Swiss Army Knife Tool Brings FIB Inline

SEM review faces the dual challenge of continuing to automatically perform defect review on increasingly smaller defects, and that of its throughput, which is directly equated with productivity, particularly since fabs must review increasingly larger amounts of wafers at a given time.

The SEM provides high-resolution images in various modes. Some portray reviewed defects, but others only indicate the existence of a defect below. If there is a defect at a filled via's bottom, SEM imaging will show this as a voltage contrast image, where the features' varying intensities indicate an electrical problem in the layers below. The next step would be to take the wafer to a focused ion beam (FIB) for local cross sectioning because, to perform root cause analysis on these defects, one must go below the surface. The problem has been that a FIB is traditionally not an inline tool, and its operation can be time-consuming.

Applied Materials (Santa Clara, Calif.) has introduced its SEMVision G2 FIB defect review and analysis tool, which integrates the FIB together with SEM and energy-dispersive X-ray (EDX) in one system for inline applications. It provides advanced SEM imaging, automatic review and monitoring applications, and integrated EDX complemented by an automated FIB and gas injection system. It is the first inline FIB on a review system, providing faster time to defect or electrical failure analysis. While there are SEM/FIB tools that are excellent at cross sectioning, they have limited SEM-related review applications. The Applied platform closes this loop, extending review capabilities, immediately revealing subsurface anomalies.

The system has a different approach to dual-beam SEM/FIB. Traditionally, SEM/FIB columns are crossbeams — both beams point at the same spot on the wafer. In the platform, the columns are parallel; thus, unlike others where the wafer may tilt and rotate around the same spot, here the wafer moves between the SEM and the FIB.

The configuration's primary advantage is shorter working distance. Having the SEM and FIB columns closer to the wafer is key to higher resolution. The wafer stage moves between the SEM and the FIB, and as quickly as with the crossbeam approach. Consecutive FIBs slice on the same location. In the case of a 150 nm diameter copper via, it is possible to slice through it, step by step, and find voids or defects inside. Twenty consecutive slices can be performed automatically at 10 nm intervals in under three minutes, with the wafer going back and forth between the SEM and the FIB. The SEM-FIB transfer and imaging is automatic and transparent.

The tool can perform embedded defect root cause analysis by imaging — electrical failure characterization — on defects not on the wafer surface, and therefore not reviewable unless cross-sectioned, which makes the FIB valuable. There are several means to detect electrical failure, traditionally by using e-beam inspection. Then there is electrical testing using a probe to test actual structure electrical performance. If an indication of a failed structure is indicated, the SEM can view the defect using voltage contrast imaging, then it can be analyzed by cutting with the FIB. Another method is automatic process inspection, which allows discrete e-beam inspection on the SEM review tool, similar to e-beam, but covering only discrete areas. Finally, the inline FIB does structural process monitoring as the operator selects critical features and cross sections them, views them, and performs profile structural measurements to ensure the features' fidelity meets requirements.

Within a flow, these applications provide different inputs. A gallery of high-magnification images of the defects is selected for review and automatic defect classification (ADC). The SEM takes three images simultaneously, with two external and one internal detectors. The external detectors highlight topography definition by capturing the slower electrons, while the internal detector highlights material contrast and edges. The combined information cannot be obtained from a traditional crossbeam configuration. The top three images in the Figure show review images of defects below the surface; their causes can only be revealed after cross sectioning using the FIB, seen in bottom images.

Defect review tools using SEM face the challenge of reviewing electrical defects caused by subsurface defects. Integrated FIB closes the loop. (Source: Applied Materials)

The SEMVision G2 FIB works like a Swiss Army knife, providing all necessary tools for extending review capability to identifying the cause of a defect, not just show the image indicating its existence. It complements this with ADC and characterizes defects through its material analysis capabilities.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt