ARTICLE IN PRESS
Radiation Physics and Chemistry 71 (2004) 713–715
X-ray radiation source for total dose radiation studies
D. Biselloa,*, A. Candeloria, A. Kaminskia, A. Litovchenkoa,
E. Noahb, L. Stefanuttia
a
Dipartimento di Fisica, INFN Sezione di Padova, Universita di Padova, Via Marzolo 8, Padova I-35131, Italy
b
Imperial College, London SW7 2AZ, UK
1. Introduction
3. The X-ray emission field
Low-energy (p60 keV) X-ray tube sources are currently used for investigating the tolerance to ionizing
radiation in total dose tests of unpackaged semiconductor devices and circuits, due to their higher dose rates
(100–1000 rad(Si)/s) and lower costs than 60Co g-ray
sources (Palkuti and LePage, 1982; ASTM), and to the
radioprotection advantage that the radiation can be
switched off. In this study we report on the X-ray field
emission characteristics, measured by silicon diodes, of
the irradiation facility installed at the INFN National
Laboratory of Legnaro (Padova, Italy) for total dose
tests of electronics devices and circuits.
X-ray emissions from tungsten anodes have been
widely investigated in the literature. For operating
voltages between 20 and 60 kV, they consists of the
bremsstrahlung component with an energy spectrum up
to Y keV, where Y is the tube operating voltage in kV,
plus the multi-line L characteristic radiation of tungsten,
which contains 12 lines for the La ; Lb ; and Lg series
between 8.3 and 11.7 keV (Birch et al., 1982). The X-ray
spectra emitted from the tube is additionally filtered for
total dose tests by 0.15 mm Al in order to attenuate the
low-energy (o8 keV) component (ASTM). An example
of X-ray spectra calculated by XCOMP (Nowotny and
Hofer, 1995) for a typical tube operating condition in
total dose tests is shown in Fig. 1, where both the
bremsstrahlung and characteristic radiation components
are taken into account.
The X-ray emission field from the tube has been
measured by a square silicon diode calibrated in
accordance to the ASTM recommendations, with active
area A¼ 5 5 mm2 and thickness d ¼ 300 mm, positioned on the chuck of a commercial semiautomatic
probe station under the X-ray tube. Silicon diodes are
considered for X-ray dose rate measurements in total
dose tests because silicon is the reference material in
microelectronics devices. During the X-ray field measurements, the diode was reverse biased at full depletion
and the active area was grounded, as well as the guardring contact to prevent border effects on the current
induced by X-rays in the diode active volume V=A d.
The dose rate from the X-ray tube at the diode surface
referred to silicon (Dr ) is determined by Dr ¼
CIx =ðqEeh VrÞ; where C is a calibration constant to report the average dose rate in the diode active
volume to the surface value (Noah, 2000), which
2. The X-ray irradiation system
The total dose test facility is the RP-149 Semiconductor Irradiation System from Seifert (Ahrensburg,
Germany) equipped with a standard tube for X-ray
diffraction analysis: maximum power 3000 W, maximum
voltage 60 kV, tungsten anode, 0.3 mm Be inherent
filtration, take off angle 6 , and focus dimensions
0.4 12 mm2. The tube voltage and current can be set
with kV and mA resolution, respectively. The tube is
located inside a shielding cabinet and it can be moved
with an accuracy of 30 mm along the x-, y- (motorized)
and z- (manual) axis of 15, 20 and 450 cm, respectively,
for accurate positioning on the tested device.
*Corresponding author. Tel.: +39-049-8277216; Fax: +39049-8277237.
E-mail address: [email protected] (D. Bisello).
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.radphyschem.2004.04.071
ARTICLE IN PRESS
D. Bisello et al. / Radiation Physics and Chemistry 71 (2004) 713–715
714
Photon spectra
normalized to the maximum value
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
50
Energy (keV)
Fig. 1. X-ray spectrum normalized to the maximum value calculated by XCOMP (Nowotny and Hofer, 1995), anode material W,
take-off angle 6 , tube voltage 50 kV, inherent filtration 0.3 mm Be, additional filtration 0.15 mm Al, distance from the tube aperture
15 cm.
