AI Lessons Series 001:  MicroX3™ NanoX3™

3.1 Introduction to Atomic Layer Deposition (ALD)

• Overview:

• Atomic Layer Deposition (ALD) is a cutting-edge thin-film deposition technique that enables atomic-level precision in material growth. This method is particularly known for its ability to deposit materials one atomic layer at a time, allowing unparalleled control over film thickness and uniformity.

• Key Features:

• Monolayer Control: The hallmark of ALD is its capability to deposit material in monolayers, where each cycle of the process adds a single layer of atoms or molecules to the surface. This results in exceptionally precise film thickness control.

• Conformality: ALD excels at coating complex, high-aspect-ratio structures uniformly, making it indispensable in semiconductor manufacturing, where fine features and intricate geometries are common.

• Smoothness: ALD can produce surface finshes that are atomically smooth

• Isotropy: ALD enables highly isotropic deposition over large surface areas

3.3 ALD Precursors and Reactants

• Precursor Selection:

  • The choice of precursor in ALD is crucial, as it must possess specific properties such as high volatility to ensure it can be easily transported into the reaction chamber, thermal stability to avoid decomposition before reaching the substrate, and high reactivity with the surface to ensure complete coverage.

• Common Precursors:

  • Trimethylaluminum (TMA): Commonly used for depositing aluminum oxide (Al2O3\text{Al}_2\text{O}_3Al2​O3​), TMA is a highly reactive precursor that ensures uniform film growth.

  • Diethylzinc (DEZ): Used for depositing zinc oxide (ZnO\text{ZnO}ZnO), DEZ offers excellent control over film properties, making it ideal for various electronic applications.

• Equation (Deposition Reaction for Aluminum Oxide):  

  • 2Al(CH3)3+3H2O→Al2O3+6CH42 \text{Al(CH}_3\text{)}_3 + 3 \text{H}_2\text{O} \rightarrow \text{Al}_2\text{O}_3 + 6 \text{CH}_42Al(CH3​)3​+3H2​O→Al2​O3​+6CH4​

  • Explanation:

    • This reaction involves TMA reacting with water to form aluminum oxide, a common dielectric material, along with methane (CH4\text{CH}_4CH4​) as a byproduct.

3.5 Conformality and Thickness Control

• Conformality:

  • One of ALD's defining characteristics is its ability to achieve excellent conformality, even on substrates with complex 3D geometries. This makes it particularly useful for depositing films on high-aspect-ratio structures such as trenches, vias, and other intricate features common in advanced semiconductor devices.

• Equation (Film Thickness): 

  • t=N⋅dmonolayert = N \cdot d_{\text{monolayer}}t=N⋅dmonolayer​

  • Explanation:

    • t represents the total film thickness.

    • N is the number of ALD cycles performed.

    • dmonolayerd_{\text{monolayer}}dmonolayer​ is the thickness of the single atomic layer deposited during each cycle.

• Applications:

  • ALD is especially advantageous in semiconductor manufacturing, where uniform thin films are required on features with deep trenches and narrow gaps, such as those found in memory devices and advanced logic circuits.

3.7 ALD Applications: Semiconductors and Beyond

• Semiconductor Manufacturing:

  • Gate Dielectrics: ALD is employed to deposit high-k dielectrics, which are crucial for reducing power consumption and improving the performance of transistors in modern microprocessors.

  • Passivation Layers: Thin oxide layers deposited via ALD protect semiconductor surfaces from environmental degradation, enhancing the longevity and reliability of electronic devices.

• Energy Devices:

  • Solar Cells: In thin-film solar cells, ALD is used to deposit buffer layers that improve efficiency by enhancing light absorption and reducing recombination losses.

  • Batteries: ALD-coated electrodes in batteries offer improved cycling stability and longer lifespans, addressing critical challenges in energy storage technology.

• Visualization:

  • Diagrams showing ALD’s application in semiconductor devices and energy storage technologies, highlighting the process's versatility and precision.

3.9 Area-Selective ALD (AS-ALD)

• Overview:

  • Area-Selective ALD (AS-ALD) is a technique that enables selective deposition of materials on specific areas of a substrate while avoiding others. This technique is especially important for advanced semiconductor manufacturing, where precise patterning is required without the need for additional lithography steps.

• Techniques:

  • Patterning by Inhibition: Selective deposition is achieved by using self-assembled monolayers (SAMs) that prevent deposition on certain areas of the substrate.

  • Patterning by Activation: Involves modifying the surface chemistry to enhance deposition in specific regions, often through selective surface treatments that increase reactivity.

• Visualization:

  • An example of a patterned substrate showing how AS-ALD can achieve selective material deposition, illustrating the precision and control possible with this advanced technique.