Abstract:
An optoelectronic device includes a semiconductor substrate, having front and back sides and having at least one cavity extending from the back side through the semiconductor substrate into proximity with the front side. At least one optoelectronic emitter is formed on the front side of the semiconductor substrate in proximity with the at least one cavity. A heat-conducting material at least partially fills the at least one cavity and is configured to serve as a heat sink for the at least one optoelectronic emitter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/194,298, filed Jul. 20, 2015, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor devices, and particularly to optoelectronic devices and their manufacture. 
     BACKGROUND 
     Effective heat dissipation is one of the major challenges in design of high-power optoelectronic emitters, such as VCSELs. Such devices generate large amounts of heat in the emitter active regions, resulting in high emitter junction temperatures, which tend to reduce VCSEL efficiency and lead to a reduced optical power output at a given drive current, shift the emission wavelength, degrade the quality of the laser modes, and reduce operating lifetime and reliability. In VCSEL array devices, inefficient heat dissipation causes temperature non-uniformity among emitters, leading to variations in emitter optical power and wavelength across the array. 
     In some designs, the VCSEL chip is thinned substantially in order to reduce the thermal resistance between the emitter junction and the heat sink on the back side of the chip. For good heat dissipation, however, the chip must be made very thin (typically on the order of 100 μm or less), which weakens its mechanical strength and causes difficulties in handling both the semiconductor substrate and the chip, and in the packaging of the chip. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved optoelectronic emitters and methods for their manufacture. 
     There is therefore provided, in accordance with an embodiment of the present invention, an optoelectronic device, which includes a semiconductor substrate, having front and back sides and having at least one cavity extending from the back side through the semiconductor substrate into proximity with the front side. At least one optoelectronic emitter is formed on the front side of the semiconductor substrate in proximity with the at least one cavity. A heat-conducting material at least partially filling the at least one cavity is configured to serve as a heat sink for the at least one optoelectronic emitter. 
     In a disclosed embodiment, the heat-conducting material in the optoelectronic device includes an electrically-conducting material, which serves the at least one optoelectronic emitter as an electrical contact. 
     In some embodiments, the at least one cavity is filled with at least two fill materials, wherein the at least two fill materials include an electrically-conductive film deposited over an interior surface of the at least one cavity in ohmic contact with the semiconductor substrate, and a thermally-conductive material deposited over the electrically-conductive film. 
     In some embodiments the at least one optoelectronic emitter includes an array of emitters on the front side of the semiconductor substrate, and the at least one cavity includes an array of cavities on the back side of the substrate, wherein the array of cavities is aligned with the array of emitters. Additionally or alternatively, the cavities have lateral dimensions that vary over the array proportionally to a local heat load over the array. 
     In the disclosed embodiments, the at least one optoelectronic emitter includes a vertical-cavity surface-emitting laser (VCSEL). 
     In still other embodiments, the optoelectronic device includes an etch stop layer deposited between the semiconductor substrate and the at least one optoelectronic emitter, wherein the at least one cavity extends through the semiconductor substrate to the etch stop layer. 
     There is also provided, in accordance with an embodiment of the present invention, a method for manufacturing an optoelectronic device. The method includes forming an optoelectronic emitter by deposition and patterning of epitaxial layers on a front side of a semiconductor substrate, etching a cavity through a back side of the semiconductor substrate, so that the cavity extends through the substrate into proximity with the epitaxial layers of the optoelectronic emitter, and at least partially filling the cavity with a heat-conducting material so as to serve as a heat sink for the optoelectronic emitter. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-C  are schematic sectional illustrations of a VCSEL array with embedded heat sinks at different stages of the manufacturing of the array, in accordance with an embodiment of the invention; 
         FIGS. 2A-B  are schematic sectional illustrations of a VCSEL array with embedded heat sinks at different stages of the manufacturing of the array, in accordance with another embodiment of the invention; 
         FIG. 3  is a flowchart that schematically illustrates a manufacturing process of a VCSEL array with embedded heat sinks, in accordance with an embodiment of the invention; and 
         FIGS. 4A-B  are schematic top views of arrays of VCSELs and embedded heat sinks, in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reducing thermal resistance for more effective heat dissipation is a major challenge in high power VCSELs. In addition, lowering the electrical resistance in the current path of the VCSEL is useful in further reducing power consumption and heat generation. 
