Microfabrication by Focused Ion Beam Micromachining

Proceedings of the 2nd International Conference on Mechatronics, ICOM’ 05 10-12 May, Kuala Lumpur, Malaysia

Trimming of Atomic Force Microscope Probe Tip by Ion Milling M. Y. Ali1 and B. H. Lim2 1

Department of Manufacturing and Materials Engineering, Faculty of Engineering International Islamic University Malaysia, Jalan Gombak, 53100 Kuala Lumpur, Malaysia 2 Precision Engineering and Nanotechnology Centre, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 [email protected]

ABSTRACT This paper discussed the trimming of atomic force microscope (AFM) probe tips to minimize measurement and imaging errors. Commonly used AFM tip was trimmed using focused ion beam (FIB) micromilling to achieve high aspect ratio and sharpness. The aspect ratio of trimmed tip was up to 10. Trimmed tip was used for measurement and imaging of high aspect ratio microstructures using tapping mode of AFM. Same microstructures were scanned with AFM under same imaging conditions using an original unmodified tip. The images and scanning results were compared. It showed that the trimmed tips were superior in imaging both shallow and deep high aspect ratio trenches. Keywords: AFM, Probe, FIB, Micromilling, Ion milling

1.

INTRODUCTION

Recently much effort has been given into miniaturization technologies and researchers around the world were aiming at higher resolutions and precision for the purpose of making things smaller [1, 2]. However, the measurement of these microstructures and systems was one of the main challenges nowadays. AFM was playing the major role in micro and nano measurement and characterization. This measurement tool also used miniaturized integrated tip cantilever. The AFM was a lens-free microscope where a sharp tip was mounted on the end of a micro-cantilever. When the tip scanned the surface of any substrate, small interaction force between the tip and the surface caused the cantilever to deflect. It finally revealed the topography of the specimen in three dimensions. Depending on the size of the tip, this imaging was able to measure down to the atomic level [3]. The commercially available standard single crystal silicon or silicon nitride pyramidal tips were commonly used in AFM measurement [4]. But these tips were not sharp enough for imaging surfaces with high precision finishing and or deep trenches with steep sidewalls. So, it was necessary to increase the aspect ratio of the AFM tip for more accurate reproduction of topographic details of scanned surfaces. In the following subsections trimming of AFM tip is discussed first. Then the evaluation of the trimmed tips is discussed.

2.

ION MILLING OF SINGLE CRYSTAL SILICON

Although numerous experiments and research were done on ion milling of silicon, those were used as guidelines [5-9]. Whenever using any machine especially for micromachining, it was essential to characterize the machine first with the same substrate material. This was to select the optimal machining parameters for different cases. In this research, a 50 keV dual focused e -/Ga+ beam (Micrion 9500EX) integrated with an energy dispersive x-ray (EDX) system (Oxford Link ISIS300) and scanning electron microscope (SEM) was used. After estimating the optimal machining condition, AFM probe tip of silicon was micromiiled for trimming and sharpening. 2.1 Preliminary Studies of Ion Milling For any particular acceleration voltage (e.g., 50 keV in this case), the basic parameters of ion milling were ion dose, beam current, dwell time, and pixel spacing. The beam current was also expressed by aperture size. For the above machine, the aperture size of 75, 100, 150, 250, and 350 µm

