Abstract:
The invention relates to a light emission device, comprising at least two light-emitting semiconductor chips and a substrate. At least one first semiconductor chip ( 12 ) is fitted on the substrate and a second semiconductor chip ( 14 ) is fitted on the first semiconductor chip ( 12 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application Ser. No. 60/931,577 filed May 24, 2007. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a light emission device, comprising at least two light-emitting semiconductor chips and a substrate, at least one first semiconductor chip is fitted on the substrate and a second semiconductor chip is fitted on the first semiconductor chip. 
     BACKGROUND OF THE INVENTION 
     Numerous attempts have been made to increase the emission intensity of light emitting diodes. It has thus been proposed to arrange a plurality of very small light emitting diode semiconductor chips closely adjacent to one another in order to attain a higher luminous efficiency. A problem in this case, of course, is the heat dissipation, since the closely adjacent arrangement of the semiconductor chips means that the latter become very hot at corresponding power. 
     It has therefore been attempted to separate the light emission from the thermal radiation if possible and to cause the thermal radiation to be radiated principally toward the rear and, by contrast, the light emission toward the front. 
     Such considerations also give rise to the so-called flip-chip technique, which involves fitting one or a plurality of semiconductor chips onto a light-transmissive substrate, which may be made of sapphire or the like, by way of example. 
     A reflector layer, for example made of silver, extends on that side of the chip or chips which is remote from the sapphire, both the contact-connection and the heat dissipation being effected by means of very thick posts, so-called bumps, which make contact with the chip in a suitable manner. 
     In the case of this solution, the intention is then for as much heat as possible to be dissipated toward the rear if possible, while light, amplified by the silver reflection layer, is intended to leave the light emission device through the sapphire substrate toward the front. 
     Endeavors have also been made for a relatively long time to increase the optical efficiency of light emission devices. For this purpose, combinations of converging lenses and reflectors are usually used, which concentrate the emitted light and are intended to reduce the generation of spurious light and scattered light. 
     It has recently been proposed, for example, in accordance with German Patent Application 10 2006 015 377 to form a semiconductor radiation source with a plurality of LED chips which are contact-connected by means of a printed circuit board brought up close, wherein a reflector in combination with a converging lens is intended to ensure a highest possible luminous efficiency, but the generated heat is nevertheless intended to be dissipated well. 
     Although a semiconductor radiation source of this type is basically already highly suitable if it is necessary to ensure a highest possible luminous efficiency, it would be desirable, however, to improve the luminous efficiency even further without increasing the emission of heat. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Therefore, the invention is based on the object of providing a light emission device which is improved with regard to the light emission without an increase in the electrical power giving rise to problems in the dissipation of heat. 
     According to the invention, it is particularly expedient that a centrally intensified light emission can be generated by the stacked arrangement of LED chips. The focusing of such a central point of brightness is significantly easier than in the case of a planar distribution of the light emission, with the result that the luminous efficiency can be significantly increased overall. 
     This opens up an extended field of application for light emission devices, for example also in dental technology, where correspondingly constructed light curing devices make it possible to rapidly polymerize in a targeted manner specific regions of a polymerizable dental restoration that have not yet fully cured sufficiently. 
     Surprisingly, the light emission can be significantly increased by the stacked fitting of chips, use being made of the fact that the chip itself is transmissive to light. The transmissivity is particularly good for the same materials used since then the same lattice constant is present there. The invention provides for forming a very thin and light-transmissive adhesive layer between the semiconductor chips of the chip stack, said adhesive layer also having good thermal conductivity and therefore dissipating the heat of the upper chips well. 
     According to the invention, it is also possible to make use of the fact that a significant part of the light emission in semiconductor chips typically takes place via lateral light exit areas. The latter are preferably arranged so as not to impair the light emission obliquely toward the front. This can be achieved for example by the stack of semiconductor chips having an essentially pyramidal construction. The respective upper semiconductor chips are set back in each case relative to the respective lower semiconductor chips, that is to say are offset inward toward the optical axis. This is accompanied by a reduction of the dimensions of the upper semiconductor chips relative to the respective lower semiconductor chip, the reduction preferably being effected in both directions. 
     In a manner known per se, the semiconductor chips may be square and formed essentially as thin laminae. Semiconductor chips having different dimensions are readily available, such that it is possible in this respect to have recourse to commercially available components which can be combined in a particularly favorable manner according to the invention. 
     In this case, it is also possible to make use of the fact that the semiconductor chips typically have a Lambertian radiating characteristic that makes it possible, precisely upon superposing a plurality of such radiating characteristics that are spaced apart somewhat from one another in the vertical direction, that is to say in the light emission direction, to increase the light emission in the region of the optical axis in relative terms. 
