Recently, Iain Black, VP of manufacturing engineering and innovation at Philips Lumileds, stated that the complexity of LED products and applications and the diverse array of customers in the illumination market make low-cost LED manufacturing a tremendous challenge. “There are numerous applications and thousands of LEDs that need to be matched together,” he said.
As a result, Black stated that Lumileds is moving toward more standardized LED products that become differentiated toward the end of the manufacturing process. “This will simplify the front-end processes, where the capital outlay is the greatest, while also reducing the number of different die types.”Black continued, “We still will have custom solutions in cases of very large customers or niche products, but the practice of customizing single-die emitters for Lighting is no longer practical.” In addition, the company is building highly-configurable manufacturing lines to achieve the necessary flexibility.
LED manufacturing supply and demand
The changing global landscape of LED manufacturing was discussed by Karen Savala, president of SEMI America. SEMI has estimated that there are 142 LED fabs in operation worldwide, up from 64 facilities in 2006. Capacity in 2012 is estimated at 1.57 million (4-inch-equivalent) wafers per month. Taiwan has the greatest LED chip-manufacturing capacity, followed by Japan and China.
Savala commented that some of the recent build-up in China has been put on hold due to falling LED prices and oversupply concerns. “We see some overcapacity, but we believe this will be a short-term issue,” she said. Savala also provided an update of sapphire substrate prices, which had fallen to a new low of $10 per 2-inch substrate by the end of 2011.
Industry standards provide one path to cost reduction in LED manufacturing. Savala noted that currently there are over 30 different 6-inch sapphire-wafer products on the market. In 2010, SEMI organized the HB-LED standards committee and there are currently four task forces for LED manufacturing. The mission is to develop geometric standards for 6-inch sapphire wafers (currently in the ballot stage); define substrate-carrier geometries and interfaces for automation; investigate the allowable impurities and defect levels for 6-inch sapphire wafers; and identify appropriate ESH (environmental, safety and health) guidelines for LED manufacturing.
Manufacturing roadmap on schedule
Jim Brodrick, the US DOE’s solid-state lighting program manager, stated that the industry is essentially on schedule with the roadmap to achieve LED efficacy of 176lm/W in 2012 with a price of $6/klm (cool white). Brodrick cited two priorities in the roadmap, which are the development of flexible and cost-effective manufacturing methods for LED modules, light engines and luminaires as well as high-speed, non-destructive test equipment and standardized test procedures for key stages in the manufacturing process.
Brodrick emphasized that cost reduction, while maintaining high-quality manufacturing, all comes down to one metric: binning yield. The DOE is funding programs in upstream process control, non-destructive testing, manufacturing automation, and advanced packaging schemes that can lead to higher binning yields. In packaging, Brodrick said that higher levels of component integration are needed, and that LED companies will in general move to wafer-scale packaging for cost reasons.
He noted that of the DOE’s total SSL program, worth $114 million in 2011, 31% is dedicated to OLED-related activities. The roadmap calls for a reduction in OLED luminaire manufacturing cost from around $230/klm in 2012 to under $20/klm in 2020. “High-speed, low-cost, thin-film deposition for OLED production probably will require new tool platforms,” said Brodrick. He added that developments in large-screen OLED-based displays, including large-scale deposition methods and automation, should benefit the OLED lighting market. Some of the focus areas for cost reduction include better utilization of materials, reduced organic layer cost and improved encapsulation methods.
Wafer size increases
Jacob Tarn, president of TSMC Solid State Lighting Ltd, said that many high-productivity processes can be transferred from semiconductor to LED manufacturing. Firstly, he said, there is a need for an integrated development environment in LED manufacturing that includes device and process simulation, similar to what exists in semiconductor manufacturing. “The GaN industry has not had enough databases to build the infrastructure, so many developments have been empirical,” he stated. TSMC plans to bring fully automated 8-inch manufacturing processes to LED manufacturing. Then, final processing – including metallization, passivation, phosphor coating and lens molding – can be performed, followed by wafer dicing and testing. In terms of process and equipment control, mainstays of the semiconductor process control world, including run-to-run control, data mining and equipment tracking, can be applied to LED manufacturing.
Beyond these steps, Tarn suggested that perhaps optics can be developed for multiple LED emitters, and that control functions might be integrated at the LED chip or package level.Raja Parvez, CEO of Rubicon Technology, then talked about the advantages of progressing to larger-diameter sapphire substrates as well as recent industry trends. He said that there are essentially five major producers of sapphire wafers, who all added capacity in the last year, leading to significant price reduction. However, he contends that few manufacturers can provide high-quality large-diameter substrate material and that at the wafer level, flatness and defect-free manufacturing are much more difficult to achieve.
Improved wafer flatness has been correlated with more consistent lithographic results and greater consistency in brightness and color of LEDs. To date, Rubicon has shipped over 230,000 6-inch polished sapphire wafers.
Thomas Uhrmann of EV Group cited lithographic patterning as a key cost-reduction area for LED manufacturers. Some of the parameters that affect patterning yield include warpage and poor visibility of alignment marks due to the light-scattering properties of LED wafers. In this area, 1× steppers compete with proximity aligners.
In LED fabs with different wafer sizes, tools must be capable of handling different wafer sizes with minimal changeover time. In evaluating equipment purchases, Uhrmann advised that users should evaluate not only the cost of ownership of the tool (in $ per wafer) but also its footprint efficiency (wafers per hour per square meter).
Many LED manufacturers are now pursing vertical LED structures, which may be capable of higher light-extraction efficiency than lateral designs, while providing good heat conduction to the submount assembly or package, according to Uhrmann. For these structures, EV Group has developed a variety of eutectic bonding and transient liquid-phase bonding processes capable of withstanding high temperature cycling in the latter process steps.
Ilkan Cokgor of Everlight Electronics focused his talk on processes for plastic leaded chip carrier (PLCC) packages. PLCCs are low-cost surface-mount packages that have not developed a reputation for reliability in LED packaging until recently. These packages distribute heat and light through spatially distributed LEDs.
Cokgor pointed to several recent improvements in PLCC packages. These include a modified primary optic; a new alloy nitridation process that improves the crystal structure of the phosphor material and improves brightness; and new sidewall etching and patterned-sapphire-substrate processes on the chips. The PLCC also uses a die-attach material with higher thermal conductivity (0.8 W/mK) and a higher efficiency reflector.
Ravi Bhatkal of Cookson Electronics discussed some of the thermal challenges associated with using package, submount and board materials that all have different coefficients of thermal expansion. Fast temperature changes induce more thermal stress and can lead to creep failures. He suggested a combination of relevant material stack design, thermal modeling and advanced thermal cycling tests to provide a mapping function between accelerated test results and estimated useful lifetime.