Patent Publication Number: US-2015060907-A1

Title: Led lamp using blue and cyan leds and a phosphor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 13/436,613, filed on Mar. 30, 2012, incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to phosphor-converted (PC) light emitting diode (LED) lamps that generate white light and, more particularly, to a PC LED lamp that uses LEDs emitting at two different peak wavelengths to improve the color rendering and luminous efficacy of the lamp. 
     BACKGROUND 
     Commercially available LEDs typically emit a single color (peak wavelength). Some experimental LED chips have been produced that can produce multiple peak wavelengths. However, such LEDs are fabricated using special techniques that add significant expense, making them not yet commercially viable. 
     The most common white-light LED uses a 450 nm blue LED that excites a relatively broad emission YAG:Ce phosphor deposited over the LED. The peak emission wavelength of YAG is about 535-565 nm (yellow-green). The blue light leaks through the YAG layer, and the combination of the blue and yellow light (phosphor converted blue light) creates white light. Such a di-chromatic light source produces relatively poor color rendering, since the highest color rendering is produced with a broad-spectrum source, such as the sun or a radiating black body. 
     The color rendering index (CRI) is a quantitative measure of the ability of a light source to faithfully reproduce the colors of certain sample patches in comparison with an ideal or natural light source, such as the sun, at the same color temperature. The sun has a CRI of 100. 
     It is known to use a blue LED to excite a combination of red, yellow, and green phosphors to produce a broader spectrum having a high CRI. However, combining and depositing phosphors to produce a consistent emitted color is difficult. Phosphors are also relatively expensive and reduce the efficiency of the LED. Additionally, the environmental impact of mining phosphors is significant and growing due to the increased use of phosphors. 
     Another problem with phosphors is that the color cannot be controlled once the phosphor is deposited over the LED. 
     It is known to combine red, green, and blue LEDs in a module to produce white light, but each LED needs its own current source, and the different LEDs react differently to heat. Accordingly, such LED modules have better CRI but lack color stability and are expensive. 
     What is needed is a technique for fabricating a white light source that improves CRI, luminous efficiency, and color control without the drawbacks discussed above. 
     SUMMARY 
     In one embodiment, thousands of micro-LEDs (e.g., 25 microns per side) are deposited on a substrate, and electrical connections are made using a transparent conductor layer. Some of the LEDs are formed to emit a peak wavelength of 450 nm (blue), and some are formed to emit a peak wavelength of 490 nm (cyan). The two types of LEDs are GaN based and can be driven at the same current. The two types of LEDs react similarly to heat. The ratio of the two types of LEDs is controlled to achieve the desired overall color emission of the LED lamp. In one embodiment, a yellow phosphor, such as YAG, is then deposited on the LEDs, or a remote yellow phosphor layer is used. YAG phosphor is most efficiently excited at 450 nm and has a very weak emission at 490 nm. The blue LED efficiently excites the yellow phosphor to produce white light having blue and yellow components, and the cyan LED adds a green component to increase the CRI of the light emission and luminous efficiency. 
     In one embodiment, a red phosphor is mixed with the YAG phosphor or overlies the YAG phosphor to create a warmer white light. If a red phosphor and yellow phosphor were used with different peak excitation wavelengths, then controlling the blue/cyan ratio or separately controlling the blue and cyan LEDs with currents controls the relative red/yellow contributions of the phosphors to the white light. 
     In another embodiment, the relatively broad emission YAG phosphor is not used. Instead, a combination of red and non-YAG yellow phosphors are deposited at least over the blue LEDs or used remotely. The blue wavelength is optimized for the phosphor. The cyan LEDs, which may not be covered by any phosphor, obviates the need for a green phosphor to broaden the spectrum, improving the efficiency of the lamp and improving color consistency. 
     This dual LED approach has been shown to improve luminous efficiency and generate a higher CRI due to the broader spectrum generated Improving the luminous efficiency is due to lower phosphor usage and generating an emission that better conforms to the luminous efficiency function of the human visual system. 
