Atomic force microscopy (AFM) is a versatile tool for nanoscale patterning of semiconductor materials, offering precise control over feature size and placement without requiring masks or resists. AFM-based nanolithography techniques include dip-pen nanolithography (DPN), local anodic oxidation (LAO), and mechanical scratching, each with distinct mechanisms, resolution limits, and material compatibility. These methods enable direct-write patterning at scales difficult to achieve with conventional lithography, making them valuable for prototyping and specialized semiconductor applications.

Dip-pen nanolithography (DPN) utilizes an AFM tip coated with a molecular ink to deposit material onto a substrate through capillary transport. The resolution of DPN depends on tip sharpness, ink viscosity, and environmental conditions, with achievable line widths ranging from 15 nm to 100 nm. DPN is particularly effective for patterning organic molecules, polymers, and biomaterials on semiconductors like silicon, GaAs, and graphene. The writing speed is relatively slow, typically 0.1–10 µm/s, due to the reliance on molecular diffusion. However, its strength lies in multi-material patterning by switching inks during the process. DPN is compatible with ambient conditions but struggles with high-throughput demands compared to parallel lithography techniques.

Local anodic oxidation (LAO) leverages the AFM tip to induce electrochemical reactions on semiconductor surfaces. A voltage applied between the conductive tip and the substrate generates oxyanions, forming oxide patterns with sub-10 nm resolution in optimal conditions. LAO is widely used for patterning silicon, metals, and transition metal dichalcogenides (TMDCs). The oxidation rate depends on humidity, voltage, and tip speed, with typical speeds of 1–100 µm/s. LAO produces durable oxide structures but is limited to materials that form stable oxides. Unlike DPN, LAO does not require additional inks, simplifying the process for dielectric patterning. However, it is less versatile for non-oxidizable materials.

Mechanical scratching, or nanoshaving, involves physically removing material from the substrate using the AFM tip’s mechanical force. This method achieves resolutions of 5–50 nm, depending on tip geometry and material hardness. It is effective for patterning soft semiconductors like organic films, layered materials (graphene, MoS2), and polymer resists. Scratching speeds range from 0.5–20 µm/s, limited by tip wear and substrate deformation. Unlike DPN or LAO, this method does not introduce foreign materials but may generate debris or surface damage. It is unsuitable for hard materials like SiC or diamond due to excessive tip wear.

Comparing these techniques in resolution, LAO generally outperforms DPN and scratching, with sub-10 nm features achievable under controlled conditions. DPN offers moderate resolution (15–100 nm) but excels in multi-material deposition. Mechanical scratching provides intermediate resolution (5–50 nm) but is highly material-dependent. In terms of speed, all AFM-based methods are slower than parallel lithography techniques, with LAO being the fastest among them. However, AFM methods eliminate the need for complex resist processes, making them advantageous for rapid prototyping.

Material compatibility varies significantly. DPN works well with organic and biological materials but requires functionalized surfaces for optimal adhesion. LAO is restricted to oxidizable semiconductors and metals, while mechanical scratching is best suited for soft or layered materials. None of these methods require vacuum conditions, unlike electron-beam lithography, allowing operation in ambient or liquid environments.

When compared to other direct-write methods like nanoimprint or focused ion beam lithography, AFM-based techniques offer superior resolution in some cases but lack scalability. Nanoimprint lithography achieves sub-20 nm resolution at high speeds but requires master templates. Focused ion beam patterning reaches similar resolutions to AFM but may introduce ion implantation damage. AFM methods avoid such collateral damage, making them preferable for delicate materials.

In semiconductor research, AFM-based nanolithography is particularly valuable for creating quantum dots, nanowire interconnects, and nanoscale sensors. For example, LAO has been used to fabricate silicon oxide templates for quantum confinement structures, while DPN enables precise placement of molecular dopants on 2D materials. Mechanical scratching has been applied to define nanoribbons in graphene for electronic devices.

Despite their advantages, AFM-based techniques face challenges in throughput and tip wear. Parallel tip arrays and automated patterning systems are being developed to address speed limitations. Advances in tip materials, such as diamond-coated probes, have extended the lifespan of tools used in mechanical scratching.

In summary, AFM-based nanolithography provides unique capabilities for semiconductor patterning at the nanoscale, complementing conventional lithography methods. DPN offers molecular precision, LAO delivers high-resolution oxide patterning, and mechanical scratching enables direct material removal. Each technique has distinct trade-offs in resolution, speed, and material compatibility, making them suitable for specialized applications where traditional lithography falls short. Continued improvements in probe technology and process automation may expand their role in semiconductor manufacturing.