Probing the Morphology and Evolving Dynamics of 3D Printed

Letter www.acsami.org

Probing the Morphology and Evolving Dynamics of 3D Printed Nanostructures Using High-Speed Atomic Force Microscopy Chen Yang,*,†,§ Robert Winkler,‡ Maja Dukic,§ Jie Zhao,† Harald Plank,‡,∥ and Georg. E. Fantner*,§ †

State Key Laboratory of Robotics and Systems, Harbin Institute of Technology, Yikuang Street 2, 150080 Harbin, China Graz Centre for Electron Microscopy, Steyrergasse 17, 8010 Graz, Austria § Laboratory for Bio- and Nano-Instrumentation, Interfaculty Institute of Bioengineering, École Polytechnique Fédérale de Lausanne, Batiment BM 3109 Station 17, 1015 Lausanne, Switzerland ∥ Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria ‡

S Supporting Information *

ABSTRACT: Focused electron beam induced deposition (FEBID) has been demonstrated as a promising solution for synthesizing truly threedimensional (3D) nanostructures. However, the lack of morphological feedback during growth complicates further development toward higher spatial fabrication precision. Here, we show that by combining in situ high speed atomic force microscopy (HS-AFM) with FEBID, morphologies in multistep fabrication process can be accessed. More importantly, the proposed method enables simultaneous imaging and fabrication operation, which opens new possibilities to investigate evolving mechanical properties of the deposit. The experiments indicate an exponential increase law of the mechanical resistance, meaning that a mechanically stable state establishes around 4 min after deposition. KEYWORDS: focused electron beam induced deposition, high-speed atomic force microscopy, morphology feedback, evolving dynamics, in situ characterization

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outcome have been widely investigated.12−14 Second, simple strategies to provide feedback about the real time deposition have been proposed, such as using the sample current. The measured sample current embeds information regarding the deposition thickness, thus could be employed as a simple height indicator.15 However, this method is only suitable for simple structures like nanofibers, and it is ill suited for providing the overall picture of the deposit morphology. In situ SEM characterization provides some measure of control, but does not allow to accurately quantify the dimension of the deposit in Z-direction. Therefore, atomic force microscopy (AFM) and transmission electron microscopy (TEM) have become routine methods to provide meaningful information about the printed nanostructures. Unfortunately, both of them fail to function in multistep nanoprinting process, since switching between ex situ characterization and nanofabrication without losing registration precision is almost impossible. Integrating AFM into scanning electron microscope (SEM) chamber, as reported in earlier studies,16,17 could serve as a promising solution. The slow image acquisition speed of AFMs in general, and in vacuum in particular, requires further efforts to achieve real combination of in situ measurement and nanofabrication.

hree-dimensional (3D) nanoprinting, a class of nanoscale additive manufacturing techniques, is attracting increasing interest. Representative approaches have been reported recently based on scanning probe microscopy (SPM),1 focused electron beam,2 feedback-controlled nanopipette,3 two-photon polymerization,4 and self-assembly techniques.5 Among them, focused electron beam induced deposition (FEBID) has been the most widely employed method for fabricating free-standing nanostructures,6,7 in part because it is resist-free, direct-write and achieves sub 10 nm resolution.8 When printing via FEBID, an injected gaseous precursor consisting of organometallic molecules is locally decomposed by a nanoscale focused electron beam. The beam-induced dissociation gives rise to a solid deposit that adheres on the substrate while the volatile byproducts are pumped away. As a consequence, by sequentially scanning the beam in the X−Y plane, patterns with the desired morphology can be formed in the out-of-plane (Z) direction. Achieving high fidelity multilayer material deposition has become one of the major challenges in FEBID. Motivation of solving this issue comes from advanced applications such as nanoelectronic devices,9 nano-optics,10 and quantum computing,11 where geometric dimensions and mechanical stability are of great concern. Hence, multiple efforts have been made to reach this goal. First, as FEBID itself is a complex process, the effects of patterning parameters (scan strategy, pixel dwell time, beam current, etc.) on the printing © XXXX American Chemical Society

