Technology Excellence for Specialty Markets(and Etching Basics)

CorporateIntroduction

Semiconductor Equipment Manufacturing

USA manufacturingFocus on high growth specialty marketsLab-to-Fab solutionsMultiple production facilitiesISO-9000/9001

Plasma-Therm, Florida, USA Rev-Tech, Florida, USAAdvanced Vacuum Lomma, Sweden

Relocated toSt. Petersburg, FL

Plasma-Therm long history of excellence

1975 1990 20001974

Origin:RF power supplies

Acquired by

2009

Industry first single wafer production plasma etch reactor

First ever Photomask plasma etch system introduction

1983 1994

Incorporated

Privately owned and operated company

with headquarters in USA

Critical machining capabilities

2010 2011 2013

Industry first Plasma dicing

System in production

Product Line extension

3

2014

Etch and Deposition Solutions Leveraged Across Markets and Materials

Photonics FailureAnalysis

R&DPowerMEMSNEMS

SolarSolidState

Lighting

Wireless DataStorage

Adv.Photomask

&Imprint

Etch Solutions: ICP, RIE, PE, PHF-RIE, DRIE

Deposition Solutions: PECVD, ICP CVD

Plasma Dicing Solutions

Configuration FlexibilityFrom Lab to Fab

Single wafer Cassette-to-cassette

Open Fab IntegrationClusterLoadlock

Product Innovation

Singulator™VERSALINE®

Mask Etcher®LAPECVD™

Etchand

Deposition

Advanced Vacuum product lineupOpen load and single wafer loadlock

Vision 300 Vision 400 Apex SLR

Americas (St. Petersburg, FL)Sales and Service distributed throughout the US.1 Spares warehouse location

Supporting Over 640 Customers Worldwide

Asia-PacificJapan, China, Taiwan, Australia, Indonesia, Korea, Singapore, India, Malaysia4 Spares warehouses

EMEA France, Germany, Ireland, United Kingdom, Sweden, Israel, South Africa1 Spares Warehouse

13

Customer Recognized Service and Support

Global service organization operating 24/7Customer recognition of leadershipVLSIresearch 10 BEST/THE BEST Awards for 17 years2015 RANKED 1st in Etch & Clean Equipment Suppliers

9

Etching Basics:PlasmaReactorsMechanisms

2015

Mask

Etch Process

Etching: creating faithful reproduction of a pattern

Plasma Etching:Fundamentals, Mechanisms, Applications

Wet vs. DryWhat is plasma?Hardware/reactors overviewBasic Low-Pressure Processes

PhysicalChemical etchingIon driven etching, Ion enhanced etchingIon enhanced + Inhibitor (sidewall passivation)

Trends and TradeoffsPowerTemperaturePressure

Etching

Wet Dry

Ions Neutrals/Radicals

11

Why all the Focus on Dry Etching?What’s Wrong with Wet Etching?

Disadvantages

Limited CD geometry and control

Isotropic and/or crystallographic

Requires rinse and dry steps

Hazardous or toxic chemicals

Resist/wafer interface failure (undercutting)

Potential contamination sources

BenefitsPurely chemical

No physical damage

Can be highly selective

Wide choice of chemicals

Can etch many materials

Low capital cost

vs.

3

Plasma Basics: Why Pursue Dry Etching?

Many materials can be wet etchedAnd plasma……

requires a lot of technology to make it workinvolves many environmental health and safety aspectsis expensive

But plasma can do things wet can’tLess reactants requiredHigher purity reactantsLower contamination riskCompatible with other dry processes

Anisotropic Etching (Isotropic possible)Pattern smaller geometriesImproved CD control

Directionality: Pattern fidelity13

Plasma Basics:What is a Plasma – Tornado in a Trashcan?

Chemical “soup” , 4th state of matter

Neutrals Ions Electrons

Overall charge quasi-neutral

Most molecules/atoms not ionized

Non-equilibrium, constant creation/loss of species

Plasma electrons - small mass move fast ~9 x 10-28g

Plasma ions - big mass move slow

Plasma neutrals - big mass move slowDynamic!

14

Plasma Basics: How to make a plasma?

Add enough energy to a gas to cause ionizationElectricalThermalOther – e.g. electro-magnetic energy (radiation)

Low PressureNeon sign

Very low pressureInter-stellar space

High PressureLightningElectric welding arc

15

Plasma Basics: What is an Ion?

