A. Candelori,
, M. Ceschia,
A. Paccagnella,
J. Wyss, D. Bisello,
and G. Ghidini
Abstract--We have investigated the degradation induced by I and Si ions on 10 nm and 3 nm thick oxide MOS capacitors. 10nm oxides were biased at low electric field (

3.3 MV/cm) during irradiation up to 100 Mrad(Si). DC radiation induced leakage current (RILC) has been observed after irradiation and the differences of RILC characteristics between 10-nm and thinner oxides are discussed. In 10-nm oxides RILC is attributed to multitrap assisted tunneling, which is reduced by subsequent Fowler-
Nordheim electron injection. The density of the radiation induced positive charged defect, the positive charge recombination by
Fowler-Nordheim electron injection, and the negative charge trapping in radiation induced neutral electron traps have been also addressed. On the other side, radiation induced soft breakdown (RSB) is triggered by I ions in 3-nm oxides at low doses (<1 krad(Si)) for moderate applied electric fields (4.4
MV/cm). Silicon ion irradiation is unable to produce RSB and
RILC in 10-nm oxides, but it can generate a peculiar RILC in 3nm oxides.
Index Terms--
61.80.Jh Ion radiation effects
73.40.Qv Metal-insulator-semiconductor structures
I.
I NTRODUCTION
ILICON oxide (SiO
2
) is and will be for some years the gate S dielectric of choice in MOS devices [1]. The wide diffusion of CMOS circuits in radiation harsh environments
(high energy physics experiments [2], space [3] and medical
[4] applications), as well as the impact of processing steps on
Manuscript received September 13, 2000. This work was supported by
Istituto Nazionale di Fisica Nucleare and CNR(Italy)-Progetto Finalizzato
Materiali e Dispositivi per l'Elettronica a Stato Solido II.
A. Candelori is with Istituto Nazionale di Fisica Nucleare and
Dipartimento di Fisica, Università di Padova, via Marzolo 8, C.A.P. 35100,
Padova, Italy (telephone: +39-049-8277215, e-mail: candelori@pd.infn.it).
M. Ceschia is with Dipartimento di Elettronica e Informatica, Università di Padova, via Gradenigo 6/a, c.a.p. 35100, Padova, Italy (telephone: +39-
049-8277664, e-mail: ceschia@dei.unipd.it)
A. Paccagnella is with Dipartimento di Elettronica e Informatica,
Università di Padova, via Gradenigo 6/a, c.a.p. 35100, Padova, Italy
(telephone: +39-049-8277664, e-mail: paccag@dei.unipd.it).
J. Wyss is with Dipartimento di Ingegneria, Università di Cassino, via
DiBiasio 43, c.a.p. 03043, Cassino (FR), Italy (telephone: +39-0776-299612, e-mail: wyss@pd.infn.it).
D. Bisello is with Istituto Nazionale di Fisica Nucleare and Dipartimento di Fisica, Università di Padova, via Marzolo 8, C.A.P. 35100, Padova, Italy
(telephone: +39-049-8277215, e-mail: bisello@pd.infn.it).
G. Ghidini is with ST-Microelectronics, via Olivetti 2, C.A.P. 20041,
Agrate Brianza (MI), Italy (telephone: +39-039-6035326, e-mail: gabriella.ghidini@st.com) device reliability (x-ray [5] and electron beam [6] lithography, ion implantation for threshold voltage correction [7]) have further stimulated studies on radiation damage in thin SiO
2 layers.
3-nm and 10-nm thick gate oxides are characteristics of contemporary CMOS technologies:
- 3-nm oxides are peculiar of the current 0.18
 m technology generation;
- 10-nm oxides are considered in analog application (such as charge pump) for handling voltages higher than the power supply and in tunnel oxides for EPROM-based memories.
In our previous study [8] we reported on the radiation induced soft breakdown (RSB) characteristics of 3-nm oxides after high linear energy transfer (LET) ion irradiation. There we showed, for the first time, that I ions can cause radiation induced leakage current (RILC) also in 10-nm thick oxides. In this contribution we will extend our previous study by investigating:
1) the charge trapping properties and the RILC characteristics of 10-nm oxides, including the dependence on the gate bias voltage during irradiation and on the radiation dose (section III);
2) the conduction characteristics through 3-nm oxides for positive and negative gate voltages, after irradiation by low and high LET ions at various doses (section IV).
II.
D EVICES , I RRADIATION CONDITIONS , AND E LECTRICAL
M EASUREMENTS
A.
Devices
Tested devices were 3-nm and 10-nm thick oxide square
MOS capacitors on p-Si substrate produced by ST-
Microelectronics with gate area A=10 -2 cm 2 . The gate is a n + polysilicon contact. The oxide layer was thermally grown in steam (dry) ambient and annealed in N
2
(N
2
O) atmosphere for the 3-nm (10-nm) oxides. The MOS active area is surrounded by a n + -ring providing the minority carriers for electron injection from the substrate.
B.
Irradiation conditions and electrical measurements
Irradiation were performed at the Tandem Van de Graaff accelerator of the I.N.F.N. National Laboratory of Legnaro
(Padova, Italy). The ion species considered for irradiation
(Iodine and Silicon), their energy, LET, and fluences corresponding to 1 Mrad(Si) are listed in Table I.
1

2
Ion
Species
127 I
28
Si
Energy
(MeV)
257
158
TABLE I
I ON SPECIES
LET
(MeV
 cm 2 /mg)
61.6
8.5
Ion fluence at 1 Mrad(Si)
(ions/cm 2 )
1.00

