Reaction and formation of La-silicate
gate dielectrics on SiC substrates
Tokyo Tech. FRC 1, Tokyo Tech. IGSSE 2, Mitsubishi Electric Corp.3, ○Y. M. Lei 1, S. Munekiyo 1, K. Kakushima 2,T. Kawanago 2, Y. Kataoka 2, A. Nishiyama 2, N.
Sugii 2, H. Wakabayashi 2, K. Tsutsui 2, K. Natori 1, H. Iwai 1, M.Furuhashi 3, N.Miura 3 and S. Yamakawa 3
Tokyo Insititute of Technology
Iwai・Kakushima labortary
SiC Power Device
Material Si SiC
Eg(ev)
Bandgap1.1 3.2
EBD(106V/cm)
Breakdown field0.3 3
μ(cm2/Vs)
mobility1450 900
Vsat(106cm/s)
Saturated velocity10 22
ĸ(W/cm2K)Thermal conductivity
1.5 5
Comparison of basic parameter of silicon and 4H-SiC Trendency of power conversion intensity
SiC is promising material for high efficiency power device
M. Ohashi et al, IEEE Trans. Electron Devices, vol. 60, no. 2, pp. 528-534
2
The advantage of SiC devices over silicon :
・High operation voltage (×10times) ・High operation frequency
・Better endurance of High temperature ・Low energy loss (1/100)
Issue of SiC power devices
Most report
http://jstshingi.jp/abst/p/10/1
018/naist1.pdf
Improvement of
channel mobility
is necessary
SiC has the ability of forming SiO2 by
thermal oxidation.
Relationship of mobility and Dit
http://www.iisb.fraunhofer.de/de/mobisic.html
Inverse proportional relationship of
mobility with interface state density
Reduction of Dit is critical to make high-speed-SiC power device
3
Approach of Interface Improvement
NO,N2O gas annealing
• Effective passivation of interface trap by annealing the gate
oxide in NO,N2O gas.
SiO2/LaSiOx gate oxides on SiC
• Recently a passivation of interface trap on SiC has been found
by forming LaSiOx interface layer on SiC substrate.
Drain
LaSiOx
SiO2
TaN/W
sicSource
Interface improvement can
be achieved by La-silicate
interface gate dielectrics
[1] X. Yang, et al., ICSCRM Th-2B-5 (2013). 4
Purpose and outline of this presentation
Investigate the interface reaction of La2O3 on 4H-
nSiC substrates to find the best process for gate
dielectric application
1.Introduction
2.Interface reaction of La2O3 on 4H-nSiC substrates
3.SiN barrier layer for La-silicate dielectrics
4.Formation of La-silicate dielectrics by SiO2 capped
annealing.
5.Conclusion
5
1.Introduction
2.Interface reaction of La2O3 on 4H-nSiC substrates
3.SiN barrier layer for La-silicate dielectrics
4.Formation of La-silicate dielectrics by SiO2 capped
annealing.
5.Conclusion
6
n-SiC(0001)substrate
La2O3 Deposition(EB)
FTIR measurement
Oxidation(5%O2/95%N2)
TEM analysis
Substrate cleaning(SPM, HF)
Sample preparation La2O3/SiC
FTIR measurement n-SiC substrate
La2O3 (2nm)
n-SiC substrate
Sample 1
Sample 2
500oC
1000oC
7
SiC(without La2O3))
5%O2 anneal:500oC—1000oC
12501300 1200 11001150 1050 1000
wavenumbers(cm-1)
Ab
sorb
an
ce (
a.u
.)
(a)
1000oC
600oC
La2O32nm/SiC)
Ab
sorb
an
ce (
a.u
.)12501300 1200 11001150 1050 1000
wavenumbers(cm-1)
(b)5%O2 anneal:500
oC—1000oC
1000oC
950oC
900oC
Oxidation on SiC substrate was enhanced by La2O3 film
Accompanied by La-silicate formation
Si-O-SiLa-O-Si
Si-O-Si
8
FITR spectra of oxidized SiC surface
(SiO2)
(SiO2)
(La-silicate)
SiO2
SiC
La-silicate
SiC
SiO2
sample:La2O3 (2nm)/SiC
SiC(0001) SiC(0001)50nm 10nm
La-silicateSiO2
TEM image of La2O3/SiC oxidized at 1000oC
• Agglomeration of La-silicates above SiO2/SiC interface
• La-silicate grains positioned at bunches of SiC substrate (4o)
• Thick SiO2 of 11nm is formed (enhanced oxidation rate)
