Chemically Stable Atomic-Layer-Deposited Al2O3 Films for Processability

Abstract

Atomic-layer-deposited alumina (ALD Al₂O₃) can be utilized for passivation, structural, and functional purposes in electronics. In all cases, the deposited film is usually expected to maintain chemical stability over the lifetime of the device or during processing. However, as-deposited ALD Al₂O₃ is typically amorphous with poor resistance to chemical attack by aggressive solutions employed in electronics manufacturing. Therefore, such films may not be suitable for further processing as solvent treatments could weaken the protective barrier properties of the film or dissolved material could contaminate the solvent baths, which can cause cross-contamination of a production line used to manufacture different products. On the contrary, heat-treated, crystalline ALD Al₂O₃ has shown resistance to deterioration in solutions, such as standard clean (SC) 1 and 2. In this study, ALD Al₂O₃ was deposited from four different precursor combinations and subsequently annealed either at 600, 800, or 1000 °C for 1 h. Crystalline Al₂O₃ was achieved after the 800 and 1000 °C heat treatments. The crystalline films showed apparent stability in SC-1 and HF solutions. However, ellipsometry and electron microscopy showed that a prolonged exposure (60 min) to SC-1 and HF had induced a decrease in the refractive index and nanocracks in the films annealed at 800 °C. The degradation mechanism of the unstable crystalline film and the microstructure of the film, fully stable in SC-1 and with minor reaction with HF, were studied with transmission electron microscopy. Although both crystallized films had the same alumina transition phase, the film annealed at 800 °C in N₂, with a less developed microstructure such as embedded amorphous regions and an uneven interfacial reaction layer, deteriorates at the amorphous regions and at the substrate-film interface. On the contrary, the stable film annealed at 1000 °C in N₂ had considerably less embedded amorphous regions and a uniform Al-O-Si interfacial layer.

Figure 1

AFM images of (a) as-deposited and (b) 1000 °C vacuum-annealed AlCl₃-H₂O films from a site without a blister. The increased surface roughness is evident.

Figure 2

Optical micrographs of TMA-H₂O films annealed at 1000 °C: (a) vacuum atmosphere (the small, white dots are blisters), (b) N₂ atmosphere.

Figure 3

Changes in thicknesses and refractive indices of TMA-H₂O films as a function of immersion time in (a) SC-1 and (b) HF. The dashed lines represent the thickness changes and the solid lines represent the refractive indices. The SDs of the thickness data were on the order of maximum 1 nm. The refractive index errors can be approximated to be ±0.01. The increases in the thicknesses of the films annealed at 800 °C after 60 min were 3 and 5 nm for the SC-1 and HF treatments, respectively.

Figure 4

X-ray diffractograms of the as-deposited and N₂-annealed TMA-H₂O films. From top to bottom: 1000 °C (blue), 800 °C (green), 600 °C (red), as-deposited (black). The indexing is according to the θ-alumina diffraction data from ref (27) taken from the Inorganic Crystal Structure Database (collection code 82504).

Figure 5

SEM surface micrographs of TMA-H₂O films after various treatments. The 800 °C (a) and 1000 °C (b) annealed films displayed a distinct difference in their surface morphology. The 800 °C annealed films in SC-1 (c) and HF (e) after 60 min had developed cracks and distinct differences in the surface contrast. The 1000 °C annealed films in SC-1 (d) and HF (f) after 60 min did not display cracks. The inset in (f) displays the boundary area of the two different contrast zones. Note that image (f) has a considerably different magnification compared to that of the other images to highlight the relevant observations.

Figure 6

TEM images of the H₂O-TMA as-deposited sample. (a) Bright-field (BF) TEM taken with the smallest objective aperture selecting the zero-order beam to increase the diffraction contrast. (b) HRTEM image at the Si–Al₂O₃ interface. (c) HRTEM image at the Al₂O₃ surface. The film is clearly amorphous based on the images (a–c). No distinct phase contrast arising from a Si native oxide was seen in image (b).

Figure 7

TEM images of the sample annealed in N₂ at 800 °C. (a) Bright-field TEM (BFTEM) overview image taken with the zero-order beam shows a polycrystalline structure. The γ value of the image has been modified to highlight the microstructure of the film. The inset shows the select-area electron diffraction (SAED) pattern along the Si⟨11̅0⟩ zone axis with only Si and Al₂O₃ selected. (b) HRTEM image at the Si–Al₂O₃ interface shows two different zones of the interfacial reaction layer. (c) Dark-field scanning TEM (DF STEM) overview image shows that the Si interface is not fully uniform and the microstructure has amorphous-like regions extending to the surface. The surface of the sample seems to have amorphized due to Ga damage, which is why no surface image was included here.

Figure 8

TEM images of the sample annealed in N₂ at 1000 °C. (a) BFTEM overview image taken with the zero-order beam shows a polycrystalline structure. The γ value of the image has been modified to highlight the microstructure of the film. (b) SAED aperture was used to select Si and Al₂O₃. The sample was oriented along the Si⟨11̅0⟩ zone axis and shows a preferential orientation for the crystallized alumina with θ-alumina (201̅) planes along the Si[002] direction. HRTEM images (c, d) show the Si–Al₂O₃ interface and the Al₂O₃ surface, respectively. (e) DF STEM overview image shows that the Si–Al₂O₃ interface is uniform and that little to no amorphous regions are in the film.

Figure 9

TEM images of the sample annealed in N₂ at 800 °C and kept in SC-1 for 60 min. (a) BFTEM image taken with the zero-order beam at a defect site shows that most of the chemical attack has taken place at the substrate interface. The γ value of the image has been modified to highlight the defect in the film. HRTEM images of the defect site at the surface (b) and the substrate interface (c). The extent of damage is evident in the DF STEM image (d).