Patent Publication Number: US-2003227030-A1

Title: Light emitting semiconductor device

Description:
BACKGROUND OF INVENTION  
       [0001] 1. Field of Invention  
       [0002] The present invention relates to a light emitting device employing semiconductor(s) comprising nitride-type compound(s).  
       [0003] 2. Conventional Art  
       [0004] Green, blue, white, and other such visible-region LED lamps, and also LED lamps operating in short-wavelength regions such as the ultraviolet, have been commercialized, such LED lamps being equipped with light emitting elements in the form of LED chips employing GaN, AlGaN, InGaN, and other such semiconductors comprising nitride-type compounds. Because such semiconductor materials comprising nitride-type compounds possess excellent luminous efficiency, the LED lamps which have been developed include some which have high luminance.  
       [0005] With such LED lamps, the LED chip is first secured to a lead frame using Ag paste, and this is then sealed using epoxy resin. Whereas one factor in LED lamp reliability is the reliability of the LED chip itself, the weather resistance of this epoxy resin affects LED lamp reliability. For example, aromatic carbon—carbon double bonds present within epoxy resin may be broken down due to heat and/or irradiation by short-wavelength visible and/or ultraviolet light, causing oxidation and yellowing, and leading to decreased optical transmissivity. In particular, with LED chips such as the foregoing which employ semiconductors comprising nitride-type compounds, the light emitted is of high energy, and there is also much generation of heat due to the high operating voltages of 3 V to 5 V. There is therefore concern because of the large possibility that LED lamp reliability could be impaired due to oxidative degradation of the molded resin.  
       [0006] In order to address such concerns in more practical fashion, the present applicant carried out reliability testing of LED lamps possessing conventional structures. The LED lamp employed for this testing was such that a blue LED chip (peak emission wavelength=470 nm) wherein a multilayer semiconductor film comprising a nitride-type compound and having a P—N junction, a p-type electrode, and an n-type electrode were formed over sapphire substrate, a protective layer being formed at prescribed locations over the electrodes, was mounted with Ag paste onto a lead frame cup, epoxy resin being used during molding to produce an LED lamp that was 5 φ in size (15 mm in diameter).  
       [0007] Reliability testing was carried out with respect to low-temperature operation, high-temperature high-humidity operation, low-temperature storage, and high-temperature high-humidity storage, 100 samples being employed for each. Conditions for the various tests were as indicated in TABLE 1, below.  
                               TABLE 1                           Ambient                       Conditions       Relative   Relative           (Temperature/   Electricity   Operating   Emitted       Test   Humidity)   Supplied   Voltage   Luminance                  Low-   −40° C.   30 mA   95%-98%   125%-130%       Temperature       Operation       High-   60° C./90%   20 mA   95%-98%   60%-70%       Temperature       High-Humidity       Operation       Low-   −40° C.   None   95%-98%    98%-100%       Temperature       Storage       High-   60° C./90%   None   95%-98%    98%-100%       Temperature       High-Humidity       Storage                  
 
       [0008] Electricity supplied and ambient conditions (temperature/humidity) in the various tests described below are as indicated here.  
       [0009] Relative operating voltage and relative emitted luminance are defined as percents of values measured for operating voltage and emitted luminance prior to the start of testing at a time when 20 mA of electricity was supplied at room temperature, TABLE 1 showing, for each test, relative operating voltage and relative emitted luminance after undergoing testing for 2,000 hours.  
       [0010] Relative operating voltage was, for all tests, almost completely unchanged from initial values, being 95% to 98% of the initial value after 2,000 hours. Relative emitted luminance exhibited a trend toward improving over the course of the low-temperature operation test, and exhibited a trend toward deteriorating over the course of the high-temperature high-humidity operation test. Particularly for the high-temperature high-humidity operation test, emitted luminance was 60% to 70% of initial values, this representing a large deteriorating trend. There was almost no change over the course of the low-temperature storage test and the high-temperature high-humidity storage test.  
       [0011] It is clear from the foregoing test results that the way in which emitted luminance changes varied depending on the ambient conditions (temperature/humidity) and on whether or how much electricity was supplied during testing. Whereas there was actually a trend toward improvement at low temperature, deterioration was particularly marked with supply of electricity at high temperature.  
