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

Goals of the Review Demonstrate that we know how to measure persistent charge Choose preferred experiment setups Choose items to purchase Generate actions 10 April 2019

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 10 April 2019

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. 10 April 2019

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. 10 April 2019

Effects of Persistence Example of persistence in NICMOS data 10 April 2019

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 10 April 2019

Geometry of Detector Pixel from Solomon (1998; PhD Thesis) 10 April 2019

Independent Variables Previous flux Previous fluence Temperature Bias Time since last exposure Exposure time 10 April 2019

NGST Requirements NDC0200 (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. 10 April 2019

Persistence Experiment Requirements (from NDC1200) 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). 10 April 2019

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 10 April 2019

Proposed Experiment 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 10 April 2019

Proposed Experiment Duration Dominant step in terms of schedule is step 1 Time estimate: 10 days Extended scope: Thermal annealing Forward biasing 10 April 2019

Proposed Experiment Designs Standard TFST hardware (dewar, Leach controller, etc.) Light source, approximately spatially flat, i.e. integrating sphere or white card 10 April 2019

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 10 April 2019

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 10 April 2019

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 10 April 2019

Costs (Shopping List) Light source: can be just about anything, including ambient light/thermal radiation in lab Pinhole masks 10 April 2019

Risks Experiment should be routine 10 April 2019

Actions Add wavelength as a variation (probably do not need all permutations of this). Look into buying pinhole masks. 10 April 2019