Abstract:
A VCSEL structure having thermal management. The structure may be designed for conveyance of heat from the active layer primarily through one of the mirrors to a material that removes heat externally away from the structure. Thermal management may involve various configurations of heat removal for various VCSEL structures. The structures may be designed to effect such respective configurations for heat removal.

Description:
[0001]    The invention pertains to laser light sources and particularly to vertical cavity surface emitting lasers. More particularly the invention pertains to long wavelength lasers.  
           [0002]    A vertical cavity surface emitting laser (VCSEL) may include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL may be driven by a current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack. The first contact may instead be on top of the first mirror stack in a coplanar arrangement.  
           [0003]    VCSEL mirror stacks are generally formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an AlAs/GaAs or AlAs/AlGaAs material system wherein the different refractive index of each layer of a pair may be, for example, achieved by altering the aluminum content in the layers. In some devices, the number of mirror pairs per stack may range from 20 to 40 to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of pairs may increase the percentage of reflected light.  
           [0004]    In many VCSELS, conventional material systems may perform adequately. However, new products are being developed requiring VCSELs to emit light having long wavelengths. VCSELs emitting light having a long wavelength are of great interest in the optical telecommunications industry because of the low fiber dispersion at 1310 nanometers (nm) and the low fiber loss at 1550 nm. For instance, a long wavelength VCSEL may be obtained by using a structure having an InGaAs/InGaAsP active region. When an InGaAs/InGaAsP active region is used, an InP/InGaAsP material system should be used for the mirror stacks in order to achieve a lattice match relative to the InP substrate. The lattice matching between the substrate and the layers should be substantially close to ensure a true single crystal film or layer growth.  
           [0005]    In the InP material based system, it is difficult to achieve a suitable monolithic DBR-based mirror structure having a reasonable thickness because of the insignificant difference in the refractive indices in this material system. As a result, many layers, or mirror pairs, are needed in order to achieve a useful reflectivity. Useful reflectivity may be 99.8 percent or greater. Numerous attempts have been made to address the problem of very thick mirror structures. One attempt included a wafer bonding technique in which a DBR mirror is grown on a separate substrate and bonded to the active region. This technique has had only limited success and also the interface defects density in the wafer fusion procedure may cause potential reliability problems. Other approaches to making satisfactory long wavelength VCSELs have been fraught with one problem or another. For instance, lattice matched InP based mirrors used for 1550 nm VCSELs may have a host of problems in growth, processing and optical performance. The low index contrast of InGaAsP and InAlGaAs tends to lead to a requirement of extremely thick (ten microns or thicker) DBRs of 45 or more mirror periods or layer pairs. The AlGaAsSb or AlGaPSb systems associated with an InP substrate may be difficult to grow by MOCVD, and for good contrast, may still require at least 25 mirror pairs to achieve adequate reflectivity for VCSEL operation. Heat dissipation of heat from a VCSEL having very thick top DBR stacks is difficult. This is particularly the case of InP related materials for long wavelength VCSEL operation. For InP based material systems, since index contrasts are relatively small as compared to GaAs based counterparts, the DBR stacks tend to be much thicker to provide reasonable reflectivity. Consequently, large amounts of heat may be contained in the device. The invention provides a solution.  
           [0006]    Additionally, for some VCSEL structures, thermal management is of critical importance for laser characteristics, especially for VCSEL structures containing high thermal impedance in InAlGaP or InAlGaAs active/cavity regions. Effective heat extraction from an active/cavity region should to be considered in the design of a VCSEL structure containing such material systems.  
         SUMMARY  
         [0007]    The invention may involve a vertical cavity surface emitting laser having an InP substrate, a first mirror situated on the substrate, an active region situated on the first mirror, a gain guide formed on the active region and a second mirror situated on the gain guide.  
           [0008]    To have a low thermal impedance structure, the thickness and the size of interconnect metal are important for effective heat removal. According to a simulation, a thick and large area interconnect metal is beneficial in thermal management. Also, the covering of the aperture with high thermal conduction materials transparent to lasing wavelengths is very important. While heat transfer occurs laterally mostly via top DBR with low thermal conductive covering (such as SiO 2 ), heat can also be transferred effectively via a conductive covering (such as GaP, SiN, AlN, BN, SiC, diamond, and the like). Since the thickness of the top or upper DBR may be smaller than the bottom DBR thickness, it may generate less heat and better convey heat away from the active region. A top layer on the upper DBR may act as an effective thermal bridge (or a shunt path of heat) to a thick interconnect metal. If the structure requires locating such a layer closer to the active region, a hybrid VCSEL structure having two stacks for an upper mirror may be designed. The heat conductor may be connected to the lower stack or part of the top or upper mirror or DBR. On the other hand, the upper part or stack of the top mirror or DBR may have dielectric material pairs having a high thermal conductivity. Then, the heat conductor may be connected to the top of the upper part or stack of the upper or top mirror or DBR of the VCSEL structure.  
           [0009]    To recap some features of thermal management, heat removal may be done with the following approaches. The use of thick interconnect metal may be used for effective heat removal. One may use high thermal conductivity covering materials transparent to the lasing wavelength on certain portions of the structure. There may be the use of a hybrid VCSEL structure having a heat removal mechanism located closer to active region. The use of high thermal conductivity dielectric DBR pair material in a hybrid VCSEL may be utilized for heat removal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0010]    [0010]FIG. 1 illustrates a vertical cavity surface emitting laser;  
         [0011]    [0011]FIG. 2 reveals an illustrative example of a long wavelength InP material based VCSEL;  
         [0012]    [0012]FIG. 3 reveals an illustrative example of a long wavelength VCSEL having a two part top mirror;  
         [0013]    [0013]FIG. 4 shows a structure of a VCSEL incorporating an enhanced oxidized layer approach;  
         [0014]    [0014]FIG. 5 shows a structure having thermal management of heat through the top mirror into thermally conductive material for conducting heat away from the structure.  
         [0015]    [0015]FIG. 6 reveals the structure of FIG. 5 except that the thermally non-conductive layer on the top mirror is replaced with a thermally conductive layer.  
         [0016]    [0016]FIG. 7 show a structure with a two part top mirror having a dielectric stack on the lower part of the top mirror, and thermally non-conductive layer between the upper stack and the contact.  
         [0017]    [0017]FIG. 8 shows the same structure of FIG. 7 except that the thermally non-conductive layer is replaced by a thermally conductive layer.  
         [0018]    [0018]FIG. 9 shows the same structure of FIG. 10 except that the dielectric stack is thermally conductive.  
         [0019]    [0019]FIG. 10 shows the same structure of FIG. 9 except that the material for conducting the heat away for the structure is extended to be in contact with the dielectric stack.  
         [0020]    [0020]FIG. 11 shows the same structure of FIG. 5 except that the material for conducting heat away from the structure is extended downward to be in contact with the sides of the top and bottom mirrors.  
         [0021]    [0021]FIG. 12 shows a thermally managed structure having a mesa for the top mirror with a coplanar contact layer on the bottom mirror. 
     
    
     DESCRIPTION  
       [0022]    [0022]FIG. 1 is a representation showing a perspective illustration of a structure for a vertical cavity surface emitting laser  11 . A substrate  12  may be disposed on an electrical contact  14 . A first mirror stack  16  and a bottom graded index region  18  may be progressively disposed, in layers, on substrate  12 . A quantum well active region  20  may be formed and a top graded index region  22  may be disposed overactive region  20 . A top mirror stack  24  may be formed over the active region and a conductivity layer  26  may form an electrical contact. A current  51  may flow from upper contact  26  to lower contact  14 . Current  51  may pass through active region  20 . Upward arrows in FIG. 1 illustrate the passage of light  52  through an aperture  30  in upper contact  26 . The downward arrows illustrate the passage of current  51  downward from upper contact  26  through upper mirror stack  24  and the active region  20 . An ion implantation  40  may form an annular region of electrically resistant material. A central opening  42  of electrically conductive material may remain undamaged during the ion implantation process. As a result, current  51  passing from upper contact  26  to lower contact  14  may be forced to flow through conductive opening  42  and thereby be selectively directed to pass through a preselected portion of active region  20 . Current  51  may flow through bottom mirror stack  16  and substrate  12  to lower contact  14 . Current  51  going through active region  20  may result in a generation of light  52  with in a cavity constituted between top and bottom mirrors  16  and  24 . Light  52  may be eventually emitted by structure  11  out of aperture  30  as shown by the upward pointing arrows.  
