LED industry requires versatile equipment to suit different needs (MAGAZINE)

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This article was published in the June 2011 issue of LEDs Magazine. View the Table of Contents and download the PDF file of the complete June 2011 issue.

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Comparing the LED industry with the IC industry shows many parallels, but also many differences. Both industries are driven by the need to increase device performance while lowering the overall manufacturing cost. While the IC industry follows Moore’s law by scaling mainly the transistor size, which results in increased integration density and performance, the LED industry does not have the possibility of scaling. However, increasing the overall LED efficiency enables the reduction of real estate required to produce the same lumen output. Since LED makers are at very different efficiency numbers right now, strong competition is present. One result is that manufacturers are conservative with high-end equipment spending until they settle on the applications on which they will focus.

FIG. 1. Another difference between the LED and IC industries is that the LED industry currently has little standardization. In this sense, the LED industry is very comparable with the IC industry many years ago. A key reason for the lack in standardization is the vast variety of applications for LEDs across different market segments. To compete efficiently, LED manufacturers rely on highly customizable and proprietary process flows for manufacturing.

More recently, the strong growth in the LED market has triggered the entry of major IC manufacturers into the LED industry. Therefore, differences between the two industries are expected to blur. The transfer of manufacturing and statistical methodology from the IC industry is adding considerable value for the LED manufacturers. Indeed, changes are needed to adapt IC-specific tools, such as statistical process control, to the LED industry, where the parameter range is entirely different, but the toolbox is already present.

Customized process flows

The fast market pace and the lack of standardized process flows have major implications for LED manufacturers and equipment suppliers alike. As already mentioned, the main way in which LED manufacturers achieve competitiveness is via highly-customizable process flows to fit all kinds of LED applications. LED manufacturers often use very different process flows for different applications and LED designs. This in turn means that standardization and optimization of individual processes is difficult.

For the equipment manufacturers, tools must be highly flexible and customizable to satisfy the individual customer needs. For example, different LED chip designs – such as lateral, vertical, flipchip or thin-film-f lip-chip designs – demand differences in the manufacturing process flow and hence in equipment. Depending on the LED design, different substrate sizes or even different substrate materials may be used that challenge the equipment in an entirely different way. Therefore, a close collaboration between LED manufacturers and equipment suppliers is essential to address individual manufacturing challenges.

The battle for improved LED efficiency is mainly fought between the high-end LED manufacturers that are investing considerable effort in research and development. On the other side stand newly-established companies that have just entered the market in the last couple of years. Gaining ground in this battle is essential if the newcomers want to remain competitive. A comparatively small percentage increase in overall LED efficiency can considerably reduce the required chip area for a certain amount of lumens. This is achieved by increasing the extraction efficiency as well as improving the current injection and internal quantum efficiency. The two latter aspects are seen as an essential step for low-cost, low-droop, high-efficiency luminaires for general lighting.

Collaboration and standards

Many different routes can be taken to improve the overall LED efficiency. However, limited resources restrict the number of routes that can be evaluated. Equipment suppliers – mainly for frontend, wafer-level equipment – are seen increasingly as development partners for the exploration of novel LED manufacturing solutions. The experience of equipment suppliers, and knowledge gained in other markets, can add considerable value for LED manufacturers, significantly shortening their development cycles.

One example of collaborative development is the NIL-COM Consortium. Companies and research organizations throughout the nanoimprint lithography (NIL) supply chain are collaborating to enhance the whole value-chain needed for the cost-effective production of photonic crystals. Stimulating the supply chain – for individual processes or for the overall LED process flow – could accelerate improvements in cost reduction and efficiency for many LED manufacturers.

For the IC industry, SEMI standards create standardization of manufacturing equipment, in the sense that equipment requirements are similar or equal among different IC manufacturers. For the LED industry this is not the case. While high-volume manufacturers of HB-LEDs rely mostly on fully-automated solutions, small- and medium-scale manufacturers use mainly manual equipment.

To satisfy the needs of all sizes of manufacturers, equipment suppliers need to have a very complete portfolio, ranging from manual to semi-automated to fully-automated, high-volume equipment. Therefore, highly versatile and universal equipment platforms are needed to suit the varying demands of LED manufacturers. In order to fulfill this demand, field-proven and highly-developed equipment from the IC industry can be used as a basis on which to develop special features that are LED specific and manufacturer specific.

Optical lithography

Optical lithography is the preferred patterning technology for high-brightness LEDs, where minimum feature sizes are in a range of 5 μm. From a cost-of-ownership perspective, full-field lithography with mask aligners is preferred over competitive patterning technologies, such as projection steppers, where capital expenditure is high and throughput is low.

FIG. 2. EVG 620HBL full-automated mask
aligner dedicated for HB-LED manufacturing Starting with EV Group’s well-developed and field-proven EVG 620 mask aligner – used in IC packaging, MEMS and compound-semiconductor manufacturing – different LED-specific features have been implemented. One of the obstacles for GaN-based LED manufacturing on sapphire is the materials’ optical transparency. Etching steps, down to the buried n-GaN layers, are therefore optically hard to resolve. The development of special optics to align such low-contrast structures is essential. Furthermore, LED wafers are highly bowed after the epitaxial growth process. A special wafer-chuck design guarantees efficient flattening of sapphire wafers and hence a constant print-gap between LED wafer and photomask. In this way, non-contact processing with superior overlay accuracy of multiple masks results in a high yield. The EVG 620HBL (Fig. 2) delivers highest-in-class throughput of up to 165 wafers per hour in aligned mode (220 wafers per hour in first-print) and autonomous operation of 125 wafers for low cost-of-ownership.

Wafer bonding and NIL

Wafer bonding is most commonly used for active-layer transfer. In this process, the growth substrate – GaAs for AlInGaP-based and sapphire for InGaN-based LEDs – is substituted with another carrier that offers higher thermal conductivity and improved optical properties. With the EVG 560HBL (Fig. 1), EV Group has introduced the first wafer-bonding equipment dedicated solely to the LED market. The tool is designed for high yield enabled by optimized pressure and temperature distribution, combined with a throughput of up to 176 bonds/hour (2-inch wafer equivalent) using multi-substrate bonding. Sapphire wafers often show high thickness variation. The EVG tool's proprietary thickness-compensation feature ensures homogenous pressure distribution over each individual wafer pair for optimum performance.

Nano-imprint lithography (NIL) can be applied for the simultaneous generation of structures ranging in size from microns down to a few nanometers. Such small features are used in LEDs for improved light extraction, either via photonic-crystal structures, or nano-patterned sapphire substrates. Nano-patterned sapphire substrates efficiently reduce the density of threading dislocations, showing promising results that could close the so-called green gap with highly-efficient InGaN LEDs. Developments in stamp material and manufacturing technology enable a cost-effective and high-throughput imprinting process on the wafer scale. Soft stamps are generally used for LED applications to conform to surface roughness and spikes stemming from the epitaxial process.

Future developments

Looking to the future, close collaboration of LED manufacturers and equipment suppliers will become more essential. On the one hand, collaboration helps LED manufacturers to get equipment specifically designed or adapted for their needs. On the other hand, equipment suppliers can be valuable development partners for novel LED technologies, efficiently shortening development cycles for new products and process flows. In addition, equipment developers are preparing right now for increased fab automation as well as increasing wafer sizes. Economies of scale and automated, single-wafer processing are expected to further enhance yields and hence reduce manufacturing costs.

Serving such a rapidly-changing market, where different technologies are still seeking pole position, requires equipment manufacturers to stay flexible in order to meet the different requirements and specifications of LED manufacturers.