• CMP = Chemical Mechanical Polishing
  • it is a solid phase etching because uses a solid to remove a solid

• A slurry containing SiO₂ abrasive particles is put on the pad (see also the tubes in the upper picture used for slurry dispense)
• The wafer is placed on a carrier
• The carrier is then moved on the pad, and the friction between the pad and wafer is realised by applying a force (see lower picture)
• Also the slurry is pressed into the wafer by the applied pressure
• Surface reaction (between OH- and H₂O from the slurry and Si wafer) and mechanical abrasion take place
• The by-products (Si(OH)₄) are removed
• Removal rate depends on pressure and relative velocity
• Smoothness depends on pressure, diameter and material type
• If the main mechanism is mechanical, CMP is not selective (etches everything), if the chemical reactions are the main mechanism, then it can be also selective

• Applications of CMP
  • Almost any material (metals, dielectrics)
  • Cu
    • it is not possible to etch it by plasma etching
    • it is a soft material
    • the pad can be conditioned to regain roughness

• Problems with CMP
  • Tradeoff between:
    • Polishing goals:
      • Uniformity
      • Planarity
      • Throughput
    • Cleaning goals:
      • Particles
      • Surface damage
      • Contaminants

A cleaning step follows the polishing