Persistence Experiment Preliminary Design Review 25 September 2001

Persistence Experiment Preliminary Design Review 25 September 2001 Don Figer SPACE TELESCOPE SCIENCE INSTITUTE 2/25/2021

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Goals of the Review • • Demonstrate that we know how to measure persistent charge Choose preferred experiment setups Choose items to purchase Generate actions 2/25/2021 2 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 2/25/2021 3 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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. 2/25/2021 4 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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. 2/25/2021 5 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Effects of Persistence • Example of persistence in NICMOS data 2/25/2021 6 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 In 2 O 3 near the Si. OX/In. Sb interface for Si. OX passivated In. Sb devices, although more updated information is now available from U. Rochester 2/25/2021 7 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Geometry of Detector Pixel from Solomon (1998; Ph. D Thesis) 2/25/2021 8 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Independent Variables • • • Previous flux Previous fluence Temperature Bias Time since last exposure Exposure time 2/25/2021 9 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization NGST Requirements NDC 0200 (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. 2/25/2021 10 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Persistence Experiment Requirements (from NDC 1200) Persistence Consistent with 1% worst-case photometry on NGST • 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). 2/25/2021 11 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Proposed Experiment Procedure 1. Drain depletion regions by blanking detector and allowing enough time for trapped charge to randomly bleed out of traps (24 hours? ). 2. Stabilize detector bias and temperature 3. Obtain bias/dark ramp frame 4. Set source flux 5. Reset and read detector 6. Illuminate detector for specific flux and fluence 7. Read detector N times up ramp 8. Blank off detector 9. Reset detector 10. Read detector M times up ramp to a typical NGST exposure time 11. Repeat sequence for range of source flux/fluences, temperatures, and bias voltages 12. Subtract bias/dark ramp frame from source frame 2/25/2021 12 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Proposed Experiment Variations • • Variations 1. Source flux 2. 3. 4. a. Over saturation b. Saturation c. Typical non-saturated flux/fluence d. 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: 1 a 2 c 3 a, 1 b 2 c 3 a, 1 c 2 c 3 a, 1 d 2 c 3 a, 2 a 1 c 3 a, 2 b 1 c 3 a, 2 d 1 c 3 a, 2 e 1 c 3 a, 3 b 1 c 2 c, plus combinations for wavelengths 2/25/2021 13 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Proposed Experiment Duration • • • Dominant step in terms of schedule is step 1 Time estimate: 10 days Extended scope: – – 2/25/2021 Thermal annealing Forward biasing 14 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Proposed Experiment Designs • Standard TFST hardware (dewar, Leach controller, etc. ) • Light source, approximately spatially flat, i. e. integrating sphere or white card 2/25/2021 15 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 2/25/2021 16 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 2/25/2021 17 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization 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 2/25/2021 18 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Costs (Shopping List) • Light source: can be just about anything, including ambient light/thermal radiation in lab • Pinhole masks 2/25/2021 19 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Risks • Experiment should be routine 2/25/2021 20 Persistence PDR

SPACE TELESCOPE SCIENCE INSTITUTE NGST Detector Characterization Actions • Add wavelength as a variation (probably do not need all permutations of this). • Look into buying pinhole masks. 2/25/2021 21 Persistence PDR