Timing resolution from very thin LGAD sensors tested on particle beam down to 12 ps

Abstract

The paper reports on the timing resolution obtained with the Low-Gain Avalanche Diode (LGAD) sensors for extreme fluences at the DESY Test Beam Facility with 4 GeV/c electrons. The LGADs adopt an $n$-in-$p$ technology with a $p^{+}$-type boron gain implant, co-implanted with carbon to mitigate acceptor deactivation. The substrate thickness of the sensors varies from 20 $μ$m to 45 $μ$m, with an active area spanning from 0.75 $\times$ 0.75 to 1.28 $\times$ 1.28 mm$^{2}$. A set of 30 $μ$m sensors irradiated with neutrons at fluences between 4 $\times$ 10$^{14}$ and 2.5 $\times$ 10$^{15}$ n$_{\textrm{1 MeV eq.}}$cm$^{-2}$ were tested on the beam. The gain was measured between 7 and 40 across all non-irradiated sensors in the study, and between 7 and 30 in irradiated sensors. The experimental setup consisted of a 45 $μ$m-thick trigger sensor with an active area of 3.6 $\times$ 3.6 mm$^{2}$, four device-under-test (DUT) planes, and a Photonis micro-channel plate photomultiplier tube (MCP) as a time reference. The timing resolution was calculated from Gaussian fitting of the difference in times of arrival of a particle at a DUT and the MCP, using the constant fraction discrimination technique. A timing resolution of 26.4 ps was achieved in 45 $μ$m sensors, and down to 16.6 ps in 20 $μ$m sensors. The combination of two 20 $μ$m LGAD sensors reached a timing resolution of 12.2 ps. A timing resolution of below 20 ps was obtained in all irradiated 30 $μ$m sensors.

Figures (17)

• Figure 1: Specifications for the LGAD sensors studied: an $n$-in-$p$ design with a peak boron ($p^{+}$-type) doping concentration of $\sim$10$^{16}$$\textrm{at}~\textrm{cm}^{-3}$ in the gain implant.
• Figure 2: Schematic images of EXFLU SP and LP devices used for timing performance studies: the EXFLU0 SP (a) and LP (b) devices with an active area of 1.28 $\times$ 1.28 mm$^{2}$ and 1.0 $\times$ 1.0 mm$^{2}$ per pad, respectively, and the EXFLU1 SP (c) and LP (d) devices with an active area of 1.28 $\times$ 1.28 mm$^{2}$ and 0.75 $\times$ 0.75 mm$^{2}$ per pad, respectively.
• Figure 3: The breakdown trends for the non-irradiated EXFLU samples. The two 20 $\rm{\upmu}\textrm{m}$ EXFLU1 sensors differ by gain implantation method, with the CHBL mode leading to a higher breakdown voltage.
• Figure 4: The distribution of signal amplitudes for the 20 $\rm{\upmu}\textrm{m}$ CBL LGAD sensor under 150 V external bias at 18$^{\circ}$C, with the selected signal events indicated above amplitudes of 25 mV. The shape is typical for all samples.
• Figure 5: The breakdown curves for the irradiated 30 $\rm{\upmu}\textrm{m}$ samples, performed at -20$^{\circ}$C, highlighting the increasing breakdown with higher fluences due to the onset of acceptor removal. The SEB region is also shown here, above the 405 V threshold for 30 $\rm{\upmu}\textrm{m}$ sensors.