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.

Professors Michael Thompson and Christopher Ober from Materials Science and Engineering at Cornell University – both pioneers in the use of lasers to expedite baking of photoresist and understanding photoresist activation chemistry — are perfecting the unique advantages of laser heating versus current state-of-art hotplate bake used in chip patterning processes.

Caption: Using a continuous wave laser focused to a line and scanned over the surface, laser-spike annealing is capable of heating up a silicon substrate to its melting temperature in millisecond time frames.

They say the new laser-based approach provides significant improvements. Historically, a lengthy bake of the wafer at low temperatures has been required to avoid degradation of the photoresist properties. Driven by industry demand for continued scalability of ICs, Cornell researchers have determined that heating at much higher temperatures for millisecond times using continuous wave lasers not only activates the necessary photoresist chemical reactions at higher throughput, but also improves the pattern fidelity and line-edge roughness over conventional methods by limiting the chemical diffusion.

“Faster, higher fidelity pattern transfer in the fab means better chip performance at reduced cost. This new laser method can deliver a breakthrough in thermal processing for the industry,” said Obert. “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.”

Among the many challenges that the industry will face in the move to 450mm is how to limit cost while continuing to improve the materials, equipment and processes needed to fabricate the rapidly shrinking features. 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 of 80-150°C for approximately one minute 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 to take place 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.

Laser spike annealing is already in use for dopant activation in silicon, as explained in this article published in Solid State Technology last year. 

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