X-ray CCD with low noise charge injection. Gregory Prigozhin, Barry Burke1, Marshall Bautz, Steve Kissel, Beverly LaMarr MIT Kavli Institute for Astrophysics and Space Research 1MIT Lincoln Laboratory

Outline: Motivation for adding an input structure. Device architecture, input structure, its operation. Input structure performance, measurement details and results. Applications of the input structure at low signal levels: quick CTI and column nonuniformity estimation, CTI improvement, calibration of charge-volume relation, charge pumping. Conclusions

Charge injection in CCID-41 1. After long discussions we chose serial injection because it provides excellent uniformity (but is slow). 2. In timing diagram the difference with usual fill-and-spill is that the input gate is clocked to maintain the voltage difference and prevent charge flow back into the diode.

Signal and noise as function of gate voltage. ASTRO-E2 electronics controls IG offset with a DAC. The smallest step is 3.8 mV, full span 0.98 V, corresponding to a maximum of ~4000 e-. At very low signal input node capacitance, 0.43 fF is much smaller than determined by S3 area. Due to 2D effects charge occupies very small region under the gate. When it expands, it reaches chan-stop boundaries, and capacitance goes up. Shape of the noise curve is very similar to theoretical prediction of sqrt(kTC/2), but values are higher. The reason for discrepancy not clear.

“Evaporation” of electrons from the potential well

Injection pattern for CTI improvement Charge injection pattern is programmable. “Grid” pattern reduces charge transfer losses due to radiation damage: Charge is injected in each column of every 54th row. Injected charge (temporarily) fills radiation-induced traps. Filled traps will not degrade charge transfer inefficiency. Result is better spectral resolution. Charge moves right during injection Input Register Charge moves down during readout Rows filled by charge injection

Energy resolution of irradiated proton irradiated chip Before irradiation: FWHM=132 eV After irradiation without injection: FWHM=210 eV

Energy resolution after irradiation with charge injection Before irradiation: FWHM=132 eV After irradiation with charge injection: FWHM=144 eV

Measuring charge loss in a column using charge injection. If a train of 8 rows is injected, the first one loses charge to empty traps. The following rows can serve as a reference, thus charge loss can be measured. The result is compared to the loss determined with Fe55 X-rays. Charge injection measurement needs just a few frames. X-ray measurement needs thousands. It also does not require a calibration source.

A train of 8 rows filled with charge, image averaged over 15 frames. Scatter in the first row determined by number of traps in a column is significantly larger than scatter in the following rows determined by injection noise.

Measurement of charge-volume relationship Charge loss is proportional to the number of traps in the volume V occupied by the charge packet. Thus, measuring loss as a function of charge Q is equivalent to measuring V(Q) dependence. This function is important in modeling CTI as a function of signal. The result (V ~ Q 0.51) is really close to square root dependence at Q >180electrons

CCID-41 in the ASTRO-E2 detector assembly. Frame transfer 1024x1024 array Has 4 output readout nodes Readout noise below 2 electrons rms at 42 kHz Charge injection capability Depletion depth approximately 65 microns

Conclusion We have implemented a charge injection structure for injecting miniscule charge packets (hundreds and even tens of electrons) with a capacitance of 0.43 fF. It has very low noise, about 6 electrons at signal level below 1000 electrons. Using certain patterns of charge injection we were able to significantly improve energy resolution of proton irradiated CCD. Charge injection allows to calibrate column-to-column CTI nonuniformities very efficiently. It is an extremely valuable tool for conducting many experiments, like measuring charge-volume relationship or charge pumping.