Are Diamonds a MEMS' Best Friend?

Next-generation military and civilian communication systems will require technologies capable of handling data/ audio, and video simultaneously while supporting multiple RF systems operating in several different frequency bands from the MHz to the GHz range [1]. RF microelectromechani-cal/nanoelectromechanical (MEMS/NEMS) devices, such as resonators and switches, are attractive to industry as they offer a means by which performance can be greatly improved for wireless applications while at the same time potentially reducing overall size and weight as well as manufacturing costs.     

Science and Technology of High Dielectric Constant Thin Films and Materials Integration for Application to High Frequency Devices

Thin films of Ba_1?x Sr _x Ti_1+y O_3+z (BST), were fabricated using both by RF-magnetron sputtering and MOCVD to demonstrate application to high frequency devices. Precise control of composition and microstructure is critical for the production of (Ba _x Sr_1?x )Ti_1+y O_3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high frequency devices. Here we review results on composition-microstructure-electrical property relationships of polycrystalline BST films produced by magnetron sputter deposition that are appropriate for microwave devices such as phase shifters. BST films with a multilayer structure were also developed with different Ti-elemental ratio in each layer to minimize losses and leakage current. Interfacial contamination from C and H species was studied and implications on electrical properties are highlighted. Finally, York's group at the University of California-Santa Barbara successfully integrated our BST films onto phase shifter arrays. The results show potential of BST films in such applications. Results from initial work on the integration of Cu-electrodes with BST films are also presented.     

MOCVD (BaxSr1−x)Ti1+yO3+z (BST) thin films for high frequency tunable devices

Abstract

We have investigated the structural and electrical characteristics of (Ba_xSr_1?x)Ti_1+yO_3+z (BST) thin films synthesized at 650°C on Pt/SiO₂/Si substrates using a large area, vertical metalorganic chemical vapor deposition (MOCVD) reactor equipped with a liquid delivery system. Films with a Ba/Sr ratio of 70/30 were studied, as determined using X-ray fluorescence spectroscopy (XRF) and Rutherford backscattering spectrometry (RBS). A substantial reduction of the dielectric loss was achieved when annealing the entire capacitor structure in air at 700°C. Dielectric tunability as high as 2.3:1 was measured for BST capacitors with the currently optimized processing conditions.     

Development of Materials Integration Strategies for Electroceramic Film-Based Devices Via Complementary In Situ and Ex Situ Studies of Film Growth and Interface Processes

We review our studies of film growth and interface processes performed using complementary in situ and ex situ characterization techniques that provide valuable information critical to the development of materials integration strategies for the fabrication of electroceramic film-based devices. Specifically, we review our work performed using in situ time-of-flight ion scattering and recoil spectroscopy (TOF-ISARS) / X-ray photoemission spectroscopy (XPS) / spectroscopic ellipsometry (SE), in conjunction with ex situ TEM and other techniques to study film growth and interface processes critical to the fabrication of non-volatile ferroelectric memories (NVFRAMs), dynamic random access memories (DRAMs), and high frequency devices based on high-K thin films. TOF-ISARS involves three distinct but closely related experimental methods, namely: ion scattering spectroscopy, direct recoil spectroscopy and mass spectroscopy of recoiled ions, which provide monolayer-specific information on film growth and surface segregation processes. Spectroscopic Ellipsometry enables investigation of buried interfaces. XPS provides valuable information on the chemistry of surface and interfaces. Specifically, we discuss: a) studies of oxidation of Ti-Al layers and synthesis and properties of La 0.5 Sr 0.5 CoO 9 /Ti-Al heterostructured layers for integration of PZT capacitors with Si substrates, and b) studies of BaSr x Ti 1 m x O 3 layer integration with Si substrates relevant to DRAMs, high frequency devices and high-K gate oxides for integrated circuits. This review shows the power of combined in situ / ex situ analytical techniques to provide valuable information for material integration strategies for electroceramic thin film-based devices.     

Electrodes for ferroelectric thin films

Abstract

Platinum and ruthenium oxide (RuO₂) deposited by ion beam sputter-deposition are evaluated for use as electrodes for PZT thin film capacitors. The effect of deposition temperature, film thickness, and the presence of oxygen on hillock formation in platinum is discussed. It is shown that the hillock density in Pt/Ti/SiO₂/Si films can be significantly reduced by properly controlling the processing conditions and film thickness. Stress measurements correlate with the experimental observation that depositing thinner platinum films (<800 Å) is an effective means of reducing hillock formation. The use of an intermediate deposition temperature 200–250°C also helps minimize hillock formation. Diffusion of the Ti adhesion layer into and/or through the platinum was significantly reduced by replacing the Ti with a TiO_x adhesion layer. RuO₂ electrodes are compared to Pt in terms of resistivity, surface morphology, microstructure and film orientation.     

