UV-LED Overview Part III: Evolution and Paper Technical Manufacturing

By Jennifer Heathcote This article is the third installment in a three-part series designed to consolidate key principles and technical information regarding the science and engineering behind UV-LEDs. If you have not read “Part I—Operation and Measurement” (July/August 2010) or “Part II—Curing Systems” (Sept/Oct 2010), you may want to do so before continuing with Part III.

ight Emitting (LEDs) are trying to understand and optimize the engineered to produce discrete core technology as well as implement infrared, visible or ultraviolet reliable manufacturing methods. The L(UV) wavelengths when a DC voltage purpose of this paper is to provide a is applied. The type of output, overall general history of LED development as performance and operating efficiency well as an overview of typical processes of any given die is directly related employed in chip production. It is only to its material composition and the with an understanding of how far the manufacturing methods employed. As technology has come, as well as what a result, there has been a concerted is involved in producing LED emitters, that one can truly appreciate the It is only with an understanding of how far the potential of where the technology has yet to take us. technology has come as well as what is involved in producing LED emitters that one can truly Discovery of Jöns Jacob Berzelius (1779-1848) appreciate the potential of where the technology was a successful Swedish has yet to take us. recognized for his great achievements effort for the last 60 years to identify in the field of electrochemistry. new semiconductive compounds, His penchant for methodical improve chip structures and optimize experimentation led to his discovery processing techniques. Much of this of previously unknown elements, activity has been concentrated in including cerium, selenium and the longer wavelength visible and thorium. In 1824, while attempting infrared ranges; and has directly led to artificially manufacture diamonds, to improved yield rates, lower power Berzelius synthesized consumptions, brighter colors and (SiC). Silicon Carbide is a strong and increased operating efficiencies as well chemically inert material having low as a steady decline in unit costs. density, high surface hardness and By comparison, UV-LED technology minimal thermal expansion. is relatively new, and engineers and Seven decades after the invention developers are still hard at work of Silicon Carbide, a Pittsburgh-based

SPRING 2011 RADTECH REPORT 43 American entrepreneur, Edward diodes emitting at 470 nm never Goodrich Acheson (1856-1931), exceeded 0.03% efficiency. As a developed a low-cost, high-volume result, the use of Silicon Carbide in manufacturing process to produce the production of LEDs was mostly

Technical Paper Technical the material. Acheson was granted abandoned in the 1990s for better a patent for his process in 1893 and performing and more efficient subsequently marketed the material to compounds. In more recent years, the abrasives and grinding industries SiC is once again being used in under the trademark of Carborundum. conjunction with more complex alloys In the early 1900s, the British- to produce regular and high-brightness Nick Holonyak, Jr., “father” of the Light based Marconi Company sent a young visible LEDs.1 Emitting Diode. electronics engineer by the name of Henry Joseph Round (1881-1966) to Evolution of LED Electric; International Business America to research developments Compounds Machines; American Telephone and in technology. One of Round’s In the decades following the Telegraph; Monsanto and Hewlett- projects involved the evaluation of discovery of electroluminescence, Packard, as well as in the labs of Carborundum and its use in early researchers identified and various universities. From these crystal-detector . Round evaluated many naturally occurring efforts, the first infrared (870-980 subjected Silicon Carbide crystals to semiconductive materials. By the nm)-emitting LEDs based on Gallium various experiments, including ones 1950s, the general consensus was Arsenide (GaAs) were reported in in which he applied a direct current to that any serious advancement in 1962. Nick Holonyak, Jr., who was the material. He noticed that whenever the technology would require the consulting at GE, is given credit for the a current was applied, the crystals development of new man-made invention and is affectionately know emitted light. This was the very first compounds. These compounds as the “father of the light emitting observation of electroluminescence. would be engineered to embody diode.” Texas Instruments became the Round’s findings were published in specific electrochemical and first company to use a small volume the Feb. 9, 1907, edition of the journal emissive properties; operate at of these diodes commercially at a Electrical World. higher efficiencies; and consume less reported price of $130 each.1 Due to In 1928, a Russian scientist, power. An entire portfolio of new technical advancements, infrared inventor and radio technician by the materials was necessary with the LEDs now yield a unit price that is name of Oleg Losev reported his own diode performance determining which miniscule in comparison to what Texas electroluminescence observations. compounds would be used for which Instruments initially paid. As a result, He conducted a series of experiments applications. infrared LEDs are extensively used on Silicon Carbide rectifiers used in Formal research and development in video games, remote controls and radio circuits and discovered that projects involving LED communication systems. the light emissions could be switched started in the 1950s in the labs of After the initial successes with very quickly. Losev believed that Radio Corporation of America; General Gallium Arsenide infrared LEDs, the phenomenon could be further developed for use as a light relay. He is credited with being the first individual to envision a practical application for electroluminescence. It wasn’t until the late 1960s that the first LEDs made from Silicon Carbide became available. The devices emitted a blue spectral output but had a poor 0.005% electrical-to-optical power-conversion efficiency. Despite all the best efforts and technological advancements, pure Silicon Carbide Modern versions of red LEDs.

