Semiconductor manufacturing, like any business, must make a profit, but profitability need not come at the expense of the environment. The latest advances in the vacuum and abatement technologies, used to remove and neutralize process wastes and by-products, are allowing manufacturers to reduce operating costs, energy consumption and carbon footprint all at the same time. In addition, integration of vacuum and abatement capabilities in optimized, process-specific systems is also reducing capital expenditures and installation times. The combined effect on total cost-of-ownership allows manufacturers to respect both the planet and the bottom line.
Mike Czerniak, Edwards, Clevedon, North Somerset, UK
The semiconductor industry is not the only industry that has developed a highly refined economic model of its manufacturing process; other industries that share an intense focus on profitability include flat panel display (FPD), light emitting diode (LED), and photovoltaic (PV) manufacturing. Notwithstanding the focus on profitability, these industries have historically also been cognizant of the potential effects of the manufacturing processes on the global environment, and been careful to comply with relevant regulations. Energy consumption has been part of the industry roadmap for much of the last ten years.
Energy consumption per cm2 of silicon declined steadily over most of the past decade, until increases in device and process complexity, such as the introduction of high-k metal gates, forced up the number of process steps. Energy consumption per cm2 actually increased from 2008 to 2009 (Fig. 1). Other new processes can be expected to put additional upward pressure on energy consumption. For example, extreme ultraviolet (EUV) lithography will use three to five times the energy of immersion lithography.
|Figure 1. Energy consumption (kWh/cm2) has declined steadily over the last decade. The uptick from 2008 to 2009 is attributed to increasing device and process complexity. It brings increased pressure on manufacturers to restore the downward trend. (Data from Korean Semiconductor Industry Association . Other territories show similar trends)|
Vacuum and abatement account for a significant portion of the power used in semiconductor manufacturing. Vacuum pumps represent as much as 20% of total fab energy consumption. Exhaust gas abatement systems, which use hydrocarbon fuels or electricity to heat and oxidize process gases, also contribute substantially to fab energy consumption. The latest developments in vacuum and abatement technology provide significant reductions in energy consumption and, because of the large contribution these processes make to the total energy budget, will likely play an important role in manufacturers’ efforts to meet their environmental goals, without adversely affecting productivity or profitability.
Vacuum pumps offer a number of opportunities to improve energy efficiency. Non-contact seals, improved bearing design and lubrication (oil flow, oil type, size, and preload), and better gear design (tooth profile and size) can all reduce frictional losses. Port and rotor profiles and the number of pumping stages can be optimized for specific compression and pumping performance profiles. Cooling systems can be designed for efficient heat transfer and minimal water consumption, while maintaining reliability with specific attention to temperature sensitive components. The motor and inverter can be matched to load requirements. As an example, comparing the current generation of Edward’s harsh duty dry pumps (iXH) with the previous generation (iH), the combination of improvements in all of these areas has halved ultimate pressure, reduced energy consumption by 5% to 13% (size dependant) and significantly decreased the overall size of the pump.
Exhaust gas abatement systems
The simplest approach to combustion is an open flame combustor in which the exhaust stream is introduced into an unconfined flame. With this approach, conditions can vary widely within the combustion zone, generally requiring a larger combustor and higher fuel consumption to guarantee the reduction of hazardous components to acceptable levels. Inward fired combustion, a more efficient alternative, uses a hollow cylinder of porous ceramic material to create a closely controlled, isothermal combustion zone. Air and fuel flow inward through the ceramic where they provide flameless combustion on the ceramic surface. The flow of exhaust gas is confined completely within the combustion zone. The ability to maintain uniform temperature throughout the zone maximizes fuel efficiency and ensures complete combustion of the exhaust stream. Careful attention to combustor design can provide significant energy savings. For instance, current generation systems can treat up to 600slm of exhaust gas, depending on the application and model, and typically consume half the fuel gas required by the previous generation.
