Atomic layer deposition chemistry: recent developments and future challenges

New materials, namely high-k (high-permittivity) dielectrics to replace SiO(2), Cu to replace Al, and barrier materials for Cu, are revolutionizing modern integrated circuits. These materials must be deposited as very thin films on structured surfaces. The self-limiting growth mechanism characteristic to atomic layer deposition (ALD) facilitates the control of film thickness at the atomic level and allows deposition on large and complex surfaces. These features make ALD a very promising technique for future integrated circuits. Recent ALD research has mainly focused on materials required in microelectronics. Chemistry, in particular the selection of suitable precursor combinations, is the key issue in ALD; many interesting results have been obtained by smart chemistry. ALD is also likely to find applications in other areas, such as magnetic recording heads, optics, demanding protective coatings, and micro-electromechanical systems, provided that cost-effective processes can be found for the materials required.     

Molecular hydrogen tweezers: structure and mechanisms by neutron diffraction, NMR, and deuterium labeling studies in solid and solution

The mechanism of reversible hydrogen activation by ansa-aminoboranes, 1-N-TMPH-CH(2)-2-[HB(C(6)F(5))(2)]C(6)H(4) (NHHB), was studied by neutron diffraction and thermogravimetric mass-spectroscopic experiments in the solid state as well as with NMR and FT-IR spectroscopy in solution. The structure of the ansa-ammonium borate NHHB was determined by neutron scattering, revealing a short N-H···H-B dihydrogen bond of 1.67 ?. Moreover, this intramolecular H-H distance was determined in solution to be also 1.6-1.8 ? by (1)H NMR spectroscopic T(1) relaxation and 1D NOE measurements. The X-ray B-H and N-H distances deviated from the neutron and the calculated values. The dynamic nature of the molecular tweezers in solution was additionally studied by multinuclear and variable-temperature NMR spectroscopy. We synthesized stable, individual isotopic isomers NDDB, NHDB, and NDHB. NMR measurements revealed a primary isotope effect in the chemical shift difference (p)Δ(1)H(D) = δ(NH) - δ(ND) (0.56 ppm), and hence supported dihydrogen bonding. The NMR studies gave strong evidence that the structure of NHHB in solution is similar to that in the solid state. This is corroborated by IR studies providing clear evidence for the dynamic nature of the intramolecular dihydrogen bonding at room temperature. Interestingly, no kinetic isotope effect was detected for the activation of deuterium hydride by the ansa-aminoborane NB. Theoretical calculations attribute this to an "early transition state". Moreover, 2D NOESY NMR measurements support fast intermolecular proton exchange in aprotic CD(2)Cl(2) and C(6)D(6).     

Aldimine 2,6-bis(imino)pyridine iron(II) and cobalt(II)/methyl aluminoxane catalyst systems for polymerization of <Emphasis Type="Italic">tert-</Emphasis>butylacrylate

Aldimine 2,6-bis[(imino)methyl]pyridine iron(II) (1, 4, and 6) and cobalt(II) (3 and 5) complexes bearing bulky cycloaliphatic (bornyl and myrtanyl) or aromatic (naphthyl) terminal groups have been applied successfully, after activation with methyl aluminoxane (MAO), as catalysts for the polymerization of tert-butylacrylate. For comparison reasons, complex 2 that contains the ketimine ligand, 2,6-bis[(−)-cis-myrtanylimino)ethyl]pyridine (BMEP), has also been utilized. All studied complexes showed moderate polymerization activities, and they produced high molar mass syndiorich-atactic polymers. Surprisingly, the aldimine-based catalyst systems showed comparable activities compared with the corresponding ketimine complex (2), and they produced high molar mass polymers. In addition, complexes with bulky terminal cycloaliphatic substituents on the tridentate aldimine ligands showed higher polymerization activity compared with the aromatic ones (6). Polymerization activity and polymer molar masses are dependent on the ligand framework.     

Platinum and Palladium Complexes Bearing New (1R,2R)‐(–)‐1,2‐Diaminocyclohexane (DACH)‐Based Nitrogen Ligands: Evaluation of the Complexes Against L1210 Leukemia

The reaction of (1R,2R)‐(–)‐1,2‐diaminocyclohexane ( 1 ) [DACH] with the aldehyde (1R)‐(–)‐myrtenal ( 2 ) in MeOH afforded the bidentate diimine ligand, (1R,2R)‐(–)‐N¹,N²‐bis{(1R)‐(–)myrtenylidene}‐1,2‐diaminocyclohexane ( 3 ) in a high yield. Reduction of 3 using LiAlH₄ led to the formation of the desired ligand ( 4 ) (1R,2R)‐(–)‐N¹,N²‐bis{(1R)‐(–)myrtenyl}‐1,2‐diaminocyclohexane. Treatment of compound 4 with K₂PtCl₄ or K₂PdCl₄ yielded the corresponding platinum(II) and palladium(II) complexes, Pt‐5 and Pd‐6 , respectively. The reaction of compound 3 with K₂PtCl₄ gave the diimine complex Pt‐7 . The cytotoxic activity of the complexes Pt‐5 , Pd‐6 and Pt‐7 was tested and compared to the approved drugs, cisplatin ( Cis ‐Pt ) and oxaliplatin ( Ox‐Pt ). The complexes ( Pt‐5 , Pd‐6 and Pt‐7 ) inhibit L1210 cell line proliferation with an IC₅₀ of 0.6, 4.2, and 0.7 μL, respectively as evidenced by measuring thymidine incorporation.     

