Name: in situ transmission electron microscopy with biasing and fabrication of asymmetric crossbars based on mixed-phased a-vox
Uploaded: 2020-05-13T13:00:00.000+00:00
Duration: 9 min 49 s
Description: Watch this Scientific Journal Video about In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx at JoVE.com

This work was performed in part at the Micro Nano Research Facility at RMIT University in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors acknowledge the facilities, and the scientific and technical assistance of the RMIT University&#39;s Microscopy, Microanalysis Facility, a linked laboratory of the Microscopy Australia. Scholarship support from the Australian Postgraduate Award (APA)/Research Training Program (RTP) scheme of the Australian government is acknowledged. We thank Professor Madhu Bhaskaran, Associate Professor Sumeet Walia, Dr. Matthew Field, and Mr. Brenton Cook for their guidance and helpful discussions.

1. Fabrication process and electrical characterization
<ol>
	<li>Use standard image reversal photolithography9 to pattern bottom electrode (BE layer 1) with photoresist&#160;of the devices using the following parameters:
	<ol>
		<li>Spin coat the photoresist at 3,000&#160;rpm, soft bake it at 90 &#176;C for 60&#160;s, expose with 25&#160;mJ/cm2 with a 405&#160;nm laser, bake at 120 &#176;C for 120&#160;s, perform flood exposure with 21 mW/cm2 and a 400&#160;nm laser, develop using developer, and rinse with deionized water.</li>
	</ol>
	</li>
	<li>Deposit 5&#160;nm titanium (Ti) for adhesion and 15&#160;nm of platinum (Pt) on top with an electron beam evaporator system with&#160;the substrate patterned on layer 1.</li>
	<li>Lift-off the deposited metals by placing the substrate in an acetone bath for ~20&#160;mins. Then, apply ultrasonic vibrations for 2&#160;min, and rinse with acetone and isopropyl alcohol (IPA) to complete the BE patterns. Repeat if the lift-off is not clean (Figure 1A, step 1).</li>
	<li>Pattern the functional oxide layer (layer 2) with photolithography on top of the BE as described in step 1.1.</li>
	<li>Deposit ~100&#160;nm of a-VOx and 5&#160;nm of Ti on top of layer 2 using a sputtering system10.</li>
	<li>Lift-off the functional oxide by placing the substrate in an acetone bath and applying pulsed ultrasonic vibrations manually with 2&#8211;3&#160;s pulses to finalize the functional oxide patterns. Repeat the procedure if the patterns are not clean. (Figure 1A, step 2 and step 3)</li>
	<li>Similarly, complete the top electrode (TE) (layer 3) patterns with Ti_20&#160;nm/Pt_200&#160;nm using image reversal photolithography, electron beam evaporation, and the lift-off process described in step&#160;1.&#160;(Figure 1A, step 4) 
	NOTE: This completes the fabrication of the cross-point device, Figure 1B.</li>
	<li>Perform electrical and temperature analysis on the fabricated device to understand its resistance switching performance.
	<ol>
		<li>Use the source meter with two-probe direct current (DC) I&#8211;V measurement system and a probe station for electrical measurements.</li>
		<li>Always maintain the relevant current compliance to avoid damaging the devices.</li>
		<li>To analyze the device&#39;s current behavior, perform voltage-controlled analysis and apply voltage sweeps starting with a low voltage of 0.1&#160;V in positive bias and increasing slowly until electroforming is observed. 
		NOTE: Electroforming is a one-time event at which a few nanometer-wide conductive filaments are formed within the initially insulating functional oxide at a particular voltage, which depends on intrinsic material properties and device dimensions. At this point, a sudden drop in resistance or increase in current is observed on the current-voltage graph due to a formed conductive path.</li>
		<li>After electroforming, apply bidirectional voltage sweeps to achieve volatile threshold switching performance. Adjust the voltage to achieve a high ON/OFF ratio. In this case, a switching ratio of ~10 was attained.</li>
		<li>Analyze the current-voltage characteristics at different temperatures from room temperature to 90 &#176;C increasing in 10&#160;&#176;C steps and reverse back to room temperature using a temperature-controlled stage.</li>
	</ol>
	</li>
</ol>
2. Gridbar and biasing chip mounting
<ol>
	<li>Design the FIB optimized gridbar in CAD software and manufacture using standard machining techniques in-house for mounting the biasing/heating chips used for in&#160;situ TEM experiments, as shown in Figure 2. 
	NOTE: Figure 2A shows separate parts of the gridbar to mount three chips simultaneously in the square shaped trenches. Figure 2B shows the zoomed squared trench section designed to fit the commercially available in&#160;situ biasing/heating chips for TEM.</li>
	<li>Clean the biasing chip by placing it in a glass Petri dish filled with acetone and gently rotate for 2 min. Then remove the chip and place it in a Petri dish filled with methanol and gently rotate for 2 min. Finally, blow dry with low pressured nitrogen. 
