Abstract:
The invention provides a method of generating an alignment feature ( 200, 300 ) in an optoelectronic assembly ( 10, 700 ) which enables another part, for example an optical fibre ( 80 ), external to the assembly to be aligned to a device ( 40, 400 ) within the assembly ( 10, 700 ) but without needing to be attached to the device ( 40, 400 ). The method involves using the device ( 40, 400 ) to define a position for its own alignment feature to which the part can register, thereby aligning the part ( 80 ) to the device ( 40, 400 ). When the device ( 40 ) is an emissive device, radiation emitted therefrom is used to delineate a position for the alignment feature. When the device ( 400 ) is a detecting device, the feature is defined with assistance of an apparatus ( 500 ) whose beam is guided in response to output from the device ( 400 ) to delineate a position for the alignment feature. The position of the alignment feature can be first defined in a system of layers ( 100 ) responsive to radiation from the device ( 40 ) or from the apparatus ( 500 ), and then transferred from the system of layers ( 100 ) by etching processes into a wall ( 70 ) of the assembly ( 10, 700 ) to provide a recess therein into which the part ( 80 ) can register for aligning to the device ( 40, 400 ).

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method of alignment in an optoelectronic assembly, in particular, but not exclusively, to a method of generating an alignment feature in an optoelectronic assembly which enables another part, for example an optical fibre, to be aligned to a device within the assembly. 
     Conventionally, a micro fabricated optoelectronic device, for example a single chip photodetector or laser source device, is packaged within an associated hermetically sealed package for protection. An optical fibre for conveying electromagnetic radiation to or from the device is attached to the package and penetrates through the package to contact directly onto the device. Alternatively, the optical fibre penetrates through the package to align remotely to the device, a secondary structure physically connecting to the device and receiving an end of the fibre thereby securing the fibre in alignment with the device. Alignment of the optical fibre to the device can often be difficult to achieve especially when the fibre is a monomode fibre having an associated core diameter in the order of a few ricrometres. When the device is a III-V compound device, for example a gallium-indiumarsenide laser source, alignment of the fibre to the device can be critical; an alignment error in the order of 0.1 μm can adversely affect coupling efficiency of the device to the fibre. There also arises an issue of mechanical stability where the optical fibre abuts onto the device; relative movement therebetween over time of tens of nanometres can adversely effect transmission efficiency from the device into the fibre or detection efficiency of the device to incoming fibre-borne radiation. 
     There are a number of conventional approaches to assist with aligning optical fibres to associated devices. 
     In a first conventional approach, there is provided an optoelectronic device housed within a hermetically sealed package, the device having etched thereinto a channel for receiving a core of an optical fibre, the fibre passing from a region exterior to the package through the package to terminate on the device. Assembly of the fibre to the device requires considerable operator skill and damage can occur to the device if the fibre is misdirected during assembly, for example the fibre scraping and severing metal electrodes of the device. Location of the core in the channel can occur by mechanical abutment although optically transparent bonding agents, for example Norland Inc. optically transparent UV curing adhesive, can be advantageously added to obtain a robust joint. 
     In a second alternative approach, an epitaxial alignment structure is formed onto the device to provide lateral abutment edges onto which the core can register. However, this second approach suffers the same disadvantages of the first approach in that a skilled operator is required for manipulating the core to align it precisely to the device without causing damage thereto. 
     In a granted U.S. Pat. No. 4,892,377, the inventor discloses an approach to accurately align an array of optical fibres with corresponding optical components such as waveguides. The fibres are fixed in accurately etched V-grooves formed into a substrate connected to the optical components and can be secured thereto using solder. Such an approach requires skilled operators to manipulate the fibres into the V-grooves to obtain a satisfactory alignment providing acceptable matching. 
     The inventor has appreciated that it is desirable to accurately align fibres to devices without having to physically bond or abut the fibres to the devices. Superficially, such an approach would seem unworkable because each fibre would have to be accurately aligned to an associated intermediate region and a corresponding device would also have to be accurately aligned to the intermediate region thereby fixing the fibre spatially with respect to the device; this would result in a build up of tolerances which would be more difficult to control than the first and second conventional approaches described above. 
     The inventor has evolved a method of alignment in an optoelectronic assembly which addresses alignment problems associated with spatially defining a fibre&#39;s position with respect to an associated device within the assembly without having to bond the fibre directly to the device within the assembly as in the prior art. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of alignment in an optoelectronic assembly, the assembly including one or more optoelectronic devices and interfacing means for interfacing from the one or more devices to one or more corresponding optoelectronic components external to the assembly, the method characterised in that it includes the steps of: 
     (a) defining one or more regions of the interfacing means to which the one or more devices are responsive to or emissive towards, thereby rendering the one or more regions delineated for processing purposes; 
     (b) processing the assembly to generate one or more alignment features at the one or more regions, the one or more features operable to assist with aligning the one or more external components relative to their respective one or more devices within the assembly; 
     (c) aligning the one or more components to their respective features so as to be operable to emit towards or receive radiation from their respective one or more devices; and 
     (d) applying attaching means for attaching the one or more external components to the assembly when aligned to their respective features, the one or more components thereby optically aligned to their corresponding one or more devices within the assembly. 
     The method provides the advantage that the one or more devices are capable of defining positions of their respective alignment features in the interfacing means, thereby assisting to align the one or more external components to the devices without the position of the interfacing means needing to be initially precisely defined with respect to the one or more devices. 
