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Nuclear Instruments and Methods in Physics Research B 210 (2003) 221–226
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Comparison of the ion induced charge collectionin Si epilayer and SOI devices
Toshio Hirao a,*, Hidenobu Mori b, Jamie Stuart Laird a,Shinobu Onoda b, Hisayoshi Itoh a
a Department of Materials Development, Radiation Engineering Division, Japan Atomic Energy Research Institute,
1233 Watanuki, Takasaki, Gunma 370-1292, Japanb Graduate Engineering, Applied Science, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan
Abstract
It is known that the single-event phenomena (SEP) are the malfunction of micro electronics devices caused by the
impact of an energetic heavy ion. Improving the tolerance of devices to the SEP requires a better understanding of basic
charge collection mechanisms on the timescales of the order of picoseconds. In order to better elucidate these mech-
anisms, we measure the fast transient current resulting from heavy ion strikes with a fast sampling data collection
system and a heavy ion microbeam line at JAERI.
In this paper we report on differences in both the transient current and charge collection from 15 MeV carbon ions on
silicon-on-insulator, Si epilayer and bulk pþn junction diodes and charge transportation under MeV ion injection is
discussed.
2003 Elsevier B.V. All rights reserved.
Keywords: Single-event phenomena; Heavy-ion microbeam; Collected charge; Funneling effect
1. Introduction
Single-event upset (SEU) is triggered when
electric charges induced by energetic ion incidence
upon an integrated memory circuit are collected at
its electrode and the amount of collected charges
exceeds a value known as a critical charge ofthe circuit within a very short time period. There-
fore, for the improvement of SEU tolerance, un-
derstanding of the charge collection mechanism is
necessary which includes charge generation, re-
* Corresponding author. Tel.: +81-27-346-9324; fax: +81-27-
346-9687.
E-mail address: hirao@taka.jaeri.go.jp (T. Hirao).
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reser
doi:10.1016/S0168-583X(03)01011-5
combination and transport along ion tracks in
semiconductor [1,2].
Measurements of SEU current transients and
accurate evaluation of the amount of collected
charges are strongly required. Because SEU cur-
rent transients vary in a picosecond time scale
depending on the location of ion impingement ona device, such sophisticated experimental tech-
niques are required as accurate positioning of in-
cident ion at a desired area of the circuit and
precise waveform measurement of the high speed
transient currents.
Several attempts have been made for the direct
measurements of extremely fast strikes on the
diodes. Charge collection mechanisms such as
ved.
222 T. Hirao et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 221–226
drift, funneling and diffusion effects have been
studied [3–8].
In this paper, we present the results of current
transient measurements for the silicon-on-insula-tor (SOI) pþn junction diode and compare them
with those obtained for Si epilayer and Si bulk
one. Effectiveness of the SOI device for SEU
hardening is discussed.
Al-Electrode (0.5µm)
n epi-lay3 µm
400
µm
Back contact
Nd=1x1016
(b) Si Epi p+n j
Cross Sectiona
SiO2
Al-Electrode
Buried
n+layerp+layer
(a) SOI p+n
625
µm3
µm
Back contact
SSiO2
Cross
Fig. 1. Schematic of silicon pþn junction SOI diode and silicon ep
(a) Structure of silicon SOI pþn junction diodes, (b) structure of silic
2. Experiment method
Experiments were performed on three kinds oftest diodes that were designed to have low capa-
citances. The first sample was SOI pþn junction
diodes with junction area of 50 lm in diameter and
depth of 0.5 lm. The top and cross sectional views
of the diodes are shown schematically in Fig. 1(a).
er
SiO2
cm-3
n +Substrate
uncton diode
l View
Top Silicon Layer
Al-Electrode(Contact)
oxide
n+layer
n+Substrate
junction diode
SiO2iO2
Sectional View
ilayer pþn junction diode for single-event upset experiments.
on pþn junction epilayer diode.
T. Hirao et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 221–226 223
The thickness of the n-type top Si layer with the
resistivity of 1–5 X cm, the buried oxide layer, and
the n-type Si substrate with the resistivity of 1–50
X cm are 3.0, 0.19 and 630 lm, respectively. Thesecond sample was a silicon pþn junction diode
with an epilayer structure, as shown in Fig. 1(b).
