Senior Project - Electrical Engineering - 2012

1. Senior Project - Electrical Engineering - 2012 Coupling of Audio Signals into AFM Images Matthew Manning Advisor: Prof. Catravas Abstract: Atomic force microscopy is capable of imaging samples with nanometer scale features. However, given the sensitive nature of the system, the measurements are easily influenced by external vibrations. This project investigated the system response of an AFM to audio excitations. Images were produced in the presence of audible noise, and frequency recovery was attempted via digital signal processing methods. The impulse response of the system was measured for three operating modes of the AFM: attractive, repulsive and intermittent contact modes. The results reflect the linearity/nonlinearity of the system and can provide insight into the imaging process. Design: Data Analysis: • Single frequency excitation signals produced sinusoidal artifacts in the images; Several digital filters were developed to minimize effects of aliasing and reduce noise • Reference images were used for background noise reduction • System was characterized by measuring • impulse response in different modes: • Attractive, Repulse and Intermittent • To further test hypotheses of linearity/nonlinearity, convolution was utilized to derive theoretical results for comparison • Results from complex music signals analyzed with same methodology • Signal Delivery: • Impulses, single frequencies and music signals delivered to system in different operating modes • Speakers rated to 48Hz utilized due the to low sampling rate of AFM (Fs = 1024s/sec) • Model MULTI75AI probes used for imaging; useful for imaging in different AFM modes • Imaged on LEXAN Polycarbonate samples for minimization of noise caused by sample surface System Response: A Corrupted Image Actively Imaging Within Noise Shield Impulse-Like Incident Signal y(t) h(t) x(t) B C Repulsive Attractive Intermittent Results: • Impulse responses in repulsive, attractive and intermittent contact mode are shown at the right. • Each mode of operation produced a different impulse response. The shortest response was found in repulsive mode; the longest in intermittent contact mode. This is qualitatively consistent with the non-linear characteristics of the force-distance curve. • Convolution of incident sinusoids excitations with the measured impulse response produced results that resembled the measured output, yet contained noticeable differences. Differences were also observed in the images (shown far right) obtained in the three modes for a music signal excitation. This implies that a “small signal” linearized model was not valid for the experimental parameters chosen. • It is of interest for an AFM user to know when a “small signal” model is or is not valid when interpreting imaging results. The approach described here has the potential to provide such information. Music Signal- Repulsive Impulse Response - Repulsive Music Signal- Attractive Impulse Response - Attractive Impulse Response - Intermittent Music Signal- Intermittent Future Work: Acknowledgements: • Improved filter design • Improved system characterization (higher sampling rate, large range frequency sweeps) • Implementing lithography methods to mimic ambient noise induced error, used as a physical writing method • Prof. Palmyra Catravas • Union ECE Department • Armin Knoll, IBM Zurich • Nanofabrication Group, IBM Zurich