A Novel Atomic Force Microscope Control System Based on PC104 and DSP Embedded System*

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Physics Procedia 33 ( 2012 ) 1497 – 1501

doi: 10.1016/j.phpro.2012.05.244

2012 International Conference on Medical Physics and Biomedical Engineering

A Novel Atomic Force Microscope Control System Based on PC104 and DSP Embedded System*

Bohua Yin, Daixie Chen, Yunsheng Lin, Mingzhang Chu, Li Han

Department of Micro-nano Fabrication Technology Institute of Electrical Engineering, CAS

Beijing 100190, China [email protected]

* This work is supported by funding of CAS :YZ200831 to Y. Lin Abstract

In order to achieve large scanning range, this article presents a new type high-speed AFM system. According to the need of rapid data transmission and operation, the AFM control system structure was composed of PC104 and DSP hardware model. Because of using a large displacement flexure stage as the sample stage, this AFM system is capable of providing a scan range of 100 100um image with 50Hz line-scan speed. The capacitor displacement sensors are used as x-y position during AFM scan image. We realized a new scanning method based on positioning control. The scanning images are more precision and less distortion than general open-loop x-y scanning image.

keywords: AFM, PC104, DSP, Sinusoidal wave, High speed scanning.

1. Introduction

The atomic force microscope [1,2] (AFM) has become a popular surface watching instrument at nanometer scale in physics, materials, and biological research fields. However, the AFM has its inherent limitation of slow scan speed which holds up watching the dynamic process of some special biological experiments like DNA unbinding. A typical image is collected over a period of 30s or even several minutes, which is much slower than the millisecond time resolution required for the visualization of macromolecular process [3].

So several research team begin to develop high speed AFM system independently. Georg Schitter [4] manufactured 10um wide cantilevers which combine high resonance frequencies with low spring

Open access under CC BY-NC-ND license.

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constants (160–360 kHz with spring constants of 1–5 pN/nm). Andrew Humphris [5] designed a micro resonant scanner which works non its resonance frequency mode to archive high speed scan image.

The above high speed AFM system has same limitation of small scan range and used for special situation, especially in biological fields. Actually, the high speed AFM has powerful potential application in the semiconductor, microfabrication and new information storage research. In this paper, we mainly discuss and develop a novel high speed AFM control system with large scan range based on PC104 and digital signal processor (DSP). The schematic diagram of the high speed AFM control system is shown in Fig.1.

2. Controller Descripion

2.1 Data Transfer Mode

To meet the need of heavy data transfer between PC104 and master computer during the high speed scanning motion, a 100M network transfer mode based on TCP/IP is adopted. Furthermore, the network transfer mode makes it possible to operate the AFM by remote control.

2.2 Nanometer Position Function

The common AFM system has no positioning measuring system in x-y direction, so the scanning image distortion could not be avoided, although some nonlinearity correction methods can eliminate the hysteresis effects at a certain extent. In our sample stage of flexure structure[6], the precision capacitive sensors (PI Company) are mounted at x, y directions to read out x-y direction’s displacement timely. The capacitive sensors have the resolution of well under one nanometer, so every point coordination of the scan image can be identified precisely in the range of 100um 100um. The nanometer positioning function is capable of correcting the nonlinearity error which is inherent in common peizo actuators.

2.3 Achieve the Sinusoidal Driving Signal

Because the triangular scanning signal contains a lot of higher harmonic components[7], the scanner’s resonance is tend to be excited when scanning at high line-scan rates with these triangular wave

Master Compute

r

PC104 Slave

DSP

ISA BUS

PID Controller

16bit

D/A Lowpass

Filter DC Motors

Module

Fig.1 High Speed AFM Control System Schematic diagram

AFM SCANNER

A/D Convertors

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signals. The sine wave scan pattern is very fit for high speed AFM imaging system because the sine wave has no higher harmonic components. Driven by sine mode signal, when the scan direction is reversed, the change of the driving signal is continue and smooth, not like triangular signal’s abrupt change. In order to get the smooth sinusoidal scan signal with minimal step voltage abrupt, a 16-bit digital-to-analog (D/A) module (AD669, ) is used to generate the sine wave as the high speed scanning signal of x direction. In this design, the minimal output voltage step of the AD669 is 0.3mV. The embedded program of DSP controller decided the amplitude (scan range) and frequency (line scan rate) of the sine wave. The electrical schematic of the sinusoidal generator is shown in fig.2.

