Fast roughness measurements

Piezoresistive silicon microprobes with an integrated stylus tip bridge the gap between scanning force microscopy and tactile profilometry with profilometers. The microprobes enable roughness measurements in structures with a high aspect ratio and depths of up to 5 mm and widths of up to 50 μm (see PTB News 3.2014). Moreover, due to their extremely small inert mass of only 0.1 mg, such microprobes in principle allow high traverse speed and sampling rates. Compared to the probing force of 750 μN recommended for profilometers, the probing force required to prevent the probe from lifting off from the surface due to dynamic forces during a measurement is only 28 μN for the 5 mm long microprobes. Within the scope of a BMBF project, this topic was investigated in more detail. For example, a roughness standard was measured with a silicon microprobe and with a conventional profilometer, and the results were compared. For this purpose, roughness measurements were carried out at high traverse speeds of up to 15 mm/s whilst applying high probing forces of 100 μN on purpose. The profiles obtained show a good agreement between the two methods – even in the details.

In addition, the wear of the tip was investigated. With increasing wear, the tip can no longer measure the grooves with great accuracy. This leads to bias and wrong roughness parameters in the measured profile. Investigations at moderate traverse speeds up to 100 μm/s and very small probing forces of 6 μN which were carried out on a roughness standard have shown that the silicon tip wears out continuously and thus flattens. Even after a measurement length of 54 m, however, the radius of these flattened tips measured less than 1 μm and was thus still smaller than the radius of conventional diamond tips (2 μm). At small probing forces and moderate displacement rates of less than 100 μm/s, a reliable tactile probing and comparability of profiles and parameters with those obtained by means of conventional procedures are ensured. At the maximum possible traverse speed of these microprobes, which amounts to 15 mm/s, wear, however, increases in such a way that after a measurement length as short as 3 m, the radius of the tip reaches more than 2 μm. After 300 m of probing, this radius increases to a value of 3 μm due to wear. At very high probing speeds, stylus tips which are more resistant to wear are therefore required.

In another BMBFfunded project, it is planned to investigate a tip-testing measurement standard for the exact determination of the tip shape. This is necessary in order to eliminate the influence of the tip's shape from the measured profile by means of morphological filtering. This would, for the first time, enable traceable microform measurements with piezoresistive microprobes. With its three interferometric measuring axes, PTB's “Profilscanner” (see PTB News 1.2012) provides ideal conditions for such measurements.