Patent Application
20110195583
1. Field of Invention
The present invention relates to a method of forming a wavelength converting layer.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
III-nitride LEDs are often combined with wavelength converting materials such as phosphors or dyes. An LED combined with one or more wavelength converting materials may be used to create white light or monochromatic light of other colors. All or only a portion of the light emitted by the LED may be converted by the wavelength converting material. Unconverted light may be part of the final spectrum of light, though it need not be. Examples of common devices include a blue-emitting LED combined with a yellow-emitting phosphor, a blue-emitting LED combined with green- and red-emitting phosphors, a UV-emitting LED combined with blue- and yellow-emitting phosphors, and a UV-emitting LED combined with blue-, green-, and red-emitting phosphors.
A common approach is to coat the LED with the phosphor, using an organic binder to adhere the phosphor particles to the LED. Organic binders can cause performance degradation at high temperature, and can even cause LED failure.
One alternative to powder phosphor adhered to the LED with an organic binder is a pre-formed sintered ceramic phosphor attached to the LED. One example of such a device, illustrated in FIG. 1, is described in U.S. Pat. No. 7,341,878, which is incorporated herein by reference. “Semiconductor structure 130 including a light emitting region is bonded to ceramic phosphor 52 by bonded interface 56. Contacts 18 and 20 are formed on semiconductor structure 130, which are connected to package element 132 by metal interfaces 134.” Though
Processing of pre-formed ceramic phosphors may be expensive. In addition, it can be difficult to form thin pre-formed ceramic layers.
It is an object of the invention to provide a wavelength converting layer that is adhered to a surface without an organic binder.
In embodiments of the invention, a layer of wavelength converting material is formed by supplying energy to a particle of wavelength converting material and causing the particle to contact a surface such that the energy causes the particle to adhere to the surface. In some embodiments, the wavelength converting material is a phosphor and the surface is a surface of a semiconductor light emitting device. In some embodiments, the energy is supplied by heating or accelerating the particle.
In embodiments of the invention, a wavelength converting material is formed on a surface. No binder material is required to adhere the wavelength converting material to the surface. In some embodiments, the surface is a surface of a semiconductor light emitting device. Though the examples below include III-nitride light emitting diodes, embodiments of the invention may include other semiconductor devices such as laser diodes, and devices made from other materials systems such as other III-V devices, III-phosphide devices, III-arsenide devices, II-VI devices, and Si-based devices. Also, though the examples below include phosphor, other appropriate wavelength converting materials may be used.
To form the device illustrated in
Interconnects (not shown in
After the semiconductor structure is bonded to mount 30, all or part of the growth substrate may be removed by any suitable technique for the particular growth substrate material. For example, an Al2O3 substrate may be removed by laser lift-off. After the growth substrate is removed, the semiconductor structure may be thinned, for example by photoelectrochemical (PEC) etching. The exposed surface of the n-type region may be textured, for example by roughening or by forming a photonic crystal.
A phosphor layer 32 is formed on the surface of n-type region 12 by one of the methods described below in the text accompanying
Phosphor layer 32 is formed by supplying enough energy to phosphor particles to cause the particles to adhere to the surface of LED 15, then bringing the particles in contact with LED 15. The phosphor particles may adhere to the surface without a subsequent anneal.
In some embodiments, enough heat is supplied to the phosphor particles to cause them to become molten. The particle size is selected such that the amount of heat required is insufficient to appreciably raise the temperature of the surface of LED 15. When the molten phosphor particles contact the surface of LED 15, they solidify. The process is repeated until the desired thickness of phosphor layer 32 is reached.
In some embodiments, kinetic rather than thermal energy is supplied to the phosphor particles. The phosphor particles are accelerated until they have a similar amount of kinetic energy as the thermal energy required to melt the particles. The surface of LED 15 is then bombarded with the high speed phosphor particles which adhere to the surface of LED 15. The particle size is selected such that the high speed phosphor particles do not damage the surface of LED 15.
In some embodiments, a combination of thermal and kinetic energy is used to cause the phosphor particles to adhere to the surface of LED 15.
Phosphor particles 38 are provided to chamber 34 by a phosphor dispersing assembly 36 which separates each phosphor particle from other phosphor particles. Particles 38 are then passed through a beam of electrons 40 generated between anode 46 and cathode 48. Electrons 40 charge phosphor particles 38. Unused phosphor particles 39 are collected by catch 44 and potentially returned to assembly 36.
The charged phosphor particles 42 are directed by an electric field generated between plates 50 and 54 toward the target surface, LED 15 in
In some embodiments, charged phosphor particles 42 are passed through a beam 58 of infra-red radiation such that they become molten and solidify on contact with the target surface, LED 15 in
In some embodiments, charged phosphor particles 42 are accelerated by the electric field, then heated by infra-red beam 58, such that the combined kinetic and thermal energy of each particle when it contacts the target surface is enough to cause the particle to adhere to the target surface.
In one example, the phosphor is YAG:Ce, which melts at a temperature greater than 1200° C. The phosphor particles are between one and five microns in diameter. The particles are heated with an infra-red source at a wavelength that the YAG:Ce absorbs until they become molten. For example, a 1 kW-class standard industrial CO2 laser with an intensity on the order of 1 kW/cm2 to 100 kW/cm2 may be used. The amount of time the phosphor particles are exposed to the beam may depend on the melting point of the phosphor, the size and speed of the phosphor particles, and the beam geometry. In some embodiments, the phosphor particles are exposed to the beam for milliseconds. When the molten droplets contact the surface of the LED, they solidify. The LED surface is bombarded with molten YAG:Ce droplets until a phosphor layer of the desired thickness is formed.
Alternatively, the YAG:Ce particles may be accelerated by the electric field to a speed of approximately 1 km/sec. At that speed, a particle that is three microns in diameter has about 50 mJ of kinetic energy, which is about the amount of energy required to melt the particle. The LED is bombarded with accelerated particles, which deform and adhere to the LED on contact, until a phosphor layer of the desired thickness is formed.
In some embodiments, other wavelength converting layers are combined with phosphor layer 32. For example, a second phosphor layer formed by the methods described in the text accompanying
In some embodiments, phosphor layer 32 is encapsulated by a transparent material such as epoxy or silicone, for example to protect phosphor layer 32 or to form a lens or other optic. The transparent material may be formed after phosphor layer 32 is formed, such that the transparent material is not required to adhere phosphor layer 32 to LED 15.
In some embodiments, phosphor layer 32 is formed on a surface that is separate from the light emitting device. For example, phosphor layer 32 may be formed on a glass or other transparent plate which is spaced apart from the light source in a display. Alternatively, phosphor layer 32 may be formed on a ceramic phosphor that is spaced apart from the light emitting device.
The wavelength converting layers described in the embodiments may have several advantages. No organic binder is required, either to bind the phosphor together or to attach a ceramic phosphor to the LED, which may eliminate problems associated with the organic binder. The wavelength converting layer may be dense and thermally conductive, which may improve the performance of the device. The thickness of the wavelength converting layer may be tightly controlled, which may improve both the performance of the device and control of the characteristics of light emitted by the device and may reduce the cost of the device by eliminating waste of wavelength converting material. The wavelength converting layer may be fairly scattering, which may improve homogeneity of light emitted by the device without significant loss of light. Processes for forming the wavelength converting layer are generally inexpensive.
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 of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.
