DR. MIKE CZERNIAK, ANDREW CHAMBERS and ADRIENNE PIERCE, Edwards, Clevedon, UK.
The move to 450mm wafers will likely result in gas flow increased on process vacuum and abatement equipment.
The semiconductor industry is gearing-up for a transition in silicon wafer diameter from 300mm to 450mm, driven by the requirement to reduce device manufacturing costs. This transition is the latest in a series of wafer size increases, as illustrated in FIGURE 1. The transition to 450mm wafer high volume manufacturing is currently predicted to occur sometime beyond 2020.
With each new generation, the corresponding process gas flows have increased in order to maintain throughput for the larger wafers and the larger process chambers they required. A similar increase in process gas flow rates is anticipated for the 300mm to 450mm transition, but with industry demands to reduce overall utility consumption while simultaneously increasing productivity, there is pressure to reduce the gas flow scaling factor relative to the 2.25 times geometrical scaling factor of the larger wafer surface area. Nevertheless, a significant increase in gas flows is still anticipated.
Impact of increased gas flows When process gases or by-products are corrosive, flammable, condensable, or contain significant quantities of solid material, nitrogen is often added into the pump mechanism itself and/or into the downstream exhaust pipe. So, higher process gas flows will drive a proportional increase in nitrogen purges added to the exhaust, which in turn, will augment the vacuum and abatement capacity required by 450mm processes. This nitrogen serves a number of important purposes:
1) diluting corrosive gases to reduce damage to pump components, 2) diluting flammable gases to avoid flammable mixtures when either an oxidant is also present or in the event of an air leak into the system, 3) diluting condensable gases to avoid condensation of damaging materials onto equipment surfaces, 4) assisting the pumping of light gases, and 5) increasing the gas velocity to ensure that entrained particulate matter keeps moving through the pump mechanism and exhaust pipe. In practice, the total nitrogen purge and dilution flow rate is scaled with the process gas flow to maintain overall system reliability or to meet a specific operational requirement, for example, dilution of a flammable gas to avoid a potential flammable mixture.
For vacuum pumps, higher process gas flows require the deployment of larger capacity pumps, increasing not only the capital cost and but also the total operating cost due to higher nitrogen and pump power consumption. Furthermore, the increased flow rates also affect the capacity requirements of point-of-use (PoU) gas abatement systems. The impact of the increased gas flow on the abatement system is similar to that on the vacuum pumps, namely, higher capital cost for greater capacity, potentially physically larger systems, higher operating costs for more fuel or electricity to heat the gas to a sufficient reaction temperature for destruction, and more water to cool the system and scrub reaction by-products from the exhaust stream.
|FIGURE 1. Historical evolution of Si wafer diameter in the semiconductor industry.
Integrated vacuum and abatement
The most obvious scenario for the 450mm transition would simply scale the capital and operating requirements of vacuum and abatement systems to match the increased gas flows. While this approach would still capture the economic efficiencies accruing from a gas flow scaling factor that is lower than the wafer surface area scaling factor, it misses other significant opportunities to enhance overall efficiency and realize even larger reductions in cost per unit area – the intended goal of the 450mm transition.
Since 450mm wafer processes are still under development, the gas flows that will ultimately be deployed are unknown at this time. However, it is possible to investigate methods to reduce nitrogen dilution or purge flow rates now, based on 300mm processes and 450mm projections. With clear process specification, the optimal pump design can be selected, and the requirement for purging of the pump mechanism itself can be reduced. For instance, the use of chemically-resistant seals would allow exposure to higher concentrations of corrosive gases. Similarly, running the pump at higher operating temperatures could allow the passage of higher concentrations of condensable gases without the build-up of solids. Examples include NH4Cl from low pressure CVD (LPCVD) nitride deposition processes and AlCl3 from metal etch processes.