       [0012] Upon investigating the causes behind the foregoing results, it was learned that the problem was not due so much to deterioration of the characteristics of the LED chip itself or degradation of the molded resin itself as it was to alteration in the closeness of contact between the molded resin and the LED chip. That is, the LED chip is in physical contact with two materials having different coefficients of thermal expansion: the epoxy resin which constitutes the resin used for molding, and the Ag paste which is used to mount it to the lead frame. There are therefore regions surrounding the LED chip which differ with respect to the closeness with which they make contact, causing occurrence of a stress distribution around the LED chip, and ultimately leading to separation of the chip. It was in particular learned that the change in closeness of contact between the LED chip and the resin used for molding was connected with temperature-related variation in stresses present throughout the resin used for molding which act on the LED chip.  
       SUMMARY OF INVENTION  
       [0013] The present invention was conceived in light of the foregoing problem, it being an object thereof to provide a light emitting semiconductor device which exhibits little change in luminance over time, without being affected by ambient temperature and/or operating conditions.  
       [0014] In order to achieve the foregoing object, in the context of a light emitting semiconductor device equipped with one or more LED chips in which one or more semiconductor layers comprising one or more P—N junctions are laminated over one or more substrates, and one or more support structures, on which at least one of the LED chip or chips is mounted and which provide electrical continuity to at least one of the LED chip or chips, at least one of the LED chip or chips being covered with resin, a light emitting semiconductor device in accordance with the present invention is characterized in that at least one of the LED chip or chips is secured by way of one or more intervening first resins to one or more mounting surfaces of at least one of the support structure or structures, and is covered by one or more second resins.  
       [0015] It is preferred that at least one thickness of at least one of the first resin or resins be not less than 5μ and not more than 10μ.  
       [0016] In the context of a light emitting semiconductor device equipped with one or more LED chips in which one or more semiconductor layers comprising one or more P—N junctions are laminated over one or more substrates, and one or more support structures, on which at least one of the LED chip or chips is mounted and which provide electrical continuity to at least one of the LED chip or chips, at least one of the LED chip or chips being covered with resin, another light emitting semiconductor device in accordance with the present invention is characterized in that at least one cavity, the perimeter at the top face of which is larger than the outer periphery of the back face of the at least one LED chip, is provided at one or more mounting surfaces of at least one of the support structure or structures; the at least one cavity is filled with one or more first resins in cured state or states; the at least one LED chip is secured over at least one of the first resin or resins by way of one or more intervening thermally conductive chip adhesives, at least one of the chip adhesive or adhesives being in physical contact with at least one of the mounting surface or surfaces; and the at least one LED chip is covered by one or more second resins.  
       [0017] In the foregoing structure(s), at least one of the first resin or resins and at least one of the second resin or resins may be the same resin.  
       [0018] With a light emitting semiconductor device constituted as described above, because resin(s) having coefficient(s) of thermal expansion on the same order are present around LED chip(s), stresses acting on LED chip(s) from resin(s) are made uniform around LED chip(s). There is accordingly little variation in the closeness with which resin(s) contacts or contact LED chip(s), reducing likelihood that resin(s) will separate from LED chip(s). As a result, there is reduced likelihood of change in the efficiency with which light from the LED chip is extracted to the exterior, and variation in emitted luminance can be kept to a minimum.  
       [0019] Furthermore, it is preferred that chip adhesive(s) has or have thermal conductivity of not less than 2.5 W/m/K.  
       [0020] Moreover, it is preferred that the foregoing chip adhesive(s) possesses or possess electrical conductivity such that the volume resistivity thereof is not more than 600 nΩm.  
       [0021] Because the foregoing chip adhesive(s) will exhibit almost no thermal expansion or contraction, it or they will contribute nothing toward the stresses on the LED chip. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0022]FIG. 1 is a sectional view of a light emitting semiconductor device in accordance with a first embodiment of the present invention.  
     [0023]FIG. 2 is a sectional view of a light emitting semiconductor device in accordance with a second embodiment of the present invention.  
     [0024]FIG. 3 is a sectional view of a light emitting semiconductor device in accordance with a third embodiment of the present invention.  
     [0025]FIG. 4( a ) and ( b ) are sectional views of light emitting semiconductor devices in accordance with other embodiments of the present invention.  
