AFM Enters the Production Environment & Test

For the past decade or so, 3-D metrology capability was not a pressing need, and being able to measure in one dimension sufficed for most requirements. This is no longer the case, and an even casual glance at the literature shows the term "3-D" popping up everywhere. The production of smaller, more complex devices demands fuller information to enable more complete control of lithography and other processes, and the road to that information lies in 3-D.

Atomic force microscopy (AFM) is a solution to meeting these new requirements. AFM is unique in that it is very physical, intuitive, and not as sensitive as other methods to extraneous parameters around the wafer. For example, a SEM or any electrical-based beam will react differently to oxides, resists, metals and nitrides. Thus, depending on the measurement's objective, the platform must be calibrated for each different process level, and can drift out of calibration. This is not so for AFM, because it treats all materials identically. AFM can sense a wide variety of forces. In non-contact mode, Van der Waals or capillary forces provide images of topography. Specialized AFM systems also can measure electrical and magnetic tip-sample interactions. In contact mode, ionic repulsion forces come into play. Because there typically is no current between the tip and the sample, an AFM can image samples that otherwise would be altered or contaminated by other metrologies. This is a main concern as CDs become smaller, and other materials more specialized.

AFMs treat all materials identically and will measure the sample, whether metal, polymer, oxide or semiconductor material. (Source: Veeco)

Until now, these capabilities have not been widely used for 3-D applications. Veeco Metrology Group (Santa Barbara, Calif.), a division of Veeco Instruments, has introduced the Dimension X3D, an AFM platform that should find application as a characterization tool and in-line monitoring system. Although they are robust and flexible systems, AFMs historically have been used as characterization tools outside the production environment because of their limited automation. The software for characterization requirements in research tools is different than that of production tools. After two years of development, production solutions have been designed, providing automation improvements that triple past throughputs and enabling the AFM platform to compete with some of the other non-optical platforms. Besides the advanced automation features, the platform has demonstrated good repeatability, providing data that goes beyond mere top-down images.

The new system provides the information necessary for data-driven decision making. It carries its own onboard references; it is insensitive to the material measured or the underlying film stack. The single platform can measure practically every feature — front-end, back-end, shallow and deep. SEMs have a long history of in-fab use, but fail to repeatably capture 3-D data. Optical systems, however repeatable, have distinct application windows — they require large test arrays that enable them to perform at a considerable rate. However, problems arise if these regularly spaced lines are absent, or the film stack is complex. An AFM suffers no such limitations; it can measure vias, contacts, regularly spaced lines, isolated lines, gates, etc.

The new system's biggest drawback is that it will require manufacturers to learn new ways. For example, there is no wide understanding of 3-D metrics. While there once was the CD, or a single number for width or depth, the industry still lacks simple, agreed upon metrics, and this will require some adjustment. However, for those characterizing new processes, the advantages may be worth the effort.

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