	NOTE: Commercially bought biasing chips, referred to as E-chips, have a photoresist coating for protection.</li>
	<li>Align the precleaned biasing chip in square trenches of gridbar, as seen in Figure 2C.</li>
	<li>Fix the grid cover on top of the biasing chip with screws to finalize the placement of the E-chip on the gridbar (Figure 2D).</li>
</ol>
3. Lamella preparation, mounting on biasing chip using focused ion beam, and in&#160;situ transmission electron microscopy
<ol>
	<li>Fabricate the samples separately as described in section 1 with a thicker BE of Ti_10&#160;nm/Pt_100&#160;nm, as seen in Figure 3A.</li>
	<li>Mount the newly prepared sample on a metal stub using conductive carbon tape and load in the FIB chamber. Apply additional tape on the sample for grounding to avoid charging problems.</li>
	<li>Load the biasing chip-mounted gridbar in the chamber at a 52&#176; tilt (see Figure 3B). This will be either perpendicular or parallel to the ion beam column depending on the stage rotation.</li>
	<li>Focus, astigmate, and align the electron beam on a sample surface using the microscope physical control panel and software on lamella preparation locations.</li>
	<li>Check the eucentric height of the focused sample location and beam coincidence for the electron beam and ion beam. 
	NOTE: The eucentric height is the position where the sample&#8217;s image does not move when the sample is tilted.</li>
	<li>Click on Auto TEM program (automatic lamella preparation program) to run it on the focused sample location using the microscope control software. The automatic program follows the sequence described below. 
	NOTE: This will complete the process for creating a TEM lamella (Figure 4). The progress of the AutoTEM program can be observed live on the desktop screen.
	<ol>
		<li>Create cross fiducial alignment markers with silicon&#160;milling and deposit a 1.5&#160;&#181;m-thick carbon protective layer over the 20 &#181;m x 5&#160;&#181;m area between alignment markers.</li>
		<li>Mill trenches on either side of carbon protective layer with a 5&#160;nA ion beam current to create the lamella.</li>
		<li>Thin the lamella with a 1&#160;nA ion beam current first and then with 300&#160;pA ion beam current to reach a 1&#160;&#181;m thickness.</li>
	</ol>
	</li>
	<li>Tilt the sample to 7&#176; to perform a J-cut on the lamella for separation from the substrate.</li>
	<li>Tilt the sample to 0&#176; (i.e., perpendicular to the electron beam column) and attach the lamella to the manipulator needle by wielding using Pt (Figure 5A).</li>
	<li>After attachment to the micromanipulator, separate the lamella from the substrate with the final cut and slowly retract the micromanipulator (Figure 5B).</li>
	<li>Focus the beam on the top edge of biasing chip on the gridbar, the lamella mounting position.</li>
	<li>Bring the lamella slowly towards the biasing chip with the manipulator needle (Figure 6A).</li>
	<li>Align the lamella in the center of the 17&#160;&#181;m gap on the top edge of the biasing chip. Slowly move it down until it barely touches the chip surface and weld the bottom edges of the lamella to the chip using Pt (Figure 6B).</li>
	<li>Cut the micromanipulator free from the lamella with silicon milling and retract the micromanipulator.</li>
	<li>Connect the top edges of the lamella with Pt traces to the two electrodes of the biasing chip for electrical connections (Figure 6C). 
	NOTE: The TE and BE are shorted at this point on both left and right sides.</li>
	<li>Thin the center region of lamella first using 300&#160;pA, and then with 100&#160;pA ion beams to make the lamella less than 100&#160;nm thick (Figure 6D) by tilting the specimen front and back by 2&#176; to ensure parallel faces and a uniform thickness.</li>
	<li>Polish out the ion beam-damaged layer with the Ga beam accelerating voltage of 5&#160;kV at an angle of 5&#176; to the surface on both faces.</li>
	<li>Remove the short connection between the top and bottom electrodes of the device with isolation cuts in the thinned region to create a current path from BE to TE through the active region (Figure 7A).</li>
	<li>Mount the biasing chip with lamella on the biasing chip holder and then load the biasing chip holder into the TEM chamber.</li>
	<li>Connect the wires from the biasing chip holder to the source meter and a control PC. 
	NOTE: Carefully place the connection wires to relieve the strain and minimize any vibrations during the experiment.</li>
	<li>Wait for the TEM chamber pressure to drop to 4e-5&#160;Torr and then focus, astigmate, and align the electron beam on a cross section of the lamella surface using the TEM control knobs.</li>
	<li>Apply voltage sweeps or constant voltage at different biasing voltages and collect the TEM micrographs in situ. 
	NOTE: Data related to diffraction patterns, electron diffraction X-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS) mapping can also be collected at different biasing voltages in situ.</li>
</ol>

