SemiLEDss Corp. Builds Cost-effective LED Lighting with Cu Chip

By KEN LIU

Perhaps revolutionizing costly LED lighting, the blue kind Nichia Corp. of Japan invented in 1993 by building GaN structure on sapphire wafer, an Al2O3 composite that has since been the mainstay of LED chip material, and adopting copper alloy wafer, a crucial technology of silicon chip manufacturing, Trung T. Doan, chairman and chief executive officer (CEO) of LED chipmaker SemiLEDs Corp, is fulfilling the corporate mission to supply the globe with affordable LED.

“Sapphire wafer is very expensive in energy-input relative to copper alloy,” explains Doan, a veteran in the silicon chip industry, in an interview with CENS Lighting Magazine. The company's data shows that a two-inch sapphire wafer takes over 200 kilowatt-hour of electricity to make, hence an energy glutton, despite being hard, transparent and resistant to heat and erosion.

Also tapping copper alloy's other advantages, Doan says copper alloy is superior in thermal conductivity relative to sapphire, showing a bar graph of thermal conductivity of six wafer substrates, where sapphire is only around 50 W/m-K compared with copper alloy's 400 W/m-K.

Better Cooling
Doan notes that better thermal conduction of copper alloy junctions cools faster than LED devices to achieve longer life cycle and improved performance, pointing out the criticality of thermal-conductivity of wafer compared to cooling of LED lighting's heat sink.

A man involved in microchip technology since 1981, Doan started with Intel, and then Philips before being a vice president of process development at Micron Technology Inc. and as vice president of Applied Global Services Product Group at Applied Materials Inc.

In January 2005, Doan began leading SemiLEDs, the world's No.2 high-power LED chipmaker by shipment volume. Bending a 4-inch copper wafer, he says that SemiLEDss is the only one using copper alloy wafer in volume production, stressing, despite sapphire being the mainstream LED wafer substrate, that copper alloy is an optimal substrate for being a mature process proven in microchip manufacturing.

On copper-alloy wafers, the company builds GaN epitaxy layers in vertical way, creating a unique chip called Metal Vertical Photon Light Emitting Diode (MvpLED).

Vertical structure assures MvpLED chip's excellent electric conductivity while copper alloy has superb thermal conductivity, resulting in an ideal high power device by allowing larger, freer current flows than horizontal structures.

“Our LED technology is the Holy Grail for lighting and we focus on general lighting because that's our passion,” Doan stresses.

Zero Sapphire Consumption
The Taiwan-headquartered chipmaker controlled by American investors puts out over 10 million good dies, or chips, on copper-alloy wafers monthly at its three factories, two in Taiwan and one in China. The chip “does not consume” sapphire because the aluminum oxide is removed and recycled.

Packages using sapphire LEDs undergo correlated color temperature (CCT) shift exceeding 20% near the blue end of the spectrum versus 2% CCT for SemiLEDs' LEDs, according to the company's data.

Chips are packaged onto silicon substrates at Silicon Base Development Inc. (SiBDI) in northern Taiwan, another process borrowed from microchip manufacturing. “Many semiconductor processes can be used on LEDs, like photomask, etching and deposition for being similar,” Doan stresses.

According to SiBDI executives, with silicon's thermal conductivity at a satisfactory 150 W/mK, single-chip packages have thermal resistance lower than 5C-per-watt, and usually thermal conductivity is trickier with single-chip, high-power packages than multi-chip, chip-array packages.

SiBDI puts eight of its LEDs on a typical 7.9mm x 7.9mm package due to superior thermal conductivity, according to SiBDI executives, while Cree's module holds only four high-power chips.

The relative high prices of LEDs keep them from being popular consumer lighting items, which gives reason for Doan and SiBDI executives to cheer. According to SiBDI executives, a typical 220-lumen, 700mA LED the company makes goes for NT$30 or roughly NT$0.14 per lumen, compared with NT$0.2 per lumen of typical multichip packages from other suppliers.

Doan believes size of LED wafers is inversely proportional to cost of LED applications. Bigger wafers have higher circumference-area ratio or less etch frames per area, hence bigger wafers result in lower cost. “Working in semiconductors gives you insider's view of the connection between wafer size and defect ratio, while defect kills semiconductor wafers as does LED wafers,” he says, adding that LED is migrating to bigger wafers as is semiconductor due to necessity to lower cost.

Optimistic
Optimistic towards the LED lighting market, Doan thinks such lighting is becoming more popular. “Many streetlights have been installed in Hsinchu and upscale restaurants this year; while LED is still expensive, it is more affordable now.”

Doan bases his optimism not only on improving LED technologies but also on inherent trends of the consumer-electronics markets. “Few cellphones existed in India even five years or a decade ago but 100 million are in use now. As prices drop volume goes up, and this trend is happening faster in LED lighting than we thought.”

Without specifying exact time and date for when LEDs will become mainstream, Doan says such lighting is becoming popular very fast as cost drops on higher volume.

Citing compact fluorescent lamp as good example, Doan says that around 10 years ago CFL prices began dropping from peak because of increased supply. Once LED bulbs are priced similar to CFLs, the market will take off very fast and it is likely to happen over the next few years. Believing wafer size is decisive, Doan says LED cost has dropped four times while four-inch wafers began replacing two-inch wafers, and will further drop when bigger wafers become feasible. It's only a matter of time.

“Moreover, CFLs have relatively poorer lighting quality, can't operate in cold temperature, and are difficult to install with rheostat, plus LEDs are scalable,” he comments.