Nuclear Instruments and Methods in Physics Research B 210 (2003) 221–226

www.elsevier.com/locate/nimb

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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