3.1.

Readout Noise

A set of 50 dark frames were collected and the per-pixel standard deviation was computed to obtain a measurement of the readout noise. The whole device was read out with a minimal integration time of 0.2 s. The contribution from dark current can be suppressed by setting the transfer gate voltage ($V_{TG}$) to zero, prohibiting charge transfer from the PPD to the sense node. The measured readout noise distribution is presented in Fig. 3. With the dark current suppressed, the median readout noise is $3.3{\mathrm{e}}^{-}\mathrm{RMS}$ with 90% of pixels reporting a value $<3.6{\mathrm{e}}^{-}\mathrm{RMS}$. This is within the SXI requirement of $<5{\mathrm{e}}^{-}\mathrm{RMS}$. Including the contribution from dark current over the minimal integration time (0.2 s), the readout noise is $4.2{\mathrm{e}}^{-}\mathrm{RMS}$ at $-40°\mathrm{C}$.

Fig. 3

CIS221-X readout noise distribution with dark current suppressed.

3.2.

Dark Current

With the image sensor in darkness, four sets of images were taken at increasing integration times. Using the mean pixel values for each set, the per-pixel dark current was calculated. Figure 4(a) shows large scale non-uniformity in the dark current performance across the image sensor. There is a “glow” coming from the outer column edges of the image area and the bottom row edge where it connects to the $10\mu \mathrm{m}$ pixel variant region. This “glow” extends further within the region of the image area that is not covered by the OBF. Also visible is an offset between the background dark current values of the OBF and non-OBF halves of the sensor, as well as a number of hot pixels across the whole image area.

Fig. 4

(a) CIS221-X dark current map with the ${\mathrm{OBF}}_{50\times 50}$ and $\text{non}\text{-}{\mathrm{OBF}}_{50\times 50}\text{pixel}$ regions outlined (red). (b) Dark current distributions of the whole OBF and non-OBF image areas (black) and the ${\mathrm{OBF}}_{50\times 50}$ and $\mathrm{non}\text{-}{\mathrm{OBF}}_{50\times 50}\text{pixel}$ regions (red). Dark current data were collected with the image sensor cooled to $-40°\mathrm{C}$.

These results are further illustrated by the dark current distributions shown in Fig. 4(b). The “glow” can be seen in the very long tails of the OBF/non-OBF histograms (black). To probe the background dark current within the two halves of the image area, $50\times 50\text{pixel}$ regions were selected far from the “glow.” The respective histograms (red) have much shorter tails, indicating the effect of the “glow” has been minimized though hot pixels are still present. To exclude the impact of the hot pixels in the measurement, the distributions were fit to a Gaussian function. The corresponding mean dark currents are $12.4\pm 0.06{\mathrm{e}}^{-}/\text{pixel}/\mathrm{s}$ (${\mathrm{OBF}}_{50\times 50}$) and $35.2\pm 0.09{\mathrm{e}}^{-}/\text{pixel}/\mathrm{s}$ ($\mathrm{n}\text{on}\text{-}{\mathrm{OBF}}_{50\times 50}$). This is unexpectedly high for a PPD pixel and exceeds the beginning-of-life THESEUS requirement of $<10{\mathrm{e}}^{-}/\text{pixel}/\mathrm{s}$ at $-40°\mathrm{C}$.

An explanation for the larger than anticipated dark current, the offset between the OBF and non-OBF halves of the image area and the “glow” are not provided here but are subject to an on-going investigation.

3.3.

Non-Linearity

The pixel photo-response was assessed using a light-emitting diode (LED). Sets of images were collected with the integration time fixed and the illumination time (LED “ON” time) varied. The signal generated by the pulsed LED was stable to within $\pm 4{\mathrm{e}}^{-}$. To ensure statistical accuracy, at least 40 images were collected for each illumination time. Figure 5 shows the mean signal measured for increasing illumination time. Fitting the linear portion of the slope to a straight line and calculating the residuals provides a measure of the non-linearity. For the energy range 0.3 to 5 keV (82 to $1370{\mathrm{e}}^{-}$), the non-linearity is $<1\%$, meeting the SXI requirement.