         [0023]    [0023]FIGS. 2, 3 and  4  reveal several illustrative examples of long wavelength InP based VCSEL structures. A long wavelength may range from about 1200 nm through about 1800 nm. FIGS. 2, 3 and  4  are not necessarily drawn to scale. Structure  13  of FIG. 2 may be a full epitaxial proton implantation version. It may have an InP substrate  15 . On substrate may be formed a lower or bottom mirror  17 . Mirror  17  may be a distributed Bragg reflector (DBR) having a stack of pairs  31  of layers  33  and  35  of materials. Each pair  31  may be about one-half wavelength thick. Each of the layers  33  and  35  may be about one-fourth wavelength thick. The thicknesses may be optical wavelengths of the light emitted from structure  13 , for the respective materials of layers  33  and  35 . The two layers,  33  and  35 , of each pair  31  may be composed of materials having different indexes of refraction. For example, layer  33  may be InAlGaAs and layer  35  may be InAlAs. These layers and pairs may be repeated in a mirror stack. Other pairs of materials for layers  33  and  35  may include InGaAsP and InP, InAlGaAs and InP, GaAsSb and AlAsSb, and GaAsSb and InP, respectively. There may also be other material pairs that may be appropriate for making DBR mirror  17 .  
         [0024]    Situated on bottom mirror  17 , may be formed an active region or cavity  19 . Region  19  may have between one and five quantum wells. On active region  19  may be formed an upper or top mirror  23 . DBR mirror  23  may have the same structure of pairs  31  of layers  33  and  35  as that in bottom mirror  17 .  
         [0025]    Proton implantation may be applied laterally at the lower part of mirror  23  to make a gain guide  21  to provide current guidance and confinement in VCSEL structure  13 . A center portion on the top of mirror  23  may be masked with a material resistant to proton implantation. Then a proton implantation may be applied to the top of structure  13  resulting in an isolation  25 . Since the indexes of refraction of each material of the pairs of layers are close to each other, then many more pairs  31  may be required to build the mirror with the needed 99.8 percent reflectivity. Consequently, top mirror is a quite thick epitaxial DBR. Thus, rather high energy is required to achieve proton implantation down far enough in mirror  23  to result in an effective isolation  25 .  
         [0026]    The mask may be removed from the central portion of top mirror  23 . Another mask may be applied to the top mirror  23  with an opening for applying a contact metal  37  on the top of mirror  23 . Structure  13  may be moved so the resultant contact metal  37  may be in the form of a ring. The mask may be removed after deposition for the contact metal  37 . Another mask may be placed on a portion of the contact metal and a passivation layer  27  may be deposited on the top of structure  13 . The mask may be removed and another mask may be formed on the center portion of passivation layer  27 . A layer of contact metal may be applied on the masked top of structure  13 . The mask from the center portion of passivation layer may be removed with the remaining contact metal resulting in a ring-like contact  29  connected to contact metal  37 . Contact metal may be deposited on the bottom side of substrate  15  to result in a second contact  39  for VCSEL structure  13 .  
         [0027]    [0027]FIG. 3 shows a VCSEL structure  50  which may a regarded as a hybrid proton implantation version. As like structure  13  of FIG. 2, a mirror  17  may be formed on an InP substrate  15 . The structure and materials used in the pairs  31  of layers  33  and  35  may be the same as those in structure  13 . An active region on cavity  19 , like that of structure  13 , may be formed on mirror  17 . An active region or cavity  19  may be formed on bottom mirror  17 . On cavity  19 , a first part  43  of mirror  47  may be formed on active layer or cavity  19 . The material of pairs  31  of mirror part  43  may be the same as the pairs of bottom mirror  17  of this structure  50 .  
         [0028]    Proton implantation may be applied laterally in a lower portion of mirror part  43  to make a gain guide  41  to provide current guidance and confinement in VCSEL structure  50 . Mirror part  43  may have fewer pairs  31  of layers  33  and  35  than bottom mirror  17  of this structure  50  or top mirror  23  of structure  13 . One reason for the shorter mirror stack  43  may be to effect a proton implantation that results in an isolation  44  requiring much less energy than the proton implantation required for making isolation  25  in structure  13 .  
