ALIGNMENT OF FIBRE ARRAYS

Abstract: In order accurately to align an array of optical fibres (10) with optical components such as waveguides (14) in a block of insulative material (12) on a substrate (2) both in the plane of the substrate and in a height dimension normal to the substrate, the fibres are fixed in accurately formed V-grooves (8), and the block (12) has an array of accurately formed solder pads (16) in one to one correspondence with an array of solder pads (4) on the surface of the substrate (2) the array of solder pads (4, 16) being secured to one another by solder drops (6) containing controlled amounts of solder and thus being precisely dimensioned. The fibres (10) may be secured in V-grooves (8) with solder. Some of the solder pads (4) may be formed as conductive tracks electrically interconnecting with electrical components formed in block (12) and integrated circuits formed in substrate (2).

Description: This invention relates to the alignment of an array of one or more optical elements, principally optical fibres, with an array of one or more optical components arranged in one or more blocks or material. For the purpose of this specification, 'optical component' is intended to means any light transmission component such as a laser diode, any light receiving component such as a photodiode, or any light or light waveguide conducting component, such as a conductive strip, arranged in a block of material, usually a plastics insulative material. For the purpose of this specification, 'optical element' is intended to mean a light conducting element such as optical fibre although it also includes a light waveguide and any of the devices encompassed in the term optical component as above defined. A problem which has existed in the past is that of alignment of an array of separate optical fibres with respective optical components in order that an efficient light coupling be achieved between the ends of the fibres and the optical components. One prior method involved locating the optical fibres at predetermined locations on a substrate of silicon and then positioning the block or blocks containing the optical components in alignment with the fibres by a manual positioning procedure while at the same time bonding the block or blocks to the substrate by a suitable adhesive. The problem with this method of alignment is that it is time consuming and a complex and highly skilled operation. To overcome this problem, the present invention adapts a technique which is well known in the art of connecting and aligning an integrated circuit chip on a substrate which is known as controlled collapse reflow chip bonding (CCRCB). This technique which is, for instance, disclosed in Patent Application WO 87/01509 involves aligning the chip in the two dimensional plane of the substrate by aligning an array of solder bumps on a registering array of solder pads on the co-operating faces of the substrate and chip. In the present invention this techique is adapted to the problem of alignment of optical components and optical elements by controlling relative to a substrate surface by close control over the size of solder drops employed in the CCRCB technique. According to the invention there is provided a method of aligning an optical fibre or an array of optical fibres with an optical component or an array of optical components included as part of a block, said method comprising, providing a substrate having a first major plane surface, forming on said first major plane surface a first array of solder wettable pads, each of a predetermined area surrounded by areas which are solder repellent, forming a groove or an array of grooves in said substrate, the groove or each groove being aligned with, and being located in an accurate lateral relationship to, said first array of solder wettable pads, providing said block with said corresponding optical component or an array of optical components, said block having a second major plane surface, forming on said second plane surface a second array of solder wettable pads, each of a predetermined area surrounded by areas which are solder repellent, said second array of solder wettable pads being located relative to each other at positions in one-to-one correspondence with corresponding positions of said solder wettable pads of said first array, depositing controlled amounts of solder on some or all of said solder pads of said first and said second arrays, mounting said block above said substrate with said first and second major plane surfaces facing each other, reflowing said solder and thereby forming solder drops of predetermined dimensions, said solder drops securing said block at a predetermined height above said first major plane surface and positioning the block laterally and longitudinally in relation to said substrate, and locating and securing said optical fibre or said array of optical fibres in said groove or said array of grooves defined in said substrate thereby to align said optical fibre or said array of optical fibres with said optical component or said array of optical components. Thus in accordance with the invention it is possible to align the block and blocks containing optical components with optical elements (optical fibres) both in the plane of the substrate and in the height dimension, since the dimensions of the solder drops on the solder pads can be precisely controlled. The dimensions of the solder pads can be very precisely controlled by photolithographic techniques, and the thickness of solder deposited on the pads can be controlled by evaporation techniques involving forming an accurately dimensioned mask by photolithographic techniques. In this way the height of the blocks above the substrate can be predicted within a margin of error of 0.5um or less, which is sufficient for the purpose of optical alignment of optical fibres with optical components. The substrate may have formed therein electrical components which are electrically connected with the solder pads and hence to electrical components within the block or blocks of optical compnents. Embodiments of the invention will now be described with reference to the accompanying drawings, wherein:- Figure 1 is a fragmentary cross-sectional view of an optical coupling assembled in accordance with the invention; Figure 2 is a view of a step in the assembly of the optical coupling of Figure 1; and, Figure 3 to 6 are perspective views of further optical couplings assembled in accordance with the invention. Referring to Figure 1, a slice of silicon 2 forms a substrate and has formed thereon an array of precisely positioned and dimensioned solder pads 4, formed by photolithographic techniques. The solder pads 4 are covered with precisely metered amounts of solder 6 by an evaporation technique involving using an accurate dimensioned mask formed by photolithographic techniques. The silicon surface surrounding the pads is non-wettable or repellent to solder. In addition, an array of V-shaped grooves 8 are formed which extend to one edge of the substrate dimensioned for receiving therein the ends of an array of optical fibres 10. An optical device 12 formed as a block of material includes optical components comprising ports (waveguides) 14. The lower surface of the block includes an array of solder pads 16 formed in the some manner as solder pads 4 and positioned so as to be in one to one correspondence or registration with pads 4. In order to assemble the optical coupling of Figure 1, the two arrays of solder pads 4, 16 are brought into coarse alignment and the assembly is raised in temperature until the solder 6 becomes molten. The molten solder will by surface tension effects align the two sets of pads and will form drops of precisely determined dimensions which will establish the height of device 12 above the plane of the substrate to an accuracy of less than 0.5µm, which is sufficient to establish an accurate optical coupling between fibres 10 and the optical components. On cooling of the solder, the device 12 is rigidly secured to the substrate. The fibres 10 may be secured into the V-grooves 8 by solder or other suitable adhesive before or after the substrate 2 is attached to the optical device. Verticle alignment of the fibre cores to the optical waveguides is achieved as a consequence of the original photolithography of the V-grooves which establishes the height of the fibre cores to the same as that of the optical waveguides 12. Referring now to Figure 2 a substrate of suitable material 20 has etched into its surface a series of V-grooves 22 and in precise alignment to these an array of solder wettable pads 24. The ends 26 of the V-grooves are also accurately fixed in the same photolithographic step and can be used as an end stop for the fibres 28 in order to set the final spacing of fibres to the optical device. The optical device 30 has a corresponding array of solder wettable pads 32. Solder can be evaporated or sputtered to either the V-groove substrate 20 or the optical device or both. The fibres may be fixed in the V-groove up to the end stop prior to attachment to the optical device or this can be done after the two are joined. On inverting the substrate 20 as shown and reflowing the solder a final fibred assembly is produced. Referring to Figure 3 two sets of V-grooves 34, which may be continuous are etched into a suitable substrate 36. Solder wettable pads 38 are again accurately located on the substrate 36 and optical device 40 to provide the final alignment. Also included on the V-groove slab are conductive tracks 42 which have solder wettable areas 44 corresponding to the optical device electrode location. After reflowing the interconnecting solder and insertion of the fibres into the V-grooves the device electrodes can be accessed by wire bonding to the protruding conductive tracks 42. In addition electrical components may be performed in the substrate 36 by known integration techniques in electrical connection with tracks 42. Figure 4 shows a similar assembly to Figure 3 including solder connected tracks 42 but in this instance the tracks terminate in plated through holes 46. The holes provide a positive location for the assembly onto a suitable package base 48 with protruding legs 49. Reflowing of solder applied to the holes gives electrical and mechanical connection without any further assembly required. The whole assembly may then be protected by encapsulation. Figure 5 which is a modification of the assembly of Figure 4, shows a method of attaching the conductive tracks 52 to the package legs 54 without the necessity for plated through holes by providing flat end surfaces 56 for legs 54 registering with tracks 52. Solder or conductive adhesive is applied to the area of contact of end surfaces 56 and tracks 52 and the inverted assembly is bonded as shown. Plastic encapsulation may be used to protect the whole assembly. The assembly shown in Figure 6 incorporates a number of optical devices 60 as well as fibre arrays 62. The optical devices 60 each is formed as a block and contains optical components aligned with one another and secured in position by the use of solder aligned pads. Such a system can be assembled in one operation and encapsulated in suitable material to protect it. The foregoing description shown the invention to be an inexpensive and labour independent method of aligning optical devices and fibres to sub micron accuracy as well as providing for electrical connections to active devices without the use of wire bonding techniques. Standard photolithographic and solder reflow techniques are used and the final assembly can be encapsulated and packaged in conventional configurations.

