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Reliable, Repeatable Wafer and Tool Dispositioning in 300 mm Fabs Bruce Johnson, Rebecca Pinto, Ph.D, and Stephen Hiebert , KLA-Tencor Corporation
Advances in wafer fabrication along with rising economic pressures on chipmakers have created greater challenges in the dispositioning of wafers and process tools. Such a climate has rendered it nearly impossible for manual disposition inspection to deliver even adequate results in a manufacturing environment or for process tool requalification. Manual inspectors often miss gross process problems, passing wafers downstream, where they will later be scrapped or create yield loss.Automated disposition, on the other hand, can integrate into a fab’s defect analysis infrastructure to enable better yield learning. These advantages, plus an automated system’s capability for high sampling, make it suitable for a low cost of ownership inspection strategy.
Introduction
Advanced fabs require accurate and rapid disposition decision-making during manufacturing, as well as a quick assessment of tool and process module output. Operators at manual or semi-automated inspection stations have historically done much of this, but these methods have been ineffective for quite some time. Manual inspections are expensive, and the results are well known to be unreliable. This is especially true for advanced 300 mm manufacturing, where vanishingly small device features, factory automation, and large wafer surfaces challenge the ability of the operator to assess the wafer and lot; these conditions place large quantities of valuable wafers at risk. There are cases in most fabs where an operator has missed process or tool errors which have resulted in litho hot spots, CMP underpolish, scratches, underetch, splashback, coating failures, and many other types of gross errors. These can happen randomly on one wafer in a lot, on some pattern of wafers within a lot (such as every other wafer due to process tool chamber/stage configuration), or on a whole production lot. Most fabs have had significant yield hits from lots 64
Spring 2006
Yield Management Solutions
which were, for example, not coated with resist, but which were not recognized or sampled by the inspector and ultimately had to be scrapped. The cost of a small inspection error – missing a significant, but challenging-to-detect process error – can be very high. In fabs with large product mixes, such as a foundry or a development line, each lot may represent all of the material for a specific customer. The loss of that lot, especially if it happens late in the device manufacturing process, can be devastating to both the fab and its relationship with the fab’s customer. For some smaller fabless semiconductor customers, it can almost be fatal because of the cycle time hit on a key part. Yet, these failures do happen with surprising regularity when fabs are not able to sample at the level and sensitivity required to capture critical excursions consistently and early on. Manual inspection has resulted in many cases of missed problems and resultant loss because of its inability to reliably find important defects. Automated inspection, on the other hand, is well suited to performing the disposition job. Its major strengths are: • Good sensitivity to detect defects of all types • Consistent results from tool to tool, day to day, and fab to fab • High throughput to adequately sample every lot