The present invention relates to semiconductor devices, and more particularly to light emitting diodes mounted to a submount in a junction-down configuration.
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. 4 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. A substantial amount of the light generated in the active region 12 may be transmitted into the substrate and absorbed by the epoxy 26.
Junction-down (or “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. Junction-down 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. Junction-down mounting of SiC-based LEDs may improve the effect of certain chip-shaping techniques known in the art. Junction-down 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 junction-down mounting is illustrated in FIG. 5. Namely, when a chip is mounted junction-down 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. Alternatively, the conductive die attach material 26 may comprise a solder such as Sn or Au/Sn in which case the chip is bonded to the submount 24 by thermocompression bonding.
Thermocompression bonding is a technique whereby a device is mounted to a substrate or submount using heat and pressure, thereby creating a conductive bond between the device and the submount. Typically, a vacuum collet is used to pick up the device and physically place it in contact with a submount that is formed of a material with which the solder used may form an alloy. Once the device is in contact with the submount, force is applied to the device through the collet. Through a combination of heat and pressure, the solder becomes alloyed with the submount and the device is welded in place. In order to form such a bond, the device must include a metal pad layer made of a metal such as Sn that will form an alloy bond with the submount when heat and pressure are applied. Other metals and alloys having a sufficiently low melting point are Au/Sn, Pb/Sn, and Ag/Sn may be used. Some suitable submount materials are silver and gold.
Typical thermocompression processes utilize a minimum force of about 30 to 50 g to cause the die to become bonded to the submount. However, this force may cause some of the molten bond metal to squeeze out and form a shunt circuit between the n-type substrate and the submount around the p-n junction, degrading the device operation.
Accordingly, as illustrated in FIG. 5, the conductive die attach material 26 can 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 design of LEDs may be desirable for improved junction-down mounting.