In-situ monitoring of optical near-field material processing by electron microscopes

Transcription

1 Appl Phys A DOI /s INVITED PAPER In-situ monitoring of optical near-field material processing by electron microscopes David J. Hwang Bin Xiang Sang-Gil Ryu Oscar Dubon Andrew M. Minor Costas P. Grigoropoulos Received: 16 December 2010 / Accepted: 15 September 2011 Springer-Verlag 2011 Abstract Lasers are efficient tools in a variety of micro/nanoscale material processing applications. Even though optical imaging techniques offer convenient in-situ monitoring, their spatial resolution is frequently not sufficient for inspecting the detailed phenomena occurring in micro/nano structures, hence requiring additional characterization tools. Besides the inconvenience, critical processing parameters cannot be readily determined ex situ. In this study, an example of an in-situ monitoring technique for micro/nanoscale laser processing is demonstrated by combining the optical near-field apparatus with a scanning electron microscopy (SEM). In-situ process monitoring under true optical near-field configuration is realized through or- D.J. Hwang S.-G. Ryu C.P. Grigoropoulos ( ) Department of Mechanical Engineering, University of California, Berkeley, USA cgrigoro@me.berkeley.edu B. Xiang O. Dubon A.M. Minor Department of Materials Science and Engineering, University of California, Berkeley, USA B. Xiang A.M. Minor National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA, USA O. Dubon Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA C.P. Grigoropoulos Advanced Energy Technologies Department, EETD, Lawrence Berkeley National Laboratory, Berkeley, CA, USA Present address: D.J. Hwang Department of Mechanical Engineering, State University of New York, Stony Brook, NY, USA thogonal probe manipulation and combined probe-sample translation and tilting apparatus. Catalyst behavior under a chemical vapor deposition (CVD) gas environment coupled with near-field illumination is monitored in an environmental SEM. 1 Introduction Lasers have proven to be efficient tools in a variety of micro/nanoscale material processing applications. Even though optical imaging techniques offer convenient insitu monitoring, their spatial resolution is frequently not sufficient for inspecting the detailed phenomena occurring in micro/nano structures, hence requiring subsequent ex-situ characterization tools such as atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Besides the inconvenience, critical processing parameters cannot be readily determined, leading to difficulties in optimizing the processing parameters as well as introducing multiple sample transfer tasks between the processing system and the ex-situ characterization setup. Maintaining consistent processing conditions while the sample is being re-located multiple times is a significant practical difficulty. Combining the optical near-field apparatus with a SEM for in-situ monitoring of micro/nanoscale processing has been demonstrated through incorporating an optical fiber with laser confinement structures implemented at the fiber end. Truly in-situ SEM monitoring of the samples under laser processing has been achieved with full compatibility of the dual beam system, including the use of the focused ion beam (FIB), the electron beam (SEM), and the gas nozzle assembly originally designed for electron-beam

