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Rauheitsmessungen in Mikrodüsen mit dem Profilscanner

01.12.2013

Durch die Weiterentwicklung des Profilscanners sind in der PTB erstmals Rauheitsprofil-Messungen in Mikrodüsen mit einem Durchmesser von 100 µm und weniger möglich.


A traceable profilometer (Profile-Scanner) was developed by PTB for high-aspect-ratio microstructure surface profile metrology. The slender cantilever makes measurements inside of micro holes with diameters of 100 µm and less feasible. A vertical resolution up to 0.1 nm can be obtained by the cantilever and tip radii of less than 0.1 µm ensure high lateral resolution. Three laser interferometers with 1 nm resolution arranged perpendicularly to each other in the head of the profilometer provide metrological traceability. During measurement, a XYZ-piezostage with a motion range of 800 µm × 800 µm × 250 µm drive the cantilever to characterize the artefact surface with a constant probing force. Small probing forces down to 1 µN prevent the tip from scratching the artefacts surface. Constituted of a motorized rotary stage with continuous 360 degrees and three motorized linear coarse positioning stages with a movement range of 50 mm × 50 mm × 12 mm (x × y × z), the coarse motion stages make programmed batch measurements possible.

To prove the metrological capabilities of the profilometer, comparison measurements of roughness standards with other metrological instruments in PTB were performed. The same measurement positions, measurement parameters and data processing methods, including sampling spacing and filter selections, were used to ensure optimum conditions for the comparison. According to DIN EN ISO 4288, an evaluation length of 4 mm or 1.25 mm is demanded for roughness measurements. However, both evaluation lengths exceed the maximal scan range of the profilometer, which is 800 µm limited due to the stage. Stitching was used to overcome this problem and to ensure the same evaluation length as for tactile measurements. The comparison results indicate that all of the measurement results agree well within their measurement uncertainty ranges, and the error of the profilometer for roughness arithmetical mean deviation Ra measurements is within ±10 nm (2σ). See table 1 for the detailed results of the comparison measurements, where Ra is the arithmetical mean deviation of the profile, Rz is the arithmetical mean of the single roughness depths of successive sampling lengths, and Rmax is the largest single roughness depth in the total measuring length.

Besides the comparison measurements, profile and roughness measurements inside a micro hole of 100 µm diameter were successfully performed. The micro holes, drilled into a steel plate 12 mm in diameter (see Fig. 1a), are arranged in a 5 x 3 matrix. The depth of the holes was 500 µm. The roughness profile inside one of the micro holes was measured using a 1.5 mm long piezoresistive cantilever. The steel plate was mounted in such a way that the cylindrical axis of the micro hole was parallel to the cantilever. Two optical microscopes, one imaging the top view of the cantilever, the other imaging from the left rear side of the cantilever, were used to observe the relative positions of cantilever and measured micro hole. Figure 1b shows the view from the top optical microscope. It can be seen that the cantilever is inside the hole during measurement. During measurements the cantilever moves along the axis of the cylindrical micro hole with a scanning speed of 20 µm/s. The probing force of the microprobe during the measurement is 7 µN. A 340 µm long profile is measured with 3580 sampling points and the measurement was repeated eight times. The measuring position and two repeated measured profiles are shown in Fig. 2a. Using a short wavelength noise filter λs = 2.5 µm, the measured profiles were evaluated using the Reference Software RPTB of the PTB [1] for roughness measurements. An arithmetic mean roughness of Ra = 845 nm at a standard deviation of σ = 2.0 nm was obtained. Besides the inner sidewall surface roughness, other geometrical parameters like the form of the micro holes in the entry region are also critical for the function of microstructures. A measurement of the surface profile at the holes entry was therefore also performed. The measuring position and measurement result are given in Fig. 2b.

Table 1: Comparison of the measurement results of the Profile-Scanner and the PTB reference facility for three roughness standards

Roughness standard No

Parameter

PTB reference value

U(k = 2)

Profile-Scanner value

U(k = 2)

1

Ra

                60.2 nm ± 3%

                60.4 nm ± 5%

Rz

              317.9 nm ± 4%

              322.4 nm ± 6%

Rmax

              392.4 nm ± 9%

              410.9 nm ± 13%

2

Ra

              0.312 µm ± 2%

              0.309 µm ± 3%

Rz

                1.69 µm ± 2%

                1.67 µm ± 3%

Rmax

                1.92 µm ± 2%

                1.90 µm ± 4%

3

Ra

              0.505 µm ± 2%

              0.502 µm ± 3%

Rz

              1.586 µm ± 2%

              1.581 µm ± 3%

Rmax

              1.594 µm ± 2%

              1.598 µm ± 2%

 

 

(a)(b)

Fig. 1 The measurement of a micro hole by the Profile-Scanner: (a) the measurement artefact, a steel plate with micro holes of 100 µm diameter, (b) the cantilever in a micro hole during a measurement.

(a)(b)

Fig. 2 Profiles measured by the Profile-Scanner inside a micro hole of 100 µm diameter. (a) inner sidewall roughness profile, (b) form of the micro hole entry region.


References