Received: May 31, 2017 Accepted: July 12, 2017 Published: July 12, 2017 A

DOI: 10.1021/acsami.7b07762 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. HS-AFM integrated in a Nova 200 DBM for in situ FEBID characterization. (a) Conceptual schematic of the integrated HS-AFM and FEBID. The gaseous precursor is injected via the nozzle of the gas injection system (GIS). The physisorbed molecules are locally decomposed by a focused electron beam and form a solid deposit on the substrate surface. The self-sensing cantilever, driven by a tripod scanner, scans over the deposit for implementing high resolution in situ measurement. (b) Scanning electron microscopy (SEM) images of self-sensing cantilever, the scale bar represents 200 μm. (c) Photograph of the HS-AFM showing the tripod scanner and cantilever readout circuit. (d) Image of the combined instruments inside the vacuum chamber. The AFM is inserted into a small gap between electron beam and sample surface. (e) SEM view of cantilever and GIS nozzle.

above-mentioned space requirements (Figure 1c). In this way, the cantilever can be placed at the very front end of the HSAFM and is easily be seen by the SEM. Figure 1d shows an infrared photograph of the instruments inside the DBM chamber. In the working configuration, HS-AFM, electronbeam, and GIS nozzle are in close proximity. Figure 1e shows the SEM view of the cantilever location with respect to the nozzle and substrate surface. The scan speed performance of the custom-built HS-AFM was demonstrated by imaging a FEBID based single frame printed “EPFL” logo (see the Supporting Information for deposition condition) at different line rates. The dominant mechanical resonance of the HS-AFM is the bending of the Zscanner at 11.3 kHz. This limited the line-rate of the scan to approximately 120 Hz. In practice, because of the limitation imposed by the data rate of the controller (Nanoscope IV controller, Veeco Instruments), the maximum scan rate was set to 100 Hz, with a imaging resolution of 256 × 128 pixels. This corresponds to a single frame time of ∼1.3 s and a tip velocity of 0.86 mm/s. As a comparison, for traditional AFMs with a typical scan rate of 1 Hz, the imaging time will be more than 2 min. AFM imaging results with scan rates of 64 and 100 Hz are illustrated in Figure 2a. The logo has the in-plane dimensions of 3.3 × 0.9 μm2 and a height of 23 nm. Figure 2b shows the matched in-plane features in AFM and SEM images. HS-AFM has an advantage of providing reliable height information on the nanostructure, while SEM has better in-plane resolutions than our HS-AFM in contact mode (see the Supporting Information for details). Correlating the HS-AFM and SEM images provide the best of both worlds (Figure 2b). Probing the morphology of 3D printed nanostructure plays a key role in multistep nanoprinting. Currently in FEBID, the deposited height after each step can barely be estimated from the SEM image due to the typically nanometer thin nature in multilayer fabrication. Also, SEM imaging of individual layer would be extremely time-consuming as a clearance time has to be introduced prior to SEM imaging without significant surface

In this work, we have implemented both in situ and online characterizations of 3D printed nanostructures by using a customized high-speed AFM (HS-AFM) inside the SEM chamber. The conceptual schematic of our method is shown in Figure 1a. HS-AFM techniques18−20 have achieved a significantly improved temporal resolution, thus enabling both static and dynamic characterizations of the deposit. The increased imaging speed not only improves throughput but also allows for the first time the observation of evolving dynamics of the printed structures just after depositing. Stand-alone HS-AFM has proven to be a powerful tool for either studying dynamic process or observing large areas in small amounts of time.21,22 However, integrating it into the vacuum chamber of SEM or dual beam microscope (DBM) is challenging because of space constraints, and requires different design approaches compared to conventional HS-AFMs. Simultaneous operations of electron beam (or ion beam), gas injection system (GIS), energy-dispersive X-ray spectroscopy (EDX) system and HS-AFM requires the latter to be compact enough to fit into the small gap between electron beam column and the sample surface. The optimized working distance for dual beam system we used in this study (NOVA 200, FEI) is 4.9 mm. As a result, typical AFM cantilever detection optics are ill suited. For increased flexibility in sample type and size, we chose a tip-scanning configuration rather than the samplescanning configuration used in most HS-AFMs. In this manner, the AFM scanning motion is independent of the FEBID, hence eliminating any potential coupling issue. Furthermore, to facilitate AFM/SEM coordination, the cantilever should be visible in the SEM’s field of view even for high magnifications. On the basis of these considerations, piezoresistive self-sensing cantilevers were chosen (Figure 1b). In these cantilevers, embedded piezoresistive strain sensors measure the cantilever deflection directly, without the need for a laser-based optical detection system.23 We implemented a tripod scanner similar to Yang et al.,24 but with a protruding cantilever mounting position (referred to as “nose” in the following) to satisfy the B