2 D.J. Hwang et al. and focused-ion-beam based deposition [1, 2]. In-situ monitoring has made it possible to monitor a variety of lasermaterials interactions such as laser-assisted cleaning, local melting of nanostructures, and local chemical vapor deposition (CVD) in conjunction with in-situ probe status monitoring and on-site repair [1 3]. However, this previous work was based on an optical far-field technique by adapting a lensed fiber design. As a consequence, the spatial confinement of the laser beam was still governed by the diffraction limit ( half of the coupled wavelength), and monitoring under the SEM environment had limitations in analyzing details of nanoscale phenomena. In this study, we present preliminary results to further expand of the in-situ monitoring capabilities by realizing a true optical near-field configuration in a SEM apparatus. 2 Coupling of true optical near-field in a SEM for in-situ laser nanoprocessing 2.1 Experimental setup for true optical near-field illumination in an environmental SEM The experimental setup to achieve true optical near-field in a SEM is shown schematically in Fig. 1. The relative position of the probe end with respect to the sample surface under processing is precisely controlled using vacuum compatible XYZ piezo stages holding the probe and translating in an orthogonal configuration. The precision of the piezo stages is critical since maintaining the sample-probe gap within 10 s 100 s nm (depending on the probe design) is necessary to ensure optical near-field illumination. In addition, the sample and probe manipulation systems had to be located on a single SEM holder so that the sample-probe gap distance would remain unchanged while frequently shifting and tiling the sample in the course of the experiments. The necessity of sample tilting can be understood from the probe alignment and actual experimental procedures as shown in Fig. 2. A normal viewing angle of the SEM (i.e. parallel to the sample surface) is needed for the precise adjustment of the probe-sample gap distance (Fig. 2(a)). On the other hand, the use of a small tilt angle is convenient for in-situ monitoring of sample surface events with the SEM (Fig. 2(b)), and the probe-sample gap distance as set should not be disturbed during the frequent transit between the two different viewing angles to keep track of the gap distance control. The optical near-field probe was fabricated using a dielectric probe design by pulling a single mode fiber of 5 µm core diameter and probe end radius was around 150 nm [4]. A compression fitting type vacuum feedthrough was utilized to pass the fiber through the SEM Fig. 1 Experimental setup for true optical near-field illumination coupling into an environmental SEM. A pulled optical fiber based near field probe is manipulated by XYZ piezostages in an orthogonal fashion, and the manipulating systems of the sample and the probe are located on a single platform in order to maintain the probe-sample gap distance. The chamber was filled with 0.9% SiH 4 gas for laser-induced CVD experiments. (a) Schematic diagram and (b) photograph of actual setup chamber and delivered the laser source under vacuum. A continuous wave laser beam of 532 nm wavelength was coupled for in-situ laser processing. To quantify the laser power emitted from the fiber probe, two power meters were used to measure the ratio of the coupled laser power with respect to the output emerging from the probe apex, before locating the probe in the optical near-field configuration. The actual output power was estimated by measuring the coupled power multiplied by the calibrated output/input ratio. The in-situ experiments were conducted inside an environmental SEM (Hitachi) with 0.9% SiH 4 (diluted in N 2 gas). The gas was introduced at a pressure level of 0.75 torr and flow rate of 100 sccm, and injected toward the sample surface using 1 mm inner diameter tubing at a fixed distance of 2 mm. The samples used for these experiments were Au nanoparticles of 80 nm diameter (Ted Pella Inc.) dispersed onto either a (100) silicon wafer or a sputtered silicon film of 4 µm thickness on fused silica wafer.

3 In-situ monitoring of optical near-field material processing by electron microscopes Fig. 2 Realization of a true optical near-field illumination in a SEM. (a) Normal SEM view (Parallel to Sample) for probe-sample gap adjustment, and (b) tilted SEM view for in-situ sample monitoring. Probe manipulation is under orthogonal configuration, and sample & probe manipulators are located on a single holder to maintain the probe-sample gap while frequent switching between (a) and(b) Fig. 4 Combination of multiple Au nanoparticles by enhanced trapping force at elevated laser power ( 150 mw) upon probe translation. Other conditions are similar to those in Fig. 3 Fig. 5 In-situ visualization of unstable Au-Si molten alloy on silicon substrate. It is shown that the molten droplet expansion over the surface has large extent as it rapidly shrinks after removing laser illumination. Laser power of 200 mw was applied for 3 seconds before laser is off Fig. 3 Experimental verification of optical near-field illumination in an environment SEM. Heating of a single Au nanoparticle ( 80 nm diameter) leads to melting and Au-Si alloy formation in a SiH 4 gas environment (0.9% diluted in N 2, filled at 0.75 torr), followed by trapping based motion upon probe translation. A continuous wave laser of 532 nm wavelength was used resulting in 90 mw of emanating laser power from the probe end 2.2 Experimental results and discussion The first task based on the aforementioned experimental apparatus was to confirm the illumination on the sample surface was indeed optical near-field illumination. Figure 3 shows that at a calibrated laser output power of 90 mw a single Au nanopaticle became mobile, presumably be- cause it was converted to a Au-Si molten alloy under the SiH 4 gas environment. By translating the probe laterally the single particle moved, possibly assisted by a thermal gradient related optical trapping mechanism with a molten alloy state on the silicon surface. At a higher elevated laser power ( 150 mw output), the translation of the laser spot trapped a number of the surrounding Au nanoparticles, leading to combination and a larger Au nanoparticle formation (Fig. 4). Figures 5, 6, 7 correspond to the investigation of laserinduced chemical vapor deposition (CVD) via a single catalytic Au particle by maintaining the laser illumination for a longer time under the CVD gas environment as compared to the previous examples. In Fig. 5, it is shown that after continuous laser illumination for 3 seconds at fixed laser position with 200 mw incident laser power, the shape of a heated Au nanoparticle started to violently fluctuate and expand to a large extent (approximately ten times the original Au particle size) presumably at a molten eutectic state (molten Au-Si alloy). This is confirmed by the fact that the

