Atomic layer deposition (ALD) is a precise thin-film deposition technique that enables atomic-level control over film thickness and composition. It is particularly valuable for depositing metallic and nitride thin films used in advanced semiconductor devices, where conformality and uniformity are critical. ALD relies on self-limiting surface reactions between gaseous precursors and a substrate, allowing for layer-by-layer growth. This article focuses on the ALD of metallic films (Cu, Ru, W) and nitride films (TiN, TaN), covering precursor chemistry, nucleation challenges, and applications in interconnects and gate electrodes.

Metallic thin films such as copper (Cu), ruthenium (Ru), and tungsten (W) are widely used in semiconductor manufacturing for interconnects and electrodes. ALD of these metals presents unique challenges due to the difficulty in finding suitable precursors and achieving smooth, continuous films. Copper ALD typically employs precursors like Cu(I) and Cu(II) compounds, including Cu(hfac)2 and Cu(acac)2, with reducing agents such as hydrogen or alcohols. However, Cu ALD suffers from poor nucleation on dielectric surfaces, often requiring seed layers or surface treatments to promote adhesion. Ru ALD is more robust, with precursors like RuCp2 and RuO4 paired with oxygen or ammonia to achieve smooth, conductive films. Ru films exhibit excellent nucleation behavior and are increasingly used as diffusion barriers and liner layers in interconnects. Tungsten ALD, using WF6 and Si2H6 or B2H6 as precursors, is critical for gate electrodes and contacts due to its low resistivity and thermal stability. However, fluorine contamination from WF6 can degrade device performance, necessitating post-deposition treatments.

Nitride films, such as titanium nitride (TiN) and tantalum nitride (TaN), are essential for gate electrodes, diffusion barriers, and metal-insulator-metal capacitors. TiN ALD commonly uses TiCl4 or TDMAT with ammonia plasma or thermal NH3 as the nitrogen source. The choice of precursor affects film properties, with TiCl4-based processes yielding lower resistivity but requiring higher deposition temperatures. TaN ALD employs precursors like PDMAT or TBTDET with NH3 plasma, producing films with excellent diffusion barrier properties. A key challenge in nitride ALD is controlling stoichiometry and minimizing impurities, as carbon and oxygen incorporation can degrade electrical performance. Plasma-enhanced ALD (PEALD) is often employed to lower deposition temperatures and improve film quality.

Nucleation is a critical challenge in ALD of metallic and nitride films, particularly on non-metallic surfaces. Poor nucleation leads to island growth, increased roughness, and higher resistivity. To address this, surface functionalization or seed layers are often employed. For example, Cu ALD benefits from organic adhesion promoters or thin Ru seed layers. Similarly, TiN nucleation on SiO2 can be improved using plasma treatments or Al2O3 nucleation layers. The initial cycles of ALD are crucial, as they determine the continuity and electrical properties of the final film. In situ monitoring techniques, such as quartz crystal microbalance (QCM) and spectroscopic ellipsometry, help optimize nucleation and growth.

In semiconductor applications, ALD metallic and nitride films are indispensable for advanced interconnects and gate stacks. Copper interconnects in back-end-of-line (BEOL) processing require ALD diffusion barriers like TaN or Ru to prevent Cu migration into dielectrics. As device dimensions shrink below 10 nm, the conformality of ALD becomes essential for lining high-aspect-ratio vias and trenches. Ru is particularly promising as a liner material due to its low resistivity and ability to direct-plate Cu without a seed layer. In gate electrodes, TiN and TaN serve as work function metals in high-k metal gate (HKMG) stacks, where precise thickness control is necessary to tune threshold voltages. ALD enables the deposition of ultra-thin, uniform layers that meet these requirements.

Emerging applications of ALD metallic and nitride films include resistive RAM (RRAM) and dynamic random-access memory (DRAM). TiN and TaN are used as electrodes in RRAM devices, where their interfacial properties influence switching behavior. In DRAM, W ALD is employed for capacitor electrodes due to its compatibility with high-temperature processing. Additionally, ALD Ru is being explored for interconnect scaling beyond the 5 nm node, where traditional barrier/liner schemes face limitations.

The development of new precursors and processes continues to advance ALD of metallic and nitride films. Non-halide precursors are being investigated to eliminate halogen contamination, while low-temperature processes are being optimized for thermally sensitive substrates. Area-selective ALD is another growing area, aiming to reduce lithography steps by depositing films only on desired regions. Despite progress, challenges remain in achieving low resistivity, high purity, and excellent nucleation on diverse surfaces. Future research will focus on integrating these films into novel device architectures while maintaining scalability and manufacturability.

In summary, ALD of metallic and nitride thin films is a cornerstone of modern semiconductor fabrication, enabling the deposition of highly conformal, uniform layers for interconnects and gate electrodes. Precursor chemistry, nucleation control, and process optimization are critical to achieving high-performance films. As device scaling continues, ALD will play an increasingly vital role in meeting the demands of advanced semiconductor technologies.