New developments at Physikalisch Technische Bundesanstalt in three-dimensional atomic force microscopy with tapping and torsion atomic force microscopy mode and vector approach probing strategy

Abstract

A new three-dimensional atomic force microscopy (3D-AFM) for true 3D measurements of nanostructures has been developed at Physikalisch Technische Bundesanstalt (PTB), the national metrology institute of Germany. In its configuration, two piezo actuators are applied to drive the AFM cantilever near its vertical and torsional resonant frequencies. In such a way, the AFM tip can probe the surface with a vertical and/or a lateral oscillation, offering high 3D probing sensitivity. For enhancing measurement flexibility as well as reducing tip wear, a vector approach probing (VAP) method is applied. The sample is measured point by point using this method. At each probing point, the tip is approached toward the surface in its normal direction until the desired tip-sample interaction is detected and is then immediately withdrawn from the surface. Preliminary experimental results show promising performance of the PTB system. The measurement of an IVPS 100 sample using a flared AFM tip showed a repeatability of its 3D profiles better than 1 nm (p-v). A single crystal critical dimension reference material having features with almost vertical sidewalls was also measured using a flared AFM tip. These results show that the feature has average left and right sidewall angles of 89.5 and 89.4, respectively. However, the nonuniformity of the feature width within the measurement window of 1 μm may be up to 10 nm. The standard deviation of the average middle CD values from 10 repeated measurements is 0.1 nm. In addition, an investigation of long-term measurement stability was performed on a PTB photomask. The results changed at a rate of about 0.00033 nm per line, which confirms the high measurement stability and the very low tip wear of the system.