Roy H. Geiss

Nanoscale elastic-property measurements and mapping using atomic force acoustic microscopy methods

We describe a dynamic atomic force microscopy (AFM) method for measuring the elastic properties of surfaces, thin films and nanostructures at the nanoscale. Our approach is based on atomic force acoustic microscopy (AFAM) techniques and involves the resonant modes of the AFM cantilever in contact mode. From the frequencies of the resonant modes, the tip–sample contact stiffness k* can be calculated. Values for elastic properties such as the indentation modulus M can be determined from k* with appropriate contact-mechanics models. We present the basic principles of AFAM and explain how it can be used to measure local elastic properties with a lateral spatial resolution of tens of nanometres. Quantitative results for a variety of films as thin as 50 nm are given to illustrate our methods. Studies related to measurement accuracy involving the effects of film thickness and tip wear are also described. Finally, we discuss the design and use of electronics to track the contact-resonance frequency. This extension of AFAM fixed-position methods will enable rapid quantitative imaging of nanoscale elastic properties.

Elastic-property measurements of ultrathin films using atomic force acoustic microscopy

Atomic force acoustic microscopy (AFAM), an emerging technique that affords nanoscale lateral and depth resolution, was employed to measure the elastic properties of ultrathin films. We measured the indentation modulus M of three nickel films approximately 50, 200, and 800 nm thick. In contrast to conventional methods such as nanoindentation, the AFAM results remained free of any influence of the silicon substrate, even for the 50 nm film. X-ray diffraction and scanning electron microscopy results indicated that the films were nanocrystalline with a strong preferred (111) orientation. Values of M ranged from 210 to 223 GPa, lower than expected from values for bulk nickel. The reduced values of the elastic modulus may be attributed to grain-size effects in the nanocrystalline films.

Resistivity dominated by surface scattering in sub-50 nm Cu wires

Electron scattering mechanisms in copper lines were investigated to understand the extendibility of copper interconnects when linewidth or thickness is less than the mean free path. Electron-beam lithography and a dual hard mask were used to produce interconnects with linewidths between 25 and 45 nm. Electron backscatter diffraction characterized grain structure. Temperature dependence of the line resistance determined resistivity, which was consistent with existing models for completely diffused surface scattering and line-edge roughness, with little contribution from grain boundary scattering. A simple analytical model was developed that describes resistivity from diffuse surface scattering and line-edge roughness.

Continuous Measurement of Atomic Force Microscope Tip Wear by Contact Resonance Force Microscopy

devices, [ 3 ] and manufacturing. [ 4 ] In these applications, tip size and shape critically affect the accuracy, resolution, and reliability of measurements and processes. [ 5 ] However, during tip–sample contact the tip can wear and break, undermining the utility of the instrument. [ 6 ] Thus, the development of wear-resistant probes, protocols for their testing and a fundamental understanding of their wear process is of vital importance. Although wear-resistant probes continue to advance, [ 7 , 8 ] tribological test methods and the collection of data for theoretical models still require measurements ex situ to the scanning process. Scanning electron microscopy (SEM) [ 8–12 ] and blind reconstruction while scanning on highaspect-ratio reference samples [ 8 , 12 , 13 ] have both been used to correlate scanning history with tip geometry changes. Such ex-situ approaches are slow and can cause additional wear, fracture, and contamination. More recently, periodic force– displacement adhesion measurements have provided a less disruptive means of monitoring changes in contact area after scanning a fi nite distance. [ 8 , 11 , 12 ] Still, adhesion measurements require interruption of the scan, and quantitative determination of a contact radius can be strongly affected by geometry, contamination, and environmental conditions. Here, we demonstrate how contact resonance force microscopy (CR-FM) methods enable quantitative in-situ evaluation of tip wear by measurement of instantaneous changes in contact radius while scanning Si cantilevers on a Si substrate. It is found that CR-FM measurements do not adversely affect the wear process, and the results compare favorably with ex-situ techniques. Overall, CR-FM is shown to be an effective tool for detecting subnanometer changes in the contact radius while also revealing novel information about tip symmetry and wear rate. Contact resonance force microscopy experiments and analysis are well described in the literature. [ 14 ] Briefl y, an AFM tip is brought into contact with a sample, and the sample or cantilever is vibrated out of plane over a frequency range that excites a fl exural resonance. Due to tip–sample interactions, the contact resonance of a given eigenmode