     Embodiments of the present invention that are described herein provide a new VCSEL structure with embedded heat sink, which enhances heat dissipation without compromising mechanical strength. After moderate thinning of the VCSEL wafer, cavities, such as vias or trenches, are etched into the back side of the chip, extending from the back side through the substrate almost all of the way to the epitaxial layers below the VCSEL emitters. These cavities are then filled with metal or another material with high thermal conductivity (i.e., substantially higher conductivity than that of the semiconductor substrate), thus creating pillars through the substrate that serve as embedded heat sinks in the VCSEL chip. The thermal resistance between these embedded heat sinks and the emitters is low, thus facilitating efficient heat dissipation without compromising mechanical strength. If filled with metal, the pillars can also serve as electrodes for driving the VCSELs. 
     Typically, the embedded heat sinks are positioned directly under the emitters in order to reduce emitter junction temperatures. In VCSEL arrays, the sizes, positions and densities of the heat sinks can be adjusted in order to control the temperature profile across the array. Thus, the embedded heat sinks not only reduce temperature, but also facilitate a more uniform temperature distribution and thus improve the uniformity of optical power and wavelength across the array. 
     The embodiments of the present invention that are described herein provide a method for forming efficient, embedded heat sinks for VCSELs. The method comprises forming cavities, such as vias or trenches, from the back side of the semiconductor substrate (also referred to as a semiconductor wafer) carrying the VCSELs, with the cavities reaching close to the epitaxial layers of the VCSELs, and filling the cavities with material possessing thermal conductivity substantially higher than that of the semiconductor substrate. When the fill material also possesses high electrical conductivity, the filled cavities will, in addition to conducting heat away from the VCSELs, also serve as electrical contacts to these VCSELs. Heat sinks comprising materials that are electrically insulating may be made in a substantially identical fashion. 
     The method uses standard semiconductor processing techniques and is compatible as an add-on to existing VCSEL array designs and manufacturing methods. The embedded heat sinks and the methods for their manufacturing are applicable to substantially all common top-emitting VCSEL structures, such as etched mesa, proton-implanted, dielectric-apertured, and buried heterostructure designs. The techniques described herein can be used in manufacturing both VCSEL arrays and singlets, including both long-wavelength and short-wavelength VCSELs based on various semiconductor materials, including GaAs, InP, ZnSe, and GaN, inter alia. 
     In the disclosed embodiments, an array of VCSELs is formed on the front side of a semiconductor substrate using standard processes of epitaxial layer growth and patterning, after which the back side of the substrate is moderately thinned. An array of cavities, such as vias or trenches, is then defined by photolithography on the back side of the thinned substrate, followed by an appropriate etching process (either dry etch or wet etch) to create the cavities. The etched cavities are fully or partially filled with metal or another appropriate heat-conducting material to create embedded heat sinks under the emitters that were formed in previous process steps on the front side of the substrate. These steps are followed by standard process steps of substrate demounting, annealing, dicing, and chip packaging, as are known in the art. 
     The cavities that contain the embedded heat sinks can have either straight or tapered profiles, and their lateral dimensions can be smaller than, the same as, or larger than the VCSEL emitters themselves. (By lateral dimensions we are referring to the dimensions parallel to the front and back surfaces of the semiconductor substrate.) The cavities are typically, but not necessarily, aligned with the positions of the VCSEL emitters. Such alignment effectively brings the heat sink and back-side electrodes closer to the VCSEL emitter active regions, thus reducing both the thermal resistance and the electrical resistance between the emitters and the embedded heat sinks/electrodes. 
     For further reduction of thermal and electrical resistance between the heat sink and the VCSEL, it is desirable that the inner ends of the embedded heat sinks be as close as possible to the active areas of the VCSELs, without disturbing their distributed Bragg reflector (DBR) structures. To control the separation between the inner ends of the embedded heat sinks and the epitaxial layers of the VCSELs, an etch-stop layer may be formed on the semiconductor substrate before depositing the epitaxial layers. The etch-stop layer will ensure that the cavities are not etched too deeply. 
     To prevent thermal stresses in the chip, it is also desirable that the material that is used to fill the cavities for the embedded heat sinks have a coefficient of thermal expansion that is similar to that of the semiconductor substrate. 
