NearNear--Threshold IonThreshold Ion--Enhanced Enhanced Silicon EtchingSilicon EtchingSilicon EtchingSilicon Etching

H. Shin, W. Zhu, V. M. Donnelly, and D. J. EconomouS , u, o e y, a d J co o ouUniversity of Houston

Acknowledgements: DOE Plasma Science Center NSF andAcknowledgements: DOE Plasma Science Center, NSF and Varian Semiconductor Equipment Associates

November 2, 2011

AVS 58h International Symposium, Nashville, TN, USA

Outline2

Control of ion energy distribution (IED)gy ( )

Experimental set-up for near-threshold etching

Results and Discussion Etching threshold, etching rate & yield Sub threshold etching Sub-threshold etching Photo-assisted etching

Summary Summary

Control of IED 3

• In 2010 AVS, we presented how to control IED using a boundary electrode*.

Biasable

* H. Shin et al, PSST 20 055001 (2011)0.05

Controllable ion energy b h DC bi

ICP coil

boundary electrode

0.03

0.04by synchronous DC bias

20µs 80µs

Variable DC synchronous bias BE bias during

afterglow

Faraday shield 0.02

IED

7 mTorr 14 mTorr

ONµ

OFF

Vp during active glow

Detailed IEAconfiguration

0 5 10 15 20 25 30 35 400.00

0.01 28 mTorr 50 mTorr

• The IEDs were obtained by a differentially pumped repelling field energy analyzer.

0 5 10 15 20 25 30 35 40

Energy (eV)

energy analyzer. • We can make IED with two peaks apart from each other placing Eth in the middle.

Time resolved electron temperature 4

4.04.55.0

OFF

ON

7mTorr 14mTorr 28mTorr 50mTorr

2 02.53.03.5

OFF

T e(eV

)

0.51.01.52.0 Electrons are cooled

by diffusion

0 10 20 30 40 50 60 70 80 90 1000.0

time (s)

• At higher pressure T is lower during

• Broadening of IED is due to collisions in pre-sheath and a function of Te.

• At higher pressure, Te is lower during active glow, but higher during afterglow.• We attribute the narrower IED width to the low Te

• Te and Vp changes while biasing.

• Time averaged IED is a convolution of IEDs during the bias.

• Late afterglow biasing is more beneficial for narrow IED.

g

PS2-TuA9, W. Zhu

Benefits of such control of IED5

• We reviewed what affects IED and learnt how to control it.

• Using the precise control of IED with a narrow width weUsing the precise control of IED with a narrow width, we investigated ion-assisted plasma etching in a plasmaenvironment.

• Such precise control of IED is applicable to high selectivity etching and precise etching (e.g. atomic layer etching).

• Conventional plasma reactors have a broad IED and high plasma potential.

• Next we present a surprising and important discovery of sub-threshold etching in plasma thanks to our control of IEDIED.

Bi bl

Setup  for etching study 6

rf coil for ICP

Biasable boundary electrode

Faraday shield

“p-type” Si sample

Periscope for

Spectrometer

Cooling water

Periscope for OES

• Si 2881 Å was used to monitor etching.

photodiode

• Etched depth was measured by IR laser interferometry.• Ion flux was measured for etching yield.

IR laser 1.31µmyield.• A load-lock is ready for clean and reproducible environment.

Si removal with a well‐controlled IED7

6

1. Eth not by extrapolation5

6

14mTorr 28mTorr 50mTorr

7000.03

0.04

0.05 50mTorr

2. Universal etching rate

4

0 m

Torr

mis

sion 60mTorr

500

600 ER(E) at 50mTorr

0.00

0.01

0.02

etching rate relation

3 Y(E)

3

min

) @50

R o

r Si e

300

4000 5 10 15 20 25 30 35

Y(40eV)=0.41Eth

3. Y(E)

4. Sub-1

2

ER

(Å/m

ativ

e E

R

200

300

Y(0eV)=0

Y(30eV)=0.14

threshold etching

0

1

Rel

a

0

100

UNIVERSITY of HOUSTON Plasma Processing Lab

0 1 2 3 4 5 6 7 8E1/2 (eV1/2)

No spontaneous chemical etching 8

30nm SiO2 mask

p‐type Si

50mTorr 1% Cl2 Ar pulsed plasma with synchronous bias, 40V.