Ne

Ne+ + e-

+

10 protons 10 electrons (2 + 8)Charge balanced

10 protons 9 electrons (2 + 7)Extra positive charge

IonAtom

EjectedElectron

Simplified view

e-

Electron

Impact

Plasma Basics: Formation of Radicals by Electron Impact (e.g. Cl2)

Cl2 Cl + Cl

Electron

Impact

Charge balancedHappy to share an electron

Full outer shell

Neutral but “unhappy”Hungry to have just one more electron

to complete the outer shell

+

Cl RadicalsCl2 Molecule

e-

Simplified view

ne= electrons concentration

no= neutral concentration

ni+ = positive ion concentration

f(E) = distribution of energy

= residence time

excitation power

excitation frequency

gas flow rate

nature of gas pumping speed

chamber design

Plasma Basics: What makes up plasma?Focus on RF Generated Plasma

7

Plasma Basics: Simple Plasma Processes

Ion productionX + e- → X+ + e- + e-

DissociationAB + e- → A + B +e- (formation of neutrals and radicals)

Ion loss mechanismsX+ + e- → X (unlikely)X + e-→ X- (electron attachment, e.g. SF6)

X+ + wall (e-) → X (a major ion loss mechanism)

Light emissionX + e- → X* + e- (excitation)X* → X + h (photon emission)

Emission at specific energy (wavelength)

19

Many more process than those shown

Used for endpoint

Throttle Valve(controls pressure)

Basic Vacuum Processing System

Pressure (Torr, mbar) Volume (l, cm3, ft3)

Pumping Speed

PlasmaProcessModule

TurboMechanical

Pump

PressureGauge Feedback

Input

Exhaust

MFC = mass flow controller

We’ll talk about plasma a bit later…

MFCMFC

Gas Flow

Loadlock

20

Throttle Valve(controls pressure)

Basic Vacuum Processing System

PlasmaProcessModule

PressureGauges

Feedback

Exhaust

MFC = mass flow controller

We’ll talk about plasma in just a minute…

MFCMFC

Gas Flow

TurboMechanical

PumpLoadlock

Slot valve

21

PressureGauge

Plasma Basics: Equipment Configuration

NeedLow pressure

Gas

Electrical energy

Generator - Power source

Matching Network (to be discussed later)

Minimizes reflected power

Increases power dissipation in the discharge

Protects the generator

Acts as blocking capacitor

22

Blocking Capacitor

Plasma

RFGenerator

MatchingNetwork

Plasma Reactors:Low Density Plasma Sources

23

RIE is a confusing name –uses plasma (reactive species, ions)

RIE technology includes other configurations

RIE is a specific configuration (|| plate)RIE = CCP

ICP = RIE ICP

PE = Plasma Etching

MatchingNetwork

RF

Pump

Gas Inlet

CCP = RIE = Reactive Ion Etching

RFMatchingNetwork Pump

Gas Inlet

PoweredElectrode

Only one RF supplyNo independent control

of ions and radicals

Summary Etching is a Balance(ICP is a Good Compromise)

Neutrals - Chemical

Chemical controlled with ICP power

Ions - Physical

Physical controlled with substrate bias power

Inhibitors complicate the balance24

Plasma Reactors:High Density Plasma Sources

25

B

Magnetically Enhanced

MERIE ECR

ICP

TCP

Two RF suppliesMore control of ions and radicals

Plasma Etching Processes

Many simultaneous processesModeling gas dynamics, electromagnetics, reaction kinetics, surface interactions, etc.

Excellent references to the study of plasma etching

Plasma Etching: Fundamentals and Applications, M. SugawaraHandbook of Plasma Processing Technology, edited by S. Rossnagel, J. Cuomo, and W. WestwoodPlasma Etching: An Introduction, D. Manos and D. Flammhttp://www.chm.bris.ac.uk/~paulmay/misc/msc/msc1.htmD. Flamm, Pure & Appl. Chem., Vol. 62, No. 9, pp 1709-1720, 1990

26

Source: Oehrlein and Rembetski

Sputter Etching (“physical” etching)

High energy ions impact substrate surface and eject material -billiard ball effect

Yield/Etch Rate typically decreases with increasing process pressure

Etch Rate typically highest at low pressure (mTorr)

