Thomas Edward Buchheit

On the measurement of the plasticity length scale parameter in LIGA nickel foils

This paper presents measurements of the plasticity length scale for polycrystalline LIGA nickel foils produced by electroplating in a sulfamate bath. The micro-bend test method developed by Stolken and Evans [Acta Mater. 46 (1998) 5109] is used to obtain a composite length scale parameter, lc that is primarily associated with rotational gradients. The micro-bend test utilizes the measurement of the curvature change associated with elastic spring-back of bent micro-beams in the extraction of a composite length scale parameter, lc. The length scale is measured to be ∼5.6 μm for foils between 25 and 175 μm thick. This is in the range (3–5 μm) reported from prior micro-bend experiments on nickel foils (loc. cit.). The measured length scale is also similar to values reported previously for polycrystalline copper wires using torsion tests in which rotation gradients dominate.

Application of generalized non-Schmid yield law to low-temperature plasticity in bcc transition metals

In this work, a generalized yield criterion that captures non-Schmid effects is proposed and implemented into a finite element crystal plasticity model to simulate plastic deformation of single and polycrystals. The parameters required for the constitutive formulation were calibrated to deformation experiments on single crystals. This model is used to investigate the effects of non-Schmid effects on the predictions of the stress–strain response and texture evolution in body-centered-cubic (bcc) metals. The non-Schmid contributions are required to accurately predict the stress–strain response of single crystals, and the concomitant non-associativity of the flow also increases the tendency of localization in polycrystal deformations.

Nanoindentation study of plasticity length scale effects in LIGA Ni microelectromechanical systems structures

This paper presents the results of a nanoindentation study of the effects of strain gradient plasticity on the elastic-plastic deformation of lithographie, galvanoformung, abformung (LIGA) Ni microelectromechanical systems (MEMS) structures plated from sulfamate baths. Both Berkovich and North Star/cube corner indenter tips were used in the study to investigate possible effects of residual indentation depth on the hardness of LIGA Ni MEMS structures between the micro- and nanoscales. A microstructural length scale parameter, l = 2.2 μm, was determined for LIGA nickel films. This is shown to be consistent with a stretch gradient length-scale parameter, l s , of approximately 0.9 μm.

Predicting the orientation-dependent stress-induced transformation and detwinning response of shape memory alloy single crystals

The present investigation examines three models that predict the orientation dependence of the stress-induced transformation strain in shape memory alloys (SMAs). The merits of each model are con-sidered in light of experimental results for three SMAs: NiTi, Cu-Ni-Al, and Ni-Al. Published experimental results fit model predictions well in most cases; the few exceptions can be accounted for by factors not included in the present models. As part of the comparison of model results with experimental observations, Ni-Al stress-strain curves generated by one of the models are found to closely match experimental stress-strain curves for the [001], [011], and [111] stress axis orientations. Finally, the predicted transformation stress anisotropy is analyzed in detail to examine the effect of detwinning of the stress-induced martensite.

Mechanical and metallographic characterization of LIGA fabricated nickel and 80%Ni-20%Fe Permalloy

A table top servohydraulic load frame equipped with a laser displacement measurement system was constructed for the mechanical characterization of LIGA fabricated electroforms. A drop in tensile specimen geometry which includes a pattern to identify gauge length via laser scanning has proven to provide a convenient means to monitor and characterize mechanical property variations arising during processing. In addition to tensile properties, hardness and metallurgical data were obtained for nickel deposit specimens of current density varying between 20 and 80 mA/cm{sup 2} from a sulfamate based bath. Data from 80/20 nickel-iron deposits is also presented for comparison. As expected, substantial mechanical property differences from bulk metal properties are observed as well as a dependence of material strength on current density which is supported by grain size variation. While elastic modulus values of the nickel electrodeposit are near 160 GPa, yield stress values vary by over 60%. A strong orientation in the metal electrodeposits as well as variations in nucleating and growth morphology present a concern for anisotropic and geometry dependent mechanical properties within and between different LIGA components.

