Companies seek to constantly improve their production methods. HBLED manufacturers are faced with many challenges including vacuum and abatement. Mike Czerniak, Product Marketing Manager, Exhaust Gas Management at Edwards discusses the benefits of combustion based abatement technology.

The technology used to manufacture high brightness LEDs (HBLEDs) made from gallium nitride (GaN) has developed rapidly over the last dozen years, and market growth has continued, despite the current economic slowdown. This is a combination of developments in LED technology, the size and contrast benefits LEDs offer in flat panel applications and adesire by both governments and individual consumers around the world to reduce energy consumption due to concerns about global warming.

Current market opportunities driving the expansion in LED production already include laptop computers, as well as liquid crystal displays (LCD) backlighting used in LED TVs and automotive applications. The LED consumer lighting market is also showing growth as LED efficiency has increased to over 100 lumens per watt. For LED technology to become truly competitive in massmarket applications, however, it must still achieve even greater reductions in the initial cost per lumen.

LED manufacturers hope to achieve this cost reduction through economy of scale that comes with high volume production. Increasing production is dependent upon demand. LED manufacturing strategies are focused on addressing the technical aspects of high-volume production, while also minimizing both the capital expenditures and operating costs of production equipment.

As manufacturing processes scale to higher volumes, the relative contribution of variable costs to costof- ownership (CoO) often grows as fixed costs are distributed over the increased volume of production. In addition, ramping LED volume production also increases the demands on both vacuum and abatement systems, which must handle higher volumes of production gases or manufacturing by-products, respectively.

The GaN compound semiconductor manufacturing processes used to produce LEDs require large flows of hydrogen and ammonia. Because of the small size of the hydrogen molecule, this gas is difficult to pump, especially in high volumes. At the same time, depending on the abatement technology in use, ammonia can produce solid byproducts during the waste removal process that tend to clog the bath, recirculation pump and drain pipes in conventional wet scrubbers, requiring them to be cleaned every month or two, during which time the production tools are idle, reducing productivity and increasing tool CoO.

 



Figure 1: HBLEDs are replacing incandescent bulbs in automobiles.

 

This article discusses the pumping and abatement requirements needed to accommodate the use of hydrogen and ammonia in HBLED production, and highlights an abatement technology that can significantly reduce manufacturing cost of ownership. It also presents a new approach to implementing vacuum and abatement systems within a process tool that can help to further reduce system CoO.

 

Vacuum Pumping

Pumping hydrogen, regardless of the process involved, presents certain challenges because of the gas’s small molecular size and low viscosity, which is approximately half that of nitrogen. While these characteristics are not an issue when pumping at low pressures, they become a problem when the gas is compressed towards atmospheric pressure in a vacuum pump. At these pressures, due to its low viscosity, hydrogen tends to leak back through pump clearances, reducing the effective pumping speed.

Hydrogen also has a much higher thermal conductivity (seven times greater) than heavier gases such as nitrogen. As a result, systems pumping hydrogen typically have a different thermal profile and different component dimensions than those pumping nitrogen. A pump optimized to deal with hydrogen ideally operates with clearances set to account for the thermal properties of the gas, and integrates a progressive nitrogen purge capacity to offset the challenges of pumping this lighter gas at higher pressures. Finally, since hydrogen is flammable if mixed with an oxidant such as air, the pump exhaust must be appropriately managed to avoid ignition.

 



Figure 2: Edwards’ iXH dry vacuum pump has been specifically designed to handle the high volumes of ammonia and hydrogen involved in HBLED production

 

As mentioned earlier, pumps used in the metalorganic chemical vaporphase deposition (MOCVD) processes used to manufacture LEDs also handle high volumes of ammonia. This means they must be highly corrosive-resistant. Modular dry pumps have proven to address the challenges of pumping both ammonia and hydrogen in a production environment, while also offering potential CoO savings. Edward’s iXH pump (Fig. 2), for example, has been specifically designed to meet the challenges associated with pumping high volumes of ammonia and hydrogen with enhanced purge flow, temperature-controlled operating range, light gas performance and corrosive resistance and improved powder handling. In addition, Edwards’ pumps incorporate a unique, proprietary seal technology that helps to prevent ammonia leaks. These capabilities reduce maintenance intervals and extend pump life, thereby helping to reduce overall tool CoO. The vacuum pump offers further CoO savings through reduced energy consumption.

 

Exhaust Gas Abatement

In LED manufacturing, the extraction, safe handling and disposal of gases from MOCVD processes significantly contribute to manufacturing CoO. The conventional approach to exhaust gas management uses a wet scrubbing technology that adds significant costs to the manufacturing process in terms of energy use, water consumption and treatment.