9
Enhanced growth rate over thermal oxidation
SiC(0001) 10nm
11.8nm
sample:La2O3 (2nm)/SiCafter oxidation in 1000℃ 5%O2 30min
K. Kita, et al., ECS Trans, vol. 61(2), p. 135-142 (2014).
Higher oxidation rate of 1.5 order of magnitude can been
achieved by La2O3 capped oxidation
However, rough interface/surface may degrade reliability
This study
100
10-1
10-2
10-3
1300oC 1200oC 1100oC 1000oC
Gro
wth
rat
e co
nst
ate(
nm
/min
)
Temperature(oC)
SiO2 Growth rate of thermal oxidation
Oxidation rate comparison
10
1.Introduction
2.Interface reaction of La2O3 on 4H-nSiC substrates
3.SiN barrier layer for La-silicate dielectrics
4.Formation of La-silicate dielectrics by SiO2 capped
annealing.
5.Conclusion
11
n-SiC substrate
La-silicate
n-SiC substrate
SiO2
SiO2
n-SiC substrate
La2O3
SiN
La2O3 /SiC reaction
La2O3 /SiN reaction
n-SiC substrate
La2O3
SiN barrier layer against agglomeration
La-silicate formation by reaction of La2O3 and SiN layer
(SiN: oxygen barrier to suppress oxidation of SiC)12
n-SiC(0001)substrate
EB-La2O3 deposition
FTIR measurement
FTIR measurement
SPM,HF cleaning
PECVD-SiN deposition
AFM, TEM analysis
annealing(5%O2/95%N2)
SiN(2nm, 1.5nm, 0nm)
n-SiC substrate
Sample preparation with SiN barrier
La2O3(4nm)
Sample 500oC ~1000oC
13
La2O3 4nm/SiC)
Ab
sorb
an
ce (
a.u
.)
12501300 1200 11001150 1050 1000
wavenumbers(cm-1)
(C)O2 anneal:500
oC—1000oC
1000゜C
950゜C
900゜C
La2O3 4nm/SiN2nm/SiC)
Ab
sorb
an
ce (
a.u
.)
12501300 1200 11001150 1050 1000
wavenumbers(cm-1)
(A)O2 anneal:500
oC—1000oC1000゜C
950゜C
900゜C
La2O3 4nm/SiN1.5nm/SiC)
Ab
sorb
an
ce (
a.u
.)
12501300 1200 11001150 1050 1000
wavenumbers(cm-1)
(B)O2 anneal:500
oC—1000oC1000゜C
950゜C
900゜C
Formation of La-silicate confirmed above 900oC
Slight suppression of SiO2 with thicker SiN
La-O-Si-O
Si-O-Si
SiN 2nmSiN 1.5nmWithout SiN
Si-O-Si
La-O-Si-OLa-O-Si-O
FTIR spectra of La2O3/SiN/SiC
14
Si-O-Si
10nm
SiC(0001)50nm
Sample: SiO2/La2O3 4nm/SiN 2nm/SiC
Annealing temperature:1000oC
La2O3/SiN/SiC interface section TEM image
SiC(0001)10nm
La-silicate SiO2
• Agglomeration of La-silicates above SiO2/SiC interface
• Thick SiO2 layer is formed (enhanced oxidation rate)
• Little suppression of oxidation variation 15
RMS: 1.80nm RMS: 2.35nm
Oxides surface roughness reduction with SiN barrier layer
has been confirmed by atomic force measurement
La2O3 4nm/SiN2nm/SiC) La2O3 4nm/without SiN/SiC)
RMS:1.80nm RMS:2.35nm
(a) (b)
Reduction of oxides surface roughness
SiN(2nm)
n-SiC
substrate
La2O3(4nm)
n-SiC
substrate
La2O3(4nm)
16
1.Introduction
2.Interface reaction of La2O3 on 4H-nSiC substrates
3.SiN barrier layer for La-silicate dielectrics
4.Formation of La-silicate dielectrics by SiO2 capped
annealing.
