Trade openness, export diversification, and political regimes

Recent studies have challenged the view that trade openness leads to more specialization in countries’ trade. Using a panel of 116 countries over 35 years, we show that openness can be positively associated with both specialization and diversification, depending on the measure used. Moreover, for developing countries in our sample, the effect of openness on trade structure depends on the type of political regime: in autocracies openness is linked with specialization, whilst in democracies it is related to diversification via export sophistication

Atomic layer deposition of silicon-based dielectrics for semiconductor manufacturing: Current status and future outlook

The fabrication of next-generation semiconductor devices has created a need for low-temperature (≤400 °C) deposition of highly-conformal (>95%) SiO2, SiNx, and SiC films on high-aspect-ratio nanostructures. To enable the growth of these Si-based dielectric films, semiconductor manufacturers are transitioning from chemical vapor deposition to atomic layer deposition (ALD). Currently, SiO2 films deposited using ALD are already being integrated into semiconductor device manufacturing. However, substantial processing challenges remain for the complete integration of SiNx films deposited by ALD, and there are no known processes for ALD of SiC at temperatures that are compatible with semiconductor device manufacturing. In this focused review, the authors look at the status of thermal and plasma-assisted ALD of these three Si-based dielectric films. For SiO2 ALD, since low-temperature processes that deposit high-quality films are known, the authors focus primarily on the identification of surface reaction mechanisms using chlorosilane and aminosilane precursors, as this provides a foundation for the ALD of SiNx and SiC, two material systems where substantial processing challenges still exist. Using an understanding of the surface reaction mechanisms, the authors describe the underlying reasons for the processing challenges during ALD of SiNx and SiC and suggest methodologies for process improvement. While both thermal and plasma-assisted SiNx ALD processes have been reported in the literature, the thermal NH3-based ALD processes require processing temperatures >500 °C and large NH3 doses. On the other hand, plasma-assisted SiNx ALD processes suffer from nonuniform film properties or low conformality when deposited on high-aspect-ratio nanostructures. In the SiNx section, the authors provide a broad overview of the currently known thermal and plasma-assisted SiNx ALD processes using chlorosilane, trisilylamine, and aminosilane precursors, describe the process shortcomings, and review the literature on precursor reaction pathways. The authors close this section with suggestions for improving the film properties and conformality. In the case of SiC, the authors first outline the limitations of previously reported SiC ALD processes and highlight that unlike SiO2 and SiNx plasma-assisted ALD, no straightforward pathway for low-temperature plasma-assisted growth is currently apparent. The authors speculate that low-temperature ALD of SiC may require the design of completely new precursors. Finally, they summarize the progress made in the ALD of C-containing SiNx and SiO2 films, which may provide many of the benefits of SiC ALD in semiconductor manufacturing. In closing, through this review, the authors hope to provide the readers with a comprehensive knowledge of the surface reactions mechanisms during ALD of Si-based dielectrics, which would provide a foundation for future precursor and process development

A Three-Step Atomic Layer Deposition Process for SiN_<i>x</i> Using Si₂Cl₆, CH₃NH₂, and N₂ Plasma

We report a novel three-step SiN_<i>x</i> atomic layer deposition (ALD) process using Si₂Cl₆, CH₃NH₂, and N₂ plasma. In a two-step process, nonhydrogenated chlorosilanes such as Si₂Cl₆ with N₂ plasmas lead to poor-quality SiN_<i>x</i> films that oxidize rapidly. The intermediate CH₃NH₂ step was therefore introduced in the ALD cycle to replace the NH₃ plasma step with a N₂ plasma, while using Si₂Cl₆ as the Si precursor. This three-step process lowers the atomic H content and improves the film conformality on high-aspect-ratio nanostructures as Si–N–Si bonds are formed during a thermal CH₃NH₂ step in addition to the N₂ plasma step. During ALD, the reactive surface sites were monitored using in situ surface infrared spectroscopy. Our infrared spectra show that, on the post-N₂ plasma-treated SiN_<i>x</i> surface, Si₂Cl₆ reacts primarily with the surface −NH₂ species to form surface −SiCl_<i>x</i> (<i>x</i> = 1, 2, or 3) bonds, which are the reactive sites during the CH₃NH₂ cycle. In the N₂ plasma step, reactive −NH₂ surface species are created because of the surface H available from the −CH₃ groups. At 400 °C, the SiN_<i>x</i> films have a growth per cycle of ∼0.9 Å with ∼12 atomic percent H. The films grown on high-aspect-ratio nanostructures have a conformality of ∼90%