     [0026]FIG. 5( a ) and ( b ) are sectional views of light emitting semiconductor devices in accordance with different embodiments of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0027] Below, embodiments of the present invention are described with reference to the drawings.  
     [0028] &lt;First Embodiment&gt; 
     [0029]FIG. 1 is a sectional view of an LED lamp in accordance with a first embodiment of the present invention. As light emitting element, this LED lamp  100  has an LED chip  104  wherein multilayer semiconductor film containing a nitride-type compound and comprising a P—N junction is formed over sapphire substrate. Below, a procedure for assembling LED lamp  100  is described.  
     [0030] A dispenser is used to apply a prescribed amount of first resin  103  to cup  102  of lead frame  101  which is secured to a die bonder (not shown), and LED chip  104  is placed on top of this first resin  103 . With the assembly in this state, first resin  103  is cured by heating in accordance with prescribed conditions.  
     [0031] p-Pad electrode  105   a  and n-pad electrode  105   b  are then formed at the principal plane of LED chip  104 , electrical connection between respective electrodes  105   a  and  105   b  and lead frame  101  being accomplished by means of wires  106   a  and  106   b.  Molding is then carried out using second resin  107 . The same resin was used for first resin  103  and second resin  107 , an epoxy resin (Ablestik Product No. 2017M) being used in the present embodiment. There is no limitation with respect to this or these resin(s), provided only that it or they is or are consistent with the intent of the present invention.  
     [0032] If the coefficients of thermal expansion of first resin  103  and second resin  107  are very different, differences in coefficient of thermal expansion, adhesive strength, and/or hardness during supply of electricity and/or changes in ambient temperature will cause balances affecting relaxation of internal stresses arising in connection with resin expansion and contraction to be disturbed, causing changes in the closeness of contact between LED chip  104  and resins  103  and  107  as well as concomitant changes in the efficiency with which light is extracted to the exterior, and making emitted luminance extremely likely to exhibit variation. Furthermore, in a worst-case scenario, separation at interface(s) where contact is made with first resin  103  and second resin  107  could serve as trigger, inducing separation of resin from LED chip  104 , and moreover, potentially serving as cause for cracking of resin and/or breaking of wires as the assembly attempts to relax the internal stresses which would be produced in the resin(s) in accompaniment to such separation.  
     [0033] Furthermore, thermal conductivity of resin being typically in the neighborhood of one order of magnitude smaller than that of Ag paste, since dissipation of heat in the present embodiment described above will be worse than would be the case had LED chip  104  been secured to the cup using Ag paste, and since this could lead to decreased reliability, it is preferred that the thermal resistance of the first resin be lowered by making the thickness of the first resin somewhat small. In addition, because making this thickness too small would interfere with ability to preserve the balance in the stresses from the resin which act on LED chip  104 , it is necessary that this thickness be made somewhat large. Establishing an appropriate value for this resin thickness is important for maintaining high reliability. Now, based on experiments carried out by the present applicant, a thickness of not less than 5μ and not more than 10μ is optimal for the first resin. Where LED chip  104  employs a structure formed over an insulating substrate, such a value represents a preferred value for the thickness of the first resin which is used to secure it to the cup of lead frame  101 .  
     [0034] 100 samples were employed for each a low-temperature operation test and a high-temperature high-humidity operation test, and relative emitted luminance characteristics over time were measured up to 2,000 hours. TABLE 2, below, shows relative emitted luminance after 100 hours, 500 hours, 1,000 hours, and 2,000 hours. As a comparative example, results are also shown for a structure similar to that of the present embodiment in all respects except for the fact that the LED chip was secured to the cup using Ag paste. In the low-temperature, operation test carried out on the first embodiment, relative emitted luminance after 100 hours had improved relative to the value at the start of testing, being 110% to 115% thereof, and this value was thereafter maintained in stable fashion, such state remaining unchanged throughout measurements made up to 2,000 hours.  
