connected to a respective gluing basin 39, 39 ′ are dug respectively in the two distal parts of

1 location 7 of the chip.

The two axial ends of the chip 1 can thus be permanently fixed against the surface of the platform 3. The advantage of such fixing of the ends of the chip is to be located as close as possible to the interface between the waveguides and the optical fibers, which freezes the positioning of the critical area for optical transmission. The fixing of the ends of the chip against the surface of the platform advantageously prevents any defect in the flatness or curvature of the chip.

When assembling the component according to the invention, it is particularly advantageous to perform a suction, to press and hold each end of the chip while the bonding of each end of the chip is carried out against the platform.

Similarly, it is planned to bond the optical fibers positioned in the axial grooves by means of transverse bonding grooves.

FIGS. 3 and 5 thus show that the two series of axial grooves 4 and 4 ′ for positioning optical fibers cross transverse grooves 40 and 40 ′ connected to gluing basins 49 and 49 ′, respectively. Preferably, a single transverse groove 40 connects the entire series of respective axial grooves 4, to one or two bonding basins 48 and 49.

The method of procedure provides for permanently fixing the fibers positioned in the axial grooves 4 by placing glue in the corresponding basin 48 or 49 and causing it to diffuse or letting it flow into the transverse groove.

40.

According to the preferred method, it is planned to press the fibers 5 and 5 'by suction to position them perfectly in the hollows of the grooves 4 and 4', before fixing them by placing glue in each basin 48,49,48 ', 49' and each bonding groove 40 and 40 '. In sum, the invention provides for multiple arrangements of transverse grooves and suction openings helping to perfectly adjust the assembly of an optical component formed by transferring an optical integrated circuit chip to a connection platform. fiber optics.

Finally, as illustrated in FIG. 5, in the production of an optical component according to the invention, provision is advantageously made for combining the various arrangements of grooves and suction openings, as explained above. The synthetic example of FIG. 5 shows a half of the platform symmetrical along the Y-Y axis, the surface of which has at location 7, the following structures:

- a median bonding basin 39 ', connected to a large series of transverse grooves 30' of bonding, alternating with blind grooves, the grooves being separated by intermediate ridges,

a succession 10 of simple transverse grooves separated by intermediate ridges,

a wide suction opening 29 communicating with a series of transverse grooves 20, hollowed out in the vicinity of assembly openings 9.9 ′, and

- Another bonding basin 39 connected to a series of bonding grooves 30 alternating with blind grooves 31, arranged at one axial end of the location 7 of the chip. In the axial extension of the location 7 of the chip, the end of the surface of the platform 3 is hollowed out by a series of axial grooves 4 connecting with:

- A transverse groove 40 of bonding connected to bonding basins 48 and 49, and with, - openings 50 to 54 of fiber suction distributed over two transverse rows A and B separated axially. It is clear from this preferred exemplary embodiment that the invention advantageously makes it possible to combine:

- the massive central bonding of the chip, - the plating and bonding of the ends of the chip against the platform, and

- the suction and bonding of the optical fibers in position in the axial grooves of the platform.

The maximum number of grooves 10, 20, 30, 30 'dug at the location 7 of the chip makes it possible to minimize the contact surface between the chip 1 and the platform 3.

The essential advantage of the invention is to allow an assembly of optical component insensitive to the presence of dust, which allows the production, connection and assembly of optical components, industrially in premises of common cleanliness class, or even on the site.

Other advantages, variants, embodiments and improvements will appear to a person skilled in the art without departing from the scope of the present invention, the object of protection being defined in the claims below.

Claims

1. Miniature optical component formed by transfer of a chip (1) comprising guides (2) of axial optical waves (OZ), on a platform (3) hollowed out by grooves (4,4 ') axial positioning of optical fibers (5.5 ′), characterized in that a portion of the surface of the platform corresponding to the location (7) of the chip transfer, is hollowed out by at least one series (10) of grooves parallel (11,12) alternating with intermediate ridges (11 ¹ ) having a reduced flat top surface.

2. Optical component according to claim 1, in which the series or series (10) of parallel grooves and intermediate ridges, arranged at the location (7) of the chip, are arranged transversely (OY) to the optical axis ( OZ).

3. Optical component according to claim 1 or 2, wherein the top of the intermediate ridges (11 ') is narrower than the opening of the grooves (11,12) dug at the location of the chip.

4. Optical component according to one of claims 1 to 3, in which the parallel grooves and the intermediate ridges, formed at the location of the chip, have transverse dimensions of the order of half to twice the transverse dimension of the chip.

5. Optical component according to one of claims 1 to 4, wherein each axial end portion of the location

(7) of the chip on the surface of the platform (3), is hollowed out by a series (30) of parallel grooves and intermediate ridges reducing the contact surface between the respective end of the chip (1) and the platform (3).

6. Optical component according to one of claims 1 to 5, in which the series or series of parallel grooves (20), hollowed out at the location of the chip, communicate with at least one opening (29) passing through the platform. shape (3).

7. Optical component according to claim 6, in which the transverse dimension of the grooves (20) communicating with the opening, is substantially less than the transverse dimension of the chip (1).

8. Optical component according to claim 6 or 7, wherein the opening (29) communicating with the parallel grooves (20) is formed at the bottom of a cavity (29) in the form of a truncated pyramid hollowed out opposite the platform surface (3).

