C. Z. Zhao

Hole traps in silicon dioxides. Part I. Properties

As the downscaling of gate oxides continues, trap density in the oxide bulk will reduce, but positive charges formed near to the SiO/sub 2//Si interface become relatively important. For gate oxides used in industry, hole trapping is the most important process for positive charge formation. Apart from as-grown hole traps, we recently reported that new hole traps were generated by electrical stresses. Information on these hole traps, however, is still limited. In part I of this work, properties of both generated and as-grown hole traps are investigated. For the first time, it will be clearly shown that generated hole traps consist of two components; cyclic positive charges (CPC) and antineutralization positive charges (ANPC). The charging and discharging rates of CPC are similar, while the neutralization of ANPC is much more difficult than its charging. Differences between them are also observed in generation kinetics and dependence on measurement temperature. Efforts will be made to explain their differences in terms of energy levels and to link them with positive charges reported in earlier works. We will also show that as-grown traps, regardless their distance from the interface, are not responsible for either ANPC or CPC. This is to say that generated hole traps are not the same as as-grown traps and their differences will be highlighted. In part II, hole trap generation mechanisms will be investigated.

Stress-Induced Positive Charge in Hf-Based Gate Dielectrics: Impact on Device Performance and a Framework for the Defect

A Hf-based dielectric has been selected to replace SiON for CMOS technologies. When compared with SiON, Hf dielectrics can suffer from higher instability. Previous attentions were focused on electron trapping, and positive charging received less attention. The objective of this paper is to study the impact of positive charging on device performance and to provide a framework for the defect. Three components of threshold voltage instability Delta Vth are unambiguously identified for pMOSFETs, i.e., loop, loop-shift, and up-loop. The loop dominates Delta Vth at a relatively short time (< 1 s). After stressing for a longer time, the whole loop is shifted in the negative direction. Unlike the loop, the up-loop cannot readily be recharged after recovery. In addition to the generated interface states, three different types of positive charges are formed in the Hf-based stacks, i.e., cyclic positive charges (CPC), antineutralization positive charges (ANPC), and as-grown hole trapping (AHT). Each type of defect has its unique signatures and properties. CPC can repeatedly be charged and discharged by alternating the gate bias polarity. ANPC is more difficult to neutralize, whereas AHT is harder to charge. Both the generated interface states and the AHT saturate at longer stress time, but ANPC does not. ANPC reduces at higher measurement temperature, but CPC is insensitive to temperature. The relation between each type of defect and each component of Delta Vth is clarified.

Determination of capture cross sections for as-grown electron traps in HfO2/HfSiO stacks

A major challenge for replacing gate SiON with HfO2 is the instability and reliability of HfO2. Unlike the SiON, there can be substantial amount of as-grown electron traps in HfO2. These traps can cause instability in the threshold voltage and contribute to the dielectric breakdown. Despite the early efforts, our understanding of them is incomplete. Agreement on their capture cross sections has not been reached and the reported values spread in a large range of 10−12–10−19cm2. The objective of this paper is to determine their capture cross sections unambiguously, which requires knowing the gate current and the electron fluency for filling the trap. A key part of this work is to estimate the trapping-induced transient gate current following the application of a pulse to the gate. This is achieved by numerical simulation. It is found that trapping can reduce the gate current by two orders of magnitude and the gate current can drop substantially within microseconds. The results show the presence of two disti...

Hole-traps in silicon dioxides. Part II. Generation mechanism

After studying the properties of hole traps in Part I, attention is turned to the physical processes responsible for generating hole traps in Part II of this work. The applicability of four models to hole-trap creation will be examined. These are the trapped hole-electron recombination model, the electrical field energy model, the hole injection model, and the hydrogen model. To testify these models, stresses have to be carried out not only under substrate hole injection (SHI), but also under Fowler-Nordheim injection (FNI). By combining FNI with SHI, we will be able to control hole fluency independent of the electron-induced hydrogen release. This allows us to determine how important hydrogen is for hole-trap generation. Although it was reported that hydrogen could play a major role in positive charge generation for devices with an Al gate or without a gate, we will show that hydrogen does not dominate hole-trap generation, when poly-si gated devices are stressed under our test conditions. Unambiguous results will also be given to show that key predictions of the recombination model and the electrical field energy model are not observed here. In this paper, the most important process for hole-trap generation is found to be the direct interaction of injected holes with the oxide.

