Cari F. Herrmann

Multilayer and functional coatings on carbon nanotubes using atomic layer deposition

Atomic layer deposition (ALD) can be used to deposit ultra-thin and conformal films on flat substrates, high aspect ratios structures and particles. In this paper, we demonstrate that insulating, multilayered and functionalized ALD coatings can also be deposited conformally on carbon nanotubes. Multilayered coatings consisting of alternating layers of dielectric and conductive materials, such as Al2O3 and W, respectively, are deposited on conductive multi-walled carbon nanotubes. This coated carbon nanotube can function as a nanoscale coaxial cable. Thin layers of Al2O3 ALD are also used as a seed layer to functionalize nanotubes. A carbon nanotube was made highly hydrophobic using an Al2O3 ALD seed layer followed by the attachment of perfluorinated molecules.

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.

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.

The role of repulsive interactions in molecular bromine adsorption and patterning of Si(100)-2×1

Abstract Scanning tunneling microscopy (STM) was used to investigate the role of repulsive interactions in the adsorption and patterning of molecular bromine on the Si(100) surface. At room temperature and low coverage, chemisorption of bromine occurs dissociatively on the same side of adjacent dimers of the same row. Using the STM tip as a probe, we demonstrate the existence of repulsive interactions at adjacent sites on the Si(100)-2×1 surface. These repulsive interactions also contribute to the arrangement of adatoms on the surface. In particular, we report the presence of a stable c(4×2) surface phase that results after exposing the Si(100) surface to bromine under certain conditions. This phase involves adsorption on non-neighboring dimers and is stabilized by repulsive interactions that force bromine adatoms to occupy alternating dimers within rows with an out-of-phase occupancy between adjacent rows.

Hydrophobic coatings using atomic layer deposition and non-chlorinated precursors

This paper describes an alternative method of depositing hydrophobic coatings on MEMS devices using atomic layer deposition (ALD) and non-chlorinated hydrophobic precursors. First, a thin film of Al/sub 2/O/sub 3/ is deposited via ALD and is used as a seed layer to prepare and optimize the MEMS surface for the attachment of the hydrophobic precursors. Subsequently, non-chlorinated alkylsilanes are chemically bonded to the surface hydroxyl groups on the ALD seed layer. This technique results in a dense and ordered hydro-phobic film with a water contact angle of 108/spl plusmn/2/spl deg/. Using MEMS cantilever beam arrays, hydrophobic ALD coated beams were determined to have an adhesion energy of 0.11/spl plusmn/0.03 mJ/m/sup 2/ at 100% humidity as compared to the same beams without coating of 12/spl plusmn/1 mJ/m/sup 2/.

Atomic layer deposition of Al/sub 2/O/sub 3//ZnO nano-scale films for gold RF MEMS

Atomic layer deposition (ALD) was used to create an Al/sub 2/O/sub 3//ZnO thin film for gold capacitive RF MEMS switches. These films exhibited a widely tunable range of physical properties, allowing the creation of a material capable of dissipating trapped charges and maximizing the on-capacitance of the switch. Predicted pull-down voltages of the ALD-coated switches underestimated the experimental findings due to residual stresses in the ALD film and annealing of the gold during the ALD deposition. Switch cycles to failure were measured using a 10 dBm, 10 GHz, CW signal with a bipolar actuation voltage of 25-55 V. Preliminary testing showed lifetimes of 400 million cycles using 50/50 ALD Al/sub 2/O/sub 3//ZnO films, with ultimate failure due to moisture-induced stiction and particulate contamination, not dielectric charging. The insertion loss and isolation for the switches was typically <0.35 dB and > 25 dBm, respectively, over a 10-25 GHz frequency range.

Ultra-thin multilayer nanomembranes for short wavelength deformable optics

Due to their unique optical properties, deformable multilayer membranes are of interest for adaptive optics. In this paper, we present the fabrication and characterization of a deformable membrane with nanometer scale thickness. This work explores the creation of adaptive optics out of multilayer thin films deposited using atomic layer deposition (ALD). SOIMUMPs, ALD, flip-chip assembly, and xenon difluoride (XeF/sub 2/) etching are used in the fabrication process. An assembled device is presented along with an electro-static characterization and interferograms to examine the deformable nature of the membrane surface. To our knowledge these are the first microstructures created with the ALD technique.

Multilayer coating method for x-ray reflectivity enhancement of polysilicon micro-mirrors at 1.54-Å wavelength

A poly-silicon piston micro-mirror array, which has been enhanced with a multilayer coating to exhibit special reflective properties at Cu Kα emission line of 1.54 Å is presented. The micro-mirror array is fabricated using the MEMSCAP PolyMUMPs process and packaged in a ceramic package. The packaged array is coated using atomic layer deposition with an Al2O3/W multilayer. The first Al2O3 layer is thicker than for a normal bilayer pair and prevents the mirror coating from creating an electrical short. This device was tested before and after coating. The snap-down voltage was reduced by half, but qualitatively the mechanical motion remained similar. The fabrication process presented for the Cu Kα wavelength at 1.54 Å can be easily adapted to other optical MEMS and for other wavelengths.

A novel method for fabricating capacitive micromachined ultrasonic transducers with ultra-thin membranes

Production of ultra-thin membranes facilitates the development of miniature capacitive micromachined ultrasonic transducers (CMUTs), which have great potential in biomedical imaging applications. We introduce a novel process, incorporating atomic layer deposition (ALD) and diffusion bonding, for the fabrication of CMUTs with ultra-thin membranes. First, an Al/sub 2/O/sub 3/ layer is deposited on an upper silicon wafer by ALD. Next, a gold layer is deposited on the Al/sub 2/O/sub 3/ layer and patterned to create circular cavities. Then the whole structure is transferred to a bottom wafer by diffusion bonding and the upper silicon wafer is etched away to release the Al/sub 2/O/sub 3/ membrane. Finally, another gold layer is deposited on the membrane for wiring and membrane excitation. Initial results show high quality membranes can be produced using this process with highly conformal surface qualities and extremely thin dimensions (<300 nm). Based on the dimensional characteristics created by this process, we simulate the performance of these transducers using equivalent circuit analysis. The results show that this new fabrication method provides another avenue for optimizing CMUT performance, especially in power savings, sensitivity and potentially increased reliability. Work to test the fabricated elements is currently under way.

Low stress atomic layer deposited alumina for nano electro mechanical systems

A new fabrication process, test structures, and measurements of material properties are presented for atomic layer deposited (ALD) alumina (Al/sub 2/O/sub 3/) nanoelectromechanical systems (NEMS). ALD Al/sub 2/O/sub 3/ has similar electrical and mechanical properties to silicon nitride (Si/sub x/N/sub y/) and can be used in many of the same applications. Yet ALD Al/sub 2/O/sub 3/ is an advantageous material to use over Si/sub x/N/sub y/ due to the low deposition temperature, which allows for integration with CMOS processing. Also, ALD Al/sub 2/O/sub 3/ has a high chemical resistance to Si etchants. In this work the stress in ALD Al/sub 2/O/sub 3/ is measured to be /spl sigma/=401/spl plusmn/32 MPa and /spl sigma/=445/spl plusmn/66 MPa for 100 nm and 50 nm thick Al/sub 2/O/sub 3/ films, respectively. The measured Youngs modulus is in the range of 110-120 GPa.