Jonas Lauridsen

We present an evaluation of the environmental impact of chemical vapor deposition (CVD) of the hard coating material titanium nitride (TiN) using Life Cycle Assessment (LCA). We identify the use of electricity and TiCl4 as the two main environmental hotspots that should be in focus when improving the process to become more environmentally sustainable. We show the high impact of the energy mix on the sustainability of the CVD process and a higher sustainability for a process using NH3 than for a process using N2. The latter can be explained by a much more efficient deposition chemistry. We point to challenges in LCA of CVD, such as which impact category should be the target and guide the work, and how this can affect the design and development, a lack of representative data, and a need to widen the system perspective to include the relevant product systems. We foresee that this type of LCA of CVD can serve as a guide on how to design more sustainable CVD processes and reactors.

Studies of the surface chemistry of the first few cycles of atomic layer deposition (ALD) using in situ and time-resolved operando techniques are attractive for realizing, understanding and obtaining true mechanistic information during the deposition. However, the latter techniques are yet to be applied to ALD of metal nitrides. Here we present a surface-chemistry investigation through a time-resolved ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the initial growth of titanium nitride. The Ti 2p, O 1s, N 1s, C 1s and Si 2p core-level spectra recorded at different stages during the deposition show that chemisorption occurs immediately on the silicon dioxide surface due to the interaction of tetrakis(dimethylamido)titanium(IV) (TDMAT) with the surface. A delay in nucleation on the TDMAT-terminated surface was observed during the NH3 pulse. The intensity of the Ti 2p and N 1s core levels began to increase after four ALD cycles, showing that the surface was coated with Ti and N atoms and no Si signals were observed with time. The results show that ligand exchange reactions take place before transamination reactions. This was verified using the periodic changes in the intensity and peak positions of the above-mentioned spectra and complemented by residual gas analysis using mass spectrometry. These results can provide insights into the ALD surface growth of not only TiN but also other metal nitrides.

Aluminum titanium nitride (AlxTi1-xN, 0<x<1) is a critical hard coating material for cutting tools due to its exceptional hardness, thermal stability, wear resistance, and oxidation resistance, all of which are influenced by its Al content. A common method to swiftly estimate the Al content is by the size of the AlxTi1-xN unit cell – determined by X-ray diffraction – and the equilibrium cell size of the binary constituents AlN and TiN in the framework of Vegard's law. However, the measured unit cell size does not merely depend on Al content as AlxTi1-xN is a metastable phase that may exhibit crystalline defects, non-ideal mixing of the binaries, differences in bonding characteristics, strain relaxation, and phase separation. We seek to investigate the magnitude of these effects and the resulting uncertainty in Vegard’s-law-based estimation by comparing estimated Al content in with elemental composition of AlxTi1-xN coatings – grown industrially by chemical vapor deposition on cemented tungsten carbide substrates – using energy-dispersive X-ray spectroscopy, hard X-ray photoelectron spectroscopy, time-of-flight elastic recoil detection analysis, and Rutherford backscattering spectrometry. Our results demonstrate that the uncertainties in Al content estimated by Vegard's law are relatively small, rendering this method a good first-order approach for compositional analysis of the metal sub-lattices in AlxTi1-xN. Moreover, our data indicate that a multimethod strategy is required for precise composition determination that includes both the metal and the non-metal sublattices and impurity level.

Different approaches are used in tailoring properties of thin films to meet requirements for specific applications. This study comprises work done on atomic layer deposition (ALD) of AlxTi1-xN employing the co-evaporation approach using tris-dimethylamido aluminum (TDMAA, Al(NMe2)3) and tetrakis(dimethylamido)titanium(IV) (TDMAT, Ti(NMe2)4), and ammonia (NH3) plasma. High Al-content, low impurity (O and C, both <5 at.%) films with uniform grain size distribution and dense morphology were deposited. The as-deposited films were X-ray amorphous, but mixed crystallographic phases were observed when the films were annealed at 700 °C. The deposited aluminum rich AlxTi1-xN films show an alternative way for ternary material depositions.

Dialkylamido compounds, such as tris-dimethylamido aluminum (TDMAA, Al(NMe2)3) and tetrakis-dimethylamido titanium (TDMAT, Ti(NMe2)4) are interesting precursors for depositing nitrides using atomic layer deposition (ALD) due to their high volatility and reactivity at low temperatures. In this study, we explored surface chemistry using mass spectrometry and discovered that the surface mechanisms involved β-hydride elimination and ligand decomposition, as well as transamination and hydrogenation reactions which facilitate ligand exchange. This is mainly based on the -N(Me)2 and HN(Me)2 detected during both TDMAA and NH3 pulses, and CH4 signals detected during the NH3 pulse stage. The expected reductive elimination of the two dimethylamido ligands, via a direct nitrogen-nitrogen coupling reaction was not observed, suggesting that it is less thermodynamically favorable compared to reduction by NH3. Arrhenius analysis between 150 and 300 °C found activation energies (Ea) = 27-30 kJmol-1 and pre-exponential factors (A) = 3-5 s-1 suggesting for the reaction between TDMAA and NH₃.

Aluminum nitride (AlN) is a semiconductor with a very wide band gap and a potential dielectric material. Deposition of thin AlN films is routinely done by several techniques, including atomic layer deposition (ALD). In this study, we deposited AlN using ALD with trimethylaluminum (TMA) as the Al precursor and ammonia (NH3) with and without plasma activation as the N precursor in the temperature range from 100 to 400 °C while monitoring the surface reactions using mass spectrometry. Our results, combined with recent quantum chemical modelling, suggest that the surface chemistry of the deposition process is chemisorption of TMA followed by reductive elimination of the methyl groups to render mono methyl aluminum species. The NH3 chemisorption is done by ligand exchange to form CH4 and an -NH2 terminated surface.