RESEARCH

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      1.c.      Biologically Inspired Materials Design

2.      MEMS Technology

      2.a.      Surface Processes in MEMS Technology

      2.b.      Integration of Novel Materials into MEMS Technology

3.   Fundamental Aspects of Surfaces

      3.a.      Thin Organic Films on Semiconductor Surfaces

      3.b.      Growth and Surface Chemistry of Silicon Carbide

      3.c.      Hydrogen Interaction with Buckminsterfullerene (C₆₀)


1.         Nanotechnology and NEMS

The frontier of modern technology is in the quest for smaller, faster and cheaper devices, and for new devices to sense and control our physical and chemical environment. Over the past two decades, silicon fabrication technology has been extended to encompass mechanical, optical, and fluidic structures. Silicon micro-electromechanical systems (MEMS) have revolutionized inertial sensing and are now enabling micro total analysis systems, which potentially will transform drug discovery, diagnosis, and the detection of bio-warfare agents. Nano-electromechanical systems (NEMS) are now emerging as processes are being developed for fabricating structures in the nanometer dimensional range. While offering vastly expanded capabilities, NEMS present engineers with unprecedented challenges in materials processing, device design, fabrication and integration.

1.a.      Nanomanufacturing

Two approaches are utilized for accessing the nanometer domain. One, often referred to as the “top-down” approach, is derived from standard microfabrication paradigm of thin film deposition, lithographic patterning and etching. Current methods for patterning NEMS include both electron- and focused-ion beams as well as specialized techniques such as spacer lithography. The latter approach can extend the resolution far below the resolution limit of the lithographic process, with the in-plane dimension of the structure set by a film deposition rather than an etching step.

The second approach, called “bottom-up” or synthetic approach, has complementary capabilities. Nanostructures (nanowires, nanotubes, nanopores, etc.) of the desired material and desired size/morphology with high yield can be produced by this approach. Thus, it promises to be a viable solution both to the heterogeneous integration problem and to the batch fabrication problem. It also enables the realization of completely new approaches to device architectures, well beyond the shear scale down of microscale devices.


Si nanowire array grown by VLS (by Di Gao)


nanopore array by electrochemical etching of aluminum oxide (by Ilaria Lombardi).

However, these structures do not function in a useful manner, unless they are interfaced with other devices or external probes. In collaboration with Profs. Roger Howe and Peidong Yang, our research aims to explore innovative solutions to the challenges of batch fabrication of nanodevices and interfacing them with external probes.

1.b.      Chemical Sensing

In the nanometer size domain, it becomes possible to create structures that exhibit extremely high resonant frequencies (in GHz range) while possessing very small force constants, thanks to their extremely small active mass. These structures also display unprecedented sensitivity to force or to added mass, possibly even down to the single atomic collision event. Our research aims to use these features of nano regime to develop functionalized nanowire-based resonant devices for electromechanical and optical detection of chemical and biological molecules.

1.c.      Biologically Inspired Materials Design

Geckos have the remarkable ability to run at any orientation on just about any smooth or rough, wet or dry, clean or dirty surface (see images below). The adhesion mechanism has been the subject of speculation for more than 100 years, but definitive measurements have been difficult to carry out. The research in our group aims to use scanning probe microscopy to understand and quantify the biomechanics of gecko feet. By understanding the nanomechanics of interactions with surfaces, we hope to design novel adhesives which stick to just about any surface, are self-cleaning, have controlled attach/detach forces, are reusable with long life, and can be intrinsically biocompatible through use of biologically inert materials. In parallel, efforts are underway to design synthetic nanohair array to mimic the amazing adhesive properties of gecko feet. This effort is in collaboration with Prof. Ron Fearing (EECS department at UCB).

Tokay gecko feet adhesive system (courtesy of Anne Peattie, Integrative Biology department, UCB)

2.         MEMS Technology

The integration of miniature mechanical components with microelectronic components has spawned a new technology known as microelectromechanical systems (MEMS). It promises to extend the benefits of microfabrication to sensing and actuating functions (see examples below). In particular, there is a growing interest in developing processes that use silicon as a mechanical material. One such process, called surface micromachining, consists of deposition and selective etching of multiple layers of structural and sacrificial (spacer) thin films, and is one of the core technological processes underlying MEMS. Surface micromachines typically have lateral sizes of 10 - 500 mm, with thicknesses of about 100 nm - 2 mm, and are offset 100 nm - 2 mm from the substrate. Some of the issues limiting the development of MEMS technology are: a) the narrow materials base; b) the limited knowledge of the mechanical and hydrodynamic behavior of materials at the micro- and nanoscale; c) the dominance of interfacial forces over body forces. Strong adhesion, friction and wear are major problems limiting the realization of many micromechanical devices.

