Facility for Heavy-Ion Irradiation of Semiconductors
at RIKEN RI-Beam Factory
Tadashi KAMBARA* and Atsushi YOSHIDARIKEN Nishina Center, Wako, Saitama 351-0198, Japan
Abstract
RIKEN RI-Beam Factory (RIBF) provides fast Kr and Ar ions to private companies in Japan for the SEE evaluations of space-use semiconductors.
The samples can be irradiated in the atmosphere, and LET and flux can be selected from wide range of values.
We present the irradiation facility and measurements of the beam characteristics, and discuss beam-impurity nuclides produced in the upstream
materials through radiochemical analyses and ion-transport simulations.

RIKEN RI-Beam Factory (RIBF)
RIBF [1] is a heavy-ion accelerator facility which features high intensity beams of ions of allelements with energy up to 345 MeV/u and various radioactive isotope beams (RI beams).

Irradiation
Beamline

RRC

RIBF is mainly used for experiments in nuclear physics, materials sciences, chemistry, andradiation biology.
In 2014, RIBF started to provide fast heavy ions up to 95 MeV/u for fee to private companies
which evaluate single-event effects (SEEs) of space-use semiconductors.

Beam and Facility for Semiconductor Irradiation•••40Ar and 84Kr beams have been inpractical use, while 136Xe and 197Au
beams are being tested.
Samples are irradiated in air: easy
access, quick exchange and simple
electric connections to the device
under test.
LET can be set between 2.2 and 41
MeV/(mg/cm2) by selection of the ion
species and with an energy degrader.

Beam Energy(MeV/u)Ions

LET in Si[MeV/(mg/cm2)]

Acceleratedby RRC

Max. atsample

Min. atsample

At Braggpeak

40Ar9581.22.2

18.7

84Kr

70

41.8

11.7

41

136Xe

39

17.3

47.2

70

197Au

18.4

4.6

94.1

95

Beamline for material irradiation
At about 4 m upstream, a gold foil
diffuses the beam spot and a pair ofwobbler magnets bend the beam at
60 Hz to trace a circle so that auniform dose distribution is formed.

Determination of degrader thickness vs. LET
The degrader-thickness dependence of IC2 current is measured andfitted by calculations of SRIM-2013 [2].
Using the best-fit result to the Bragg-peak measurement, the beamenergy at the sample is determined within 5um accuracy of the Althickness.
The actual degrader thickness for the desired LET is then calculatedwith the SRIM code.

Before irradiation, the beam passes through•a vacuum separation window (60-mmφ, 75-mm thick Kapton) and•two fixed transmission-type detectors to measure the beam flux:(1) a plastic scintillator (100 mm x 100 mm, 0.5-mm or 0.1-mm thick) to countall ions, and(2) a parallel-plate ionization chamber (IC1: 50-mm diameter) for high flux.

•LET is adjusted with energy-degrader (12 Al-foils, total thickness up to 3 mm
by about 5-mm step).

At the clients’ irradiation position:(1) Si DE-E detectors (total thickness 150 - 4000 mm) and(2) a shallow ionization chamber (IC2: 14-mm diameter, 2-mm thick)can be inserted for beam measurements.
Clients control ion flux and LET from outside of the radiation controlled area
via LANBeam flux calibration
IC1 output current is calibrated by
the ion count rate of the scintillator.
Flux is continuously monitored
during irradiation: scintillator isused below 7104 ions/cm2/s, andIC1 is used above it.
Clients can select ion flux between
10 to 107 ions/cm2/s.

Uniformity of flux distribution

Energy and LET distribution
Energy spectra of ions are measured by the Si detectors at different
degrader thicknesses.

The beam is scanned by the Si
detectors at the irradiation
position.
The uniformity is within ±5% in
a 5-cm diameter area and within±1.8 % in a 3-cm diameter area.

Beam Impurities Originating from Upstream Reactions
Nuclear reactions in the upstream materials, such as the window, the energy-degrader, the transmission-type detectors and air, produce fast nuclides that
contaminate the beam and affect the LET distribution. We studied the production probabilities of impurity nuclides in the Kr beam by a radiochemical
method and compared them with a simulation.

Irradiation of test samples

Simulation

We irradiated Si wafer and acrylic plate with the 84Kr beam at the clients’irradiation position. The two materials were selected to distinguish theradionuclides produced upstream from those produced in the sample.
The degrader was 586-mm thick and the irradiation time was 10 min each.

Si-wafer

Acrylicplatesize

100 mmφ75 mm
80 mmthickness

0.5 mm1 mm

4.51010

5.51010

0.33 mm

0.5 mmSampleNumberof Ions
Range

We simulated the nuclear reactions inthe irradiations with Particle andHeavy Ion Transport code System [3](PHITS) which traced each resultant
nucleus from its production to
stopping, and compared the resultswith the gamma-ray measurements toevaluate the validity of PHITS.

Probabilities of nuclides stopped in the sample

Gamma-ray measurements
Gamma-ray spectra of the samples were taken with the Gedetectors from 7 min to 3 mon after the irradiations.
PHITS predicts that the impurity beam consists mainly of projectile-like
nuclides with 20 < Z ≤ 39 , whereas those produced in the sample extend tohigher Z, and reproduces the overall features of the measured probabilities ofthe RI nuclides in the samples. We think it validates the use of PHITS tosimulate our beam and irradiation, and we go further into simulations withdifferent thickness of degraders.

Degrader thickness dependence of LET distribution
About 61 species (24Na to 104Ag) were identified in the Si wafer and 49
species (24Na to 93mMo) in the acrylic plate.
Production probabilities were obtained from the radioactivity.

Z ≤ 36: similar between Si and acrylic, mainly impurity-beam nuclides
Z > 42: only nuclear reactions in the Si sample,
37 ≤ Z ≤ 42: similar or higher in the acrylic, both origins may contribute.

According to PHITS, the high-LET impurity is degraded projectile-like
nuclides and accounts for about 1 % of the beam at maximum forintermediate thicknesses of the degrader.

Summary
RIKEN RI Beam Factory provides fast heavy ions to private companies in Japan to simulate cosmic-ray induced SEE in space-use semiconductor devices.
Kr and Ar ions are in use with a uniform dose distribution in the 5-cm-diameter area and the LET selectable from 2.2 to 41 MeV/(mg/cm2).
Samples can be irradiated in air and the clients can control the LET and ion flux from outside of the radiation-controlled area.
Production probabilities of impurity-beam RI nuclides were measured by radiochemical analyses of irradiated test samples.
Heavy-ion transport simulations reproduced the overall features of the measured probabilities, and shows that high-LET impurity nuclides produced in theupstream materials accounts for about 1 % at maximum of the intensity of the Kr-beam.

References
1. H. En’yo, Nuclear Physics News, Vol. 25, No. 3 (2015) 5, http://www.rarf.riken.jp/index_e.html.
2. J. F. Ziegler: http://www.srim.org/.
3. T. Sato et al., “Features of Particle and Heavy Ion Transport code System (PHITS) version 3.02”, J. Nucl. Sci. Technol.
https://doi.org/10.1080/00223131.2017.1419890, https://phits.jaea.go.jp/index.html.