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Figure 2. Correlated HS-AFM and SEM imaging. (a) High-speed AFM imaging results of a single layer “EPFL” logo fabricated via FEBID. The scan rate was set to 64 and 100 Hz, with pixel resolutions of 384 × 192 and 256 × 128 pixels, respectively. The scale bar in the images is 1 μm. (b) Correlative HS-AFM/SEM images providing the truly 3D morphology of the nanostructure.

contamination. In this situation, the combined HS-AFM/ FEBID however, enables high fidelity morphology feedback, which paves the way to multistep FEBID process control. As a demonstration and proof-of-concept experiment, we monitored the growth process of a 3D nanostructurea miniature replica of the Swiss mountain Matterhorn (Figure 3f).25 To build up a replica at the nanoscale via FEBID, the whole structure was sliced into 25 layers according to the contour map (Figure 3a), whereby each layer has the same altitude intercept. The whole structure (Figure 3b) was deposited layer by layer using the platinum based precursor (see the Supporting Information for deposition condition). In between the deposition steps, AFM images were taken a total of 8 times. The first 7 AFM images were taken after every third layer, whereas the final imaging step was implemented after the last 4 layers. All images were acquired with a scan rate of 20 Hz (with a resolution of 640 × 640 pixels) for increased resolution and reduced tip−sample force. Figure 3c shows the process of building the Matterhorn replica. To quantify the layer growth, cross section curves are extracted and plotted in Figure 3d, illustrating how the structure was built step by step. The peak of the miniature mountain is 185 nm, with in-plane dimensions of 3 × 3 μm2. The incremental height of every three layers are plotted in Figure 3e. Information from the last image is not included here as it contains the deposition of 4 layers. The mean value of each cross section curve ranges from 18.9 nm (the fifth image) to 24.6 nm (the fourth image). As a result, for a single deposition layer, the mean height (as indicated by the curve of Figure 3e) varies from 6.3 to 8.2 nm. Figure 3g shows the 3D render of Matterhorn replica. Moreover, the HS-AFM/FEBID combination also allows for completely new type of synchronized experiments, thus enabling the investigation of evolving properties of FEBID nanostructures. FEBID materials grown from organometallic precursors typically consist of 2−5 nm large metal nanograins, which are homogeneously embedded in an amorphous carbon matrix with contents around 85% for the here used Pt based precursor. The latter stems mostly from nondissociated precursor molecules, reaction products and carbon impurities from the sample surface.26 These defects and other chemical processes may lead to evolving physical properties after

Figure 3. In situ probing of the topography in three-dimensional additive patterning. (a) Contour map of the Matterhorn, including 25 height lines (b) SEM image of the finished miniature Matterhorn replica. (c) Layer series showing the process of 25-layer patterning. The first 7 AFM images are taken after every 3 layers being deposited, while the eighth image contains information on the remaining 4 layers. The scale bar of the image is 1 μm. (d) Line profiles illustrating the miniature Matterhorn growth. (e) Height distribution of the imaged layers. (f) Photograph of the Matterhorn in Switzerland (photographer, Andrew Bossi; source, Wikimedia). (g) Three-dimensional AFM image of the Matterhorn replica.