2.4 Scanning Method of Position-based Sampling Process

The sinusoidal driving signal reduces the mechanical resonance, but has nonlinearity character by itself. So, in our AFM system, we use a new line scan method different from general equal time intervals sampling. The line scan principle based on position is shown in fig.3. During the AFM scan process, the sample stage is driven by the sinusoidal wave generated by DSP. Each scan line is divided by equal distance, such as X1, X2, X3, X4, and so on. According to the position’s coordination of the x direction, the AFM topography information of Xn point is sampled. The distance between two adjacent point (Xn- Xn-1 ) is equal, but the time of the two adjacent point is not equal. Actually, according to the need of scan range, the sampling point’s number can be divided up to 512 or 1024 which is set by PC104 embedded program. Using this equal interval distance scan method, we can realize AFM image scanning with the minimal distortion

TMS320-6713 HV-AMPLIFIER PIEZO-STAGE

Fig.2 Schedule diagram of sinusoidal driving signal generator

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3. Experiments Results And Conclusion

3.1 Grating Scanning Experiments

In the AFM system, the PC104 control board is in charge of the proportional integral parameters setting, approaching sample surface by driving the DC motors, and get the sample surface topography image by controlling the A/D convertors. The Linux software system is adopted in the PC104 board, which is high effective in running the AFM control program with a real-time operation requirement. With this AFM system, firstly, the result shown in Fig4(a), we make a raster-scanning motion by common triangle wave driving signal at a rate of 28 lines per second. Obviously, the mechanical resonance happened, which results in the artifact image in the left of Fig.4 (a). In the Fig.4 (b), the grating image acquired by position-based sinusoidal scanning mode is shown, at a rate of 50 lines per second.

3.2 . Some Conclusion and discussion

Fig.4 Grating scanning images

(a) Triangle wave scanning mode. (b) Sine wave scanning mode

(a) (b)

X4

X3

X2

X1

t

1

t

1

t

1

t

1

Sampling Point Position (um)

X

Fig.3. Sampling mode based on positioning control

Sampling Time During Scanning (s)

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In this paper, we demonstrate the new AFM control system based on PC104 and DSP architecture.

From the above experiments, it can be concluded that the sinusoidal wave is fit for high speed AFM application. Position-based sampling mode is benefit to reduce image distortion brought by piezoelectric actuators hysteresis. To further improve the AFM image resolution in the vertical and horizontal direction, we will focus on establishing PI control model and improving A/D speed in the future research work.

4. Acknowledgment

This research work was supported by equipment development foundation of Chinese Academy of Science. The project is: High Speed AFM on Nanometer Biomedical Application. The author thanks Dr.

Wang Lina for her help with the sample preparation and image scanning.

References

[1] G.Binnig, C.F.Quate, C.Gerber, Phys.Rev. Lett.56 (1986) 930.

[2] G.Schitter, R.W.Stark, A.Stemmer. Fast contact-mode atomic force microscopy on biological specimen by model-based control. Ultramicroscopy 100 (2004) 253–257

[3] A. D. L. Humphris, M. J. Miles, and J. K. Hobbsb. A mechanical microscope: High-speed atomic force microscopy;

APPLIED PHYSICS LETTERS 86, 034106 (2005)

[4] Georg E. Fantner, Georg Schitter, Johannes H. Kindt; Components for high speed atomic force microscopy;

Ultramicroscopy 106 (2006) 881–887

[5] Georg Schitter, Philipp J. Thurner, Paul K. Hansma. Design and input-shaping control of a novel scanner for high-speed atomic force microscopy. Mechatronics 18 (2008) 282–288.

[6] Johannes H. Kindt, Georg E. Fantner, Jackie A. Cutroni. Rigid design of fast scanning probe microscopes using finite element analysis. Ultramicroscopy 100 (2004) 259–265

[7] Andrew Humphris, Max McConnell and David Catto; A High-Speed Atomic Force Microscope Capable of Video-Rate Imaging. Microscopy and Analysis 20(2):S29-S31 (UK), 2006