Barton C. Prorok

Recent advances in peptide probe-based biosensors for detection of infectious agents

Recent biological terrorism threats and outbreaks of microbial pathogens clearly emphasize the need for biosensors that can quickly and accurately identify infectious agents. The majority of rapid biosensors generate detectable signals when a molecular probe in the detector interacts with an analyte of interest. Analytes may be whole bacterial or fungal cells, virus particles, or specific molecules, such as chemicals or protein toxins, produced by the infectious agent. Peptides and nucleic acids are most commonly used as probes in biosensors because of their versatility in forming various tertiary structures. The interaction between the probe and the analyte can be detected by various sensor platforms, including quartz crystal microbalances, surface acoustical waves, surface plasmon resonance, amperometrics, and magnetoelastics. The field of biosensors is constantly evolving to develop devices that have higher sensitivity and specificity, and are smaller, portable, and cost-effective. This mini review discusses recent advances in peptide-dependent rapid biosensors and their applications as well as relative advantages and disadvantages of each technology.

Correction for longitudinal mode vibration in thin slender beams

This letter reports on a correction to the theoretical prediction of longitudinal mode vibration in thin, slender beams. Thin magnetostrictive strips were fashioned from Metglas™ and subjected to a modulated magnetic field to determine resonant frequency and acoustic wave propagation speed. The results indicated that current analytical solutions were not adequate to predict behavior. Numerical simulations were performed that adjusted Poisson’s ratio until the acoustic wave speed matched that measured in the experiments. The results indicated that the current equations, formulated using the plane-strain modulus, should be modified by using the plane-stress or biaxial modulus.

Characterization of aging effects in lead free solder joints using nanoindentation

The mechanical properties of a lead free solder are strongly influenced by its microstructure, which is controlled by its thermal history including solidification rate and thermal aging after solidification. Due to aging phenomena, the microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal and/or thermal cycling environments. Through uniaxial testing of miniature bulk solder tensile specimens, we have previously demonstrated that large changes occur in the stress-strain and creep behaviors of lead free solder alloys with aging. Complementary studies by other research groups have verified aging induced degradations of SAC mechanical properties. In those investigations, mechanical testing was performed on a variety of sample geometries including lap shear specimens, Iosipescu shear specimens, and custom solder ball array shear specimens. While there are clearly aging effects in SAC solder materials, there have been limited prior mechanical loading studies on aging effects in actual solder joints extracted from area array assemblies (e.g. PBGA or flip chip). This is due to the extremely small size of the individual joints, and the difficulty in gripping them and applying controlled loadings (tension, compression, or shear). In the current work, we have explored aging phenomena in actual solder joints by nano-mechanical testing of single SAC305 lead free solder joints extracted from PBGA assemblies. Using nanoindentation techniques, the stressstrain and creep behavior of the SAC solder materials have been explored at the joint scale for various aging conditions. Mechanical properties characterized as a function of aging include the elastic modulus, hardness, and yield stress. Using a constant force at max indentation, the creep response of the aged and non-aged solder joint materials has also been measured as a function of the applied stress level. With these approaches, aging effects in solder joints were quantified and correlated to the magnitudes of those observed in testing of miniature bulk specimens. Our results show that the aging induced degradations of the mechanical properties (modulus, hardness) of single grain SAC305 joints were similar to those seen previously by testing of larger “bulk” solder specimens. However, due to the single grain nature of the joints considered in this study, the degradations of the creep responses were significantly less in the solder joints relative to those in larger uniaxial tensile specimens. The magnitude of aging effects in multi-grain lead free solder joints remains to be quantified. Due to the variety of crystal orientations realized during solidification, it was important to identify the grain structure and crystal orientations in the tested joints. Polarized light microscopy and Electron Back Scattered Diffraction (EBSD) techniques have been utilized for this purpose. The test results show that the elastic, plastic, and creep properties of the solder joints and their sensitivities to aging are highly dependent on the crystal orientation. In addition, an approach has been developed to predict tensile creep strain rates for low stress levels using nanoindentation creep data measured at very high compressive stress levels.

Conversion of metal carbides to carbide derived carbon by reactive ion etching in halogen gas

The excellent tribological properties, very low friction coefficient, ~0.05, of the recently discovered carbide derived carbon (CDC) films have shown them to be excellent candidates in many applications where friction and wear are dominating issues in performance. In this work we examine the feasibility of employing a reactive ion etching process (RIE) with chlorine gas at low temperature, as opposed to the current high temperature chlorination process, in achieving the conversion of metal carbide films into amorphous carbon films. The overall goal is develop a process that is friendlier to microfabricated devices towards employing the tribological properties of CDC films in such devices. We examine this RIE processing using both bulk scale and thin film specimens. These metal-carbide specimens are subjected to a halogen containing ion plasma at reduced pressure in order to leach out the metal, resulting in an amorphous carbon film, a so-called carbide-derived carbon (CDC) process. This reactive ion etching process has been used to produce carbon layers on multiphase carbide materials containing silicon and titanium. The resulting carbon layers have been characterized using a variety of techniques. The results on the bulk scale specimens, via Raman spectrometry, indicated that RIE processing can indeed achieve conversion, while results of the thin films indicated that although conversion occurred poor adhesion of the films to the substrate resulted spallation during friction testing attempts.