140
Dose rate (rad(Si)/s)
120
100
13.9 mm
80
60
15.5 mm
40
16.1 mm
20
20.6
0
-20
-15
-10
(a)
-5
0
5
X position (mm)
10
15
20
10
15
20
140
Dose rate (rad(Si)/s)
120
7.2 mm
100
80
60
9.2 mm
40
11.4 mm
20
19.4 mm
0
-20
(b)
-15
-10
-5
0
5
Y position (mm)
Fig. 2. Dose rate from the X-ray tube as a function of the position along the x-direction perpendicular (a) and the y-direction parallel
(b) to the anode-cathode line at different distances from the tube aperture: 10 cm (squares), 15 cm (diamonds), 20 cm (triangles), and
40 cm (circles). Experimental conditions: tube voltage 50 kV, tube current 10 mA, filtration 0.15 mm Al, take off angle 6 . The
horizontal lines delimited by two open circles correspond to the intervals for the 10% dose rate variation with respect to the maximum
value.
ARTICLE IN PRESS
D. Bisello et al. / Radiation Physics and Chemistry 71 (2004) 713–715
accounts for the dose profile in the silicon diode as a
function of the depth; Ix is the diode current induced by
X-rays obtained by subtracting the diode current with
and without the X-ray radiation; q is the electron charge;
Eeh ¼ 3:6 eV is the mean energy to create and electron
hole pair in silicon; r is the silicon density.
The X-ray dose rate fields from the tube operating at
50 kV and 10 mA, measured by moving the probe station
chuck on which the silicon diode is positioned, along
the x- (perpendicular to the anode–cathode line) and
y- (parallel to the anode–cathode line) axes, are shown
in Fig. 2. Measurements have been repeated at different
distances between the tube aperture (E11 cm down to
the tube focus) and the diode surface. As expected,
the X-ray emission is symmetric along the direction
perpendicular to the anode–cathode line, but unsymmetrical along the anode–cathode direction due to the
standard X-ray tube geometry: electrons impact perpendicularly on the anode and induce a cylindrical (not
spherical) symmetric photon emission. The effect of the
take-off angle (6 ) is seen on the position of the
maximum emission along the AC directions, which
slightly decreases (from 1 to 4 mm) by increasing the
distance from the tube. Data in Fig. 2 shows that
uniform irradiation with 10% accuracy can be obtained
on 13.9 7.2 mm2 (20.6 19.4 mm2) area at 10 cm
(40 cm) from the tube aperture, even if the dose rate
715
is reduced by a factor 8.6 in this distance range.
Depending on the DUT dimension, the dose rate
can be further increased (decreased) by decreasing
(increasing) the distance from the source. Finally,
we remark that the 10% accuracy estimation is a worst
case condition, because the diode averages the dose
rate on its 5 5 mm2 active area when positioned
at (x,y).
References
American Society for Testing and Materials (ASTM). Standard
guide use of an X-ray tester (E10 KeV photons) in ionizing
radiation effects testing of semiconductor devices and
microcircuits. ASTM designation F 1457–99.
Birch, R., Marshall, M., Peaple, L.H.J., 1982. Theoretical and
measured L X-rays from a solid tungsten target. Phys. Med.
Biol. 27, 1119–1129.
Noah, E., 2000. X-ray dosimetry: determination of ionization
dose delivered by the Seifert Rp-149 X-ray machine.
Imperial College, London, UK.
Nowotny, R., Hofer, A., 1995. Ein program fur die berechung
von diagnostiscen Rontgenspekten. Fortschr. Rontgenstr.,
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dose testing. IEEE Trans. Nucl. Sci. 29, 1832–1837.