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ZEHRA PARLAK Ph.D.
Candidate, Research Assistant Work Phone: (404) 385-2051
Education:
Research Experience:
Teaching Experience:
Research Projects:
Atomic force microscope (AFM) is a
tool which is mainly used for topography imaging with nanometer resolution.
AFM uses a micromachined cantilever beam as a probe to detect the surface. Not only topography imaging but also subsurface
and mechanical properties imaging are possible by using AFM. My research
interests are; to model current AFM techniques for quantitative subsurface
imaging and to improve mechanical properties imaging by AFM.
I.
Quantitative Nanoscale Subsurface Imaging
by UAFM
Atomic force microscopy (AFM) has been initially developed for topography imaging. Nevertheless,
several methods, including ultrasonic methods combined with the AFM have made
this instrument useful for characterization of mechanical properties of
surfaces at the nanoscale while maintaining its nondestructive nature.
In most of these methods, basically AFM cantilever is vibrated in ultrasonic frequencies while the cantilever tip is in contact with surface with a DC force. If the cantilever is well-known, contact stiffness can be measured by detecting the resonance frequencies during the contact. To measure the contact stiffness by using resonances, we used an Atomic Force Acoustic Microscopy (AFAM) set-up (Fig. 1).
Fig.1. Schematic of AFAM set-up
Although contact stiffness –which is
represented as a spring resulted from the deformation of the tip and
substrate- can be detected with high accuracy, models are required to
interpret these results. Thus, we created a 3D FEA model with ANSYS (Fig. 2)
which can simulate scan of an AFM tip on a substrate with subsurface
structures.
Fig.2. Meshing of FEA model
We performed various convergence
analyses to verify the model. But an experimental verification is needed for
the case with finite size subsurface structures. In the experiment, a smooth
sample with well-known subsurface structure is necessary for verification.
Thus, we fabricated a cavity from the side of a silicon sample by using
Focused Ion Beam (FIB). Simulations are performed for different subsurface
cavity radii and contact stiffness is measured on these locations. The
comparison of experimental data and FEA is shown in Fig. 3.
Fig.3. Comparison of experimental data and simulation
The agreement between the
experiments and simulation results shows that the 3-D model can be used to
simulate subsurface imaging by AFM on substrates with single and multiple
subsurface structures. By using this 3-D model; we investigated the effects
of different force levels and materials on the contact stiffness. We can
analyze if an electromigration defect can be detected by using UAFM. You can
check our journal paper1 for more detailed information.
1. Z. Parlak and F. L. Degertekin, Journal of Applied Physics 103, 114910-8(2008).
II.
Controlling
Interaction Forces in Tapping Mode AFM by FIRAT Probe
Tapping mode (TM) AFM is the preferred mode of operation when imaging polymers and biological samples since the lateral forces are reduced. Due to the complex frequency spectrum of cantilevers, obtaining interaction forces during TM AFM is challenging and requires extensive processing, filtering and mathematical reconstruction. Interaction forces in TM AFM may provide information on mechanical properties of the sample. In 2005, an alternative AFM probe for interaction forces imaging is proposed in Degertekin group. This probe consists of a micromachined beam with a sharp tip on the center (Fig. 4a). Unlike AFM cantilevers, this probe has high cut-off frequency and low Q; consequently broad frequency response. It is fabricated on quartz and diffraction gratings are deposited under the beam which provides interferometric displacement detection. The beam can be biased to an interferometrically sensitive point or controlled during the taps by using the electrical inputs of beam and grating.
Fig.4. a)Schematic
b)Side view of FIRAT probe.
One of the problems of TM AFM is the instability that is caused by attractive forces of the surface. To solve this issue, one may apply higher oscillation amplitudes or use stiffer probes. But these methods may cause tip-sample deformation and inaccurate topography imaging. We propose an active control scheme which can be used while performing TM AFM imaging by FIRAT probe. In this scheme, an active tip control signal is applied to the beam and tip is retracted while it is in contact with the surface. The signal is set zero while attractive forces are effective on the beam. As a result, probe is softened during contact but stiff and stable while it is not in contact. For more detailed information, you can check our journal paper2. 2. Z. Parlak, R. Hadizadeh, M. Balantekin, and F. Levent Degertekin, Ultramicroscopy, vol. 109, pp. 1121-1125, 2009. Journal Publications: Z. Parlak, R. Hadizadeh, M. Balantekin, and F. Levent Degertekin, "Controlling tip�sample interaction forces during a single tap for improved topography and mechanical property imaging of soft materials by AFM," Ultramicroscopy, vol. 109, pp. 1121-1125, 2009. Z. Parlak and F. L. Degertekin, "Contact stiffness of finite size subsurface defects for atomic force microscopy: Three-dimensional finite element modeling and experimental verification," Journal of Applied Physics, vol. 103, pp. 114910-8, 2008. A. G. Onaran, M. Balantekin, W. Lee, W. L. Hughes, B. A. Buchine, R. O. Guldiken, Z. Parlak, C. F. Quate, and F. L. Degertekin, "A new atomic force microscope probe with force sensing integrated readout and active tip," Review of Scientific Instruments, vol. 77, pp. 023501-7, 2006.
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