44 RADTECH REPORT SPRING 2011 development projects quickly expanded to include Aluminum Gallium Arsenide (AlGaAs) and Gallium Arsenide Phosphide (GaAsP). By 1967, the first visible-spectrum, red emissions Paper Technical were produced by a team at IBM using AlGaAs. IBM soon integrated red LEDs made with GaAsP into circuit boards and mainframe computers as status and function indicators. Modern versions of the devices are shown on page 2. In the 1960s, General Electric became the first company to commercially sell red LEDs at a price of $260 each.1 In an effort to reduce costs, Monsanto focused on manufacturing and opened a dedicated facility in 1968 to begin Message-changing highway billboard with red, green and blue high-brightness LEDs. high-volume production. Research expanded to Gallium LEDs were actively being developed Phosphide (GaP) in the hopes of from AlGaAs, GaAsP, GaP and GaAsP; making more efficient red and green however, the industry had not yet LEDs. AT&T utilized green LEDs determined the optimal structure for in the late 1970s for backlighting blue emitters. (GaN) touch-tone phone keypads. They was thought to be the best candidate selected red LEDs for use as status and in the late 1960s RCA began indicators for multiline office working on the development of blue LED-backlit Liquid Crystal Display (LCD) phones. Hewlett-Packard and Texas LEDs made from this material. RCA television. Instruments developed GaAsP LEDs made its first observations of GaN Fortunately for the UV-curing for the alphanumeric displays in luminescence centered at 475 nm in industry, work on GaN was resurrected digital calculators and watches. 1971. Additional doping of the material by Japan in the 1980s. In 1989, Monsanto continued experimenting with magnesium generated 430 nm Japanese researchers produced the first with various manufacturing blue and violet LEDs. GaN junction with magnesium doping techniques and managed to achieve Despite some initial success, RCA’s and in 1992 a GaN UV-LED with an emissions in the red, orange, management team became discouraged efficiency of around 1% was born. The yellow and green spectrums. While by the lack of commercially viable Nichia Chemical Industries Corporation electronics applications utilizing progress and the magnitude of in Japan has been a leading driver of LEDs progressively expanded, there additional research required. RCA development using GaN, GaInN and remained various challenges regarding terminated its work on Gallium Nitride GaP compounds doped with impurities low diode efficiencies, limited in the mid-70s. Even today, 40 years to produce high-brightness blue, green, brightness, high power consumption after RCA embarked on its quest to white and UV-LEDs. Commercially, and the inability to view LED emitters develop a flat screen television made Gallium Nitride LEDs are most widely in direct sunlight. entirely of LEDs, the dream of an all used for backlighting electronic devices In a much more forward thinking LED television is still a bit elusive. such as mobile and smart phones; pursuit, the director of the Materials While red, green and blue LEDs are personal data assistants; and global Research Division at RCA was widely used for large, electronic positioning systems. High-brightness intrigued by the possibility of a flat highway billboards and commercial LEDs are increasingly being used in screen television where red, green and signage, LEDs are limited to indicator automotive applications; traffic and blue LEDs would be instantly switched lights and high-brightness backlighting pedestrian crossing signals; backlighting in various combinations to create video in all general consumer flat screen of LCD and rear projection television images. At the time, red and green televisions. See photos above. displays; and general lighting.