2.a.      Surface Processes in MEMS

i)          Tribology of MEMS

In MEMS technology, one utilizes the materials and processes of the integrated circuit (IC) to fabricate miniature mechanical components, such as beams, membranes, valves, pumps, and gears.  Due to the large surface area to volume ratio of most MEMS devices, interfacial forces dominate over body forces.  This is sometimes seen when microstructure surfaces spontaneously stick together, and remain adhered.  When these adhesive forces dominate over restoring forces, adhesion or stiction occurs leaving the micromechanical surfaces to permanently adhere to each other or to the substrate.  Current research in the Maboudian group explores chemical modification of MEMS surfaces to address in-use stiction (stiction occurring during operation of the MEMS), friction and wear.  Specific projects include vapor phase processes for anti-stiction monolayers, thermal stability of anti-stiction monolayers, alkene based monolayers and hard coatings for MEMS. We are also interested in understanding how these interactions are modified in fluidic environments, an issue of relevance in microfluidic devices.         


Lateral Friction Micro-Instrument (Top View)

ii)         Effect of Surfaces in Electrokinetic Phenomena for Microfluidic Systems

While microsystems today rely on fluid transport by using electrical or hydrostatic means, relatively few studies have been undertaken to understand performance-limiting phenomena in practical microfluidic systems.  A joint effort with the Matheis group is focusing on the characterization of the effect of surface charge on the electrokinetic flow behavior in microfluidic channels.  Of interest is the relationship between the flow rate and the applied voltage on different surfaces of microfluidic channels.  The surfaces of interest include polymer, glass and silicon.  Since our lab has developed a unique technique of depositing silicon carbide, we would also like to investigate the possibility of using silicon carbide for biological application.

2.b.      Integration of Novel Materials into MEMS Technology

i)          Silicon Carbide Based MEMS

Silicon remains the dominant material system in many technologies, including M/NEMS. However, our capabilities are enhanced if we can effectively expand the materials base beyond that of silicon. Research in our group has aimed at realizing silicon carbide (SiC) based MEMS. The wide energy band gap, high thermal conductivity, large break down field, and high saturation velocity of SiC makes this material an ideal choice for high temperature, high power, and high voltage electronic devices. In addition, its chemical inertness, high melting point, extreme hardness, and high wear resistance make SiC an attractive candidate to fabricate sensors and actuators capable of performing in harsh environments. The research efforts in the group include schemes: (i) to reduce the deposition temperature; (ii) to achieve high selectivity etching of SiC; (iii) to develop surface micromachining technology based on SiC; (iv) to investigate metal/SiC interfaces; and (v) to realize a series of SiC-based sensors for harsh environment applications such as high temperature, high acceleration, and high radiation. The latter is in collaboration with Prof. Albert Pisano (mechanical engineering department at UCB), and Prof. Roger Howe (EECS department at Stanford University).


SiC-based Comb-drive Resonators

ii)        Novel Metal Deposition Techniques and Microreactors

Potential application of metals in MEMS technology range from refractive/conductive coatings for optical MEMS, microswitches and microrealys, to catalytically active coatings in microchemical reactors. Our group is investigating electrochemical processes, in particular galvanic displacement methodologies, for selective metallization of silicon microstructures. In galvanic displacement, metal ions from solution are reduced to the metal, with the subsequent oxidation (and typically dissolution) of the substrate. This simple method deposits metals selectively onto the oxidizable substrate and does not require an external voltage source or a reducing agent in solution like electrodeposition and electroless plating, respectively. . Metals that are of interest in the semiconductor industry include copper group metals (Au, Ag, and Cu), catalysts (Pt, Pd, and Rh) and catalysts/refractory metals (Ru,Ir, and Re). To date, we have observed the displacement of Cu, Au, Ag, Pt, Ru, and Rh on single crystalline Si. One of our goal is to use galvanic displacement to deposit metal films in microreactors, where selective, conformal deposits can be achieved by immersion of silicon microstructures into the plating solution. This effort is currently in collaboration with Prof. Iglesia (chemical engineering department at UCB).

3.         Fundamental Aspects of Surfaces

3.a.      Thin Organic Films on Semiconductor Surfaces

Understanding and manipulating the structure and chemistry of thin organic films are important in such diverse fields as chemical separation applications, corrosion chemistry, electronic and optical devices, chemical sensors, and tribology. Self-assembled monolayers (SAM) are unique among thin organic films in that they are robust and possess a high degree of intrinsic order and uniformity as a result of their adsorbate-surface and adsorbate-adsorbate interactions. These attributes make SAM ideal model systems for studying organic surfaces and their chemistry at the molecular level.

i)         Formation Kinetics of Alkyltrichlorosilane Self Assembled Monolayers

Time-resolved atomic force microscopy, contact angle analysis and X-ray photoelectron spectroscopy were employed to study the effect of cyclic heating and cooling of a partial octadecyltrichlorosilane (OTS) monolayer on the oxidized Si(100) surface. A reversible structural change of the partial monolayer between high density and low density two-dimensional liquid phases was observed as a function of temperature at constant coverage. These studies show that the monolayer exists in a highly mobile hydrogen-bonded state akin to the equilibrium state of Langmuir films at the air-water interface. The lifetime of the mobile state was measured to be on the order of several minutes, after which grafting and cross-linking immobilize the monolayer. The results show that the mobile state permits large scale rearrangements of the molecules within the monolayers. Currently, we are using electric force microscopy to characterize the electrical properties of these monolayers; we are also investigating their thermal stability using a variety of techniques such as X-ray photoelectron spectroscopy.