Fig. 5

(a) CIS221-X photo-response measured at $-40°\mathrm{C}$ fit to a straight line and (b) the corresponding residuals.

3.4.

Image Lag

The incomplete transfer of charge from the PPD to the sense node during readout is known as image lag. While continuously collecting frames of a fixed integration time, a LED was triggered off for five frames and then on for five frames. The first of the five LED “ON” frames records less signal than the other four and provides a measurement of the lag. Figure 6(a) shows the resultant mean leading-edge image lag as a percentage of signal. At least eight sets of images were collected for each signal to ensure statistical accuracy in spite of any LED instability. For comparison, the lag performance of the CIS221-X “Variant #1” and “Variant 2” pixels are presented alongside that of “Variant #3” (the subject of discussion so far). As measured previously in an FSI device,⁹ “Variant #1” and “Variant #2” exhibit excessive lag while “Variant #3” has near-zero lag ($<0.1\%$) for all signal values used. This can be attributed to the larger transfer gate in this variant.

Fig. 6

(a) Mean leading-edge image lag of the CIS221-X “Variant #1,” “Variant #2,” and “Variant #3” $40\mu \mathrm{m}$ pixels expressed as a percentage of signal. (b) CIS221-X “Variant #3” lag variation (fit to a power law function) and standard deviation against signal.

Though the “Variant #3” average image lag is near-zero, each pixel of the image area has a distinct lag value which may vary significantly from the mean. The standard deviation of the per-pixel lag values is shown in Fig. 6(b). Also shown is the lag variation (calculated as the standard deviation divided by the mean signal) which has been fit to a power law function, showing significant variation at low signal which rapidly decreases as the signal increases. This will result in a degraded energy resolution at lower photon energies.

3.5.

Per-Pixel Gain

The CIS221-X was exposed to x-ray fluorescence using an x-ray tube and a Mn target. The resultant spectral response is shown in Fig. 7, revealing the $\mathrm{Mn}-{\mathrm{K}}_{\alpha }$, $\mathrm{Mn}-{\mathrm{K}}_{\beta }$ emission lines as well as other spectral features originating from the aluminum target support and the vacuum chamber walls. Measuring the position of the $\mathrm{Mn}-{\mathrm{K}}_{\alpha }$ peak in ADU gives a conversion gain of $10.41\pm 0.005\mathrm{eV}/\mathrm{ADU}$.

Fig. 7

CIS221-X spectral response to x-ray fluorescence of a Mn target.

Since each CIS221-X pixel has its own amplifier, each pixel also has a distinct conversion gain. If a sufficient number of x-ray events are recorded by each pixel, the per-pixel gain can be calculated according to the method outlined above. Using 10,000 x-ray exposures with an integration time of 1 s, the per-pixel gain was measured for the whole image area. The measurement error for each pixel is $\sim 0.01\mathrm{eV}/\mathrm{ADU}$ ($\sim 0.001\%$). Figure 8(a) shows non-uniformity in gain at both small and large scales. Most noticeably, the gain is higher at the outer columns of the image area, especially for the non-OBF region.

Fig. 8

(a) CIS221-X gain map with the ${\mathrm{OBF}}_{50\times 50}$ and $\mathrm{non}\text{-}{\mathrm{OBF}}_{50\times 50}\text{pixel}$ regions outlined (red). (b) Gain distributions of the whole OBF and non-OBF image areas (black) and the ${\mathrm{OBF}}_{50\times 50}$ and $\mathrm{non}\text{-}{\mathrm{OBF}}_{50\times 50}\text{pixel}$ regions (red).

The large scale non-uniformity is reflected in Fig. 8(b) and the shoulder of values around $11\mathrm{eV}/\mathrm{ADU}$ in the OBF and non-OBF histograms (black). Comparing these two distributions reveals an offset in their per-pixel gains. Examining the same $50\times 50\text{pixel}$ regions (red) as defined in Sec. 3.2, the gain variation is $0.64\pm 0.003\%$ (${\mathrm{OBF}}_{50\times 50}$) and $0.67\pm 0.001$% (non-${\mathrm{OBF}}_{50\times 50}$). The tails of the histograms can be attributed to hot pixels and have been excluded in the measurement by fitting the distributions to Gaussian functions.