The epilayer thickness was 3 lm and its doping
level was 4 1015 cm3. The third sample was sili-
con pþn junction bulk diodes with junction area of
50 lm in diameter and depth of 0.5 lm.
The pþ region, a circuit region of 50 lm in dia-
meter and 0.5 lm in depth, was formed by boronimplantation with a dose 1 1015 ions/cm2. The
surface on test diodes except contact regions was
covered with a 0.5 lm thick field oxide. The
backside of the Si substrate was also contacted
with aluminum.
To obtain an accurate positioning of a single ion
hit on the samples, we used heavy ion microbeam
connected to a 3 MV tandem accelerator with afocused spot size of about 1 lm [9]. The samples
were mounted on a chip carrier with a 50 Xdouble-end microstripline [10]. The stripline was
connected with 50 X co-axial cable to a 40 GHz
bias tee and a sampling oscilloscope was used to
digitize the transient signal. The experimental set-
up is schematically shown in Fig. 2. The ion flu-
ence was adjusted with an attenuator installed onthe injection beam line of the accelerator. The
Bias Tees
Amplifier
Vacuum ChamberQuadrupole
Focusing Lens
SEU diode
Bias Supply
50Ω
Delay Line (47
Fig. 2. Schematics of a single-event tran
number of incident ions was counted using a
Faraday cup or a semiconductor detector placed in
the irradiation chamber.
3. Results and discussion
Fig. 3 shows the waveforms of the transient
currents induced by 15 MeV carbon ions in the
silicon bulk and SOI test samples under reverse
bias condition from 3 to 30 V. The peak-height
and the fall time width of the current pulse in-crease with increasing bias voltage. In the case of
silicon bulk diode, as shown in Fig. 3, the peak-
height of transient current waveform with a bulk
substrate is lager than that of the transient current
with SOI. The value of peak-height with a bulk
substrate shows a value three times higher than
that induced in the SOI diodes. The peak-height,
the full width at half-maximum (FWHM), falltime Tf (90–10%) and charge collection time Tc (fullwidth at 10%) of the transient currents for carbon
ion strikes are tabulated in Table 1. Collected
charges obtained by integrating the prompt com-
ponents of the transient currents are also listed in
the table.
Fig. 4 shows the relationship between bias
voltage and the amount of charge collected in thethree kinds of test samples, which are derived from
Oscilloscope
Signal
Trigger
Microslits
Preslits
ns)
sient current measurement system.
-200
0
200
400
600
800
1000
1200
1400
0(a)
(b)
0.5 1 1.5 2Time [ ns ]
Sample: Si-Bulk
-30 V
-25 V
-20 V
-15 V
-10 V
-5 V
-3 VTra
nsie
nt C
urre
nt [
µA
]
Time [ ns ]
Tra
nsie
nt C
urre
nt [
µA
]
-200
0
200
400
600
800
1000
1200
1400
-20 V
-5 V
-3 V
Sample: SOI 3 µm p+n junction diode
1.00 0.5 1.5 2.0
Fig. 3. The transient current as a function of bias voltage as
measured using a 15 MeV carbon ion on silicon bulk and SOI
samples.
224 T. Hirao et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 221–226
the transient current waveform in Fig. 3. In the
case of SOI pþn junction diode, as shown in Fig. 4,
the collected charge is held constant. On the other
hand, in the case of the epilayer diode and bulkdiode, the collected charge increases with increas-
ing bias voltage. In the case of silicon epilayer
diode, the collected charge increase rapidly at the
reverse bias voltage above 15 V. This reason re-
sults in the breakdown due to the avalanche effect
increasing of leaking current generated by heavy
ion injection in the silicon devices.We introduce a funneling factor defined by
QT=QD, in order to make clear the effect of the
funneling, where QT is the total amount collected
charge which can be obtained from the time inte-
gral of the current waveform by applying a cor-
rection factor related to the attenuation of the
signal intensity through the cable. QD is the charge
calculated for the depletion layer. The idealamount of charges, QC, collected from the deple-
tion layer are theoretically derived by the follow-
ing equation,
QC ¼ q LET qðSiÞ L=W ; ð1Þ
where q is unit charge (1.602 1019 C), LET
(linear energy transfer) in MeV/(mg/cm2), q(Si) thedensity of silicon in g/cm3, L the depletion layer
thickness and W the energy necessary for creating
one electron–hole pair in silicon. When the fun-
neling factor is less than unity, charges are col-
lected mostly from the depletion layer. Whereas,
when the funneling factor is greater than unity,
additional charges are collected from the substrate
by the funneling effects. We obtained funnelingfactors for various bias conditions as shown in
Table 1. The funneling factor is 2.0–4.0 for 15
MeV carbon ion. From obtained results, the
charge collection length could be estimated with
the relationship between the funneling factor and
the depletion layer.
