Surface micromachining is another technique used for making MEMS

• What is it?
  • a sacrificial layer beneath another layer is etched (completely removed from the final structure), thus releasing the upper layer, which will remain connected to the wafer only in some regions
• It is called "surface" because it takes place on the wafer surface (compared to bulk, where the whole wafer thickness is etched)
• Why is it used?
  • Bulk micromachining requires bigger areas due to anisotropic wet etching (the lateral etch is big)
  • Parts of the structure can be released and move laterally, thus it is useful in making actuators
  • Can be integrated with IC
• Materials used
• low-stress film  polysilicon deposited by LPCVD
  • it is annealed because annealing changes the type of stress from compressive to tensile due to crystallization (contraction), giving the possibilty to obtain free of stress material if the annealing time is chosen well
• Si₃N₄ - increased hardness
• sacrificial layer - removed without etching the structural layer
• Al, photoresist, SiO₂ 
• SiO₂ is prefered because of high temp deposition, high selectivity for HF(polysilicon)
• Applications: cantilever 
  • used to sense chemicals
  • the cantilever is heated periodically, to create vibrations (due to difference in thermal coefficient) 
  • frequency of vibrations is measured and the mass of chemical particles can be calculated

• Surface micromachining is used in making a cantilever, a simple process flow is shown below
  • the structural layer is the polysilicon, which is released by etching and removing totally the sacrificial layer below, which in this case is silicon oxide.

• Problems: surface tension forces pull the poly layer, when the liquid is dried
• Solutions: freezing, additional bumps to cantilever beam