Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Heterojunction formed between semiconductor materials which...
Reexamination Certificate
2002-06-27
2004-06-08
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Heterojunction formed between semiconductor materials which...
C257S190000, C257SE21122, C257SE33054, C438S022000, C438S046000, C438S507000
Reexamination Certificate
active
06747298
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor devices, and more particularly to devices for use in mounting semiconductor devices to a submount in a flip-chip configuration.
BACKGROUND OF THE INVENTION
GaN-based light emitting diodes (LEDs) typically comprise an insulating or semiconducting substrate such as SIC or sapphire on which a plurality of GaN-based epitaxial layers are deposited. The epitaxial layers comprise an active region having a p-n junction that emits light when energized. A typical LED is mounted substrate side down onto a submount, also called a package or lead frame (hereinafter referred to as a “submount”).
FIG. 1
schematically illustrates a conventional LED having an n-type SiC substrate
10
, an active region
12
comprising an n-GaN-based layer
14
and a p-GaN-based layer
16
grown on the substrate and patterned into a mesa. A metal p-electrode
18
is deposited on the p-GaN layer
16
and a wire bond connection
28
is made to a bond pad
20
on the p-electrode
18
. An n-electrode
22
on the conductive substrate is attached to metallic submount
24
using a conductive epoxy
26
. In the conventional process, the conductive epoxy
26
(usually silver epoxy) is deposited on the submount and the LED is pressed into the epoxy
26
. The epoxy is then heat cured which causes it to harden, providing a stable and electrically conductive mount for the LED chip. Light generated in the active region
12
is directed up and out of the device. However, a substantial amount of the generated light may be transmitted into the substrate and absorbed by the epoxy
26
.
Flip-chip mounting of LEDs involves mounting the LED onto the submount substrate side up. Light is then extracted and emitted through the transparent substrate. Flip chip mounting may be an especially desirable technique for mounting SiC-based LEDs. Since SiC has a higher index of refraction than GaN, light generated in the active region does not internally reflect (i.e. reflect back into the GaN-based layers) at the GaN/SiC interface. Flip chip mounting of SiC-based LEDs may improve the effect of certain chip-shaping techniques known in the art. Flip chip packaging of SiC LEDs may have other benefits as well, such as improved heat dissipation, which may be desirable depending on the particular application for the chip.
One problem with flip-chip mounting is illustrated in FIG.
2
. Namely, when a chip is flip-chip mounted on a conductive submount or package using conventional techniques, a conductive die attach material
26
is deposited on the chip and/or on the submount
24
, and the chip is pressed into the submount
24
. This can cause the viscous conductive die attach material
26
to squeeze out and make contact with the n-type layers
14
and
10
in the device, thereby forming a Schottky diode connection that short-circuits the p-n junction in the active region with predictably undesirable results. Thus, improvements in the flip-chip mounting of LEDs may be desirable.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide for bonding of flip-chip mounted light emitting devices having an irregular configuration. Certain embodiments of the present invention bond a light emitting diode having a shaped substrate to a submount by applying forces to the substrate in a manner such that shear forces within the substrate do not exceed a failure threshold of the substrate. Such bonding may, for example, be provided by thermosonic and/or thermocompression bonding. In certain embodiments of the present invention, a light emitting diode is bonded to a submount is by applying force to a surface of a substrate of the light emitting diode that is oblique to a direction of motion of the light emitting diode to bond the light emitting diode to the submount.
In particular embodiments of the present invention, force is applied to the shaped substrate by mating a collet to the surface of the substrate oblique to the direction of motion and moving the collet in the direction of motion. Such a mating of the collet may be provided by seating a collet having a mating surface corresponding to an oblique surface of the substrate so that the mating surface of the collet contacts the oblique surface of the substrate. In certain embodiments of the present invention, the mating surface of the collet is a fixed surface relative to a body of the collet. In other embodiments of the present invention, the mating surface of the collet is a moveable surface relative to a body of the collet.
In still further embodiments of the present invention the collet is seated by placing the collet over the light emitting diode and applying a vacuum pressure to the collet. In certain embodiments of the present invention, the light emitting diode is a gallium nitride based light emitting diode having a silicon carbide shaped substrate. In particular, the silicon carbide shaped substrate may have a cubic portion and a truncated pyramidal portion adjacent the cubic portion. In such cases, force is applied to sidewalls of the truncated pyramidal portion of the silicon carbide substrate.
In additional embodiments of the present invention, a collet for bonding light emitting diodes having a shaped substrate to a submount is provided. The collet has a body having a chamber therein and an opening in communication with the chamber and adapted to receive the light emitting diode. The collet also includes means for mating surfaces of the collet to surfaces of the shaped substrate that are oblique to a direction of motion of the collet.
In certain embodiments of the present invention, the means for mating surfaces of the collet to the shaped substrate is provided by fixed surfaces of the collet defining the opening and disposed at an angle corresponding to an angle of the surfaces of the shaped substrate that are oblique to the direction of motion. Furthermore, the body may include a top portion and vertical side portions defining the chamber and an opening for inducing a vacuum in the chamber.
In further embodiments of the present invention, the body includes spaced apart side portions defining the chamber. In such embodiments, the fixed surfaces of the collet may be provided by angled surfaces at a terminus of the side portions. Furthermore, the side portions may be spaced apart a distance corresponding to a dimension of the surfaces of the shaped substrate that are oblique to the direction of motion. In such embodiments, the body may also include a top portion and an opening for inducing a vacuum pressure in the chamber. The side portions may then be vertical side portions.
In still further embodiments of the present invention, the body includes a top portion and vertical side portions defining the chamber and an opening for inducing a vacuum in the chamber. In such embodiments, the side portions may be horizontal side portions that extend from the vertical side portions and are spaced apart from the top portion.
In additional embodiments of the present invention, the means for mating surfaces of the collet to the shaped substrate is provide by surfaces of the collet defining the opening and that are moveable with respect to the body and are configured to adjust to an angle corresponding to an angle of the surfaces of the shaped substrate that are oblique to the direction of motion. In such embodiments, the body may include a top portion and vertical side portions defining the chamber and an opening for inducing a vacuum in the chamber. Furthermore, the body may include spaced apart horizontal side portions. In such a case, the moveable surfaces of the collet may be provided by moveable end portions of the horizontal side portions, the moveable end portions being configured to substantially conform to the angle of the shaped substrate and are spaced apart a distance corresponding to a dimension of the surfaces of the shaped substrate that are oblique to the direction of motion.
In certain embodiments of the present invention, the moveable end portions are hinged to rotate about an end of the horizontal side po
Andrews Peter S.
Bharathan Jayesh
Edmond John
Mohammed Anwar
Negley Gerald H.
Cree Inc.
Myers Bigel Sibley & Sajovec P.A.
Nelms David
Nguyen Dao H.
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