     Advantageously, in step (a), the interfacing means includes a wall of the assembly, the wall bearing a system of layers responsive to radiation emitted from the one or more devices within the assembly for defining the one or more regions in the system of layers, the layers providing a template for the formation of the alignment features in step (b). Inclusion of the system of layers enables the one or more devices to define positions of their corresponding one or more alignment features, the system of layers responsive to radiation emitted from the one or more devices. 
     Conveniently, the system of layers comprises: 
     (a) an etch resist layer through which the wall is processable to generate the one or more features therein; and 
     (b) a photochromic layer operable to be activated by externally applied radiation and capable of being rendered locally transmissive in response to radiation received thereat from the one or more devices, 
     thereby enabling the externally applied radiation to define regions in the resist layer corresponding to the one or more features, the resist layer providing a template for formation of the one or more features. Use of the photochromic layer provides a simplified method of alignment which does not need to be conducted in darkroom conditions. 
     Alternatively, the system of layers includes multiplying means for frequency multiplying radiation emitted from the one or more devices within the assembly, thereby generating corresponding relatively shorter wavelength radiation for defining the one or more regions in one or more layers of the system. Inclusion of the multiplying means enables one or more of the devices emitting infra-red radiation having a wavelength in the order of 1300 to 1550 nm to form a latent image in the system of layers, the latent image processable for ultimately forming a template in the system of layers through which the wall can be etched to generate the alignment features therein. 
     The multiplying means preferably comprises a multiplying layer including potassium titanyl phosphate which is operable to emit radiation at a relatively shorter wavelength when stimulated by relatively longer wavelength radiation emitted from the one or more devices. Use of potassium titanyl phosphate material enables infra-red radiation output from the one or more devices within the assembly to be transformed to radiation within the visible electromagnetic radiation spectrum to which other of the layers of the system are responsive. 
     In order to provide adequate spatial resolution for delineating the alignment features, the multiplying layer beneficially comprises a continuous film of potassium titanyl phosphate. 
     Advantageously, the system of layers includes a photoresponsive layer responsive to radiation emitted from the one or more devices within the assembly subject to frequency multiplication in the multiplying means, the photoresponsive layer processable to define a first template which is transferable to an etch resist layer of the system, the etch resist layer forming a second template through which the wall is processable to generate the one of more features therein. Use of the photoresponsive layer enables radiation within the visible range to define features in the photoresponsive layer which can be transferred from that layer by ultra violet (UV) radiation exposure to a photoresist layer, the photoresist layer being required to withstand etching gases or solutions where alignment features are not to be formed into the wall. 
     Conveniently, where direct responsivity to infra-red radiation is required in the system of layers, the system includes a photoresponsive layer directly responsive to radiation emitted from the one or more devices within the assembly, the photoresponsive layer processable to define a first template which is transferable to an etch resist layer of the system, the etch resist layer forming a second template through which the wall is processable to generate the one of more features therein. 
     The method of the invention according to the first aspect described in the foregoing is modified where the assembly incorporates one or more devices which are not radiation emissive. Preferable, the interfacing means includes a wall of the assembly, the wall bearing a system of layers responsive to radiation received thereon from a source external to the assembly, the radiation from the source being guided by a response from one or more of the devices within the assembly to define the one or more regions in the system of layers, the layers providing a template for the formation of the one or more features in step (b). Use of the external source compensates for the one or more devices being responsive to radiation but not radiation emissive. 
     Conveniently, the system of layers includes a photoresponsive layer responsive to radiation emitted from the external source, the photoresponsive layer processable to define a first template which is transferable to an etch resist layer of the system, the etch resist layer forming a second template through which the wall is processable to generate the one of more features therein. The photoresponsive layer can, for example, be a photoemulsion responsive to radiation emitted from the external source, the source not restricted to being emissive at infra-red radiation wavelengths in the order of 1300 to 1550 nm. 
     Beneficially, the etch resist layer includes an UV-responsive organic resist layer. Such resist layers are conventionally used in semiconductor fabrication processes for providing a template for etching processes. 
     Where problems of resist adhesion and robustness are experienced, for example when undertaking isotropic acid etching using buffered hydrofluoric (HF) acid, the etch resist layer advantageously further includes a UV-responsive organic resist layer and also a silicon nitride layer into which the second template in the organic resist layer is transferable to form a third template in the silicon nitride layer through which the one or more features are generated. Silicon nitride is better able to resist etchant attack than organic resist for many conventionally used silicon etchants. 
     Conveniently, there are several alternative etching processes which can be used for forming the alignment features in the wall; the features can be generated by one or more of anisotropic wet etching, isotropic wet etching, dry plasma etching or dry reactive ion etching processes. Preferably, the one or more features are recesses into which the one or more external components are registerable for aligning to their respective one or more devices within the assembly. 
     Where the assembly is used for one or more of receiving and emitting infra-red radiation in the order of 1300 to 1550 nm wavelength, the wall is conveniently fabricated from silicon transmissive to infra-red radiation. 
     For ease of fabricating the assembly, the attaching means is advantageously a substantially transparent UV-curable adhesive. 