A new paradigm in thin film indentation

A new method to accurately and reliably extract the actual Youngs modulus of a thin film on a substrate by indentation was developed. The method involved modifying the discontinuous elastic interface transfer model to account for substrate effects that were found to influence behavior a few nanometers into a film several hundred nanometers thick. The method was shown to work exceptionally well for all 25 different combinations of five films on five substrates that encompassed a wide range of compliant films on stiff substrates to stiff films on compliant substrates. A predictive formula was determined that enables the film modulus to be calculated as long as one knows the film thickness, substrate modulus, and bulk Poissons ratio of the film and the substrate. The calculated values of the film modulus were verified with prior results that used the membrane deflection experiment and resonance-based methods. The greatest advantages of the method are that the standard Oliver and Pharr analysis can be used, and that it does not require the continuous stiffness method, enabling any indenter to be used. The film modulus then can be accurately determined by simply averaging a handful of indents on a film/substrate composite.

Measuring the thin film elastic modulus with a magnetostrictive sensor

This paper reports on the measurement of the thin film Youngs modulus for Au, Cr, Cu, Al and SiC materials by a magnetostrictive sensor. Youngs modulus of these films was determined by monitoring the sensors resonant frequency, both before and after deposition. By measuring the film thickness, this frequency shift can be directly related to the film elastic modulus. All thin films were sputter deposited at room temperature with various thicknesses. The determined Youngs modulus values were comparable to those found in the literature. An error analysis was performed and parameters such as film thickness and film density were found to dominate the measurement technique. The measurement error was also found to decrease as film thickness increased and was negligible (~6% or less) for films 0.1 µm thick or greater. The error was also found to be approximately half of that reported by competing techniques. The technique was found to be simple, quick and inexpensive to employ in assessing the thin film elastic properties of both metals and ceramics.

Exploration of aging induced evolution of solder joints using nanoindentation and microdiffraction

Due to aging phenomena, the microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal and/or thermal cycling environments. In our ongoing studies, we are exploring aging phenomena by nano-mechanical testing of SAC lead free solder joints extracted from PBGA assemblies. Using nanoindentation techniques, the stress-strain and creep behavior of the SAC solder materials are being explored at the joint scale for various aging conditions. Mechanical properties characterized as a function of aging include the elastic modulus, hardness, and yield stress. Using a constant force at max indentation, the creep response of the aged and non-aged solder joint materials is also being measured as a function of the applied stress level. With these approaches, aging effects in actual solder joints are being quantified and correlated to the magnitudes of those observed in testing of miniature bulk specimens. In our initial work (ECTC 2013), we explored aging effects in single grain SAC305 solder joints. In the current investigation, we have extended our previous work on nanoindentation of joints to examine a full test matrix of SAC solder alloys. The effects of silver content on SAC solder aging has been evaluated by testing joints from SACN05 (SAC105, SAC205, SAC305, and SAC405) test boards assembled with the same reflow profile. In all cases, the tested joints were extracted from 14 × 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter) that are part of the iNEMI Characterization of Pb-Free Alloy Alternatives Project (16 different solder joint alloys available). After extraction, the joints were subjected to various aging conditions (0 to 12 months of aging at T = 125 C), and then tested via nanoindentation techniques to evaluate the stress-strain and creep behavior of the four aged SAC solder alloy materials at the joint scale. The observed aging effects in the SACN05 solder joints have been quantified and correlated with the magnitudes observed in tensile testing of miniature bulk specimens performed in prior studies. The results show that the aging induced degradations of the mechanical properties (modulus, hardness) in the SAC joints were of similar order (30-40%) as those seen previously in the testing of larger “bulk” uniaxial solder specimens. The creep rates of the various tested SACN05 joints were found to increase by 8-50X due to aging. These degradations, while significant, were much less than those observed in larger bulk solder uniaxial tensile specimens with several hundred grains, where the increases ranged from 200X to 10000X for the various SACN05 alloys. Additional testing has been performed on very small tensile specimens with approximately 10 grains, and the aging-induced creep rate degradations found in these specimens were on the same order of magnitude as those observed in the single grain joints. Thus, the lack of the grain boundary sliding creep mechanism in the single grain joints is an important factor in avoiding the extremely large creep rate degradations (up to 10,000X) occurring in larger bulk SAC samples. All of the aging effects observed in the SACN05 joints were found to be exacerbated as the silver content in the alloy was reduced. In addition, the test results for all of the alloys show that the elastic, plastic, and creep properties of the solder joints and their sensitivities to aging are highly dependent on the crystal orientation. The observed mechanical behavior changes in joints are due to evolution in the microstructure and residual strains/stresses in the solder material, and measurements of these evolutions are critical to developing a fundamental understanding of solder joint aging phenomena. As another part of this work, we have performed an initial study of these effects in the same SAC305 solder joints that were tested using nanoindentation. The enhanced x-ray microdiffraction technique at the Advanced Light Source (Synchrotron) at the Lawrence Berkeley National Laboratory was employed to characterize several joints after various aging exposures (0, 1, and 7 days of aging at T = 125 C). For each joint, microdiffraction was used to examine grain growth, grain rotation, sub-grain formation, and residual strain and stress evolution as a function of the aging exposure. The entire joints were scanned using a 10 micron step size, and the results were correlated with changes in the mechanical response of the joint specimens measured by nanoindentation.

