Laser spike anneal for photoresists outperforms hotplate bake
Researchers at Cornell University have developed a new laser-based method for ultra-fast anneal of thin photoresist films. The research, sponsored by Semiconductor Research Corporation (SRC), has shown that the new anneal outperforms a state-of-the-art hotplate bake for both 193nm and EUV lithography applications.
Figure 1: Using a continuous wave laser focused to a line and scanned over the surface, laser-spike annealing heats up a silicon substrate to its melting temperature in millisecond time frames.
Laser spike annealing is already in use for dopant activation in silicon (see J. Hebb et al., "Dual-beam laser spike annealing for advanced logic applications," http://bit.ly/VLRWCg). Now, Professors Michael Thompson and Christopher Ober from Cornell are perfecting the unique advantages of laser heating vs. current state-of-art hotplate bake used in chip patterning processes.
Historically, wafers required a lengthy bake at low temperatures to avoid degradation of the photoresist properties. Heating at much higher temperatures for millisecond times using continuous wave lasers not only activates the necessary photoresist chemical reactions at higher throughput, the researchers have determined, but also improves the pattern fidelity and line-edge roughness by limiting the chemical diffusion.
Figure 2: Patterns generated under 13.5nm EUV exposures using a) hot-plate PEB and b) laser PEB on a commercially available EUV resist. Critical dimensions were 52.1nm and 47.8nm for hot-plate and laser patterns respectively, with a target half pitch of 50nm. For these comparable SEM images, laser patterns show ~15% enhancement in sensitivity and ~15% reduction in linewidth roughness compared to hot-plate patterns. PEB temperatures and durations were 85??C for 60 sec (hot-plate) and 72W (~225??C) for 2ms (laser). (SEM image courtesy of GlobalFoundries)
"This new laser method can deliver a breakthrough in thermal processing for the industry," said Ober. "Until now, lithography progress has been held back by the traditional methods for heating the resist that were regarded by many as already optimized. The laser proves otherwise."
As one of the key benefits of the laser-based bake process, Cornell Ph.D. candidate Byungki Jung has shown significant improvements in line-edge roughness for both current 193nm immersion lithography as well as for next-generation 13nm EUV lithography.
Currently, photoresists are exposed and then baked on a hot plate at low temperatures (80-150??C) for approximately 1min to activate the resist chemistry and create a solubility differential between exposed and unexposed parts of the resist, which delineates the post-develop pattern. Prolonged heating, especially at higher temperatures, causes excessive chemical diffusion which degrades the image quality.
The novel application of laser spike heating ??? for a duration of milliseconds only ??? preserves the polymer integrity at much higher temperatures (up to 800??C) and provides a means to maximize resist sensitivity while minimizing pattern roughness, thereby facilitating enhanced scalability. ??? P.S.
Solid State Technology | Volume 55 | Issue 9 | November 2012