     In a second aspect of the present invention, there is provided an assembly fabricated by a method according to the first aspect of the invention, the assembly characterised in that it comprises one or more optoelectronic devices incorporated within a housing, the housing including the interfacing means in the form of a wall, the wall including the one or more features to which the one or more external components are registerable for aligning the components to their respective one or more devices. 
     Conveniently, the wall is fabricated from &lt;100&gt;-cut single-crystal silicon and is wet anisotropically etchable to form pyramidal recesses therein for providing the one or more features to which the external components are registerable. The pyramidal features provide a beneficial characteristic that they are self-limiting in size in a wet anisotropic etching process, thereby counteracting a need to monitor etching critically to counteract overetching, for example as can occur when wet isotropic etchants are employed. 
     Preferably, in order to obtain a more ruggedized assembly, the wall can be recessed within the housing, thereby exposing a greater surface area of the housing to which the attaching means can bind for anchoring the one or more external components to the assembly. 
     Conveniently, the one or more external components include one or more optical fibres, each fibre having its core protruding from its cladding where the core is registerable into the one or more features. Such alignment of the cores to the features provides an enhanced degree of coupling efficiency between the one or more external components and corresponding one or more devices within the assembly. 
     In a third aspect of the present invention, there is provided an array of assemblies, each assembly according to the second aspect of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example only, with reference to the following diagrams in which: 
     FIG. 1 is a schematic illustration of an optoelectronic assembly including an optical fibre aligned to the assembly according to a method of the invention; 
     FIG. 2 is an illustration of a system of layers used for fabricating the assembly in FIG. 1; 
     FIG. 3 is an illustration of an alignment recess etched into the assembly for receiving the fibre in FIG. 1; 
     FIG. 4 is an illustration of a core of the fibre in FIG. 1 locating into the recess of FIG. 3; 
     FIG. 5 is a cross-sectional illustration of a core of the fibre in FIG. 1 locating into an isotropically etched recess of the assembly in FIG. 1; 
     FIG. 6 is an illustration of a configuration for use according to a method of the invention; 
     FIG. 7 is an illustration of an alternative projector for use in the configuration in FIG. 6 according to a method of the invention; 
     FIG. 8 is an illustration of an alternative assembly according to the invention including a recessed plate enabling the assembly to provide improved support for an optical fibre attached to the alternative assembly; and 
     FIG. 9 is an illustration of the assembly in FIG. 1 during fabrication thereof with etch resist and photochromic layers applied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown an optoelectronic assembly indicated by  10 . The assembly  10  comprises a silicon substrate  20  including an upper major face onto which is formed a 1 μm-thick silicon nitride insulating layer supporting 1 μm-thick aluminium electrode connection pads, for example a pad  22 , connected to associated aluminium conductive tracks, for example a track  24 . 
     The assembly  10  further comprises a &lt;100&gt; crystal orientation silicon cap  30  into which a recess has been anisotropically wet etched, and also a silicon end plate  70 . In the assembly  10 , the cap  30 , the substrate  20  and the plate  70  are mutually fusion bonded, soldered or adhesively bonded; the recess of the cap  30  forms a hermetically-sealed cavity for housing a micro fabricated infra-red laser device  40  of conventional design. The device  40  is solder bump-bonded onto the tracks of the substrate  20 , thereby enabling electrical connection to be made to the device  40  from the pads; during fabrication, the device  40  is bonded onto the tracks before the cap  30  and the plate  70  are bonded to the substrate  20  and to one another. 
     The device  40  incorporates an emitting region  50  from which, in operation, a beam  60  of infra-red radiation is emitted which propagates to illuminate a region  65  of the silicon end plate  70 . The plate  70  includes a major external planar surface onto which a monomode fibre  80  is abutted and bonded, registering with its monomode core  85  into a recess etched into the major surface corresponding to the region  65 . The plate  70  further comprises a major inner planar surface facing towards the device  40 , the inner surface being substantially parallel to the external surface. 
     A first method of fabricating the assembly  10  and aligning the fibre  80  thereto will now be described in overview with reference to FIG. 1, the method comprising a series of sequential steps of: 
     STEP 1: fabricating the substrate  20  with its silicon nitride layer and associated tracks and pads as a first piece part; 
     STEP 2: fabricating the device  40  using conventional micro fabrication techniques as a second piece part; 
     STEP 3: fabricating the cap  30  as a third piece part from a section of a &lt;100&gt;-cut silicon wafer, fabrication involving initially delineating a first etch window photo-lithographically in the section, and then anisotropically selectively etching the section through the window in a mixture of isopropanol and potassium hydroxide (KOH) solution to generate the recess in the section; 
     STEP 4: scribing a &lt;100&gt;-cut silicon wafer having polished front and rear surfaces to provide the plate  70  as a fourth piece part; 
     STEP 5: assembling the first and second piece parts together, namely bump bonding the device  40  onto the tracks on the substrate  20 , thereby enabling the device  40  to be electrically driven from the pads on the substrate  20 ; 
     STEP 6: assembling the third piece part onto the assembled first and second piece parts, namely fusion bonding, soldering or adhesively bonding the cap  30  onto the substrate  20  to form a cavity for the device  40 ; 
     STEP 7: assembling the fourth piece part to the assembled first, second and third piece parts, namely fusion bonding, soldering or adhesively bonding the plate  70  onto edges of the cap  30  and the substrate  20  as illustrated in FIG. 1 to provide a hermetically sealed housing for the device  40 ; a metallisation layer together with a window layer (not shown) can optionally be included between the plate  70  and the cap  30  and the substrate  20 ; 
     STEP 8: coating the external surface of the plate  70  with a photosensitive etch-resistant system of layers; 
     STEP 9: activating the device  40  via the pads so that it emits the beam of radiation  60  onto the plate  70  thereby exposing the region  65  thereon and also corresponding regions of the system of layers in close proximity to the region  65 ; 
     STEP 10: developing the system of layers to leave a second etch window through the layers in the vicinity of the region  65 ; 
     STEP 11: anisotropically etching a pyramidal-form recess into the external surface of the plate  70  where it is exposed through the second window in the system of layers; 
     STEP 12: stripping off the system of layers to leave the external surface of the plate  70  with its etched recess exposed; 
     STEP 13: at an end of the fibre  80 , stripping off a short length of fibre cladding to leave the core  85  protruding; 
     STEP 14: offering the protruding core  85  to the recess and locating it therein; and 
     STEP 15: applying optically-transparent bonding agent, for example UV curable Norland Inc. adhesive, onto the end of the fibre and the plate  70  and then UV curing it to bond and anchor the fibre  80  to the plate  70 . If an interfacing lens is located into the recess and the fibre then registered near to the lens, an air interface may be required on a non-collimating side of the lens which is therefore optionally filled with the curable adhesive. 
