Japan – Tackling the Challenges of Today and Tomorrow
Multi-discipline cooperation will be key to the successful development of 300mm clean manufacturing environments and processes.
By John Haystead
On the silicon islands of the Pacific Northwest, it may appear that the focus of contamination control research is strictly aimed at the next generation of IC technology, but this is in fact, not the case. As in the U.S., a heated and continuous debate rages in Japan over where and how contamination control attention should be focused, and when new approaches and technology should be introduced.
Smack in the center of this activity is the Ultra Clean Society (UCS) of Japan, which was organized in October 1988 to help transition the futuristic ultraclean technology concepts proposed by Professor Tadahiro Ohmi of Tohoku University (Sendai, Japan) from the academic environment into actual volume semiconductor manufacturing. In addition, however, the UCS continues to sponsor the development of new and innovative technologies for immediate use in the semiconductor industry. Today there are 240 companies supporting the society, including not only Japan`s semiconductor manufacturers, but also process equipment vendors and component suppliers. Non-Japanese members of the society include Sematech, Advanced Micro Devices, Varian, IMEC, Meissner & Wurst, and Unit Instruments.
Chaired by Professor Ohmi himself, the Society conducts two to three technical symposia in Japan each year to promote and disseminate the ultraclean technology activities developed by both industry and academia. In addition, it publishes a regular technical journal which includes English-language summaries of each paper.
One of the most important activities of the UCS, however, according to UCS Secretary General, Mr. Yoriyuki Murakawa, is its work on the development of standards for ultraclean semiconductor manufacturing in Japan. There are currently four working groups targetting materials, components, equipment and cleanrooms, and the committee formally releases summaries of its work every two years. For the moment, the UCS is not actively pursuing the adoption of its standards work beyond Japanese industry.
Although the UCS`s activities are funded by its member companies, at present, the society does not itself fund independent research and development work. This part of the organization`s goal is handled by the closely-associated, but independent company, Ultraclean Technology Research Institute (UCRI) Tokyo. The President of UCRI, Dr. Takahisa Nitta (formerly of Hitachi Ltd), is also the Vice Chairman of the UCS. UCRI`s objective is to commercially develop new ultraclean process equipment and manufacturing systems technology with the support of industry investment partners.
Just formed in April of this year, UCRI is not currently structured to manufacture equipment itself, but instead provides consulting services to semiconductor equipment manufacturers in Japan and the U.S. Among its current projects, the company is working on an ultraclean, room temperature wet processing station designed to be incorporated into a cluster tool environment. Developed with technical guidance from Dr. Ohmi, UCRI has shipped the first machine to Tohoku University where it is being evaluated. The company has also begun discussions with US companies for further testing.
Dr. Ohmi presented the results of the prototype room temperature wet station development work at the UCS`s latest meeting last month in Tokyo. A potential replacement for RCA wet cleaning, the system uses a combination of ozonized DI water (5ppm), megasonic shower cleaning, and a solution of HF/H2O2/H2O with surfactant to remove wafer contaminants. According to Ohmi, consumption of ultrapure water and chemicals is reduced to 5% and 1% respectively of current levels and, unlike RCA clean, the system does not generate chemical vapor contamination.
Another ongoing UCRI project is for a high performance ultraclean gas panel for dry plasma processing such as oxidation furnaces. Again, the concept was developed by Tohoku University but testing has been done by industry.
Timing Is Everything
While individual ultraclean products and concepts are viewed with great interest, the overall UCS philosophy of a totally clean manufacturing environment is a long way from being totally designed and built into today`s fabs. Hiroshi Nakagawa, Managing Director of UCRI, and also a member of the UCS International Cooperation Committee, says “industry is gradually accepting the UCS philosophy of ultraclean manufacturing”, but he also recognizes that some of the elements of the concept are sometimes seen as prohibitively expensive. “The tendency is to view adoption of the philosophy one step at a time, but when people are only looking at one slice of the problem, they may see what appear to be unreasonable costs. Instead, they should think in terms of total cost of ownership, then the proposal is very reasonable,” says Nakagawa.
