3.1

Frame rate

The frame rate is largely set by the chosen resolution (8 to 14 bits), the shutter mode and how many rows are to be read (windowing). Table 2 lists the maximum frame rates for whole array readout in rolling shutter mode at each resolution.

Table 2.

Maximum rolling shutter frame rates for each resolution

Using windowing operation, with fewer rows readout (time per row * number of rows), can give much higher frame rates, while the readout for GS in DDS mode will give a lower frame rate, often little more than half the rolling shutter frame rate.

3.2

Relation between integration time and frame rate

The CIS120 uses the external signal PIXEL_SEQ_INIT to drive the pixel sequencer and starts the pixel read sequencer as shown in Figure 4. Integration on each line starts when PIXEL_SEQ_INIT pulse goes down. Integration on each line continue while other lines are addressed and only stops when the line is addressed again on the following frame. Therefore, using the default setting the integration time on each line is equivalent to the max frame time.

Further, there are multiple applications such astronomy where long integration times are required. It is relatively simple to set integration times longer that the default max frame rate by adding delays which can be introduced in different ways such as for example introducing a delay between the ADC of one line and the pixel access of the following line as shown below (Figure 5) or introducing a fix delay between the end of the last line readout and before addressing the first line of the next frame as shown in the next slide. Another option would be to reduce the clocks frequency. By default pixel clock is 4MHz and ADC and readout clocks are both 130MHz but they can be reduced individually.

Figure 5.

Achieving short integration time by re-setting rows.

There are other applications where it is required very small integration times, shorter than the one defined by the default maximum frame rate. In this case, the CIS120 can be configured so the integration time on a row can be reduced by resetting it during pixel access of a different row.

The idea is to extend the signal PIXEL_SEQ_INIT and while is held high access by SPI to any other different row. by keeping PIXEL_SEQ_INIT high longer the pixel is held in reset mode preventing any light integration which means that any row that is accessed during this period will be re-set and will start integration short after PIXEL_SEQ_INIT goes down again.

5.1

ETF

The electrical transfer function (ETF) is characterised by measuring the output in counts generated by inputting a known difference in electrical signal between the reference and signal levels. The ETF pixel timing is used for this test, so that no optical signal contributes to the measurements. The VREFR supply is clocked between 2 different levels with a signal that is synchronised to when the reference (shr) and signal (shs) are sampled. This enables a known voltage difference to be applied between reference and signal. The output is measured for different signal levels to form the ETF characteristic as shown in Figure 4. ETF measured for four different ADC Ramps (gain 1 to 4) which are configurable by SPI. The designation of gain 1-4 is arbitrary and for reference only.

Gain 4 defines the default ADC ramp slope used for 12 bit operating mode as allows the sensor to saturate close to the ADC count saturation providing an optimum match between pixel range and ADC input range. Gain 2 defines the default ADC ramp slope for 14 bit operation which allows lowering the LSB below the expected readout noise so that can be measure it. Therefore, it is also used to measure the noise in any ADC bit mode.

Figure 7.

ETF curve for 4 gains (cases) measured on CIS120 for 12 bit mode

5.2

X-Ray CCF

An iron-55 (Fe55) low level X-ray source is used during image acquisition to provide a known signal level in the pixels and the Charge to Code Factor (CCF) is the conversion gain (in ADU/e^-) from electrons collected by the photodiode to the output of the device. Due to the sparse nature of the x-ray events it is only possible to calculate the CCF as an average of every pixel, not on a pixel by pixel basis. Figure 8 shows the CCF measured in rolling shutter for 12 bit (which is the default operating mode) and similar plots would be expected in global shutter 12 bit simple mode. However, a discrepancy is observed which is attributed to the fact that global shutter simple mode is measured at 10 bit ADC resolution, which is the operating mode required for the Copernicus CO2M mission.

Figure 8.

X-Ray CCF histogram in Rolling shutter mode for gain 2 (left) and gain 4 (right)

Figure 9.

X-Ray CCF histogram in Global shutter mode for gain 2 (left) and gain 4 (right)

Figure 10-

Rolling shutter noise histogram for gain 2 with CCF=0.1307ADU/e- and gain 4 with CCF=0.0384

Table 3 -

X-Ray CCF Results

5.3

Noise

The rolling shutter noise results are close to the expected value. The global shutter simple mode (without DDS) noise is as expected considerably higher than in rolling shutter results since CDS is not implemented and therefore the KTC noise cannot be removed.

Table 4

Noise results

5.4

Dark Signal and Dark-signal Non-Uniformity (DSNU)

Due to the very low dark signal value obtained at -23°C the results from this run of test carried out with the main device from wafer #1 used for this characterisation are not reliable enough. Instead, a set of results from a device from a different wafer #2 previously characterised for dark signal at ambient temperature, 21°C, is presented in this section.

Figure 11.

Global shutter noise histogram for gain 2 with CCF=0.1291ADU/e- and gain 4 with CCF=0.0096 ADU/e-

Table 5 shows the dark signal in rolling shutter and global shutter which are within expectation at 21°C. It is important to point out that the global shutter results are significantly higher, 391 e-/pix/s, than in rolling shutter, 44 e-/pix/s. This higher value comes from the timing regime used for resetting the sense node. In rolling shutter mode the sense node is reset just prior to the transfer of charge from the photodiode. In global shutter however the sense node is not cleared before the TRAGL (transfer global) pulse, Figure 12. This means that the dark signal generated on the photodiode is added to the dark signal generated on the sense node before being measured. Because the sense node is unpinned the dark signal rate is around 3 times higher. However, this is not a significant issue on applications where integration time is short.