10 9
7.35

10 9
Due to the different characteristics of the 10-nm and 3-nm oxides, different experimental procedures were adopted in the two cases.
1) 10-nm oxides
10-nm oxides were irradiated by I ions up to 100 Mrad(Si) with a dose rate of 4 Mrad(Si)/min. During irradiation devices were biased with a positive (Vbp=3 V), zero (Vb 0 =0 V), or negative (Vbn=-4.5 V) gate voltage. The oxide electric field for the positive (Eox_p) and negative (Eox_n) gate bias can be determined as [9],[10]:
Eox_p=Vbp/tox=3MV/cm (1)
Eox_n=(Vbn+Vcorr)/tox =3.3 MV/cm (2) where tox is the oxide thickness and Vcorr=1.2 V for Eox_n takes into account the difference between the gate and substrate work functions. Consequently, |Eox_p| and |Eox_n| are comparable within 10%.
For each dose and bias condition up to three capacitors were irradiated in order to improve the data statistics. After irradiation devices were characterized by: a) 100 kHz C-Vg measurements by using a HP4284A; b) a series of positive (Vg>0) and negative (Vg<0) Ig-Vg measurements with ramp rate 375 mV/s by using a HP4145B.
The electrical characterization started 10 hours after the end of the irradiation, after transporting the devices kept under bias in the main laboratory.
2) 3-nm oxides
3-nm oxides were irradiated biased with a positive gate voltage Vbp=1.33 V in cumulative steps up to the dose of: a) 10 krad(Si) with dose rate 500 rad(Si)/min for I ions; b) 212 Mrad(Si) with dose rate 4 Mrad(Si)/min for Si ions.
Charge trapping is negligible in 3-nm oxide and a measurement protocol, simpler than the one adopted for 10-nm devices, was applied. Positive and negative Ig-Vg measurements with ramp rate 60 mV/s were performed immediately after each irradiation step by a HP4155A, without extracting the devices from the experimental chamber. The ramp rate was lower than for 10-nm oxides to better detect the
Ig-Vg curve instabilities after the oxide soft breakdown [11].
III.
10NM OXIDES
A.
Radiation induced positive charge
The midgap gate voltage shift (

V mg
) of the C-Vg measurement is correlated to the charge density

Qox trapped in the oxide at position Xox (i.e., the charge centroid) measured from the gate [12] by:

V mg
=-
 Qox·Xox/( 
0
·  ox
)=-
 Qeff·tox/( 
0
·  ox
) (3) where

0
and
 ox
are the vacuum and the oxide relative dielectric constants, respectively.

Qeff is the effective trapped charge, i.e., the charge trapped at the SiO
2
/Si interface causing the same

V mg
of the charge

Qox in Xox.
The radiation induced effective positive charge

Qeff_p, determined by

V mg
measurements is shown in Fig. 1.

Qeff_p increases with the dose, and the experimental data can be fitted by the relation

Qeff_p = A
 ln (D)+B. Parameters
A and |B| (see Fig. 1) depend on the irradiation bias conditions, being maximum under Vbn and minimum under
Vb 0 , respectively.
12
11
Vbn: Qeff_p = 3.43·10 ·Ln(D)-5.42·10
11
10
11
Vbp: Qeff_p = 2.61·10 ·Ln(D)-3.94·10
11
8
h
6
11
Vb : Qeff_p = 2.06·10 ·Ln(D)-1.88·10
11
4
2
0
0 20 40 60 80 100
Dose (Mrad(Si))
Fig. 1. Positive effective trapped charge after I ion irradiation for devices biased at Vbp=3 V (open squares), Vb 0 =0 V (dashed circles), and Vbn=-4.5
V (closed triangles). The solid lines are fittings of the experimental data by

Qeff_p=A
 ln (D)+B; the values of the A and B parameters are also reported.

Qeff_p is higher for the negative gate bias for all radiation doses. This effect is peculiar of thin oxides (tox

10 nm) kept under bias after irradiation. Holes are trapped close to the cathode during irradiation, i.e., their centroid is expected to be closer to the SiO
2
/Si and gate/SiO
2
interface for the positive and negative gate bias, respectively. Consequently from eq.(3)

V mg
is expected to be higher for Vbp than for Vbn, in agreement with usual observations on thicker oxides. The opposite is observed in Fig. 1, where C-Vg measurements were performed 10 hours after irradiation, with the gate bias always applied. In fact, trapped holes are removed by thermal detrapping and by electron tunneling [13] after irradiation.
Hole removal at the SiO
2
/Si interface by electron tunneling from the Si substrate is enhanced under positive gate bias and
|

V mg
| is lower [13]-[17] accordingly with data in Fig. 1. This effect was observed also in 8-nm oxides irradiated by 8 MeV electrons and characterized three hours after irradiation [18].
Similar

Qeff_p values are found for the Vb 0 and Vbp bias.
Actually, the oxide electric field has the same direction for both bias values, but it is lower for Vb 0 being Eox 0

0.4
MV/cm. Hence, the positive trapped charge can be reduced by the high initial recombination of the radiation induced electron-hole pairs under the low field condition, that is, under
Vb 0 .
B.
Positive charge recombination by Fowler-Nordheim electron injection
We have evaluated the kinetics of positive charge

3 recombination by Fowler-Nordheim (FN) electron injection in the oxide [19]. The time evolution of the gate voltage for the
FN constant current Ig=100 nA is shown in Fig. 2 for devices biased at Vbp during irradiation. The first measured value
(after 1 s) for irradiated samples is lower than in the as received MOS, due to the positive trapped charge (Qp) which reduces the potential barrier for electron injection and increases the electron tunneling probability [18],[20]-[28]. As expected, the initial Vg value decreases by increasing the radiation dose due to the Qp increase.
In the as received device Vg continuously decreases as a function of time, owing to positive charge accumulation in the oxide. On the contrary, in irradiated devices Vg increases as the electron injection proceeds, due to Qp recombination and electron trapping in radiation induced neutral electron traps
[18],[20]-[23],[29]-[32]. After irradiation Vg becomes larger than in the as received device after 20-70 s depending on the radiation dose. Vg grows faster for higher radiation doses and the highest Vg corresponds to the largest dose after 800 s: these results indicate that the negative charge trapping and the neutral electron trap density grow with the radiation dose.
9.4
9.3
9.2
9.1
As received
9.0
8.9
100 Mrad(Si)
50 Mrad(Si)
20 Mrad(Si) 8.8
8.7
1 10 100 1000
Time (s)
Fig. 2. Time dependence of the gate voltage during FN constant current electron injection from the substrate (Ig=100 nA) before (as received) and after the I ion irradiation for devices biased at Vbp=3 V during irradiation.
C.
Ig-Vg characteristics and radiation induce leakage current
The positive charge recombination, the electron trapping in radiation induced neutral traps and the modifications of the conduction through the 10-nm oxides after irradiation have been also investigated by two series of 12 consecutive Ig-Vg measurements. Each series consists of four groups of three positive (p) or negative (n) Ig-Vg sweeps up to a gate current
|Ig|=1

