Improved photonic integrated circuit and method for the fabrication thereof

Abstract

The present invention relates to a photonic integrated circuit, comprising a cavity provided in a substrate, a waveguide arranged in or on the substrate, support pillars mechanically supporting an optical element, and soldering being arranged in the cavity between the support pillars for mechanically bonding the optical element to the substrate. In particular, the structural arrangement of the support pillars in combination with the soldering allows for improved connection of the optical element to the photonic integrated circuit upon flip chipping.

Description

[0001] IMPROVED PHOTONIC INTEGRATED CIRCUIT AND METHOD FOR THE FABRICATION

[0002] THEREOF

[0003] BACKGROUND

[0004] 1. Field

[0005] The present invention relates to a photonic integrated circuit (PIC) with improved mechanical bonding (or fixation) of an optical element arranged thereon and a (flip chip) method for the fabrication thereof, preferably for the purpose of attaining optical coupling.

[0006] One field of technology in which the application is PICs is particularly promising, is that of augmented reality (AR). AR is the integration of computer-generated visual elements, sounds and other stimuli with the real-world environment of the user in real time. In this rapidly developing field of technology, small and efficient red, green, and blue (RGB) lasers are key to the development of AR projections applications, such as AR glasses. Challenges that AR projection applications are faced with include miniaturization, integration, and energy efficiency. Additionally, the fabrication method needs to be ready for mass volumes for AR projections applications to be commercially successful.

[0007] 2. Description of Related Art

[0008] The optical connection of optical elements onto a PIC requires precise alignment of the optical paths of the optical elements with the associated optical paths within the PIC. Several techniques are known to this purpose. According to a first prior art technique, the optical element is mounted upright and electrically connected to the integrated circuitry through wire bonding. Wires are welded onto the chip metallized contact pads to provide electrical connection between the optical element and the PIC. In another technique referred to as “flip chip method”, soldering bumps are deposited onto the chip contact pads located on a top side of said chip. Then, the chip is flipped over such that its top side faces down, while controlling the position of the chip contact pads with respect to that of the contact pads on the PIC. The solder bumps are for instance then melted using hot air reflow to complete the interconnect. Lastly, an underfilling step takes place, in which an electrically-insulating adhesive is injected to fill the empty spaces between the chip and the PIC in order to improve the mechanical bonding.

[0009] In practice, the PIC may comprise a substrate in which a cavity is provided and a waveguide arranged in the substrate and comprising an end facet ending in the cavity. In such related art, typically two support pillars are provided in the cavity for mechanically supporting the optical element. These two support pillars are typically elongated in their shape and are arranged parallel with respect to each other, along their long axis. Both support pillars may then support one end of the optical element after flip chipping. This known structural arrangement of the support pillars, however, creates a small space for both mechanical and electrical bonding after flip chipping. In this small space, typically both an underfilling material for mechanical bonding and a soldering for electrical connection are received. The typical underfilling materials are yet sensitive to ionization when exposed to high intensities of optical power and may hence suffer degraded mechanical properties when exposed to high levels of light. When the chip to be flipped is an optical element emitting light like for instance a laser diode, the influence of light emitted from said optical element on the underfilling material may thus create longevity / reliability issues. Therefore, there is a need for a PIC for light applications with high intensities of optical power while offering a reliable mechanical bonding as well as a method for fabricating such a PIC. Additionally, unreliable mechanical bonding limits the accuracy of the placement of the optical element, that is, the optical alignment of the optical output to the waveguide. Accurate alignment of the optical path of the optical output to the waveguide improves the efficiency with which the light can be coupled into the waveguide.

[0010] SUMMARY

[0011] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0012] According to a first aspect, a photonic integrated circuit (PIC) is provided, comprising a cavity provided in a substrate to receive in said cavity optical elements of different dimensions, a waveguide arranged in or on the substrate, support pillars provided in the cavity for mechanically supporting an optical elements of different dimensions, said support pillars forming at least two rows and at least three columns, an optical element comprising an optical output for emitting light to the end facet of the waveguide, said optical element being arranged on a plurality of the support pillars acting as effective support pillars and soldering being arranged in the cavity as substantially filling a space defined both in between the optical element and the substrate, and in between adjacent effective support pillars for mechanically bonding the optical element to the substrate. In particular, the structural arrangement of the support pillars in combination with the soldering allows for improved connection of the optical element to the photonic integrated circuit upon flip chipping.

[0013] In this way, the optical element is reliably bonded to the substrate. Soldering in the context of optical elements with high energy levels is regarded as a reliable material which is not sensitive to the light emitted by the optical element, thus increasing the reliability of the mechanical coupling / bonding of the (flipped) optical element onto the substrate compared to prior art solutions with underfilling suffering from ionization and hence degraded mechanical properties when exposed to high intensities of optical power. The soldering may act as the sole mechanical bonding means. In other words, the soldering arranged in the cavity is arranged such as to ensure on its own the mechanical bonding of the optical element. No other mechanical bonding material like an underfilling is then required. The soldering may be the sole filler of a space under the optical element delimited laterally by the support pillars. The reliable mechanical bonding allows for accurate alignment of the optical path of the optical output to the waveguide. This improves in turn the efficiency with which the light can be coupled into the waveguide. In the context of AR projection applications but more generally as well, efficiency is typically a critical factor. By using soldering to create a reliable mechanical bonding, miniaturization, integration, and energy efficiency can thus be achieved. By support pillar is meant a supporting part standing alone in the cavity. Further by using a generic arrangement of pillars in a (wide) generic cavity, manufacturing costs can be kept low. By generic is meant not dimensioned in view of a single optical device but suitable for optical elements of different dimensions (i.e. different widths and / or different lengths). According to a preferred embodiment, the soldering may have been heated to form said mechanical bond. The support pillars may be arranged such that by capillarity the heated soldering has flown along the effective support pillars to bond the optical element, the effective support pillars and the substrate. After flip chipping, the soldering is heated, to the effect that capillary forces cause the soldering to creep up between the substrate, support pillars and the optical element. The arrangement of the support pillars may be such that it allows the soldering to creep up between the substrate, support pillars and the optical element to form a reliable mechanical bonding (and electrical connection) once cooled down again. It is noted that the pillars may be dimensioned based on one or more of the following: the size of the optical element, the accuracy of the fabrication process of said pillars (etching accuracy), trade-off between achieved support and volume of soldering.

