SummaryWe report on results on the preparation of thin (<100 nm) aluminum oxide (Al2O3) films on silicon substrates using thermal atomic layer deposition (T-ALD) and plasma enhanced atomic layer deposition (PE-ALD) in the SENTECH SI ALD LL system. The T-ALD Al2O3 layers were deposited at 200 °C, for the PE-ALD films we varied the substrate temperature range between room temperature (rt) and 200 °C. We show data from spectroscopic ellipsometry (thickness, refractive index, growth rate) over 4” wafers and correlate them to X-ray photoelectron spectroscopy (XPS) results. The 200 °C T-ALD and PE-ALD processes yield films with similar refractive indices and with oxygen to aluminum elemental ratios very close to the stoichiometric value of 1.5. However, in both also fragments of the precursor are integrated into the film. The PE-ALD films show an increased growth rate and lower carbon contaminations. Reducing the deposition temperature down to rt leads to a higher content of carbon and CH-species. We also find a decrease of the refractive index and of the oxygen to aluminum elemental ratio as well as an increase of the growth rate whereas the homogeneity of the film growth is not influenced significantly. Initial state energy shifts in all PE-ALD samples are observed which we attribute to a net negative charge within the films.
Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) applying a capacitively coupled plasma source where trimethylgallium (TMGa) as the gallium precursor and oxygen (O2) plasma were used in a substrate temperature (Ts) in range of 80–200 °C. TMGa exhibits high vapor pressure and therefore facilitates deposition at lower substrate temperatures. The Ga2O3 films were characterized by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) measurements. The SE data show linear thickness evolution with a growth rate of ∼0.66 Å per cycle and inhomogeneity of ≤2% for all samples. The refractive index of the Ga2O3 thin films is 1.86 ± 0.01 (at 632.8 nm) and independent of temperature, whereas the bandgap slightly decreases from 4.68 eV at Ts of 80 °C to 4.57 eV at 200 °C. XPS analysis revealed ideal stoichiometric gallium to oxygen ratios of 2:3 for the Ga2O3 layers with the lowest carbon contribution of ∼10% for the sample prepared at 150 °C. The permittivity of the layers is 9.7 ± 0.2 (at 10 kHz). In addition, fixed and mobile oxide charge densities of 2–4 × 1012 and 1–2 × 1012 cm−2, respectively, were observed in the C-V characteristics. Moreover, the Ga2O3 films show breakdown fields in the range of 2.2–2.7 MV/cm. Excellent optical and electrical material properties are maintained even at low substrate temperatures as low as 80 °C. Hence, the TMGa/O2 PEALD process is suitable for electronic and optoelectronic applications where low-temperature growth is required.
In this work, we report on aluminum oxide (AlO) gas permeation barriers prepared by spatial ALD (SALD) at atmospheric pressure. We compare the growth characteristics and layer properties using trimethylaluminum (TMA) in combination with an Ar/O remote atmospheric pressure plasma for different substrate velocities and different temperatures. The resulting AlO films show ultralow water vapor transmission rates (WVTR) on the order of 10 gmd. In notable contrast, plasma based layers already show good barrier properties at low deposition temperatures (75 °C), while water based processes require a growth temperature above 100 °C to achieve equally low WVTRs. The activation energy for the water permeation mechanism was determined to be 62 kJ/mol.
The authors report the preparation of transparent conductive gas permeation barriers based on thin films of tin oxide (SnOx) grown by spatial atomic layer deposition (ALD) at atmospheric pressure. They present a comparative study using tetrakis(dimethylamino)tin(IV) and various oxidants (atmospheric pressure oxygen plasma, ozone, and water) at process temperatures in the range of 80–165 °C. Specifically, for oxygen plasma or ozone as oxidant, the authors confirm self-limited ALD growth with a growth per cycle (GPC) of 0.16 and 0.11 nm for 80 and 150 °C, respectively, comparable to the classical vacuum-based ALD of SnOx. On the contrary, for water-based processes the GPC is significantly lower. Very notably, while SnOx grown with water as oxidant shows only a very limited electrical conductivity [10−3 (Ω cm)−1], atmospheric pressure oxygen plasma affords SnOx layers with an electrical conductivity up to 102 (Ω cm)−1. At the same time, these layers are excellent gas permeation barriers with a water vapor transmission rate as low as 7 × 10−4 g m−2 day−1 (at 60 °C and 60% rH). ALD growth will be demonstrated at substrate velocities up to 75 mm/s (i.e., 4.5 m/min), which renders spatial plasma assisted ALD an excellent candidate for the continuous manufacturing of transparent and conductive gas permeation barriers based on SnOx.
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