Abstract:
The integration of micro-optical elements on a semiconductor substrate, particularly elements, such as glass-material optical components, such as lenses, beam splitters, mirrors and polarizing elements. A micro-optical device comprising a silicon substrate, and at least one glass-material micro-optical element mounted on the substrate. The device also includes a glass-material stand, for example, a grass frit, interposed between the micro-optical element and substrate. The stand is fuse-bonded to both the substrate and the micro-optical element. The substrate has, within a groove, a recess bounded by one or more walls, and the stand is dimensioned to fit within the recess and be positioned by one or more walls of the recess.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the integration of micro-optical elements on a semiconductor substrate, particularly glass-material optical components such as lenses, beam splitters, mirrors and polarising elements that are to be mounted on a silicon substrate. 
     2. Discussion of the Background Art 
     Opto-electronic devices, for example laser transmitter and receiver units for use in fibre optic transmission systems, and laser diode units for use in applications as diverse as optical data storage and laser range finding systems, often combine a solid state optical source or detector with a number of glass-material micro-optical elements, such as a collimating lens, beam splitter, polarising element or optical waveguide. These elements may have to be secured in a particular orientation with respect to an opto-electronic element. In particular, the position of a collimating micro-lens or a focussing micro-lens may need to be held to an accuracy of less than 1 μm for up to 25 years for the performance of a product to be maintained. 
     Because silicon processing is a well-established and economical technology, micro-optical elements are often secured to a machined and etched silicon substrate, formed as one of many from a larger silicon wafer. Electrical connections and insulating areas can also be conveniently formed on such a substrate by well-known means. One way in which a bond can be made between the micro-optical elements and the substrate is by using an epoxy adhesive, for example an ultraviolet or thermal curing epoxy adhesive. Although it is possible to obtain good long term dimensional stability with such adhesives, a problem has been noted that such cured epoxy adhesives can move or deform in subsequent processing of the substrate, or due to temperature excursion experienced during normal operation of the product, and environmental factors. For example, when a solder connection is made to a contact pad on the substrate, the cured adhesive can be raised to a temperature of 320° C. to 350° C. This can cause the position of the secured micro-optical element to shift, thus affecting adversely the performance of the opto-electronic device. 
     In many applications it is also necessary that the opto-electronic element is able to survive mechanical shocks that could dislodge a micro-optical element secured to the substrate. In practice, the micro-optical element should be able to withstand a static force of at least about 0.5 N. 
     It is an object of the present invention to provide a micro-optical device, and a method for forming such a device, that addresses these issues. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a micro-optical device, comprising a semiconductor material substrate, and at least one glass-material micro-optical element mounted on said substrate, characterised in that the device includes a glass-material stand interposed between said micro-optical element and said substrate, said stand being fuse-bonded to both the substrate and the micro-optical element. 
     Also according to the invention, there is provided an opto-electronic device, comprising at least one opto-electronic element and a micro-optical device, characterised in that the micro-optical device is according to the invention and in that the opto-electronic element is secured to said substrate and coupled optically with said at least one micro-optical element. 
     It has been found that the fuse-bonding of the stand to the adjacent substrate and micro-optical element forms a secure and stable mount of the micro-optical element to the substrate, with very low creep over time. The glass transition or melting point of the glass material stand will be related to a suitable fuse-bond temperature and bonding time. The fuse-bond temperature can be chosen to be lower than that of the micro-optical element, so that the element is essentially unaffected by the fuse-bonding process. In addition, the fuse temperature can be chosen to be sufficiently higher than temperatures expected in subsequent processing, for example from soldering of electrical contacts, so that the fuse-bonded stand is essentially unaffected by such subsequent temperatures. 
     In a preferred embodiment of the invention, the micro-optical element is a lens arranged to couple optical radiation into (or out of) one or more opto-electronic elements. 
     Prior to formation of the micro-optical device, the stand may be a partially fused glass-material frit. This is a convenient way to form an glass-material stand that has a particular desired shape. The frit is sufficiently robust so that it may be handled, for example by a micro-manipulator such as a vacuum pick-up tool, and then placed in position on the substrate. The fuse-bonding process may then further fuse together the frit, so that this becomes a fused frit. 
     In a preferred embodiment of the invention, the substrate has a recess in a surface thereof, said recess being bounded by one or more walls. The stand may then conveniently be dimensioned to fit within the recess and be positioned by one or more walls of said recess. 
     In its simplest form, the recess may be a circular recess, in which case there may be just one circular wall. In a preferred embodiment of the invention the recess is in a groove with opposite side walls that position the stand in a first direction. The recess and the stand may then have therebetween an alignment feature that aligns the stand in a second direction transverse to the first direction. 
     For example, the alignment feature may be a ridge on one of the stand or the recess, and a groove matching the ridge on the other of the stand or the recess. 
