Materials Transactions, Vol. 52, No. 3 (2011) pp. 331 to 335 Special Issue on New Trends for Micro- and Nano Analyses by Transmission Electron Microscopy #2011 The Japan Institute of Metals
Particle Size and Morphology of Iridium Oxide Nanocrystals in Non-Volatile Memory Device Wei-Chih Li1 , Writam Banerjee2; *1 , Siddheswar Maikap2 and Jer-Ren Yang1; *2 1
Department of Materials Science and Engineering, College of Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan, R. O. China 2 Department and Graduate Institute of Electrical Engineering, College of Engineering, Chang Gung University, No. 259, Wen-Hwa 1st Road, Kwei-Shan Tao-Yuan, Taiwan, 333, Taiwan, R. O. China The aim of this work is to investigate the particle size and morphology of the nanocrystals in the non-volatile memory device by annular dark field (ADF) scanning transmission electron microscopy (STEM) and conventional transmission electron microscopy (CTEM) techniques. With respect to TEM investigation, statistical analysis on STEM image can acquire the size and density of nanocrystals more accurately than on TEM image. In addition, ADF STEM images successfully provide powerful evidences revealing the structure of IrOx nanocrystals as a core shell structure, where the inner structure is rich in Ir and the outer area is abundant in O or Al. This method could be one of the efficient way for examining the nanocrystals with a complicated cored structure. [doi:10.2320/matertrans.MB201020] (Received September 2, 2010; Accepted December 13, 2010; Published February 9, 2011) Keywords: particle size, morphology, iridium oxide nanocrystals, core shell structure, non-volatile memory device, scanning transmission electron microscopy
1.
Introduction
Non-volatile memory (NVM) devices using high- nanocrystals as floating gate have attracted considerable attention because of its excellent memory performance and high scalability, which can keep the charge after 104 times read and write.1) Through the sputtering process to prepare nanolaminate structures as well as post deposition annealing (PDA) heat treatment, nanocrystals can be formed and aligned horizontally parallel to the silicon substrate. This technique has been performed widely for the fabrication of NVM device.2) Considering the nature of the true thickness, epitaxial orientation relationships and morphology of each layer, one can examine the sample through tilting the orientation to an exact zone of silicon substrate from cross-sectional observation. The plane-view examination can provide the information of particle size, density and morphology as well. The above works can be done under TEM examination. Recently Choi et al.3) found that the density of IrO2 nanocrystals was lower than the calculated value by the relation stated by Tan et al.4) Having regard to the previous research work, care must be taken for the result differences when the work is performed by using TEM investigation. Kim et al. examined the structural characteristics of the asdeposited and annealed IrO2 films by glancing angle X-ray diffraction.5) The crystallographic characteristics of the nanolaminates over a large area were obtained with almost no preferred orientation. The results indicated that the asdeposited multilayer memory structure provided an almost amorphous environment for the growth of nanocrystals before annealing. For the films annealed at 750 and 1000 C, the diffraction peaks could be indexed as the (110), (111), (200), (220) and with only trace amounts of *1Graduate
Student, Chang Gung University author, E-mail:
[email protected]
*2Corresponding
the (101) and (211) orientation of tetragonal IrO2 . For the reasons mentioned above, the growth of nanocrystals should be in different orientation and it therefore cannot be measured accurately due to the invisible condition. In addition, the basic principle for the crystallization method from amorphous solids is to control the crystallization kinetics by optimizing the heat treatment conditions so that the amorphous phase crystallizes completely into a crystalline material with ultrafine crystallites.6) The chemical compositions of the amorphous state and the transformation mechanism determine the phases and morphologies of nanocrystals.6,7) Three different types (polymorphous, eutectic, and primary crystallization) of conventional crystallization of amorphous solids are similarly referred to the nanocrystallization of amorphous solids in different systems.7) Thus, the achieved structure for nanocrystals should be a spherical core-shell structure.8) However, there is no strong evidence to support this point of view. STEM HAADF images are independent of objective lens defocus and sample thickness9) and can provide atomic resolved images;10) as a result, it would be more accurate to calculate the density and particle size by STEM. This work is performed in STEM by using different camera lengths, which obtain the images called high-angle (HA), medium-angle (MA) and low-angle (LA) annular dark field (ADF) images, respectively. 2.
