Lecture 7 - ETCHING & WET PROCESSING
Chapter 11 & 14.1 in Campbell11.1 Wet Etching
14.1 Cleaning
11.3 Basic Regimes of Plasma Etching
11.4 High-Pressure Plasma Etching
11.5 Ion Milling
11.6 Reactive Ion Etching
11.7 Damage in Reactive Ion Etching
11.8 High-Density Plasma (HDP) Etching
11.9 Liftoff
MNT 2016; Lecture 7
Wet etching - key features• Batch process
• High selectivity
• Simple setup (wet bench or fume hood)
Problems:
• Hard to control etch rate
• Isotropic => Not good for small features
• Particulates => High defect levels
• Chemical waste
Anisotropy
A = 1 - RL/RV
Wet etching of SiO2
• Etch in diluted HF
Overall reaction is:
SiO2 + 6HF → H2 + SiF6 + 2H20
• High selectivity to Si (approx 100:1)
• Use buffered solution (BHF or BOE) to control pH and HF conc.
NH4F ↔ NH3 + HF
• Etch rate in BHF approx 70 nm/min
- higher for CVD or sputtered SiO2
14.1 Wafer cleaning
Things to remove:• Organics ( resist residues, polymers, . . . )
• Metallic particles (from plasma chambers, etchers, implanters, . . . )
• Thin oxides
14.1 Wafer cleaning• Clean water = Deionized water (DI water)
Resistivity ~ 15 MΩ cm
TOC ~ 10-6 (Total Organic Content or Total Organic Carbon)
• RCA - cleanTemp 60 - 80 CSC1 (Standard Clean 1) NH4OH : H2O2 : H2O approx 1:1:5Removes organic residues
SC2 (Standard Clean 2) HCl : H2O2 : H2O approx 1:1:6Removes metallic residues
• Piranha / Caro's / Seven-upTemp 100 - 130 CH2SO4 : H2O2 approx 2:1Removes particles, organics, resist residues
11.9 Liftoff
• Simple method to create metal lines or other structures
- Easy to set up in the lab
• Often used when no suitable etch is available
- Common in III-V technology
• Resist pattern first - then deposit metal or other film
• Re-entrant resist profile wanted
• Films best deposited by evaporation, not sputtering
• Works best on "simple" structures,
- Low yield on complex and dense patterns
11.3 Plasma etching / dry etching
– Why?• Anisotropy
• Few particles
• Little chemical waste
• Repeatable etch rate
• Endpoint detection possible
Problems:
• Hard to etch some materials
• Low etch rate
• Selectivity
• Plasma damage / Charge build-up
DC-plasma reactor
RF-plasma reactor
• Dissociatione* + AB ⇔ A + B + e
• Atomic ionizatione* + A ⇔ A+ + e + e
• Molecular ionizatione* + AB ⇔ AB+ + e + e
• Atomic excitatione* + A ⇔ A* + e
• Molecular excitatione* + AB ⇔ AB* + e
* is exited state
Glow discharge processes
Etch mode
Full name Configuration Typical pressure
Comments
Plasma etching
Barrel type reactor 1Torrλ < 1 mm
Mostly isotropicResist ashing
RIE Reactive Ion Etching
Parallel plate reactor 10 mTorrλ > 1 mm
Anisotropic
HDP High Density Plasma
ICP (Inductively Coupled Plasma)ECR (Electron Cyclotron Resonance)Various magnetic enhancement schemes
10-3 Torr Increased etch rateLow DC-bias possibleLow pressure
IBE Ion Beam Etching
Ar+ ion beam 10-4 Torr AnisotropicEtch any materialNo selectivity
RIBE Reactive Ion Beam Etching
Ion beam of CHF3+, Cl2+, SiCl4+, …
10-4 Torr As IBE but increased selectivity or etch rate
CAIBE Chemically Assisted Ion Beam Etching
Ar+ ion beam, O2 or other gas added close to sample
10-4 Torr As IBE but increased selectivity or etch rate
Etching toolsPE
RF
Plasma Etcher
RFNo electrical field in center
100-1000 mTorr
IBE, RIBE, CAIBE• Some materials will not form volatile compounds
=> Only way to etch is by ”ion milling”
• No selectivity
• Low etch rate
• Charge build-up => add electron flood gun to neutralize
Etching toolsPE
RFRF
IBERIBE
CAIBE
Ion Beam Etcher (Ion miller)
RFUbeam
Uacc
Angled etchingpossible
0.1 mTorr
Etching toolsPE
RF
RF
RIE
RFIBE
RIBECAIBE
RF
Reactive Ion Etcher
10-500 mTorr
Reactive Ion Etch Self DC-bias
Velect.