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were attached to beam current of 209 pA, 569 pA, 2.07 nA, 8.6 nA and 13.7 nA respectively. Ion dose was the number of ions delivered per unit area of the substrate. So, higher ion dose implied deeper milling depth. Dwell time was the period of time that a beam was stationary at each spot. Pixel spacing was the distance between two adjacent dwell points. With the factorial combination of these parameters, experiments were conducted by milling square boxes of 4 µm x 4 µm on silicon substrate which were basically silicon probe holders. The observations of these experiments were: 1. The milling depth was linearly proportional to the ion dose. An ion dose of 1 nCµm-2 was found to be enough to mill a depth of 1 µm on single crystal silicon. So the selection of the ion dose depends on the expected depth of milling. 2. The higher the beam current (bigger aperture size) the faster the milling rate was. But at higher beam current the spread of the Gaussian beam profile was wider and eventually the structural details of the components was poor. In this studies the medium sized aperture of 75, 100, and 150 µm were used. 3. Dwell time had a direct effect on the quality of mill. If the dwell time was increased, the amount of redeposited material was also increased. For shorter dwell time the small redeposits was sputtered again by the frequent repeated scan. Moreover, if the milled cavity was too deep then the sputtered material deposited without being ejected. A dwell time of 10 µs was found appropriate for above mentioned aperture sizes. 4. Pixel spacing must be small enough so that the neighbouring beam profiles overlapped with each other with a significant intensity level. A pixel spacing of 10 nm was found to provide uniform milling and better surface topography. The optimal parameters found by these experiments are listed in Table 1. These experimental results were guidelines when using this machine for sputtering single crystal silicon. However, for another substrate material, the same experiment will have to be repeated to find optimal sputtering parameters. 2.2 Trimming of AFM Probe Tip AFM tips of 15 µm height and 1-2 aspect ratio were used for trimming. The material of the tip was single crystal silicon. The goal was to achieve high aspect ratio by sharpening. FIB micromilling was performed using annular milling pattern which left a smaller area at the centre to define the tip. The shapes of the annular areas were circular, square, and triangular to produce various tips. The mill patterns were generated and loaded to the FIB system in a standard format [10]. The tip was mounted with the tip axis parallel to the FIB column i.e., tip pointing towards the FIB column. By aligning the centre of the pattern at the peak of the tip, sputtering was performed with appropriate parameters. In this experiment, the sputtering parameters were 75 µm aperture, 1-2 nCµm-2 ion dose, 5 µs dwell time, and 10 nm pixel spacing. These parameters were estimated by preliminary studies as discussed in section 2.1. Fig. 1 shows an AFM tip before and after trimming. The trimmed tip has a 1.5 µm x 1.5 µm sized square base and a height of about 13 µm. During trimming, the apex of the tip was untouched to preserve the original height. As this process required no special fixtures and only one single milling was enough, the accuracy of trimming process was high.

Table 1. Optimal ion milling parameters for single crystal silicon using 50 keV Ga+ FIB. Parameters

Value 1 nCµm-2 75, 100, 150 5-10 10

Ion dose (per µm depth) Aperture size (µm) Dwell time (µs) Pixel spacing (nm)

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(a)

(b)

Fig. 1. SEM micrograph of AFM tips (a) before trimming (b) after trimming by FIB micromilling. Ion milling parameters: 50 keV Ga+ FIB, 75 µm aperture, 2 nCµm-2 ion dose, 5 µs dwell time, and 10 nm pixel spacing.

3. EVALUATION OF THE TRIMMED AFM TIP The performance of the trimmed AFM tip was evaluated by imaging a test sample of microstructure. The test sample was 10x10 array of 1µm x 1µm sized square holes as shown in Fig. 2. The gap between the holes was 0.5 µm. This array was fabricated by FIB micromilling with 150 µm aperture, 1 nCµm-2 ion dose, 3 µs dwell time, and 10 nm pixel spacing. This micrograph was taken by the SEM integrated with the FIB. According to the delivered ion dose and in-shitu SEM imaging, the estimated depth of the holes was 700 nm. This array served as a basis for comparison of the topographic images obtained by AFM with two different tips. At first 5µm x 5µm sized square areas on the test sample were scanned by AFM tapping mode with a commercially available tip (untrimmed, aspect ratio 2). Other scanning parameters were 260 kHz frequency, 0.5 kHz scan rate, and 1.2 v set voltage. The image obtained by this scanning is shown in Fig. 3a. Then the test sample was scanned again by tapping mode of AFM with trimmed tip (aspect ratio 8). All scanning parameters were the same as used for untrimmed tip. The image obtained by using the trimmed tip is shown in Fig. 3b. In can be observed from Fig. 3 that the untrimmed tip was not able to produce the exact geometry of the structures. It was unable to scan at the deeper point of the structure and resulted different height and shape from the actual dimensions. However, the trimmed high aspect ratio sharp tip was able to scan at the deeper point and produced a closer replica of the structures both in shape and height. It can be noted that the depth of the holes is about 300 µm and 700 µm as measured by the untrimmed and trimmed tips respectively (Fig. 3). But the actual depth was about 700 nm as found by in-shitu SEM observation and empirical studies of ion milling. While the FIB trimmed tip was able to produce more accurate topographic images; there was some inevitable trade-off. Height variations of the test sample were to be relatively smaller thus it was only suitable for samples with very good surface finish. The trimmed high aspect ratio tips were more prone to breakage, therefore the scanning speed was significantly low. It made the scanning process more time consuming. When using the trimmed tip repeatedly, wear of the tips was also a significant issue as the tip point was very sharp. But most of the tip failures were experienced by breakage rather than wear.

