In the high luminosity LHC phase, the collider will operate at a luminosity of 1.5 × 1034/cm/s and strict requirements will be posed on subdetectors capabilities. Concerning the LHCb Upgrade2 VELO, to guarantee a good detector performance, the additional information of the hit time stamping with an accuracy of at least 50ps is needed. A very promising option today to achieve this level of timing precision is the 3D trench silicon pixel, developed by the INFN TimeSPOT collaboration. This kind of sensor would allow to build a 4D tracker, capable of excellent resolution in both space and time measurements. The latest beam test with the 3D trench sensors has been performed at the CERN SPS/H8 in 2021. By means of low-noise custom electronics boards featuring a two-stage transimpedance amplifier, it was possible to test silicon pixels and strips made with the 3D trench technology. To extrapolate the sensor time resolution, the crossing time of a particle was estimated using two MCP-PMTs as time tag, with an accuracy of approximately 7 ps. The beam-characterization was performed for both non-irradiated and irradi- ated (2.5·1016 1MeV neq/cm2) devices. Preliminary results show that the standard deviation of the core of the pixels time distribution is about 10 ps and the tilted sensor has shown an efficiency close to 100%

Borgato, F.

Brudu, D.

Cardini, A.

Cossu, G. M.

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DOI 10.1393/ncc/i2023-23084-yCommunications: SIF Congress 2022IL NUOVO CIMENTO 46 C (2023) 84Picosecond timing resolution with 3D trench silicon sensorsF. Borgato(1)(6)(∗), D. Brundu(2), A. Cardini(2), G. M. Cossu(2),G.-F Dalla Betta(5)(8), M. Garau(2)(7), L. La Delfa(2), A. Lampis(2)(7),A. Lai(2), A. Loi(2), M. Obertino(4)(9), S. Vecchi(3) and G. Simi(1)(6)(1) INFN, Sezione di Padova - Padova, Italy(2) INFN, Sezione di Cagliari - Cagliari, Italy(3) INFN, Sezione di Ferrara - Ferrara, Italy(4) INFN, Sezione di Torino - Torino, Italy(5) INFN, TIFPA - Trento, Italy(6) Dipartimento di Fisica dell’Università di Padova - Padova, Italy(7) Dipartimento di Fisica dell’Università di Cagliari - Cagliari, Italy(8) Dipartimento di Ingegneria Industriale, Università di Trento - Trento, Italy(9) Dipartimento di Scienze Agrarie, Forestali ed Alimentari dell’Università di Torino -Grugliasco, Italyreceived 30 January 2023Summary. — In the high luminosity LHC phase, the collider will operate at aluminosity of 1.5×1034/cm/s and strict requirements will be posed on subdetectorscapabilities. Concerning the LHCb Upgrade2 VELO, to guarantee a good detectorperformance, the additional information of the hit time stamping with an accuracyof at least 50 ps is needed. A very promising option today to achieve this level oftiming precision is the 3D trench silicon pixel, developed by the INFN TimeSPOTcollaboration. This kind of sensor would allow to build a 4D tracker, capable ofexcellent resolution in both space and time measurements. The latest beam testwith the 3D trench sensors has been performed at the CERN SPS/H8 in 2021. Bymeans of low-noise custom electronics boards featuring a two-stage transimpedanceamplifier, it was possible to test silicon pixels and strips made with the 3D trenchtechnology. To extrapolate the sensor time resolution, the crossing time of a particlewas estimated using two MCP-PMTs as time tag, with an accuracy of approximately7 ps. The beam-characterization was performed for both non-irradiated and irradi-ated (2.5 ·1016 1MeV neq/cm2) devices. Preliminary results show that the standarddeviation of the core of the pixels time distribution is about 10 ps and the tiltedsensor has shown an efficiency close to 100%.1. – IntroductionLooking at the future of LHC, the instantaneous luminosity will increase by a factorfive with respect to the LHC nominal value, and this will imply a degradation of thedetectors performance due to the very large pile up. Concerning the LHCb Upgrade2,2000 tracks from 40 pp interactions will cross the vertex detector (VELO) at each bunchcrossing. To guarantee good detector performance, the additional information of thehit time stamping with an accuracy of at least 50 ps is needed [1]. A viable option to(∗) E-mail: federica.borgato@pd.infn.itCreative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0) 12 F. BORGATO et al.obtain such a challenging time