The Second Generationof IR detectors for WFC3

Massimo Robberto

European Space Agency

and

Space Telescope Science Institute

Summary

1. Status of the Wide Field Camera 32. First Generation IR detectors3. Second Generation Detectors4. Preliminary results on Substrate Removed parts5. Scientific gain with Substrate Removal6. WFC3 status: an update

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 (1.7m) using two different optical channels

– UVIS: based on EEV CCD43 4k2k– IR: based on RSC H1R 1k1k

• 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

FPA64 is currently the flight-prime WFC3 IR detector

• FPA64 is HgCdTe with 1.72m cutoff produced by Rockwell Scientific Company (RSC)

• Type: - MBE grown on ZnCdTe substrate - Hawaii-1R MUX

• Operating T: 150K

• Pixel size: 18 m

• Format: 1024 1024 pixels

1014 1014 active pixels - 5 outer rows/column with reference pixels

Quantum Efficiency (80% poeak at 1.6m)

Readout noise ~15e (16 read/ramp)

FPA64 Thermal Vacuum performance

Dark current at 150K less than 0.2e/s/pix

Linear scale

Logarithmic scale

Radiation tests on FPA67 at UC Davies

Detector radiated with ~900 hits/sec/cm-2 of 62 MeV protons

The picture shows:

• Uniform distribution of proton hits

• Additional background “glow”

The illumination pattern of the radiation-induced background is similar to the short wavelength QE response of the detector!

See Poster of B. Hill for details

Second generation WFC3 detectors

proton

VIS photon

IR photon

proton

First generation (FPA64) Second generationwith substrate removed

VIS photon

IR photon

Substrate removal:1. Eliminates radiation induced glow2. Increases the QE and moves the blue cutoff at shorter wavelengths

QE gain with substrate removal

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.

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.

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.

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.

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.

Speed is defined as 1/time (in nr.of orbits)

FPA 64

Baseline

FPA 64

CR glow

High

FPA 64

CR glow

Moderate

New FPA

Baseline

New FPA

High RON

New FPA

Low RON

F110W 1.54 0.70 1.25 2.88 2.49 3.05

F160W 1.05 0.47 0.72 1.05 0.85 1.15

F126N 1.15 0.27 0.71 2.12 1.35 2.68

Speed

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

F110W F160W F126N F093M

FPA64

FPA64+1.0e/s

FPA64+0.25e/s

SR-BL

SR-highRON

SR-lowRON

NIC3-ACSSpeed normalized toNIC3/ACS

Absolutespeed

Discovery efficiency

Discovery Efficiency

0.00

5.00

10.00

15.00

20.00

25.00

30.00

F110W F160W F126N F093M

FPA64

FPA64+1.0e/s

FPA64+0.25e/s

SR-BL

SR-highRON

SR-low RON

NIC3-ACS

FPA 64

Baseline

FPA 64

CR glow

High

FPA 64

CR glow

Moderate

New FPA

Baseline

New FPA

High RON

New FPA

Low RON

F110W 7.33 3.32 5.97 13.74 11.88 14.56

F160W 5.00 2.23 3.44 5.00 4.03 5.48

F126N 5.48 1.27 3.40 10.13 6.43 12.79

defined as speed field of view (arcmin)

Normalized toNIC3/ACS

absolute

Ultra-deep sensitivity

Ultra-deep sensitivity

28.00

28.50

29.00

29.50

30.00

30.50

mA

B

mAB(F098M) in 200 orbitsmAB(F110W) in 200 orbitsmAB(F160W) in 200 orbitsmAB(UD)

limiting magnitude reached with SNR=10 in 200 orbitsFPA 64

Baseline

FPA 64

CR glow

High

FPA 64

CR glow

Moderate

New FPA

Baseline

New FPA

High RON

New FPA

Low RON

k

e

y

mAB(F110

W)29.86 29.43 29.75 30.20 30.12 30.23

mAB(F160

W)29.65 29.21 29.45 29.65 29.54 29.70

mAB(F098

M)29.12 28.45 28.93 29.67 29.52 29.74

Optimal distribution of orbits

UDF-like problem: how to distribute 600 HST orbits between z, J and H

FPA 64

Baseline

FPA 64

CR glow

High

FPA 64

CR glow

Moderat

e

New FPA

Baseline

New FPA

High

RON

New

FPA

Low

RON

Deep field

NIC3

or ACS

Wide

field

Nic3

or ACS

Nr of orbits

F100W94.2 69.9 83.4 93.7 85.9 97.7 104.3 233.0

Nr of orbits

F160W138.0 104.1 144.6 257.2 252.7 259.5 104.7 233.7

Nr of orbits

F098M/

F850LP

367.8 426.0 372.0 249.1 261.5 242.8 391.0 133.3

mAB 29.45 28.86 29.27 29.79 29.66 29.84 29.11 28.52

0.0

100.0

200.0

300.0

400.0

500.0

600.0

Nr of orbits F110W

Nr of orbits F160W

Nr of orbits F098M

The UDF sensitivity is controlled bythe z-band filter F098M/F850LP

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.

Hubble Space Telescope discovery efficiency

HELP IS ON THE WAY!