A, which is low enough to prevent oxide stressing.
The two series were:
In: 1n, 2n, 3n, 1p, 2p, 3p, 4n, 5n, 6n, 4p, 5p, 6p;
Ip: 1p, 2p, 3p, 1n, 2n, 3n, 4p, 5p, 6p, 4n, 5n, 6n.
The negative and positive Ig-Vg measurements of the In series for devices irradiated at 100 Mrad(Si) under Vbn are shown in
Fig. 3 and Fig. 4, respectively.
In Fig. 3, the FN injection before irradiation appears at
|Vg|>7 V and for |Vg|

7 V the gate current is lower than 8 pA.
After irradiation the first Ig-Vg sweep (i.e., 1n) is leftwards shifted due to Qp, which is recombined by electrons injected during the 1n measurement [18],[20]-[23]. A high excess DC leakage current appears in curves 2n and 3n between 2 V and 9
V, that is, before and inside the FN region. The curve 4n, which is the first negative measurement after a positive sweep
(3p), presents a transient current higher than 10 -11 A even at
|Vg|=1 V due to radiation induced donor defects in the oxide close to the gate/SiO
2
interface [33]. These defects are positively charged before 4n (i.e. after 3p) and become neutral by electron trapping, when electrons are injected from the gate in the SiO
2
layer during the 4n sweep. By repeating negative
Vg sweeps (5n and 6n), the transient current disappears and the excess DC leakage current decreases, but it remains higher than in the as received devices, even in the FN regime. After
1n, all the negative Ig-Vg sweeps are rightwards shifted with respect to the as received curve for |Vg|

9 V due to negative charge trapping in the oxide. In fact, electrons injected during the 1n sweep are trapped in radiation induced neutral defects causing negative charge trapping in the oxide. The negative trapped charge is stable at room temperature and no further electron trapping/detrapping was detected after 1n.
10 -6
10
10
10
-7
-8
-9
3n
4n
5n
6n
As received
1n
2n
10 -10
10 -11
10 -12
0 1 2 3 4 5 6 7 8 9 10
|Vg| (V)
Fig. 3. Negative Ig-Vg curves of the In series for devices biased at Vbn=-4.5
V during the 100 Mrad(Si) I irradiation: 1n (closed squares), 2n (closed diamonds), 3n (close triangles), 4n (open squares), 5n (open triangles), 6n
(open triangles). The solid line is the as received negative Ig-Vg sweep.
Positive Ig-Vg curves show characteristics similar to those found for negative sweeps (Fig. 4), but the DC leakage current in irradiated devices is smaller than in the previous case. The
1p and 4p curves, which are the first positive measurements after the negative 3n and 6n sweeps, respectively, present a transient current higher than 10 -11 A even at |Vg|=1 V. This excess current is due to radiation induced donor defects in the oxide close to the Si substrate [33], which are positively charged before 1p and 4p, and become neutral after substrate electron injection during 1p and 4p sweeps. By repeating positive Vg sweeps (2p, 3p, 5p, 6p), the transient current disappears. All the positive Ig-Vg measurements are rightwards shifted with respect to the as received curve for
|Vg|

8.5 V due to the oxide negative trapped charge which is stable at room temperature. Similar results have been found for

4 the other radiation doses and bias conditions.
10 -6
10
10
10
-7
-8
-9
4p
5p
6p
As received
1p
2p
3p
10 -10
10 -11
10 -12
0 1 2 3 4 5 6 7 8 9 10
Vg (V)
Fig. 4. Positive Ig-Vg curves of the In series for devices biased at Vbn=-4.5 V during the 100 Mrad(Si) I irradiation: 1p (closed squares), 2p (closed diamonds), 3p (close triangles), 4p (open squares), 5p (open triangles), 6p
(open triangles). The solid line is the as received positive Ig-Vg sweep.
10 -6
10 -7
10 -8
I ions
Si ions
10 -9
10 -10
10 -11
10 -12
3 4 5 6 7 8 9 10
|Vg| (V)
Fig. 5. Ig-Vg curve 2n of the In series for devices biased at Vbn=-4.5 V after
20 Mrad(Si) Iodine (close circles) and Silicon (open circles) irradiation. The solid line is the as received negative Ig-Vg curve.
The DC excess current we measured in both negative and positive sweeps has some similarities with the radiation induced leakage current (RILC) and will be addressed with the same name. RILC has been quantitatively simulated by inelastic single-trap assisted tunneling (S-TAT) in 4-nm oxides
[34]. In that work it was shown that only neutral traps close to the center of the oxide thickness effectively contribute to the
DC RILC. Other studies found that the S-TAT model can be applied to oxide thickness up to

6 nm, taking into account that direct tunneling current predominates for tox

3 nm [35] and oxide charge trapping is negligible for tox

6 nm [36]. In fact, in 10-nm oxides the tunneling probability to/from a trap 5 nm far from the interfaces is so low, that S-TAT could not support any DC current. Even by increasing the trap density we should observe only an increase of the negative trapped charge, which electrostatically hampers further electron injection. Hence multi-trap assisted tunneling (M-TAT) must be considered to justify RILC in 10-nm oxides, that is, electrons tunnel across the oxide through two or more traps.
Noticeably, DC stress induced leakage current (SILC) similar to Fig. 3 and Fig. 4 has never been observed in 10-nm devices after electrical stresses [37]-[41].
This means that a critical trap density is needed for M-TAT, which can only be produced by ionizing radiation. How LET affects the M-TAT generation has been studied by using different ions. The 2n curves of the In series for two MOS capacitors biased at Vbn and irradiated with 20 Mrad(Si) by I and Si ions are shown in Fig. 5. RILC is observed after I but not after Si ion irradiation owing to the different LET values.
Only dense ion tracks can produce locally a high defect density, enabling M-TAT. We cannot exclude that Si ions may induce RILC at higher doses, but we must remark that RILC has been never observed after electron and Si ion irradiation of
8-nm and 10-nm oxides [18],[22],[23].
D.
RILC kinetics and annealing
We define the excess RILC current as

