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

The surface chemistry of the atomic layer deposition of metal thin films

In this perspective we discuss the progress made in the mechanistic studies of the surface chemistry associated with the atomic layer deposition (ALD) of metal films and the usefulness of that knowledge for the optimization of existing film growth processes and for the design of new ones. Our focus is on the deposition of late transition metals. We start by introducing some of the main surface-sensitive techniques and approaches used in this research. We comment on the general nature of the metallorganic complexes used as precursors for these depositions, and the uniqueness that solid surfaces and the absence of liquid solvents bring to the ALD chemistry and differentiate it from what is known from metalorganic chemistry in solution. We then delve into the adsorption and thermal chemistry of those precursors, highlighting the complex and stepwise nature of the decomposition of the organic ligands that usually ensued upon their thermal activation. We discuss the criteria relevant for the selection of co-reactants to be used on the second half of the ALD cycle, with emphasis on the redox chemistry often associated with the growth of metallic films starting from complexes with metal cations. Additional considerations include the nature of the substrate and the final structural and chemical properties of the growing films, which we indicate rarely retain the homogeneous 2D structure often aimed for. We end with some general conclusions and personal thoughts about the future of this field.

Synthesis and Crystal Structures of New Strontium Complexes with Aminoalkoxy and β-Diketonato Ligands

New heteroleptic strontium complexes were synthesized using substitution reaction of bis(trimethylsilyl)amide of Sr(btsa)₂·2DME with aminoalkoxide and β-diketonate ligands. The complexes [Sr(bdmp)(btsa)]₂·2THF (1), [Sr(bdeamp)(btsa)]₂ (2), [Sr(dadamb)(btsa)]₂ (3), [Sr(bdmp)(hfac)]₃ (4), [Sr(bdeamp)(hfac)]₃ (5), [Sr(dadamb)(hfac)]₃ (6), and [Sr₃(dadamb)₄(tmhd)₂] (7) were prepared and characterized by means of various analysis techniques such as Fourier transform infrared, NMR, thermogravimetric analysis, and elemental analysis. Complexes 1-3 were further structurally confirmed by single-crystal X-ray crystallography, and they displayed dimeric structures in which strontium atoms were connected by alkoxide oxygen atoms of the μ₂ type. Compound 1 has a trigonal prismatic structure, whereas 2 and 3 have a distorted square pyramidal structure. In complexes 5-7, trimeric structures were obtained with strontium atoms connected by μ₃-O bonds of alkoxide oxygen atoms and μ₂-O bonds of alkoxide and β-diketonate oxygen atoms. The crystal structures of 5, 6, and 7 showed distorted capped octahedral geometry, while 7 (middle Sr atom) displayed a distorted trigonal prism geometry. Complexes 5-7 displayed ∼70% mass loss in the temperature range from 25 to 315 °C.

Synthesis and Characterization of Strontium N,N-Dimethylaminodiboranates as Possible Chemical Vapor Deposition Precursors

The reaction of SrBr₂ with 2 equiv of sodium N,N-dimethylaminodiboranate (DMADB; Na(H₃BNMe₂BH₃)) in Et₂O at 0 °C followed by crystallization and drying under vacuum gives the unsolvated strontium compound Sr(H₃BNMe₂BH₃)₂ (1). Before the vacuum-drying step, the colorless crystals obtained by crystallization consist of the diethyl ether adduct Sr(H₃BNMe₂BH₃)₂(Et₂O)₂ (2). If the reaction of SrBr₂ with 2 equiv of Na(H₃BNMe₂BH₃) is carried out in the more strongly coordinating solvent thf, the solvate Sr(H₃BNMe₂BH₃)₂(thf)₃ (3) is obtained. Treating the thf adduct 3 with 1,2-dimethoxyethane (dme), bis(2-methoxyethyl) ether (diglyme), or N,N,N',N'-tetramethylethylenediamine (tmeda) in thf affords the new compounds Sr(H₃BNMe₂BH₃)₂(dme)₂ (4), Sr(H₃BNMe₂BH₃)₂(diglyme) (5), and Sr(H₃BNMe₂BH₃)₂(tmeda) (6), respectively, in greater than 60% yields. Treatment of 3 with 2 equiv of the crown ether 12-crown-4 affords the charge-separated salt [Sr(H₃BNMe₂BH₃)(12-crown-4)₂][H₃BNMe₂BH₃] (7). Crystal structures of all the Lewis base adducts are described. Compounds 2-6 all possess chelating κ²-BH₃NMe₂BH₃-κ² groups, in which two hydrogen atoms on each boron center are bound to strontium. Compound 6 is dinuclear because each metal atom is also coordinated to one hydrogen atom on a BH₃NMe₂BH₃^- ligand that chelates to the neighboring metal center. Compound 7 possesses an unusual κ¹-BH₃NMe₂BH₃^- group owing to the near-complete encapsulation of the Sr atom by two 12-crown-4 molecules; the other BH₃NMe₂BH₃^- anion is a charge-separated counterion. When they are heated, the diglyme and tmeda compounds 5 and 6 melt without decomposition and can be sublimed readily under reduced pressure (1 Torr) at 120 °C. The diglyme and tmeda adducts are some of the most volatile strontium compounds known and are promising candidates as CVD precursors for the growth of strontium-containing thin films.

Heteroleptic strontium complexes stabilized by donor-functionalized alkoxide and β-diketonate ligands

Heteroleptic complexes of strontium () were prepared by employing β-diketonates and donor-functionalized alkoxides as coordinating ligands. The results illustrate the effect of alkoxide substituent groups on the overall structures of the complexes. The presence of a terminal methoxy group in the alkoxide ligands leads to the formation of trimeric complexes , whereas the substituents on the amine nitrogen prove to have less influence in determining the structure. The attempts to increase steric bulkiness of the aminoalkoxide ligands by introducing ethyl groups on the amine nitrogen and to the alkoxy carbon did not lead to a structural change from the dimeric form in but resulted in structurally interesting strontium complexes. In trimeric complexes , the three strontium atoms were held together by two μ3-O bonds using alkoxide oxygen atoms and two μ2-O bonds using a combination of alkoxide and β-diketonate ligand oxygens. The strontium metal centers in these complexes exhibit seven-coordination states in and , whereas exhibits one six-coordinated and two seven-coordinated strontium metals in its structure. All of the complexes were characterized using FT-NMR, FT-IR, elemental analyses, and thermogravimetric (TG) analyses.

Mechanochemical synthesis of an organometallic compound: a high volume manufacturing method

Chemical vapor deposition (CVD) precursor chemicals are held to some of the highest purity levels in industry. Many metal reagents form stable, unbreakable adducts with the coordinating solvents that are necessary for solvating highly polar reagents. These adducts are undesirable and must be removed prior to usage. Herein we describe a mechanochemical approach to the synthesis of bis(n-propyltetramethylcyclopentadienyl)strontium that eliminates the use of strongly coordinating solvents. This method overcomes the solubility problems of the two reagents without the formation of stable, unbreakable adducts. We utilize a unique reactor geometry that facilitates mechanochemical syntheses while simplifying handling and allowing for "one pot" production. The synthesis was scaled to five hundred gram lot sizes in a six liter reactor. This technique is applicable to many syntheses and is linearly scalable - limited only by reactor size.