Imec has demonstrated that the photo-speed of metal-oxide resist (MOR) can be improved significantly when oxygen concentration is raised beyond atmospheric levels during the EUV post-exposure bake (PEB) step. Faster photo-speed means the resist reaches target dimensions at a lower EUV dose, which directly improves EUV scanner throughput and reduces exposure cost. Gas composition in the PEB chamber has not been widely treated as an EUV optimization knob, which makes the announcement significant, but it remains to be seen whether this can be industrialized.
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Scientists from Imec discovered that raising oxygen levels from 21% (ambient air) to 50% during the EUV PEB step results in a 15% – 20% increase in photo-speed, which means that the metal-oxide resist can reach its target dimension at a lower EUV dose. Lower dose cuts exposure time, which in turn increases an EUV scanner's throughput per hour and can reduce the cost of EUV step per wafer and ultimately per chip, though do not expect the lowered exposure cost to have a significant impact on the cost of the final product. The improvement was confirmed for both experimental MOR formulations and commercially available MOR materials, according to Imec.
Metal-oxide resists have become leading candidates for advanced process technologies that rely on Low-NA EUV and eventually on High-NA EUV lithography as their high-resolution capability, lower line-edge roughness, and favorable dose-to-size characteristics outperform those of chemically amplified resists (CARs), which are widely used today. The high resolution and reduced LER directly translate into better pattern transfer capability for the smallest features of critical layers set to be printed using High-NA EUV litho systems. Now, Imec’s findings suggest that MOR's performance can be amplified with environmental conditions during the PEB step.
It should be noted that post-exposure bake is one of the most sensitive steps in the entire lithography flow. PEB activates and drives reactions triggered by photons during the exposure, so small variations of temperature, heating ramp rate, bake time, and atmosphere can have drastic effects on critical dimension (CD), line-edge roughness (LER), and stochastic defect levels, which means that one combination of settings can lead to yield improvement, another can be a yield killer. Changing gas composition inside the PEB module is a big deal not only from the pure semiconductor manufacturing flow point of view, but also from such points of view as long-term material stability, tool oxidation, and safety considerations, just to mention a few.
In standard EUV production environments, wafers are exposed in a vacuum and then transferred to a bake module operating under normal cleanroom air containing 21% of oxygen. So, to conduct its experiments, Imec developed a special tool called BEFORCE*, which isolates wafer handling and baking from the surrounding fab environment. The system integrates gas injection and blending capabilities along with built-in photo-speed metrology, which enabled researchers to regulate oxygen contents in the chamber while controlling photoresist performance. To put Imec's discovery into use, foundries will have to ask their fab tool makers to replicate what BEFORCE does during the PEB step.
"This is just a first result from the BEFORCE tool: the controlled gas composition provides an additional knob to study the origins of environmental effects on the lithographic variability of MOR materials," said Ivan Pollentier, Senior Researcher at imec. "Equipment manufacturers can use these insights as a guideline to adapt their tools for improved EUV lithography throughput and stability."
*Bake and EUV system with FTIR and Outgas measurement for Resist evaluation in Controlled Environment.
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