Given the cost issues, Japanes manufacturers question whether the time is right to introduce a next-generation system. One Japanese company is reportedly already constructing a fab on Kyushu island that it claims is 60% based on ultraclean concepts, but as pointed out by Nakagawa, a clear distinction between an ultraclean and conventional installation is sometimes difficult. “Sometimes a gas system or DI water is based on ultraclean technology, for example, but the cleanroom is conventional.”
Also, while the goal remains the same, the UCS itself acknowledges that the ultimate implementation mechanism for ultraclean manufacturing is still evolving with both alternative approaches to be weighed, and missing technological elements to be invented. For example, according to Nakagawa, “although we know future trends will be toward closed systems, we don`t yet know what the final environmental approach will be. SMIF could be a candidate but also cluster tool technology is being developed.”
In the meantime, the extent to which ultraclean manufacturing is implemented in production fabs is driven by the relatively modest contamination control requirements of current devices. “But,” says Nakagawa, “as we approach the 1 Gigabit structure levels, we expect the concept will be incorporated widely.”
Mini`s Offer No Panacea
Only two companies are currently using minienvironment technology in Japan: LSI Logic and IBM. Significantly, neither is a Japanese company, and their decision to use minienvironments is believed to have been largely made by their American management. In strong contrast, native Japanese companies are still using the ballroom cleanroom approach.
According to Dr. Takeshi Hattori, general manager, ultraclean process technology, ULSI R&D laboratories, Sony Corp., (Atsugi, Japan), and Chairman of UCS`s International Planning & Coordinating Committee, there are several reasons for this, one being that “minienvironments are not currently suitable for volume production.” Another, however, is more fundamental. “Minienvironments do not solve the biggest problem affecting yield in modern semiconductor manufacturing today, let alone tomorrow.” Hattori is referring to process equipment-generated contamination, and it is this problem that consumes the current attention of the Japanese semiconductor industry.
Although ranked ninth in the Japanese semiconductor industry, Sony generates $2.5B in semiconductor business annually. Fifty percent of its devices are used in Sony`s own products with the remaining sold to other OEMs. Its principle semiconductor products are SRAMs (IBM being its biggest customer), followed by CCDs. Sony is also the world`s largest producer of GaAs laser diode devices and also manufactures small dimension LCDs using conventional 8″ wafer technology.
The shortfall of minienvironments, says Hattori, is that current designs only protect wafers from particulate and molecular contamination existing in the cleanroom air flow. “They don`t address the problem of the outgassing of contaminants from the containment/transport system materials themselves, and, more importantly, they do not address the problem of process-equipment generated particles and other contaminants.”
As observed by Hattori, although modern fab cleanrooms are far from clean relative to molecular contamination, for current geometry devices, the problem is still primarily particulates, even though the challenges of particulate control in cleanroom air have largely been solved. “The particles aren`t getting get any smaller, and today`s filters are more than adequate to the task.”
It`s The Process ….!
In Japan`s fabs, according to Hattori, the challenge remains the particles generated by the process equipment. “Of the many potential contamination sources, particle generation within process equipment is by far the most frequent cause of yield loss — not people and not the air.” For this problem, minienvironments pose additional challenges rather than solutions since smaller environments mean contaminants become more concentrated near the wafers. In addition, to remove accumulated contaminants from the process equipment, there must be easy and efficient access.
There`s no question that contamination control research work at Tohoku University is also now closely associated with improvements to process equipment cleanliness. As part of these efforts, the university is building an extension to its existing “supercleanroom” facility. According to Dr. Mizuho Morita, Associate Professor, Integrated Intelligent Device Laboratory, Department of Electronic Engineering, Tohoku University, with the new cleanroom extension, they will now have a full 0.01 micron device production line. A minimum process system exists in the earlier cleanroom which was built in 1986. The current program began in 1994 and will extend for ten years. The building will be completed this month with new process equipment, including reactive ion etching, installed in September and operational capability planned for a year from then. The purpose of the facility is to develop and demonstrate a high-performance production line.