Ig=Ig-Ig o
where Ig o and Ig are the gate current before and after irradiation, respectively. The positive (i.e., measured at positive Vg)

Ig was measured at Vg=6.4 V in the 2p curve of the Ip series; the negative one at Vg=-7 V in the 2n curve of the In series. In both cases RILC was measured just after the positive charge recombination.
10 -9
10 -10
10
-11
Negative excess current
10 -12
Positive excess current
Vbn
Vbp
Vb 0
10 -13
1 10 100 1000
Dose (Mrad(Si))
Fig. 6. Positive (triangles) and negative (circles) excess current as a function of the dose for devices biased at Vbp=3 V (open symbols), Vb
0
=0 V(dashed symbols) and Vbn=-4.5 V (closed symbols) during I irradiation.
The

Ig kinetics with radiation dose is shown in Fig. 6. The negative

Ig is larger for Vbn biased devices, while the positive

Ig is larger for the Vbp biased capacitors. The lowest

Ig values are always observed for the Vb 0 bias. This bias dependence of RILC in 10-nm oxide, we attributed to the
M-TAT microscopic mechanism, is opposite to that previously reported for 4-nm and 6-nm oxides [11],[42], where RILC was attributed to the S-TAT microscopic mechanism and the maximum RILC occurred at |Eox|=0 MV/cm. Quite curiously, the 10-nm oxide RILC bias dependence is instead similar to the radiation induced soft breakdown (RSB) found in irradiated 4-nm oxides [11], where the minimum RSB current is observed at |Eox|=0 MV/cm. The soft breakdown (SB) conduction in electrically stressed oxides has been attributed to percolative conduction through neighboring traps [43] and to multiple tunneling [44], that is, M-TAT. This result suggests that two or more defects are involved in the 10-nm oxide

5
RILC.
On the other side, the RILC increase with the oxide field indicates that the ion generated current discharging across the oxide contributes to enhance the local defect density. A close correlation appears between this result and what was previously found for radiation induced single event gate rupture (SEGR) [45],[46], where the applied field has a leading role in generating the conductive path across the oxide.
RILC follows a super-linear relation with the dose, at least for the highest dose values (Fig. 6). Again, this behavior differs from the results found in 4-nm and 6-nm oxides, where both RSB and RILC linearly increased with the dose [11],[42].
Possibly, in heavily damaged 10-nm oxides the generation of conductive paths can take advantage from the overlap of two or more tracks, which can produce high defect density needed for the establishment of these paths.
As shown in Fig. 3 and Fig. 4, RILC decreases when electrons are injected in the oxide. This process has been followed for the different radiation doses, as shown in Fig. 7.
Here the negative

Ig has been evaluated from the 2n
(

Ig_2n) and 6n (

Ig_6n) curves of the In series. The decrease of the negative

Ig is slightly larger in the Vbp biased devices.
We have calculated also the relative variation log (|

Ig_2n|/|

Ig_6n|), whose values are listed in Table II. All values lie in the range 0.14-0.27, even though |

Ig_2n| varies by two decades in Fig. 7. This suggests that the

Ig variations slightly depend on radiation dose and bias voltage.
10 -9
10 -10 unavailable for M-TAT are simply filled with electrons. Once the bias is removed and electrons are re-emitted from the defects, the leakage current rapidly returns to the original value. Assuming the E' center related model proposed by
Walters and Reisman for positive and negative charge trapping in SiO
2
[13],[17],[47] the dipolar and amphoteric neutral electron/hole trap may be a good candidate to be the defect responsible for RILC.
Dose (Mrad(Si)
10
20
50
TABLE II
E XCESS CURRENT DECREASE
Vbn
0.181
0.175
0.146
Vbp
Log(|

Ig_2n|/|

Ig_6n|)
0.204
0.251
0.266
Vb 0
0.184
0.182
0.178
100 0.138 0.270
75
70
65
60
55
50
B
A
Read at Vg=-7V
45
40
0.1
1 10 100 1000
Time (s)
Fig. 8. Gate current as a function of time at Vg=-7 V for devices biased at
Vbp=3 V after 100 Mrad(Si) I irradiation up to 100 s (measurement A) and
1000 s (measurement B). Measurements A and B were performed on the same device waiting 1 minute between acquisitions.
10 -9
Vbn 10 -11
Vbn
Vbp
Vb 0
10 -12
0 20 40 60 80 100
Dose (Mrad(Si))
Fig. 7. Negative excess current as a function of the I radiation dose for devices biased at Vbp=3 V (open symbols), Vb
0
=0 V (dashed symbols) and
Vbn=-4.5 V (close symbols), during the I irradiation, determined by the 2n,

Ig_2n, (circles) and 6n,

Ig_6n, (triangles) curves of the In series.
The RILC time decay has been followed in detail by measuring the Ig-time characteristics at Vg=-7 V after 100
Mrad(Si) (see Fig. 8). Curve "B" has been measured starting 1 minute after the end of measurement "A". A two-slope curve always appears: |Ig| rapidly decays during the first second and then the slope decreases. The initial point of curve "B" is higher than the final point but lower than the initial value of curve "A". These results indicate that, during electron injection, the density of tunneling sites decreases due to electron trapping and/or defect annealing. The small difference between curves "A" and "B" suggests that most of the defects
10
10
-10
-11
Vbp
0.1
1 10 100 1000
Time (s)
Fig. 9. Gate current as a function of time at Vg=-7 V for devices biased at
Vbp=3 V and Vbn=-4.5 V after 100 Mrad(Si) I irradiation up to 1000 s.
We remark that the main component of SILC after electrical stresses for tox

10 nm is a transient current, caused by charging/discharging of stress generated traps near the oxide interfaces: in a log-log plot SILC decreases linearly with time
[33], [38]-[40] and for 10-nm oxides the SILC DC component is extremely low (

10 -12 A/cm 2 ) [41]. On the contrary, Fig. 9 shows in a log-log plot that RILC only slightly decreases with time. RILC is not a displacement current but a true DC current