Fig. 5 shows the relationship between bias and
charge collection length. As shown in Fig. 5, thecharge collection length increases with increasing
bias voltage. Also the charge collection length with
used samples is about 6 lm, whereas the penetra-
tion depth of 15 MeV carbon ions in Si is about 14
lm. This indicates that the collection length is 43%
of projection length in silicon.
Fig. 6 shows the relation between bias and an
inverse charge collection length. As shown in Fig.6, an inverse charge collection length decreases
with increasing bias voltage in a similar manner to
the reciprocal of the electric field in the depletion
Table 1
Summary of experimental results for three kinds in silicon diodes
Bias [V] FWHM
[ns]
Tf [ns] Tc [ns] peak-height
[lA]
QT [fC] QD [fC] F K
Bulk
)3 0.203 0.750 0.875 297 120 41.7 2.81
)5 0.203 0.688 0.828 347 144 54.5 2.63
)10 0.188 0.531 0.688 471 174 77.1 2.25
)15 0.172 0.500 0.625 596 189 92.8 2.04
)20 0.156 0.422 0.516 760 202 109 1.85
)25 0.156 0.375 0.500 942 242 122 1.89
)30 0.141 0.375 0.469 1195 272 132 2.06
Epi
)3 0.172 0.421 0.500 563 136 30.4 4.49
)5 0.156 0.406 0.469 623 142 37.4 3.81
)10 0.141 0.406 0.469 696 150 50.8 2.96
)15 0.109 0.328 0.391 826 160 61.3 2.60
)20 0.125 0.422 0.484 890 205 70.3 2.92
)22.5 0.141 0.516 0.578 990 260 74.4 3.49
SOI
)3 0.357 0.429 0.690 204 94.8 47.7 1.98
)5 0.262 0.405 0.619 253 102 58.3 1.78
)10 0.214 0.476 0.643 302 105 79.4 1.32
)15 0.190 0.286 0.452 342 101 95.8 1.05
)20 0.190 0.309 0.500 342 100 110 –
0
2
4
6
8
10
0 -5 -10 -15 -20 -25 -30
Bias Voltage [ V ]
C-ion , 15 MeV
Cha
rge
Col
lect
ed L
engt
h [
µm ]
SOI Sample
Si-Epi Sample
Si-Bulk Sample
Fig. 5. Relation between the bias voltage and charge collection
length for carbon ion incidence.
0
100
200
300
400
500
0 -5 -10 -15 -20 -25 -30
Bias Voltage [ V ]
C-ion, 15MeV
SOI Sample
Si-Ep Sample
Si-Bulk
Col
lect
ed C
harg
e [
fC ]
Fig. 4. Relation between the bias voltage and collected charge
for carbon ion incidence.
T. Hirao et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 221–226 225
0
0.1
0.2
0.3
0.4
0.5
0 -5 -10 -15 -20 -25 -30
Bias Voltage [ V ]
1/QT( SOI )
1/QT( Bulk )
1/QT( Epi )
1/E
0.5
0.4
0.3
0.2
0.1
0
Rat
io
1/E
[ 1
/V/µ
m ]
Fig. 6. Relation between the bias voltage and inverse QT (SOI,
EPI, Bulk) for carbon ion incidence.
226 T. Hirao et al. / Nucl. Instr. and Meth. in Phys. Res. B 210 (2003) 221–226
layer, however the reverse of charge collection
length with SOI and epilayer diodes are not de-
pendent on electric fields.
4. Conclusion
By applying focused heavy-ion microbeams and
a bandwidth digitizing sampling technique, we
measured the effect of the structures of silicon pþn
junction diodes on the waveform of single-event
transient current and the collected charge. Wefound that SOI structure was effective to increase
the radiation hardness electronic devices used in
the space environment.
Acknowledgements
The authors would like to acknowledge Mr.
Takada and members of JAERI tandem accele-
rator group who spent many long hours operating
the accelerator for us.
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