Decentralized environmental controls bring minienvironments to the forefront
“Working in concert with standard cleanroom delivery and conditioning systems, minienvironment technologies have emerged as the model for 21st century device processing.”
By Brian Milhous
In the space of the last 10 years, contamination control in the semiconductor manufacturing environment has undergone many developments. Early cleanroom technology simply isolated potential contamination sources from operators and processing facilities while providing large-scale ambient controls. As processing technologies evolve, geometries shrink, wafer sizes increase, and production controls get tighter, new environmental control concepts are being implemented that will play a significant role in enabling production of advanced semiconductor devices.
The major industry trend has been toward decentralization, that is, providing HVAC, filtration and process air control closer to the point-of-process — single process steps or processing tools are surrounded to isolate and segment a larger environment into smaller more manageable areas.
In many situations, wafer production at different processing stages requires special levels of environmental control. At one stage, specific environmental control parameters may be tighter than at another stage. For example, ambient temperature and humidity control must be controlled within an increasingly narrow range for the successful application of wafer-resist coating as smaller device size geometries are being constructed. However, during wafer storage this is not as critical as removal of moisture.
At different stages, new environment controls need to be employed to increase production yields. In the future, one of the key challenges will be determining the extent of advanced environmental controls required, how to integrate those controls and which stages will benefit the most from them. Today, simply applying tight environment controls over the entire production facility is cost prohibitive and may not have the same benefits at each processing stage during wafer and device production.
Minienvironments come in various configurations. Working in concert with standard cleanroom delivery and conditioning systems, minienvironment technologies have emerged as the model for 21st century device processing. Traditional large-scale cleanroom technology will continue to provide the baseline ambient control of the physical environment. But, meeting today`s and tomorrow`s process requirements will be achieved by using complimentary and increasingly sophisticated, tool-specific, integrated point-of-process environment control technologies.
The production environment can be viewed as not one aggregate process, but as many individual processing areas. The modern fab has evolved into a complex arena of multiprocessing stages that demand huge real-estate requirements to house large, specialized production tools and specialized wafer delivery systems.
Modern fabs will continue to be populated with various types of minienvironments in the increasingly larger production environment. The minienvironments will encompass manually operated solutions for wafer delivery to a process stage and various automated systems that enclose and deliver wafers to a specific processing phase. Some minienvironments will be only passive solutions, such as enclosures to surround process tools from the ambient environment. Other minienvironment applications will be active solutions that monitor various equipment parameters and provide the necessary control to isolate or decrease potential yield reducing effects. All of the various applications that divide the processing environment into smaller more manageable and controllable stages can be referred to as minienvironments.
Three factors have driven the increasing use of minienvironments in semiconductor processing. First, the overall size of processing areas, which use stringent environment control, has risen dramatically. The physical areas that require control of contaminants and ambient environmental factors was once a fraction of the total processing area being used today for semiconductor production. Currently, more process sequences require tighter control of environmental variables to meet yield and performance results. The full-scale demand on traditional cleanroom technologies, which include conventional HVAC systems, particle and chemical filtration systems, and process air delivery systems, has risen dramatically and requires greater loads on the systems with skyrocketing costs.
Manufacturers are now investigating the potential benefits of shifting tighter environmental control onto the individual equipment suppliers of process, metrology, sorting and other production tools. For example in lithography processing, extremely precise temperature, humidity and chemical filtration control specifications can challenge the cost effectiveness of a traditional cleanroom design. Minienvironment architecture can support the relaxing of ambient control in large-scale control systems while individual equipment suppliers work toward integrating tool-specific environment control at load/un-load stations, and within process chambers.
Second, minienvironment technologies can provide increased production flexibility within the fab environment. Increased flexibility is especially beneficial to foundry and ASIC-device producers. These manufacturers that have a shifting array of products require rapid and cost-effective modularity within their fab architecture to meet the demands of their customers.
Finally, as advances in device production technology have moved into submicron geometries, contamination sources have extended beyond the particle level. Molecular contamination, in the form of metallics, organics and inorganics is being increasingly viewed as a critical enabling technology for sub-0.5-micron processing. As 0.35-, 0.25- and 0.18-micron device geometries become standard, particle control is simply viewed as a given throughout the process staging. The traditional particulate control via airborne filtration control is no longer enough. Controlling the environment at the molecular level will require using new isolation technologies at various processing stages and even during wafer storage. One new technology application is purging a wafer carrier with an inert gas, such as nitrogen, to reduce the effects of oxygen and moisture during the process staging. These emerging technology applications will be directly tied to the extensive application of sealed wafer pods and small, integrated minienvironment solutions.
Minienvironments, in their various configurations, have proven to be an effective solution for cleanroom technology applications. In addition, minienvironments have provided measurable yield improvements through tighter environmental control of ambient variables and molecular concerns.
Passive vs. active SMIF
The SMIF (Standard Mechanical Interface) approach to automated wafer handling, for example, is gaining strong momentum particularly in new fab construction that uses 100 to 200 mm wafer processing. SMIF is also well-positioned to support the manufacturing requirements for 300-mm wafer processing solutions. However, it highlights one of the key questions in minienvironment implementation within cleanrooms for future advanced processing: how and when to utilize passive- versus active-SMIF minienvironment systems.
Active systems include those solutions that monitor and provide integrated precision environmental control of contamination sources including particulate filtration, temperature, humidity, and molecular control or a combination of any of these variables. These active systems can offer significant benefits beyond passive control using only filtration or environmental isolation control. Both active and passive solutions are being developed and implemented in various process tool designs in 200 and 300 mm wafer applications.
The semiconductor industry faces many challenges in the control of processing environment variables. The use of traditional cleanroom technologies will continue, but new isolation technologies will become more pervasive. Minienvironment solutions in various configurations will provide the next phase of cleanroom design and system integration issues especially for today`s advanced wafer processing demands. With both the size and complexity of the wafer processing requirements needed for next-generation devices, the impact of environmental variables and their interaction at the molecular level is a major concern.
Where and when specific minienvironment configuration solutions can support yield gains, and how extensively they can be implemented cost effectively will continue to be one focus of the semiconductor industry well into the next century. n
Brian Milhous is a marketing communications professional at Semifab Inc. in Hollister, CA, a manufacturer of process environmental control systems used in the semiconductor industry. He has previously worked in marketing at Intel, Analog Devices and Motorola. He holds a Bachelor of Science degree from San Jose State University and holds a US patent for packaging design work he developed in 1980.