E Erik Langereis

Ultralow surface recombination of c-Si substrates passivated by plasma-assisted atomic layer deposited Al2O3

Excellent surface passivation of c-Si has been achieved by Al2O3 films prepared by plasma-assisted atomic layer deposition, yielding effective surface recombination velocities of 2 and 13cm∕s on low resistivity n- and p-type c-Si, respectively. These results obtained for ∼30nm thick Al2O3 films are comparable to state-of-the-art results when employing thermal oxide as used in record-efficiency c-Si solar cells. A 7nm thin Al2O3 film still yields an effective surface recombination velocity of 5cm∕s on n-type silicon.

Plasma-assisted atomic layer deposition of Al2O3 moisture permeation barriers on polymers

Thin Al2O3 films of different thicknesses (10–40nm) were deposited by plasma-assisted atomic layer deposition on substrates of poly(2,6-ethylenenaphthalate) (PEN), and the water vapor transmission rate (WVTR) values were measured by means of the calcium test. The permeation barrier properties improved with decreasing substrate temperature and a good WVTR of 5×10−3gm−2day−1 (WVTRPEN=0.5gm−2day−1) was measured for a 20nm thick Al2O3 film deposited at room temperature using short purging times. Such ultrathin, low-temperature deposited, high-quality moisture permeation barriers are an essential requirement for the implementation of polymeric substrates in flexible electronic and display applications.

Low Temperature Plasma-Enhanced Atomic Layer Deposition of Metal Oxide Thin Films

Many reported atomic layer deposition (ALD) processes are carried out at elevated temperatures (>150°C), which can be problematic for temperature-sensitive substrates. Plasma-enhanced ALD routes may provide a solution, as the ALD temperature window can, in theory, be extended to lower deposition temperatures due to the reactive nature of the plasma. As such, the plasma-enhanced ALD of Al 2 O 3 , TiO 2 , and Ta 2 O 5 has been investigated at 25-400°C using [Al(CH 3 ) 3 ], [Ti(O i Pr) 4 ], [Ti(Cp Me )(O i Pr) 3 ], [TiCp*(OMe) 3 ], and [Ta(NMe 2 ) 5 ] as precursors. An O 2 plasma was employed as the oxygen source in each case. We have demonstrated metal oxide thin-film deposition at temperatures as low as room temperature and compared the results with corresponding thermal ALD routes to the same materials. The composition of the films was determined by Rutherford backscattering spectroscopy. Analysis of the growth per cycle data and the metal atoms deposited per cycle revealed that the growth per cycle is strongly dependent on the film density at low deposition temperatures. Comparison of these data for Al 2 O 3 ALD processes in particular, showed that the number of Al atoms deposited per cycle was consistently high down to room temperature for the plasma-enhanced process but dropped for the thermal process at substrate temperatures lower than 250°C.

Conformality of plasma-assisted ALD: physical processes and modeling

For plasma-assisted atomic layer deposition (ALD), reaching conformal deposition in high aspect ratio structures is less straightforward than for thermal ALD due to surface recombination loss of plasma radicals. To obtain a detailed insight into the consequences of this additional radical loss, the physical processes in plasma-assisted ALD affecting conformality were identified and investigated through Monte Carlo simulations. The conformality was dictated by the recombination probability r, the reaction probability s, and the diffusion rate of particles. When recombination losses play a role, the saturation dose depended strongly on the value of r. For the deposition profiles, a minimum at the bottom of trench structures was observed (before reaching saturation), which was more pronounced with larger values of r. In turn, three deposition regimes could be identified, i.e., a reaction-limited regime, a diffusion-limited regime, and a new regime that is recombination-limited. For low values of r, conformal deposition in high aspect ratio structures can still be achieved, as observed for several metal oxides, even for aspect ratios as large as 30. For high surface recombination loss probabilities, as appears to be the case for many metals, achieving a reasonable conformality becomes challenging, especially for aspect ratios >10.

Surface chemistry of plasma-assisted atomic layer deposition of Al2O3 studied by infrared spectroscopy

The surface groups created during plasma-assisted atomic layer deposition (ALD) of Al2O3 were studied by infrared spectroscopy. For temperatures in the range of 25–150°C, –CH3 and –OH were unveiled as dominant surface groups after the Al(CH3)3 precursor and O2 plasma half-cycles, respectively. At lower temperatures more –OH and C-related impurities were found to be incorporated in the Al2O3 film, but the impurity level could be reduced by prolonging the plasma exposure. The results demonstrate that –OH surface groups rule the surface chemistry of the Al2O3 process and likely that of plasma-assisted ALD of metal oxides from organometallic precursors in general.