fabrication.27 According to earlier studies, electrical conductivity of FEBID deposit decreased dramatically as a function of time, followed by a relatively stable saturation value.28 Inspired by this, an important issue on the mechanical property of the deposit is to understand whether it evolves after fabrication. To this end, mechanical information on the deposit has to be obtained within seconds after fabrication. One way to overcome this challenge is to synchronize the operation of HS-AFM and FEBID. As we use our HS-AFM in contact mode, a constant load governed by the feedback loop is applied to the nanostructure, leading to shear loading when the cantilever scans over the FEBID deposit. This load will cause scratching into the nanostructure. Consequently, a larger material removal amount corresponds to a lower scratching resistance. We then used this as a measure of the mechanical stability of the FEBID deposit. For the purpose of making the scratching effect obvious, a stiff cantilever (see the Supporting Information for details) was chosen to implement the following experiments. The set point of feedback loop was minimized (0.2 V in our case) without deteriorating the image quality. This results in a load of approximately 2.6 μN. Parameters used for coordinating FEBID and HS-AFM consists of two subsets: point pitch (spacing between patterning points) and dwell time from the FEBID side; pixel resolution, scan rate and scan range from HSAFM side. All of them work together to time the AFM imaging just after the nanoprinting process. The tip of the cantilever we used was slightly set back from the front end of the cantilever. C


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right corner at time t0, meanwhile FEBID began from rectangle 1 (see the Supporting Information for deposition condition). After finishing the deposition (at t = t1, i.e., Two seconds after t0) of the first round, FEBID stopped and the electron-beam moved to the waiting position, which was outside the scan area (6.5 × 6.5 μm2). The cantilever was close to the deposits at this time, but still with a safety distance. The scan rate was set to 20 Hz, with an imaging resolution of 640 × 640 pixels. The first (up-trace) and second (down-trace) imaging happened around 25 and 35 s after the deposition. At time t2, the cantilever and electron-beam moved back to their starting positions. Then the next round started again. Based on this protocol, we achieved automatic printing-imaging-printing cycles. The whole process contained 5 nanoprinting and 10 imaging actions. Figure 5a shows the 3D up-trace imaging results of each deposition (for the down-trace images see Figure S2), in which the red arrows indicate where the deposition happened in each round. It can be seen that in the first round 5 rectangles were deposited. While in the second round the deposition happened in 4 positions. In the last round, materials were only deposited in the position of rectangle 5. The experiment was designed in this way to collect more height information just after the deposition. (It should be noted that in the fourth round, deposition happened at position 3 and 5, not at position 4 and 5 as might be expected. The reason is that we wanted to see whether the position of deposition could lead to some differences in the trends. Therefore, we made this change and deposited at position 4 and 5.) According to Figure 5b, the height of each deposit after the first printing is relatively constant, with a mean

To allow deposition of the FEBID structure in close proximity to the AFM tip apex, we cut off the front part of the cantilever using focused ion beam (FIB) milling (Figure S1). The strategy that coordinated operations of FEBID and AFM imaging is illustrated in Figure 4. The AFM started to scan from the lower

Figure 4. Schematic of the coordination strategy for simultaneous AFM imaging and FEBID. AFM imaging starts from the lower right corner at time t0. Meanwhile, FEBID process begins from the rectangle 1. The deposition finishes in 2 s and then e-beam moves to the waiting position, which is out of the scan range. The up-trace AFM imaging happens 25 s after the deposition and the down-trace imaging happens 35 s after the deposition. At time t2, AFM probe and e-beam moves back to the starting positions for the next round of experiment. The whole process contains 5 depositions and 10 imaging actions.

Figure 5. (a) AFM images (up trace) of the FEBID rectangles after each deposition. (b) Stacked column of height change after each deposition. (c) Height variation of each rectangle as a function of time. (d) Mean scratched value as a function of time. The fitted curve exhibits an exponential decrease of the scratched value with time, indicating the fabricated structure tends to achieve mechanical stability after ∼4 min of the deposition (the deposition happens at t = 0 s). D


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Moreover, FEBID is known as a rapid process, leading to chemical nonequilibrium states right after deposition, which has been clearly revealed via in situ electric measurements.30 Although dynamic measurements concerning chemistry and internal structural composition right after fabrication are still challenging, there are practically no results available in the literature. In this condition, further characterization via, for example, conductive AFM has to follow to correlate mechanical, volumetric, and functional properties of FEBID materials. This is another focus of the future work. In conclusion, we have achieved high fidelity in situ characterizations of a multistep FEBID process by integrating a custom HS-AFM into the DBM. This combination will enable the spatial information about the just deposited nanostructure to be directly used in controlling the FEBID process parameters for the following layers. As a proof-of-concept test, slice by slice additive tomography was achieved via in situ probing the growth of a nanostructureminiature Matterhorn replica. Furthermore, by coordinating operations of HS-AFM and FEBID, we could shed light on evolving physical properties (such as mechanical stability) that happen at short time scales post deposition. We believe that this in situ and online monitoring approach will ramp up the thorough understanding of FEBID parameters and thus enable highly reliable fabrication of nanostructures with strict morphological requirements.