<ol>
	<li>Kozma, R., Pino, R. E., Pazienza, G. E., Kozma, R., Pino, R. E., Pazienza, G. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Advances in Neuromorphic Memristor Science and Applications. , 9-14 (2012).</li><li>Pan, F., Gao, S., Chen, C., Song, C., Zeng, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+progress+in+resistive+random+access+memories:+Materials,+switching+mechanisms,+and+performance.">Recent progress in resistive random access memories: Materials, switching mechanisms, and performance.</a> Materials Science and Engineering: R: Reports. 83, 1-59 (2014).</li><li>Zhou, Y., Ramanathan, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mott+Memory+and+Neuromorphic+Devices.">Mott Memory and Neuromorphic Devices.</a> Proceedings of the IEEE. 103 (8), 1289-1310 (2015).</li><li>Nirantar, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+Situ+Nanostructural+Analysis+of+Volatile+Threshold+Switching+and+Non-Volatile+Bipolar+Resistive+Switching+in+Mixed-Phased+a-VOx+Asymmetric+Crossbars.">In Situ Nanostructural Analysis of Volatile Threshold Switching and Non-Volatile Bipolar Resistive Switching in Mixed-Phased a-VOx Asymmetric Crossbars.</a> Advanced Electronic Materials. 5 (12), 1900605 (2019).</li><li>Rupp, J. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Different+threshold+and+bipolar+resistive+switching+mechanisms+in+reactively+sputtered+amorphous+undoped+and+Cr-doped+vanadium+oxide+thin+films.">Different threshold and bipolar resistive switching mechanisms in reactively sputtered amorphous undoped and Cr-doped vanadium oxide thin films.</a> Journal of Applied Physics. 123 (4), 044502 (2018).</li><li>Ahmed, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inducing+tunable+switching+behavior+in+a+single+memristor.">Inducing tunable switching behavior in a single memristor.</a> Applied Materials Today. 11, 280-290 (2018).</li><li>Nili, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoscale+Resistive+Switching+in+Amorphous+Perovskite+Oxide+(a-SrTiO3)+Memristors.">Nanoscale Resistive Switching in Amorphous Perovskite Oxide (a-SrTiO3) Memristors.</a> Advanced Functional Materials. 24 (43), 6741-6750 (2014).</li><li>Ahmed, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transparent+amorphous+strontium+titanate+resistive+memories+with+transient+photo-response.">Transparent amorphous strontium titanate resistive memories with transient photo-response.</a> Nanoscale. 9 (38), 14690-14702 (2017).</li><li>Reuhman-Huisken, M. E., Vollenbroek, F. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+optimized+image+reversal+process+for+half-micron+lithography.">An optimized image reversal process for half-micron lithography.</a> Microelectronic Engineering. 11 (1), 575-580 (1990).</li><li>Taha, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Insulator-metal+transition+in+substrate-independent+VO2+thin+film+for+phase-change+devices.">Insulator-metal transition in substrate-independent VO2 thin film for phase-change devices.</a> Scientific Reports. 7 (1), 17899 (2017).</li><li>Booth, J. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Correlating+the+Energetics+and+Atomic+Motions+of+the+Metal-Insulator+Transition+of+M1+Vanadium+Dioxide.">Correlating the Energetics and Atomic Motions of the Metal-Insulator Transition of M1 Vanadium Dioxide.</a> Scientific Reports. 6, 26391 (2016).</li><li>Lee, S., Ivanov, I. N., Keum, J. K., Lee, H. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Epitaxial+stabilization+and+phase+instability+of+VO2+polymorphs.">Epitaxial stabilization and phase instability of VO2 polymorphs.</a> Scientific Reports. 6, 19621 (2016).</li></ol>

The authors have nothing to disclose.

This paper explains the protocol for in&#160;situ biasing with transmission electron microscopy including the fabrication process for the device, gridbar designing for biasing chip mounting, lamella preparation and mounting on the biasing chip, and TEM with in&#160;situ biasing.
The fabrication methodology of cross-point devices, which can be easily scaled up to crossbar structures, is explained. The Ti capping of vanadium oxide is essential to incorporate amorphous vanadium oxide, because it dissolves in water during the fabrication steps after a-VOx deposition. Devices are fabricated with two different sizes for electrical testing, 4 &#181;m x 4&#160;&#181;m and 6 &#181;m x 6&#160;&#181;m. The contact electrode used here is Pt, a noble metal that degrades minimally over the fabrication period. Due to this and to avoid the uniform crystallization of vanadium oxide in the device structure, the electrode annealing step typically used was omitted in this fabrication method. A complete fabrication flow and the device structure schematic is presented Figure 14.
For in&#160;situ experiments, the devices are fabricated separately with thicker BE as explained in step 1 of the lamella preparation and mounting on biasing chip section. This is done to avoid the deposition of Pt particles on the functional layer during connections. The change in thickness of the BE is not expected to have an effect in device switching.
The commercially available biasing chips (e.g., E-chip) for in&#160;situ biasing with TEM have four biasing electrodes available for connection and a 17&#160;&#181;m wide gap for mounting the lamella, as shown in Figure 7B. A customized gridbar is designed to mount the biasing chips, as this arrangement allows access to the scanning transmission electron microscopy (STEM) detector in the FIB chamber for precise cutting and connection of the lamella mounted on the biasing chip. This is especially required for the precise isolation cuts explained in step&#160;17 of the lamella preparation and mounting on the biasing chip section. For the lamella preparation and mounting process, the sequence of Pt connections, lamella thinning, and making isolation cuts (steps 14&#8211;17 of lamella preparation and mounting) are the most critical to achieve a clean lamella. Here, the Pt connecting traces are performed before the thinning process to avoid deposition of Pt particles on the functional oxide layer, which can ruin electrical attributes.
As the lamella is prepared using Ga ion milling, some undesirable Ga contamination is expected in the final lamella. However, lamella polishing is performed to significantly reduce the damage caused by the Ga beam. Another drawback of this protocol is that the dimensions of the lamella are significantly smaller (in nanoscale) compared to the actual device (a few microns). Due to this, variability can be observed in the electrical characterizations of the actual device and the lamella-based device.
Despite of that, this protocol offers a significant advantage over the existing techniques as it provides visual verification of every step of the lamella preparation and during the in situ biasing. As all the steps can be seen visually in real time, failures are detected and rectified immediately. There are no hidden aspects in the process and troubleshooting is simply by visual observation unless there are any instrument specific issues.
The presented methodology has a notable impact in the field of material science and resistive switching devices compatible with high vacuum conditions. The protocol can explain the electrical results and operation mechanisms based on visually observed nanostructural changes in&#160;situ. This protocol will influence next generation of nanoelectronics, logic circuits, neuromorphic devices, and material sciences to reveal the underlying operation mechanisms and predict the practical applicability of novel structures and materials.

