Frank W. DelRio

Conformal hydrophobic coatings prepared using atomic layer deposition seed layers and non-chlorinated hydrophobic precursors

Ultrathin and conformal films deposited using atomic layer deposition (ALD) can enhance the reliability and performance of micro-electro-mechanical systems (MEMS) devices. Al2O3 ALD films are particularly useful because the Al2O3 ALD surface chemistry is very favorable and amenable to growth on a wide variety of substrates. Al2O3 ALD can be utilized to deposit robust and reliable hydrophobic coatings. A thin Al2O3 ALD film is deposited and is used as a seed layer to prepare and optimize the MEMS surface for the subsequent attachment of the hydrophobic precursors. Once the Al2O3-coated surface is prepared, non-chlorinated alkylsilanes are chemically bonded to the surface hydroxyl groups on the ALD seed layer. The film growth was monitored using an in situ quartz crystal microbalance, Fourier transform infrared spectroscopy and ex situ Auger electron spectroscopy. This deposition technique results in a dense and conformal hydrophobic film with a water contact angle of 108 ± 2°. When annealed in air to 300 °C for 10 min, the hydrophobic ALD films remained hydrophobic with a contact angle greater than 90°. Using MEMS cantilever beam arrays, hydrophobic ALD-coated beams were determined to have an adhesion energy of 0.11 ± 0.03 mJ m−2 at 100% humidity as compared with an adhesion energy of 12 ± 1 mJ m−2 for the same beams without any coating.

The effect of nanoparticles on rough surface adhesion

Particulates can strongly influence interfacial adhesion between rough surfaces by changing their average separation. In a cantilever beam adhesion test structure, a compressive zone exists just beyond the crack tip, which may act to deform such particles. To explore this phenomenon quantitatively, we compared finite element method calculations of the interface to load-displacement experiments of individual particles. Below a certain threshold density, we show that the stress distribution at the interface is sufficient to deform individual particles. In this regime, the adhesion is controlled by the intrinsic surface roughness and under dry conditions is mainly due to van der Waals forces across extensive noncontacting areas. Above this threshold density, however, the particles introduce a topography that is more significant than the intrinsic surface roughness. As a result, the interfacial separation is governed by the particle size and the adhesion is lower but stochastic in nature. We demonstrate that ...

Fracture strength of micro- and nano-scale silicon components

Silicon devices are ubiquitous in many micro- and nano-scale technological applications, most notably microelectronics and microelectromechanical systems (MEMS). Despite their widespread usage, however, issues related to uncertain mechanical reliability remain a major factor inhibiting the further advancement of device commercialization. In particular, reliability issues related to the fracture of MEMS components have become increasingly important given continued reductions in critical feature sizes coupled with recent escalations in both MEMS device actuation forces and harsh usage conditions. In this review, the fracture strength of micro- and nano-scale silicon components in the context of MEMS is considered. An overview of the crystal structure and elastic and fracture properties of both single-crystal silicon (SCS) and polycrystalline silicon (polysilicon) is presented. Experimental methods for the deposition of SCS and polysilicon films, fabrication of fracture-strength test components, and analysis...

Properties of atomic-layer-deposited Al2O3/ZnO dielectric films grown at low temperature for RF MEMS

Al2O3/ZnO alloy films were grown at 100°C using atomic layer deposition (ALD) techniques. It has been previously established that the resistivity of these films can be tuned over a wide range by varying the amount of Zn in the film. Al2O3/ZnO ALD alloy films can therefore be designed with a dielectric constant high enough to provide a large down-state capacitance and a resistivity low enough to promote the dissipation of trapped charges. The material and electrical properties of the Al2O3/ZnO ALD films were investigated using Auger electron spectroscopy (AES), nanoindentation, and mercury probe measurements. Chemical analysis using AES confirmed the presence of both Al and Zn in the alloys. The nanoindentation measurements were used to calculate the Youngs modulus and hardness of the films. Pure Al2O3 ALD was determined to have a modulus between 150 and 155 GPa and a hardness of ~8 GPa, while the results for pure ZnO ALD indicated a modulus between 120 and 140 GPa and a hardness of ~5 GPa. An Al2O3/ZnO ALD alloy displayed a modulus of 140-145 GPa, which falls between the two pure films, and a hardness of ~8 GPa, which is similar to the pure Al2O3 film. The dielectric constants of the ALD films were calculated from the mercury probe measurements and were determined to be around 6.8. These properties indicate that the Al2O3/ZnO ALD films can be engineered as a property specific dielectric layer for RF MEMS devices.