[0014] According to a preferred embodiment, a PIC may comprise more than six support pillars. Preferably, the PIC may comprise at least six support pillars forming at least two rows and three columns. In this way, more contact surface can be provided under the optical element for the soldering, to further improve the mechanical bonding of the (flipped) optical element onto the substrate. The structural arrangement of the support pillars may yet be rearranged and / or optimized in alternative ways to facilitate the connection of an optical element of different dimensions. Preferably the soldering may have prior to reflow a fish bone shape filling an entire surface between the support pillarswhen arranged in two rows and multiple columns. According to a preferred embodiment, the support pillars have an elongated shape and a circular cross -section. A round shape of the support pillars avoid wetting issues in the pillar definition process and adds surface that can be covered with gold and / or solder. The support pillars may further have an elongated shape. The pillars may allow support while leaving a large space for soldering, i.e. keeping a large ratio of the surface of the optical element for thermal and electrical contact.

[0015] According to a preferred embodiment, the height of the support pillars may be such that the point and / or direction of the light emitted at the optical output of the optical element is (vertically) aligned with the optical axis of the waveguide at the end facet. In this way, the (vertical) alignment of the optical element with the waveguide can be precisely set. This allows an optimization and increase of the efficiency which is desirable for the (AR) application. In particular the support pillars may not all have the same height as the optical element may not have a uniformly flat surface. According to embodiments, the respective heights of the support pillars may then be dimensioned to match the surface of the optical element to be mechanically supported by said support pillars such that the point and / or direction of the light emitted at the optical output of the optical element is (vertically) aligned with the optical axis of the waveguide at the end facet. According to a preferred embodiment, the soldering may be arranged as substantially filling the entire space both in between adjacent effective support pillars, and in between the optical element and the substrate. Preferably, at least 80% of this space may be filled. More preferably, the soldering is arranged as substantially filling an entire space between the support pillars. In this way, a monolithic arrangement of the mechanical bonding between the support pillars, the optical element and the substrate can be ensured. The volume of soldering to be reflown may be dimensioned such as to allow solder to reflow upwards to the optical element due to surface tension.

[0016] According to a preferred embodiment, the PIC may comprise a conductive layer arranged at the bottom of the cavity and in between the support pillars. In this way, electrical connection may be realised as well as mechanical bonding through the soldering. Preferably, the conductive layer may be made of any one or more of the following: gold (Au), platinum (Pt) or aluminium (Al). Further conductive materials and / or stacks thereof (i.e. metallization stacks) may be envisaged without inventive step including combinations of the above metals and / or other alloys. Among the possible metallization stacks arranged around the pillars may be any one or more of the following: an adhesion layer, an optional gold layer, soldering, a metal stack associated with flip chipped optical element (diode). An adhesion layer may comprise typically tantalum (Ta) and, either platinum (Tt) if a gold layer is present or Titanium if not gold layer is used. In this way, a reliable and efficient electrical connection can be provided due to the excellent electrical conductivity, corrosion resistance, durability and solderability of these materials. Alternatively or additionally, wire bonding may be used.

[0017] According to a preferred embodiment, a metal or stack of metals may be provided in between the conductive layer and the substrate. In this way, the adhesion of the conductive layer to the substrate can be improved.

[0018] According to a preferred embodiment, the soldering is arranged as substantially filling the entire space in between the optical element and the conductive layer. The quantity of soldering arranged in between the optical element and the conductive layer (after reflowing the soldering) may be such as to ensure mechanical bonding of the optical element to the conductive layer, to the support pillars, and thus to the rest of PIC. In this way, both electrical and mechanical connection may be established by the soldering at once, combining both mechanical strength, appropriate electrical conductivity and durability as soldering is not sensitive to light. The soldering may also provide a thermal connection between the optical element and the conductive layer and / or the substrate. Typically the flipped side of the optical element is the side generating most heat. The soldering may thus provide a mechanical connection, in combination with either an electrical or thermal connection, or both. According to a preferred embodiment, the support pillars may be monolithic with the substrate. In this way, both the alignment of the optical element via the support pillars with the waveguide in / on the substrate and the strength of the integrated circuit can be ensured. According to a preferred embodiment, the substrate may be made of a semiconductor material. Preferably, the substrate is made of silicon dioxide (Si Ch). According to a preferred embodiment, the waveguide is made of trisilicon tetranitride (SisNr). In this way, the refractive indices of the materials are suitable for forming an efficient waveguide.

[0019] According to a preferred embodiment, the optical element is a laser diode. In this way, high brightness and high energy efficiency may be achieved. Preferably, the optical element emits light in the visible spectrum, more preferably red, green or blue light. In this way, combining the light output of a plurality of optical elements may allow for colour mixing and may result in white light emission. Alternatively, the optical element may emit light in the non- visible spectrum, typically in Infrared.

[0020] According to a preferred embodiment, the photonic integrated circuit may further comprise n cavities, n waveguides arranged in or on the substrate and comprising n respective end facets ending in n respective cavities, n optical elements, arranged in the n respective cavities and each comprising an optical output for emitting light respectively to the end facet of its respective waveguide, n groups of support pillars provided, respectively, in the n cavities, for mechanically supporting the respective optical element arranged in that respective cavity and soldering being arranged in the n cavities between the n groups of support pillars and the optical element of the respective cavities, for mechanically bonding the n optical elements to the substrate. In this way, the photonic integrated circuit may comprise any number of optical outputs appropriate for the application. It is noted that multiple pillar heights may be used in the different cavities as long as the pillars height allows for the alignment of the respective optical element of that cavity with its respective waveguide. In this way, optical elements of different manufacturers may be accommodated on a single substrate.

[0021] According to a preferred embodiment, the n+1 waveguides may be arranged such as to output their respective light into free space at the same physical location. In this way, several optical elements may contribute to an optical output that is perceived as a single optical output.

[0022] According to a preferred embodiment, the n+1 optical elements may emit light at different wavelengths. In this way, emission spectra with different emission peaks can be obtained. This, in turn, allows for color mixing to be achieved by tuning the wavelengths of the output of the individual optical elements.