Resistance change oxide memories are increasingly used as the building block for novel memory and logic architectures due to their compatible switching speed, smaller cell structure, and the ability to be designed in high capacity three-dimensional (3D) crossbar arrays1. To date, multiple switching types have been reported for resistive switching devices2,3. Common switching behaviors for metal oxides are unipolar, bipolar, complementary resistive switching, and volatile threshold switching. Adding on to the complexity, single cell has been reported to show multifunctional resistive switching performance as well4,5,6.
This variability means that nanostructural investigations are needed to understand the origins of different memory behaviors and corresponding switching mechanisms to develop clearly defined condition-dependent switching for practical utility. Commonly reported techniques to understand the switching mechanisms are depth profiling with X-ray photoelectron spectroscopy (XPS)7,8, nanoscale secondary ion mass spectroscopy (nano-SIMS)6, nondestructive photoluminescence spectroscopy (PL)8, electrical characterization of different size and thickness of functional oxide of devices, nanoindentation7, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS) on cross-sectional lamella in a TEM chamber6,8. All the above techniques have provided satisfactory insights about the switching mechanisms. However, in most of the techniques, more than one sample is required for analysis, including the pristine, electroformed, set, and reset devices, to understand the complete switching behavior. This increases experimental complexity and is time consuming. Additionally, the failure rates are high, because locating a subnanoscale filament in a device a few microns in size is tricky. Therefore, in&#160;situ experiments are important in nanostructural characterizations to understand operation mechanisms, as they provide evidence in real-time experiments.
Presented is a protocol for conducting in&#160;situ TEM with electrical biasing for metal-insulator-metal (MIM) stacks of asymmetric resistive switching cross-point devices. The primary goal of this protocol is to provide a detailed methodology for lamella preparation using a focus ion beam (FIB) and in&#160;situ experimental setup for TEM and electrical biasing. The process is explained using a representative study of asymmetric cross-point devices based on mixed-phased amorphous vanadium oxide (a-VOx)4. Also presented is the fabrication process of cross-point devices incorporating a-VOx, which can be easily scaled up to crossbars, using standard micro-nano fabrication processes. This fabrication process is important as it incorporates in crossbars a-VOx which dissolves in water.
The advantage of this protocol is that with only one lamella, nanostructural changes can be observed in TEM, unlike the other techniques, where a minimum of three devices or lamellae are required. This significantly simplifies the process and reduces time, cost, and effort while providing reliable visual evidence of nanostructural changes in real-time operations. Additionally, it is designed with standard micro-nano fabrication processes, microscopy techniques, and instruments in innovative ways to establish its novelty and address the research gaps.
In the representative study described here for a-VOx-based cross-point devices, the in&#160;situ TEM protocol helps to understand the switching mechanism behind apolar and volatile threshold switching4. The process and methodology developed for observing nanostructural changes in a-VOx during in&#160;situ biasing can be easily extended to in situ temperature, and in situ temperature and biasing simultaneously, by just replacing the lamella mounting chip, and to any other material including two or more layers of functional material in a metal-insulator-metal sandwiched structure. It helps reveal the underlying operation mechanism and explain electrical or thermal characteristics.