     Although fabrication of the assembly  10  as a single item is described, the method can be extending to use parallel batch processing techniques so that several such devices can be processed simultaneously according to the invention. 
     The first method provides the advantage that, after assembling the substrate  20 , the cap  30 , the device  40  and the plate  70  together to fix their relative spatial positions, the device  40  is capable of defining its own recess position on the plate  70  which precisely aligns with the beam  60  emitted from the device  40 . Thus, none of the substrate  20 , the cap  30 , the device  40  and the plate  70  need to be particularly accurately mutually aligned during assembly, accuracy of position of the recess relative to the device  40  being achieved by allowing the beam  60  to define a precise position for the recess to ensure efficient coupling of radiation into the fibre  80 . This advantage is of considerable commercial importance because it circumvents a need to manufacture component parts which are mutually matched to a high degree of accuracy, thereby reducing cost of manufacture. 
     The system of layers in STEPS 8 to 12 is peculiar to the method and will be described in further detail with reference to FIG.  2 . The system of layers is indicated by  100 . It comprises in sequence from the major external surface of the plate  70  indicated by  120 : 
     (a) an organic resist layer  130 , for example a Hoechst AZ series organic resist such as AZ1505, in a range of 1 to 1.5 μm thick; 
     (b) a conventional silver-based proprietary photoemulsion layer  140  in the order of 0.5 to 5 μm thick which is responsive to radiation in the visible electromagnetic radiation spectrum having a wavelength range from 200 nm to 800 nm; and 
     (c) a frequency-multiplier layer  150  in the order of 1 to 20 μm thick, the frequency-multiplier layer  150  comprising a unitary layer of potassium titanyl phosphate. 
     Operation of the system of layers  100  will now be described with reference to FIG. 2, operation corresponding to STEP 9 to STEP 15 in the first method described above. The device  40  is activated to generate the beam of infra-red radiation  60  having a wavelength in the order of 1300 to 1550 nm. The beam  60  propagates from the device  40  to the inner surface of the plate  70  at the region  65 , the inner surface indicated by  110 . The beam  60  propagates through the plate  70  to exit into the resist layer  130  through which it propagates to the photoemulsion layer  140 ; photons in the beam  60  have insufficient quantum energy to substantially affect the resist layer  140 . The layer  140  is insensitive to infra-red radiation and requires shorter wavelength radiation having a wavelength of less than 800 nm to form a latent image therein. The beam  60  thus propagates through the photoemulsion layer  140  without affecting it and is finally absorbed in the frequency-multiplier layer  150 . 
     The layer  150  performs a frequency multiplying operation. Photons of radiation having a relatively longer wavelength in the order of 1300 to 1550 nm incident on the layer  150  stimulate therein the generation of photons having a relatively shorter wavelength in the order of 650 nm which are subsequently emitted from the layer  150 . It takes several photons of the relatively longer wavelength to be absorbed to generate a photon of the relatively shorter wavelength. 
     Thus, infra-red radiation photons absorbed where the beam  60  is incident on the layer  150  result in the generation of relatively shorter wavelength photons in the visible range which are emitted locally to the layer  140  and result in the formation of a latent image therein in the vicinity of where the beam  60  propagates through the layer  140  in an outwards direction from the device  40 . 
     When the device  40  has been activated for a sufficiently long duration to form a latent image in the photoemulsion layer  140 , the device  40  is de-activated. The layer  150  is then removed to expose the layer  140 ; removal of the layer  150  is performed in dark-room conditions to avoid further exposure of the layer  140  and disturbing the latent image formed therein. The layer  140  is then developed using a conventional proprietary photographic process appropriate for the layer  140  material. Development of the layer  140  provides a darkened opaque region in the vicinity of the region  65  because the layer  140  functions in negative sense. The resist layer  130  is then exposed to ultra-violet (UV) radiation of substantially 250 nm wavelength through the layer  140  which hardens the resist layer  130  by polymerisation except in the vicinity of the darkened opaque region where the resist layer  130  remains locally relatively soft. Once this exposure has occurred, residual traces of the layer  140  are dissolved away and the resist layer  130  is then developed in a solvent, for example a proprietary solvent mixture comprising methyl isobutyl ketone and isopropanol, to leave a window in the resist layer  130  in the vicinity of the region  65 , a remainder of the resist layer  130  remote from the region  65  remaining intact in a hardened polymerized state. 