[0023] According to a preferred embodiment, the amount of optical elements and their emission wavelength may be chosen such that white light emission is obtained. Preferably n may be equal to two, and the emission wavelengths of the three optical elements may be chosen as to correspond to the red, green and blue emission wavelengths. In this way, white light emission may be achieved. According to a second aspect of the invention, a (flip chip) method for fabricating a PIC is provided. In the first step of the method, a substrate, a cavity provided in the substrate to receive optical elements of different dimensions, a waveguide arranged in or on the substrate and comprising an end facet ending in the cavity and support pillars provided in the cavity for mechanically supporting optical elements of different dimensions, said support pillars forming at least two rows and at least three columns, are provided. In the second step of the method, soldering is applied in between the support pillars. In the third step of the method, an optical element is flipped into the cavity and onto a plurality of the support pillars acting as effective support pillars. In the final step of the method, the optical element is mechanically connected to the substrate, wherein mechanically connecting the optical element to the substrate comprises reflowing the soldering in a molten state to substantially fill a space both in between the optical element and the substrate, and in between adjacent support pillars. In this way, a PIC can be fabricated with an improved mechanical bonding of the optical element to the substrate. The processing method may further be generic and use the same generic substrate for a wide variety of optical elements. According to a preferred embodiment, providing a substrate comprises providing a bottom substrate layer, preferably obtained by thermally growing a bottom substrate layer onto a wafer, preferably a silicon (Si) wafer. According to a preferred embodiment, providing a waveguide arranged in or on the substrate comprises depositing a layer of waveguide material; and depositing a top substrate layer onto the layer of waveguide material. In this way, the position of the waveguide, in particular its height can be controlled with accuracy during deposition. This ensures proper alignment during flip chipping. According to a preferred embodiment, providing a cavity provided in the substrate and a waveguide comprising an end facet ending in the cavity comprises creating an opening from above at a specific location, preferably by means of subsequent lithography and etching processes. In this way, the facet of the waveguide can be fabricated with accuracy.

[0024] According to a preferred embodiment, providing support pillars provided in the cavity comprises trench etching to create support pillars within the substrate. In this way, the height of the support pillars can be determined with precision during trench etching, ensuring an appropriate height and thus alignment during flip chipping.

[0025] According to a preferred embodiment, the method further comprises depositing a conductive layer in between the supports pillars prior to applying soldering and reflowing said soldering, wherein preferably the conductive layer is made of gold (Au) and / or wherein preferably the soldering is, after reflowing, arranged as substantially filling the entire space in between the optical element and the conductive layer.

[0026] According to a preferred embodiment, providing support pillars further comprises providing more than two support pillars, preferably at least four support pillars forming at least two rows and two columns; and / or providing support pillars with a height such that the direction of the light emitted at the optical output of the optical element is aligned with the optical axis of the waveguide at the end facet.

[0027] According to a preferred embodiment, providing an optical element comprises providing a laser diode, preferably red, green, or blue light laser diode.

[0028] According to a preferred embodiment, the method further comprises providing n cavities in the substrate, n being an integer larger or equal to 1; providing n waveguides in or on the substrate and comprising n respective end facets ending in the n respective cavities; providing n optical elements, each comprising an optical output for emitting light respectively; providing n groups of support pillars provided, respectively, in the n cavities, for mechanically supporting the n respective optical elements arranged in the n respective cavities; applying soldering in the n cavities between the support pillars of the n respective groups of support pillars; flip chipping the n optical elements into the n respective cavities and onto the support pillars of the n respective groups of support pillars such that the optical output of each optical element emits lights to the end facet of its respective waveguide; and reflowing the soldering, for mechanically bonding the n respective optical elements to the substrate. According to a preferred embodiment, the method further comprises further providing the n+1 waveguides arranged such as to output their respective light into free space at the same physical location, preferably for causing white light emission, wherein more preferably the n=l optical elements emit light at different wavelengths

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 illustrates a top view of a photonic integrated circuit (PIC) at a fabrication stage before reflow of the soldering and obtention of a PIC according to an embodiment.

[0031] FIG. 2A illustrates a longitudinal cross-sectional view of the PIC of FIG. 1.

[0032] FIG. 2B illustrates a longitudinal cross-sectional view of a photonic integrated circuit (PIC) according to an embodiment, as obtained after reflow of the soldering in the PIC of FIG. 1 and 2A.

[0033] FIG. 3A illustrates a lateral cross-sectional view of the PIC of FIG. 1 and 2A.

[0034] FIG. 3B illustrates a lateral cross-sectional view of the PIC of FIG. 2B after reflow of the soldering.

[0035] FIG. 4 illustrates a top view of a PIC, according to another embodiment.

[0036] FIG. 5 illustrates a flowchart of a method for fabricating a PIC according to any of FIG. 1-4.

[0037] FIG. 6A-I further illustrate the steps of FIG. 5 in a more detailed manner providing a step-by-step fabrication method for a PIC according to embodiments of the present invention.

[0038] Throughout the drawings and the detailed description, unless otherwise described or provided, the same or like drawing reference numerals will be understood to refer to the same or like elements, features, and structures. The drawings may not be to scale, and the relative size, proportions, and depiction of elements in the drawings may be exaggerated for clarity, illustration, and convenience.

[0039] DETAILED DESCRIPTION

[0040] FIG. 2B and 3B illustrate, respectively, a longitudinal cross section view and a lateral cross section view of a photonic integrated circuit (PIC) 100 according to an embodiment. As already stated, same drawing reference numerals will be understood to refer to the same elements and FIG. 1 , 2A, 2B, 3 A and3B will now be described together as much as possible. Figures 1, 2A and 3 A illustrate respectively, a top view, a longitudinal cross section view and a lateral cross section view of a photonic integrated circuit 100’ at an intermediate stage during fabrication of the PIC 100, i.e. before obtention of the PIC 100 according to an embodiment.

[0041] The PIC 100’, respect. 100, comprises a substrate 10 and a (open) cavity 20 provided in the substrate 10. By cavity is meant in the context of the application, a hole in the substrate, for example obtained by etching to a certain depth in the substrate at a specific location. By substrate is meant a semiconductor material layer having typically predetermined refractive index properties rendering it suitable for photonic applications. In FIG. 1, 2A, 2B, 3A and 3B, the substrate 10 may be arranged as a layer having a thickness o onto a wafer 15 having a thickness k (shown in particular in FIG. 3). The substrate 10 may be a silicon dioxide (SiOz) layer and the wafer 15 may be a silicon (Si) underlayer. Yet other embodiments may be envisaged with other suitable materials and / or in which the substrate 10 and the wafer 15 are one and the same. In FIG. 1, 2A, 2B,3A and 3B, the cavity 20 may be vertically etched in the substrate 10 up to the wafer 15. The cavity 20 may have a depth o, a width g and a length f. Yet other embodiments may be envisaged in which the depth of the cavity 20 may be smaller than the thickness o of the substrate.

