Excellence in Nanomechanical Test Instrumentation

MEMS

Control of friction and wear of moving parts is a key issue for the reliability of Microelectromechanical (MEM) Devices. A common problem is that they are fabricated from Si wafers using techniques originally developed by the microelectronics industry. Unlubricated Si02 surfaces (Si with its native oxide) are very hydrophilic and therefore suffer from high friction and wear, and unwanted adhesion. Solid lubrication approaches are being developed to combat this - and low friction, hard coatings that are resistant to abrasive wear are being developed which lower the surface energy. The NanoTest is being used to assess potential overcoat formulations, such as DLC, ta-C and other more novel materials.

Case study: scratch resistance of MEMS overcoat

Optimum deposition conditions necessary for improved scratch resistance of tetrahedral amorphous carbon (ta-C) films produced by FCVA plasma technology can be determined by nano-scratch testing. Ta-C films have been developed for MEMS applications including capacitive sensors and protective coatings for micromachined components. Although they have a high sp3 fraction of carbon atoms resulting in high hardness, these films can be highly stressed and their suitability for tribological applications needs to be evaluated.

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Experimental conditions: Multi-pass scratch test with 3μm end radius probe scanning over a 150μm track at a scan speed of 2μm/s. In the scratch pass after 20μm the load is ramped at 2.5mN/s. 3 repeat tests were performed to test the reproducibility of scratch behaviour. The replot shown was from the second of these.

The nano-scratch data can be automatically corrected for any initial slope/topography and compliance in the NanoTest software. This enables us to very clearly define several key transition points that are confirmed by microscopy. These are Le-p (critical load for the onset of yield), Lc1 for the onset of edge cracking and Lc2 for total film failure. The figure shows an image taken with the NanoTest integrated optical microscope which confirms the excellent reproducibility of the NanoTest nano-scratch technique. The nano-scratch tests show the ta-C film has acceptable durability under sliding contact with Lc2 at a much higher load than Lc1.

Other NanoTest MEMS applications include:-

  • Bending of micro-beams and cantilevers
  • Fatigue of micro-beams and cantilevers (using patented nano-impact technology)
Further Reading

 

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