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Graduate Student Seminar - 10/13/2017

"The Role of Plasma Species and Sample Composition on Dense Amorphous Carbon Layer Formation and Polymer Etching Behavior"

by Adam Pranda

Friday, October 13, 2017 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Advisor:  Professor Gottlieb Oehrlein

Numerous polymer etching models have been previously developed to correlate the structure or composition of the polymer to the plasma etching behavior. A key assumption in these models is that the polymer structure remains homogenous as it is etched. For applications in photoresist pattern transfer, this assumption is not valid since high-energy ion bombardment results in the formation of a heterogeneous structure consisting of a 2-3 nanometer thick dense amorphous carbon (DAC) layer on the polymer surface which mediates the overall etch rate. In this work, we implement a physical model which takes into account the ion penetration depth in photoresist materials to provide a basis for an ellipsometric model which we use to describe the evolution of the photoresist structure in real time. From the modeling, we have identified a correlation where a molecular structure consisting of a greater ratio of carbon carbon-type bonding results in a more optically dense DAC layer, which limits the ion flux that reaches the bulk layer, and thus leads to a lower steady-state etch rate. In the presence of any reactive species in the plasma, such as oxygen or fluorocarbon, there is an additional component to the etch rate due to chemical sputtering which results in an increase in the etch yield of the DAC layer. Once the DAC layer is sufficiently depleted, the ion flux reaching the bulk layer increases and thus the bulk etch rate increases as well. Utilizing the experimental results, we seek to arrive at an etching model that can be applied in the development of new photoresist materials that attain a target steady-state etch rate.

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