5.2 Electron beam lithography

Electron beam lithography takes nanofabrication to the next level. It uses a focused beam of electrons to create super tiny patterns on special materials. This technique can make features way smaller than regular light-based methods.

The process isn't simple though. Electrons scatter in weird ways when they hit stuff, which can mess up the patterns. Scientists use fancy math and clever tricks to deal with these issues and make amazingly small structures.

Electron Beam Fundamentals

Electron Beam Properties and Direct-Write Technique

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• Electron beam consists of a focused stream of high-energy electrons accelerated through an electric field
• Direct-write technique enables precise pattern creation by guiding the electron beam across a resist-coated substrate
• Beam diameter typically ranges from 1-100 nm, allowing for nanoscale feature creation
• Electron beam energy usually falls between 10-100 keV, influencing penetration depth and scattering behavior
• Computer-controlled beam deflection system directs the electron beam to create desired patterns

Resolution and Scattering Effects

• Resolution in electron beam lithography reaches sub-10 nm scales, surpassing optical lithography limitations
• Factors affecting resolution include beam spot size, resist properties, and electron scattering
• Forward scattering occurs as electrons enter the resist, causing beam broadening
• Backscattering results from electrons reflecting off the substrate, leading to additional resist exposure
• Monte Carlo simulations model electron trajectories to predict scattering effects and optimize exposure parameters
• Resist contrast and development processes also impact achievable resolution

Resist Interaction

Resist Exposure Mechanisms

• Electron beam interacts with resist molecules, breaking chemical bonds or inducing crosslinking
• Positive resists become more soluble in developer after exposure (PMMA, ZEP)
• Negative resists become less soluble in developer after exposure (HSQ, SU-8)
• Exposure process involves energy deposition, secondary electron generation, and chemical changes in the resist
• Resist sensitivity determines the required electron dose for proper pattern formation

Proximity Effect and Dose Control

• Proximity effect results from electron scattering, causing unintended exposure of nearby areas
• Backscattered electrons contribute significantly to proximity effect, especially in dense patterns
• Proximity effect correction algorithms adjust electron dose to compensate for scattering-induced variations
• Dose control involves optimizing electron beam current and exposure time for each pattern feature
• Base dose determination requires consideration of resist sensitivity, substrate material, and feature size
• Dose modulation techniques include variable shape beam and character projection to improve throughput

Process Considerations

Beam Scanning Strategies and Pattern Generation

• Vector scan mode directs the beam only to areas requiring exposure, minimizing write time
• Raster scan mode covers the entire writing field, suitable for dense or complex patterns
• Hybrid scanning combines vector and raster approaches for optimized pattern generation
• Beam blanking prevents unintended exposure during beam repositioning
• Stitching techniques enable large-area patterning by combining multiple write fields
• Pattern fracturing divides complex designs into simple geometric shapes for efficient beam control

Throughput Optimization and System Limitations

• Throughput in electron beam lithography limited by sequential nature of exposure process
• Write time depends on total exposure area, required dose, and beam current
• Strategies to improve throughput include increasing beam current and employing multiple beams
• Multibeam systems use arrays of electron sources to parallelize exposure process
• Character projection technique exposes pre-defined shapes in a single shot, reducing write time for repetitive patterns
• Resist sensitivity improvements and advanced scanning strategies contribute to throughput enhancement
• Trade-offs exist between resolution, throughput, and pattern complexity in system optimization