Ioannis Chasiotis

A new microtensile tester for the study of MEMS materials with the aid of atomic force microscopy

An apparatus has been designed and implemented to measure the elastic tensile properties (Youngs modulus and tensile strength) of surface micromachined polysilicon specimens. The tensile specimens are “dog-bone” shaped ending in a large “paddle” for convenient electrostatic or, in the improved apparatus, ultraviolet (UV) light curable adhesive gripping deposited with electrostatically controlled manipulation. The typical test section of the specimens is 400 μm long with 2 μm×50 μm cross section. The new device supports a nanomechanics method developed in our laboratory to acquire surface topologies of deforming specimens by means of Atomic Force Microscopy (AFM) to determine (fields of) strains via Digital Image Correlation (DIC). With this tool, high strength or non-linearly behaving materials can be tested under different environmental conditions by measuring the strains directly on the surface of the film with nanometer resolution.

The mechanical strength of polysilicon films : Part 2. Size effects associated with elliptical and circular perforations

Abstract A systematic study of failure initiation in small-scale specimens has been performed to assess the effect of size scale on “failure properties” by drawing on the classical analysis of elliptically perforated specimens. Limitations imposed by photolithography restricted the minimum radii of curvature of the specimen perforations to one micron. By varying the radius of curvature and the size of the ellipses, the effects of domain size and stress concentration amplitude could be assessed separately to the point where the size of individual grains (∼0.3 μm ) becomes important. The measurements demonstrate a strong influence of the domain size under elevated stress on the “failure strength” of MEMS scale specimens, while the amplitude, or the variation, of the stress concentration factor is less significant. In agreement with probabilistic considerations of failure, the “local failure strength” at the root of a notch clearly increases as the radius of curvature becomes smaller. Accordingly, the statistical scatter also increases with decreasing size of the (super)stressed domain. When the notch radius becomes as small as 1 μm the failure stress increases on average by a factor of two relative to the tension values derived from unnotched specimens. This effect becomes moderate for larger radii of curvature, up to a radius of 8 μm (25 times the grain size), for which the failure stress at the notch tip closely approaches the value of the tensile strength for un-notched tensile configurations. We deduce that standard tests, performed on micron-sized, non-perforated, tension specimens, provide conservative strength values for design purposes. In addition, a Weibull analysis shows for surface-micromachined specimens a dependence of the strength on the specimen length, rather than the surface area or volume, which implies that the sidewall geometry, dimensions and surface conditions can dominate the failure process.

Mechanical measurements at the micron and nanometer scales

Abstract Experimentation at the micron level requires specific tools and methods. It will be illustrated how some of these tools have to be combined to achieve this goal. Because the determination of strains at the micron and nanoscales has been explored with the aid of probe microscopy, attention needs to be devoted to the limitations of digital image correlation. In this context it is illustrated how the results of the correlation method are affected by several process parameters, such as subset size, out-of-plane deformation, displacement gradients and scanning noise introduced in measurements. We also present measurements of material fracture on small, elliptically perforated (MEMS) specimens under tension via specially constructed equipment. Of particular interest is how the failure strength of polycrystalline silicon (grain size ∼0.3 μm) is influenced by the magnitude of the notch radius (1–8 μm) and the stress concentration factor (3–10). It is demonstrated that when the notch radius falls below 3 μm, the strength of the material is no longer governed by the critical stress criterion that controls failure initiation for larger radii (e.g. 8 μm or larger). In fact, the stress gradient plays a significant role in the failure process, which is explained in terms of the statistical spatial distribution of small flaws or cracks and the size of the zone at the notch tip under high stress. Failure stresses increase by a factor of two or more at a characteristic size of 1 μm.

Novel method for mechanical characterization of polymeric nanofibers

A novel method to perform nanoscale mechanical characterization of highly deformable nanofibers has been developed. A microelectromechanical system (MEMS) test platform with an on-chip leaf-spring load cell that was tuned with the aid of a focused ion beam was built for fiber gripping and force measurement and it was actuated with an external piezoelectric transducer. Submicron scale tensile tests were performed in ambient conditions under an optical microscope. Engineering stresses and strains were obtained directly from images of the MEMS platform, by extracting the relative rigid body displacements of the device components by digital image correlation. The accuracy in determining displacements by this optical method was shown to be better than 50 nm. In the application of this method, the mechanical behavior of electrospun polyacrylonitrite nanofibers with diameters ranging from 300 to 600 nm was investigated. The stress-strain curves demonstrated an apparent elastic-perfectly plastic behavior with elastic modulus of 7.6+/-1.5 GPa and large irreversible strains that exceeded 220%. The large fiber stretch ratios were the result of a cascade of periodic necks that formed during cold drawing of the nanofibers.

Young's modulus, Poisson's ratio and failure properties of tetrahedral amorphous diamond-like carbon for MEMS devices

The elastic and failure mechanical properties of hydrogen-free tetrahedral amorphous carbon (ta-C) MEMS structures were investigated via in situ direct and local displacement measurements by a method that integrates atomic force microscopy (AFM) with digital image correlation (DIC). On-chip MEMS-scale specimens were tested via a custom-designed apparatus that was integrated with an AFM to conduct in situ uniaxial tension tests. Specimens 10 µm and 50 µm wide and of 1.5 µm average thickness were used to measure the elastic properties while 340 µm wide tension specimens with a central elliptical perforation resulting in a stress concentration factor of 27 were tested to investigate local effects on material strength. The Youngs modulus, Poissons ratio and tensile strength were measured as 759 ± 22 GPa, 0.17 ± 0.03 and 7.3 ± 1.2 GPa, respectively. In an effort to understand the effect of local defects and assess the true material strength, the local failure stress at sharp central elliptical notches with a stress concentration factor of 27 was measured to be 11.4 ± 0.8 GPa. The AFM/DIC method provided for the first time local displacement fields in the vicinity of microscale perforations and these displacement fields were in accordance with those predicted by linear elasticity.

