Bulk Silicon CCDs, Point Spread Functions, and Photon Transfer Curves: CCD Testing Activities at ESO Mark Downing, Dietrich Baade, Sebastian Deiries, (ESO/Instrumentation Division), Paul Jorden (e2v technologies). 13 Oct 20091DfA 2009: Bulk CCD, PSF, & PTC. Agenda: Bulk Silicon CCDs PSF Photon Transfer Curves

The CCD Silicon Family System designers can now choose the silicon thickness that best suits their application. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.2 16 um standard silicon ohm-cm.

The CCD Silicon Family System designers can now choose the silicon thickness that best suits their application. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.3 16 um standard silicon ohm-cm. 40 um deep depletion ohm-cm.

The CCD Silicon Family System designers can now choose the silicon thickness that best suits their application. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.4 16 um standard silicon ohm-cm. 40 um deep depletion ohm-cm. 70 um bulk silicon - > 3000 ohm-cm.

The CCD Silicon Family System designers can now choose the silicon thickness that best suits their application. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.5 16 um standard silicon ohm-cm. 40 um deep depletion ohm-cm. 70 um bulk silicon - > 3000 ohm-cm. 150 um high-rho - > 3000 ohm-cm.

The CCD Silicon Family System designers can now chose the silicon thickness that best suit their application. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.6 16 um standard silicon ohm-cm. 40 um deep depletion ohm-cm. 70 um bulk silicon - > 3000 ohm-cm. 150 um high-rho - > 3000 ohm-cm. 300 um high-rho - > 3000 ohm-cm.

Bulk Silicon CCD 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.7

Why the Interest in Bulk Silicon? Pin and mechanically compatible with existing detector family. Upgrades are plug and play →No rewiring of cryostats, →No modification to controllers (to provide high voltages), →Standard clock and bias voltages used →No re-writing of timing patterns. Improve observing efficiency in the “red” without major costs in manpower, controller, instrument down time or schedule risk. Objective - to prove performance is as good as current CCDs. One engineering and one science grade loaned by e2v for test and evaluation. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.8

Nothing unusual in performance and as good as other family members 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.9 ParameterResults (-120 ºC) Comment DeviceBulk Silicon Science Serial Number Type NumberCCD44-82 Extensively used at ESO Pixel Size15μm Number of Pixels2048 x 4096 Noise (50 kpix/s)< 2.5 e- rmsGain of 0.6 e-/ADU Noise (225 kpix/s)< 4 e- rmsGain of 1.6 e-/ADU Linearity (500e- to 100 ke-) < ± 0.2% Photon Transfer Curve method - not fully optimized Dark Current (e-/pixel/hour) < 0.2 Limited by extraneous sources and not CCD Cosmic hit event rate (events/min/cm²) 3.0 Vertical CTE EPER – Extended Pixel Edge Response Horizontal CTE

ESO-e2v QE agree Good agreement between e2v and ESO results. Determining QE depends on knowing gain precisely and care must be taken as: –calculated gain varies with signal level when using the photon transfer curve (SPIE 2006 Downing et. al.). Recommend using binning (2x2 or 4x4) to determine gain. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.10

ESO-e2v QE agree Good agreement between e2v and ESO results. Determining QE depends on knowing gain precisely and care must be taken as: –calculated gain varies with signal level when using the photon transfer curve (SPIE 2006 Downing et. al.). Recommend using binning (2x2 or 4x4) to determine gain. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.11

ESO-e2v QE agree Good agreement between e2v and ESO results. Determining QE depends on knowing gain precisely and care must be taken as: –calculated gain varies with signal level when using the photon transfer curve (SPIE 2006 Downing et. al.). Recommend using binning (2x2 or 4x4) to determine gain. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.12

PRNU Measured with 7nm Bandwidth PRNU shows very low fringing in the “red”. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC um bulk silicon

PRNU Measured with 7nm Bandwidth PRNU shows very low fringing in the “red”. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC um bulk silicon 40 um deep depletion 16 um standard silicon

PRNU Measured with 7nm Bandwidth PRNU shows very low fringing in the “red”. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC um bulk silicon 40um deep depletion 16 um standard silicon 900nm Images 16um Std Si40um DD 70um Bulk 5 – 95% Histogram Scaling

PRNU Measured with 7nm Bandwidth PRNU shows very low fringing in the “red”. At shorter wavelengths (< 400 nm) PRNU is a little worse due to thinning and laser annealing. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC um bulk silicon 40um deep depletion 16 um standard silicon 350nm Images 16um Std Si40um DD 70um Bulk 5 – 95% Histogram Scaling

Very acceptable cosmetics at -120 DegC 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.17 BlemishSpec.Number Hot pixels> 100e-/pixel in 1 hour dark14 Hot Columns> 100 bad pixels in 1 hour dark0 Dark pixels< 50% response112 Dark Columns> 100 bad pixels0 Traps> 200e-2 Traps

At higher temperatures, hot pixels become a problem. Hot pixels are highly temperature dependent. At < -120 DegC, hot pixels are not a problem and the device is of excellent scientific grade DegC is ESO’s standard operating temperature for all CCD44-82s at the observatories. Hot pixels are due to impurities in the silicon. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.18 Temperature (DegC) Average Dark Current (e-/pix/hr) Number of Hot Pixels (1 hour dark) Comments Average dark current dominated by hot pixels

