Scanning probe devices, such as the atomic force microscope (“AFM”) have proven to be excellent tools for imaging a wide range of materials such as metals, semiconductors, minerals, polymers, and biomaterials. In an AFM, forces are measured by means of a cantilever that deflects when forces act on it. The deflection of the cantilever is sensed by a detection system, commonly by focusing an incident beam as a spot onto the cantilever and directing the reflected beam onto a segmented detector. Specialized AFMs called “force pullers” have been built for the purpose of pulling on molecules to determine the structure and dynamics of those molecules.
Since its introduction, the AFM and its cantilever sensor have become increasingly more advanced, measuring decreasingly smaller forces and utilizing decreasingly smaller cantilevers. This has introduced problems relating to the sensitivity of the instrument. There is a need to provide greater sensitivity to accommodate the smaller cantilevers and smaller forces that scientific investigators need to either measure samples or manipulate them. Similar detection techniques are also used to monitor the motion of the optical probes used in near-field scanning optical microscopes, scanning ion-conductance microscopes, and a variety of other scanning probe microscopes. The growing field of nanotechnology also provides ample motivation for the precision measurement of the position and/or motion of a wide variety of objects down to the nanometer scale and below.
The development of new small cantilevers with resonance frequencies two orders of magnitude higher than conventional cantilevers make the detection mechanism and the cantilever response much faster than necessary for conventional AFM systems. The speed of AFMs depends on the response time of the detection mechanism (cantilever and readout), the actuator (scanner), the feedback electronics and the piezo driver electronics. These components together form a feedback loop in which the performance of the overall system is affected by phase delays and resonances in any of these components. As the resonance frequencies of new, small cantilevers reach frequencies around 280 kHz even for a soft cantilever (0.006 N/m) in liquid, the new mechanical bandwidth is set by the scanner, and by the mechanical superstructure. Therefore, to further improve the capabilities of the AFM, special attention has to be given to the mechanical design of the scan and detection unit.
One of the main speed determining factors in an AFM system is the scanner, which is generally made with piezo crystals as the actuating components. In many commercially available systems piezo tubes are used to generate the displacement in x, y and z directions. The active part of the scanner consists of a tube made out of piezoelectric material segmented into different sections. Tube scanners use the principle of mechanical amplification to transform the small expansion of the piezos to a larger scan range. Scan ranges of commercial scanners can range from 0.6 μm to 100 μm. This principle reduces the need for large capacitance piezos and reduces the requirements on the amplifier. However, it also results in a weak mechanical structure and therefore a low mechanical resonance frequency (˜800 Hz). This is one of the primary speed limits of commercial tube scanners.
Another disadvantage of all kinds of piezos is their nonlinearity in operation. Piezos exhibit a large position hysteresis, up to 30%, with respect to the activating voltage. Piezos are also unstable in their position over time, changing its expansion even with a constant actuation voltage. These nonlinearities are a severe problem for the use of piezos as scanners for AFM as they distort the image, resulting in image drift and making it hard to find the same spot on the sample after zooming in. The hysteresis has to be accounted for either by a mathematical model to correct the actuation voltage or by controlling the actual piezo position in a closed loop feedback. Some commercial scanners model the piezo behavior, and changes in the actuating voltage are made by the controlling software. This approach has several disadvantages:                scanner parameters have to be measured for each individual scanner, with up to thirty parameters needed to model the piezo sufficiently;        the behavior of the piezo is dependent on the DC offset, scan range and scan frequency; and        position creep is unaccounted for by the modeling.For the user of the AFM this results in:        image warping (images are expanded in some directions and compressed in others);        change of image center when zooming in or out; and        image drift        incorrectly measured sizes of the objects.However, this approach does not need any sensors and all the modeling can be done by the software and the digital signal processor.        
The following references relate to the background of this invention: (1) C. F. Quate, et al., Atomic Force Microscope, Phys. Rev. Lett. 56 (1986) 930; (2) D. Rugar, et al. Atomic Force Microscopy, Phys. Today 43 (10) (1990) 23; (3) Atomic resolution with the atomic force microscope on conductors and nonconductors, J. Vac. Sci. Technol. A 6 (1988) 271; (4) G. Schitter, et al., Robust 2DOF-control of a piezoelectric tube scanner for high speed atomic force microscopy, Proceedings of the American Control Conference, Denver, Colo., Jun. 4-6, 2003, pp. 3720; (5) D. A. Walters, et al., Short Cantilevers for Atomic Force Microscopy, Rev. Sci. Instrum. 67 (1996) 3583; (6) M. B. Viani, et al., Small cantilevers for force spectroscopy of single molecules, J. Appl. Phys. 86 (4) (1999) 2258; (7) T. Ando, A high-speed atomic force microscope for studying biological macromolecules, Proc. Natl. Acad. Sci. USA 98 (22) (2001) 12468; (8) Humphris, A D L, Hobbs, J K and Miles, M J, Ultrahigh-speed scanning near field optical microscopy capable of over 100 frames per second, Apl. Phys. Let. 2003, 83:6-8; (9) J. B. Thompson, et al., Assessing the quality of scanning probe microscope designs, Nanotechnology 12 (2001) 394; (10) T. E. Schaffer, et al., Characterization and optimization of the detection sensitivity of an atomic force microscope for small cantilevers, Journal of Applied Physics, (84), (No. 9) (2001), 4661; (11) T. E. Schaffer, et al., An atomic force microscope using small cantilevers, SPIE—The International Society for Optical Engineering, (3009) (1997) 48; (12) T. E. Schaffer, et al, Studies of vibrating atomic force microscope cantilevers in liquid, Journal of Applied Physics, (80) (No. 7) (1996) 3622. See also the following U.S. patents: U.S. Pat. No. 5,825,020—Atomic force microscope for generating a small incident beam spot, U.S. Pat. No. RE034489-Atomic force microscope with optional replaceable fluid cell, and U.S. Pat. No. 4,800,274—High resolution atomic force microscope. The foregoing publications and patents are all incorporated herein by reference.