Controlled Formation and Characterization of Dithiothreitol-Conjugated Gold Nanoparticle Clusters

We report a systematic study of the controlled formation of discrete-sized gold nanoparticle clusters (GNCs) by interaction with the reducing agent dithiothreitol (DTT). Asymmetric-flow field flow fractionation and electrospray differential mobility analysis were employed complementarily to determine the particle size distributions of DTT-conjugated GNCs (DTT-GNCs). Transmission electron microscopy was used to provide visualization of DTT-GNCs at different states of aggregation. Surface packing density of DTT and the corresponding molecular conformation on the Au surface were characterized by inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy. Results show that DTT increases the aggregation rate of gold nanoparticles (AuNPs) up to ≈100 times. A mixed conformation (i.e., combining vertically aligned, horizontally aligned, and cross-linking modes) exists for DTT on the Au surface for all conditions examined. The primary size of AuNPs, concentration of DTT, and the starting concentration of AuNPs influence the degree of aggregation for DTT-GNCs, indicating that the collision frequency, energy barrier, and surface density of DTT are the key factors that control the aggregation rate. DTT-GNCs exhibit improved structural stability compared to the citrate-stabilized GNCs (i.e., unconjugated) following reaction with thiolated polyethylene glycol (SH-PEG), indicating that cross-linking and surface protection by DTT suppresses disaggregation normally induced by the steric repulsion of SH-PEG. This work describes a prototype methodology to form ligand-conjugated GNCs with high-quality and well-controlled material properties.

Etching Process Effects on Surface Structure, Fracture Strength, and Reliability of Single-Crystal Silicon Theta-Like Specimens

The etching processes used to produce microelectromechanical systems (MEMS) leave residual surface features that typically limit device strength and, consequently, device lifetime and reliability. In order to optimize MEMS device reliability, it is therefore necessary to determine the effects that these etching processes have on MEMS component strength. The microscale theta specimen, which is shaped like the Greek letter Θ, acts as a tensile test specimen when loaded in compression by generating a uniform tensile stress in the central web region of the specimen. Three sets of single-crystal silicon theta specimens are fabricated using two deep reactive ion etching recipes and a temperature-controlled cryogenic plasma etching recipe, each set resulting in a different specimen surface structure. The resulting strength distributions are analyzed in two ways. First, the strength data are fit to a three-parameter Weibull distribution function to determine the lower bound, or threshold strength, of each distribution. Second, the strength data are used in conjunction with various loading schemes to assess their effect on the lifetime spectrum of the device. In both approaches, the theta specimen is used to great effect to gain quantitative insight into the role of etching-induced surface features on the manufacturing yield and operational reliability of MEMS components.

Elastic, adhesive, and charge transport properties of a metal-molecule-metal junction: the role of molecular orientation, order, and coverage

