Working Group 5.11

Nanoforce Metrology for Tactile Sensors

• Overview dimensional standards for micro structures [270 kB]
• Depth-Setting standards for measurement ranges from 1 µm to 5 mm
• Micro-force standards

Depth Setting Standards for Measurement Ranges from 1 µm to 5 mm

Microsystem technology makes it possible to miniaturize existing systems and to manufacture novel systems. For the dimensional production and quality control of these systems, measuring instruments and methods are used, which allow quasi two-and-a-half-dimensional measurements: contact stylus instruments, interference microscopes, scanning white-light microscopes, fringe projection measuring instruments, stereo image processing, photogrammetry, etc. The required measurement ranges extend laterally or vertically over some millimeters. For the calibration of the vertical measurement axis of these devices, depth setting standards with depths into the millimeter range are needed. The conventional glass standards whose grooves have a circular arc profile [1] which have proved to be suitable for the calibration of contact stylus instruments can be manufactured with depths greater than 10 µm only at great expense. Etched grooves show surface roughness values of a few nanometers already at depths of some micrometers and thus in most cases are not smooth enough. At the PTB, diamond-turned depth setting standards with groove depths up to 900 µm have been developed, which are easy to handle and relatively small and thus handy [2].

Figure 1: sketch and photo of the 900 µm-depth-setting standard

The material used is very pure copper (OFHC-Cu) as it provides the best surface qualities (arithmetical mean deviation Ra = 1 nm, waviness Wt = 50 nm, flatness 50 nm over 20 mm measurement length). The copper discs used are 90 mm in diameter and 10 mm thick. After provision of the disc with rotationally symmetrical grooves, the disc is subjected to wire EDM to cut it into twelve segments of equal size which are then chemically nickel-plated to a thickness of 5 µm. The surface hardness of the standards then is about 500 HV and wear-resistant even when used with conventional contact stylus instruments with their relatively great probing forces in the millinewton range. As a result of nickel plating, however, the surface roughness rises to arithmetical mean deviations of Ra = 9 nm. Limited by the layer thicknesses which can be obtained in practice, profiles up to 100 µm depth can also be cut directly into the Ni layer and be used as standards without further plating. So the original, somewhat smaller roughness obtained by turning remains. The minimum V-angle of the grooves is 70°, the groove width in the tread is 0.3 mm and the region between the V-grooves is 0.4 mm long. As the grooves can be made with arbitrary depths, their grading can be matched to certain measuring instruments. Moreover, any V-angles greater than 70° and, as a result, smaller edge slopes can be produced. Even optical measuring instruments such as fringe projection measuring instruments relying on continuous stripe tracking for evaluation can be calibrated with such grooves. In three parallel profile sections a, b and c, the groove depth is measured and stated. A profile section over all seven grooves is 8 mm long. The uncertainties achieved for grooves 10 to 1000 µm deep are in the range from 20 to 55 nm and are typical of these diamond-turned depth setting standards.

The calibration of topography measuring instruments using the diamond-turned depth setting standards

Which uncertainty of measurement is achieved when the novel diamond-turned depth setting standards are used for calibration? For this question to be answered it is assumed that the measuring instrument to be calibrated has similar properties as the contact stylus instrument (Form Talysurf L120) sed for calibration at the PTB. Then an uncertainty of measurement for the measuring instrument to be calibrated of U = 78 nm is obtained in the 1 mm measurement range.

New depth-setting standards with grooves up to 5 mm depth

For dimensional measurements on micro systems, devices with vertical measuring ranges of some millimeters are increasingly used [3]. For the calibration of these instruments depth-setting standards have been developed in the PTB within the scope of a cooperation with industry. To improve the surface quality, a machine finishing of the grooves has been applied following the Ni-coating process. The surface roughness attained was Ra = 2.0 nm and the waviness of the surface Wt = 15 nm. The hardness of the surface (500 HV) corresponds to that of a steel of medium hardness. Due to their low surface roughness, the standards can be used for the calibration of both tactile and optical surface measurement instruments.

Figure 3: Diamond-turned depth-setting standard with groove depths up to 5 mm

Die Normale sind aus sauerstofffreiem Kupfer gefertigt und mit einer verschleißmindernden Nickelschicht überzogen. Zur Verbesserung der Oberflächenqualität wurden die Rillen nach dem Beschichten noch einmal fein nachbearbeitet. Die erzielte Oberflächen-Rauheit beträgt R_a = 2,0 nm, die Welligkeit der Oberfläche W_t = 15 nm und die Härte der Normale (500 HV) entspricht der eines Stahls mittlerer Härte.

Aufgrund der sehr geringen Rauheit der Oberfläche können diese Normale sowohl für die Kalibrierung von optischen als auch taktilen Messgeräten eingesetzt werden.

Die Kalibrierung der Rillentiefen (5 µm, 50 µm, 450 µm, 1 mm, 2 mm, 5 mm) erfolgte mit einem Tastschnittgerät, das mit Hilfe von Endmaßstufen kalibriert wurde und zum Vergleich mit einem Dreiwellenlängeninterferometer. Die Übereinstimmung beider Messgeräte ist für die vier tiefsten Rillen (5 mm, 2 mm, 1 mm und 0,45 mm) besser als 100 nm (s. Tabelle 1).

Table 1: Comparison of the groove depths measured by means of a stylus instrument and by means of an interferometer for two different depth-setting standards

It can be seen that the tactile stylus and the interferometric measurements agree very well. The maximum deviation between the two instruments is < 100 nm. This means that for the first time, high-quality diamond-turned depth-setting standards up to 5 mm groove depth are available for the precision calibration of topography measuring instruments.
With the novel diamond-turned depth setting standards handy standards are for the first time available for the calibration of topography measuring instruments with measurement ranges in the millimeter range. The calibration can be carried out very efficiently because all grooves can be measured in a single measurement. The length of the profile to be measured is only 8 mm for the 900 µm - standard and only 16 mm for the 5 mm - standard.
The standards are made from OFHC copper and plated with a wear-reducing nickel coat. The hardness of the standards (500 HV) is equal to that of a steel of average hardness.
In the case shown here, the grading of the groove depths is matched to the measurement ranges of a contact stylus instrument but the manufacturing procedure permits any desired grading. Nominal values can be complied with with tolerances of a few thousandths.