ALD Process
Atomic layer deposition is a vapor-phase thin-film deposition technique that builds material one atomic layer at a time through sequential, self-limiting chemical reactions between gaseous precursors and wafer surface functional groups. Unlike conventional Chemical Vapor Deposition (CVD) where multiple precursors are co-injected simultaneously and react continuously in the gas phase, ALD physically separates chemical half-reactions into discrete, alternating exposure steps separated by inert gas purging phases ($\text{N}_2$ or $\text{Ar}$). Because surface chemisorption naturally halts once all available active surface reactive sites are saturated, ALD delivers atomic-scale thickness control, sub-angstrom repeatability, and flawless $100\%$ conformal step coverage across ultra-high-aspect-ratio ($> 100:1$) 3D architectures such as FinFETs, Gate-All-Around (GAA) nanosheets, and 3D NAND memory trenches.
The fundamental mechanism of atomic layer deposition relies on self-limiting surface saturation kinetics. In an ideal ALD half-cycle, precursor gas molecules impinge on the wafer and chemisorb onto active surface functional groups (such as hydroxyl $-\text{OH}$ or amine $-\text{NH}_2$ sites). The chemisorption process follows Langmuir adsorption kinetics:
where $\theta$ is fractional surface site coverage, $S_0$ is the initial sticking coefficient, $F_{\text{flux}}$ is precursor incident molecular flux, and $\Gamma_{\text{sat}}$ is maximum saturated surface site density. Once all reactive sites are occupied, steric hindrance between bulky organic ligand groups prevents further precursor adsorption, causing the reaction rate to drop to zero ($\mathrm{d}\theta/\mathrm{d}t = 0$). Extending the precursor pulse duration cannot deposit excess material, providing digital self-limiting control.
The four-step ALD sequence eliminates gas-phase parasitic CVD reactions through intermediate inert purges. In the initial precursor exposure phase, volatile metal precursor vapor (such as trimethylaluminum $\text{Al(CH}_3)_3$, TMA) is pulsed into the reactor chamber to form a chemisorbed sub-monolayer. Next, high-purity inert gas ($\text{N}_2$ or $\text{Ar}$) sweeps the chamber during Purge A, exhausting unreacted precursor molecules and weakly physisorbed species. In the subsequent co-reactant exposure, oxygen- or nitrogen-containing reactant vapor (such as $\text{H}_2\text{O}$, $\text{O}_3$, or $\text{NH}_3$) is pulsed to react with the chemisorbed metal complex, eliminating organic ligands as volatile byproducts ($\text{CH}_4$). Finally, Purge B flushes reaction byproducts and excess co-reactant out of the reactor, regenerating active $-\text{OH}$ surface termination sites for the next cycle.
Operating within the ALD thermal process window ensures true self-limiting growth per cycle (GPC). Every precursor-reactant chemistry exhibits a characteristic temperature window where Growth Per Cycle (GPC, typically $0.5\text{--}1.2\ \text{\AA/cycle}$) remains constant and independent of substrate temperature. Below the lower thermal boundary ($T < T_{\text{min}}$), low surface thermal energy causes precursor condensation or incomplete chemical reaction kinetics that reduce film quality. Above the upper thermal limit ($T > T_{\text{max}}$), precursor molecules thermally decompose via parasitic CVD pyrolysis or desorb prematurely from the surface, destroying self-limiting conformality.
Atomic layer deposition provides unmatched 100% conformal step coverage across deep high-aspect-ratio nanostructures. Because ALD precursors do not react until they contact an unreacted surface site, gas molecules diffuse deeply into ultra-narrow high-aspect-ratio ($> 100:1$) trenches and 3D Gate-All-Around (GAA) nanosheet channels without suffering line-of-sight shadowing or entrance pinch-off. Knudsen diffusion governs precursor transport in nanoscale cavities ($d_{\text{feature}} < 20\text{ nm}$), requiring pulse durations to scale with the square of the aspect ratio ($t_{\text{pulse}} \propto \text{AR}^2$) to achieve saturated coverage across all internal vertical sidewalls.
| Thin-Film Material | Primary Metal Precursor | Co-Reactant & Oxidizer | ALD Temperature Window | Growth Per Cycle (GPC) | Primary Semiconductor Application |
|---|---|---|---|---|---|
| Aluminum Oxide ($\text{Al}_2\text{O}_3$) | Trimethylaluminum (TMA) | $\text{H}_2\text{O}\text{ or }\text{O}_3$ | 150°C – 320°C | $0.9\text{--}1.1\ \text{\AA/cycle}$ | Gate dielectric cap, passivating liner, and etch stop |
| Hafnium Oxide ($\text{HfO}_2$) | $\text{HfCl}_4\text{ or TDMAH}$ | $\text{H}_2\text{O}\text{ or }\text{O}_3$ | 200°C – 350°C | $0.8\text{--}1.2\ \text{\AA/cycle}$ | Leading-edge High-$k$ metal gate dielectric ($k \approx 22$) |
| Titanium Nitride ($\text{TiN}$) | $\text{TiCl}_4\text{ or TDMAT}$ | $\text{NH}_3\text{ or Plasma }\text{N}_2/\text{H}_2$ | 350°C – 450°C | $0.2\text{--}0.5\ \text{\AA/cycle}$ | Metal gate workfunction electrode and Cu barrier layer |
| Ruthenium Metal ($\text{Ru}$) | $(\text{EtCp})_2\text{Ru}$ | $\text{O}_2\text{ or Plasma }\text{H}_2$ | 250°C – 350°C | $0.4\text{--}0.6\ \text{\AA/cycle}$ | Sub-2nm interconnect liner and seedless direct plating |
| Silicon Dioxide ($\text{SiO}_2$) | $\text{BDEAS}\text{ or 3DMAS}$ | $\text{O}_3\text{ or Plasma }\text{O}_2$ | 100°C – 300°C | $0.7\text{--}1.0\ \text{\AA/cycle}$ | SAQP / SADP self-aligned spacer oxide deposition |
Plasma-Enhanced ALD and Area-Selective Deposition extend processing to lower thermal budgets and bottom-up patterning. While thermal ALD relies on substrate thermal energy, Plasma-Enhanced ALD (PEALD) uses radiofrequency (RF) plasma to generate reactive radicals ($\text{O}^*$, $\text{N}^*$, $\text{H}^*$), enabling high-density dielectric and metallic film growth at low temperatures ($< 150^\circ\text{C}$) compatible with temperature-sensitive photoresist and back-end metallization. Area-Selective Deposition (ASD) deploys Self-Assembled Monolayers (SAM) or plasma passivation inhibitors that block nucleation on dielectric surfaces while permitting growth on metal surfaces, achieving self-aligned bottom-up feature synthesis without lithographic cut masks.
Achieving sub-angstrom thin-film precision requires viewing atomic layer deposition as a self-limiting-surface-saturation-steric-hindrance-and-purge-dynamics lens. By balancing precursor chemisorption kinetics, purge boundary layer fluid dynamics, steric molecular footprint limitations, and reactor thermal uniformity, semiconductor foundries synthesize atomic-precision high-$k$ gate stacks, ultra-thin barrier liners, and multi-patterning spacers. Rigorous ALD execution ensures that leading-edge 3D transistors, high-density memory cells, and advanced packaging interconnects achieve flawless step coverage, low leakage currents, and high manufacturing yield across billions of nanoscale devices.