The elastic, adhesive, and charge transport properties of a metal-molecule-metal junction were studied via conducting-probe atomic force microscopy (AFM) and correlated with molecular structure by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The junctions consisted of Co-Cr-coated AFM tips in contact with methyl-terminated alkanethiols (CH(3)(CH(2))(n-1)SH, denoted by C(n), where n is the number of carbons in the molecular chain) on Au substrates. AFM contact data were analyzed with the Derjaguin-Muller-Toporov contact model, modified by a first-order elastic perturbation method to account for substrate effects, and a parabolic tunneling model, appropriate for a metal-insulator-metal junction in which the thickness of the insulator is comparable to the Fermi wavelength of the conducting electrons. NEXAFS carbon K-edge spectra were used to compute the dichroic ratio R(I) for each film, which provided a quantitative measure of the molecular structure as a function of n. As n decreased from 18 to 5, there was a change in the molecular phase from crystalline to amorphous (R(I) --> 0) and loss of surface coverage, and as a result, the work of adhesion w increased from 82.8 mJ m(-2) to 168.3 mJ m(-2), the Youngs modulus of the film E(film) decreased from 1.0 to 0.15 GPa, and the tunneling barrier height phi(0) - E(F) decreased from 2.4 to 2.1 eV. For all n, the barrier thickness t decreased for small applied loads F and remained constant at approximately 2.2 nm for large F. The change in behavior was explained by the presence of two insulating layers: an oxide layer on the Co-Cr tip, and the alkanethiol monolayer on the Au surface. X-ray photoelectron spectroscopy confirmed the presence of an oxide layer on the Co-Cr tip, and by performing high-resolution region scans through the film, the thickness of the oxide layer t(oxide) was found to be between 1.9 and 3.9 nm. Finally, it was shown that phi(0) - E(F) is strain-dependent, and the strain at which the film is completely displaced from under the tip is -0.17 for all values of n.

Stress mapping of micromachined polycrystalline silicon devices via confocal Raman microscopy

Stress mapping of micromachined polycrystalline silicon devices with components in various levels of uniaxial tension was performed. Confocal Raman microscopy was used to form two-dimensional maps of Raman spectral shifts, which exhibited variations on the scale of the component and on the scale of the microstructure. Finite element analysis models enabled direct comparison of the spatial variation in the measured shifts to that of the predicted stresses. The experimental shifts and model stresses were found to be linearly related in the uniaxial segment, with a proportionality constant in good agreement with calculations based on an opto-mechanical polycrystalline averaging analysis.

Nanomechanical properties of polyethylene glycol brushes on gold substrates.

A necessary step in advancing the use of polyethylene glycol (PEG) surface coatings in critical biotechnological applications such as cancer treatments is to provide direct and reliable nanoscale property characterization. Measurements for such characterization are currently provided by scanning probe methods, which are capable of assessing heterogeneity of both surface coverage and properties with nanoscale spatial resolution. In particular, atomic force microscopy (AFM) can be used to detect and quantify the heterogeneity of surface coverage, whereas atomic force spectroscopy can be used to determine mechanical properties, thereby revealing possible heterogeneity of properties within coatings. In this work, AFM and force spectroscopy were used to characterize the morphology and mechanical properties of thiol-functionalized PEG surface coatings on flat gold substrates in aqueous PEG solution. Thiol-functionalized PEG offers a direct and simple method of attachment to gold substrates without intermediate anchoring layers and therefore can be exploited in developing PEG-functionalized gold nanoparticles. AFM was used to investigate the morphology of the PEG coatings as a function of molecular weight; the commonly observed coverage was in the form of sparse, brushlike islands. Similarly, force spectroscopy was utilized to study the mechanical properties of the PEG coatings in compression and tension as a function of molecular weight. A constitutive description of the mechanical properties of PEG brushes was achieved through a combinatorial analysis of the statistical responses acquired in both compression and tension tests. Such a statistical characterization provides a straightforward procedure to assess the nanoscale heterogeneity in the morphology and properties of PEG coverage.

Growth of Silicon Carbide Nanoparticles Using Tetraethylorthosilicate for Microelectromechanical Systems

Silicon carbide (SiC) is a wide bandgap semiconductor of interest in microelectromechanical systems. We demonstrate the growth of SiC nanoparticles using silicon dioxide (SiO 2 ) films deposited from tetraethylorthosilicate [TEOS, Si(OC 2 H 5 ) 4 ]. High-temperature annealing allows residual carbon to diffuse to and react with the silicon substrate to form SiC nanoparticles (rather than a uniform SiC film). The growth of the SiC nanoparticles can be controlled by annealing conditions or eliminated by flowing oxygen during the film deposition. These particles are revealed by a standard wet etchant and provide an effective method to reduce adhesion between micromachined surfaces.