Ion

Sputtering

27

Sputter Etching

Characterized by non-volatile byproductsFluorine based chemistry (e.g. LiF, CaF: tungsten, titanium,, silicon)Chlorine based chemistry (CuCl2)No easily made byproducts (e.g. gold, platinum)

High bond energies (e.g. GaN, Al2O3), characterized by low selectivity

28

- Sputter etch

- Unknown- Etchable at RT

- May be able to etch

- Gaseous

H

Li Be

Na Mg

K Ca

Rb Sr

Lu

Y

BaCs

Sc

Ta

Cd

Ir

Rh

Co

OsW

TcMo Ru

Fe

Hf

Nb

Re

Mn

Zr

CrVTi Zn Ge

Si

FN Ne

ClS

Au

Al

C

Pb

O

Se

Ar

Rn

Te

Pt Hg

Ga

B

Sn

P

KrBr

Pd

Ni

Ag

Cu

Tl

In

Bi

As

Sb

He

At

XeI

Po*

1/IA

2/IIA

3/IIIB 4/IVB 5/VB 6/VIB 7/VIIB 9 10

VIIIB

13/IIIA

12/IIB11/IB

18/VIIIA

17/VIIA16/VIA15/VA14/IVA

8

Sputtering Process

Ions create a cascade of collisions on the surface.

Multiple collisions eject (or 'sputter') atoms from the surface.

Sputter yield depends on :Target material (how tightly bound and density)

Incoming ion mass (typically Ar, He too light, Xetoo expensive, hard on turbos)

Energy of the incoming ion

29

Material SputterYield*

Silver 3.4Gold 2.8

Copper 2.3Platinum 1.6

Nickel 1.5Iron 1.3Al 1.2Si 0.5

Al2O3 0.05

*600 eV Ar ions Source: Angstrom Sciences

www.angstromsciences.com/ref.sputtering-yields

Note: Si has very low sputter yield, so does graphite

Sputter Etching – Purely physicalCommon Reactor Configuration

“RIE” mode (common configuration)

Sputtering mode with inert gas (usually Ar)

Low etch rates but can remove non-volatile materials

Profile may be directional (but rarely vertical)

Ion energy, flux and radical conc. not independent

Low selectivity- most materials sputter equally

Mask choice is important but not always a problem. May produce desired result

“Dirty”– Process products not volatile and accumulate on chamber and feature surfaces

Typically 13.56 MHz

Inert Gas (e.g. Ar)

SubstrateRF

MatchingNetwork

CapacitivePlasma

30

Angular Dependence of Sputter Yield

Results in enhanced etch rate at angle of max. value (θmax)

Etch profile will develop at this angle

“faceting” of mask

Angle (degrees)0 90

Max. sputter yield angle

31

θmax

Spu

tter R

ate

θmax

Substrate

Sputter Etching and Development of Sloped Etch Profile

As mask material is sputtered away it preferentially is lost at the corner (some part of the corner equals θmax)

32

Example: resist after etching

Not perfectly 90° at corner

CD loss

Angle approaches angleof max. sputter rate

θ

Resist erosion

Sputter Etching: Mask Profile Can Affect Feature Profile

Lateral erosion of resist mask can cause feature size shrinkage or roughness

33

GaAs

Photoresist

Another description is that forward sputtering will always round a corner

Silicon

Photoresist

SiC

Oxide

Angled ProfileSometimes it’s not a problem, it’s a solution!

Starting resist profile

Desired result (patterned sapphire for LED manufacturing

Beginning of mask faceting

Mask slope transferring to feature

34

Sloped Masks Make Vertical Etching Difficult

Selectivity = ERmask/ ERfilm = tan(θfilm)/tan(θmask)Use to predict slope.

35

Selectivity Mask θmask Best Profile1 75 75.01 80 80.01 85 85.02 75 82.42 80 85.02 85 87.53 75 84.93 80 86.73 85 88.3

10 75 88.5

Sputter Etching: Can create “grass” or sidewall redisposition

Micro-maskingOften redeposited mask materialDepends on mask material, bias, ion massTends to be less farther from masking material

Not the only source of micromasking (e.g. excessive passivation, to be discussed later)

36

37

Sidewall redepositionGold Etching – Process Example

Gold

PRRe-sputter

“Dog ears”After mask removal

After “etch”

Sputter Etching: Can be a source of “trenching”

Deflected ionsIncreased etch rate at mask edgeDepends on mostly on bias and ion mass