     In one embodiment, the micro-LEDs are combined together in a liquid medium (as an LED ink) and screen printed, slot-die coated, sprayed, or otherwise deposited onto a metal layer on a substrate. The LED chips, in one embodiment, have a reflective metal (e.g., silver) electrode (an anode or cathode electrode) covering the bottom surface of the chip and a distributed metal electrode on the top surface of the chip to allow light to pass through the top surface of the chip. The LEDs self-orient themselves as they sink through the liquid medium so that the bottom electrode tends to face the metal layer on the substrate. Since the large metal bottom electrode has a density that is much greater than the GaN semiconductor, the LEDs tend to orient with their bottom electrode side down. Self-orientation may be accomplished in various other ways. 
     The structure is then heated (or otherwise cured) to evaporate the liquid medium (a solvent) and bond the silver electrode to the metal layer on the substrate. In one example, the metal layer on the substrate is an uncured aluminum layer, deposited by, for example, slot-die coating, and the layer is heated to cure the aluminum layer while bonding the LED electrodes to the aluminum. 
     In another embodiment, the blue and cyan LEDs are separately printed onto the substrate&#39;s metal layer in different areas or zones so that they can be separately electrically contacted and energized. The yellow phosphor may be printed over only the blue LEDs or over a combination of the blue and cyan LEDs in a separate zone. The YAG or yellow phosphor may be mixed with a red phosphor. Some of the blue and cyan LEDs may not be covered by the phosphor. The three groups of LEDs may then be separately controlled by different current sources to select the amount of blue, cyan, and phosphor wavelengths to generate a target color. 
     Correlated color temperature (CCT) is a specification of the color appearance of the light emitted by a lamp, relating its color to the color of light from a black body reference source (a Planckian reference) when heated to a particular temperature, measured in Kelvin (K). 
     Luminous efficacy of radiation (LER) is luminous flux divided by radiant flux, where the luminous flux is a measure of perceived brightness to a human eye (taking into account the luminosity function of the eye), and the radiant flux is the total flux output of the light source independent of wavelength. The SI units for LER are lumens per Watt. The maximum LER value is 683 lm/w at 555 nm (green light), where the human eye is most sensitive. The theoretical maximum for white light depends of the color temperature and spectral power distribution (SPD). 
     The lamp is engineered for the warmest CCT, such as 2700K. Then, by increasing the amount of blue and/or cyan, the color is cooled to higher CCTs. For example, at one setting, the lamp may produce a CCT of 2700K with a CRI of 94 and a LER of 288 lm/W. Then, by increasing the current to the blue and/or cyan LEDs not covered by the phosphor, the light can be cooled to 4000K or 6000K, for example. All points on the Planckian locus may be possible. Dynamically adjusting the light throughout the day for a circadian rhythm is achievable. 
     The phosphor layer may be replaced by a quantum dot layer. 
     In another embodiment, much larger and conventional 450 nm and 490 nm LED chips (e.g., 0.5 mm per side) may be positioned on the substrate using pick and place tools or using another method. The usage of the phosphor and separate controlling of the blue and cyan LEDs may be the same as described above to achieve a higher CRI and improved LER. 
     The 490 nm LEDs may be replaced with GaN LEDs emitting a longer peak wavelength (closer to green), such as 507 nm, to obtain a high scotopic response, since the scotopic response typically peaks at 507 nm. This would result in the lamp appearing brighter than actually measured. In other words, it would increase the pupil lumens. 
     Other peak emissions of the two types of LEDs may be used to improve CRI and LER. 
     Thus, by using the inventive technique, there is less energy loss due to phosphor conversion, the luminous efficacy is improved, the CRI is increased, and the color can be controlled. 
     Other embodiments are described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a micro-LED chip, emitting a peak wavelength of, for example, either 450 nm or 490 nm. The LED has a reflective metal bottom electrode and a distributed top electrode that allows light to escape. 
         FIG. 2  illustrates LED ink (containing the micro-LED chips) being printed over an uncured aluminum layer on a substrate. 
         FIG. 3  is a top down view illustrating the 450 nm and 490 nm LED chips being randomly mixed over a small section of the substrate. 
         FIG. 4  is a top down view illustrating how the 450 nm and 490 nm LED chips may be separately printed in zones, such as strips, and a combination of 450 nm and 490 nm LEDs may be printed in other zones, to allow the different zones to be separately controlled by current sources to control the overall color. 
         FIG. 5  is a cross-sectional view along line  5 - 5  in  FIG. 3  illustrating the LED chips having their bottom electrodes bonded to an aluminum layer and their top electrodes contacted by a transparent conductor overlying the LED chips, connecting the LED chips in parallel. 
         FIG. 6  is a cross-sectional view along line  6 - 6  in  FIG. 4  illustrating the LED chips with their top electrodes contacted by separate strips of a transparent conductor for connecting the three strips of LEDs to three different current sources for separately controlling the strips of LED chips. 
         FIG. 7  illustrates a remote phosphor over the structure of  FIG. 6 . 
         FIG. 8  illustrates a phosphor deposited directly on the LED chips of  FIG. 6 . The LED chips may first be encapsulated for protection and improved light extraction. 
         FIG. 9  is a top down view of a small portion of the substrate illustrating mixed LED chips coated with a transparent conductor, connecting the LED chips in parallel, and coated with a phosphor. 
         FIG. 10  is a top down view of a small portion of the substrate illustrating separately deposited groups of LED chips connected to different current sources, where only some of the LED chips are covered with a phosphor. 
         FIG. 11  is a flowchart identifying various steps used to form an LED lamp in accordance with one embodiment of the invention. 
         FIG. 12  illustrates a fabrication system used to form LED lamp panels. 
         FIG. 13  illustrates multiple LED lamp panels being interconnected to form a luminaire, such as for being mounted in a ceiling for general illumination. 
     
    
    
     Elements that are similar or identical in the various figures are labeled with the same numeral. 
     DETAILED DESCRIPTION 
       FIG. 1  is a cross-sectional view of a micro-LED chip  10  emitting a peak wavelength of either 450 nm or 490 nm, depending on the structure of its active layer. The LED chip  10  is manufactured as part of a wafer containing many thousands of LEDs. The wafer may be broken along etched lines into small LED chips with sides approximately 25 microns. Since the growth substrate is removed, the chip  10  may be only a few microns thick, such as 3-10 microns thick. Forming GaN-based LEDs to emit light with peak wavelengths of 450 nm and 490 nm is well known. 
     Typically, the GaN-based LED semiconductor layers are grown on a sapphire substrate or other suitable growth substrate. N-type layers  12  are epitaxially grown, followed by an active layer  14 , followed by p-type layers  16 . The growth substrate is then removed, and ohmic contact to the n-type layers  12  is made by depositing a large, reflective metal alloy (or multiple metal layers) on the n-layers  12  to form a bottom electrode  18 . Ohmic contact to the p-type layers  16  is formed by depositing a metal alloy on the p-layers  16  to form a top electrode  20 . The top electrode  20  may be patterned to be dots, lines, a radial pattern, or other pattern to allow light to escape while distributing current to the p-layers  16 . In an alternative embodiment, the large reflective electrode is formed over the p-layers  16 , and the patterned electrode is formed over the n-type layers  12 , where the light escapes through the n-type layers  12 . 
     By forming the active layer with different stoichiometries of AlInGaN, the peak wavelength can be adjusted to be anywhere between blue and green. In the present disclosure, the wavelength of 450 nm will be referred to as blue, and the wavelength of 490 nm will be referred to as cyan, even though 490 nm is near the transition from blue to cyan. The 490 nm wavelength was chosen since it fills in a wavelength gap when a blue LED is used to excite a YAG phosphor (having a peak emission wavelength of about 575 nm). The 450 nm wavelength was chosen since the YAG phosphor is most efficiently excited by 450 nm LED chips emitting other than 450 nm and 490 nm may be used, depending on the phosphor used or other factors. Other types of phosphors can be used besides YAG. For example, a non-YAG yellow phosphor may be used in combination with a red phosphor. YAG will be used in the various examples. 
     Instead of phosphor, a quantum dots layer may be used in all embodiments to create white light or any other color light. 
     In one embodiment, the blue and cyan micro-LED chips  10  are mixed in a liquid medium (a solvent) to form an LED ink, whose viscosity is relatively low to allow the LED chips  10  to easily move through the medium. The ratio of the blue and cyan LED chips  10  in the medium is controlled to achieve the desired overall color of the LED lamp. 
