This invention relates to the field of light emitting devices, and in particular to a light emitting device with improved efficiency by providing a highly reflective coating on the substrate that supports the light emitting device.
The need for energy efficient illumination has accelerated the demand for efficient light emitting devices, particularly in view of the fact that solid state light emitting devices are now able to provide white light by mixing light of multiple wavelengths.
In a typical solid state multiple wavelength light emitting device, an active element of a light emitting structure provides light of a first wavelength, and a wavelength conversion element converts some of this light into light of a second wavelength. The wavelengths provided by the light of the light emitting element and the wavelength conversion elements, and the proportions of each, are designed/selected so as to produce a desired combination of these lights, such as a white light at a desired color temperature (CCT).
FIG. 1A illustrates a conventional structure of a multi-wavelength light emitting device. The device comprises a stack of elements 110, which includes an active element 112 that emits light of a first wavelength, and a wavelength conversion element 114, typically comprising a phosphor, that converts some of light at the first wavelength to light of a second wavelength. The materials of the elements 112 and 114 are selected so as to provide desired light output characteristics based on a mix of the light of the two wavelengths. The stack 110 is built upon a substrate 120, which is typically a ceramic, such as alumina, and encapsulated by an optical element 130.
Although most of the emitted and converted light is output from the optical element 130, some of the light 101 is internally reflected or otherwise misdirected, resulting in a loss of efficiency. In most lighting applicants, the desired light output is toward a particular direction, and reflectors are used to re-direct light toward that direction. A technique commonly used to direct the light output of an LED is to include the LED chip within a reflector cup. However, with the advent of die on ceramic (DoC) LED packaging, the use of a reflector cup is not generally economically viable.
A number of techniques have been proposed for increasing the efficiency of a light emitting device on a substrate. U.S. Patent Application 2010/0038655, “REFLECTIVE LAYER FOR LIGHT EMITTING DIODES”, filed 13 Nov. 2008 for Chen et al., for example, discloses the placement of a reflective layer 140 upon the substrate and adjacent the stack 101, as illustrated in FIG. 1B. In this manner, the internally reflected light 101 is reflected back up to the lens element 130, thereby potentially reducing the loss of efficiency caused by this internally reflected light 101. However, at least a portion of the light 101 that is reflected into the stack 110 is likely to be absorbed by either the elements of the stack 110 or the substrate 120.
As illustrated in FIG. 1C, if the substrate 120 is transparent, or semi-transparent, a reflective layer 150 can by positioned below the substrate 120 so as to reflect any light (102 of FIG. 1A) that would otherwise escape through the substrate 120. However, because most substrates 120 are not transparent, per se, some light will be lost/absorbed with each transit through the substrate 120.
In U.S. Patent Application 2008/0062701, “LIGHT EFFICIENT LED ASSEMBLY INCLUDING A SHAPED REFLECTIVE CAVITY AND METHOD FOR MAKING SAME”, filed 29 Aug. 2007 for Harrah et al., a reflective cavity 160 is formed for mounting a plurality of stacks 110 within the light emitting device, as illustrated in FIG. 1D. However, this arrangement does not allow the stack 110 to be mounted on the substrate 120.
It would be advantageous to provide a method for producing a light emitting device with enhanced luminance output. It would also be advantageous to provide such a light emitting device with enhanced luminance output. It would also be advantageous to minimize light loss and/or absorption through the substrate used to mount the light emitting and light conversion elements.
These advantages, and others, can be realized by providing a submount that includes a substrate and a metallization layer having circuit traces and a planar dielectric layer that fills regions between the circuit traces. The planar dielectric layer serves to minimize the amount of light lost/absorbed by the substrate and preferably reflects the internally reflected light back toward the desired light output element. To facilitate efficient manufacture, a dielectric paste is applied over the metallized layer, cured, and then planed to expose at least portions of the metal conductors for the subsequent coupling to the light emitting stack. Pedestal elements are preferably provided at select locations on the circuit traces to facilitate this coupling while allowing the remainder of the circuit traces to be covered with the dielectric layer.
Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.