The variable-entrance slit system (or Micro-slit) is now commonly used as a critical diaphragm component in many spectrophotometers whose principal function is to analyse the molecular fingerprint of liquid samples. This micromachined structure (Fig. 1) consists of a central aperture plate supported by a pair of flexible beams which allow light of different wavelengths to pass through the slit. The aperture plate is coated with a thin (500 nm) aluminium coating which serves the function of masking any light around the aperture.
Characterisation of the adhesion of this Al coating to its Si substrate is difficult owing to the small size of the aperture plate. The Nano-Scratch Tester (NST) from CSM Instruments has been used to accurately measure the scratch resistance by making progressive load scratches over the load range 0 - 10 mN with a 5 ìm diamond tip. Fig. 1 shows two such scratches made on each side of the central slit. Subsequent optical microscopy along the scratch paths allows the critical failure points to be observed: first failure consists of cracking at the sides of the path (Fig. 2(a)), whereas final failure is seen as delamination of the coating from the substrate (Fig. 2(b)). Such measurements confirm the use of the NST as a useful tool for characterising coatings in-situ on ultra-small devices where low loads and high positioning accuracy are indispensible.
Figure 1.Optical micrograph of a typical Microslit structure showing the central aperture plate supported by a pair of flexible beams of thickness 80 ìm. The zoomed image shows two scratches made on each side of the central slit (scratch direction from left to right).
Figure 2.Optical micrographs of first failure (a) where initial cracking occurs and final failure (b) where the aluminium coating completely delaminates from the Si substrate. These images correspond to one of the scratches shown in Fig. 1.