Etch Dry and Wet Etches

Index Overview Wet etch/Dry etch Etch characteristics / requirements Oxide, Nitride, Silicon and Al Cleaning Summary 11/10/2018

Background One of the material removal techniques Either in liquid or gaseous phase (Wet etch or Dry etch) gas ==> plasma etch A “lot” of 200 mm wafers in a ‘cassette” Teflon ©IBM web site 11/10/2018

Wet Etch Unit: Sample Image ©IBM web site Note: Clean room floor “Bunny suite” 11/10/2018

Sample Image Wet Deck. Arizona State University semi-automatic 11/10/2018

Background Etching: usually after Photo Needs to be selective To photoresist To hard mask To the bottom material (in most cases) Blind Etch vs “Stop on” etch Planarization Etch Isotropic vs Anisotropic should handle high Aspect Ratio clean cost 11/10/2018

Wet vs Dry etch Wet chemical etch: Usually isotropic In some single crystals, chemical etch occurs in only one direction ==> Anisotropic Dry etch: Plasma : Ion impact mechanical removal, gas reaction, chemical removal Can be anisotropic or isotropic. Controllable 11/10/2018

Requirements Uniformity: Macro scale: across the wafer uniformity, plasma confinement, wet etch chemical replenishment Micro Scale: Micro loading or etch loading Mass transfer/ Boundary layer controlled vs kinetics controlled process proximity correction 11/10/2018

Wet Etch SiO2 Si3N4 Si Al Resist Silicon Oxide, Silicon Nitride Aluminum, Chromium Photoresist (develop) Silicon Oxide: HF Buffered HF (Buffered Oxide Etch or BOE) NH4F + HF BHF+Glycerin to protect aluminum HF+HCl (increases etch rate) Temperature as controlling element 11/10/2018

Wet Etch SiO2 Si3N4 Si Al Resist Etch rates in um/min Etch rate strongly depends on type of oxide: Thermal oxide has low etch rate BSG has low etch rate PSG, CVD oxide high etch rate 11/10/2018

Wet Etch Steps SiO2 Si3N4 Si Al Resist Soak (etch) in tank controlled temp, composition, close loop control double rinse (usually DI) counter current spin dry 11/10/2018

Nitride Etch SiO2 Si3N4 Si Al Resist HF or Buffered Oxide Etch HF+HCl Hot Phosphoric acid etch etch rate of oxide is low of Si is lower of resist is high! Use oxide as a mask 11/10/2018

Nitride Etch SiO2 Si3N4 Si Al Resist Boiling Point of Phosphoric Acid Replace water as necessary Nitride edge > 30 min usually 11/10/2018

Silicon Etch SiO2 Si3N4 Si Al Resist KOH on single crystal Poly crystal etches in random (i.e. All )direction Hard Mask with silicon nitride Or silicon oxide thermal oxide vs CVD oxide Use low conc and temp otherwise oxide will also etch fast! 11/10/2018

Silicon Etch SiO2 Si3N4 Si Al Resist Adding boron reduces the etch rate Etch rate vs Conc 11/10/2018

Al Etch Resist Etch (Strip) SiO2 Si3N4 Si Al Resist Phosphoric, nitric and acetic acid mixture Al has some Si and Cu added to help electro migration; Si helps a bit in electro migration, but helps more in spiking (kind of pre-dissolves) Si does not dissolve well in the mix Heat it to remove non uniformities H2CrO4, H3PO4, H2O mix also used Resist Etch (Strip) Resist stripping, usually < 30 mins Chemistry depends on resist composition (organic/acetone based) 11/10/2018

Wet Etch / Cleaning Cleaning silicon wafers RCA or SC1/SC2 (modified now) SC1 : ammonia, peroxide, water :(APM, Huang A): high temp SC2: HCl, peroxide, water (HPM, Huang B): high temperature Gentler cleaning preferred now a days (i.e. Lower concentration/ lower temp) dilute HF etch added between SC1 and SC2 After etch/clean, rinse with 18 Mohm-cm DI water SC1 Organic, complexes some metals SC2 Sodium and other metal complexes 11/10/2018

Wet Etch Summary Wet etch is low temperature, high selectivity Charge issues: ESD (electrostatic discharge) in dry etch Increased reliability for wet etch Of late, photochemical reactions and electrochemical dissolution are becoming important in wet etch and wet cleaning Cheap equipment cost Fast etch rate, Any material can be etched Surfactant to enhance wetting Batch processing is easy Inline monitoring/End Point Detection is not easy Is the only method for cleaning equipment/labware 11/10/2018

Wet Etch Summary Isotropic Transport limited in small trenches “loading” in large trenches may ‘give back’ some uniformity Poor control of dimension for sub micron, etch rate in general Running cost high (very clean wet chemicals with out Na or other metals are very expensive) 11/10/2018