Nanomechanical characterization of SAC solder joints - reduction of aging effects using microalloy additions

In this work, we have examined the ability of microalloy additions (dopants) to reduce aging effects in solder joints by nanoindentation testing of several sets of doped/non-doped alloys. The investigated solder joint alloys included SAC105, SAC105+Ni, and SAC105+Mn. For the doped alloys, the base SAC105 solder in the PBGA component solder balls was modified by microalloying an additional small amount (<; 0.05%) of the dopant material (Ni, Mn, etc.). The tested joints were extracted from 14 × 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter) that were part of the iNEMI Characterization of Pb-Free Alloy Alternatives Project. After extraction, the joints were then subjected to various aging conditions at high temperature (0-6 months aging at T = 125 °C). After aging, the joints were loaded in the nanoindentation system, and the load-deformation behavior during indentation was used to characterize the mechanical properties of the solder joints for various aging conditions including modulus, hardness, and yield stress. Using constant force at max indentation, we have also measured the creep response of the aged and non-aged solder joint materials for various stress levels. With this approach, we have been able to quantify aging effects in joints and compare the behavior of the standard and doped alloys. Our results have shown that addition of the microalloy elements significantly reduced the aging degradations of the mechanical properties and creep resistance in the joints. For example, the average reduction of the effective elastic modulus of SAC105 joints was 32.9% with 180 days of aging, while the analogous average reductions were 7.0% and 8.1% for the SAC105+Ni and SAC105+Mn joints. Similarly, the average hardness (yield stress) degradation for the SAC105 joints was 45.9%, while those for the SAC105+Ni and SAC105+Mn were 10.5% and 10.7%, respectively. Finally, the creep rate increased 69.1X for the aged SAC105 joints, while the creep rate degradations for the SAC105+Ni (6.5X) and SAC105+Mn (9.0X) were an order of magnitude smaller. Our findings have also suggested that the addition of Ni was slightly more effective than Mn for the SAC105+X dopants.

The mechanical properties of freestanding near-frictionless carbon films relevant to MEMS

Amorphous, diamond-like carbon films with a mixture of sp2 and sp3 hybridizations have exhibited excellent material properties such as chemical stability, wear resistance and optical transparency resulting in their wide use as protective coatings in numerous applications. The hydrogenated forms of these films, a-C:H, and specifically the near-frictionless carbon (NFC) films developed at Argonne National Laboratory have exhibited the lowest ever recorded friction coefficient, 0.001, and ultra-low wear rates of 10−11−10−10 mm3 N−1 m−1, even under dry sliding conditions and at very high contact pressures (Robertson 2002 Diamond-like amorphous carbon Mater. Sci. Eng. R 37 129). Application of these films to sliding or rotating microelectromechanical systems (MEMS) would open up an entirely new class of commercialized MEMS devices. With this in mind, this paper reports on thin-film mechanical property measurements of the NFC films relevant to MEMS. The membrane deflection experiment was employed to subject microfabricated freestanding films to pure tension and measure mechanical properties such as Youngs modulus, residual stress and fracture strength. Youngs modulus was consistently measured at 35.13 ± 2.29 GPa. The fracture strength varied from 0.12 GPa to 0.90 GPa and the residual stress state was compressive and ranged from 79 MPa to 310 MPa. Width and thickness effects of the membranes were also observed in this work, where fracture strength increased with decreasing membrane width and thickness. Weibull analysis of the fracture strength is also presented in the paper.

Mechanical Properties of Ultrananocrystalline Diamond Thin Films for MEMS Applications

Microcantilever deflection and the membrane deflection experiment (MDE) were used to examine the elastic and fracture properties of ultrananocrystalline diamond (UNCD) thin films in relation to their application to microelectromechanical systems (MEMS). Freestanding microcantilevers and membranes were fabricated using standard MEMS fabrication techniques adapted to our UNCD film technology. Elastic moduli measured by both methods described above are in agreement, with the values being in the range 930 and 970 GPa with both techniques showing good reproducibility. The MDE test showed fracture strength to vary from 3.95 to 5.03 GPa when seeding was performed with ultrasonic agitation of nanosized particles.