Scatterometry for the measurement of metal features

Scatterometry, an optical metrology based on the principle of diffraction, has received a considerable amount of attention in the literature in the past decade. By measuring and analyzing the light scattered, or diffracted, from a patterned periodic sample, the dimensions of the sample itself can be measured. Applications of the technique have included the characterization of photomasks, the monitoring of focus, dose and the post exposure bake process, and even the characterization of three-dimensional features such as contact hole and DRAM arrays. Like other optical metrologies, scatterometry measurements are rapid, non-destructive and highly repeatable. The most widely reported scatterometry data have been for measurements of developed photoresist and etched poly-Si gratings. Although these types of features represent a considerable portion of a typical process, other types of materials, such as metal layers, are also commonly present, and are important to characterize. In this work scatterometry measurement data from metal layers will be presented. The scatterometer used in this investigation was the well-known 2- (Theta) variety, where the scatter signature is obtained by measuring the diffraction efficiency of a particular order as the incident angle is varied. The analysis involved comparing the measured data to a library of theoretical scatter signatures (generated a priori from rigorous coupled wave theory) to find the best match. The measurements were performed at two process steps: pre-etch, where the patterned features were resist gratings on uniform metal layers, and post-etch/strip, where the patterned features were etched metal gratings. The composition of the metal stack starts with a silicon substrate, upon which is deposited 3000 A of oxide, followed by 500 A of Ti, 6000 A of AlCu and 250 A of TiN. A BARC layer and then the photoresist were deposited on this stack for patterning. The nominal patterned dimensions were 350 nm lines in 800 nm pitch gratings. Overall the sample set for this study included some 24 wafers with various AlCu layer thicknesses, providing a broad wafer split for analysis. Measurement results for linewidth and the various layer thicknesses will be presented, and comparisons to AFM measurements will be made. Results from assessing the repeatability of the 2-(Theta) scatterometer will also be presented, and indicate sub-nanometer precision (6-(sigma) ) for linewidth measurements. In addition, the practical aspects of the method, such as the modeling time required to generate the signature library as well as measurement speed and throughput will be presented.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.