Rare earth-based high-k materials for non-volatile memory applications

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Abstract

A study of a La-based high-k oxide to be employed as active dielectric in future scaled memory devices is presented. The focus will be held on LaxZr1−xO2−δ (x = 0.25) compound. In order to allow the integration of this material, its chemical interaction with an Al2O3 cap layer has been studied. Moreover, the electrical characteristics of these materials have been evaluated integrating them in capacitor structures. The rare earth-based ternary oxide is demonstrated to be a promising candidate for future non-volatile memory devices based on charge trapping structure.

Introduction

Over the last few years, NAND flash memories are experiencing explosive growth because of new and more demanding applications are constantly added, partly due to the need for low power solid-state storage and partly due to rapidly declining market prices [1]. More evident from the ITRS roadmap [2], NAND flash scaling is currently progressing at a much faster rate than complementary metal-oxide-semiconductor (CMOS) logic technology. Scaling of conventional floating gate flash memories, both in NOR or NAND architectures, presents tremendous challenges. In NOR flash, fundamental limitations including the junction breakdown and short channel effects have essentially squeezed out the device design space below 45 nm node. For NAND flash, the tight spacing, floating gate interference and the need for sufficient gate control (gate coupling ratio) have also ruled out the maintenance of the conventional floating gate design below 32 nm node. Due to the requirements of continuous device scaling, the implementation of new architectures together with the integration of innovative materials is needed. Charge trapping devices, exploiting high dielectric constant (high-k) materials as blocking oxide and innovative tunneling barrier engineering, could constitute a possible way for scaling flash memories. In particular, high-k materials are very interesting for non-volatile memories based on metal gate/high-k/SiO2/Si3N4/Si (TANOS) structures since they can be stacked as blocking oxide in substitution of standard oxide–nitride–oxide stack. Alternatively, such dielectrics can also be employed as charge trapping layer instead of Si3N4. When used as blocking oxide, the high-k material should block charge loss from the charge trapping layer without trapping charge itself. In addition, it should have a high enough barrier to prevent charge injection from the gate. However, most high-k materials present low band-gap values and hence they result unsuitable for this purpose. Some high band-gap materials (e.g., Al2O3) do not show a sufficiently high dielectric constant to provide the acceptable coupling ratio required for planar device architecture. On the other hand, even high-k materials that are suitable as ultra-thin gate dielectric for logic applications can trap charges when film thickness is increased up to the values required for blocking dielectric operations. Moreover, materials with sufficiently high permittivity may tend to have low breakdown field and may suffer from early dielectric breakdown during programming and erasing operation. The study of high-k materials as blocking oxide requires the finding of an accurate balance between all the electrical and the process integration requirements, which together constitute a very challenging task for memory industry.

This work focuses on high-k materials to be implemented as blocking oxide. For the 32–22 nm technology node, the equivalent oxide thickness (EOT) of this layer should be between 4 and 6 nm, while for the leakage currents a value around 1 × 10−14 A/cm2 at low fields is requested. In principle, materials with a dielectric constant in the 10–20 range can fulfill these requirements. Due to its excellent conformality, very good thermal stability, moderately high dielectric constant (k  10 after crystallization), large band-gap and good resistance to leakage currents [3], Al2O3 is a well-known high-k material which might be properly integrated as blocking oxide in 32 nm TANOS memories. For 22 nm node, dielectric materials with higher k values are currently under investigation. In the frame of this paper, a comparison between Al2O3 and a La-based high-k dielectric is shown. Actually, alloying two binary high-k oxides like La2O3 [4] and ZrO2 [5] might result in a ternary compound with optimized dielectric properties and thermal stability [6]. In this work the LaxZr1−xO2−δ (x = 0.25) (LZO) compound will be considered. In particular, this ternary oxide has been developed in a research tool at MDM, deposited on 8″ patterned Si wafers, and then transferred to the Numonyx pilot line for device finishing and characterization.

Section snippets

Experimental

LZO films have been grown by atomic layer deposition (ALD) using (iPrCp)3La, (MeCp)2ZrMe(OMe) (SAFC Hitech) and O3 at 300 °C. Details about the growth process and the extended characterization of the chemical, structural and dielectric properties of this ternary oxide are extensively reported in Ref. [7]. La atomic fraction x = La/(La + Zr) = 0.25, was tuned by altering the ALD pulse number in the growth cycle, and estimated by X-ray photoelectron spectroscopy and electron energy loss spectroscopy.

Film integrability

LZO might intermix with the other layers of the stack due to possible thermal instability issues. This fact would certainly constitute a main integration problem. Therefore, chemical interaction between LZO and Al2O3 has been studied in as-grown stacks and also after thermal treatments in N2 atmosphere at 900 °C. In order to obtain a well-defined bi-layer, different Al2O3 thicknesses have been taken into account. In particular, 40 and 80 Å thick films have been deposited on LZO. ToF-SIMS depth

Conclusions

In this work we have shown the integration and the electrical analysis of LZO in 8″ test structures for the evaluation of this material, as possible interpoly dielectric or blocking oxide candidate, for next generation of non-volatile memories. Amongst the different problems that might occur during integration, the La and Zr contamination issue has been solved by capping the LZO layer with a 80 Å thick Al2O3 film. In this way, a satisfactory integration in an industrial pilot line has been

Acknowledgment

This work is partially supported by the European FP6-Program “REALISE” (Grant No. IST-NMP 016172).

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