4.1

Panchromatic sensor architecture

The PAN sensor shall be able to perform a TDI function with up to 25 physical pixels. The sensor has four modes of operation:

The pixel array size is 8000x25 (pitch: 13μm). Together with the associated 8000 columns performing CDS and TDI functions, pixels are controlled by a buffered decoder. The 8000 columns outputs are multiplexed by group of 500 on 16 outputs. This operation is controlled by horizontal shift registers.

Fig. 3.

PAN sensor architecture

A column contains:

After proper sampling of the light information onto the photodiode capacitor, the signal is handled by the read out chain. Pixel reset and signal levels are multiplexed to the column bus, and sampled by a CDS stage.

This stage performs a subtraction of the reset level to the signal level, and feed the ktC noise free sample to the adder. When additions are completed, signals of 500 columns are multiplexed to an output buffer.

Fig. 4.

PAN sensor column architecture

4.2

Multispectral sensor architecture

The XS sensor has four identical lines having a pitch a 1.04 mm (same pitch between line 1 and PAN pixel 1). Horizontal pitch is 52μm. The four lines are operated together, having their integration time starting and stopping synchronously (snapshot). Each line is then read one by one during the next integration period.

Fig 5 gives the architecture of the sensor.

Fig. 5.

XS sensor architecture

Each pixel has a local amplifier and pseudo differential sample and hold stages. The four pixels of a column are then multiplexed to two outputs, one for the signal level and one for the reset level. Correlated reset and signal levels are multiplexed per group of 500, separately for reset en signal level. The subtraction by the system of the two outputs should give a ktC noise free signal.

A column contains:

Because specifications between PAN and XS sensors are fundamentally different, column circuits are quite different as well. The reset and signal levels subtraction is done by the preamplifier. In order to reduce other source of noise (coupling…), the sensor provides pseudo-differential outputs. This sensor is essentially made of four linear arrays divided in four [500*4] arrays, due to the extra spacing required between lines. This allows an alternative approach for the pixel design.

Fig. 6.

XS sensor column architecture

4.3

Sensor Technology

Target technology is the XFAB 0.35μm standard CMOS flow. The die area, which is larger than the maximal reticule area, requires that the sensor is fabricated using “stitching” technology. The device is divided into several basic blocks which are repeated and aligned together during photocomposition.

Both sensors are packaged in a single ceramic pin grid array (PGA) made of alumina (Al₂O₃).

Fig. 7.

View of the sensor in PGA package.

5.1

Quantum efficiency

In order to optimize quantum efficiency, the wafer epitaxial layer is 7 μm thick with a peak QE at around 600 nm. The measurement results show QE x FF in the range [48%-53%] for spectral band [400nm-900nm]. These results are in line with a predicted high fill factor [2].

PAN-XS QE x FF average performances

5.2

Conversion factor.

PAN conversion factor results are independent versus NTDI as shown in Fig. 8. XS and PAN results are closed to predicted values.

Fig. 8.

PAN conversion factor versus N_TDI

PAN-XS conversion factor performances

5.3

Photo response non uniformity.

Fig. 9. shows the average intra-stitch block PRNU versus nTDI for the three gain modes. It shows a maximum PRNU value of about 6.5%.

Fig. 9.

Average intra-stitch block PAN PRNU versus nTDI for different gain modes

Remarkable is the increase with nTDI for the high gain mode. The effect is probably caused by a variation of some capacitive load with nTDI.

Fig. 10. shows the extracted PRNU values for the three gain modes of the XS and all PRNU definitions regarding stitching technology.

Fig. 10.

XS PRNU values for the three gain modes.

A maximum value of less than 1.5% is obtained.

5.4

Noise

The noise is measured in the voltage domain. Conversion gain figures are used to express the noise in electrons.

XS noise results are slightly different from noise predictions.

XS noise versus gain

PAN noise is higher than predicated, but still below the specification of 30 e- for 2 out of 3 gain modes, even for nTDI 25. Fig. 11. shows the results graphically. The slope of the noise curves is clearly different than predicted

Fig. 11.

PAN noise versus TDI number

5.5

Dark current and dark signal non uniformity.

Only the dark current values for the longest exposure and the highest temperature are significant. At 80 degC, 3.36 mV is measured, or 168 mV/s dark current with a standard deviation of 147 mV/s (DSNU). This indicates that the dark current is far below the specification of 1 nA/cm2 (~1 V/s) at 15 degC.

5.6

Dynamic range and non linearity.

The PAN linearity was measured for all NTDI and gain configurations.

In each case, linearity is better than 2% in the performance irradiance range [E1-E3] and better than 10% in the useful irradiance range [E1-E4].

The saturation level is a function of NTDI. For low value of NTDI, saturation is due to pixel saturation level. For high NTDI value, CDS and adder saturate before pixel level.

Fig. 12.

Preliminary PAN pixel response curves with high preamp gain low adder gain (NTDI=8)

Similar to the response measurements on the PAN, measurements were carried out for the XS.

5.7

Fixed pattern noise (FPN).

For nTDI equals 8, 12, 16, 20 and 25, the FPN has been measured on a complete image without calibration mode. As expected, the dominant source of FPN is the TDI stage. But calibration mode keeps the nominal mode and optimization and characterization of this calibration function are in progress.

Fig. 13.

Global PAN FPN without calibration

Preliminary promising results using calibration mode present a large decrease of PAN FPN. For NTDI=8, a 3mV rms global FPN level was measured with calibration mode compared to 70 mV rms without calibration mode.

XS FPN has been measured on a complete image, per row, per output and per both row and output. As expected, the dominant source of FPN for this case is the buffer associated with each pixel.

XS FPN