                       TABLE 2                              Low-Temperature   High-Temperature High-       Elapsed   Operation Test   Humidity Operation Test                                 Time   First   Comparative       Comparative       (hours)   Embodiment   Example   First Embodiment   Example                                         100   110%-115%   125%-130%   95%-98%   90%-95%       500   110%-115%   125%-130%   92%-95%   85%-90%       1,000   110%-115%   125%-130%   85%-90%   70%-80%       2,000   110%-115%   125%-130%   80%-85%   60%-70%                  
 
     [0035] On the other hand, in the high-temperature high-humidity operation test, while there was a steady decrease in emitted luminance, even after 2,000 hours the relative emitted luminance obtained was 80% to 85%. At the comparative example, while there was the same trend of changing emitted luminance over time as at the first embodiment, relative emitted luminance after 2,000 hours was 125% to 130% in the low-temperature operation test but had dropped to 60% to 70% in the high-temperature high-humidity operation test, the deterioration in emitted luminance over time being more gradual for the LED lamp of the first embodiment than for that of the comparative example. Furthermore, with respect to the change represented by the trend toward increased emitted luminance during the low-temperature operation, since—e.g., where a full-color display is being manufactured or the like—this could disturb balance in luminescence or brightness of other colors, excessive increase thereof would not necessarily be a good thing.  
     [0036] &lt;Second Embodiment&gt; 
     [0037]FIG. 2 is a sectional view of an LED lamp in accordance with a second embodiment of the present invention. As light emitting element, this LED lamp  200  has an LED chip  204  wherein multilayer semiconductor film containing a nitride-type compound and comprising a P—N junction is formed over sapphire substrate. Below, a procedure for assembling LED lamp  200  is described.  
     [0038] A dispenser is used to apply a prescribed amount of first resin  203  to cup  202  of lead frame  201  which is secured to a die bonder (not shown). Formed in the center at the bottom of cup  202  is a cavity  205  which is slightly larger than the outer periphery of LED chip  204 , first resin  203  being applied so as to fill the region at the interior of this cavity  205 . The surface of this first resin  203  is made smooth, and first resin  203  is heated and cured before proceeding. In addition, chip adhesive  206 , to which filler has been added, is applied to the cured first resin  203  and to the bottom of cup  202 , LED chip  204  is placed over the applied chip adhesive  206 , and this is heated and cured.  
     [0039] p-Pad electrode  207   a  and n-pad electrode  207   b  are then formed at the principal plane of LED chip  204 , electrical connection between respective electrodes  207   a  and  207   b  and lead frame  201  being accomplished by means of wires  208   a  and  208   b.  Molding is then carried out using second resin  207 . The same resin was used for first resin  203  and second resin  209 , an epoxy resin (Ablestik Product No. 2017M) being used in the present embodiment. There is no limitation with respect to this or these resin(s), provided only that it or they is or are consistent with the intent of the present invention.  
     [0040] In the structure of the present embodiment, first resin  203  is arranged beneath chip adhesive  206 . Chip adhesive  206  contains filler in an amount that is 70 wt % thereof, and thermal expansion and contraction thereof is less than that of first resin  203 . Accordingly, chip adhesive  206  does not contribute to the resin stresses acting on LED chip  204 , and first resin  203  relaxes stresses acting on LED chip  204  from second resin  209 .  
     [0041] While the shape of cavity  205  in the present embodiment is such as to constitute a cylindrical platform, such shape is not limited thereto but may be any shape permitting uniform relaxation of stresses from resin(s) that act on the LED chip  204  which is placed thereover. That is, it being sufficient that there be symmetry with respect to the center of the principal plane of the LED chip  204  which is placed thereover, such shape may, for example, be that of a truncated cone or may be hemispheric.  
     [0042] The depth of cavity  205  correspond to the thickness of first resin  203 . As this first resin  203  relaxes stresses from second resin  209  that act on LED chip  204 , it will be sufficient if the thickness thereof is not less than 20μ, but there is no particular limitation with respect thereto. While somewhat on the large side due to limitations associated with machining accuracy of lead frame  201 , the thickness is 100μ in the present embodiment.  
     [0043] In the present embodiment, LED chip  204  is made to adhere to the insulating substrate by way of chip adhesive having thermal conductivity of not less than 2.5 W/m/K. Employed as such chip adhesive  206  was a mixture containing epoxy resin as base resin and Ag paste (e.g., Chemist CT220HK manufactured by Toshiba Chemical or T3007S manufactured by Sumitomo Metal Mining) employing Ag having thermal conductivity of not less than 170 W/m/K as filler. Because of the high thermal conductivity of the Ag paste, heat produced by LED chip  204  during supply of electricity to LED chip  204  is quickly dissipated to lead frame  201  by way of the Ag paste, permitting improvement in reliability.  
