Abstract:
Very thin flash modules for cameras are described that do not appear as a point source of light to the illuminated subject. Therefore, the flash is less objectionable to the subject. In one embodiment, the light emitting surface area is about 5 mm×10 mm. Low profile, side-emitting LEDs optically coupled to solid light guides enable the flash module to be thinner than 2 mm. The flash module may also be continuously energized for video recording. The module is particularly useful for cell phone cameras and other thin cameras.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to solid state flashes for cameras or illumination lights for video cameras and, in particular, to very thin flashes and illumination lights for small cameras. 
       BACKGROUND 
       [0002]    Flash modules in cellular telephones that incorporate a camera are now commonplace. Such flash modules must be thin since a desirable feature of cell phones is their small size. Many types of cell phone cameras also take digital movies, which may require the illuminator to be constantly on while the video is being recorded. The illuminator for flashes and video illumination will be referred to as a flash for simplicity. As used herein, the term camera refers to a still picture camera and/or a video recorder. 
         [0003]    To improve reliability, reduce cost, and reduce size, it is known to replace the conventional, non-solid state flash bulbs with a high power light emitting diode (LED) that emits white light. 
         [0004]    Bulbs (e.g., xenon) are relatively large and use a curved mirror to direct light toward the subject. This results in a relatively deep bulb module that is objectionable when extreme thinness is desirable. Such a module is also relatively expensive and requires a special high voltage generator, which is large. The arc produced in the bulb is an intense point of light that concentrates all the light output power in substantially a point source. 
         [0005]    LEDs used as flashes are also typically mounted in a mirrored bowl. The LEDs output light in a Lambertian pattern, and the curved mirror redirects light toward the subject. Even though LED dice are very thin, the dice are typically mounted on a submount and packaged. The package contains a reflector and lens, and the overall package typically has a thickness on the order of 5 mm. The package is mounted on a printed circuit board, which adds more thickness to the module. The LED flash is substantially a point source, since the LED die is about 1×1 mm. 
         [0006]    The intense point of light emitted by the bulb or LED is objectionable to the subject being photographed. If the light were diffused by a diffusing lens separated from the point light source, the thickness of the overall light module would greatly increase, and the extreme diffusing needed for a point source would inherently result in much of the point source light output being absorbed by the diffuser or reflected back toward the light source. 
         [0007]    What is needed is an extremely thin illuminator for cameras, especially cell phone cameras, where the illuminator light output is spread over a relatively large area. 
       SUMMARY 
       [0008]    Various side-emitting LED designs are described herein for creating a very thin flash for a camera. A small reflector above the LED die causes the light to be efficiently emitted within a relatively narrow angle generally parallel to the flash module&#39;s light output surface. No lenses are used to create the side emission. The LEDs have a low profile, allowing the entire flash module to be less than 1 mm. 
         [0009]    One or more of the side emitting LEDs are positioned within a thin light guide with reflective walls, except for a top opening. The bottom of the light guide has features that reflect the light upward. The light emitted through the top opening of the light guide is sufficiently diffused such that the light source appears to the subject to be spread out over a relatively large surface area, such as 0.5-2 cm 2  or greater. Since the LED light emitted from a die is not directly seen by the subject, there is no intense point source of light seen by the subject. 
         [0010]    For further diffusion, a thin diffuser layer may be positioned over the light guide. To increase the brightness in the direction of the subject, a brightness enhancement layer may be used. 
         [0011]    In one embodiment, the LED die(s) used in the flash module emit blue light, and one or more types of phosphors are located over the LED to emit light with red, green, and/or yellow components so that the resulting combination of emissions results in white light of any desired color temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a cross-sectional view of a low profile, side-emitting LED used in a flash module in accordance with an embodiment of the invention. 
           [0013]      FIG. 2  is a cross-sectional view of a flash module cut through the LEDs in accordance with an embodiment of the invention. 
           [0014]      FIG. 3  is a top down view of the module of  FIG. 2 . 
           [0015]      FIG. 4  illustrates a cell phone camera incorporating any of the flash modules described herein. 
