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Combinatorial RF Magnetron Sputtering Combinatorial RF Magnetron Sputtering for Rapid Materials Discovery: for Rapid Materials Discovery: Methodology and ApplicationsMethodology and Applications

Philip D. RackPhilip D. Rack, Jason D. , Jason D. FowlkesFowlkes, and, and YuepengYuepeng DengDengDepartment of Materials Science and Engineering Department of Materials Science and Engineering

University of TennesseeUniversity of Tennesseeprackprack@@utkutk..eduedu

Definitions

Plasma - partially ionized gas containing an equal number of positive and negative charges, as well as some other number of none ionized gas particlesGlow discharge - globally neutral, but contains regions of net positive and negative chargeMost thin film processes utilize glow discharges, but “plasmas” and “glow discharges” are often used interchangeably

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Plasma Properties

Plasma Density (n) – number of species/cm3107 – 1020

Typical glow discharges and arcs have an electron and ion density ~ 108 – 1014

DC Glow Discharge

Before application of the potential, gas molecules are electrically neutral and the gas at room temperature will contain very few if any charged particles. Occasionally however, a free electron may be released from a molecule by the interaction of, for example, a cosmic ray or other natural radiation, a photon, or a random high energy collision with another particle.

0V

A → A+ + e-

hv

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DC Glow DischargeWhen a large voltage is applied between the electrodes, say 100 V/cm, any free electrons which may be present are rapidly accelerated toward the anode. They quickly attain high velocity (kinetic energy) because THEY HAVE SUCH LOW MASS. Since kinetic energy can be related to temperature, the electrons are “hot” - they achieve extremely high temperatures because of their low mass, in an environment of heavy, slow-moving “cold” gas molecules.

100V

A+

e-

- +

slow“cold”

“hot”fast

Cathode Anode

DC Glow DischargeElectrons begin to collide with gas molecules, and the collisions can be either elastic or inelastic.

Elastic collisions deplete very little of the electron’s energy and do not significantly influence the molecules because of the great mass difference between electrons and molecules: Mass of electron = 9.11 e-31 kg, Mass of Argon = 6.64e20 kg.Inelastic collisions excite the molecules of gas or ionize them by completely removing an electron. (The excitation - relaxation processes are responsible for the glow)

100V

e- (100eV) + A

- +

e- (100eV) + A

e- (100eV) + A

e- (<100eV) + A*

elastic

inelastic

Cathode Anode

e- (100eV) + A e- (<100eV) + A+ + e-inelastic

A* A + photon (glow)

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DC Glow Discharge

Other electron/particleinelastic events

e- (100eV) + AB e- (<100eV) + A + B + e-inelastic

e- (100eV) + AB e- (<100eV) + A+ + B + 2e-inelastic

e- (100eV) + AB e- (<100eV) + A+ + B- + e-inelastic

Dissociation/Fragmentation

DissociativeIonization

Dissociative Ionization with Attachment

DC Glow Discharge

Newly produced electrons are accelerated toward the anode and the process cascades (Breakdown).

100V- +

A+e-e-

e-A+e

-

e-

A+e-

e-

A+e-

e-

A+e-

e-

A+e-

e-

A+e-

e-

Cathode Anode

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DC Glow Discharge

With sufficient voltage, the gas rapidly becomes filled with positive and negative particles throughout its volume, i.e. it becomes ionized.

100V- +

Cathode Anode

DC Glow Discharge

Positive ions are accelerated toward the negative electrode (cathode). Collision with the cathode causes the emission of secondary electronswhich are emitted from the cathode into the plasma.

100V- +

A+e-

Cathode Anode

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Secondary Electron Coefficient

Secondary Electron Coefficient (δ) vs Incident Electron Energy

Secondary Electron Coefficient (γi)vs Incident Ion Energy

DC Glow Discharge

Free electrons from secondary emission and from ionization are accelerated in the field to continue the above processes, and a steady state self-sustaining discharge is obtained.

