Characterization Of Persistence

To help avoid the troubles associated with persistence, we collected experimental data in an effort to better understand the character of this phenomenon. In the case of NIRC2, persistence was analyzed while the instrument was being developed in the Caltech laboratories and additional measurements were conducted after the instrument was delivered to the Keck II telescope. Several experiments were setup to quantify the characteristics, including decay time, various source inputs that can create persistence, and whether or not amelioration techniques would help remove the unwanted signal.

3.1 Decay Time

Persistence is often quantified in terms of the percentage of charge present at period of time after the source is removed. However, since persistence is actually a current source, the percentage measurement is dependent on the integration time of the latency image. Thus all of our measurements are divided by the integration time and quoted in terms of current with a unit of electrons per second. Persistence can be thought of as "virtual dark current" and can be compared to a minimum amount of unwanted current, the lower limit being the amount of true dark current.

We performed several experiments where the detector was exposed to high flux sources of various wavelengths and the amount of persistence signal was measured as function of time after the illuminating source was removed. The amount of trapped charge has an upper limit [4], so increasing the flux beyond a certain point does not increase the persistence effect. We also found that the wavelength of the incoming flux has no measurable effect on the persistence characteristics over the wavelength range tested, which was 1.1 ^m to 4.1 ^m. This independent wavelength result is consistent with tests on other types of InSb arrays [5]. The decay time measurements are summarized in Tables 1 and 2. The decay time data are plotted in Fig. 2, which includes a fit to the data and the true dark current limit measured after the detector was maintained in a dark environment for greater than 72 hours.

Tables 1 and 2. Decay time measurements for persistence current.

Persistence Current Decay Data Set 1

Integration Time (sec)

Time from Illumination Removal (sec)

Persistence Current (e"s/sec)

100

54

19.680

100

270

2.480

100

486

1.360

100

702

0.920

100

918

0.640

128

2994

0.250

256

3258

0.250

512

3778

0.211

1024

4810

0.160

1024

6874

0.121

1024

7906

0.113

Persistence Current Decay Data Set 2

Integration Time (sec)

Time from Illumination Removal (sec)

Persistence Current (e-s / sec)

1

6

92.000

2

16

38.000

4

28

19.000

8

44

13.000

16

68

9.500

32

108

6.375

64

180

3.938

128

316

2.500

256

580

1.406

512

1100

0.773

1024

2132

0.398

Figure 2. Persistence current, Ip, decay rate. Charge persistence leaks into the measured signal of the detector at a rate that is proportional to the time after the illuminating source has been removed. The persistence current appears as a nearly straight line when plotted with log(t) vs log(Tp). The decay rate of current is proportional to 1/t. The dark current limit is overlaid on this plot, which was measured after 72 hours of no light on the detector. These data indicate that a waiting period of greater than 10,000 seconds is required to allow the persistence to reach an acceptable level.

Figure 2. Persistence current, Ip, decay rate. Charge persistence leaks into the measured signal of the detector at a rate that is proportional to the time after the illuminating source has been removed. The persistence current appears as a nearly straight line when plotted with log(t) vs log(Tp). The decay rate of current is proportional to 1/t. The dark current limit is overlaid on this plot, which was measured after 72 hours of no light on the detector. These data indicate that a waiting period of greater than 10,000 seconds is required to allow the persistence to reach an acceptable level.

3.2 Source Flux and Fluence

Persistence can result in saturation of the array with too long of an exposure time or from high levels of incident flux on the array, independent of integration times. We compared the level persistence from both types of sources in order to determine which of these are more important to avoid. Fluence is defined as flux multiplied by integration time. Figure 3 plots a comparison of the persistence current verses source intensity in units of full-well percentage. The saturation full-well capacity of the Aladdin-3 array is approximately ~25,000 e-s. The data points plotted using the square symbols were a result of keeping the integration time constant while increasing the incident flux. The data plotted with the circle shaped symbols resulted from a relatively low incident flux while letting the detector saturate with increasing integration times. In both plots the persistence was measured immediately after the illumination was removed using an integration time of 10 sec. This comparison shows that incident flux is a more significant factor in generating latent charge than letting the array saturate from longer integrations. The data presented in Fig. 3 was acquired at 2.20 ^m (K-band) and data acquired at 1.25 ^m (J-band) was virtually the same. Longer wavelength versions of this test were not performed due to the difficulty of achieving a low incident flux environment at thermal infrared wavelengths.

10% 100% 1000% 10000% 100000% log Percentage of Full Well

10% 100% 1000% 10000% 100000% log Percentage of Full Well

-«- IncreasingFlux(lntegration time constant) -*- Increasing Integration (flux constant)

Figure 3. Comparison of incident flux and fluence (fluxxintegration time) effects on persistence current. With integration time held constant (1 sec) a light source was adjusted with varying ND filters to increase the flux incident on the array to beyond saturations levels (square symbols). Another test was performed by keeping the flux source constant and increasing integration times to beyond the saturation point (circular symbols). The residual current was measured in each case and plotted above. The plot shows that flux alone is a stronger source of persistence than fluence.

3.3 Removing the Trapped Charge with Continuous Resets

There is anecdotal evidence that with certain types of arrays, the persistence can be removed or the decay time can be improved if the array is flushed, which is defined as series of reset-reads. We tested this technique by comparing decay times with and without flushing applied. After illuminating the array with the identical source, the persistence level was measured every hour, except in one set of data in which the array was reset and readout continuously in the period between samples. Figure 4 plots this comparison and shows that this type of attempt at ameliorating the charge not only failed but seemed to make things worse, presumably by raising the dark current level through internal heating of the array.

0.01

0.001

0.01

0.001

x

. A-A—

• •

1000 10000 log time (sec)

100000

-With Flush -♦-Without Flush

Figure 4. Effects of flushing (continuous reset/reads) on persistence decay. This plot not only indicates that continuous reset-reads fail to improve persistence decay, but actually make things worse. Images were saved each hour with continuous reads occurring between data sets in one series, and no reads or resets in the other. The decay rate was not significantly different between the two but the dark current floor became worse when flushing. This may be due to local heating effects on the detector. The Aladdin-3 was maintained at 29.0 K during these tests.

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