Abstract:
A rear light entry photodetector chip is secured face-down with solder on to the front face of a ceramic submount provided with a pair of electrically conductive vias. A frame-shaped mass of solder seals the chip to the submount to provide a hermetic enclosure protecting sensitive semiconductor surface areas of the photodetector chip where electric fields are liable to be present in the vicinity of a pn or metal/semiconductor junction.

Description:
BACKGROUND OF THE INVENTION 
     It is known that, if exposed to an undesirable environment, the performance of a semiconductor photodetector can become degraded even though sensitive surface areas of the photodetector, particularly where an electric field is present during operation of the photodetector in the vicinity of a p-n junction or a junction between semiconductor material and metal, are protected from the ambient by a deposited dielectric layer. For this reason a number of applications for photodetectors have required such photodetectors to be hermetically packaged in order to reduce the risk of occurrence of this kind of performance degradation. The packaging must include provision, both for electrical connections with the photodetector from outside the package, and for the feeding into the package of the optical signal that the photodetector is designed to detect. One way of effecting such provision is to provide a window in the package wall, as for instance described in GB 2 208 944A. If the photodetector is to receive light emitted from the end of an optical fiber, the construction of the package may be such as to make the minimum separation between the end of the fiber and the photodetector so great as to require some form of lens coupling between them. This may be provided by a lens within the package, by a lens outside the package, or by using a lens as part of the package window, as described in GB 2 208 944A. As an alternative to the use of a window, a wall of the package may be penetrated in a hermetically sealed manner by some form of light guide, as for instance described in GB 2 064 862A. In the case of the particular structure described in GB 2 064 862A, the outer end of the light guide is butted against the optical fiber, but a clearly valid alternative is to replace the light guide with an optical fiber having properties similar to, or identical with, those of the optical fiber whose output it is desired shall be detected by the photodetector. 
     The use of the optical fiber/guide hermetic feedthrough, by definition requires an hermetic seal around the optical fiber/guide, and this is typically expensive to manufacture and difficult to test for leaks because the fiber is usually provided outside the enclosure with mechanical protection in the form of a plastics protective coating. The use of a window in place of the hermetic fiber-guide feedthrough avoids these particular problems, albeit at the expense of adding significantly to the complexity and expense of the provision of a mechanical link between the photodetector chip and the optical fiber whose output is to be detected by the chip. 
     SUMMARY OF THE INVENTION 
     The present invention is concerned with an alternative form of hermetic enclosure which does not involve any requirement for an optical fiber/guide hermetic feedthrough, and which affords the possibility of a relatively simple and cheap form of establishing mechanical coupling of the end of an optical fiber so as direct light emitted therefrom on to the photosensitive area of the photodetector. 
     According to the present invention there is provided a photodetector assembly including a rear light entry planar construction of semiconductor photodetector chip bonded face down upon a ceramic substrate so as to constitute a hermetic enclosure with opposed walls constituted respectively by the chip and the ceramic substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There follows a description of photodetector assemblies embodying the present invention in preferred form. The description refers to the accompanying drawings in which FIG. 
     FIG. 1 is a schematic cross-section of the photodetector area of the photodetector, 
     FIGS. 2 and 3 are schematic front and rear views of the photodetector submount, 
     FIG. 4 is a schematic cross-section of the photodetector and submount sub-assembly, 
     FIGS. 5 and 6 are fiber-pigtailed and connectorised versions respectively of assemblies incorporating the sub-assembly of FIG. 4, and 
     FIG. 7 is a schematic front view of an alternative form of photodetector submount. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The basic structure of the rear light-entry planar semiconductor photodiode chip depicted in FIG. 1 is created on a tin-doped InP substrate  10 . On the substrate are epitaxially grown an n-type InP buffer layer  11 , an intrinsic InGaAs absorption layer  12 , and an n-type InP layer of capping material  13 . The capping layer is masked, and a portion  14  of it, and of the underlying absorption layer  12 , is converted into p-type material by the diffusion of zinc from a source (not shown) of ZnAs. Except for a small region of the remaining n-type capping material that surrounds the p-type region of portion  14 , the rest of the capping layer material is removed. A layer  15  of silicon oxynitride is deposited to augment silicon oxynitride regions left by previous processing, and this (composite) layer  15  is patterned to define contact areas prior to the sputtering of platinum/gold contact metal which is patterned to define a central contact  16   a  contacting the p-type material and, encircling contact  16   a , a rectangular frame-shaped outer contact  16   b  contacting the n-type material of the absorption layer  12 . The inner and outer contacts  16   a  and  16   b  are coated respectively with preforms  17   a  and  17   b  of 80:20 gold:tin solder conforming in shape with their underlying contacts  16   a  and  16   b . This solder is conveniently deposited using an e-beam evaporator and lift-off photolithography. Optionally, the rear face of the substrate  10  is provided with an antireflection coating  18 . 
     The front and rear faces of a ceramic submount for the photodiode of FIG. 1 are depicted respectively in FIGS. 2 and 3, while a section through the submount, with the photodiode already mounted, is depicted in FIG.  4 . For convenience of illustration, the photodiode chip is represented in section in FIG. 4, and in succeeding Figures; by three layers topped with the solder preforms  17   a  and  17   b , the three layers comprising the photodiode substrate  10 , the optional anti-reflection coating  18  on the rear face of the substrate  10 , and a composite layer  19  that incorporates the whole structure of the completed photodiode of FIG. 1 that is formed on the front face of its substrate  10 . 
