AFM probe structures with integrated interferometric sensing andelectrostatic actuation
F. Levent Degertekin, A. Guclu Onaran, Hamdi Torun, Mujdat Balantekin, Krishna
Sarangapani, and Cheng Zhu
Georgia Institute of Technology
G.W. Woodruff School of Mechanical Engineering
801 Ferst Dr. NW, Atlanta, GA 30332, U.S.A.
E-mail: levent@gatech.edu
In this talk, we summarize our efforts in development of novel AFM probes and actuators
based on extended use of micromachining techniques with a focus on applications in liquidmedia. The first type of device uses a surface micromachined membrane structure as force
sensor which is directly actuated using the built-in electrostatic actuator [1]. This enables fast
actuation of the probe tip limited only by the membrane dynamics. The motion of the tip ismeasured with high sensitivity using an integrated optical interferometer. Membrane
structures suitable for in-liquid operation are fabricated on transparent substrates made ofquartz or glass. A reflective metal grating is formed on the surface which also serves as one of
the actuator electrodes. The membrane is made of dielectric layers, a silicon nitride - siliconoxide stack, or a polymer such as parylene over a sealed gap. The top metal actuator electrode– optical reflector layer is buried in this dielectric layer for electrical isolation in conductive
liquids such as buffer solutions. To illustrate application of these probes in single moleculemechanics experiments, they were used to measure unbinding forces between L-selectinreconstituted in a polymer-cushioned lipid bilayer on the membrane and an antibody adsorbed
on an AFM cantilever. Piconewton range forces between single pairs of interacting molecules
were measured from the cantilever bending while using the membrane as an actuator. The
integrated diffraction-based optical interferometer of the probe was demonstrated to have <10fm/√Hz noise floor for frequencies as low as 3 Hz with a differential readout scheme. Withsofter membranes, this low noise level would be suitable for direct force measurements
without the need for a cantilever. Furthermore, the probe membranes were shown to have 0.5µm actuation range, with a flat response up to 100 kHz enabling measurements at high speeds[2]. We also describe a second type of device, the acoustic radiation force (ARP) actuator, forfast imaging in liquids. The ARP actuator uses focused acoustic waves at RF frequency range(100-300MHz) to induce localized forces on AFM cantilevers in liquids. The actuator has an
actuation bandwidth in excess of 1MHz and it can be used with any type of AFM cantilever
without a need for any magnetic or piezoelectric film. ARP actuator has been integrated to acommercial AFM system and fast tapping mode imaging without a Z-piezo has been
demonstrated. Furthermore, single molecule force spectroscopy experiments were conductedusing the same system [3].
1. 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, 77, 023501 (7pages), 2006.
2. H. Torun, J. Sutanto, K.K. Sarangapani, P. Joseph, F.L. Degertekin and C. Zhu,
“Micromachined membrane-based active probe for biomolecular mechanics measurement,”
submitted to Nanotechnology, 2006.
3. A.G. Onaran and F.L. Degertekin, “A fluid cell with integrated acoustic radiation pressure
actuator for atomic force microscopy”, Review of Scientific Instruments, 76, 103703, (6pages) 2005.