6 through the oxide by radiation induced defects.
Finally, no random telegraph noise peculiar of RSB activation [11] has been detected in the Ig-time measurements of Fig. 8 and Fig. 9. In the RSB conduction of thinner oxides few (if not just one) conductive paths are active across the oxide, each of them driving a substantial part of the total Ig current. In the 10-nm RILC case, many paths are active, each driving a tiny amount of the total Ig. When a conductive path switches on-off, it is clearly detectable by RSB but invisible in
RILC. In both cases the electron transport across some traps is likely but the global tunneling probability must be much lower for RILC (higher for RSB). The need of extremely high ion doses to generate RILC in 10-nm oxides indicates that a very high defect density must be produced, which is coherent with a picture needing a multi-trap transport from cathode to anode.
Only for very high defect density, the probability of having several traps aligned along a conductive path is high enough to give rise a measurable leakage current.
E.
Negative charge density and centroid
The density (

Qox_n) and the centroid measured from the gate (Xox_n) of the oxide negative charge due to electron trapping in radiation induced neutral traps (see subsection
III.B) have been determined after the In or Ip series by
DiMaria's method [48]:
 Qox=Cox/A·( 
V
FN
-

V
FN
+ ) (4)
Xox/tox=1/(1-

V
FN
/

V
FN
+ ) (5) where

V
FN
=V
FN
-V
FNo
and

V
FN
+ =V
FN
+ -V
Fno
+ . V
FNo
+ (V
FNo
) and V
FN
+ (V
FN
) are the gate voltages needed to sustain a positive (negative) FN current density, Jg + =100

A/cm 2 (Jg =-
100

A/cm 2 ) before and after irradiation, respectively.

Qox_n (see Fig. 10) linearly increases with the radiation dose up to 5.9

10 11 electrons/cm 2 and 6.3

10 11 electrons/cm 2 after 100 Mrad(Si) dose for the negative and positive biased devices, respectively.

Qox_n is slightly higher for Vbp than for Vbn, at least at high doses, but without any dependence on the Ip or In series. The effects of gate bias during irradiation on

Qox_n disappear at low doses.
The normalised centroid of the negative charge (Xox_n/tox) is shown in Fig. 11, zoomed on the central part of the oxide: 0 corresponds to the gate/SiO
2
interface. Data are slightly dispersed for low doses, but the centroid is always close to the center of the oxide and slightly shifted to the SiO
2
/Si interface
(Xox_n/tox

0.55) at least for the higher doses. This suggests that the radiation induced neutral electron trap distribution may be not perfectly symmetric, even if it is close to the uniform one (Xox/tox=0.50).
The ratio

Qeff_n/

Qeff_p, that is, the probability that an effective negative trapped charge (due to electron trapping in radiation induced neutral traps) is generated with respect to a positive one, is shown in Fig. 12.

Qeff_n can be deduced from equation (3):

Qeff_n=
 Qox_n·Xox_n/tox (6) by using the data shown in Fig. 11 and Fig. 12.

Qeff_p has been previously shown in Fig. 1.
e
8
6
4
0.60
0.50
Vbp In
Vbp Ip
Vbp In
Vbp Ip
2
Vbn In
Vbn Ip
0
0 20 40 60 80 100
Dose (Mrad(Si))
Fig. 10. Negative trapped charge density as a function of the I radiation dose for devices biased at Vbp=3 V (squares), Vb 0 =0 V (circles), and Vbn=-4.5 V
(triangles) after the In (solid symbols) and Ip (open symbols) measurement series.
0.70
0.40
0 20 40 60 80 100
Dose (Mrad(Si))
Fig. 11. Negative trapped charge normalized centroid as a function of the I radiation dose for devices biased at Vbp=3 V (squares), Vb 0 =0 V (circles), and Vbn=-4.5 V (triangles) after the In (solid symbols) and Ip (open symbols) measurement series.
0.5
0.4
0.3
0.2
0.1
Vbn In
Vbn Ip
Vp In
Vbp Ip
Vbn In
Vbn Ip
0.0
0 20 40 60 80 100
Dose (Mrad(Si))
Fig. 12.

Qeff_n/

Qeff_p values as a function of the I radiation dose for devices biased at Vbp= 3 V (squares), Vb 0 = 0 V (circles), and Vbn=-4.5 V
(triangles) after the In (solid symbols) and Ip (open symbols) measurement series.

Qeff_n/

Qeff_p increases by increasing the radiation dose as a consequence of the positive charge saturation effect. This can imply that at higher doses there are not really more electron traps than hole traps, but rather more negative charge

7 in relation to positive charge. This may be due to the difference in trapping cross sections for holes and electrons
(i.e., hole trapping saturates first due to large cross section while electron trapping continues to grow by increasing the dose further). This can also support the E' model proposed by
Walters and Reisman [17]: E' centers first trap a hole to become positively charge, than trap an electron to become neutral, and finally trap another electron to become negatively charged. conduction can determine the different RSB characteristics for
Vg>0 and Vg<0.
10 krad(Si)
IV.
3NM OXIDES
The positive and negative Ig-Vg measurements of 3-nm oxides before and after I irradiation are shown in Fig. 13.
Before irradiation, the main contribution to Ig comes from the electron direct tunneling current [44],[49] and the shape of the
Ig-Vg curves is quite different from the FN current found in
10-nm oxides.
10 -7
10 -8
10 -9
10 -10
10 -11
10 -12
As received
10 krad(Si)
As received
10 -13
-3.5
-2.5
-1.5
-0.5
0
0.5
1.5
2.5
Vg (V)
Fig. 13. Negative (left side) and positive (right side) Ig-Vg measurements before and after 100 rad(Si), 300 rad(Si), 1 krad(Si), 3 krad(Si) and 10 krad(Si) I ion irradiation, for devices biased at Vbp=1.33.
The gate current increases even just after the first radiation steps, i.e., 100 rad(Si) and 300 rad(Si). Variations are seemingly larger in the negative characteristics and increase with the dose. The instabilities of the negative Ig-Vg curve after 3 krad(Si) are characteristics of RSB [11], due to the activation and deactivation of the conductive spots. These effects are not clearly visible on the positive Ig-Vg sweeps, due to the high positive current.
Nevertheless, the positive and negative excess currents (Fig.
14) can both be fitted by a power law |

