Dark Current Experiment Preliminary Design Review 25 September 2001 Bernie Rauscher

1. SPACE TELESCOPE SCIENCE INSTITUTE Dark Current Experiment Preliminary Design Review 25 September 2001 Bernie Rauscher

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

3. Definition of Dark Current • Dark current is the signal that accumulates linearly in the absence of any light source • Important contributors to dark current include • Charge diffusion, • Thermal generation-recombination (GR) of charges within the semiconductor, & • Small leakage currents • Strictly speaking, dark current does not include effects such as amplifier glow and multiplexer glow. • Nevertheless, these effects have much the same effect as dark current and therefore require good characterization.

4. The importance of Reference Pixels • Experience by G. Finger, D. Hall, and others has shown that reference pixels are necessary for measuring dark current at ultra-low background levels • Reference pixels shall be used in a manner TBD to suppress correlated noise and drifts to a level well below the rms noise in each frame. • The next two slides illustrate the importance of reference pixels.

5. Darkcurrent Measurementusing open In bump bonds • Triangles:measured integration ramp • Diamonds:dead pixels • Open In bump bonds are used to monitor drifts • Squares:drift corrected integration ramp • darkcurrent at 28.5 K:0.017 e/s/pixel This slide courtesy of Gert Finger, ESO

6. Darkcurrent Measurementusing open In bump bonds • Triangles:measured integration ramp • Open In bump bonds are used to monitor drifts • Squares:drift corrected integration ramp • darkcurrent at 25 K:0.004 e/s/pixel

7. NGST Requirements from NGST Doc. 641 • NGST does not have a dark current requirement. Rather, total rms noise including contributions from read noise and dark current in a 1000 seconds exposure should satisfy the aforementioned requirements.

8. It shall be possible to measure the dark current of a detector meeting the NGST Project’s requirements to 10% accuracy. Assume 1/√2 of total = 2.5 e- in a 1000 s exposure is shot noise on dark current idark = 3.12510-3 e-/s/pixel This implies a requirement on light-tightness of the detector head enclosure =3.12510-4 e-/s/pixel Dark Current Experiment Requirements

9. There are implied requirements on per sample read noise The procedure (described later) will be to take samples at ~15 minute intervals for several (~4) hours and to fit a line. For this experiment, the per-sample rms noise requirement is =4.9 e- Reference pixel corrections shall be made so that drifts are negligible compared to rms noise Dark Current Experiment Requirements Least Square Line Fitting

10. Proposed Experiment Procedure • Blank off detector • If detector has been warm or exposed to light in past 24 hours, drain depletion regions by allowing enough time (TBD) for trapped charge to randomly bleed out of traps (24 hours?). • Stabilize detector bias and temperature • Obtain bias/dark MULTIACCUM ramp exposure as follows

11. Summary of Requirements

12. Proposed Experiment Variations • Variations • The following would be decided early on and frozen • Length of ramp • Number of samples (TBD) at each point on ramp • Select 1 pixel and do not clock (Cannot use reference pixels when doing this) • The following would be done for each detector • Temperature: 3 levels covering NGST range • Bias levels: 2 levels covering NGST requirement =6104 e- and goal =2105 e- for well capacity • Sample spacing along ramp: 2 spacings ~7.5 min and ~15 min to allow separating intrinsic dark current from glow • Combinations: 12 combinations per detector -> Approx. 1 week per SCA. • Dewar is big enough for 4 SCAs, could save time by testing 4 detectors at a time.

13. Proposed Experiment Duration • Dominant step in terms of schedule is length of ramp • Time estimate: 6 days per SCA (excluding cooldown/warm up) • Extended scope: • Single pixel readout using custom hardware reference • Use of off-chip pre-amp

14. Proposed Experiment Designs • Standard TFST hardware (dewar, Leach controller, etc.) • Blank off “top” for detector enclosure • Blank off “top” must include a light-tight vent to allow pumping. Alternatively, a slide can be closed once the system is fully under vacuum.

15. Data Reduction/Analysis Procedure • Data collected in passthrough mode • Reference pixel correction (TBD) • Fit slope on per-pixel basis

16. Expected Performance (Accuracy) • Requirements have been set to allow ~10% accuracy measuring NGST’s goal total noise • May be limited by light leaks

17. Schedule • Dark current is a fundamental parameter that directly effects sensitivity. • Should be one of the first “real” experiments done in lab.

18. Costs (Shopping List) • “Top” for detector head

19. Risks • System may not be light tight • Potential for major reworks in detector head. Can mitigate this risk by using aluminized tape for critical joints if found to be necessary. • Bias may be insufficiently stable • Can mitigate this with simple custom voltage reference if found to be necessary.

20. Actions