Truly next-generation technologies, such as Dr. Ohmi`s ultrapure gas handling, ultrapure water supply or prototype processing equipment will not be a part of Tohoku`s existing production operation, however, Dr. Ohmi has submitted a proposal to the Japanese Ministry of Education to build an advanced technology center and cleanroom facility at the university which will be used to further develop this technology. According to Dr. Ohmi, “the prognosis for this proposal is good and, if approved, will lay the foundation for developing and testing the next-generation clean manufacturing environment.”
Some of the concepts for the next generation cleanroom/clean process environment are now being developed in Dr. Ohmi`s mini-supercleanroom laboratory. These include: Ohmi`s new room temperature, wet process station, a “clean” CMP station, an excimer laser lithography process and advanced, specialty gas handling and delivery systems including tube passivation and welding techniques, end magnetic media analysis. Mr. Yasuyuki Shirai presented the laboratory`s latest work on an “Ultraclean Gas Tubing System for Excimer Laser Lithography” at the UCS`s symposium in Tokyo last month. This work addresses the corrosive properties of Fluorine gases with a newly-developed fluorine passivation (FeF2) technique.
By Any Name
In Japan, any discussion of “minienvironment” systems inevitably leads to a discourse on the merits of Standard Mechanical Interface (SMIF) technology, the result being that the discourse usually quickly ends. The often cited reason is shared by Dr. Kiyoshi Demizu, Chief Engineer of Mitsubishi`s USLI Laboratory, (Hyogo, Japan). Demizu sees SMIF-type technology as a barrier to high-volume, factory automation systems.
Mitsubishi is Japan`s 4th ranked semiconductor company operating seven fabs in Japan, Europe and the U.S. as well as a joint fab in Taiwan. Says Demizu, “when you`re concerned about volume production, factory automation systems and tools are critical. They must be able to easily access and manipulate product. Technicians must also be able to easily and safely maintain the equipment.”
Regarding SMIF, Sony`s Hattori shares Demizu`s concerns, but adds that in the broader definition of the word, minienvironments are in fact the inevitable technological approach of the future. He points out that, even today, there has been in effect a slow movement toward minienvironments as the amount of “clean” space around process equipment continues to shrink and people are more and more restricted from the environment with “through-the-wall” approaches etc. “As next-generation process requirements come into place, people will not even be able to survive in the actual clean processing environment because of the need to remove all moisture from the area.”
While many of the challenges of process equipment cleanliness must await further development, Mitsubishi`s Demizu points to two principle contamination control points in the current Mitsubishi manufacturing process that will also need to be addressed in any next generation facility. The first is oxidation occurring at the interface layers between wafer process steps and the second is contamination of wafers while they wait in stockers in the cleanroom.
As described by Demizu, their approach to the first problem will be to contain the most oxidation-sensitive process steps within an extremely dry-air environment which will eliminate all moisture and hydrocarbon formation. Automated material handling systems will transport the wafers to and from the ultraclean process stations within a standard cleanroom environment.
Another approach advocated by Tohoku University`s Ohmi in the past was the use of ultraclean transport tunnels between process steps incorporating a steady flow of ultraclean nitrogen to carry away molecular contaminants and moisture. Mitsubishi`s Demizu, however, shys away from the use of nitrogen in his more open-access approach because of the potential oxygen deprivation danger to cleanroom workers. In a recent interview with CleanRooms, Dr. Ohmi, himself, agrees that the dry-air approach is equally acceptable, but on the other hand, is also supportive of SMIF-type containerized transport systems, referencing his recent work on a nitrogen-infused cassette system.
To deal with the problem of wafer con tamination in stockers, Demizu proposes separate stocker areas washed continuously with “fresh” dry, highly-filtered outside air separate from the standard moisture and hydrocarbon contaminated cleanroom air. Demizu anticipates this technology will be initially introduced in Mitsubishi`s first 12 inch fab facility planned for roughly 1999.
