The Second Generation of IR detectors for WFC3

1. The Second Generationof IR detectors for WFC3 Massimo Robberto European Space Agency and Space Telescope Science Institute

2. Summary • Status of the Wide Field Camera 3 • First Generation IR detectors • Second Generation Detectors • Preliminary results on Substrate Removed parts • Scientific gain with Substrate Removal • WFC3 status: an update

3. 1. The Wide Field Camera 3 (WFC3) • WFC3 is the last imager built for the Hubble Space Telescope (HST). It will replace the WFPC2 camera. • WFC3 provides diffraction limited imaging from the UV (2000 Å) to the near IR (1.7mm) using two different optical channels • UVIS: based on EEV CCD43 4k2k • IR: based on RSC H1R 1k1k • WFC3 optics deliver • UVIS: 0.05" 0.05" pixel scale ~3.3' 3.3' field of view • IR: 0.135" 0.135" pixel scale ~ 2.2' 2.2' field of view • Status of WFC3 • WFC3 has successfully conducted thermal vacuum performance characterization test at the NASA Goddard Space Flight Centter (GSFC) in September 2004 • The instrument is currently at GSFC for partial de-integration and fix of residual liens • In the forthcoming months WFC3 will be re-integrated, tested, calibrated, and readied for mission I&T

4. FPA64 is currently the flight-prime WFC3 IR detector • FPA64 is HgCdTe with 1.72mm cutoff produced by Rockwell Scientific Company (RSC) • Type: - MBE grown on ZnCdTe substrate - Hawaii-1R MUX • Operating T: 150K • Pixel size: 18 mm • Format: 1024 1024 pixels 1014 1014 active pixels - 5 outer rows/column with reference pixels

5. FPA64 Thermal Vacuum performance Dark current at 150K less than 0.2e/s/pix Linear scale Logarithmic scale Readout noise ~15e (16 read/ramp) Quantum Efficiency (80% poeak at 1.6mm)

6. Radiation tests on FPA67 at UC Davies The illumination pattern of the radiation-induced background is similar to the short wavelength QE response of the detector! Detector radiated with ~900 hits/sec/cm-2 of 62 MeV protons The picture shows: • Uniform distribution of proton hits • Additional background “glow” See Poster of B. Hill for details

7. Second generation WFC3 detectors First generation (FPA64) Second generation with substrate removed proton VIS photon IR photon proton VIS photon IR photon • Substrate removal: • Eliminates radiation induced glow • Increases the QE and moves the blue cutoff at shorter wavelengths

8. QE gain with substrate removal

9. IR Detector Procurement Progressing Smoothly at Rockwell • Procurement of substrate-removed IR detectors calls for two lots: • 6 layers (wafers) in each lot, 4 FPAs on each layer = 48 potential FPAs • Plan is to package and test at least one device from each layer, 18 FPAs total • Lot One status: • All layers have been grown and processed • One hybridized device from each layer is now into substrate-removal steps • One additional hybridized device has small edge chip; set aside in stable state • Substrate-removal process to be followed by passivation and wirebonding • RSC test results mid-May through mid-June; DCL results few weeks later • Lot Two status: • All layers have been grown, are partway through layer processing • RSC test results and deliveries to DCL begin early July • The additional 6 planned devices will be built from the highest performance layers.

10. Review of Radiation-Induced Background Issue • A radiation-induced background has been seen in radiation testing of the WFC3 IR detectors. • Testing indicated that the likely cause is particle-induced luminescence in the thick CdZnTe substrate. • The best estimate of the effect on sensitivity/discovery efficiency in orbit is a modest degradation (~25%), but this estimate is very uncertain. • Since the time of the initial WFC3 IR detector procurement, Rockwell has demonstrated the ability to remove the substrate of their HgCdTe arrays.

11. Substrate-Free Devices • Late last year Rockwell began substrate removal from 5 existing WFC3 detector die. • Four devices have been delivered to date. • 5th device is being delivered this week. • Primary goal was to demonstrate that devices without the substrate do not suffer from the radiation enhanced background seen in earlier radiation testing. • Second goal was to demonstrate improved QE performance of devices without substrates. • Two substrate-free devices were radiation tested in the same manner as earlier tests.

12. Second Generation IR Detector Lots • Rockwell has begun fabrication of two new lots of detectors for WFC3. • There are six wafers per lot, with four devices per wafer. • The wafers are being fabricated with the same prescription used to produce the current WFC3 flight detectors. • All detectors will have substrates removed. • Rockwell will package and deliver up to nine devices from each lot.

13. Plan For Flight FPA Replacement • The first devices from Lot 1 are currently in substrate removal, with the first devices expected to be available in early June. • 2nd Lot wafers are approximately two months behind Lot 1. • Devices will be delivered through the summer and tested in the DCL. • The best will be selected for packaging by Ball and eventual installation into WFC3 for flight. • Success in this development will eliminate the radiation-induced background risk and could substantially improve the sensitivity of the IR channel as well.

14. Speed is defined as 1/time (in nr.of orbits) Absolute speed Speed normalized to NIC3/ACS

15. Discovery efficiency defined as speed  field of view (arcmin) absolute Normalized to NIC3/ACS

16. Ultra-deep sensitivity limiting magnitude reached with SNR=10 in 200 orbits

17. Optimal distribution of orbits UDF-like problem: how to distribute 600 HST orbits between z, J and H The UDF sensitivity is controlled by the z-band filter F098M/F850LP

18. Conclusions • FPA64 with radiation induced background: • the worse case scenario (1e/s/pix of extra dark current) has a major impact on speed (more than a factor of 2) • New FPA with substrate removed: • High QE in the blue offers a major improvement in speed • Higher readout noise is acceptable, especially for broad-band surveys (e.g. UDF) • Dramatic increase of discovery efficiency in the J band (up to ~25 times vs. NIC3). • F160W remains slower than F110W, mostly due to relatively narrow passband.

19. Hubble Space Telescope discovery efficiency

20. HELP IS ON THE WAY!