Development of CCDs for the SXI

1
Development of CCDs for the SXI
We have developed 2 different types of CCDs for
the SXI in parallel. . Advantage gtThey are
successfully employed for current
satellites. Disadvantage gtThe maximum
thickness of the depletion layer is 100 mm
1, P-type CCDs, fabricated on p-type Si wafer.
2, N-type CCDs, fabricated on n-type Si wafer.
Advantage gtmobility of major carrier is 3
times larger gthigher resistivity of silicon
wafer N-type CCDs have 2-3 times thicker
depletion layer than p-type. Disadvantage
gtThey have never been employed onboard
detectors. gtThe performance for X-rays are
hardly investigated.
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Background of developing CCDs with thick
depletion layer
L. Strüder et al. developed pn-CCDs with 300 mm
thickness of the depletion layer, and it had
already employed for XMM-Newton
Steven Holland et al. fabricated MOS CCDs with
high resistivity n-type Si wafer and
successfully obtained a 300 mm thick depletion
layer.
Development of the n-type MOS CCDs is under way
with a collaboration of Osaka U., Kyoto U., NAOJ
(National Astronomical Observatory of Japan) and
HPK (Hamamatsu Photonics K.K.).
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Specification of test device
Specification (design value) Pixel size 14.5 mm
x 15 mm Format 328 x 320 Wafer thickness 200
mm Depletion layer 200 mm Illuminated method FI
(Front illuminated)
Schematic of cross section
Depletion layer 200 mm
electrode
Fully depleted CCD
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Basic performance
Operation temp -70oC Readout speed 60 kHz
Spectrum of 55Fe
X-ray image
1 pixel size is 14.5 x 15 um
n-type CCD Energy resolution 143 3
eV Readout noise 7 electrons
p-type CCD 135 eV 5 electrons
Basic performance was comparable to that of
p-type CCD
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Responsivity for soft X-ray
spectrum of oxygen K line
We investigated the responsivity for soft X-ray
by using Oxygen K line 527 eV Energy
resolution FWHM 81 1 eV We confirmed that
this CCD completely collects the signal
charges Because this peak has no low energy tail.
Oxygen K line 527 eV
counts
Pulse height ADU
This CCD have good responsivities for soft X-ray
down to 0.5 keV.
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Thickness of the depletion layer
Detection efficiency based on http//www-cxro.lbl
.gov/optical_constants/
We measured the thickness of depletion layer from
the detection efficiency.
Solid line 172 mm
Dotted line 70 mm
Spectrum of 109Cd
Detection efficiency
Energy keV
Thickness of the depletion layer 172 13 mm
Energy resolution 574 eV at 22.4 keV.
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Future plan
To improve the efficiency below 0.5 keV, We have
developed the BI (Back illuminated) CCDs. We will
evaluate the performance of this device.
Schematic of cross-section
Large imaging area (3 x 6 cm) Photograph of
engineering model
BI
Depletion layer
electrode
FI