«UNIVERSITY OF CALIFORNIA Santa Barbara A Micro/Nano-Fabricated Gecko-Inspired Reversible Adhesive A Dissertation submitted in partial satisfaction of ...»
Having qualified this test methodology on simple structures where it was simple to build an analytical or numerical model, the characterization technique was then extended to more complex structures. For application to the platform pillar structures highlighted in this work please refer to the paper Single High Aspect Ratio Pillar Support Structures in chapter IV.
An even more complex structure used as a high sensitivity mass sensor. The sensor makes use of parametric resonance to detect small changes in the resonance frequency, which can then be related to a mass shift. The device is explained in great detail in ref (89). Thorough characterization of the device was desirable and measurements of the spring constant out-of-plane and in-plane were made. The inplane measurement was the first of its kind. The device consists of a backbone connected to two springs at either end of the device, Fig. A-6. In the middle portion of the device along the backbone there are a series of non-interdigitated comb fingers. Using the scanning probe microscopy of the nanoindenter high resolution images of these fingers can be taken, Fig. A-6. These images also allow for exact placement of the nanoindenter tip, a nice feature of using the actual tip for imaging.
Figure A- 6 Optical image of the non-interdigitated comb drive oscillator obtained in the nanoindenter (left). Scanning probe image (SPM) obtained using the 5 μm cone tip in the nanoindenter imaging the non-interdigitated portion of the comb fingers (right).
The out-of-plane mode of flexure was tested by placing the indenter tip on the backbone of the structure and performing a load-controlled indent. Interpreting the slope of the indent to be the spring constant of the structure a value of 12.5 N/m was determined. Given this particular device’s primary operation mode, the in-plane spring constant of the structure was of particular interest. To ascertain this another functionality of the nanoindenter was explored.
Originally designed for scratch testing, making a lateral movement of the tip with a constant normal load, the nanoindenter is equipped with a 2-dimensional transducer able to move and sense in the orthogonal and transverse directions.
Making use of the transverse translocation feature lateral spring constant measurements were performed.
Instead of placing the indenter tip on the backbone of the structure the tip was placed within in a slot of the backbone. A desired load function must then be supplied to the transducer. This load function must include both a normal load component and a lateral displacement component, Fig. A-7. The normal load is typically held constant while a lateral displacement is applied to the tip.
Figure A- 7 Typical load function for performing a lateral compliance test.
The normal load is ramped to a specified value (top) to then remain constant while a lateral displacement is applied (bottom).
While the transducer is applying a normal load and moving transversely, the normal displacement and transverse load is monitored. The outputted data then contains all four of these data sets plotted against time, Fig A-8. In order to determine the lateral spring constant of the device the slope of the lateral load versus displacement can be determined. Here care must be taken to only analyze the lateral data while the normal load and displacement is constant. As can be seen in figure Athe normal load and displacement rises and drops with a periodicity corresponding to the spacing of the backbone of the device, thus adding another force component as the tip rises. While the normal force is constant there is only the lateral force component. Relating this lateral force to the lateral displacement, over the constant force region, a spring constant of 9 N/m was determined for the device.
Figure A- 8 Outputted data versus time for a typical lateral force test:
Normal force (top-left), (top-right) Normal displacement, (bottom-left) Lateral force, (bottom-right) Lateral displacement. The slope of the lateral force versus lateral displacement for the constant normal force section was taken to be the