Developing a large-range video-rate atomic force microscope (LRVR-AFM) to observe nanoscale processes in real time.
Combining the high throughput capability of the NI PXI Express platform and the flexibility and speed of two FPGA platforms with high-speed data acquisition modules to control multiple piezo actuators, minimize probe-sample interaction forces, and visualize measurements in the form of high-resolution microscopy images.
Iman Soltani Bozchalooi - MIT
A.C. Houck -
K. Youcef-Toumi - MIT
Atomic force microscopy (AFM) is the only microscopy technique capable of providing subnanometer resolution in air, vacuum, or buffer solutions. This powerful microscope helps researchers observe atoms and gain unprecedented insight into the nanoworld. Although an AFM is a powerful instrument, mechanical constraints and problems with probe-sample interactions have limited its use to static or quasistatic observations. Enhancing the speed and accuracy of AFM would unlock the ability to observe nanoscale processes in real time and allow AFM to be used in a myriad of applications, ranging from semiconductor manufacturing to molecular biology. Several aspects of AFM operation such as controls, signal processing, data acquisition and handling, instrumentation, and mechanical design need to meet stringent performance criteria to accommodate the requirements of high-speed imaging. Large scanning ranges should accompany high speed and accuracy to fully realize the scientific potential of this new instrument.
To achieve this goal, we designed and built a LRVR-AFM. The device includes five precisely controlled nanopositioners, a laser beam deflection sensing system, high-speed control, and synchronized data logging and image plotting platforms. Using this unique powerful instrument, we can study dynamic processes with nanometer resolution over scanning ranges of several hundred microns. This instrument leads to many new possibilities for novel scientific research and empowers us to explore processes at the nanoscale like never before.
Taking advantage of the abstract and high-level graphical programming environment of NI LabVIEW software helped us focus our time and efforts on the challenging developmental aspects of the work. Using textual programming environments such as VHDL to implement our complex controllers on FGPAs would have made implementing and testing our LRVR-AFM setup extremely difficult and unnecessarily convoluted. Furthermore, the possibility for seamless incorporation of technologies developed in other programming environments (such as The MathWorks, Inc. MATLAB® software and C) was also a great benefit because we could fuse our expertise in various programming environments on a unified LabVIEW platform. The NI PXI Express platform, featuring a high-speed controller along with multiple FPGAs and high-speed data acquisition periphery also helped us meet our requirements.
We used an NI PXIe-1085 chassis with an NI PXIe-7966R NI FlexRIO FPGA module for our LRVR-AFM. We also used an NI 5781 baseband transceiver to regulate probe-sample interaction forces through a PID controller, shape the command signals that drive two independent out-of-plane piezo actuators through high-order infinite impulse response (IIR) filters, and acquire and transfer line and frame synchronization signals along with the laser deflection and PID command signals to the PXI Express controller for AFM image formation. We used the NI 7851R FPGA module to control a stepper motor for probe-sample engagement, drive the lateral piezo actuators for sample scanning, and shape the lateral scan signals to avoid excitation of scanner dynamics. Both FPGAs are used to excite the out-of-plane and lateral piezo actuators for system identification and IIR filter parameters update.
We operated the AFM with LabVIEW. We used the NI LabVIEW FPGA Module and NI LabVIEW Digital Filter Design Toolkit to implement the IIR filters on the FPGAs. We incorporated the MATLAB .m file scripts prepared for the IIR filter parameter updates into the LabVIEW platform with the NI LabVIEW MathScript RT Module. We implemented various functions using several LabVIEW toolkits such as the NI LabVIEW Advanced Signal Processing Toolkit, the NI LabVIEW Vision Development Module, and the NI LabVIEW System Identification Toolkit. We also used NI Multisim software and NI Ultiboard software to design and layout the deflection sensing and signal filtering circuitry used in the LRVR-AFM. We found the simulation and layout annotation features of this software to be especially helpful as we worked through several prototype stages and optimized the circuitry to meet our speed and accuracy requirements.
We could effectively combine the performance of slow/large-range and fast/short-range piezo actuators to achieve the high-speed and large-range capabilities of the LRVR-AFM scanner. Our system has a 120 μm lateral range and 7 μm out-of-plane range, with nearly 100 kHz out-of-plane and 10 kHz lateral scan bandwidth, making it the largest range high-speed atomic force microscope in the world.
MATLAB® is a registered trademark of The MathWorks, Inc.
Iman Soltani Bozchalooi