[0042] A waveguide 30 is arranged in the substrate 10 and comprises an end facet 40 ending in the cavity 20. The end facet 40 may debouch in the cavity 20. In the vicinity of the end facet 40, the waveguide 30 may extend along an optical axis A extending substantially perpendicular to the end facet 40, i.e. substantially perpendicular to a (vertical) wall of the cavity 20 at which the waveguide 30 debouches. The waveguide 30 may be made of trisilicon tetranitride (Si iNr) and may be arranged at a height 1 within the substrate 10. An optical waveguide, also referred to as just waveguide, in general may refer to an optical system comprising a higher refractive index material, sometimes referred to as the core, surrounded by a lower refractive index material, sometimes referred to as the cladding. Total internal reflection may confine, in one or more directions, a propagating electromagnetic wave to the region of the higher refractive index material. The higher refractive index material thus guides the wave, and by itself also may be referred to as the waveguide. This latter definition will be adhered to throughout the description. In the present embodiments, the substrate may be the lower refractive index material, while the waveguide may be the higher refractive index material. Different orientations of the higher refractive index material with respect to the lower refractive index material may lead to the formation of different types of waveguides. The used type of waveguide may be a slab waveguide or planar waveguide but is not limited thereto. Also, other types of such as ridge waveguides, rib waveguides, stripe waveguides, triplex waveguides, wire waveguides, strip-loaded waveguides, slot waveguides, buried waveguides, diffused waveguides, ARROW waveguides or SWG waveguides may be used. Additionally, the skilled person may apply any other type of currently existing or future waveguide to the technology described in the invention.

[0043] An optical element 50 is arranged in the cavity 20 and comprises an optical output 60 for emitting light to the end facet 40 of the waveguide 30. The optical element 50 may be a laser diode (chip). The optical element 50 may emit light in the visible spectrum, in particular red, green, or blue light. The optical element 50 may have a top surface 51, a bottom surface 52, and side surfaces 53 and 54. The (chipped) optical element 50 may have a width d, a length e and a given height (no reference sign). The width g, respect, length f, of the cavity 20 should be larger than the width d , respect, length e, of the optical element 50 in order that the optical element 50 may be received in said cavity 20. Typically optical elements 50 (laser diodes) come in different sizes. To improve scalability and industrialisation, the width f and length g of a cavity may thus be dimensioned to accommodate a large range of optical elements.

[0044] A first contact pad (not represented) may be arranged on the top side surface 51 of the optical element 50 for connection with further external circuitry (power supply, control circuitry among others). When flip chipped, the top side surface 51 may face downwards, i.e. may face towards the bottom of the cavity 20. The optical output 60 may be arranged on a side surface 53 at a height n from the top surface 51. A second contact pad (not represented) may be arranged on surface 52 of the optical element 50 for wire bonding connection with further external circuitry (power supply, control circuitry among others). In a different embodiment, the second contact pad (not represented) may be arranged on surface 51 of the optical element 50.

[0045] As illustrated in FIG. 1 and 2A, 2B, the optical output 60 of the optical element 50 and the end facet 40 of the waveguide 30 may be facing each other such that the light at the optical output 60 from the optical element 50 may be received at the end facet 40 to further travel through the waveguide 30. The output emission direction of the optical element 50 may thus, in use, be aligned with the optical axis A of the waveguide 30. A free space may be arranged between the output 60 and the end facet 40. In particular, the output 60 of the optical element 50 may be, in use, at a distance a from the end facet 40 of the waveguide 30 when seen along the optical axis A of the waveguide, such that light output at the output 60 travels the distance a over free space before reaching the end facet 40 of the waveguide. FIG. 3A represents a lateral cross section view at the optical output 60 according to section axis B as represented in FIG. 1. It is here noted that the height of the optical element 60 should not per se be limited by the one of the substrate 10, it is rather the dimension n which may be related to the dimension 1 of the waveguide 30 to ensure the alignment of the optical element 50 and the waveguide 30. In FIG. 1-4, the optical element / laser chip 50 may be entirely contained within the volume of the cavity 20. However other embodiments (FIG. 6 for example) may have a laser chip 50 extending beyond the top surface of the substrate. A skilled person would then envisage without inventive step different technical solutions to encapsulate the cavity area (not represented here) such as to keep at least the free space a dust and moisture free.

[0046] Support pillars 70 are further provided in the cavity 20 for mechanically supporting the optical element 50 and for achieving the above-mentioned alignment between the optical output 60 and the end facet 40. The support pillars 70 may be elongated structural elements on top of which the optical element 50 is arranged. These support pillars 70 may extend, (vertically), along a depth of the substrate ( / photonic chip), from the bottom of the cavity 20 up to a predetermined height m. The height m of the support pillars 70 may be selected such that (m + n), n being the height of the output 60 from the top surface 51 , amounts to 1, the height of the waveguide from the bottom of the cavity (i.e. from the bottom of the substrate in FIG. 1-3). The height of the support pillars 70 may thus be selected to ensure the vertical alignment of the output 60 and the waveguide 30.

[0047] As illustrated in FIG 1 , 2A and 3 A, at an intermediate stage of fabrication, soldering (also called solder material in the erst of text) 80 is further arranged in the cavity 20 of PIC 100’ between the support pillars 70. Soldering 80 is meant for mechanically bonding / fixing the optical element 50 to the substrate 10. By soldering is meant a fusible metal alloy, typically used to solder electronic components together. In particular soldering 80 becomes fluid when (re)heated / reflown and returns to a solid once cooled. Heating / reflowing of the soldering may be performed using a laser. After reflow of soldering 80 in the PIC 100’ of FIG. 1, 2A and 3A, the PIC 100 according to the present invention is obtained. PIC 100 is illustrated then in FIG. 2B and 3B. Soldering in the context of optical elements with high energy levels is regarded as a reliable material which is not sensitive to the light emitted by the optical element, thus increasing the reliability of the mechanical bonding of the (flipped) optical element 50 onto the substrate 10.

[0048] As previously explained, the use of an underfilling like in the related art is undesirable in the particular application. Resins typically used in flip chip technology as underfilling are indeed sensitive to the light energy levels typically used for lasers. Such resins would typically suffer from ionization when interacting with light from an optical element like a laser. The subsequent vapor would then degrade the mechanical properties of the resin over time. The present solution without such underfilling does not suffer from such disadvantages.

