Tool/Fab Support Strategies

Investigating the use of clean dry air to run fab lithography

Wayne Curcie and Marcel Choudhury, Infineon Technologies; and
Thomas Stagg, Alliance Engineering

Semiconductor fab clean dry air (CDA) system designs have received increasing attention because of the requirements of photolithography tools. Technology developments in the IC industry have led to the use of lithography tools with higher purity, flow, and pressure requirements, placing new demands on CDA system design. This article discusses traditional CDA system design, the role of vendors in supplying CDA to fabs, and lithography tool requirements. It presents measures that existing and future fabs can take to meet their CDA needs and discusses the potential impact of those measures on the manufacturing process.

Figure 1: Schematic diagram of a typical central plant CDA system located inside the central utility building.

A Typical CDA System

CDA is used in many applications throughout the fab and support areas, including in pneumatic controls and tools, purging equipment, air cylinders for machine actuation, product cleaners and blow-off devices, and air-driven pumps. A fab's CDA system is typically located in the central utility plant and is configured along the lines of the simplified schematic in Figure 1. The system is normally designed to provide –80° to –100°F dew-point air with 0.01–0.003-µm filtration. Delivered pressure to the point of use is generally 100 psig. Generally, multiple compressors are needed to generate CDA, and an additional unit (n + 1) serves as a standby. Design data from many fabs indicate that CDA consumption can vary significantly from 25 to 50 std cu ft/min per 1000 sq ft of production cleanroom area. CDA consumption in newer fabs seems to be closer to a nominal 40 std cu ft/min per 1000 sq ft of cleanroom area.

Figure 2: Breakdown of CDA use by fab area.

Typically, more than 80% of the CDA system is used to support manufacturing equipment, while the remaining 20% is used for instrument air and utility applications. In the manufacturing area, wet and lithography applications are the largest consumers of CDA, each using 15% of the facility's total supply. Empirical data from one factory indicate that actual nominal CDA consumption is 55% of the manufacturer's design-flow requirements. Surprisingly, the correlation between wafer starts and CDA use is higher than that for many bulk process gases. Figure 2 breaks down a typical fab's CDA consumption.

Requirements of Modern Photolithography Tools

Photolithography is the most demanding of all fab processes. Depending on the specific model, a lithography tool can require low-parts-per-billion concentrations of organic and inorganic species, a high air-pressure supply, and high airflow.

Some lithography steppers have high purity requirements in order to prevent lens contamination caused by photochemical reactions. While lenses can be cleaned, the cleaning process is difficult, time- consuming, and disruptive to the production process. Moreover, lenses can be cleaned only a limited number of times before their coating is damaged and replacement becomes necessary. Lens replacement can cost several million dollars.

As device geometries shrink, the potential for lens contamination increases. The shorter wavelengths and higher light intensities used to manufacture devices with small linewidths increase the incidence of photochemical reactions. While specific organic and inorganic species of concern differ from one manufacturer's tool to another and from one technology generation to another, <1-ppm levels of total organic carbon or total hydrocarbons, low parts-per-billion levels of organic gases, and moderate parts-per-billion levels of specific volatile inorganic species can contaminate lenses. Species of concern include ammonia compounds, amines, sulfates, and phosphates. In many cases, finding a lab capable of performing contamination analyses can be challenging.

Typically, CDA systems produce air at 100 psig. However, scanner tools require higher CDA pressure of 125 psig for optimal performance. Furthermore, scanners require twice as much airflow as steppers because they use active air mounts for vibration isolation and air bearings for stage positioning. New-generation track tools, on the other hand, require five times more CDA than tools installed five years ago because they incorporate hot plates where CDA is used to remove heat rapidly. These data are based on total design flow specifications rather than actual tool data; preliminary data indicate that actual scanner-tool CDA use is approximately half of the tool's rated airflow.

Clearly, new fabs must consider the requirements of critical lithography tools when they set out to design a CDA supply system. However, even new fabs may be forced to procure such a system before they have detailed information on the process equipment they will be installing. When new tools are to be used in an existing facility, the CDA system may have to be upgraded or otherwise modified. The demand for increased CDA pressure and flow can be particularly challenging for systems that may already be highly utilized.

Supplying CDA to the Fab and the Tool

CDA Distribution Systems. CDA piping distribution systems are commonly arranged in a center-spline configuration. Some older fabs use a perimeter-loop design, while some very large new fabs may use a double-spline design. Tubing is typically constructed of cleaned-for-oxygen-service (CFOS) copper, although CFOS stainless-steel tubing has become more common. While copper tubing is sufficient for the quality of air required, it is prone to quality problems (in part because of oxidation). Copper also requires brazing, but brazing should not be performed in clean areas. Although more expensive than copper tubing, stainless steel may be preferable because it is easier to install, requiring orbital welding instead of brazing. Because stainless steel is easier to work with than copper, it is particularly advantageous for large-diameter tubing. The total installed cost of stainless steel can be similar to that of copper. Labor time and costs of installation seem to vary regionally depending on the experience of the local workforce.

