# A High Full Quality CMOS Image Sensor for Space Applications
## 3 Experimental Results
### Radiation  
Figure 10 shows a simplified block diagram and actual pictures of the radiation test setup. The TID and DDD test setups are similar except for radiation source and exposure time. In the radiation chamber, the CIS chip is placed at a set distance from the radiation source, which is determined by the amount of radiation delivered to the chip per hour and electrical devices except for the CIS chip are protected from radiation by lead blocks. For the TID test, gamma rays generated by cobalt-60 (\({}^{60}\)Co) were used as a radiation source and the amount of radiation exposed to the chip was set to 236 rad per hour. Proton particles with a fluence of 1.48 \(\times\) 10\({}^{10}\) particles/cm\({}^{2}\) are accelerated with an energy of 45 MeV by a MC-50 cyclotron, and the CIS chip was exposed to the accelerated proton particles for the DDD test.  
\begin{table}
\begin{tabular}{c c c} \hline  & **MTF (Horizontal)** & **MTF (Vertical)** \\ \hline Metal covering & 0.402 & 0.406 \\ Without covering & 0.131 & 0.406 \\ Improvement (\%) & 307 & 0 \\ \hline \end{tabular}
\end{table}
Table 2: Measured MTF results with metal covering.  
Figure 10: _Cont._  
Figure 9: Measured MTF at 550 nm wavelength: (**a**) Horizontal MTF; (**b**) Vertical MTF.  
Three important performance measures (QE, power consumption, and dark current) are evaluated by the radiation test. While radiation did not affect QE and power consumption of the CIS, dark current was significantly increased depending on the size of in-pixel capacitor. The radiation damage effect on an in-pixel MOS capacitor on the dark current performance can be explained with the measurement results shown in Figure 11. The CIS with two different sizes of in-pixel MOS capacitors was fabricated and was tested for the TID and DDD. Figure 11a shows the test results for the CIS with an in-pixel MOS capacitance of 16.3 fF; the average dark current of CISs was 133.15 e\({}^{-}\)/s before the radiation test and it increased to 490.5 and 622.78 e\({}^{-}\)/s after the TID and DDD tests, respectively. With an in-pixel MOS capacitance of 19.6 fF, as shown in Figure 11b, the dark current significantly increased to 931.59 and 2585 e\({}^{-}\)/s for the TID and DDD test, respectively. Only a 20% increase in in-pixel MOS capacitance deteriorates the dark current performance by about two times for the TID and by four times for the DDD. The possible reasons for the sharp increase in dark current are the threshold voltage shift of the MOS capacitor due to the trapped charges in the oxide area and Shockley-Read-Hall (SRH) generation by the radiation induced defects. The measurement results indicate that there exists a critical trade-off between the FWC and the radiation damage effect on the dark current. With the multiple charge transfer technique, the FWC can be significantly enhanced by increasing the size of an in-pixel MOS capacitor, however, the dark current performance after radiation, especially for the DDD effect, can be severely worsened. Therefore, in-pixel capacitance should be carefully chosen based on an understanding of the trade-off.  
Figure 10: Radiation test setup: (**a**) Simplified block diagram; (**b**) Displacement damage dose (DDD) test with metal shielding to protect the other electrical parts; (**c**) DDD test setup without metal shielding; (**d**) Front view of total ionizing dose (TID) test setup using lead blocks; (**e**) Overall TID test setup including radiation source.  
Table 3 compares the radiation performance of this work with that of commercial image sensors which were tested with the similar radiation conditions. Dark current increment observed after irradiation of the commercial sensors varies from about 200% to 2700%, which is similar to the findings in this work.