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Metal Bonding Delivers High-power AlGaInP-on-silicon LEDs

Wafer bonding of AlGaInP LED epilayers to silicon substrates using metallic interlayers results in very-high-power devices, write Pai Hsiang Wang and colleagues from United Epitaxy Company.
United Epitaxy Company (UEC) has released the world s first 10 watt high-power LED. The device is made by bonding an AlGaInP LED epiwafer to a silicon substrate, after which the light-absorbing GaAs substrate is removed. A high-reflectivity metal layer is also inserted between the LED structure and the silicon substrate, in order to reflect the light that would otherwise reach the light-absorbing silicon substrate. Using both metal reflector and metal bonding technologies, a single high-power light emitter with a chip size of 2.5 x 2.5 mm2 can provide a total flux of more than 200 lumens.

The introduction of high-brightness AlGaInP red, orange and yellow LEDs in the early 1990s, followed by AlGaInN blue, green and white devices, has created many new applications - such as traffic signals, automotive lighting and full-color outdoor displays - for these highly efficient solid-state lighting sources.

Highly efficient LEDs have also started to replace conventional incandescent light bulbs and halogen lamps. However, although LEDs are more reliable than these other light sources, the extent of replacement has been quite low. One of the major reasons for this is that the price, in lumens per dollar, is too high. Incandescent bulbs cost about $0.001 per lumen, while LEDs can only achieve around $0.1 per lumen, which is about two orders of magnitude higher.

The other reason for the low penetration rate of LEDs into conventional lighting is that the total flux from a standard 5 mm LED package operating at 20 mA is only about 1 lumen. Many of the applications that use conventional incandescent bulbs or halogen lamps need a few hundred or even several thousand lumens. Hundreds or thousands of LEDs are therefore needed to achieve the same total light output, and the total assembly size can then be too large for some applications.

Improving performance

There are several different approaches to solving the above problems. First, if the luminous efficiency of a single 0.3 x 0.3 mm2 single emitter can be improved, the dollars per lumen value can be reduced. Luminous efficiency can be increased in several ways, including improvements in material quality, better design of LED epiwafer structure, using non-absorbing substrates, or using chip shaping or surface texturing to achieve higher light-extraction efficiency.

Second, the current injected into the LED per unit chip area (i.e. the current density) can be increased. If the light output versus drive current is quite linear up to higher current values without saturation, then the total flux from a single LED driven by higher current with the same chip size should be several times higher compared with operating just at 20 mA. The maximum current that can be injected into the LED chip is determined by two main factors. The operating current has to remain below the level that causes significant degradation, and also below the level at which the efficiency (in lm/W) starts to fall rapidly. LEDs are normally operated at a current density of around 20 A/cm2. However, some of the latest high flux LEDs can be operated at 70-80 A/cm2.

Removing heat

Another approach is to adopt chip and package designs that provide better heat dissipation. AlGaInP and AlGaInN LED epiwafers are grown on GaAs and sapphire (or SiC) substrates, respectively. As shown in the table, GaAs and sapphire substrates have relatively low thermal conductivity values of 44 and 35 W/m.K, respectively.

There are three different approaches to remove heat and lower the junction temperature more effectively. The first technique is to thin the substrate, while the second is to flip the chip, which positions the light-emitting p-n junction close to the heat sink. The third approach is to remove the original substrate that was used for growing the LED epilayers, and then transfer the epitaxial structure to an electrically and thermally conductive substrate.

The first approach is the easiest, but it is difficult to thin the substrate to less than 100 µm. The second approach is now used by Matsushita and Lumileds to improve both luminous efficiency and heat dissipation in GaN LEDs. However, the flip-chip process is complicated and expensive.

Many companies, including UEC, VPEC and AET in Taiwan, Osram Opto in Germany, Sanken in Japan and Oriol in the US, have used the third approach and announced the successful development of metal bond-ing (MB) LEDs, in which metal layers are used to bond the LED to a conductive substrate. This approach is now emerging as the most cost-effective method to make high-power LED chips.

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