Persistence Experiment Preliminary Design Review 25 September 2001 Don Figer

1. SPACE TELESCOPE SCIENCE INSTITUTE Persistence Experiment Preliminary Design Review 25 September 2001 Don Figer

2. Goals of the Review • Demonstrate that we know how to measure persistent charge • Choose preferred experiment setups • Choose items to purchase • Generate actions

3. Definition of Persistence • Persistence is the portion of the signal that is produced by light/particle sources in previous images. Anything that liberates charge into the conduction band can result in latent charge, i.e. a bright star or a cosmic ray. • It does not include electronic “ghosts” or crosstalk between electronic readout channels. • Also called: Latent Charge or Memory Effects

4. Definition of Persistence • Example of persistence in 10 sec. “dark” exposures. • Persistent charge adds significant noise, even after waiting many minutes after the initial exposure.

5. Effects of Persistence • Example of persistence in 60 sec. “science” exposure. • Note the nearly vertical wide band of signal superposed on the science scene. • Some charge is missing from original exposure. • Some charge is added to new exposure. • Results are inaccurate photometry and increased systematic errors.

6. Effects of Persistence • Example of persistence in NICMOS data

7. Source of Persistence • Negative charge is trapped in sites with excess positive charge (hole sites) • Sites are a result of incomplete (charged) molecular bonds • Could be in semiconductor, or could be at interface layers where dissimilar materials chemically bond, i.e. between the semiconductor and passivation layer • Solomon (1998) claims that the dominant traps are In2O3 near the SiOX/InSb interface for SiOX passivated InSb devices, although more updated information is now available from U. Rochester

8. Geometry of Detector Pixel from Solomon (1998; PhD Thesis)

9. Independent Variables • Previous flux • Previous fluence • Temperature • Bias • Time since last exposure • Exposure time

10. NGST RequirementsNDC0200 (from NGST Doc. #641) • Note that the requirement is in violation of the NGST requirement on photometric accuracy, i.e. 1% worst-case photometry. Taking an example, an after image of 0.1% of a very bright object will produce photometric errors far in excess of 1% of a faint object that happens to be located in the same area of the detector as the bright object. • Note that the specification is incomplete in that it does not specify time since previous exposure, saturation level of previous exposure, or integration time. One might hope to define a new specification that encapsulates “typical” NGST operating conditions.

11. Persistence Experiment Requirements(from NDC1200) • We can require a measurement accuracy that produces no more than 10% error in our measurement in the case that the total system noise goal is dominated by error due to latent charge. In this case, latent charge would produce 2.5 e- per pixel, and our measurement accuracy would have to be 1.2 e- per pixel. Of course, it would be very challenging to make such precise measurements, given that this accuracy is 10% of the read noise goal (in quadrature).

12. Proposed Experiment Procedure • Drain depletion regions by blanking detector and allowing enough time for trapped charge to randomly bleed out of traps (24 hours?). • Stabilize detector bias and temperature • Obtain bias/dark ramp frame • Set source flux • Reset and read detector • Illuminate detector for specific flux and fluence • Read detector N times up ramp • Blank off detector • Reset detector • Read detector M times up ramp to a typical NGST exposure time • Repeat sequence for range of source flux/fluences, temperatures, and bias voltages • Subtract bias/dark ramp frame from source frame

13. Proposed Experiment Variations • Variations • Source flux • Over saturation • Saturation • Typical non-saturated flux/fluence • Starvation • Temperature: 5 levels (a through e, c optimal) covering NGST range • Bias levels: 2 levels covering NGST requirement (a) and goal (b) for well capacity • Wavelength (not clear how many variations) • Combinations: 1a2c3a, 1b2c3a, 1c2c3a, 1d2c3a, 2a1c3a, 2b1c3a, 2d1c3a, 2e1c3a, 3b1c2c, plus combinations for wavelengths

14. Proposed Experiment Duration • Dominant step in terms of schedule is step 1 • Time estimate: 10 days • Extended scope: • Thermal annealing • Forward biasing

15. Proposed Experiment Designs • Standard TFST hardware (dewar, Leach controller, etc.) • Light source, approximately spatially flat, i.e. integrating sphere or white card

16. Data Reduction/Analysis Procedure • Subtract dark/bias ramp frames from illuminated and blanked ramp frames • Plot recaptured charge rate versus time during blanked ramp frame for range of source flux/fluences, detector temperatures, and bias voltages

17. Expected Performance (Accuracy) • For saturated source exposure, 0.1% is equivalent to 50-100 e-, a level easily measured • Ultimately, we will be limited by uncertainties due to read noise and dark current

18. Schedule • Because read noise and dark current noise will dominate most accurate measurements, the persistent charge experiment should be done after the system is optimized • Estimate that the test set would take about 15 days

19. Costs (Shopping List) • Light source: can be just about anything, including ambient light/thermal radiation in lab • Pinhole masks

20. Risks • Experiment should be routine

21. Actions • Add wavelength as a variation (probably do not need all permutations of this). • Look into buying pinhole masks.