Paradee Leerungnawarat

Department of Materials Science and Engineering

Introduction

What is SiC?

Why SiC?• SiC has many advantages over Si.• It is widely used for high power, high temperature electronic devices.

Why High-Density Plasma?• It is very difficult to wet etch (practicality).• Conventional Reactive Ion Etching produces very low etch rates

3C-SiC 4H-SiC

6H-SiC

Silicon Carbide Crystal Structure

Silicon carbide properties

Properties 3C-SiC 6H-SiC 4H-SiC Si

Indirect Indirect Indirect IndirectBandgap (300 K, eV)

2.3 3.0 3.2 1.12

Electron Mobility (cm2/V-s)

@ room temperature, doping~1016 cm-3

750 370 800 1100

Hole Mobility (cm2/V-s)

@ room temperature, doping~1016 cm-3

40 90 115 420

Breakdown field (MV/cm)

@ doping~1016 cm-3

>1.5 3.2 3.0 0.6

Thermal Conductivity (W/cm-K)

@ room temperature, doping~1017 cm-3

3.2 4.9 3.7 1.5

Electron drift velocity (107 cm/s) 2.5 2.0 2.0 1.0

Dielectric constant, εε 9.7 9.7 9.7 11.8

SiC Power Electronics

• The much wider bandgap and breakdown voltage for SiC relative to Si makes it attractive for high temperature, high power device applications.

• To date, high power MOSFETs, rectifiers, thyristors and IGBTs have all been demonstrated in SiC, with far superior breakdown voltage and on-resistance compared to Si.

• To make many of these devices, it is necessary to have low damage, dry etch processes with high anisotropy.

Theoretical and Experimental Performance of SiCAnd Si Schottky Rectifiers.

One Application: Via-Hole Etching

VIA

2-4 mil THINNEDWAFER

EMITTER FINGERS GROUNDING PAD

FLIP-SIDEPLATED METAL

•Requirements : High SiC Etch rate: High selectivity for SiC over mask material

Reactor SiC Gas Condition at highest ER ER

(Å-min-1)

Ref

RIE 6H CHF3/O2 20 sccm, 200 W, 0% O2 32 24SF6/O2 410CF4/O2 278NF3/O2 483

3C SF6 150W, 80 mTorr 700 26

6H,4H NF3 225 mTorr, 95-110 sccm, 275 W 1500 23

ECR 6H CF4/O2 500W, -100V, 17.5%O2, 50 sccm 800 28

3C,6H CF4/O2 650W, -100 V, 17%O2, 50sccm 700 29

6H 20SF6/10Ar 750W, 250 rf, 2 mTorr, 30 sccm 4500 34

3C,6H SF6/O2 1200W, 1 mTorr, 4 sccm 2500 27

6H 10Cl2/5Ar 1000W, 150 rf, 1.5 mTorr, 15 sccm 2500 3010Cl2/5H2 1000W, 150 rf, 1.5 mTorr, 15 sccm 10004IBr/4Ar 1000W, 250 rf, 1.5 mTorr, 15 sccm 1100NF3 800W, 100 rf, 1mTorr, 10 sccm 1600SF6 800W, 100 rf, 1 mTorr, 10 sccm 450

ICP 6H Cl2/Ar or /He 100 Cl2, 750 W, 250rf, 5 mTorr 100 34Cl2/Xe 13%Cl2, 750 W, 250rf, 5 mTorr 260IBr/Ar 10%Ar, 750 W, 250 rf, 5 mTorr 800ICl/Ar 66%Ar, 750 W, 250 rf, 5 mTorr 250

6H NF3/O2 or Ar 100%NF3, 750W, 250 rf, 5mTorr 4000 35

Previously ReportedEtch rates of SiC

High-Density Plasma

• In this work we employ a form of high density plasma (Inductive Coupled Plasma) and several different F2-based gas chemistries.

• HDP provides high ion flux, low ion energy and efficient plasma dissociation : low damage, high rate etching.

• We examined the active plasma species by Optical Emission Spectroscopy (OES) in order to understand the etch mechanism.

• We also examined several different mask materials (Al, ITO and Ni).

ICP Schematic

~

13.56 MHzrf source

Gas Distribution

AluminaChamber

PoweredElectrode

Plasma

Sample

~

2 MHz PowerSupply

n

n nn

nn

n

neee

ei

ie

nn

n n

s

abs des

n s

n

diff

i

acc

s s

s ssputsput

n n

i

s s

abs des

s-n

abs

n

i

s

sput

n s

n

Plasma Region

SubstrateBy products (Volatile products)

Etching Mechanisms

Sputtering Chemical Reaction

Ion-AssistedPlasma Etching

Keys to SiC Etching

• Must break strong Si-C bonds in order to allow etch products to form.

• Expect SiFx and CFx to be the etch products (These are much more volatile than SiClx and CClx, and hence F2- based plasma chemistries are favored over Cl2-based chemistries).