[0049] In the prior art, only two long pillars have typically been used. As illustrated in FIG. 1 , more than two support pillars 70 may be present. The support pillars 70 may form as illustrated two rows and a plurality of columns. The support pillars 70 may be, preferably regularly, distributed in the cavity 20. In this way, the contact surface between pillars 70 and optical element 50 may be reduced while creating more contact surface under the optical element 50 for the soldering 80. By providing more room in between the support pillars 70 for the soldering 80, the mechanical bonding of the (flipped) optical element 50 onto the substrate 10 may be improved. Additional support pillars 75 identical to support pillars 70 may be provided over the whole cavity to allow supporting optical elements of different sizes. As the support pillars 75 in FIG 1-3B are not used to support the optical element 50 contrary to the support pillars 70 which are effectively supporting the optical element, the support pillars 70 will be referred to as effective support pillars and the support pillars will be referred to as non-effective support pillars. Which pillars are effective and non-effective may be determined by the size of the optical element 50. The structural arrangement of the support pillars 70 and 75 shown in FIG. 1-3B is yet only an example and the support pillars 70, 75 may yet be rearranged and / or optimized differently to facilitate the connection of an optical element 50 of different dimensions. The amount of required support pillars 70, that is the length of the row or equivalently the number of columns, may be in particular determined by the length of the optical element 50 they support. In FIG. 1-3B, the support pillars 70 may be identical, and may have a height m, a length h perpendicular to the optical axis A, and a width i along the optical axis A. The length h may be adapted depending on the number of rows in order to allow a variety of optical devices to supported. Similarly, the width i may be adapted depending on the number of columns in order to allow a variety of optical devices to supported. In the embodiment shown in Figure 1 , the length m may be in order of several times the width i. Such an embodiment allows with only two rows to support optical elements of different widths. In FIG. 1-3B, the optical element 50 may be substantially smaller than the cavity 20 and non-effective support pillars 75 (on the right of FIG. 1) may be arranged in a portion of the cavity beyond the length e of the optical element 50 and the free space a. These pillars 75may thus not support said optical element 50. Such an embodiment may correspond to a support pillar arrangement suitable for a large variety of optical elements 50 with different dimensions d*e. In some embodiments depending on the size of the optical element 50, extra (optional, non-effective) support pillars 75 may thus present, however, any embodiment should at least comprise effective support pillars 70 under the optical element 50.

[0050] In the PIC 100’, the soldering 80 may be arranged as substantially covering an entire surface at the bottom of the cavity 20 and in between the support pillars 70. Initially, when multiple columns of pillars 70 are present, in PIC 100’, the soldering 80 may then have a fish bone shape filling the entire space between the support pillars arranged in at least two rows and multiple columns. The space left in FIG. 1 between the support pillars 70 and the soldering 80 may then be discarded. Further, as illustrated in Fig 2A and 3 A, an initial top surface 81 of the soldering 80 may extend lower than the height m of the support pillars 70.

[0051] After reflow as shown in FIG 2B and 3B, the soldering 80 may be arranged as substantially filling the space in between the optical element 50 and the bottom of cavity 20 (i.e. the substrate 10). In this way, a monolithic arrangement of the mechanical bonding between the optical element 50 and the substrate 10 is ensured.

[0052] Intervals may be created between adjacent support pillars 70 in the same row, and between adjacent support pillars 70 in the same column. Adjacent pillars 70 in a row may be spaced apart by an interval b in their width direction (along the optical axis A), while adjacent pillars 70 in a column may be spaced apart by an interval c in their length direction. In embodiments in which the optical element 50 is supported by only a portion of the pillars, acting as effective pillars 70, more soldering 80 then needed for reflowing may be present. This may allow redistribution of soldering 80 to the heated area under the optical element 50. For instance, at least areas that are located in the intervals c and intervals b between effective support pillars 70 and under the optical element 50 may be substantially entirely filled by soldering 80 after reflow of the soldering 80. As illustrated in Fig 2B and 3B, a top surface 82 of the soldering 80 after reflow may extend in these areas up to the height m of the support pillars 70 and thus up to the bottom surface 51 of the optical element 50. In non-heated areas, the top surface 82 may remain the same as the initial surface 81 while in transition areas, the top surface 82 may lower than the initial surface 81 due to the migration by capillarity of soldering towards the heated area under the optical element 50.

[0053] The solder material may be made of a thermally conductive material. This allows for localized heating to raise the temperature through the solder material. In this way, effective melting and subsequent upward flow can be achieved using localized heating.

[0054] The support pillars 70 may be monolithic with the substrate 10. This means that the support pillars 70 and the substrate 10 may form one single entity, or one indivisible unit. In this way, both the alignment of the of the optical element 50 via the support pillars 70 with the waveguide 30 in / on the substrate and the strength of the PIC may be ensured.

[0055] As illustrated in the cross sections FIG. 2A, 2B and 3 A and 3B, a conductive layer 90 may be present in between the substrate 10 and the soldering 80. The conductive layer 90 may facilitate the electrical connection of the optical element to further electrical elements (not represented), including a power supply and control means for controlling the optical element 50. The conductive layer 90 may be made of gold (Au). The conductive layer 90 may be arranged to provide one or more separate electrical paths. A conductive layer 90 made of gold provides a reliable and efficient electrical connection due to its excellent electrical conductivity, corrosion resistance, durability and solderability. After reflow, the soldering 80 may be arranged as substantially filling the entire space in between the optical element 50 (in particular between the pads on the top surface 51) and the conductive layer 90. In this way, both electrical and mechanical connection may be established by the soldering 80 at once, combining both mechanical strength, appropriate electrical conductivity and durability as soldering is not sensitive to light. The soldering 80 may also provide a thermal connection between the optical element 50 and the conductive layer 90 and / or the substrate 10. The soldering 80 may thus provide a mechanical connection, in combination with either an electrical or thermal connection, or both. An additional layer of metal or stack of metals (not represented) may be added in between the conductive layer and the substrate. In this way, the adhesion of the conductive layer to the substrate can be improved.

[0056] FIG. 4 illustrates a top view of a PIC, according to another embodiment. Compared to FIG. 1-3B, FIG. 4 relates to a PIC 400 in which three PICs 100A-100C, each PIC being as in FIG. 2B-3B, have been included in the same substrate 10. The PICs 100A -100C may only differ from each other in the type of optical element 50 that they each may include: The first, second, and third optical elements 50A-50C may be laser diodes, emitting red, green and blue light, respectively. In this way, combining the light output of a plurality of optical elements 50A-50C may allow for colour mixing and may result in white light emission. It is noted that different numbers of optical elements and different choices of wavelengths may also result in an overall emission that is perceived as white. For example, when using two optical elements, blue and yellow emission spectra may together result in perceived white light emission. In a different example, four optical elements may be used, of for instance red, green, blue and a fourth color, to obtain an overall emission that is perceived as white.