     The invention also provides a method of forming a micro-optical device, said device comprising a semiconductor material substrate, at least one glass-material micro-optical element and at least one glass-material stand, characterised in the method comprises the steps of: 
     i) bringing together the micro-optical element and the substrate so that a stand is interposed between the micro-optical element and the substrate; 
     ii) heating the stand so that this becomes fuse-bonded to both the substrate and the micro-optical element, thereby mounting said micro-optical element to the substrate; and 
     iii) then allowing the stand to cool down so as to fix the bond between the stand and said micro-optical element and between said stand and the substrate. 
     Step ii) may then be accomplished by applying heat through the substrate towards the stand. For example, a heater may be affixed to a rear surface of the substrate, so that heat conducted through the substrate causes the fuse-bonding of the stand. 
     The invention further provides a method of forming an opto-electronic device, said opto-electronic device comprising at least one opto-electronic element and a micro-optical device, characterised in that said micro-optical device is formed according to the method of forming a micro-optical device according to the invention, and the opto-electronic device is then formed by the step of securing said opto-electronic element to said substrate so that said opto-electronic element is coupled optically with at least one of said fuse-bonded micro-optical elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail, and by way of example only, with reference to the accompanying drawings, in which 
     FIG. 1 is a schematic plan view of an opto-electronic device, having a micro-optical device according to the invention in the form of a ball collimating lens fuse-bonded via a stand to a silicon substrate and with a laser diode arranged to direct light into the lens; 
     FIG. 2 is a cross-sectional schematic view of the lens, stand and substrate during the fuse-bonding process; and 
     FIG. 3 is a perspective view of the stand and substrate prior to placement of the stand on the substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a schematic plan view of an opto-electronic device  1 , here a hybrid chip that would be used in an optical transmitter unit in a fibre-optic transmission link. The device  1  is constructed on a silicon substrate  2  that is about 6 mm square and up to 2 mm thick. The substrate  2  has been etched and plated so that a number of opto-electronic, electronic and optical components can be secured to the substrate to form the working device  1 . 
     The device  1  is based around a laser diode chip  4  held on a chip submount  6 , which is itself bonded to a metalised ground strip  8  and extends across the floor  9  of a rectangular recess  10  in which the laser diode  4  is mounted. The recess  10  has along one side an upwardly tapering wall that is divided into two equal portions  11 , 12  either side of a groove-like channel  13  that extends in line with an optical axis  14  of the laser diode  4 . A micro-optical element in the form of a ball lens  16  is mounted to the substrate  2  within the channel  13 . In the context of the present invention, a micro-optical element is one having dimensions of about 5 mm or less. In many instances, a micro-optical element will have dimensions of the order of 100 μm to 1 mm. 
     Optical radiation, for example near infrared radiation  18  is collimated by the lens  16  and directed towards other optical elements, here a polarising isolator  20 , beam splitter  21 , two optical filters  22 , 23  and two prism/photodiode assemblies  24 , 25 . Another prism/photodiode assembly  26  captures optical radiation  28  emitted by a back facet  29  of the laser diode  4  to monitor the optical power of the laser diode  4 . A number of electrical components  27  are also secured to the substrate  2 . The operation of the various optical and electronic components  20 - 27  is conventional, is not central to an understanding of the invention, and so will not be described in detail. 
     Reference is now also made to FIG. 2, which shows a cross section through the device  1  taken along lines II—II, showing the process by which the ball lens  16  is secured to the substrate  2 . A stand in the form of a partially fused glass frit  30  is first interposed between the ball lens  16  and a base surface  32  of the channel  13 . The channel  13  and glass frit  30  are shown in more detail in FIG.  3 . The frit  30  is a stand in the sense that the frit raises the ball lens  16  above the underlying surface of the substrate  2 . The use of a stand is particularly helpful, as the thickness can be chosen to place the lens  16  at the correct height above the substrate  2  relative to the laser diode  4 . 
     The base  32  of the channel  13  lies in a recess that has two equal rectangular halves, i.e., recesses  33  and  34 , separated by a triangular cross section ridge  35  that extends between opposite side walls  36 , 37  of the channel  13  in a direction transverse to the channel side walls  36 , 37  and also transverse to the optical axis  14 . Recess  33  is separated from the base  9  of the recess  10  in which the laser diode  4  is mounted, by a downwards tapering step  38 . A similar step  39  separates recess  32  from a flat surface  40  on which the polarizing isolator  20  is mounted. 
