Metal Film Thickness Standards Enable NIST-Traceable Calibration
Alexander E. Braun, Senior Editor -- Semiconductor International, 6/1/2001
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Metal film thickness and sheet resistance standards in the form of aluminum films on a silicon wafer are now available from VLSI Standards Inc. (San Jose). These standards are traceable to NIST and are useful for manufacturers seeking compliance with ISO 9000 and QS 9000 quality protocols.
NIST-traceable standards are increasingly necessary for the precise calibration of diverse metal film thickness measurement tools. Before these standards were available, manufacturers had to use bare silicon standards for the calibration of sheet resistance and resistivity tools, which could then be applied to metal thickness monitoring. The problem is, however, that contact probes — such as a four-point probe — are application-specific. This means that one kind of probe type should be used for silicon and another for metal film measurements. Ideally, a process-relevant metal standard should be used for sheet resistance tool calibration to enable calibration of the actual probes used for the production measurement of metal films.
The new metal standards have a calibrated 40 mm diameter sputtered aluminum region in the center of either a 200 or 100 mm diameter silicon wafer (Figure). Delicate measurements are performed that are then traceable to NIST. This traceability is achieved through an unbroken chain of measurement comparisons between the VLSI standard and NIST-calibrated standards. When establishing a standard, it is necessary to measure the mean value as well as the uncertainty in calibration measurements associated with that value. With these metal film standards, a combination of sheet resistance and topography metrology is used to certify the film thickness and sheet resistance values, and to evaluate spatial uniformity and total measurement uncertainty. The Al-FTS and Al-RS standards are available in nominal thickness values of 0.5, 1.0 and 2.0 µm in both 200 and 100 mm wafer diameters.
From the Nanoscale Physics Facility
The Nanoscale Physics Facility at the National Institute of Standards and Technology (NIST, Gaithersburg, Md.) was established to permit physicists to manipulate and arrange atoms, one by one, into desired patterns on a metallic surface.
According to Joseph Stroscio, the project leader, one of the motivations behind the facility is to enable the U.S. electronics industry to manufacture smaller, faster, and more powerful and versatile communications devices and computers.
Semiconductor manufacturers are fast approaching the boundary where quantum effects become a concern, and the Nanoscale Physics Facility is exploring these effects in nanoscale devices by building structures atom by atom.
One of the research group's latest achievements is an arrangement of cobalt atoms on a copper surface (Figure). The structure was made with a scanning tunneling microscope at 2.3 K. This kind of structure allows the comparison of experimental electron energy level structure to theoretical models.
For additional information on inspection, measurement and test, go to www.semiconductor.net/imt