       FIG. 2  illustrates a substrate  24 , which may be a thin, flexible stainless steel sheet or a polymer sheet. The fabrication process may be a roll-to-roll process performed at atmospheric pressure. An uncured aluminum layer  26  is deposited over the substrate  24  such as by slot-die coating, spraying, printing, etc. The aluminum layer  26  is formed of particles of aluminum in a solvent. 
     A printing or spraying mechanism  28 , containing the LED ink  30 , is moved relative to the substrate  24  to deposit a thin layer of the LED ink  30  on the aluminum layer  26 . The relatively high mass of the bottom electrodes  18  naturally orient the LED chips so that the bottom electrodes  18  face the aluminum layer  26 . The structure is then heated (cured) by an overhead heater or laser to evaporate the solvents and fused the aluminum particles together to form a high conductivity metal layer. This also creates an ohmic bond between the bottom electrodes  18  and the aluminum layer  26 . 
     There will typically be many thousands or millions of LED chips  10  deposited over the aluminum layer  26 . 
       FIG. 3  is a simplified top down view of a small portion of the substrate  24  showing a random distribution of the blue LED chips  32  and the cyan LED chips  34 . In the example, the ratio of blue LED chips  32  to cyan LED chips is two to one. 
     Although the peak wavelengths of LED chips varies somewhat even within the same wafer, the average peak wavelength of the blue LED chips  32  is about 450 nm, and the average peak wavelength of the cyan LED chips  34  is about 490 nm. 
     In another embodiment, illustrated in  FIG. 4 , the blue LED chips  32  are deposited separately, such as by slot-die coating or screen printing, to be in specified zones on the substrate  24 . The zones may be narrow strips or other shapes that allow the light from adjacent zones to mix to create a uniform color when viewed a certain distance from the lamp. The cyan LED chips  34  are also deposited separately to be in specified zones. There may also be a mixture of blue and cyan LED chips deposited separately. In the example of  FIG. 4 , the three groups of LED chips are deposited as very narrow strips (shown oriented vertically). Each strip may have many LED chips across its width, although the LED chips are shown to be linearly oriented along the length of each strip for simplicity. 
       FIG. 5  is a cross-sectional view along line  5 - 5  in  FIG. 3 .  FIG. 5  illustrates a dielectric layer  36  deposited over the LED chips  32 / 34  by printing or spraying. The dielectric layer  36  partially encapsulates the chips and mechanically fixes the chips in place. The top portion of the dielectric layer  36  is then etched away, such as by a wet etchant, to expose the top electrodes of the LED chips  32 / 34 . A transparent conductor layer  38  is then deposited over the LED chips  32 / 34  to connect them in parallel. The transparent conductor layer  38  may be deposited in zones to interconnect a group of LED chips  32 / 34  in parallel, and the various transparent conductor layer zones are effectively electrically isolated so they can be driven by different current sources. This also increases reliability and reduces the current needed to be delivered per current source. The transparent conductor may be deposited by screen printing, where the screen is patterned to print only through non-masked portions of the screen. 
     In another embodiment, the substrate  24  is a dielectric film, and the aluminum layer  26  is formed in electrically isolated zones. By also forming the transparent conductor layer  38  as zones, groups of parallel-connected LED chips can be connected in series on the same substrate to obtain any voltage drop across the series string. 
     Metal bus bars  40  are deposited at intervals to provide high conductivity paths from the current sources to the transparent conductor layer  38 . 
       FIG. 6  is a cross-sectional view along line  6 - 6  in  FIG. 4 . Instead of the transparent conductor layer  38  being blanket deposited over the mixture of LED chips  32 / 34 , as in  FIG. 5 , the transparent conductor layer  38  is selectively deposited, such as by screen printing, to only interconnect the LED chips in a single vertical strip in parallel, where the LED chips were separately deposited in the strips. This allows the blue LED chips  32  in the rightmost strip to be separately controlled by a current source, allows the cyan LED chips  34  in the leftmost strip to be separately controlled by another current source, and allows the mixture of LED chips  32 / 34  in the center strip to be separately controlled by another current source. 
