William A. Osborn

Experimental determination of mode correction factors for thermal method spring constant calibration of AFM cantilevers using laser Doppler vibrometry.

Mode correction factors (MCFs) represent a significant adjustment to the spring constant values measured using the thermal cantilever calibration method. Usually, the ideal factor of 0.971 for a tipless rectangular cantilever is used, which adjusts the value by 3% for the first flexural mode. An experimental method for determining MCFs has been developed that relies on measuring the areas under the first few resonance peaks for the flexural mode type. Using this method, it has been shown that MCFs for the first flexural mode of commercially available atomic force microscope cantilevers actually vary from 0.95 to 1.0, depending on the shape and end mass of the cantilever. Triangular shaped cantilevers tend to lower MCFs with tipless versions providing the lowest values. Added masses (including tips) tend to increase the first flexural modes MCF to higher values with large colloid probes at the high extreme. Using this understanding and applying it to the recently developed laser Doppler vibrometry thermal calibration method it is now possible to achieve very accurate and precise cantilever spring constant calibrations (uncertainties close to ±1%) with commonly available commercial cantilevers such as tipped rectangular and triangular cantilevers, and colloid probes.

Designing a standard for strain mapping: HR-EBSD analysis of SiGe thin film structures on Si

Patterned SiGe thin film structures, heteroepitaxially deposited on Si substrates, are investigated as potential reference standards to establish the accuracy of high resolution electron backscattered diffraction (HR-EBSD) strain measurement methods. The proposed standards incorporate thin films of tetragonally distorted epitaxial Si₁-xGex adjacent to strain-free Si. Six films of three different nominal compositions (x=0.2, 0.3, and 0.4) and various thicknesses were studied. Film composition and out-of-plane lattice spacing measurements, by x-ray photoelectron spectroscopy and x-ray diffraction, respectively, provided independent determinations of film epitaxy and predictions of tetragonal strain for direct comparison with HR-EBSD strain measurements. Films assessed to be coherent with the substrate exhibited tetragonal strain values measured by HR-EBSD identical to those predicted from the composition and x-ray diffraction measurements, within experimental relative uncertainties of order 2%. Such films thus provide suitable prototypes for designing a strain reference standard.

Micro-scale measurement and modeling of stress in silicon surrounding a tungsten-filled through-silicon via

The stress in silicon surrounding a tungsten-filled through-silicon via (TSV) is measured using confocal Raman microscopy line scans across the TSV both before and after etch removal of an oxide stack used as a mask to define the TSV during fabrication. Stress in the silicon arose in response to both athermal deposition and thermal expansion mismatch effects. The complex three-dimensional stress and strain field in silicon surrounding the TSV is modeled using finite element analysis, taking into account both athermal and thermal effects and the elastic anisotropy of silicon. Comparison of the measurements and model results shows that no one component of the stress tensor correlates with the Raman peak shift generated by the deformed silicon. An analysis is developed to predict the Raman shift in deformed silicon that takes into account all the components of the stress or strain tensor; the results of the model are then used as inputs to the analysis for direct comparison with measured peak shifts as a fun...

Assessing strain mapping by electron backscatter diffraction and confocal Raman microscopy using wedge-indented Si

The accuracy of electron backscatter diffraction (EBSD) and confocal Raman microscopy (CRM) for small-scale strain mapping are assessed using the multi-axial strain field surrounding a wedge indentation in Si as a test vehicle. The strain field is modeled using finite element analysis (FEA) that is adapted to the near-indentation surface profile measured by atomic force microscopy (AFM). The assessment consists of (1) direct experimental comparisons of strain and deformation and (2) comparisons in which the modeled strain field is used as an intermediate step. Direct experimental methods (1) consist of comparisons of surface elevation and gradient measured by AFM and EBSD and of Raman shifts measured and predicted by CRM and EBSD, respectively. Comparisons that utilize the combined FEA-AFM model (2) consist of predictions of distortion, strain, and rotation for comparison with EBSD measurements and predictions of Raman shift for comparison with CRM measurements. For both EBSD and CRM, convolution of measurements in depth-varying strain fields is considered. The interconnected comparisons suggest that EBSD was able to provide an accurate assessment of the wedge indentation deformation field to within the precision of the measurements, approximately 2×10(-4) in strain. CRM was similarly precise, but was limited in accuracy to several times this value.

