Hydrogel tip attached quartz tuning fork for shear force microscopy
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This paper reports the first demonstration of hydrogel conical tip attachment onto quartz tuning fork (QTF) by using an elastomeric tip mold that is soft-lithographically replicated from an electrochemically etched tungsten wire. The tungsten tip of 10–100 nm radius obtained by time-controlled electrochemical etching is replicated with h-polydimethylsiloxane (h-PDMS) to make negative conical tip molds large enough to be used for QTFs. By approaching a QTF to the negative h-PDMS tip mold filled with polyethylene glycol-diacrylate (PEGDA), a PEGDA tip is attached to the QTF without using an adhesive. Then, the PEGDA tip attached QTF is employed for shear force microscopy for calibration grating and atomic layers of hexagonal silicon carbide and also compared with a silicon tip attached QTF. Exclusively for the PEGDA tip attached QTF, we demonstrate that the imaging tip could be regenerated multiple times to address issues associated with tip wear. In a stark contrast with conventional QTF probes in attachment of electrochemically etched metallic wires or microfabricated AFM cantilevers, photocuring of liquid phase prepolymer within a tip mold demonstrated herein allows adhesive-free and exclusive attachment of the imaging tip onto a QTF. The relatively large PEGDA tip enables facile operation during approach and engagement. Moreover, the organic and inorganic combination of imaging tip and resonating body offers regeneration of the imaging tip upon its degradation.
KeywordsAtomic force microscopy (AFM) Hydrogel tip Shear force Tip attachment Quartz tuning fork (QTF)
Although atomic force microscopy (AFM) probes have been typically made of silicon that is the second most abundant atom on earth, disposal of the whole probes is neither cost-effective nor environment-friendly considering the fabrication cost/time and hazardous chemicals used during fabrication. Therefore, it would be ideal if the worn or damaged imaging tip could be independently replaced. The recent study revealed that a PEGDA tip shows superior wear-resistance to silicon tip due to the extended attractive regime for the PEGDA tip, which makes the true noncontact mode imaging more favorable . In addition to the systematic wear comparison, it was shown that a worn or damaged PEGDA tip could be regenerated multiple times by chemically removing it and subsequently attaching a new PEGDA tip to a tipless silicon cantilever . The multiple regeneration was guaranteed by the etch selectivity of silicon in piranha solution that exclusively removed the PEGDA tip.
However, tipless silicon cantilevers are still not cost-effective to fabricate, and their resonance frequencies exhibit intrinsic uncertainty originating from non-uniform wafer thickness and spatiotemporal process variation during fabrication. On the other hand, QTFs typically used for timing references in wrist watches  are orders of magnitude cheaper than tipless silicon cantilevers and exhibit very uniform frequency characteristics and high quality factors. Sizes of commercial QTFs are much larger than that of tipless silicon cantilevers to guarantee much easier manufacturing and handling. In addition, the QTFs are more robust than tipless silicon cantilevers and tend to survive even after physical impact by accident. Due to the aforementioned benefits, it would be advantageous if the PEGDA tip can be attached and regenerated onto QTFs. While attaching a PEGDA tip to QTFs seems much easier than attaching a microfabricated silicon cantilever [4, 5] or an etched tungsten wire , the negative tip mold previously used for tipless silicon cantilevers  is too small for QTFs: it may prevent easy use of the probe for imaging after tip attachment. A simple and straightforward solution for this issue is to make a larger negative tip mold. However, it requires extended processing time as well as thicker wafers to make larger tip mold masters with silicon via potassium hydroxide (KOH) etching.
This paper reports the fabrication of relatively large negative elastomeric tip molds replicated from electrochemically etched tungsten wires for the first time. Using the large negative tip molds, sufficiently large but sharp PEGDA tips are fabricated and attached to QTFs by single ultraviolet (UV) exposure without using an adhesive. PEGDA tip attached QTFs are successfully employed for imaging of calibration grating and silicon carbide monolayers. In addition, the used PEGDA tip is removed by piranha solution and a new PEGDA tip is attached to demonstrate the modular AFM concept where the imaging tip can be replaced and regenerated if necessary.
Electrochemical etching of tungsten wires
Tip mold fabrication and hydrogel tip attachment
Figure 2b shows a 3D drawing of the tip attachment setup with the mold compression jig, where a CCD camera monitors the attachment process from below. With this setup and the process aforementioned, the tip can be exclusively attached to QTFs at a desired position without using an adhesive. Figures 2c, d compare a tungsten tip fabricated by electrochemical etching and a replicated PEGDA tip pair of which radii are 26 and 29 nm, respectively. It should be noted that the sub-30 nm radius PEGDA tip is the best result to date. However, such a high-quality PEGDA tip is not routinely obtained due to uncertainties in the tip mold fabrication and compression processes. The average PEGDA tip radius is around 250 nm with 10% variability. If necessary, plasma sharpening  can be used to further reduce the tip radius. The optional bi-axial compression may also be necessary to achieve a tip radius smaller than 50 nm. The relationship between PDMS mold compression and PEGDA tip sharpness enhancement was studied in the previous study, reporting the improvement of a tip radius by 10 folds with the 15% increase of bi-axial compression . Although the initial dimension of the tip mold and mold materials are different, the underlying physics is identical when applying the same strain, so we expect similar enhancement.