• p‐type Si is known not to etch by Cl or Cl2 spontaneously [Mogab and Levinstein (1979), Ogryzlo et al (1990), Flamm (1990)] Th b th h ld 2/3 f I i t d t hi t 40V b t d tti

UNIVERSITY of HOUSTON Plasma Processing Lab

• The sub‐threshold was 2/3 of Ion‐assisted etching at 40V, but no undercutting.• The sub‐threshold etching is NOT due to spontaneous chemical (isotropic) etching.

Pure Cl2 plasma also shows the sub threshold etching

9

the sub‐threshold etching• Energetic Ar metatstables (11.55 and 11.72 eV for 3P2 and 3P0) could have lead to surface reaction

0.35

have lead to surface reaction.• This excludes possibilities of the Ar metastables as a source for the spurious etching.

0.25

0.30

882

0.15

0.20

tens

ity a

t 2

0 00

0.05

0.10

Si in

t

100% Cl2 CW plasma at 40mTorrBE bias for 50µs at 10kHz

0 1 2 3 4 5 6 70.00

Boundary electrode bias1/2

UNIVERSITY of HOUSTON Plasma Processing Lab

Low energy (<Eth) ions etching10

@7mTorr to minimize charge exchange

CVpVA

VB

@7mTorr, to minimize charge exchange

A B-5V

Sample (VC)

Sample bias was used to repel ions

1

Current (mA)

1.4

repel ions

• The low energy ions can create low energy electrons when they are

-30 -20 10 20 30

Voltage (V)

0.8

1.0

1.2hi

ng R

ate

energy electrons when they are neutralized by an Auger process.

• Vp hardly changed during this measurement

4

-2

0.4

0.6

Rel

ativ

e Et

ch measurement

• Fast neutrals by charge exchange can be safely ignored at low pressure.Ions

Radicals Radicals-4

0.0

0.2

UNIVERSITY of HOUSTON Plasma Processing Lab

• We did turn off ALL the ion flux but still have the same weird etching.

RadicalsPhotons

RadicalsPhotons

Photo‐assisted etching (PAE) 11

2 0

2.5 Si +30V (I=-3.5mA) 0V (I=-0.1mA) -30V (I=0.7mA)

1.5

2.0

SiSiCl

Si

(arb

. uni

ts)

1.0

Si

Inte

nsity

The sub-threshold etching remains the same under no

The sub-threshold etching remains the same under no

2500 2600 2700 2800 2900

0.5 the same under no ion bombardment.the same under no ion bombardment.

2500 2600 2700 2800 2900

Wavelength (Å)

• With negative 30V (ion-assisted etching with E=30+Vp), the etching is more.• With positive 30V (no ions) the etching is the same as with no bias

UNIVERSITY of HOUSTON Plasma Processing Lab

• With positive 30V (no ions), the etching is the same as with no bias. • The sub-threshold etching is due to photons which always exist in plasma.

0 35

Validating proof of PAE 12

0.25

0.30

0.35

w/o grid correcte w/ grid

at 7mTorr

0.15

0.20

emis

sion

0 00

0.05

0.10Si e

•Comparison of the etching rate with grids (3% Cl2 in CW) to the earlier

0 1 2 3 4 5 6 7 80.00

E1/2(eV1/2)

measurement without the grids (1% Cl2 in PP).•With our best effort of calibration, the sub-threshold etching with the grid coincides well with the one without the grid → validation of the grid experiment and reconfirm of no ion effect on the sub thresholdof no ion effect on the sub-threshold. • This is the first time to report the photo-assisted etching (PAE) in a plasma environment.

Further investigation of PAE13

(b)

• Etching for 12min in Ar plasma with 3% Cl i CW d (300W)3% Cl2 in a CW mode (300W).

• Same plasma and neutrals but different light illumination using an g gopaque and transparent (>170nm) quartz roof.

