The implementation of double patterning presents certain challenges, including elevated mask and lithography expenses, and constraints on circuit layout, which can affect design complexity, device performance, variability, and density.
Double patterning mitigates diffraction effects in optical lithography, which arise because the minimum dimensions of advanced process nodes are significantly smaller than the 193nm wavelength of the light source. These diffraction effects compromise the accuracy of deep sub-micron patterns, causing blurring and potentially preventing the replication of small features from the mask onto the wafer.
To mitigate diffraction effects, such as shorts and opens, the original mask is partitioned into two distinct masks, designated as Mask A and Mask B. This approach, however, escalates mask costs (lithography expenses) due to the implementation of double patterning and introduces additional Design Rule Checking (DRC) constraints.
To address the increasingly significant diffraction challenges associated with each new process node, several reticle enhancement techniques have been developed.
Phase-shift masks, introduced at the 180nm process node, modify the phase of light passing through specific mask regions. This alteration influences diffraction patterns, thereby mitigating the defocusing effects associated with mask dimensions smaller than the illumination wavelength. A drawback of phase-shift techniques is the increased complexity and cost of mask fabrication.
Optical Proximity Correction (OPC) techniques involve pattern distortion on the mask to counteract diffraction effects. This is achieved, for example, by adding small "ears" to the corners of a square feature on the mask, ensuring sharp definition on the wafer. While effective, this technique introduces layout restrictions, computational overhead in design, and increases the time and cost associated with mask production..
