Flat, stress-free substrates with great surface quality will help to drive mass adoption of solid-state lighting. Such platforms reduce the impact of edge effects, allow the MOCVD deposition process to run more efficiently and ultimately cut the cost of LED chips, says Rubicon’s Raja Parvez.

 

Consumer demand for energy-efficient, environmentally safe lighting will help to drive a solid-state lighting revolution. The US market research firm Strategies Unlimited predicts that LED lighting will grow from $5 billion in 2010 to $20 billion in 2015, and then continue at a compound annual growth rate of 33 percent. By 2020 this market has the potential to consume more than 100 billion LEDs, according to Jed Dorsheimer from the financial firm Canaccord Genuity.

This tremendous growth in the deployment of solid-state lighting will be underpinned by a shift to larger diameter sapphire substrates. Large wafers are essential to accommodate the increasing LED chip size while maintaining high chip yield over the wafer surface. Moving to these larger substrates is not just a challenge for sapphire substrate makers – it also impacts the entire LED supply chain, which must scale up its operations to meet the growing demand for lower-cost chips.



Rubicon’s sapphire growth process produces boules that can be processed into a wide variety of wafer sizes

Economies of scale

Many leading LED chip manufacturers have already  moved from 2-inch to 4-inch substrates, and they arenow announcing their intentions to migrate to 6-inch.The benefit of this move – and what is possible bymoving to even larger sizes – is illustrated in Table 1.

One benefit of turning to bigger wafers is that more LED chips can fit along the outer perimeter due to reduced curvature, especially for larger high-brightness chips. This so-called ‘edge-effect’ provides incrementally more chips than just a raw calculation of geometrical area. For example, the surface area of a 6-inch wafer is nine times that of a 2-inch wafer, but it can yield between ten and twelve times as many chips.

Switching to growth on larger substrates also leads to more efficient film deposition. That’s because large diameter wafers provides more efficient utilization of useable surface area in the MOCVD reactor, thereby increasing chip productivity. According to a recent study by the German tool-maker Aixtron, if a platter in an MOCVD tool is filled with seven 6-inch substrates rather than 42 substrates that are 2-inch in diameter, throughput increases by 52 percent. These significant gains in cost reduction mirror the savings made in the silicon industry as it has scaled production to larger and larger wafers.

An additional incentive for moving to larger wafers is the existing legacy equipment and expertise with large wafers. Many LED companies have a heritage in the optoelectronics or semiconductor industries, and are already possess large diameter equipment and process expertise for 6- and 8-inch LED production. What’s more, they understand the logistical issues involved in migration to large diameter wafers.



According to analysis by Yole Developpément, 6-inch sapphire will provide the foundation for the majority of LED chips by the middle of this decade

Meeting the needs

From the perspective of a sapphire substrate maker,increasing wafer size is not trivial. LED makers needsubstrates that are flat, uniformly polished, and have asurface free from contamination. Meeting these tightproduct specifications becomes increasingly difficultwith larger substrate sizes.

Another key requirement made by LED manufacturers is for the sapphire substrates to be stress free. During the MOCVD process these wafers are subjected to temperature cycles of up to 1200° C, which can introduce stress and ultimately lead to small cracks.

At Rubicon, headquartered in Bensenville, IL, we have developed proprietary crystallization and wafer processing technologies to produce stress-free crystals in an unconstrained environment most mimicking nature. We use this unconstrained process to manufacture 6-inch wafers that not only meet industry requirements, but exceed them. What’s more, our wafers offer an excellent surface for MOCVD growth, thanks to our expertise in both equipment customisation and polishing process development. Such efforts are needed, because slicing, lapping and polishing this material is not easy – sapphire is the second hardest naturally occurring material on earth after diamond, and cutting and polishing it requires specialized equipment and process expertise.

We are able to produce our substrates in high volume, and the variation between them is minimal. This is a major asset for LED makers, who can only turn in a profit if they can churn out wafers with very small variations in properties such as emission wavelength and brightness. Delivering on these fronts means that the wafer yields a high proportion of LEDs that meet their key specs, such as those for colour and efficacy.

How large can we go?

The key to producing high-quality, large diameter wafersis an end-to-end stable process that starts with crystalgrowth and goes right through to wafer polishing. It isimpossible to make such wafers without a high-quality,single sapphire crystal boule. We, along with some ofour competitors, are well placed in this regard becausewe are vertically integrated. This means that we canensure quality from the raw material, aluminium oxide, to finished polished wafers in a controlled manner. One of our biggest accomplishments is the fabrication of the world’s largest single-crystal sapphire boules, which can be as large as 200 kg, with no variation of quality as the boule size increases. Our process expertise has enabled us to reach volume production of high-quality 6-inch diameter sapphire wafers and R&D volumes of 8-inch diameter sapphire wafers. According to the French market research firm Yole Developpément, the proportion of LEDs made on sapphire substrates that are 6-inch in diameter will increase from 5 percent this year to 16 percent in 2012 and more than 55 percent in 2015. Sampling of 8-inch wafers at the research and development stage has also commenced at several LED chipmakers.



Thanks to edge effects, a four-fold increase in wafer diameter that results from a doubling of substrate diameter increases chip production quantities by up to a factor of five. Numbers are estimated, based on typical dimensions for a high-brightness chip



Weighing in at 200 kg, Rubicon’s single-crystal sapphire boule is claim to be the largest in the world

Alternative substrates

The industry continues to explore alternative substratesfor the growth of GaN LEDs, such as those made from silicon, which are cheaper. Progress has been made, but devices grown on this platform still have a performance that lags that of GaN-on-sapphire LEDs.

One of the challenges that the R&D community has struggled with for more than a decade is the fundamental barriers imposed by the large mismatch in the thermal coefficient of expansion of GaN and silicon. In addition, there is intermixing between this pair of materials during MOCVD growth. This introduces a multitude of crystal defects, leading to physical cracking of the epitaxial layers. One consequence of this is a compromise in the long-term reliability of LED lamps.

In comparison, GaN-on-sapphire LEDs are proven in the industry, thanks to field-demonstrated, long operating service life (long-term reliability). During the last two decades, billions of these LED chips been successfully tested in multiple applications. This has led the industry to recognize sapphire as the only commercially viable, ‘field proven’ substrate for LEDs. At this time, more than 80 percent of LEDs are built on a sapphire foundation.

More and more of these LEDs are going into general illumination, but this technology will not dominate until the cost of ownership of this form of lighting is competitive with today’s incumbent, the compact fluorescent bulb. According to Dorsheimer, LEDs account for 40 percent of the bill of materials for LED light bulb. Chips costs will fall as LED makers move to larger diameters that aid operational efficiency and yield, and this will spur affordability of LED bulbs.

We believe that this growth in solid-state lighting is a tremendous opportunity for every sapphire manufacturer. Our recent completion of two state-of-the-art manufacturing facilities that deliver large diameter sapphire wafers in high volume to customers puts us in a great position to dominate the sapphire market.

To date, we have shipped 100,000 wafers with a 6-inch diameter that have formed the foundation for the manufacturer of 2 billion LED chips. And we are well positioned to increase these shipments, thanks to the construction of our new Batavia, IL, facility that focuses on crystal growth, and the new facility in Malaysia that focuses on polishing large diameter wafers in high volume.

This latter facility was recently qualified by a major customer for 6-inch sapphire wafers, the diameter that will be used to drive significant penetration of LED bulbs in the lighting market.

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