           [0016]      FIG. 5  is a cross-sectional view of a flash module where the LED is mounted upside down into a cavity in a light guide to reduce the thickness of the module. 
           [0017]      FIG. 6  is a partially transparent top down view of the module of  FIG. 5 . 
           [0018]      FIGS. 7 ,  8 , and  9  are related partial cross-sectional views of additional embodiments of flash modules. 
       
    
    
       [0019]    Elements that are similar or identical in the various figures are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0020]    Embodiments of the present invention include flash modules comprising low profile side-emitting LEDs in conjunction with thin light guide designs for providing a relatively large light emitting surface. The flash is less objectionable than a point source flash yet provides equivalent light output power, and the flash module is much thinner than prior art flash modules. 
         [0021]      FIG. 1  is a cross-sectional view of one embodiment of a thin, side-emitting LED  10 . Other embodiments of thin, side-emitting LEDs that may be used in the flash module embodiments can be found in U.S. application Ser. No. 11/423,419, entitled Low Profile Side Emitting LED, filed Jun. 9, 2006, by Oleg Shchekin et al., assigned to the present assignee, and incorporated herein by reference. 
         [0022]    The active layer of the LED  10  in one example generates blue light. LED  10  is formed on a starting growth substrate, such as sapphire, SiC, or GaN. Generally, an n-layer  12  is grown followed by an active layer  14 , followed by a p-layer  16 . The p-layer  16  is etched to expose a portion of the underlying n-layer  14 . Reflective metal electrodes  18  (e.g., silver, aluminum, or an alloy) are then formed over the surface of the LED to contact the n and p layers. When the diode is forward biased, the active layer  14  emits light whose wavelength is determined by the composition of the active layer (e.g., AlInGaN). Forming such LEDs is well known and need not be described in further detail. Additional detail of forming LEDs is described in U.S. Pat. No. 6,828,596 to Steigerwald et al. and U.S. Pat. No. 6,876,008 to Bhat et al., both assigned to the present assignee and incorporated herein by reference. 
         [0023]    The semiconductor LED is then mounted on a submount  22  as a flip chip. A flip chip is a chip with all electrical terminals on the “bottom” surface of the chip for direct bonding to a submount or other mounting structure. The submount  22  contains metal electrodes  24  that are soldered or ultrasonically welded to the metal  18  on the LED via solder balls  26 . Other types of bonding can also be used. The solder balls  26  may be deleted if the electrodes themselves can be ultrasonically welded together. 
         [0024]    The submount electrodes  24  are electrically connected by vias to pads on the bottom of the submount so the submount can be surface mounted to metal pads on a printed circuit board (PCB)  28 . Metal traces on the circuit board  28  electrically couple the pads to a power supply. The submount  22  may be formed of any suitable material, such as ceramic, silicon, aluminum, etc. If the submount material is conductive, an insulating layer is formed over the substrate material, and the metal electrode pattern is formed over the insulating layer. The submount  22  acts as a mechanical support, provides an electrical interface between the delicate n and p electrodes on the LED chip and a power supply, and provides heat sinking. Submounts are well known. 
         [0025]    To cause the LED  10  to have a very low profile, and to prevent light from being absorbed by the growth substrate, the growth substrate is removed, such as by CMP or using a laser lift-off method, where a laser heats the interface of the GaN and growth substrate to create a high-pressure gas that pushes the substrate away from the GaN. In one embodiment, removal of the growth substrate is performed after an array of LEDs are mounted on a submount wafer and prior to the LEDs/submounts being singulated (e.g., by sawing). 
         [0026]    After the growth substrate is removed, a planar phosphor layer  30  is positioned over the top of the LED for wavelength-converting the blue light emitted from the active layer  14 . The phosphor layer  30  may be preformed as a ceramic sheet and affixed to the LED layers, or the phosphor particles may be thin-film deposited, such as by electrophoresis. The phosphor ceramic sheet may be sintered phosphor particles or phosphor particles in a transparent or translucent binder, which may be organic or inorganic. The light emitted by the phosphor layer  30 , when mixed with blue light, creates white light or another desired color. The phosphor may be a YAG phosphor that produces yellow light (Y+B=white), or may be a red and green phosphor (R+G+B=white). 
