This invention, in general, relates to the manufacture of light emitting diodes, and in particular, to apparatus and methods by which one or more thin homogeneous phosphor layers can be uniformly applied to LED chips to control the color, brightness, and variability of radiation they emit
Unacceptable color consistency and uniformity are significant problems within the high brightness LED market because of the methods by which such LEDs are manufactured. The most common method for making white light from an LED is by the deposition of phosphor onto a blue LED. When excited by a specific frequency of light, the phosphor has a broadband fluorescence emission into the green, yellow, and potentially, red regions of the light spectrum. The combination of blue light from the LED in conjunction with the emission spectrum from the phosphor results in white light. For example, U.S. Pat. Nos. 5,813,753 and 5,998,925 disclose light emitting devices in which a blue LED is disposed in a reflective cup and surrounded by material including phosphors. If the phosphor is not excited uniformly by the blue light from the LED, the resultant emission can have substantial color variation. This effect occurs because of a non-uniformity in the thickness of the phosphor-containing material surrounding the LED and consequent spatially non-uniform absorption of blue light and emission of red and green light. If the thickness of the phosphor is not closely controlled, the color temperature of the white light can have large deviations from nominal.
Conventional techniques for depositing phosphor on an LED assembly mainly consist of combining the phosphor with an epoxy-based encapsulant, then either photo curing or heat-curing the encapsulant. The deposition method of the phosphor-encapsulant mix typically involves pushing the material through a syringe onto the LED. During processing (mixing, deposition, curing), the phosphor particles inconsistently settle within the mixture, leading to a large amount of variability in white light spectrum from unit to unit. Additionally, the phosphor-encapsulant thickness is typically on the order of the die thickness or greater. Light exiting substantially normal to the die surface has a different path length relative to light exiting the die at increasing angles from the normal. This results in both color and intensity nonuniformities thereby reducing the utility of the light source for demanding applications.
It is therefore a primary object of the invention to provide methods and apparatus for manufacturing LEDs that emit bright and spectrally uniform light.
It is another object of the invention to provide methods and apparatus for uniformly depositing a homogeneous mixture containing phosphor on a LED assembly.
It is yet another object of the invention to provide a fast and efficient method for uniformly depositing homogeneous matrix of phosphor on LED arrays.
It is yet another object of this invention to provide LEDs that emit white light.
Still another object of this invention is to provide LEDs that emit green light.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter, when reading the following detailed description in connection with the drawings.
This invention provides apparatus and methodology for uniformly depositing a homogeneous matrix containing light converting materials such as phosphors or quantum dots onto an LED or LED array so that the resulting LED assemblies emit light in a substantially spectrally uniform manner. The method includes mixing predetermined ratios of powder, e.g. phosphor or quantum dots, in a support matrix that holds and maintains the powder such that it is distributed uniformly within it.
A carrier is then introduced into the phosphor powder matrix to decrease the viscosity of the mixture for further processing (e.g. pumping through distribution channels).
The lower viscosity mixture is then deposited onto LEDs by atomizing and spraying the mixture in one or more uniform thin layers.
The type and amounts of phosphor powder within the matrix and number of layers applied to the LED(s) are predetermined so that the LED(s) emit light of predetermined spectral properties. In one aspect of the invention, the mixture and number of layers are applied such that the LED(s) emit substantially uniform white light.
In an aspect of the invention, the mixture of carrier and phosphor matrix is repeatedly agitated after mixing and prior to deposition to maintain the homogeneity of the phosphor matrix and carrier mixture.
In another aspect of the invention, after deposition onto the LED(s), the one or more thin layers of phosphor are encapsulated onto the LED(s) with a thin transparent material that keeps the phosphor matrix layer(s) substantially stable and stationary on the LED(s).
In yet another aspect of the invention adapted for batch processing of LED arrays, the position of the LED arrays is actuated in an automated manner with respect to the deposition components and process.
The structure, operation, and methodology of the invention, together with other objects and advantages thereof, may best be understood by reading the following detailed description in connection with the drawings in which each part has an assigned numeral or label that identifies it wherever it appears in the various drawings and wherein:
The present invention relates to apparatus and methods for depositing homogeneous mixture of a matrix and preselected phosphor powder mixture as a uniform layer on an LED chip to reduce variability in the color of radiation emitted by the resultant LED assembly. The structure and properties of the final LED product using the inventive methodology also forms part of the invention. The methodology may best be understood by now referring to
The support matrix material is selected so that it functions appropriately during the remaining steps of the deposition process, does not interfere with the spectral characteristics of the LED device, and also remains stable at the LED's operating temperature. An example of such material is NuSil Silicone Elastomer R-2615 (10:1 silicone to hardener mix), which remains soft when mixed with the phosphor powders, is optically clear, and is not adversely effected at the operating temperature of the 460 nm LED previously referenced. The support matrix prevents migration of the phosphor powders within the matrix. NuSil Silicone has a viscosity on the order of five to ten thousand centipoises.
