Thank you for joining us in this special blog series from Particle Measuring Systems on mitigating defects caused by airborne molecular contamination in lithography processes. This time, we look at stochastic defects and where they can happen in photolithography functional areas. You can find the previous blog in this series here.
Stochastic Defects
Here’s a refresher. Stochastic defects are:
- Myriad pattern and overlay defects that originate in the litho FA actinic processes.
- Random time-dependent variations in light exposure and resist chemistry.
Stochastic defects can occur whenever there is as in acid-catalyzed, chemically amplified resist (CAR) material that is unique to the DUV Photolithography FA. They can occur as a direct result of degradation of photo resist material in the highly controlled pre-Bake, Coat, post-exposure-Bake (PEB), and Develop processes within the Litho Track.
Where does Airborne Molecular Contamination come from?
In DUV, acid-catalyzed photo resist is an integral part of the actinic processes in litho. In the case of “positive-patterning” lithography:
- Resist (made of solvents) coats the wafer substrate.
- UV light converts the resist into water-soluble acids.
- The acids are visible through an actinic lens during the exposure time in the exposure tool.
- Polymerized, water-soluble areas of resist are then removed by dissolution in the aqueous develop solution.
In the case of negative-pattering, the pattern of UV light in the exposure tool is the image-negative of the positive-patterning image from the final desired resist structures.
When basic AMCs (the most common of which is ammonia) come into contact with the acid catalyst intended to react with the surface resist, UV light exposure is slower and less effective than intended for use in the extremely precise UV light pattern. Many types of pattern defects and overlay defects are caused by this unwanted resist-catalyst reaction when AMC are not controlled and below ~1 ppb.
What do Lithography Defects look like?
These Lithography Defects can take the form of bridging between lines, missing contact holes, line opens or merged contact holes. Recent work has shown that they are more common than simple extrapolation of Critical Dimension variation, assuming a normal distribution. Stochastic defects currently limit the usable resolution of EUV tools, but are not the result of AMC reactions on the exposed wafer inside the vacuum environment of the EUV tool. The EUV area still requires general environmental and transport-path AMC control.
One of the first pattern defects found to be directly caused by the presence of reactive amines reacting with resist, called T-Topping (see image), was first researched and documented in 1997 using IMS as the most suitable choice for real-time in-situ AMC monitoring.
For these reasons, most Photolithography Functional Areas now set upper control limits of ~1 ppbv Total Amines, creating the obvious need for sub ppbv or ppt detection limits.
What’s next?
Moving into the 2020s, the emphasis has been made to implement even longer exposure and scanning in the DUV litho processes to accommodate the double and quadruple exposure needed per IRDS. This change is to further prevent the occurrence of the stochastic defects when using the prescribed slower reacting stabilized resist. Over the past few decades, extensive research has been performed for more stable resist and resist additives that can endure longer queue times in both pre- and post-exposure and bake to eliminate loss of photo-acid by surface evaporation. There is a need for more control of deleterious environmental contamination, including better detection and real-time response to undesired AMC concentration levels, the most prevalent of which are the amines class of AMCs. Real-time AMC monitoring with the precision, accuracy, and ease-of-use performance capabilities of the ASII, is becoming more essential as these advanced EUV processes are developed and sustained into the current decade.
Thank you for reading this lithography-focused blog series. You can find the full paper to download and read here. Next time, we’ll look at AMC Control in Precision Optical Lens Manufacturing. See you then!
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RELEVANT CONTENT
Airborne Molecular Contamination (AMC) Control in Advanced Lithography Applications
Airborne Molecular Contamination Monitoring Optimized for Lithography
AirSentry® II Molecular Contamination Analyzers Calibration and Troubleshooting
AMC Airborne Molecular Contamination Control in Clean Manufacturing Environments
