7.2 Pixel Characterization Measurements
7.2.1 MSDD Front-End Input Capacitance
The DSSC system runs at a frequency where the white series noise is dominant, the ENC is thus inversely proportional to the input capacitance (equation 3.47) and thus very important. The input capacitance of the MSDD front-end has been measured for various chips and assemblies using the internal charge injection circuit. A measurement procedure has been developed which uses the MIM caps which can be statically added to the input node to estimate the input capacitance. A set of three capacitors are implemented here, these are originally intended to decrease the gain of the front-end for higher photon energies (> 1 keV).1. The procedure is very simple: based on observing the degradation of the gain when adding a known capacitance Cadd, the input capacitance can be calculated. The pixel injection circuit is used to measure the gain once without any added capacitance and the procedure is repeated with the exact same settings despite for adding a known capacitor to the input. The exact gain for this measurement is irrelevant, the capacitance can be calculated from the relative degradation. The exact value cannot be determined because Cadd is not exactly know due to process variations but a sufficient estimate can be retrieved. If we call a the nominal gain without added capacitance and aaddedC the gain with an added capacitance, we can use the following relations to calculate the capacitance:
1
7.2 Pixel Characterization Measurements 0.2 0.5 1 pF TGain Cin Vinj 10 fF Cadd 0.2;0.5;1 pF 40 42 44 46 48 50 52 54 56 58 60 62 0 50 100 150 200 ADU
Pixel Injection Setting no Cadd
Cadd = 0.2 pF Cadd = 0.5 pF Cadd= 1 pF
Figure 7.2: Example of the measurement method used to estimate the input capacitance. The gain settings for all measurements are the same despite for added input capacitances. From the slope degradation, the input capacitance can be estimated.
a∝ C1 → r = aaddedC
a =
Cin Cin+Cadd
The input capacitance can thus be estimated by: Cin=
r
r− 1Cadd (7.21)
According to equation 5.33, the injected charge depends on Cin. The low gain injection mode is used for the measurement, because the effect of the small injection capacitance of 10 fF can be neglected for the expected input capacitance of > 200 fF for a sufficiently good estimation.
Figure 7.2 shows an example of the measurement for a single pixel. By adding a 1 pF capacitor to the input, the gain (slope) degrades by a factor of ≈ 2 which leads to a capacitance of ∼ 1 pF. Figure 7.3 shows a map and histogram of the F1 chip bump bonded to an MSDD sensor.
The mean capacitance of 900 fF extracted with this measurement from the F1 and MM4 chips is far more than the expected 400 − 500 fF. The extra capacitance stems from the DEPFET cascode PMOS transistor which is connected to the input. The n-well of this PMOS transistor is unfortunately also connected to the input which could have been avoided. The extra capacitance is mainly attributed to this fact.
The MM6 chip shows a significantly smaller input capacitance of ∼ 270 fF. (figure 7.4). The DEPFET front-end is not included, the input transistor is smaller and the landing pad of the bump (last metal in the ASIC) has been made smaller than the recommended size for test purposes. The two peaks in the histogram in figure 7.4 are separated by the capacitance of the sensor anode which is estimated as ∼ 60 fF with the applied measurement method. The MSDD mini matrices have a size of (8 × 8), whereas the MM ASICs have a size of 8 × 16. The upper half of the sensor matrix only comprises landing pads and no sensor pixels.
For F1, a re-fabrication would be possible, where the DEPFET front-end would be disconnected by patching only one of the metal layers. Additionally, the bump landing pads could be shrunk to decrease the capacitance further.
0 0.2 0.4 0.6 0.8 1 1.2 F1 MSDD FE Cin [pF] F1 MSDD FE Cin [pF] 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 0 200 400 600 800 1000 1200 1400 1600 1800 F1 MSDD FE Cin [pF] F1 MSDD Cin [pF] Entries 4096 Mean 0.8963 RMS 0.07 F1 MSDD FE Cin [pF]
Figure 7.3: MSDD front-end input capacitance measurement for an F1 flipped to an MSDD sensor. The F1 ASIC includes the DEPFET front-end and therefore has an increased input capacitance.
0 0.2 0.4 0.6 0.8 1 1.2 0.265362 0.267397 0.283084 0.214657 0.364081 0.231376 0.34833 0.237686 0.206453 0.271203 0.354663 0.288259 0.297658 0.266124 0.269343 0.320369 0.272618 0.31888 0.286059 0.343442 0.297136 0.280811 0.273487 0.236621 0.134425 0.282631 0.297145 0.287031 0.327052 0.238 0.373837 0.301366 0.277416 0.314711 0.280929 0.29723 0.282478 0.27833 0.285994 0.29004 0.239528 0.260505 0.287003 0.282505 0.266709 0.26398 0.29948 0.291957 0.282411 0.271671 0.29872 0.300104 0.302276 0.289414 0.303012 0.285277 0.32589 0.311236 0.204612 0.263728 0.345853 0.298589 0.100729 0.256352 0.203973 0.220358 0.236102 0.208828 0.221033 0.211612 0.226054 0.224158 0.216603 0.219739 0.239769 0.219706 0.196866 0.229113 0.227705 0.211586 0.229266 0.218453 0.234087 0.218831 0.198049 0.253147 0.242087 0.27336 0.222926 0.212972 0.2079 0.213323 0.219551 0.210547 0.220919 0.227199 0.216163 0.209387 0.220919 0.239304 0.208974 0.22202 0.28346 0.221653 0.230799 0.222305 0.214428 0.217518 0.218635 0.234047 0.211684 0.212454 0.216607 0.222894 0.218083 0.213078 0.225569 0.224642 0.224943 0.818103 0.48703 0.585871 MM6 MSDD FE Cin [pF] (8x16 pixels) MM6 MSDD FE Cin [pF] (8x16 pixels) 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0 5 10 15 20 25 MM6 MSDD FE Cin [pF] F1 MSDD Cin [pF] Entries 128 Mean 0.254 RMS 0.04512 MM6 MSDD FE Cin [pF]
Figure 7.4: MSDD front-end input capacitance measurement for an MM6 chip which does not feature the DEPFET front-end, flipped to an (8 × 8) MSDD sensor. The upper 64 pixels only have a landing pad on the sensor, there is no sensor pixel. The two peaks in the histogram correspond to the capacitance of the MSDD anode which contributes ∼ 60 fF. The pixels in the map which have a capacitance of > 400 fF (including the white pixels) lead to wire bond pads for testing and thus have a higher capacitance.
7.2 Pixel Characterization Measurements
Assembly Measured Cin Front-End
F1 on the probe station ∼0.92 pF MSDD + DEPFET
F1 flipped to a 64 × 64 MSDD ∼0.90 pF MSDD + DEPFET
MM4 flipped to an 8 × 8 MSDD ∼0.90 pF MSDD + DEPFET
MM6 flipped to an 8 × 8 MSDD ∼0.25 pF MSDD
Table 7.1: Measured input capacitances. The measurement method relies on a reference MIM cap in the pixel, the fact that the flipped F1 has a lower capacitance than on the probe station can be attributed to fabrication mismatch.