Direct measurement of electrostatic fields using single Teflon nanoparticle attached to AFM tip
© Chang et al.; licensee Springer. 2013
Received: 12 September 2013
Accepted: 23 November 2013
Published: 7 December 2013
A single 210-nm Teflon nanoparticle (sTNP) was attached to the vertex of a silicon nitride (Si3N4) atomic force microscope tip and charged via contact electrification. The charged sTNP can then be considered a point charge and used to measure the electrostatic field adjacent to a parallel plate condenser using 30-nm gold/20-nm titanium as electrodes. This technique can provide a measurement resolution of 250/100 nm along the X- and Z-axes, and the minimum electrostatic force can be measured within 50 pN.
07.79.Lh, 81.16.-c, 84.37. + q
KeywordsElectrostatic Teflon Nanoparticle Atomic force microscopy
Measuring the electrical properties of devices at the micro/nanoscale is an important issue in the semiconductor industry and in materials science [1–4]. Electrical modes in scanning probe microscopy (SPM)  have become an essential tool in characterizing the electrical properties at the surface of samples, providing spatial resolution and sensitivity at the micro/nanoscale. Several methods have been developed for the measurement of surface electrical properties and local surface potential, such as electrostatic force microscopy [2, 3] and Kelvin probe force microscopy [6, 7]. The basic principle behind these techniques  is applying a direct current (DC) bias between the conductive probe and the sample to facilitate the recording of variations in the electrostatic force between the probe and sample. These signals are then analyzed in order to interpret the associated surface electrical properties. Jenke et al.  used a Pt-coated Si tip with a radius of about 380 nm to probe the electrostatic force generated above embedded nanoelectrodes in the vertical (Z) direction. The electrostatic force acting on a grounded conductive tip within an electrostatic field can also be characterized. In this approach, the electrostatic force acting on the atomic force microscopy (AFM) tip comprises Coulombic, induced charge, and image charge forces [9–11]. However, only the Coulombic force is capable of directly revealing the electrical properties of the sample because the two other terms are the result of the AFM tip effect. Kwek et al.  glued a charged microparticle to an AFM cantilever to investigate the relative contributions of the Coulombic, induced charge, and image charge forces in the electrostatic force acting on the charged particle; however, the diameter of the charged particle was approximately 105 to 150 μm, which is unsuitable for measurement at the nanoscale.
The process of fabricating the sTNP tip
The experimental setup of the deposition of charge to the sTNP tip
Measurement of the electrostatic fields
The charged sTNP tip was then used for the measurement of f-d curves to determine the electrostatic field beside the top electrode of the parallel plate condenser (Figure 1). The sTNP tip is located slightly inward at the end of the AFM cantilever; therefore, the end of the AFM cantilever is susceptible to striking the edge of the top electrode when the distance between the AFM tip and the electrode is within 10 μm. To overcome this situation, 21 spots spaced at 0.25 μm along the X-axis at a distance of 10 to 15 μm are selected for the measurement of the f-d curves in order to derive the electrostatic field. As shown in Figure 1, the edge center of the condenser was plotted as the origin of the X- and Z-axes. DC voltage (Vapp) of ±25 V was applied on the top electrode, and the bottom electrode was left grounded. Each curve measurement was conducted for distances of 15 μm along the Z-axis, from 6 μm below to 9 μm above the top electrode. The ramp rate and the ramp size of each f-d curve were 2 Hz and 15 μm, respectively. The sTNP tip did not come into contact with the substrate during the measurement of electrostatic fields. The measurement of f-d curves was conducted using the force mapping function in the JPK SPM software.
Simulation of the electrostatic field
The electric field was simulated using finite element method in Ansoft Maxwell simulation software  to estimate the electrostatic field. The current model deals only with the electric field in the Z direction from −10 to approximately 10 μm. After designing the model, the maximum length of elements was set at 0.4 μm; this was sufficient to provide accurate solutions to model at that scale. The Maxwell program automatically fits the mesh to estimate the electrostatic field.
Results and discussion
where FC is the Coulombic force that resulted from the external field acting on the charged particle, Fimage is the image force caused by the attraction of the particle to its net charge image, and Fpol is the force created by the attraction between the field-induced dipolar charge (polarization) in a particle in an electrostatic field and its dipole image in the electrode.