<table><tbody><tr><td>Resist processing system</td><td>EV group</td><td>EVG 101</td><td></td></tr><tr><td>Acetone</td><td>Chem-Supply</td><td>AA008</td><td></td></tr><tr><td>Biasing Chip - E-chip</td><td>Protochips</td><td>E-FEF01-A4</td><td></td></tr><tr><td>Developer</td><td>MMRC</td><td>AZ 400K</td><td></td></tr><tr><td>Electron beam evaporator - PVD 75</td><td>Kurt J Leskar</td><td>PRO Line - eKLipse</td><td></td></tr><tr><td>Focused Ion beam system</td><td>Thermo Fisher - FEI</td><td>Scios DualBeam&lt;sup&gt;TM&lt;/sup&gt; system</td><td></td></tr><tr><td>Hot plates</td><td>Brewer Science Inc.</td><td>1300X</td><td></td></tr><tr><td>Magnetron Sputterer</td><td>Kurt J Leskar</td><td>PRO Line</td><td></td></tr><tr><td>Mask aligner</td><td>Karl Suss</td><td>MA6</td><td></td></tr><tr><td>Maskless Aligner</td><td>Heildberg instruments</td><td>MLA150</td><td></td></tr><tr><td>Methanol</td><td>Fisher scientific</td><td>M/4056</td><td></td></tr><tr><td>Phototresist</td><td>MMRC</td><td>AZ 5412E</td><td></td></tr><tr><td>Pt source for e-beam evaporator</td><td>Unicore</td><td></td><td></td></tr><tr><td>The Fusion E-chip holder</td><td>Protochips</td><td>Fusion 350</td><td></td></tr><tr><td>Ti source for e-beam evaporator</td><td>Unicore</td><td></td><td></td></tr><tr><td>Transmission Electron Microscope</td><td>JEOL</td><td>JEM 2100F</td><td></td></tr></tbody></table>

in situ transmission electron microscopy with biasing and fabrication of asymmetric crossbars based on mixed-phased a-vox

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different materials show variability in resistive switching properties due to the intrinsic nature of the material used, leading to discrepancies in the field because of underlying operation mechanisms. This highlights a need for a reliable technique to understand mechanisms using nanostructural observations. This protocol explains a detailed process and methodology of in&#160;situ nanostructural analysis as a result of electrical biasing using transmission electron microscopy (TEM). It provides visual and reliable evidence of underlying nanostructural changes in real time memory operations. Also included is the methodology of fabrication and electrical characterizations for asymmetric crossbar structures incorporating amorphous vanadium oxide. The protocol explained here for vanadium oxide films can be easily extended to any other materials in a metal-dielectric-metal sandwiched structure. Resistive switching crossbars are predicted to serve the programmable logic and neuromorphic circuits for next-generation memory devices, given the understanding of the operation mechanisms. This protocol reveals the switching mechanism in a reliable, timely, and cost-effective way in any type of resistive switching materials, and thereby predicts the device&#39;s applicability.

The results achieved using this protocol for the a-VOx cross-point devices are explained in Figure 8. Figure 8A shows the TEM micrograph of the intact lamella. Here the diffraction patterns (inset) indicate the amorphous nature of the oxide film. For the in situ&#160;TEM measurements,&#160;controlled voltages were applied starting from 25&#160;mV to 8&#160;V in 20&#160;mV steps with the bottom electrode (BE) positively biased and top electrode (TE) grounded. Figure 8B shows that at 4&#160;V a localized crystalline region formed in the oxide layer. Here, d-spacing was 0.35&#160;nm, as shown in the high-resolution TEM (HRTEM) and diffraction patterns (insets). This d-spacing corresponds to the (011) plane of the VO2&#8211;M1 phase10,11. Figure 8C shows the multiple localized crystal islands within the oxide layer at 5&#160;V. These crystal islands were oriented in different directions with respect to the substrate. Two different d-spacings can be observed in the corresponding FFT and HRTEM (insets): 0.35&#160;nm and 0.27&#160;nm. A spacing of 0.27&#160;nm corresponds to the VO2&#8211;A phase, while 0.26&#160;nm corresponds to the VO2&#8211;M1 phase12. Considering the aberration defects and tilt correction limits of the instrument, the observed 0.27&#160;nm d-spacing likely corresponds to the mixed phase of VO2&#8211;M1 and VO2&#8211;A. Figure 8D shows the Moir&#233; fringes at 6&#160;V. There are multiple nucleation sites in the lamella. Here FFT and HRTEM (insets) provide further evidence of the different orientations of the VO2&#8211;M1 crystal islands. After 6&#160;V, the lamella is completely crystallized with multiple orientations only with the electrical bias without any conventional annealing.
This is the first demonstration of a-VOx thin film crystallizing into localized c-VO2 islands with electric bias. The strong evidence for the presence of c-VO2 islands in a-VOx devices after biasing at higher voltage proves the resistive switching characteristics (Figure 2 of cited reference4) and the switching mechanism (Figure 6 of the cited reference4) for asymmetric cross-point devices based on mixed-phased a-VOx.
The results show the application of the explained protocol. Here the in&#160;situ nanostructural changes were captured in remanence of voltage sweeps at different voltages with the high-resolution TEM (HRTEM) micrographs and corresponding diffraction patterns.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61026/61026fig1.jpg" /> 
Figure 1: Fabrication flow and cross-point device structure schematic. (A) Fabrication flow incorporating Ti capping to protect a-VOx film from dissolving in water. (B) Schematic of cross-point device structure. This figure has been modified from Nirantar et al.4. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61026/61026fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/61026/61026fig2.jpg" /> 
Figure 2: Custom-made FIB optimized gridbar for in&#160;situ mounting of TEM chips. (A) Individual parts of the gridbar. (B) Squared trench for in&#160;situ TEM chip placement. (C) Aligned biasing chip for in&#160;situ TEM in the squared trench. (D) Mounted biasing chip on the gridbar. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61026/61026fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/61026/61026fig3.jpg" /> 
Figure 3: Cross section stacks for in&#160;situ samples and FIB chamber setup of biasing chip. (A) Cross section stacks of devices prepared separately for in&#160;situ biasing using TEM sample. (B) Gridbar setup in the chamber to allow access to the scanning electron microscopy (STEM) detector for precision cutting and connections on the lamella. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61026/61026fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/61026/61026fig4.jpg" /> 
Figure 4: Processing steps of Auto TEM. (A) Alignment markers and protection layer deposition. (B) Trenches formed with rough milling using a 5&#160;nA current. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61026/61026fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/61026/61026fig5.jpg" /> 
Figure 5: The process of lamella separation from the substrate. (A) Manually made J-cut and attached lamella to the manipulator needle. (B) Extracted lamella through the trenches after the final separation cut. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61026/61026fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/61026/61026fig6.jpg" /> 
Figure 6: Lamella mounting on the biasing chip process. (A) Manipulator bringing the attached lamella to the biasing chip. (B) Lamella attached to the biasing chip. (C) Connections with platinum traces between the electrodes of the biasing chip and the region of interest of the lamella. (D) Sub-100&#160;nm thinned center region of the lamella. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61026/61026fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/61026/61026fig7.jpg" /> 
Figure 7: Final isolation cuts and the current path in the lamella and micrograph of the FIB optimized biasing chip. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61026/61026fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/61026/61026fig8.jpg" /> 
Figure 8: In&#160;situ electrical transmission electron microscopy. (A) Original lamella. The inset shows the FFT of the functional layer. (B) Micrograph after 4&#160;V biasing. FFT inset shows c-VO2 (M1) phase with (011) plane and HRTEM inset shows the fringes separation as 0.35&#160;nm. (C) Micrograph after 5&#160;V. FFT and HRTEM insets show multiple nucleation sites and different orientations of the same c-VO2&#8211;M1. (D) Micrograph after 6&#160;V. FFT inset shows different orientations of same c-VO2 &#8211; M1 phase. HRTEM inset shows the formation of Moir&#233; fringes. This figure has been modified from Nirantar et al.4. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61026/61026fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx at JoVE.com