A Cleanroom Vendor Perspective
Established in 1923, Takasago Thermal Engineering Group (Tokyo, Japan) is Japan`s largest cleanroom company. Their business is mainly large cleanrooms, constructed using modular components. Over 80% of their cleanroom business is in the IC industry, followed by LCD displays and pharmaceuticals.
Takasago does not currently operate a U.S. branch, but, in contrast, according to General Manager, Dr. Takao Okada, S.E. Asia is seen as a growing market area for them. Asian operations include Singapore, Indonesia, China, Thailand and Malaysia although, according to Okada, most of their current business in S.E. Asia is in cleanroom components rather than complete design and construction.
According to Dr. Soichiro Sakata, Takasago`s Research & Development Center Manager, “most of their advanced research work is done in cooperation with Tohoku University and in general relates to the need to improve process cleanliness. Although Takasago generally views Ohmi`s work as next generation technology — “we focus on our near-term business and don`t expect to be working on supercleanroom technology anytime soon” — they have also discovered that cleanroom technology alone is sometimes not adequate to deal with the challenges faced by their customers, such as the readherence of organic (hydrocarbon) contaminants from cleanroom air.
Says Takasago engineer, Hideto Takahashi, in Japan, although the main interest in the IC industry may be shifting from particles to molecular-level contamination, particles remain of principle concern in the LCD cleanroom. More specifically, the concern is the electrostatic characteristics of the glass substrates which are easily charged and hence attract particles.
One benefit of Takasago`s research work in this area was in proving the benefits of deelectrifying the LCD glass with grounding. Japanese LCD manufacturers are already taking advantage of the relationship of surface resistivity and organic contamination derived from this work by not only grounding substrates during process stages, but by reducing the amount of time that the substrates wait between processes in the cleanroom atmosphere.
Another non-traditional cleanroom technology developed by Takasago is a new type of ionizer using “soft x-rays” (wavelengths over approximately 1 angstrom) and vacuum UV-ray irradiation to ionize gas molecules by means of photon absorption. According to Engineer, Hitoshi Inaba, the “clean” neutralization technology via photon irradiation allows the neutralization process to be conducted in close proximity to semiconductor devices without the problems of ozone generation and electromagnetic noise of conventional ionizers. This means the devices can be included in process equipment as well as wafer transport tunnels. A number of systems have already been sold in Japan which have significant application in both ULSI and LCD manufacturing applications.
Regarding an ultimate solution to the problem of process-equipment contamination, Mitsubishi`s Demizu states simply and candidly, “nowadays I have no idea,” but adds he has great faith in ongoing cooperative R&D efforts. “Cooperation is key,” says Demizu, advocating extensive future work between process equipment vendors, materials suppliers, factory automation, and contamination control companies to develop the semiconductor manufacturing capabilities that will be required for 300mm wafer processing. Demizu points to Mitsubishi`s work with nine other Japanese semiconductor companies in the recently-formed, SEMA TECH-like, SELETE organization whose goal is to fund research work in 300mm process equipment.
Sony`s Hattori agrees stressing that success will require the collaboration of all the participants. “It`s a huge task, but without cooperation, we will not be able to develop these complete solutions,” says Hattori. In addition to technological cooperation, standardization is also seen as a critical element since, without standardization, the cost of modifying equipment to implement any kind of minienvironment system will be cost prohibitive. Says Hattori, “Maybe the next-generation, 300 mm factory will be the opportunity for participants to get together in terms of minienvironment standardization requirements.” n
Next Month: “Asia” Part II, Singapore`s “Next Lap”.
Prototype room temperature wet station equipment undergoes test and evaluation at Tohoko University.
View of Dr. Ohmi`s mini-supercleanroom facility where multiple ultraclean development projects are underway.
The supercleanroom extension will be completed in August.