In Situ Infrared Absorption Study of Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride.

Despite the success of plasma-enhanced atomic layer deposition (PEALD) in depositing quality silicon nitride films, a fundamental understanding of the growth mechanism has been difficult to obtain because of lack of in situ characterization to probe the surface reactions noninvasively and the complexity of reactions induced/enhanced by the plasma. These challenges have hindered the direct observation of intermediate species formed during the reactions. We address this challenge by examining the interaction of Ar plasma using atomically flat, monohydride-terminated Si(111) as a well-defined model surface and focusing on the initial PEALD with aminosilanes. In situ infrared and X-ray photoelectron spectroscopy reveals that an Ar plasma induces desorption of H atoms from H-Si(111) surfaces, leaving Si dangling bonds, and that the reaction of di-sec-butylaminosilane (DSBAS) with Ar plasma-treated surfaces requires the presence of both active sites (Si dangling bonds) and Si-H; there is no reaction on fully H-terminated or activated surfaces. By contrast, high-quality hydrofluoric acid-etched Si3N4 surfaces readily react with DSBAS, resulting in the formation of O-SiH3. However, the presence of back-bonded oxygen in O-SiH3 inhibits H desorption by Ar or N2 plasma, presumably because of stabilization of H against ion-induced desorption. Consequently, there is no reaction of adsorbed aminosilanes even after extensive Ar or N2 plasma treatments; a thermal process is necessary to partially remove H, thereby promoting the formation of active sites. These observations are consistent with a mechanism requiring the presence of both undercoordinated nitrogen and/or dangling bonds and unreacted surface hydrogen. Because active sites are involved, the PEALD process is found to be sensitive to the duration of the plasma exposure treatment and the purge time, during which passivation of these sites can occur.

Density functional theory study on the full ALD process of silicon nitride thin film deposition via BDEAS or BTBAS and NH3.

A detailed reaction mechanism has been proposed for the full ALD cycle of Si3N4 deposition on the ß-Si3N4(0001) surface using bis(diethylamino)silane (BDEAS) or bis(tertiarybutylamino)silane (BTBAS) as a Si precursor with NH3 acting as the nitrogen source. Potential energy landscapes were derived for all elementary steps in the proposed reaction network using a periodic slab surface model in the density functional approximation. Although the dissociative reactivity of BTBAS was slightly better than that of BDEAS, the thermal deposition process was still found to be an inherently high temperature process due to the high activation energies during the dissociative chemisorption of both precursors and the surface re-amination steps. These results underline the need to develop new precursors and alternative nitrogen sources when low temperature thermal silicon nitride films are targeted.

New volatile strontium and barium imidazolate complexes for the deposition of group 2 metal oxides.

We report the synthesis, characterization, and experimental density function theory-derived properties of new volatile strontium and barium imidazolate complexes, which under atomic layer deposition conditions using ozone as a reagent can deposit crystalline strontium oxide at 375 °C.

Vapor-Phase Cleaning and Corrosion Inhibition of Copper Films by Ethanol and Heterocyclic Amines.

Cleaning and passivation of metal surfaces are necessary steps for selective film deposition processes that are attractive for some microelectronic applications (e.g., fully aligned vias or self-aligned contacts). For copper, there is limited knowledge about the mechanisms of the copper oxide reduction process and subsequent passivation layer formation reactions. We have investigated the in situ cleaning (i.e., oxidation and reduction by vapor-phase species) and passivation of chemical-mechanical polishing (CMP)-prepared Cu films in an effort to derive the mechanisms associated with selectively tailoring the surface chemistry. By monitoring the interaction of vapor-phase ethanol with the surface species generated after ozone cleaning at 300 °C, we find that the optimum procedure to remove these species and avoid byproduct redeposition is to use atomic layer deposition (ALD)-like binary cycles of ethanol and N2, with active pumping. We have further explored passivation of clean Cu using benzotriazole and 2,2'-bipyridine in an ALD environment. Both molecules chemisorb on clean Cu in an upright orientation, with respect to the metal surface at temperatures higher than the melting point of the organic inhibitors (100 ≤ T < 300 °C). Both molecules desorb without decomposition from clean Cu above 300 °C but not from Cu2O. Previous studies related to the passivation of Cu surfaces using heterocyclic amines have focused on solution-based or ultrahigh vacuum applications of the passivation molecules onto single crystalline Cu samples. The present work explores more industrially relevant vapor-phase passivation of CMP-cleaned, electroplated Cu samples using ALD-like processing conditions and in situ vapor-phase cleaning.

Coordination Chemistry, Structure, and Reactivity of Thiouracil Derivatives of Tungsten(0) Hexacarbonyl: A Theoretical and Experimental Investigation into the Chelation/Dechelation of Thiouracil via CO Loss and Addition.