SPRING 2011 RADTECH REPORT 45 applications are shown at left while various packages of UV-LED chips are provided in the photos

Technical Paper Technical to the right. Despite the negligible peak irradiance of the first UV-LED chips (1992), the discovery was a significant breakthrough in the evolution of semiconductor technology as it confirmed that High-brightness LED traffic signal (left) LEDs were not limited and crossing signal (right). to infrared and visible Development work has also spectrums. As is the case continued on AlGaInP high-brightness with most cutting edge LEDs in the rest of the visible technologies, it often Packaged UV-LEDs from Integration Technology (top), Nichia (bottom). spectrum. Alloys of these are now the takes years or decades dominant materials used for high- to transition from initial LED curing systems commercially brightness emissions in the red (625 discovery to viable application. The viable for an increasing number of nm), orange (610 nm) and yellow evolution of UV-LEDs has been no applications. (590 nm) spectral range. It should be different. It took more than a decade UV Process Supply and Phoseon emphasized that UV-LEDs packaged after the first lab-created ultraviolet are credited with commercializing for use in curing systems still represent diode for UV-LED systems to begin the first UV-LED systems. Other a very small portion of the total volume appearing on the market (2003). companies such as Integration of UV, white and high-brightness LEDs; The early systems were not very Technology Limited, Lumen Dynamics, however, since the technologies are powerful and the applications were Summit UV, Sun LLC and Clearstone, similar, the design and manufacturing limited. Fortunately, the overall to name a few, entered the market of UV-LED chips directly benefits output, performance and efficiencies soon afterward. Today, the majority of from improved performance and have increased every year. Starting conventional UV system developers higher volume production of white around 2008, UV-LED systems began have embarked on some level of and high-brightness diodes. Examples generating sufficient UV output at a research or commercialization of of common high-brightness LED reasonable price point to make UV- irradiators made with UV-LEDs. Large

High-brightness Formula One rain tail light (left) and high-brightness headlight (above) are examples of common high- brightness LED applications.

46 RADTECH REPORT SPRING 2011 investments are being made despite the reality that not all applications are Figure 1 currently suited for UV-LED curing and many system configurations Timeline for evolution of LEDs are still quite expensive. In lieu of Paper Technical these challenges, the overall chip and packaging technology continues to evolve and, in time, prices and performance will also improve. This has encouraged more semiconductor manufacturers, chip packaging companies and UV system developers to enter or consider entering the market. This increase in competition will likely advance the technology much further in a much shorter period of time. To summarize, engineers and research have been formulating and experimenting for conductive compound, Silicon Carbide Every single stage and step of the nearly 60 years with an ever increasing (SiC), is illustrated in Figure 1. production process—from raw material portfolio of man-made semiconductor selection through to packaging—is compounds that emit infrared, Semiconductor Manufacturing critical. Each has the potential to visible and, most recently, ultraviolet The spectral emission, electrical- introduce defects or foreign matter, wavelengths. These compounds have to-optical power-conversion efficiency, resulting in lower efficiencies and been engineered to embody specific unit cost, quality, yield rate and insufficient performance in the final electrochemical and emissive properties lifetime hours of each die depend not dies. As a result, numerous quality while operating at increasingly higher only on the substrate material, but also assurance checks are performed and efficiencies and consuming less on each additional processing step and most processes take place in Class power. Since each compound emits a manufacturing method employed. As a 1 and Class 10 clean rooms. Each different wavelength and/or amount of result, significant investment has gone and every step of the manufacturing energy, an entire portfolio of materials into developing and controlling the process, therefore, is a distinct and is necessary in order to produce production of LEDs. challenging engineering and quality dies for various applications. The Actual manufacturing yield rates improvement project that takes years, final diode performance determines vary significantly depending on the if not decades, to perfect. which compounds are used for which type of LED. Yield rates for standard An overly simplified description of applications. infrared and visible LEDs have this process can be segmented into It should be emphasized that improved drastically over the past three parts that focus on the ingot, the necessary research is far from 50 years as most of the production wafer and die. It should be noted exhausted, and there are presently issues have been resolved. Yield that there are many variations to the complex alloys still in development, rates for white and visible spectrum production methods described; not including GaN/SiC, GaN/Sapphire, high-brightness LEDs are increasing all steps have been included in this GaN/Silicon, GaN/B–Si/pSiC, GaN rapidly; although, there is still room for document; and each manufacturer Glass, GaN/ZnO, GaN/AIN, GaN/B-GaN/ improvement. The current quality of likely incorporates proprietary pAIN and GaN/Ge, to name a few. Very UV-LED production, unfortunately, is processing that is not generally preliminary research into materials not very good and much optimization known to the public. As a result, the that emit shorter wavelength UV-A, work remains to be done. With UV- following sections are meant to give UV-B and UV-C diodes is also currently LEDs, there is a significant amount of the reader some general insight into underway. A timeline summarizing scrap generated during production that a typical LED manufacturing process key breakthroughs in development has a direct impact on the final die cost and are by no means meant to be fully since the discovery of the first semi- and performance. comprehensive.