A+e- e-

e-A+e-

e-

Cathode

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Sputtering

( ) ( )cosI Iθ θ=

Target

0

45

90

Target

Dark space

Positive Glow

V010

V

Self

bias

Ar+ Ar+

SubstrateAngular Distribution

Rf Magnetron Sputtering System

Load-lockchamber

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Source 1

substrate

Source 2

Source 3

Simultaneous 3 Source Sputtering

Angular Distribution

( ) ( )cosnI Iθ θ=

Target

0

45

n~10

Modified Cosine Distribution

05

1015202530354045

0 1 2 3 4 5 6 7 8Position (cm)

Film

Thi

ckne

ss (n

m) Ni Uniform Film

Cu Uniform

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

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Process Modeling

{ } ( )360 32.7

0 01[ ] 360 90

3 30 0

21 1 1 1

cos sin1 cos cos

sin

2

nP

j k j ki i

total ij i j ij

i j

d d PtSC nVqd d

n n rr

θ θ φ θφ θ

θ φ θθ

π

= == =

= = = =

+

= = ∆ ∆

∫ ∫∫ ∫∑ ∑ ∑ ∑

Ф

θsubstrate

r

Input variables:Source power, voltage, current

Material sputter yieldSource tilt angle

Substrate positionSource time

Determines substrate composition as a function of position

MATLab Process ModelBinary Composition Profile

Al

ZrCu

Ternary Composition Profile

AlZr

Cu

Thickness Modeling

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Recent Applications

Ternary Phase Diagrams: Fe-Ni-CrWith Pharr et al.

Catalysts for Carbon Nanofiber Growth: Cu-Ni

With Simpson et al.Bulk Metallic Glass Alloy Development

With Liaw et al.Ultraviolet Emitting Materials: Y3Al5O12:Gd

Fe-Ni-Cr Ternary Phase Diagrams

Co-sputter Fe (160W), Cr (60W), Ni (60W) onto (1 –1 0 2) single crystal sapphire substratesAnneal 200, 400, 600, 800 oC for 2 hoursRapid synchrotron fluorescence and XRD measurementsFuture Work: Nano-indentation

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Non-Equilibrium Ternary Phase Diagrams (Fe-Ni-Cr)

Fe

NiCr

synchrotron beam12 keV

0.25 x 1 mm2

CCD (diffraction)

detector

Fluorescence detector

With Pharr et al.

Modeled versus Measured Composition

Cr

Fe Ni

Cr

Fe Ni

Measured Composition Space Modeled Composition Space

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Phase Diagram Temperature Evolution

As deposited 200oC 400oC

600oC 800oC Equilibrium

Phase Analysis versus Temperature

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Particle Size

0

1020

3040

50

0 200 400 600 800Annealing Temperature (oC)

XRD

Par

ticle

Siz

e (n

m)

a-Mn bcc sigma fcc

Carbon Nanofiber CatalystCu-Ni Alloy

0102030405060708090

-5 -4 -3 -2 -1 0 1 2 3 4 5Distance from Center (cm)

Ato

mic

Per

cent

(Cu

& N

i)

ModeledResults

MeasuredResults

Ni Cu

Alloy Strip Deposited on Si

With Klien et al.

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Carbon Nanofiber CatalystCu-Ni Alloy

10 at% 10 at% CuCu

20 at% 20 at% CuCu

37 at% 37 at% CuCu

80 at% 80 at% CuCu

90 at% 90 at% CuCu

The morphology and shape of vertically–aligned carbon nanofibers are a strong function of the composition of the Cu–Ni catalyst particle that acts as the nucleation site for individual fiber growth. An optimum fiber geometry is realized at 80% Cu.

250nm

With Klien et al.

Glass substrate

ITO (1500A)

Si (1500A)PVX

Lab 5: Al-Cu Phase Diagram

MSE 300

0 10 20 30 400

10

20

0

5

10

15

20Magnetron Sputtering Configuration

x [cm]y [cm]

z [c

m]

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Al-Cu Phase Diagram

Atomic % On Substrate

0 2 4 6 8 10cm

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Test Setup

Base pressure = 6x10-7 TorrOperating pressure = 5 mTorrQ (Ar) = 25 sccmPower Al = 200WPower Cu = 34WS (Al) = 1.2S (Cu) = 2.3Gun Tilt (Al,Cu) = 7,7 mmBias = 0 VDCSustrate Height = 7.5 cmTemp = 30 ºCTime = 1.5 hr