38

Bias Power200 W 250 W 300 W

Chemical Etching

Dominated by neutral reactive species (atoms, radicals, molecules)

Radicals react with film surfaceReaction is spontaneous – ion assistance not required

Purely isotropic (nondirectional) and low damage

Reaction by-products are volatile

Volatile ProductNeutral

Chemical Etching

39

Etch Product Volatility Guidance

40Source: C. Cardinaud et.al., Applied Surface Science 164, pp72-83 (2000)

Chemical Etching: Common Reactor Configurations

CCP = RIE= Reactive Ion EtchingLower electrode poweredRun at low power to avoid

unwanted sputtering

PE = Plasma EtchingUpper electrode powered

RF + MatchPump

Gas Inlet

Pump

Gas Inlet

RF + Match

ICP = Inductively Coupled Plasma Etching

Substrate bias and highdensity plasma

41

Gas Inlet

RF + Match

RF + Match

Low density plasmas

High density plasmas

Chemical EtchingReactor Configuration

PE configurationLittle ion bombardment – almost “purely” chemical etchLeast flexible configuration

RIE = CCP configurationHigh pressure operation to minimize ion reactions Low plasma/radical density (low etch rates)RF power/ion bombardment not independent

ICP configurationHigh plasma/radical density (high etch rates)Can operate with no bias – almost purely chemical etchPlasma density/ion bombardment are independent

42

Low density plasmas

High density plasmas

Chemical Etching: Examples

Spontaneous etch reactions (examples)Si with SF6 (F radical)SiNx with SF6 (F radical)GaAs with Cl2 (Cl radical) (Interestingly NOT etching Si with Cl2)Al with Cl2 (Cl radical)HC polymers (e.g. PR) with O2 (O radical)

Choice of chemistry can provide high selectivitySiNx/SiO2 using SF6

GaAs over InGaAs or AlGaAs using Cl2

43

Chemical Etching: Examples

Silicon pillar, SF6

GaAs, HCl(crystallographic)

Silicon trench, SF6

InP, HBr

GaAs, Cl2

44

GaAs, Cl2

Chemical Etching: Characteristics

45

High etch rates possibleHigh radical concentrations possible in plasma (particularly ICP)

Higher density at higher pressureRate limited by mass flow balance (what goes out, must come in)

Load dependent etch rateEtch rate lower as etched area increases

Consequence of mass flow balance

Frequently classic “bulls-eye” uniformity profileEdge of substrate etches faster than center

Same # radicals at substrate edge but less area

Chemical Etching: Edge Effect

Edge has less area for available reactant

flux

substrate

film

Etc

h ra

te

Center EdgeEdge

Gas Phase Reactants

46

Chemical Etching: Reduction of edge effect

Other solutions:High pump speeds, dilution

substrate

film

Gas Phase Reactants

Etc

h ra

te

Center EdgeEdge

substrate

film

Physical barrier limits speciesaccess to edge

47

uncorrected

corrected

Modest changes in gas phase chemistry can have strong impact on etching

In this case, adding ~15% O2increases etch rate ~4xSi + F → SiFx

Chemical Etching: Controlling Reaction Kinetics

Formation of F: CF4 + e- → CF3 + F + e-

(electron impact) CF3 + e- → CF2 + F + e-

Radical recombination: CF3 + F → CF4(loss of F) CF2 + F → CF3

Addition of O2: CF2 + O → COF + F(generation of F) COF + O → CO2 + F

Si R

elat

ive

Etch

Rat

eO2 %

10 20 30 40 500

0.2

0.4

0.6

0.8

1.0

060

Relative F Intensity0.2

0.4

0.6

0.8

1.0

0

703.7 nm line intensity(F etching Si)

Silicon etch rate

Addition of O2 to CF4

Source: C.J. Mogab, A.C. Adams, D.L. Flamm, J. Appl. Phys. 49, 3796 (1978)

48

Chemistry Choice - Volatility of By-product(s)

49

Source: T.P. Chow, A.J. Steckl, Electron Devices, IEEE Transactions, 11, 1983

Refractory metals and silicides

Chlorine based

Log

10P

(m

m)

Fluorine based

Fluorides ChloridesAlF3 1291 AlCl3 178 sub

Cu2F2 1100 Cu3Cl3 1490TiF4 284 TiCl4 136WF6 18 WCl6 347

Use vapor pressures when available or boiling points as a guide when they

are not

Chemistry Choice - Volatility of By-product(s)

Chemistry Primary Films Etched

Cl – based ( e.g. Cl2, BCl3, SiCl4) Al (+ alloys), Ti, TiN, W, Cr, Hf, W, Mo

F – based (e.g. SF6, CF4, CHF3) Ti, W, TiW, WN, Mo, Ta, silicides

50

Other factors play a role:• masking materials• additive gases, • film quality,• desired results,• etc.