Circular Coinduction with Special Contexts

Coinductive proofs of behavioral equivalence often require human ingenuity, in that one is expected to provide a "good" relation extending one's goal with additional lemmas, making automation of coinduction a challenging problem. Since behavioral satisfaction is a 0 2 -hard problem, one can only expect techniques and methods that approximate the behavioral equivalence. Circular coinduction is an automated technique to prove behavioral equivalence by systematically exploring the behaviors of the property to prove: if all behaviors are circular then the property holds. Empirical evidence shows that one of the major reasons for which circular coinduction does not terminate in practice is that the circular behaviors may be guarded by a context. However, not all contexts are safe. This paper proposes a large class of contexts which are safe guards for circular behaviors, called special contexts, and extends circular coinduction appropriately. The resulting technique has been implemented in the CIRC prover and experiments show that the new technique can prove many interesting behavioral properties fully automatically

Circular Coinduction with Special Contexts

Coinductive proofs of behavioral equivalence often require human ingenuity, in that one is expected to provide a "good" relation extending one's goal with additional lemmas, making automation of coinduction a challenging problem. Since behavioral satisfaction is a 0 2 -hard problem, one can only expect techniques and methods that approximate the behavioral equivalence. Circular coinduction is an automated technique to prove behavioral equivalence by systematically exploring the behaviors of the property to prove: if all behaviors are circular then the property holds. Empirical evidence shows that one of the major reasons for which circular coinduction does not terminate in practice is that the circular behaviors may be guarded by a context. However, not all contexts are safe. This paper proposes a large class of contexts which are safe guards for circular behaviors, called special contexts, and extends circular coinduction appropriately. The resulting technique has been implemented in the CIRC prover and experiments show that the new technique can prove many interesting behavioral properties fully automatically

Atomic layer deposition of wet-etch resistant silicon nitride using di(sec-butylamino) silane and N2 plasma on planar and 3D substrate topographies

The advent of three-dimensional (3D) finFET transistors and emergence of novel memory technologies place stringent requirements on the processing of silicon nitride (SiNx) films used for a variety of applications in device manufacturing. In many cases, a low temperature (&lt;400 °C) deposition process is desired that yields high quality SiNx films that are etch resistant and also conformal when grown on 3D substrate topographies. In this work, we developed a novel plasma-enhanced atomic layer deposition (PEALD) process for SiNx using a mono-aminosilane precursor, Di(Sec-ButylAmino)Silane (DSBAS, SiH3N(sBu)2), and N2 plasma. Material properties have been analysed over a wide stage temperature range (100 – 500 °C) and compared with those obtained in our previous work for SiNx deposited using a bis-aminosilane precursor, Bis(Tert-ButylAmino)Silane (BTBAS, SiH2(NHtBu)2), and N2 plasma. Dense films (∼3.1 g/cm3) with low C, O and H contents at low substrate temperatures (&lt;400 °C) were obtained on planar substrates for this process when compared to other processes reported in the literature. The developed process was also used for depositing SiNx films on high aspect ratio (4.5 : 1) 3D trench nanostructures to investigate film conformality and wet-etch resistance (in dilute hydrofluoric acid, HF : H2O = 1 : 100) relevant for state-of-the-art device architectures. Film conformality was below the desired levels of &gt;95% and is attributed to the combined role played by nitrogen plasma soft saturation, radical species recombination and/or ion directionality during SiNx deposition on 3D substrates. Yet, very low wet-etch rates (WER ≤2 nm/min) were observed at the top, sidewall and bottom trench regions of the most conformal film deposited at low substrate temperature (&lt;400 °C), confirming that the process is applicable for depositing high quality SiNx films on both planar and 3D substrate topographies

Insight into the removal and reapplication of small inhibitor molecules during area-selective atomic layer deposition of SiO2

As the semiconductor industry progresses towards more complex multilayered devices with ever smaller features, accurately aligning these layers with respect to each other has become the bottleneck in the advancement to smaller transistor nodes. To avoid alignment issues, area-selective ALD can be employed to deposit material in a self-aligned fashion. Previously, we demonstrated area-selective ALD of SiO2 in three-step (i.e. ABC-type) ALD cycles comprising an acetylacetone (Hacac) dose (step A), a bis(diethylamino)silane (BDEAS) precursor dose (step B), and an O2 plasma exposure (step C). In this work, the mechanisms of the removal and reapplication of the inhibitor molecules during area-selective ALD were studied, with the aim of enhancing the selectivity of the process. In situ infrared (IR) spectroscopy shows that the O2 plasma exposure does not completely remove the adsorbed Hacac species (i.e. acac adsorbates) at the end of the cycle. The persisting species were found to contain fragments of Hacac molecules, which hinder subsequent inhibitor adsorption in the next ALD cycle, and thereby contribute to a loss in selectivity. Alternatively, it was found that an H2 plasma is able to completely remove all acac species from the surface. An improvement in selectivity was achieved by using a four step ALD cycle that includes an H2 plasma step, allowing the nucleation delay to be prolonged from 18 ± 2 to 30 ± 3 ALD cycles. As a result, 2.7 ± 0.3 nm SiO2 can be deposited with a selectivity of 0.9, whereas only 1.6 ± 0.2 nm can be achieved without the H2 plasma step. This work shows that the addition of a dedicated inhibitor removal step before the reapplication of the inhibitors can significantly improve the selectivity