Degradation of electron irradiated MOS capacitors A. Candelori a , A. Paccagnella a, *, A. Scarpa a , G. Ghidini b , P.G. Fuochi c a Dipartimento di Elettronica e Informatica, Universita Á di Padova, via Gradenigo 6a, 35131 Padova, Italy b SGS-Thomson Microelectronics, via C. Olivetti 2, 20041 Agrate Brianza, Italy c CNR-FRAE, via Gobetti 101, 40129 Bologna, Italy Received 30 April 1998; received in revised form 13 August 1998 Abstract We have investigated the degradation of MOS structure due to high energy electron irradiation as a function of radiation dose and gate bias applied during the irradiation. Devices have been characterized by current±voltage measurements, in order to study charge accumulation also at the gate interface. Three types of oxide charge have been observed: the unstable positive charge, due to trapped holes induced by the electron irradiation; the negative charge in the oxide bulk, deriving from capture of electrons injected during electrical measurements in radiation generated traps; and border traps, at both oxide interfaces. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Degradation of MOS devices can be induced by elec- trical stresses or by exposure to ionizing radiation [1, 2]. Historically, radiation damage has usually been studied by measuring the capacitance±voltage (C±V) curves of MOS capacitors, or the MOSFET current±voltage characteristics [3]. Hence, much attention has been devoted to the silicon/oxide interface. The gate/oxide interface, which looks like the dark side of the irra- diated MOS, has seldom been investigated [4]. On the other hand, studies aiming to evaluate the degradation of electrically stressed oxides have focused on the measurement of the oxide trapped charge, by using ex- perimental methods sensitive to both interfaces. In this contribution, we intend to study irradiated oxides, by using experimental methods typically used to study oxide properties after electrical stresses. Other motiv- ations support our work: (i) non-volatile memories rely on current injection across a tunnel oxide for proper operations, and they can be aected by radiation damage, for instance in space environment; (ii) deep- submicron lithography will soon use highly ionizing radiation, such as X-ray or electron beam, and the impact of radiation damage on the oxide charging needs to be clari®ed; (iii) oxide damage due to Fowler± Nordheim (FN) injection is enhanced at the anodic interface, and it should be compared with the uniform damage generated in the oxide by the ionizing radi- ation. In this work, we have investigated the degradation of MOS capacitors exposed to high energy electrons as a function of radiation dose and gate voltage during ir- radiation. We have measured the modi®cations of the current±voltage (I g  V g ) curves in the FN conduction regime, in order to determine the oxide trapped charge in the dierent experimental conditions considered. 2. Experimental set-up Tested devices are 1 mm 2 -area MOS capacitors with n + -polysilicon gate. The 20 nm thick SiO 2 layer was thermally grown in wet atmosphere on a p-Si sub- strate. Each capacitor is surrounded by an n + guard- ring short-circuited to the substrate contact. Devices were irradiated by using an 8 MeV electron beam from a linear accelerator (LINAC). Three dierent bias voltages (V b ) were applied to the MOS gate during irradiation: V  b = 4.5 V, V 0 b = 0 V, or Microelectronics Reliability 39 (1999) 227±233 0026-2714/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0026-2714(98)00231-5 PERGAMON * Corresponding author. Tel.: +39-049-8277664; fax: +39- 049-8277699; e-mail: paccag@dei.unipd.it.