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Presented here is a protocol for analyzing nanostructural changes during in&#160;situ biasing with transmission electron microscopy (TEM) for a stacked metal-insulator-metal structure. It has significant applications in resistive switching crossbars for the next generation of programmable logic circuits and neuromimicking hardware, to reveal their underlying operation mechanisms and practical applicability.

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different ...

in-situ-transmission-electron-microscopy-with-biasing-fabrication

Nanyang Technological University

Research

JoVE Journal

Engineering

4.0K Views.  RMIT University. Hier wordt een protocol gepresenteerd voor het analyseren van nanostructurele veranderingen tijdens in situ biasing met transmissie elektronenmicroscopie (TEM) voor een gestapelde metaal-isolator-metaalstructuur. Het heeft belangrijke toepassingen in resistieve schakelbalken voor de volgende generatie programmeerbare logische circuits en neuromimicking hardware, om hun onderliggende werkingsmechanismen en praktische toepasbaarheid te onthullen.

Video: In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Scanning-probe Single-electron Capacitance Spectroscopy

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the electronic quantum structure of small systems - including individual atomic dopants in semiconductors. Here we present a capacitance-based method, known as Subsurface Charge Accumulation (SCA) imaging, which is capable of resolving single-electron charging while achieving sufficient spatial resolution to image individual atomic dopants. The use of a capacitance technique enables observation of subsurface features, such as dopants buried many nanometers beneath the surface of a semiconductor material1,2,3. In principle, this technique can be applied to any system to resolve electron motion below an insulating surface.
As in other electric-field-sensitive scanned-probe techniques4, the lateral spatial resolution of the measurement depends in part on the radius of curvature of the probe tip. Using tips with a small radius of curvature can enable spatial resolution of a few tens of nanometers. This fine spatial resolution allows investigations of small numbers (down to one) of subsurface dopants1,2. The charge resolution depends greatly on the sensitivity of the charge detection circuitry; using high electron mobility transistors (HEMT) in such circuits at cryogenic temperatures enables a sensitivity of approximately 0.01 electrons/Hz&frac12; at 0.3 K 5.

Scanning-probe single-electron capacitance spectroscopy facilitates the study of single-electron motion in localized subsurface regions. A sensitive charge-detection circuit is incorporated into a cryogenic scanning probe microscope to investigate small systems of dopant atoms beneath the surface of semiconductor samples.

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the ...

Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input bias voltage. However, the tradeoff for the improved switching performance is a relatively long settling time to reach each gap height in response to various applied voltages. Transient applied bias waveforms are employed to facilitate reduced switching times for electrostatic fringing-field MEMS actuators with high mechanical quality factors. Removing the underlying substrate of the fringing-field actuator creates the low mechanical damping environment necessary to effectively test the concept. The removal of the underlying substrate also a has substantial improvement on the reliability performance of the device in regards to failure due to stiction. Although DC-dynamic biasing is useful in improving settling time, the required slew rates for typical MEMS devices may place aggressive requirements on the charge pumps for fully-integrated on-chip designs. Additionally, there may be challenges integrating the substrate removal step into the back-end-of-line commercial CMOS processing steps. Experimental validation of fabricated actuators demonstrates an improvement of 50x in switching time when compared to conventional step biasing results. Compared to theoretical calculations, the experimental results are in good agreement.