Calibration grating imaging
In shear force microscopy (SFM), the QTF is mechanically driven by a dithering piezo-actuator (PZT) at its in-plane anti-phase resonance, such that the tip’s motion is predominantly lateral . This enables shear (or frictional) force interaction of the tip with the sample surface. Under the shear force, the piezoelectric property of the QTF itself generates a current proportional to the deflection of the QTF prongs. Figure 3c represents operational schematic of the SFM, which utilizes a vertically aligned QTF with a PEGDA tip attached to the end of one of two prongs. Due to the high Q-factor of the QTF, the resonance frequency shift (\(\Delta f\)) of the QTF probe is measured by a phase-locked loop (PLL) and used as the set point for the z-feedback control of the sample stage. While a PEGDA tip scans over a sample surface, surface topography can be precisely measured by monitoring the vertical displacement of the sample stage to maintain \(\Delta f\) at the set point. The QTF electrical signal is detected with two amplifiers (A1: AD8626, Analog Devices) with the trans-impedance gain (Rf) of 1 × 108 V/A, from which a differential signal is amplified (A2: AD8228, Analog Devices) with a gain of 10. Therefore, the output of the SFM pre-amplifier is a voltage signal proportional to the QTF oscillation amplitude. The signal from the SFM pre-amplifier is demodulated by using a lock-in amplifier at the dithering piezo drive frequency, yielding the amplitude (A) and phase (Φ) of the QTF oscillation. After the resonance frequency and vibration amplitude are measured as a function of the tip-sample distance during approach and retract cycle, noncontact-mode shear force imaging is performed on the calibration sample (~ 50 ± 10 nm thick vanadium rhombus array on a quartz substrate): see Fig. 3d. Figure 3e shows a line profile of the calibration sample along the white dashed line A–A′ in Fig. 3d.
Result and discussion
Comparison of commercial silicon tip and PEGDA tip
By setting a proper setpoint, topography of the 6H-SiC monolayer can be obtained along with amplitude and phase errors. Figure 4e shows a topographic image of the 6H-SiC monolayer sample where the setpoint, scan speed and scan area are 8 Hz, 0.5 µm/s and 1 µm × 1 µm, respectively. Quick side note here, shear force exerting on the tip during the imaging can be estimated to be ~ 67 nN, which may be not sufficient to detach the tip from the QTF. During routine imaging with such a condition, we have not observed the detachment of hydrogel tips from QTFs.
Figure 4f shows line profiles extracted from the black dashed lines in Fig. 4e where each step is equal to half of the lattice constant of the silicon carbide (i.e., 7.5 Å). Figure 4g represents histograms of the transition slope for the 3rd step between the 3rd and 4th atomic layers of the 6H-SiC where all 256 linescan data are used. Each histogram is fitted to Gaussian to obtain average and standard deviation, yielding 8.57 ± 0.91 and 10.30 ± 1.77 nm/µm for the silicon tip and the PEGDA tip, respectively. Although the PEGDA tip exhibits larger variation, its average slope is 1.2 times steeper than that for the silicon tip. The steeper transition with the PEGDA tip may be because the meniscus effect around the tip is more dominant for the silicon tip. In addition, the larger variation of the transition slope may be attributed to mechanical deformation of the relatively soft PEGDA tip. From a practical point of view, the silicon tip attached QTF is more challenging to handle than the PEGDA tip attached QTF mainly due to the relatively small size of the imaging tip. If a small tip attached QTF is slightly tilted during engagement to a sample surface, other corners of the QTF would touch the sample prior to the attached tip and damage the sample surface or produce image artifacts.
In this paper, we introduce an unconventional approach to fabricate hydrogel (PEGDA) tip attached QTFs for shear force atomic force microscopy. Using the h-PDMS tip mold replicated from an electrochemically etched tungsten wire, a PEGDA tip can be spontaneously attached to QTFs upon UV exposure without using an adhesive. PEGDA tip attached QTFs are implemented to image calibration grating and atomic layers of hexagonal silicon carbide samples in an SFM platform, demonstrating the advantages of the PEGDA tip in noncontact shear force imaging over a silicon tip. The proposed tip attachment method also offers repeated tip regeneration without damaging the QTF when tip wear or damage is not tolerable for imaging. We believe that other AFM applications beyond topographic imaging can be explored with the PEGDA tip attached QTFs in near future.
JK and JL developed the idea. JK and JL designed, fabricated, and characterized PEGDA tips and PEGDA tip attached QTF probes while AJ and KP conducted the SFM measurements and imaging processes using the developed probes. JK mainly analysed the results with AJ’s assistance, and JK and JL wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
This research was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (NRF-2017R1A2B3009610 and NRF-2017R1A4A1015564) was also supported by the National Science Foundation (CBET-1605584) and the University of Utah Funding Incentive Seed Grant. AJ also acknowledges financial supports from the University of Utah’s Sid Green Fellowship and the National Science Foundation Graduate Research Fellowship (No. 2016213209).
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