Hot Pixels and Annealing

While the increase in mean dark rate with proton irradiation is important, the dark current non-uniformity (hot pixels) is generally a bigger concern for astronomical applications. Some pixels show very high dark current up to several times the mean dark rate. Depending on the particular collision sequence, protons of the same energy may produce varying degrees displacement damage. Moreover, if a defect is created in a high electric field region, the contaminated pixel can show very large dark current as a result of field-enhanced emissions [4].

Hot pixels accumulate as a function orbit time. Figure 1 shows the evolution of the distribution of pixels over time at different dark rates for one ACS/WFC CCDs. As the proton-induced damage increases, the mean dark current (the peak of the distribution) and the hot pixels population (the tail of the histogram) increase. According to Hopkinson, et al. [5] a simple fit can be made assuming a Gaussian main peak whose half-width increases approximately as the square root of the fluence, and an exponential tail whose amplitude is proportional to fluence.

Table 3. Dark rate increase as a function of time for HST CCDs.

WFC

HRC

STIS

WFPC2

eVpix/hr/year 1.5 WFC1

2.1

3.3 (side 1)

2.0 (1993- 1998)

2.0 WFC2

2.2 (side 2)

~ 0 ( >1998)

Temperature (°C) -77

-81

-83 (side 1)

-88

<-83 (side 2)

oA/zm-i

e-/pix/sec

Figure 1. Distribution of dark pixels for (bottom) one WFC CCD at launch (2002), and after 1,2 and 3 years on orbit.

e-/pix/sec

Figure 1. Distribution of dark pixels for (bottom) one WFC CCD at launch (2002), and after 1,2 and 3 years on orbit.

Sirianni, et al. [6] discussed the evolution of hot pixels in ACS CCDs and found that the number of new hot pixels with a dark current higher than the mean dark current increases every day by few to several hundreds depending on the threshold used (see Table 4). In order to partially anneal the hot pixels, all HST instruments perform monthly anneal, by raising the CCD temperature to approximately +20 C for few hours. Although it is still not clear why significant annealing is observed at such low temperatures (the most common defects in silicon anneal at much higher temperatures), we can report a few interesting findings (see [7] for more details):

• The annealing rate strongly depends on the dark rate of the hot pixel. Very hot pixels show a higher anneal rate than warmer pixels and there is no impact on the average dark current level [6].

• The same anneal rate can be obtained at colder temperatures. In at least four instances, in occurrence of HST safing events, the temperature of ACS CCDs were raised to -10°C for periods ranging between 24 and 48 hours. After these periods the population of hot pixels decreased by the same amount as in a normal anneal cycle at +20°C.

• The anneal rate does not seem to be related to time. Since launch the anneal time for ACS CCDs have been reduced first from 24 to 12 hrs and more recently from 12 to 6 hr without impacting the annealing effectiveness.

• For any particular hot pixel, a complete anneal is a rare event. Most of the annealed hot pixels significantly reduce their dark current level but never rejoin the population of normal (Gaussian distribution) dark pixels.

• Several hot pixels show evidence of reverse annealing. While most of the hot pixels show some degree of healing in response to an annealing procedure, some of them may be activated to "hot" by the same procedure. This occurred to pixels that have been damaged by radiation and that, depending on the anneal cycle, they may lose or regain the status of "hot pixel". Figures 2 and 3 show examples of normal and reverse annealing respectively.

Table 4. Anneal rate for different HST CCDs.

ACS/WFC

ACS/HRC

STIS

WFPC2

Temp

-77

°C

-81

°C

-83 °C

-88

°C

Annealing Temp

-10 to + 20

-10 to + 20

+5

+30

Annealing

6 to 24 hr

6 to 24 hr

24 hr

24 hr

Duration

Threshold

%

+/-

%

+/-

% +/-

%

+/-

> 0.02

0.55

0.02

0.64

0.02

0.80

0.05

> 0.04

0.70

0.07

0.84

0.07

> 0.06

0.78

0.04

0.84

0.04

> 0.08

0.82

0.03

0.87

0.03

> 0.10

0.84

0.02

0.85

0.02

0.77 0.05

> 1.0

0.55

0.15

0.64

0.15

Figure 2. Signal level of hot pixel # 136 as a function of time (days). Vertical dashed lines mark the annealing dates. The horizontal line marks the threshold for hot pixel definition. The signal level is in e-/pix/sec at -77 C. The pixel is damaged just after

Figure 3. Signal level of hot pixel # 174 as a function of time (days). The pixel is damaged after day 350. Following anneal cycles partially heal the pixel whose dark current jumps between discrete levels in correspondence of anneal cycles.

day 800 and it is fully healed after the first annealing cycle.

Figure 3. Signal level of hot pixel # 174 as a function of time (days). The pixel is damaged after day 350. Following anneal cycles partially heal the pixel whose dark current jumps between discrete levels in correspondence of anneal cycles.

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