     According to the invention, it is thus possible to significantly increase the proportion of the directly radiated light with respect to the proportion of the reflected light, with the result that reflector faults and the like are also less important. 
     In an advantageous configuration of the invention it is provided that at least one upwardly facing area of a lower semiconductor chip is not covered by the semiconductor chip situated thereon and remains free in this respect. Said area serves on the one hand for light emission; on the other hand this provides the possibility of providing a connection area for bonding, such that per semiconductor chip at least two connection areas are available for the electrical supply of the semiconductor chips. According to the invention, it is provided that the lower area is formed in a suitable manner for the heat dissipation. For this purpose, the lower semiconductor chip is at least not smaller and preferably larger than the semiconductor chip situated above it. Insofar as the semiconductor chip situated above it extends partially beyond the semiconductor chip situated underneath, this is effected on a spatially limited scale, in which case the sum of the self-supporting areas of the upper semiconductor chip should preferably make up less than 20% and in particular less than 10% of the total area. 
     It goes without saying that here semiconductor chip should be understood to mean a die in which it is also possible to combine a plurality of electrical structures which can also be connected to a plurality of connection areas. 
     It is particularly favorable if a converging lens is fitted above the topmost semiconductor chip. 
     The emitted light radiation can thereby be focused in a suitable manner, it also being readily possible according to the invention for the emitted light to be conducted into an optical waveguide such as a light guiding rod. 
     According to the invention, it is preferred for the arrangement of the semiconductor chips to be free of reflectors and reflection layers, apart from a reflection layer arranged below the bottommost semiconductor chip. Said reflection layer reflects the downwardly emitted radiation essentially completely and has an effect upward, it being preferred for the semiconductor chip arrangement to be surrounded by a ring reflector by means of which the radiation reflected in this way is additionally projected toward the front and focused. 
     According to the invention it is also readily possible to configure the stacked semiconductor chips such that they emit light in different wavelength ranges. For this purpose, it is possible to make use of the fact that each semiconductor chip is provided with connection areas. The electrical driving can thus be effected independently of one another, such that color changes can also readily be realized. 
     According to the invention, in the case of the preferred pyramidal construction of the semiconductor chip arrangement, use is also made of the effect that the bottommost semiconductor chip exhibits the highest light emission and, accompanying this, evolution of heat. The heat arising there can be dissipated very well on account of the direct mounting onto a substrate having a thermal conductivity, while the quantity of heat introduced in the case of semiconductor chips arranged further up is also smaller according to the smaller dimensions of said chips. 
     Since a compact arrangement of semiconductor chips is possible according to the invention, it is preferred to provide the provision of free areas for bonding on a printed circuit board directly adjacent to the semiconductor chips. For this purpose, the substrate preferably has a projection on which the bottommost semiconductor chip is fitted and up to the lateral area of which the printed circuit board is brought with its free area. 
     It is favorable in this connection if the structural height of the printed circuit board essentially corresponds to the height of the substrate base projection. 
     In a further advantageous configuration it is provided that the second as relatively upper semiconductor chip leaves free an upwardly facing area of the first as relatively lower semiconductor chip, and that at least the left-free area of the lower semiconductor chip and at least one area of the upper semiconductor chip are light-emitting, the lower semiconductor chip being fitted on the substrate. 
     In a further advantageous configuration it is provided that lateral light emission areas extend laterally at the chips, which are offset with respect to one another. 
     In a further advantageous configuration it is provided that a lateral light emission area of the upper semiconductor chip extends, relative to a lateral light emission area of the lower semiconductor chip, in a manner shifted toward the center of the semiconductor chip. 
     In a further advantageous configuration it is provided that the semiconductor chips are constructed essentially pyramidally with respect to one another. 
     In a further advantageous configuration it is provided that the semiconductor chips are at least partly transmissive to emitted radiation. 
     In a further advantageous configuration it is provided that the light emission device has a reflector fitted below the lower semiconductor chip, and that no reflector is provided between the lower, a middle, and the upper semiconductor chip. 
     In a further advantageous configuration it is provided that the light emission device has a plurality of semiconductor chips and a reflector is fitted below the bottommost semiconductor chip, which extends in particular laterally beyond the semiconductor chip. 
     In a further advantageous configuration it is provided that the semiconductor chips are connected to one another by means of a reflection-free adhesive layer having a thermal conductivity having approximately the same magnitude as the thermal conductivity of the semiconductor chips, the thickness of the adhesive layer being less than 100 μm and preferably less than 50 μm. 
     In a further advantageous configuration it is provided that a plurality of semiconductor chips which emit light having different spectral wavelengths are arranged one on top of another. 
     In a further advantageous configuration it is provided that the semiconductor chips are at least partly electrically connected in series with one another. 
     In a further advantageous configuration it is provided that the semiconductor chips are at least partly electrically connected in parallel with one another. 
