Light emitting device having a refractory phosphor layer

A light emitting device and method of producing the same is disclosed. The light emitting device includes a transparent thermal conductor layer, a refractory phosphor layer provided on the transparent thermal conductor layer, and a light emitting semiconductor arranged to emit light toward the transparent thermal conductor layer and the refractory phosphor layer.

BACKGROUND

The present disclosure relates to light emitting devices, and more particularly, to semiconductor light emitting devices having a refractory phosphor layer.

Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources, such as incandescent lamps and fluorescent light sources. LEDs have substantially higher light conversion efficiencies than incandescent lamps and longer lifetimes than both types of conventional light sources. In addition, some types of LEDs now have higher conversion efficiencies than fluorescent light sources and still higher conversion efficiencies have been demonstrated in the laboratory. Furthermore, LEDs require lower voltages than fluorescent lamps, and therefore, are better suited for applications in which the light source must be powered from a low-voltage source, such as a battery or an internal computer DC power source.

Unfortunately, LEDs produce light in a relatively narrow spectrum band. To replace conventional lighting sources, LEDs that generate light that appears to be “white” to the human observer are required. A light source that appears to be white and that has a conversion efficiency comparable to that of fluorescent light sources can be constructed from a blue light emitting semiconductor covered with a layer of phosphor that converts a portion of the blue light to yellow light. If the ratio of blue to yellow light is chosen correctly, the resultant light source appears white to a human observer. In applications requiring high power illumination, however, the phosphor layer may overheat. The heat, if not sufficiently dissipated, may cause premature degradation of the phosphor layer, decreasing the device's performance and life-span.

To prevent the phosphor layer from overheating, many contemporary devices are designed with the phosphor layer mounted further away from the light emitting semiconductor. This approach, however, creates additional problems. Increasing the distance between the phosphor layer and the light emitting semiconductor increases the size of the device, and thus, can increase the manufacturing cost of the device. Moreover, such a design does not effectively address the heat dissipation issue as it does not provide any means for dissipating the heat away from the phosphor layer.

Accordingly, although contemporary LEDs have proven generally suitable for their intended purposes, they possess inherent deficiencies which detract from their overall effectiveness and desirability. As such, there exists a need for small, high-power “white light” LEDs with a system for dissipating heat from the phosphor layer.

SUMMARY

In one aspect of the disclosure, an apparatus includes a transparent thermal conductor layer, a refractory phosphor layer provided on the transparent thermal conductor layer, and a light emitting semiconductor arranged to emit light toward the transparent thermal conductor layer and the refractory phosphor layer.

In another aspect of the disclosure, a light emitting device includes a refractory phosphor layer fused onto a transparent layer having a thermal conductivity greater than that of the refractory phosphor layer.

In a further aspect of the disclosure, a method for manufacturing a light emitting device includes depositing at least one phosphor mixture onto a transparent substrate, wherein the phosphor mixture includes a phosphor powder, a glass frit, and a binder.

In yet a further aspect of the disclosure, a method for manufacturing a light emitting device includes fusing a refractory phosphor layer onto a transparent layer.

It is understood that other aspects of light emitting devices will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only in examples of various aspects of light emitting devices by way of illustration. As will be realized, the various aspects of light emitting devices disclosed herein are capable of modification in various other respects, all without departing from the spirit and scope of the present disclosure. Accordingly, the drawings and detailed description that follow are to be regarded as illustrative in nature and not as restrictive.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appended drawings is intended as a description of various aspects of light emitting devices and is not intended to represent all ways in aspects of the present invention may be practiced. The detailed description may include specific details for the purpose of providing a thorough understanding of various aspects of light emitting devices; however, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and components are summarily described and/or shown in block diagram form in order to avoid obscuring the concepts of the present invention.

Furthermore, various descriptive terms used herein, such as “provided on” and “transparent,” should be given the broadest meaning possible within the context of the present disclosure. For example, when a layer is said to be “provided on” another layer, it should be understood that that one layer may be deposited, etched, attached, or otherwise prepared or fabricated directly or indirectly above that other layer. Also, something that is described as being “transparent” should be understood as having a property allowing no significant obstruction or absorption of electromagnetic radiation in the particular wavelength (or wavelengths) of interest.

FIG. 1is a cross-section view illustrating an example of a light emitting device100. In this example, the device may include a blue light emitting semiconductor102provided within a recessed housing104. The light emitting semiconductor102may be driven by a power source (not shown) that is electrically coupled to the light emitting semiconductor102via electrically conductive traces (not shown). The recessed housing104may be formed by boring a cavity106, such as a conical cavity, for example, in a layer of a material, such as ceramic, resin, polyphthalamyde, polycarbonate, or some other suitable material. An inner wall108of the recessed housing106may be coated with a reflective material such as, for example, aluminum, sliver, or a suitable plastic impregnated via injection molding with titanium dioxide. The cavity106may be filled with an index-matching material, such as silicone, or with an oxygen reducing gas, such as nitrogen, for example. Thereafter, a phosphor layer110may be provided on the recessed housing104, covering the cavity106.