Anisotropic elastic properties of nanocrystalline nickel thin films

The elastic properties of a nickel film approximately 800 nm thick were measured with nanoindentation, microtensile testing, atomic force acoustic microscopy (AFAM), and surface acoustic wave (SAW) spectroscopy. Values for the indentation modulus (220–223 GPa) and Young’s modulus (177–204 GPa) were lower than predicted for randomly oriented polycrystalline nickel. The observed behavior was attributed to grain-boundary effects in the nanocrystalline film. In addition, the different measurement results were not self-consistent when interpreted assuming elastic isotropy. Agreement was improved by adopting a transversely isotropic model corresponding to the film’s 〈111〉 preferred orientation and reducing the elastic moduli by 10–15%. The SAW spectroscopy results indicated that the film density was 1–2% lower than expected for bulk nickel, consistent with models for nanocrystalline materials. Similar reductions in modulus and density were observed for two additional films approximately 200 and 50 nm thick using AFAM and SAW spectroscopy. These results illustrate how complementary methods can provide a more complete picture of film properties.

Imaging of the grain-to-grain epitaxy in NiFe/FeMn thin-film couples

It is known that a metastable crystalline thin‐film structure can often be stabilized by choosing a suitable substrate. For single‐crystal thin films, this pseudomorphic growth has been attributed to the stabilization of the metastable phase by epitaxial growth. However, in the polycrystalline structure, the mechanism is less clear. In this study, we have observed through plane‐view and cross‐section transmission electron microscopy (TEM) that the metastable fcc FeMn phase was stabilized by the underlaying NiFe layer. The continuous moire fringes across the FeMn/NiFe interface as revealed by the cross‐section TEM image indicate the existence of the epitaxial relationship between FeMn and NiFe. The epitaxial strain involved has also been found to be confined locally within each grain and was accommodated by the coherent strain across the interface. Consistent with the epitaxial strain observation, it is also found that this metastable fcc FeMn phase exists at the interface of NiFe and FeMn, and away from this interface, the stable A12 cubic FeMn phase is formed. Microdiffraction study showed that the epitaxially grown FeMn lattice was rotated through a small angle relative to the NiFe lattice. This rotation suggests that, in addition to the conventional lattice‐dilation strain‐epitaxy mechanism observed in single crystals with small misfit, there may be a rotation‐epitaxy mechanism in the polycrystalline thin‐film epitaxial structure.It is known that a metastable crystalline thin‐film structure can often be stabilized by choosing a suitable substrate. For single‐crystal thin films, this pseudomorphic growth has been attributed to the stabilization of the metastable phase by epitaxial growth. However, in the polycrystalline structure, the mechanism is less clear. In this study, we have observed through plane‐view and cross‐section transmission electron microscopy (TEM) that the metastable fcc FeMn phase was stabilized by the underlaying NiFe layer. The continuous moire fringes across the FeMn/NiFe interface as revealed by the cross‐section TEM image indicate the existence of the epitaxial relationship between FeMn and NiFe. The epitaxial strain involved has also been found to be confined locally within each grain and was accommodated by the coherent strain across the interface. Consistent with the epitaxial strain observation, it is also found that this metastable fcc FeMn phase exists at the interface of NiFe and FeMn, and away from t...

Transmission Electron Diffraction From Nanoparticles, Nanowires and Thin Films in an SEM With Conventional EBSD Equipment