SPRING 2011 RADTECH REPORT 47 The Ingot characteristics of the original seed The manufacture of LED material (2-d). See photo at left. semiconductors starts with the Before moving to the next stage production of a long, cylindrical ingot, of production, a diamond saw is Technical Paper Technical also called a boule. The ingot is typically used to remove the cone and the formed to diameters of 2, 4, 6, 8 or 12 remaining cylindrical ingot is ground inches and can be grown to any desired and polished. length. The material properties are uniform throughout the ingot, and the The Wafer crystalline structure contains relatively A diamond saw is used to slice few impurities or contaminants. This is the ingot into crystalline wafers or achieved through precision temperature Fully formed ingots. substrates that are less than 250 and pressure control in a clean microns thick (10 mils). This is environment and by using only the the ingot takes the shape of a cone illustrated in Figure 3. After slicing, highest purity of raw materials. that tapers from the seed outward the wafer is subjected to a lapping Formation of the ingot begins when to the desired diameter (2-c). The and/or etching process in order to carefully remove unnecessary or high-grade raw materials are mixed crystals continue to grow in length damaged particles from the surface. together in a specially designed reactor as long as the process conditions are Lapping is a physical process that involves moving the wafer over a plane Figure 2 of liquid abrasive, while etching is a chemical process intended to dissolve Forming the ingot unwanted material. Both are used to ensure a smooth surface. Next, the thickness and flatness of each wafer is measured, and the wafers are sorted and polished in order to improve the efficiency and quality of downstream processing and make the wafers more receptive to additional layers of semi- conductive material. Any imperfections created when chamber as illustrated in Figure 2. maintained. The applied temperatures the ingot is formed and sliced or when When elevated temperatures and and pressures, as well as the rotation the wafer is lapped, etched or polished pressures are applied, the materials and extraction speed, all have a direct will lead to a poorly functioning or combine to form a uniform molten effect on the diameter and the quality non-functioning LED. Unclean wafers solution that is covered with a layer of the ingot. When the desired growth also have a negative effect on the of liquid boron oxide to prevent is reached, the seed and bonded ingot performance of the final diodes. As vaporization. A rod or chuck is then are removed from the chamber. The a result, all wafers are cleaned using lowered toward the solution (2-a). On result is an ingot with all the physical the end of the chuck is a tiny, purified seed that contains the exact properties Figure 3 of the mixture. When the seed reaches the solution, a chemical bond begins to Wafers being sliced from ingot form between the seed and the molten material (2-b). As the rod is rotated and withdrawn at a slow and constant rate of speed, some of the mixture begins to cool and solidify on the seed. The top of

48 RADTECH REPORT SPRING 2011 containing the metallic contact Figure 4 pattern is placed over the wafer. The entire wafer is then exposed to UV Wafer processing—liquid phase epitaxy light. All exposed areas of the resist

