Metal etcher qualification using angular scatterometry

The deployment of angular scatterometry as a powerful and effective process control methodology has recently included the measurement of etched metal features in a typical complex Aluminum stack. With the control of metal process steps taking a more critical role in silicon manufacturing, a fast, reproducible and accurate methodology for measuring CD and depth is necessary. With the half-pitch of the metal pattern being as low as the minimum device feature, etch rate measurements on above-micron test structures are hardly indicative of the pattern-dependent etch profiles and behavior. Angular scatterometry offers a non-destructive, fast and powerful approach for measuring the profiles of the yield-relevant array features in metal applications. In this work we demonstrate the application of angular scatterometry to the qualification of metal etchers. Etch depth is difficult to control and must be inspected with slow techniques such as profilometry. In addition to the slow response time and sparse radial sampling, contact profilometry is susceptible to residual resist and polymer residue as well as to the variations in the TiN ARC layer affecting the measurement of the Aluminum etch rates. We show that the choice of a suitable profile model and accurate knowledge of the optical properties allow scatterometry to overcome all of these traditional challenges. We demonstrate that angular scatterometry is sensitive to the parameters of interest for controlling metal etchers, specifically etch depth, CD and profile. Across an experimental design that introduced intentional variations in these parameters, angular scatterometry results were able to track the variations accurately. In addition, profile results determined through scatterometry compare favorably with cross-sectional SEM images and measurements. Measurement precision results will also be presented.