Scatterometry for post-etch polysilicon gate metrology

The use of scatterometry as a rapid and non-destructive technique for the characterization of grating structures has received significant attention in recent years. The problems of mask alignment, overlay, latent image monitoring, and others have been investigate using scatterometry. Research is currently underway at TI which extends the state of the art by using scatterometry for the measurement of very small dimension gate-level polysilicon gratings on a thin oxide under-layer. The measurements were taken after etch and cleanup on typical process development test wafers used at TI and were acquired using a Bio-Rad CDS-2 scatterometer. A variety of gratin pitches and grating line widths were measured and compared to duplicate measurements obtained from top-down SEM. A total of 2,975 non-repeated measurements were collected on each metrology tool. The large range of line width result in overlapping measurements for each pitch, so linearity with pitch can be evaluated. The results from the scatterometer are in excellent agreement with the SEM in nearly all cases. For certain line widths, regardless of the grating pitch, a discrepancy can be seen between the measurement for the scatterometer and the SEM. The reason for this is currently under investigation. If side wall angle is treat as a fixed parameter, the discrepancies are removed. Example measurements and corresponding theoretical traces will ge shown for process development samples measured at TI. Some result from an extended gauge study will also be shown to provide estimates of the precision of the scatterometric measurements.