Bulk and Surface Mircomachining - UMD ECE Class...
Transcript of Bulk and Surface Mircomachining - UMD ECE Class...
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Bulk and SurfaceMircomachining
Tuan Dang and Anita QuabiliENEE416
October 6, 2009
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Wet Bulk Micromachining
• Wet (etchant): aqueous solution that eatsaway Silicon.
• Bulk: usually Silicon wafer where wetecthants remove material.
• Micromachining: method of fabricatingmicrostructures using wet etchingtechniques.
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Wet Etching Process
Silicon
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Isotropic Wet Etching
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Anisotropic Wet Etching
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Bulk Applications
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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
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Surface Micromachining
• Bulk Micromachining: a silicon wafer isselectively etched to produce
microstructures
• Surface Micromachining is based on depositing and etching layers on top ofthe silicon substrate
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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
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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, knockingthe metal particles loose– This deposits a layer of
metal onto the unmaskedpart of the wafer
• Requires very lowpressure
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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 sothat the etchedmolecules don’tredeposit themselvesonto the wafer
• Is less anisotropicthan sputtering
• Requires lessenergy thansputtering
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Reactive Ion EtchantsO2, SF6, CF4CCl4, CF4, NF3, SF6CF4, C2F6, C3F8, CHF3CF4, C2F6, CHF3, SF6CCl4, Cl2, BCl3C2Cl2F4, CF4Cl2
• Organic Materials:• Polysilicon:
• Silicon Dioxide:• Silicon Nitride:
• Aluminum:• Titanium:
• Tungsten:
Different etchants require different temperaturesand pressures to be effective
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Pros
• High cost• Poor selectivity (compared to wet etching)• Potential radiation damage
Cons
• Anisotropic• Capable of small feature sizes (<100nm)• Can be used for many layers• Insensitive to crystalline structures
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Surface Applications