Critical Dimension

Imaging and measurements of semiconductor structures with critical dimensions is another challenging application for AFM. The image in Fig. 1 shows a 3D view of the features, which are 30nm in width and 100nm in height.

Fig. 1. Lines with 30nm pitch and 100nm height.

The basic array parameters (pitch and step height) are evaluated correctly when regular AFM probes are used. However, the edges of the lines presented in the images are affected by a convolution of the AFM tip shape. In other words, the shape of the AFM tip limits the accuracy of the measured shape of the sample features. The problem of the tip-shape convolution becomes even more severe with increases in height and decreases in spacing between features. The use of AFM probes with small opening angles (such as FIB-made AFM probes) is required in such cases.

The Hi'Res-C AFM probes can provide an improvement in accuracy from the standard HQ:NSC AFM probes. Very great care must be taken when using the Hi'Res-C AFM probes on such samples, as damage to the AFM tip is easily incurred. Light tapping conditions are preferable, which implies a low resonance amplitude (start at 0.2V up to 1.2V) and a set-point ratio of about 0.9 to 1. Scan rate should start at below 1Hz. Scan size should start out at 50nm (250nm maximum). It is also important, in general, when imaging samples with a large span in the vertical dimension to use large gains. Please refer to the High Resolution page for details.