TCT measurements of strip devices

The strip devices have been measured with a multi-channel Transient Current Tech-nique (TCT) set-up at the Hamburg University. The TCT basically consists in the recording and analysis of time resolved current waveforms induced by laser pulses. A description of the TCT technique and its applications are given in appendix A.

The TCT set-up used to perform these measurements is made of a Picoquant pulsed laser that generates light pulses with duration shorter than 100 ps and with a repetition rate of 1 kHz. Infra-red and red lasers of wavelength equal to 1060 and 660 nm respectively have been used.

Red and infra-red light have different absorption lengths in silicon. Red light has an absorption length of 3µm generating electron-hole pairs just below the illuminated surface. Infra-red light instead has an absorption length of about 1 mm, that is much larger than the sensor thickness. For this reason it can be considered to generate electron-hole pairs uniformly along its path, similarly to a mip. The laser light is focused to the sensor surface with a spot size of the order of σ∼10µm. The strip sensors are mounted on a ceramic substrate which in turn is mounted on an x-y movable table with 0.1µm precision on both directions, allowing to probe the sensor along the strip direction and across it. The ceramic plate substrate can be mounted in such a way that the illuminated surface is the one with segmented electrodes or flipped to illuminate the sensor from the back side.

4.1. LGAD FOR TRACKING 59 About 15 strips and the bias ring are wire-bonded to a readout line connected to ground. The signals of two strips in the central part of the sensor are read out after being amplified by Femto current-sensitive amplifiers [54] with 2 GHz bandwidth and a gain of 40 dB corresponding to an amplification factor of 100. The readout is performed via a Tektronix DPO7254 oscilloscope [55] with 2.5 GHz bandwidth and 40 GS/s sampling rate. The back side of the sensor is read out as well after decoupling the high voltage with a Pulse Lab 5531 bias-T. The bias is provided by a Keithley 6517 high voltage power supply [56]. A sketch of the set-up is shown in figure 4.6.

Ch1Ch2

- HV

GND

Ch3 n+

p p+

p+

e h

Figure 4.6: Sketch of the multi-channel TCT setup at the Hamburg University in the front-side illumination configuration.

The strip sensors have been first tested with back-side illumination using 660 nm laser light, to allow this, the backside metallization has been partially removed. This configuration is useful to study the CM process. As the red laser only penetrates few micrometers, the ionization is localized just below the p⁺ electrode, so that holes are immediately collected and the signal is given by the electrons that drift towards the n⁺ electrode. If there is charge multiplication, the electrons will initiate an avalanche after reaching the multiplication region generating secondary charges that induce an additional contribution to the waveform. This extra component is present only in case of CM due to the drift of holes towards the opposite electrode. Compared to the signal of a sensor without charge multiplication, the signal of a sensor with gain is expected to be larger and lasting longer because the additional contribution is given by holes that have a lower drift velocity. In figure 4.7 the TCT waveforms of an LGAD and a reference strip sensor without CM layer for this configuration are shown. The readout is performed on the back-side electrode that is biased at 300 V. The waveform of the LGAD sensor almost perfectly overlaps with the one of the reference sensor.

The measurement has been repeated for all the strip sensors listed in table 4.1 at different bias voltages approaching the breakdown and pointing the laser at different positions with respect to the strip center. All the measurements lead to the conclusion

that the devices do not present charge multiplication.

Figure 4.7: TCT current waveform on the back-side electrode of an LGAD and a reference strip detector biased at 300 V for back illumination with 660 nm laser light.

The strip devices have been also measured in the front-side illumination con-figuration with 1060 nm laser light. Each strip has an opening on the aluminium metallization that runs along the strip center and the light is injected through this window into the sensor. In this configuration the IR laser light produces electron-hole pairs along the whole sensor thickness imitating the behaviour of amip.

The current waveform of two strips and the back-side electrode is integrated to get the total collected charge. Figure 4.8 shows the charge collected on the back-side electrode in a scan across the strips with step size of 2µm at 300 V by two LGAD sensors and a reference one. The opening on the metallization is about twice as big as the laser spot σ, when the laser point to the center of the strip the tails of the laser hit the metal and are reflected. As a fraction of the light is reflected and does not penetrate into the sensor, a lower charge is collected when the laser is focused on the strip center. This effect is less pronounced for sensor 6894-W14-AC2 where a better focusing was achieved. The effect on the strips boundaries might be caused by small differences in the surface topology, which is characterized by shallow trenches due to the p-stop implantation. This might cause differences in the absorption and transmission properties of the light which might be also sensitive to the laser focus variation.

The charge measured on the LGAD sensors is compatible with the one measured on the reference sensor showing that the CM mechanism did not occur on the LGAD strip sensors from this CNM run. LGAD devices are expected to collect a charge

4.1. LGAD FOR TRACKING 61

Figure 4.8: Integrated charge as a function of y position for a scan with focused 1060 nm laser light through an opening in the metallization of the strips using back-side readout. The two LGAD samples have been measured at different times and it was not possible to reproduce the laser configuration.

about a factor 10 (the target gain) larger than the one collected by the reference device. The plot of figure 4.8 instead shows an agreement between the LGAD and reference sensors, and, as explained above, discrepancies are probably due to the shadowing of the laser light by the metal layer. In addition the charge is observed to be constant with bias voltage after full depletion, see figure 4.9, while in case of CM the gain is expected to increase with voltage since it would lead to a larger electric field in the multiplication region.

Similar results have been observed on the other samples of table 4.1 measured with the same set-up, in particular no differences have been observed between samples from different wafers with different diffusion of the n⁺ layer or between samples with different implant geometries. These measurements point to the failure to obtain strip devices with gain in the CNM run 6827. Further studies on pixel devices from the

Figure 4.9: Integrated charge as a function of bias voltage for 1060 nm laser light front illumination in the center of a strip using back-side readout. The two LGAD samples have been measured at different times and it was not possible to reproduce the laser configuration.

same production are shown in the following section.