Electron bombardment radiation damage in Tektronix CCDs

 

Alice Reinheimer
CCD Product Group
Tektronix, Inc.

Feb. 2, 1993

 

Two types of damage are caused by electron radiation...

1) Flatband voltage shift

¥X-rays create electron-hole pairs in the gate insulator.

¥The holes are trapped leaving a net positive charge.

¥The positive charge changes the potential under the gate (unless the device is pinned). ¥A changed potential means well capacity is lost.

 

2) Increase in interface state density

¥Interface states make it easier for electrons to "jump" from the valence band to the conduction band.

¥Dark current is proportional to the interface state density.

¥Interface states can also trap charge

 

MPP operation can do away with some ill effects

¥ Inversion of the surface populates the interface states with holes, suppressing dark current generation.

¥ But... a flatband voltage shift can bring an MPP device out of inversion.

Hypothesis:

If damage is due to a flatband voltage shift then the amount of charge that can be transferred through one damaged pixel is limited. The charge will be limited to the same amount no matter how many damaged pixels are transferred through. The spilled charge will be present in the first empty pixel following a block of charge.

If damage is due to an increase in interface state density, then some charge will be captured and reemited later in each damaged pixel. The amount of spill present in the first empty pixel will depend on how many damaged pixels the signal charge is transferred through.

Results:

Dose (C/cm²)          Number of damaged rows         5000 e^- CL level

Undamaged                             ------                       630 ke/pixel
     2x10^-3                                  61                               530 ke/pixel
     2x10^-3                                518                               200 ke/pixel
   2.5x10^-4                                518                               530 ke/pixel

 

CTL = charge lost / (signal charge * number of transfers)
            Initial (undamaged) CTL_i = 15x10^-6 per transfer

 

              charge lost - [CTL_i* signal charge * number of transfers]
CTL_r = -----------------------------------------------------------------------------
                         signal charge * number of damaged rows

 

Dose (C/cm²)       Number of damaged rows        Normalized CTL_r

     2x10^-3                               61                               .12 (C/cm²)^-1
     2x10^-3                              518                              .16 (C/cm²)^-1
    2.5x10^-4                             518                         .12 (C/cm²)^-1

Conclusion

CTL depends linearly on the number of damaged rows signal is transferred through. Therefore there is no flatband voltage shift.