Bottom-up approach for traceable calibration of tip geometry of stylus profilometer awarded “Paper of the Year 2022”

Stylus profilometry is a widely applied technique for the measurement of form, contour and roughness of surfaces. The geometry of a stylus tip is an important influence factor of measurements, as the measured profile is the dilated result of surface features by the tip geometry. Thus, the tip geometry sets a bandwidth limit of the profilometer in roughness measurements and biases the measurement results in form and contour metrology. To correct its contribution, the geometry of the stylus tip needs to be characterized accurately.

To solve these challenging problems, a novel approach for accurate and traceable calibration of stylus tip geometry has recently been developed. The approach consists of several steps, as illustrated in figure 1. In the first step (S1), the geometry of an AFM tip is calibrated to a specific line width standard (IVPS100-PTB) whose geometry is traceably calibrated to the lattice constant of single crystalline silicon. In the second step (S2), the stylus tip to be calibrated is measured by the calibrated AFM tip in an AFM, thus its tip geometry can be accurately determined after the contribution of the AFM tip geometry is corrected from the measured AFM image. Once calibrated, the stylus tip can be applied in measurements of microstructures and surfaces in the third step (S3), where measurement results in turn can be corrected using the characterized stylus tip geometry. In such a way, the stylus tip geometry and its measurement results can be finally made traceable to the lattice constant of silicon, using this bottom-up approach.

The concept was experimentally verified for a commercial stylus type RFHTB-50 (Mahr). The stylus tip radius is measured as 1.72 μm. It is significantly smaller than its nominal value of 2 μm, indicating the need for calibration. The application of the calibrated stylus tip is demonstrated in measurements of microspheres. For three spheres with nominal radii of 150 μm, 250 μm and 500 μm, their radii are measured as 150.8596 μm, 248.9966 μm and 499.9548 μm, respectively. The standard deviation of five repeat measurements reaches 0.5 nm, 0.5 nm and 0.6 nm for three microspheres, respectively, indicating excellent measurement repeatability of the metrology tool.

Compared to the conventional characterization method using e.g. a kind of a tip characterizer, the proposed novel method has several advantages:

1. It avoids the risk of damage at the sharp edges of conventional tip characterizers
2. It is capable of directly characterizing the 3D geometry of the stylus tip
3. It avoids the ambiguity in determining profile segments in reconstructing the tip geometry
4. It can be achieved with high accuracy and is traceable to a natural constant—the lattice constant of crystalline silicon.

The publication, “Bottom-up approach for traceable calibration of tip geometry of stylus profilometer”, has been awarded “Paper of the Year 2022” by the journal “Surface Topography: Metrology and Properties” (https://iopscience.iop.org/journal/2051-672X/page/Awards).

Figure 1. Concept of the new methodology for characterizing stylus tip geometry.