Caeleste’s LAP2010A pixels are exceptionally 60Co radiation hard
24 April 2011, BD
Caeleste
has the background and experience to design radiation tolerant pixels and
circuitry. The “LAP2010A” pixels array are designed for and used in in medical
X-ray. The LAP2010A pixel array contains 6 variants of pixels.
4 samples
of the LAP2010A prototype pixel arrays were irradiated with 0, 100, 200 and
500Gy, in a qualified 60CO facility, 25°C, 128Gy/h, no bias
applied. Dark currents are recorded and normalized to pixel size
10µm. Measurement results are shown below, all at RT, in e-/s.
We observe no significant dark current increase for these pixels. This is an exceptional achievement as for the first time, pixels that have not measureable dark current increase
Comparison with state of the art
The plot superimposes our LAP results with the few published[[1],[2],[3]] gamma TID (total ionizing dose) results in relevant other CMOS imagers. The compilation has to be interpreted with a margin of error, as radiation conditions, measurement conditions, bias conditions etc. all differ. For some cases we had to make assumptions on the actual conversion factor mV/s to e-/s.
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Figure 10
compilation of relevant literature on Gamma TID. Remark: the point
“0.0” is added on the logarithmic X-axis to
indicate time 0 or beginning of life
(BOL).
In the plot, the “LAP default” is the actual pixel being used in the medical X-ray applications. It is a classic 5 T charge integrating pixel.
LAP DS, LAP
DX and LAPXS are variants of the default LAP pixel. Of these especially
the LAP XS is relevant. It has due to its specific design a dark current
that is 5 times lower than the default pixel, and can maintain that low level
virtually unchanged even after 500 Gy (50kRad).
And at higher TID?
Present results are recorded up to 50kRad. We
did not record higher doses for budget and opportunity reasons. We welcome third parties that want to
irradiate and evaluate the LAP2010 device at much higher doses, say 1MRad and
beyond.
Other forms of radiation tolerance
We
demonstrated exceptional gamma (and X-ray) TID tolerance. However, Caeleste pixels and arrays are
tolerant for other forms of radiation too:
Proton and heavy ion total dose damage susceptibility
This form of radiation damage is generally understood as an inherent bulk Si property, creating isolated localized defects in the diode’s sensitive volume. As the photosensitive volume is quite constant across technologies and pixels design, even from CCD to CMOS, so is the damage effect.
Single event or single event upsets (SE, SEU) and Single event latchup
(SEL)
SE(U) is the temporary failure of the
circuit due to the localized deposition of a large charge packet due to a heavy
particle absorption. Such large charge
packet can “upset” analog or digital circuitry.
Ideally circuits should be insensitive.
If not, the imager should recover in maximally one frame period. Caeleste can design its imager with the
highest possible SE tolerance.
A specific case of SE is Single Event Latchup (SEL). The triggering of CMOS latchup can be prevented by proper choice of technology options and circuit design style.
[1] J.Bogaerts, B.Dierickx, C.Van Hoof, “Radiation effects in CMOS Active Pixel Sensors”, RADECS, ESTEC Noordwijk, 2000
[2] B.Dierickx, J.Bogaerts, “STAR250 Radiation-Tolerant APS for Star Tracker Applications”, CNES Atelier, Toulouse, 2002 (same data also in later publications)
[3] M.Innocent, “A radiation tolerant 4T pixel for space applications”, IISW, Bergen, 2009