         [0029]    On mirror part  43 , another mirror part  45  may be formed. Mirror parts  43  and  45  constitute upper DBR mirror  47 . Mirror part  45  is a dielectric mirror stack (DBR)  45  that may be like a mesa or an island situated on lower mirror part or portion  43  of upper mirror  47 . Mirror stack  45  may have, as examples, 3 to 4 pairs of TiO 2  and SiO 2 , 2 to 3 pairs of Si and Al 2 O 3 , or 4 to 5 pairs of TiO 2  and Al 2 O 3 , respectively. The dielectric stack may cover the light aperture of VCSEL structure  50  and not block emitted light.  
         [0030]    Formed around dielectric stack  45  may be a ring of contact metal as a first contact  46  for VCSEL structure  50 . Contact  46  may be deposited in a manner similar to that of contact  37  for structure  13 . A second contact metal may be deposited on the bottom of InP substitute  15  as a second contact  39  for VCSEL structure  50 . A disadvantage of structure  50  is the process for making it is complicated by the making of stack  45  and related issues such as, for instance, stress in dielectric DBR stack  45 .  
         [0031]    [0031]FIG. 4 shows VCSEL structure  60  which may be regarded as a full epitaxial oxide version. Lateral oxidation in upper mirror  23  may be resorted to for isolation and current confinement. On InP substrate  15 , a lower DBR mirror  17  may be formed. Mirror  17  may have a stack of pairs  31  of layers  33  and  35  having material like that of mirror  17  in structure  13  of FIG. 2. An active region or cavity  19  may be formed on bottom DBR mirror  17 . Active region  19  may have one to five quantum wells. The material of active region  19  may include material similar to that of region  19  in structure  13 . A top mirror  23  may be formed on active region or cavity  19 . Mirror  23  may have a structure of pairs  31  of layers of material like that of mirror  23  in structure  13 .  
         [0032]    One thing different about structure  60  that is different from structure  13  is that one or two of the layers of a pair  31 , near active region  19  in mirror  23 , may have a high content of aluminum. In other words, these layers are oxidizable and may be oxidized laterally under certain environmental conditions such as high water vapor and temperature. The result may be lateral oxidation of layer  48  forming a gain guide  49  and providing isolation for VCSEL structure  60 . The oxidation of layer  48  may be effected from the edge of mirror  23 , via an isolation trench or vertical trenches. Isolation  25  and a gain guide  49  as provided by proton implantation in structure  13  may be absent in structure  60 . Contact metal  37  and passivation layer  27  may be formed on the top of upper DBR mirror  23  in the same manner as formed for structure  13 . An electrical contact  29 , connected to contact metal  37 , may be made in the same manner as that for structure  13 . Also, contact material may be deposited on the bottom of InP substrate  15  to provide a second electrical contact for VCSEL structure  60 .  
         [0033]    [0033]FIG. 5 shows thermal management of a VCSEL structure  70 . Structure  70  may be similar to structure  60  of FIG. 4 with respect to substrate  15 , contact  39 , lower mirror  17 , active region  19 , upper mirror  23 , and layer  48  with aperture  49 . Heat may be significant from active region  19 . Heat  71  and the direction of its movement may be indicated by the arrows, some of which are labeled “ 71 ”, although all of the arrows are meant to be labeled “ 71 ” but might not be so as to maintain an uncluttered figure. These comments may also be applicable to FIGS. 6 through 12. The arrows, however, are not meant to represent the velocity of magnitude of hear  71 .  
         [0034]    Formed on mirror  23  may be a contact  72  which is thermally conductive. Formed on top of mirror  23  may be a thermally non-conductive layer capping  73 . The layers  33  and  35  which alternate through mirror  23 , except for layer  48 , may be one of the combinations of materials noted above for an InP based system. Layers  33 ,  35  and  48  may be effectively thermally conductive, depending on the amount of thermal conductivity and depending in part on the material in the respective layers. Heat  71  may emanate from active region  19  having a cavity through mirror  23 . Since layer  73  may be effectively non-conductive, heat  71  may move outward as it approaches layer  73 . Metal contact  72  (which may be ring-like on the top surface of mirror  23 ) may be effectively thermally conductive and conduct heat  71 . Formed on contact  72  may be a thermally conductive material  74 . Material  74  may be gold or other like metal. It may also be a non-metal, thermally conductive material. Heat  71  may flow into contact  72  and material  74 . Also, heat  71  may flow from mirror  23  into material  74 . Material  74  may be part of a heat sink or interconnect for contact  72 . Heat  71  may flow from material  74  to a heat sink, interconnect, or the like external to device structure  70 . This scheme of thermal management of structure  70  may be applicable to structure  11 ,  13  and  60  shown in FIGS. 1, 2 and  4 , respectively.  
