Assist bar Optical Proximity Correction (OPC) has been demonstrated to increase across pitch performance and depth-of- focus of semi-dense to isolated lines1. As the sub-resolution assist feature (SRAF) or assist bar’s size increases, so does its desired lithographic effect, as well as its undesired printability. In other words, when large assist features are required at isolated pitches, the assist features may print. A frequency-preserving assist bar solution is the most preferred one, but difficult to realize for opaque assist features due to printability. The concept of frequency-preserving Gray Assist Bar OPC has been introduced as a method to extend imaging performance for small features across a wide rage of duty ratios3. In this paper, we will present the experimental validation of this concept. The Gray Assist Bar mask was manufactured using a two-level lithography process, and the optical properties have been characterized using a Woollam VUV VASE system. Additional metrology was performed using an AFM (SNP9000) and CD SEM (KLA8250XR). Exposures on a 0.75NA 193nm scanner clearly show the expected effects. The use of the Gray Assist Bar features reduces the through pitch critical dimension (CD) variations significantly and can hence be regarded as an ‘Optical Proximity Correction’. The isofocal inflection point of aerial images is shifted in cases with Gray Assist Bars, resulting in flatter bossung curves and a larger depth of focus (DOF) for the various features through pitch at their target size. This results in larger overlapping process windows. The Gray Assist Bars has also shown a very low printability, even with aggressive off-axis illumination (OAI) settings.
Date of creation, presentation, or exhibit
Department, Program, or Center
Microelectronic Engineering (KGCOE)
Lafferty, Neal; Vandenberghe, Geert; Smith, Bruce; and Lassiter, Matthew, "Gray assist bar OPC" (2004). Accessed from
RIT – Main Campus