     The assembly  10  is then covered in a protective resin or photoresist except for the plate  70  with its resist layer  130  which is left exposed for etching purposes, the resin or photoresist capable of withstanding anisotropic silicon etches such as KOH/isopropanol solution mixtures. The resin protects, for example, the tracks and pads of the assembly  10  from the anisotropic etchants. 
     The assembly  10  is then immersed in an anisotropic silicon etch solution, for example a KOH/isopropanol mixture, which has access to the plate  70  through the window in the vicinity of the region  65 . The solution etches a self-limiting pyramidal recess in the surface  120  of the plate  70  as illustrated in FIG. 3; the recess is indicated by  200  and comprises four &lt;111&gt; crystal plane surfaces, for example a surface  210 . 
     When etching of the recess  200  has occurred, the assembly  10  is removed from the etch solution, the resin or resist protecting the assembly  10  and the resist on the surface  120  are then stripped to yield the assembly  10  with its recess  200  precision aligned to the device  40 . 
     An end of the fibre  80  with its core  85  protruding at the end is then offered up to the surface  120  so that the core  85  locates into the recess  200  as illustrated in FIG.  4 . In FIG. 4, the core  85  abuts at its peripheral circumferential edge onto surfaces of the recess  200  at points P 1  to P 4  as illustrated, for example, the core  85  abuts at its peripheral edge to the surface  210  at the point P 2 . 
     When the core  85  is correctly abutted into the recess  200 , the fibre  80  in the vicinity of the plate  70  and the surface  120  are covered in a quantity of optically transparent bonding resin, for example Norland Inc. optical UV-curable resin type N65, which is then UV (ultra violet radiation) cured, thereby forming a mechanical bond of the fibre  80  to the assembly  10 . Abutment of the core  85  within the recess  200  assists to ensure that precision alignment to the device  40  is substantially maintained even if the bonding resin shrinks or changes dimension slightly due to ageing processes. 
     The layers  130 ,  140  in FIG. 2 are chosen to be sufficiently thin to counteract lateral scattering of the beam  60  as it propagates therethrough. Conversely, they are chosen to be sufficiently thick to enable the window to be reliably defined in the resist layer  130 , and to survive anisotropic etching necessary to form the recess  200 . If scattering is isotropic through the layers  130 ,  140 , a limited degree of scattering can be tolerated because the recess  200  will be substantially correctly centered to the region  65 . 
     In the aforementioned first method, the multiplying layer  150  need not be included for forming the recess if the device  40  is substituted with a corresponding device emitting radiation in a wavelength range to which the layer  140  is substantially responsive. In the aforementioned first method, wet isotropic etching can alternatively be used in STEP 11 instead of wet anisotropic etching. Suitable isotropic etchants for etching the plate  70  through the window in the resist layer  130  include buffered hydrofluoric acid. In FIG. 5, such wet isotropic etchants result in the formation into the surface  120  of a rounded substantially-hemispherical recess indicated by  300  into which the core  85  can locate; the core  85  is located around its entire peripheral edge onto an inside surface of the recess  300 . Wet isotropic etching tends to be dimensionally less accurately controllable compared to wet anisotropic etching on account of isotropic etching rate being dependent upon spatially localised etchant flow rates. The self-limiting characteristic of wet anisotropic etching is thus capable of providing more accurately centered recesses compared to recesses formed by isotropic etching processes where self-limiting does not occur. 
     As an alternative to using wet isotropic or anisotropic etchants, gaseous plasma or reactive ion etching can be employed in STEP 11 although the effects of plasma field distortion around the assembly  10  has to be allowed for when such dry etching is employed. Plasma or reactive ion etching can be isotropic or anisotropic depending upon etching gas pressure employed and electric field distributions in the vicinity of the assembly  10  during such etching. Suitable etching gases can include hydrogen-halide compounds, or a mixture of oxygen and carbon-halide compounds. 
     In the system  100  shown in FIG. 2, the photoemulsion layer  140  and the frequency-multiplier layer  150  can alternatively be replaced by a single layer of infra-red radiation sensitive photoemulsion. Such infra-red photoemulsions are employed in the manufacture of infra-red films as used in road traffic speed cameras and are a conventional proprietary product. When such an infra-red sensitive photoemulsion is used, the device  40  is capable of exposing the emulsion directly without the need for frequency multiplication in the layer  150 . 
     Although the method including STEPS 1 to 15 is suitable for the assembly  10  incorporating the device  40  which is radiation emissive, the method can be modified to cope with a situation where the device  40  in the assembly  10  is replaced by a detecting device, for example a photodetector responsive to infra-red radiation at a wavelength of substantially 1550 nm. 
     In FIG. 6, there is illustrated a configuration indicated by  500  including the device  10  fabricated according to STEPs 1 to 6 of the first method but incorporating a photodetecting device  400  instead of the device  40 . The configuration  500  further includes a projector indicated by  550 , a plinth  610  and an actuator mechanism  600 . The projector  550  and the mechanism  600  are mounted onto the plinth  610 . The mechanism  600  incorporates a platform onto which the assembly  10  is mounted. FIG. 6 is not to scale and certain parts therein are shown in relatively exaggerated size for clarity. 
