High-energy neutrons effect on strained and non-strained SOI MuGFETs and planar MOSFETs

Abstract

A comparative investigation of high-energy neutrons effect on strained and non-strained devices with different geometries is presented. Both single-gate planar and multiple-gate (MuG) silicon-on-insulator (SOI) devices are considered. Device response to the neutron irradiation is assessed through the variations of threshold voltage and transconductance maximum. The difference between strained and non-strained device response to the high-energy neutrons exposure is clearly evidenced. The reasons for such a difference are discussed. Analysis of the experimental results allows for suggesting that strain relaxation is one of the probable causes.

Introduction

Requirements for the nano-scaled devices identified by ITRS [1] imply the employment of both new device architectures (to improve short-channel effects (SCE) control) and new channel materials (to boost carrier mobility). This motivated numerous studies of multiple-gate (MuG) FETs and high-mobility materials (strained Si, Ge, etc.) and their combinations. From another side, enhanced electronic penetration in transport and medical domains demands for higher reliability levels than previously acceptable. Moreover, harsh environment aspects previously considered only for some specific “niche” applications, now concern a much wider range of applications. These create a demand for an in-depth assessment of reliability issues in “novel” devices, which, however, are not yet widely explored.

During the last decade, radiation effects on the behavior of MuGFETs were widely studied. Some works were devoted to the investigation of heavy-ions [2] and proton irradiation [3] on strained MuGFETs and SOI MOSFETs, which however, did not point out strain-related specificity in the device response to irradiation. At the same time, [4] demonstrates the strain relaxation in SiGe under proton irradiation and [5] reveals the tensile strain relaxation under 2 MeV-electron irradiation with 10¹⁵ e/cm² fluence. Then, [6] emphasizes that proton radiation induced mobility degradation depends on the strain technique.

Unfortunately, the effect of neutrons on microelectronic devices, and particularly on MOSFETs, is usually considered to be negligible and hence rarely discussed. However, since neutrons with wide energetic spectrum (from about 0.1 eV to about 10 GeV [7]) are present both at high altitudes and at the ground level, their effect on microelectronic devices is worth of in-depth investigation. Indeed, the neutron component representing about half of the dose equivalent at avionic altitudes and 10% at sea level [7], was identified as a crucial radiation source for advanced technologies with strongly downscaled devices [7]. Furthermore, understanding of the effects caused by neutrons is of interest for nuclear reactors and high-energy physics large-scale experiments at CERN, where strong neutron background can lead to the detectors and readout electronic destruction. While the part of high-energy neutrons in these cases is rather low [8], they cannot be neglected. Next to that, since neutron is an uncharged particle, the main neutron-induced damages in semiconductor devices are generally considered to be related to the non-ionizing effects [9] and thus are almost not discussed for MOSFETs. Degrading mainly carrier lifetime, mobility, etc., these effects are of major importance for bipolar devices. Nevertheless, neutrons are believed to be capable to produce ionization damages through indirect or secondary processes [9].

Indeed, our previous works have shown that degradations in MuGFETs and SOI MOSFETs behavior caused by high energy neutrons and γ-rays with about the same dose are largely similar [10]. In additional to the total-dose effects discussed in [10], [11], neutrons, being known to cause displacement damages, could be suspected to result in strain relaxation.

In this work, we study the effect of high-energy neutrons on strained MuGFETs and MOSFETs and compare them with non-strained devices fabricated in the same technological run.