EEV44-82 CCDs Performance overview
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Oct/10/1999, C. Cavadore
 

Generality
 

The company EEV supplied ESO with many custom designed CCD called EEV44-82. This CCD has been used in many systems at ESO, such as WFI, VLTTC2, UVES, VIMOS, CES …

This device has 2 readout ports. The CCD can be operated either in dual readout mode using two ports, or single port readout. Binning along the X and Y direction is possible using any kind of binning factor provided that the scene is not oversatured. A fast wiping of the device can be achieved by using a dump gate close to the serial register.

This table summarizes main CCD geometrical dimension.
 

EEV 44-82	Full Frame / Frame transfer capable
Type : Backside, Single layer AR	Pixel size 15x15 mm 100% photosensitive
Number of photosensitive pixels	2048 x 4102 [HxV]	Number of outputs : 2
Size of the photosensitive area	30.72 x 61.53 mm
Horizontal pre-scan pixels	50

This device is 3 sides buttable. For each device, the theoretical non sensitivity side area is 100 microns and the Left-Right gaps are 450 microns each.
 
 

Noise

The following table shows noise performances achieved with FIERA at different readout speeds and conversion factors (e^-/ADU).
 
 

These noise figures may vary slightly according to the device used.

Quantum efficiency

EEV has optimized the QE to get good QE in the B and U bands. Nevertheless, depending on the device, the QE in this range may vary slightly. The explanation is the following: the Quantum Efficiency of these devices is strongly related to of the thickness backside Anti-Reflection coating and this thickness may vary according to the device (Hafnium Oxide 50nm). ESO measured 12 devices and used to achieve a statistic QE plot (figure A). This is a criterion also to sort science grade devices : the best QE in the blue (except for application requiring a higher red sensitivity).
 
 

Fringing
 

Fringing is an issue for spectroscopic purposes. ODT has measured this effect with 8 devices. The resulting parameter is called PRNU across the chip (Photo-response non-uniformity). The near IR PRNU depends upon the fringing effect. This effect is related to the thinning of the CCD, and also to the aperture of the incoming beam, the more this beam is open, the less the fringing is visible (figure B). In the blue part of the spectrum the PRNU degrades also due to the backside p+ implementation laser annealing. The images (figure C) show qualitatively these effects. Figure C is a statistical analysis over 8 CCDs.
 
 

Cosmetic defects

The different kind of defects, which degrade the cosmetic quality of the device, can be split in three parts :

• Defects visible in the bias images (mainly hot pixels)
• Defect visible with a low light level flat field (traps, figure F)
• Defects visible in the median stack of 5 1h exposed dark frame (mainly oversatured hot pixels, trails, figure D,E)

A defect is a pixel value above or below 5 sigmas from the mean of the neighborhood pixels.
These defects are based upon the grading of the device, typically a good science grade is one with less than 5 defective columns. The amount of defects visible on long dark exposure is strongly related to the CCD temperature. An operating CCD temperature of –120C is used to minimize these effects.

Also the CCD device is made of 1024x512 blocks, and the area boundaries show sometime a 1% QE variation over 1 row/column due to photolithography stepper mismatches. These small defects flat-field out perfectly.

Trap defects are a determining factor for assigning devices to certain applications. These kind of defects cannot be suppressed because they are created during chip fabrication, and are not temperature dependant.

Finally, the amount of the "defects" induced by cosmic rays should not be more than the one provided by natural radioactivity. The CCD package and the head of the detector do not add additional hits. This value is typically between 1 and 1.5 event/min/cm².
 
 

Charge transfer efficiency

ESO measurements shows horizontal CTE of 0.9999995 (six 9s, 5) and vertical CTE of 0.9999988 (almost six 9s). It means in the worst case : A pixel having 1000 electrons located on the opposite side to the readout port (X=2048, Y=4100) will lose 6 electrons, once the charge packet will reach the readout node (photon shot noise is about 31e^- at that level).