5.Conclusion
17
SiO2 capped annealing
n-SiC substrate
La2O3
SiO2
n-SiC substrate
La2O3
n-SiC substrate
SiO2
n-SiC substrate
SiO2
n-SiC substrate
SiO2
SiO2
La-silicate
O2 annealing
O2 annealing
Before
SiO2 capped annealing
O2 annealing
SiO2 La2O3
Huge reduction of
oxygen diffusion at
oxidating surface
SiC-Sub
Suppression of oxidation variation by increasing the diffusion length
of oxygen to reduce the amount of oxygen reaching oxidating surface
18
P(O2)
L
n-SiC(0001)substrate
EB-La2O3 deposition
SPM,HF cleaning
PECVD-SiO2 deposition
TEM analysis
annealing(5%O2/95%N2)1000℃n-SiC substrate
Sample preparation for SiO2 capped annealing
La2O3 (4nm)
Sample
Sputter-W(50nm) deposition
PMA in F.G for 30 min
Backside Al contact
Al
W
SiO2 (40nm)
19
• Suppression of agglomeration by SiO2 capped annealing
• Some protrusion of La-silicates presented at the interface
SiC(0001) SiC(0001)
Sample: SiO2 40nm/La2O3 4nm/SiC
50nm 10nm
Annealing temperature:1000oC
SiO2La-silicate
SiO2/La2O3/SiC interface section TEM image
protrusion
20
La2O3
O2
O*
・ Thicker La2O3 may suppress the oxidation variation by
generating more radical oxygen
・optimal oxygen pressure may change due to change of
annealing temperature.
Suppression of carbon generation with
higher temperature and low P(O2)
[2]
[2] Y. Song et al., J. Am. Ceram. Soc., 88 [7], p. 1864–1869 (2005)
Modification of SiO2 capped annealing process
Catalytic effect by La2O3 to
generate radical oxygen [1]
[1] K. Kakushima, et al., J. Solid-State Electronic, vol. 54, p. 720-723 (2010).
SiO2
La2O3 SiC-SubSiO2
SiC-Sub
5 20 50 100
W/SiO2(40nm)/La2O3(10nm)/SiC
Oxygen ratio (%)
Hyst
eres
is v
olt
age
ran
ge
(V)
1.6
1.2
0.8
0.4
0
1MHz
L/W =50/50μm
C-V, Hysteresis on different O2 partial pressure
-5 -1 3 7 1511 19 23 270
20
40
60
100
80
120
Gate voltage (V)
Cap
acit
ance
(n
F/c
m-2
)
W/SiO2(40nm)/La2O3(10nm)/SiC
L/W =50/50μm
100%O2
Annealing ambient
50%O220%O25%O2
1MHz
Electrical properties improved by increasing
O2 partial pressure within annealing period
5%100%
22
5% 100%
C-V on annealing temperature of 1050℃
n-SiC substrate
La2O3 (4nm)
Before
Al
W
SiO2 (40nm)
Modification of SiO2 capped annealing process
n-SiC substrate
La2O3 (10nm)
After
Al
W
SiO2 (40nm)
}Annealing in
5%O2, 1000℃Annealing in
O2, 1050℃}
23
4H-SiC (1120) 100nm
La-silicate SiO2W
W50nm/SiO235nm/la2O310nm/SiC(gate oxide SiO2/La2O3 1050℃ oxidation)
SiO2/La2O3/SiC interface section TEM image
A uniform La-silicate layer has been achieved by
SiO2 capped annealing in O2 ambient, 1050℃24
100nm 10nm
Light color region distributed in La-silicate layer
4H-SiC (1120) 4H-SiC (1120)
SiO2/La2O3/SiC interface section TEM image
25
Electron diffraction pattern for region 1,2
Both black and light color region in La-silicate
layer are indicated to be La-silicate grain region26
10nm4H-SiC (1120)
*1 *2
c
o
u
n
t
s
c
o
u
n
t
s
c
o
u
n
t
s
c
o
u
n
t
s
500 600 700 800 900 1000
500 600 700 800 900 1000 500 600 700 800 900 1000
500 600 700 800 900 1000
(1) (2)
(3) (4)
Energy loss (eV) Energy loss (eV)
Energy loss (eV) Energy loss (eV)
O
La
O
La
O
La
O
10nm4H-SiC (1120)
*1 *2*4
*3
EELS analysis result of 4 gate oxide region
・Lower Lanthanum density has been detected in light color reigion. ・Existence of Lanthanum atom in thermal growth region (4) has been confirmed by EELS
27
Hypothesis for light color region in La-silicate
La-silicate SiO2
SiO2
*4
*3
SiC-substrate
*2*1
SiO2 protrusion in La-silicate layer by facet
dependent oxidation of SiC substrate 28
