5.4 Scanning probe lithography

Scanning probe lithography takes nanofabrication to the atomic level. Using tools like atomic force microscopes, we can manipulate individual atoms and molecules to create incredibly tiny structures. It's like painting with the smallest brush imaginable.

This technique opens up new possibilities for making super-small electronic devices and sensors. By precisely placing atoms and molecules, we can build things that were impossible before. It's slow but allows for incredible precision and control.

Scanning Probe Microscopy Techniques

Atomic Force and Scanning Tunneling Microscopes

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• Atomic force microscope (AFM) utilizes a sharp tip attached to a cantilever to scan sample surfaces
  • Measures forces between tip and sample using laser deflection off cantilever
  • Operates in contact, non-contact, and tapping modes for various applications
• Scanning tunneling microscope (STM) employs a conducting tip to measure tunneling current
  • Requires conductive samples for imaging
  • Provides atomic-scale resolution of surface topography and electronic structure
• Both AFM and STM offer high-resolution imaging capabilities (atomic scale)
  • AFM resolution typically 1-10 nm laterally, <1 nm vertically
  • STM resolution can reach 0.1 nm laterally, 0.01 nm vertically
• Scanning probe techniques enable surface manipulation at nanoscale
  • Move individual atoms (STM)
  • Modify surface chemistry (AFM)

Applications and Limitations

• AFM applications include surface characterization, force measurements, and nanomanipulation
  • Used in materials science, biology, and semiconductor industries
• STM primarily used for conductive materials and surface science studies
  • Provides insights into electronic properties and atomic arrangements
• Resolution limited by tip sharpness, environmental factors, and sample properties
  • Thermal drift and vibrations can affect image quality
  • Tip convolution effects may distort features smaller than tip radius
• Scanning speed constraints limit real-time imaging capabilities
  • Typical scan rates range from seconds to minutes per image
• Sample preparation requirements vary between techniques
  • AFM can image in air, liquid, or vacuum
  • STM typically requires ultra-high vacuum for best results

Dip-Pen Nanolithography

Principle and Process

• Dip-pen nanolithography (DPN) transfers molecules from AFM tip to substrate
  • Utilizes water meniscus formed between tip and surface
  • Enables direct writing of nanoscale patterns
• Process involves coating AFM tip with "ink" molecules
  • Ink can be proteins, DNA, polymers, or small molecules
  • Tip moves across surface, depositing ink through water meniscus
• Pattern resolution determined by multiple factors
  • Tip sharpness (typical radius 10-50 nm)
  • Environmental conditions (humidity, temperature)
  • Ink properties (viscosity, surface tension)
• Writing speed and feature size controllable
  • Slower writing typically produces smaller features
  • Feature sizes range from 10 nm to several micrometers

Applications and Advantages

• Chemical modification of surfaces at nanoscale
  • Create functional nanopatterns for biosensors or molecular electronics
  • Deposit multiple "inks" in single patterning process
• Advantages over traditional lithography techniques
  • Direct writing without masks or resists
  • Ambient condition operation (no vacuum required)
  • Compatibility with wide range of materials (metals, polymers, biomolecules)
• Applications in nanofabrication and surface engineering
  • Protein nanoarrays for drug screening
  • DNA templating for nanoelectronics
  • Patterned self-assembled monolayers for surface functionalization

AFM-Based Lithography Methods

Local Oxidation and Mechanical Patterning

• Local oxidation nanolithography uses AFM tip to induce electrochemical reactions
  • Applied voltage between tip and substrate creates localized oxide patterns
  • Commonly used on silicon and metal surfaces
  • Feature sizes down to 10 nm achievable
• Mechanical patterning involves physically modifying surface with AFM tip
  • Indentation: pressing tip into soft materials to create patterns
  • Plowing: dragging tip across surface to form trenches or grooves
  • Resolution limited by tip sharpness and applied force
• Applications of local oxidation and mechanical patterning
  • Fabrication of nanoscale electronic devices (quantum dots, single-electron transistors)
  • Creation of templates for selective deposition or etching
  • Surface texturing for tribological studies

Nanoshaving and Nanografting Techniques

• Nanoshaving removes material from self-assembled monolayers (SAMs)
  • AFM tip applies higher force to displace molecules in specific areas
  • Creates nanoscale patterns within existing molecular layers
  • Useful for studying molecular interactions and surface properties
• Nanografting combines nanoshaving with simultaneous deposition
  • Removed molecules replaced by different molecules from solution
  • Enables creation of complex, multi-component nanopatterns
  • Applications in biosensing and molecular recognition studies
• Control parameters for nanoshaving and nanografting
  • Applied force (typically 10-100 nN)
  • Scan speed (0.1-10 μm/s)
  • Tip sharpness and geometry
• Advantages of nanoshaving and nanografting
  • In situ patterning in liquid environments
  • High spatial resolution (down to 10 nm features)
  • Compatibility with delicate biological molecules