Tutorial 3 Derek Wright Wednesday, February 2 nd, 2005
Photoresist Application Exposure/ Developing Deposition/ Growth or Etching Photoresist Etching Building Devices
Etching Physical Etching –Relies on momentum of particles to sputter material off the substrate Chemical Etching (Wet/Dry) –Relies on a chemical reaction between the etchant and the material at the substrate
Isotropic vs. Anisotropic Etching Isotropic Etching –Material is etched away in all directions at an equal rate Anisotropic Etching –Material is selectively etched in a specific direction A = 1 – (ER L /ER V ) –ER L = lateral etch rate, ER V = vertical etch rate
Isotropic vs. Anisotropic Etching A = 1 – (ER L /ER V ) –Isotropic:A = 0 –Anisotropic:0 < A < 1 –Directional:A = 1 A = 0 0 < A < 1 A = 1
Isotropic vs. Anisotropic Etching
Physical Etching Sputtering of the surface material Reactive Ion Etching is great for this because it is very anisotropic high aspect ratio trenches The ions don’t react with the surface, they just slam into it Can cause substrate damage NONSELECTIVE
Dry Chemical Etching PECVD causes reactive species to form in the plasma The reactive species interact with the surface and form volatile products The volatile products evaporate from the surface SELECTIVE
Wet Chemical Etching The substrate is dunked in a solution containing a material-specific solvent Very low damage to the substrate Concentrations of solvents need to be very tightly controlled Creates a lot of toxic waste SELECTIVE
Typical Wet Etchants
Bulk Micromachining “Bulk” means substrate Make a structure on top of the substrate Etch away the substrate underneath the structure Left with a “floating” structure above a well of missing substrate Key process in creating MEMS
Bulk Micromachining
Surface Micromachining Often a layer will be deposited (usually an oxide) with the intention of removing it later –“Sacrificial Layer” This allows “floating” structures to be created on top of the substrate without machining the substrate
Surface Micromachining
MEMS Examples
Photoresist Application Exposure/ Developing Deposition/ Growth or Etching Photoresist Etching Building Devices
Photoresist Removal When photoresist is exposed, some of it is washed away with developer revealing the mask pattern Some process is done to the remaining pattern –Deposition, growth, etching… Once that step is done, the remaining hardened photoresist must be removed: –Photoresist Etching
Photoresist Removal The hardened photoresist is etched away A wet etch is used to minimize surface damage PECVD methods can be used with O 2 gas, but usually only one wafer at a time –ASHING
Plasma Process Control Optical Emission Spectroscopy –Look at the wavelengths and intensities coming out of the plasma Interferometry –Shine a laser onto the substrate and measure the phase changes Mass Spectroscopy –Measures the types and energies of ions present in the plasma Electrical Measurements –Measure the power output of the generator
Chemo-Mechanical Polishing Every time a new layer is added to the substrate, it becomes uneven over the surface –Lumpy Even if you deposit a layer of oxide to even things out it’s still lumpy CMP allows planarization of the wafer before the next layer is added Very important in metal layers to have predictable line resistances
Chemo-Mechanical Polishing Wafer is held face down in a “carrier” Polishing pad is underneath “Slurry” is sprayed between the two –Slurry would be a somewhat chemically reactive solution with bits of hard material in it to scrape and eat away at the surface The wafer and pad are pushed together and spun around to ensure even wearing of the surface
Chemo-Mechanical Polishing
CMP is a relatively rough process and if it’s not done very carefully it can cause defects Tight process control must be used Wafers must be cleaned once they’ve been through CMP
Chemo-Mechanical Polishing Without CMP, the divisions between metal layers would be wobbly (lumpy) instead of straight
Chemo-Mechanical Polishing
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