Experimental Procedures
To remove native oxide from the surface of the n-type Si (100) substrate, it was cleaned by the standard RCA process. After the cleaning of the n-type Si substrate, a 2 nm thick layer of silicon dioxide (SiO2 ) was grown after annealing at 850 C for 5 s. An approximately 2 nm-thick high- Al2 O3 film was grown by RF-sputtering as a tunneling oxide layer. Then iridium oxide (IrOx ) layers with thicknesses of 2 nm were grown by RF-sputtering. After the deposition of the
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IrOx nanolayers, the high- Al2 O3 film (10 nm) was grown by RF sputtering as a blocking oxide. To form the IrOx nanocrystals from the IrOx nanolayer, a PDA treatment at 900 C for 1 min in N2 ambient was performed. Finally, the top IrOx patterns (gate area: 4:9 10 4 cm2 ) were deposited by RF-sputtering on the blocking oxide layer as a metal electrode gate. A schematic annealed structure of the novel IrOx metal nanocrystals memory device in an n-Si/SiO2 / Al2 O3 /IrOx /Al2 O3 /IrOx structure is shown in Fig. 1. In order to minimize the artificial effects on sample preparation,11) specimens for cross-sectional (XTEM) and planeview TEM analysis in this work were prepared by using a combined mechanical and ion-beam thinning technique. To characterize the microstructure and thickness of all films, high-resolution transmission electron microscope (FEI Tecnai F20) equipped with a high angle annular dark field (HAADF) detector. The ADF STEM images are recorded with free of image processing. 3.
Results and Discussion
Figure 2(a) reveals a cross-sectional TEM image of the annealed structure where excellent interfaces between each layers can be observed, in agreement with the schematic structure shown in Fig. 1(b). The nanocrystals embedded in Al2 O3 layer were aligned as a band and almost parallel to the interface of Si and SiO2 . The nominal thicknesses of SiO2 / Al2 O3 bilayered tunneling oxide barrier, IrOx nanocrystals, Al2 O3 blocking oxide layer as well as IrOx metal gate have been measured to be 5, 2, 10 and 90 nm, respectively. EDX
Fig. 1 Schematic diagram of (a) as-deposited and (b) annealed memory device. The novel IrOx nanocrystals have fabricated in the device.
Fig. 2 (a) Cross-sectional TEM image exhibits the memory structure; (b) EDX analysis on IrOx nanocrystals from cross-section; (c) plane-view TEM image and (d) EDX analysis on the IrOx nanocrystals from plane side.
Particle Size and Morphology of Iridium Oxide Nanocrystals in Non-Volatile Memory Device
spectrum taken from the cross-section side shows the main composition of IrOx nanocrystals is Ir and O with dilute Al and Si elements, as shown in Fig. 2(b). It is believed the Si signal is arisen from the neighbor layer, SiO2 , due to the EDX electron probe expanding the interaction volume effect. In order to observe the particle size and density of the nanocrystals, planar observation parallel to the Si substrate is needed. To prepare the plane-view sample, one has to completely remove the metal gate and silicon substrate to get a plane-view sample as thin as possible. Since the top IrOx metal gate electrode was prepared by a pattern structure, it’s easy to find an area with no gate electrode to prepare the TEM sample. As to the way for removing the silicon substrate, it takes time to polish the sample turned yellow-light transparent as 3 and 1 mm diamond lapping films were used, followed by ion milling. A trick to check the thickness of the wedge-shape sample in BF TEM image mode is with no band counters and thickness fringes and thus presence thickness of the sample utilized in this study was roughly estimated to be 10 nm if silicon substrate can be moved out clearly. Figure 2(c) displays a plane-view image with an overall area, approximately 300 nm. HAADF STEM image is strongly depending on sample thickness and specimen with thicker area may show high intensity which does not necessarily indicate a high atomic number.12) Since thickness of the sample is thin and uniform, the effect of sample thickness on our experimental results could be negligible. Composition of IrOx nanocrystals is also confirmed from the EDX spectrum, shown in Fig. 2(d). Al and Ir are two relatively stronger signals than Si, indicating the IrOx nanocrystals are composed with Ir, Al and O elements. With a higher magnification of TEM plane-view image, Fig. 3(a), the nanocrystals show the morphology with and without lattice fringe