MECHANICAL CHARACTERIZATION OF THIN FILMS USING A MEMS DEVICE INSIDE SEM Changhong Cao, Brandon Chen,Tobin Filleter*, and Yu Sun* University of Toronto, Canada In addition, larger forcesare also required during tension to displace meso-scaled2D films(vs. displacing 1D nanostructures) due to larger cross-sectional areas. A single nanowire/nanotube typically has a diameter of several nanometers and tens of nanometers, while 2D films can easily cover hundreds of nanometers or a few microns. The significantly larger force requirement demands the actuator beams to be much stiffer than the ones for tensile testing of 1D nanomaterials. Based on these requirements, V-beam electrothermal actuators were chosen in this work.
ABSTRACT A MEMS device was developedfor mechanical characterization of 2D ultra-thin films.The device utilizes electrothermal actuators to apply uniaxial tension. The robust design makes the device capable of withstanding both dry and wet transfer of 2D ultra-thin film materials onto the suspended structures of the device. Fracture stress of thin graphene oxide (GO) films was measured.
INTRODUCTION
In order to monitor the evolution of failure of materials during tension, the MEMS device must be made SEM and/or TEM compatible. In this case, heat released from the electrothermal actuator needs to be well dissipated to minimize heat-induced drift of SEM images. High temperatures can also introduce thermal stress into the nanomaterial under test. Finally, the alignment of the two ends of the actuation shuttles where the edges of 2D films are anchored must be well controlled. A slight misalignment can cause failure of material transferand/or artifacts in measurement results. Device design should be symmetrical on the two sides of the sample under test, and hence, any unwanted stress from one side during fabrication can be counteracted to keep both actuation shuttles on the same plane.
Two-dimensionalthin films such as graphene and graphene oxide (GO)have been shown to possess outstanding mechanical behavior[1, 2], promising applications in composites, batteries, and electronics [3-5]. For mechanical characterization, conventional tensile stagesare used to test macroscopic films (micrometers thick and above), and nanoindentationwas previouslyused to characterize mechanical properties of monolayer films [6, 7]. These existing experimental techniques are not able to characterize mesoscale films (tens of nanometers thick) whichare important for applications in energy storage [8] and electronic devices [9]. Strength characterization of multilayer thin films by indentation requires anin-direct analysis including the knowledge of interfacial properties between layers; however, such mechanisms are not currently well understood. Indentation methods also have the limitation of probing a smalllocal film area. Moreover, 2D films are in general too small in size to be tested on conventional tensile testers due to both geometric as well as force resolution limitations. Therefore, a MEMS device for tensile testing nanometer thick thin film nanomaterials is required for strength characterization.
DEVICE DESIGN, FABRICATION, AND CALIBRATION The MEMS device consists of two symmetrical sets of V-beam electrothermal actuators, as shown in Fig. 1. The2µm gap between the two actuation shuttles enable SEM imaging in a confined region at high magnifications. Each electrothermal actuator contains eight pairs of V-beams (500µm long, 10µm wideand 10µm thick) and seven pairs of heat sink beams (100µm long, 10µm wideand 10µm thick). Two groups of heat sink beams were used to dissipate heat generated evenly and holdthe shuttle straight in the sample testing area. Features on the device were intentionally made wide and thick to tolerate the transferof different types of thin films.Finite element analysis was conducted to guide the device design.
Unlike MEMS testing of 1D nanomaterial (nanotubes and nanowires), which generally relies on nanomanipulation to place a sample onto the testing platform[10, 11], MEMS for mechanical characterization of 2D thin films requiredifferent transfer approaches. Take the transfer of graphene as an example. Graphene transfertechniquescan be classified into two major categories: dry hard contact transfer and multi-stage chemical wet etching transfer. Lee et al.[2] transferred graphene onto holey transmission electron microscopy (TEM) grids by hard pressing the graphene/PDMS onto the target substrate to form an intimate contact between graphene and the substrate. Suk et al.[12] introduced PMMA as a handle layer to protect graphene film during the transferring process, and then etched PMMA away when graphene is securely adhered to the substrate, during which the target substrate needs to be fully immersed in analcohol solution. In order to survive these transferring processes, the device must be robust.
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The device was fabricated using the MicralyneMicraGEM-Si fabrication process schematically described in Fig. 2. Two SOI wafers were DRIE etched separately and then bonded using conductive adhesives. After patterning and metalizing, devices were released from the main wafer. In order to achieve potential TEM compatibility, the top surface area of the whole device was intended to be maximized. Thus, when back etching is conducted to form a through window, stress exerted by its own weight can be minimized.
