By Chris Muller, senior member of IEST
To those charged with establishing and maintaining the appropriate controlled environments for leading-edge semiconductor manufacturing, airborne molecular contam-ination (AMC) can appear to be a moving target. Sulfur dioxide, ozone, and organics from outside air, as well as acids, bases, dopants, and organics from sources inside the fab, may all have to be considered in a successful AMC control program. But which contaminants should you choose? What control levels should be considered? Fortunately, there is a resource that facility and process engineers can use to learn about critical issues relative to AMC and its effects on semiconductor manufacturing.
The road(map) to knowledge
The objective of the International Technology Roadmap for Semiconductors (ITRS)1 is to ensure advancements in the performance of integrated circuits and to provide an assessment of the semiconductor technology requirements. This assessment, called roadmapping, is a cooperative effort of the global industry manufacturers and suppliers, government organizations, consortia, and universities.
The ITRS identifies the technological challenges and needs facing the semiconductor industry over the next 15 years. It is sponsored by the European Semiconductor Industry Association (ESIA), the Japan Electronics and Information Technology Industries Association (JEITA), the Korean Semiconductor Industry Association (KSIA), the Semiconductor Industry Association (SIA), and the Taiwan Semiconductor Industry Association (TSIA). SEMATECH is the global communication center for this activity. The ITRS team at SEMATECH also coordinates the USA region events.
Roadmap architects-the working groups
The ITRS process encourages discussion and debate throughout the semiconductor community about the requirements for success. The key factor in the success of this roadmap is obtaining consensus on industry drivers, requirements, and technology timelines.
The Technology Working Groups (TWG) are the organizations that “build” the roadmaps. These representatives assess the state of technology and identify areas that may provide solutions. The TWG members also indicate opportunities for new research and innovation. Those working groups that discuss AMC and associated control requirements are Factory Integration (FI), Lithography (Litho), and Yield Enhancement (YE). Of these, the Yield Enhancement TWG and, specifically, the Wafer Environmental Contamination Control (WECC) sub-TWG, actively discuss, identify, and deal with issues relative to AMC for cleanroom and wafer environments.
Factory Integration. Factory integration focuses on integrating all the factory components needed to efficiently produce the required products in the right volumes on schedule while meeting cost targets. In the 2005 Roadmap, the Factory Integration chapter2 provides technical requirements and also proposed potential solutions for factory operations, production equipment, material handling, factory information and control systems, and facilities. This chapter also provides FI-related challenges from key focus areas, such as AMC, that need to be addressed in order to keep up with the technology generation changes and, at the same time, maintain the decades-long trend of 30 percent per year reduction in cost per function.
Factory Integration has cross-TWG issues relating to lithography and metrology for AMC relative to the reticle (reticle storage and in the litho equipment), but defers to the Yield Enhancement TWG to maintain AMC technical requirements and facility cleanliness levels. There are also AMC concerns relative to interconnect, front-end processing, and environmental safety and health.
Lithography. The key requirements of Lithography for manufacturing integrated circuits as summarized in the 2005 Roadmap include critical dimension control, overlay, defect control, and low cost.3 The Lithography TWG considers the control of AMC critical to maximize yield by minimizing local poisoning of resist and minimizing the formation of progressive defects on masks during exposure, and expects yield enhancement to become a major challenge, as critical defect sizes become smaller than the limits of optical detection.
For some processes, such as advanced lithography, very small quantities of “high molecular weight/high boiling point” (eg, C6 to C30) hydrocarbons are detrimental because of increased adherence to the exposed surfaces and the potential for photochemical degradation to leave non-volatile residues on lenses, masks, mirrors, etc. For the same reason, other potential impurities, such as siloxanes or organophosphates, can also be very detrimental in extremely small quantities.
Maintaining the rapid pace of shrinking device geometries requires overcoming the challenge of improving and extending optical projection lithography technology while simultaneously developing alternative, next-generation lithography technologies. Extending optical projection lithography and developing next-generation lithographic technology requires advances in, among others, resist materials and processing equipment, as well as mask-making, mask-making equipment, and materials.
Yield Enhancement. Yield enhancement is defined in the 2005 Roadmap as the process of improving the baseline yield for a given technology generation, from R&D yield level to mature yield. The Yield Enhancement Chapter4 and, specifically, the focus topic of Wafer Environmental Contamination Control (WECC) are where guidelines and technology requirements for AMC can be found.
