1. T. Dziomba
  2. P.N. Luskinovich
  3. V.A. Zhabotinskiy
  4. P. Krebs
  5. H.-U. Danzebrink
  1. Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
  2. NANOmeter Standard GmbH, Astrid-Lindgren-Str. 19, 81829 München, Germany

The calibration of SPM scan axes is conventionally achieved with a set of step height and lateral standards. In addition or as an alternative to these, 3D standards with landmarks have been introduced on the market in the past few years. All these passive transfer standards rely on the principle that they are fabricated in a way that they are stable in their dimensions over many years.

While different kinds of actuators, mainly based on piezo-materials, have been realized to achieve the lateral scanning and the tracing of surface features throughout the history of SPM, such concepts have been pursuit for the realization of active lateral or vertical transfer standards in some research projects only without becoming a commercial product. This is due to the fact that conventional piezo-materials show significant hysteresis, creep and ageing, making them unsuitable for SPM calibration unless they are operated with position sensors with closed-loop position control. Such additional equipment, however, would increase the overall size of the active standard so that it would no longer fit onto the sample stage of many SPMs, and secondly, make it rather expensive with its control electronics compared to sets of well-established passive standards.

The company NANOmeter Standard GmbH has developed novel active standards based on lithiumniobate monocrystals. This material is known to show a much more linear behaviour without significant drift and is considered to be largely hysteresis-free when voltage ramps are applied. Consequently, additional sensors are not necessary, thus allowing a rather compact design. Both lateral standards (expansion in one lateral direction) and vertical standards are available (Figure 1). The active standards are sold together with a precision high-voltage amplifier and the control software that gives the user maximum freedom to define the voltage profiles to be applied to the standard and thereby control its elongation. In the experiments described here, a bipolar voltage profile symmetrically around 0 V is applied. The voltage actually applied to the active standard is measured with a calibrated high-precision voltmeter by Fluke. While this voltmeter measures up to +/-1000 V, the standards themselves can typically be operated up to +/-2000 V.

While the expansion of these novel standards has been widely tested with laser- interferometers also developed by NANOmeter Standard, a number of measurement campaigns with the SIS Nanostation II (now called N8 Neos by Bruker) non-contact AFM have been performed at PTB to assess the quality of these novel standards under real SPM conditions and to calibrate them (Figure 1). The SIS Nanostation II is an instrument of category B according to VDI/VDE 2656-1 (ISO/DIS 11952) that is carefully calibrated with Met.LR-SPM calibrated step height and lateral standards as well as 3D standards and has been thoroughly characterized throughout its 10 years of operation.

Fig. 1 Active dynamic standards under the AFM head of the SIS Nanostation II.

Left: Lateral standard mounted in a square-shaped metal block of 50 mm x 50 mm to ease alignment to the scanner axes, total height of standard 19.5 mm

Right: Vertical standard with a diameter of 38 mm and a total height of ~15 mm

For the measurement of the expansion of the vertical active standards, two different measurement schemes have been applied: Firstly, a trapezoidal voltage profile was applied to the active standard in a way that several steps appear in every scanned AFM profile so that the AFM image seems to contain a measurement of several parallel bars (Figure 2). The duration of the generated trapezoidal profile and the AFM scanrate are carefully adjusted so that the offset of the bars’ positions from one scanline to the next is minimized. These images can then be analyzed routinely by ISO 5436-1 and histogram method, thus following VDI/VDE 2656-1 (ISO/DIS 11952), just like conventional step height measurements. The AFM scanrate is rather low with 0.05 lines/s only, as the high voltage cannot be switched faster. For this reason, a second scheme is applied: Every 16 scanlines, the sign of the voltage applied to the active standard is reversed so that steps parallel to the fast scan direction are generated. For analysis, the AFM image is rotated 90 degrees and then analysed routinely with regard to the step height. This requires the instrument to be very stable throughout the recording of the image; such a series is therefore recorded only after the series according to the first scheme has been completed when remaining drift is expected to be minimal, and the scanrate is chosen comparatively high with 0.6 lines/s to shorten the recording time and thereby the influence of remaining potential z drifts. Both measurement schemes have so far yielded a good agreement of the active standards’ sensitivity coefficients within the errors of the linear regression.

In order to characterize the lateral active standards, a TGZ1 by NT-MDT is mounted on top of the lateral standard with the orientation of the grating bars in y-direction and perpendicular to the expansion direction of the active standard that is aligned to x-direction (the fast scan direction) of the AFM. Similar to the second measurement scheme of the vertical standards, every 50 scanlines, the polarity of the voltage applied to the active standard is inverted so that the grating on top of it is moved in x-direction while image scanning continues. After another 50 lines, the voltage is inverted again so that the TGZ1 grating bars move in the opposite x- direction and are again at their initial x-positions in the AFM image. The difference of the bars’ edges is thus the measure of the elongation of the active lateral standard (Figure 3). Such AFM images with multiple changes of voltage polarity are individually recorded for at least 10 different voltages covering the maximum range of +/-1000 V.

Figures 2 and 3 show plots of the measured vertical and lateral displacements versa the applied voltage for an active vertical and lateral standard. The linear regressions show a very high degree of linearity (regression coefficients up to 0.99999 and 0.99994); the standard deviation of the 10 data points from the fitted line is as small as 0.10 nm and 0.88 nm, respectively.

While a very good linearity could be proven for these dynamic active standards, the long-term stability of their sensitivity coefficient c is still under investigation. It could be shown already that the control / high-voltage amplifier is rather sensitive to electro-magnetic noise so that care needs to be taken in this respect when using these standards.

Fig. 2 Characteristics of active vertical standard Nr. 50/4 (measurement scheme 1)

Inset on the right: Example of a measurement with trapezoidal voltage profile of +/-892 V (1784 V peak-to-peak) applied to the vertical standard resulting in apparent stripes of (54.3 ± 0.9) nm height

Fig. 3 Characteristics of active lateral standard Nr. 3

Inset on the right: Example of a measurement with a total of 256 scanlines, inversion of the voltage every ~51 lines, resulting in a lateral displacement d of (251.0 ± 1.5) nm for a peak-to-peak voltage of 1981.1 V