Experiment

ICP reactor2 mTorr15 sccm

Gas : NF3, BF3, SF6 or PF5ICP source power: 750, 1000, 1250,1500 WRf chuck power: 150, 250, 350, 450 WOptical Emission Spectroscopy (OES)

Etcheddepth

measurementStylus Profilometer(Tencor)

RoughnessMeasurement Atomic Force Microscope

1. 4H-SiC2. Al3. Ni4. ITO

Mask withApiezon wax

ICP-Power (W)750 1000 1250 1500

Etc

h Y

ield

(at

om/io

n)

0

1

2

3

4

PF5BF3SF6NF3

750 1000 1250 1500

Etc

h R

ate

(Å/m

in)

1000

4000

6000

8000

10000 PF5BF3SF6NF3

250W rf, 2 mTorr, 15 sccm

250W rf, 2 mTorr, 15 sccm

Etch rate in different ICP Power in four different gases

Inte

nsity

(A

rb. U

.)

3

6

9100 BF3

1500W ICP5W rf35mTorr

Wavelength (nm)

200 400 600 800

Inte

nsity

(A

rb. U

.)

3

6

9100 PF5

1500W ICP5W rf35mTorr

B

SiFx

F

F

F F

P SiFx

F

F F

Inte

nsity

(A

rb. U

.)

5

10

15

20100NF3

1500W ICP5W rf35mTorr

Wavelength (nm)

200 400 600 800

Inte

nsity

(A

rb. U

.)

2

4

6100SF6

1500W ICP5W rf35mTorr

F

F F

F F

F F

SiFx

Chemical Bond Strength(KCal/mol)

NF3 66.4

SF6 78.3

PF5 126

BF3 154

Dissociation Energiesand OES Spectra

rf-power (W)

150 250 350 450

dc s

elf-

bias

(-V

)

200

300

400

500PF5

BF3

SF6

NF3

150 250 350 450

Etc

h ra

te (Å

/min

)

0

1000

2000

3000

4000

5000

6000

7000

PF5

BF3

SF6

NF3

750W ICP

Etch rate and dc self-biasin different rf power

SiC

Control : RMS = 2.5 nm

PF5: RMS = 2.0 nm BF3 : RMS = 0.8 nm

NF3 : RMS = 0.6 nmSF6 : RMS = 1.4 nm

1500 W Source Power250 W rf chuck power1 minute

Surface roughness data from AFM

ICP-Power (W)750 1000 1250 1500

RM

S (n

m)

0

1

2

3

4PF5

BF3

SF6

NF3Control

Surface roughness in difference gases

• Surface may actually become

smoother due to the angular dependence of ion milling which leads to faster removal of sharp features on the surface.

θθ

+

ER

0 90θθ

ICP power (W)

0 200 400 600 800 1000 1200

Etc

h R

ate

(Å/m

in)

0

1000

2000

3000

4000

5000

dc

self

-bia

s (-

V)

150

200

250

300

350

400

450

500

UV w/o UVdc self-bias

Etching in 10SF6/5Arw and w/o UV

rf power (W)

50 100 150 200 250 300

Etc

h R

ate

(Å/m

in)

0

1000

2000

3000

4000

dc s

elf-

bias

(-V

)

150

200

250

300

350

UV w/o UVdc self-bias

ICP power (W)

0 200 400 600 800 1000

Etc

h R

ate

(Å/m

in)

0

400

800

1200

1600

dc s

elf-

bias

(-V

)150

200

250

300

350

400

450

UV w/o UVdc self-bias

Etching in 10Cl2/5Arw and w/o UV

rf power (W)

50 100 150 200 250 300

Etc

h R

ate

(Å/m

in)

0

200

400

600

800

1000

dc s

elf-

bias

(-V

)

100

150

200

250

300

350

UV w/o UVdc self-bias

ICP power (W)

0 200 400 600 800 1000

RM

S (

nm)

0.5

1.0

1.5

2.0

2.5

w UV illuminationw/o UV illumination

ICP power (W)

0 200 400 600 800 1000

RM

S (

nm)

0.5

1.0

1.5

2.0

2.5

w UV illuminationw/o UV illumination

10SF6/5Ar 10Cl2/5Ar

Surface roughness of SiC at 200 W rf chuck power

SEM: SiC surface, etches with SF6

W/O UV

With UV

ICP Power (W)

800 1000 1200 1400 1600

Etc

h R

ate

(µm

/min

)

-1

0

1

2

SiCAlNiITO

rf Power (W)

100 200 300 400 500E

tch

Rat

e (µ

m/m

in)

-1

0

1

2SiCAlNiITO

Etch

Deposit

Etch

Deposit

10NF3/5O2

250 W rf chuck power 750 W ICP power

Etch rate of different metal masks

Etc

h R

ate

(µm

/min

)

-0.5

0.0

0.5

1.0SiCAlNiITO

%NF3

(Total flow NF3/O2 is 15 sccm)

20 40 60 80 100

Sele

ctiv

ity

0

3

6

9

12

15

18

21SiC/AlSiC/NiSiC/ITO

Infinite

Etching

Deposition

Etch rate and selectivity in various gas ratio

SUMMARY

• NF3 achieved the highest etch rate up to 8000 Å/minwhich is a result of its higher dissociation efficiency in theICP source.• With high O2 addition to NF3 , very high etch selectivityfor SiC over Al achieved due to oxidation of Al.• With very low O2 concentration, Ni mask shows betterperformance.• UV illumination can increase the etch rate of SiC in Cl2discharge but not in SF6 discharge.• UV illumination can improve surface morphology of SiCin SF6 discharge but not in Cl2 discharge.