At higher temperatures, hot pixels become a problem. Hot pixels are highly temperature dependent. At < -120 DegC, hot pixels are not a problem and the device is of excellent scientific grade DegC is ESO’s standard operating temperature for all CCD44-82s at the observatories. Hot pixels are due to impurities in the silicon. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.19 Temperature (DegC) Average Dark Current (e/pix/hr) Number of Hot Pixels (1 hour dark) Comments Average dark current dominated by hot pixels DegC-100DegC-120DegC

Hot pixels are fixable. Note no hot pixels at the edges. e2v are working on it to have the whole device as good as the edges. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.20 Overscan Pixels Overscan Pixels Overscan Pixels Edges have no hot pixels CCD Top CCD Bottom -80DegC 1 hour dark

Effect of cosmic rays become worse with thicker devices The thicker the device, the more chance that a cosmic ray will affect more than one pixel. –Standard Silicon - mostly single pixel events –Deep Depletion – mix of single and multiple pixel events –Bulk – mostly multiple pixel events Expect number of events to scale with thickness. –Deep Depletion Cosmic hit event rate: ~ 1.8 events/min/cm² –Bulk Cosmic hit event rate: ~ 3.0 events/min/cm² –~ ratio of 70/40 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.21 16um Std Si40um DD 70um Bulk

PSF 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.22

PSF depends on CCD design and wavelength PSF depends on: 1.Thickness of the undepleted region (X UNDEP ) at the back of the CCD. 2.The strength of the electric field to draw the electrons into the potential well depends on:  (Vc – Vsub)  X THICK 3.Wavelength and depth of penetration of the photon  Blue photons are in general absorbed nearer the silicon surface and have farther to travel.  While red photons penetrate on the average deeper into the silicon. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC nm 600nm UV VIS 900nm RED CCD Backside Cross Section of CCD Electric Field Extent Collection phase CCD Frontside Potential Well Vc VsubX THICK X UNDEP

PSF can be improved by increasing the number of collection phases and the voltage across the CCD PSF can be improved by : Increasing the extent (i.e. reduce undepleted region at back of CCD) and strength of the electric field by:  Increasing the collection phase voltage (Vc).  Decreasing the substrate voltage (Vsub).  Increasing the number of collecting phases (2 for 3 phase device or 3 for 4 phase device). 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC nm 600nm UV VIS 900nm RED CCD Backside Cross Section of CCD Electric Field Extent Collection phase CCD Frontside Potential Well Vc VsubX THICK Virtual Knife Edge 1um Spot Sub Window Pixel Spot scanning used to measure PSF

13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.25 70um Bulk CCD 40um MIT/LL Hi Rho16um e2v Std Si40um e2v Deep Depletion PSF Results Vc

Photon Transfer Curves 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.26

Photon Transfer Family of Curves 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.27 70um Bulk CCD16um e2v Std Si Increasing the collection phase voltage to improve the PSF impacts the well depth. 40um e2v Deep Depletion Gain ~ 11e/ADU

16um Standard Silicon is well behaved; text book 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.28 70um Bulk CCD40um E2v Deep Depletion 16um e2v Std Si Gain ~ 11e/ADU Bloomed Full Well Surface Full Well BFW=SFW Vc Optimum full well = ~ 2V

40 um Deep Depletion starts to show interesting behaviour 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.29 70um Bulk CCD16um E2v Std Si 40um e2v Deep Depletion Gain ~ 11e/ADU Bloomed Full Well Surface Full Well BFW=SFW Vc Note interesting behaviour at 2-5V.

Interesting behaviour up to where surface full well dominate Behaviour enables a larger full well and different optimum full well voltage to standard silicon; 6V versus 2V. No explanation yet for the behaviour. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.30 Between 2-4V, starts to bloom at low signal level but then recovers at higher levels. No blooming Blooming Minimal blooming At 6V, no blooming is observed. No blooming Blooming

70 um Bulk is even more pronounced 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.31 16um E2v Std Si40um E2v Deep Depletion Gain ~ 11e/ADU 70um Bulk CCD

Full Well versus Collection Phase Voltage Choose trade between PSF improvement and well depth 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.32 70um Bulk CCD Vc Optimum full well = ~ 6V

Conclusion Performance at -120DegC of noise, gain, linearity, cosmetic, dark current, and CTE is as good as previous e2v CCD44-82s. Below -120DegC, hot pixels are not a problem. Bulk delivers better QE in the “red” and much less fringing. With PSF of ~ 1 pixel, the bulk CCD is very suitable for not too demanding optical designs. PSF can be improved by increasing collection phase voltage or running at a lower active substrate voltage. When increasing collection phase voltage, one has to be careful about change in well depth. As the resistivity of the silicon is increased, the photon transfer curve becomes more interesting and does not agree with the text books. 13 Oct 2009DfA 2009: Bulk CCD, PSF, & PTC.33

END Many thanks 13 Oct DfA 2009: Bulk CCD, PSF, & PTC.