Design and development of a novel 3D-AFM head

In dimensional nanometrology tasks of measuring surface geometries get more and more complex. Especially for atomic force microscopy (AFM) these tasks shift from simple and relatively flat samples to narrow structured samples with steep sidewall angles and dense pattern. In this field, state of the art AFM is the so-called critical dimension (CD)- and three-dimensional (3D)-AFM. A special CD-cantilever is used as a probe to detect two orthogonal probing directions, here denoted as x and z. This probing is sufficient for structures, where the probing direction (xz-plane) is in the direction of the surface normal, such as for 1D-gratings. For slopes and curved geometries, however, the direction of the surface normal has an additional y-component. Consequently, crosstalk appears, because the CD-cantilever is not selective for the tip displacements in y- and z-directions, leading to tip slipping motion and even tip breakage.

A kind of a novel 3D-Nanoprobe, a structured CD-cantilever has been developed at PTB. It is selective for probing in all three spatial dimensions (x, y, z). The 3D-Nanoprobe is realised by combining compliant flexure hinges (FH1 and FH2) with stiff head sections (HS1 and HS2), and the movement of these head sections are designed to be specific for the tip displacement, as shown in Fig.1 and [1]. The head section 2 (HS 2) is optimised to reduce the crosstalk and shows no change in inclination angle for a tip displacement in z-direction and the change in inclination angle is only sensitive for displacements in x- and y-directions. A semi-analytic model was used to analyse the mechanical properties of the probe, based on which the design is optimised for isotropic stiffness, equal sensitivity, and separated lower eigenfrequencies.

As an example, a 3D-Nanoprobe is realised based on a commercially available CD-cantilever (type CDR 70 – Team Nanotec company). The ratio of the probe’s stiffness was optimised from 8:8:1 (x:y:z) to 0.9:0.9:1 (x:y:z) and the sensitivity was improved by a factor of 106, 128 and 1.6 for the x-, y- and z-direction, respectively. The tip displacement of the 3D-Nanoprobe can be detected by two dual optical lever (Fig.1 below, Spot 1 und 2) or by combining one dual optical lever and an interferometer.



Figure 1: Working principle of the 3D-Nanoprobe for two dual optical lever. The bending and torsion angles of the 3D-Nanoprobe are the inclination angles  in yz–plane and β in xy-plane respectively, which can be used to detect the tip displacement (Spot 1 and Spot 2) (Figure modified from [1]).

Reference

[1] Thiesler, J., Tutsch, R., Fromm, K., and Dai, G., “True 3D-AFM sensor for nanometrology”, Measurement Science and Technology, vol. 31, no. 7, 2020. doi:10.1088/1361-6501/ab7efd.