The robust device design of fringing-field electrostatic MEMS actuators results in inherently low squeeze-film damping conditions and long settling times when performing switching operations using conventional step biasing. Real-time switching time improvement with DC-dynamic waveforms reduces the settling time of fringing-field MEMS actuators when transitioning between up-to-down and down-to-up states.

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input ...

In Situ Time-dependent Dielectric Breakdown in the Transmission Electron Microscope: A Possibility to Understand the Failure Mechanism in Microelectronic Devices

The time-dependent dielectric breakdown (TDDB) in on-chip interconnect stacks is one of the most critical failure mechanisms for microelectronic devices. The aggressive scaling of feature sizes, both on devices and interconnects, leads to serious challenges to ensure the required product reliability. Standard reliability tests and post-mortem failure analysis provide only limited information about the physics of failure mechanisms and degradation kinetics. Therefore it is necessary to develop new experimental approaches and procedures to study the TDDB failure mechanisms and degradation kinetics in particular. In this paper, an in situ experimental methodology in the transmission electron microscope (TEM) is demonstrated to investigate the TDDB degradation and failure mechanisms in Cu/ULK interconnect stacks. High quality imaging and chemical analysis are used to study the kinetic process. The in situ electrical test is integrated into the TEM to provide an elevated electrical field to the dielectrics. Electron tomography is utilized to characterize the directed Cu diffusion in the insulating dielectrics. This experimental procedure opens a possibility to study the failure mechanism in interconnect stacks of microelectronic products, and it could also be extended to other structures in active devices.

In Situ Time-dependent Dielectric Breakdown in the Transmission Electron Microscope: A Possibility to Understand the Failure Mechanism in Microelectronic Devices

The time-dependent dielectric breakdown (TDDB) in on-chip interconnect stacks is one of the most critical failure mechanisms for microelectronic devices. This paper demonstrates the procedure of an in situ TDDB experiment in the transmission electron microscope, which opens a possibility to study the failure mechanism in microelectronic products.

The time-dependent dielectric breakdown (TDDB) in on-chip interconnect stacks is one of the most critical failure mechanisms for microelectronic devices. ...

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities &#8211; high spatial resolution and richness of microstructural data &#8211; with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

The use of electron channeling contrast imaging in a scanning electron microscope to characterize defects in III-V/Si heteroexpitaxial thin films is described. This method yields similar results to plan-view transmission electron microscopy, but in significantly less time due to lack of required sample preparation.

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and forging. Adoption of this technique is proceeding despite disagreement concerning the underlying mechanism responsible for EAD. The experimental procedure described herein enables a more explicit study compared to previous EAD research by removing thermal effects, which are responsible for disagreement in interpreting previous EAD results. Furthermore, as the procedure described here enables EAD observation in situ and in real time in a transmission electron microscope (TEM), it is superior to existing post-mortem methods that observe EAD effects post-test. Test samples consist of a single crystal copper (SCC) foil having a free-standing tensile test section of nanoscale thickness, fabricated using a combination of laser and ion beam milling. The SCC is mounted to an etched silicon base that provides mechanical support and electrical isolation while serving as a heat sink. Using this geometry, even at high current density (~3,500 A/mm2), the test section experiences a negligible temperature increase (&#60;0.02 &#176;C), thus eliminating Joule heating effects. Monitoring material deformation and identifying the corresponding changes to microstructures, e.g. dislocations, are accomplished by acquiring and analyzing a series of TEM images. Our sample preparation and in situ experiment procedures are robust and versatile as they can be readily utilized to test materials with different microstructures, e.g., single and polycrystalline copper.

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

Experimental Methods for Trapping Ions Using Microfabricated Surface Ion Traps

Ions trapped in a quadrupole Paul trap have been considered one of the strong physical candidates to implement quantum information processing. This is due to their long coherence time and their capability to manipulate and detect individual quantum bits (qubits). In more recent years, microfabricated surface ion traps have received more attention for large-scale integrated qubit platforms. This paper presents a microfabrication methodology for ion traps using micro-electro-mechanical system (MEMS) technology, including the fabrication method for a 14 &#181;m-thick dielectric layer and metal overhang structures atop the dielectric layer. In addition, an experimental procedure for trapping ytterbium (Yb) ions of isotope 174 (174Yb+) using 369.5 nm, 399 nm, and 935 nm diode lasers is described. These methodologies and procedures involve many scientific and engineering disciplines, and this paper first presents the detailed experimental procedures. The methods discussed in this paper can easily be extended to the trapping of Yb ions of isotope 171 (171Yb+) and to the manipulation of qubits.

This paper presents a microfabrication methodology for surface ion traps, as well as a detailed experimental procedure for trapping ytterbium ions in a room-temperature environment.

Ions trapped in a quadrupole Paul trap have been considered one of the strong physical candidates to implement quantum information processing. This is due ...

