scaling of hollow cathode magnetrons for metal...

GEC99C01

SCALING OF HOLLOW CATHODE MAGNETRONS FORMETAL DEPOSITIONa)

Gabriel Fontb)

Novellus Systems, Inc.San Jose, CA, 95134 USA

and

Mark J. KushnerUniversity of Illinois

Dept. of Electrical and Computer EngineeringUrbana, IL, 61801, USA

October 1999

a) Work supported by Novellus and SRCb) Present Address: University of Texas at San Antonio

AGENDA

GEC99C15

University of Illinois Optical and Discharge Physics

• Introduction to Ionized Metal PVD and Hollow Cathode Magnetrons

• Description of Model

• Plasma properties of IMPVD of Cu using a HCM

• Comparison to Experiments

• Deposition and Target Erosion

• Concluding Remarks

COPPER INTERCONNECT WIRING

GECA99C12


• The levels of interconnect wiring in microelectronics will increase to 8-9over the next decade producing unacceptable signal propogation delays.

• Innovative remedies such as copper wiring are being implemented.

Ref: IBM Microelectronics

UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS

IONIZED METAL PHYSICAL VAPOR DEPOSITION (IMPVD)

CACEM98M03

• In IMPVD, a second plasma source is used to ionize a large fraction of the the sputtered metal atoms prior to reaching the substrate.

• Typical Conditions: • 10-30 mTorr Ar buffer • 100s V bias on target • 100s W - a few kW ICP • 10s V bias on substrate

TARGET(Cathode)

MAGNETS

ANODESHIELDS

PLASMA ION

WAFER

INDUCTIVELYCOUPLEDCOILS

SUBSTRATE

SECONDARYPLASMA

BIAS

e + M > M+ + 2e

NEUTRALTARGET ATOMS

+


IMPVD DEPOSITION PROFILES

CACEM98M04

• In IMPVD, a large fraction of the atoms arriving at the substrate are ionized.

• Applying a bias to the substrate narrows the angular distribution of the metal ions.

• The anisotropic deposition flux enables deep vias and trenches to be uniformly filled.

SiO2

METALATOMS

METALIONS

METAL

HOLLOW CATHODE MAGNETRONS

GEC99C02


•• Hollow Cathode Magnetrons (HCM) are typically cylindrical devices withinverted metal cups and floating shields. Production tools have 300 mmsubstates.

MAGNETS

IRON PLATE

IRON RING

CATHODE

SHIELD (FLOATING)

SUBSTRATE (FLOATING)

SHIELD (GROUND)

GROUND

GAS INLET

PUMPRADIUS (cm)

0 3 6 912 369 120

HE

IGH

T (c

m)

10

5

15

20

25

35

30

• Typical Operating Conditions:

• Pressure: 5-10 mTorr

• B-field: 100-300 G atcathode

• Voltage: 300-400 V

• Substrate: Floating or biased

• Buffer gas: Ar, 50-100 sccm

• Power: 2-5 kW

HOLLOW CATHODE MAGNETRONS-IMPVD

GEC99C03


•• HCM - IMPVD devices use magnetic confinement in the cathode for high iondensities and sputtering, and cusp B-fields to focus the plasma.

ION CONFINEMENT AND SPUTTERING

PLASMA FOCUSING

DIFFUSION (SOME GUIDING)

• B-Field Vectors • B-Field Magnitude

HYBRID PLASMA EQUIPMENT MODEL

GECA99C13


LONG MEAN FREE PATH (SPUTTER)

Φ(r,θ,z,φ) V(rf), V(dc)

PLASMA CHEMISTRY MONTE CARLO SIMULATION

ETCH PROFILE MODULE

FLUXES

E(r,θ,z,φ),

B(r,θ,z,φ)

ELECTRO- MAGNETICS

MODULE

CIRCUIT MODULE

I,V (coils) E

FLUID- KINETICS

SIMULATION

HYDRO- DYNAMICS MODULE

ENERGY EQUATIONS

SHEATH MODULE

ELECTRON BEAM

MODULE

ELECTRON MONTE CARLO

SIMULATION

ELECTRON ENERGY EQN./

BOLTZMANN MODULE

NON- COLLISIONAL

HEATING

SIMPLE CIRCUIT

MAGNETO- STATICS MODULE

= 2-D ONLY = 2-D and 3-D

EXTERNAL CIRCUIT MODULE

MESO- SCALE

MODULE

VPEM: SENSORS, CONTROLLER, ACTUATORS

SURFACE CHEMISTRY

MODULEFDTD

MICROWAVE MODULE

Bs(r,θ,z)

S(r,θ,z),

Te(r,θ,z)

µ(r,θ,z)

Es(r,θ,z,φ) N(r,θ,z)

Φ(r,θ,z)

R(r,θ,z)

Φ(r,θ,z)

R(r,θ,z)

J(r,θ,z,φ) I(coils), σ(r,θ,z)

Es(r,θ,z,φ)

v(r,θ,z)

S(r,θ,z)

PHYSICS MODELS USED IN HCM SIMULATIONS

GECA99C14


• Monte Carlo secondary electrons emitted from cathode

• Fluid bulk electrons (Te from energy equation with Boltzmann derivedtransport coefficients)

• Continuity, momentum and energy for all heavy species (multi-fluid, slipand temperature jump boundary conditions)

• Long-mean-free path transport for sputtered atoms with sputter-heating

• Implicit Poisson solution simultaneous with continuity and momentum forelectrons

• Species in Model:

e, Ar, Ar(4s), Ar+, Cu(2S), Cu(2D), Cu(2P), Cu+


DESCRIPTION OF SPUTTERING MODEL

PEUG99M03

• Energy of the emitted atoms (E) obeys the cascade distribution, an approximation to Thompson’s law for Ei ≈ 100’s eV:

F(E) =2 1+

Eb

ΛE i

2EbE

Eb + E( )3 , E ≤ ΛE i

0, E > ΛEi

where

Λ = 4m imT / m i + mT( )2

subscripts: b ~ binding, i ~ ion, T ~ target.

