This article was originally published in the April/May 2012 issue of our sister publication LEDs Magazine and is republished here with permission.
May 21, 2012 -- LED manufacturers must choose the appropriate materials and processes to fight low yields. This is especially true in wafer bonding for vertical LEDs.
LED manufacturers in different regions of the world are confronting similar challenges concerning wafer bonding, particularly in the processing of vertical light-emitting diode (VLED) chip architectures. VLEDs offer certain key advantages over their lateral LED counterparts, though the lateral approach is a simpler manufacturing. This article covers the differences -- in terms of processing and the optimization of light output -- between vertical and lateral LEDs.
Both LED designs begin with the epitaxial growth of gallium nitride (GaN) on a sapphire substrate, but that is the end of their similarities. In a lateral LED design, the sapphire remains a part of the GaN LED stack. Since sapphire is an insulator, both contacts to the LED diode structure must be formed at the topside of the LED die, taking up valuable device real estate.
A simple back-of-the-envelope calculation of surface loss for a 4” LED wafer, assuming 300 x 300µm die and 100 x 100µm wire-bonding pads, reveals that each diode contact, to p-doped and n-doped GaN, consumes about 10% of the wafer surface.
In contrast, VLEDs are formed by full-wafer deposition of a metal-film stack, followed by wafer bonding with a carrier substrate. Since one electrical contact is the bonding layer itself and hence buried inside the LED stack, VLEDs immediately save the aforementioned 10% of real estate. In addition, electrical injection is more efficient for VLEDs, where lateral LED have difficulties, especially with higher current density.
Optimizing light output
Optimizing the LED’s real estate and electrical efficiency is only one aspect of the process: getting the light output from the LED remains a challenge. In GaN-based LEDs, the crystal planes of the GaN lead to a concentrated light emission normal to the sapphire's c-plane, i.e., normal to the LED surface. In lateral LED designs, photons also couple into the transparent sapphire wafer, so that light is also emitted from the LED’s sidewalls. Since losses are higher, efficiency is decreased.
To increase light output in VLEDs, a metallic mirror is deposited prior to the metal bonding layers. The mirror will redirect emitted light to the LED surface. Light extraction is further optimized by creating a resonant cavity, and with surface roughening. Light extraction efficiency improves and the light is well directed to the user.
Added complexity with wafer bonding
If the benefits are so profound, why don’t all manufacturers produce VLEDs? One reason is a complex patent situation. In addition, LED makers must thoroughly understand the wafer bonding step to achieve high process yield. In VLEDs, the bonding layer is multifunctional. As electrical contact to the p-GaN, the bonding layer needs high conductivity to reduce ohmic losses. As the heat transfer layer between the LED and the heat sink, the bonding layer needs to have high thermal conductivity.
From a material standpoint, many eutectic metal systems (e.g. gold/tin, Au/Sn) or diffusion solders (e.g. gold/indium, Au/In) fulfill these requirements. However, each presents different processing requirements. The metal system determines the bonding temperature. Because the sapphire substrate and the carrier substrates have quite different coefficients of thermal expansion (CTE), a metal system with low bonding temperature will keep strain at a more manageable level. The selection of these layers is beyond the scope of this article, but typically metal layers such as platinum, aluminum, and gold, or combinations of these materials, are used.
Next, adhesion and diffusion barriers have to be chosen to contain the diffusive metals from the injection contacts or mirror layer of the LED structure. The correct choices will result in a high-yield layer transfer process.
GaN-on-silicon: the rookie
The potential use of GaN-on-silicon in LED manufacturing is an exciting prospect that seems likely to come to fruition in the next several years. Announcements by Osram Opto Semiconductors, Samsung LED (now Samsung Electronics), and Bridgelux have indicated that companies are 2-3 years from entering mass production, with laboratory LED efficiencies comparable to LEDs on sapphire. With a silicon substrate, wafer bonding provides one of the enabling steps of transferring the LEDs after growth.
Wafer bonding is a sophisticated process that requires extensive knowledge of material science. However, given the right material selection and process expertise, it can prove enabling when bringing up stable, next-generation LED manufacturing processes.
Thomas Uhrmann, Ph.D., is Business Development Manager, EV Group (EVG). Learn more about the company at http://www.evgroup.com/en.