One of the most important developments in vacuum and abatement components has been the full implementation of "green" or idle mode capabilities, which reduce power consumption during idle periods. The capability was first introduced early in the decade and it had become standard on most semiconductor products by 2007. However, full realization of its potential has occurred only lately as manufacturers have developed the means to effectively control switching among various modes (addressed below under fab integration).
|Figure 2. Three different idle modes provide different levels of energy savings, allowing manufacturers to maximize savings by fine tuning operations while preserving sufficient response time to meet specific process requirements.|
Vacuum pump green modes include a deep mode, which reduces power to a control level only, a 98% savings, but requires time for pump temperature to recover; medium mode, which reduces power consumption 37% while maintaining inlet pressure and takes less time for pump temperature to recover; and CVD mode, which reduces power consumption 12% while maintaining both inlet pressure and pump temperature, and permits the most rapid recovery (Fig. 2). (Power savings are illustrative of typical results and drawn specifically from the Edwards iXH610 pump). Assuming a time distribution of 70% normal and 30% idle, green mode vacuum pumps can produce over $360,000 in annual utility savings and 1,778 tons reduction in CO2 emissions in a typical 300mm semiconductor fab, and up to $400,000 in annual utility savings and 2,000 tons reduction in CO2 emissions for a typical Gen 8 LCD flat panel fab.
|Figure 3. Idle mode operation also reduces energy consumption of combustion based exhaust gas abatement systems.|
Abatement systems can also benefit from green mode operation with equally significant savings, in one study reducing heat load from 4.58kW/hr to 0.44kW/hr, fuel consumption from 18 lpm to 2 lpm and water usage from 2 lpm to 0 lpm, while requiring recovery time of <10s (Fig. 3) .
Integration: system level
Integration of vacuum and abatement technologies offers the opportunity to design modular, configurable solutions optimized for environmental impact as well as safety, tool compatibility, footprint, and low installation and ownership costs. Vacuum/abatement system integration saves valuable fab real estate by minimizing the wiring and piping required, while preserving optimal pumping efficiency. Integrated systems can offer standard modules that can be easily configured to meet specific customer space and configuration requirements.
Intelligent integration of the abatement process can also reduce its environmental impact. For example, several processes that generate condensable by-products require the post-pump addition of large flows of hot nitrogen to keep these materials from condensing in the exhaust pipe work. By keeping the pipe work short and heating it all the way to the abatement module in an integrated system, the post-pump purge can often be eliminated for additional gas and energy savings.
Comparison of an integrated vacuum/abatement system (Edwards Zenith) with the costs of an ad hoc solution having similar performance/capability typically yields savings of as much as 50% in installation time, 50% in contractor installation costs, 40% in system footprint and 20% in ongoing energy costs.
Integration: process level
Vacuum pumps must be customized to meet the requirements of the specific process. For instance, the pump may need to handle high particle loads, or corrosive materials. It may need to be designed for optimal efficiency when pumping light gases such as hydrogen. It may need to operate at a certain temperature, or with a foreline trap, to prevent the deposition of process materials on pump surfaces where they can increase friction and decrease pumping efficiency. Ultimately, the choice of pump and the power requirements of the vacuum system are determined by the process requirements. Clean processes, such as etch and implant, can generally use low power pumps, while more challenging processes, such as CVD, may require high torque motors, pipe heaters, heating jackets and other power hungry capabilities to ensure reliability.
Combustion abatement systems also have process specific requirements, not only to optimize energy consumption, but also to avoid the unintentional creation of noxious compounds in the combustion process. Each process can pose specific abatement challenges and opportunities. For instance, abatement for nitride based compound semiconductor processes must handle high flows of flamable hydrogen in the presence of other toxic and pyrophoric gases, while preventing the formation of closely regulated nitrous oxides (NOx). On the plus side, a properly designed system for this application can use the ammonia and hydrogen in the exhaust gas as fuel for the combustion process. Ensuring complete combustion of widely varying hydrogen flows requires careful control of the air flow into the combustor.
In addition to simply meeting the performance and safety requirements of a particular process, appropriately designed systems matched to specific process requirements can yield significant energy savings. In one example, a current generation FPD fab had 50 process tools, each using multiple pumps to efficiently evacuate the very large process chamber. Replacing four smaller previous generation pumps (Edwards iH60K) with two larger, current generation pumps (Edwards iXH1220 plus a PXH600) reduced power consumption per process tool from 22.4 kW to 13.2 kW, a 41% reduction. Additional savings also accrue from advanced design features such as the ability to operate at reduced pump speeds when the process requires less vacuum.
Integration: fab level
As described previously, green mode operation holds the potential for significant energy savings, contingent upon successful integration of control infrastructure in the fab. Internal green mode switching has been standard on most semiconductor vacuum and abatement systems since 2007. These systems include a µTIM connector with pins assigned for the switching signal. Equipment manufacturers are now beginning to provide output signals to control the vacuum and abatement systems that support their process tools. The next level of integration will be the implementation of the proposed E30/37 standard, a much more flexible and comprehensive communication protocol, which will allow the manufacturing execution system (MES) to control the idle mode status of process and ancillary equipment through active utility control (AUC).