rf excitation of the stm eudean sun, uc berkeley, eecs joonhee lee, xiuwen tu dr. wilson ho im-sure...
TRANSCRIPT
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RF Excitation of the STM
Eudean Sun, UC Berkeley, EECSJoonhee Lee, Xiuwen TuDr. Wilson Ho
IM-SURE Fellow:Graduate Students:
Faculty Mentor:
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Scanning Tunneling Microscope Angstrom Resolution Microscope Tunneling Current
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Quantum Tunneling
JT – tunneling current
VT – DC bias
s – tip-sample distance
Order of magnitude change in JT for every angstrom change in s.
sAVJ TT2/1exp
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Scanning Feedback loop
Piezoelectric drives to position sample Ceramic materials that distort with voltage for high-precision
positioning
Maintain constant tunneling current by adjusting tip-sample distance
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RF Excitation via a Coil 800MHz – 2.0GHz, 100MHz steps Resonance
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Experimental Data
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The Model SolidWorks, AutoCAD, Ansoft’s HFSS – High
Frequency Structure Simulator Finite Element Method, like FEMLAB
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The Model cont’d Parts:
Inner radiation shield Sample holder Sample RF coil Crosspiece Tip
Excitation 2mA current
Solution Frequency sweep Fields along polylines
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Preliminary Results Resonance due to radiation shield
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Making a Better Model Added parts:
Outer shield
Coaxial cable
Rails / Grabber
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Making a Better Model cont’d New excitation
Wave port Finer mesh
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Making a Better Model cont’d Rebuilt all parts in HFSS
Transferring between SolidWorks/AutoCAD and HFSS was inconsistent
Refined solution setupAdded parametric analysis to plot E field
across sample for four different tip-sample distances: 1e-6, 1e-5, 1e-4, 1e-3 in.
Increased data points across polylines to 10,000 for plotting fields.
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The New Model…in Color
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Results Frequency Sweep
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Results cont’d Parametric Analysis
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Results cont’d
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Results cont’d
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Results cont’d Field plots
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Results cont’d
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Problems Resonance in simulation at 1.2GHz,
1.4GHz, 2.0GHz primarilyResonance in experiment at 800MHz,
1.2GHz, 1.3GHz, 2.0GHz primarily Parametric analysis shows large E field
differences between 1e-3 in, 1e-4 in, and 1e-5 in, but not a big jump between 1e-5 in and 1e-6 in.
“Out of memory”
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Potential Fixes Better geometry Finer meshes
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Limitations Model complexity
Can’t include everything, but what parts will affect resonance most?
Computer speed 2.66GHz Pentium 4 – 20 hours to complete one
analysis
RAM 1GB physical + 4GB virtual memory “Out of memory”, literally, when finer meshes applied
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Acknowledgements Dr. Wilson Ho Joonhee Lee Xiuwen Tu IMSURE Program