 



Figure 3: Cost Comparison of Operating Costs for Combustion-Based Systems vs. Wet Scrub Abatement Systems

 

Combustion-based abatement offers an attractive alternative to wet scrubbing. While capital costs are approximately the same, combustionbased abatement systems offer greatly reduced operating costs, as illustrated in Figure 3. Their large input flow capability, typically equivalent to three to five process tool exhausts, eliminates capacity constraints and helps minimize capital expenditures. At the same time, their use of the exhaust gases as the main fuel in the abatement process significantly reduces energy costs. In addition, innovative reaction chemistries minimize the formation of unwanted by-products such as nitrogen oxides (NOx), a regulated emission, or ammonium solids, which require increased system maintenance downtime, thereby reducing tool productivity

The wet scrubbing technology typically used in the GaN MOCVD process essentially bubbles the gases through a tub of water where they are absorbed. This process, however, does not remove hydrogen, which is the most common waste gas produced in the MOCVD process. While hydrogen emissions are not regulated, allowing the gas to be vented directly into the atmosphere, there is a slight, but potential danger that static electricity could ignite the hydrogen during the abatement process, causing an explosion and fire.

The second most common gas produced as a by-product of the MOCVD process is ammonia. While water scrubbing can eliminate this gas, there is a danger, when hard water (water containing calcium or magnesium salts) is used, that the ammonia will react with the salts in the water producing ammonium solids. Hard water occurs anywhere that mountain run-off is a major source of water, which constitutes a large portion of the globe. As mentioned earlier, these ammonia solids can build up in the system, thereby increasing maintenance requirements and increasing CoO.

Combustion-based abatement technology solves both these problems by burning the exhaust gases in a controlled way. (Figure 4 provides a schematic of a typical four-stage combustion abatement system.) The only outside energy required is that to operate a small pilot light, similar to the ones used in home gas furnaces or stoves. Not only does this approach eliminate the hydrogen safety issue and the maintenance problem caused by ammonium solids, it offers significant reductions in energy costs compared to wet scrub technology.

 



Figure 4: Four Stage Combustion

In addition, the combustion-based system is air cooled, with the air flow being generated by the house extraction system. This air-cooled design ensures that combustion byproducts are efficiently transported from the system to the factory central scrubber or dust filter. An air-cooled system eliminates many of the fixed and operating costs associated with a wet process, including the cost of the water itself, the capital and operating costs associated with water pumps, the energy to run them and water treatment costs. Air-cooled systems are also simpler in design and have fewer moving parts. As a result, maintenance intervals are increased, while maintenance times and spares inventory requirements are reduced, all of which helps to further reduce system CoO.

When burning ammonia in a combustion-based abatement system, there is always the danger of creating NOx emissions, which are strictly regulated in most regions of the world. Edwards’ Atlas and Spectra G (Fig. 5) abatement systems avoid this danger by using a proprietary process that carefully controls the oxidation of the gases being burned to minimize NOx emissions.

In addition to the benefits of combustion-based exhaust systems in terms of safety, lower CoO and reduced environmental concerns, combustion-based abatement technology has a well-established track record for reliability. Hundreds of combustion-based exhaust abatement systems are deployed at a variety of companies in the semiconductor, flat panel and solar cell industries. The efficiency of its gas treatment process has been field-tested and meets the most stringent air emission regulations in Europe, the United States and Asia. It is currently experiencing a high rate of adoption by leading LED manufacturers world wide.

An Integrated Solution

The benefits of integrating related subsystems in a manufacturing tool has been proven in a variety of industries, such as semiconductor and flat panel manufacturing, to help reduce overall manufacturing CoO in a variety of ways. For example the combined vacuum and exhaust management systems provide enhanced safety, process tool compatibility, minimum footprint and reduced CoO. In addition, since they are housed in a single extracted cabinet with a single utility connection, these integrated systems provide a highly cost-effective means of providing protection should leakage occur.

Using an integrated vacuum pump and abatement system, such as Edwards’ Zenith system can reduce installation time by up to 70 percent, cut installation costs by over 60 percent and eliminate over 50 percent of facilities connections. Overall capital expenditures are also significantly reduced. In the case of phosphide MOCVD, the Zenith also significantly improves the safety when dealing with toxic gases like phosphine and arsine.

 

Author Biography:

M.R. Czerniak, Product Marketing Manager, Exhaust Gas Management, Edwards.

Mike Czerniak received his PhD at Manchester U., and started as a scientist at Philips’ UK laboratories before moving to its fab in Nijmegen, working on compound semiconductor applications. He was in marketing at Cambridge Instruments and VG Semicon; he is now the product marketing manager of the Exhaust Gas Management Division of Edwards.