Ig|=a

|Vg| b and are very close each other, at least for the 3 krad(Si) and 10 krad(Si) doses, confirming that RSB has been activated
[43],[44].
The match between the positive and negative excess currents is low for the first irradiation steps (100 rad(Si)-1 krad(Si)), suggesting that at very low doses the DC leakage current can be influenced by weak spots, whose conduction slightly depends on Vg (asymmetric conduction). The
"asymmetric conduction" may derive from statistical fluctuations of density, energy and/or spatial distribution of the defects generated along the ion track. At very low doses, when the excess DC leakage current is small, a single conductive path (driving most of the DC leakage current) with asymmetric
0.0
1.0
2.0
3.0
|Vg| (V)
Fig. 14. Excess current for positive (thin line) and negative (thick line) Ig-Vg measurements after 100 rad(Si), 300 rad(Si), 1 krad(Si), 3 krad(Si) and 10 krad(Si) I ion irradiation, for devices biased at Vbp=1.33 V.
10 -7
10 -8 212 Mrad(Si)
10 -9
10 -10
10 -11
As received
As received
10 -12
10 -13
-3.5
-2.5
-1.5
-0.5
0 0.5
1.5
2.5
Vg (V)
Fig. 15. Negative (left side) and positive (right side) Ig-Vg measurements before and after 4Mrad(Si), 10 Mrad(Si), 50 Mrad(Si) and 212 Mrad(Si) Si ion irradiation for devices biased at Vbp=1.33 V.
212 Mrad(Si)
212 Mrad(Si)
0.0
1.0
2.0
3.0
|Vg| (V)
Fig. 16. Excess current for positive (thin line) and negative (thick line) Ig-Vg measurements after 4 Mrad(Si), 10 Mrad(Si), 50 Mrad(Si) and 212 Mrad(Si)
Si ion irradiation, for devices biased at Vbp=1.33 V.
Much higher radiation doses, similar to those used in 10-nm oxides, have been considered for Si ion irradiation of 3-nm oxides. The corresponding positive and negative Ig-Vg curves are shown in Fig. 15. |Ig| increases with the dose, and the variation is again more evident for the negative sweeps.
Noticeably, the current shape after Si irradiation is quite

8 different from that observed after I ions, shown in Fig. 13. No
RSB instabilities and associated noise are now observed, suggesting a different microscopic conduction mechanism after high dose Si irradiation.
This is confirmed by the excess current characteristics shown in Fig. 16. On one side, the Si positive and negative
|

Ig| curves do not overlap and they cannot be fitted by a power law, hence indicating that no RSB has occurred. On the other side, the positive |

Ig| is higher than the negative one for devices positively biased during irradiation suggesting a
RILC-like conduction mechanism [11],[42]. Taking into account that |

Ig| values for I and Si irradiation in Fig. 14 and
Fig. 16, respectively, are comparable, but the dose range are completely different, our results show that in 3-nm oxides the I and Si ion microscopic damage distribution is different:
- high LET ions create highly damaged regions in SiO
2 localized along the particle tracks, resulting in non-linear RSB resistive paths even at low doses; in fact, RSB may be generated by a single ion;
- low LET ions generate a low defect density along the particle track, which are unable to sustain RSB conduction.
Still, by increasing the dose the gate current can reach values comparable to RSB. However, the conduction does not take place in localized conduction paths across the oxide as in RSB
[11],[43], but it is more uniformly distributed over the MOS area, similar to the RILC conduction. This difference is schematically depicted in Fig. 17.
Remarkably, the defect distribution observed after Si irradiation in 3-nm oxides has not been produced by any electrical stresses, where soft breakdown or catastrophic breakdown is reached before the onset of the phenomena observed in this work. SILC decreases in oxides thinner than 5 nm [50] and in 3-nm oxide it can modify the Ig-Vg curves only in the low voltage region (-1.5 V

Vg

0.5 V) [44], without reaching the high RILC degradation shown in Fig. 15. Even in this case, no electrical stress can produce such leakage current in ultra-thin oxides.
In 10-nm oxides, heavy ions produce positive charges whose density logarithmically grows with the radiation dose.
Contrary to what is observed in thicker oxides, the effective positive charge is higher for devices negatively biased during irradiation, due to the lower positive charge recombination by electron tunneling from the Si substrate. Positive charge is easily neutralized by Fowler-Nordheim electron injection, which also produces negative charge trapping in radiation induced neutral electron traps and points out the presence of donor-type traps at both oxide interfaces. The negative charge density linearly increases with the dose and its centroid is close to the oxide center. The donor-type defects contribute to the transient leakage current measured when the gate polarity is reversed. After high dose irradiation a DC RILC is measured, which was never observed in oxides thicker than 7-8 nm after electrical stresses or low LET irradiation. RILC kinetics are super-linear with the dose and the lowest RILC is observed for zero bias during irradiation: these results are in contrast with previous data on thinner oxides. RILC bias dependence is similar to that of RSB, suggesting that RILC conduction mechanism in 10-nm oxides should not be attributed to singletrap assisted tunneling, but instead to multi-trap assisted tunneling. RILC decreases in time during measurement, due to electron trapping in defects responsible for tunneling.
However, most of the trapped electrons can be easily reemitted by the oxide traps when gate bias is removed and
RILC reversibly reaches values close to the initial ones.
In 3-nm oxides few I ions can produce RSB conductive paths. On the other hand, Si ions cannot generate RSB even at high doses. However, the gate current can reach values comparable to that of RSB current, owing to the accumulation of defects generating a RILC-like conduction mechanism.
Even in this case, radiation appears to be a unique stress capable for introducing extremely high defect densities in thin oxides without driving them into breakdown.
( A ) ( B ) ( C )
Fig. 17. Model for the conduction in 3-nm oxides: by direct tunneling before irradiation (A), by localized low resistive path after I ion irradiation (B), by direct tunneling probability increase after Si ion irradiation (C).
V.
C ONCLUSIONS
In this paper we have investigated the radiation induced damage in 10-nm and 3-nm oxides after I and Si ion irradiation at low (

