| Since its introduction more than twenty years ago, Atomic Force Microscopy (AFM) has extended its application areas from material science to biology or biophysics, based on its capability to image/manipulate objects in various environments with sub-nanometer spatial resolution in three dimensions. AFM has become a very useful microscopic instrument and/or manipulator of biological objects, such as proteins on membranes, in their physiological conditions.;Among the two most commonly used modes, the dynamic (tapping) mode has a great advantage over contact mode when imaging soft materials, in which the AFM cantilever is actively vibrated around its resonance frequency while the tip-sample separation is regulated such that the tip lightly taps the surface only around the lowest point of its oscillation and thus minimizes potentially destructive shear and adhesive forces on the sample. The amplitude modulation is the most commonly used control method of the tip-sample separation in the dynamic mode of AFM, in which the oscillation amplitude of the cantilever is regulated at a set-point by feeding back changes in oscillation amplitude to adjust the cantilever z-position. However, in typical implementations, due to tapping dynamics of the AFM cantilevers, the transient response of the cantilever induced by changes of the tip-sample interaction force leads to greater variations in tip-sample interaction via feedback, causing excessive tapping forces and/or possible loss of tapping during the scanning, and thus greater sample distortions and imaging errors. In order to suppress the effect of noise in the system when measuring the oscillation amplitude, the bandwidth of the oscillation amplitude estimation was limited and so was the imaging rate. In addition, the low bandwidth of the actuators in conventional AFMs, such as the z-positioner and the raster scanner limits the scanning speed. Therefore, while dynamic mode AFM has many potential applications, the inability to achieve direct and precise control of the tip-sample interaction forces and the low bandwidth of actuators for the tip-sample separation control and the raster scanning have been key barriers which imaging rate and inhibit innovation leading to new applications.;In this research, the design, actuation and control of a new generation AFM probing system which enables high-speed and high-resolution imaging of samples are investigated. In order to achieve direct tip-sample interaction control during the scanning, a novel dynamic sensing and control method are implemented, in which the tip-sample interaction force of each tapping cycle is estimated and subsequently controlled for dynamic force microscopy. By employing collocated magnetic actuation of the AFM cantilever and dual-actuator tip-motion control scheme, the high bandwidth tip-motion control, whose bandwidth is comparable to that of the cantilever, the dynamics over-damped, and the motion range comparable to that of conventional z-scanner is achieved. For the high bandwidth raster scanning as well as high bandwidth tip-sample separation control, active multi-axis probing system is implemented, in which multi-axis magnetic actuators along with a multi-axis probe with one magnetic moment, specially designed and fabricated for the multi-axis actuation, achieves high bandwidth multi-axis tip-motion control along the Z axis and the X axis. In order to achieve the high resolution imaging, a low noise laser measurement system is implemented and integrated to a commercial AFM (MFP3D, Asylum research). For the implementation of the direct tip-sample interaction control and high bandwidth active multi-axis probing system, high speed programmable digital controller is developed using field programmable gate array (FPGA) whose closed loop update rate is two orders of magnitude higher than commercially available ones. The results of scanning a standard grating whose pitch is 100nm and a biological grating (repeating protein structure on purple membranes) whose lateral pitch is about 6.2 nm using the high speed AFM are presented and discussed. |
