2.1 Top-down fabrication methods (lithography, etching)

Top-down fabrication methods are crucial for creating quantum dots with precise control. These techniques, including lithography and etching, allow researchers to shape materials into nanoscale structures with specific sizes and arrangements.

Lithography uses light or particle beams to pattern a resist layer, while etching removes material to form the desired structures. These methods offer high precision but can be costly and have resolution limits, impacting quantum dot properties.

Lithography principles and processes

Lithography process steps

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• Coat the substrate with photoresist, a light-sensitive chemical that changes solubility when exposed to light
• Expose the photoresist to a pattern of intense light using a photomask or direct-write method (UV, electron beam, ion beam)
• Develop the exposed or unexposed portions of the resist to create the desired pattern by selectively dissolving the resist in a developer solution
• The patterned resist acts as a mask for subsequent etching or deposition steps to transfer the pattern onto the underlying substrate or material layers
• Strip the remaining resist from the substrate after the pattern transfer is complete, leaving the final patterned structure

Lithography resolution and light sources

• The resolution of the lithography process depends on the wavelength of the light source or the beam size
• Shorter wavelengths (UV, X-ray) and smaller beam sizes (electron, ion) enable the creation of smaller features
• Photolithography uses UV light and a photomask to create patterns with resolutions typically >100 nm
• Electron beam lithography (EBL) and ion beam lithography (IBL) use focused electron or ion beams to directly write patterns onto the resist without a mask, achieving resolutions <10 nm

Etching in quantum dot fabrication

Etching methods and mechanisms

• Wet etching uses liquid chemicals (acids, bases) to dissolve the material selectively
• Dry etching uses reactive gases or plasma to remove material through physical sputtering or chemical reactions
• Anisotropic etching techniques, such as reactive ion etching (RIE), create vertical sidewalls and high aspect ratio structures by combining physical and chemical etching mechanisms
• The choice of etching method and parameters (etchant chemistry, temperature, time) depends on the material being etched and the desired etch rate, selectivity, and profile

Etching for quantum dot geometry control

• Etching selectively removes material from the substrate or deposited layers to create the desired quantum dot nanostructures
• The pattern of the resist mask and the etching conditions control the size, shape, and arrangement of the quantum dots
• Various quantum dot geometries can be created, such as:
  • Nanowires: elongated, one-dimensional structures
  • Nanopillars: vertical, high aspect ratio columns
  • Nanoholes: inverted, pit-like structures
• Precise control over the etching process is crucial for achieving uniform and reproducible quantum dot arrays with well-defined dimensions and spacing

Lithography methods for quantum dot fabrication

Photolithography

• High-throughput, low-cost method that uses UV light and photomasks to create patterns
• Resolution is limited by the wavelength of light (typically >100 nm), making it suitable for larger quantum dots or arrays
• Advantages: fast, scalable, and compatible with existing semiconductor manufacturing processes
• Limitations: insufficient resolution for ultra-small quantum dots (<10 nm)

Electron beam lithography (EBL)

• Uses a focused electron beam to directly write patterns with high resolution (<10 nm)
• Enables the creation of smaller quantum dots and more complex geometries compared to photolithography
• Advantages: high resolution, flexibility in pattern design, and direct-write capability
• Limitations: slower and more expensive than photolithography, potential for electron scattering and proximity effects

Other advanced lithography techniques

• Ion beam lithography (IBL): uses a focused ion beam to pattern the resist, offering high resolution and direct milling of the substrate, but can cause surface damage and ion implantation
• Nanoimprint lithography (NIL): a mechanical patterning method that uses a mold to transfer patterns onto the resist, enabling high-resolution and high-throughput fabrication, but requires the creation of a high-quality mold
• Extreme UV (EUV) lithography: uses shorter wavelength light (13.5 nm) to achieve higher resolutions than conventional photolithography, but requires specialized light sources and optics

Top-down fabrication: Advantages vs limitations

Advantages of top-down fabrication

• Precise control over the size, shape, and position of quantum dots, enabling the creation of ordered arrays and complex geometries
• Compatibility with existing semiconductor manufacturing processes and infrastructures, allowing for the integration of quantum dots with other electronic and photonic devices
• High uniformity and reproducibility, which is crucial for the consistent performance and scalability of quantum dot-based devices
• Ability to create site-controlled quantum dots with deterministic placement, facilitating the development of quantum information processing and sensing applications

Limitations of top-down fabrication

• Resolution is limited by the lithography techniques, which may not be sufficient for creating ultra-small quantum dots with sizes below 10 nm
• Fabrication process can introduce defects, surface states, or contamination that affect the optical and electronic properties of the quantum dots, requiring additional surface passivation or post-processing steps
• More expensive and time-consuming compared to bottom-up approaches, due to the need for sophisticated equipment and multiple processing steps
• Scalability may be limited by the throughput of the lithography and etching tools, as well as the availability of high-quality substrates and materials
• Challenges in achieving high quantum yields and narrow emission linewidths compared to bottom-up synthesized quantum dots, due to the influence of surface states and defects introduced during fabrication