Wafer environmental control4
Wafer environment control includes the ambient space around the wafer at all times, whether the wafers are open to the cleanroom air or stored in PODs/FOUPs (front-opening unified pods). AMC needs to be controlled in the front end and back end of line operations in semiconductor fabs. This control may be achieved fabwide or at certain critical processes, potentially also at different levels for different processes.
Outgassing from materials of construction in the cleanroom, wafer processing equipment, and wafer environmental enclosures, as well as fugitive emissions from chemicals used in wafer processing, are considered to be the main sources of AMC. Acid vapors in the air have been linked with the release of boron from HEPA filters, and the impact of amines on deep ultraviolet (DUV) photoresists are well known examples of AMC affecting wafer processing. Hydrocarbon films of only a few monolayers may lead to loss of process control, especially for front-end processes. The impact of AMC on wafer processing can only be expected to become more deleterious as device dimensions decrease.
There is a need for better AMC monitoring instrumentation in the cleanroom to measure AMC at the part-per-trillion levels, and low-cost, routine monitoring may be required as devices approach molecular dimensions. Not all process steps will be impacted by AMC, however; the potential for AMC to impact new processes should be considered in all process integration studies.
The 2001 ITRS recommendations for AMC levels could be found in Tables 95a and 95b of the Yield Enhancement chapter.5 These tables outlined the technology requirements for wafer environmental contamination control (WECC) for the near term and long term, respectively. The WECC technology requirements were categorized by manufacturing materials or environment, and indicated target levels of ambient acids, bases, condensables, dopants, and metals for specific process steps (see Table 1). For AMC, the entire cleanroom was considered to be the wafer environment.
Table 1: 2001 Technology requirements for wafer environmental contamination control
The Yield Enhancement chapter of the 2003 Roadmap6 provided both the short-term and long-term technology requirements for the WECC, as before (now found in Tables 114a and 114b). The wafer environment was still considered to be the cleanroom ambient, but there were a couple of significant changes. There was now a differentiation between AMC in the gas phase and that which deposited on surfaces (surface molecular condensable or SMC). This is shown in Table 2.
Table 2: 2003 Technology requirements for wafer environmental contamination control
The Yield Enhancement chapter of the 2005 edition of the ITRS4 addresses AMC control based on (in part) the three major advances in semiconductor manufacturing that were becoming fully integrated into state-of-the-art fabs, namely device geometries of 65 nm and smaller, 300 mm wafers, and cop-per processing.
As mentioned above, previous versions of the Roadmap treated the entire fab as the wafer environment; however, increasingly automated wafer handling and processing means that the wafer is rarely, if ever, exposed to the ambient cleanroom environment. Instead, the wafer environment has shrunk to mini-environments, process tools, PODs, and FOUPs. To acknowledge this, and as an attempt to reduce operation costs for the same level of AMC control, WECC has been divided into process areas. For the Yield Enhancement chapter of the 2005 ITRS, WECC requirements are categorized by manufacturing materials or environment (Tables 115a and 115b) and are summarized here in Table 3.
Table 3: 2005 Technology requirements for wafer environmental contamination control
Other significant changes include:
■ AMC monitoring for condensable organics is now relative to compounds with GC/MS retention times greater than or equal to benzene, calibrated to hexadecane.
■ A requirement for the control of acids in the cleanroom ambient, lithography processing, and for exposed aluminum and copper wafers and reticles.
It should be noted that these new AMC requirements are minimum requirements, and contaminant targets apply up to the point of use (defined as the entry point of the wafer process chamber within the process tool) and not to the ambient cleanroom environment. Tighter limits might be more efficient.
The sources of AMC originate both from outside and inside the fab. The outside AMC sources origins are: industrial (e.g. organics, acids), automobile and truck traffic, agricultural (eg ammonia, hydrogen sulfide), and other pollution types. Also, when not properly designed or controlled, the semiconductor factories exhausts themselves turn to a major external source for AMC (re-entrained back to the fab). Internal AMC sources include: outgassing from construction materials (e.g. polymers, coatings, concrete), accidental releases of exhausts and chemicals used in the cleanroom for maintenance/cleaning/hook-up activities.
Many of these contaminants can cause corrosion on wafers and facility materials, and damage to borosilicate glass fiber filters (with subsequent release of boron into the cleanroom). In addition, the cleanroom personnel are a source of AMC. The timely identification of AMC sources determines which control measures are appropriate in order to prevent wafer damage.
