Incident Angle Change Caused by Different Off-Axis Illumination in Extreme Ultraviolet Lithography

Extreme ultraviolet lithography (EUVL) is believed to be possible patterning technology which can make 22 nm and below. EUV uses a reflective mask so that the mask is shined with the oblique incident light. Thus, the study of incident angle effect is very important. Currently, 6 degree oblique incidence is main stream, but 5 degree incident angle is also studied for 0.25 NA. Incident angles larger than 6 degree are also considered for larger NA. This incident angle will affect many things, eventually to the line width. Shadow effect also strongly depends on the incident angle. This shadow effect in the EUVL mask is an important factor that decreases the contrast of the aerial image and causes a directional problem, thus it will make line width variation. The off-axis illumination (OAI) will be used with conventional on-axis illumination to make much smaller patterns. This OAI will split the main beam and change the incident angle. We found that if the incident angle increased with higher degree of coherence, the aerial image went worse. The CD difference between the horizontal and the vertical pattern is also dependent on the degree of coherence even though it is small.