resolution is the 3D trench silicon pixel developed by theINFN TimeSPOT Collaboration. In the following, the results obtained from the beam-characterization of these sensors at CERN SPS/H8 are presented, for both non-irradiatedand irradiated (2.5 · 1016 1MeV neq/cm2) devices.2. – The TimeSPOT 3D sensorsWithin the TimeSPOT (Time and SPace real-time Operating Tracker) project, 3Dsilicon sensors with trench-shaped electrodes were designed, fully simulated and charac-terized [2, 3]. Unlikely planar sensors, with 3D technology the electrodes of both dopingtypes are penetrating partially or entirely through a high-resistivity silicon substrate,perpendicularly to the wafer surface. As a consequence, the electric field is parallel tothe wafer surface, starting from one electrode type going to the other. This design al-lows having extremely fast signals and robustness to bulk damages thanks to the shortinter-electrode distance [4].For the TimeSPOT silicon sensors, the shape of the electrodes was chosen in orderto maximize the timing performance: the goal is going towards uniformity in both theweighting field and the charge carriers velocity, aiming at the saturation regime thanksto the high and uniform electric field. The TimeSPOT sensors are 55μm× 55μm pixelswith an active thickness of 150μm; the substrate is made of p-type silicon.3. – Devices under test and the SPS/H8 beam test setupThe devices under test (DUT) are pixels and triple strips (three strips made of 10pixels read out together). The characterization was performed both for sensors irradiatedwith neutrons up to 2.5 · 1016 1MeV neq/cm2 and non-irradiated ones. To read out thefast current signals, all the sensors are connected to custom front-end electronics basedon a trans-impedance amplifier (TIA) scheme with two amplification stages.These sensors were characterized with a 180GeV/c pion beam with 8mm of transversesize at the CERN SPS/H8. A scheme of the setup is displayed in fig. 1 (left). Twosensors were placed in a Radio-Frequency shielded box: the device upstream was fixedto a support with piezo-electric stages (“sensor 1” in fig. 1 (left)), that allowed movingthe board in the transversal plane with respect to the beam. The DUT was mounted ona fixed support that could be rotated (“sensor 2” in fig. 1 (left)). Moreover, this secondsensor could be operated down to -40◦C by means of a polystyrene box filled with dryice: in this way, it was possible to test the irradiated devices as well. The time-tag ofthe setup was provided by two MCP-PMTs, placed outside the box; their average timeresolution is approximately 5 ps.The full setup was read out with an 8GHz analog bandwidth, 20GSa/s 4-channeloscilloscope (the Rhode&Schwartz RTP084). The two silicon sensors and the two MCPswaveforms are recorded and analyzed in order to determine the time of arrival of thecrossing particle. The trigger of each data acquisition is the logic AND of the upstreamsensor and the two MCP-PMTs waveforms.4. – Waveforms analysis and resultsIn order to determine the time of arrival of the particles crossing both 3D silicondevices and the MCP-PMTs, different algorithms are considered. Namely, the Splinemethod (interpolating the rising edge of the pulse, taking as the reference time the onecorresponding to the 20% of the signal amplitude), the Leading Edge method (takingthe reference time as the one corresponding to a threshold of 15mV), and the Referencemethod (subtracting from each waveform a delayed copy of itself, then taking the 50%-amplitude time as the reference one).PICOSECOND TIMING RESOLUTION WITH 3D TRENCH SILICON SENSORS 3Fig. 1. – The setup in the SPS/H8 experimental area (left). Maximum amplitudes distributionfor the irradiated pixel (centre) and the non-irradiated pixel (right) for different bias voltages.4.1. Amplitude distributions . – The trend of the maximum amplitudes of the pixelwaveforms as a function of the bias voltage is a useful observable in order to check thecharge collection performance of the sensors. In the case of the non-irradiated sensor(fig. 1, right), the Landau-shaped distribution is well distinguishable from the noise peakeven for low bias voltages, ensuring a good sensor performance in a wide range of biasvoltages.Regarding the irradiated pixel (fig. 1, centre), a higher bias voltage is necessary torecover the expected