Working Group 5.11

Nanoforce Metrology for Tactile Sensors

Collaborative Research Centre 516" "Design and Manufacturing of Active Microsystems"

PTB subproject B6 in the field of geometrical metrology is aimed at investigating the potential of measuring procedures and measuring systems for use in microsystem technology (MST) and at assisting project partners in solving metrological problems. Within the scope of the project, the technology required for the setting-up and calibration of miniaturized dimensional measuring systems for process optimization and manufacture-oriented quality assurance is to be developed.

The following results were obtained in the third application period (1-7-2003 to 30-06-2007).

Investigation of the probing process at small probing forces

Target was the investigation of the probing process of the mechanical probing of micro structures with low hardness (especially SU-8). In order to measure the probing forces a nanoforce measuring device was set-up and metrologically characterised.

Nanoforce measuring device

The thermal drift of the nanoforce measuring device (distance between nanopositioning device and balance pan) was measured to 2,7 µm/K. In order to reduce the thermal drift of the nanoforce measuring device it is operated in an thermally isolated chamber. The chamber temperature is damped by 30 L of water inside the chamber. The transient response time constant of the chamber is 148 minutes. Twelve hours after closing the chamber the mean thermal drift amounts to 0.1 K/d. Thus in a typical measurement time intervall of 1 hour the thermal drift amounts to 4 mK. This leads with the above determined value of the thermal drift of the set-up to a positioning drift of 11 nm/h.

Besides the thermal length drift of the set-up there is also a temperature related drift of the force measured by the compensation balance of 0.38 µN/K. Is this systematic effect compensated, a resolution of the nanoforce measuring device of 1.5 nN over 2 hours is measured.

Bending stiffness of AFM-Cantilevers

A typical application of the nanoforce measuring device is the measurement of very small bending stiffnesses of scanning force microscopes. The stiffness of four v-shaped AFM cantilevers of the Thermomicroscopes Corp. with a nominal stiffness of 0.03 N/m were measured. The stiffness was determined in a force range of 1 nN to 800 nN. The measured stiffnesses were 0.0650 N/m, 0.0650 N/m, 0.0639 N/m and 0.0734 N/m, the standard deviations were between 0.0002 N/m and 0.0006 N/m. These results confirm the relatively large uncertainty in the statement of the nominal stiffness of AFM cantilevers. The residuals of the applied linear regression were in a range of ± 10 nN. This value demonstrated the achieved high quality of the nanoforce mearuring device.

New 3d micro probe probing pin for undercut structures

A modified probing pin for the 3d micro probe was developed. The shank was built out of a 7 mm long capillary whose end was angled by 40 degrees.