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Measuring the thickness of soft materials with greater precision

24 June 2013

Micro-metallic components are often replaced by polymer counterparts, but polymers yield under pressure, and so thickness measurement becomes imprecise.

Image: PTB
Image: PTB

This is particularly so when using conventional contact stylus instruments.

Scientists at the Physikalisch-Technische Bundesanstalt (PTB) in Germany have now come up with a new formula that will enable thickness measurements of soft polymer layers on hard substrates using stylus instruments, to be corrected. Their initial targets have been the widely used photoresist, SU-8, and Ormocomp, a polymer used in the manufacture of micro-optics.

With these soft materials, the contact force, the stylus tip radius and the traverse speed have an influence on the deformation of the component and thus on the measurement result.

In order to model these systematic errors, the scientisit focused on nonlinear strain hardening, which takes into account elastic, plastic and viscous deformations. In future work, the viscoelastic-plastic model will be extended so that the ageing of polymers can also be taken into account.

While the team at PTB expect their work eventually to reinforce current measurement standards, they say it does provide industry with an immediate practical method of correcting these measurement errors.

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