Mechanical deformation and failure of electrospun polyacrylonitrile nanofibers as a function of strain rate

The mechanical deformation of 12μm long electrospun polyacrylonitrile (PAN) nanofibers with diameters of 300–600nm was investigated. The nanofibers were subjected to cold drawing in atmospheric conditions and at strain rates between 10−2 and 10−4s−1. The ultimate strain of the PAN nanofibers was 60%–130% varying monotonically with the strain rate. On the contrary, the fiber tensile strength, ranging between 30 and 130MPa, varied nonmonotonically with the slowest drawing rate resulting in the largest ductilities and fiber strengths. At the two faster rates, the large fiber ductilities originated in the formation of a cascade of ripples (necks), while at the slowest strain rate, the nanofibers deformed homogeneously allowing for the largest engineering strengths and extension ratios.

Anodic oxidation during MEMS processing of silicon and polysilicon: native oxides can be thicker than you think

The thickness and surface roughness of the native oxide on undoped and P-doped single crystal silicon and polycrystalline silicon (polysilicon) were measured after exposure to aqueous hydrofluoric acid (HF) in the presence of localized metallization of sputtered Au or Pd. Both P-doping and the presence of metallization led to an increase in the thickness of the native surface oxide and an increased surface roughness after HF exposure. An external positive (negative) potential during HF immersion increased (decreased) the rate of what is clearly electrochemical i.e., anodic corrosion. The presence of the sputtered metallization promoted anodic corrosion, particularly in HF and particularly for P-doped silicon. Porous silicon can be formed under these conditions, due to dissolution of the anodically produced surface oxide. Subsequent oxidation of the porous silicon can lead to thick surface oxide layers.

Mechanics of thin films and microdevices

This paper discusses the latest developments in nanomechanics of thin films with applications in microelectromechanical systems (MEMS) and microelectronics. A precise methodology that combines in situ atomic force microscopy (AFM) surface measurements of uniaxially tension-loaded MEMS specimens and strain analysis via digital image correlation (DIC) achieving 0.1 pixel spatial displacement resolution is presented. By this method, the mechanical deformation of thin films was obtained in areas as small as 4 /spl times/ 4 /spl mu/m and with 1-2 nm spatial displacement resolution supporting the derivation of interrelations between the material microstructure and the local mechanical properties. This methodology provided for the first time the values of Youngs modulus and Poissons ratio from specimens with cross-sections as small as 2 /spl times/ 6 /spl mu/m. The value of properties derived via AFM/DIC demonstrated very limited scatter compared to indirect mechanical property measurement methods. The application of this technique on nonuniform geometries resolved nanoscale displacement and strain fields in the vicinity of ultrasharp elliptical perforations achieving very good agreement with finite element models. Furthermore, the stochastic and deterministic material failure properties described via Weibull statistics and fracture toughness, respectively, are illustrated for brittle thin films. Failure initiated at notches was found to be influenced by the local radius of curvature and the stress concentration factor. Precise fracture toughness values for MEMS materials were obtained from MEMS specimens with atomically sharp cracks. These studies were supported by measurements of displacements/strains conducted for the first time in the vicinity of mathematically sharp cracks via the AFM/DIC method. The method can be applied to a variety of thermomechanical reliability problems in multilayered thin films and inhomogeneous/anisotropic materials.

Realization of low-stress Au cantilever beams

A significant challenge in the fabrication of thin-film (<1 µm) Au MEMS devices is maintaining planarity after removal of the sacrificial layer. Out-of-plane deformations are driven by residual stress gradients in the Au films. It was found that the baking time and temperature of the sacrificial photoresist layer, as well as the thermal history once the Au was deposited, combined to determine the stress gradient within the Au film. In this technical note we provide the complete details of the optimized procedures to fabricate planar thin-film Au devices with unattached ends that are characterized by low residual stresses.

MEMS platform for on-chip nanomechanical experiments with strong and highly ductile nanofibers

A new nanoscale tension testing platform with on-chip actuation and the unique capability for nanoscale mechanical characterization of highly deformable and strong nanostructures is presented. The specimen force and extension measurements are based on optical imaging, supported by digital image correlation, which allows the resolution of 20 nm specimen extensions and force measurements better than 30 nN, without the use of high-resolution electron microscopy. The breakthrough of this nanomechanical testing platform is the ability to study the mechanical behavior of nanostructures subjected to a wide range of forces (30 nN–300 µN) and displacements (20 nm–100 µm), which are significantly beyond the limits of typical surface micromachined MEMS with on-chip actuators, such as comb-drives and thermal actuators. The utility of this device in experimental nanomechanics is demonstrated by investigating the mechanical behavior of electrospun polyacrylonitrile nanofibers with diameters of 200–700 nm subjected to strains as high as 200%. The mechanical property measurements were compared to and agreed well with off-chip measurements by an independent testing method, which validates the capability of this on-chip testing platform to characterize strong and highly ductile nanomaterials.