Atomic Layer Deposition (ALD) for Semiconductor Fabrication

Key Takeaways

• Precise Thin Film Growth: ALD enables controlled deposition of atomic layers, offering exceptional control over thickness and composition, which is vital for applications like semiconductors.

• Microelectronics Advancements: ALD's conformal coverage in intricate geometries, uniform films, and versatility drive innovations in microelectronics, MEMS devices, and protective coatings.

• Semiconductor Fabrication Solutions: ALD addresses silicon manufacturing challenges, creating high-k gate oxides, transition-metal nitrides, metal films, and diverse layers, enhancing device performance and reliability.

Diagram of the atomic layer deposition (ALD) cycle

Atomic layer deposition (ALD) is used to grow thin films serving a broad spectrum of applications. This specialized method is a variant of the chemical vapor deposition (CVD) technique. In ALD, gaseous reactants, referred to as precursors, are introduced into a reaction chamber to facilitate the formation of the desired material through chemical surface reactions. ALD enables the fabrication of various microelectronics and semiconductors. Read on as we delve into ALD and discuss its importance in microelectronic fabrication.

ALD vs. CVD at a Glance

The Atomic Layer Deposition (ALD) Process

The ALD process involves the growth of a film on a substrate by exposing the substrate's surface to alternating gaseous species, often referred to as precursors or reactants. Unlike chemical vapor deposition, these precursors are never simultaneously present within the reactor. Instead, they are introduced as a series of sequential pulses that do not overlap. In each of these pulses, the precursor molecules engage with the surface in a self-limiting manner, causing the reaction to halt once all available surface sites have been utilized. Consequently, the interaction between the precursor and the surface dictates the maximum material deposition achievable on the surface following a single exposure to all precursors, constituting an ALD cycle. By manipulating the cycle count, the potential arises to uniformly and precisely cultivate materials on diverse complexity and size substrates.

1. In a classic ALD process model, a substrate is subjected to two reactants, A and B, in a sequential and non-overlapping fashion. Unlike processes such as CVD, where thin-film growth occurs in a continuous manner, ALD governs each reactant's interaction with the surface in a self-limited manner. 

2. Reactant molecules are only capable of reacting with a finite number of active surface sites. As soon as all these sites are engaged within the reactor, growth comes to a standstill. 

3. The residual reactant molecules are then purged, paving the way for the introduction of reactant B. 

4. By alternating between exposures of reactants A and B, the gradual deposition of a thin film transpires.

Notably, within the ALD process, it is imperative to allocate sufficient time for each reaction step, ensuring the attainment of full adsorption density. At this point, the process reaches saturation. The duration required for this accomplishment hinges on two pivotal factors: the precursor pressure and the sticking probability.

Comparing ALD and CLD

When contrasting ALD with CLD, it is worth noting that CVD operates as a continuous process where all reactants are introduced simultaneously to construct the film. In contrast, ALD follows a distinct path by executing two half-reactions consecutively. An additional distinction between ALD and CVD revolves around the manner in which films are deposited. In the case of ALD, films are meticulously grown, one atomic layer at a time, embracing precision and control. In contrast, CVD possesses a broader scope, capable of depositing films with varying thicknesses spanning a wider range. ALD stands as a subset of CVD, nestled within the realm of deposition techniques driven by chemical reactions. ALD's unique methodology involves layer-by-layer growth, achieved through alternating pulses of source gasses. This meticulous approach empowers exquisite control over thickness. In most other CVD techniques, source gasses flow concurrently while an external energy source, such as high temperature or plasma, aids the reaction process.

Key Advantages of ALD

ALD presents a variety of key advantages:

• ALD's ability to deposit films layer by layer provides exceptional control over film thickness. This precision is especially crucial in applications such as semiconductors, where nanometer-scale variations can significantly impact device performance.