which indicates that among of them are in different orientation. According to the plane-view TEM images, the nanocrystals exhibit a density higher than 1012 cm 2 and a mean diameter of 3.20 nm. Approximately 40% of nanocrystals belonged to a diameter between 3 and 3.5 nm. It is believed that the density and size evaluation of nanocrystals performed in TEM mode would lose the accuracy owing to the invisible criterion consideration. In contrast, STEM image depends strongly on the atomic number of the scattering atoms, no contrast reversal with respect to defocus and sample thickness,9,10) thus offering additional analytical capabilities to calculation. Figure 4 shows experimental ADF images recorded using four different detector inner angles: 72, 51, 28 and 14 mrad. For comparison, Fig. 4(a)–(b) were recorded with inner angles of higher than 50 mrad and can be classified into HAADF images. Figure 4(c) and (d) can be distinguished by comparing a medium-angle ADF (MAADF) with a lowangle ADF image (LAADF) image. Figure 4(a) cannot reveal the morphology clearly because the noise increases with increasing inner collecting angles. As discussed before, the HAADF images of Fig. 4(a) and (b) definitely indicate a high atomic number within the inner area of nanocrystals. In HAADF STEM image, the contrast is given by the sum of the intensity of thermal diffuse scattering (TDS), so that each atom ejects TDS electrons and the cross section for TDS is different among atom species.13) As a result, the intensity in
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Fig. 3 (a) TEM plane-view image showing the morphology of the IrOx nanocrystals embedded in the Al2 O3 matrix; (b) grain size statistical results measured from TEM image as shown in (a).
HAADF STEM image depends on the atomic number and the concentration of atoms in the specimen.14) These images reveal the existence of the core shell structure, exhibiting bright contrast. The results indicate in the inner area and dark contrast in outer area as compared with the atomic number (considering the atomic number Al13 , O8 and Ir77 ). Thereby, structure of nanocrystals can be explained as follows. The morphologies and composition are dominated by the transformation mechanism, which is related to the thermodynamics and kinetics theory.15,16) Driving force of the reaction is intended to reduce the system energy as active energy is served to overcome the energy barrier during the heat treatment. For the inner area of nanocrystals, a spontaneous reduction reaction of Ir oxides and suboxides occurred, leading to the creation of a large amount of element Ir atoms. Therefore, the process of nanocrystals primary precipitates on Ir nuclei corresponding to the reduction of Ir oxides. Meanwhile, some diffusing Ir atoms bond to existing nuclei and incorporated with Al and O atoms to form the outer area
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Fig. 4 ADF STEM images of the IrOx nanocrystals acquired with different inner collecting angles: (a) 72; (b) 51; (c) 28 and (d) 14 mrad. The inset figure in (c) shows a clear core-shell structure of IrOx nanocrystals.
it is apparent that the density of nanocrystals is higher than that estimated from TEM image. The mean diameter of the particle size is 3.71 nm. The outer area of the nanocrystals is with same composition as the Al2 O3 matrix and hardly distinguished by diffraction contrast image. Therefore, STEM images can provide more precise information for the evaluation of particle size and density of nanocrystals. 4.
Fig. 5 Grain size statistical results measured from STEM image as shown in Fig. 4(c).
Conclusions
IrOx nanocrystals have been examined by using STEM, and illuminated to be with a core-shelled structure. The composition of inner area is rich in Ir element and the outer area is abundant in O or Al element. Moreover, STEM images can provide more precise information for the evaluation of particle size and density of nanocrystals in the present work. REFERENCES
of nanocrystals. The final size of nanocrystals depends on the number of nucleation site and coarsening process.16) The results provide a strong evidence to elucidate that the IrOx nanocrystals would be a core-shell structure. Figure 5 presents the statistical result of the size of IrOx nanocrystals from STEM image shown in Fig. 4(c). Comparing to the TEM and STEM images (Fig. 3(a) and Fig. 4(c)),
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