[0057] The PIC 400 comprises in addition to a first cavity 20A, a second cavity 20B and a third cavity 20C provided in the same substrate 10. A first, second and third waveguide 30A-30C are arranged in or on the substrate and each comprise an end facet 40A-40C ending its respective cavity 20A- 20C. Each optical element 50A-50C may be arranged in its respective cavity 20A-20C, respectively, and may comprise a respective optical output 60A-60C for emitting light to the end facet 40A-40C of its respective waveguide 30A-30C. Respective groups of more than two support pillars 70A-70C may be provided in respective cavities 20A-20C, respectively, for mechanically supporting their respective optical element 50A-50C arranged in that respective cavity 20A-20C. Soldering 8OA-8OC is respectively arranged in the respective cavities 20a-20C, between the respective support pillars 70A-70C and their optical element 50A-50C of the respective cavities 20a-20C, for mechanically connecting the respective optical elements 50A-50C to the substrate 10. The waveguides 30A-30C are further arranged such as to output their respective light of the red, green, and blue laser diode into free space at the same physical location O for output emission, causing white light emission 410. The paths of the waveguides 30A-30C may thus be curved to create separate respective light paths, each from an end facet of a different cavity and all converging at the output emission location O. The ends of the waveguides 30A-30C connected to the output emission location O, may thus extend parallel to each other along at least a last portion thereof. Preferably the output emission of the PIC 400 at location O may be into free space, for instance for AR projection. The PIC 400 may then be integrated within googles or any head mounted device for AR visualization.

[0058] FIG. 5 illustrates a flowchart of a method for fabricating a PIC according to any of FIG. 1-4. The method 500 for fabricating a photonic integrated circuit comprises the succession of steps 510 to 540. First step 510 comprises providing a substrate 10, a cavity 20 provided in the substrate 10, a waveguide 30 arranged in or on the substrate 10 and comprising an end facet 40 ending in the cavity 20 and support pillars 70 provided in the cavity 20. Then Step 520 comprises applying soldering 80 to the substrate 10, in between the support pillars 70. Next step 530 comprises flipping an optical element 50 into the cavity 20 onto the support pillars 70. Finally, step 540 comprises mechanically connecting the optical element to the substrate by reflowing the soldering 80 in a molten state to fill a space in between the support pillars 70, the optical element 50 and the substrate 80. The volume filled by the soldering 80 may be delimited above by the optical element 50 and below by the substrate 10 and / or by the conductive layer 80 if present.

[0059] It is further noted that when three cavities and three optical elements are combined onto a single substrate as illustrated in FIG. 4, any one or more corresponding steps of steps 510-540 of the PIC 100A-100C may preferably be performed in parallel to save production time and materials.

[0060] Alternatively, all steps 510-540 of a given PIC may be performed before the corresponding steps for another PIC are performed.

[0061] FIG. 6A-I further illustrate the steps of FIG. 5 in a more detailed manner providing a step-by-step fabrication method for a PIC according to embodiments of the present invention. All FIG. 6A-6I are cross sectional longitudinal views similar to FIG. 2 such that reference will be made to elements previously described in view of FIG. 1-3.

[0062] FIG. 6A illustrates a first step 610 comprising thermally growing a substrate layer 10, i.e. a low(er) refractive index material layer, onto a wafer layer 15, typically a silicon (Si) wafer. The low(er) refractive index material may be SiOz-

[0063] FIG. 6B illustrates a following step 620 comprising depositing a layer of a high(er) refractive index material 11 onto the thermally grown layer 10 of the low(er) refractive index material. The high(er) refractive index material 11 may be trisilicon tetranitride (SisNr). At this stage, the waveguide 30 may be created by the deposition of the material 11.

[0064] FIG. 6C illustrates a following step 630 comprising depositing a second layer of the same lower refractive index material 10 onto the deposited higher refractive index material 11. The thermally grown lower refractive index material and the deposited lower refractive index material could alternatively be different. The higher refractive index material may be Si iNr.

[0065] FIG. 6D illustrates a following step 640 comprising opening the second layer of low refractive index material (acting as top cladding) using typically lithography and etching to expose at a first area 12 the high refractive index material 11.

[0066] FIG. 6E illustrates a following step 650 comprising trench etching (using typically lithography and etching) through the substrate 10 and the waveguide 30 to expose support pillars 70 still covered by some material 11.

[0067] FIG. 6F illustrates a following step 660 comprising removing the exposed material 11 by lithography and etching, and depositing a conductive layer 90 at the bottom of the exposed cavity 20 around the support pillars 70. FIG. 6G illustrates a following step 670 comprising applying soldering 80 on top of the conductive layer 90. Soldering may be applied in between the support pillars and around them to cover a first surface relatively larger than a minimum bonding surface needed by the optical element 50 to be reliably bonded.

[0068] FIG. 6H illustrates a following step 680 comprising flip chipping an optical element 50 with a top surface 51 coming onto the support pillars 70, such that the contact pads may face the soldering 80. FIG. 61 illustrates the final following step 690 comprising reflowing the soldering in a molten state to fill the space in between the support pillars 70, the conductive layer and the contacts pads 55 of the optical element 50. By capillary, the soldering 80 may be rearranged to substantially fill the (entire) space between the conductive layer 90 and the top surface 51 of the optical element 50 comprising the connect pads 55 to ensure a reliable mechanical bonding of the optical element 50. Due to surface tension, the soldering 80 rises along the support pillars 70 to connect to the (contact pads of the) optical element 50 (reducing thus the surface covered to the profit of an increased height). The initial volume of soldering 80 is thus redistributed to cover vertically the distance between the conductive layer 90 and the optical element 50. At the end of step 690, soldering 80 no longer covers the first initial surface, but at least covers the minimum bonding surface with the optical element 50 required to ensure the reliable mechanical bonding of that element. The reflowing of the soldering may be obtained through laser soldering using a laser source for heating / melting the soldering 80.

[0069] Steps 610-660 may be sub-steps of step 510 of FIG. 5. Steps 670-690 may illustrate respectively steps 520-540 of FIG. 5.

[0070] Whilst the principles of the invention have been set out above in connection with specific embodiments, it is understood that this description is merely made by way of example and not as a limitation of the scope of protection which is determined by the appended claims.