Leasing Vendor CDA Systems. Presumably because of the need to focus capital and personnel resources on the production of devices, fabs have increasingly leased CDA systems from gas vendors in recent years. While some fabs purchase CDA or N₂ to back up an existing central plant CDA system, others receive all their CDA from outside suppliers.

In general, leased systems should be considered carefully to ensure that they meet all site requirements, such as noise levels, energy efficiency (particularly when an extended factory ramp-up is planned), and flexibility. In particular, leased systems can complicate the use of lithography tools. They tend to be less flexible than on-site systems, and because they are typically procured as a complete package of services, they have longer lead times than the traditional central plant system. Hence, fabs considering a vendor-supplied CDA system must define required use and purity requirements ahead of time. Often, leased gas systems are not procured as part of the capital building and site construction project and therefore do not meet site construction requirements.

Figure 3: Leased vendor CDA system located on a gas pad outside the facility. (The numbered boxes indicate the sequence of backup steps in the event of system failure.)

Case Study A

The schematic diagram in Figure 3 offers an example of a leased vendor system installed at an Infineon fab. The system was a unique design intended to provide a reliable, cost-effective supply of CDA by using redundant compressors, a single dryer, and nitrogen backup from liquid nitrogen (LN₂). The equipment was located outside the facility in the vendor gas-pad area. A potential drawback with such a configuration is that a single large CDA compressor is less energy efficient during the initial factory ramp-up than is the compressor setup in the central plant configuration shown in Figure 1. In this case, however, a very fast ramp-up of DRAM products was planned and executed.

A critical flaw was discovered in mid-1998 during the factory ramp-up when a CDA outage occurred. While the system shifted to the use of nitrogen during the outage, as it was designed to do, a set of lithography tools were not able to continue operating. These tools did not use a separate gas supply for the lens purge and were not able to function with nitrogen because of the difference in refractive index between N₂ and CDA, which affected the lens focus. While planned and unplanned N₂ backup events occurred only twice a year, they suspended production in the lithography area, crippling manufacturing. This critical flaw was the result of several converging factors: a lack of understanding of the lithography tools' unique requirements, the use of nitrogen as a backup supply, and a bottleneck in the lithography area that persisted for the duration of the factory ramp-up.

Figure 4: Leased vendor CDA system with a separate CDA train. Redundant feeds run on normal power, and there are separate controls and cooling units.

The system in use was successfully upgraded to provide a separate CDA train, as shown in the schematic diagram in Figure 4, and has performed well ever since. This upgrade was carefully planned and executed to avoid affecting fab operations.

Case Study B

Case study B involves the same fab several years later. A fully ramped, built-out factory, it has a stable level of wafer starts and processes increasingly complex devices with progressively more numbers of layers.

Lithography-Tool Purity Requirements. The fab, which uses the system shown in Figure 4, operates with traditional equipment and components and meets lithography-tool purity requirements. Even at parts-per-billion purity levels, acceptable organic and inorganic concentrations are achievable with conventional dryers (thermal swing adsorption, hot-air regenerable units). However, testing has indicated that special care must be taken at all times when working on the system to prevent the introduction of organics. For example, new equipment (e.g., receivers), piping, and other components must be purged and tested extensively before being placed into service. Furthermore, parts and materials, such as gaskets, temporary hoses, hydrocarbon-based instrument cleaners, and liquid fitting sealants, must be carefully monitored using field quality control methods to ensure that they meet facility specifications.

Initially, analytical testing was time-consuming and expensive. Samples were carefully collected and sent off-site for analysis, which took one week and cost $3000 per test. After QA/QC resources were coordinated and materials to be analyzed were carefully selected, the facility was able to reduce testing time to 48 hours and lower costs by 50%.

Flow and Pressure Requirements. While the CDA system has met the fab's purity needs successfully, flow and pressure requirements have continued to be a challenge, especially as older equipment has been replaced with new-generation tools that have higher flow and pressure requirements.

The first lithography tools requiring high-pressure air were installed with booster pumps—redundant air amplifiers that boost the outlet air pressure using additional flow from the CDA source. While these units are inexpensive (less than $10,000 apiece), they require relatively high levels of maintenance and must be rebuilt every six months, which costs up to $5000 per unit per year. Incurring that cost may be a reasonable approach for a fab with a few lithography tools, but when more tools are involved, the increased maintenance and flow requirements compel the fab to consider alternatives.