Frustrated Lewis Pair Chemistry of Chiral (+)‐Camphor‐Based Aminoboranes

Dimethylamino‐(+)‐camphorenamine reacted with an equimolar amount of Piers’ borane, HB(C₆F₅)₂, to give the corresponding iminium–hydroborate zwitterionic salt. Being in equilibrium with the parent enamine–HB(C₆F₅)₂ N–B pair, this salt was able to split hydrogen heterolytically, hydrogenating the iminium group in the molecule. Detailed studies revealed that the hydrogen splitting in this reaction proceeded through an intermolecular pathway leading to a bornylamine–HB(C₆F₅)₂ adduct. When the starting enamine is present in excess over HB(C₆F₅)₂, the produced bornylamine–HB(C₆F₅)₂ adduct breaks up, eliminating free bornylamine and forming the initial camphorenamine– HB(C₆F₅)₂ pair. This results in hydrogenation of the camphorenamine framework in a catalytic fashion.     

Molecular tweezers for hydrogen: synthesis, characterization, and reactivity

The first ansa-aminoborane N-TMPN-CH2C6H4B(C6F5)2 (where TMPNH is 2,2,6,6-tetramethylpiperidinyl) which is able to reversibly activate H2 through an intramolecular mechanism is synthesized. This new substance makes use of the concept of molecular tweezers where the active N and B centers are located close to each other so that one H2 molecule can fit in this void and be activated. Because of the fixed geometry of this ansa-ammonium-borate it forms a short N-H...H-B dihydrogen bond of 1.78 A as determined by X-ray analysis. Therefore, the bound hydrogen can be released above 100 degrees C. In addition, the short H...H contact and the N-H...H (154 degrees) and B-H...H (125 degrees) angles show that the dihydrogen interaction in N-TMPNH-CH2C6H4BH(C6F5)2 is partially covalent in nature. As a basis for discussing the mechanism, quantum chemical calculations are performed and it is found that the energy needed for splitting H2 can arise from the Coulomb attraction between the resulting ionic fragments, or "Coulomb pays for Heitler-London". The air- and moisture-stable N-TMPNH-CH2C6H4BH(C6F5)2 is employed in the catalytic reduction of nonsterically demanding imines and enamines under mild conditions (110 degrees C and 2 atm of H2) to give the corresponding amines in high yields.     

Speciation of activators in electroluminescent thin films: an EXAFS study of cerium and terbium doped strontium sulfide

Extended X-ray absorption fine structure (EXAFS) was used to determine the local structure of the luminescent centers in Tb³⁺ and Ce³⁺ -doped strontium sulfide thin films deposited by Atomic Layer Epitaxy (ALE). The rare earths were observed to enter mainly the substitutional sites but at the same time a part of the atoms form RES clusters (RE = rare earth). The presence of both substitutional sites and RES clusters has been observed for the first time in this study for rare earths in II–VI compounds.     

In-situ investigations on the SILAR-growth of ZnS films as studied by tapping mode atomic force microscopy

Tapping mode atomic force microscopy (TM-AFM) has been successfully used for in-situ imaging of the deposition of ZnS films with the successive ionic layer adsorption and reaction (SILAR) method. The films were deposited in-situ using the commercial TM-AFM liquid cell as a flow-through reactor. The potential of TM-AFM has been used to study the growth of ZnS on different substrates up to 50 SILAR cycles. Reactants and rinsing water were alternately exchanged in the cell by a computer controlled valve system. In comparison to earlier work performed with the conventional AFM operated in contact mode, imaging artefacts introduced by lateral shear forces can be largely eliminated with TM-AFM. On glass the roughness is observed to decrease initially until typical island formation takes place at a larger number of deposition cycles. On mica island formation can be observed right from the beginning of the process and the roughness increases with increasing number of deposition cycles.     

Investigation of ZnS thin films on Si(100) by phase detection imaging and Young’s modulus microscopy

In this study phase detection imaging (PDI) and Young’s modulus microscopy (YMM) have been used for investigation of ZnS thin films deposited on Si(100) substrates with successive ionic layer adsorption and reaction (SILAR). The additional information obtained by these techniques allows a better interpretation of the topographic images in the case of the determination of the number of grains, their distribution and surface coverage. Moreover, the comparison between PDI and YMM is discussed in order to shed more light on the contrast mechanism in phase detection imaging.