Photolithography is a crucial process in microfabrication, using light to transfer patterns onto substrates. It involves precise steps, from coating with photoresist to etching, enabling the creation of tiny electronic components and sensors. The process relies on specialized materials and equipment for accurate pattern transfer.

Photoresists, the key materials in lithography, come in positive and negative varieties. Their composition and photochemical reactions determine how they respond to light exposure. Understanding these materials and their interactions is essential for achieving high-resolution patterns in various applications.

Photoresists and Lithography Fundamentals

Principles of photolithography

• Photolithography process transfers patterns onto substrates using light-sensitive materials

  1. Substrate preparation cleans and primes surface
  2. Photoresist application coats substrate with light-sensitive polymer
  3. Soft baking removes excess solvent and improves adhesion
  4. Mask alignment positions photomask over substrate
  5. UV exposure selectively alters photoresist solubility
  6. Development removes exposed (positive) or unexposed (negative) areas
  7. Hard baking stabilizes remaining photoresist pattern
  8. Etching or deposition modifies exposed substrate areas
  9. Photoresist removal strips remaining resist

• Key components enable precise pattern transfer

  • Light source emits specific wavelengths (UV)
  • Photomask contains desired pattern to be transferred
  • Photoresist changes solubility when exposed to light
  • Substrate serves as base material for pattern creation

• Applications in microfabrication span various industries

  • Integrated circuit production forms complex electronic components
  • MEMS fabrication creates tiny sensors and actuators
  • Microfluidic device creation enables lab-on-a-chip technologies
  • Flat panel display manufacturing produces high-resolution screens

Composition of photoresists

• Positive photoresists become more soluble when exposed to light
  • Composition tailored for high resolution and process control
    • Base resin (novolac) provides structural support
    • Photoactive compound (diazonaphthoquinone) changes solubility
    • Solvent allows for thin film application
  • Properties enable precise pattern formation
    • Increased solubility upon exposure allows selective removal
    • Higher resolution achieves smaller feature sizes
    • Better process control improves reproducibility
• Negative photoresists become less soluble when exposed to light
  • Composition designed for durability and adhesion
    • Base polymer (epoxy or polyisoprene) forms crosslinked structure
    • Photoinitiator triggers polymerization reaction
    • Solvent facilitates application and film formation
  • Properties suit certain applications
    • Decreased solubility upon exposure creates durable patterns
    • Better adhesion to substrates improves pattern stability
    • Higher chemical resistance withstands harsh processing steps

Photochemical reactions in patterning

• Positive photoresist reactions alter molecular structure
  • Photochemical decomposition of diazonaphthoquinone releases nitrogen
  • Formation of carboxylic acid increases polarity
  • Increased solubility in alkaline developer allows selective removal
• Negative photoresist reactions initiate polymerization
  • Photoinitiator activation absorbs light energy
  • Free radical generation triggers chain reaction
  • Cross-linking of polymer chains reduces solubility
  • Decreased solubility in organic solvents creates stable patterns
• Quantum yield impacts process efficiency
  • Efficiency of photochemical reactions determines sensitivity
  • Impact on exposure time and energy requirements affects throughput

Advanced Lithography Concepts

Resolution limits in lithography

• Resolution limits determine smallest achievable feature size
  • Rayleigh criterion: $R = k_1 \frac{\lambda}{NA}$ relates key parameters
    • R: minimum feature size sets lower bound for patterns
    • $\lambda$: wavelength of light influences resolution directly
    • NA: numerical aperture of the optical system affects light focusing
    • $k_1$: process-dependent factor accounts for resist properties
• Challenges in advanced lithography push physical limits
  • Diffraction effects blur pattern edges at small scales
  • Depth of focus limitations restrict usable resist thickness
  • Proximity effects cause unwanted exposure in nearby areas
  • Line edge roughness introduces variations in feature dimensions
• Advanced techniques overcome traditional limitations
  • Extreme ultraviolet (EUV) lithography uses 13.5 nm wavelength
  • Electron beam lithography directly writes patterns without masks
  • Nanoimprint lithography mechanically deforms resist
  • Multi-patterning techniques combine multiple exposures
• Emerging technologies explore novel approaches
  • Directed self-assembly utilizes block copolymer properties
  • Block copolymer lithography creates regular nanoscale patterns
  • Plasmonic lithography exploits near-field optical effects