• Traditional deposition techniques struggle to uniformly coat complex structures, leading to uneven films. ALD overcomes this challenge by ensuring conformal coverage even in intricate geometries, making it suitable for microelectronics and microelectromechanical systems (MEMS) devices.

• ALD's self-limiting reactions lead to highly uniform films free from defects and thickness variations. This homogeneity is vital for applications like thin-film transistors and protective coatings.

•  ALD accommodates various materials, from oxides and nitrides to metals and organic compounds. This versatility allows for the creation of novel material combinations and functional structures.

• ALD's conformal nature extends to high-aspect-ratio structures like nanopores and nanotubes, enabling innovations in energy storage and catalysis.

• Compared to traditional methods, ALD's controlled process reduces material waste and energy consumption, aligning with sustainable manufacturing practices.

ALD for Microelectronics

As mentioned, ALD is used extensively in silicon and microelectronics manufacturing. It is a vital part of a variety of processes, discussed further below:

• Gate Oxides: ALD addresses the challenge of tunneling current in MOSFETs by depositing high-κ oxides (like Al₂O₃, ZrO₂, and HfO₂). High-k dielectrics are used as gate insulators to counteract the undesirable effects of quantum tunneling in transistors. This enables thicker gate dielectrics to reduce tunneling currents while maintaining the required capacitance density.

• Transition-Metal Nitrides: ALD creates metal barriers and gate metals using transition-metal nitrides (e.g., TiN, TaN). These materials encase copper interconnects, preventing unwanted diffusion. 

• Metal Films: ALD enables Cu interconnects, W plugs, and seed layers. It also supports the creation of noble metals for memory capacitors and facilitates high- and low-work function metals for dual-gate MOSFETs.

• Precision in Magnetic Recording Heads: ALD's precise insulation layer creation benefits magnetic recording heads, enhancing magnetized patterns on hard disks. A precision-controlled insulation thickness improves recording quality.

• Shaping DRAM Capacitors: ALD plays a vital role in fabricating DRAM capacitors, contributing to higher memory density. It aids in scaling capacitor features while maintaining capacitance levels, which is crucial for semiconductor size reduction.

• Dielectric Barrier Layers: ALD creates precise and conformal dielectric barrier layers. These barrier layers are crucial for isolating and protecting sensitive components from external contaminants, moisture, and electrical interference.

• Passivation Layers: Semiconductor devices often require passivation layers to shield them from environmental factors, prevent corrosion, and ensure long-term functionality. ALD-generated passivation layers offer exceptional conformality, even on complex and intricate device structures.

• Copper Barrier and Seed Layers: ALD creates barrier layers for copper interconnects in ICs. These barrier layers prevent copper diffusion into surrounding materials, preventing electrical short circuits and enhancing the overall reliability of interconnect structures. Additionally, ALD deposits seed layers, promoting improved adhesion and uniformity during subsequent copper electrodeposition steps.

• Encapsulation and Moisture Barrier Films: ALD-deposited moisture barrier films offer an effective solution for semiconductor packaging, particularly in applications where hermetic sealing is challenging. These films inhibit moisture ingress, safeguarding sensitive components from degradation and extending the lifespan of the devices.

• MEMS Devices: MEMS require intricate structures and precise coatings. ALD provides the means to deposit thin films with atomic precision, enabling the creation of functional and reliable MEMS devices.

• 3D Integration and Through-Silicon Vias (TSVs): ALD contributes to the fabrication of through-silicon vias, a key component in three-dimensional integrated circuits (3D ICs). ALD ensures the conformal deposition of insulating and barrier layers within TSVs, facilitating efficient signal propagation and heat dissipation across vertically stacked layers.

• Package-Level Barrier Films: In advanced packaging techniques like fan-out wafer-level packaging (FOWLP), ALD is utilized to deposit barrier films that protect sensitive components from moisture, mechanical stress, and external contaminants. These barrier films contribute to the reliability and performance of the packaged devices.

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