The practical limitation to reducing nitrogen purge flows is often the pipeline that connects the pump exhaust to the PoU gas abatement system, which can be very long. Because the gas in this pipe is at approximately atmospheric pressure, the risk of corrosion is greater than at the reduced pressure found in the vacuum foreline and the gas is often diluted to avoid damage to the equipment. Likewise, condensable gases are most prone to condense into solids and block the pipe when the pressure is higher, and the typical response to this problem is to increase the nitrogen purge flow rate. A flammable gas may be diluted to prevent fire or explosion if the gas leaks into the fab environment.
|FIGURE 2. Integrated vacuum and abatement.|
A viable solution is to make the pump exhaust pipe very short, which minimizes the volume of enclosed gas and reduces the cooling that occurs over extended pipe runs, as illustrated in FIGURE 2. While it is common practice to actively heat long pump exhaust runs, the power used for heating can exceed the power consumed by the pump itself. In addition, bends, valves and joints all need to be properly heated and insulated otherwise they act as sites for solids accumulation and blockages. To minimize the risk of flammable, toxic or hazardous gas escaping into the fab environment, the vacuum pumps and gas abatement can be housed in a common extracted enclosure held at lower pressure than the fab, ensuring that any leaking gas does not escape into the fab. By monitoring the cabinet extraction, the presence of a gas leak can be detected and suitable alarms raised for remedial action, while the leaking gas remains confined within the equipment enclosure and factory personnel are protected from exposure.
The opportunity to optimize exhaust pipe configuration and system operation while minimizing nitrogen purging, equipment size and utility costs, constitutes a major benefit for adopting an “integrated system” approach to vacuum and abatement. An expertly integrated design can resolve potential conflicts between reducing utility costs and ensuring operational safety in the presence of flammable or reactive gases. New approaches to safe equipment operation can be better implemented, and more readily accepted, in the context of an integrated system, in which the various elements are designed to be complementary and mutually supportive. The integrated supervisory monitoring and control system can be designed to ensure safe operation, monitor leak integrity, detect equipment malfunction, and shut-off process gas if a critical situation arises. The integrated system also offers better alignment with sub-system performance requirements under specific process conditions driving optimized BKMs (Best Known Methods). For example, during a deposition versus a clean process, and a purge only mode can be added when appropriate.
Additional benefits of sub-fab equipment integration include:
- Reduced overall equipment footprint compared to traditional “stand-alone” configurations.
- Reduced utilities hook-up requirements, since facilities connections can be shared between vacuum and abatement systems, reducing hook-up cost and installation time.
- Easier implementation of “Green Mode” utility-saving strategies – the process tool only needs to send one set of signals to the integrated system controller to trigger low power or idle mode operation.
- Overall system ownership and the reduction of unintended consequences.
Integrated vacuum and abatement systems, such as the one shown in figure 3, are installed and operating at more than fifteen 300mm fabs across the full spectrum of process applications. The higher overall costs and productivity requirements for 450mm processing make the arguments for adopting integrated systems even more compelling. The industry has a unique opportunity to incorporate and enhance these cost-saving system considerations in the 450mm fab. Collaborative 450mm initiatives are at the forefront of “changing the game” to reduce the higher overall costs while improving the value of wafer throughput and reducing the manufacturing cost by area of device produced.
The transition from 300mm to 450mm is underway and it is likely to be accompanied by significant increases in not only process gas flows, but also the consequential nitrogen purges used in process vacuum pumps and downstream exhaust systems. Simply increasing pump and abatement capacity will result in increased capital and operating costs proportional to the increased flow. An alternative strategy is to integrate the vacuum pumps and gas abatement into a combined system with optimized exhaust pipe configuration, process exhaust temperature control, a common extracted housing, single utilities connection points and a single supervisory control system. These integrated systems with better tool communications interfaces have the potential to increase the value of the 450mm transition by further reducing operating costs.
DR. MIKE CZERNIAK, is a product marketing manager, ANDREW CHAMBERS, is technical manager and ADRIENNE PIERCE is business development manager with Edwards, Clevedon, UK.