Calibrated Atomic Force Microscopy
Why are atomic force microscope (AFM) standards needed?
Atomic force microscopes (AFM's) are being increasingly used as metrology tools in a variety of industrial applications, thus driving an increasing demand for accuracy in these instruments. Some properties commonly measured in the industrial setting are feature spacing (pitch), feature height (or depth), feature width (critical dimension), and surface roughness. To achieve high accuracy in AFM measurements, the scales of an instrument should be calibrated. The use of a calibration standard is normally the most straightforward and appropriate means of doing this.
What standards are now available?
An example of a popular AFM standard is a three dimensional "grid" or "waffle" pattern which can be used as a three axis magnification standard. Many presently available AFM standards are calibrated using stylus instruments and optical techniques. The effectiveness of this approach, however, is limited by the differences in the working ranges of the various techniques and by questions of methods divergence (i.e., difference in instrumental response to a sample for different measurement techniques).
What is NIST's role?
As the repeatability of many commercial instruments continues to improve, the importance of accurate calibration standards, available over the useful range of AFM measurements, will increase. The Calibrated Atomic Force Microscope (CAFM) project is a NIST program which is striving to respond to this growing demand by developing an AFM which will be used to calibrate AFM standards.
What approach is NIST taking?
The central goal of the project is the development of an AFM which has metrology traceable to the wavelength of light for all three axes. To accomplish this, a flexure x-y translation stage, heterodyne laser interferometers, and a digital-signal-processor based closed-loop feedback system are used to control the x-y scan motion. The z-axis translation is accomplished using a piezoelectric actuator with an integrated capacitance sensor, which is calibrated using a heterodyne laser interferometer. When fully developed, this instrument will be a calibration tool for scanned probe microscope standards. Specifically, our first certified calibrations are expected to be of combined pitch/height or three-dimensional magnification standards.
In this side view of the C-AFM system (see Figure on the left), the rear of the AFM head and the cantilever, which is elevated above the sample on the specimen platform is visible in the foreground. The displacement measuring interferometers can be seen behind the specimen platform. In the top view (see Figure on the right), with the AFM head removed, the specimen platform and the x-y interferometers can be seen more clearly.
Where does the C-AFM project stand?
The CAFM The C-AFM is now in a fourth generation of development. The basic design concept of the system has proven to be very successful. Uncertainty budgets have been developed for pitch, height, and width measurements, and we are continuing to reduce these uncertainties. For C-AFM pitch measurements, the standard uncertainty (k = 1) typically ranges from 0.5 nm for intervals in the deep sub-micrometer range up to 7 nm for pitch values on the order of 10 µm. The standard uncertainty of C-AFM height measurements ranges from 0.4 nm for sub 10 nm heights up to several nanometers for heights approaching 1 µm. Although the uncertainty of width measurements depends significantly on the specific tip and tip correction algorithm used, we have demonstrated top width measurements of near vertical features with a standard uncertainty of 7 nm.
What will the next steps be?
The C-AFM has been used to perform both pitch and height measurements for external users—including several commercial suppliers of secondary standards. Plans for the near future include the measurement of a NIST Standard Reference Material (SRM) for AFM scale calibration. The pitch and height of the SRM's will be measured on the NIST C-AFM. We expect this standard to play an important role in the continued development of AFM's as metrology instruments. We also plan to evaluate other types of characterizers and tips to further improve the accuracy of C-AFM width measurements.
 J. A. Kramar, R. Dixson, N. G. Orji, “Scanning Probe Microscope Dimensional Metrology at NIST,” to be published in Meas. Sci. Technol. (2010).
 R. Dixson, D. A. Chernoff, S. Wang, T. V. Vorburger, S. L. Tan, N. G. Orji, J. Fu, “Interlaboratory Comparison of Traceable Atomic Force Microscope Pitch Measurements,” SPIE Proceedings Vol. 7729, 77290M (2010).
 T. V. Vorburger, A. Hilton, R. G. Dixson, N.G. Orji, J. A. Powell, A. J. Trunek, P. G. Neudeck, P. B. Abel, “Calibration of 1 nm SiC step height standards,” SPIE Proceedings Vol. 7638, 76381D-1 (2010).
Physical Measurement Laboratory (PML)