No abstract

A grain-size-dependent reduction in the room-temperature thermal conductivity of nanocrystalline yttria-stabilized zirconia is reported for the first time. Films were grown by metal-organic chemical vapor deposition with controlled grain sizes from 10 to 100 nm. For grain sizes smaller than approximately 30 nm, a substantial reduction in thermal conductivity was observed, reaching a value of less than one-third the bulk value at the smallest grain sizes measured. The observed behavior is consistent with expectations based on an estimation of the phonon mean-free path in zirconia. © 2000 American Institute of Physics. ͓S0003-6951͑00͒05034-8͔The efficiency of gas turbine engines is dictated by the maximum sustained operating temperature of their typically Ni-or Co-based alloy turbine rotors. The development of new, higher temperature, high-strength, lightweight alloys is desirable. 1 However, recent studies have concluded that significant near-term progress in increasing turbine engine operating temperatures is more likely to come from the development of improved thermal barrier coatings ͑TBCs͒, typically yttria-stabilized zirconia ͑YSZ͒, than from the design of new alloys. 2 New processing techniques that result in TBC microstructures with lower thermal conductivity could lead either to higher operating temperatures of turbine engines, resulting in greater efficiency, or thinner coatings for the same operating temperature, which would reduce overall weight. Nanocrystalline YSZ coatings are of interest because they offer the possibility of lowering thermal conductivity, and may also provide additional benefits for TBC applications because of the possibility of improved toughness and ductility compared to that of coarser-grained ceramics. 3,4 The low thermal conductivity of YSZ ͑ϳ2.3 W/mK for high-density, polycrystalline material with a yttria-content of 10 mol. % at 20°C 5 ͒ is due primarily to phonon scattering by vacancies on the material's highly defective oxygen sublattice. 6 The potential for reduced thermal conductivity in nanocrystalline coatings arises from the predicted enhanced phonon scattering due to the presence of numerous closely spaced grain boundaries. For example, Klemens and Gell 6 have theoretically predicted that the room temperature thermal conductivity of 10 nm grain-sized YSZ containing 7 wt. % Y 2 O 3 will be decreased more than 50% compared to 1 m grain-sized YSZ of the same composition. The goal of the present study was to experimentally determine the effect of grain size on the room-temperature thermal conductivity of YSZ, thus contributing to the fundamental understanding of grain-size-dependent phonon scattering processes.Nanocrystalline YSZ films were grown by metal-organic chemical vapor deposition ͑MOCVD͒ using a low-pressure, horizontal, cold-walled deposition system. Yttrium b-diketonate ͓Y͑thd͒ 3 ͔ and zirconium t-butoxide ͓ZrOC͑CH 3 ͒ 4 ͔ 7 were chosen as precursor materials. Highpurity nitrogen was used as the precursor carrier gas. The precursors were mixed with high-purity ...

The challenges of reducing gate leakage current and dielectric breakdown beyond the 45 nm technology node have shifted engineers' attention from the traditional and proven dielectric SiO 2 to materials of higher dielectric constant also known as high-k materials such as hafnium oxide ͑HfO 2 ͒ and aluminum oxide ͑Al 2 O 3 ͒. These high-k materials are projected to replace silicon oxide ͑SiO 2 ͒. In order to address the complex process integration and reliability issues, it is important to investigate the mechanical properties of these dielectric materials in addition to their electrical properties. In this study, HfO 2 and Al 2 O 3 have been fabricated using atomic layer deposition ͑ALD͒ on ͑100͒ p-type Si wafers. Using nanoindentation and the continuous stiffness method, we report the elastomechanical properties of HfO 2 and Al 2 O 3 on Si. ALD HfO 2 thin films were measured to have a hardness of 9.5 Ϯ 2 GPa and a modulus of 220 Ϯ 40 GPa, whereas the ALD Al 2 O 3 thin films have a hardness of 10.5 Ϯ 2 GPa and a modulus of 220 Ϯ 40 GPa. The two materials are also distinguished by very different interface properties. HfO 2 forms a hafnium silicate interlayer, which influences its nanoindentation properties close to the interface with the Si substrate, while Al 2 O 3 does not exhibit any interlayer.For the past 40 years the microelectronics industry has relied on the scaling down of device size in order to improve the performance, functionality, and bit density of chips, as described by Moore's law. As microelectronics is transitioning into deep nanotechnology, the drawback of the increasing miniaturization of devices is the increase of gate leakage current and oxide breakdown. 1 To reduce the gate leakage current and breakdown field across the gate insulator, researchers are looking into high-k dielectric materials. High-k materials such as HfO 2 and Al 2 O 3 will increase the transistor drive current and the transistor switching speed. 2 HfO 2 is predicted to replace SiO 2 , SiO x N y , and Si 3 N 4 as the gate dielectric of complementary metal oxide semiconductor ͑CMOS͒ devices at the 45 nm technology node and beyond. HfO 2 and Al 2 O 3 have dielectric constants of approximately k = 25 and 8, respectively, 3 which compare favorably with k = 3.9 for SiO 2 . Various deposition techniques have been used to deposit high-k materials. Among these growth techniques are metallorganic chemical vapor deposition ͑MOCVD͒, 4-6 pulsed laser deposition ͑PLD͒, 7 and atomic layer deposition ͑ALD͒. 4-6,8 MOCVD and PLD require a high temperature during processing and film fabrication. 9 For example, a minimum temperature of 600°C is required to deposit HfO 2 with MOCVD, whereas HfO 2 crystallizes once the temperature reaches 600°C. 10 ALD is a chemical reactionbased deposition technique that requires only relatively low temperatures. ALD provides absolute film deposition uniformity ͑atomic layer by atomic layer͒, precise composition control, high conformality, and completely self-limiting surface reactions, which makes ALD the most...

No abstract

Whenever a nanoindent is placed near an edge, such as the free edge of the specimen or heterophase interface intersecting the surface, the elastic discontinuity associated with the edge produces artifacts in the load–depth data. Unless properly handled in the data analysis, the artifacts can produce spurious results that obscure any real trends in properties as functions of position. Previously, we showed that the artifacts can be understood in terms of a structural compliance, Cs, which is independent of the size of the indent. In the present work, the utility of the SYS (Stone, Yoder, Sproul) correlation is demonstrated in its ability to remove the artifacts caused by Cs. We investigate properties: (i) near the surface of an extruded polymethyl methacrylate rod tested in cross section, (ii) of compound corner middle lamellae of loblolly pine (Pinus taeda) surrounded by relatively stiff wood cell walls, (iii) of wood cell walls embedded in a polypropylene matrix with some poorly bonded wood–matrix interfaces, (iv) of AlB2 particles embedded in an aluminum matrix, and (v) of silicon-on-insulator thin film on substrate near the free edge of the specimen.