We describe a new scanning electron microscope (SEM) method for obtaining and analyzing the crystallographic structure and orientation of nanoparticles and ultrathin films with conventional electron backscatter diffraction (EBSD) equipment. Thinned electron-transparent samples and nanoparticles or nanowires suspended on transparent carbon film substrates were positioned immediately below the pole piece of the SEM by use of a custom made holder. This allowed electron diffraction patterns in transmission from nanodimensioned samples to be captured in an SEM by a conventional EBSD camera obviating the need for a transmission electron microscope (TEM). The resulting diffraction patterns were displayed with EBSD software on the computer screen. In this fashion, we obtained diffraction patterns from nanoparticles as small as 20 nm in diameter and from nanowires with widths as small as 30 nm. Patterns were also obtained from free-standing thin film metal samples sandwiched in copper grids and from thin metal samples deposited on silicon nitride window substrates. In these samples, orientation mapping was facilitated by scanning the electron beam in the same manner that is used in reflection EBSD. By detecting the spatial variation in high-angle transmitted-electron scattering from a nanoparticle or thin free-standing film, we obtained improvements in pattern contrast, signal to noise ratio, and lateral spatial resolution compared to the same factors observed with conventional electron backscatter diffraction (EBSD). The correlation between transmitted energy spread and image contrast can be qualitatively explained with the Monte Carlo simulations shown in figure 1. The improvements are thought to be due in part to a strongly reduced contribution from diffuse scattering. The self-energyfiltering capacity of electron transmission through nanoscale specimens can result in usable SEM-based diffraction patterns from individual grains having lateral dimension as small as approximately 10 nm. In figure 2, we show an example where good patterns were obtained in transmission from a 40 nm thick nickel film deposited on a silicon nitride substrate. We were not able to obtain well-defined EBSD patterns from this sample with the conventional EBSD reflection mode. In figure 3, we give examples of transmission electron diffraction patterns from iron cobalt nanoparticles (diameter of 10 nm), in figure 4, from Al2O3 nanoparticles (diameter 20 nm), and in figure 5, from GaN nanowires with 30 nm to 50 nm wide hexagonal cross-sections. In figure 6, we show an orientation map from transmission electron diffraction patterns from an aluminum film, 150 nm thick. Finally, we have observed that for diffraction patterns formed by transmitted electrons, the quality of the specimen surface has a much less significant effect on pattern contrast than it does for EBSD patterns formed by reflected electrons.

Transmission EBSD in the Scanning Electron Microscope

We demonstrate in this article an exciting new method for obtaining electron Kikuchi diffraction patterns in transmission from thin specimens in a scanning electron microscope (SEM) fitted with a conventional electron backscattered diffraction (EBSD) detector. We have labeled the method transmission EBSD (t-EBSD) because it uses off-the-shelf commercial EBSD equipment to capture the diffraction patterns and also to differentiate it from transmission Kikuchi diffraction available in the transmission electron microscope (TEM). Lateral spatial resolution of less than 10 nm has been demonstrated for particles and better than 5 nm for orientation mapping of thin films. The only new requirement is a specimen holder that allows the transmitted electrons diffracted from an electron transparent sample to intersect the EBSD detector. We briefly outline our development of the technique, followed by descriptions of sample preparation techniques and operating conditions. We then present examples of t-EBSD patterns from a variety of specimens, including particles of diameter

Elastic constants and dimensions of imprinted polymeric nanolines determined from Brillouin light scattering

Elastic constants and cross-sectional dimensions of imprinted nanolines of poly(methyl methacrylate) (PMMA) on silicon substrates are determined nondestructively from finite-element inversion analysis of dispersion curves of hypersonic acoustic modes of these nanolines measured with Brillouin light scattering. The results for the cross-sectional dimensions, under the simplifying assumption of vertical sides and a semicircular top, are found to be consistent with dimensions determined from critical-dimension small-angle x-ray scattering measurements. The elastic constants C(11) and C(44) are found to be, respectively, 11.6% and 3.1% lower than their corresponding values for bulk PMMA. This result is consistent with the dimensional dependence of the quasi-static Youngs modulus determined from buckling measurements on PMMA films with lower molecular weights. This study provides the first evidence of size-dependent effects on hypersonic elastic properties of polymers.

An Electrical Method for Measuring Fatigue and Tensile Properties of Thin Films on Substrates

A novel approach for measuring thermal fatigue lifetime and ultimate strength of patterned thin films on substrates is presented. The method is based on controlled application of cyclic joule heating by means of low-frequency, high-density alternating current. Such conditions preclude electromigration, but cause cyclic strains due to mismatch in coefficients of thermal expansion between film and substrate. Strain and stress are determined from measurement of temperature. Fatigue properties are a natural fit to testing by alternating current. Stress-lifetime (S-N) data were obtained from patterned aluminum lines, where stress amplitude was varied by changing current density, and lifetimes were defined by open circuit failure. Electron microscopy and electron backscatter diffraction observations of damage induced by a.c. testing suggested that deformation took place by dislocation mechanisms. We also observed rapid growth of grains – the mean diameter increased by more than 70 % after a cycling time of less than six minutes – which we attribute to strain-induced boundary migration. Ultimate strength was determined by extrapolating a modified Basquin relation for high cycle data to a single load reversal. A strength estimate of 250 ± 40 MPa was determined based on a.c. thermal fatigue data. In principle, an electrical approach allows for testing of patterned films of any dimension, provided electrical access is available. Furthermore, structures buried beneath other layers of materials can be tested.