In-Situ predictive endpoint for dual damascene trench etch depth control for composite dielectric films

As device technology is moving into a new era with Copper and new material for BEOL manufacturing for faster chip in-situ control becomes an important area for the equipment manufacturer. Integrated Rate Monitor (iRM™) using light beam interferometry to monitor In-Situ changes in the dielectric film thickness when exposed to plasma. iRM™ enhances detection sensitivity for Dual Damascene (DD) Trench Etch, when No Middle Etch Stop Layer (NMESL) is a preferred scheme. Most of the work on iRM™ is focused single layer etch underneath the Photoresist (PR) pattern, e.g. PR on Low k (Carbon Doped Oxide-CDO) film. We also understood how iRM™ would behave if we have a composite layer to etch like PR on TEOS/low k (CDO) stack. Various types of blanket wafer, single and composite stack pattern wafers were etched to achieve successful trench depth control for DD process. Film stack used is TEOS Hard Mask on CDO or FSG with Middle Etch Stop Layer (MESL). iRM™ was introduced on dielectric etcher (Applied Materials eMAX™ chamber) in addition to the Optical Emission Endpoint (OES) for etching DD NMESL Trench. Good and repeatable Trench Depth control was achieved. It provided an accurate, reliable and production worthy etch depth control for NMESL. Currently not many equipment manufacturers have the capability of doing In-Situ Predictive Endpoint for Cu BEOL films (CDO/FSG), hence iRM™ provides a breakthrough in DD plasma etch technology. To establish the iRM™ for a specific film the Refractive Index (RI) of the film is required. RI (n) for blanket wafers is calculated based on the etch rate using thickness measurement tool and period (Δt) of the sinusoidal waveform observed in the iRM™ trace. To calculate RI for pattern wafers, etch rate is measured based on trench depth from X-SEM. Once the RI is determined and the iRM™ trace optimized, every subsequent wafer etched will achieve an accurate and reliable trench depth control as defined in the iRM™ algorithm. To implement iRM™ on a composite film stack, a few approaches were taken. Each method was tested to check its repeatability and accuracy. iRM™ etch depth accuracy is measured by the e- tch depth shown in the iRM™ screen and the actual trench depth as shown in the X-SEM pictures. We have shown that with composite film stack, we can achieve the targeted etch depth with iRM™ allowing to etch DD trenches without the need of a MESL which increases the k-value of the film.