amplitude distribution of the non-irradiated sensor, which is howeverrecovered for bias voltages higher than 100V in absolute value.4.2. Detection Efficiency . – The TimeSPOT 3D sensors have a geometrical efficiencyof approximately 80% due to the trench-shaped electrodes which are inactive volumes.Thus, it is relevant to check the dependence of the efficiency on the tilt angle with respectto the normal incidence.A single pixel is placed centered and upstream with respect to a triple strip: thedetection efficiency is estimated counting how many events triggering a signal in the pixelwould generate a signal in the triple strip as well. To do so, the distribution of the timeof arrival of the triple strip with respect to the two MCP-PMTs is considered. Fittingthe distribution with a double gaussian and a constant term (fig. 2, left), efficiency isevaluated as the number of events populating the peak over the total number of triggeringevents. The efficiency at normal incidence is compatible with the expected geometricalcalculations, and at tilt angles larger than 10◦ the efficiency is close to 99% for−100V biasvoltage (fig. 2, right). The irradiated triple strip fully recovers the efficiency performancefor a bias voltage of −130V.Fig. 2. – Time of arrival of the triple strip with respect to the two MCP-PMTs for the non-irradiated pixel at normal incidence (left). Detection efficiency as a function of the tilt angle forthe irradiated and non-irradiate triple strip (right).4 F. BORGATO et al.Fig. 3. – Distribution of the time of arrival of the non-irradiated pixel with respect to the averageof the two MCP-PMTs (left). Time resolution of the non-irradiated pixel as a function of thebias voltage (centre). Comparison of the time resolution of the irradiated and non-irradiatedpixel as a function of the bias voltage (right).4.3. Timing performance. – In order to extrapolate the time resolution, the differenceof the time of arrival of the particle in the pixel under test and the average of the timeof arrival in the two MCP-PMTs is considered. As it is possible to notice in fig. 3 (left),where a typical time distribution for the non-irradiated pixel is displayed, the distributionis not perfectly symmetric and the sum of two gaussians was chosen as the fit functionto include the contribution of the tails. The effective time resolution is obtained byproperly weighting the standard deviations of the two gaussians. The time resolutionas a function of the bias voltage for the three different analysis methods is displayed infig. 3 (centre). The best result obtained is 11 ps with a bias voltage of −100V with theReference method. Also, the Spline and Leading Edge methods show promising results,all below 30 ps.The irradiated sensor shows excellent time resolution as well, reaching the result ofapproximately 10 ps at a −150V bias voltage. Figure 3 (right) displays the comparisonbetween the time resolution obtained with the irradiated and non-irradiated sensor as afunction of the bias voltage.Concluding, in both the irradiated and non-irradiated pixels there is no dependenceof the time resolution on the bias voltages below −20V.Moreover, a study was performed considering two neighbouring pixels tilted in orderto have tracks crossing both. It was observed that combining the contributions of the twopixels it is possible to recover the time resolution expected for a single pixel at normalincidence.5. – ConclusionsThe high luminosity conditions in future hadron colliders pose strict requirements tothe detectors performances. Concerning tracking detectors, the TimeSPOT 3D sensorshave all the characteristics to overcome such challenges, with a time resolution of theorder of 10 ps and a detection efficiency close to 100% for both the irradiated and non-irradiated sensors.REFERENCES[1] LHCb Collaboration, Framework TDR for the LHCb Upgrade II, CERN-LHCC-2021-012, LHCB-TDR-023 (2021).[2] Anderlini L. et al., JINST, 15 (2020) P09029.[3] Brundu D. et al., JINST, 16 (2021) P09028.[4] Parker Sherwood, Da Vià Cinzia and Dalla Betta Gian-Franco, Radiation Sensorswith 3D Electrodes (CRC Press), 2019.

Picosecond timing resolution with 3D trench silicon sensors

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Picosecond timing resolution with 3D trench silicon sensors

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