       FIGS. 7 and 8  illustrate the structure of  FIG. 6  being used with a phosphor layer. The structure of  FIG. 5  may be used instead. 
     In  FIG. 7 , in one embodiment, a phosphor layer  50  may be a mixture of a YAG phosphor and a red phosphor, or a layer of a YAG phosphor and a layer of a red phosphor. The phosphor layer  50  is deposited on a transparent substrate  52 , such as glass. The phosphor layer  50  is separated from the LED chips  32 / 34 , so is a remote phosphor. The phosphor layer  50  may be formed as separate pieces and affixed over the LED chips  32 / 34  using any technique, such as by a transparent adhesive or a mechanical assembly. Providing a remote phosphor layer  50  improves mixing of the LED light, reduces the effects of light and heat on the phosphor, and allows the manufacturer to select a pre-formed phosphor layer having a desired characteristic to be used in the LED lamp to achieve a color specified by the customer, while not changing the LED chips  32 / 34 . 
     The YAG/red phosphor will emit light having well defined characteristics. The phosphor emission is shown by the light rays Y/R. The blue LED chips  32  may be controlled by a current source to be brighter to add more blue to the white light output to increase the CCT. The blue emission is shown by the light rays B. The cyan LED chips  34  may be controlled by a current source to be brighter to add more green to the white light output. The cyan emission is shown by the light rays C. Other phosphors may be used instead of YAG to create white light. 
     If all the LED chips were connected in parallel, such as in  FIG. 5 , the ratio of the cyan LED chips  34  to the blue LED chips  32  is selected to boost the 490 nm output of the LED lamp a certain amount to create a broader spectrum of light to improve the CRI of the lamp. 
       FIG. 8  illustrates the structure of  FIG. 6  with the phosphor layer  50  deposited directly over the LED chips  32 / 34 , such as by spraying or printing. The use of the cyan LED chips  34  improves the CRI and LER of the LED lamp and allows the overall color to be dynamically controlled by changing the currents supplied to the various zones of LED chips. 
     Although the YAG phosphor is excited by the 490 nm emitted by the cyan LED chips  34 , the excitation efficiency is not as great as with the 450 nm LED chips  32 . Therefore, in one embodiment, the phosphor does not cover the cyan LED chips  34  but only covers the blue LED chips  32  to maximize efficiency. The cyan LED chips  34  then just supply the 490 nm wavelength that is substantially lacking in the YAG phosphor emission. 
     In another embodiment, the phosphor layer  50  comprises a yellow phosphor and a red phosphor that are energized by the blue LED chips  32 . The cyan LED chips  34  contribute a cyan component to increase CRI, and the cyan LED chips  34  are optionally not covered with a phosphor. The blue LED chip  32  peak wavelength may be tuned to the peak excitation wavelength of the phosphors used without compromising CRI. 
     Since the blue and cyan LED chips have very similar structures, they can be driven by the same current, allowing them to be connected in parallel. 
     The peak emission of the cyan LED chip  34  may be anywhere within the cyan (blue-green) range while still achieving the benefits of the invention. The peak wavelength of the blue LED chip will typically be the most efficient energizing wavelength for the phosphor. The optimum peak wavelengths of the two types of LED chips are selected to improve CRI and LER and depend on the particular phosphors used and CCT or chromaticity to be generated. In one embodiment, the two LED type have a range between about 430 nm and 507 nm. 
       FIG. 9  is a top down view of a portion of a completed LED lamp with either a remote phosphor layer  50  or a phosphor layer  50  deposited directly over the LED chips. All the LED chips in  FIG. 9  are driven in parallel by the current source  54 , since the transparent conductor layer  38  connects together the top electrodes of all the LED chips. CRI is increased by the addition of the cyan LED chips  34  since some of the cyan emission passes through the phosphor layer  50 . 
       FIG. 10  is a top down view of a portion of another embodiment of a completed LED lamp with either a remote phosphor layer  50  or a phosphor layer  50  deposited directly over certain LED chips. In  FIG. 10 , the transparent conductor layer  38  is deposited in strips to only interconnect selected LED chips in parallel. If the aluminum layer  26  were segmented, the LED chips connected to the same segment may form a group of parallel LED chips that are connected in series with other groups of parallel LED chips. 