Probing the Structure and Dynamics of Interfacial Water with Scanning Tunneling Microscopy and Spectroscopy

Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal structure and probing the hydrogen-bond (H-bond) dynamics of the water molecules adsorbed on solid surfaces are fundamental issues of water science, which remains a great challenge owing to the light mass and small size of hydrogen. Scanning tunneling microscopy (STM) is a promising tool for attacking these problems, thanks to its capabilities of sub-&#197;ngstr&#246;m spatial resolution, single-bond vibrational sensitivity, and atomic/molecular manipulation. The designed experimental system consists of a Cl-terminated tip and a sample fabricated by dosing water molecules in situ onto the Au(111)-supported NaCl(001) surfaces. The insulating NaCl films electronically decouple the water from the metal substrates, so the intrinsic frontier orbitals of water molecules are preserved. The Cl-tip facilitates the manipulation of the single water molecules, as well as gating the orbitals of water to the proximity of Fermi level (EF) via tip-water coupling. This paper outlines the detailed methods of submolecular resolution imaging, molecular/atomic manipulation, and single-bond vibrational spectroscopy of interfacial water. These studies open up a new route for investigating the H-bonded systems at the atomic scale.

Here, we present a protocol to investigate the structure and dynamics of interfacial water at the atomic scale, in terms of submolecular resolution imaging, molecular manipulation, and single-bond vibrational spectroscopy.

Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal ...

Chemistry

Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared on transparent membranes which are very fragile and difficult to manipulate. We present processes for the fabrication of samples with magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for Lorentz transmission electron microscopy and magnetic transmission x-ray microscopy studies. The samples are prepared on silicon nitride membranes and the fabrication consists of a spin coating, UV and electron-beam lithography, the chemical development of the resist, and the evaporation of the magnetic material followed by a lift-off process forming the final magnetic structures. The samples for the Lorentz transmission electron microscopy consist of magnetic nanodiscs prepared in a single lithography step. The samples for the magnetic x-ray transmission microscopy are used for time-resolved magnetization dynamic experiments, and magnetic nanodiscs are placed on a waveguide which is used for the generation of repeatable magnetic field pulses by passing an electric current through the waveguide. The waveguide is created in an extra lithography step.

A protocol for the fabrication of magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for transmission electron microscopy (TEM) and magnetic transmission x-ray microscopy (MTXM) studies is presented.

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared ...

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We demonstrate not only lateral control but, importantly, also control over the angular alignment of adjacent layers. The core of the technique is represented by a home-built transfer setup which allows the user to control the position of the individual crystals involved in the transfer. This is achieved with sub-micrometer (translational) and sub-degree (angular) precision. Prior to stacking them together, the isolated crystals are individually manipulated by custom-designed moving stages that are controlled by a programmed software interface. Moreover, since the entire transfer setup is computer controlled, the user can remotely create precise heterostructures without coming into direct contact with the transfer setup, labeling this technique as &#8220;hands-free&#8221;. In addition to presenting the transfer set-up, we also describe two techniques for preparing the crystals that are subsequently stacked.

In this work we describe a technique that is used to create new crystals (van der Waals heterostructures) by stacking ultrathin layered 2D materials with precise control over position and relative orientation.

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We ...

Sample Preparation and Experimental Design for In Situ Multi-Beam Transmission Electron Microscopy Irradiation Experiments

There is a need to understand materials exposed to overlapping extreme environments such as high temperature, radiation, or mechanical stress. When these stressors are combined there may be synergistic effects that enable unique microstructural evolution mechanisms to activate. Understanding of these mechanisms is necessary for the input and refinement of predictive models and critical for engineering of next generation materials. The basic physics and underlying mechanisms require advanced tools to be investigated. The in situ ion irradiation transmission electron microscope (I&#179;TEM) is designed to explore these principles.
To quantitatively probe the complex dynamic interactions in materials, careful preparation of samples and consideration of experimental design is required. Particular handling or preparation of samples can easily introduce damage or features that obfuscate the measurements. There is no one correct way to prepare a sample; however, many mistakes can be made. The most common errors and things to consider are highlighted within. The I&#179;TEM has many adjustable variables and a large potential experimental space, therefore it is best to design experiments with a specific scientific question or questions in mind.
Experiments have been performed on large number of sample geometries, material classes, and with many irradiation conditions. The following are a subset of examples that demonstrate unique in situ capabilities utilizing the I3TEM. Au nanoparticles prepared by drop casting have been used to investigate the effects of single ion strikes. Au thin films have been used in studies on the effects of multibeam irradiation on microstructure evolution. Zr films have been exposed to irradiation and mechanical tension to examine creep. Ag nanopillars were subjected to simultaneous high temperature, mechanical compression, and ion irradiation to study irradiation induced creep as well. These results impact fields including: structural materials, nuclear energy, energy storage, catalysis, and microelectronics in space environments.

Sample preparation techniques are outlined with specific considerations for in situ ion irradiation TEM experiments. Ion species, energy, and fluence are discussed with methods for how to select and compute them. Finally, procedures for conducting an experiment are described and accompanied by the representative results.

There is a need to understand materials exposed to overlapping extreme environments such as high temperature, radiation, or mechanical stress. When these ...

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VO_x

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