Paper PS-MoM10
BEOL Pattern Flop Over as a Challenge to Shrink Feature Critical Dimension Continuously

Monday, October 20, 2008, 11:20 am, Room 304

As feature critical dimension (CD) shrinks toward the limit of Moore’s law, many problems, including pattern flop over, become serious challenges in the Back-End-Of-Line (BEOL) plasma etch development. A clear understanding of the origin and control of pattern flop over is extremely desirable since it will cause failure in product development. BEOL pattern flop over is due to several different reasons. One of the root causes is the high Aspect Ratio (AR) of mask/dielectric lines. As the technology node moves from one generation to the next, the feature pitch size shrinks faster than the feature vertical dimension (trench/via depth) to allow the integration density to increase. Consequently, mask/dielectric lines with higher AR in BEOL are required. When the AR exceeds the critical value (approximately 3) flop over can happen very easily. The mask/dielectrics lines bend over and sometimes touch each other. Different flop over phenomena have already been observed at 22 nm node look-ahead research. Photoresist feature delamination has been noted post resist development because of the smaller pitch size. Two of the most significant mechanisms, mask flop over and dielectrics flop over, occur during the plasma etching process. The soft organic materials in the litho stacks can bend over and lead to mask flop over when the AR is high. This flop over partially shadows the trench and forms distorted dielectric lines. The dielectrics line itself can also flop over at high aspect ratios, which is most likely due to a combination of high AR and low material strength. In addition, wet treatment post plasma patterning can cause significant flop over due to capillary forces. One of the possible ways to avoid pattern flop over, is to improve the mechanical properties of the mask and dielectric materials. With our research efforts, we hope we can understand and solve this challenge in time to move closer toward the limit of Moore’s law.