Actinic Phase Defect Detection for Extreme Ultraviolet Lithography Mask with Absorber Patterns

Abstract

An actinic (at-wavelength) dark-field inspection tool was developed for detecting phase defects over the full area of a multilayer coated extreme ultraviolet lithography (EUVL) mask blank. This tool was also used for inspecting EUVL mask with absorber patterns. To understand the effect of absorber patterns on dark-field inspection signals, a test mask with programmed phase defects and absorber lines with periodic patterns was fabricated and inspected. Simulated and experimental results showed that the absorber lines increased the background intensity of inspection signals, whereas the intensity of the phase defect signals remained almost unchanged. The level of the background intensity of the inspection signal for absorber lines (128–180 nm wide) was approximately 10 times higher than that for mask blank signals without absorbers. Although increased background resulted in a decrease in the defect's signal-to-background ratio, the signal from the phase defect was still brighter in comparison to its surrounding area.