The second major step of the method is to add a carrier to the mixture to make it of sufficiently low viscosity for further processing, allowing it to be distributed within the deposition system and finally deposited in spray form on the LED device(s). The preferred carriers are preferably powerful solvents, such as Xylene, added in sufficient amounts to make the phosphor matrix and carrier mixture close in viscosity to the carrier itself.
To keep the phosphor matrix evenly distributed within the mixture, the third major step of the inventive process is to agitate the mixture until it is ready for deposition. In an embodiment of the invention, the phosphor matrix and carrier mixture is distributed into a fluid cup (see
The fourth major step of the inventive process is to atomize and spray the atomized mixture of phosphor matrix and carrier onto an LED assembly in one or more layers. The number of layers depends on the characteristics of the phosphor mixture and desired spectral output of the device. This step may be accomplished with a spray head (such as that described in relation to
The matrix also increases the light extraction efficiency from the LED chip because the silicone material of which it is composed has an index of refraction (1.41) greater than that of air and closer to the index of refraction of the LED substrate (2.40 in the case of Gallium Nitride based LEDs). Also, the phosphor layers deposited in this manner conform more closely to the shape of the chip thereby not altering the apparent size of the emitting area thus resulting in brighter LEDs with desired spectral outputs possible compared with other processes that do alter the apparent size of the emitting area while trying to achieve preferred colors.
In a particularly preferred process, the yellow and red phosphors are preferably mixed and applied independently. The reason for this is that the density of the yellow phosphor vs. the red phosphor is different, so it is easier to maintain a homogeneous matrix by first depositing the yellow phosphor, and then depositing the red phosphor to keep the phosphors from potentially otherwise separating. This approach produces much more controllable and consistent results than if two phosphors of differing density are mixed and applied together.
To improve the efficiency and uniformity of the deposition process, a step may be added (further described in reference to
Now referring to
Referring still to
Again referring to
In an embodiment of the invention, the deposition system of
In addition, the above described deposition system may include ultrasonic transducers in the flow lines to ensure that the matrix remains homogenous, and heat may be introduced into the delivery lines and other elements of the circulatory system to aid in the evaporation of the carrier.
Reference is now made to
Reference is now made to
In alternative embodiments, batch processing may be accomplished with other types of stage arrangements. For example, rotary stages are possible as shown in
Alternative embodiments may also include an in-process spectral monitoring system as shown in
Whether processed separately or in one of the described batch processes, the color temperature of an LED can be adjusted in a controllable manner by suitably adjusting the thickness of the phosphor containing layer.
Examples of white LED devices manufactured in accordance with the invention comprise LED linear arrays available in lengths of 4-inches and 10-inches with input powers ranging from 3-15 Watts and other properties as set forth in the following table:
The foregoing white LEDs were achieved by applying yellow and red phosphors to blue emitting LEDs as described earlier.
An example of a green emitting LED was achieved by starting with a blue LED at approximately 460 nm, and coating the LED with a phosphor that emits green light when excited by the blue light. A typical green phosphor has the following chemical composition: SrGa2S4.
Having described the invention with reference to particular embodiments, other variations will occur to those skilled in the art based on its teachings. For example, any of the batch processing arrangements can also readily be automated and controlled via computer 22 as previously described. The inventive phosphor deposition process can also be applied to LEDs that emit in the UV. With a UV chip, different phosphors are used, but the concept remains the same. Additionally, the inventive deposition process can be used with a class of light converting materials known as quantum dots, such as those marketed by Evident Technologies, 216 River Street, Suite 200, Troy, N.Y. 12180. The efficiency of quantum dots is currently low, but quantum dots have the potential to replace phosphors for LED light conversion. In powder form, they can be used in the inventive deposition process much the same as the phosphors, but would have the advantage of being tunable because they are based on semiconductor nanocrystals that are designed to translate light of one wavelength into light of another wavelength, allowing them to act as both filters and converters. Thus, quantum dots would substitute for the phosphor powder in the foregoing process. Therefore, it is intended that all such variants be within the scope of the invention as defined by the claims.