Steven M. George

Dept. of Chemistry, Univ. of Colorado

Boulder, Colorado 80309

Steven.George@Colorado.edu

Thermal Atomic Layer Etching (ALE)

of Silicon Using an Oxidation and

“Conversion-Etch” Mechanism

Thermal Atomic Layer Etching (ALE)

Using Sequential Surface Reactions

2

Thermal Si ALE Using an Oxidation and

“Conversion-Etch” Mechanism

Oxidation

Ligand-Exchange & Conversion

FluorinationRepeat

(C)

Si

AlF(CH3)2

Si(CH3)4

SiO2

Al(CH3)3

H2O

O2

Etched

Thickness

Si

(A)

(B)HF

Al2O3 Al2O3

Si

SiO2

AlF3

Si

SiO2

Repeat

Al2O3

SiO2

3

Outline

1. Thermal Al2O3 ALE Using Fluorination &

Ligand-Exchange

2. Thermal SiO2 ALE Using “Conversion Etch”

3. Thermal Si ALE Using Oxidation &

“Conversion Etch”

4

100 ALE Cycles

Mass change per cycle

= -15.9 ng/cm2

Etch rate = 0.51

Å/cycle

Al2O3 ALE Using Al(CH3)3 and HF

Y. Lee, J.M. DuMont & S.M. George, Chem. Mater. 28, 2994 (2016). 5

Mass Loss During Al(CH3)3 and HF

Exposures for 3 Cycles of Al2O3 ALE

Constant mass changes

with each reactant

exposure

Y. Lee, J.M. DuMont & S.M. George, Chem. Mater. 28, 2994 (2016). 6

Al2O3 ALE via Fluorination &

Ligand Exchange

7

Outline

1. Thermal Al2O3 ALE Using Fluorination &

Ligand-Exchange

2. Thermal SiO2 ALE Using “Conversion Etch”

3. Thermal Si ALE Using Oxidation &

“Conversion Etch”

8

SiO2 ALE at 300°C Using Al(CH3)3 & HF

0.31 Å/cycle

0.20 Å/cycle

0.15 Å/cycle

0.027 Å/cycle

Pre

ssu

re

J.W. DuMont et al., ACS Appl. Mater. Interfaces 9, 10296 (2017). 9

Reactions during “Conversion-Etch”

Conversion of SiO2 to Al2O3:

1.5SiO2 + 2Al(CH3)3 Al2O3 + 1.5Si(CH3)4

Fluorination of Al2O3:

Al2O3 + 6HF 2AlF3 + 3H2O

Etch of AlF3:

AlF3 + 2Al(CH3)3 3AlF(CH3)2

10

Pressure-Dependence of TMA Reaction

During “Conversion-Etch”

TMA converts Si-O species

to Al-O species.

1.5SiO2 + 2Al(CH3)3

Al2O3 + 1.5Si(CH3)4

TMA etches AlF3 layer.

AlF3 + 2Al(CH3)3

2AlF(CH3)2

Difference Spectra

J.W. DuMont et al., ACS Appl. Mater. Interfaces 9, 10296 (2017). 11

Pressure-Dependence of HF Reaction

During “Conversion-Etch”

HF Fluorinates Al-O

Species

Al2O3 + 6HF

2AlF3 + 3H2O

Difference Spectra

J.W. DuMont et al., ACS Appl. Mater. Interfaces 9, 10296 (2017). 12

Mechanism of Thermal SiO2 ALE by

“Conversion-Etch”

TMA removes AlF3 &

converts SiO2 to Al2O3

HF converts Al2O3 to AlF3

13

Outline

1. Thermal Al2O3 ALE Using Fluorination &

Ligand-Exchange

2. Thermal SiO2 ALE Using “Conversion Etch”

3. Thermal Si ALE Using Oxidation &

“Conversion Etch”

14

Spectroscopic Ellipsometry

Measurement of Both Top SiO2 & Si

15

Si ALE Using O2-HF-TMA Sequence

-30

-20

-10

0

10

-10

-5

0

5

10

15

0 20 40 60 80

Si T

hic

kn

es

s C

ha

ng

e (

Å)

