END-POINT PREDICTION FOR POLYSILICON PLASMA ETCH VIA OPTICAL-EMISSIONINTERFEROMETRY

In semiconductor manufacturing, the completion or ''endpoint'' of a pl asma etch is typically controlled as a timed process, or monitored by use of optical emission spectroscopy. As etch process requirements and complexity increase (sequences of discrete recipe changes during the etch), consistency in timing changes is becoming an important issue. N ewly developed full wafer interferometry sensors enable real-time moni toring of the entire wafer surface during an etch. In this article, en dpoint prediction (or film thickness estimation) for etching polysilic on is considered. For the etch of a specific film structure, the inter ferometric signal is cyclical with a known number of cycles. Similar t o the phase angle of a cosine function, data points of an interferomet ric curve can be associated with a linear phase function. If the phase at endpoint is known, in situ film thickness can be obtained by deter mining the phase of the incoming signal in real-time. Simulations show that the proposed film thickness estimation algorithm has good accura cy in the face of etch rate drift and variation in him structure. We a lso find that estimation is more robust if shorter wavelength signals are used. Finally, experimental data verifies that remaining film thic kness can be-estimated within reasonable accuracy, and endpoint effect ively predicted for blanket and patterned films. These methods provide the critical information needed to make control decisions (e.g., when to switch to a more selective chemistry) based on reaching a desired known film thickness. (C) 1997 American Vacuum Society.