     [0044] Note that if the thickness of the Ag paste layer is too small, there is a possibility that breakage of the Ag paste layer could occur in the vicinity of the outer peripheral region of LED chip  204  during relaxation of stresses from the resin(s) which act on LED chip  204 . It is accordingly preferred that thickness of the Ag paste layer be not less than 5μ and not more than 20μ. The upper limit of this thickness is the limiting value at which relaxation of stresses from first resin  203  can still be effectively maintained. The same applies to the embodiments described below.  
     [0045] The filler used in chip adhesive  206  is not limited to Ag, it being preferred for example that Au, Cu, BeO, AlN or any other substance having thermal conductivity of not less than 170 W/m/K be used as filler.  
     [0046] 100 LED lamps prepared in accordance with the method described above were employed for each a low-temperature operation test and a high-temperature high-humidity operation test, and the change in emitted luminance over time was measured. TABLE 3 shows relative emitted luminance after 100, 500, 1,000, and 2,000 hours.  
     [0047] In the low-temperature operation test, relative emitted luminance after 100 hours had improved relative to the value at the start of testing, being 102% to 105% thereof, and this value was thereafter maintained in stable fashion, a relative emitted luminance of 98% to 103% being obtained at 2,000 hours.  
                       TABLE 3                       Elapsed       High-Temperature       Time   Low-Temperature   High-Humidity       (hours)   Operation Test   Operation Test                                            100   102%-105%   95%-98%        500   102%-105%   99%-102%       1,000   102%-105%   98%-101%       2,000   102%-105%   98%-103%                  
 
     [0048] Because the thickness of first resin  203  can be made larger in the present embodiment than was the case in the first embodiment, further improvement in relaxation of stresses acting on the LED chip from the resin(s) is permitted, increasing reliability.  
     [0049] &lt;Third Embodiment&gt; 
     [0050]FIG. 3 is a sectional view of an LED lamp in accordance with a third embodiment of the present invention. As light emitting element, this LED lamp  300  has an LED chip  304  wherein multilayer semiconductor film containing a nitride-type compound and comprising a P—N junction is formed over n-Si substrate. Below, a procedure for assembling LED lamp  300  is described.  
     [0051] A dispenser is used to apply a prescribed amount of first resin  303  to cup  302  of lead frame  301  which is secured to a die bonder (not shown). Formed in the center at the bottom of cup  302  is a cavity  305  which is slightly larger than the outer periphery of LED chip  304 , first resin  303  being applied so as to fill this cavity  305 . The surface of this first resin  303  is made smooth, and first resin  303  is heated and cured before proceeding. In addition, chip adhesive  306 , to which filler has been added, is applied to the cured first resin  303  and to the bottom of cup  302 , LED chip  304  is placed over the applied chip adhesive  306 , and this is heated and cured.  
     [0052] Thereafter, p-pad electrode  307  is formed at the principal plane of LED chip  304 , and moreover, n electrode  309  is formed at the back surface of the substrate of LED chip  304 . p-Pad electrode  307  is electrically connected to lead frame  301  by way of wire  308 , and n electrode  309  is electrically connected to lead frame  301  by way of chip adhesive  306 .  
     [0053] Molding is then carried out using second resin  310 . The same resin was used for first resin  303  and second resin  310 , a BA resin (bisphenol-A-type resin) being employed as molding resin for the light emitting diode in the present embodiment.  
     [0054] In the structure of the present embodiment, Ag paste was used as chip adhesive  306 . As mentioned during description of the second embodiment, Ag paste permits heat to be dissipated quickly, improving reliability of the LED lamp. The Ag paste moreover provides electrical connection between LED chip  304  and lead frame  301 . As substitutes for this Ag paste, it is also possible to use chip adhesives  306  wherein Cu, Au, or other such filler having high thermal conductivity and electrical conductivity is added to epoxy resin or the like.  