In some cases, existing CDA system pressure can be increased to meet higher-pressure requirements (depending on such factors as compressor selection, system demand versus capacity, and distribution-piping pressure drop). However, it is important to consider the energy and capacity impact of such an approach. Boosting the outlet pressure decreases system capacity. In the case under investigation here, increased wafer starts and tool upgrades eventually pushed CDA consumption to within 5% of system capacity.

Since boosting system pressure was not feasible, several other options, as presented in Table I, were weighed:

• The use of a separate lithography compressor system was the approach favored by the tool vendor. Because it is a small, segregated system, such a system can accommodate changing tool requirements. However, the system involves high capital and operating costs, can pose placement difficulties (particularly in the case of existing facilities), and entails production downtime during installation. Despite these challenges, an Infineon joint-venture factory overseas has implemented this approach successfully.

• Another viable option was the use of electric compressors to boost CDA supply pressure to the tools. Such a system must be designed to maintain CDA purity and minimize maintenance. This approach has reportedly been used successfully.

• Other potential options included separating less-critical utility users from the system to increase the availability of CDA from the main supply and upgrading the main CDA supply. However, these options were relatively expensive and did not offer the potential advantage of separating the critical lithography tools from the main CDA system.

In an attempt to avoid or delay costs associated with implementing any of these options, a project was initiated to reduce overall CDA use by 12.5%. Such a reduction would allow the existing CDA system to supply the needs of the site during the most challenging summer months. The project evaluated CDA use throughout the factory and support areas, focusing on relatively inefficient applications and leaks. Reductions were achieved primarily by repairing minor leaks, optimizing and standardizing regulator settings, and reducing or eliminating the CDA used as combustion air and purging in tool-exhaust abatement units. These efforts succeeded in reducing the site's CDA use by almost 10% and eliminating the immediate need for equipment installations or upgrades. CDA conservation efforts are ongoing, and demand is periodically assessed and forecasted based on production plans.

Case Study C

The CDA options investigated in case study B were evaluated to determine their applicability to a new DRAM fab. The design that was finally selected consists of a high-pressure main CDA supply system with N₂ backup similar to that shown in Figure 3 and a separate, very small, high-purity lens-purge air-supply system. The fully redundant lens-purge air-supply system is intended to serve a set of lithography tools that cannot operate with N₂ backup. Nevertheless, depending on the specific process and tools, it may also be feasible to use high-purity nitrogen for lens purging in some cases.

It has been estimated that boosting the main air pressure costs less than $50,000 per year, and at a relatively inexpensive $100,000, the lens-purge air system was more cost-effective than a cylinder supply system and small enough to function with the main system or cylinders as backup.

Conclusion

The competitive DRAM semiconductor market manufactures small, high-capacity devices at a low cost per bit, leading the industry to develop lithography tools with high throughput and fine resolution. Such tools, in turn, require supplies of CDA at a higher flow, pressure, and purity than in the recent past. These requirements have driven and will continue to drive CDA system design.

Many system design options exist. Leased vendor-supplied systems should be carefully considered, specified, and procured. Current and potential lithography tool requirements should be thoroughly documented. Ultimately, the most appropriate systems depend on specific site conditions and preferences, such as the products manufactured, whether the fab is a new or existing one, the availability and cost of capital, tool suppliers, internal lens-purge methods, the anticipated number of tools to be used, and energy costs.

Wayne Curcie is a senior staff engineer at Infineon Technologies (Richmond, VA), where he has served for five years. He has engineering responsibility for various process support systems and QA/QC functions. Before joining the company, he was a project manager and lead process engineer at IDC. He also worked at IBM as an ultrapure water and wastewater treatment systems engineer. Curcie has published and presented articles on semiconductor process support systems and is a member of the American Institute of Chemical Engineers. He received a BS in chemical engineering from Syracuse University in New York. (Curcie can be reached at 804/952-7880 or wayne.curcie@infineon.com.)

Marcel Choudhury is a senior staff engineer at Infineon Technologies, where he has worked for four years. His focus is in the lithography equipment engineering group, where he is responsible for step-and-scan exposure tools. Before joining the company, he worked at Motorola in the lithography/automation group and at Mitsubishi Semiconductor America in wafer fabrication automation. Choudhury received a BS in mechanical engineering from North Carolina State University in Raleigh, NC, and an MS in mechanical engineering from the Georgia Institute of Technology in Atlanta. (Choudhury can be reached at 804/952-7122 or marcel.choudhury@infineon.com.)

Thomas Stagg is a process engineer at Alliance Engineering (Richmond, VA). His responsibilities have included providing design engineering services for process support systems in the semiconductor manufacturing industry. Previously he worked at Infineon Technologies Richmond as a process support systems engineer. He received a BS in chemical engineering from Virginia Polytechnic Institute and State University in Blacksburg, VA. (Stagg can be reached at 804/952-8048 or stagg.external@infineon.com.)