     The projector  550  incorporates an optical unit  560  incorporating mirrors which are capable of focussing radiation both at infra-red radiation wavelengths in the order of 1300 to 1550 nm and also at visible radiation wavelengths around 560 nm. Moreover, the projector  550  further incorporates a beam directing mirror  570 , a first laser source  580  operable to emit a beam of radiation in the order of 1300 to 1550 nm wavelength and a second laser source  590  operable to emit a beam of radiation at around 560 nm wavelength. The mirror  570  is pivotally mounted at a point P at one end thereof and is controllably movable between a first position as illustrated in FIG. 6 and a second position where it is retracted as shown by a dotted line  620 . 
     The assembly  10  has in sequence the layer of resist  130  deposited onto the major exterior surface  120  of the plate  70  followed by the conventional photoemulsion layer  140 . The configuration  500  is maintained in darkroom conditions to avoid forming a latent image in the layer  140  prior to irradiation from the sources  580 ,  590 . 
     The mechanism  600  is operable to controllably move the assembly  10  laterally in directions x and y with respect to the projector  550  as shown in FIG. 6; the direction x is in a sense out of the plane of the paper, directions x, y, z are mutually orthogonal, and the directions y, z are in the plane of the paper. 
     The projector  550  is operable to project images of the first source  580  and the second source  590  onto a region  410  of the device  400  and onto the external surface  120  of the plate  70  respectively. These images are formed at different distances from the projector  550 , such distances determined by the second source  590  being situated further from the mirror  570  relative to the first source  580  therefrom. The sources  580 ,  590  are solid-state laser devices which provide point objects for the optical unit  560  in the order of 1 to 3 μm diameter. 
     A second method of fabricating the assembly  10  including the device  400  will now be described with reference to FIG. 6, the method employing the configuration  500  and including the following steps: 
     STEP A: Fabricating the assembly  10  including the device  400  according to STEPs 1 to 6 of the aforementioned first method. Then, making electrical connections to the pads of the assembly  10  to enable a signal to be conveyed therefrom to a control unit (not shown). The control unit is also connected to the mechanism  600  and the projector  550 . 
     STEP B: Rotating the mirror  570  to the second position  620  by instruction from the control unit. Then, activating the first source  580  to emit a beam of infra-red radiation at a wavelength of around 1550 nm towards the optical unit  560  thereby projecting an image of the source  580  towards the device  400 . Next, instructing from the control unit the mechanism  600  to move the assembly  10  laterally with respect to the projector  550  until the image is incident on the region  410  which causes a received signal to be generated by the assembly  10  at its pads. These signals are conveyed via connections through the mechanism  600  to the control unit. 
     STEP C: Deactivating the first source  580 . Next, rotating the mirror  570  to the first position as illustrated in FIG. 6, and then activating the second source  590  to generate a beam of radiation at around 560 nm wavelength which is reflected by the mirror  570  to the optical unit  560  thereby projecting an image of the second source  590  onto the exterior surface  120  of the plate  70 . Where a beam associated with the second source&#39;s  590  image propagates through the layer  140 , it forms a latent image therein. Because images of the sources  580 ,  590  are formed along an optical axis common to the sources  580 ,  590 , the latent image is formed in a region of the layer  140  to which the device  400  is receptive. The second source  590  is then deactivated. 
     STEP D: Removing the assembly  10  from the mechanism  600  and developing the latent image in the layer  140  using a proprietary developer; this is undertaken in dark-room conditions where ambient radiation in the visible spectrum is excluded. Where the film  140  is exposed to the image of the second source  590 , there results in the layer  140  an opaque region. The assembly  10  is then exposed to general UV irradiation which hardens the layer  130  except in a region of the layer  130  corresponding to the opaque region in the layer  140 . 
     STEP E: Stripping away the layer  140  and then developing the resist layer  130 . After development, hardened regions of the resist remain with an exception of the region shadowed by the opaque region in the layer  140  where a window through the resist layer  130  is formed to the surface  120 . 
     STEP F: Coating the assembly  10  in resin or resist except for the plate  70  and next immersing the assembly in a wet anisotropic etch solution, namely a KOH/isopropanol mixture, to etch a recess into the plate  70  corresponding to the window. 
     STEP G: Stripping the resin/resist and the resist layer  130  using appropriate solvents when anisotropic etching of the assembly  10  has been completed. The monomode fibre  80  with its cladding removed at one end thereof to expose a length of monomode core  85  protruding therefrom is then offered up to the assembly  10  so that the protruding core  85  locates into the recess. UV-curable substantially transparent optical bonding adhesive is then applied over the end of the fibre  80  and the plate  70  to rigidly maintain the protruding core  85  of the fibre  80  aligned to the recess and anchored relative thereto. This completes connection of the assembly  10  to the optical fibre  80 , the assembly  10  responsive to infra-red radiation conveyed along the fibre  80  to the assembly  10 . If more efficient coupling is desired from the fibre  80  into the device, a lens is included in the recess and the end of the core  85  is then aligned to the lens; the UV curing adhesive is then applied over the fibre  80  to anchor it to the assembly  10 . 
