Sapphire Substrate Advances Lead to Brighter LEDs at Lower Costs

As LED manufacturers relentlessly strive to produce greater light output at a lower cost, the most significant advance in cost per lumen in recent years has been the adoption of patterned sapphire substrates (PSS).

When a pattern is etched onto polished sapphire — the material used as a substrate in the vast majority of LED chips — total light extraction efficiency (LEE) can be increased by as much as 30 percent. This can happen in two different ways:

(1)    By encouraging lateral growth of the epitaxial layers, thereby reducing epitaxial defect density and increasing the light emission of the active quantum well layers

(2)    By reducing light loss, through creation of a photon scattering effect that allows more of the light generated to escape

Generally, the patterns consist of shapes — cones, domes, pyramids — created in a hexagonal pattern on the surface of the sapphire through dry plasma etching. Pattern features may be 0.65 to 2 microns in height, and the pitch (the distance between the centers of adjacent features) may be 1.5 to 3 microns.

SSL Design Blog Post Image

These pattern designs are developed independently by each of the LED manufacturers to meet the needs of their unique epitaxial recipes and are considered proprietary technology. Because of this, no standard library of patterns exists. The critical dimensions to increasing LEE include the shape and size of the pattern features and the aspect ratio — the ratio of height to width. Deeper patterns tend to be associated with greater LEE, but can be difficult to make if conditions are not well-controlled.

In addition to developing their PSS recipes in-house, LED manufacturers originally performed the patterning operations themselves. While most still conduct some of their patterning operations in-house, third-party patterning became more available in 2010 for two-inch wafers, and later four-inch wafers. It was then that LED companies had the option of outsourcing at least a portion of their patterning activity.

Concurrently with the development of patterned wafers, LED chip manufacturers have been slowly migrating to larger substrates for greater efficiency. Larger wafers provide several benefits, including:

  • Increased throughput for each reactor run, effectively increasing capacity without adding additional MOCVD reactors or additional floor space
  • Reduced edge loss
  • Reduced wafer handling

The yield for PSS at larger diameters is affected by the flatness of the wafer, and bowing of the wafer can cause inconsistent etching and lower yields.

So what’s next for PSS? Can we get even more luminous efficiency with this technology?

Nanoscale patterning has been extensively studied for its potential impact on light extraction efficiency due to both its significantly increased pattern density and its impact on internal quantum efficiency from the improvement of epitaxial quality. As we continue to test new patterns and sizes, like nanoscale patterning, it is clear that the industry hasn’t reached a wall in what is possible for light output. Further advances in PSS technology will provide improvements in light extraction efficiency, contributing to the continued market success of LED technology.

Substrate Update: It’s All About Patterning & Large Diameter Wafers

yole_developpement_logoMarket research firm Yole Developpement recently published a new report on front-end manufacturing trends for LEDs. Their latest report gives us some very good news about the sapphire market. Semiconductor Today reported on Yole’s analysis. Here are some big take-aways:

  • There is increased demand for larger-diameter sapphire wafers, with big players (such as LG, Sharp or Osram) moving to 6” wafers and Taiwanese players moving to 4” wafers.
  • LED chip makers demand more patterned sapphire substrates (PSS). PSS are now mainstream in the market with an 87% share as of Q1 2014.
  • While some companies (such as Soraa and Toshiba) have begun mass production of gallium nitride-on-silicon (GaN-on-Si) and GaN-on-GaN LEDs, market penetration of these alternative substrates will depend on future improvements in terms of performance and cost.  Without these improvements, alternative substrates will not be able to fully compete with sapphire-based LEDs.

What does this mean for sapphire makers? LED chip manufacturers are looking to gain production efficiencies, lower costs, and increase performance for their LEDs.  As the adoption for LED lighting increases, they need to make more and better performing LEDs. Large diameter sapphire wafers enable more throughput for each run of the MOCVD reactor, making better use of the reactor “real estate” and decreasing the cost per unit of area processed. Depending on the type of MOCVD reactor used, LED chip manufacturers using six-inch wafer platforms may achieve up to 48% greater usable area per reactor run as compared to two-inch wafers.

What does PSS offer? First, PSS helps improve epitaxial growth by promoting growth of the GaN in parallel to the substrate surface. This helps reduce the number of dislocations, called the dislocation density, which can degrade performance of an LED.  Secondly, patterning can help extract as much as 30 percent more light from an LED.  This is particularly advantageous for high brightness LEDs (HB LEDs) that are used in LED lighting applications.

LED chip manufacturers have been buying smaller 2-inch and 4-inch PSS from outside suppliers for years.  The next step in the evolution in the market is the migration to large diameter PSS. Already a pioneer in the development of large diameter sapphire substrates, Rubicon Technology has developed capabilities for large diameter PSS making it possible to manufacture 6-inch and even 8-inch PSS. Rubicon is already gaining traction in the PSS market.  The company recently reported in their Q1 2014 earnings call that they received their first order for PSS and have samples out to more than a dozen LED chip manufacturers.

For more information about the report from Yole, visit

For Further Reading

Semiconductor Today, Substrates shaping trends in LED front-end manufacturing,, Larger Wafers, Larger Yield – The Numbers Behind Large Diameter Sapphire Wafers and Yield,, Large Diameter Patterned Sapphire Substrates Explained,, Sapphire Substrates for LED: The Big Move Toward 6″ Has Already Started,

How Do They Do It? From Sapphire to LED Infographic

You’ve heard a lot about LEDs, but did you know that a tiny piece of sapphire – the pure, colorless industrial variety, not the blue gemstone – is in more than 80% of LEDs? Sapphire is the foundation for the LED chip, just as silicon is for a computer chip.  Rubicon Technology has put together an infographic that describes the sapphire manufacturing process and where sapphire is found in an LED. The bottom of the infographic features examples of products that feature LEDs for lighting. Click on the infographic below to see it larger.

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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,

Yole Developpement Webinar, Alternative Substrates for LED Manufacturing,

Solid State Technology, Beyond sapphire: LED substrates from GaN to ZnO, SiC, and Si,

Solid State Technology, The demise of sapphire wafers?