3.9 The ATLAS Upgrade
4.1.2 Charge collection study of pixel devices
The CNM LGAD production run 6827 also contains pixel sensors compatible with the ATLAS readout chips FE-I3 and FE-I4. The charge collection of LGAD pixel devices when exposed to a90Srβ-source has been measured and compared to the one of reference devices without CM layer.
The full module assembly has been performed at IFAE. The sensors have been bump-bonded to FE-I3 and FE-I4 chips, glued on PCBs and wire-bonded. The chip readout is performed through the USBPix setup [57], an FPGA based readout system able to configure and readout both the FE-I3 and FE-I4 chips. The USBPix is connected to a DAQ PC where the STControl data acquisition software is used.
The readout chips do not allow a direct measurement of the charge or of the current waveform. Each readout pixel collects the signal coming from the sensor that is first amplified and then compared to an adjustable threshold. In case the signal is larger than the threshold two values are returned, the time of arrival or time stamp and the Time Over Threshold (TOT), both times are measured in units of 25 ns, that is the LHC bunch crossing clock. The TOT dynamic range is different for the FE-I3 and FE-I4 chips, being of 8 and 4 bits respectively.
4.1. LGAD FOR TRACKING 63 The front-end chips can be tuned, calibrating the gain of global and local (one per pixel) amplifiers and of the discriminator threshold, to set the threshold value to a given value of charge. Additionally, the discharge time of the feedback capacitor can be adjusted to target a chosen TOT value.
It is also possible to perform a calibration of each readout channel in order to obtain a pixel-by-pixel TOT to charge conversion. Due to the different local tuning parameters the same TOT value may correspond to different charges for different pixels. With this calibration it is possible to correct the local effect and to use the TOT as an estimator for the collected charge. This calibration is based on a charge injection circuit built in the chip. The value of the injection capacitance can differ from the nominal value by 10% [58] which propagates to the final charge uncertainty.
Before exposing the sensor to a radioactive source to perform the charge collection measurements, a current-voltage (IV) characterization of each sample has been done, see figure 4.10.
Bias[V]
0 50 100 150 200 250 300 V350
A]µLeakageCurrent[
0 10 20 30 40 50
Reference FEI3 LGAD FEI3 Reference FEI4 LGAD FEI4
Figure 4.10: Current-Voltage relation of the studied pixel devices after assembly.
The FE-I4 LGAD sensor shows a breakdown voltage lower than 50 V, while the FE-I3 LGAD and both reference sensors can be operated up to few hundreds of volts.
Because of the low voltage capabilities of the FE-I4 LGAD sensor it has been excluded from the measurements since CM is expected to appear at larger bias voltage.
The FE-I3 LGAD and reference sensors have been tuned to the same parameters, that are TOT = 30 for a collected charge of 20 ke− with a threshold set to 3.2 ke−. The result of the tuning procedure is shown in figure 4.11 where it is visible a larger noise for the device with CM layer that turns into a larger spread of the TOT and threshold distributions with respect to the reference sensor. The larger noise of the LGAD sample is in any case much lower than the threshold and will not affect in
a relevant way the efficiency of the device. In addition, since the TOT to charge conversion will be applied to each readout channel, the broader TOT distribution will be neutralized by the conversion to equivalent charge.
TOT [25 ns] 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000
Entries
Figure 4.11: Distribution of the TOT, threshold and noise for all the readout channels of the LGAD FE-I3 (dashed black) and of the reference (solid red) devices after tuning.
The two FE-I3 sensors are therefore set to the same working configuration and exposed to the radiation of a90Srβ-source. A scintillator is placed below the sensor to provide a trigger signal to the readout system during the source scans. These scans have been repeated for increasing values of bias voltage until breakdown is reached.
In a pixel device the electron-hole pairs cloud produced by an ionizing particle is often not confined into the volume of a single pixel generating a signal in two or more neighbouring readout channels. For each trigger the TOT values returned by the chip are converted into equivalent charge and the hits of neighbouring pixels are clustered together. The result is a cluster charge distribution that is fitted with the convolution of a Landau and a Gauss distributions. From each fit, the collected charge MPV is taken. The MPVs of the collected charge are plotted against the bias voltage for both the LGAD and reference sensors in figure 4.12, where the bands indicate the uncertainty due to the charge calibration capacitor.
The reference device reaches a saturation at a bias voltage V ∼50 V with a col-lected charge of about 22 ke−, as expected from the depletion voltage. The LGAD device shows a similar behaviour saturating atV ∼50 V with a collected charge com-patible with the one of the reference device within the charge calibration uncertainties.
Moreover, as explained in the previous section, the gain is expected to increase with increasing bias voltage due to the larger electric field in the multiplication region, but