Development of high speed atomic force microscopy and its applications

Atomic Force Microscopy (AFM) is a member of scanning probe microscopy family that allows characterizing the surfaces in 3D at nanoscale. The operation of AFM is based on scanning the surface using a cantilever with a sharp tip. Atomic interaction forces between the tip and the sample is used as a feedback signal. The interaction force is controlled by a PID controller using Z scanner. The surface is raster scanned and movement of Z scanner at each position is recorded to have the surface topography. Atomic Force Microscopes are widely used in many fields such as material science, biophysics, nanotechnology and industrial process control. In addition to common areas of use such as obtaining surface topography, mechanical properties, electrical or magnetic properties, AFM is a technique that allows many experiments to be carried out in high resolution in their natural environment. AFMs can work in air, vacuum or liquid environments with less effort according to other surface characterization methods. However, AFM scanning speed limits the usability and application areas. The surface imaging for a given area with standard AFMs takes at least a few minutes. The dynamic phenomena occurring on the surfaces occur much faster than this speed. This makes it impossible to follow dynamic phenomena and understand their mechanisms. For this reason, the idea of High Speed AFM (HS-AFM) has begun to attract the attention of scientists since 1993. Hansma and Ando groups independently worked on HS-AFMs since 1993. High-speed Atomic Force Microscopes (HS-AFM) enables the investigation of structural changes of materials and dynamic phenomena occurring on the surfaces. HS-AFMs can contribute to the understanding of dynamic systems by allowing to investigate the changes occurring on surfaces with electrochemical reactions or to examine biological molecules in their physiological environments. In addition, AFMs can be actively used in industrial process control. Many characterization techniques based on indirect methods, such as fluorescence microscopy, requires labelling the sample with fluorescent stains. HS-AFM allows making direct characterization on the surfaces that decreases the degree of freedom on an experiment that makes much easier to interpret the results. A HS-AFM should be capturing the data as simultaneously as possible with the changes taking place on the sample. Many dynamic events on the surfaces occur in liquid environment, so it must be working in liquid without a decrease in quality or speed. The surface needs to be characterized at molecular resolution to understand the mechanism better. Therefore, a HS-AFM image quality should be sub nanometer resolution. Finally, and most importantly, it has to be as non-invasive as possible. If not the tip may damage the sample while scanning or change the natural flow of the dynamic event which in turn makes it impossible to understand the dynamic mechanism. There are many AFM imaging modes such as dynamic, contact, noncontact and many others derived from these modes. Dynamic (amplitude modulation) scanning mode is predominantly used. The cantilever is excited to oscillate at its fundamental resonance frequency and cantilever oscillation amplitude is used as a feedback signal. Oscillation amplitude is kept constant at a set point using a PID control loop during the scanning. In this thesis, the first large area HS-AFM development in our country was aimed. Despite the 24-year history of the HS-AFM idea, there is still no satisfactory HS-AFM developed in the world. There are many research groups and companies working on HS-AFM development. Ando and Hansma groups developed HS-AFMs that are dedicated especially for biophysics researches. The developed HS-AFMs have very limited scan range (~1μm2) that makes it useless for many application fields. We focused on developing a large area HS-AFM in this thesis that can be used in material science as well for surface characterization. We developed a HS-AFM that can take the images at 8 frames/sec (f/s) of a 15μm2 scan area. Development of HS-AFM requires different backgrounds from ordinary Atomic Force Microscopes. It takes a few minutes to have complete image with standard AFMs. Therefore, we need to increase the speed at least 1000 times. The speed is mainly limited by the mechanics. HS-AFM development requires development of high resonance frequency flexure based scanners. An XY scanner can be driven at %10 percent of first resonance frequency safely. In this thesis, we aimed to scan 1000 lines/sec. Therefore, the XY scanner resonance frequency needs to be around 10 kHz. The most important limitation is the bandwidth of the Z scanner. Real video rate AFM requires a few megahertz bandwidth. HS-AFM development also requires a high-speed AFM controller. AFM controller is responsible for keeping the tip-sample force constant, calculating the cantilever oscillation amplitude at each