Bulk and Surface Mircomachining Tuan Dang and Anita Quabili ENEE416 October 6 2009 Wet Bulk Micromachining Wet etchant aqueous solution that eats away Silicon Bulk usually Silicon wafer where wet ecthants remove material Micromachining method of fabricating microstructures using wet etching techniques Wet Etching Process Silicon Isotropic Wet Etching Anisotropic Wet Etching Bulk Applications Pros Inexpensive Done in bulk Good for making micromechanical parts holes trenches pits and MEMS Cons Not easily incorporated with microelectronic devices Cannot be layered on the same wafer Surface Micromachining Bulk Micromachining a silicon wafer is selectively etched to produce microstructures Surface Micromachining is based on depositing and etching layers on top of the silicon substrate Dry Etching Process The main purpose of dry etching is to avoid isotropic etching associated with wet techniques Realized using plasma etching systems Methods Ion bombardment Chemical Reactive Environment Vacuum chamber Ion Bombardment Sputtering Also known as Ion Beam Etching Is performed in a low pressure vacuum chamber Energetic gas particles are introduced into the chamber Gas particles hit the metal at the cathode knocking the metal particles loose This deposits a layer of metal onto the unmasked part of the wafer Requires very low pressure Reactive Ion Etching Reverse polarity Is like sputtering except the cathode is under the wafer and the anode is on top of the wafer The gas particles hit the wafer and physically etch away the unmasked portion of the wafer The wafer is usually held at an angle so that the etched molecules don t redeposit themselves onto the wafer Is less anisotropic than sputtering Requires less energy than sputtering Reactive Ion Etchants Organic Materials O2 SF6 CF4 Polysilicon CCl4 CF4 NF3 SF6 Silicon Dioxide CF4 C2F6 C3F8 CHF3 Silicon Nitride CF4 C2F6 CHF3 SF6 Aluminum CCl4 Cl2 BCl3 Titanium C2Cl2F4 CF4 Tungsten Cl2 Different etchants require different temperatures and pressures to be effective Pros Anisotropic Capable of small feature sizes 100nm Can be used for many layers Insensitive to crystalline structures Cons High cost Poor selectivity compared to wet etching Potential radiation damage Surface Applications
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