Roya Maboudian

Critical Review: Adhesion in surface micromechanical structures

We present a review on the state of knowledge of surface phenomena behind adhesion in surface micromechanical structures. After introducing the problem of release-related and in-use adhesion, a theoretical framework for understanding the various surface forces that cause strong adhesion of micromechanical structures is presented. Various approaches are described for reducing the work of adhesion. These include surface roughening and chemical modification of polycrystalline silicon surfaces. The constraints that fabrication processes such as release, drying, assembly, and packaging place on surface treatments are described in general. Finally, we briefly outline some of the important scientific and technological issues in adhesion and friction phenomena in micromechanical structures that remain to be clarified.

Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides

Significance A new class of heterostructures consisting of layered transition metal dichalcogenide components can be designed and built by van der Waals (vdW) stacking of individual monolayers into functional multilayer structures. Nonetheless, the optoelectronic properties of this new type of vdW heterostructure are unknown. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe spatially direct absorption but spatially indirect emission in this heterostructure, with strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN dielectric layers into the vdW gap. The generic nature of this interlayer coupling is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties through customized composite layers. Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells, and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide heterostructures can be designed and built by assembling individual single layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components, and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe a large Stokes-like shift of ∼100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment having spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. This coupling at the hetero-interface can be readily tuned by inserting dielectric layers into the vdW gap, consisting of hexagonal BN. Consequently, the generic nature of this interlayer coupling provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.

Alkyltrichlorosilane-based self-assembled monolayer films for stiction reduction in silicon micromachines

We have investigated the potential of self-assembled monolayer (SAM) coatings for the purpose of adhesion reduction in microelectromechanical systems (MEMS). Two types of SAM coatings, derived from the precursor molecules octadecyltrichlorosilane [CH/sub 3/(CH/sub 2/)/sub 17/SiCl/sub 3/, OTS] and 1H,1H,2H,2H-perfluorodecyltrichlorosilane [CF/sub 3/(CF/sub 2/)/sub 7/(CH/sub 2/)/sub 2/SiCl/sub 3/, FDTS], were applied to polycrystalline silicon microstructures in a liquid-based process. Due to the hydrophobicity of these coatings, the water capillary forces responsible for the phenomenon known as release-related stiction are eliminated, and SAM-coated cantilever beams 2 /spl mu/m thick, 2 /spl mu/m above the substrate, and up to 2 mm in length emerge dry and free standing when removed from the final water rinse. The effects of SAM coating on adhesion encountered during device operation, termed in-use stiction, were characterized using arrays of cantilever beams of varying lengths. Structures made with a polycrystalline silicon of 3-nm rms roughness gave apparent works of adhesion of 30 and 8 /spl mu/J/m/sup 2/ for the OTS and FDTS SAM coatings, respectively, in comparison to 56 mJ/m/sup 2/ for standard oxide-coated structures. These results demonstrate that OTS coating reduces adhesion by more than three orders of magnitude over the conventional process and that the fluorinated SAM can lessen it further by four times. With regard to thermal stability, both SAM coatings can withstand heat treatment for 5 min at 450/spl deg/C in an N/sub 2/ ambient. In air, the OTS film begins to degrade at 150/spl deg/C while the fluorinated coating remains intact up to 400/spl deg/C. Therefore, both types of SAM coatings are compatible with several MEMS packaging techniques, with the FDTS monolayers exhibiting superior stiction and thermal stability properties to those derived from OTS. Furthermore, the FDTS formation does not require any chlorinated solvents such as carbon tetrachloride, which has been banned from industrial use, making the latter coating an industrially viable antistiction treatment.

Self-assembled monolayers as anti-stiction coatings for MEMS: characteristics and recent developments

Despite significant advances in surface micromachining technology, stiction remains a key problem, severely limiting the realization and reliability of many micro-electro-mechanical systems (MEMS) devices. In this article, we focus on self-assembled monolayers as release and anti-stiction coatings for MEMS. Their formation mechanism, the microstructure coating process, and the characteristics of the coated microstructures are described, followed by a discussion of the current limitations, areas for improvements and recent progress for this coating technology.

Surface processes in MEMS technology

Abstract This review focuses on the problem of adhesion in microelectromechanical systems and on the application of the principles and techniques of surface science to understand and manipulate, at the atomic level, the interfacial forces which are responsible for strong adhesion in microdevices. Various approaches developed for reducing both release and in-use adhesion are discussed. They include surface roughening and chemical passivation. Results of ultrahigh vacuum studies on well-characterized surfaces are compared to experiments on test structures, such as cantilever beam arrays, which are specifically designed to study the surface properties of real micromechanical devices under normal operating conditions.

Hole selective MoOx contact for silicon solar cells

Using an ultrathin (∼ 15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of ∼ 6.6 eV, which is significantly higher than that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

Silver Dendrites from Galvanic Displacement on Commercial Aluminum Foil As an Effective SERS Substrate

A silver galvanic displacement process on commercial aluminum foil has been carried out to produce cost-effective SERS substrates. The process is based on an extremely simple redox process where aluminum is oxidized while silver ions are reduced, yielding a final silver dendritic structure that offers a large surface area-to-volume ratio. XPS measurements confirmed the metallic nature of the formed dendrites. SERS substrates were fabricated by spreading of the dendrites on double side Scotch tape attached to a paper slide. Three different thiols were incubated to achieve SAM formation on the Ag dendrites and measured by Raman spectroscopy. The obtained spectra presented well resolved bands and provided valuable information regarding the orientation of the thiols. The high Raman intensity also demonstrates the high enhancement capacities of the produced silver structures. The overall method is cost-effective and allows the use of silver dendrite paste for the mass production of SERS-active substrates, including on flexible substrates and/or via screen printing.

Evidence of Structural Strain in Epitaxial Graphene Layers on 6H-SiC(0001)

The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC (0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows us to resolve submonolayer growth, including individual, localized C=C dimers in a diamondlike carbon matrix for AES C/Si ratio of approximately 3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio >6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.

Metal-catalyzed crystallization of amorphous carbon to graphene

Metal-catalyzed crystallization of amorphous carbon to graphene by thermal annealing is demonstrated. In this “limited source” process scheme, the thickness of the precipitated graphene is directly controlled by the thickness of the initial amorphous carbon layer. This is in contrast to chemical vapor deposition processes, where the carbon source is virtually unlimited and controlling the number of graphene layers depends on the tight control over a number of deposition parameters. Based on the Raman analysis, the quality of graphene is comparable to other synthesis methods found in the literature, such as chemical vapor deposition. The ability to synthesize graphene sheets with tunable thickness over large areas presents an important progress toward their eventual integration for various technological applications.

Tribological Challenges in Micromechanical Systems

Despite much progress in surface micromachining technology, adhesion, friction and wear remain key issues, severely limiting the realization and reliability of many microelectromechanical systems (MEMS) devices. In this article, we focus on the use of molecularly thin organic films as release and anti-stiction coatings for MEMS. The various classes of organic films explored for MEMS are reviewed here, followed by a discussion of the current limitations and areas for improvements for this coating technology.