The Impact of the Use of Strongly Absorbing Etch Hard Masks and Shallow Topography to Alignment Signal Strength in 28 nm Technology Nodes and Beyond

Abstract

As the design rule of the semiconductor fabrication continue to shrink in the lateral dimensions, the vertical dimension becomes thinner and the requirement on the planarization becomes higher. In the front end of the line (FEOL), the shallower topography after STI CMP has become significantly thinner than 20 nm, adding the use of strongly absorbing hard mask, such as, amorphous carbon, the signal for alignment detection at the poly layer becomes significantly less than 1% in ASML's wafer quality metric. To get better signal, one will be forced to use an extra mask step to clear out some of the oxide in the STI trenches in the alignment and overlay marks to create necessary topography. In the back end of the line (BEOL), the use of strongly absorbing hard masks, such as, amorphous carbon or titanium containing compound, can also adversely affect alignment signals. At the contact layer, the alignment signal strength can be reduced to around the detection limit, ~1%. At the via layers, due to the use of titanium containing hard mask, the alignment signal to can also be significantly reduced by 1 order of magnitude. In this paper, we will use alignment mark signal simulation to analyze the impact of the shallow topography and the use of typical hard mask to the alignment signal.