Exothermic reactions in Co/Al nanolaminates

Vapor-deposited Co/Al nanolaminates having a net equiatomic stoichiometry exhibited rapid, high-temperature synthesis. When ignited locally, 7.5-μm-thick Co/Al foils were characterized by self-sustained propagating reactions with flame front speeds between ∼0.5 and 9 m/s. Speed was largely affected by bilayer thickness and premixed volume fraction, consistent with previous studies of other exothermic multilayers. Cobalt aluminide foils reacted by self-propagating synthesis developed a variety of rough surface morphologies characterized by a large amplitude (peak-to-valley ∼1.0 μm), multiperiod, wavelike structure. High-temperature reaction was also stimulated by rapid global heating. Tests revealed low ignition temperatures (Tig)∼240–460 °C compared with previous Co–Al powder reactant experiments. Ignition temperature was influenced by bilayer thickness. All foils ignited by global heating and those reacted in a self-propagating mode developed a single-phase B2 (CsCl-type) crystal structure. Thick Co/Al n...

Micromechanical testing of MEMS materials

This paper discusses tensile testing techniques and results derived as part of a broader microstructure-properties investigation of structural materials used in surface micromachined (SMM), and LIGA MEMS technologies. SMM techniques produce devices on the surface of a silicon wafer, with critical dimensions as small as 1–2 μm, using a subtractive multilayer film deposition process. Two structural materials have been investigated: SUMMiT™ polysilicon and amorphous Diamond (a-D). Mechanical properties presented in this paper on these SMM structural materials were obtained from a direct tensile testing method using the lateral force measurement capability of a nanoindentation system. LIGA, a German acronym extracted from the words “Lithographie, Galvanoformung, Abformung,” is an additive process in which structural material is electrodeposited into a polymethyl-methacrylate (PMMA) molds realized by deep x-ray lithography. LIGA tensile specimens of several different materials have been evaluated using a mini-servohydraulic load frame, designed to test specimens of sizes similar to structural components. In this paper, tensile test results from LIGA fabricated Ni and Ni-alloys and examples of their correlation to processing and microstructure will be presented.

Understanding and Tailoring the Mechanical Properties of Liga Fabricated Materials

LIGA fabricated materials and components exhibit several processing issues affecting their metallurgical and mechanical properties, potentially limiting their usefulness for MEMS applications. For example, LIGA processing by metal electrodeposition is very sensitive to deposition conditions which causes significant processing lot variations of mechanical and metallurgical properties. Furthermore, the process produces a material with a highly textured lenticular rnicrostructural morphology suggesting an anisotropic material response. Understanding and controlling out-of-plane anisotropy is desirable for LIGA components designed for out-of-plane flexures. Previous work by the current authors focused on results from a miniature servo-hydraulic mechanical test frame constructed for characterizing LIGA materials. Those results demonstrated microstructural and mechanical properties dependencies with plating bath current density in LIGA fabricated nickel (LIGA Ni). This presentation builds on that work and fosters a methodology for controlling the properties of LIGA fabricated materials through processing. New results include measurement of mechanical properties of LIGA fabricated copper (LIGA Cu), out-of-plane and localized mechanical property measurements using compression testing and nanoindentation of LIGA Ni and LIGA Cu.

A nano-indentation study on the plasticity length scale effects in LIGA Ni MEMS structures

This paper presents the results of a nano-indentation study of the effects of strain gradient plasticity on the elastic-plastic deformation of LIGA Ni MEMS structures plated from sulfamate baths. Both Berkovich and North Star indenter tips were used in the study to investigate possible effects of residual indentation depth between the micro and nano scales on the hardness of LIGA Ni MEMS structures. A microstructural length scale parameter, l^ = 0.89 μm, was determined for LIGA nickel films. This is shown to be consistent with a stretch gradient length scale parameter, ls, of ∼0.36 μm.

Strength of polysilicon for MEMS devices

The safe, secure and reliable application of microelectromechanical systems (MEMS) devices requires knowledge about the distribution in material and mechanical properties of the small-scale structures. A new testing program at Sandia is quantifying the strength distribution using polysilicon samples that reflect the dimensions of critical MEMS components. The strength of polysilicon fabricated at Sandias Microelectronic Development Laboratory was successfully measured using samples 2.5 microns thick, 1.7 microns wide with lengths between 15 and 25 microns. These tensile specimens have a freely moving hub on one end that anchors the sample to the silicon die and allows free rotation. Each sample is loaded in uniaxial tension by pulling laterally with a flat tipped diamond in a computer-controlled nanoindenter. The stress-strain curve is calculated using the specimen cross section and gage length dimensions verified by measuring against a standard in the SEM.