Noncontact Atomic Force Microscopy - Yale School of Engineering ...

M. Teresa Cuberes
Ultrasonic Nanolithography on Hard Substrates
Laboratory of Nanotechnology, University of Castilla-La Mancha, Almadén, Spain.
P.I-34
Ultrasonic- AFM techniques provide a means to monitor ultrasonic vibration at the
nanoscale and open up novel opportunities to improve nanofabrication technologies [1].
Ultrasonic Force Microscopy (UFM) relies on the mechanical-diode cantilever response
when ultrasonic vibration is excited at the tip-sample contact [2]. Strictly, UFM is a
dynamic AFM (Atomic Force Microscopy) implementation in which the tip-sample
contact is periodically broken at ultrasonic frequencies. In the presence of ultrasonic
vibration, the tip of a soft cantilever can dynamically indent hard samples due to its
inertia. Fig. 1 (a) demonstrates scratching of a silicon sample in the presence of ultrasonic
vibration, using a cantilever with nominal stiffness of 0.11 Nm -1 and a SiN tip. In the
absence of ultrasound, it was not possible to scratch the Si surface using such a soft
cantilever. Interestingly, no debris is found in the proximity of lithographed areas.
Fig. 1 (b) displays ultrasonic curves -UFM response for increasing ultrasonic
excitation amplitude- recorded during UFM imaging (curve (a)) and during scratching of
silicon (curves c-d). UFM imaging is carried out without substrate modification by using
a lower tip-sample load. When increasing the load, the silicon substrate is scratched
provided a sufficiently high ultrasonic amplitude is excited. By monitoring the ultrasonicinduced
cantilever responses during manipulation, information about the substrate
modification can be obtained. As the substrate is being scratched, the UFM responses
change. Apparently, for a fixed load, a critical ultrasonic amplitude is needed to induce
modification (see Fig 1 (b)). In the poster, the advantages of ultrasonic nanolithography,
and the shape of UFM curves recorded during manipulation will be discussed in detail.
a) b)
Figure 1. AFM scratching of Si(111) in the presence of normal surface ultrasonic vibration of ~
5 MHz using a pyramidal SiN cantilever tip with nominal stiffness 0.11 Nm -1 (a) Nanotrenches
formed in 50, 70 and 100 cycles respectively, at a load of ~ 40 nN and maximum ultrasonic
amplitude Am = 500 mV (b) UFM response for a triangular modulation of the ultrasonic
excitation (see lowest curve). Load: (i) ~ 26 nN. (ii-iv) ~ 40 nN. Am=500 mV.
[1] M. T. Cuberes, J. of Phys.: Conf. Ser. 61 (2007) 219.
[2] M. T. Cuberes in “Applied Scanning Probe Methods”, B. Bhushan and H. Fuchs (ed.), Springer 2009.
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