Interferometric tool extends reach of deep UV

SAN JOSE, CA - During the annual SPIE Microlithography conference in February, scientists from the IBM Almaden Research Center (San Jose, CA) described results obtained using NEMO, an interferometric tool for deep UV optical immersion lithography. They have created distinct and uniformly spaced patterns with ridges and spaces that are only 29.9-nm wide, less than one-third the size of the 90-nm features now in production in the semiconductor industry. The features are also smaller that the 32-nm size that the industry currently holds as the limit for optical lithography.

The new result indicates that a “high-index immersion” variant of deep UV lithography may provide a path for extending Moore’s Law further within the optical realm. Light passing through a higher-index material acts as if it has a shorter wavelength and can be focused more tightly. Resolution in immersion lithography is limited by the lowest refractive index of the final lens, fluid and photoresist materials. In the NEMO experiments reported at the meeting, the lens and fluid had indices of about 1.6, and the photoresist’s index of refraction was 1.7. Future research is aimed at developing lens, fluid and photoresist materials with indices of refraction of 1.9, which would enable even smaller features to be imaged.

“Our goal is to push optical lithography as far as we can so the industry does not have to move to any expensive alternatives until absolutely necessary,” said Robert Allen, manager of lithography materials at Almaden Research Center. “This result is the strongest evidence to date that the industry may have at least seven years of breathing room before any radical changes in chip-making techniques would be needed.”

NEMO was designed and built at IBM Almaden using materials developed by its collaborator, JSR Micro (Sunnyvale, California). It uses two intersecting laser beams to create light-and-dark interference patterns with spacings closer than can be made with current chip-making apparatus. The system is intended to enable relatively inexpensive researching, testing and optimizing of the various high-index fluids and photoresists being considered for use in those future DUV systems that would create such fine features.

“We believe that high-index liquid imaging will enable the extension of today’s optical lithography through the 45- and 32-nanometer technology nodes,” said Mark Slezak, technical manager of JSR Micro. “Our industry faces tough questions about which lithography technology will allow us to be successful below 32 nanometers. This new result gives us another data point favoring the continuation of optical immersion lithography.”

The concept of design for manufacturing (DFM) was a recurrent theme, both in technical sessions and on the exhibit floor. In an invited talk, Luigi Capodieci of Advanced Micro Devices said that Moore’s Law of exponential growth has not been limited to circuit density on semiconductor chips, but has also applied to the growth of pages in manuals of semiconductor design rules. There was only one page in 1987 but there are 250 pages today; according to Moore’s Law, he joked, there will be 2000 pages upon reaching the 22-nm node.

As feature sizes on semiconductor chips have become smaller than the wavelengths of light they are written with, the complexity of design rules has grown to ensure that circuits, once manufactured, actually do perform according original design specifications. The complexity of design rules, however, can also lead to designs that are too complex to manufacture while also achieving acceptable yields. DFM seeks to address such issues by bringing the designer and lithographer communities together in the design process, often assisted by software tools that make the design step more of an iterative modeling and feedback process.
- Hassaun A. Jones-Bey