Dissertation defense -- Erhan Yenilmez, Nov 30th

Erhan Yenilmez yenilmez at stanford.edu
Tue Nov 29 14:48:20 PST 2005


Erhan Yenilmez

Research Advisor: Professor H. Dai

Carbon Nanotube Tips for Scanning Probe Microscopy

Wednesday 30 November, 2005
@10:00 A.M.

CIS-X Building, Room 101

Scanning probe microscopy (SPM) is a technique for characterizing the  
topography of surfaces and detecting forces at nanometer scale. There  
is need for new kinds of probes to meet the metrology challenges as  
the size of devices at this scale shrink, new nanostructured  
materials are introduced and new experiments involving SPM are proposed.

Commonly used probes for SPM are sharp crystal tips etched on  
silicon. Carbon nanotubes that are cylindrical structures of carbon  
with a few nanometers in width and micrometers in length have been  
introduced as ultra sharp and long lasting probes for SPM. The  
structure and the aspect ratio of carbon nanotubes give them  
desirable mechanical and chemical properties as SPM probes.

In this work we investigate a process where we can grow carbon  
nanotube tips on a wafer of crystal silicon tips using chemical vapor  
deposition (CVD). We would like to have one individual nanotube  
protruding from the apex and pointing within a narrow solid angle  
along the axis of each silicon tip. This corresponds to a narrow  
window in growth conditions, which yields one nanotube tip for each  
of the probes on the wafer while avoiding a dense growth of nanotubes  
around the apex. We identify the key elements to reach this yield as  
a 10nm layer of silicon oxide on the surface of the cantilever, a  
nominally 2 Angstroms thick film of cobalt catalyst and a reduction  
of the catalyst before CVD. We choose ethanol over methane as a  
carbon feedstock gas for CVD to achieve reproducible results.  This  
wafer scale process has reproducibly been shown to work with over 90%  
yield. There is still room for improvement of the orientation of the  
as grown nanotube tips.

A method to shorten as grown nanotube tips to a desired length is  
discussed. The force calibration mode of an atomic force microscope  
is used to controllably bend and buckle an individual nanotube  
between the probe cantilever and the surface. An external voltage is  
applied to cut the end of the mechanically deformed nanotube. This  
method enables us to make nanotube tips with desired length  
especially in the 100nm to 500nm range. However, the success rate of  
this cutting process depends on the initial orientation of the  
nanotube, and is below 50% for a given batch of as grown nanotube tips.

The topographic imaging potential of carbon nanotube tips is  
demonstrated by imaging a few nanometers wide gap on a cut nanotube  
lying on a surface.

We introduce magnetic force microscopy (MFM) capability for carbon  
nanotube tips by coating them with cobalt. Commercially available MFM  
tips are pyramid shaped silicon tips coated with a magnetic thin  
film. Our metal-coated nanotube tips confines the magnetic material  
at the tip in a cylindrical volume. This gives a higher resolution in  
MFM imaging compared to silicon tips where the magnetic material is  
spread on the surface of a pyramid. We have imaged features as small  
as 20nm are imaged using these tips on an experimental magnetic  
recording media, which is one of the best resolutions in MFM reported  
so far in literature. The yield of the metal coating process is 100%  
since the electron beam evaporated metal does not damage the nanotube  
tips if it is incident along the axis of the nanotube tip.

Micrometers long and a few nanometers thick as grown nanotube tips do  
not display high imaging quality. We increase the thickness of these  
tips by coating with metal to make high aspect ratio tips. We show  
that these metal-coated tips can be manipulated to point to a desired  
direction using focused ion beam. We demonstrate the high-aspect  
ratio imaging capability of these tips on micrometers deep holes on  
micro-machined surfaces and tall structures biological samples.


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