OTS-based Self-Assembled Monolayers

ii)          Formation of Alkanethiol Monolayers on Ge(111) and InP(100)

The latest advent of micromachining that interlace electronic devices with mechanical actuators has prompted a new interest in a robust chemistry to passivate Ge surfaces. He et al. reported self-assembly of alkyl monolayers on Ge(111) surface by Grignard reaction. However, long reaction times are required to form such a monolayer (e.g., seven days to form an octadecyl, n = 18, layer). In search of a simpler approach, we have discovered that alkanethiols (CH₃(CH₂)_n-1SH) react with Ge surface, producing an alkyl monolayer. Compared to the laborious pathways described for alkyl monolayers on Ge, we have found that alkanethiols readily react with HF-treated Ge surface at room temperature to form a tightly packed monolayer. The presence of the monolayer is confirmed using contact angle analysis, X-ray photoelectron spectroscopy and high-resolution electron energy loss spectroscopy. Currently, we are exploring the growth of alkanethiols on III-V compound semiconductors such as InP.

3.b.      Growth and Surface Chemistry of Silicon Carbide

i)           Reactor Modeling

Reactor modeling is necessary to understand the chemical and physical phenomena involved during SiC deposition using single precursors and to optimize the processing conditions. In particular, five processes take place during the deposition: mass transport of the precursor towards the deposition surface, precursor adsorption, surface reaction, byproducts desorption and mass transport of the byproducts to the gas bulk. A reactor model has been developed using quantum chemical methods to take into account all these processes and to estimate the unknown constants of various reactions (e.g. precursor adsorption).

ii)          Surface Chemistry

In recent years, SiC has shown potential as a new material for electronic devices. Characteristics motivating the incorporation of SiC into electronic devices include a wide band gap and high thermal conductivity. Additionally, SiC is being explored as a means for achieving hard, robust coatings for demanding chemical and mechanical applications. SiC overlayers exhibit low friction and wear, as well as excellent thermal and chemical stability at high temperatures. In order to utilize these properties in future technological applications, further investigation into both the atomic structure and surface reactivity of SiC is needed.

Hydrogen is commonly encountered at various stages of film processing. For example, it is used as carrier gas during the film deposition or later on during etching. For these reasons, we have investigated the interaction of hydrogen with SiC. The low-energy electron diffraction (LEED) pattern of the SiC(0001) (3x3) surface reconstruction is found to undergo a change from (3x3) to (1x1) upon exposure to atomic hydrogen (and deuterium). Using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES) and LEED, we have determined that this change is due to disordering and etching of the uppermost Si layers. With increasing deuterium exposure at 320 K, depletion of the Si adlayer and formation of SiD surface species is observed. At high deuterium exposure, observation of the C-D stretch mode indicates the onset of bulk silicon carbide etching. SiD₂ and SiD₃ surface species, known intermediates in the Si etching process, are observed with deuterium exposure at 180 K.

Current efforts on this project involve investigation of the interaction of ammonia with SiC surface to better understand the mechanism of doping.

3.c.      Interaction of Hydrogen with Buckminsterfullerene

It has long been known that interstellar and circumstellar dusts show emission features ranging from 3300 cm^-1 to below 800 cm^-1. Sharp IR bands are observed at 2920, 1610, and 880 cm^-1, with a broad, intense emission envelope near 1300 cm^-1. Additional features include a recurrent mode at 3050 cm^-1, a weak mode near 1450 cm^-1, and distinct shoulder near 1150 cm^-1. More recently, a small but increasing number of carbon-rich astronomical objects reveal an unidentified emission feature in the far-IR at 490 cm^-1. It is now generally accepted that most of these features are due to IR fluorescence from large, carbon-rich molecular species.


Orion emission nebula 1500 light yrs. away

Soon after the initial discovery of C₆₀ and the development of laboratory methods for synthesizing it in bulk quantities, theoretical work suggested hydrogenated C₆₀ as a potential candidate for unidentified interstellar emission features. Yet, few experimental efforts have been undertaken to substantiate this claim.

We have investigated the interaction of atomic hydrogen with buckminsterfullerene (C₆₀), using high-resolution electron energy loss spectroscopy (HREELS) in ultra-high vacuum. The energy loss spectra of partially hydrogenated C₆₀ multilayers reveal vibrational features previously observed in infrared emission from interstellar and circumstellar dust clouds, including a broad loss envelope between 1150 and 1310 cm^-1, followed by a band at 1620 cm^-1 in remarkable agreement with the canonical interstellar spacing of 300 cm^-1. Additionally, a major C-H stretching band near 2900 cm^-1 is observed which compares well to the galactic center absorption spectrum. These results suggest hydrogenated C₆₀ as a potential candidate for unidentified interstellar emission features.