We report on the performance of an analog application-specified integrated circuit (ASIC) developed for the front-end electronics of the X-ray CCD camera system (SXI: Soft X-ray Imager) onboard the ASTRO-H satellite. The ASIC consists of four identical channels and they simultaneously process the CCD signals at the pixel rate of 68kHz. Delta-Sigma modulator is adopted to achieve effective noise shaping and obtain a high resolution decimal values with relatively simple circuits. We will implement 16 ASIC chips in total in the focal plane assembly. The results of the unit test shows that it works properly with moderately low input noise of <30μV at the pixel rate of 80kHz. Power consumption is sufficiently low of 150mW. Dynamic range of input signals is +-20mV that covers effective energy range of the CCD chips of SXI (0.2-20keV). The integrated non-linearity of 0.2% satisfies the same performance as the conventional CCD detectors in orbit. The radiation tolerance against total ionizing dose (TID) effect and single event latch-up (SEL) has also been investigated. The irradiation test using 60Co gamma-rays and proton beam showed that the ASIC has sufficient tolerance against TID up to 200 and 167krad respectively, which thoroughly exceeds the expected operating duration in the planned low-inclination low-earth orbit. The irradiation of the Fe ion beam also showed no latch-up nor malfunctions up to the fluence of 4.7x10^7ions. The threshold against SEL is larger than 1.68MeVcm^2/mg, which is sufficiently high enough that SEL events should not be a major cause of instrument downtime.
We have developed application specific integrated
circuits(ASICs) for multi-readout X-ray CCDs in order to improve their
time resolution. ASICs with the size of 3mm × 3mm were fabricated by employing a Taiwan
Semiconductor Manufacturing Company(TSMC) 0.35 μm CMOS technology.
The number of channels is 4 and the each channel consists of a
preamplifier, 5-bit DAC and delta-sigma analog-to-digital converters
(ADCs). The measured equivalent input noise at the
pixel rate of 19.5 kHz and 625 kHz are 36 μV and 51 μV,
respectively. The power consumption is about 110 mW/chip at 625 kHz pixel rate,
which is about 10 times lower than that of our existing system.
We now expect to employ an ASIC as the readout system of X-ray CCD camera onboard the next Japanese X-ray astronomy satellite. We tested the
readout of the prototype X-ray CCDs by using ASICs and the total-dose effects of ASICs. We describe the overview of our ASICs and test results.
We report on the development of high-speed and low-noise readout system of X-ray CCD camera with ASIC and the Camera Link standard.
The ASIC is characterized by AD-conversion capability and it processes CCD output signals with a high pixel rate of 600 kHz, which is ten times quicker than conventional frame transfer type X-ray CCD cameras in orbit.
There are four identical circuits inside the chip and all of them process CCD signals simultaneously. ΔΣ modulator is adopted to achieve effective noise shaping and obtain a high resolution decimal values with relatively simple circuits.
The results of the unit test shows that it works properly with moderately low input noise of ~70 μV at pixel rate of 625 kHz, and ~40 μV @ 40 kHz.
Power consumption is sufficiently low of <120 μuV @ 1.25 MHz. We have also developed the rest of readout and driving circuits. As a data acquisition scheme we adopt the Camera Link standard in order to support the high readout rate of the ASIC.
In the initial test of the CCD camera system, we used the P-channel CCD developed for Soft X-ray Imager onboard next Japanese X-ray astronomical satellite. The thickness of its depletion layer reaches up to 220 μm and therefore we can detect the X-rays from 109Cd with high sensitivity rather than N-channel CCDs. The energy resolution by our system is 379 (±7)eV (FWHM) @ 22.1 keV, that is, ΔE/E=1.8% was achieved with a readout rate of 44 kHz.
The Soft X-ray Imager (SXI) is the X-ray CCD detector system on board the NeXT mission that is to be launched around 2013. The system consists of a camera, an SXI-specific data processing unit (SXI-E) and a CPU unit commonly used throughout the NeXT satellite. All the analog signal handling is restricted within the camera unit, and all the I/O of the unit are digital.