         [0027]    With a YAG phosphor (i.e., Ce:YAG), the color temperature of the white light depends largely on the Ce doping in the phosphor as well as the thickness of the phosphor layer  30 . 
         [0028]    A reflective film  32  is then formed over the phosphor layer  30 . The reflective film  32  may be specular or diffusing. A specular reflector may be a distributed Bragg reflector (DBR) formed of organic or inorganic layers. The specular reflector may also be a layer of aluminum or other reflective metal, or a combination of DBR and metal. A diffusing reflector may be formed of a metal deposited on a roughed surface or a diffusing material such as a suitable white paint. The phosphor layer  30  also helps to diffuse the light to improve light extraction efficiency. 
         [0029]    Although side-emitting lenses are sometimes used to divert all light emitted by an LED&#39;s top surface into a circular side-emission pattern, such lenses are many times the thickness of the LED itself and would not be suitable for an ultrathin flash. 
         [0030]    Processing of the LED semiconductor layers may occur before or after the LED is mounted on the submount  22 . 
         [0031]    Virtually all light emitted by the active layer  14  is either directly emitted through the sides of the LED, or emitted through the sides after one or more internal reflections. 
         [0032]    In one embodiment, the submount  22  has a thickness of about 380 microns, the semiconductor layers have a combined thickness of about 5 microns, the phosphor layer  30  has a thickness of about  200  microns, and the reflective film  32  has a thickness of about 150 microns, so that the LED plus the submount is less than 1 mm thick. Of course, the LED  10  can be made thicker. The length of each side of the LED is typically less than 1 mm. 
         [0033]    If the LED need not be ultra-thin, the efficiency of the side emission may be increased by adding a clear wave guiding layer over the n-layer  12 , a scattering layer over the phosphor layer incorporating reflective particles or a roughed/prism surface, and a dichroic mirror or a one-way mirror below the phosphor layer  30  so that light downwardly reflected by the reflective film  32  is not absorbed by the semiconductor layers. 
         [0034]      FIG. 2  is a cross-sectional view of a flash module  34  incorporating three LEDs  10 , and  FIG. 3  is a top down view of module  34  with the LEDs  10  exposed. In the top down view of  FIG. 3 , the LEDs  10  would typically not be well outlined if a diffuser sheet were used. 
         [0035]    The LEDs  10  are mounted on a thin PCB  28 . The PCB  28  may provide the base of the module  34 , or there may be a separate support structure for the module  34 , such as a reflective box. The submount electrodes  24  ( FIG. 1 ) are ultrasonically bonded to conventional metal traces on the PCB  28  and terminate in connection pads  35  at the edge of the module  34 . Any type of connector, such as pins, solder pads, plugs, etc., may be used, and the connectors may be at any location. The metal traces on the PCB  28  interconnect the LEDs  10  in any suitable manner such as serially and/or in parallel. A current source (not shown) is electrically coupled to the pads  35 , and may be part of the module  34 . 
         [0036]    A solid transparent light guide  36  has cavities  37  in it through which the LEDs  10  are inserted. The light guide  36  may be a plastic (e.g., PMMA). Since the side-light emitting portion of the LEDs  10  can be about 0.25-0.5 mm thick, the thickness of the light guide  36  may be about 0.3-0.5 mm thick. Molded into the bottom of the light guide  36  are small indentations  38 , such as prisms, that reflect the light upward. The indentations may be arranged periodically or distributed to maximize the uniformity of the light emitted through the top surface of the light guide  36 . The indentations may instead be formed by etching or sand blasting to create a roughened bottom surface. 
         [0037]    The light guide  36  has reflective walls  40  and a reflective bottom surface  42 . A reflective film on the surface of the light guide  36  may be used as the reflectors, or the reflectors may be separate pieces that form a box in which the light guide  36  is positioned. The reflective film may be Enhanced Specular Reflector (ESR) film available from  3 M Corporation. The side light generated by the LEDs  10  is reflected within the light guide  36  and leaked out by the indentations  38  to create a substantially uniform brightness pattern across the top surface of the light guide  36 . 
