7.3 Selectivity and Anisotropy in Plasma Etching

Plasma etching is a crucial technique in semiconductor manufacturing, allowing precise control over material removal. Selectivity and anisotropy are key factors that determine the quality and precision of etched features, enabling the creation of complex microstructures.

Understanding the mechanisms behind selective and anisotropic etching is essential for optimizing plasma etching processes. By manipulating gas composition, pressure, RF power, and temperature, engineers can fine-tune etch characteristics to achieve desired outcomes in semiconductor device fabrication.

Selectivity and Anisotropy in Plasma Etching

Selectivity and anisotropy in plasma etching

• Selectivity
  • Ratio of etch rates between different materials (Si vs. SiO2)
  • Ability to preferentially etch one material over another enables precise pattern transfer
  • Critical for achieving desired etch profiles and protecting underlying layers (gate oxide)
• Anisotropy
  • Directionality of the etching process determines vertical vs. lateral etching
  • Characterized by the ratio of vertical etch rate to lateral etch rate (aspect ratio)
  • Essential for achieving high aspect ratio features (deep trenches) and maintaining pattern fidelity
  • Enables fabrication of complex 3D structures (MEMS devices)

Mechanisms of selective and anisotropic etching

• Chemical selectivity
  • Etchant species (F radicals) react preferentially with certain materials based on chemical composition
  • Dependent on factors such as bond strengths and activation energies (Si-Si vs. Si-O bonds)
  • Influenced by the chemical nature of the etchant and the materials being etched (SF6 for Si etching)
• Ion-assisted etching
  • Energetic ions (Ar+) bombard the surface, enhancing vertical etching through physical sputtering
  • Ion bombardment can break chemical bonds and remove material anisotropically
  • Contributes to anisotropy by promoting vertical etching over lateral etching (reactive ion etching)
• Passivation layers
  • Formation of protective layers (fluorocarbon polymers) on sidewalls during etching inhibits lateral etching
  • Can be formed by deposition of etch products or intentionally introduced species (C4F8)
  • Promotes anisotropy by blocking lateral etching while allowing vertical etching to proceed

Process parameters for etch characteristics

• Gas composition
  • Ratio of reactive species (CF4) to passivating species (C4F8) affects selectivity and anisotropy
  • Higher concentration of reactive species promotes selectivity by increasing chemical etching
  • Higher concentration of passivating species enhances anisotropy through sidewall protection
• Pressure
  • Lower pressures (<10 mTorr) result in longer mean free paths and more directional ion bombardment
  • Higher pressures increase collisions and scattering, reducing anisotropy but improving uniformity
• RF power
  • Higher RF power (>100 W) increases ion energy and flux, enhancing anisotropy through physical sputtering
  • Excessive RF power can lead to physical damage and reduced selectivity
• Substrate temperature
  • Higher temperatures (>100°C) can increase chemical reaction rates and reduce selectivity
  • Lower temperatures (<50°C) can promote passivation layer formation and improve anisotropy

Design of plasma etching processes

• Material selection
  • Choose etchant gases (Cl2 for Al, SF6 for Si) and chemistry based on the materials being etched
  • Consider the selectivity requirements between the target material and underlying layers
• Process parameter optimization
  • Adjust gas composition, pressure, RF power, and temperature to achieve desired selectivity and anisotropy
  • Use design of experiments (DOE) techniques to systematically explore the parameter space
• Etch stop layers
  • Incorporate etch stop layers (SiO2) with high selectivity to the target material (Si)
  • Allows for precise control of etch depth and protects underlying structures
• Mask design
  • Design etch masks with appropriate materials (photoresist, hard masks) and dimensions
  • Consider the selectivity between the mask and the target material to ensure pattern fidelity
  • Optimize mask thickness and profile to withstand the etching process