Increase of space width roughness in directed self-assembly patterning arising from shrinking stress in the remaining poly(methyl methacrylate)

The mechanism for the increase in space width roughness (SWR) after selective poly(methyl methacrylate) (PMMA) etching to polystyrene (PS) is investigated for directed self-assembly patterning. Considering the cross-sectional image obtained from field-emission scanning electron microscopy, the authors propose a physical model where the shrinking stress in the remaining PMMA with thickness variation induces PS-line collapse and this collapse increases SWR. Linear finite-element calculations show that this proposed model can explain two experimental results; namely, the SWR exhibits a peak that depends on the thickness of the remaining PMMA and the low-frequency component of SWR becomes larger after etching. To prove our model more directly, PMMA was shrunk by electron beam (EB) irradiation and the change in SWR was examined. SWR with residual PMMA increased with an increasing EB-dose amount, while SWR without residual PMMA did not show a clear increasing trend. These results clearly show the shrinking stress in the remaining PMMA with thickness variation increases SWR. To suppress the SWR increase during etching, the etching condition was also modified based on our model. Intrinsic-shrinking stress in PMMA was reduced by a carbon-oxygen mixture plasma. In addition, the thickness variation of the remaining PMMA was suppressed by the reduction of the reactive ion etching lag with a dual-time modulated (DTM) plasma. Eventually, an SWR less than the incoming value was realized by using a carbon-oxygen mixture plasma with DTM. Published by the AVS.