2 MeV electron irradiation effects on the electrical characteristics of metal–oxide–silicon capacitors with atomic layer deposited Al₂O₃, HfO₂ and nanolaminated dielectrics

Abstract

The effects of 2 MeV electron irradiation on the electrical characteristics of atomic layer deposited (ALD) high permittivity (high-k) layers of Al₂O₃, HfO₂ and a nanolaminate of them are evaluated. Metal–oxide–semiconductor capacitors with a nominal dielectric physical thickness of 10 nm were fabricated on different p-type and n-type silicon substrates. The capacitance–voltage (C–V) and current–voltage (I–V) characteristics of the different structures are analyzed as a function of electron irradiation. A progressive negative shift of the C–V characteristics is observed with increasing electron irradiation, indicating the generation of effective positive charges. Similar generation rates for effective trapped charges and interface states are obtained for all the different high-k dielectric layers studied. The hysteresis of the C–V curves after irradiation increases in the case of Al₂O₃ samples, for HfO₂ decreases while the irradiation has little impact on the hysteresis of the nanolaminate stack. A progressive increase of the leakage current with electron irradiation dose is observed for all the studied dielectrics. The analysis of the current–voltage characteristics measured at different temperatures point to Poole–Frenkel as the dominant conduction mechanism. Under the studied conditions, no impact of electron irradiation fluence on dielectric breakdown voltage has been appreciated.

Introduction

A number of high permittivity (high-k) dielectrics have been investigated as candidates to replace the SiO₂ as gate dielectric in complementary metal–oxide–semiconductor (CMOS) technologies, being Al₂O₃ and HfO₂ among the most studied ones [1], [2], [3], [4]. Apart from CMOS technologies, high-k dielectrics are also of strong interest for a wide range of micro/nanoelectronics applications, including, dynamic random access memories (DRAM) [5], emerging nanodevices [6], [7], organic light emitting diodes [8], as a surface passivation layer on high-efficiency crystalline silicon solar cells [9], and for a variety of microelectromechanical systems [10]. For all these applications, atomic layer deposition (ALD) has shown great potential to meet the requirements concerning dielectric properties, large area uniformity, conformality and accurate thickness control. Moreover, taking advantage of ALD unique properties, some works have also studied the possibility to deposit alternate layers of different high-k materials (nanolaminates). This can be useful to tailor the dielectric properties of the stack, reduce leakage currents in the presence of possible crystallization processes or improve the interface in contact with the semiconductor [11], [12], [13].

The integration of new materials and processes requires the study of different aspects for technology assessment. In recent years, a lot of work has been devoted to physical and electrical characterization, as well as to reliability issues, of alternative high-k dielectrics [14], [15], [16]. However, much less is known about their behaviour in radiation-harsh environments. The study of ionizing radiation effects on high-k dielectrics is of special interest for space applications [17], but also for high energy physics experiments and even to gain insight into possible effects of some advanced micro/nanofabrication processes like e-beam or X-rays lithography.

Since the early days of microelectronics, radiation-induced effects on conventional SiO₂ dielectrics for CMOS technologies have been investigated [18], [19]. Total ionizing dose (TID) damage, with charge trapping in the gate dielectric and interface states generation at the silicon interface, have been the main radiation-induced degradation mechanisms for a wide range of SiO₂ layer thickness [20]. A trap assisted tunnelling phenomenon, the radiation-induced leakage current (RILC), was observed for SiO₂ layers with thickness in the range of about 3–8 nm [21], [22]. Fortunately, the continuous miniaturization process has led to radiation-harder SiO₂ layers, with reduced charge trapping and flat band voltage shifts [23], and with thickness below the trapped-hole tunnelling limit (around 3 nm) [20].

The introduction of high-k dielectrics brings some uncertainty in terms of their radiation hardness. Moreover, due to the dependence of TID damage on the physical dielectric thickness, the radiation response could be an issue for such thicker high-k dielectric layers used in place of SiO₂ [24]. Although some works have already been published on radiation effects on a few high-k dielectrics, these have been mostly limited to irradiations with heavy ions or X-rays, and only separated studies addressing either capacitance–voltage or current–voltage characterization have been generally considered [24], [25], [26], [27], [28], [29].

In this work, the effects of different doses of 2 MeV electron irradiation on the electrical characteristics of ALD-deposited layers of Al₂O₃, HfO₂ and a nanolaminate of them are evaluated. Metal–oxide–semiconductor (MOS) structures with a nominal high-k dielectric physical thickness of 10 nm were fabricated on p-type and n-type silicon substrates. The capacitance–voltage and current–voltage characteristics of the different structures are analyzed as a function of electron irradiation dose paying special attention to the study of the effective trapped charges, generation of interface states, presence of hysteresis and electrical conduction through the layers.