Dark current

Every material at a temperature much above absolute zero will be subject to thermal noise within. For silicon in a CCD, this means that when the thermal agitation is high enough, electrons will be freed from the valence band and become collected within the potential well of a pixel. When the device is readout, these dark current electrons become part of the signal, indistinguishable from astronomical photons. Thermal generation of electrons in silicon is a strong function of the temperature of the CCD, which is why astronomical use generally demands some form of cooling (McLean, 1997b). Figure 3.6 shows a typical CCD dark current curve, which relates the amount of thermal dark current to the CCD operating temperature. Within the figure the theoretical relation for the rate of thermal electron production is given.

102 r

8 101

RATE (300°k) _ nA cm-2 - 5.86 x 109 T2/8 exp [-Eg/2kT]

-110 -100 -90 -80 -70 -60 -50 -40 TEMPERATURE (°C)

Fig. 3.6. Experimental (symbols) and theoretical (line) results for the dark current generated in a typical three-phase CCD. The rate of dark current, in electrons generated within each pixel every second, is shown as a function of the CCD operating temperature. Eg is the band gap energy for silicon. From Robinson (1988a).

Dark current for a CCD is usually specified as the number of thermal electrons generated per second per pixel or as the actual current generated per area of the device (i.e., picoamps cm-2). At room temperature, the dark current of a typical CCD is near 2.5 x 104 electrons/pixel/second. Typical values for properly cooled devices range from 2 electrons per second per pixel down to very low levels of approximately 0.04 electrons per second for each pixel. Although 2 electrons of thermal noise generated within a pixel every second sounds very low, a typical 15 minute exposure of a faint astronomical source would include 1800 additional (thermal) electrons within each CCD pixel upon readout. These additional charges cannot, of course, be uniquely separated from the photons of interest after readout. The dark current produced in a CCD provides an inherent limitation on the noise floor of a CCD. Because dark noise has a Poisson distribution, the noise actually introduced by thermal electrons into the signal is proportional to the square root of the dark current (see Section 4.4).

Cooling of CCDs is generally accomplished by one of two methods. The first, and usually the one used for scientific CCDs at major observatories, is via the use of liquid nitrogen (or in some cases liquid air). The CCD and associated electronics (the ones on or very near the actual CCD itself, called the head electronics) are encased in a metal dewar under vacuum. Figure 3.7 shows a typical astronomical CCD dewar (Brar, 1984; Florentin-Nielsen, Anderson, & Nielsen, 1995). The liquid nitrogen (LN2) is placed in the dewar and, although not in direct physical contact with the CCD, cools the device to temperatures of near -100° C. Since LN2 itself is much colder than this, CCDs are generally kept at a constant temperature (±0.1° C) with an on-board heater. In fact, the consistency of the CCD temperature is very important as the dark current is a strong function of temperature (Figure 3.6) and will vary considerably owing to even modest changes in the CCD temperature.

A less expensive and much less complicated cooling technique makes use of thermoelectric cooling methods. These methods are employed in essentially all "off-the-shelf" CCD systems and allow operation at temperatures of -20 to -50° C or so, simply by plugging the cooler into an electrical outlet. Peltier coolers are the best known form of thermoelectric cooling devices and are discussed in Martinez & Klotz (1998). CCD operation and scientific quality imaging at temperatures near -30° C is possible, even at low light levels, due to advances in CCD design and manufacturing techniques and the use of multipinned phase operation (see Chapter 2). Other methods of cooling CCDs that do not involve LN2 are discussed in McLean (1997a).

G10SPACER (3, SPACED 120° APART)

COPPER TANK

ALUMINUM DEWAR BODY

SLOTTED

LN2 FILL TUBE THERMAL STRAP COPPER WIRE (15 BUNDLES) G10SPACER (3, SPACED 120° APART)

G10 SUPPORT (3, SPACED 120° APART)

COPPER RADIATION SHIELD

PRINTED CIRCUIT BOARD

IM I.D. VACUUM PORT

LN2 FILL PORT

GAS EXHAUST PORTS (3)

GRANVILLE PHILLIPS 'CONVECTRON' VACUUM SENSOR

IM I.D. VACUUM PORT

LN2 FILL PORT

GAS EXHAUST PORTS (3)

G10SPACER (3, SPACED 120° APART)

DESICCANT CONTAINER

CONNECTOR PLATE

ELECTRONICS BOX

1.75 LITERS LN2, ANY POSITION

VACUUM CONNECTORS

SWING LINK

COLD PLATE

HEATER RING

WINDOW SHUTTER SHUTTER HOUSING

Fig. 3.7. A typical CCD dewar. This is the Mark-II Universal dewar originally produced in 1984 at Kitt Peak National Observatory. The dewar held 1.75 liters of liquid nitrogen providing a CCD operating time of approximately 12 hours between fillings. This dewar could be used in up-looking, down-looking, and side-looking orientations. From Brar (1984).

DESICCANT CONTAINER

CONNECTOR PLATE

ELECTRONICS BOX

1.75 LITERS LN2, ANY POSITION

VACUUM CONNECTORS

SWING LINK

COLD PLATE

HEATER RING

WINDOW SHUTTER SHUTTER HOUSING

Fig. 3.7. A typical CCD dewar. This is the Mark-II Universal dewar originally produced in 1984 at Kitt Peak National Observatory. The dewar held 1.75 liters of liquid nitrogen providing a CCD operating time of approximately 12 hours between fillings. This dewar could be used in up-looking, down-looking, and side-looking orientations. From Brar (1984).

The amount of dark current a CCD produces depends primarily on its operating temperature, but there is a secondary dependence upon the bulk properties of the silicon used in the manufacture. Even CCDs produced on the same silicon wafer can have slightly different dark current properties.

Today's CCDs are made from high purity epi wafers produced with low occurrences of integrated circuit error. These factors have greatly reduced many of the sources of dark current even at warmer temperatures. As with most of the noise properties of a given CCD, custom tailoring the CCD electronics (such as the bias level and the readout rate) can produce much better or much worse overall dark current and noise performance.

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