The cause of the higher gain in the outer columns and the offset between the OBF and non-OBF regions has not yet been determined. A possible explanation is that the dark current non-uniformity (see Sec. 3.2) is influencing the gain measurement. Section 3.3 shows that the CIS221-X non-linearity grows with increasing signal, which implies the gain would also behave non-linearly. The non-uniformity of the dark current means that the gain is measured at different signal values for each pixel. It would therefore be expected that the per-pixel gain distribution would reflect the same non-uniformity as the dark current, as shown in Fig. 8(a). This possible explanation could be validated by repeating the per-pixel gain measurement as above using a lower integration time, minimizing the influence of dark current.

3.6.

Energy Resolution

Energy resolution is an important x-ray image sensor parameter and is often used as a benchmark for performance. The energy resolution of a CIS can be expressed as¹⁴

Experimentally, energy resolution is measured as the full width half maximum (FWHM) at a specific spectral peak. When measuring the CIS221-X energy resolution, hot pixels were first excluded using a mask derived from the dark current map (see Sec. 3.2). Then, the x-ray events were graded according the XMM-Newton/EPIC grading procedure¹⁵ and all but the single-pixel events were discarded. This reduced the impact of charge sharing between pixels. The resultant spectrum was fit to a Gaussian function and the energy resolution was calculated as

After measuring the per-pixel gain (see Sec. 3.5), it is possible to correct for the gain variation before measuring the energy resolution. As shown in Fig. 9, this results in a smaller FWHM. Before correction, the CIS221-X energy resolution at the $\mathrm{Mn}-{\mathrm{K}}_{\alpha }$ emission line (5898 eV) is $161.7\pm 0.7\mathrm{eV}$, while after correction it is $130.2\pm 0.4\mathrm{eV}$.

Fig. 9

CIS221-X single-pixel event spectra using a Mn target before and after per-pixel gain correction.

To examine the energy resolution at different photon energies, the image sensor was exposed to x-ray fluorescence of Al (1487 eV), Ti (4510 eV), Mn (5898 eV), and Cu (8047 eV) targets. The corresponding FWHMs are shown in Fig. 10 alongside the Fano limit and the theoretical energy resolution according to Eq. (1). The data before and after per-pixel gain correction are well modeled by Eq. (1) with ${\sigma }_{\text{gain}}=0.64\%$ and ${\sigma }_{\text{gain}}=0\%$, respectively, validating the per-pixel gain measurement and correction. The total noise was calculated

Fig. 10

CIS221-X energy resolution as measured at the Al (1487 eV), Ti (4510 eV), Mn (5898 eV), and Cu (8047 eV) emission peaks. Also shown is the Fano limit and the theoretical energy resolution based on measured CIS221-X noise performance at $-40°\mathrm{C}$.

3.7.

Quantum Efficiency

The image sensor’s quantum efficiency (QE) was measured using the PTB Laboratory at the BESSY II synchrotron radiation source (Helmholtz-Zentrum Berlin). The OBF and non-OBF sides of the device were illuminated separately, and the recorded signal was assessed against a well characterized reference diode. Figure 11 shows the measured QE at various photon energies alongside a layer-model based on an approximate understanding of the image sensor and OBF material compositions. Across the measured energy range, the model fits the data well and the QE performance is as expected. Importantly, the OBF pixels report a $\mathrm{QE}>70\%$ for 0.5 to 1.8 keV photons, outperforming the SXI OBF pixel requirement of $>60\%$ for 0.5 to 1.5 keV. The non-OBF pixels report a $\mathrm{QE}>80\%$ for 0.3 to 1.8 keV, matching the non-OBF pixel requirement for the energy range measured. However, further testing up to a photon energy of 5 keV is necessary to fully confirm whether the non-OBF pixels meet the SXI QE requirement.

Fig. 11

CIS221-X quantum efficiency at different photon energies alongside a layer-model based on an approximate understanding of image sensor/OBF material composition.