In situ reaction mechanism studies of plasma-assisted atomic layer deposition of Al2O3

Reaction mechanisms during plasma-assisted atomic layer deposition (ALD) of Al2O3 from Al(CH3)3 and O2 plasma were studied by time-resolved quartz crystal microbalance measurements, mass spectrometry, and optical emission spectroscopy. Al(CH3)3 chemisorption on the oxide surface after the plasma pulse releases CH4 products while from the detection of CO, CO2, and H2O in the O2 plasma it is established that surface –CH3 groups are predominantly removed by O radical-driven combustionlike reactions. Also a second pathway exists for –CH3 removal driven by H2O generated in this plasma step. These reaction pathways are expected to be generic for plasma-assisted ALD of oxides from metal organic precursors.

Optical emission spectroscopy as a tool for studying, optimizing, and monitoring plasma-assisted atomic layer deposition processes

In this note it is demonstrated that optical emission spectroscopy (OES) is an easy-to-implement and valuable tool to study, optimize, and monitor thin film growth by plasma-assisted atomic layer deposition (ALD). The species in the plasma can be identified through the analysis of the light emitted by the plasma. OES provides therefore information on the reactant species delivered to the surface by the plasma but it also yields unique insight into the surface reaction products and, as a consequence, on the reaction mechanisms of the deposition process. Time-resolved measurements reveal information about the amount of precursor dosing and length of plasma exposure needed to saturate the self-limiting half reactions, which is useful for the optimization of the ALD process. Furthermore, time-resolved OES can also be used as an easy-to-implement process monitoring tool for plasma-assisted ALD processes on production equipment; for example, to monitor reactor wall conditions or to detect process faults in real time.

Low-Temperature Deposition of TiN by Plasma-Assisted Atomic Layer Deposition

Titanium nitride (TiN) films were deposited by a plasma-assisted atomic layer deposition (PA-ALD) process, based on TiCl 4 precursor dosing and remote H 2 -N 2 plasma exposure, at temperatures ranging from 100 to 400°C. The plasma, the PA-ALD process, and the resulting TiN material properties were extensively investigated. The plasma was studied by optical emission spectroscopy and Langmuir probe, revealing an ion density of 10 9 cm -3 and an electron temperature of 3.5 eV just above the substrate. Under floating conditions there is thus a considerable ion flux towards the substrate per ALD cycle with a typical ion energy of ∼ 15 eV. TiN film growth was studied by in situ spectroscopic ellipsometry, revealing self-limiting surface reactions for the complete temperature range. At 100°C the growth rate of 0.3 A/cycle was found to be significantly lower than the growth rate of 0.6 A/cycle at 400°C. The stoichiometry of the films varied with the plasma exposure time, while the Cl content was mostly affected by the deposition temperature (2.1 atom % at 100°C to 0.07 atom % at 400°C). Resistivities as low as 71 μΩ cm were obtained at a temperature of 400°C, while at 100°C a fair resistivity of 209 μΩ cm was reached. These results show that PA-ALD with TiCl 4 and H 2 -N 2 plasma is well suited for low-temperature deposition of high-quality TiN films.

Synthesis and in situ characterization of low-resistivity TaNx films by remote plasma atomic layer deposition

Remote plasma atomic layer deposition (ALD) of TaNx films from Ta[N(CH3)2]5 and H2, H2-N2, and NH3 plasmas is reported. From film analysis by in situ spectroscopic ellipsometry and various ex situ techniques, data on growth rate, atomic composition, mass density, TaNx microstructure, and resistivity are presented for films deposited at substrate temperatures between 150 and 250°C. It is established that cubic TaNx films with a high mass density (12.1gcm−3) and low electrical resistivity (380μΩcm) can be deposited using a H2 plasma with the density and resistivity of the films improving with plasma exposure time. H2-N2 and NH3 plasmas resulted in N-rich Ta3N5 films with a high resistivity. It is demonstrated that the different TaNx phases can be distinguished in situ by spectroscopic ellipsometry on the basis of their dielectric function with the magnitude of the Drude absorption yielding information on the resistivity of the films. In addition, the saturation of the ALD surface reactions can be determined...

In situ spectroscopic ellipsometry study on the growth of ultrathin TiN films by plasma-assisted atomic layer deposition

The growth of ultrathin TiN films by plasma-assisted atomic layer deposition (PA-ALD) was studied by in situ spectroscopic ellipsometry (SE). In between the growth cycles consisting of TiCl4 precursor dosing and H2–N2 plasma exposure, ellipsometry data were acquired in the photon energy range of 0.75–5.0eV. The dielectric function of the TiN films was modeled by a Drude-Lorentz oscillator parametrization, and the film thickness and the TiN material properties, such as conduction electron density, electron mean free path, electrical resistivity, and mass density, were determined. Ex situ analysis was used to validate the results obtained by in situ SE. From the in situ spectroscopic ellipsometry data several aspects related to thin film growth by ALD were addressed. A decrease in film resistivity with deposition temperature between 100 and 400°C was attributed to the increase in electron mean free path due to a lower level of impurities incorporated into the films at higher temperatures. A change in resist...