4 D.J. Hwang et al. Fig. 6 In-situ visualization of silicon crystal growth at extended laser illumination. Laser power of 200 mw was applied for 10 seconds before laser is off In order to further examine the effect of the laser illumination on the directional Si nanowire growth, the laser illumination spot was intentionally broadened to 20 µm diameter by setting a larger probe-sample gap distance with increased laser power ( 350 mw). A thin Si film ( 4 µm thick) sputtered on fused silica was used to easily reach the growth temperature even at the reduced local laser intensity. AsshowninFig.7, while the direct laser illumination spot (the central region of obtained feature) was entirely covered with Si deposits, directional Si nanowires were grown in the surrounding area (i.e. indirectly heated region. We do not believe that this growth occurred through direct laser illumination but through conducted heat from the direct laser illumination spot. This result implies that enhanced laser absorption in Au catalysts by plasmonic resonance [6] is not necessarily favorable to achieve stable catalytic nanowire conditions under the VLS mechanism due to unwanted instabilities in molten state and/or laser-induced temperature gradients. 3 Summary and outlook Fig. 7 Gradient distribution of silicon nanowire growth. The laser illumination spot is filled with direct silicon CVD and growth of small silicon nanowires are found in the surrounding area. A laser power of 350 mw was applied for 80 seconds and 80 nm Au nanoparticles wereassembledonthinsifilm( 4 µm thick) on fused silica wafer Au-Si alloy significantly shrinks in volume when the laser illumination was removed from the spot as seen in the figure, demonstrating a highly unstable and dynamic behavior of the Au-Si alloy molten droplet on the substrate. Control of the molten catalyst behavior has been reported as a critical factor in achieving directional epitaxial growth (i.e. silicon nanowire) by the vapor-liquid-solid (VLS) growth mechanism [5]. Upon further extension of the laser illumination period for 10 seconds, a rapid isotropic solid growth was observed (Fig. 6). The isotropic growth is attributed to the continuous precipitation of Si components from the molten droplet as the concentration of Si reaches the solubility limit through the VLS nanowire growth mechanism. However, the directional nanowire growth was not achieved. Possibly this was due to a high degree of instability of eutectic molten droplet behavior under the given laser illumination configuration and gas pressure. Further improvement in laser processing monitoring resolution has been achieved by coupling optical near-field illumination into an electron microscope. In contrast to previous efforts, true optical near-field illumination was realized by adopting an advanced apparatus capable of an arbitrary viewing angle while maintaining a constant probe-sample gap distance. Using an environmental SEM, complex Au catalyst behavior was explored under CVD gas environment. More rigorous in-situ adjustment of laser parameters is currently underway in order to elucidate the detailed mechanisms involved in laser-assisted growth. Nevertheless, the herewith presented results demonstrate that laser-materials interactions during growth can be monitored with in-situ electron microscopy. Currently, significant research activity is under way based on coupling an optical near-field setup into a TEM. It is expected that the in-situ TEM apparatus will offer unprecedented capabilities for observing nanostructures under localized laser illumination. Acknowledgements The authors gratefully acknowledge support by DARPA/MTO under grant N , and DOE/STTR under grant 95632B10-I. Research performed at the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, was supported by the Scientific User Facilities Division of the Office of Basic Energy Sciences, U.S. Department of Energy under Contract # DE-AC02-05CH DJH and CPG acknowledge support by the SINAM NSEC. References 1. D.J. Hwang, N. Misra, C.P. Grigoropoulos, A.M. Minor, S.S. Mao, In situ monitoring of laser cleaning by coupling a pulsed laser beam

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