Mauro Alessandri

Effects of nitridation by nitric oxide on the leakage current of thin SiO2 gate oxides

We have studied the effects of nitridation with nitric oxide on the leakage current of thin (<8 nm) gate oxides. Under gate injection of electrons the oxide leakage current behavior reflects the trend of the electrical thickness and flatband voltage and it can be modeled by the Fowler–Nordheim relation. Conversely, a different behavior is observed for electron injection from the substrate. The leakage current during substrate injection is strictly related to the nitrogen that, as observed by secondary ion mass spectrometry, is located at the SiO2/Si substrate interface.

The impact of the nitridation process on the properties of the Si–SiO2 interface

Abstract A newly developed technique for the simultaneous measurement of the oxide–silicon interface properties and of minority carrier lifetime in the silicon volume was used for a systematic study of the nitridation process of oxide films.This technique is based on the surface recombination velocity measurements, and does not require the formation of a capacitor structure, so it is suitable for the measurement of as-grown interface properties. Oxides grown both in dry and in wet environments were prepared, and nitridation processes in N2O and in NO were compared to N2 annealing processes. The effect of nitridation temperature and duration were also studied, and processes of rapid thermal oxidation (RTO) and nitridation (RTN) were compared to conventional furnace nitridation processes. Surface recombination velocity was correlated with nitrogen concentration at the oxide–silicon interface obtained by secondary ion mass spectroscopy (SIMS) measurements. Surface recombination velocity (hence surface state density) decreases with increasing nitrogen pile-up at the oxide–silicon interface, indicating that in nitrided interfaces surface state density is limited by nitridation. NO treatments are much more effective than N2O treatments in the formation of a nitrogen–rich interface layer and, as a consequence, in interface state reduction. X-ray photoelectron spectrometry (XPS) analyses were used to extend our correlation to very thin oxides (3 nm).

Surface morphology of nitrided thin thermal SiO2 studied by atomic force microscopy

In this work the surface morphology of nitrided silicon dioxide is extensively studied using atomic force microscopy. Nitridation is obtained by thermal annealing in nitriding atmosphere in conventional furnace, immediately after thermal oxidation of silicon substrates. The characterisation performed concerns the oxide surface, as well as the region where nitrogen is incorporated, the latter exposed using a diluted HF solution. Significant differences in the morphology of the nitrided layer are observed, which are a function of the nitridation process applied. They allow us to correlate the morphology to the nitrogen incorporation mechanisms that have occurred.

Point defects in thermal SiO2 layers: Thermally stimulated luminescence and corona oxide electrical characterization

Point defects in thin (≈100 A) SiO2 layers thermally grown on silicon were investigated by thermally stimulated luminescence (TSL) and corona oxide characterization of semiconductor measurements. A comparison is proposed between layers grown by steam and N2 diluted steam processes. The effects of post-growth annealing treatments in N2O, NO, and N2 atmospheres and of ion irradiation (As or P) have been investigated. A good correlation between the results obtained by the two different techniques has been found, suggesting a common structural origin of defects responsible for TSL active traps and total oxide charge.

Influence of charge trapping on oxide scaling down

Abstract The aim of this work is the characterization, in terms of trapped charge and charge to breakdown, of the quality of an oxide with reduced thickness. A comparison between two evaluation methods, the widely used exponentially ramped current stress (ERCS) and the constant current stress (CCS), is established obtaining contradictory results. A measurement of the charge trapped in the oxide bulk is performed by sensing the modification of the Fowler–Nordheim barrier under constant current stress. Using this technique it is possible to correlate the charge trapping characteristics with the charge to breakdown and to explain the inconsistencies.

Nitridation of gate and tunnel oxides employed in CMOS-ULSI technology

Nitridation treatments are very important in CMOS technology because of their capability of improving the gate and tunnel oxide reliability. In this work we report on N2O and NO annealing of pre-oxidised samples showing physical and electrical characteristics of the thin oxides. The difference between the physical behaviours of N2O and NO oxides is evidenced and related to their different electrical properties.

AlCu Pitting Prevention in Post Etch Cleaning

Three different cases of wet polymer removal in presence of AlCu are described: namely a) oxide passivation opening, b) vias etch and c) pad patterning. The occurrence of pitting at such levels is analyzed and linked to halogens presence. Moving from these results, a proposal to limit such kind of defects is given, focusing on a trimmed DIW rinsing step and on a possible AlCu passivation through a plasma ashing process.

Investigation of Defectivity Coming from the Back Side of Wafers during AlCu Polymer Removal Processes Performed in a Batch Spray Tool

The defectivity of a batch process for polymer removal in AlCu BEOL technology has been investigated, reasearching the defectivity source and studying the composition of the defects. Different solutions for the minimization of this defectivity have been found, working on the positioning of the wafers or additional cleanings. The complete eradication of the defectivity has also been demonstrated performing the process on a single wafer tool.

Post CMP Wet Cleaning Influence on Cu Hillocks

Up to date, it is commonly reported in literature that the amount of copper hillocks is dependent on a) the total amount of residual Cu oxide after Cu CMP, and b) the Cu nitridation. The present work describes how that is only partially true: hillocks depend also on the kind of oxide and on the roughness of the surface after wet treatment. This contribution is even more important than what reported in literature. A tentative model for this behavior is proposed.

Study of TiW Conditioning through Different Wet and Dry Treatments to Promote Ni Electroless Growth

Electroless Ni deposition is often used in presence of Cu, Ti and Au. Recently TiW has also started to be employed, but with this alloy the Ni deposition is not always neat. In our work we investigate the effect of different wet treatments on the Ni growth by means of XPS analyses and SEM inspections. It is found that an oxidized surface inhibits the activator deposition. The de-oxidized Ti atoms in TiW on the other hand are believed to act as the principal sites for Pd seed deposition and subsequent Ni growth.