Measuring with (razor-)thin layers

Thin-film systems with thicknesses from a few 10 nanometres up to several micrometres have been successfully applied as sensors at PTB for several years. After using such layers successfully – e. g. as sensors for measuring the electrical resistance or conductance of aqueous solutions – PTB focused its developments on thin-film sensors which can be deposited on components with uneven, rough surfaces and on the most diverse materials.

These films are applied in vacuum by means of sputtering: atoms are ejected out of a suitable electrode material (target) by ions, and these atoms are deposited onto a substrate as a thin layer. Hereby, the layer has a very strong bond to the substrate, which, for sensors, is a great advantage compared to an adhesive film. Given the increasing need for highprecision sensors, further advantages are that measurements can be carried out with high resolution and, especially, directly at the working point and under the influence of harsh ambient conditions. Besides the development of 3D-compatible insulation layers, also the structuring procedures had to be adapted. After a few development steps, it is now possible to manufacture structures that are precise to the micrometre on such uneven, rough surfaces by exposing a photoresist to a UV laser and a 4-axis precision robot system. Hence, resistance strain gauges (e. g. for force sensor systems) were fixed directly onto a cylindrical test piece for force measurement of up to 10 kN. Compared to affixed strain gauges, the sputtered ones showed clear advantages with regard to sensitivity as well as with regard to the step function response of the sensor which emphasize the huge potential of this kind of bonding.

In future, this technique is to be further refined to be able to deposit a dense, flawless and well-bonded insulation layer onto various substrate materials and varying geometries. Hereby, particular attention will be paid to a perfect insulation layer between the electrically conducting substrate and the sensor layer.

This is the key to a universally applicable technology, since numerous applications are based on a metallic – and, thus, electrically conducting – uneven, rough body material.