Real-time virtual metrology and control of etch rate in an industrial plasma chamber

Plasma etch is a semiconductor manufacturing process during which material is removed from the surface of semiconducting wafers, typically made of silicon, using gases in plasma form. A host of chemical and electrical complexities make the etch process notoriously difficult to model and troublesome to control. This work demonstrates the use of a real-time model predictive control scheme to control plasma etch rate in the presence of disturbances to the ground path of the chamber, which are representative of maintenance events. Virtual metrology (VM) models, using plasma impedance measurements, are used to estimate the plasma etch rate in real time for control, with a view to eliminating the requirement for invasive measurements. The VM and control schemes exhibit fast set-point tracking and disturbance rejection capabilities. Etch rate can be controlled to within 1% of the desired value. Such control represents a significant improvement over open-loop operation of etch tools, where variances in etch rate of up to 5% can be observed during production processes due to disturbances in tool state and material properties.