     In a further advantageous configuration it is provided that the semiconductor chips have at least one electrical connection zone on their top side in the region of the left-free area. 
     In a further advantageous configuration it is provided that the substrate is electrically insulating and carries a reflection layer. 
     In a further advantageous configuration it is provided that the bottommost semiconductor chip is applied to an electrically at least semiconducting substrate, and that an electrical connection area of the bottommost semiconductor chip is formed by the substrate. 
     In a further advantageous configuration it is provided that the upper semiconductor chip is smaller than the lower semiconductor chip, in particular the size difference corresponding approximately to twice the thickness of each semiconductor chip. 
     In a further advantageous configuration it is provided that semiconductor chips fitted one on top of another are rotated relative to one another, in particular at an angle of, in particular, 45°. 
     In a further advantageous configuration it is provided that the substrate has a projection, the dimensions of which essentially correspond to the dimensions of the lower semiconductor chip and on which the lower semiconductor chip is fitted. 
     In a further advantageous configuration it is provided that an additional reflector, in particular a ring reflector, surrounds the semiconductor chips fitted one on top of another, the orientation of which reflector is oblique or parabolic in a manner known per se and which reflector reflects the lateral light emission of the semiconductor chips toward the front. 
     In a further advantageous configuration it is provided that a converging lens is fitted before the semiconductor chips, said converging lens being supported in particular on the reflector. 
     In a further advantageous configuration it is provided that the space between the semiconductor chips and the underside of the converging lens is filled by a transparent or translucent, liquid or gel-type light-transmissive substance, in particular silicone gel. 
     In a further advantageous configuration it is provided that the electrical connection areas are adjacent to free areas of a printed circuit board, to which areas they are electrically connected by means of bonding wires. 
     In a further advantageous configuration it is provided that a printed circuit board extends laterally alongside the semiconductor chips in a manner at least partly surrounding said semiconductor chips, and in particular runs with conductor tracks below a ring reflector. 
     Further advantages, details and features emerge from the following description of two exemplary embodiments of the invention with reference to the drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a schematic view of one embodiment of a semiconductor radiation source according to the invention in a perspective illustration; 
         FIG. 2  shows a view corresponding to  FIG. 1  of a further embodiment of a semiconductor radiation source according to the invention; 
         FIG. 3  shows a partly sectional, likewise perspective illustration of a semiconductor radiation source in the embodiment in accordance with  FIG. 2 , the illustration showing the embodiment in the installed state. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  reveals a light emission device  10  according to the invention in one embodiment. This light emission device  10  has a plurality of semiconductor chips, namely a lower semiconductor chip  12 , a middle semiconductor chip  14  and an upper semiconductor chip  16 . In this case, the lower semiconductor chip  12 , which may also be referred to as first semiconductor chip, is fitted onto a substrate, which can be seen from  FIG. 3  by way of example. 
     The substrate may be configured in any suitable manner. It preferably dissipates heat that is particularly preferably metallic. However, it is also possible, in a solution with lower power, to use a plastic casting material as substrate, which then fixes the spatial arrangement of the semiconductor chips. 
     In the exemplary embodiment illustrated, the first or lower semiconductor chip  12  is formed as a lamina having an essentially square parallelepiped form. Such semiconductor chips or LED chips are readily commercially available. 
     In the exemplary embodiment illustrated, the lower semiconductor chip  12  and the middle semiconductor chip  14  have the ratio between edge length and thickness of approximately 8:1. They are identical in size and placed one on top of another, but rotated relative to one another. The rotation is effected at an angle of approximately 45°, but a rotational angle of 30° or 60° would also readily be possible. 
     As a result of the rotation, the middle semiconductor chip  14  leaves four triangular areas as left-free areas, the areas  20 ,  22  and  24  being evident from  FIG. 1 . The area remaining there suffices to provide in each case an electrical connection area  26 ,  28  and  30 , which are in each case provided on the very far outer part and in circular fashion and are prepared for bonding. 
     In the exemplary embodiment illustrated, four connection areas are provided per chip, and the chip carries a total of three structures which can be driven independently of one another and which emit light, with the result that any driving arrangements are possible. As an alternative, provision may also be made for providing only two structures which can then be electrically isolated from one another and are connected in each case to a pair of connection areas. 
     In the exemplary embodiment illustrated, the upper semiconductor chip  16  is smaller than the middle and lower semiconductor chips  14  and  12 . 
     Its edge length/thickness ratio is likewise approximately 8:1, with the result that it is also thinner than the other two chips  12  and  14 . 
     It is rotated once again by 45° relative to the middle semiconductor chip  14 , with the result that it extends edge-parallel to the lower semiconductor chip  12 . This arrangement gives rise to further left-free areas  32 ,  34 ,  36  and  38 , which in each case have a triangular construction and also serve for contact-connection via corresponding connection areas  40 ,  42 ,  44  and  46 . 
