News Article

Mirror Adhesion Technique Boosts LED Chip Brightness

Donald Huo and colleagues from Epistar Corporation describe a new mirror adhesion technique for producing high-brightness and high-power AlGaInP LED chips.
One of the main reasons that high-brightness LEDs have yet to penetrate a variety of new applications is that the cost per lumen is still too high. There are two ways to expand the market: improving the external light efficiency; or reducing the manufacturing cost.

Improvements in epitaxial growth and device processing have pushed LED internal quantum efficiencies close to the limit, particularly for AlGaInP devices. Extracting more internally generated light from the device is the major challenge for the future.

There are many ways to improve extraction efficiency in AlGaInP devices. These include replacing the absorbing GaAs substrate with a non-absorbing substrate such as GaP, changing the chip geometry, using surface texturing or growing a semiconductor DBR.

Although DBR layers can reflect most of the light normal to the incident direction, they are non-reflective at oblique angles of incidence, which results in optical losses.

Besides such light trapping issues, AlGaInP and AlGaInN LED epitaxial films are grown on GaAs and sapphire substrates, respectively. These are poor materials for thermal dissipation, limiting the device s drive current and its resulting light output.

In 1994, Hewlett-Packard introduced the first commercial AlGaInP LED on a transparent GaP wafer fabricated using direct wafer bonding. Since then, several interesting metal-bonding techniques have been successfully developed by Osram, Sanken, Oriol, VPEC and UEC. These involve the transfer of epitaxial layers to an electrically and thermally conductive substrate.

However, very few of those devices are in mass production and commercially available. The problems may be due to the difficulties in preventing reactions between the reflector metals, the solder materials and the epitaxial layers, while at the same time overcoming adhesion problems.

Omnidirectional mirror adhesionEpistar Corporation has recently developed a novel technique, omnidirectional mirror adhesion (OMA), to mass-produce a new type of AlGaInP LED. The extraction efficiency of the new device is improved by reflecting the downward light using a metal mirror. OMA LEDs are made using existing equipment and there is virtually no restriction on output capacity.

To eliminate the chance of reactions between the reflector metal and the solder materials, a high-temperature stable organic film is used as the bonding agent instead of a solder metal. Also, n and p ohmic contacts are fabricated on the same side of the chip to provide more freedom of substrate selection and to build a platform for flip-chip applications.

Table 1 compares direct wafer bonding, eutectic metal bonding and OMA techniques. One of the key advantages of OMA is the relatively low bonding temperature.

Figure 1 shows an OMA high-brightness AlGaInP LED chip. To fabricate the device, an ITO film with a thickness of 3000 Å is deposited on top of the AlGaInP-on-GaAs epiwafer. Then a film of commercially available organic material is laid on top of the ITO layer. This wafer is then bonded to a silicon substrate covered with an Al layer that acts as the reflector. After bonding, the light-absorbing GaAs substrate is removed by chemical etching, and a 3000 Å thick ITO film is deposited to form the n-side transparent electrode. The p contact area is then patterned and etched to stop at the ITO layer. The p and n ohmic and bonding metals are then deposited. After probing, the bonded device wafer is sawn into individual discrete devices.

The advantage of the OMA technique is that the reflectivity of the metal mirror is extremely high, even after bonding and ohmic contact formation. With metal bonding, on the other hand, the reflectivity of the mirror may be reduced during the bonding process. As a result, OMA improves the light extraction efficiency.

Device characterizationFigure 2 shows the L-I (luminosity-current) characteristics of a high-brightness OMA AlGaInP LED chip with a size of 12 x 12 mil2 (approximately 300 x 300 µm2) together with a standard absorbing-substrate (AS) device. The light output of the OMA chip, which saturates at around 200 mA, is 3 times brighter than the AS type, which saturates at around 160 mA. On the other hand, an AlGaInP LED chip with a semiconductor DBR has a light output 1.7 times higher than the AS-type chip.

The reliability of OMA chips was tested at 50 mA and 55 ºC for 1000 h; the results were comparable to standard AS-type chips with only slight changes in light output after 1000 h operation. Epistar has also performed harsh thermal cycle testing for evaluation of the bonding interface. The results indicate that despite the use of an organic adhesion layer, the bonding interface is very robust and no failures have been observed.

Large-area chipsRecently, large-sized power LED chips have also attracted lots of attention. They have found applications in outdoor signage, traffic signals, automotive exterior lighting, and medium- to large-sized LCD backlighting. They will eventually be used in general illumination. Epistar has just completed the development of two different sizes of LED chip with areas of 22 x 22 mil2 (approximately 560 x 560 µm2) and 40 x 40 mil2 (approximately 1 x 1 mm2). Figure 3 shows the top view of two OMA high-power AlGaInP LED chips, driven with a low current to show the quality of the lighting areas.

Table 2 shows the luminosity of the high-power AlGaInP LEDs at three different chip sizes. The largest chip (1 x 1 mm2) generated a light output of 44.5 lm at a drive current of 500 mA, and had a saturation current of 1.5 A. In reliability tests, the 22 x 22 mil2 OMA high-power chips were driven at 150 mA on a PC board at 25 ºC for 1000 h. The results again showed excellent behavior, with the light output dropping to 95% after 100 h operation and to about 92% after 1000 h.

Future workHaving successfully developed this high-power AlGaInP LED chip technology, Epistar commenced mass production of the devices in the fourth quarter of 2003. To further improve the drive current limitation, the company has modified the OMA technique to replace a large portion of the silicon substrate with plated copper; this improves the thermal conductivity for higher-power applications. The new AlGaInP high-power LED devices with copper-plated silicon substrates will be ready for sampling in early 2004.
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