Volatile metal borohydride complexes: synthesis and characterization of new chemical vapor deposition precursors
The complex sodium methylborohydride, Na(H3BCH3), can be prepared in high yield (83%) by the addition of trimethylboroxine, (H3C)3B3O3, to sodium aluminum hydride, NaAlH4. The subsequent reaction of two equivalents of sodium methylborohydride with the alkaline earth bromides; MgBr2, CaBr2, SrBr2, and BaBr2 in 1,2-dimethoxyethane, DME, affords the new alkaline earth methylborohydride DME adducts: [Mg(H3BCH3)2(DME)]2, Ca(H3BCH3)2(DME)2, Sr(H3BCH3)2(DME)3, and Ba(H3BCH3)2(DME)3. [Mg(H3BCH3)2(DME)]2 sublimes between 80 and 90 °C at 10 mTorr while the larger alkaline earth methylborohydrides do not sublime up to 120 °C. [Mg(H3BCH3)2(DME)]2 is an asymmetrically bridged dimer in the solid state where each Mg center has a terminal κ2H-methylborohydride, a bridging κ2H-methylborohydride, a bridging κ1H-methylborohydride, and a chelating DME. The other alkaline earth methyborohydrides have two κ3H-methylborohydrides and two chelating DME for the Ca complex and three chelating DME for the Sr and Ba complexes. Rare earth methylborohydride THF adducts are prepared by the reaction of a rare earth chloride (Sc, Y, Nd, Gd, Er) with 3 to 4 equivalents of sodium methylborohydride in THF. The scandium and yttrium complexes are isolated by sublimation at 50 °C while the neodymium, gadolinium, and erbium complexes are isolated by sublimation at 60 °C. In the solid state, scandium methylborohydride has three κ3H-methylborohydrides and one coordinated THF. The yttrium, gadolinium, and erbium complexes crystallize as charge separated ion pairs: [RE(H3BCH3)2(THF)4] [RE(H3BCH3)4], where the cation has two κ3H-methylborohydrides and four coordinated THF and the anion consists of four κ3H-methylborohydrides. The neodymium complex is a methylborohydride bridged dimer, [Nd(H3BCH3)3(THF)2]2, where each Nd center has two κ3H-methylborohydrides, two bridging κ2H,κ2H-methylborohydrides and two THF. In addition to the THF adducts, the neodymium DME adduct, Nd(H3BCH3)3(DME)1.5, has also been synthesized by a similar method. This complex can be sublimed under vacuum at 115 °C. The Er complex has been used in preliminary CVD experiments which demonstrate the ability to grow thin films between 250 and 350 °C using this new precursor. The synthesis of sodium aminodiboranates with sterically bulky or electron withdrawing substituents on nitrogen has been achieved by the treating amine-borane with either BH3•THF or by thermolysis at elevated temperatures followed by the addition of BH3•THF, which produced µ-aminodiborane. The µ-aminodiborane can then be ring opened with NaH, similar to what has been reported by Keller for the synthesis of sodium N,N-dimethylaminodiboranate. Implementing this method, the sterically bulky aminodiboranates: sodium N-isopropyl-N-methylaminodiboranate, sodium N,N-diisopropylaminodiboranate, sodium cis-2,6-dimethylpiperidinyldiboranate, sodium tert-butylaminodiboranate, and sodium N-isopropylaminodiboranate have been prepared. The aminodiboranates with electron withdrawing substituents on nitrogen: sodium N-benzylaminodiboranate, sodium N-benzyl-N-methylaminodiboranate, and sodium 2,2-difluoroethylaminodiboranate were also able to be prepared by the addition of BH3•THF to the appropriate amine-borane followed by treatment with sodium hydride. Unfortunately, these aminodiboranates decompose at room temperature. Magnesium cis-2,6-dimethylpiperidinyldiboranate was able to be synthesized by treatment of MgBr2 with two equivalents of sodium cis-2,6-dimethylpiperidinyldiboranate in diethyl ether followed by sublimation at 50 °C under vacuum. The hydrolysis/thermolysis product µ-(cis-2,6-dimethylpiperidinyl)diborane is, however, present in the sublimate due to similar volatility to the desired magnesium product. Synthesis of magnesium N,N-diisopropylaminodiboranate was attempted by ball milling MgBr2 and sodium N,N-diisopropylaminodiboranate followed by sublimation at 65 °C. Interestingly, primarily decomposition products, N,N-dimethylimine and magnesium borohydride, Mg(BH4)4, were observed by 11B NMR in the reaction mixture. Static chemical vapor deposition (CVD) has been successfully used to deposit conformal thin films of hafnium diboride, HfB2, and iron metal from hafnium borohydride, Hf(BH4)4, and iron pentacarbonyl, Fe(CO)5, respectively. Microtrenches with aspect ratios greater than 10:1 were able to be completely infilled with HfB2 or iron and macrotrenches were able to be coated with thin films of HfB2 which has a 40% step coverage at an aspect ratio of 1000:1. HfB2 thin films deposited by static CVD have a Hf:B ratio similar to films deposited using Hf(BH4)4 in an actively pumped, low pressure CVD system; although the relative hydrogen content of the film deposited by static CVD was greater. Iron thin films deposited by static CVD have an iron composition as high as 97% with approximately 1.5% carbon and oxygen each.
inorganic synthesis, chemical vapor deposition, precursor, thin films, rare earth metal, lanthanide, borohydride
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign