Student Projects
Contact Information
University: Massachusetts Institute of Technology
Team Members:
Iman Soltani Bozchalooi, PhD candidate 2014
Andrew Careaga Houck, MS candidate 2015
Faculty Advisor: Kamal Youcef-Toumi
Email address: isoltani@mit.edu , achouck@mit.edu
Submission language: English
Project Information
Title: Building the world's Largest range high-speed atomic force microscope
(LRVR-AFM)
Description: The Large Range Video Rate Atomic Force Microscope (LRVR-AFM) is an instrument that enables observation of dynamic processes at the nanoscale in real time. With a maximum lateral scan range of 120 um and out-of-plane range of 7 um, the LRVR-AFM is currently the largest range video-rate atomic force microscope in the world.
Products:
Hardware -
PXIe-1085 chassis with PXIe-8135 controller
PIXe-7966R FPGA with NI 5781 baseband transceiver
PXI-7851 FPGA
Software -
NI LabVIEW 2013
MATLAB Script
FPGA Module
Digital Filter Design Toolkit
System Identification Toolkit
Advanced Signal Processing Toolkit
Vision Development Module
Multisim and Ultiboard 2013
The Challenge:
Atomic force microscopy (AFM) is the only microscopy technique capable of providing sub-nanometer resolution in air, in vacuum or in buffer solution. This powerful microscope has helped researchers observe atoms and gain unprecedented insight into the nano-world. Although AFM is a very powerful instrument, mechanical constraints and problems of probe-sample interaction have limited its use to static or quasi-static observations (~1 line per second). Recently, efforts in high-speed AFM have been made. These instruments, although faster, have very small imaging ranges, limiting their utility.
Our challenge in this project was to create a new instrument capable of both high speed and large range. To achieve this goal we have designed and built a large range video rate atomic force microscope (LRVR-AFM). The device includes five precisely controlled nano-positioners, a laser beam deflection sensing system, high-speed control, and synchronized data logging and image plotting platforms. The LRVR-AFM utilizes a variety of software and hardware platforms from National Instruments, and is currently the largest range high-speed atomic force microscope in the world.
Using this unique and powerful instrument, we are now able to study dynamic processes with nanometer resolution over scanning ranges of several hundred microns. This instrument will enable many new possibilities for novel scientific research, and will allow us to explore processes at the nano-scale like never before.
The Solution:
AFMs capture images by scanning a sample while in contact with a micro cantilever. The deflection of the cantilever is measured as it moves across the surface, and this information is used to form the image. LabVIEW, together with NI hardware, is used to control the LRVR-AFM and capture images at very high speed. Figure 1 demonstrates our AFM design and the individual components.
Figure 1: LRVR-AFM design overview
Imaging begins by bringing the sample up into contact with the cantilever. This is done by a stepper motor, which drives a vertical linear stage that holds the sample scanner. The stepper is controlled by a PXI-7851 FPGA, and is shown in Figure 2.
Figure 2: Stepper motor approach mechanism
Next, the sample is raster scanned by the sample positioner. The key to the simultaneous high speed and large range offered by the LRVR-AFM is the multi-actuated sample scanner, which uses a combination of large-range/slow and small-range/fast actuators to achieve desired performance. Unwanted scanner dynamics are identified before imaging and compensated through properly designed IIR filters using NI software and hardware. Figure 3 shows our multi-actuated sample scanner.
Figure 3: Multi-actuated sample scanner
Deflection of the cantilever is measured with a laser optics system. A laser beam is reflected off the back of the probe, and the movement of the reflected spot is measured on a photodiode circuit. Our optical system features a very small focused spot size (3 um), allowing us to use the smallest cantilevers currently available. Furthermore, for minimal noise performance the laser source is RF modulated at 300 MHz. The detection circuit is shown in Figure 4, and the optical path of the laser is shown in Figure 5. The detection circuit has a bandwidth of 3 MHz, and has very low noise. It was designed using Multisim and Ultiboard software from NI.
Figure 4: Photodiode circuit
Figure 5: Optical path diagram
The instrument is controlled with a series of LabVIEW VIs. LabVIEW, along with various toolkits and modules, is used for all aspects of instrument operation, including system identification of sample scanner dynamics, compensator design, stepper control, control of probe-sample interaction, piezo command shaping, etc. Figure 6 shows the front panel of the central VI used during imaging.
Figure 6: Main LRVR-AFM operation VI front panel
The abstract and high-level graphic programming environment of LabVIEW allowed us to focus our time and efforts on the challenging developmental aspects of the work. Utilizing textual programming environments such as VHDL to implement our complex controllers on FGPAs would have made implementing and testing our video rate atomic force microscopy setup extremely difficult and unnecessarily convoluted. Furthermore, the possibility for seamless incorporation of technologies developed in other programming environments (such as MATLAB) was also a great benefit, as it enabled us to fuse our expertise in various programming environments on a unified LabVIEW platform. To meet our ambitious goal of video-rate atomic force microscopy, we needed a data logging and plotting platform with a throughput of ~20 MHz, an out-of-plane feedback control bandwidth of ~100 kHz and a lateral control bandwidth of ~10 kHz, all with minimal latency. All these requirements could be met through the PXIe hardware platform, featuring a high-speed controller, along with multiple FPGAs and high-speed data acquisition periphery. The combination of flexible, easy-to-use software and powerful hardware from National Instruments helped us achieve our ambitious goals. The examples and training resources available to help us best utilize the NI software and hardware platforms were also of paramount importance. Along with the seamless compatibility of NI hardware and software, the NI support team and applications engineers helped our team members find the appropriate resources for various tasks to speed up the project progress.
Our LRVR-AFM has been in development in approximately 6 months, and is currently in the beta prototype phase. There are several improvements in the LRVR-AFM that will be developed in the coming months, including incorporation of cantilever holders for different imaging modes, and scanner sensing to allow closed-loop position control.
Nominate Your Professor
We would like to nominate our advisor, Professor Kamal Youcef-Toumi, as an outstanding educator. This project would not have been possible without his guidance. Prof. Youcef-Toumi specializes in system dynamics and control, and teaches several undergraduate and graduate courses in these fields here at MIT. National Instruments software and hardware are frequently used in the lab components of these courses, giving students hands-on experience with LabVIEW and other essential NI products. In addition to his impact in the classroom, Prof. Youcef-Toumi strives to benefit the wider research community. One example of this is his publication "A Tutorial on Fixed Point Implementation of IIR Filters with Emphasis on FPGA Targets." This tutorial paper helps researchers with fixed-point implementation of IIR filters - a challenging task that slows down many researchers unnecessarily as they begin their work in the field. The paper demonstrates the implementation of these techniques on NI hardware. Kamal Youcef-Toumi is an excellent example of a professor who is energetically educating and motivating the next generation of engineers.