Si
O O
O
O
Si
O O
O
OO
OO
O
SiBO
BO
Si Si
Si
BO
BOSi
O O
O
O
Si
O O
O
OO
O
O
O
La NBO
BO
Si Si
La
NBO
BO
La atomBO
NBOSiO4-tetrahedron
BO: bridging oxygen atom
Si-O-Si bonding La-O-Si bonding
NBO:non-bridging oxygen atom
Atomic configuration of La-silicate
La atoms act as the
network modifier to
relax the SiO4 network
The interface between La-silicate/SiC may be modified by
Lanthanum atom in thin grown SiO2
29
Conclusion
• Oxidation with La2O3 capped SiC substrate• Enhanced oxidation rate with radical oxygen atoms generated
by La atoms• Grain growth, agglomeration of La-silicates• Surface roughness due to step enhanced oxidation
• SiN barrier layer for La2O3/SiC substrates• Formation of LaSiON interface layer• Reduction of gate oxides surface roughness
• SiO2 capped annealing for La2O3/SiC substrates• Formation of uniform La-silicate layer• Interface modification with Lanthanum atom relax the SiO4
network of thermal growth layer
30
Thank you very much
Spot 4
Spot 3
Spot 2
Spot 1
Si
CO
La
50 250 450 650 850
c
o
u
n
t
s
Energy loss (eV)
10nm4H-SiC (1120)
*1 *2*4
*3
FFT測定結果
格子のデータが一致しています18
sample:La2O3 (2nm)/SiCafter oxidation in 1000℃ 5%O2 30min
ステップ制御エピタキシー
C-face
Si-face
(1120)face
SiC酸化スピードの異方性
オフ角
テラスステップ
(0001)面
テラス面とステップ面の反応スピードが違う
テラスステップ
ステップ ステップオフ角;4°
[4]
[4] Y. Song et al., J. Appl. Phys. 95 (2004) 4953.
4H-SiC
膜厚不均一
SiO2膜厚不均一性の説明
Tokyo Institute of Technology
160
40
0
Cap
acit
ance
(n
F/cm
2)
80
Gate voltage (V)20-2-4-6 4 6 8 10 161412
120
: 1MHz
: 500kHz
: 100kHz
: 10kHz
W/TEOS-SiO2/La2O3(4nm)/SiC950oC oxidation50mm/50mm
Gate voltage (V)20-2-4-6 4 6 8 10 161412
: 1MHz
: 500kHz
: 100kHz
: 10kHz
W/TEOS-SiO2/La2O3(4nm)/SiC1000oC oxidation50mm/50mm
Gate voltage (V)20-2-4-6 4 6 8 10 161412
: 1MHz
: 500kHz
: 100kHz
: 10kHz
W/TEOS-SiO2/La2O3(4nm)/SiC1050oC oxidation50mm/50mm
950oC 1000oC 1050oC
950oC, 1000oCの場合にはslow trapが大きい。
・ La2O3が完全にsilicateに変わっていない
・界面、絶縁膜のバルク欠陥の除去が不十分
1050oCの場合に、その問題は改善
35
熱処理温度増加によるCV特性の変化
La-silicate
n-SiC substrate
La2O3 (10nm)
SiO2 (40nm)
Sufficient oxygen
insufficient oxygen
n-SiC substrate
La-silicate
SiO2 (40nm)
n-SiC substrate
La2O3 (10nm)
SiO2 (40nm)Fixed
charges
Incomplete oxidation by insufficient oxygen generate
fixed negative charges at la2O3/La-silicate interface
SiO2 La2O3
SiC-SubLa2O3
SiC-Sub
SiO2
O2
O*
SiO2 La2O3 SiC-Sub
・ Thicker La2O3 may suppress the oxidation variation by generating more radical oxygen・Prohibition of carbon generation reaction with higher temperature
Suppression of carbon generation with
higher temperature and low P(O2)
[2]
[2] Y. Song et al., J. Am. Ceram. Soc., 88 [7], p. 1864–1869 (2005)
Modification of SiO2 capped annealing process
Huge reduction of
oxygen diffusion at
oxidating surface
Catalytic effect by La2O3 to generate radical oxygen
radical oxygen
oxygen molecule
21
[1]
[1] K. Kakushima, et al., J. Solid-State Electronic, vol. 54, p. 720-723 (2010).
La2O3
O2
O*
・ Thicker La2O3 may suppress the oxidation variation by
generating more radical oxygen
・optimal oxygen pressure may change due to change of
annealing temperature.
Suppression of carbon generation with
higher temperature and low P(O2)
[2]
[2] Y. Song et al., J. Am. Ceram. Soc., 88 [7], p. 1864–1869 (2005
Modification of SiO2 capped annealing process
Catalytic effect by La2O3 to
generate radical oxygen [1]
[1] K. Kakushima, et al., J. Solid-State Electronic, vol. 54, p. 720-723 (2010).
SiO2
La2O3 SiC-SubSiO2
SiC-Sub
SiC(0001) 10nm
protrusion
[2]