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

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

Microelectronic Engineering 87 (2010) 290–293 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.c...

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Microelectronic Engineering 87 (2010) 290–293

Contents lists available at ScienceDirect

Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee

Rare earth-based high-k materials for non-volatile memory applications M. Alessandri a, A. Del Vitto a,*, R. Piagge a, A. Sebastiani a, C. Scozzari a, C. Wiemer b, L. Lamagna b, M. Perego b, G. Ghidini a, M. Fanciulli b,c a

Numonyx Italy S.r.l., Via C. Olivetti, 2, 20041 Agrate Brianza (MI), Italy Laboratorio Nazionale MDM, CNR INFM, Via C. Olivetti, 2, 20041 Agrate Brianza (MI), Italy c Dipartimento di Scienza dei Materiali, Università degli Studi Milano-Bicocca, 20125 Milano, Italy b

a r t i c l e

i n f o

Article history: Received 23 March 2009 Received in revised form 17 June 2009 Accepted 22 June 2009 Available online 26 June 2009 Keywords: Non-volatile memory Atomic layer deposition High-k dielectric Rare earth oxide

a b s t r a c t 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 LaxZr1xO2d (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. Ó 2009 Elsevier B.V. All rights reserved.

1. 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

* Corresponding author. Address: Advanced Dielectrics and Thermal Processes Development Engineer, Advanced R&D - NVM & Derivatives, Numonyx Italy S.r.l., Via C. Olivetti, 2, 20041 Agrate Brianza (MI), Italy. E-mail address: [email protected] (A. Del Vitto). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.06.022

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  1014 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

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10

2

(a)

(b)

1

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Intensity (arb. units)

high dielectric constant (k  10 after crystallization), large bandgap and good resistance to leakage currents [3], Al2O3 is a wellknown 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 LaxZr1xO2d (x = 0.25) (LZO) compound will be considered. In particular, this ternary oxide has been developed in a research tool at MDM, deposited on 800 patterned Si wafers, and then transferred to the Numonyx pilot line for device finishing and characterization.

10

0

10

-1

10

-2

10

-3

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-4

30

Si LaO ZrO AlO SiO3

0

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100

Time (s)

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50

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2. 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. The films have been deposited on 800 Si(1 0 0) patterned wafers in the 50–200 Å thickness range. In order to prevent La and Zr contamination in the front end of the line due to wafer processing, LZO films should be capped. For this purpose, a thin Al2O3 film has been deposited by ALD in ASM Pulsar 3000TM reactor using TMA and H2O (as described in Ref. [3]). In one case, H2O was substituted by O3 in order to evaluate the eventual beneficial effect of a stronger oxidizing precursor on the electrical properties of the stack. The thickness of the Al2O3 layer should be minimized in order to enhance the quality of the information that can be obtained on the high-k layer from the electrical characteristics of the capacitors. The stack thermal stability and structural evolution upon postdeposition rapid thermal annealing (RTA) at 900 °C in N2 have been studied by time of flight-secondary ion mass spectrometry (ToF-SIMS), total reflection X-ray fluorescence (TXRF), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Current–voltage (J–V) and capacitance–voltage (C–V) curves have been measured on capacitors obtained by 3-masks test pattern. 3. Results and discussion 3.1. 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 profiles performed on Al2O3/LZO stacks show that a slight diffusion of ZrO occurs through a 40 Å thick Al2O3 cap (not shown). Differently, capping the LZO film with an 80 Å thick Al2O3 film avoids undesired metal diffusion up to the surface (Fig. 1a). After annealing only a slight intermix occurs, as reported in Fig. 1b: the ZrO signal increases within the Al2O3 region and AlO signal increases within the LZO region. Nevertheless, no Zr or La contaminants were revealed by means of TXRF on the surface of the wafer either before or after RTA. After annealing both Al2O3 and LZO films are completely crystallized

Fig. 1. ToF-SIMS depth profiles of (a) as deposited and (b) annealed at 900 °C in N2LZO/Al2O3 stacks.

