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Calibration Standards for Scanning Probe Microscopes

Scanning probe microscopes allow to obtain unique insights into the microworld of surfaces. So far, they have been used mainly as imaging systems. Now, they are becoming real measuring instruments. This development will be accelerated by novel calibration standards, which allow economic calibration and direct traceability.

Detail of a step-height standard. The reference structure in the centre is a two-dimensional array of «holes» with identical nominal depths covering an area of approximately 240 μm × 240 μm. This pattern is a stepheight standard with calibrated mean step height (deviations from the mean value are less than 1 nm). In the corners, parts of other one- and two-dimensional structures with identical nominal step heights are to be seen, which can also be used in conventional measuring instruments such as mechanical or optical profilometers. All structures are located on an Si chip of 5 mm × 7 mm, which is fastened to a standard sample holder with 12 mm diameter. Photo: NanoSensors, Wetzlar, Germany

For the quantitative determination of microscopic geometries, some metrology institutes have specially designed scanning probe microscopes (SPMs) at their disposal. These SPMs are usually equipped with laser interferometers. In view of the sophisticated equipment required, this is not a suitable procedure for industrial and scientific applications. Calibrations with the aid of standards, as they are common practice for other measuring instruments used in dimensional metrology, would be an adequate solution.

Though material measures with microscopic dimensions are already available, these have mainly been developed for scanning electron microcsopes and not especially for SPMs. A recently completed EU project has improved of this unsatisfactory situation. Within the scope of the project, SPM calibration standards were manufactured by two companies, IBSEN (Denmark) and NanoSensors (Germany), and calibrated by the NPL (UK) and PTB. In addition, a calibration software was developed by the Dansk Institut for Fundamental Metrologi. Since the lateral dimensions and structural arrangements are adapted, the standards can be measured by optical methods. Compared with an SPM calibration, the measuring time is drastically reduced from about two hours to a few minutes. This also considerably reduces the calibration costs.

The following kinds of standards with etched structures were manufactured for calibration and testing the image quality of SPMs: one flatness standard, six step-height standards (8 nm to 2,4 μm) and five lateral two-dimensional standards (100 nm to 10 μm). The standards were calibrated with interference microscopes and laser diffractometers. In result a complete set of standards for the calibration of SPMs has been furnished. For the first time, industry and science are in a position to comply with the quality requirements according to DIN ISO 9000 and DIN EN 45 000 also for SPMs used as measuring instruments.

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