         [0035]    [0035]FIG. 6 shows thermal management of a VCSEL structure  80 . This structure may be similar to structure  70  of FIG. 5 except that layer  73  may be replaced with an effectively thermally conductive capping layer  81 . Heat  71  may emanate from active region  19  into and through mirror  23  in a manner similar to that in structure  70 . However, heat  71  may go into layer  81  and be conducted outwardly toward and into contact  72 . Heat  71  may flow from contact  72  into material  74 . Also, heat  71  may flow from mirror  23  into material  74 . Heat  71  may flow from material  74  to a heat sink, interconnect, or the like external to device structure  80 . This scheme of heat management of structure  80  may be applicable to structures  11 ,  13  and  60  shown in FIGS. 1, 2 and  4 , respectively.  
         [0036]    [0036]FIG. 7 shows a structure  90  having a two-part top mirror  47 . Structure  90  may be similar to structure  50  of FIG. 3, except that current confinement and device isolation provided by a laterally oxidized layer  48  with an aperture  49  which may be similar to layer  48  and aperture  49  of structure  60  in FIG. 4. Heat  71  may be generated by active region  19 . Heat  71  may be thermally conducted by mirror part  43  of mirror  47  as discussed above relative to mirror  23  in structure  70  of FIG. 5. The flow of heat  71  may be from region  19  through mirror part or stack  43 . Layer  91  and dielectric stack  45  may be essentially thermally non-conductive. Thus, heat  71  flows away from the center to effectively thermally conductive electrical contact  92  and material  93 . Heat  71  may flow into material  93  from contact  92  and mirror stack  43 . Material  93  may be essentially thermally conductive. Material  93  may be gold or other metal. Also, material  93  may be non-metallic. Heat  71  may flow from material  93  to a heat sink, interconnect, or the like external to structure  90 . This scheme of thermal management of structure  90  may be applicable to structure  50  of FIG. 3.  
         [0037]    [0037]FIG. 8 shows a structure  100  that may be similar to structure  90  of FIG. 7, except that structure  100  has an essentially thermally conductive layer  101  in lieu of layer  91  of structure  90 . Active region  19  may dissipate heat  71  into stack  43  of top mirror- 47 . Heat  71  may go through stack  43  into layer  101 , contact  102  and effectively thermally conductive material  103 . Stack  43  of top mirror  47  may be effectively non-thermally conductive. So heat  71  may move from the center of structure  100  towards and into layer  101 . Heat  71  from layer  101  may flow into contact  102 . From contact  102 , heat  71  may flow into material  103 . Material  103  may be gold or some other metal. Also, material  103  may be non-metallic. Heat  71  may flow from material  103  to a heat sink, interconnect or the like external to structure  100 . This scheme of thermal management of structure  100  may be applicable to structure  50  of FIG. 3.  
         [0038]    [0038]FIG. 9 shows a structure  110  that may be similar to structure  100  of FIG. 8 except for stack  114  of mirror  47 , which may be essentially thermally conductive, in contrast to stack  45  of structure  100  in FIG. 8 which may be essentially thermally non-conductive. Heat  71  generated my active region  19  may move through mirror  43 . From mirror  43 , heat  71  may move into stack  114 , effectively thermally conductive layer  111 , contact  112  and effectively thermally conductive material  113 . Heat  71  from layer  111  may move into contact  112 . Heat  71  from contact  112  may move into material  113 . Material  113  may be gold or other metal. Alternatively, material  113  may be non-metallic. Heat  71  may flow from material  113  to a heat sink, interconnection, or the like external to structure  110 . Heat  71  entering stack  114  may be dissipated into the ambient environment of structure  110 .  
