LED Lighting Spotlight: Patterned Sapphire Substrates

In the ongoing quest to make LEDs more efficient, LED chip manufacturers have developed patterned sapphire substrates (PSS).  In fact, most high-brightness LEDs are made using PSS. There are very few resources online that explain patterned sapphire substrates. Here’s a brief explanation.

PSS helps extract more light from LEDs.  A lot of light bounces back into the LED when using a polished sapphire substrate.  Researchers discovered that patterning the surface of the substrate by etching nano-scale patterns helps more light, in the form of photons, escape, improving the light generated or extracted by the LED.  It is reported that patterning can improve the extraction of light by as much as 30%.

A second important point is that patterning also improves the epitaxial growth process.  The nano-patterned surface can have a positive effect on the nitride semiconductor growth process by promoting growth of the GaN in parallel to the substrate surface, called lateral growth.  This also helps reduce the number of dislocations, the dislocation density, that can degrade performance.

LED chip manufacturers originally developed PSS.  The patterns are quite proprietary and helped the LED chip companies differentiate themselves.  Today, the sapphire industry has joined in and sapphire wafer manufacturers have begun to put patterns on sapphire wafers in partnership with the LED chip manufacturers. The patterning work is concentrated with smaller wafers in the 2 to 4-inch diameter range, but manufacturers of large diameter wafers like Rubicon Technology are beginning to develop PSS for larger wafers.

Most patterning is based on a proprietary design from the LED chip manufacturer.  The patterns can vary from cones, pyramids and flat tops and can be organized in hexagonal or trigonal patterns.  Some basic design rules based on height and height/pitch ratio have emerged, but so far, no standards exist.  Currently the most popular pattern is a cone shape, but these patterns change frequently.  Here are some sample patterns.

Sample patterns:

Sample patterns for PSS

 

 

 

 

 

 

 

Sample Pattern for PSS

 

 

 

 

 

 

For Further Reading

Semiconductor Today, Patterned sapphire for nitride enhancements, http://www.semiconductor-today.com/features/SemiconductorToday_SeptOct_PatternedSapphire.pdf

Compound Semiconductor, New Wet Process For LEDs On Patterned Sapphire Boosts Efficiency, http://www.compoundsemiconductor.net/csc/news-details.php?cat=news&id=19734296

Compound Semiconductor, Rubicon Orders Multiple Profilers For Sapphire Production, http://www.compoundsemiconductor.net/csc/news-details.php?cat=news&id=19735318

 

Alternative Substrates for LEDs – Not Ready for Prime Time

Over the past several months, there has been some industry chatter about alternative substrates for the production of LEDs.  In fact, the dialog has been more heat than light until now if you think about it in terms of a filament in an incandescent light bulb.

Large Diameter Sapphire Wafer (source: Rubicon Technology)

Sapphire substrates have been established for quite some time as the base material for LED chips.  Today, more than 80% of LEDs are based on sapphire substrates with the remainder based on SiC and a few other materials.  But the big question is whether an alternative substrate like silicon or GaN can offer the performance and cost advantages of sapphire.

Last week, market research firm Yole Developpement held a webcast, Alternative Substrates for LED Manufacturing, to examine the alternatives, the technical challenges and the conditions for success.  You can access the archive here.

According to Yole analyst Eric Virey, the principal benefit of using Si as an LED substrate would be the ability to leverage larger 8” wafers and use fully depreciated and highly automated CMOS fabs.  But “the jury is still out,” he said, “regarding a massive industry transition from sapphire to silicon.  At the end of the day, this is a cost game; manufacturing yields are a major cost contributor to LED, and they pose specific challenges to the use of silicon.”

These challenges range from a lattice mismatch and thermal expansion coefficient mismatch, to melt back and blue light absorption. Sapphire outperforms silicon on all of these factors, and each is having a negative impact on LED chip yields from silicon.  Virey commented silicon and/or GaN must meet the performance of sapphire to be successful. To date, that hasn’t happened.

In the meantime, the sapphire substrate manufacturers have made great strides to making large diameter substrates that help LED manufacturers drive down costs and increase yields to support the aggressive cost targets of SSL.  For example, Rubicon Technology has shipped more than 230,000 large diameter sapphire wafers with this number growing.

Where are efforts now? Virey mentioned during the webinar that almost all LED manufacturers are exploring alternative substrates, although most are doing so only as a defensive strategy. Toshiba and Bridgelux have been working with silicon as a substrate. In July, the companies announced Toshiba would begin silicon-based LED production in October 2012, but there has been no further word. Plessey Semiconductors and Lattice Power also announced they would enter production in 2012.

LED Magazine reports that silicon-based substrates are “no sure thing” in their latest SSL Technology Update video blog.  Associate editor Nicole Pelletier said, “A number of companies plan to ride the incumbent sapphire technology. At The LED Show back in August, LED market leader Nichia said it had investigated and dismissed the possibility of using silicon.”

At the conclusion of the webcast, Virey agreed.  “If the technology hurdles are cleared, LED on silicon will be adopted by some LED manufacturers, but not necessarily become the standard.”

For Further Reading

LED Magazine, SSL Technology Update: October 22, 2012, http://ledsmagazine.com/features/9/10/11

Yole Developpement Webinar, Alternative Substrates for LED Manufacturing, http://www.i-micronews.com/consult_webcast.asp?uid=97

Solid State Technology, Beyond sapphire: LED substrates from GaN to ZnO, SiC, and Si, http://www.electroiq.com/articles/sst/2012/05/beyond-sapphire-led-substrates-gan-zno-sic-si.html

Solid State Technology, The demise of sapphire wafers? http://www.electroiq.com/articles/sst/print/vol-55/issue-6/columns/leds/the-demise-of-sapphire-wafers.html