Charge trapping behavior of SiO2-Anodic Al2O3–SiO2 gate dielectrics for nonvolatile memory applications

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Abstract

In this study, Anodic Aluminum Oxide (AAO) is used as the storage layer instead of silicon nitride in a silicon–SiO2–Si3N4–SiO2–Silcon (SONOS) structure in flash memory devices and a double IV method is utilized to investigate its charge trapping behavior on SiO2–Al2O3–SiO2 (OAO) triple stacks. The experimental results show that more initial charge density can be charged into the centroids of neutral charge trapping sites using post deposition annealing (PDA) treatment in OAO triple stacks. Under stress bias testing, OAO triple stacks exhibit a larger shift of threshold voltage than do ONO type devices. The trap energy levels of the bulk electron traps in high k AAO materials were determined. The initial trapping charges are produced during the fabrication process and are changed into centroids of neutral charge trapping sites after PDA treatment, which are responsible for charge trapping/de-trapping during programming/erasing. All devices show that the trapping centroid moves to the top oxide with an increase of stress time.

Introduction

Scaling down the interpoly dielectrics of conventional floating gate flash memory technology is close to its limitations with respect to scaling the cell size and program/erase voltages [1], [2]. The reliability issue hinders the development of conventional floating gate memory devices owing to the constriction of operational voltages. However, high voltage operation has exceeded reliability issues, such as dielectric hot carrier degradation and punch-through avalanche effects. Moreover, high voltage operation would result in junction breakdown, so that this limited the lateral scaling down in device size. In order to achieve high density, where device fabrication can no longer be simply scaled to progressively smaller sizes, SONOS structure would be considerable effort to develop replacement dielectrics for memory devices. SONOS, a charge trapping device, has been given more attention than the conventional floating gate one in recent nonvolatile memory applications because of its numerous advantages such as excellent scalability, repetitive electrical program/erase capability, radiation hardness, and low power consumption [3]. However, SONOS lacks reliable retention properties at elevated temperature because of the very thin tunnel oxides with thicknesses of 2–3 nm and the moderate activation energy of nitride traps. Thicker tunnel oxides improve the retention performance but compromise the programming speed of the devices [4], [5], [6]. These problems have blocked the progress of SONOS flash memory. Researchers have reported that various storage dielectrics of flash cells could solve these problems [7], [8], [9]. High-k dielectrics, such as Al2O3 or HfO2 are promising candidates to replace conventional nitride charge trapping layers [7], [8], [9], [10]. Sugizaki et al. [8] reported that Al2O3 film showed better retention characteristics with a low charge loss than Si3N4 and HfO2 films as a storage layer in the oxide-storage dielectrics-oxide structure. And their report indicated that almost no charge loss was observed for either programmed bit-1 or erased bit-2 [8].

Al2O3 film is a promising candidate as a charge trapping layer due to its large bandgap value of 8.9 eV, large conduction band-offset of Si/Al2O3 (=2.8 eV), dielectric constant value of 9, and high thermal stability, making it a possible replacement for the traditional silicon nitride layer in ONO (oxide–nitride–oxide) structures [3]. Additionally, the dielectric constant of aluminum oxide is higher than that of silicon nitride, which could reduce the operating voltage. Another advantage is that there is a weaker lateral migration of trapped electrons in Al2O3 storage dielectrics because of its deeper trap level [7], [8]. Therefore, it is an attractive candidate for the next generation of flash memory devices, especially for multi-bit memories [8], [11], [12]. Post rapid thermal annealing (RTA) could improve the OAO device reliability [13]. Therefore, post deposition annealing (PDA) plays an important role on OAO flash memory devices.

In our research, we focus on using anodic aluminum oxide (AAO) to replace SiN as storage dielectrics. Composed to other fabricated technologies, the preparation of AAO has some advantages, such as: (i) higher defect density resulting from the fabrication process; (ii) it is fabricated at room temperature and has lower fabrication cost than other technologies. This paper is organized into two parts. The first focuses on investigating the electrical characteristics of AAO film. The second investigates the charge trapping behavior of the OAO triple stacks after applying stress bias.

Section snippets

Experimental

The key features of OAO film with SiO2–Al2O3–SiO2 triple stacks are as follows. A tunnel oxide with a thickness of 2 nm was grown in a furnace on a p-type Si substrate. Subsequently, a thicker Al2O3 layer with a thickness of 7 nm was grown by anodic oxidation in sulfuric acid under an applied voltage at room temperature. The top dielectric oxide layer (block oxide) was deposited in the PECVD system with SiH4 and N2O gas mixtures. Post annealing treatment was carried out in a rapid temperature

Characteristics of Anodic Aluminum Oxide (AAO) film

The analysis of FTIR spectrum on AAO film reveals an O–Al–O bending mode (650–700 cm−1) and an Al–O stretching mode (750–850 cm−1) as shown in Fig. 1 [14]. The broad features include the overlap of these two modes with distinct peaks at 690 and 745 cm−1. The absorption band at 610 cm−1 is due to Al–O stretching in the condensed AlO6 octahedral. The slight spectral feature is present at 1345 cm−1, indicating Al=O in the film. According to the FTIR results, the anodic Al2O3 oxide film is a complicated

Conclusion

In this study, characteristics of OAO triple stacks are demonstrated using IV and CV techniques. The OAO triple stacks show better performance than ONO structures in terms of improvement in the charge trapping efficiency, large shift of threshold voltage, and decreased effective charge density. The results of our experiment can be summarized as follows. The shift of threshold voltage after applying a bias depends on the effective trapping charge density. The AAO film shows better charge

Acknowledgement

This work was supported by the National Science Council of the Republic of China under contract number NSC 96-2221-E-006-082-MY2.

References (23)

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