Nanomechanical Properties of Nanostructures and Scale Effects

Abstract

Structural integrity is of paramount importance in all devices. Load applied during the use of devices can result in component failure. Cracks can develop and propagate under tensile stresses, leading to failure. Knowledge of the mechanical properties of nanostructures is necessary for designing realistic micro/nanoelectromechanical systems (MEMS/NEMS) and biological micro/nanoelectromechanical systems (BioMEMS/BioNEMS) devices (Bhushan 2016). Elastic and inelastic properties are needed to predict deformation from an applied load in the elastic and inelastic regimes, respectively. The strength property is needed to predict the allowable operating limit. Some properties of interest are hardness, elastic modulus, bending strength, fracture toughness, and fatigue strength. Many of the mechanical properties are scale dependent. Therefore, these should be measured at relevant scales. Atomic force microscopy and nanoindenters can be used to evaluate the mechanical properties of micro/nanoscale structures. Commonly used materials in MEMS/NEMS are single-crystal silicon and silicon-based materials, e.g., SiO₂ and polysilicon films deposited by low-pressure chemical vapor deposition. Single-crystal SiC deposited on large-area silicon substrates is used for high-temperature micro/nanosensors and actuators. Amorphous alloys can be formed on both metal and silicon substrates by sputtering and plating techniques, providing more flexibility in surface-integration. Electroless deposited Ni-P amorphous thin films have been used to construct microdevices, especially using the so-called LIGA (Lithographie, Galvanoformung, Abformung) techniques. Micro/nanodevices need conductors to provide power, as well as electrical/magnetic signals to make them functional. Electroplated gold films have found wide application in electronic devices because of their ability to make thin films and process simply. Polymers, such as poly(methyl methacrylate) (PMMA), poly(dimethylsiloxane) (PDMS), and polystyrene are commonly used in BioMEMS/BioNEMS such as micro/nanofluidic devices because of the ease of manufacturing and reduced cost. Many polymers are biocompatible, so they may be integrated into biomedical devices. This chapter presents a review of mechanical property measurements on the micro/nanoscale of various materials of interest and stress and deformation analyses of nanostructures.

Appendix A: Fabrication Procedure for the Double Anchored and Cantilever Beams

The starting materials for the polymer microbeams are PPMA (molecular weight 250,000, Scientific Polymer Products), PMMA (molecular weight 75,000, Sigma-Aldrich), PS (melt flow index 4.0, Sigma-Aldrich) and PS/Clay solutions in anisole (Acros Organics). The clay additive in PS/Clay is Cloisite® 20A surface modified natural montmorillonite (Southern Clay Products, Inc.) with a thickness of approximately 1 nm and lateral dimensions of 70–150 nm. The surface modification of the clay additive improves the dispersion of the nanoparticles in the polymer matrix, thus improving the properties of the composite. To prepare the nanocomposite, the clay was first dispersed in the PS matrix by melt compounding at a concentration of 10% wt/wt (clay/PS). The composite was then dissolved in anisole and sonicated for at least 8 h to dissolve the polymer and redisperse the particles.

Double anchored polymer microbeams were fabricated using a soft lithography based micromolding process along with standard photolithography (Palacio et al. 2007a). The process involves selectively filling a poly(dimethyl siloxane) (PDMS) mold with the polymer of interest followed by transfer of the resulting structures to a prefabricated substrate. This substrate is a silicon wafer with a layer of SU-8 25 negative tone photoresist (MicroChem Corp.) patterned by photolithography to create 25 µm wide channels, which is the resulting length of the suspended beams. (The second number 25 in the SU-8 designation relates to a viscosity appropriate for a given thickness.)

A patterned PDMS mold with the desired polymer beam geometry was fabricated from a photoresist master. Briefly, a layer of SU-8 5 photoresist was spin coated on silicon, and photolithography was used to define 5 μm wide photoresist features separated by 45 μm gaps. A 10:1 ratio of T-2 PDMS translucent base and curing agent (Dow Corning) were mixed thoroughly and poured over the photoresist master to transfer the pattern into the PDMS. The mold was then placed in a vacuum dessicator to remove bubbles . The sample was removed from the vacuum periodically, and a razor blade was used to remove surface bubbles. After the bubbles were completely removed, the PDMS mold was allowed to cure at room temperature for 48 h before removing it from the wafer.