In this study, Fpol acting on the sTNP was due mainly to the thin layer of water adsorbed on the surface of the tip due to the large dielectric constant of water (ϵwater = 80). To eliminate the influence of the water layer, the measurement of the electrostatic field was conducted under N2 conditions (RH < 5%), such that Fpol acting on the sTNP could be disregarded; a plastic O-ring was placed between the scanner and sample to allow the injection of N2 into the O-ring. Charges deposited on the sTNP under N2 conditions can last (variation smaller than 5%) for over 90 min, and the measurement process can be completed within 10 min. In this study, the dissipation or generation of charge did not occur during charge verification, despite the fact that the charged sTNP tip touched the grounded metal surface under N2 conditions. Figure 4b presents the three f-d curves at X = 11 μm under N2 conditions when Vapp = +25, 0, and −25 V were applied to the top electrode, and the bottom electrode remained grounded. The Z-axis component of FE acting on the sTNP tip can be revealed in the measured f-d curves (Figure 4b), expressed as FE(Vapp). FE(0 V) acting on the sTNP tip is due mainly to Fimage, which is always attractive to the top electrode of the condenser. The FC(+25 V) is the attractive force acting on the negative-charged sTNP tip, such that FE(+25 V) is smaller than FE(0 V) above Z = 0 μm. FC(+25 V) always attracts the negative-charged sTNP tip, regardless of whether the sTNP tip is above or below the top electrode at Z = 0 μm. This results in the charged sTNP tip being trapped at Z = 0 μm, preventing it from moving forward during the measurement of the f-d curves, as shown in Figure 4b. FC(−25 V) is a repulsive force acting on the negative-charged sTNP tip, such that FE(−25 V) is larger than FE(0 V) above Z = −2.6 μm; however, it is smaller below Z = −2.6 μm due to the attractive force induced from the bottom electrode.
In the future, the pyramidal shape of the Si3N4 tip could be modified using a focused ion beam system to create a cylindrical shape in order to avoid the possibility of experimental fluctuations resulting from the shape of the tip. This probe could be employed to scan surface topographies by mapping f-d curves, and the interaction force between the charged Teflon particle and sample would give a direct indication of the local electric field and properties of the sample.
In summary, this paper reported the direct measurement of the electrostatic field beside a parallel plate condenser using a charged sTNP on an AFM tip. Experimental results were then compared with those obtained through simulation. A sTNP tip was fabricated by attaching a single 210-nm Teflon nanoparticle at the vertex of a Si3N4 AFM tip and was charged via contact electrification. The lateral/vertical resolution of the electrostatic force measurement is 250/100 nm, respectively. The minimum Fele that can be measured using this method is less than 50 pN. This technique provides a novel means of studying the electric properties of electrical devices. The AFM tip is able to hold a single charged nanoparticle, making it possible to directly quantify the local electric/magnetic field, charge distribution, and electrostatic force of a sample surface using an AFM system. The charged sTNP tip could find a wide application in electrical research at the nanoscale.
JMC received his M.S. degree in engineering and system science from National Tsing Hua University, Hsinchu, Taiwan in 2005. He is currently working towards finishing his Ph.D. at the Institute of NanoEngineering and Microsystems, National Tsing Hua University, Hsinchu, Taiwan. WYC is currently working towards finishing a Ph.D. degree at the Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan. FRC is a professor at the Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan. FGT is a professor at the Department of Engineering and System Science, National TsingHua University, Hsinchu, Taiwan. He received his Ph.D. degree in mechanical engineering from the University of California, Los Angeles (UCLA), under the supervision of Prof. C-M Ho and C-J Kim in 1998. He is currently the Deputy Director of Biomedical Technology Research Center of NTHU and Chairman of the ESS department. He has written five book chapters, including ‘Micro droplet generators’ in MEMS Handbook (CRC) and ‘Technological aspects of protein microarrays and nanoarrays’ in Protein Microarrays (Jones and Bartlett), and he has published more than 80 SCI Journal papers and 240 conference technical papers in MEMS, bio-N/MEMS, and micro/nanofluidic-related fields. He has received 32 patents. FGT is a member of ASME, APS, and ACS. He has received several awards, including the Mr. Wu, Da-Yo Memorial Award from National Science Council, Taiwan (2005–2008), five best paper/poster awards (1991, 2003, 2004, 2005, and 2009), NTHU new faculty research award (2002), NTHU outstanding teaching award (2002), NTHU academic booster award (2001), and NSC research award (2000).
Atomic force microscopy
Coulombic force resulting from the external field acting on the charged sTNP tip
Z-axis component of net electrostatic force acting on the sTNP tip
Electrostatic force field of the condenser
Scanning electron microscope
Scanning probe microscopy
Single Teflon nanoparticle
- sTNP tip:
Single 210-nm Teflon nanoparticle attached to the vertex of an insulated Si3N4 AFM tip
DC voltage applying on the top electrode.
This work was supported by grants from the National Science Council of Taiwan under the programs NSC102-2627-M-007-002, NSC100-2120-M-007-006, NSC 99-2120-M-007-009, NSC100-2627-M-007-013, and NSC 99-2627-M-007-002.
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