HOE enhances AFM performance through its features of compact configuration, small size, and high sensitivity. This unique capability and easy optical adjustment of HOE can make the AFM system much more desirable than the conventional optical lever method.
For understanding the optical properties of HOE, theoretical analyses by ray-tracking method describe the optical model of holographic pickup head. For the consistency of the theoretical analysis, the software simulation is made to verify the optical properties of holographic pickup head. Through comprehensively studying the effect between the optical components and light spot, the optical system for the AFM is adaptively optimized.
The performances of the holographic pickup head are experimentally verified. The derivations of translational S-curve with different objective lens show the relationship between depth of focus and linear region. The relationships between angular signal and angle for bending and torsion are demonstrated. The peak to peak voltage of translational S-curve decreases gradually with the increase of water thickness. The linear region increases gradually with the increase of water thickness. During the experiment, the thermal fluctuation method simplifies the procedure of spring constant calibration.
Besides, the cantilever motion measurements are examined the translational and angular detection functions with high sensitivity as 0.45 nm/mV and 0.95 μrad/mV, respectively.
For suppressing the spurious peaks in liquid, the integration of two damping plates (PEEK) and the steel structures is adopted in the probe holder. The spurious peaks are less than 4.3% and 14.5% of the resonant peaks in air and water. The spectrum of stimulated cantilever is clear and significant without any spurious peaks. This result shows that the spurious peaks can be suppressed by the probe holder effectively.
The practicality of the proposed methodology is demonstrated through the AFM scanning process to verify the stability and high resolution of the developed HOE-based AFM in air and water. Single atomic step (0.34 nm) of graphite is detected by force error signal and tracking error signal. In addition, the function of HOE-based profiler is also verified by directly measuring calibration grating with laser beam. Through successfully detecting the profiles of microcircuit and tuberose epidermis tissue, it testifies the measurement feasibility of the HOE-based profiler in micron-scale applications in scientific and engineering fields.
For observing dynamic processes in liquid, an area of future research should be considered is the high speed and high resolution AFM. Therefore, the objective lens with high numerical aperture and the high resonant frequency cantilever should be utilized.
Besides, the angular signal of holographic pickup head is proposed for improving the sensitivity. As a result, the tracking error signal is necessary for high speed AFM and torsion mode AFM.
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Appendix A
Specification of HOE unit
Appendix B
Specification of AD624
Appendix C
AFM probe: PPP-NCH
Appendix D
AFM probe: PPP-CONT
Appendix E
AFM probe: CSC38/Cr-Au
Appendix E
Test grating: TGQ1
Appendix F
Calibration grating: TGZ3