Burden of Progress Continues to Fall to Optical

Arguably the most noteworthy change to the 2005 edition of the International Technology Roadmap for Semiconductors (ITRS) — introduced last month in Seoul, Korea — is its departure from the concept of "technology node" as the main pace setter for the IC industry (see "No More Technology Nodes in New ITRS "). Although the latest edition acknowledges an independent measure of the technology pace of DRAM, microprocessor and flash products, the DRAM half pitch remains a convenient shorthand. At the 32 and 22 nm half pitch, for example, the Lithography working group considers optical lithography to still be a potentially viable solution. Along with next-generation considerations, the 2005 ITRS updates the expectations and challenges associated with extending photolithography to such an extent.

A significant change for lithography since the 2003 edition of the ITRS is the new criteria for evaluating near-term potential solutions. Candidates must address leading-edge requirements in at least two geographic regions, and all infrastructure must be ready at the time needed. It also helps if a candidate can show its extension across at least three technology generations. With this in mind, 193 nm systems (including immersion) have won out over 157 nm lithography and electron-beam lithography using masks for the next two technology generations. If fluids and lens materials can be developed with a high index of refraction (n), immersion may extend into the 32 and 22 nm generations. At those generations, however, extreme ultraviolet (EUV) lithography is considered the most probable solution, although nanoimprint and maskless lithography are also considered potential solutions.

But along with the extended confidence in optical's abilities, more emphasis has been placed on immersion's challenges as well. Both immersion and EUV lithography are mentioned as Grand Challenges in the 2005 ITRS — "difficult challenges" deemed significant enough to mention on a broader scale. For volume production to be realized with immersion lithography, it is critical to gain more control of defects and staining developed within the immersion environment, and to improve immersion resists and topcoats. Then, extending immersion down to the 32 nm half pitch would require development of resists, immersion fluids and lens materials with very high indexes of refraction.

EUV has long been the favored post-optical candidate, and has the most momentum behind it. But to get EUV lithography where it needs to be for volume production, there are several issues that remain to be resolved from both technical and economic standpoints, including mask blank defects, lightsource, resist, optics performance, optics contamination, and the protection of the mask without the use of a pellicle.

The 2005 Lithography chapter provides more detail on several key elements, including resolution enhancement techniques, design- for-manufacturing, and next-generation requirements — adding a table describing the requirements for imprint templates.

The lithography-specific difficult challenges for the near term are detailed in the Table . For lithography beyond the 32 nm half pitch, challenges also include such issues as defect-free masks for both imprint and EUV lithography, phase-shifting masks for EUV, metrology and defect inspection, cost control and return on investment (ROI) and various resist material concerns.

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