Based on the evolution of traditional atomic layer deposition (ALD) technology, plasma-enhanced atomic layer deposition (PEALD) has achieved technological breakthroughs in two key dimensions by introducing radio frequency plasma sources. First, radical reactions activated by plasma extend the process temperature window from the conventional ALD range of 200?400 ℃ to 30?300 ℃, which significantly enhances compatibility with thermally sensitive substrates. Second, plasma-induced surface activation increases the growth rate to 0.05?1 ?/cycle while maintaining atomic-level thickness uniformity. These dual advantages of low-temperature processing and accelerated growth make PEALD particularly advantageous in advanced manufacturing fields, including flexible electronics, photovoltaic passivation layers, and three-dimensional packaging. Contemporary ALD research primarily focuses on three-dimensional structure-property relationships: 1) the correlation between precursor chemical systems (metalorganic compounds/halides) and intrinsic film properties; 2) the regulatory mechanisms of process parameters (temperature, pulse sequence, purge efficiency) on interfacial reactions; 3) the coupling mechanisms between microstructural characteristics (crystalline phase composition, defect density, stress state) and functional properties (dielectric constant, transmittance). Notably, despite PEALD’s breakthrough in deposition kinetics, significant knowledge gaps remain regarding the dynamic evolution of amorphous/crystalline structures during ultra-thick film deposition (>1000 nm) and their impact on optical properties. Specifically, nonlinear relationships exist between the evolution of optical constants and the cross-scale surface morphology transitions (layer-by-layer growth, island formation, surface roughening) from initial ultrathin films (10 nm) through submicron (300 nm) to ultra-thick (>1000 nm) film systems. Investigating the co-evolution of optical properties and surface microstructure in the 10?1250 nm thickness regime is crucial for advancing PEALD applications in high-precision optical coatings and graded-index devices.

We fabricate Si(100) substrates with two distinct surface morphologies through chemical mechanical polishing, where the substrate exhibiting fractal characteristics in surface topography is designated as S-1, while that lacking fractal features is labeled as S-2. Ultrathin (~10 nm) to ultra-thick (~1250 nm) Al₂O₃ films are deposited via PEALD using trimethylaluminum (TMA) and oxygen plasma. The process parameters are maintained at constant values: plasma power (200 W), oxygen flow rate (150 cm³/min), and reaction temperature (100 ℃). The deposition cycle consists of sequential steps: TMA pulse (150 ms), purge (10 s), oxygen plasma pulse (6 s), and purge (10 s), with 70, 176, 352, 705, 2130, 3650, and 9250 cycles. Film thickness and refractive index are determined through spectroscopic ellipsometry analysis (240?1240 nm spectral range) using a multi-layer fitting model comprising: effective medium approximation (EMA) layer, Al₂O₃ layer, SiO_x interface layer, and Si substrate. Surface microtopography characterization is performed via atomic force microscopy (AFM) in tapping mode with operational parameters: scanning rate (1 Hz), scanning lines (256), and multiple scanning areas (1 μm×1 μm, 5 μm×5 μm, 10 μm×10 μm, 50 μm×50 μm). Crystallographic analysis is conducted using X-ray diffraction (XRD) in the 2θ range of 35°?90° with 0.02° step increments, employing Jade software for peak deconvolution and removal of silicon substrate diffraction artifacts. For samples with 70, 176, 352, and 705 deposition cycles, additional X-ray reflectivity (XRR) measurements are performed using a Bruker^TM D8 Discover system (incidence angle range: 0°?4°, step size: 0.004°), with subsequent GenX3 software modeling to extract film thickness and density parameters.

Figure 2 illustrates the evolution of deposition rate, refractive index, and density of Al₂O₃ thin films as functions of deposition cycles in the PEALD process. It is observed that both the density and refractive index of the films exhibit continuous enhancement with increasing thickness, stabilizing beyond a thickness of 500 nm. Notably, no significant crystallization is detected even when the film thickness reaches 1250 nm. The effective filling of substrate polishing marks by PEALD-deposited Al₂O₃ films is demonstrated in Fig. 4. However, starting from 705 deposition cycles, distinct characteristic microstructures emerge on the film surface. Their dimensional growth with increasing film thickness significantly elevates surface roughness and modifies the evolution of power spectral density (PSD), as shown by the scanning results in Fig. 5 and Fig. 6. These findings collectively indicate the unsuitability of PEALD for fabricating thick Al₂O₃ films. The growth kinetics of the films conform to the ABC-type evolutionary model, with corresponding fitting parameters systematically presented in Table 1 and Table 2. The comparative analysis of Figs. 8 and 9 reveals that the initial growth stages of PEALD-deposited Al₂O₃ films are critically influenced by substrate surface topography, where substrates with higher initial roughness induce premature transitions in growth modes. Figure 10 further demonstrates that the substrate’s morphological influence persists beyond the nucleation phase, which continues to govern the PSD evolution of submicron-scale films. Remarkably, even when the film thickness attains micrometer dimensions, the PSD characteristics remain partially constrained by the underlying substrate morphology.

We employ PEALD to deposit Al₂O₃ thin films (10?1250 nm) on two Si substrates with distinct surface topographies. Systematic investigations are conducted on the evolution of film growth rate, refractive index, and density as functions of deposition cycles. Key observations are as follows. 1) The growth rate demonstrates a gradual decrease followed by convergence with increasing deposition cycles, while both density and refractive index exhibit progressive enhancement before stabilization. 2) Characteristic microstructures emerge on film surfaces, with dimensional expansion proportional to film thickness. 3) Spectral analysis of PSD demonstrates effective retention of low-frequency substrate characteristics, while progressive masking occurs in medium-to-high frequency ranges. Notably, the medium-to-high frequency PSD components of films display an ascending trend with deposition cycles, which indicates gradual decoupling from substrate influences. The evolution of surface roughness exhibits substrate-dependent behavior: S-1 substrates with lower initial roughness demonstrate increasing roughness values due to microstructure development, whereas S-2 substrates with higher initial roughness show surface smoothing effects. Post-deposition analysis reveals that although medium-to-high frequency PSD components follow systematic evolutionary patterns across different substrates, complete elimination of substrate-induced PSD variations remains unattainable. The residual PSD discrepancies in deposited films are fundamentally attributed to the persistent influence of underlying substrate topography characteristics.