From Precursor Chemistry to Gas Sensors: Plasma-Enhanced Atomic Layer Deposition Process Engineering for Zinc Oxide Layers from a Nonpyrophoric Zinc Precursor for Gas Barrier and Sensor Applications.

The identification of bis-3-(N,N-dimethylamino)propyl zinc ([Zn(DMP)2 ], BDMPZ) as a safe and potential alternative to the highly pyrophoric diethyl zinc (DEZ) as atomic layer deposition (ALD) precursor for ZnO thin films is reported. Owing to the intramolecular stabilization, BDMPZ is a thermally stable, volatile, nonpyrophoric solid compound, however, it possesses a high reactivity due to the presence of Zn-C and Zn-N bonds in this complex. Employing this precursor, a new oxygen plasma enhanced (PE)ALD process in the deposition temperature range of 60 and 160 °C is developed. The resulting ZnO thin films are uniform, smooth, stoichiometric, and highly transparent. The deposition on polyethylene terephthalate (PET) at 60 °C results in dense and compact ZnO layers for a thickness as low as 7.5 nm with encouraging oxygen transmission rates (OTR) compared to the bare PET substrates. As a representative application of the ZnO layers, the gas sensing properties are investigated. A high response toward NO2 is observed without cross-sensitivities against NH3 and CO. Thus, the new PEALD process employing BDMPZ has the potential to be a safe substitute to the commonly used DEZ processes.

Potential Precursor Alternatives to the Pyrophoric Trimethylaluminium for the Atomic Layer Deposition of Aluminium Oxide.

New precursor chemistries for the atomic layer deposition (ALD) of aluminium oxide are reported as potential alternatives to the pyrophoric trimethylaluminium (TMA) which is to date a widely used Al precursor. Combining the high reactivity of aluminium alkyls employing the 3-(dimethylamino)propyl (DMP) ligand with thermally stable amide ligands yielded three new heteroleptic, non-pyrophoric compounds [Al(NMe2 )2 (DMP)] (2), [Al(NEt2 )2 (DMP)] (3, BDEADA) and [Al(NiPr2 )2 (DMP)] (4), which combine the properties of both ligand systems. The compounds were synthesized and thoroughly chemically characterized, showing the intramolecular stabilization of the DMP ligand as well as only reactive Al-C and Al-N bonds, which are the key factors for the thermal stability accompanied by a sufficient reactivity, both being crucial for ALD precursors. Upon rational variation of the amide alkyl chains, tunable and high evaporation rates accompanied by thermal stability were found, as revealed by thermal evaluation. In addition, a new and promising plasma enhanced (PE)ALD process using BDEADA and oxygen plasma in a wide temperature range from 60 to 220 °C is reported and compared to that of a modified variation of the TMA, namely [AlMe2 (DMP)] (DMAD). The resulting Al2 O3 layers are of high density, smooth, uniform, and of high purity. The applicability of the Al2 O3 films as effective gas barrier layers (GBLs) was successfully demonstrated, considering that coating on polyethylene terephthalate (PET) substrates yielded very good oxygen transmission rates (OTR) with an improvement factor of 86 for a 15 nm film by using DMAD and a factor of 25 for a film thickness of just 5 nm by using BDEDA compared to bare PET substrates. All these film attributes are of the same quality as those obtained for the industrial precursor TMA, rendering the new precursors safe and potential alternatives to TMA.

PEALD of SiO2 and Al2O3 Thin Films on Polypropylene: Investigations of the Film Growth at the Interface, Stress, and Gas Barrier Properties of Dyads.

A study on the plasma-enhanced atomic layer deposition of amorphous inorganic oxides SiO2 and Al2O3 on polypropylene (PP) was carried out with respect to growth taking place at the interface of the polymer substrate and the thin film employing in situ quartz-crystal microbalance (QCM) experiments. A model layer of spin-coated PP (scPP) was deposited on QCM crystals prior to depositions to allow a transfer of findings from QCM studies to industrially applied PP foil. The influence of precursor choice (trimethylaluminum (TMA) vs [3-(dimethylamino)propyl]-dimethyl aluminum (DMAD)) and of plasma pretreatment on the monitored QCM response was investigated. Furthermore, dyads of SiO2/Al2O3, using different Al precursors for the Al2O3 thin-film deposition, were investigated regarding their barrier performance. Although the growth of SiO2 and Al2O3 from TMA on scPP is significantly hindered if no oxygen plasma pretreatment is applied to the scPP prior to depositions, the DMAD process was found to yield comparable Al2O3 growth directly on scPP similar to that found on a bare QCM crystal. From this, the interface formed between the Al2O3 and the PP substrate is suggested to be different for the two precursors TMA and DMAD due to different growth modes. Furthermore, the residual stress of the thin films influences the barrier properties of SiO2/Al2O3 dyads. Dyads composed of 5 nm Al2O3 (DMAD) + 5 nm SiO2 exhibit an oxygen transmission rate (OTR) of 57.4 cm3 m-2 day-1, which correlates with a barrier improvement factor of 24 against 5 when Al2O3 from TMA is applied.

Unearthing [3-(Dimethylamino)propyl]aluminium(III) Complexes as Novel Atomic Layer Deposition (ALD) Precursors for Al2 O3 : Synthesis, Characterization and ALD Process Development.

Identification and synthesis of intramolecularly donor-stabilized aluminium(III) complexes, which contain a 3-(dimethylamino)propyl (DMP) ligand, as novel atomic layer deposition (ALD) precursors has enabled the development of new and promising ALD processes for Al2 O3 thin films at low temperatures. Key for this promising outcome is the nature of the ligand combination that leads to heteroleptic Al complexes encompassing optimal volatility, thermal stability and reactivity. The first ever example of the application of this family of Al precursors for ALD is reported here. The process shows typical ALD like growth characteristics yielding homogeneous, smooth and high purity Al2 O3 thin films that are comparable to Al2 O3 layers grown by well-established, but highly pyrophoric, trimethylaluminium (TMA)-based ALD processes. This is a significant development based on the fact that these compounds are non-pyrophoric in nature and therefore should be considered as an alternative to the industrial TMA-based Al2 O3 ALD process used in many technological fields of application.