Traceable Profile Scanner with a Piezoresistive Cantilever

Characterizing high-aspect-ratio surface features of micro components is a challenge for micro- and nanometrology. These components are too delicate for stylus profilers. The Atomic Force Microscope (AFM), which is widely used in the nanotechnology area, is a two-and-half dimension measurement instrument and can only measure relatively flat surfaces. The limitations of AFMs come from the small measurement range along the z-axis (less than 10 µm for most commercial AFMs), the relatively low height of the cantilever tips (normally smaller than 20 µm), the slow scanning rate (smaller than 50 µm/s in many cases) and the mostly used optical beam detection method. To overcome these limitations, a traceable profile scanner with a very long piezoresistive cantilever has been developed.  The slender silicon cantilever with an integrated piezoresistive strain gauge for signal read-out was designed together with the Forschungsinstitut für Mikrosensorik und Photovoltaik GmbH (CiS) Erfurt and Institute for Semiconductor Technology of Braunschweig Technical University. The cantilevers are 1.5, 3 and 5 mm long, 30, 100 and 200 µm wide, and the tips are about 70 µm high. With a well-shielded design of the cantilever holder (see Fig. 1), the noise of the cantilever is about 3.7 nm in a bandwidth of 10 mHz to 1 kHz, and its sensitivity is about 250 V/m. One aim of the set-up of the profile scanner and cantilever is to achieve a measurement uncertainty of less than 10 nm. Because of its high scanning rate of up to 1 mm/s, the profile scanner can also be applied for other complicated measurement tasks such as determining the influence of the cantilever scanning rates on the measured thickness of viscous materials.

The profile scanner system is shown in Fig. 2. The contact force between cantilever and artifact surface keeps constant during the measurement. This is achieved by using the cantilever signal as input for the z-axis positioning controller. The piezoresistive tactile sensor can be moved by a commercial x-y- and z-piezo driven stage. The moving range is 800 µm × 800 µm × 250 µm (x × y × z). The contact force between cantilever tip and artifact can be set down to 1 µN.

On the head of the system, three laser interferometers with 1 nm resolution are arranged perpendicular to each other to provide metrological traceability. Their measurement beams intersect on the cantilever tip end for an Abbe error-free measurement. During measurements the reading of the three laser interferometers and the cantilever signal are synchronized. After a coordinate transformation of the laser interferometer data, the xyz coordinates of the measured profile is achieved and can be stored.

The artifact is placed on a coarse positioning stage with a movement range of 12.5 mm × 12.5 mm × 12 mm (x × y × z). The three linear coarse stages are mounted on a rotation stage so that the artifact can be rotated by 360 degrees around the z-axis. All coarse stages are equipped with motorized drives, so that the positioning can be realized ​​without opening the shielding hood. In addition, the entire head of the system can be moved about 100 mm in the z-direction to measure large artifacts up to 8 cm × 10 cm × 10 cm.

The measurement and control software is written in LabWindows^TM and allows an easy operation of the instrument. Once the measurement parameters (scan range, number of sampling points, control parameters etc.) are set, the cantilever can approach to the surface automatically and performs a measurement with constant contact force between the cantilever and the artifact surface.

Some first measurements have been performed to characterize the properties of the cantilever and scanning system. A Zerodur optical flat was measured five times at the same place to evaluate the flatness along the x-axis (see Fig. 3). For the five measurements the scan range was 800 µm, contact force 3.5 µN, scan speed 50 µm/S, and the scan was recorded with 4096 data points. The Ra values of the profiles are approximately 6 nm. Later more measurements, especially comparison measurements with other instruments will be executed to assess the performance of the profile scanner.

Fig. 1 The slender cantilever on the cantilever holder

Fig. 2 The profile scanner system


Fig. 3 Five measurements of a Zerodur optical flat at the same position. Ra values calculated are approximately 6 nm.