No abstract

High aspect ratio (HAR) structures found in three-dimensional NAND memory structures have unique process control challenges. The etch used to fabricate channel holes several microns deep with aspect ratios beyond 50:1 is a particularly challenging process that requires exquisitely accurate and precise control. It is critical to carefully analyze multiple aspects of the etch process, such as hole profile, tilt, uniformity, and quality during development and production. X-ray critical dimension (XCD) metrology, which is also known as critical dimension small-angle x-ray scattering, is a powerful technique that can provide valuable insights on the arrangement, shape, and size of periodic arrays of HAR features. XCD is capable of fast, nondestructive measurements in the cell-area of production wafers, making XCD ideal for in-line metrology. Through several case studies, we will show that XCD can be used to accurately and precisely determine key properties of holes etched into hard mask, multilayer oxide/nitride film stacks and slit trenches. We show that the measurement of hole and slit tilt can be achieved without the aid of a structural model using a Fast Tilt methodology that provides sub-nanometer precision. Measurements were performed across several production wafers to determine the etch uniformity and quality. Particular attention was given at the edge of the wafers to account for large variations observed. In addition, we used a detailed physical model to characterize the HAR structures beyond linear tilt. This approach provides a more complete picture of the etch quality.

Memory hole (MH) etching may be one of the most critical and challenging processes in three-dimensional flash processing and integration. As the holes get deep, hole critical dimensions (CDs) can vary significantly during the top-down etch processing, the hole shape can deviate from a round hole, and the centerline of the hole can deviate from a vertical line. These and other complex behaviors depend on not only the process conditions (chemistry, plasma power, bias, temperature, etc.) but also the process chamber history [e.g., radio frequency (RF) hours from preventative maintenance] and how the process chambers are conditioned. In addition, the etch behavior depends on the tier thickness (ONON or OPOP), materials, deposition conditions, and overall wafer topography and shapes. The etch behavior is further related to the die positions on the wafer (center or middle versus edge dies) and the position in the memory cell (close to the edge of the cell or near the center of the cell). To monitor and control the etch behavior, many metrology methods have been developed; these include the etch-back top-view scanning electron microscopy (SEM), tilt SEM, transmission electron microscopy (TEM), and high voltage scanning electron microscopy (HVSEM). These methods are time consuming, and some are destructive, but they have been deemed the ground truth for determining variations in MH etch processes. We report a methodology that combines optical scatterometry (SCD) and small-angle x-ray scatterometry measurements to provide reliable CD profile measurements while meeting the fast-turnaround sampling requirements of high-volume manufacturing.