• Etched depth was compared using a• Etched depth was compared using a step profilometer.

VUV is Responsible for PAE 14

• Under the opaque roof, the p-type Si etching rate is much smaller due to smaller light illumination.

140160

Opaque (p-Si) Transparent(p-Si)

• n-type Si showed more overall etching but less effect of photo-assisted etching80

100120140

pth

(nm

)

assisted etching.

• With the quartz roof, the etching rate is 105 Å/min, which is only a fraction 20

406080

Etc

hed

de

y(<9%) of what would’ve been expected at the same conditions (>1200 Å/min) (3% Cl2, 300W).

1 2 30

20E

Position (#)

•This implies the photo-assisted etching is dominated by the photons blocked by quartz VUV photons below 170nmphotons blocked by quartz, VUV photons below 170nm.

Efficiency of VUV for etching15

Streller et al, Journal of Electron Spectroscopy and Related Phenomena (1996)

• It was reported by Streller et al that VUV <130nm is much more efficient to etch GaAs in Cl2 system in their study using synchrotron.

• Strong VUV lines (104.8nm and 106.6 nm) exist in Ar plasma. 52mW/cm2 in ArICP over 50 and 250nm [Woodworth et al JVST A (2001)]

SEM of the sub‐threshold etching1616

DG 0.1 L/0.1S 0VDG 0.1 L/0.1S 0V

~15nm~110nm

Xe 50mTorr 10minAr 50mTorr 10min

• Under no ion-assisted etching regime, we see micro-trenches by PAE.Chec

• Like ions, VUV photons glanced off the sidewall lead to microtrenches (???).Chec

17SEM of sidewalls in halogen etching

♠♠ ♥

Cl2 HBrCl2 HBr

400W ICP; 80W rf biasing; 100sccm

♠♠

400W ICP; 20W rf biasing; 175sccm Cl2Longer time of Cl2 etching♠ Longer time of Cl2 etching♠ Mahorowala et al JVST B (2002)

♥ Vyvoda et al JVST B (2000)

2.5min 5min

105nm

90nmArAr

120nm210nm

Xe & Ar comparison 50mTorr 40V DG0 1L/0 1S

2.5min 5minXeXe

Xe & Ar comparison 50mTorr 40V DG0.1L/0.1S

110nm80nm

180nm 360nm

Summary19

Using our ability to control IED for exploring plasma etching near threshold, we showed a definitive evidence of photo-assisted t hi (PAE) i l t hi i tetching (PAE) in plasma etching environment.

PAE is dominated by VUV photons.

The PAE could be an impediment to etching with atomic precision processing for smaller device fabrication in the future.

The PAE could also be a cause of some of etching artifacts in chlorine containing plasma etching (e.g. sloped sidewalls and

i t hi )micro-trenching).

UNIVERSITY of HOUSTON Plasma Processing Lab

Th kThank you

BACKUPBACKUP

22

Vp by synchronous active glow bias

1.4

1.6 20% pulsed 100sccm Ar/Cl2/TRGp-type Si, synchronous boundary biasHV 1500 G 10^9 ti l bi

@50mTorr Ar

Vp by synchronous active glow bias

1.2

1.4

ti l bity (a

.u.)

HV=1500, G=10^9 activeglow bias• Active glow sync bias does not produce more

0.8

1.0 active glow sync bias after glow sync bias

ion

inte

nsit produce more

etching until afterglow ion peak reaches the

0.4

0.6

2881

em

iss reaches the

threshold.

• Vp during the

0 0

0.2~12V=Active glow VpSi

2 p gactive glow can be deduced.

0 5 10 15 20 25 300.0

Bias voltage (V)

200Current measured at the sample

200 Bias during afterglow

100) A ti l+20V100

I (m

A)

Active glow

0 0V

0 20 40 60 80 100 Time (s)

1.6 20% pulsed 100sccm Ar/Cl2/TRG

1.4

a.u.