The phosphor layer110is used in combination with the light emitting semiconductor102to create light with a range of color temperatures and spectral composition. The phosphor layer110may include a mixture of silicone and phosphor particles, which are evenly dispersed and suspended within the silicone. The phosphor particles may be of different colors (e.g., yellow, red, blue) in order to enhance a color rendering index of the light produced by the device100. The phosphor layer110may have a round disk-like shape in order to provide a uniform radiation pattern.

During operation, the light emitting semiconductor102may emit a blue light. A portion of the blue light may be absorbed by the phosphor particles of the phosphor layer110and the remaining blue light may pass through the phosphor layer110. Once the blue light is absorbed by a phosphor particle, the phosphor particle may emit a light of its respective color. This secondary emission of colored light from the phosphor particle, also known as a Stokes shift, is optically mixed with the remaining blue light, and the mixed spectra thus produced is perceived by the human eye as being white.

Unfortunately, the Stokes Shift process for converting blue light to other wavelengths in the phosphor is not 100% efficient. Each photon of blue light absorbed by the phosphor particle may not always produce a photon of a different wavelength. This lost energy is absorbed by the phosphor and is emitted as heat into the phosphor layer110. For small devices, this generated heat is very small and typically has no significant effect on the performance of the device. But for more powerful devices, such as those exceeding 1 watt in consumed electrical power, the amount of heat generated inside the phosphor layer becomes significant if it is not sufficiently dissipated. Excessive heat may consequently degrade the phosphor layer and reduce its efficiency that is, the phosphor layer will still absorb the same amount of radiant optical power, but will emit less light. As a result, the luminance may decrease and the color temperature may shift from white to blue, adversely affecting the performance of the device100. In order to dissipate the heat generated within the phosphor layer110, a heat dissipating structure may be integrated into the light emitting device, as shown inFIG. 2.

FIG. 2is a cross-section view illustrating an example of a device200having a heat dissipating structure with a refractory phosphor layer214. Light emitting semiconductor202, recessed housing204, reflective inner wall208, and cavity206ofFIG. 2correspond to the light emitting semiconductor102, recessed housing104, reflective inner wall108, and cavity106ofFIG. 1, respectively, and as such, their respective descriptions are omitted. The heat dissipating structure of device200may include a transparent layer210, a metal housing216, a metal substrate218, and fins220. The metal housing216, the metal substrate218, and the fins220may all be composed of a heat conductive material, such as copper, aluminum, aluminum nitride, or diamond, for example.

The phosphor layer214may be fused onto the transparent layer210so as to form an integrated glass-like layer. The transparent layer210may be a transparent and heat conductive material, such as, for example, glass, sapphire, or diamond. The phosphor layer214, after being fused onto the transparent layer210, may be a refractory glass-like layer including phosphor particles of one or a plurality of colors (e.g., yellow, red, green). The process of fusing the phosphor layer214onto the transparent layer210is described in detail later with reference toFIG. 3.

Once fused, the phosphor layer214and the transparent layer210may be provided on the recessed housing204, covering the cavity206. AlthoughFIG. 2shows the phosphor layer214as being located over the transparent layer210, the order of the layers may be reversed such that the phosphor layer214is located below the transparent layer210.

Optionally, a mirror212, such as a Bragg mirror (DBR), may be provided below the transparent layer210and the phosphor layer214. The mirror212, for example, may be composed of alternating titanium dioxide and silicon dioxide layers of a particular thickness. The mirror212may be designed to transmit short wavelength light (e.g., blue) that is emitted by the light emitting semiconductor202, but reflect the longer wavelength light (e.g., red, yellow) emitted by the phosphor layer214. This prevents the light rays emitted by the phosphor layer214from entering the cavity206where they may potentially become lost and instead reflects such rays out of the device200. As such, the mirror212may improve the efficiency of the device200.

The recessed housing204including the light emitting semiconductor202and the phosphor layer214, the transparent layer210, and the mirror212may be provided within a metal casing composed of the metal housing216and the metal substrate218. The metal housing216may be bonded to the metal substrate218by capacitance discharge welding or some other suitable method. The recessed housing204may be bonded to the metal substrate218by some suitable chemical and/or mechanical bonding method. Once within the metal casing, the phosphor layer214, the transparent layer210, and the mirror212may be secured to the recessed housing204by some method suitable to hermetically seal the cavity206. For example, the layers214,210,212may be crimped to the recessed housing204by mechanically folding over the edges of the metal housing216, as shown inFIG. 2. By hermetically sealing the device200in such a manner configures the device200to withstand extreme fluctuations in temperature, pressure, and other environmental conditions.

In addition to providing a hermetic seal, crimping the layers214,210,212via the metal housing216ensures that the phosphor layer214and the transparent layer210are thermally coupled to the metal housing216, which itself is thermally coupled to the metal substrate218and the fins220, establishing the thermal conduction circuit of the heat dissipating structure.

