Transcript Diapositiva 1

Reliability of ZrO2 films grown
by atomic layer deposition
D. Caputo, F. Irrera, S. Salerno
Rome Univ. “La Sapienza”, Dept. Electronic Eng.
via Eudossiana 18, 00184 Rome (Italy)
S. Spiga, M. Fanciulli
Laboratorio MDM-INFM, via C. Olivetti 2
20041 Agrate Brianza (Italy)
Aim of the work
• To verify the reliability of ZrO2 films embedded in
MOS structures as gate dielectric
• Electrical characterization by means of I-V curves
and C-V measurements in as grown conditions and
after constant current stress (CCS)
• Extraction of defect density
Why high-k dielectrics?
• SiO2 thickness below 2 nm is required in the 2005 technology node
• Substitution of SiO2 with oxides with higher dielectric constant
• Equivalent Oxide Thickness EOT = eSiO2/ehigh-k tox
• Candidates: Al2O3, Gd2O3, ZrO2, HfO2, . . . .
Requirements
Good chemical stability, amorphous network, large energy gap and
high band offset with silicon and of course … lower leakage current
Devices under test
Al
ZrO2
SiO2
Silicon substrate
Vgate
Silicon substrate is 2-3 W cm
1.2 ±0.1 nm thick native SiO2 layer
19.1±0.3 nm thick ALCVD ZrO2 layer
About the leakage in ZrO2 films …..
10
-8
ZrO EOT = 4 nm
2
2
inj
2
Current (A/cm )
Q = 1.22 C/cm
10
10
-9
SiO EOT= 5.5 nm
2
-10
fresh
0
0.5
1
Voltage (V)
1.5
2
2
Current (A/cm )
Fresh Samples
Current-voltage measurements
10
-6
10
-7
10
-8
10
-9
Fowler-Nordheim
10
-10
10
-11
10
-12
Poole-Frenkel
?
-4
-3
-2
-1
0
Voltage (V)
1
2
3
Fresh Samples
FOWLER-NORDHEIM regime
-42
2
-44
2
ln J/E (A/V )
-43
-45
-46
-47
-48 -6
1 10
-6
1.2 10
-6
-6
1.4 10
1.6 10
1/E (cm/V)
1.8 10
-6
If meff = 0.5 m0 then Al-ZrO2 barrier is 0.3 eV
Current (A/cm2)
Fresh Samples
Weak accumulation condition (0<Vgate<1 V)
10
-8
10
-9
t-1
10
-10
10
-11
10
-12
1
10
100
Time (s)
1000
Transport dominated by charge trapping and de-trapping
Stressed Samples:
Current-voltage measurements
2
Current (A/cm )
Cumulative Constant Current Stress @ 1nA/cm2
10
-6
10
-7
10
-8
10
-9
10
10
Q
inj
fresh
-10
17.05 mC/cm
2
118.2 mC/cm
2
618.2 mC/cm
2
3020 mC/cm
2
-11
-3
-2
-1
0
Voltage (V)
1
2
Stressed Samples:
Time behavior of the defect density extracted
from PF conduction
6
5
N(t)/N(0)
4
3
2
t
1/2
1
0
10
5
10
6
10
7
10
8
10
9
10
10
Stress time (s)
Scattering Induced Degradation (SID) model
Stressed Samples:
Current-voltage measurements
2
Current (A/cm )
Cumulative Constant Current Stress @ 1nA/cm2
10
-6
10
-7
10
-8
10
-9
10
10
Q
inj
fresh
-10
17.05 mC/cm
2
118.2 mC/cm
2
618.2 mC/cm
2
3020 mC/cm
2
-11
-3
-2
-1
0
Voltage (V)
1
2
Stressed Samples:
Weak accumulation condition (0< Vgate < 1V)
• I(t)= q.N.A.(2tb)-1 tunneling front model **
q electron charge
N defect density
A area
b a constant
• the same defect states are involved in the trapping-detrapping
process, i.e the Fermi level remains almost constant with the
applied voltage
** S. Manzini, A. Modelli, “Insulating films on semicond.”, Elsevier Science, 112, (1983)
D.J. Dumin, J.R. Maddux, IEEE Trans. on Electron Dev., 40, 986, (1993).
2 10
19
1.6 10
19
1.2 10
19
8 10
18
4 10
18
-3
Defect density (cm )
Weak accumulation condition (0< Vgate < 1V)
t1/2
1
10
100
1000
2
Q (mC/cm )
inj
Defect density greater than 1019 cm-3 pins the Fermi level
Stressed Samples:
Current-voltage measurements
2
Current (A/cm )
Cumulative Constant Current Stress @ 1nA/cm2
10
-6
10
-7
10
-8
10
-9
10
10
Q
inj
fresh
-10
17.05 mC/cm
2
118.2 mC/cm
2
618.2 mC/cm
2
3020 mC/cm
2
-11
-3
-2
-1
0
Voltage (V)
1
2
Strong accumulation condition (Vgate > 1V)
Unlike in SiO2 the FN treshold voltage shifts
1.7
2
Voltage @ 1 nA/cm (V)
1.8
1.6
1.5
1.4
1.3
0.1
1
10
100
2
Q (mC/cm )
inj
1000
Capacitance measurements: fresh samples
2
Capacitance (nF/cm )
1000
3 Hz
200 Hz
100 kHz
800
600
400
er ≈ 22
200
0
-4
-3
-2
-1
0
1
Voltage (V)
CFB = 168 nF/cm2 VFB = 450 mV
2
3
Nbulk ≈ 1019 cm-3
Capacitance measurements: low level stress
2
Capacitance (nF/cm )
800
C
700
600
Q =17 mC/cm
500
inj
V
400
2
FB
300
Q =5.53 mC/cm
2
inj
Q =1.8mC/cm
2
inj
200
Q =0.576 mC/cm
inj
100
0
2
fresh
0
0.5
1
1.5
Voltage (V)
2
Negative shift of VFB due to trapping of negative charge
A decrease of capacitance in the strong accumulation region
due to variation of the interfacial SiOxZr layer thickness
Capacitance measurements: high level stress
2
Capacitance (nF/cm )
800
700
600
500
2
V
400
Q =1022 mC/cm
inj
2
FB
Q =618.2 mC/cm
inj
300
2
200
Q =118.2 mC/cm
100
Q =17 mC/cm
0
inj
2
inj
0
0.5
1
1.5
Voltage (V)
2
Positive shift of VFB due to trapping of positive charge
A tendency to saturation in the strong accumulation region
Conclusions
 Electrical characterization of fresh and stressed ZrO2
films deposited onto native SiO2 on Si by ALCVD
 Densities of native bulk defects were estimated in the
order of 1019 cm-3
 The density of bulk defects follows a square root time
dependence (Scattering Induced Degradation model)
 Leakage current of ZrO2 films is lower than that of
thicker SiO2 films
 Deposition technology needs to be improved