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