cycle, generating high speed scanning signal, capturing the multichannel image data at correct positions and transferring the high bandwidth data to the PC. AFM control software is also a big challenge for HS-AFM. 20MB/sec raw image data needs to be processed and shown real-time. HS-AFM requires high resonance frequency small cantilevers that are commercially available nowadays. HS-AFM also requires special optics for cantilever deflection measurements. Within the scope of this thesis, flexure based HS-AFM scanner with 15μmx15μmx3μm scanning area was developed and manufactured. Finite element analysis (FEA) tools were used for mechanical simulations. The XY scanner resonance frequency was measured to be approximately 7kHz. We developed a dual Z scanner to increase the bandwidth of Z scanner. First flexure guided Z scanner resonance frequency was about 155kHz. We used a high bandwidth stack piezo actuator for the first stage. Second Z scanner resonance frequency measured to be around 1.6MHz. We used single crystal PMN-PT piezo layer to have larger displacement and higher resonance frequency. FPGA-based high-speed control electronics was developed. Analog front-end is optimized for minimal phase shifts. An FPGA based PID controller was developed to keep the force constant. This is inevitable for low invasiveness. In addition, a novel multistage dynamic PID controller was developed. It allows scanning faster even the rough surfaces without any damage. A single cycle AC signal amplitude detector was developed to measure the cantilever oscillation amplitude. A novel direct digital synthesizer (DDS) based scan signal generator and data capture system was developed. It allows manipulating the scan signal waveform and data acquisition points. In addition, DDS based scan signal generator allows very sensitive scanning speed control as an advantage. The scanner is driven by sinusoidal waveform and data is acquired at equidistant positions. This allows the scanner to scan at high speed without stimulating the resonance frequency. A high bandwidth USB 2.0 data transfer was implemented that allows 40Mb/sec to transfer the image data in real-time. A high bandwidth HS-AFM piezo driver was developed to drive the HS-AFM scanner. We used stack piezo actuators that has higher blocking force and larger capacitance. Therefore, special piezo drivers were developed to drive the high capacitance piezo stacks. Piezo driver is capable of driving a piezo with 1800nF at 1000Hz from -30 to 130V swing. In addition, Z channel piezo driver was optimized for 150 kHz bandwidth. XY scanner has higher resonance frequency in XY axis but generally, it has smaller resonance frequency at Z-axis because it is not too stiff at Z direction. When the Z scanner moves, it generates an impulsive force that excites the Z resonance frequency of XY scanner that decreases the operation bandwidth. This is why we implemented a counterbalance method to solve this issue. A dummy counter equivalent Z piezo and dummy weight were added to opposite side that moves simultaneously with Z scanner. Therefore, counter impulsive forces cancel each other. Sample-scan HSAFMs has an important drawback; it requires very limited sample sizes. We worked on a tip-scan HS-AFM system to scan larger samples at high speed. Optical simulation tools are used to develop HS-AFM optics. Beam tracking lens method is simulated to measure the beam tracking errors. A laser beam tracking system was developed to compensate the laser beam tracking errors. Generally, cantilevers are resonated with another small piezo but it is an indirect excitation method. It has some drawbacks such as spurious peaks on resonance curves. As a part of this thesis, we developed a novel direct cantilever excitation method, which is based on the radiation pressure force. When a light beam is incident and reflected from the end of the cantilever, the momentum of the light is transferred to the cantilever. We used a single laser and a fiber to excite and measure the displacement on a low temperature AFM system. We have demonstrated the performance of the radiation pressure excitation in AFM/MFM by imaging atomic steps in graphite, magnetic domains in CoPt multilayers between 4-300 K. In summary, we developed a HS-AFM, which allows scanning 15μm2 area at 1000 lines/second speed. Experiments were carried out in air and liquid media with the system. We imaged calibration test sample, blu-ray disc, FeS oxidation and gold electroplating on an aluminum surface at high speeds. Calibration sample was imaged at 8fps without clear hysteresis or creep effect. We moved the sample while scanning without any problem. Moving the sample while scanning can give us opportunity to find the region of interest very quickly. Oxidation of FeS single crystal was imaged at 4fps. Formation of oxide layers were imaged in real time. An aluminum coated silicon surface was coated with gold by electrodeposition technique and coating process was imaged in-situ with HS-AFM in real time.