• The sampling of sputtered atom energy E from the cascade distribution gives

E =EbΛE i RN

Eb + ΛE i 1− RN( )

where RN is a random number in interval [0,1].

ion flux

fast neutral flux

θ

ArAl Al

Target

wafer

HCM- Cu IMPVD: ELECTRON DENSITY, TEMPERATURE

GECA99C04


• E-Density(6.6 x 1012 cm-3)

• E-Temperature(4.8 eV)

• Electron densities in excess of1012 cm-3 are generated inside andin the throat of the cathode.

• Fractional ionizations are ≤≤ 10%.

• Electron temperatures peak at 4-5eV in the cathode, and are a feweV downstream.

• Ar, 6 mTorr, 150 sccm, 325 V, 160 G

HCM- Cu IMPVD: ELECTRON SOURCES

GECA99C05


• E-beam(3 x 1017 cm-3s-1)

• Bulk(5 x 1018 cm-3s-1)

• Secondary electron emission andbeam ionization produce electronsources in the cathode.

• Ionization in the bulk plasma,however, is the major electronsource.

• Ar, 6 mTorr, 150 sccm, 325 V, 160 G

HCM- Cu DENSITIES

GECA99C06


• Cu(1.9 x 1013 cm-3)

• Cu+

(1.5 x 1012 cm-3)

• Neutral copper [Cu(2S) + Cu(2D)]undergoes rapid ionization andrarefaction in the throat of thecathode.

• The majority of neutral copper isin metastable Cu(2D).

• Fractional ionization of Cu in thebulk is 10's %.

• Ar, 6 mTorr, 150 sccm, 325 V, 160 G

HCM- GAS TEMPERATURE

GECA99C07


• Average Gas Temperature (max = 1530 K)

• Gas heating occurs dominantly by symmetriccharge exchange between Ar neutrals andions, and produces significant rarefaction.

• Slip between neutrals and disparate rates ofcharge exchange produce differences in theirmaximum temperatures:

Ar: 1546 K Cu: 900 K

• Ar, 6 mTorr, 150 sccm, 325 V, 160 G

ELECTRON DENSITY AND TEMPERATURE vs EXPERIMENT

GECA99C09


MODEL

EXPERIMENT

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 2 4 6 8 10 12

RADIUS (cm)

EXPERIMENT

MODEL

ELE

CT

RO

N T

EM

PE

RA

TU

RE

(eV

)

• Electron Density • Electron Temperature

• Langmuir probe measurements of electron density (2.8 cm abovesubstrate) show densities and temperatures more highly peaked on axis.

• The model underpredicts the "jetting" of plasma from the throat of thecathode which likely results from long-mean-free path effects not capturedby the ion model.

• Ar, 6 mTorr, 150 sccm, 325 V, 160 G

ELECTRON DENSITY, TEMPERATURE vs BIAS

GECA99C18


• HCM devices have significantly "less steep" I-V characteristics comparedto conventional magnetron discharges.

• For example, downstream plasma densities and power deposition scalenearly linearly with bias. Electron temperatures are weak functions ofbias.

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

320 340 360 380 400 420 440

CATHODE BIAS (v)

EXPERIMENT

MODEL

EL

EC

TR

ON

T

EM

PE

RA

TU

RE

(e

V)

0

5

10

15

20

25

320 340 360 380 400 420 440

CATHODE BIAS (v)

EXPERIMENT

MODEL

ELE

CT

RO

N D

EN

SIT

Y (

101

1 cm

-3)

HCM- ION AND CU FLUXES TO THE SUBSTRATE

GECA99C08


10

8

6

4

2

0

FLU

X (

101

7 c

m-2

s-1

)

Ar+

Cu (Direct)

Cu+Cu(2D)

Cu(2S)

• Ion flux to the substrate isdominated by Ar+

• Direct non-thermal Cudominates the Cu fluxes tothe substrate.

• The Cu flux is 25-30%ionized.

• Ar, 6 mTorr, 150 sccm, 325 V, 160 G

Cu DEPOSITION RATE

GECA99C10


MODEL

EXPERIMENT ( x 1.33)

• Cu Deposition Rate (A/min)

• The off-axis peak in directsputter flux produces an off-axis peak in the depositionrate.

• Ar, 6 mTorr, 150 sccm, 325 V, 160 G

TARGET EROSION

GECA99C11


-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0 5 10 15

EXPERIMENT MODEL

POSITION ALONG TARGET (cm)

EROSION

DEPOSITION

CORNER

0

L

HCM TARGETB-FIELD

• Magnetron trapping and the resulting large ion fluxes to the target occursat the side walls.

• Net deposition of sputtered Cu occurs on the end walls while the sidewalls experience net erosion.

• Ar, 6 mTorr, 150 sccm, 325 V, 160 G

CONCLUDING REMARKS

GEC99C16


• Hollow Cathode Magnetrons (HCM) are capable of producing metaldeposition with plasma densities of 1012 cm-3.

• The HCM operates somewhat as a remote plasma source with moderateelectron temperatures near the substrate compared to the cathode interior.

• The configuration of the magnetic field is important in at least tworespects:

• Jetting of plasma at throat of cathode• Erosion profile inside the cathode

• Small HCMs having higher power densities experience significantrarefaction which ultimately limits their capability to produce high plasmadensities and deposition rates.

• Large HCMs (10-20 cm diameter) which avoid these problems have beendesigned and built based on these scalings studies.

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