3.3 MV/cm) and moderate (

4.4 MV/cm) oxide fields during irradiation, respectively.
VI.
R EFERENCES
[1] S. M. Sze, Modern Semiconductor device physics, John Wiley & Sons,
1998.
[2] G. Anelli, M. Campbell, M. Delmastro, F. Faccio, S. Florian, A.
Giraldo, E. Heijne, P. Jarron, et al., "Radiation tolerant VLSI circuits in standard deep submicron CMOS technologies for the LHC experiments: pratical design aspects," IEEE Trans. Nucl. Sci., vol. 46, pp. 1690-1696,
December 1999.
[3] A. H. Johnston, "Radiation effects in Advanced Microelectronics
Technologies," IEEE Trans. Nucl. Sci., vol. 45, pp. 1339-1353, June
1998.
[4] C. Conneely, B. O'Connell, P. Hurley, W. Lane and L. Adams,
"Startegies for millirad sensitivity in PMOS dosimeters," IEEE Trans.
Nucl. Sci., vol. 45, pp. 1475-1480, June 1998.
[5] R. A. B. Devine, "Radiation induced structural changes in amorphous
SiO
2
: I. point defects," Jpn. J. Appl. Phys., vol. 31, pp. 4411-4421,
December 1992.
[6] J. M. Aitken, "Radiation induced trapping centers in thin silicon dioxide films," J. Non-Cryst. Solids, vol. 40, pp. 31-47, 1980.
[7] R. A. B. Devine, F. Ferrieu and A. Golanski, "A study of Ar implantation induced defects in SiO
2
," Nucl. Instr Meth., vol. 209/210, pp. 1201-1206, 1983.
[8] M. Ceschia, A. Paccagnella, M. Turrini, A. Candelori, G. Ghidini and J.
Wyss, "Heavy ion irradiation of thin oxides," IEEE Trans. Nucl. Sci., vol. 47, pp. 2648-2655, December 2000.

9
[9] K. F. Schuegraf and C. Hu, "Hole injection SiO
2
breakdown model for very low voltage lifetime extrapolation," IEEE Trans. Electr. Dev., vol.
41, pp. 761-767, May 1994.
[10] E. Rosenbaum and L. F. Register, "Mechanism of stress-induced leakage current in MOS Capacitors," IEEE Trans. Electr. Dev., vol. 44, pp. 317-323, Febraury 1997.
[11] M. Ceschia, A. Paccagnella, S. Sandrin, G. Ghidini, J. Wyss, M. Lavale and O. Flament, "Low field leakage current and soft breakdown in ultrathin gate oxides after heavy ions, electrons or x-ray irradiation," IEEE
Trans. Nucl. Sci., vol. 47, pp. 566-573, June 2000.
[12] T. P. Ma and P. V. Dressendorfer, Ionizing radiation effects in MOS devices and circuits , John Wiley & Sons, 1989.
[13] T. R. Oldham, Ionizing radiation effects in MOS oxides , World
Scientific, 2000.
[14] J. M. Benedetto, H. E. Boesh, F. B. McLean and J. P. Mize, "Hole removal in thin-gate MOSFETs by tunneling", IEEE Trans. Nucl. Sci., vol. 32, pp. 3916-3920, December 1985.
[15] T.R. Oldham, A. J. Lelis and F. B. McLean, "Spatial dependence of trapped holes determined from tunneling analysis and measured annealing ", IEEE Trans. Nucl. Sci., vol. 33, pp. 1203-1209, December
1986.
[16] H. E. Boesh, F. B. McLean, J. M. Benedetto and J. M. McGarrity,
"Saturation of threshold voltage shift in MOSFET's at high total dose",
IEEE Trans. Nucl. Sci., vol. 33, pp. 1191-1197, December 1986.
[17] M. Walters and A. Reisman, "Radiation-induced neutral electron trap generation in electrically biased insulated gate field effect transistor gate insulators," J. Electronchem. Soc., vol. 138, pp. 2756-2762, September
1991.
[18] A. Candelori, A. Paccagnella, M. Cammarata, G. Ghidini and P. G.
Fuochi, "Fowler-Nordheim characteristics of electron irradiated MOS capacitors," IEEE Trans. Nucl. Sci., vol. 45, pp. 2383-2390, December
1998.
[19] M. Lenzlinger and E. H. Snow, "Fowler-Nordeim tunneling into thermally grown SiO
2
," J. Appl. Phys., vol. 40, pp. 278-283, January
1969.
[20] A. Scarpa, A. Paccagnella, F. Montera, A. Candelori, G. Ghibaudo, G.
Pananakakis, G. Ghidini and P. G. Fuochi, "Modifications of Fowler-
Nordheim injection characteristics in

