Automated SEM Offset Using Programmed Defects Oliver D. Patterson, Andrew Stamper IBM Semiconductor Research and Development Center 2070 Route 52, Mail Stop: 46H Hopewell Junction, NY 12533 USA
Roland Hahn KLA-Tencor 20 Corporate Park Drive, Suite C Hopewell Junction, NY 12533 USA
Abstract - Defect inspection plays a large role in the development and manufacture of semiconductor technologies. Defects detected in today’s inspections tools are generally a fraction of a micron and require SEM review to analyze and justify corrective measures. It is very important that the review SEM drives to the exact location of the defects as a FoV (Field of View) of 2µm is necessary to provide the resolution needed for defect redetection without the inefficiencies associated with repeated ‘zooming’ of the image. A methodology which allows quick and accurate alignment of the review SEM to the defects in the results file is presented. This methodology uses a special structure containing programmed defects. The methodology is illustrated using the challenging example of PWQ wafers.
spot size. Also, the coordinate accuracy of bare wafer inspection degrades with higher throughput. The following offsets can be corrected by using an deskew: translation, scaling, rotation and non-orthogonality. To perform an efficient defect deskew, a set of reference defects needs to be selected, relocated and marked on the wafer. Defects detected by the inspection tool are sometimes not visible to the review SEM. When they are, the visible ones are not always well distributed across the wafer as required for an ideal deskew. Deskew is especially difficult for Focus Exposure Matrix (FEM), Process Window Qualification (PWQ) and Process Window Centering (PWC) inspections [1]. The nature of these inspections results in very high defect density and large defects in the higher modulations, making it difficult to reliably locate a suitable set of defects for deskew. In this paper, we propose the use of programmed defects (PD) to assist in the deskew process. This methodology is described in Section II. Application of this methodology to a number of PWQ wafers for comparison with current methods is discussed in Section III.
Keywords- SEM Alignment, defect offset, review SEM,
deskew I. INTRODUCTION
Optical defect inspection plays a large role in the development and manufacture of semiconductor technologies. Tens of optical inspections are strategically interlaced throughout the process sequence in order to detect, quantify and classify defectivity affecting the wafer. Because of the small feature size of today’s technologies, and in turn the small size of critical defects, SEM review is almost always necessary to classify the defects. Redetection of defects by review SEM has become particularly challenging in recent years, again because of the small size of the typical defect. Robust wafer alignment and a common die corner are two necessary factors for successful defect review. Despite excellent review SEM stage accuracy, a small offset between defects across the wafer still exists. This is because of variability in the calibration wafers, temperature, identification of the center of a defect and other factors. Therefore a third parameter, the defect deskew, is also necessary. The process of calculating the defect deskew, also termed ‘defect deskew’, may be performed automatically or manually. In addition to correcting offset within a wafer, defect deskew also compensates for a systematic offset between different inspection tools and modes. For example, the coordinate accuracy of darkfield inspection tools gets worse with larger
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II. METHODOLOGY
Traditionally, PDs have been used for calibrating the sensitivity of inspection techniques such as e-beam and brightfield inspection [2,3]. This paper introduces a special structure, called the SEM Alignment Structure, which contains PDs at key levels throughout the process. These include active, deep trench, gate-stack, contact and all the metal and via levels. A small area of the structure layout around the PD at the active and contact levels is shown in Fig. 1. A small area of the metal 1 structure layout around the PD and a corresponding wafer image are shown in Fig. 2. This structure is 58um x 58um so that it can easily fit within the scribe line. The structure must have a repetitive pattern so that it can be inspected in array mode. A random mode inspection will not work for 1x1 reticles, which are common in development, because the PD appears on the same location in each reticle field. The PDs for each level are stacked on top of each other so that only a single PD is detected at each level. Since brightfield can sometimes