         [0039]    Material  113  of structure  110  may be replaced with material  123  extending inward to be in contact with the edge of stack  124 , as shown in structure  120  of FIG. 10. Heat  71  of stack  124  (same as stack  114 ) may flow into thermally conductive material  123  which may have the same characteristics as material  113  of structure  110 . Thermally conductive layer  121  and contact  122  are like layer  111  and contact  112  of structure  110 , respectively. Heat  71  may flow from material  123  to a heat sink, interconnection, or the like. The other heat  71  flows in structure  120  may be similar to those of structure  110  of FIG. 9.  
         [0040]    These schemes of thermal management for structures  110  and  120  may be applicable to structure  50  of FIG. 3. FIG. 11 shows a structure  130  having a material  134  which may be put around mirror  23  and possibly mirror  17  for dissipation of heat  71  from active region  19 . Structure  130  may be similar to structure  70  if FIG. 5, except for the substitution of material  134  in place of material  74 . Capping layer  73  may effectively be non-thermally conductive but can be replaced with a thermally conductive capping layer. Heat  71  may flow from active region  19  into mirrors  17  and  23 , and material  134 . Mirrors  17  and  23  may utilize pairs  31  of layers  33  and  35  having effectively thermally conducting materials. Heat  71  may flow from mirrors  17  and  23  into contact  72  and material  134 . Heat  71  may flow from contact  72  to material  134 ′. Material  134  may be gold or another metal or it may be a non-metallic material.  
         [0041]    Thermally conductive or non-conductive passivation material (not shown) may be placed between material  134  and certain portions of mirrors  17  and  23 , active region  19 , and/or substrate  15 . Thermally non-conductive layer  73  may be replaced with a thermally conductive layer to thermally manage heat  71  in another manner. Heat  71  may flow from material  134  to a heat sink, interconnection, or the like. Thick interconnect metal and a highly thermally conductive cap/passivation layer may be located at active region  19  for another scheme of thermal management. The various schemes of thermal management of heat  71  in structure  130  may be applicable to structure  11 ,  13 ,  50 ,  60 ,  70 ,  80 ,  90 ,  100 ,  110  and  120  of FIGS. 1 through 10.  
         [0042]    The above noted thermal management schemes may be applicable to coplanar, mesa and other types of structures, with various approaches for isolation, and current and optical confinement. Another illustrative example, as in structure  140  of FIG. 12, the various schemes of thermal management noted above may be applicable to a coplanar or mesa structure, where one contact in the mesa structure may be near the top or bottom of bottom mirror  17 . Thermal management may be applicable to other variants of the structures disclosed. Structure  140  may have a contact  146  placed on an intra cavity contact layer  145 . However, this contact could instead be on the bottom side of substrate  15 . Layer  145  may be situated on the top of bottom mirror  17  which in turn is on substrate  15 . Another contact  142  may be situated on top mirror  23 . Between or in the center of circular contact  142  may be a capping layer  141 . Layer  141  may be thermally conductive. However, layer  141  could be substituted with a layer that is effectively thermally non-conductive. Situated on contact  142  may be a thermally conductive material  144 . Material  144  could be in contact with mirror  23  adjacent to contact  142 . But in FIG. 4 is a passivation layer  147  which may be thermally conductive. Alternatively, layer  147  could be effectively thermally non-conductive. Isolation and/or current confinement (including possible optical confinement) may be provided by partially and laterally oxidized layer  48  with aperture  49 . However, isolation and/or current confinement (including possible optical confinement) may be provided by implantation  40 ,  21 ,  25 ,  41 , and  44 , as shown in FIGS. 1, 2 and  3 , respectively. Device  140  isolation may additionally or instead be provided by a trench or other technique.  
         [0043]    Heat  71  may emanate from active region  19  and go through top mirror  23  to layer  141 , contact  142  and passivation layer  147 . Heat  71  may flow from layer  141  to contact  142 . Heat  71  from passivation layer  147  and contact  142  may go into material  144 . Heat  71  may flow from material  144  to a heat sink, an interconnection, or the like. Material  144  may be applied to the side or edge around the perimeter of mirror  23  for greater heat  71  dissipation. The approach to thermal management of structure  140  may be applicable to the other structures disclosed in the present description.  
         [0044]    Although the invention has been described with respect to at least one illustrative embodiment, may variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.