[0071] Further embodiments are described in the following clauses:

[0072] 1. A photonic integrated circuit, comprising: a substrate; a cavity provided in the substrate; a waveguide arranged in or on the substrate and comprising an end facet ending in the cavity; an optical element arranged in the cavity and comprising an optical output for emitting light to the end facet of the waveguide; support pillars provided in the cavity for mechanically supporting the optical element; soldering arranged as filling a space in between the optical element, the support pillars and the substrate, in order to mechanically bond the optical element to the substrate. 2. The photonic integrated circuit of clause 1 , wherein the soldering has been heated to form said mechanical bond and wherein the support pillars are arranged such that by capillarity the heated soldering has flown along the support pillars to bond the optical element, the support pillars and the substrate.

[0073] 3. The photonic integrated circuit of clause 1 or 2, comprising more than two support pillars, preferably at least four support pillars forming at least two rows and two columns, wherein preferably the soldering has a fish bone shape filling the entire space between the support pillars when arranged in two rows and multiple columns.

[0074] 4. The photonic circuit of any of the above clauses, wherein the support pillars have an elongated shape and a circular cross -section.

[0075] 5. The photonic integrated circuit of any of the above clauses, wherein a height of the support pillars is such that the direction of the light emitted at the optical output of the optical element is aligned with the optical axis of the waveguide at the end facet.

[0076] 6. The photonic integrated circuit of any of the above clauses, wherein the soldering is arranged as substantially filling the space in between the support pillars, the optical element and the substrate, preferably at least 80 % of the space.

[0077] 7. The photonic integrated circuit of any of the above clauses, further comprising a conductive layer arranged at the bottom of the cavity and in between the support pillars, wherein the conductive layer is preferably made of any one or more of the following: gold (Au), platinum (Pt) or aluminium (Al).

[0078] 8. The photonic integrated circuit of the previous clause, further comprising a metal or stack of metals in between the conductive layer and the substrate.

[0079] 9. The photonic integrated circuit of any of the last two clauses, wherein the soldering is arranged as substantially filling a space in between the support pillars, the optical element and the conductive layer, preferably at least 80% of the space. 10. The photonic integrated circuit of any of the above clauses, wherein the support pillars are monolithic with the substrate.

[0080] 11. The photonic integrated circuit of any of the above clauses, wherein the substrate is made of a semiconductor material, preferably silicon dioxide (Si Ch); and / or wherein the waveguide is made of trisilicon tetranitride (SisNr).

[0081] 12. The photonic integrated circuit of any of the above clauses, wherein the optical element is a laser diode; and / or wherein the optical element emits light in the visible spectrum, preferably red, green, or blue light.

[0082] 13. The photonic integrated circuit of any of the above clauses, further comprising n additional cavities, n being an integer larger or equal to 1 ; n additional waveguides arranged in or on the substrate and comprising n respective end facets ending in the n respective cavities; n additional optical elements, each arranged in z respective cavity of the n respective cavities and each comprising an optical output for emitting light respectively to the end facet of its respective waveguide; n additional groups of support pillars, each group provided, respectively, in a respective cavity of the n cavities, for mechanically supporting the respective optical element when arranged in each respective cavity; soldering being arranged in the n cavities between the support pillars of the n groups of support pillars and the optical element of the respective cavities, for mechanically bonding the n optical elements to the substrate.

[0083] 14. The photonic integrated circuit of the previous clause, wherein the n+1 waveguides are arranged such as to output their respective light into free space at the same physical location.

[0084] 15. The photonic integrated circuit of any of the last two clauses, wherein the n+1 optical elements emit light at different wavelengths.

[0085] 16. The photonic integrated circuit of any of clauses 13-15, wherein the number n of optical elements and their emission wavelength are chosen such that white light emission is obtained, wherein preferably n = 2 and the emission wavelengths of the optical elements are chosen as corresponding to red, green and blue light. A method for fabricating a photonic integrated circuit, comprising: providing a substrate, a cavity provided in the substrate, a waveguide arranged in or on the substrate and comprising an end facet ending in the cavity and support pillars provided in the cavity; applying soldering in between the support pillars; flipping an optical element into the cavity onto the support pillars; and mechanically bonding the optical element to the substrate; wherein mechanically bonding the optical element to the substrate comprises reflowing the soldering in a molten state to fill a space in between the optical element, the support pillars and the substrate. The method of the previous clause, wherein providing a substrate comprises providing a bottom substrate layer, preferably obtained by thermally growing a bottom substrate layer onto a wafer, preferably a silicon (Si) wafer. The method of any of the above method clauses, wherein providing a waveguide arranged in or on the substrate comprises: depositing a layer of waveguide material; and depositing a top substrate layer onto the layer of waveguide material. The method of any of the above method clauses, wherein providing a cavity provided in the substrate and a waveguide comprising an end facet ending in the cavity, comprises creating an opening from above at a specific location, preferably by means of lithography and / or etching processes. The method of any of the above method clauses, wherein providing support pillars provided in the cavity comprises trench etching to create support pillars within the substrate. The method of any of the above method clauses, further comprising depositing a conductive layer in between the supports pillars prior to applying soldering and reflowing said soldering, wherein preferably the conductive layer is made of gold (Au) and / or wherein preferably the soldering is, after reflowing, arranged as substantially filling the entire space in between the optical element and the conductive layer. The method of any of the above method clauses, wherein providing support pillars further comprises: providing more than two support pillars, preferably at least four support pillars forming at least two rows and two columns; and / or providing support pillars with a height such that the direction of the light emitted at the optical output of the optical element is aligned with the optical axis of the waveguide at the end facet. The method of any of the above method clauses, wherein providing an optical element comprises providing a laser diode, preferably red, green, or blue light laser diode. The method of any of the above method clauses, further comprising: providing n additional cavities in the substrate, n being an integer larger or equal to 1 ; providing n additional waveguides in or on the substrate and comprising n respective end facets ending in the n respective cavities; providing n additional optical elements, each comprising an optical output for emitting light respectively; providing n additional groups of support pillars, each group provided, respectively, in a respective cavity of the n cavities, for mechanically supporting the respective optical element when arranged in each respective cavity; applying soldering in the n cavities between the support pillars of the n respective groups of support pillars; flip chipping the n optical elements into the n respective cavities and onto the support pillars of the n respective groups of support pillars such that the optical output of each optical element emits lights to the end facet of its respective waveguide; and reflowing the soldering, for mechanically bonding the n respective optical elements to the substrate. The method of the previous clause, further providing the n+1 waveguides arranged such as to output their respective light into free space at the same physical location, preferably for causing white light emission, wherein more preferably the n+1 optical elements emit light at different wavelengths.

