Cost, performance challenge both established and advanced lithography

by Katherine Derbyshire, Contributing Editor, Solid State Technology

Conferences like the recent SPIE Advanced Lithography conference focus on the leading edge — e.g.,EUV, double patterning, immersion lithography — yet process technologies remain commercially important long after the leading edge has moved on. In a presentation to investors, ASML CEO Eric Meurice noted that KrF-based lithography is used in more than 30% of the layers in 65nm node DRAMs. That number is likely to increase in the coming years as KrF displaces i-line exposure, itself used for about 40% of 65nm node DRAM layers. While these relatively mature technologies are not used for performance-critical applications, they still have a substantial effect on manufacturing yield and cost.

According to Plamen Tzviatkov, business director for advanced photoresists at Fujifilm Electronic Materials, fabs are especially interested in simplifying their KrF processes. A leading-edge process might use several different resists, optimized for different pattern structures: one for isolated lines, one for dense lines, one for contact hole arrays. The resist stack might include a bottom antireflective coating (BARC) in addition to the imaging layer itself. Yet these additional layers add cost and process complexity. Each resist increases the number of chemicals and process recipes that the fab must manage, while each unique process recipe complicates the scheduling of both wafer tracks and steppers.

Thus, Tzviatkov said, resist suppliers like Fujifilm are developing resists that can serve multiple process layers. For example, the company’s GKR5212 resist can print 220nm contact holes, dense line/space patterns, and isolated lines [see Fig. 1].

Similarly, in ion implant applications using thin resists, the GKRS-6757A resist can print dense lines, isolated lines, and isolated trenches with 125nm critical dimensions, while not requiring a BARC layer [see Fig. 2]. Innovations like these can substantially reduce the cost of KrF processes.

That’s not to say that the leading edge is not important, however. As fabs contemplate device production with immersion lithography, requirements for immersion resists are coming into focus. During early immersion development, fabs and exposure tool suppliers worried about lens contamination due to leaching of resist components into the immersion fluid. Tool suppliers mandated the use of resist topcoats. Yet leaching is a chemical problem, Tzviatkov pointed out, and is one that chemical companies like Fujifilm can and should address. Fujifilm took a topcoat-free approach to immersion resists, thereby simplifying the resist process and reducing chemical costs.

Yet merely avoiding leaching is not enough to produce a successful resist. To match dry lithography’s performance, an immersion resist must achieve comparable imaging results while avoiding such immersion-specific defects as watermarks and pattern collapse. Tzviatkov attributes Fujifilm’s success in this area to a proprietary “magic additive” that allows effective optimization of the resist’s surface properties.

Immersion resist development has been especially challenging because resist requirements have been something of a moving target. Exposure tool suppliers only recently released leaching specifications. Measurement standards for immersion defects have yet to be established, Tzviatkov said, so results presented by different laboratories cannot be compared. Though researchers are making progress, the ultimate destination remains somewhat unclear.

In the end, both mature and leading-edge processes have the same goals: superior imaging performance with minimal defects at the lowest possible cost. As processes mature, superior imaging becomes routine, allowing fabs to focus on defects and costs. To prosper, suppliers must make the transition along with them. — K.D.

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