irradiated MOS devices," IEEE
Trans. Nucl. Sci., vol. 45, pp. 1390-1395, June 1998.
[21] A. Candelori, A. Paccagnella, A. Scarpa, G. Ghidini and P. G. Fuochi,
"Degradation of electron irradiated MOS capacitors," Microel. Reliab., vol. 39, pp. 227-233, 1999.
[22] A. Candelori, A. Paccagnella, M. Cammarata, G. Ghidini and M.
Ceschia, "Electron irradiation effects on thin MOS capacitors," J. Non-
Cryst. Solids., vol. 245, pp. 238-244, 1999.
[23] A. Candelori, A. Paccagnella, G. Raggi, J. Wyss, D. Bisello and G.
Ghidini, "High energy Si ion irradiation effects on 10 nm thick oxide
MOS capacitors," J. Non-Cryst. Solids , vol. 280, pp. 193-201, 2001.
[24] J. A. Lopez-Villanueva, J. A. Jimenez-Tejada, P. Cartujo, J. Bausells and J. E. Canceller, "Analysis of the effects of constant-current Fowler-
Nordheim-tunneling injection with charge trapping inside the potential barrier," J. Appl. Phys., vol. 70, pp.3712-3720, October 1991.
[25] A. El-Hdiy, G. Salace, C. Petit, M. Jourdain and D. Vuillaume,"Study of defects induced by high-electric-field stress into a thin gate oxide (11 nm) of metal-oxide-semiconductor capacitors," J. Appl. Phys., vol. 74, pp.1124-1130, July 1993.
[26] P. S. Ku and D. K. Schroder,"Charges trapped throughout the oxide and their impact on the Fowler-Nordheim current in MOS devices," IEEE
Trans. Electr. Dev., vol. 41, pp.1669-1672, September 1994.
[27] S. Elrharbi, M. Jourdain and A. Meinertzhagen,"Effect of tunneling electrons in Fowler-Nordheim regime on the current-voltage characteristics and model of degradation of metal-oxide-semiconductor capacitors," J. Appl. Phys., vol. 76, pp.1013-1020, July 1994.
[28] S. Elrharbi and M. Jourdain,"Influence of the oxide charge build-up during Fowler-Nordheim stress on the current-voltage characteristics of metal-oxide-semiconductor capacitors," J. Non-Cryst. Solids, vol. 187, pp.175-180, 1995.
[29] J. M. Aitken and D. R. Young,"Electron trapping by radiation-induced charges in MOS devices," J. Appl. Phys., vol. 47, pp.1196-1198, March
1976.
[30] A. Reisman, C. K. Williams and J. R. Maldonado,"Generation and annealing of defects in silicon dioxide," J. Appl. Phys., vol. 62, pp.868-
874, August 1987.
[31] M. Walters and A. Reisman,"The distribution of radiation-induced charged defects and neutral electron traps in SiO
2
, and the threshold voltage shift dependence on oxide thickness," J. Appl. Phys., vol. 67, pp.2992-3002, March 1990.
[32] A. Aassime, G. J. Sarrabayrouse, G. Slace and C. Petit, "Irradiation effects on the high field behaviour of very thin silica layers," Solid State
Electr., vol. 41, pp.945-949, 1997.
[33] A. Meinertzhagen, C. Petit and M. Jourdain, "Stress-induced leakage current reduction by a low field of opposite polarity to the stress field,"
J. Appl. Phys., vol. 84, pp.5070-5079, November 1998.
[34] L. Larcher, A. Paccagnella, M. Ceschia and G. Ghidini, "A model of radiation induced leakage current (RILC) in ultra-thin gate oxides,"
IEEE Trans. Nucl. Sci., vol. 46, pp. 1553-1561, December 1999.
[35] T. Endoh, T. Chiba, H. Sakuraba, M. Lenski and F. Masuoka, "A qualitative analysis of stress-induced leakage currents and extraction of trap properties in 6.8 nm ultrathin silicon dioxide films," J. Appl. Phys., vol. 86, pp.2095-2099, August 1999.
[36] M. Itsumi, Y. Sato, S. Nakayama and S. Muramoto, "Current increase in thin gate-oxide (3.5-7.0 nm) metal-oxide-silicon structures with the boron-doped polycrystalline-silicon gates biased negatively," J. Appl.
Phys., vol. 82, pp.4629-4636, November 1997.
[37] D. J. Dumin and J. R. Maddux, "Correlation of stress-induced leakage current in thin oxides with trap generation inside the oxides," IEEE
Trans. Electr. Dev., vol. 40, pp.986-993, May 1993.
[38] E. F. Runnion, S. M. Gladstone, R. S. Scott, D. J. Dumin, L. Lie and J.
C. Mitros, "Thickness dependence of stress-induced leakage currents in silicon oxide," IEEE Trans. Electr. Dev., vol. 44, pp.993-1001, June
1997.
[39] S. M. Gladstone and D. J. Dumin, "Thickness dependence of thin oxide wearout," Solid State Electr., vol. 42, pp.317-324, 1998.
[40] T. Endoh, K. Shimizu, H. Iizuka and F. Masuoka, "A new write/erase method to improve the read disturb characteristics based on the decay phenomena of stress leakage current for flash memories," IEEE Trans.
Electr. Dev., vol. 45, pp.98-104, June 1998.
[41] B. De Salvo, G. Ghibaudo, G. Pananakakis, B. Guillaumot and G.
Reimbold, "A general bulk-limited transport analysis of a 10 nm-thick oxide stress-induced leakage current," Solid State Electr., vol. 44, pp.895-903, 2000.
[42] M. Ceschia, A. Paccagnella, A. Cester, A. Scarpa and G. Ghidini,
"Radiation induced leakage current and stress induced leakage current in ultra-thin gate oxides," IEEE Trans. Nucl. Sci., vol. 45, pp. 2375-
2382, December 1998.
[43] E. Miranda, J. Suné, R. Rodriguez, M. Nafria, X. Aymerich, L. Fonseca and F. Campabadal, "Soft breakdown conduction in ultrathin (3-5 nm) gate dielectrics," IEEE Trans. Electr. Dev., vol. 47, pp.82-89, January
2000.
[44] M. Depas, T. Nigam and M. M. Heyns, "Soft breakdown in ultra-thin gate oxide layers," IEEE Trans. Electr. Dev., vol. 43, pp. 1499-1504,
September 1996.
[45] F. W. Sexton, D. M. Fleetwood, M. R. Shaneyfelt, P. E. Dodd, G. L.
Hash, L. P. Schanwald, R. A. Loemker, K. S. Krisch, et al. “Precursor ion damage and angular dependence of single event gate rupture in thin oxides”,
IEEE Trans. Nucl. Sci., vol. 45, pp. 2509-2518, 1998.
[46] A. H. Johnston, G. M. Swift, T. Miyahira and L. D. Edmonds
“Breakdown of gate oxides during irradiation with heavy ions”, IEEE
Trans. Nucl. Sci., vol. 45, pp. 2500-2508, 1998.
[47] P. M. Lenahan et al. “Identification of Atomic Scale Defects Involved in
Oxide Leakage Currents”, 2000 IEEE Integrate Reliability Workshop,
IEEE #00TH8515 , pp. 112-115.
[48] D. J. DiMaria,"Determination of insulator bulk trapped charge densities and centroids from photocurrent-voltage characteristics of MOS structures," J. Appl. Phys., vol. 47, pp.4073-4077, September 1976.
[49] B. Riccò, G. Gozzi and M. Lanzoni, "Modeling and simulation of stressinduced leakage current in ultrathin SiO
2
films," IEEE Trans. Electr.
Dev., vol. 45, pp.1554-1560, July 1998.
[50] A. I. Chou, K. Lai, K. Kumar, P. Chowdhury and J. C. Lee, "Modeling of stress-induced leakage current in ultrathin oxides with the trapassisted tunneling mechanism," App. Phys. Lett., vol. 70, pp.3407-3409,
1997.