Mask Roughness Induced LER
Brittany Marie McClinton1 and Patrick Naulleau2
National Science Foundation and Department of Energy
As next generation lithography techniques such as extreme-ultraviolet lithography (EUVL) push to ever smaller critical dimensions, achieving the stringent requirements for line-edge and -width roughness (LER/LWR) is increasingly challenging. For this reason, discovering the principle causes leading to LER demands immediate attention.
Until recently, LER has been considered a resist-limited effect. Now, however, it is better understood that system-level effects can greatly influence LER. Thus, in order to achieve adequately low LER levels for next generation projection lithography, we must understand not only the basic material properties of the resist, but also how resist effects and mask effects can each propagate down to and contribute to the LER that is ultimately printed. Much research has already been conducted to fully characterize photoresists available to next generation lithography methods. Other studies have documented how the LER on the mask couples to the printed LER. What remains incompletely understood, however, is the extent to which system-level effects such as mask surface roughness, defocus, and illumination conditions are currently factoring into LER limits and how to distinguish mask effect from resist effect in practice.
Because EUVL is a band-limiting imaging process (ie., an aperture limits the spatial frequencies it passes), the mask surface roughness (which is geometrically related to phase roughness) is directly coupled to speckle-induced intensity variations at the image plane, and can significantly alter LER measurements of the printed image [1,2]. Moreover, because EUVL is dependent on reflective optics and masks, the geometrical relation of mask roughness to phase roughness is amplified by a factor of 2 due to reflection. As defocus is introduced, mask phase errors even further couple into intensity variation, or speckle. As expected, when coherence is increased, speckle is enhanced, and the mask phase error-induced speckle increases proportionately.
Preliminary results below show that for a particular type of illumination (in this case, disk of sigma .5) at a defocus of -50 nm, there exists a ‘forbidden’ zone of correlation lengths of roughness on the mask where the LER reaches a maximum. For small correlation lengths below this peak value (high frequency scatter), we suspect that more of the scattered light is thrown out of the pupil thereby reducing speckle and LER. This is in contrast to long correlation lengths above the peak value (low frequency scatter), where all of the scattered light is caught by the pupil. This detrimental effect, however, is mitigated by the slower rate of change of the induced phase across the rough mask since the mask itself is smoother from the slope error perspective.
Figure 1: Simulated LER values through focus for various correlation widths in mask coordinates (a 4X system).
Figure 2: Looking closely at one defocus setting (in this case -50 nm), there exists a ‘forbidden’ zone of correlation lengths of roughness on the mask where the LER reaches a maximum
- N. Beaudry and T. Milster, “Effects of object roughness on partially coherent image formation,” Opt. Lett. 25, 454–456 (2000).
- N. Beaudry and T. Milster, “Effects of mask roughness and condenser scattering in EUVL systems,” in Emerging Lithographic Technologies III, Y. Vladimirsky, ed., Proc. SPIE 3676, 653–662 (1999).
1CXRO / LBNL
2CXRO / LBNL