     The submount, which is indicated generally at  20 , is a ceramic submount of a type which has electrically conductive metallic-filled vias  21   a  and  21   b  extending through the thickness of the submount from the front surface  20   a  to the rear  20   b . Such vias are created by filling through-holes in the submount, while it is still green (un-fired), with a metal loaded paste of a refractory metal such as tungsten, and then firing the submount. Ceramic submounts of this type are known for example from EP 0 547 807, and can for instance be purchased from Sumitomo Metal Ceramics Inc. or NTK, Technical Ceramics Division of NGK Spark Plug Company Ltd 
     The fully fired submount is provided, on its front face  20   a , with areas  22   a  and  22   b  of metallisation. These areas of metallisation cover the respective vias  21   a  and  21   b , and have configurations corresponding to those of the p-type and n-type contacts  16  and  17  of the photodiode respectively. The rear face  20   b  of the submount  20  is provided with two areas  23   a  and  23   b  of metallisation respectively contacting vias  21   a  and  21   b , to which areas terminal tags  24   a  and  24   b  are secured with braze metal  25 . 
     The photodiode chip is applied to the submount with a tack bond which is effected at a temperature at which the solder is hot enough to be tacky rather than fully molten. The assembly is placed in a chamber (not shown) in order to remove any moisture from the space, contained between the photodiode chip and submount, that is encircled by the frame-shaped solder preform  17   b . To this end, this space may be first evacuated, and then back-filled with an inert gas such as dry nitrogen or argon. The flushing of moisture from the space may be facilitated by arranging for there to be a vent provided by a break (not shown) in the frame of the solder preform  17   b . After the residual moisture has been flushed from the space, the solder is heated to a higher temperature sufficient to cause it to become fully molten, at which stage the vent is closed by capillary flow. In this way a hermetically sealed enclosure is formed containing the regions of the photodiodes particularly sensitive to degradation, namely the surface area of semiconductive material where an electric field may be present during operation of the photodetector in the vicinity of its p-n junction or a semiconductor/metal junction. 
     In the foregoing specific description the frame-shaped solder preform  17   b  performs (after it has been flowed) the dual function of seal for the hermetic enclosure, and of providing electrical connection between the n-type contact of the photodiode chip and metallisation on the submount. In an alternative configuration, not shown, these functions are separated so that the frame-shaped solder preform that performs the sealing function (after it has been flowed) encircles both the electrical connection with the p-type contact, and that made with the n-type contact. In either instance the frame-shaped solder preform that performs the sealing function is not necessarily rectangular, but may for instance be circular. Another variant involves applying the solder preforms to the submount rather than to the photodiode chip. Such solder can, in this instance, conveniently be applied by screen printing. 
     The sub-assembly of FIG. 4, or variant thereof, may be incorporated into a fiber pigtailed assembly as depicted in FIG. 5 or a connectorised assembly as depicted in FIG.  6 . 
     In the case of the fiber pigtailed assembly of FIG. 5, a length of plastics protective coated optical fiber  50  has its plastic protective coating stripped for a short distance back from one end to expose bare fiber  51 . The end of this bare fiber is secured in position in abutment with the rear face of the photodiode chip (in abutment with its antireflection coating  18  if such coating is provided) by means of a fillet  52  of adhesive, for instance epoxy. The sub-assembly is then potted in a mass  53  of plastics encapsulant, and then a further mass  54  of plastics material is applied around the fiber  50 , where it emerges from the mass  53 , in order to limit in-service bending strain in this region. 
     In the case of the connectorised assembly of FIG. 6, the sub-assembly is potted in a mass  60  of clear plastics encapsulant, and this is contained within a connector shell  61  which is typically also made of plastics, but may alternatively be a metal connector shell. The clear plastics encapsulant is formed with a bore  62 , for receiving a ferrule terminated fiber (not shown). The depth of insertion of such a ferrule is limited by an internal flange  63  and co-axial, with the bore  62  is formed a lens  64  for focusing light emitted from the ferrule terminated fiber upon the photosensitive area of the photodetector. 
     FIG. 7 depicts an alternative form of submount for a photodiode which is distinguished from that of FIGS. 2 and 3 primarily in that its ceramic  70  is provided, while still in its green state, with a set of V-slots  701  for mechanical registration purposes when the submount is later to be encapsulated. In the connectorised assembly such mechanical registration avoids the necessity for more time-consuming registration techniques such as active optical alignment or visual alignment. The layout of the metallisations on both faces of the ceramic have also been changed, together with the shape of the terminal taps and the location of the via providing electrical connection with the n-type contact of the photodetector chip (not shown in this Figure). Thus in this instance the via  71   b  that provides electrical connection with the n-type contact lies outside the frame-shaped area of metallisation on the front surface of the submount contacting the frame-shaped solder preform of the photodiode chip. Accordingly metallisation  72  includes, in addition to its frame portion, a short half-dumbbell  721  extending to cover via  71   b . In this way any departure in planarity in the immediate vicinity of via  71  does not affect the quality of the seal provided by the frame-shaped solder preform of the photodiode chip. The configuration of the areas of metallisation  73   a  and  73   b , and those of the two terminal tags  74   a  and  74   b , are also changed in a manner that provides a greater separation between the terminal tags that meets an industrial standard.