0t
VDC-bias
~Electron surplusRF
Etching toolsPE
RF
RF
RIE
RF
RFICP / RIE
RFIBE
RIBECAIBE
High Density Plasma (HDP)• High ion and radical density at low pressure
• Decouple DC-bias from ion density
• Long mean free path => anisotropic etch at 10-30 V bias
• HDP used for thin sensitive layers
or
thick layer where high etch rate is needed
• Various configurations: ICP (Inductively Coupled Plasma)
ECR (Electron Cyclotron Resonance)
Magnetic enhancement
RF
RF
Inductively Coupled Plasma Etcher
1-500 mTorr
RF
Inductively Coupled Plasma Etcher
Plasma etching mode
RF
Inductively Coupled Plasma Etcher
Reactive Ion Etching mode
Process gases (examples)
Physical etching gas
DilutantWafer backside coolingOrganics, Photoresist
Si, III-V, Metals
III-V
Si (SiN) SiO2
Ar
He
O2
Cl2 BCl3 SiCl4HBr CH4 H2
SF6 NF3 CF4 C4F8 CHF3
Plasma etching of Si, SiO2, Si3N4
• Fluorine-based plasma often used
Si + 4F → SiF4
• Add carbon to control formation of hydrocarbons
=> Use CF4 ,CHF3 , …
• Add O2 to reduce carbon concentration => CO2
• ”Fluorine rich” or ”Carbon rich” plasma
Chlorine-based plasmas• Use Cl2, CCl4, SiCl4, BCl3• Undoped & low doped Si etched anisotropically (ion bombardment
needed)
• N-type Si and metals etched by Cl without ion bombardment =>
Sidewall passivation needed for anisotropy =>
Use suitable mix of polymer forming gas (CCl4, SiCl4, BCl3) and Cl2• Cl does not etch SiO2 => very good selectivity
• Cl-residues can form HCl when exposed to air (H2O in air) =>
Post-etch treatment might be needed to prevent corrosion
(”mouse bites” in aluminium lines)
SEM photo of sidewall passivation after etching in a HCl/O2/BCl3 plasma.The photoresist and the silicon post have been removed by wet etching.
Etching of III-V compounds• Group III Elements – Al, Ga, In
Group V Elements – P, As, Sb, N
• Binary (GaAs, InP), ternary (AlGaAs, InGaSb) and quaternary (AlGaInP,
InGaAs,P) alloys and compounds
• Etch Chemistries:
Chlorine based chemistries used extensively
Bromine based processes used occasionally
Iodine based chemistries used infrequently
Fluorine based chemistries not used – Group III fluorides involatile
Bosch process for
deep etching in Si
Plasma etcher Tool #228
Resistasher / stripper
O2 N2
Optical emission EPD
Reactive Ion Etcher Tool #419
ResistSiO2Si3N4
O2 H2 Ar CF4
Etching e.g.
The Applied Opus AdvantEdge metal etch system featuresa process-optimized 5-chamber configuration for etching advanced Flash and DRAM devices.
Inductively Coupled Plasma Etcher #404
III-V, MetalsAr O2 H2
Cl2 SiCl4 CH4
Ch1 Etching e.g.
Resist SiO2 Si3N4
Ar O2 H2 N2NF3 CF4 CHF3
Ch2 Etching e.g.
Laser EPD
Load lock
Ion Beam Etcher Tool #412
Metal oxides
Ar
Etching e.g.
SIMS EPD
Cryo etch
Load lock
Inductively Coupled Plasma Etcher #307
O2 SF6 C4F8
Etching Si using Bosch process
Optical emission EPD
Load lock
500 eV C+ implanted in Si