The camera unit and SXI-E are connected by multiple LVDS lines, and SXI-E and the CPU unit will be connected by a SpaceWire (SpW) network. The network can connect SXI-E to multiple CPU units (the formal SXI CPU and neighbors) and all the CPU units in the network have connections to multiple neighbors: with this configuration, the SXI system can work even in the case that one SpW connection or the formal SXI CPU is down.
The main tasks of SXI-E are to generate the CCD driving pattern, the acquisition of the image data stream and HK data supplied by the camera and transfer them to the CPU unit with the Remote Memory Access Protocol (RMAP) over SpW. In addition to them, SXI-E also detects the pixels whose values are higher than the event threshold and both adjacent pixels in the same line, and send their coordinates to the CPU unit. The CPU unit can reduce its load significantly with this information because it gets rid of the necessity to scan whole the image to detect X-ray events.
X-ray CCDs have superior spatial resolution of ~20μm and moderate energy resolution of ~130 eV(FWHM)
at 5.9 keV. On the other hand, the number of pixels assigned to each readout node is generally so large that it
takes several seconds to process a frame data of the entire chip. Relatively low pixel readout rate in order to
keep readout noise low also limits the timing resolution of X-ray CCDs. Although a large number of readout
nodes is essential to improve the timing resolution, size and power consumption of conventional readout circuits
prevent us from being implemented in X-ray CCD camera systems onboard satellites. We are developing an
application specific integrated circuits (ASIC) for multi readout of X-ray CCD signals. The ASIC with the size
of 3mm×3mm has four channels of readout electronics that employs the delta-sigma (ΔΣ) digitization. The
fabrication process is a 0.35μm complimentary metal-oxide semiconductor (CMOS) process provided by Taiwan
Semiconductor Manufacturing Company (TSMC). The equivalent input noise was about 33μV and the power
consumption was about 70mW per chip at the pixel rate of 44 kHz. When we used the X-ray CCD whose
sensitivity was 3 μV/e-, the equivalent noise charge was 10.8e- and the energy resolution was 168 eV(FWHM)
at 5.9 keV. The noise level of our ASIC is comparable to that of the conventional readout systems.
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.
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 (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.
Delta Sigma digitizers generally have excellent linearity, precision and noise rejection. They are especially well
suited for implementation as integrated circuits. However, they are rarely used for time bounded signals like
CCD pixels. We are developing a CCD video digitizer chip incorporating a novel variant of the Delta Sigma
architecture that is especially well suited for this application. This architecture allows us to incorporate video
filtering and correlated double sampling into the digitizer itself, eliminating the complex analog video processing
usually needed before digitization.
We will present details of a multichannel ASIC design that will achieve spectroscopic precision and linearity
while using much less energy than previous CCD digitizers for technical applications such as imaging X-ray
spectroscopy. The low conversion energy requirement together with the ability to integrate many channels will
enable us to construct fast CCD systems that require no cooling and can handle a much wider range of X-ray
intensity than existing X-ray CCD systems.
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.
We present the current status of soft X-ray calibration of X-ray CCD cameras, X-ray Imaging Spectrometer (XIS), onboard Astro-E2. We perform soft X-ray calibration of four front illuminated (FI) CCD cameras and two back illuminated (BI) CCD cameras, among which four cameras will be selected to be installed on the satellite. The calibration aims to measure the quantum efficiency and re-distribution function of the CCDs as a function of incident X-ray energy. A soft X-ray spectrometer is used to measure these items. In addition, we employ a gas proportional counter and an XIS engineering unit as reference detectors for the quantum efficiency measurement. We describe how we calibrate the absolute quantum efficiency of the XIS using these instruments. We show some of the preliminary results of the calibration including quick look results of BI CCD cameras.