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Fig. 2. SEM micrograph of the test sample (10 x 10 arrays of 1µm x 1µm sized square holes) for the evaluation of trimmed AFM tip. Micromilling parameters: 50 keV Ga+ FIB, 150 µm aperture, 1 nCµm-2 ion dose, 3 µs dwell time and 10 nm pixel spacing.

(a)

(b)

Fig. 3. AFM micrograph of the test sample microstructures shown in Fig. 2. (a) image taken by using untrimmed AFM tip of aspect ratio 2, (b) image taken by using trimmed AFM tip of aspect ratio 8. Scanning parameters: 5µm x 5µm scan size, 260 kHz frequency, 0.5 kHz scan rate, and 1.2 v set voltage.

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4.

CONCLUSIONS

The fabrication process described in this paper was ion milling to fabricate sharp high aspect ratio AFM probe tips to produce image of microstructures at nanometric scale. This research showed: 1. 2. 3.

4. 5. 6. 7.

Trimming by FIB micromilling was a unique process to fabricate high aspect ratio AFM tip. Using the developed process, AFM tip of 5-10 aspect ratio was fabricated. FIB technology provided the versatility in micromachining and providing a means to trim AFM tips to image at nanometric scale. Process parameters were to be selected carefully. Higher ion dose caused over-milling and damage the tip at the base. Use of larger aperture may mill the apex of the tip and reduced the height. The preliminary studies of ion milling was essential to fabricate high aspect ratio probe tips and at the same time to avoid over-milling. This high aspect ratio tip was found to scan at the deeper point and produced images with higher accuracy and clarity. Trimmed tip was fragile and needed to use at low scanning speed. Although the process was time consuming, high aspect ratio AFM tips were important for some special applications.

ACKNOWLEDGMENT The kind help from the Precision Engineering and Nanotechnology Centre at Nanyang Technological University, Singapore has been much appreciated.

REFERENCES [1] S. W. Park, H. T. Soh, C. F. Quate and S. I. Park, “Nanometer scale lithography at high scanning speeds with atomic force microscope using spin on glass,” Applied Physics Letters, vol. 67, no. 16, pp. 2415-2417, 1995. [2] P. A. Fontaine, E. Dubois and D. Stivenard, “Characterisation of scanning tunneling microscopy and atomic force microscopy-based techniques for nanolithography on hydrogen-passivated silicon,” Journal of Applied Physics, vol. 84, no. 4, pp. 1776-1781, 1998. [3] D. M. Eigler and E. K. Schweizer, “Positioning single atoms with a scanning tunneling microscope,” Nature, vol. 344, pp. 524-526, 1990. [4] A. Folch, M. S. Wrighton and M. A. Schmidt, “Microfabrication of oxidation-sharpened silicon tips on silicon nitride cantilever for atomic force microscopy,” Journal of Microelectromechanical Systems, vol. 6, no. 4, pp. 303-306, 1997. [5] P. Sigmund, Sputtering by Particle Bombardment I, R. Behrisch (Ed.), Springer, 1981. [6] M. J. Vasile, J. Xie and R. Nasar, “Depth control of focused ion beam milling from numerical model of the sputter process,” Journal of Vacuum Science Technology. B, vol. 17, no. 6, pp. 3085-3090, 1999. [7] M. Y. Ali and N. P. Hung, “Surface roughness of sputtered silicon, part i: surface modeling,” Materials and Manufacturing Processes, vol. 16, no. 3, pp. 297-313, 2001. [8] M. Y. Ali and N. P. Hung, “Surface roughness of sputtered silicon, part ii: model verification,” Materials and Manufacturing Processes, vol. 16, no. 3, pp. 315-329, 2001. [9] N. P. Hung, M. Y. Ali, Y. Q. Fu, N. S. Ong and M. L. Tay, “Surface integrity and removal rate of sputtered silicon,”, Machining Science and Technology, vol. 5, no. 2, pp. 239-254, 2001. [10] Y. Q. Fu, B. K. A. Ngoi, A. S. Ong and B. H. Lim, “Data format transferring for FIB microfabrication,” International Journal of Advanced Manufacturing Technology, vol. 16, pp. 600-602, 2000.

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