TWO-LAYER PLATE MECHANICAL METAMATERIALS

We present the fabrication and characterization of plates made from two ultrathin layers and capable of withstanding repeated extreme mechanical deformation. The ultrathin layers, one flat and the other corrugated, are less than 100 nm thick and created using atomic layer deposition, forming a joined structure similar to conventional honeycomb sandwich plates. The structures exhibit a high bending stiffness while still possessing a low area density of approximately 0.5 g/m2. Finite element simulations show that the bending stiffness initially increases quadratically with the height of the plate, but saturates at very large heights. Measurements of bending stiffness using an atomic force microscope agree qualitatively with the simulations. INTRODUCTION Cellular solids offer low weight and unique mechanical and thermal properties, which explains their longstanding use for a variety of structural applications in the transportation and construction industries [1]. In the last decade, a novel class of cellular solids known as mechanical metamaterials has been introduced [2-3]. These solids have carefully designed and tightly controlled periodic 3D geometries, usually at the microor nanoscale, which lead to unique combinations of mechanical properties, allowing them to occupy new areas on the Ashby charts [4-6]. Recently, we introduced the concept of plate mechanical metamaterials [7]—cellular plates with carefully controlled periodic geometry and unique mechanical properties—as well as its initial realization in the form of freestanding corrugated plates made out of an ultrathin film. In particular, we used atomic layer deposition (ALD) and microfabrication techniques to make robust plates out of a single continuous ALD layer with cm-scale lateral dimensions and a thickness as low as 25 nm, creating the thinnest freestanding plates that can be picked up by hand [7]. Our continuous plates differ from “bulk mechanical metamaterials” reported by other groups (e.g., Refs. [2-6]), which form a lattice that is easily penetrated by air and are typically made using much slower fabrication techniques, such as nanoscale 3D printing [4-5] or evaporation-driven self-assembly [6]. At the macro scale, many types of periodic cellular plates are used. In particular, honeycomb lattices and sandwich structures, which consist of two face sheets attached to a periodic core, have become ubiquitous in construction, aerospace, scientific instrumentation (e.g., optical tables), and other industries that require lightweight rigid plates [8,9]. Sandwich structures possess a high bending stiffness and very low areal density (mass per unit area). However, macroscale sandwich plates generally cannot sustain sharp bending deformations without permanent damage [9]. Here, we report fabrication and characterization of fully suspended plate metamaterials made from two layers of nanoscale thickness, whose geometry and properties are reminiscent of honeycomb sandwich plates (Fig. 1). The two layers are offset from each other but at the same time are connected using hexagonal vertical walls, which prevent shear of the two layers with respect to one another. As a result, the two-layer plates offer much higher bending stiffness than the single-layer structures we reported earlier [7], while still possessing extremely low weight and mechanical robustness. The increase in the bending stiffness is expected, and its mechanism is similar to that used in conventional honeycomb sandwich plates. However, in contrast to sandwich composite plates, our nanoscale two-layer mechanical metamaterials can sustain extremely large deformations without fracture, fully recovering their original shape and not displaying any signs of internal damage. Figure 1: (a) SEM image of a 1-mm-long and 0.5-mm-wide cantilevered plate made from a two-layer mechanical metamaterial. Because the freestanding ALD layers are highly transparent in SEM, they are colored for clarity. (b) Schematic illustrating the periodic geometry of the two-layer plate. The bottom layer is planar and continuous while the top layer is corrugated and includes etch holes for release. (c) SEM showing the detail of the cantilever edge. The layers are approximately 60-nm-thick and are colored differently for clarity. The plate height, i.e., the spacing between the two layers, is nominally 2 microns, but in practice varies slightly in a fully released structure. FABRICATION The plates were fabricated from ALD aluminum oxide (alumina) as illustrated in Fig. 2. First, we deposited a 60-nm-thick planar layer of aluminum oxide (Al2O3) on a cleaned double-side polished silicon wafer. The deposition was performed at 250 oC using water and trimethylaluminum precursors in the ALD tool. Next, a sacrificial layer of amorphous silicon (a-Si) was deposited at 175°C on the front side using plasma-enhanced chemical vapor deposition (PECVD). The thickness of the a-Si layer, which determines the height of the finished plate and the spacing between the ALD film layers, varied between 1 and 3 microns in different fabrication runs. The a-Si layer was then patterned using photolithography and anisotropic reactive ion etching (RIE) to obtain the hexagonal honeycomb pattern shown in Fig. 1b. The hexagonal honeycomb geometry was chosen because it exhibits approximately isotropic bending stiffness [1,7]. Subsequently, another 60-nm-layer of ALD alumina was deposited to form the corrugated layer. The second ALD layer was then patterned using photolithography and inductively coupled plasma (ICP) RIE to define the width and length of the cantilever as well as to open the etch holes for removing the a-Si sacrificial layer later on. Next, an approximately 500-nm-thick layer of silicon nitride was deposited 200 μm a)