Dark Current

EEV44-82 devices do not have MPP implant. The dark current is temperature dependent as shown in figure G. The CCD must be cooled at a temperature less than –95C for science purposes to achieve a dark current less and equal to 10 e-/pixel/hour :
 
 

Amplifier glowing

To avoid amplifier glowing, voltages less than 25 volts must be set to the VOD node of the chip.

Remanence effects

Remanence effects may occur if a flat field (Mean>10000e-) is taken prior to the dark frame. The resulting effect resembles an increase of dark current, especially at temperatures lower than 180K. ODT is currently investigating to cure this problem. Figure H, shows a set of 24 dark frames exposed each one-hour (CCD temperature : 160K). This set was taken after the CCD was exposed to the ambient light. It requires four 1 hour darks frames to eliminate of the remanence and to reach the actual dark current value (about 5e-/pix/hour).
 
 

Bright Star/bright spots remanent might be also visible when doing long exposure afterward. Figure Ha shows an oversaturated star, with intense vertical blooming. Figure Hb shows one hour dark exposure taken just after at –120C. The blooming and central spot ghost are still visible. If additional images are taken, it disappears after 4 hours.  From that study, It means that remanance has a global and local effects.
 
 

	T+1h	T+2h	T+3h
Central Bulb remanent (e-)	8	6	2
Blooming remanent (e-)	2	1	0

Linearity

By optimizing the voltages applied to the CCD readout amplifier, a good linearity can be achieved in the range of 0-100Ke^- without degrading noise performance. The table below summarizes a typical linearity performance from an EEV device at 225Kps.
 
 
 

Linearity
	Speed (kps)	Non-Linearity		Range (ADU)	saturation level (e-)
		Peak-peak (%)	Rms (%)
Port A (left bottom)	225	+0.20 / -0.35	0.15	0-62000	96k
Port B (right bottom)	225	+0.20 / -0.35	0.15	0-62000	95k
Left and Right Port (225kps)

 

Best performance is achieved with p-p linearity of ? 0.35%. Special voltage optimization is performed to get ? 0.5% at least across the full range.
 

Flatness

ODT has developed a special measurement device to acquire full chip surface profile. A typical value is typically ? 6 microns at 150K, a maximum value is ? 10 microns p-p.
 
 

MTF

According to ODT’s latest measurements, the CCD shows no serious degradation from the Nyquist curve. The very sharp hits resulting from the cosmic ray impacts show that 95% of the energy released is within one pixel.
 
 

Binning eclipse effect on saturated stars

When using binning more than 1x1, a saturated star appears with a strange black hole in the middle. The CCD output amplifier is, in this particular state, completely out of range and behaves strangely. As there is, anyway, no useful information in the center of a saturated star (no photometry/astrometry feasible), this effect is more a cosmetic issue (which could be eliminated by software) than scientific.
 
 

Blooming effect
 

To avoid loss of fill-factor, this CCD has not antiblooming implant. This causes vertical trails over bright and saturated stars. The table below gives a crude indication of the order of magnitude for point sources. L1 denotes the number of pixels concerned toward the serial register, L2 the one in the opposite direction. Measurements made with binning 1x1.
 
 

Direction	Overexposure factor
	2	4	5.8	10	25	50	100	1000
L1(pixels)	0	0	1	2	8	19	39	344
L2(pixels)	0	0	1	3	5	8	13	136

Cross talk effect
 

This CCD allowing two port readout in the same time could exhibit cross talk between two channels. It could be visible when a bright star is present over the CCD. This effect has been measured at ESO. If an over saturated bright spot is projected over one side of the CCD, ghost having an additional fix level of 100 e- is visible on the other side, with the same shape of the original bright spot. This is, for all imaging purposes, a negligible effect.
 
 

figure L : Cross ta with a saturated star and 2 port readout
NEGATIVE IMAGE.

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