         [0038]    Over the top of the light guide  36  is placed an optional diffuser sheet  44 , which helps fill in the small dark spots over the LEDs  10  with light and diffuses any bright spots over the indentations  38 . The diffuser sheet  44  may be about 0.1 mm thick. 
         [0039]    Brightness enhancement films (BEFs)  46  and  48  are positioned over the diffuser sheet  44  to redirect light generally normal to the surface. BEFs are well known for redirecting light through a selected angle. One type of BEF has prism surface features that refract light toward the subject. BEF  46  may limit the horizontal emission angle, while BEF  48  may limit the vertical emission angle. Each BEF  46  and  48  may be about 0.062 mm thick. 
         [0040]    In one embodiment, the overall thickness of the module  34  is 0.62 mm. The typical total thickness may range between 0.3 mm and 2 mm. The light emitting surface area of the flash module  34  may be virtually any practical size such as 0.5-2  2  cm or greater. 
         [0041]    In another embodiment, the light guide is shaped as a wedge, where light is inherently reflected upward due to the angled bottom surface. No light scattering indentions are then required to redirect the light. 
         [0042]      FIG. 4  is a simplified view of a cell phone camera  50 , which may take video or still pictures. The camera  50  represents any type of camera (digital or film), whether it takes still pictures or video. The cell phone camera  50  has a conventional keypad  52 , display  54 , and camera lens  55 . All aspects of the camera  50  may be conventional except for the flash module  34 . The flash module  34  is preferably at least 5 mm wide to spread the brightness over an area much larger than a point source. In one embodiment, the module  34  light emitting surface is greater than 5×10 mm. In another embodiment, only one LED is used, and the module  34  is 5×5 mm. The thickness of the module  34  does not change with its light emitting surface area. 
         [0043]      FIG. 5  is a cross-sectional of another embodiment of a flash module  60 , and  FIG. 6  is a see-through top down view of the module  60  of  FIG. 5  showing the location of the LED die. 
         [0044]    A solid transparent light guide  62 , such as formed of a polymer or glass, has a cavity  64  formed in it, where the cavity has the approximate dimensions of a side-emitting LED  66  (similar to LED  10  in  FIG. 1 ). Multiple LEDs may be inserted in additional cavities for increased light output. The side-emitting LED  66  emits most of its light at a low angle 360 degrees around the LED die. Thus, most of the light from the LED  66  is directly transmitted into the light guide  62 . A reflector (not shown) may be located around the side walls of the light guide  62  to prevent light escaping from the side walls. 
         [0045]    A bottom reflector  68  reflects light in the light guide  62  upward. The bottom reflector  68  extends below the LED  66  so that a separate reflector attached to the surface of the LED  66  is optional. The light guide  62  may have facets, a roughened surface, or other deformities to allow light to leak out the top surface of the light guide  62 . The light guide  62  may even be wedge shaped. Many techniques are well known to uniformly leak light out of a light guide. 
         [0046]    A diffuser sheet  70 , which may be a translucent film, diffuses the light from the light guide  62  to increase the uniformity of light across the surface. At least one brightness enhancement film (BEF)  72  redirects light into a narrower angle to increase the brightness within that angle. 
         [0047]    The LED  66  is mounted on a submount  74 . The submount  74  may be formed of ceramic, silicon, an insulated metal plate, or other suitable material. Metal pads on the LED  66  are bonded to corresponding pads on the submount  74  (solder balls  76  are shown). The LED  66  is preferably a flip-chip to minimize thickness. 
         [0048]    The submount  74  has terminals  78  that connect to an LED driver  80  that either provides a pulse of current to the LED  66  for a flash, to take a still picture, or provides a continuous current for taking a video. Conventional control circuitry  81  in the camera determines whether the operation is a flash or continuous illumination. For a flash, the LED driver  80  comprises a boost regulator that charges a capacitor then discharges the capacitor energy into the LED as a high power pulse. Preferably, the light output of the flash module is at least 10-15 lux.sec, where over 100 lux is emitted by the module for about 0.1 second. The total light energy while the camera shutter is open for a single picture is the relevant figure of merit for the flash. Flash LED drivers are commercially available, such as from Micrel, Inc., that can supply 1 amp to the LED. 