Physical and chemical components: individually slow but together increase the etch rate and anisotropy

Combination of radicals and ions etch wafer

Ions striking the surface enhance reactions and product desorption

Ion Enhanced Etching

Ion Enhanced

ProductIon

Neutral

51

Ion Enhanced Etching:Common Reactor Configurations

RF + MatchPump

Gas Inlet

Gas Inlet

RF + Match

RF + Match

CCP = RIE configurationLow pressure operation to maximize ion formation Low plasma/radical density (low etch rates)Ion density/ion energy not independent

ICP configurationHigh ion densityPlasma density/ion bombardment are independent

ICP power → ion and radical densitiesBias power → Ion energy

52

Ion Enhanced EtchingExperiment shows role of ions/radicals

Illustrates the dramatic affect of combining radicals and ions

Source: J.W. Coburn, H.F. Winters, J. Appl. Phys. 50, 3189 (1979)

XeF2 gasonly

Ar+ ion

beam

XeF2gas+

Ar+ ionbeam only

> 10x etch rate increase

ModelsReactive spot (energy addition)

Tachi (1985)Damage & enhancement

Coburn & Winters (1978)

53

Radicals Radicals + Ions Only Ions

spontaneous

Ion enhanced

sputtered

Other early studies of ion effects

Al with Ar+ + F2Low volatility of AlF3 , Al alone sputters faster

Si etching with Ar+ CF4CF4 is not adsorbed on Si surface

Si etching with Ar+ Cl2SiCl4 volatile

Source: J.W. Coburn, H.F. Winters, J. Appl. Phys. 50, 3189 (1979)

Source: J.W. Coburn, H.F. Winters, J. Appl. Phys. 50, 3189 (1979)

Source: Pabst et al., Int. J. Mass Spect. Ion Phys. 20, 191 (1976)

54

Ion Enhanced EtchingIons contribute to anisotropy

Example: XeF2 etching of Si

Anisotropy 1

Higher anisotropy with higher energy ions

Penalty for anisiotropy by increasing ion flux or energy –but typically lower selectivity, trenching, damage

55

Source: P. Martin, Vacuum Technology & Coating, , 28, (Nov. 2009)(Not the original reference)

XeF2 Flow rate

Ion Enhanced Etching Examples

GaN with Cl2: ions assist desorption of GaClx etch products

SiC with SF6: ions assist formation of SiFx and CFx etch products

SiO2 with CF4: ions assist formation of Si and C etch products

Al2O3 with BCl3: ions assist formation of AlClx etch products

56

Ion Enhanced Etching

Ion bombardment is necessary to break strong bondsSi – O 8.2 eVAl – O 9.3 eV

By products must still be volatile (e.g. SiF4, AlCl3)

Vertical ion bombardment →vertical etchMask erosion may result in sloped profile (similar to sputter etch)

Choice of radicals can provide selectivitye.g. etching SiO2 with CF4 (CF2 and CF3 radicals)Selectivity less than Chemical etch and greater than Sputter etch

Loading effects < chemical etchRate rarely mass flow limited

57

Ion Enhanced Etching + Inhibitor(Sidewall passivation)

Inhibitor (often polymer forming) can passivate surfacesMinimal ion bombardment on sidewalls to remove inhibitorSurface protected from etching

Passivation removed from horizontal surfaces by ion bombardmentSurface open for etching (chemical or ion assisted)

ProductIonReactant

Inhibitor

Inhibitor

58

undercut

Most highly anisotropic processes fall in this category

Inhibitor introduced intentionallySi etch using SF6 plus C4F8 Al etch using BCl3/Cl2 plus CH4InP etch using HBr or Cl2 plus N2

Inhibitor introduced “unintentionally”Etching using photo resist mask

Resist etch by-product is polymer precursorEtch result depends on resist loadingAnisotropic etch with resist mask, isotropic with “hard” mask

e.g. GaAs via etch with resist mask

Ion Enhanced Etching + Inhibitor

Source: after M. Mieth and A. Barker, Semiconductor International, 5, 222 (May, 1984)