Figure 12.

Pixel sequencer timing for single global shutter. You can see that Resmem(m) is only pulsed prior to reading the reference and not the signal.

Table 5

Dark signal results

For applications where it is required to achieve low dark signal in simple global shutter mode there is the possibility of resetting the sense node prior to the TRAGL, as proven in Figure 13 where it has been measured a dark signal of 47 e^-/pix/s, in line with rolling shutter mode. However, since the pulse for resetting the sense nodes is created by the internal pixel sequencer, there will be an impact on the frame rate as the external signal required to be used will affect other internal signals.

Figure 13.

Dark signal results for Global shutter and Rolling shutter. Also included are results from global shutter while not transferring charge to the sense node and without transferring charge from the photodiode

The DSNU of rolling shutter mode is in line with expectation at 34 e^-/pix/s. the higher dark signal in global shutter mode means that the DSNU is also higher. In rolling shutter mode there is no identifiable DSNU pattern in the images. In global shutter mode there is a slight ramp up the image caused by charge still being generated at a fast rate on the sense node while the device is being read out. Row 2048 will have longer to generate this charge than row 0 (see Figure 14 and Figure 15).

Figure 14-

Rolling shutter dark signal histogram.

Figure 15-

Global shutter dark signal histogram.

Table 6

DSNU results

5.5

Saturation level (Qsat), Linear saturation level (Qlin) and Non-linearity

The Qsat obtained in global shutter and rolling shutter are very similar as expected with a difference of global shutter less than 1ke^-.

QLin is defined here at the point where the non-linearity exceeds 10%. The Qlin value is expected to be below that of Qsat and in general results are in line with expectation. The median device Qlin results, after optimisation, are slightly higher in rolling shutter mode than global shutter mode but only by around 1.2ke^-. The non-linearity results are calculated as the maximum linearity error of a pixel over the range 5% to 95% of the calculated Qlin value.

During the bias optimization it has been found that the device requires to be operated with different biases in global shutter mode with respect to rolling shutter mode in order to maximise Qsat and Qlin. VREFR has to be increase from 2.9V to 3.1V and VHIGH from 2.80 V to 2.95V.

Table 7

Qsat, Qlin and Non-lin results

Figure 16-

Rolling shutter (left) and Global shutter (right) Qsat maps.

5.6

Lag

Table 8 shows the lag results at three different signal levels. The lag results are around the expected values for rolling and global shutter, less than 1%, with global shutter experiencing more Lag. It was found during the optimisation of Qlin described in section 5.5 that the increase of VREFR and VHIGH also helped improve the global shutter lag.

Table 8

Lag results

5.7

QE

Preliminary QE measurements have been carried out on the devices. As can be seen the QE measured follows quite well the profile of the predicted QE although it is slightly lower. At this stage, it is not clear if this discrepancy is due to a backthinning process variability or to the calibration of the camera.

Figure 17.

QE predicted and QE measured on a back illuminated CIS120 device.

5.8

Summary EO results

Table 9 summarises the EO results presented in the previous sections from device from wafer #1 in RS 12 bit mode and simple GS in 10 bit mode at -23°C, except dark signal and DSNU, tested at 20°C on a different characterization device, from wafer #2.

Table 9.

Summary EO results.

6.1

Proton Irradiation

Two devices underwent proton irradiation of 5x10¹⁰p/cm² and as expected the main difference is in dark signal. Results shows an increase in dark signal of approximately 45 times which match relatively well with heritage.

Dark signal without the contribution from the sense node has also been investigated and shows that at post irradiation the increase in dark signal is approximately 7 times lower than with the sense node which is in line with the results obtained with other Teledyne e2v CMOS sensors on previous programs.

A small increase in x-ray CCF and similar reduction in QSat has also been observed but we are carrying out further investigation into whether extra hot pixels are having effect on x-ray. Post irradiation histogram shows an extended tail of hot defects

Table 10.

Pre and post proton radiation EO results obtained in simple GS at -23°C from device #3.

Figure 18.

Dark signal pixels distribution pre and post irradiation

6.2

Heavy Ion test

Heavy Ion test has been performed to investigate the susceptibility of the CIS120 to latchup and Single Event Upset (SEU). As shown in Table 11, two devices were tested up to 62.5 MeV·cm²/mg LET.

Table 11.

Different Heavy Ion test runs performed on two devices.

In order to detect latchup all the sensor biases were monitored and the compliance limit set to around 10 times. No latchup were detected on any of the two devices up to 62.5 MeV·cm²/mg LET. During all irradiations, the devices continued imaging correctly implying also no obvious signs of SEU neither up to 62.5 MeV·cm²/mg LET. The devices were also tested with and without the PLL and no difference was detected. Therefore, it can be concluded that the PLL is also immune to latch-up and SEU up to 62.5 MeV·cm²/mg LET.

After these harsh test was performed, two usual changes were observed; An increase in dark signal over 100 times, which is expected due to the large ionising doses induced by the heavy Ion radiation. An increase of noise was also observed which can be explained due to the increase in dark signal shot noise.