Ion energy control in an industrial ICP etch chamber without bias power usage

This investigation focuses on process control for commercial inductively coupled plasma (ICP) etchers under manufacturing conditions. Plasma processes close to active zones of surface-sensitive devices are critical, demanding minimal damage caused by ion bombardment and so an excellent process understanding and control. In order to get the ion energy below the surface damage limit, RF biasing at the substrate is switched off. The plasma process then works in a downstreamlike mode. Without Faraday shielding, capacitive coupling must always be considered. Also with very low bias power, the ion energy can still be too high. Without bias power, bias matchbox capacitances are used as control elements for the ion energy. To ensure a high reliability for this control solution in a running production line, a combined RF and plasma model of the entire system with this special setup is presented and validated. The etch rate shows that the RF peak voltage measurement in the bias matchbox does not represent the ion energy at the substrate. The sheath voltage provided by the model is closely related to the ion energy and shows a reasonable correlation with the etch rate of the photoresist on test wafers. This relation shows the transition of chemical etching at low ion energies to ion-assisted etching with increasing sheath voltage.