)20% pulsed 100sccm Ar/Cl2/TRGp-type Si, synchronous boundary biasHV=1500, G=10^9 activeglow bias

1.0

1.2

active glow sync bias after glow sync biasin

tens

ity (

0 6

0.8

emis

sion

0.4

0.6

~12V=Active glow V

Si 2

881

0.0

0.212V Active glow Vp

0 5 10 15 20 25 30

Bias voltage (V)

IED is determined by temporal evolution of V and T

25

14

16

Ar VpKr Vp

temporal evolution of Vp and Te3 5

4.0

Ar TeK T

10

12

)

Kr Vp Xe Vp

2.5

3.0

3.5

V)

Kr Te Xe Te

4

6

8

Vp (

V)

1.0

1.5

2.0

T e (eV

0 10 20 30 40 50 60 70 80 90 1000

2

time ( s)0 10 20 30 40 50 60 70 80 90 100

0.0

0.5

time ( s) time (s)time (s)

• Te and Vp decays slower in Xe plasma (slower diffusion cooling).• ∆Vp is similar but Te is highest in Xe plasma. • This results in broader width in Xe plasma.

IEDs of Ar, Kr and Xe pulsed plasmas26

0 4

0.5 Ar KrXe

0.3

0.4 Xe

0.2

IED

• T and V changes while

0 0

0.1Te and Vp changes while

biasing.

• Broadening of IED is due to collisions in pre sheath0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

0.0

Energy (eV)

to collisions in pre-sheath and a function of Te.

• Time averaged IED is a • Ar has the narrowest width of IED convolution of IEDs during

the bias duration.• Ar has the narrowest width of IED • Xe has the lowest Vp during active glow

Etching with different buffer gases

27

gases

8

9.0x10-8

Ar 120(11W)Kr 110(4W) Uncertain

6 0x10-8

7.0x10-8

8.0x10-8

50mTorrG=108 ; HV=1500VA)

Kr 110(4W) Xe 110(3W)

4.0x10-8

5.0x10-8

6.0x10

;

curr

ent (

A

2.0x10-8

3.0x10-8

4.0x10

PM

T c

0 10 20 30 40 500.0

1.0x10-8

0 10 20 30 40 50

BE bias (V) or ion energy (eV)

Preliminary Result of Different Etchant/Buffer Gas 

28

/

0 4 G=10^7, HV=1500,G=10^7, HV=1500V

0.3

0.4

Cor_1% Br2/Xe Cor_1% Br2/Ar

n (V

)

G 10 7, HV 1500, 50mTorr

1 0

1.2

1.4G 10 7, HV 1500V50mTorr

Cor_1% Cl2 Cor_1% Br2io

n (V

)

0.2

em

issi

on

0.6

0.8

1.0 2% Br2

881

emis

s

0 0

0.1

Si 2

881

0 1 2 3 4 5 6 7 8 9

0.2

0.4

Si 2

8

0 1 2 3 4 5 6 7 80.0

Bias0.5

0 1 2 3 4 5 6 7 8 9Bias0.5

SEM images of etched patterns for XX min. with 0V afterglow bias in different carrier gas plasma with 1% Cl2 at 50mTorr: (Top) in pulsed Ar plasma with 1% Cl2 (a) 100nmgas plasma with 1% Cl2 at 50mTorr: (Top) in pulsed Ar plasma with 1% Cl2 (a) 100nm line and 100nm space (b) 500nm line and 100nm space; (Bottom) in pulsed Xe plasma with 1% Cl2 (c) 100nm line and 100nm space (d) 500nm line and 100nm space

30

0.02 8y )

6

rom

etry

on (a

.u.

0.01

4

inte

rfer

emis

sio

Plasma off

2IR

Si e

0 10 20 30 40 50 600.00

Time (min)

0

Time (min)

• During etching with 30V BE bias, 50mTorr 1% Cl2

ReferencesReferences Mogab and Levinstein (1979), Ogryzlo et al

(1990) Fl (1990) tg ( ) g y

(1990), Flamm (1990) : no spontaneous etching of p-type Si by Cl or Cl2

Photochemical etching Photochemical etching F. A. Houle T. J. Chuang (1982) Ehrlich et al(1981) Okano et al Jackman Strellar et al More…

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