     The glass fit  30  is approximately square in plan form looking down into the channel  13  and recesses  33 , 34 , and has plane parallel upper and lower surfaces  41 , 42 . The lower surface of the frit  30  is divided by a V-groove  43  that extends fully across the width of the flit  30  between opposite parallel vertically extending side walls  44 , 45 . Running parallel to the length of the V-groove  43 , are two other parallel vertically extending front and back walls  46 , 47 . The shape of the V-groove  43  matches the profile of the triangular ridge  35  in the base  32  of the channel  13 . Similarly, the nearly square plan profile of the flit  30  is such that the side, front and back walls  44 - 47  of the frit will fit between the corresponding surfaces  36 - 39  of the channel  13  when the base  42  of the frit  30  is resting against the base  32  of the channel  13 . The clearance between the frit side walls  44 , 45  and channel side walls  36 , 37 , and the clearance between the V-groove  43  of the frit and the triangular ridge  35  on the base  32  of the channel  13 , is such that the glass frit  30  is simultaneously aligned within the channel  13  in the direction parallel to the optical axis  14 , and also in a transverse direction in the plane of the channel base  32 . 
     A micro-optical device  100  according to the invention is formed first by placing the frit  30  in the channel  13  as described above. This can be accomplished by conventional manipulator tools, for example a vacuum pick-up tool  50 . Once the frit  30  is in place, the ball lens  16  can be similarly positioned up against the upper surface  41  of the frit. At this stage in the manufacture of the opto-electronic device  1 , the position of the lens would be adjusted by moving the tool  50  until the correct optical performance for the device  1  has been determined. Once the ball lens  16  is in the correct position, heat  52  is applied to a rear surface  51  of the substrate  2  from a heating element  53 . The heat applied is of the order of 425° C. to 475° C. The heat  52  is transmitted vertically upwards through the substrate  2  towards the glass frit  30 . The glass frit is formed from partially fused silica with a high proportion of lead-oxide, so that the melting point of the glass frit  30  is below that for the glass ball lens  16 . A downward pressure  54  is applied to the ball lens  16  by the vacuum pick-up tool  50 , so that when the frit  30  begins to soften, the ball lens  16  is pressed gently into the upper surface  41  of the frit. At the same time, the lower surface  42  of the frit  30  begins to flow across the base  32  of the channel  13 . Because the substrate  2  is formed from silicon, there will be a natural silicon-oxide surface coating to exposed portions of the substrate  2 . Because of this, the heated glass frit  30  will form a good contact with the adjacent exposed portions of the substrate  2 . The heating element  53  is then switched off, allowing the substrate, glass frit  30  and ball lens  16  to cool down, whereupon the now fused glass frit  30  solidifies to bond securely to both the ball lens  16  and base  32  of the channel  13 . 
     The fuse-bonds thus formed are unaffected by subsequent processing of the opto-electronic device  1 , for example soldering of electronic components  27  on the substrate  2 . 
     In the present example the ball lens  16  has a radius of 400 μm, and the frit  30  has a thickness  61  of 300-350 μm, a width  62  of 300-350 μm and a length  63  of 400-450 μm. The base  32  of the channel  13  has a depth  64  of 100 μm beneath the level of the adjacent flat surfaces  9 , 40 , and a width  68  about 25 μm more than the width  62  of the frit  30 . The length  65  of the channel  13  is about 670 μm. The width  66  of the triangular ridge  35  is about 290 μm, and the height of the triangular ridge  35  above the surrounding base  32  of the channel  13  is about 100-125 μm. A depth of the V-groove  43  in the frit  30  is normally the same as the height of the triangular ridge  35 . 
     The channel  13  has a total depth  67  which is about 650 μm, and the thickness of the silicon substrate  2  beneath this channel is about 1 mm. 
     For components such as those described above, it has been found that a good bond can be formed between the frit  30  and adjacent substrate  2  and ball lens  16 , if the substrate is heated from underneath to about 460° C. A downward pressure of about 30 g is applied by the vacuum pick-up tool  50  against the ball lens  16 . These conditions are held for about 30 seconds, and during this time the ball lens can be manipulated into the correct position relative to the laser diode  4 . The heating of the substrate  2  then ceases, and once the substrate and frit have dropped to about 310° C., the frit solidifies to form a secure join between the ball lens  16  and substrate  2 . The pressure applied by the manipulator can then be relieved. Shear testing on ball lenses joined in this way has shown that the join can withstand lateral forces of at least 1 N, which is sufficient for most uses of an opto-electronic device. 
     The invention provides a secure joint between a micro-optical component and a semi-conductor substrate. The invention is particularly useful with integrated micro-optical elements to be mounted on a silicon substrate, and also provides a convenient way in which such a joint and opto-electronic device incorporating such a micro-optical element can be manufactured. The joint is formed at a temperature above that normally reached in subsequent processing of an opto-electronic device, and is unaffected by any solvents that might be used in a manufacturing process. Furthermore, the joint does not outgas as might a joint formed from an epoxy resin. The joint does not creep by any significant amount over a typical lifetime of an electronic device, or as a result of temperature cycling. The invention therefore provides a convenient micro-optical device and method for forming such a device.