     [0055] Furthermore, it also possible to use an LED chip wherein a multilayer semiconductor film containing a nitride-type compound and comprising a PN junction is laminated over n-GaN substrate and/or over a metal thick film on the order of 100μ to 200μ.  
     [0056] Here also, where an LED chip is formed over such an electrically conductive substrate, an electrically conductive adhesive having thermal conductivity of not less than 2.5 W/m/K and having electrical conductivity such that the volume resistivity thereof is not more than 600 nΩm is used as chip adhesive. For example, an adhesive containing epoxy resin as base resin, to which Au, Ag, Cu, or other such electrically conductive substance having thermal conductivity of not less than 170 W/m/K and having resistivity of not more than 27 nΩm is added as filler, or the like may be used.  
     [0057] Whereas the same resin was used for the first resin and the second resin at the first through third embodiments, different resins may be used therefor. In such a case, it is preferred that the difference in thermal expansivity of the two resins used be on the order of 1%. Use of such resins makes it possible to cause the stresses surrounding the LED chip to be made uniform, this being a characteristic of the present invention. Note that what is here referred to as the difference in thermal expansivity is (η1-η2)/η2, where η1 and η2 are respectively the coefficients of thermal expansion of the first and second resins.  
     [0058] Furthermore, whereas in the first through third embodiments LED lamps having structures such that, as shown in FIGS. 1 through 3, an LED chip is placed on a lead frame and this is covered with molding resin were indicated, the present invention is not limited thereto, it also being possible, for example as shown in FIG. 4( a ) and ( b ) or in FIG. 5( a ) and ( b ), to form a protruding-type light emitting semiconductor device.  
     [0059] LED lamp  400 A shown in FIG. 4( a ) is such that electrically conductive film  401  is formed over insulating substrate  400 , LED chip  404  being mounted over this electrically conductive film  401  by way of intervening first resin  402 . Formed on this LED chip  404  is pad electrode  407 , this pad electrode  407  being electrically connected to the electrically conductive film by way of wire  408 . Second resin  403  is used to carry out molding around LED chip  404  having the structure described and insulating substrate  400  which contains same, forming a protruding-type LED lamp  400 A in which the protruding portion is in the shape of a truncated prism.  
     [0060] LED lamp  400 B shown in FIG. 4( b ) differs in structure from LED lamp  400 A in that pad electrode  407  is formed at the bottom surface of LED chip  404 , electrical connection of this pad electrode  407  to electrically conductive film  401  being accomplished by way of chip adhesive  409 . The structure thereof is in other respects similar to that of LED lamp  400 A shown in FIG. 4( a ).  
     [0061] In as much as they are both of the protruding type, the structure of LED lamp  500 A shown in FIG. 5( a ) is similar to the structure of LED lamp  400 A shown in FIG. 4( a ), but the former differs from the latter in that in the former the protruding portion produced by molding of second resin  403  is in the shape of a truncated cone. Furthermore, in as much as they are both of the protruding type, the structure of LED lamp  500 B shown in FIG. 5( b ) is likewise similar to the structure of LED lamp  400 B shown in FIG. 4( b ), but the former differs from the latter in that in the former the protruding portion formed by second resin  403  is in the shape of a truncated cone.  
     [0062] At the embodiments respectively shown in FIG. 4( a ) and ( b ) and FIG. 5( a ) and ( b ), resins having thermal expansivities which are on the same order are employed as first resin  402  for mounting of the LED chip and second resin  403  serving as molding resin. This makes it possible to cause the stresses surrounding the LED chip to be made uniform, permitting attainment of an LED lamp having satisfactory characteristics.  
     [0063] The present invention may be embodied in a wide variety of forms other than those presented herein without departing from the spirit or essential characteristics thereof. The foregoing embodiments and working examples, therefore, are in all respects merely illustrative and are not to be construed in limiting fashion. The scope of the present invention being as indicated by the claims, it is not to be constrained in any way whatsoever by the body of the specification. All modifications and changes within the range of equivalents of the claims are moreover within the scope of the present invention.  
     [0064] Moreover, the present application claims right of benefit of prior filing date of Japanese Patent Application No. 2002-071348, the content of which is incorporated herein by reference in its entirety. Furthermore, all references cited in the present specification are specifically incorporated herein by reference in their entirety.