Plasma density control for reactivate ion etch variation reduction in industrial microelectronics

This dissertation addresses monitoring and control of in situ plasma density drifts in a commercial reactive ion etch (RIE) tool used for semiconductor manufacturing. We present sophisticated real-time sensor data which monitors wafer state, chemical state, plasma state, and chamber wall state evolution to gauge final etch performance and resultant pattern topography. The focus is on the implementation of a unique feedback control structure which automatically corrects for the ill effects of plasma transients induced by typical chamber preparation steps. Similar effects are commonly encountered across the manufacturing industry, some of which are well documented, but in situ compensation for these effects in order to reduce process variation has not previously been shown.; Our investigations are in two steps. First, plasma density drifts are identified as one of the sources of RIE process variation. This is accomplished using an integrated real-time sensing approach which correlates process drift with measured density drifts. Using a novel density sensor developed at the University of Michigan, we qualify which process parameters affect plasma density, and link these parameters to process state outcomes. Once density is identified as a contributing source of real-time variation, a physical model describing these effects is suggested which is consistent with these findings. This model suggests dynamic interactions of plasma species (such as Cl and F) with the chamber walls cause plasma density fluctuations which adversely affect etch performance.; Secondly, we design and implement a proportional-integral (PI) feedback control scheme, utilizing the findings of the physical model, to compensate for the undesired etch performance variations in real-time. The control architecture measures plasma density as the system output variable and compensates for disturbances to density using generator forward power as the input variable actuator. As a simplified example, if the plasma density starts to go down, the controller corrects for the drift by supplying more power to the plasma generation subsystem. Results of this control system are shown to improve first wafer effects by a factor of three and reduce overall etch depth variation from run to run by an additional 33% compared to standard manufacturing practice.