Computed-Tomographic OES for n_e and T_e Mapping in Processing Plasmas

The increasing complexity of semiconductor fabrications, with a focus on sub-nanometer precision, has led to the ongoing evolution of low-temperature plasma processes. Among the challenges faced, reducing the discontinuity of plasma sheath near the perimeter of a workpiece (e.g., wafer) for better plasma uniformity remains a long-standing issue. Nowadays, the use of focus rings has become widespread, effectively minimizing the discontinuity. Furthermore, active control methods are being invented to enhance plasma uniformity by compensating for the loss of the edge ring over the processing time, therefore necessitating real-time monitoring of plasma uniformity. However, to date, a promising diagnostic method remains elusive. In this presentation, we demonstrate a tomographic reconstruction technique to get two-dimensional information on plasma characteristics from line-integrated data of optical emission spectroscopy (OES), referred to as “Computed-Tomographic OES.” Because the number of data to be reconstructed exceeds that of the available viewport and detector, we employed the Phillips–Tikhonov regularization incorporating priori information on the system. Using our homemade measurement system comprising five detector assemblies, each consisting of a pinhole, interference filter, and 8k line scan camera, line-integrated spectral data of an argon capacitively coupled plasma were obtained. For automated data acquisition and processing, we implemented a Labview program, enabling real-time monitoring, as a reconstruction image and uniformity of Ar I emission were obtained in less than a second. We firmly believe that this demonstration will expand the application of computed-tomographic OES beyond monitoring, facilitating active control of spatial plasma uniformity—a critical factor in semiconductor processing.