Ultrananocrystalline Diamond Film as a Wear-Resistant and Protective Coating for Mechanical Seal Applications

Mechanical shaft seals used in pumps are critically important to the safe operation of the paper, pulp, and chemical process industry, as well as petroleum and nuclear power plants. Specifically, these seals prevent the leakage of toxic gases and hazardous chemicals to the environment and final products from the rotating equipment used in manufacturing processes. Diamond coatings have the potential to provide negligible wear, ultralow friction, and high corrosion resistance for the sliding surfaces of mechanical seals, because diamond exhibits outstanding tribological, physical, and chemical properties. However, diamond coatings produced by conventional chemical vapor deposition (CVD) exhibit high surface roughness (R_a ≥ 1 μm), which results in high wear of the seal counterface, leading to premature seal failure. To avoid this problem, we have developed an ultrananocrystalline diamond (UNCD) film formed by a unique CH₄/Ar microwave plasma CVD method. This method yields extremely smooth diamond coatings with surface roughness R_a = 20–30 nm and an average grain size of 2–5 nm. We report the results of a systematic test program involving uncoated and UNCD-coated SiC shaft seals. Results confirmed that the UNCD-coated seals exhibited neither measurable wear nor any leakage during long-duration tests that took 21 days to complete. In addition, the UNCD coatings reduced the frictional torque for seal rotation by five to six times compared with the uncoated seals. This work promises to lead to rotating shaft seals with much improved service life, reduced maintenance cost, reduced leakage of environmentally hazardous materials, and increased energy savings. This technology may also have many other tribological applications involving rolling or sliding contacts.     

Characterization of conduction in PZT thin films produced by laser ablation deposition

Abstract

Both direct current (d.c.) and alternating current (a.c.) conductivity measurements were undertaken on lead zirconate titanate (PZT) films synthesized by laser ablation deposition. Direct current (I) displayed an initial time dependence of the form I ∝ t^?γ (γ ∝ 0.5–1.0). The possible reasons for this time dependence are discussed. At lower temperatures, the a.c. electrical conductivity shows a frequency dependence of the form σ ∝ ω′ which is explained as electrical charge hopping. At higher temperatures, the d.c. component of electrical conductivity becomes dominant, and is accompanied by a strong low frequency dispersion of the dielectric constant. The results are compared to published data on conductivity in SrTiO₃, and discussed in terms of the latest theories for dielectric response of materials.     

Status review of the science and technology of ultrananocrystalline diamond (UNCD™) films and application to multifunctional devices

This review focuses on a status report on the science and technology of ultrananocrystalline diamond (UNCD) films developed and patented at Argonne National Laboratory. The UNCD material has been developed in thin film form and exhibit multifunctionalities applicable to a broad range of macro to nanoscale multifunctional devices. UNCD thin films are grown by microwave plasma chemical vapor deposition (MPCVD) or hot filament chemical vapor deposition (HFCVD) using new patented Ar-rich/CH₄ or H₂/CH₄ plasma chemistries. UNCD films exhibit a unique nanostructure with 2–5 nm grain size (thus the trade name UNCD) and grain boundaries of 0.4–0.6 nm for plain films, and grain sizes of 7–10 nm and grain boundaries of 2–4 nm when grown with nitrogen introduced in the Ar-rich/CH₄ chemistry, to produce UNCD films incorporated with nitrogen, which exhibit electrical conductivity up to semi-metallic level. This review provides a status report on the synthesis of UNCD films via MPCVD and integration with dissimilar materials like oxides for piezoactuated MEMS/NEMS, metal films for contacts, and biological matter for a new generation of biomedical devices and biosensors. A broad range of applications from macro to nanoscale multifunctional devices is reviewed, such as coatings for mechanical pumps seals, field-emission cold cathodes, RF MEMS/NEMS resonators and switches for wireless communications and radar systems, NEMS devices, biomedical devices, biosensors, and UNCD as a platform for developmental biology, involving biological cells growth on the surface. Comparisons with nanocrystalline diamond films and technology are made when appropriate.     

DNA-modified nanocrystalline diamond thin-films as stable,biologically active substrates

Diamond, because of its electrical and chemical properties, may be a suitable material for integrated sensing and signal processing. But methods to control chemical or biological modifications on diamond surfaces have not been established. Here, we show that nanocrystalline diamond thin-films covalently modified with DNA oligonucleotides provide an extremely stable, highly selective platform in subsequent surface hybridization processes. We used a photochemical modification scheme to chemically modify clean, H-terminated nanocrystalline diamond surfaces grown on silicon substrates, producing a homogeneous layer of amine groups that serve as sites for DNA attachment. After linking DNA to the amine groups, hybridization reactions with fluorescently tagged complementary and non-complementary oligonucleotides showed no detectable non-specific adsorption, with extremely good selectivity between matched and mismatched sequences. Comparison of DNA-modified ultra-nanocrystalline diamond films with other commonly used surfaces for biological modification, such as gold, silicon, glass and glassy carbon, showed that diamond is unique in its ability to achieve very high stability and sensitivity while also being compatible with microelectronics processing technologies. These results suggest that diamond thin-films may be a nearly ideal substrate for integration of microelectronics with biological modification and sensing.