     Parallel processing of several assemblies similar to the assembly  10  incorporating the device  400  can be undertaken to reduce cost. However, operation of the configuration  500  is essentially a serial process addressing only a single assembly at any instance of time. 
     Instead of employing wet etching to form the recess in the plate  70  of the assembly  10  incorporating the detector  400 , wet isotropic etching and gaseous plasma or reactive ion etching can alternatively be employed. 
     The configuration  500  and its associated method STEPS A to G can be modified. For example, the second source  590  can be replaced with a shorter wavelength source eissive at ultra violet (UV) radiation so that the resist layer  130  can be exposed directly during the method thereby circumventing a need to include the layer  140  and performing processing steps associated with STEP D. Making this modification avoids the need to ensure dark-room conditions during the second method. A compact excimer laser can be used as a substitute for the source  590  to provide the UV radiation. 
     Moreover, the configuration  500  can be modified to assist with forming an alignment recess in the plate  70  when the device  400  is replaced with the emissive device  40 . The first source  580  is replaced with an infra-red detector device incorporating apertures to provide it with a sensing aperture of a few μm in diameter, the device connected to the control unit. In operation, the mirror  570  is retracted into the second position  620  and the device  40  in the assembly  10  is then activated via the mechanism  600  from the control unit to emit a beam of infra-red radiation. The mechanism  600  then moves the assembly  10  relative to the projector  550  until the infra-red detector device generates a signal in response to receiving radiation emitted from the device  40  within the assembly  10 . The mirror  570  is then pivotally rotated to its first position where it reflects radiation emitted from the second source  590  to form a latent image in the layer  130 ,  140  corresponding to the recess. Development and etching process as described before in STEPS D to G are then applied to form the recess and align and anchor the fibre  80  thereto. 
     In the modified configuration  500  where the first source  580  is replaced by an infra-red detector device, the second source  590  can be replaced with a UV source, for example an excimer laser, thereby circumventing a need to include the layer  140  and to perform STEP D in the second method. 
     The projector  550  in the configuration  500  can be substituted by an alternative projector indicated by  700  and illustrated in FIG.  7 . The projector  700  incorporates a first optical assembly comprising the first source  580  and an associated collimating lens  780 ; the lens  780  is conveniently fabricated from BK 7  glass or a plastics material, although germanium and silicon can be used but then tend to make the lens  780  expensive to manufacture. The projector  700  further includes a second optical assembly comprising a second source  710  radiation emissive at a wavelength in the order of 560 nm together with an associated actuated shutter  720  moveable between an off-axis position and a blocking position  730  and an associated silica glass or quartz lens  740 . The projector  700  additionally includes a third optical assembly comprising a mirror unit indicated by  750  comprising a dichroic mirror  760  orientated with its plane at an angle of 45° with respect to optical axes of the first and second optical assemblies whose axes are mutually orthogonal. The projector  700  also incorporates a projection lens  770  operable to project images of the sources  580 ,  710  onto points Q 2  and Q 1  respectively. The point Q 2  corresponds to a point on the device  40 ,  400  within the assembly  10 . The point Q 1  corresponds to a point in the layers  130 ,  140 . 
     Operation of the projector  700  when incorporated into the configuration  500  will now be described where the assembly  10  includes the detecting device  400 . 
     Initially, the sources  580 ,  710  are not activated. The shutter  720  is moved by the control unit to the blocking position  730 . The source  580  is then activated to generate a beam of infra-red radiation which propagates from the source  580  to the lens  780  which collimates the beam to generate a collimated beam which further propagates to the dichroic mirror  760  through which it passes substantially undeflected to the lens  770  which focusses the collimated beam to generate an image at the point Q 2  on the device  400  within the assembly  10 . The control unit then moves the assembly  10  relative to the projector  700  until an output signal is generated in response to radiation received at the detector device  400 ; this corresponds to the beam from the source  580  illuminating the region  410  of the device  400 . The control unit then finely moves the assembly  10  on the mechanism  600  until the detector  400  receives radiation corresponding to a central region of the beam from the source  580 . The source  710  is then activated and the shutter  720  moved to its off-line position; the source  710  emits a beam of radiation which is collimated by the lens  740  to provide a collimated beam which propagates to the dichroic mirror  760  and is reflected thereat in a direction towards the lens  770 . The reflected beam passes through the lens  770  which forms an image at the point Q 1  in the plane of the layers  130 ,  140  thereby forming a latent image in the layer  140 . 
     Processing steps from STEP D to STEP G are then applied to form the recess in the assembly  10  for receiving the fibre  80 . 
     In a similar manner to the projector  550 , the source  580  in the projector  700  can be replaced by an infra-red detector connected to the control unit thereby enabling the configuration  500  including the projector  700  to be used for defining a recess position for the assembly  10  when it incorporates an infra-red emissive device, for example the device  40 . Likewise, the second source  710  can be replaced by an excimer laser operable to emit UV radiation, thereby circumventing a need to perform STEP D and to include the layer  140 . 