9. The optical component as claimed in claim 8, in which the cavity (29) hollowed out to provide the opening (25) communicating with the parallel grooves (20), has a depth equal to the depth of a cavity (9) hollowed out for providing an opening (8) for the assembly structure.

10. Optical component according to one of claims 1 to 9, in which at least one groove (32) is connected with at least one bonding basin (39) hollowed out in the platform (3).

11. An optical component according to claim 10, in which the transverse dimension of the groove (s) (30) connected with the bonding basin (39) is substantially greater than the transverse dimension of the chip.

12. Optical component according to one of claims 1 to

11, in which the axial grooves (4) for positioning the optical fibers (5.5 ′) communicate with at least one transverse groove (40) connected with at least one bonding basin (48,49), hollowed out in the platform shape (3).

13. Optical component according to one of claims 1 to

12, in which the axial grooves (4,4 ') for positioning the optical fibers (5,5') communicate with at least one opening (51) passing through the thickness of the platform (3).

14. Optical component according to one of claims 1 to

13, in which each axial groove (4) for positioning an optical fiber communicates with a respective opening (51, 52) passing through the thickness of the platform (3).

15. Optical component according to claim 14, wherein the openings (51,52,54,55) comiπuniquant with the axial grooves (4) are distributed over several rows spaced axially.

16. Optical component according to one of claims 13 to 15, wherein each opening (51) communicating with an axial groove (4) is formed at the bottom of a cavity (51) forming a dihedral, hollowed out opposite the surface of the platform (3).

17. Method for assembling a miniature optical component formed by transferring a chip into a substrate (1) comprising axial waveguides (2), on a platform (3) made of crystalline material hollowed out with axial grooves (4,4 ') for positioning optical fibers (5,5'), characterized in that it comprises steps consisting in:

- dig, by anisotropic etching, in a portion of the surface of the platform corresponding to the location (7) of the chip, at least one series (10) of parallel grooves (11,12) by providing intermediate ridges ( 11 ¹ ) having a reduced flat top surface, and

- transfer the chip (1) onto the platform (3), the face of the chip, referenced with respect to the waveguides (2), resting against the top surface of the ridges (11 ') formed in the surface of the platform.

18. The method of claim 17, wherein the parallel grooves (10) are hollowed out, at the location of the chip in the surface of the platform (3) of crystalline material, by anisotropic etching, transversely to the axis optical (OZ).

19. The method of claim 17 or 18, comprising other steps consisting in:

- provide, through the thickness of the platform (3), at least one opening (29) communicating with a series (20) of parallel grooves dug at the location (7) of the chip, and

- press the chip (1) against the surface of the platform (3) by sucking through the opening (29).

20. The method of claim 19, wherein the opening (29) communicating with the parallel grooves (20) is formed by digging a cavity (29) with a flat bottom, by chemical attack opposite the surface of the plate -form through a mask with a large window and by stopping the chemical attack, when the bottom of the cavity reaches a thickness less than the depth of the grooves dug in the surface of the platform.

21. Method according to one of claims 17 to 20, comprising other steps consisting in: - moving the chip (1) parallel to the platform (3), keeping the face of the chip transferred or pressed against the platform surface, and

- optimize the position of the chip relative to the platform, by measuring the optical transmission between the axial waveguides (2) of the chip and the optical fibers (4,4 ') arranged in the axial grooves (5 , 5 ') of the platform.

22. Method according to one of claims 17 to 21, comprising other steps consisting in:

- dig at least one bonding basin (39) in the surface of the platform, the basin being connected with at least one groove (32) dug at the location of the chip,

- have glue in the bonding basin (39 '), and

- spread the glue in each groove (30 ') connected to the basin, to fix the assembly of the chip (1) and the platform.

23. The method as claimed in claim 22, comprising steps consisting in:

- glue each end of the chip (1) to the platform (3) by digging respectively, in each distal portion of the location (7) of the chip on the surface of the platform, a series of grooves ( 30.30 ') connected to a bonding basin (39.39'), and

- spread the glue in each series of grooves (30 ').

24. Method according to one of claims 17 to 23, comprising other steps consisting in:

- dig at least one transverse groove (40) communicating with the axial grooves (4), and at least one bonding basin (49) connected to each transverse groove,

- position the optical fibers (5) in the axial grooves, - have glue in the bonding basin, and

- spread the glue in each transverse groove (40) to fix the positioning of the optical fibers (4) in the axial grooves (5).

25. Method according to one of claims 17 to 24, comprising other steps consisting in:

- provide through the thickness of the platform (3), at least one opening (51) communicating with the axial grooves (4), and

- Position the optical fibers at the bottom of the axial grooves by sucking through the opening (51).

26. The method of claim 25, wherein each axial groove communicates with a respective opening (51,52,54,55).

27. The method of claim 25 or 26, wherein each opening (51,52) communicating with an axial groove is formed by digging a pointed bottom cavity by anisotropic chemical attack, through a mask with a narrow window, at the opposite of the platform surface.

28. The method of claim 20 or 27, wherein the openings communicating with the grooves are formed by digging, in a single step of anisotropic chemical attack, sharp bottom cavities and flat bottom cavities.

29. The method of claim 28, wherein the pointed bottom cavities have a depth less than the depth of the flat bottom cavities.