New developments in 3D-AFM with tapping and torsion AFM mode and vector approach probing strategy

A new 3D-AFM for true 3D measurements of nanostructures has been developed. The 3D-AFM applies AFM probes with flared tips having an extended geometry near the end which enables the probing of steep and even undercut sidewalls. Two piezo actuators have been 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 so called “vector approach probing” (VAP) method has been applied. The sample is measured point by point using this method. At each probing point, the tip is approached towards the surface in its normal direction until the desired tip-sample interaction is detected and then immediately withdrawn from the surface.

Preliminary experimental results show promising performance of the developed system. The measurement of an IVPS 100 sample employing a flared AFM tip is performed, showing a repeatability of its 3D profiles of better than 1 nm (P-V). A single crystal critical dimension reference material (SCCDRM) having features with almost vertical sidewall is measured using a flared AFM tip. The standard deviation of the averaged middle CD values of 10 repeated measurements reaches 0.1 nm. However, the line width varies approx. 10 nm within the measurement range of 1 μm, showing the limited uniformity of the line structure. The averaged left and right sidewall angles are determined to be 89.5° and 89.4°, respectively. The standard deviation of 10 repeated measurements is approximately 0.1°.

In addition, an investigation of long-term measurement stability was performed on a PTB photomask. The whole investigation lasted about 30 hours during a weekend. Totally 197 measurements have been performed with each taking about 9 minutes. The measured CD values changed with a statistical rate of about 0.00033 nm per line, which confirms the high measurement stability and the very low tip wear.

Figure 1: (a) 3D view of linewidth features of a single crystal critical dimension reference material (SCCDRM) measured by the developed 3D-AFM technique, and (b) the results of a long-term stability investigation of CD measurements.