bulk and surface mircomachining - umd ece class...

Bulk and SurfaceMircomachining

Tuan Dang and Anita QuabiliENEE416

October 6, 2009

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.

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 isselectively etched to produce

microstructures

• Surface Micromachining is based on depositing and etching layers on top ofthe 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, knockingthe metal particles loose– This deposits a layer of

metal onto the unmaskedpart of the wafer

• Requires very lowpressure

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

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

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

Surface Applications