The synthesis of 2-thiouracil and 6-methyl-2-uracil derivatives of tungsten carbonyl from the reaction of photogenerated W(CO)(5)(solvent) (solvent = MeOH or THF) and the corresponding [Et(4)N][thiouracilate] is described. The crystal structure of the [Et(4)N][W(CO)(5)(2-thiouracilate)], 1, derivative is reported where the thiouracilate is found to be bound to the tungsten center via the exocyclic sulfur atom. In the solid-state structure of 1 two anions are associated by means of two hydrogen bonds between the metal bound nucleobases. These pentacarbonyl complexes stereoselectively lose cis carbonyl ligands, as is apparent from (13)C-labeling studies, to provide the endocyclic nitrogen N(1)-chelated tungsten tetracarbonyl derivatives, e.g., [Et(4)N][W(CO)(4)(2-thiouracilate)], 3. The kinetics of the loss of CO from the pentacarbonyl anions to afford the metal tetracarbonyls, and the reverse of that process, were monitored by means of in situ infrared spectroscopy in the nu(CO) region as a function of temperature. These studies reveal that the tetracarbonyl anions in CO-saturated acetonitrile ([CO] approximately 6 x 10(-)(3) M) are unstable with respect to the formation of the pentacarbonyl derivatives, i.e., the equilibrium 1 right harpoon over left harpoon 3 + CO lies to the left under an atmosphere of carbon monoxide. From the activation parameters determined for the dissociative CO loss process (DeltaH() = 82.0 +/- 3.6 kJ mol(-)(1) and DeltaS() = -44.9 +/- 9.6 J mol(-)(1) K(-)(1) for complex 1) it is apparent that the sulfur-bound thiouracilate ligand is serving as a pi-donor during CO dissociation, i.e., behaving as a cis-labilizing ligand. Ab initio geometry optimizations carried out for the process 1 right harpoon over left harpoon 3 + CO at the Hartree-Fock and DFT levels support these experimental observations. For example, complex 1 is shown to be more stable than 3 + CO and chelation via the endocyclic N(1) donor is favored over N(3) binding. Finally, the "16-electron" intermediate resulting from CO dissociation in 1 was found to possess a significantly shortened W-S interaction, presumably due to the pi-donating ability of the thiouracilate ligand.

The origin of endocyclic bond length variations in disubstituted cyclotriphosphazenes.

Intraannular ring alternation in heterogeneously substituted cyclotriphosphazenes is investigated using both ab initio and density functional methods. Comparisons of the calculated geometries for N3P3X6 (X = H, F, Cl, Me, Ph) and N3P3X4Y2 (X = F, Cl; Y = Me, Ph, X, Cl) with experimental X-ray data establish the utility of the 6-31G* basis for use with both Hartree-Fock and the B3LYP and B3PW91 functionals. Analysis of orbitals and charges shows that the bonding is best described using a polarized bond model rather than the previous explanation of asymmetric nitrogen-lone-pair donation into the phosphorus-nitrogen bond.

Unique reactivity of a tetradentate N(2)S(2) complex of nickel: intermediates in the production of sulfur oxygenates.

Complex 1 [(N,N'-dimethyl-N,N'-bis(2-sulfanylethyl)ethylenediamine)nickel(II)], previously shown to react with H(2)O(2) to produce the fully oxygenated disulfonate 5 [diaqua(N,N'-dimethyl-N,N'-bis(2-sulfonatoethyl)ethylenediamine)nickel(II)], has been explored in detail to explain the observed reactivity of this compound and to discern intermediates in the oxygenation reaction. Reaction of 1 with 1 equiv of methyl iodide results in the monomethylated square-planar nickel complex 2 [[(N,N'-dimethyl-N-(2-sulfanylethyl)-N'-(2-methylthioethyl)(ethylenediamine)nickel(II)] iodide], while a slight excess of methyl iodide results in the dimethylated complex 3 [diiodo(N,N'-dimethyl-N,N'-bis(2-methylthioethyl)ethylenediamine)nickel(II)], an X-ray structure of which has shown that the nickel ion is in an octahedral N(2)S(2)I(2) environment. Crystal data of 3: monoclinic, a = 8.865(3) A, b = 14.419(4) A, c = 14.389(6) A, beta = 100.19(3) degrees, V = 1810.2(12) A(3), space group P2(1)/n, Z = 4. The equatorial positions are occupied by the two cis-amine N-atoms and the coordinated iodides, while the axial positions are occupied by the thioether sulfur atoms. In organic solvents, the dithiolate complex 1 reacts with molecular oxygen or H(2)O(2) to produce the mixed sulfinato/thiolato complex 4 [(N,N'-dimethyl-N-(2-sulfanylethyl)-N'-(2-sulfinatoethyl)(ethylenediamine)nickel(II)], and the fully oxidized product 5. X-ray analysis of complex 4 reveals a square-planar geometry in which the nickel ion is coordinated by two cis-amine nitrogens, one thiolate sulfur donor, and one sulfinato sulfur donor. Crystal data of 4: orthorhombic, a = 11.659(2) A, b = 13.119(3) A, c = 16.869(3) A, V = 2580.2(9) A(3), space group Pbca, Z = 8. This complex is the only intermediate in the oxygenation reaction that could be isolated, and it is shown to be further reactive toward O(2) to yield the fully oxidized product 5. For a better understanding of the reactivity observed for 4, DFT calculations have been undertaken, which show a possible reaction path toward the fully oxidized product 5.