We report on the development of the X-ray CCD for the soft X-ray imager (SXI) onboard ASTRO-H. SXI CCDs are
P-channel, back-illuminated type manufactured by Hamamatsu Photonics K. K.
Experiments with prototype CCD for the SXI shows the device has a depletion layer as thick as 200μm, high efficiency for hard X-rays.
By irradiating soft X-rays to the prototype CCD for the SXI.
At the same time, we found a significant low energy tail in the soft X-ray response of the SXI prototype CCD.
We thus made several small size CCD chips with different treatment in processing the surface layers.
CCDs with one of the surface layers treatment show a low energy tail of
which intensity is one order of magnitude smaller than that of the original SXI prototype CCD for 0.5keV X-ray incidence.
The same treatment will be applied to the flight model CCDs of the SXI.
We also performed experiments to inject charge with the SXI prototype CCD, which is needed to mitigate the radiation damage in the orbit.
We investigated the operation conditions of the charge injection.
Using the potential equilibration method, charges are injected in each column homogeneously,
though the amount of the charge must be larger than 20ke-.
We have been developing fully-depleted CCDs fabricated on N-type silicon wafer in collaboration with HAMAMATSU
Photonics K.K.We have made several wafer runs to optimize the basic characteristics of the devices such
as the charge transfer efficiency (CTE), the full-well capacity and the amplifier gain, followed by the optimization
of the backside treatment to improve quantum efficiency (QE) in blue wavelengths. The optimization process is
successfully completed, and Hamamatsu recently started to deliver the 2k × 4k (15 μm pixel) four-side buttable
devices for acceptance evaluation at the National Astronomical Observatory of Japan. Based on the measured
QE in the X-ray, the depletion depth reaches 200 μm with CTE as good as >0.999995 for serial and parallel
directions and with readout noise of < 5 e- for 130 kHz readout. The size of charge diffusion is estimated to be
< 7.5 μm (one sigma) for pinhole image at wavelength of 450 nm. The device flatness is < 15-20 μm, and the
dark current is a few e-/hour/pixel at -100°C and ~ 20 e-/hour/pixel at -80°C.
We have developed X-ray charge-coupled devices (CCD) for the next Japanese X-ray astronomical satellite mission, NeXT (Non-thermal energy eXploration Telescope). The hard X-ray telescope(HXT) onboard the NeXT can focus X-rays above 10 keV. Therefore, we need to develop an X-ray CCD for a focal plane detector to cover the 0.3-25 keV band in order to satisfy the capability of the telescope. We newly developed an n-type CCD fabricated on an n-type silicon wafer to expand the X-ray energy range as a focal plane detector of the
HXT. It is possible to have a thick depletion layer of approx. 300μm with an n-type CCD because it is easy to obtain high resistivity with an n-type silicon wafer compared to a p-type silicon wafer. We developed prototypes of n-type CCDs and evaluated their X-ray performance, energy resolution, charge transfer inefficiency(CTI) and the thickness of the depletion layer of two devices, designated Pch15 and Pch-teg. We measured the thickness of the depletion layer of Pch15 to be 290±33μm. For Pch-teg, the energy resolution was 152±3eV full width at half maximum (FWHM) at 5.9 keV and the readout noise was 7.3 e-. The performance of the n-type CCDs was comparable to that of p-type CCDs, and their depletion layer were much thicker than those of p-type CCDs.
The next Japanese X-ray astronomical satellite mission, NeXT, was proposed to ISAS/JAXA following the Astro-E2 Suzaku satellite which was launched in July 2005. We develop an X-ray CCD camera system, SXI (Soft X-ray Imager), for NeXT. The Hard X-ray Telescope (HXT) onboard NeXT provides imaging capability up to 80 keV, using the multilayer-coated X-ray mirror technology, called "Supermirror", newly developed in Japan. SXI is one of the focal plane detectors of HXT, which covers the soft energy band in the 0.5-12 keV in the baseline and 0.3-25 keV in the goal. We develop p-type CCDs for the baseline of SXI because p-type CCDs have been successfully used for previous X-ray astronomical satellites. We developed a prototype of a p-type CCD for SXI, called "CCD-NeXT1". CCD-NeXT1 is a frame-transfer CCD with two readout nodes. The image area of CCD-NeXT1 consists of 2Kx2K pixels with a pixel size of 12 μm x 12 μm. We evaluated performance of CCD-NeXT1 devices, KG-4 and KG-5. The energy resolution was 141.8±0.6 eV full width at half maximum at 5.9 keV, the readout noise was 4.7±0.2 e-, the horizontal CTI was < 5.1 x 10-7, and the vertical CTI was < 2.4 x 10-7 for KG-5. The performance of KG-4 was more or less the same as that of KG-5. The thickness of the depletion layer was 82±7 μm for KG-4 and 76±6 μm for KG-5. We conclude that our technology for p-type CCDs is sufficient to satisfy the CCD performance for the baseline of SXI.