value of 11.2 nm and a standard deviation of 0.4 nm. However, subsequent printing results showed a much larger deviation although keeping the same parameters. This may relate to the shape and the density of the substrate, because the first 5 rectangles were deposited on a flat surface, whereas the others were printed on previous layers. The last deposition on rectangle 5 led to only a 2 nm growth, possibly because of drift issues. To determine the mechanical resistance of the deposits, we evaluated the nanoscratching behavior in time. The overall height information on each rectangle is provided in Figure 5c, representing the effects of both deposition and scratching. At a certain point after deposition, there is no significant height reduction appeared due to nanoscratching. This is particularly apparent for rectangle 1 and 2. The height reduction is about 0.5 nm after each AFM imaging. In contrast, right after a new deposition the AFM tip could remove up to 4.9 nm in height (with a mean value of 3.3 nm) from the first image to the second one (see Figure 5d). It is worth noting that as the rectangles were actually rounded, the removed height for each rectangle was calculated as the difference between the peak value of current measurement and that of the last measurement. Moreover, each data point in Figure 5d is the mean value of the removed height at a given time after deposition for all of 5 rectangles. (For example, the data point at 35 s is the mean value of the removed height happened 35 s after the deposition. As there are 5 rounds of deposition, it is actually the mean value of 15 data points.) The variation law of evolving mechanical stability is carefully investigated by plotting the removed height of the structures as a function of time, as shown by Figure 5d. The fitted function shows a downtrend of the removed height in a form of a single exponential, which is Rh(t ) = 6.7e−t/44.04 + 0.36

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07762. Experimental details of HS-AFM scanner, cantilever and optimizing results, and dual beam microscope and downtrace images of simultaneous imaging/nanoprinting (PDF)

(1)

where the constant item represents the removed height of each imaging when t → ∞. Practically, after around 4 min of the fabrication, the scratched height falls into 5% of the initial value. We thus believe the printed nanostructures reach a mechanically stable state. The integrated HS-AFM/FEBID, as proposed in this letter, has shown a promising capability in studying dynamic behaviors of direct-write nanodepositions. This makes it possible, for the first time, to investigate the evolving mechanical stability immediately after fabrication, and opens a door for various studies such as the morphological change with time in the low nanometer range. However, experimental results shown in Figure 5d indicate a material removal rate of 0.5 nm per pass, even after the FEBID deposit was stabilized. Therefore, for higher precision morphology characterization, a gentler imaging mode such as amplitude modulation mode (also called tapping mode) would be more suitable, thus becoming the focus of our future work. Achieving high speed tapping mode imaging in the DBM chamber however is difficult, because in a vacuum environment, the quality factor (Q factor) of the cantilever is dramatically higher than in air. As the bandwidth (BW) of an oscillating cantilever in tapping mode is proportional to the ratio of the resonance frequency f 0, and its Q factor (BW ∝ f 0/Q), the detection speed (mechanical bandwidth) of the cantilever in vacuum is also dramatically lower than in air. As a result, AFM has to be operated in an impractically low scan rate. One possible solution is to decrease the cantilever Q factor by using high damping materials,29 so that both the mechanical bandwidth and the scanning speed can be significantly improved.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: georg.fantner@epfl.ch. ORCID

Chen Yang: 0000-0002-0760-8284 Harald Plank: 0000-0003-1112-0908 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research received funding from the European Union FP-7 program Eurostars E!8213, from the European Union FP7/ 2007-2013/ERC under Grant Agreement 307338-NaMic. C.Y. acknowledges the financial support from the China Scholarship Council for his joint PhD project (Grant 201306120115).

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REFERENCES

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Probing the Morphology and Evolving Dynamics of 3D Printed

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