In this installation of our lithography-focused blog series, we discuss haze formation in lithography applications, and how to mitigate it by monitoring for AMCs. Go here for our previous blog in this AMC Contamination monitoring series.
Optical haze is the result of crystalline salts photochemically produced by the reaction between airborne acidic and basic AMC. The most abundant basic AMC contamination is ammonia (NH3) and other primary and secondary amines. A major objective in eliminating defects in litho is to prevent the formation of haze on mirrors, lenses, masks, reticles, prisms and other optical components. Here’s the general process:
- AMC contamination-caused formation of salt crystals.
- Haze forms on the surface of optical components.
- Pattern defects occur in the litho functional area (litho FA).
Who should be concerned: Manufacturers of highly engineered, precision surfaces used for optimal transmission of light.
What does optical haze do?
Once formed on optical surfaces, the salt haze will degrade the image that is intended to pass through the optics without distortions in wavelength or emission power during the actinic processes within the litho scanning tracks. Instead, the resultant image degradation causes pattern defects on the wafer. These defects lead to dimensional and overlay defects.
- Acid-Base Example: Airborne constituents of ammonia and sulfur dioxide (SO2) photochemically react to produce ammonium sulfate salts.
This reaction is further catalyzed by the presence of UV light, speeding up the production of salts and increasing optical haze on all surfaces. EUV techniques of double and quadruple exposure and slower resist reaction times will also increase the production of salts and haze. Although pellicles are available to help keep masks clean in DUV and previous generations of litho techniques, their transmission is low, thereby reducing exposure tools throughput significantly.
How do we limit optical haze?
As of 2020, the 7 nm lithography FA employs the use of DUV patterning and immersion patterning. As the industry moves forward into the 5 and 3 nm logic nodes, EUV will be used for the smallest pitches and immersion quadruple patterning for some levels. More exposures translate into higher probability of defects related to the litho process, including those caused by the presence of AMC-related anomalies in precision transmissive and reflective optical components. Therefore, real-time AMC contamination measurement and control with sub ppb sensitivity remains a primary requirement in the litho FA, which extends to optical component and sub-system manufacturing areas in order to improve defect excursions emanating from litho.
The AirSentry® II (ASII) Point of Use Analyzer provides highly sensitive detection of AMC Acids and Amines with 24/7 coverage and excellent time-resolution, providing AMC concentrations with detection limits down to 70 ppt sensitivity.
Shown above are the lower detection limits for available form factors of the Particle Measuring Systems AirSentry II product line:
- Point of Use (POU) Analyzer: Provides 24x7x365 coverage at critical single sample locations.
- The AMC Multipoint Manifold System allows automated sampling across 16 or 30 sample channels with up to 75 m of sample tubing length for 2 – 5 per day sample trends in areas of interest.
- The fully integrated AMC Mobile Cart provides real-time sampling on a mobile platform and maps concentration gradients across the fab to pinpoint sources and easily characterize new areas by a single user.
Download the full analysis of haze-causing reactions here. And look forward to our next blog on defects caused by AMC in the photolithography functional areas.