Claims: A method of aligning an optical fibre or an array of optical fibres with an optical component or an array of optical components included as part of a block, said method comprising, providing a substrate having a first major plane surface, forming on said first major plane surface a first array of solder wettable pads, each of a predetermined area surrounded by areas which are solder repellent, forming a groove or an array of grooves in said substrate, the groove or each groove being aligned with, and being located in an accurate lateral relationship to, said first array of solder wettable pads, providing said block with said corresponding optical component or an array of optical components, said block having a second major plane surface, forming on said second plane surface a second array of solder wettable pads, each of a predetermined area surrounded by areas which are solder repellent, said second array of solder wettable pads being located relative to each other at positions in one-to-one correspondence with corresponding positions of said solder wettable pads of said first array, depositing controlled amounts of solder on some or all of said solder pads of said first and said second arrays, mounting said block above said substrate with said first and second major plane surfaces facing each other, reflowing said solder and thereby forming solder drops of predetermined dimensions, said solder drops securing said block at a predetermined height above said first major plane surface and positioning the block laterally and longitudinally in relation to said substrate, and locating and securing said optical fibre or said array of optical fibres in said groove or said array of grooves defined in said substrate thereby to align said optical fibre or said array of optical fibres with said optical component or said array of optical components. A method as claimed in Claim 1, wherein the optical fibre or each of the optical fibres is secured in its groove by adhesive means. A method as claimed in Claim 1, wherein the solder pads are formed by a photolithographic process. A method as claimed in Claim 1, wherein the controlled amounts of solder are evaporated onto the substrate employing a mask formed by a photolithographic process. A method as claimed in Claim 1, wherein the substate is formed with electrical components which are electrically connected with the solder pads for interconnection to electrical components within said blocks of optical components.

Inventors: RANDLE, FREDRICK ARTHUR
Applicants: GEC-MARCONI LIMITED
Application year: 1988
Main classification: G02B6
IPC codes: G02B6