     The upper semiconductor chip  16  is of a size such that it covers a significant part of the middle semiconductor chip  14 , for example 70% of its area. 
     The middle semiconductor chip  14  emits light on the one hand at its lateral emission areas, the emission areas  50  and  52  being evident from  FIG. 1 , and on the other hand at its underside, but also in large part at its top side. 
     The light emitted below the upper semiconductor chip  16  passes through the latter and in this respect amplifies the light emitted on the top side  54  of the upper semiconductor chip  16 . In this case, it is possible expediently to make use of the fact that the thickness of the upper semiconductor chip  16  is somewhat smaller than that of the other two semiconductor chips  12  and  14 ; the attenuation when passing through the relevant chip  16  decreases as a result. 
     A corresponding light emission and passage behavior is also exhibited by the stacking of the middle semiconductor chip on the lower semiconductor chip  12 . Accordingly, the light emitted from the top side of the lower semiconductor chip  12  passes through both the middle semiconductor chip  14  and the upper semiconductor chip  16  insofar as it is not emitted in the region of the areas  20  to  24 . 
     In this case, too, light is additionally emitted at the lateral emission areas, the emission areas  56  and  58  being evident from  FIG. 1 . 
     Semiconductor chips having a spherical radiating characteristic are preferably used. The radiating characteristics of the light emission device formed from the three semiconductor chips  12 ,  14  and  16  essentially results as the computational summation of the emitted light intensities, considered over the angular deviation from the optical axis  60 . This idealized radiating characteristic, which exhibits a behavior approximated to a lobe, is in practice attenuated by the radiation absorption of the light emission of the lower semiconductor chips by the upper semiconductor chips. In the case of material identity, however, the attenuation is surprisingly markedly low, which in particular is probably attributable to the matching of the lattice constants. 
       FIG. 2  reveals a modified embodiment of a light emission device  10  according to the invention. In  FIG. 2  and also in the rest of the figures, identical reference symbols indicate identical or corresponding parts. In the case of the embodiment in accordance with  FIG. 2 , all the light conductor chips  12 ,  14  and  16  are constructed pyramidally with respect to one another. The semiconductor chip  12  is the largest and is fitted on the substrate in thermally conductively connected fashion. The middle semiconductor chip  14  is applied on said semiconductor chip  12 , and it is smaller than the semiconductor chip  12 . 
     The differences in dimensions give rise to a peripheral free edge  62 , the width of which essentially corresponds to the thickness of the semiconductor chip  14 . The connection areas  26 ,  28 ,  30  and  31  of the lower semiconductor chip  12  are again fitted in the corners, the same correspondingly also applying to the connection areas  40  to  46  of the middle semiconductor chip  14  and the connection areas of the upper semiconductor chip  16 . 
     Since the upper semiconductor chip  16  is even smaller than the middle semiconductor chip  14  and is likewise fitted on the latter, there as well a peripheral edge  64  arises, the width of which in turn corresponds to the thickness of the semiconductor chip  16 . 
     This arrangement and configuration results overall in a truncated pyramid having a pyramid angle of approximately 45°. 
     The chips  12  to  14  are each applied on one another in thermal contact. This can be realized by a very thin adhesive layer, for example, which additionally improves the heat transfer. 
       FIG. 3  reveals a corresponding light emission device  10  in a sectional state and in an installed state. The light emission device  10  is fitted on a substrate  66 , which is composed of copper, for example, that has a base projection  68 . The base projection  68  has larger dimensions than the lower semiconductor chip  12 . It is adjoined by a printed circuit board  70  carrying free areas  72  of conductor tracks which serve for making contact with the various connection areas. By way of example, the connection area  28  can be connected to the free area  72  via the bonding wire (not illustrated) 
     Supported on the printed circuit board is a ring reflector  80  as an additional reflector, which annularly surrounds the light emission device. The ring reflector  80  has, in a manner known per Se, an inner area which faces obliquely inward and reflects light emerging from the lateral emission areas, for example the emission area  56 , toward the front, that is to say in the direction of the optical axis  60 . 
     A reflector is furthermore provided between the base projection  68  and the lower semiconductor chip  12 , which reflector cannot be seen from the illustration in the drawing and likewise projects the light impinging there toward the front. 
     While a preferred form of this invention has been described above and shown in the accompanying drawings, it should be understood that applicant does not intend to be limited to the particular details described above and illustrated in the accompanying drawings, but intends to be limited only to the scope of the invention as defined by the following claims. In this regard, the term “means for” as used in the claims is intended to include not only the designs illustrated in the drawings of this application and the equivalent designs discussed in the text, but it is also intended to cover other equivalents now known to those skilled in the art, or those equivalents which may become known to those skilled in the art in the future.