Direct control of AMC inside the facility include measures such as:
■ Air washers/chemical filters in the makeup air-handling units
■ Chemical filters in the recirculating air system
■ Chemical filters for the mini-environment/stockers
■ Purging of wafer and reticle carriers
Modeling and simulation tools are required to determine and validate the most appropriate AMC control solutions. Furthermore, these tools should deliver a fair basis for estimating the cost effectiveness of the proposed solutions.
To keep AMC under control, multi-faceted programs are recommended. They include: surface molecular contamination (SMC) measurements and analyses, off-line sampling with impingers, online instrumentation, cooling traps, and adsorption tubes. The greatest challenge and the essential goal is to establish a data management system, which links these measurements to some performance/production metric that provides feedback for automatic process control and run-to-run control.
Ongoing and future work
An emerging focus area requiring innovative solutions for Factory Integration is the prevention and control of AMC. The increased importance of AMC in the fab will require revisiting contamination-control procedures with new methods and materials, which could also affect facility components used during construction. Facility operations will also require coordination with production equipment vendors to ensure proper AMC control.
Factory integration will need to specify general cleanroom conditions with respect to environmental AMC limits. Discussions between the FI and YE TWGs are ongoing to determine whose responsibility this will be.
AMC control guidelines for Lithography are based in large part on inputs from photolithography tool suppliers. All photolithography tools should have chemical filters on the makeup air to the internals of the tools. These filters have a finite lifetime, which is dependent on the contaminant loading. Providing a chemically cleaner cleanroom ambient environment will extend the life of these filters.
The Yield Enhancement chapter now has a requirement for the control of total acids, in addition to total bases, in the cleanroom ambient for lithography. For future Roadmaps, the YE TWG has been asked to put in requirements addressing specific acids-not just totals.
The use of inert environments to transport and store wafers is expected to increase with process sensitivities. Pre-gate and pre-contact clean and salicidation are cited as processes to first require this capability. Potential solutions for WECC relative to AMC include:
■ Online monitoring for AMC contaminants
■ Reduced cost of ownership for AMC control
■ Development of emergency response procedures and measures for fugitive emissions
■ Verification for AMC limits relative to metal corrosion
In the 2005 ITRS, wafer environment contamination control now includes the ambient space around the wafer at all times, whether the wafers are open to the cleanroom air or stored in PODs/FOUPs. As the list of ambient contaminants to be controlled broadens, one of the main WECC challenges is to measure AMC at specified control levels in real time. Affordable, accurate, repeatable, real-time sensors for non-particulate contamination are becoming increasingly necessary.
There is also a growing need for accurate modeling of cleanrooms for AMC sources and distribution. This is because with many current cleanroom designs, the reduced air volumes cannot dilute AMC as well as before-whether it comes from outside or inside the fab.
■ Laminar flow cleanrooms will keep contaminants more localized, but how much dilution air is needed to mitigate AMC events?
■ A cleanroom with turbulent flow spreads contaminants more quickly with a fast dilution rate.
■ An open sub-fab design allows for quick dilution of contaminants and may not require the use of direct AMC control, whereas a closed sub-fab makes direct AMC control almost obligatory.
■ Routine maintenance of process tools, pumps, and wet benches can lead to fugitive emissions.
The wafer environment contamination control tables of the Yield Enhancement Chapter provide recommended contamination levels as follows:
■ AMC as measured/monitored in the cleanroom air and/or purge gas environment
■ Surface molecular contamination (SMC) on monitoring wafers
Cost-effective integration of AMC controls into factory design and operation should incorporate a variety of measures, all the way from detection of AMC sources, through control methods, up to the active protection of the wafer environment.
The 2005 edition of the ITRS takes advantage of expertise from around the world to help identify the technical challenges related to AMC, and offers recommendations that can be used in establishing AMC control strategies and guidelines for advanced semiconductor device manufacturing.
Christopher O. Muller is the technical director for Purafil, Inc. in Doraville, Georgia, a manufacturer of gas-phase air-filtration media, filters, equipment, and air-monitoring instrumentation. He is responsible for technical support services and various research and development functions. Prior to joining Purafil, he worked in the chemical process and pharmaceutical manufacturing industries in quality assurance/quality control. He has written and spoken extensively on the subject of environmental air quality and the application/use of gas-phase air filtration, and counts over 90 papers and articles, more than 20 seminars, and seven handbooks to his credit. He can be reached at [email protected]