as shown in the stack cross TEM image (Fig. 2). XRD performed on capped stacks before and after RTA (Fig. 3) shows the crystallization of the Al2O3, while the crystallographic ordering of the LZO layer, in the cubic or tetragonal phase of ZrO2, is mainly not affected by the thermal treatment. Moreover, TEM cross section shows well-defined interfaces between the different layers. 3.2. Electrical results The electrical characteristics of metal-oxide-semiconductor (MOS) structures have been evaluated. Three different film thicknesses have been deposited, each film has been capped with Al2O3 and subsequently annealed at 900 °C. Eight-inch wafers have been processed using a three masks test pattern on which capacitors having different areas (1.1  103 and 9.18  103 cm2) have been defined. A poly-Si layer, covered by WSix, in order to decrease its resistance, has been employed as metal gate for MOS structures on p, 1 0 0 oriented, 1  1018 cm3 doped Si substrates. WSix/ poly-Si/Al2O3/pSi(1 0 0) MOS structures and TaN/Al2O3/pSi(1 0 0) structures have been also evaluated as reference. In Table 1 is reported the electrical samples description. The EOT values of LZO have been evaluated from capacitance in the accumulation region (gate voltage 5 V and frequency 10 kHz) of capacitors having the same Al2O3 thickness and different LZO thicknesses, corresponding to samples w1, w2 and w3 (Table 1). The k value attributed to LZO films is equal to 27 ± 3, in fairly good agreement with the one reported in Ref. [7], although in the present work the LZO was capped with Al2O3. This high permittivity value makes LZO a promising candidate for integration as blocking oxide in TANOS structures. The C–V curves (reported in Fig. 4) show that all the samples have

Fig. 2. Cross-sectional TEM image of LZO/Al2O3 stack annealed at 900 °C in N2 atmosphere.

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Fig. 3. XRD analysis of as-grown and annealed LZO/Al2O3 stacks. The thickness of the Al2O3 layer is 80 Å. The powder spectra of tetragonal and cubic ZrO2 are also added for comparison.

Table 1 Electrical samples description: stack films thickness and postdeposition annealing. Al2O3 deposited on wf1–wf4 was prepared using H2O-based ALD process whereas Al2O3 deposited on wf5 was prepared using O3-based process. Sample

LZO thickness target (Å)

Al2O3 thickness target (Å)

RTA

wf1 wf2 wf3 wf4 wf5

200 150 100 50 150

80 80 80 110 80 (O3-process)

900 °C 900 °C 900 °C 900 °C 900 °C

in in in in in

N2 N2 N2 N2 N2

a hysteresis due to unstable charge present in the layer. This charge can be present both in the Al2O3 and LZO layers, the contribution to the charge of the different films cannot be attributed precisely due to the stacked structure of the dielectric in the MOS capacitor. The leakage current evaluation has been performed by normalizing all the J–V curves with respect to the EOT, thus obtaining J–E curves. In this way we can have a consistent comparison between the different samples and the Al2O3 reference. In Fig. 5, J–E curves acquired on different samples are reported. J–V measurements have been carried out in positive and negative polarity in order to evaluate the fixed charge inside the films. Repeated J–V measurements in negative polarity do not reveal the presence of such fixed charge; moreover, no significant trapping during voltage ramp is observed. The Al2O3 reference is reported both with poly-Si

Fig. 5. J–E curves relative to LZO/Al2O3 stacks with poly-Si as metal gate. Curves obtained on structures including only Al2O3 and both poly-Si and metal gate are reported as reference.

and metal gate (i.e., TaN). For low voltages the leakage current can be extrapolated by considering the slope of the curve. At low voltages, the leakage current measured for LZO/Al2O3 stacks is larger for all the analyzed samples as compared to the one measured on MOS structures including only Al2O3. On the other hand, for high voltages the leakage current is lower for LZO/Al2O3 stacks. Therefore, this stack performs better in program/erase whereas Al2O3 is favored in retention. Furthermore, a very good breakdown field is measured: the film breaks down around 14–16 MV/cm, a value very close to that measured for Al2O3 films. Thus, depending on the working voltage of the memory, the electrical quality of this La-based film can be comparable and even better than that of Al2O3. Comparing samples w2 and w5, LZO films present the same electrical characteristics, although Al2O3 was grown using H2O and O3, respectively. Since the two stacks show the same electrical behavior, we can conclude that oxidant agent has in this case no appreciable impact. 4. Conclusions In this work we have shown the integration and the electrical analysis of LZO in 800 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 carried out, giving the possibility of electrically test this material on 800 test patterned wafers. Promising electrical results have been achieved; after integration LZO presents a large dielectric constant 27. This material contains instable charge as revealed by C–V hysteresis. Nevertheless, negative J–V characteristics are quite stable showing that no permanent charge trapping is occurring. Finally, LZO shows higher leakage current at low fields with respect to Al2O3 but lower leakage at higher fields. Stack quality in terms of breakdown electric field is also comparable. This ternary rare earth-based material shows promising results in terms of breakdown, dielectric constant and leakage current. In order to further investigate its electrical behavior it needs to be integrated in a complete TANOS stack. Acknowledgment

Fig. 4. C–V plots of electrical samples w1, w2 and w3. Measurements sequence is Vgate = 2/+2/2 V.

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

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