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Lithography

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In a global magazine exclusive David Ridsdale, Editor-in-Chief, EuroAsia Semiconductor reports on ASML’s newest immersion lithography tool.

If you were to look at the lithography players in the 1980's you would find a list of about ten players from across the world planning for a bright future. Around this time Philips decided to commercialise their stepper to Dutch company ASMI who continued the technology growth but had trouble finding a profit so in 1990 decided to spin out the stepper business and a new lithography company, ASML, was created.

Despite the history the company found themselves a new player in the lithography market. The earliest days of ASML depended on the technical connections and collaborations they had developed. Philips maintained a 60% stake in the beginning ensuring ASML continued to benefit from the technical interaction that began in the 1980's. This collaborative approach would go on to form the basis of future success.

Throughout the 1990's ASML continued to develop new lithography tools that were the technical equivalent of the top two lithography vendors; both Japanese. The Japanese semiconductor industry had prided itself on their global leadership at this time and the two lithography vendors were the jewel in the crown. By the end of the 1990's ASML was one of five lithography companies left in the world, leaving very few areas with the capacity of complete silicon chip manufacturers. The Japanese semiconductor industry has been well known for its insularity and resistance to outside companies so what ASML achieved from the year 2000 to the present is even more remarkable. Between 1999 and 2000, three events occurred that shaped the future success for the company. Firstly ASML purchased MaskTools, creating a lithography enhancement division. Secondly the company released a new lithography concept with the TWINSCAN dual stage system and finally in October 2000, ASML bought the last front end lithography company in the USA.

ASML has gone from strength to strength on the basis of their modular approach to their Twinscan platform that was the first lithography tool that allowed upgrades and customisation on a common platform allowing manufacturers to personalise processes creating a new differentiator. The European industry realised the importance ASML's success was to the European semiconductor community.

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European success
No matter what benchmark you use, ASML has become the number one lithography company in the world today. This is something that no-one in the industry, except ASML, appears to have expected. By 2002 ASML was the number one tool supplier and have developed an extraordinary network of partnerships, joint ventures and outsourcing that define the success of ASML. This success has a knock on effect for the whole European industry and ASML is now one of the key factors for continued success in the European semiconductor community. ASML may be global but their importance to European research is paramount. Whether it is collaboration with IMEC, Crolles or European manufacturers, ASML is active in almost every research and development in Europe as well as other programmes around the world.

ASML is a prime example of the positive impact European support for R&D can have when clearly oriented to market needs. Market forces alone simply cannot stimulate the huge collaborative effort required. The Japanese semiconductor industry were concerned enough by ASML's rapid rise that a study from Tokyo's Hitotsubashi University looked at the reasons ASML were able to surpass the Japanese companies. The study came to the conclusion that it was the way that ASML integrates with the rest of the industry. The report discusses how ASML has used forums like SPIE much more effectively than their Japanese counterparts. According to the report by Professor Hiroyuki Chuma, the Japanese companies have been reticent to share their ideas whereas ASML has made it part of their culture.

Immersing in the future
With every generation, the complexity of producing integrated circuits with more functionality increases. Semiconductor manufacturers need partners that provide technology and complete process solutions rather than traditional tool providers. Chip manufacturers make road maps to provide guidance on future technology needs and the industry felt a looming crisis a few years ago as optical lithography was reaching the end of expected limits.

The development of 157nm lithography was dropped when it was realised the cost of change outstripped the benefit. After much throwing of hands in the air it was realised that the older and well known practice of immersion lithography may be used to be able to create smaller line widths with the current 193nm lithography. The development of immersion, which places liquid between the lens and the silicon wafer, meant chipmakers could produce smaller features while using light with the same wavelength.