Claims

CLAIMS1. A photonic integrated circuit, comprising: a substrate; a cavity provided in the substrate to receive in said cavity optical elements of different dimensions; a waveguide arranged in or on the substrate and comprising an end facet ending in the cavity; support pillars provided in the cavity for mechanically supporting optical elements of different dimensions, said support pillars forming at least two rows and at least three columns, an optical element comprising an optical output for emitting light to the end facet of the waveguide, said optical element being arranged on a plurality of the support pillars acting as effective support pillars; soldering arranged as substantially filling a space defined both in between the optical element and the substrate, and in between adjacent effective support pillars, in order to mechanically bond the optical element to the substrate.

2. The photonic integrated circuit of claim 1 , wherein the soldering has been heated to form said mechanical bond and wherein the support pillars are arranged such that by capillarity the heated soldering has flown along the effective support pillars to bond the optical element, the effective support pillars and the substrate.

3. The photonic integrated circuit of claim 1 or 2, , wherein the soldering has prior to reflow a fish bone shape filling an entire surface between the support pillars.

4. The photonic circuit of any of the above claims, wherein the support pillars have an elongated shape and a circular cross -section.

5. The photonic integrated circuit of any of the above claims, wherein a height of the support pillars is such that the direction of the light emitted at the optical output of the optical element is aligned with the optical axis of the waveguide at the end facet.

6. The photonic integrated circuit of any of the above claims, wherein substantially filling the space amounts to filling at least 80 % of the space.

7. The photonic integrated circuit of any of the above claims, further comprising a conductive layer arranged at the bottom of the cavity and in between the support pillars, wherein the conductive layer is preferably made of any one or more of the following: gold (Au), platinum (Pt) or aluminium (Al).

8. The photonic integrated circuit of the previous claim, further comprising a metal or stack of metals in between the conductive layer and the substrate.

9. The photonic integrated circuit of any of the last two claims, wherein the soldering is arranged as substantially filling a space both in between adjacent effective support pillars, and in between the optical element and the conductive layer, preferably at least 80% of the space.

10. The photonic integrated circuit of any of the above claims, wherein the support pillars are monolithic with the substrate.

11. The photonic integrated circuit of any of the above claims, wherein the substrate is made of a semiconductor material, preferably silicon dioxide (Si Ch); and / or wherein the waveguide is made of trisilicon tetranitride (SisNr).

12. The photonic integrated circuit of any of the above claims, wherein the optical element is a laser diode; and / or wherein the optical element emits light in the visible spectrum, preferably red, green, or blue light.

13. The photonic integrated circuit of any of the above claims, further comprising n additional cavities, n being an integer larger or equal to 1 ; n additional waveguides arranged in or on the substrate and comprising n respective end facets ending in the n respective cavities; n additional optical elements, each arranged in z respective cavity of the n respective cavities and each comprising an optical output for emitting light respectively to the end facet of its respective waveguide; n additional groups of support pillars, each group provided, respectively, in a respective cavity of the n cavities, for mechanically supporting the respective optical element when arranged in each respective cavity;soldering being arranged in the n cavities between the support pillars of the n groups of support pillars and the optical element of the respective cavities, for mechanically bonding the n optical elements to the substrate.

14. The photonic integrated circuit of the previous claim, wherein the n+1 waveguides are arranged such as to output their respective light into free space at the same physical location.

15. The photonic integrated circuit of any of the last two claims, wherein the n+1 optical elements emit light at different wavelengths.

16. The photonic integrated circuit of any of claims 13-15, wherein the number n of optical elements and their emission wavelength are chosen such that white light emission is obtained, wherein preferably n = 2 and the emission wavelengths of the optical elements are chosen as corresponding to red, green and blue light.

17. A method for fabricating a photonic integrated circuit, comprising: a. providing a substrate, a cavity provided in the substrate to receive optical elements of different dimensions, a waveguide arranged in or on the substrate and comprising an end facet ending in the cavity and support pillars provided in the cavity for mechanically supporting optical elements of different dimensions, said support pillars forming at least two rows and at least three columns; b. applying soldering in between all the support pillars; c. flipping an optical element into the cavity onto a plurality of the support pillars acting as effective support pillars; and d. mechanically bonding the optical element to the substrate; wherein mechanically bonding the optical element to the substrate comprises reflowing the soldering in a molten state to substantially fill a space both in between the optical element and the substrate, and in between adjacent effective support pillars.

18. The method of the previous claim, wherein providing a substrate comprises providing a bottom substrate layer, preferably obtained by thermally growing a bottom substrate layer onto a wafer, preferably a silicon (Si) wafer.

19. The method of any of the above method claims, wherein providing a waveguide arranged in or on the substrate comprises: a. depositing a layer of waveguide material; and b. depositing a top substrate layer onto the layer of waveguide material.

20. The method of any of the above method claims, wherein providing a cavity provided in the substrate and a waveguide comprising an end facet ending in the cavity, comprises creating an opening from above at a specific location, preferably by means of lithography and / or etching processes.

21. The method of any of the above method claims, wherein providing support pillars provided in the cavity comprises trench etching to create support pillars within the substrate.

22. The method of any of the above method claims, further comprising depositing a conductive layer in between the supports pillars prior to applying soldering and reflowing said soldering, wherein preferably the conductive layer is made of gold (Au) and / or wherein preferably the soldering is, after reflowing, arranged as substantially filling the entire space in between the optical element and the conductive layer.

23. The method of any of the above method claims, wherein providing support pillars further comprises: a. providing support pillars with a height such that the direction of the light emitted at the optical output of the optical element is aligned with the optical axis of the waveguide at the end facet.

24. The method of any of the above method claims, wherein providing an optical element comprises providing a laser diode, preferably red, green, or blue light laser diode.

25. The method of any of the above method claims, further comprising: a. providing n additional cavities in the substrate, n being an integer larger or equal to 1; b. providing n additional waveguides in or on the substrate and comprising n respective end facets ending in the n respective cavities; c. providing n additional optical elements, each comprising an optical output for emitting light respectively;d. providing n additional groups of support pillars, each group provided, respectively, in a respective cavity of the n cavities, for mechanically supporting the respective optical element when arranged in each respective cavity; e. applying soldering in the n cavities between the support pillars of the n respective groups of support pillars; f. flip chipping the n optical elements into the n respective cavities and onto the support pillars of the n respective groups of support pillars such that the optical output of each optical element emits lights to the end facet of its respective waveguide; and g. reflowing the soldering, for mechanically bonding the n respective optical elements to the substrate.

26. The method of the previous claim, further providing the n+1 waveguides arranged such as to output their respective light into free space at the same physical location, preferably for causing white light emission, wherein more preferably the n+1 optical elements emit light at different wavelengths.

Images