material that are not blocked by the Paper Technical reticule cure underneath the light and are subsequently washed away in a developer bath. The resist that was under the mask remains on top of the semiconductor layers. It will be removed in a later step. Contact metal is then evaporated onto the wafer in the areas where the resist was removed. This is done in ultrasonic or solvent-based methods or furnace tube where dopants are a high-temperature, vacuum-sealed in order to remove any remaining dirt, forced into the upper layer(s) using chamber flooded with a mixture of dust or foreign matter. high temperature gases. A photo of one hydrogen and nitrogen gas. High Next, a process known as Liquid such furnace is shown below. temperatures are used to vaporize Phase Epitaxy is used to grow The next step is to add conductive the metal contact material inside the additional layers of semiconductive metal contacts or circuitry to the chamber. Once the metal is vaporized, material on top of each wafer. These wafer. This may be done all at once the chamber conditions are adjusted in layers often contain intentionally or built up sequentially by repeating order to condense the metal onto the added impurities or dopants that the following photoresist and vapor wafer in the areas where the resist was improve the emissive characteristics deposition process several times. First, removed. The remaining resist is then and efficiencies of the final diodes. The a light-sensitive, liquid material called stripped in a chemical bath leaving just dopants interfere with the crystalline a “photoresist” is applied to the top the epitaxial wafer and metal contacts. structure of the die resulting in an surface of the wafer while it spins. Finally, the contact pattern is annealed altered output when a voltage is Spinning ensures that the material is to the wafer in a high temperature applied. All epitaxial layers have the evenly distributed across the entire furnace over a period of several hours. same crystal orientation as the ingot; surface. Next, the wafer and applied The end result is a chemical bond however, the existence of dopants resist are baked at a low temperature between the semiconductor and the means that each layer has a different in order to harden the resist surface. contacts. An example of a processed electronic density. A mask (also called a “reticule”) wafer is provided in Figure 5. Each epitaxial layer is several microns thick, and the process involves passing each wafer underneath various reservoirs of molten material or melt as illustrated in Figure 4. A melt can contain one or several types of dopants. If multiple dopants are needed to produce the desired effect, they can either be grown on the wafer structure in sequential layers or all at once. Sometimes it is necessary to further increase the quantity of dopants or introduce new dopants into the wafer structure after the epitaxial layers are grown. This step is often administered using a continuous diffusion furnace Wafers pictured in front of furnace tube entrance.

SPRING 2011 RADTECH REPORT 49 production process was successful. 19 years since the first UV-LED was Figure 5 Each functioning LED is then binned produced in a lab; and eight years according to wavelength, peak since UV-LED curing systems were Processed wafer irradiance, and forward voltage. For first offered to the market. It has been

Technical Paper Technical additional information on integrating a long and remarkable journey that spheres and binning, refer to “UV- has involved extensive development of LED Overview Part I—Operation and previously unknown compounds; the Measurement.” creation of diverse diode structures; After the diodes are binned, and the invention and optimization of they must be packaged according precision manufacturing methods. to the intended application. The Despite all the amazing semiconductor manufacturer will often accomplishments, this is a journey package the dies itself before selling that is still far from over. During the LEDs to the market as modules. the foreseeable future, UV-LED The Die (Chip, Diode, Alternatively, some semiconductor performance will continue to improve. Semiconductor) manufacturers will sell the individual More wavelengths will become Depending on the diameter, a dies without further packaging. In this available; higher irradiances will be fully processed wafer is made up of case, the packaging is done directly generated; efficiencies will increase; thousands or even tens of thousands by the final equipment manufacturer lifetime hours will improve; and costs of individual LEDs all with the same or other supplier. The packaging of will become more attractive. These general material structure and metallic UV-LEDs was detailed in “UV-LED breakthroughs will not necessarily contact pattern. Figure 6 shows one Overview Part II: UV Curing Systems.” happen overnight, but recent LED die resting on the face of a U.S. advancements evaluated in the penny. This single die would have been Conclusion historical context of LED evolution one of many cut from a 2-, 4-, 6-, 8- or The evolution of LEDs has spanned make it hard not to be excited about 12-inch diameter wafer using a laser or 187 years since the invention of the technology’s future and all its diamond saw. Silicon Carbide; 118 years since the possibilities. w After the dies are extracted from commercialization of Carborundum; the wafer, a forward voltage is applied 104 years since the discovery of References to each and every die, and the output electroluminescence; 83 years since 1. Schubert, E. Fred. Light-Emitting the light relay was first conceptualized; Diodes. Cambridge, UK: Cambridge is measured using an integrating University Press. 2008. 56 years since the initiative to engineer sphere. It is only at this time that 2. Held, Gilbert. Introduction to Light manufacturers know whether the semiconductive man-made compounds; Emitting Diode Technology and Applications. Florida, USA: Auerbach Publications. 2009. Figure 6 3. Light-Emitting Diode. How Products Are Made, Volume 1. 2006 - 2011. Completed individual die shown in proportion to April 4, 2011. http://www.madehow. com/Volume-1/Light-Emitting-Diode- a U.S. penny LED.html 4. Silicon Wafer Processing. IISME. Summer 2000. April 4, 2011. http://iisme.org/etp/Silicon_Wafer_ Processing.pdf

—Jennifer Heathcote is general manager, North America for Integration Technology in Individual die Chicago, Ill.

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