         [0049]    A continuous light output for video requires a very robust power source since high power LEDs require over 0.5 amps to illuminate a subject for good video quality. 
         [0050]    In addition to the submount  74  acting as an electrical interface between the driver  80  and the LED  66 , the submount  74  also acts as a heat sink to remove heat from the LED  66 . The surface of submount  74  may be reflective to reflect the LED light back towards the light guide  62 . 
         [0051]    The electrical connection between the submount  74  and the LED driver  80  can be easily realized by so called flex foil interconnects, as for example JTC Flex™, as manufactured by Gould Electronics. In case the submount is too small to attach to a flex foil, wirebonding might be used to make the electrical connections to the flex foil, clamped to the side of the submount, or the submount might be placed (glued) in a hole in the flex foil, or the LED  66  might be directly attached on top of a thin flex foil. The electrical connector, such as the flex foil, may additionally serve as a heat sink to remove heat from the LED die. 
         [0052]    A metal support  82  is connected to both the light guide  62  and the submount  74  to affix the submount  74  in place and to act as an additional heat sink. The submount  74  may be adhesively affixed in place or secured by other suitable means, such as by a thermally conductive tape. In another embodiment, the submount  74  is directly affixed to the light guide  62  without any support member. 
         [0053]    The metal support  82  may be a lead frame, where the metal support is split into at least two parts, each part being an electrical connection to provide the anode and cathode voltages to the LED die, as well as providing mechanical support and heat sinking. Either a submount with vias is used for direct bonding of the submount electrodes to the lead frame, or the submount is connected to the lead frame by means of wirebonds. 
         [0054]    Since the LED  66  and submount  74  do not need any printed circuit board (PCB) for mechanical support, heat sinking, and electrical interfacing, the module  60  may be much thinner than the module  34  of  FIG. 2 . 
         [0055]    Multiple LEDs with submounts may be used to couple additional light power into the light guide  62  along one or more sides of the light guide  62 , or at the corners of the light guide  62 , or through the center line of the light guide  62 , or in any other configuration. The LEDs emit white light using a blue LED die and phosphors to contribute red, green, and/or yellow components. 
         [0056]    The thicknesses of the various layers may be the same as described with respect to  FIG. 2 , where the resulting thickness of the entire module is less than 2 mm. The module  60  may be used in the camera  50  of  FIG. 4  instead of the module  34 . The surface area dimensions may be the same as described for the module  34 . The driver  80  may be used with either module. 
         [0057]      FIGS. 7-9  are close-up views of LEDs within a cavity of a light guide  84 . The light guide  84  may be identical to the light guide  36  ( FIG. 2 ) or  62  ( FIG. 5 ). In  FIGS. 7-9 , light is more uniformly emitted by the light guide  84  since the LED is laterally spaced away from the light emitting surface portion of the light guide  84 . 
         [0058]    In  FIG. 7 , the LED  10  from  FIG. 1  is used, having a reflective film  32  formed on its surface. The LED die is mounted on a submount  22 . Reflective surfaces  88 ,  92 , and  93  prevent light from escaping except through the top surface of the light guide  84 . Layers  96  may be the same diffuser layer and BEF shown in  FIGS. 2 and 5 . There may be multiple side-emitting LEDs distributed throughout the light guide  84  for increased light output power. The resulting flash module may have the same thickness and surface dimensions as the module  34  of  FIG. 2  for use in the camera  50  ( FIG. 4 ). 
         [0059]      FIG. 8  is similar to  FIG. 7  except that the LED  98  does not have a reflective film formed on it. The reflector  92  serves the purpose of creating only side emission into the light guide  84  and preventing the photographed subject from seeing a point source of light. 
         [0060]      FIG. 9  is similar to  FIG. 8  except that the LED  98  abuts the reflector  92  to create a thinner flash module. 
         [0061]    Features of the various embodiments may be combined as desired to produce a very thin camera illuminator with a relatively large light emitting surface area. 
         [0062]    Having described the invention in detail, those skilled in the art will appreciate that given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.