In 2003, ASML shipped the first immersion tool in the industry, providing ASML an earlier entry point than the competition. With a Numerical Aperture (NA) of 0.75 to produce features smaller than 90nm, the first tool could use a light source with a wavelength of 193 nm to produce features as small as 65 nm. Although these first immersion tools would not produce features smaller than their dry counterparts versions, they gave chipmakers the opportunity to test and develop immersion technology as a volume-manufacturing tool. Chipmakers cannot jump into immersion production without first learning to use immersion tools and adapting their processes to immersion.

This collaborative approach was key to the next success of the company. ASML tested all their new immersion tools in active fabs around the world. The Japanese competitors chose to test only in laboratories meaning sceptics found results did not provide any real time outcomes. Immersion is not a new technology. Engineers working in the field of lithography have understood the impact liquid could have on the lithography process for years, but the challenges of bringing liquid into the process were too great at that time. The close collaboration with partners means ASML can deal with issues in a real environment.

The latest milestone
Not content to rest on their laurels ASML continues to drive immersion with the singular goal of maintaining technology leadership whilst creating innovations ahead of the competition. To this end their latest immersion tool, the TWINSCAN XT:1900i once again exceeds industry expectations and provides the highest NA in the industry with 1.35 NA with a 26x33mm2 field making it the highest NA ArFi in the world. Considering the industry though it would be hard to pass a NA of 1, these are impressive figures.

All the lithography companies are providing immersion lithography and the main differences for ASML is the dual staging platform that increase throughput and the methods used to keep the water between the lens and the wafer and keeping water off the wafer. ASML use a shower head that keeps the water in situ rather than a flow of water that can drop onto the wafer. Good throughput is more useful to a manufacturer than raw figures as profits are based on the number of chips you can make rather than individual quality.

Manfred Suddendorf, the Product Manager for XT:1900i at ASML pointed out that the qualities of the tool were only one part of the equation in scaling down nodes. How the tool is used by the manufacturer is just as important. Companies can use lithography tools in a relaxed manner. The machine is not run beyond its limits and operates without extra add-ons, tweaking, difficult materials and other invasive assisting methods. This is where the manufacturer comes into play. The more aggressive they approach manufacturing a particular product the more they will gain from a lithography tool. This is basically true of any tool but the difference in output in this case can be startling. (see table)

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One of the main challenges for immersion lithography has been the damage a liquid can do to the etching process, should it come into contact with the photoresist, which was developed to work in a dry environment. With the advent of immersion, the photoresist must now be water resistant. The liquid itself is a challenge. Water produces good results in the current 193 nm systems, but if the NA has to be significantly increased, a liquid with a higher refractive index than water will be needed.

Another challenge had to do with metrology, or the measurement of the wafer. In order to be able to print the patterns onto the wafer with extreme precision, the lithography equipment must make measurements of the wafer surface. By introducing water into the process, the measurements can become inaccurate. The metrology challenge has not been huge in this area as the TWINSCAN platform allows the measurements to be taken when the stage is in the dry position before liquid enters the equation.

The three points of excellence ASML is selling with the new tool are the dual stage, the modularity and the experience and readiness of the tool. ASML only build tools for high volume production. A new tool is not released unless the company is already sure it will meet industry requirements. The modularity also enables companies to re-use lithography commodities, like reticles, in different tools. ASML long term vision is paying off for them

ASML state this may be one of the last increases in immersion NA due to the limits of chemistry.

There is still a great deal of learning but results are positive with IBM and TSMC stating defects in single figures with immersion lithography. Results from patterning with immersion tend to be connected to length of time a company works with a tool. Results improve as the manufacturer understands how far they can push the tool for their own needs. It has been a big change in industry culture where ASML is now a partner rather than supplier.

ASML knows it has not being easy to achieve their recent milestones but they are under no illusions that the competition is looking to close the gaps. Industry grapevines suggest that 2006 will see both companies introducing a dual stage on their lithography tools and will be looking to improve their positions. So far the Japanese have not shipped any immersion tools as opposed to ASML's 15 shipped around the world, including Japan. When the tools cost between 20-30 million dollars each, it is plain to see why ASML continues to move further away from the competition.


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