During operation of the device200, the heat generated by the phosphor particles in the phosphor layer214may be dissipated from the phosphor layer214to the metal housing216via the phosphor layer214itself as well as the transparent layer210. The metal housing216transfers the heat to the metal substrate218, which in turn vents the heat to the outside environment via the fins220. As such, the phosphor layer214is cooled, preventing the degradation of the phosphor layer214.

FIG. 3is a flow-chart diagram300illustrating an example of a process for combining the phosphor layer214and the transparent layer210. The process begins and proceeds to block302, where various components of the phosphor layer214are mixed. For example, a specific amount of phosphor powder may be mixed with a specific amount of glass frit, organic binder, and glass flux. The phosphor powder, for example, may be of a particular color or a combination of colors (e.g., yellow, red, green) and of a particular type, such as garnet structure phosphors (e.g., yttrium aluminum garnet, terbium aluminum garnet), sulfide phosphors (e.g., zinc sulfide, strontium sulfide), selenide phosphors (e.g., cadmium selenide, zinc selenide), silicate phosphors (e.g. barium silicate, strontium silicate, calcium silicate) and alkali halide phosphors (e.g., cesium chloride, potassium bromide). The phosphor powder may contain phosphor particles having a diameter of about 3 μm to 25 μm, but is not limited thereto. The glass frit may be any suitable type of powdered glass. The organic binder may be any suitable organic dispersant that is burnt off during firing at or below 600° Celsius., and may contain compounds such as zinc oxide, lead oxide, and borax, for example. The phosphor powder, glass frit, organic binder, and glass flux may be mixed in order to effectively mix and degas the mixture so that the phosphor particles are suspended and evenly dispersed within the mixture and the mixture is substantially devoid of gas bubbles.

Once the mixture is prepared, the process proceeds to block304, where the mixture is uniformly deposited onto a transparent substrate, such as the transparent substrate210, via screen-printing, stenciling, or some other suitable method. A device, such as that used for manufacturing circuit boards, may be used for this purpose. The mixture may be deposited to cover all or a portion of the transparent layer as one continuous layer, a particular pattern, or an array of dots, for example. The thickness of the deposited mixture may be controlled to obtain a desired final thickness of the phosphor layer.

After the mixture is deposited, the process proceeds to block306, where a determination is made as to whether all of the desired phosphors are present on the transparent layer. If it is determined that not all of the desired phosphors are present on the transparent layer, then the process proceeds to block308. At block308, the mixture is dried for a predetermined amount of time, and the process proceeds back to block302where a phosphor powder of another type and/or color is mixed with the glass frit, organic binder, and glass flux. The process then proceeds down through blocks302-306until all of the desired phosphors are present on the transparent layer.

In such a case where the process undergoes an iteration for each different phosphor powder, in block304, each phosphor mixture may be deposited as a particular pattern and/or an array of dots on the transparent layer. The resulting phosphor layer may thus be a combination of patterns and/or dot arrays of different phosphor mixtures. This may be done with a specific lithographic pattern when screen-printing each mixture. The array may be such that each phosphor mixture is deposited so as not to overlap with a neighboring phosphor mixture. It may be desirable to deposit the different phosphors in such an array to decrease the absorption of light by neighboring phosphor particles of different color. Furthermore, depositing each phosphor mixture separately allows for incompatible phosphor mixtures to exist within the resulting phosphor layer, wherein the incompatible phosphor mixtures are localized within their respective areas within the array.

If at block306it is determined that all of the desired phosphors are present on the transparent layer, then the process proceeds to block310.

At block310the transparent layer with the deposited mixture is fired in a furnace where the mixture is fused to the transparent layer. The furnace, for example, may be a multi-zone belt furnace where the mixture is heated to a specific temperature (e.g., 600° Celsius.), cooled, and annealed within a period of 30-40 minutes. As the mixture melts and fuses to the transparent layer it acquires a refractive glass-like property (i.e., it becomes a refractive phosphor layer). Due to the similar inorganic compositions of the refractive phosphor layer and the transparent layer, the resulting bond between these layers may include exceptional chemical and optical characteristics.

After the refractive phosphor layer is fused to the transparent layer in block310, the process proceeds to block312where the refractive phosphor layer and transparent layer are cut into discs of a predetermined shape (e.g., circle, square) by a die cutter or a similar device. After block312, the process ends.

Prior to attaching the discs to their respective optical devices, such as the device200shown inFIG. 2, each disc may be tested for various performance characteristics (e.g., color temperature).

LEDs with a heat dissipation structure including a refractive phosphor layer may be used in numerous applications. By way of example, these LEDs may be well suited for liquid crystal display (LCD) backlighting applications. Other applications may include, but are not limited to, automobile interior lighting, light bulbs, lanterns, streetlights, flashlights, or any other application where LEDs are used.