The NeXT (New X-ray Telescope) satellite to be launched around 2010, has a large effective area in the 0.1-80
keV band with the use of the multilayer super mirror (HXT). As one of the focal plane detectors for NeXT,
we have been developing the Soft X-ray Imager (SXI). SXI consists of charge coupled devices (CCDs). In order
to increase the quantum efficiency (Q.E.) as high as possible, i.e., to detect X-rays collected by HXT as many
as possible, we developed a "fully-depleted and back-illuminated CCD" in the attempt to improve the Q.E.
of soft X-rays by the back-illuminated structure and that of hard X-rays by thickening of a depletion layer.
Thanks to a high-resistivity (over 10kΩ•cm) n-type Si, we have successfully developed Pch CCDs with very thick
depletion layer of over 300 micron, which is 4 times thicker than that of established X-ray MOS CCDs (for example
XIS, EPIC-MOS and ACIS-I). Furthermore, we have already confirmed we can thin a wafer down to 150 micron
independent of its resistivity from the experience of the development of the back supportless CCD. Based on
these successful results, we fabricated a test device of "fully depleted and back-illuminated CCD" with the high
resistivity (10kOhm cm) N-type Si thinned down to 200 micron. The pixel number and size are 512 x 512 and 24
x 24 μm, respectively. For optical blocking, we coated the surface with Al. We evaluated this test device and
confirmed the thickness of depletion layer reaches 200 micron as we expected. In this paper, we present progress in
development of these devices for SXI.
We give overview and the current status of the development of the Soft X-ray Imager (SXI) onboard the NeXT
satellite. SXI is an X-ray CCD camera placed at the focal plane detector of the Soft X-ray Telescopes for Imaging
(SXT-I) onboard NeXT. The pixel size and the format of the CCD is 24 x 24μm (IA) and 2048 x 2048 x 2
(IA+FS). Currently, we have been developing two types of CCD as candidates for SXI, in parallel. The one is
front illumination type CCD with moderate thickness of the depletion layer (70 ~ 100μm) as a baseline plan.
The other one is the goal plan, in which we develop back illumination type CCD with a thick depletion layer
(200 ~ 300μm). For the baseline plan, we successfully developed the proto model 'CCD-NeXT1' with the pixel
size of 12μm x 12μm and the CCD size of 24mm x 48mm. The depletion layer of the CCD has reached 75 ~ 85μm.
The goal plan is realized by introduction of a new type of CCD 'P-channel CCD', which collects holes in stead
of electrons in the common 'N-channel CCD'. By processing a test model of P-channel CCD we have confirmed
high quantum efficiency above 10 keV with an equivalent depletion layer of 300μm. A back illumination type
of P-channel CCD with a depletion layer of 200μm with aluminum coating for optical blocking has been also
successfully developed. We have been also developing a thermo-electric cooler (TEC) with the function of the
mechanically support of the CCD wafer without standoff insulators, for the purpose of the reduction of thermal
input to the CCD through the standoff insulators. We have been considering the sensor housing and the onboard
electronics for the CCD clocking, readout and digital processing of the frame date.
Quantum Efficiency (QE) of CCDs decreases at λ >~ 0.7 μm since photons penetrate a depletion layer of CCD. If one makes the layer thicker, the QE will be largely improved. In collaboration with HAMAMATSU Photonics, we have been developing the thicker CCDs which are implemented on the high resistivity n-type silicon wafers. We have made several wafer runs to optimize the basic characteristics of the devices such as charge transfer efficiency (CTE), full-well and node sensitivities of the amplifiers. The results obtained so far mostly satisfied the specifications imposed by astronomical observations. We also attempted to build back-side illuminated devices to realize high QE in wider wavelength. The test devices shows that the QE exceeds 60% at 1 μm, which is roughly 5 ~ 6 times improvement over ordinary CCDs. We will present the current status of the projects.
The NeXT (New X-ray Telescope) satellite to be launched around 2010, has a large effective area in the 0.1-80 keV band with the use of the multilayer super mirror. As the focal plane detector for NeXT, we have been developing the Wideband hybrid X-ray Imager (WXI) consisting of X-ray CCDs and a hard X-ray imager placed under X-ray CCDs. The X-ray CCD of WXI is required to (1) keep the high quantum efficiency up to the high energy band and (2) pass hard X-rays unabsorbed in the depletion (sensitive) layer. In order to meet these requirements, we have been developing the back supportless CCD which has the thick depletion layer, thinned Si wafer and back supportless structure. As the first step, we make the test model with thinned Si wafer in order to (1) learn the handling and thinning process and (2) confirm no change of the performance after the thinning process. The test model has the pixel number and size of 512 x 512 and 24 x 24 μm, respectively. We select the thickness of the wafer is about 200 μm considering the thinning process. We verify the depletion layer and the wafer of the test model are about 65 and 190 μm in thickness, respectively. The energy resolution is about 144 eV at 5.9 keV which is substantially comparable to the un-thinned CCD, hence we confirm our thinning process has no effect on the perfomance. In order to reduce the dead layer ompletely and increase the quantum efficiency at the high energy band, we are also developing the full depleted X-ray CCD with the high resistivity N-type Si wafer.
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