An Assessment of the Location of As-Grown Electron Traps in

Replacing SiON by high-kappa layers is a pressing issue for CMOS technologies. The presence of as-grown electron traps in HfO2 is a major obstacle, since they can induce threshold-voltage instability, reduce electron mobility, and result in early breakdown. Their location has not been clarified and is addressed in this letter. By selecting test conditions carefully and using samples with a progressive reduction of HfO2 thickness, the authors are able to rule out that traps are piled up near the HfO2/HfSiO interface. A uniform distribution throughout HfO2 does not agree with the test data, either. Results support that trapping is negligible near to one or both ends of the HfO2 layer when compared with trapping in the central region

hboxHfO_2

Positive charge formation in gate oxides is a main source for the instability of the state-of-the-art metal-oxide-semiconductor device. Despite past efforts, the relation between hydrogenous species and positive charges is not fully understood. In this work, the effects of hydrogen on positive charges will be investigated at both elevated temperature (e.g., 400°C) and room temperature. At 400°C, it is found that hydrogen can convert some defects into hole traps. Three different types of positive charges have been reported recently. They are as-grown hole traps, anti-neutralization positive charges (ANPC), and cyclic positive charges (CPC). Although an exposure to hydrogen at 400°C neutralizes all three, impacts of hydrogen on these three types of defects are markedly different. After the hydrogen-induced neutralization, the defect responsible for ANPC is fully recovered and is the same as that in a fresh device. In contrast, the defect for CPC is not fully recovered and can be reactivated easily by stress...

/HfSiO Stacks

This article investigates the H2-anneal induced positive charge generation in the gate oxide of metal-oxide-semiconductor field-effect transistors fabricated by a submicron complementary metal-oxide-semiconductor process. A significant number (∼1012 cm−2) of fixed and mobile positive charges are generated at 450 °C. Properties (reactivity, electrical and thermal stability) of these positive charges are compared with the positive charges observed in the buried oxide of silicon-on-insulator devices. The differences in these two are investigated, in terms of their transportation time across the oxide, uniformity and sources of hydrogen. Attention is paid to the role played by boron in the generation and the possible connection between the positive species observed here and the defects responsible for the positive bias temperature instability. Efforts are made to explain the difference in reactivity between the H2-anneal induced positive species and the hydrogenous species released by irradiation or electrica...

Effects of hydrogen on positive charges in gate oxides

Interface state generation is a major reliability issue for metal–oxide–semiconductor based devices. The generation can take place not only during stresses, but also after terminating the stress. Our attention is focused on analyzing the dynamic behavior of the generation after substrate hot hole injection. Despite previous efforts in this area, the generation kinetics is not fully understood, and there is insufficient information on the process limiting the generation rate. We start by showing that the normalized generation kinetics is insensitive to either the defect density or the processing condition. We then investigate the effect of various stressing parameters on the kinetics, including the oxide field strength during and posthole injection, the stress time, the energy and current of hot holes. This is followed by examining why the available models are inapplicable in our cases, including hydrogen transportation, trapped hole conversion, and coupling models. Finally, we propose both hydrogen emissi...

Hydrogen induced positive charge generation in gate oxides

Oxide breakdown is an important issue for MOS devices. It is widely believed that defects generated within the oxides are responsible for the breakdown. However, it is still not clear which type of the various generated defects is the main cause for the failure. This paper unambiguously shows that generated hole traps, low-field electron traps, and high-field electron traps with a capture cross section of 10/sup -15/-10/sup -16/ cm/sup 2/ are not the main source of the breakdown. The generated high-field electron trap with a capture cross section in the order of 10/sup -14/ cm/sup 2/ is the only defect having all the characteristics required for breaking down the oxide. This paper should provide useful information for modeling oxide breakdown.

Analysis of the kinetics for interface state generation following hole injection

Abstract There are at least two ways for creating positive charges in silicon oxides: hole trapping and the formation of positive hydrogenous species. This paper investigates the relation between them. The issues addressed include if hole traps assist in the generation of hydrogenous positive charges and how the formation of hydrogenous charges affects the hole trapping. Both reactive and non-reactive hydrogenous species are investigated and their different effects on hole traps are pointed out. It is found that there are two types of hole traps, having different relations with hydrogen.