59

Materials which act as inhibitors

Photoresist etch by-product (particularly from Cl etch)

Hydrocarbons e.g. CH4

Chlorinated hydrocarbons e.g. CHCl3 (most banned)

Freons

CHF3 ~= C4F8 > C2F6 > CF4

F:C ratio and oxidation capability of surface determines effectiveness as polymer precursor (e.g. O from SiO2 can provide oxidation source)

N2 for some processes, e.g. Al, InP

O2 can oxidize sidewall e.g. silicon (e.g. cryo black silicon process)

60

Ion Enhanced Etching + Inhibitor

Isotropic etch induced notch in feature profile

HBr/CH4/BCl3Preferentially etches InGaAs layer

HBr/N2Inhibition of InGaAs etching

Example: Multilayer InP/InGaAs/InP

61

Requires understanding of how inhibitors participate(e.g. CH4 sometimes an inhibitor, sometimes an etchant

Ion Enhanced Etching + Inhibitor

Anisotropic GaAs etch (via etch) using Ar/Cl2Resist by-products provide sidewall passivation

Polymer provides sidewall protection

62

GaAs Profile Control- Isotropic to Anisotropic

Selectivity increases with Cl2 partial pressure: etch rate and isotropy increases

Resist implicated in controlling profile

63

Cl2 partial pressure (mTorr)

GaA

s Et

ch R

ate

(um

/min

)

Etch Selectivity (GaA

s:PR)

40 um vias

Tungsten Etch Example

Beforeetch

SF6 / CF4 / Ar

F for reaction, CF4 for passivationVertical profile

Sidewall roughness transferred from mask roughness

PR

Al

W

Al

W

64

Afteretch

Process temperature 20CSelectivity to PR ~5:1

~2000 A/min

Tungsten Etching with Fluorine Chemistry

Source: Tang and Hess, J. Electrochem. Soc. 131, 115 (1984)Mark Salimian, Plasma Etching, Technologies and Processes, 1993

CF4 / O2 vs. SF6 / O2: Etch rate and Relative F Atom Density• F is primary etchant (similar activation energies) for SF6 and CF4

• O2 increase rate for CF4 /O2 with more available F. O and F compete for W sites• O2 decrease rate for SF6/O2 , interaction with SFx and W? (adding O2 doesn’t help)

Etc

h ra

te (n

m/m

in)

Rel

ativ

e F

Ato

m D

ensi

ty

(arb

. Uni

ts)

Rel

ativ

e F

Ato

m D

ensi

ty

(arb

. Uni

ts)

Etc

h ra

te (n

m/m

in)

CF4 / O2

SF6 / O2

65

Mask material is often important

RIE with SF6

PRSiO2W

PRSiNxW

SiO2W

SiNxW

Data suggests O from SiO2 attacks sulfur (S) protection layer on W

More efficient removal of inhibitor than N from SiNx

Source: Sato et al, 7th Plasma Process Symp. Proceedings Electrochem. Soc. 1988, p240Source: Mark Salimian, Plasma Etching, Technologies and Processes, 1990

66

Often involved in achieving anisotropic profilesBalance of etchant and inhibitor

Profile, uniformity sensitive to ratio Too little inhibitor, profile → isotropicToo much inhibitor, etch rate decreases, polymer formation, micro-masking

Balance can change with Aspect ratio (mass transfer issues)Temperature sensitive

Polymer deposition (affects profile) changes with temperaturePotential for deposition on chamber surfaces

Reduced at elevated temperatures

Ion Enhanced Etching + InhibitorCharacteristics

67

Excess inhibitor deposition causing grass

Al Etch – General Considerations

Typical Al process steps:Breakthrough (remove native oxide) → Main etch → Post etch treatment

Material composition issuesAl-Si: Si residues may require post F-plasma treatment (e.g. CF4) to remove unetched Si (this may impact underlayer)Al-Cu: Cu difficult to etch, susceptible to “grassing” from precipitates

Requires higher temperature and more ion bombardment Cu3Cl3 a temperature of about 200C

Al deposition quality: no O2 present Reduce Al2O3 formation during deposition: no leaks, loadlock, high purity Ar

Al2O3 formation from poor vacuum conditions (e.g. leaks, outgassing)Mask profile (sloped, rough, or poor adhesion)Corrosion (post etch)

68

Al EtchingAnother example of sidewall passivation

Photoresist erosion assists sidewall passivationResist condition (e.g. slope, bake temperature)

Surface AlCl3 reacts with moisture exposure to make HClwhich corrodes Al

Cl residues in resist & sidewall deposits also react to form HCl

TreatmentsIn-situ PR strip (O2 ash) (best)

F replacement of Cl using CF4 (O2) (possibly SF6)

CHF3 will deposit polymer protecting the wafer from moisture attack.