     Referring now to FIG. 8, there is shown an alternative assembly indicated by  800 . The assembly  800  can be fabricated using the aforementioned first and second methods. The assembly  800  includes either the device  40  or the detector device  400 . Moreover, the assembly  800  is similar to the assembly  10  except that the plate  70  is moved further inside the assembly  800  compared to the assembly  10 , thereby providing greater mechanical support to the fibre  80  when bonded into position using UV-curable adhesive after its core  85  has been aligned to an alignment recess in the plate  70 , the recess indicated by  820 . In FIG. 8, a cured region of the adhesive for anchoring the fibre  80  to the assembly  800  is indicated by  810 . 
     A third method of fabricating the assembly  10  will now be described with reference to FIG.  9 . In FIG. 9, there is shown indicated by  900  the assembly  10  fabricated according to STEPs 1 to 6 of the first method and coated at the external surface  120  of its plate  70  with a positive-tone etch resist layer  910  followed by a photochromic layer  920 . Formation and operation of photochromic layers is described in a patent GB 2208271 which is hereby incorporated by reference. The assembly  10  incorporates the emissive device  40 . 
     The third method includes the following steps: 
     STEP 1: Fabricating the assembly  10  according to STEPs 1 to 6 of the first method; 
     STEP 2: Coating the plate  70  in the layer of etch resist  910  followed by the photochromic layer  920 ; 
     STEP 3: Exposing the assembly  10  to UV radiation having a wavelength in the order of 250 nm to activate the photochromic layer  920 ; 
     STEP 4: Activating the device  40  within the assembly  10  to emit the beam  60  of infra red radiation therefrom, the beam  60  propagating to the plate  70  and the layer  910 , passing therethrough to a local region of the layer  920  corresponding to the region  65  whereat the beam  60  causes bleaching of the layer  920  thereby rendering it locally transmissive to the externally applied UV radiation. At the region, the photochromic layer  920  transmits the UV radiation to the resist layer  910  which causes softening thereof in close proximity to the region in the layer  920 ; 
     STEP 5: Stripping the photochromic layer  920  and then developing the etch resist layer  910  in an appropriate solvent to provide an etch window corresponding to the local region in the photochromic layer  920 ; 
     STEP 6: Applying STEPs 11 to 15 of the first method to form a recess and then registering the core  85  of the fibre  80  to the recess. Finally, anchoring the fibre  80  to the assembly  10  using UV curable substantially transparent adhesive. 
     The third method provides the benefit that it does not need to be conducted in dark-room conditions. Moreover, it is a simpler process than the first and second methods. However, it is only appropriate where the assembly  10  incorporates a radiation emissive device, for example the device  40 . 
     Modifications can be made to the embodiments and methods of the invention described in the foregoing without departing from the scope of the invention. For example, alternative types of photoemulsion and resist layers can be employed in the system  100 . The photoemulsion layer  140  and the resist layer  130  described in the foregoing operate in negative sense; it is possible to perform STEPS 9 to 10 and STEPS C to F using photoemulsions and photoresist materials operating in positive sense. 
     Moreover, a 0.5 μm-thick layer of silicon nitride can be included between the resist layer  130  and the external surface  120  of the plate  70 . In the STEPS 10 to 11 and STEPS E to F, the windows formed in the resist layer  130  can be transferred by reactive ion etching or a phosphoric acid etchant to form a window in the silicon nitride layer. Silicon nitride forms a more effective barrier against wet anisotropic and isotropic etches than organic resist and a layer of silicon nitride should therefore be incorporated where problems are encountered with resist robustness when executing the methods of the invention described in the foregoing. 
     Although the assembly  10  is described including either the device  40  or the detector device  400 , the assembly  10  can be enlarged to include both emitting devices and photodetectors. Moreover, several aligned recesses can be formed into the plate  70  of such an enlarged assembly. The recesses can be etched to a size appropriate for accommodating components such as microlenses; in this case, fibres can be abutted to locate onto the lenses, the lenses being located precisely into position in their respective recesses. 
     Assemblies incorporating collimating lenses in their recesses, each assembly otherwise similar to the assembly  10  including the device  40  or the detector device  400  or both, can be assembled into a matrix, thereby providing an optically interconnected array. Such arrays are potentially usable in communication systems and light optical directing and ranging (LIDAR) systems, for example for use in making air flow measurements. 
     In the foregoing, optical lenses located into the recess  200 ,  300  formed into the assembly  10  can included moulded lenses and ball lenses for generated collimated input and/or output beams from the assembly  10 . Similarly, by choice of lens focal length, converging or diverging beams can be generated. 
     In the assemblies  10 ,  800 , the plate  70  is fabricated from silicon. In modified versions of the assemblies  10 ,  800 , the plate  70  can be alternatively fabricated from germanium or an infra-red transmissive plastics material. 
     The resist layer  130  is capable of inefficient conversion of infra-red radiation directly to ultra-violet radiation for exposing itself. Likewise, the multiplier layer  150  is also capable of performing such inefficient direct conversion. When fabrication time allows, exploitation of this characteristic of the layers  150 ,  130  allows at least the layer  140  to be dispensed with thereby simplifying process steps of the first method, for example STEPS 8, 9 and 10. Such conversion is known as “3 or 4 photon conversion” and corresponds to three to four photons of infrared radiation being absorbed in the resist layer  130  or the multiplier layer  150  and causing electrons therein to be excited to higher energy states; when the electrons return to their respective ground states, some change their energy state by amounts corresponding to the quantum energy of UV photons, thereby emitting UV photons. Such UV photons are then able to cause exposure of the resist in the resist layer  130  thereby locally altering its chemical structure.