Surface mediated assembly of small, metastable gold nanoclusters

The unique properties of metallic nanoclusters are attractive for numerous commercial and industrial applications but are generally less stable than nanocrystals. Thus, developing methodologies for stabilizing nanoclusters and retaining their enhanced functionality is of great interest. We report the assembly of PPh3-protected Au9 clusters from a heterogeneous mixture into films consisting of sub 3 nm nanocluster assemblies. The depositing nanoclusters are metastable in solution, but the resulting nanocluster assemblies are stabilized indefinitely in air or fresh solvent. The films exhibit distinct structure from Au nanoparticles observed by X-ray diffraction, and film dissolution data support the preservation of small nanoclusters. UV-Vis spectroscopy, electrospray ionization mass spectrometry, X-ray photoelectron spectroscopy and electron microscopy are used to elucidate information regarding the nanocluster formation and assembly mechanism. Preferential deposition of nanocluster assemblies can be achieved on multiple substrates, including polymer, Cr, Si, SiO2, SiNx, and metal-organic frameworks (MOFs). Unlike other vapor phase coating processes, nanocluster assembly on the MIL-68(In) MOF crystal is capable of preferentially coating the external surface and stabilizing the crystal structure in hydrothermal conditions, which should enhance their storage, separation and delivery capabilities.

Accurate flexural spring constant calibration of colloid probe cantilevers using scanning laser Doppler vibrometry

Calibration of the flexural spring constant for atomic force microscope (AFM) colloid probe cantilevers provides significant challenges. The presence of a large attached spherical added mass complicates many of the more common calibration techniques such as reference cantilever, Sader, and added mass. Even the most promising option, AFM thermal calibration, can encounter difficulties during the optical lever sensitivity measurement due to strong adhesion and friction between the sphere and a surface. This may cause buckling of the end of the cantilever and hysteresis in the approach-retract curves resulting in increased uncertainty in the calibration. Most recently, a laser Doppler vibrometry thermal method has been used to accurately calibrate the normal spring constant of a wide variety of tipped and tipless commercial cantilevers. This paper describes a variant of the technique, scanning laser Doppler vibrometry, optimized for colloid probe cantilevers and capable of spring constant calibration uncertainties near ±1%.

Interactions of Adhesion Materials and Annealing Environment on Resistance and Stability of MEMS Platinum Heaters and Temperature Sensors

We evaluate the microstructural and electrical stability of Pt thin films with Ti or Ta as the adhesion layer after furnace annealing and rapid thermal annealing up to 750°C in three different environments. Test devices were made with 100 nm of Pt with a 10-nm adhesion layer. After annealing, the resistance anomalously increased for samples annealed in ultrahigh purity N2 (UHP, 99.999%), while the resistance decreased, as expected, for samples annealed in 99.95% N2 or air. The Ta/Pt film stack shows better microstructural and electrical stability compared with Ti/Pt. X-ray photoelectron spectroscopy (XPS) data indicate that diffusion of the Ti and Ta adhesion layers through the Pt film occurs in samples annealed in UHP N2, which is responsible for the remarkable increase of resistance. For samples annealed in air, the oxidation of Ti/Ta suppresses the diffusion process and expected grain growth occurs in the Pt, thus decreasing the resistance. Furthermore, XPS elemental mapping and atomic force microscope imaging shed light on void formation/dewetting seen under certain conditions.

Nanoscale specific heat capacity measurements using optoelectronic bilayer microcantilevers

We describe a technique for optically and electrically detecting and heating bilayer microcantilevers (BMCs) to high temperatures at fast heating rates for nanoscale specific heat capacity measurements. The platinum and silicon nitride (Pt−SiNx) BMCs act simultaneously as a heater, temperature sensor, and mass sensor (24 pg resolution). The calibration of the system was validated for a mass of 12.6 ng by melting point and specific heat capacity measurements of deposited aluminum nanoparticles.

Decoupling small-scale roughness and long-range features on deep reactive ion etched silicon surfaces

A methodology to decouple irregular small-scale roughness and regular long-range features on deep reactive ion etched (DRIE) silicon surfaces is presented. Height-height correlations of three different DRIE silicon surfaces are evaluated via atomic force microscopy height data and fit to an analytic, five-parameter model based on a phenomenological scaling function for the small-scale roughness and a Bessel function for the long-range features. The resulting roughness parameters are constant for all three surfaces at small lateral length scales, indicating self-affine roughness inherent to the DRIE process, but dependent on the etch process at large lateral length scales, increasing by a factor of five as the controlled portion of the DRIE process decreased. The results from the analysis are also compared to fracture strengths from recently introduced “theta” test samples with the same etch features as an example of the potential of the analysis in providing an unbiased assessment of the processing-struct...

In-situ elastic strain mapping during micromechanical testing using EBSD

Compared to more commonly used strain measurement techniques, electron backscatter diffraction (EBSD) offers improved spatial resolution and measurement sensitivity. Additionally, EBSD can provide the full deformation tensor, whereas other techniques, such as digital image correlation (DIC), are limited to only in-plane strains and rotations. In this work, EBSD was used to measure strains and rotations in-situ during testing of a single-crystal silicon micromechanical test specimen. The theta-like specimen geometry was chosen due to the complex and spatially-varying strain states that exist in the circular frame of the sample during testing, as well as the nominally uniform strains in the central web. Full-field strain maps were generated for each strain and rotation component and compared to those from finite element analyses (FEA), showing strong agreement in all cases. Additionally, potential sources of error and their impact on both measurement accuracy and uncertainty are discussed.