Immediate rinsing of wafer and removal of resist after Al etch

High temperature bake (usually not available due to PR)

Substrate

Al

Photo-resist

Sidewallfilm

Cl

AlC

O

69

Al Etching – BreakthroughRemoval of Native oxide

Surface reacts to ambient atmosphere to make 20-50Å Al2O3

Cl2 alone won’t work. Al2O3 + Cl2 → AlCl3 + O2 (makes more Al2O3)BCl3 to needed to reduce Al2O3

Al2O3 + BCl3 → AlCl3 + B2O3 (BOCl)BCl3 acts as an O2 scavengerCClx could be used but carcinogenic (Al2O3 + CClx (x ≤ 4) → AlCl3 + CO)

Even with reducing chemistry and energetic ion bombardment, the etch rate of Al2O3 is >300x less than Al

70

Al

A2O3

Cl Cl Cl Cl Cl Cl

Cl2Al + O2

Al

Al2O3

Cl Cl Cl

Al

Surface oxide Surface reoxidized

Al Etching – Main Etch

Al (unlike Al2O3) can be etched in Cl2 (or with BrCl2 reacts spontaneously at RT (without plasma or ion bombardment)

AlCl3 sufficiently volatile at 20C to desorbDesire high pumping speed (low residence ) reduces AlCl3 partial pressureHeated chamber walls (reduce re-deposition)Load lock (safety and low oxygen presence)

Highly load dependent (chemical component) may require diffusion barriers to improve uniformity)Al not etched in F-containing gases. AlF3 only volatile at elevated temperatures (i.e. chamber should be free of F)

71

Al

Cl Cl ClCl2

Cl Cl Cl

Al

Al

AlCl3 or (AlCl3)2

AlAl

Plasma EtchFeature Profile Defects

72

BowingMask Facet

TrenchingIon Reflection

UndercutUnder Passivation

TaperOver passivation

NotchingCharging

Re-entranceIon Scatter

Source: adapted from Donnelly, V. M. and Kornbilt, A., J. Vac. Sci. Technol. A 31, 050825 (2013).

Tilt-SkewIon Trajectory

Physical effects (ions)Mask facetingIon scatteringOff-angle ions

Chemical effectsOver passivationUnder passivation

Neutral beam solutions?

72

Damage EffectsProcess and device dependentMultiple types of damage can result from processes

Ion damage to lattice (e.g. ion penetration, vacancies, interstitials)

Carrier passivation (carrier concentrations)

Surface contamination (surface states) (e.g. fluorocarbon layers)

Recombination centers

Surface morphology

Insulator breakdown/leakage

Depth can be surprisingly deepSurface to many 100s Å

73

Many sources

Damage examples

H ion penetrates ~5x deeper

UV/VUV penetrates deeper with O2 plasma but fewer defects near surface

Source: Seiji Samukawa et al 2012 J. Phys. D: Appl. Phys. 45 253001

Source: Wong, Green, Liu, Lishan, Bellis, Hu, Petroff, Holtz, Merz, J. Vac. Sci. Technol. B 6 (6) 1906 (1988)

Buried Quantum Well Photoluminescence for Monitoring Etch Damage Depth

Damage ExampleInP Etching: RIE vs. ICP

CH4/H2 chemistry

75

RIE 2x P surface loss enhanced

RIE 2-3x thicker amorphous layer

RIE has lower etch rate

RIE P depleted layer ~2x ICP Source: Cardinaud, C, et al,., Appl. Surface Science, 164 (2000) 72-83

Summary - Etching Mechanisms

Volatile Product

Sputtering Ion Enhanced

ProductIon

Chemical

Neutral

Ion

Ion Enhanced + Inhibitor

Neutral

ProductIon

Neutral

Inhibitor

Inhibitor

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SummaryBasic Low Pressure Etching Mechanisms