     Multiple materials and multiple layers can be used in filling the cavities in order to achieve both good thermal conductivity and good ohmic contact. For example, a thin metal layer may be deposited on the inside walls of the cavities for good ohmic contact, with the choice of metal depending on the choice of semiconductor substrate, followed by partial or complete filling with a thicker heat-conducting layer. Alternatively, a thick metal layer with both good thermal and electrical conductivity, such as gold or copper, for example, can be used as both heat sink and electrode. Standard plating and sputtering processes may be used in applying the metal layer or layers. 
     It is desirable that all individual VCSELs in an array have the same output of radiative power. As the output of an individual VCSEL depends, among other factors, on its temperature, which in turn depends on the thermal environment of the VCSEL, the embedded heat sink array provides a way to even out the temperature differences between the individual VCSELs. The positions, lateral dimensions, and/or other features of the pattern of the embedded heat sinks can be varied in order to achieve uniform temperature distribution in the VCSEL array. For example, due to the fact that the center of a chip with a VCSEL array usually has a poorer heat dissipation than the edge of the chip, an increase of the lateral dimensions of the embedded heat sinks from the edge of chip towards its center may be used to compensate for the heat dissipation imbalance, achieving a more uniform VCSEL array temperature distribution. 
     Although the embodiments described herein and shown in the figures relate specifically to VCSELs, and particularly to VCSEL arrays, the principles of the present invention may similarly be applied in the design of other sorts of integrated high-power emitters. 
       FIGS. 1A-C  are schematic sectional views of a VCSEL array  20  with embedded heat sinks  21  at different stages of manufacture, in accordance with an embodiment of the invention.  FIG. 1A  shows VCSEL array  20  formed on a semiconductor substrate  22 .  FIG. 1B  shows VCSEL array  20  after the photolithographic definition and etching of an array of cavities  24  in a back side  26  of substrate  22 .  FIG. 1C  shows VCSEL array  20  after filling the cavities with a heat-conducting fill material, forming heat sinks  21 . In the schematic sectional illustration of  FIGS. 1A-C , as well as in subsequent illustrations, the dimensions of the features are not to scale. 
     As illustrated in  FIG. 1A , VCSEL array  20  is manufactured, using standard semiconductor processes, on semiconductor substrate  22 , starting with an epitaxially deposited etch stop layer  34  and continuing with an epitaxially deposited multilayer  36 . Three individual emitters  38 , as well as their front-side drive electrodes  40 , are formed over epitaxial layer  36 . In  FIG. 1A , semiconductor substrate  22  is shown after the process step of thinning back side  26 , and consequently the same thickness of substrate is shown in subsequent  FIGS. 1B-C . 
     In  FIG. 1B , cavities  24  have been defined by photolithography from back side  26  of substrate  22  so as to be aligned with emitters  38 . After photolithographic definition, cavities  24  are etched using standard semiconductor processes to a depth reaching close to epitaxial layers  36 , without however encroaching on these layers. Etch depth of cavities  24  is controlled by the use of etch-stop layer  34  formed on substrate  22  before depositing epitaxial layers  36 . 
     As shown in  FIG. 1C , cavities  24  are filled with heat-conducting material, forming heat sinks  21 . Heat sinks  21  are aligned with emitters  38 , with the tops of heat sinks  21  brought into close proximity with epitaxial layers  36 . The alignment with emitters  38  and proximity to epitaxial layer  36  is designed for providing efficient cooling to emitters  38 , as well as for providing low ohmic losses when heat sinks  21  also serve as back-side electrodes to emitters  38 . Additional heat-conducting material has been deposited over back side  26  as an unpatterned, continuous film  42 , serving as a common back-side electrode of VCSEL array  20 . 
       FIGS. 2A-B  are schematic sectional views of a VCSEL array  50  with embedded heat sinks  52  at different stages of manufacture, in accordance with another embodiment of the invention. In contrast to the preceding embodiment, cavities  24  in array  50  are lined with a metal layer  54  for ohmic contact. In other respects, array  50  is produced in a similar manner to array  20 . 
     As shown in  FIG. 2A , cavities  24  are, after etching, coated with metal film  54 , for example, by depositing a thin metal layer on the inner cavity surfaces, which serves as an ohmic contact to the semiconductor material of substrate  22 . A metal film  56 , as a continuation of metal film  54 , is deposited on back side  26  for ohmic contact. 
