Metallic films deposited via chemical vapor deposition (CVD) play a critical role in semiconductor manufacturing, particularly in interconnect metallization, barrier layers, and seed layers for subsequent electroplating. Key metals such as copper (Cu), tungsten (W), and aluminum (Al) are widely used due to their electrical conductivity, thermal stability, and compatibility with device integration. The CVD process enables conformal deposition, essential for high-aspect-ratio features in advanced nodes. Precursor chemistry, reduction mechanisms, and process conditions significantly influence film properties, including resistivity, adhesion, and microstructure.

**Precursor Selection and Reduction Mechanisms**
The choice of precursor is dictated by volatility, thermal stability, and reactivity. For tungsten, tungsten hexafluoride (WF6) is the most common precursor due to its high vapor pressure and efficient reduction pathways. WF6 is typically reduced by hydrogen (H2) or silane (SiH4) in the temperature range of 300–500°C. The hydrogen reduction follows the reaction:
WF6 + 3H2 → W + 6HF
Silane reduction proceeds faster at lower temperatures but may introduce silicon impurities, affecting film resistivity.

Copper CVD relies on precursors such as copper(I) hexafluoroacetylacetonate trimethylvinylsilane (Cu(hfac)(tmvs)) or copper(II) precursors like Cu(II) β-diketonates. These compounds undergo disproportionation or reduction with hydrogen or forming gas (H2/N2). The disproportionation reaction for Cu(hfac)(tmvs) occurs as:
2Cu(I)(hfac)(tmvs) → Cu(0) + Cu(II)(hfac)2 + 2tmvs
This mechanism requires precise temperature control to avoid excessive carbon incorporation.

Aluminum deposition employs precursors like trimethylaluminum (TMA) or dimethylaluminum hydride (DMAH). DMAH is preferred for high-purity films due to its clean decomposition pathway:
2Al(CH3)2H → 2Al + 3CH4 + H2
TMA requires higher temperatures and may leave carbon residues, increasing resistivity.

**Challenges in Resistivity and Adhesion**
Achieving low resistivity in CVD metallic films is hindered by impurities, grain boundaries, and incomplete precursor decomposition. Tungsten films often exhibit higher resistivity than bulk values (5.6 μΩ·cm) due to fluorine incorporation from WF6. Post-deposition annealing can reduce resistivity by promoting grain growth and removing trapped fluorine.

Copper films face challenges with carbon and oxygen contamination, which increase resistivity and degrade electromigration resistance. Adhesion is another critical issue; copper tends to diffuse into silicon or dielectrics, necessitating barrier layers such as tantalum (Ta) or tantalum nitride (TaN). Tungsten adheres well to diffusion barriers like TiN but may suffer from poor step coverage in high-aspect-ratio vias if nucleation is non-uniform.

Aluminum CVD must overcome oxidation and poor nucleation on dielectrics. DMAH-based processes yield better adhesion due to in-situ hydrogen reduction of surface oxides. However, controlling film stoichiometry is critical to avoid hillock formation during thermal cycling.

**Applications in Semiconductor Devices**
**Interconnects:** Tungsten is the standard material for via filling in logic and memory devices due to its excellent conformality and compatibility with dielectric etch stops. CVD tungsten is deposited as a liner and bulk fill, with WF6 chemistry enabling seamless integration in dual-damascene structures.

Copper interconnects, introduced at the 180 nm node, rely on CVD for seed layer deposition prior to electroplating. The seed layer must be continuous and low-resistivity to ensure uniform plating. CVD copper is advantageous for ultra-thin seed layers in advanced nodes, where physical vapor deposition (PVD) struggles with poor sidewall coverage.

Aluminum CVD is less common in modern interconnects but remains relevant for specialized applications requiring low-temperature processing, such as flexible electronics or back-end-of-line (BEOL) metallization in MEMS.

**Barrier and Seed Layers:** Tungsten CVD is used in conjunction with TiN barriers to prevent interdiffusion and improve adhesion. The TiN layer is typically deposited by atomic layer deposition (ALD) or CVD before tungsten deposition.

Copper seed layers require ultra-thin, continuous films to facilitate void-free electroplating. CVD copper offers better conformality than PVD, especially for features below 10 nm. However, impurity control is critical to maintain low resistivity and good plating uniformity.

**Process Optimization and Future Directions**
Advanced CVD techniques, such as plasma-enhanced CVD (PECVD) or pulsed CVD, are being explored to lower deposition temperatures and improve film quality. PECVD of tungsten using WF6 and H2 plasmas enables deposition at <300°C, reducing thermal budget and substrate damage.

For copper, alternative precursors with lower decomposition temperatures and reduced carbon uptake are under development. Similarly, aluminum CVD research focuses on precursors that minimize oxygen incorporation and enhance nucleation on dielectrics.

In conclusion, CVD of metallic films is indispensable for semiconductor metallization, offering precise control over film properties and conformality. Continued advancements in precursor chemistry and process engineering are essential to meet the demands of shrinking device geometries and emerging applications in 3D integration and advanced packaging.