     In the example of  FIG. 10 , the phosphor layer  50  is only formed over the middle strip of LED chips. The leftmost strip of cyan LED chips  34  and the rightmost strip of blue LED chips  32  are not covered by a phosphor. Normally, there would be many more LED chips covered by the phosphor than not covered by the phosphor to generate a proper balance of wavelengths to produce white light. Each of the three strips of LED chips is controlled by its own current source  56 ,  57 ,  58  so that the color components of the white light may be adjusted to achieve a wide range of CCT, such as 2700K to 6500 K. All points on the Planckian locus may be possible. The LED chips and phosphor are designed to provide the lowest target CCT (e.g., 2700K) and, by increasing the currents to the separate blue and cyan LED chips, the CCT can be raised. 
     In another embodiment, blue and cyan LED chips are mixed together and connected in parallel for being controlled by the same current source. The ratio of the LED chips controls the color. A second group of either mixed LED chips or only blue LED chips is covered by phosphor or overlain with a remote phosphor, and the second group of LED chips is controlled by a separate current source. By controlling the relative currents of the two current sources, a wide range of CCTs is achievable, while increasing the CRI and improving the LER. 
     The aluminum layer  26  and transparent conductor layer  38  may be segmented in any manner to connect any number of LED chips in series and parallel. 
     A quantum dot layer may be used instead of the phosphor layer  50 . 
     The current sources  56 - 58  may be automatically controlled during the day to sweep the CCT between two temperatures to mimic sunrise to sunset to correspond to the circadian rhythm. 
     In one embodiment, instead of using LED chips with a peak wavelength of 490 nm, LED chips with a peak wavelength of 507 nm are used since the scotopic response typically peaks at 507 nm. This would result in the lamp appearing brighter than actually measured by increasing the pupil lumens. 
       FIG. 11  is a flowchart summarizing various steps used for form an LED lamp in accordance with one embodiment of the invention. In step  61 , blue and cyan LED chips are deposited on a substrate. The LED chips may be mixed or separated on the substrate. The ratio of the LED chips affects the resulting color. 
     In step  62 , the LED chips are electrically interconnected and connected to current sources. The phosphor-converted LED chips in a group may be connected to one current source. The blue LED chips in a group may be connected to another current source. And, the cyan LED chips in a group may be connected to a third current source. 
     In step  63 , a phosphor is selectively applied overlying all the LED chips or only over a selected group of LED chips. The phosphor may be a remote layer instead. 
     In step  64 , the current sources are controlled to provide a desired mix of the blue, cyan, and phosphor wavelengths to achieve the desired CCT. CRI and LER may be optimized by the combination of LED chips and phosphor. 
       FIG. 12  illustrates a fabrication system used to form LED lamp panels. The substrate  24  may be moved relative to the various stations. One or more depositing stations  82  are used for depositing the LED chips and various layers in any pattern, such as by using screen printing. A heating/curing station  84  cures the various deposited materials. An etching station  86  removes materials, as desired. All processes may be performed using a roll-to-roll process at atmospheric pressures to fabricate relatively large and inexpensive LED light panels. Each panel may comprise many thousands of LED chips that are interconnected in series and/or parallel. 
       FIG. 13  illustrates four LED lamp panels  100  being interconnected to form a luminaire, such as for being mounted in a ceiling for general illumination. There may be many more panels in a single luminaire. Each panel  100  is made by any of the above-described processes and each contains many thousands of LED chips. A matrix of metal bus bars  114  is formed over the transparent conductor layer to provide high conductivity paths proximate to the LED chips. Electrical connectors  116  and  118  connect to the aluminum layer  26  ( FIG. 2 ) and metal bus bars  114  for coupling a current to the various LED chips. There may be many more connectors for each panel if there are multiple, electrically isolated segments of LED chips that are to be separately controlled. 
     The various panels  100  are then connected in any combination of series and parallel by external conductors, such as wires or part of a frame, to achieve the desired voltage and current. The luminaire may replace a standard 2×4 foot fluorescent troffer. 
     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.