Bulk Silicon CCDs, Point Spread Functions, and Photon Transfer Curves: CCD Testing Activities at ESO Mark Downing, Dietrich Baade, Sebastian Deiries, (ESO/Instrumentation

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 2009 1DfA 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, &

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• 16 um standard silicon - 100 ohm-cm.




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• 16 um standard silicon - 100 ohm-cm. • 40 um deep depletion - 1500 ohm-cm.




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• 16 um standard silicon - 100 ohm-cm. • 40 um deep depletion - 1500 ohm-cm. • 70 um bulk silicon - > 3000 ohm-cm.




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• 16 um standard silicon - 100 ohm-cm. • 40 um deep depletion - 1500 ohm-cm. • 70 um bulk silicon - > 3000 ohm-cm. • 150 um high-rho - > 3000 ohm-cm.


• System designers can now chose the silicon thickness that best suit their application.


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• 16 um standard silicon - 100 ohm-cm. • 40 um deep depletion - 1500 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


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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.


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Nothing unusual in performance and as good as other family members


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Parameter Results(-120 ºC)

Comment

Device Bulk Silicon Science

Serial Number 07382-24-01

Type Number CCD44-82

Extensively used at ESOPixel Size 15μm

Number of Pixels 2048 x 4096

Noise (50 kpix/s) < 2.5 e- rms Gain of 0.6 e-/ADU

Noise (225 kpix/s) < 4 e- rms Gain of 1.6 e-/ADU

Linearity (500e- to 100 ke-)

< ± 0.2%Photon Transfer Curve method - not fully optimized

Dark Current(e-/pixel/hour)

< 0.2Limited by extraneous sources and not CCD

Cosmic hit event rate (events/min/cm²)

3.0

Vertical CTE 0. 9999991 EPER – Extended Pixel Edge ResponseHorizontal CTE 0. 999996

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.


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ESO-e2v QE agree






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ESO-e2v QE agree






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PRNU Measured with 7nm Bandwidth

• PRNU shows very low fringing in the “red”.


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• 70 um bulk silicon




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• 70 um bulk silicon• 40 um deep depletion• 16 um standard silicon




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• 70 um bulk silicon• 40um deep depletion• 16 um standard silicon

900nm Images

16um Std Si 40um DD 70um Bulk

5 – 95% Histogram Scaling


• PRNU shows very low fringing in the “red”.• At shorter wavelengths (< 400 nm) PRNU is a little worse

due to thinning and laser annealing.


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• 70 um bulk silicon• 40um deep depletion• 16 um standard silicon

350nm Images


5 – 95% Histogram Scaling

Very acceptable cosmetics at -120 DegC


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Blemish Spec. Number

Hot pixels > 100e-/pixel in 1 hour dark 14

Hot Columns > 100 bad pixels in 1 hour dark 0

Dark pixels < 50% response 112

Dark Columns > 100 bad pixels 0

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.• -120 DegC is ESO’s standard operating temperature for all

CCD44-82s at the observatories.• Hot pixels are due to impurities in the silicon.


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Temperature(DegC)

Average Dark Current

(e-/pix/hr)

Number of Hot Pixels

(1 hour dark)

Comments

-120 0.2 14

-100 5.5 160500 Average dark current dominated by hot pixels.-80 310 661025

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.• -120 DegC is ESO’s standard operating temperature for all

CCD44-82s at the observatories.• Hot pixels are due to impurities in the silicon.


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Temperature(DegC)

Average Dark Current

(e/pix/hr)

Number of Hot Pixels

(1 hour dark)

Comments

-120 0.2 14

-100 5.5 160500 Average dark current dominated by hot pixels.-80 310 661025

-80DegC -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.


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OverscanPixels

OverscanPixels

OverscanPixels

Edges have no hot 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


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PSF


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PSF depends on CCD design and wavelength

PSF depends on:

1. Thickness of the undepleted region (XUNDEP) 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) XTHICK

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.


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400nm

600nm

UV

VIS

900nm

RED

CCDBackside

Cross Section of CCDCross Section of CCD

ElectricField Extent

Collectionphase

CCDFrontside

PotentialWell

Vc

Vsub

XTHICK

XUNDE

P

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).


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400nm

600nm

UV

VIS

900nm

RED

CCDBackside

Cross Section of CCDCross Section of CCD

ElectricField Extent

Collectionphase

CCDFrontside

PotentialWell

Vc

Vsub

XTHICK

Virtual Knife Edge

1um SpotSub Window

Pixel

Spot scanning used to measure PSF


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70um Bulk CCDGauss Fit PSF FWHM of MIT/LLCCID-20 DD CCD versus

wavelength and collection phase voltage

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

400 500 600 700 800 900

Wavelength (nm)

PS

F F

WH

M (

pix

els

)

Ph=2V

Ph=4V

Ph=6V

Ph=8V

Ph=10V

Ph=12V

40um MIT/LL Hi Rho

Gauss Fit PSF FWHM of e2v CCD44-82 Std Silicon CCD versus wavelength and collection phase voltage

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

400 500 600 700 800 900

Wavelength (nm)

PS

F F

WH

M (

pix

els

) Ph=2V

Ph=4V

Ph=6V

Ph=8V

Ph=10V

Ph=12V

16um e2v Std Si

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

400 500 600 700 800 900

PS

F F

WH

M (p

ixels

)

Wavelength (nm)

Gauss Fit PSF FWHM of e2v CCD44-82 Deep Depletion CCD versus wavelength and collection

phase voltage

Ph=2V

Ph=4V

Ph=6V

Ph=8V

Ph=10V

Ph=12V

40um e2v Deep DepletionPSF Results

VcVc

VcVc

Photon Transfer Curves


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Photon Transfer Family of Curves


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70um Bulk CCD

16um 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


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70um Bulk CCD

40um E2v Deep Depletion16um e2v Std Si

Gain ~ 11e/ADU

Bloomed Full Well

Surface Full WellBFW=SFW

Vc

Optimum full well = ~ 2V

40 um Deep Depletion starts to show interesting behaviour


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70um Bulk CCD

16um E2v Std Si40um e2v Deep Depletion

Gain ~ 11e/ADU

Bloomed Full Well

Surface Full WellBFW=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.


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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


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16um E2v Std Si 40um E2v Deep Depletion

Gain ~ 11e/ADU

70um Bulk CCD

Full Well versus Collection Phase Voltage

• Choose trade between PSF improvement and well depth


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70um Bulk CCD

Vc

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.


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END

Many thanks

13 Oct 2009 34DfA 2009: Bulk CCD, PSF, &

PTC.

Documents

Bulk Silicon CCDs, Point Spread Functions, and Photon Transfer Curves: CCD Testing Activities at ESO Mark Downing, Dietrich Baade, Sebastian Deiries, (ESO/Instrumentation