| Popis: |
This thesis proposes a novel atomic force microscopy (AFM) system based on silicon waveguide cantilever displacement sensor. The sensor consists of input and output silicon waveguide cantilevers facing each other and separated by a nano-gap. It detects displacement of input silicon waveguide from intensity modulation of optical power received at the output silicon waveguide. In addition to playing displacement sensing role, the input silicon waveguide cantilever also acts as an AFM cantilever with a nano-tip integrated to it and as electrostatically driven actuator. This approach of displacement sensing uniquely allows the use of nano-scale wide cantilever with high resonance frequency and low spring constant desired for on-chip high speed atomic force microscopy (HS AFM). It also allows self-sensing and actuation integral for parallel AFM implementation. The silicon waveguide cantilever displacement sensor has been studied to maximize responsivity and ensure single mode using Optiwave 2D and 3D FDTD optical simulation software. The effect of various waveguide dimensions, offset, and nano-gaps between the input and output waveguides are taken into account in the study. The silicon waveguide dimensions have also been designed to satisfy the requirements for HS AFM cantilevers, which are high resonance frequency and low spring constant. The design was facilitated using COMSOL simulation. The results show that optimal responsivity of 3.1%/10nm for the sensor, resonance frequency of 5.4MHz and spring constant of 0.21N/m are achievable with the proposed system. The noise performance of the proposed sensor is analysed and compared with the existing typical optical beam displacement (OBD) measurement technique. The analysis shows that the silicon waveguide cantilever yields better noise performance. The realisation of the proposed displacement sensor requires the fabrication of low-loss nano-scale silicon waveguides with nano-gaps, in order of 20nm, in a repeatable and reliable manner. This thesis presents the development of sub-micron wide silicon waveguides with small line edge roughness, and smooth and vertical sidewalls while allowing incorporation of nano-gaps. The developed fabrication method enables silicon waveguides with 6dB/cm and 7.5dB/cm transmission loss for TE and TM modes, respectively, by reducing rms roughness due to line-edge and sidewall to 2.1nm and yielding vertical sidewalls. The method also allows formation of 13nm wide nano-gaps between silicon waveguides. Also critical for the realisation of the proposed AFM system is a low-loss misalignment tolerant wide bandwidth optical coupler to facilitate coupling of light from optical fiber to sub-micron waveguide. This thesis presents the design, fabrication and characterization of inverse taper cantilever coupler. By introducing a SiO2 gap in front of taper waveguide, the coupler effectively increases misalignment tolerance to μm for 5μm spot size lensed fiber and decreases coupling loss to 2.12dB/facet for standard cleaved single mode fiber without compromising other performances. It has also demonstrated high bandwidth with no apparent loss in the wavelength range of 1520nm to 1570nm. In this thesis, a novel low temperature nanofabrication approach that enables the formation of ultra-sharp high aspect ratio and high density nanotip structures and their integration onto nanoscale cantilever beams is developed. The nanotip structure consists of a nanoscale thermally evaporated Cr Spindt tip on top of an amorphous silicon rod. An apex radius of the tip, as small as 2.5 nm, has been achieved, and is significantly smaller than any other Spindt tips reported so far. 100 nm wide tips with aspect ratio of more than 50 and tip density of more than 5 × 109 tips cm-2 have been fabricated. The HAR tips have been integrated onto an array of 460 nm wide cantilever beams with high precision and yield. In comparison with other approaches, this approach allows the integration of HAR sharp nanotips with nano-mechanical structures in a parallel and CMOS compatible fashion for the first time to our knowledge. Finally, the design and fabrication of electrostatically driven nano-cantilever actuator are presented. The design ensures the generation of ample displacement from the nano-cantilever needed for AFM application while maintaining the sensor‟s optical and mechanical characteristics designed for HS AFM application. The measurement shows that the nano-cantilevers can produce 180nm displacement at 30V driving voltage. |
