Characterization of N-in-N microstrip radiation detectors fabricated on different silicon substrates C. Fleta, M. Lozano, F. Campabadal, G. Pellegrini, J.M. Rafí, M. Ullán, G. Casse 1 and P.P. Allport 1 Institut de Microelectrònica de Barcelona (IMB), CNM-CSIC. Campus UAB, 08193-Bellaterra (Barcelona), SPAIN. 1 Oliver Lodge Laboratory, University of Liverpool, UK e-mail: Celeste.Fleta@cnm.es 1. Abstract A set of N-in-N microstrip radiation detectors has been fabricated at the IMB-CNM facilities on FZ standard, FZ oxygenated and magnetic Czochralski silicon substrates. A first electrical characterization shows that the devices have a good behavior, with low leakage currents and a full depletion voltage of about 55 V for the FZ substrates and 330 V for the CZ. 2. Introduction Silicon pixel and microstrip detectors are widely used as trackers in high-energy physics experiments thanks to their high spatial resolution and low response times. However, the hostile radiation environment in which these devices are immersed can seriously affect their performance after a few years of operation. Recent studies [1] have shown that a high concentration of oxygen atoms in the silicon lattice improves significantly its radiation hardness. The material most widely used in the fabrication of radiation detectors, Float Zone (FZ) silicon, has a low oxygen concentration, but it can be increased up to 10 17 cm -3 with high temperature diffusion processes [2]. On the other hand, the introduction of the magnetic field assisted Czochralski (CZ) method has made possible the fabrication of detector grade quality (high resistivity) CZ silicon with an intrinsically high oxygen concentration of about 10 17 -10 18 cm -3 . In order to compare their performance and radiation hardness, microstrip radiation detectors have been fabricated on three different silicon substrates: FZ standard and oxygenated and high resistivity magnetic CZ. This paper presents the results of their electrical characterization prior to their irradiation. 3. Experimental A mask set containing microstrip radiation detectors of different sizes, as well as several test structures, was designed by the University of Liverpool and the CNM- IMB. This mask set is specific for the fabrication of N- in-N (n + strips in an n-type substrate) detectors, as they require double-side processing. The fabrication was carried out at the CNM facilities. The FZ wafers processed have a thickness of 280 μm and a nominal resistivity of 1-5 kohm·cm. The CZ wafers are 300 μm thick and their resistivity is 1 kohm·cm. The area of the detectors used in this study is 3×2 cm 2 , and they consist of 128 microstrips with a pitch of 120 μm. The isolation between strips is achieved with individual p-stops and a p-spray implantation. The bias is provided by a bias ring connected to the strips via polysilicon resistors, and the active area of the detector is surrounded by a multi-guard ring structure. The electrical characterization of the devices was carried out in a class 10000 clean room, at a temperature of 20ºC. The samples were placed inside a shielded probe station that provided the electrical contact and kept them dark. An ascending ramped voltage was applied between the polarization ring and the back of the devices to obtain the curves of leakage current and capacitance versus applied voltage. The full depletion voltage of the devices was extracted from the log C-log V curves. 4. Results The full depletion voltage of the detectors fabricated on FZ substrates is slightly higher for the standard silicon (57 V) than for the oxygenated material (53 V). For the CZ magnetic wafers, this value is about 330 V. The dark currents of the detectors are low. However, the detectors fabricated on oxygenated FZ silicon show an early breakdown, some of them at voltages lower than 10 V. Although the reason for this behavior still has to be determined, the use of guard rigs should improve it. In the case of the detectors fabricated on FZ std and CZ substrates, the breakdown voltages are usually higher than the full depletion voltage, and some of the devices can withstand more than 600 V without breaking. References [1] G. Lindstrom et al. “Radiation hard silicon detectors- developments by the RD48 (ROSE) collaboration”, Nucl. Instr. and Meth. A, vol. 466, pp. 308-326 (2001). [2] L. Fonseca et al., “Silicon wafer oxygenation from SiO 2 layers for radiation hard detectors”, Microelectronics Reliability, vol. 40, pp 791-794 (2000). 59