Real-time process monitoring by optical emission spectroscopy in DRAM gate CD control

As the DRAM gate CD is a critical parameter for contact resistance for self-aligned contact as well as transistor performance, CD control by change of process parameter such as O₂ flow rate is introduced to suppress the wafer-to-wafer and lot-to-lot CD variation. During the gate CD control by changing the O₂ flow rate, it is necessary to real time monitor the plasma status to determine whether the required result would be obtained or not. By using optical emission spectroscopy (OES) and multivariate analysis, process status index (PSI) has been derived. And it can be confirmed that PSI has strong correlation with the O₂ flow rate and CD skew (= post etch CD - pre etch CD) as well. This idea was applied for monitoring mass wafers in a gate mask etch chamber. And process drift as well as first wafer effect could be observed.