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be achieved, which is above thee strain required to load thin films to failure[2, 7]. At a givenapplied voltage, the displacement difference with and a without the thin film on the device is used to calculate applied a force to the thin film using the calculated stiffness of the actuator based on Hooke’s Law [13].
Figure 2(Top) SEM image of the MEMS device. (Bottom) Zoom-in image of actuation shuttles with a 2µm gap in between as red boxed in thee top image.
Figure 3 (Top) Calibrationn results. Displacement corresponds to the distance chhange between two edges of the actuation shuttles. (Bottom) m) SEM images showing the gap distance change as a fuunction of applied voltage. (Scale Bar 1µm)
MATERIAL PREPARA ATION Graphene oxide (GO) thiin film was prepared by a similar method we reported preeviously[1]but with a larger thickness. A water solution witth GO flakes , with a carbon to oxygen ration of 4 to 1 as measured m by XPS, was drop casted onto the center of the tw wo actuation shuttles, using a custom-built robotic micropippette system[14]. A water drop containing GO flakes was formed on top of the gap region. After air drying and bakking at 90°C, a thin GO film was suspended over the two siddes of the actuation shuttles. Film thickness for each sam mple tested was measured usingatomic force microscopyy (AFM) in tapping mode. Fig. 4(a) shows a representativee topography image of a GO filmsuspended on the MEMS device. d Fig. 4(b) shows the height profile of the film acrosss the red dash line labelled on Fig. 4(a).
Figure 1 Fabrication process for constructing the MEMS tensile tester. After fabrication, devices were calibbrated under SEM imaging before and after graphene trannsfer. Transfer of graphene was used as a process to verify whether the device was able to tolerate the transfeer steps. From the calibration results summarized in Fig. 3, it can be seen that the transfer processes did not cause signiificant influence to the performance of the device.The displaacement resolution is better than 2 nm. This allows fine contrrol of tensile strain, which is critical for testing thin films beecause of their low ductility. The calibration results also match well with multiphysics simulations shown as a pollynomial line fit in Fig. 3. When a voltage of 4V is appliedd, a 25% strain can
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As expected the meso-scaled thin GO films (tens of nanometer thick) were found to have a fracture stress lower than the strength previously measured for monolayer GO of 27.3GPa [1] due to the pre-existing crack, but interestingly, the fracture stress is similar to that of two layer CVD graphene of 2-9 GPa[15]. Our present work focuses on modeling to quantify the fracture behavior of these GO films.
CONCLUSION
Figure 4 (a) AFM tapping mode topography scan of suspended GO suspended on MEMS device. Red dashed line represents where height profile was taken (scale bar: 5µm). (b) Height profile corresponding to the red dash line in (a). Thickness of the GO film was measured to be 30nm.
This paper reported an electrothermal actuator-based MEMS device for 2D thin film mechanical characterization in situ SEM. The device is robust enough to tolerate dry and wet transfer of 2D thin films such as graphene and graphene oxide (GO). The MEMS device has a displacement resolution better than 2 nm, enabling fine control of tensile strain. The measured GO films had thickness of ~30 nm, and the measured fracture stress was 7-8 GPa.
RESULTS Fig. 5(a) shows a suspended GO film with a pre-existing crack(less than 10% of the sample length)that was tested under uniaxial tension. When actuation voltages were increased (0.5V step size), the GO film was stretched until brittle failure suddenly occurred. The crack initiated at the pre-cracked tip and subsequently propagated across the entire length of the sample. Zhang et al.[15]found similar behavior for two layer graphene films. From Fig. 5(b), tensile stress was found to increase with respect to the increase of applied voltage, and drop significantly when the crack started propagating.Stress was calculated by dividing applied force by the cross section area of the sample measured via AFM (thickness) and SEM (width).
ACKNOWLEDGEMENTS Authors would like to acknowledge Canada Foundation of Innovation (CFI) and Natural Sciences and Engineering Research Council of Canada (NSERC) for funding this project and CMC Microsystems for fabrication assistance.
CONTACT *Tobin Filleter, Tel: +1416-978-5877;
[email protected]; *Yu Sun, Tel: +1416-946-0549;
[email protected];
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Figure 5 (a) SEM images showing crack propagation in a GO thin film during tensile testing. The arrow indicates pre-existing crack. Scale bar: 1µm. (b) Stress versus applied voltage data. Blue, red and yellow correspond to images with same color in (a).
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