     In  FIG. 2B , cavities  24 , coated by metal film  54 , are filled with heat conducting fill material, forming heat sinks  52  for emitters  38 . The combination of metal film  54 , providing ohmic contact, and the heat conducting material forming heat sinks  52  serves as an array of back-side electrodes to emitters  38 . As in  FIG. 1C , additional heat-conducting material has been deposited to form unpatterned, continuous film  58  on back side  26 , serving as a common back-side electrode of VCSEL array  20 . 
     It will be appreciated that in some embodiments the shapes and relative dimensions as well as the alignment of heat sinks  21  are different from those shown in the above figures. 
       FIG. 3  is a flowchart that schematically illustrates the manufacturing process of VCSEL array  20  ( FIGS. 1A-C ) with embedded heat sinks  21 , in accordance with an embodiment of the invention. A similar process can be applied, mutatis mutandis, in producing array  50  ( FIGS. 2A-B ). VCSEL array  20  itself is manufactured in a fabrication step  60 . In a thinning step  62 , semiconductor substrate  22  is moderately thinned from back side  26  ( FIG. 1A ). Steps  60  and  62  are typically carried out using techniques of thin film deposition, lithographic patterning, and wafer processing that are known in the art. 
     In a patterning step  64 , cavities  24  ( FIG. 1B ) are photolithographically defined on back side  26  of substrate  22 . Cavities  24  are etched through the back side of the substrate, using dry or wet etching techniques that are known in the art, in an etching step  66 . The cavities are then filled with a metal or other thermally conductive material to form heat sinks  21  ( FIG. 1C ), for example by plating or sputtering, in a filling step  68 . Further in step  68 , unpatterned film  42  is formed over the back side of the substrate. Step  68  may involve only a single-material filling, as in  FIG. 1C , or separate coating by metal film  56  for ohmic contact, followed by filling with thermal conducting material for embedded heat sinks  52 , as in  FIGS. 2A-B . 
     After fabrication of heat sinks  21 , substrate  22  is demounted, annealed, diced, and packaged in a process completion step  70 , using standard semiconductor manufacturing techniques. In process completion step  70 , film  42  of  FIG. 1C  (or film  58  of  FIG. 2B ) is typically brought to contact with an external heat sink and/or electrical conductors (not shown). 
       FIGS. 4A-B  are schematic top views of respective arrays  72  and  74  of VCSELs and embedded heat sinks, in accordance with embodiments of the invention. These figures compare an embodiment in which embedded heat sinks have uniform lateral dimensions ( FIG. 4A ) to an embodiment in which the lateral dimensions of embedded heat sinks  78 ,  80  are adjusted according to the thermal environment of the VCSEL which each heat sink  78 ,  80  is cooling ( FIG. 4B ). The lateral dimensions of the VCSELs and heat sinks are shown schematically as circles, with VCSELs denoted by a solid line and heat sinks denoted by a dotted line. 
     As shown in  FIG. 4A , array  72  comprises VCSELs  82 ,  84  and their associated heat sinks  76  aligned with the respective VCSELs. Although in  FIG. 4A  heat sinks  76  are shown as vias, in some embodiments the heat sinks have a different design, for example a trench-like design. In VCSEL array  72 , all heat sinks  76  have the same lateral dimensions and consequently the same thermal conductance. As VCSEL  84  inside array  72  is surrounded by additional VCSELs  82 , its temperature may be higher than that of the surrounding VCSELs, with a possible detrimental impact on its performance, as has been previously described. 
     In  FIG. 4B , array  74  comprises VCSELs  86 ,  88  and their associated heat sinks  78 ,  80 . In this embodiment, the lateral dimensions of individual heat sinks  78 ,  80  have been adjusted for the individual VCSEL environment and vary in proportion to the local heat load across array  74 . VCSELs  86  at the edge of array  74  have heat sinks  78  with the same lateral dimensions as the other heat sinks at the edge of array  74 . However, as opposed to array  72 , heat sink  80  aligned with VCSEL  88 , located in an inside position in array  74 , has lateral dimensions larger than that of heat sinks  78 , aligned with VCSELs  86  on the edge. Consequently, the thermal conductance of heat sink  80  is higher than that of heat sinks  78 , and heat sink  80  will cool VCSEL  88  more efficiently than a heat sink of smaller lateral dimensions would. The different dimensions of heat sinks  78  and  80  are chosen so as to ensure that the temperature of VCSEL  88  is similar to that of the surrounding VCSELs  86 . By the same token, as shown in  FIG. 4B , the heat sinks of the VCSELs at the corners of array may have smaller dimensions than those at the edges. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.