Resistance of siloxane-based, low-dielectric-constant ͑low-k͒ dielectrics against heat and moisture stress is clarified. The organosilica-glass ͑OSG͒ and the silicon-oxycarbide are shown to be the most reliable: the k-values are stable even after a heating test at 650°C and a pressure cooker treatment for 100 h. This stability is high enough to ensure the low-k property throughout fabricating multilevel interconnects and long-term reliability after the fabrication. This is shown to be due to the stability of Si-CH 3 bonds and Si-CH n -Si bonds incorporated in the OSG and the silicon-oxycarbide. The stability of the OSG in real low-k interlevel dielectric structure was also demonstrated using four-level interconnect test devices. The low-k property still remains even after the reliability tests, showing that the low-k interlayer dielectric structure is sufficiently resistant to heat and moisture stresses.Low-parasitic-capacitance multilevel interconnects using lowdielectric-constant ͑low-k͒ interlevel dielectrics ͑ILDs͒ are essential for high speed ultralarge-scale integrated circuits ͑ULSIs͒. Siloxanebased low-k materials, as well as carbon-based aromatic polymers, are promising as low-k ILDs with k of about 3. The siloxane-based materials are classified into the following: fluorinated-silica-glass ͑FSG, k ϭ 3.5-3.8͒, hydrogen-silsesquioxane ͑HSQ, k ϭ 2.7-3.3͒, organo-silica-glass ͑OSG, k ϭ 2.7-3.3͒. 1-6 Their typical chemical bonding structure is shown in Fig. 1a. The structure consists of siloxane networks ͑-Si-O-Si-͒ terminated by R. The R stands for F, H, and CH 3 in FSG, HSQ, and OSG, respectively. This R makes the film have low density and low moisture content, so the film has low-k properties. On the other hand, the R degrades the mechanical strength of the film because of network termination. Recently, we have proposed a new low-k material, silicon-oxycarbide, with high mechanical strength. 7,8 The structure of this material is shown in Fig. 1b. This material contains much less terminating CH 3 ͑R͒. Instead, it contains CH n ͑RЈ, n ϭ 0-2͒ bridging the siloxane networks.To assure the performance of the high-speed ULSIs, the electrical properties ͑k and breakdown voltage͒ of the low-k ILDs are needed to be resistant against heat and moisture stresses. The heat resistance is necessary to prevent degradation during the interconnect fabrication process. The ILDs are submitted to heating at over 400°C during fabrication of upper-level interconnects. The length of the heating is dependent on the process. If a conventional batch-type furnace is used for curing the low-k materials, the heating per layer is estimated to be 0.5-1 h. As a result, the lower level ILDs are subjected to many hours of heating. In addition, the moisture resistance is necessary for the long-term reliability after packaging. This is because, in real operating conditions, the ILDs are subjected to moisture penetrating from the outside the package. In the package reliability area, the effect of this moisture penetration was evaluated by accel...

Oxygen plasma resistance of low-k organosilica glasses ͑OSGs͒ is shown to be strongly dependent on the material structure: a silicon oxycarbide has a higher oxygen plasma resistance compared to a conventional OSG. The silicon oxycarbide is stable at pressures up to 300 mTorr, which is 10 times higher than those of the conventional OSG. Even at higher pressures, the degradation is much lower. Structural analysis using wet etching demonstrated that the stability at low pressures is due to a thin protecting layer: dense oxide formed by impingement of directional oxygen ions. The inside layer is shown to have the same k-value as the original film. The superior oxygen plasma resistance of the silicon oxycarbide is probably due to lower methyl group content, which provides greater volume reduction toward achieving a dense siloxane networks that protects the inside.

The barrier mechanism against copper-ion diffusion in silicon-oxide films deposited by plasma-enhanced chemical vapor deposition (PECVD) using trimethoxysilane (TMS) and nitrous oxide (N2O) chemistry (PE-TMS oxide) was studied. It was found that the flow ratio of TMS gas to N2O gas during deposition strongly affects a time-dependent dielectric-breakdown lifetime of PE-TMS oxide with a copper electrode as well as other PE-TMS oxide film properties such as electrical properties (leakage current and dielectric constant), a physical property (atomic composition), and chemical properties (chemical bonding states and wet-etching rate). The dielectric-breakdown lifetime of PE-TMS oxide film with a copper anode is a maximum at a source-gas ratio ranging from 1.7% to 3.3%. On the other hand, leakage current density, wet-etch rate, and dielectric-breakdown lifetime of PE-TMS oxide film with an aluminum electrode are degraded by increasing the source-gas flow ratio (0.83% to 12%). These results suggest that two types of degradation mode exist in the dielectric breakdown of PE-TMS oxide with a copper electrode. Namely, at low flow ratio (<1.7%), copper-induced degradation is dominant, but at high flow ratio (>3.3%), the dielectric degradation is probably not caused by copper contamination but by low-quality dielectric material. The dielectric-breakdown lifetimes of a PE-TMS oxide film (flow ratio: 3.3%) with a copper anode show an Arrhenius-type temperature dependence. That is, the activation energy of the dielectric-breakdown lifetime depends on the applied electric field and decreases from 1.8 to 0.55 eV when the applied field is increased from 0 to 5 MV/cm. As a simple kinetic model of the copper injection reaction at the anode surface, a thermally activated reaction process between two energy states—copper atom state on the anode surface and copper ion state in the dielectric material—is proposed.

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We report the first total syntheses of (+)-isolaurenidificin (1) and (−)-bromlaurenidificin (2), the latest acetogenins of the 2,6-dioxabicyclo[3.3.0]­octane class. The synthesis features a completely stereoselective one-pot epimerization-ring contraction to establish the cis configuration with respect to C10–H and C12–H of the tetrahydrofuran ring. Six stereogenic centers and an olefin geometry were constructed in a highly stereoselective manner. Absolute configurations of the natural products were deduced by the comparison of NMR data and specific rotations.