Atomic layer deposition (ALD) has emerged as a critical technique for depositing ultrathin, conformal seed layers in advanced semiconductor manufacturing. Among its most significant applications is the deposition of copper (Cu) and cobalt (Co) seed layers, which serve as foundational layers for subsequent electroplating or chemical vapor deposition (CVD) processes. These seed layers play a pivotal role in ensuring uniform filling of high-aspect-ratio features in interconnect structures, a necessity for continued scaling in integrated circuits.
The primary advantage of ALD in this context lies in its self-limiting, layer-by-layer growth mechanism, which enables precise thickness control and exceptional step coverage. Traditional physical vapor deposition (PVD) methods struggle to achieve uniform coverage in nanoscale features, particularly as interconnect dimensions shrink below 10 nm. ALD overcomes these limitations by ensuring conformal deposition even in high-aspect-ratio trenches and vias.
Nucleation behavior is a critical factor in ALD-deposited seed layers. Copper, for instance, tends to form discontinuous islands during initial ALD cycles due to high surface energy and poor wettability on diffusion barrier materials such as tantalum nitride (TaN) or titanium nitride (TiN). To enhance nucleation, surface pretreatment or the use of adhesion promoters is often employed. Plasma treatments, such as hydrogen or nitrogen plasma, can modify surface chemistry to improve Cu nucleation density. Alternatively, ultrathin intermediate layers, such as ruthenium (Ru) or manganese (Mn), have been shown to promote uniform Cu growth by reducing interfacial energy.
Cobalt seed layers exhibit superior nucleation characteristics compared to copper on typical barrier materials. The lower surface energy of Co allows for more continuous film formation at reduced thicknesses, making it an attractive alternative for advanced interconnects. Additionally, Co demonstrates better adhesion to dielectric barriers and improved electromigration resistance, which are crucial for reliability in scaled interconnects. The ALD process for Co typically employs precursors such as cobaltocene (CoCp2) or dicobalt hexacarbonyl tert-butylacetylene (CCTBA), along with reactants like hydrogen plasma or ammonia.
The transition from ALD seed layers to subsequent electroplating or CVD requires careful optimization. For electroplating, the seed layer must provide sufficient electrical conductivity to enable uniform current distribution during plating. Thin ALD seed layers, often just a few nanometers thick, may exhibit high resistivity, leading to non-uniform plating. To mitigate this, a two-step approach is frequently used: an initial ALD seed layer followed by a brief PVD or CVD step to increase thickness and reduce resistivity before electroplating.
In the case of CVD, ALD seed layers serve as catalytic surfaces that promote uniform film growth. For copper CVD, the ALD seed layer ensures that the subsequent CVD Cu film grows conformally without voids or seams. The compatibility between ALD and CVD processes is particularly advantageous in dual-damascene structures, where void-free filling is essential for minimizing resistance and preventing electromigration failures.
Applications of ALD seed layers extend across multiple interconnect generations. In backend-of-line (BEOL) metallization, they enable the fabrication of sub-10 nm vias and trenches with minimal resistance variation. The use of Co ALD seed layers has gained traction in middle-of-line (MOL) contacts, where their superior filling properties and compatibility with tungsten (W) CVD enhance contact resistance and reliability. Furthermore, ALD seed layers are being explored for emerging interconnect technologies such as hybrid bonding, where precise thickness control and surface smoothness are critical for achieving high-quality bonds at low temperatures.
Beyond traditional interconnects, ALD-deposited seed layers find utility in through-silicon vias (TSVs) for 3D integration. The high aspect ratios of TSVs demand conformal seed layers to ensure uniform plating and prevent defects such as keyholing or seam formation. ALD’s ability to deposit thin, continuous films makes it indispensable for TSV fabrication, particularly in advanced packaging schemes.
Material selection for ALD seed layers also influences device performance and reliability. Copper, while highly conductive, suffers from electromigration at scaled dimensions, prompting research into alternative materials like Co or Ru. Ruthenium, in particular, has gained attention due to its noble metal properties, which eliminate the need for a diffusion barrier in some cases. ALD Ru seed layers have demonstrated excellent compatibility with direct Cu electroplating, offering a simplified integration scheme for future interconnects.
The thermal stability of ALD seed layers is another consideration, especially in high-temperature processing steps. Copper ALD films may agglomerate or dewet during annealing, leading to increased resistivity or discontinuities. In contrast, Co and Ru exhibit better thermal stability, making them suitable for applications requiring post-deposition thermal treatments.
Process integration challenges remain, particularly in achieving low-resistivity seed layers at minimal thicknesses. The resistivity of ultrathin metal films increases significantly due to electron scattering at surfaces and grain boundaries. Strategies such as doping, alloying, or post-deposition annealing are being investigated to mitigate this effect while maintaining the benefits of ALD conformality.
Looking ahead, the continued scaling of interconnects will drive further innovation in ALD seed layer technology. The development of novel precursors with higher reactivity and lower impurity incorporation is critical for improving film quality. Additionally, advances in area-selective ALD could enable seed layer deposition only where needed, reducing material waste and simplifying patterning processes.
In summary, ALD-deposited seed layers of Cu, Co, and related metals are indispensable for modern interconnect technology. Their ability to provide conformal, ultrathin films with enhanced nucleation addresses key challenges in electroplating and CVD processes. As semiconductor devices continue to scale, the role of ALD in enabling reliable, high-performance interconnects will only grow in importance.