Next, the PDMS mold was selectively coated with the polymer solution to form the microbeams and then transferred to the substrate. Figure 6.32 is a schematic of the fabrication process used for making the polymer microbeams. As shown in Fig. 6.32a, the polymer solutions were spin coated on the PDMS mold for one minute. A 10% solution of polymer was spin coated on a mold with 5.3 μm deep features at 3000 RPM for fabrication of the beams in the bending experiments. After spin coating , the mold was brought into contact with a heated glass plate to promote adhesion of the contacting polymer materials. This process removed the polymer material from the raised surfaces of the mold, resulting in the polymer remaining only in the recessed portions of the PDMS mold. The glass plate was heated to 175 °C for all four materials. As shown in Fig. 6.32b, the selectively coated mold was then aligned with the photolithographically patterned silicon substrate so that the beams of interest (PPMA, PMMA, PS, PS/Clay) ran perpendicular to the channels defined in the photoresist. The substrate was then heated and pressure was applied to the top of the mold to transfer the material onto the substrate. The transfer temperature for PPMA, PS, PS/Clay and PMMA were 95, 125, 125, and 175 °C, respectively, and the transfer pressure for all materials was around 0.21 MPa.

Fig. 6.32

Schematic of the micromolding process used in polymer double anchored beam fabrication. a Selective patterning of PDMS mold (i) mold is spin coated with a polymer layer. (ii) mold is inverted and brought into contact with a heated glass plate (iii) mold is removed from the plate, transferring the surface polymer onto the glass (iv) mold is left with polymer only in the recessed features. b Stamping of the polymer beams (i) Selectively patterned mold (ii) mold is inverted and aligned with the photolithographically patterned substrate (iii) mold and substrate are brought into contact under heat and pressure (iv) polymer is transferred onto the photoresist and mold is removed (Palacio et al. 2007a)

After removal of the mold, two types of polymer beams were transferred onto the wafer sample. The first type is supported beams, which was used to determine the hardness, elastic modulus, and creep response. The second type is suspended beams, on which the bending experiments were performed. The double anchored beam samples studied by Palacio et al. (2007a) were 3–5 μm thick, 5 μm wide and nominally 25 μm long.

The process for fabrication of the polymer cantilever for lateral bending tests is shown in Fig. 6.33 (Palacio et al. 2007a). A PDMS mold was first cast from a photolithographically patterned SU-8/silicon master. The resulting molds consisted of 50 μm wide channels that were approximately 27 μm deep. The PDMS mold was then coated with the polymer of interest (PS or PS/Clay). The polymers were spin coated on the mold at 3000 RPM for one minute at concentrations of 15% and 10% for PS and PS/Clay, respectively. The polymer on the raised surface of the mold was removed by contacting the surface with a glass slide heated to 180 °C. The mold was then inverted and manually aligned with a silicon substrate coated with patches of polyvinyl alcohol (PVA), which acts as the sacrificial layer. The PVA was patterned using photolithography and reactive ion etching with an oxygen plasma. The substrate was heated and pressure was applied to the top side of the mold to transfer the polymer from the recessed features of the mold onto the substrate such that a 350 µm portion of it is attached to the PVA and the remainder is attached to the bare silicon surface. Transfer temperatures for PS and PS/Clay were 150 and 175°C, respectively. The process was repeated to apply another layer of beams across the length of the original layer. This was performed to provide reinforcement as it was observed that the one-layer design was not robust. The cantilever beam samples studied by Palacio et al. (2007a) were 12–27 µm thick, 60–80 µm wide and 350 µm long.

Fig. 6.33

Schematic of the polymer cantilever beam fabrication process (i) PDMS mold (ii) mold spin coated with the polymer of interest (iii) selectively filled mold after removal of surface polymer (iv) mold aligned with sacrificial layer (v) polymer features transfer to the substrate using heat and pressure (vi) mold aligned for a second time to add another layer of lines across original beams for reinforcement (vii) polymer features transfer to the substrate using heat and pressure (viii) cantilever structures after removal of the sacrificial layer (Palacio et al. 2007a)