From Radiation Induced Leakage Current to Soft Breakdown in Irradiated MOS Devices with Ultra-Thin Gate Oxide
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ABSTRACT Metal Oxide Semiconductor (MOS) capacitors with ultra-thin oxides have been irradiated with ionising particles (8 MeV electrons or Si, Ni, and Ag high energy ions), featuring various Linear Energy Transfer (LET) ranging over 4 orders of magnitude. Different oxide fields (Fbias) were applied during irradiation, ranging between flat-band and 3 MV/cm. We measured the DC Radiation Induced Leakage Current (RILC) at low fields (3-6 MV/cm) after electron or Si ion irradiation. RILC was the highest in devices biased at flat band during irradiation. In devices irradiated with higher LET ions (Ni and Ag) we observed the onset of Soft-Breakdown phenomena. Soft-Breakdown current increases with the oxide field applied during the stress. INTRODUCTION In contemporary deep submicron CMOS technologies, the thickness of the MOS gate oxide is reduced to few nm's. It is well known that electrical stresses at high fields can produce a degradation of the insulating properties of the oxide layer, resulting in catastrophic breakdown, Soft Breakdown (SB), and Stress Induced Leakage Current (SILC) [1]. In ultra-thin oxides the last two issues are of major concern. On the other side, previous results have shown that ionizing radiation can produce catastrophic breakdown. In particular single event gate rupture can be
ignited by a single ionizing particle as long as its Linear Energy Transfer (i.e., the ionizing energy lost per unit length along the particle path) and the oxide field are high enough [2-4]. Excess gate current attributed to the onset of SB phenomena has been reported as well on thin oxides [2-3]. Further, a low-field leakage current, i.e., the Radiation Induced Leakage Current (RILC), can be observed after gamma ray or electron irradiation, featuring electrical characteristics very similar to those of SILC [5, 6]. As for SILC, RILC is attributed to a trap assisted tunneling (TAT) of electrons through neutral oxide traps generated by irradiation [7]. A single trap is needed to mediate the electron tunneling across the oxide in each weak spot. In this contribution, we present new aspects of the gate leakage current after exposure to ionizing particles with high LET and for low oxide fields applied during irradiation. No effort has been made to study the radiation induced catastrophic breakdown, which can occur only for high oxide fields applied during irradiation in the oxides used in this work. EXPERIMENTAL AND DEVICES The devices used in this work were square MOS capacitors with oxide thickness t,,x=4 nm 3 on p-Si with doping concentration .3 2 2 NA=5x1 016 cm" . The polysilicon gate doping was ND=7x1 019 cm-, and gate area A=1 0 cm. Radiation stresses were performed by using four different sources (see Table I). A LINAC accelerator (CNR-FRAE, Bologna, Italy) supplied the 8 MeV electrons for high energy electron stressing, while ion irradiation were performed at the Tandem 201
Mat. Res. Soc. Symp. Proc. Vol. 592 © 2000 Materials Research Society
Van der Graaff accelerator of the Laboratori Nazionali di Legnaro, INF
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