Precision Engineering 34 (2010) 735–744 Contents lists available at ScienceDirect Precision Engineering journal homepage: www.elsevier.com/locate/precision Design and characterization of MIKES metrological atomic force microscope V. Korpelainen ∗ , J. Seppä, A. Lassila Centre for Metrology and Accreditation (MIKES), P.O. Box 9, Tekniikantie 1, 02151 Espoo, Finland article info Article history: Received 22 December 2009 Received in revised form 26 March 2010 Accepted 9 April 2010 Available online 18 April 2010 Keywords: Nanometrology Metrological atomic force microscope Laser interferometer Nonlinearity Calibration Uncertainty abstract An interferometrically traceable metrological atomic force microscope (IT-MAFM) has been developed at MIKES. It can be used for traceable atomic force microscope (AFM) measurements and for calibration of transfer standards of scanning probe microscopes (SPMs). Sample position is measured online by 3 axes of laser interferometers. A novel and simple method for detection and online correction of the interfer- ometer nonlinearity was developed. Effect of the nonlinearity in measurements is demonstrated. In the design, special attention has been paid to elimination of external disturbances like electric noise, acoustic noise, ambient temperature variations and vibrations. The instrument has been carefully characterized. The largest uncertainty components are caused by Abbe errors, orthogonality errors, drifts and noise. Noise level in Z direction was 0.25 nm, and in X and Y directions 0.36 nm and 0.31 nm, respectively. Stan- dard uncertainties for X, Y and Z coordinates are u cx = q[0.48; 0.04x; 0.17y; 1.7z; 2 time] nm, u cy = q[0.45; 0.31x; 0.07y; 0.14z; 4 time] nm and u cz = q[0.42; 3x; 7.2y; 0.18z; 2 time] nm where x, y, z are in m and time in h. Standard uncertainty for 300 nm pitch is 0.023 nm,and for 7 nm step height measurement is 0.35 nm. Uncertainty estimates are supported by an international comparison. © 2010 Elsevier Inc. All rights reserved. 1. Introduction New fields of research and industry such as nanotechnology pose significant challenges to metrology. Reliable measurements and thus traceability to the definition of the SI units are also needed in these areas. Different types of scanning probe microscopes (SPMs) are used in nanometre scale measurements. SPMs fall into three categories based on the quality of position measurements: open loop, closed loop and metrological SPMs. Open loop SPMs use voltage-driven piezo scanners with no other position sensors or lateral feedback. Closed loop SPMs have integrated position sensors and an active feedback circuit for position control. Metrological SPMs have direct traceability to the metre via integrated laser interferometers. Trace- ability for open loop and closed loop SPMs can be reached by careful calibration using calibrated transfer standards. Many types of calibration standard are available for SPMs. To achieve traceability for the measurements the calibration standards need to be calibrated. National metrology institutes (NMIs) have been active in developing traceable measurements and calibration methods [1,2] for modern microscopes. Metrological atomic force microscopes (MAFMs) have been developed by several NMIs [3–9]. Some of these systems have 2D interferometric closed loop con- trol and separate calibration for Z-axis position sensor [4–6], when ∗ Corresponding author. E-mail address: virpi.korpelainen@mikes.fi (V. Korpelainen). only few have online 3D laser interferometric control and mea- surement [7–9]. In order to guarantee commensurability of the SI units also at nanometre scale, there have been several international comparisons between the NMIs [10–13]. After the first publications on the periodic nonlinearity of homodyne [14,15] and heterodyne [16] interferometers plenty of research has been published on the phenomenon. The nonlin- earities of interferometers are carefully studied and fairly well understood [[17–23] and references therein], also several tech- niques have been developed for detection and correction of the periodic nonlinearity [[24–29] and references therein]. The nonlinearity is known to be periodical with a period equal to a submultiple of the used wavelength, . The periodical nonlin- earity of heterodyne interferometers is known to relate to several factors including elliptically and non-orthogonally polarized laser beams, rotational errors in the setup and different transmission coefficients in the beam splitter, or errors in phase detection. Depending on the setup, the nonlinearity can vary from the sub- nanometre range to a magnitude of 10 nm. It has been reported that in heterodyne systems the nonlinearity is not stable [30]. An interferometrically traceable MAFM (IT-MAFM) has been developed at MIKES. This paper describes the design and character- ization of the IT-MAFM. A novel method for the detection and online correction of periodic nonlinearity of the laser interferometer is described. The procedure can be easily repeated to compensate drift of nonlinearity and the performance is independent of the causes of the nonlinearity. The effect of interferometer nonlinear- ity on topography measured with MAFM is shown. A method for 0141-6359/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.precisioneng.2010.04.002