CCD Radiation Damage Amelioration
Sequencer Manipulations to Reduce CTI

This page reviews some sequencer manipulations appropriate for reducing radiation damage CTI of CCDs for the ACIS, HETE, Astro-E systems. The examples tend to be for the CCID-17 using ACIS electronics, but the similarity in sequencer design should allow general use of the methods. One very important difference between these instruments is the intended operating temperature of the CCDs. Currently explored techniques are:

Reduced Parallel Transfer Speed (Image to Frame)

Artificial Charge Injection (Sacrificial Charge)

Trap Filling (Squeegee)

Reduced Parallel Transfer Speed
The typical timed exposure CCD readout involves the bulk transfer of charge in the image array to the shielded frame store array in preparation of the readout of the later. This transfer involves 1026 (CCID-17) parallel row operations and typicaly requires 41 msec (ACIS). Reducing this speed can be accomplished in too numerous to describe ways, most of which require but trivial adjustments to the sequencer and need not involve new SRAM primitives. In the case of the low energy proton damaged device the reduction in CTI is quite evident.

These data were made for 9.17 second readout times - somewhat longer than the nominal 3.5 second ACIS readout rate. This is an unavoidable consequence of slower clock speeds. Another consequence is the decrease in collection area due to cosmic ray "blobs". If the smear time (ratio of transfer time to readout time) of 10% can be tolerated then a 24 x decrease in clock speed (for ACIS) should decrease the CTI by 3 for a loss of 6% of collection area due to cosmic rays.
A briefly explored variation of slow readout involves "dancing" the parallel clocks in a fashion familiar to proponents of "pocket-pumping". This also improves the CTI - but not really any more than the method illustrated above. In other words, it doesn't much matter how the details of the slow transfer are facilitated.
Details of PRAM modifications:

Sacrificial Charge The concept of sacrificial charge and its influence on CTI will be familiar to the reader. The techniques described here artificially place charge at predetermined locations in order to influence radiation damaged devices. There are 3 convenient sources of charge which may be employed: 1.) Natural (cosmic particle, x-ray, light leaks and dark current) 2.) Spurious dark current produced by clocking 3.) Charge injected via the output node. The later most is by the far the most wonderful to use but is only convenient for Astro-e sequencer users. The first 2 methods occur togther in varying degrees due to the necessary clocking operations on the one hand and the unavoidable presence of cosmic rays on the other.
Line Charge
The first technique involves placement of a single line (row) of charge at a predetermined location in the image array prior to the integration (exposure) period. Natural charge is collected in the image and frame store immediately after a readout. This charge is swept into the serial register which is held with all phases high to allow even distribution of charge. The charge is then moved backwards through the frame and image array to the desired location where it produces a line prior to the start of the next integration (exposure) period. CCD readout image Here is what the readout looks like. This ccid-17 has been damaged with 40 MeV protons. (ref) Notice the line of charge at row 800 (center of damaged area) and the cosmic rays, the charge from similar being the main contributors to the charge in the line itself. In general there is more than enough charge available from cosmic rays to generate a line of nearly full wells. An extra integration time of 2 seconds is about right but the actual time needs to be tuned for the background level. This example comes from the laboratory environment which has a lower background level than expected on orbit. A severe drawback with this technique is the loss of integration time during the frame-store readout. This means the readout time will be longer than the exposure time or, alternately put, the instrument will not have a 100% on time.
This sequencer modification requires additional SRAM to open the serial register and to move charge backwards in the parallel direction. Care must also be exercised in matching various SRAM segments to avoid the case of all three parallel phases momentarily in the same state. For this reason the actual number of SRAM segments may increase. When charge is injected via the output node the creation of the line charge is simplified but the on time (duty cyle) still suffers. One definite advantage of output node charge injection is the ease with which multiple lines can be produced.
The immediate consequence of the line charge is a restoration (partial?) of the amplitude of subsequent pixels. The degree of this improvement (for indeed it is such) depends upon the temperature as does the duration in time for which the effect of the line charge persists.
The parallel transfer rate in this case is the standard 40.00 microsecond per row. The interpretation of this amusing graph is left as an exercise for the student.

Charge Sweeping (Squeegee) The squeegee technique preserves a small number of rows, the charge in these rows is swept across the image array from top to bottom during the image to frame transfer, and then swept from bottom to top prior to exposure. Any charge in these rows is never removed except to fill traps. All the mechanisms which contribute to the artificial charge injection apply here. This technique does not impact the readout duty cylce. Experimentation shows that some number greater than 2 rows is needed, but even 2 rows does something. The success of this method depends upon long time scale traps (greater than readout time) which seems to be the case. Here is an example at -113 c:
In this case 48 rows were reserved to collect charge for the squeegee. The elimination of the long time scale traps has a lot to do with improving the resolution. It is rather a pity that the reserved rows occur at the center of the focal plane. It remains to be determined what the minimum number of required rows is.