Sputter EtchingIons strike surface and eject or sputter materialNonvolatile reaction products

Chemical EtchingNeutral dominated (atoms, radicals, molecules)Isotropic, non-directional, can be reagent limited

Ion Driven EtchingIons strike surface and enhance reactionsBoth physical and chemical componentsAllows “fast” anisotropic etching

Ion Enhanced + Inhibitor FormationPolymer forming components passivate sidewalls where ion bombardment is minimalPrevention of lateral etching

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Pressure EffectsStrong effects

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Ratio of sheath thickness and mean free path of ion

High pressure• Requires additional control

mechanism (e.g. passivation)• More neutral influence

Ion flux

Neutralflux

Ion flux

Neutralflux

Physical(sputtering)

1 10 100 1000

Ion

Ener

gy

Pressure (mTorr)

Ion Assisted(chemical and physical

Chemical(radicals)

Low pressure• Stronger ion influence• More directional (CD control)• Potential for more damage

Pressure EffectsStrong effects

Affects ion energy, density, bias, electron temperatureReduction in pressure increases electron temperature and this results in increase in DC bias (more electrons “leave” the plasma)

Chemical based processes – etch rate usually increases with pressure (longer residence time for reactants)

Ion enhanced processes – Etch rate might actually decrease with increasing pressure (lower ion energies)

Profile affected: mass transport (lag), sidewall passivation

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Power Effects

Increased ICP power increased concentration of radicalsIncreased electrode bias power increased DC bias

Increases the effect of ions (more energetic and more of them)Mask erosion increases selectivity decreases

Possible to actually have better selectivity if material of interest etch rate goes up faster than mask erosion

Wafer cooling becomes more important as either ICP or bias power increases

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Wafer gets hot with all the energy being delivered (and exothermic reactions)

Need to manage thermal budget!

Temperature Effects

Wafer temperature affects etch rate, selectivity, surface roughness and anisotropyChemical processes affected the most

Temperature , isotropy (and often roughness)Temperature , Etch rate Temperature , Selectivity (usually but not always)

Consider Si etching masked by SiO2 (example) RateF (SiO2) = 6.14 x 10-13 nFT1/2 e-0.163/kT Å/min RateF (Si) = 2.91 x 10-12 nFT1/2 e-0.108/kT Å/min Selectivity = RF(Si)/RF(SiO2) = 4.7 e +0.055/kT ( higher T, lower

selectivity)Changes in T can have large effect on selectivity (e.g. InP etching)

Photoresist etch rate increases with temperature

81Source: Flamm et al., VLSI Electronics, Vol. 8, Chap (1984)

General Process Trends

Response with Increase in Process Parameter

Process Parameter Etch Rate Selectivity(to mask)

Mechanism Trend Uniformity

Bias Power Increases Decreases More physical Weak effectElectrode Size

(power density) Decreases Increases Less physical Weak effect

Source Power Increases Increases More chemical Weak effect

Pressure Usually increases Weak effect More chemical Usually worse

Substrate Temperature Increases Depends More chemical Weak effect

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Important Caveat: material dependent

Advanced Plasma DevelopmentsInterest in removing ions & controlling neutral energies

ALE – Atomic Layer EtchingTemporalSpatial

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RF Bias

Source: adapted from Ankur Agarwal et. al., J. of Applied Physics, 106, 103305, (2009)

Neutral control –grids, ion filters

Pulsing ICP source

Pulsing bias

Multiple frequencies Pulsed gases

Temporal EtchingAtomic Layer Etching (ALE)

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Adsorb

Flush

Flush

Volatize

Time-Multiplexed EtchingAtomic Layer Etching (ALE)

85

Source: Donnelly, V. M. and Kornbilt, A., J. Vac. Sci. Technol. A 31, 050825 (2013).

Potential BenefitsMonolayer control of etchElimination of profile defectsElimination of ARDE

LimitationsExtremely low etch ratesNarrow process window to achieve monolayer control

StatusDemonstrated in Si Cl-based chemistries4 step cycle demonstrated

280 ALE cycles150sec cycle times

Spatially Multiplexed EtchingDRIE

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Source: Roozeboom, F. et al., ECS Trans. 50 (32), 73-82 (2013).

Spatial ALD ReactorSpatial DRIE Approach

Summary Usually there are Tradeoffs

Etching Rate

Selectivity

Surface Morphology

Profile

DamageUniformity

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Thank you