Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
1999-05-24
2001-04-24
Smith, Matthew (Department: 2825)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C257S079000, C257S081000, C257S082000, C257S091000, C257S099000
Reexamination Certificate
active
06222207
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to light-emitting diode lamps and more particularly to a light-emitting diode chip having a back reflector.
DESCRIPTION OF THE RELATED ART
Light-emitting diode (LED) lamps utilize LED chips that are attached to lead frames which conduct excitation signals to initiate the generation of light. LEDs are well-known solid state devices that can emit light having a predefined spectral distribution. LEDs are widely used as illuminators, indicators and displays. In a typical LED lamp, the LED chip includes a layer of epoxy that bonds the LED chip to the lead frame during a die attach process.
Some types of conventional LEDs have semiconductor layers which are transparent to the emitted light and are located between the active layer and the lead frame. Such is the case for InGaN on sapphire, Transparent Substrate-AlInGaP (AlInGaP wafer bonded to a transparent GaP substrate) and GaP LEDs, which all have substrates that are transparent to the emitted light.
A concern with conventional LED lamps is that a significant amount of emitted light from the LED chip may be absorbed by the underlying material, which lowers the light output power (LOP) of the lamp. A typical method of alleviating this concern is to include silver (Ag) in the layer of epoxy on the LED chip, with the Ag functioning as a reflector. With the inclusion of Ag in the epoxy layer, some of the light that would have been absorbed by the underlying material is reflected by the Ag and is emanated from the LED chip as output light, thereby increasing the LOP of the lamp.
However, a high power LED chip recently developed by Hewlett-Packard Company, the assignee of the invention disclosed herein, requires a bonding layer and package that has a much lower thermal resistance than the epoxy/Ag layer. The high power LED chip is described in an article entitled “High-flux high-efficiency transparent-substrate AlGaInP/GaP light-emitting diodes,” by G. E. Höfler et al.,
Electronic Letters
, Sep. 3, 1998, Vol. 34, No. 18. The high power LED chip utilizes a layer of soldering material, instead of the epoxy/Ag layer, to bond the LED chip to a lead frame or a die pad, which also serves as a heat sink. Preferably, the soldering material has a low thermal resistance to lower the operating temperature of the LED chip, which improves light output and reliability. In addition, the high power LED chip includes a layer of reflective material to function as a reflector that increases the LOP of the lamp embodying the high power LED chip. The reflective material is chosen such that the reflector has a high reflectivity with respect to emitted wavelengths (>80%) and has a good thermal conductivity.
In
FIG. 1
, a high power LED chip
10
having an LED
12
and a layer
14
of soldering material is shown. The LED is an AlInGaP LED having an active layer
16
, where light is generated in response to applied electrical energy. The generated light is emitted in all directions, as illustrated by the arrows near the active layer. Attached to the lower surface of the LED is ohmic contact
18
. The ohmic contact is covered by an Ag reflector
20
. The high power LED chip
10
also includes an ohmic contact
21
, located on the upper surface of the LED. When the LED is operating, some of the light generated by the active layer of the LED propagates away from the lower ohmic contact, emanating from the LED as output light
22
. However, some of the light propagates toward the ohmic contact. A portion of this light impinges the lower ohmic contact
18
and may be absorbed. Another portion of this light, however, impinges the Ag reflector, which operates to reflect the impinging light out of the chip. Thus, the intensity of the output light is increased by the light reflected from the Ag reflector.
The Ag reflector is located between the LED and the solder layer
14
. The soldering material in the solder layer is indium (In). The solder layer allows the LED chip to be attached, or bonded, to an external surface (not shown).
During a high temperature process, such as a die attach process which is performed at a temperature above the melting point of the solder layer
14
(for In, the melting point is approximately 156 degrees Celsius), the Ag reflector
20
and the In solder layer
14
can intermix, reducing the reflectivity of the Ag reflector from approximately 95% to approximately less than 70%. This results in an LOP reduction of approximately 15%-20% in packaged devices.
Therefore, what is needed is a solderable high power LED chip that includes a reflector that results in high reflectivity, even after the LED chip has been subjected to high temperature processes, such as the die attach process.
SUMMARY OF THE INVENTION
A solderable light-emitting diode (LED) chip and a method of fabricating an LED lamp embodying the LED chip utilize a diffusion barrier that prevents appreciable intermixing of two different layers of the LED chip during high temperature processes. The diffusion barrier is formed of a material that appreciably blocks migration between the two layers of concern when the layers are subjected to an elevated temperature. In the preferred embodiment, the two different layers of the LED chip are a back reflector and a solder layer. By preventing the intermixing of the materials of the back reflector and the solder layer, the diffusion barrier functions to impede degradation of the back reflector with respect to its ability to reflect light emitted by the LED. The diffusion barrier should block migration into and intermixing of the solder layer into the reflector such that the reflectivity of the reflective layer is not appreciably reduced (i.e., more than 10% decrease in reflectivity) at the surface between the reflector and the LED chip. The diffusion barrier should maintain structural integrity, i.e. remain a barrier to diffusion, even at the elevated temperatures necessary to melt the solder layer.
In a first embodiment of the invention, the LED chip includes a high power AllnGaP LED. However, the type of LED included in the LED chip is not critical to the invention. Any LEDs with semiconductor layers transparent to the emitted light, located between the active layer and the solder, would benefit from this invention. Attached to the back surface of the LED is a back reflector. The back surface is the surface opposite to the light-emitting surface of the LED. Preferable for AlInGaP LEDs, the back reflector is composed of silver (Ag) or Ag alloy that has been sputtered on the back surface of the LED. However, the reflector should be optimized for the wavelength of the emitted light. Good reflectors will have reflectivity >90%. Examples of other reflectors include Al or Ag for AlGaN and Au for transparent substrate-AlGaAs LEDs. The back reflector may be formed by evaporation, electroplating, or other suitable techniques. Situated adjacent to the back reflector is the diffusion layer. In this embodiment, the diffusion layer is made of nickel (Ni) or nickel-vanadium (NiV). If the back reflector is sputtered, NiV is preferred over Ni since the NiV can be sputtered on the back reflector. The use of a sputtering process to form the diffusion barrier allows the back reflector and the diffusion barrier to be formed in a single fabrication system. Alternatively, the diffusion barrier may be formed by evaporating or electroplating Ni.
The LED chip also includes a solder layer that is affixed to the diffusion layer, such that the back reflector and the solder layer are separated by the diffusion layer. The solder layer may be made of indium (In), lead (Pb), gold (Au), tin (Sn) or their alloy and eutectics. The solder layer allows the LED chip to be mounted on an integrated heat sink, also known as the slug, or a die pad during a die attach process. The die attach process involves melting the solder layer of the LED chip to physically bond the LED chip to the slug or the die pad. However, the die attach process involves exposing the back reflector, the diffusion barrier and the solder layer to a temperat
Carter-Coman Carrie
Hofler Gloria
Kish, Jr. Fred A.
Keshavan Belur
Leiterman Rachel V.
LumiLeds Lighting U.S., LLC
Ogonowsky Brian D.
Skjerven Morrill & MacPherson LLP
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