SiO

2 Th

ickn

es

s (Å

)

Si ALE Cycles

290oC

O2, 250 Torr

HF, 1 Torr

TMA, 1 Torr

O2-HF-TMA

0.4 Å/cycle

Si ALE Linear decrease in Si

thickness

Si etch rate: 0.4

Å/cycle

Surface oxide

remains constant

Surface oxide

thickness: ~11 Å

A.I. Abdulagatov & S.M. George, Chem. Mater. 30, 8465 (2018). 16

-35

-30

-25

-20

-15

-10

-5

0

0 20 40 60 80

30 Torr O2

90 Torr O2

170 Torr O2

250 Torr O2

Si T

hic

kn

es

s C

ha

ng

e (

Å)

Si ALE Cycles

0.2 Å/cycle

0.4 Å/cycle

290oC

HF, 1 Torr

TMA, 1 Torr

O2-HF-TMA

Si ALE vs. O2 Pressure

Linear decrease in Si

thickness at all O2

pressures

Etch rate increases with

O2 pressure

Higher oxidation at

higher O2 pressures

17A.I. Abdulagatov & S.M. George, Chem. Mater. 30, 8465 (2018).

Si ALE vs. TMA and HF Pressure

-25

-20

-15

-10

-5

0

0 10 20 30 40 50 60

0.5 Torr HF

1 Torr HF

1.5 Torr HF

Si T

hic

kn

es

s C

ha

ng

e (

Å)

Si ALE Cycles

0.4 Å/cycle

290oC

O2, 250 Torr

TMA, 1.5 Torr

O2-HF-TMA

TMA pressure dependence

consistent with conversion process HF pressure independence suggests that

HF fluorinates all converted Al2O3

-25

-20

-15

-10

-5

0

0 10 20 30 40 50 60

0.5 Torr TMA

1.5 Torr TMA

2 Torr TMA

0.8 Torr TMA

Si T

hic

kn

es

s C

ha

ng

e (

Å)

Si ALE Cycles

0.2 Å/cycle

0.4 Å/cycle

290oC

O2, 250 Torr

HF, 1 Torr

O2-HF-TMA

TMA Pressure Dependence HF Pressure Dependence

18A.I. Abdulagatov & S.M. George, Chem. Mater. 30, 8465 (2018).

Si ALE of Ultrathin Si Films

No difficulty etching

ultrathin Si films

Linear decrease in film

thickness

No effect from silicon

quantum confinement

0

20

40

60

80

100

0 50 100 150 200

Si T

hic

kn

es

s (

Å)

Si ALE Cycles

290oC

O2, 250 Torr

TMA, HF, 1 Torr

O2-HF-TMA

0.5 Å/cycle

19A.I. Abdulagatov & S.M. George, Chem. Mater. 30, 8465 (2018).

Applications for Thermal Si ALE

20

1. Isotropic etching of three-dimensional

structures such as silicon nanowires

2. Surface smoothening to reduce silicon line edge

roughness

3. Surface polishing, surface cleaning and

removing surface damage

Conclusions – Thermal Si ALE

1. O2 Oxidizes Si to SiO2

2. TMA Converts SiO2 to Al2O3

3. HF Fluorinates Al2O3 to AlF3

4. TMA Removes AlF3 by Ligand-Exchange

21

Extra Slides

QCM Studies in Hot Wall, Viscous

Flow Reactor

Pump

Heated Substrates

Gas Switching

Valves

Throttle

Valve

Quartz Microbalance

HeatersFlow Tube

N2

H2O

TMA

J.W. Elam et al., Rev. Sci. Instrum. 73, 2981 (2002).

FTIR Spectroscopy Studies of Surface

Species & Etched Material

Film Thickness Measurement with

Spectroscopic Ellipsometry

LightSource

Detector

Sample Insulation

Manifold

ALEChamber

Pump

Gas In

Gas out