Process for fabricating, and light emitting device resulting from, a homogenously mixed powder/pelletized compound

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of pending U.S. patent application Ser. No. 10/623,229 filed Jul. 18, 2003 entitled PROCESS FOR FABRICATING, AND LIGHT EMITTING DEVICE RESULTING FROM, A HOMOGENOUSLY MIXED POWDER/PELLETIZED COMPOUND, which is related to U.S. provisional patent application No. 60/397,082, filed Jul. 19, 2002, entitled PROCESS FOR PREPARING, AND DEVICE RESULTING FROM, A HOMOGENOUSLY MIXED POWDER/PELLETIZED COMPOUND FOR USE IN LIGHT EMITTING SEMICONDUCTOR DEVICES, the entire contents of which are incorporated herein by this reference. The Applicants hereby claim the benefits of this earlier pending provisional application under 35 U.S.C. Section 119(e).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to processes for preparing compositions used in the fabrication of light emitting semiconductor devices, and also relates to the devices resulting from the use of said process.

BACKGROUND OF THE INVENTION

LEDs are used in a variety of applications, such as displays, illumination in control panels, in traffic lights and electronic devices. An LED is a semiconductor device that converts electrical energy into optical energy. This occurs when a forward bias is applied to the device, resulting in holes in the P-type semiconductor material combining with the electrons in the N-type semiconductor material at the P-N junction. A “white” light emitting diode uses a blue light emitting diode as the source of the light. A luminous material layered over the blue light emitting diode acts to partly absorb the blue light and convert it into yellow or yellowish green light. When mixed, the blue and yellow/yellowish green light combine as white light.

A conventional method of producing white light emitting devices includes a process in which yttrium aluminate (“YAG”) is directly deposited onto a light emitting diode (“LED”) chip surface in order to produce LEDs that emit white light. However, a disadvantage of said process, and the resulting LED, is that the color temperature from such a white LED is not uniform over a plurality of angular positions due to the non-uniform thickness of the YAG over the LED chip surface. Further, this unequal distribution of color temperature occurs on a device by device basis within the same lot.

In addition, at elevated drive current, bleed-through of radiation in the ultra-violet (“UV”) range can occur. The use of the conventional process in producing these “white” light emitting devices disadvantageously results in package complexity, leading to limited ruggedness and decreased reliability of the devices.

SUMMARY OF THE INVENTION

The present invention comprises a process of mixing a luminous substance in powder form to a transferable grade molding compound in a pelletized or powder form, such as a clear epoxy, to derive a homogeneous mixture that can be pressed and sintered into solid pellets. The solid pellets are further processed so as to permit their deposition on and around a light emitting semiconductor driver so as to obtain a “white” light emitting semiconductor device. This “white” light emitting device can be used in a variety of lighting applications.

DETAILED DESCRIPTION OF THE DISCLOSED PROCESS AND DEVICE

The present invention discloses a process for adding a luminous substance in powder form to a transferable grade molding compound in pellet or powder form to derive a homogeneous mixture that can be pressed and sintered into pellets so as to obtain a “white” light emitting semiconductor device.

In an embodiment of the invention, the luminous powdered substance is a Cerium doped garnet, such as the inorganic luminous substance YAG:Ce. In this embodiment, YAG:Ce is homogeneously admixed with a pelletized molding compound, such as a clear epoxy. Although YAG:Ce has an index of refraction of about 1.84, resulting in good mixing of blue diode emissions with yellow/yellowish green converter radiation, any type of luminous substance in powder form can be used in the disclosed process. Other luminous substance powders that can be homogenously distributed within the molding compound include but are not limited to other garnets doped with rare earths.

The particle sizes of the luminous powdered substances are less than or equal to 5 microns and can be spherical or flake-like in shape. Different chromaticity can be obtained by adjusting the luminous powder's (i) percentage by weight of the combined material and (ii) micron size, before admixing and pelletizing. Because of the characteristics of the mixture, no sedimentation of the substance occurs and the mixture remains homogeneous. The powder and pellets must be stored in Nitrogen before and during production as they are susceptible to moisture. Because the luminous powder added to the epoxy changes its spiral flow, the transfer pressure must be increased 10% to 15% of manufacturer's recommended settings. As used with the Microsemi(r) PM3 package, a “color tunable” device can be produced. The disclosed process can also be used to convert the wavelength of blue, green or ultra-violet light of LEDs to other wavelengths.

By providing uniformity in concentration by weight and homogeneous disposition of the luminous substance within the molding compound, uniform distribution of color temperature can be achieved. Using the disclosed process to control the percentage weight of the luminous substance, such as YAG, to the molding compound, such as a clear epoxy, ensures uniform distribution of color temperature. Furthermore, use of the disclosed process allows a more robust injection molded package. This is accomplished by depositing the admixed substance around the LED chip atop a copper lead frame Specifically, pellets are taken from a freezer and stored in Nitrogen until they reached room temperature. They are then removed from the Nitrogen box and placed in a feed bowl to be loaded into a mold shuttle for transfer. The feed bowl and mold shuttle are purged with nitrogen to keep the pellets dry. The use of this process advantageously results in no UV bleed through at elevated drive current.

More specifically, the powder/pelletized composition includes, among other things, the following parts:

(a) molding compound, such as a clear epoxy;

(b) luminous substances, such as YAG:Ce;

(c) thixotorpic agent, such as pyrogenic silicic acid, to thicken the epoxy casting resin;

(d) mineral diffuser, such as CaF2, for optimizing the luminous pattern of the composition;

(e) processing adjuvant, such as glycol ether or surface modifiers based on silicone, to improve the compatibility between the epoxy and the luminous substance;

(f) hydrophobic agent, such as liquid silicone wax to modify the pigment surface; and

(g) adhesion promoters, to improve the adhesion between the pigments and the epoxy.

The innovative teachings of the present invention are described with particular reference to the disclosed embodiment. However, it should be understood that the embodiment provides only one example of the many advantageous uses and innovative teachings herein. Various alterations, modifications and substitutions can be made to the disclosed invention without departing in any way from the spirit and scope of the invention.