Applications of Scanning Transmission Electron Microscopy (STEM) in the New Generation of High-K Gate Dielectrics

doi:10.15541/jim20140110

Abstract

Abstract:

Scanning transmission electron microscopy (STEM) Z-contrast image has some advantages such as high image resolution (directly revealing the real positions of atoms in crystal), high compositional sensitivity and directly interpretable images, it becomes a powerful tool for investigating the microstructure of materials at atomic scale. In this review, the formation mechanisms, methods and features of the Z-contrast STEM images are introduced, and its applications in the new generation of high-k gate dielectrics (e.g., Hf-based metals oxides, rare-earth oxides and epitaxial perovskite oxides) are also reviewed. After aberration-correction the spatial resolution of the Z-contrast STEM images is as high as the sub-Å level, this technique is invaluable for characterizing the interfacial structures between high-K gate dielectrics and semiconductors. The related results are also introduced.

Key words: scanning transmission electron microscopy (STEM), Z-contrast STEM images, high-K gate dielectrics, interfacial microstructures, review

CLC Number:

ZHU Xin-Hua, LI Ai-Dong, LIU Zhi-Guo. Applications of Scanning Transmission Electron Microscopy (STEM) in the New Generation of High-K Gate Dielectrics[J]. Journal of Inorganic Materials, 2014, 29(12): 1233-1240.

Figures/Tables 11

Fig. 1 Schematic diagram for the formation of STEM image[6]

Fig. 2 Potential for high-angle scattering of Si atomic columms The specimen in (a) consists of an array of atomic columns (〈110〉 Si for example), in which the potential for high-angle scattering can be represented by an object function consisting of weighted spikes, as shown in (b). The experimental image can be interpreted as a convolution of the experimental probe and the object function, as in (c)[8]

Fig. 3 Different contrast transfer functions of a STEM (VG Microscopes HB501UX STEM operated at 100 kV with a probe size of ~ 0.22 nm) measured simultaneously at (a) coherent and (b) incoherent imaging conditions by using a small bright field detector and a large annular detector, respectively Plots show the very different transfer functions for the two detectors, the bright field detector showing contrast reversals and oscillations characteristic of coherent phase contrast imaging, the dark field detector showing a monotonic decrease in transfer with spatial frequency characteristic of incoherent imaging[6]

Fig. 4 Aberration corrector and improvement of Z-STEM image resolution [11] (a) An Aberration corrector consisting of 60 optical components, (b) experimental Z-STEM image of Si in [110] orientation taken from VG STEM HB501 STEM with a point resolution of ~ 0.2 nm, un-resolving the dumbbell structure of silicon, and (c) Z-STEM image of Si in [110] orientation taken from VG STEM HB501 STEM equipped with Nion aberration corrector, resolving clearly the dumbbell unit structure of silicon with a point resolution of 0.13 nm and increasing the ratio of signal-to-noise

Fig. 5 Images of Si/HfO2 (a) and HfO2/HfSiO (b,c) gate stacks (a) Cross-sectional Z-STEM image of an Si/HfO2 gate stack. The SiO2/HfO2 interface is seen in the center of the image[16]; (b) Cross-sectional HRTEM image of a HfO2/HfSiO gate stack viewed from the [110] direction of Si substrate, and (c) HAADF image of the same stack[17]

Fig. 6 Cross-sectional HRTEM image of the epitaxial Pr2O3 film (6 nm thick) grown on Si(111) substrate, and the film can be overgrown epitaxially with high quality Si (111) film[20]

Fig. 7 Z-STEM image of a-Si/Y2O3/c-Si stack (a) The formation of SiOxHy interfacial layer was due to the exposure of Y2O3 film to atmospheric conditions; (b) The interfacial layer of SiOxHy was not visible due to the Y2O3 film in-situ capped by a layer of amorphous Si during the deposited process[21]

Fig. 8 Cross-sectional HRTEM image of a SrTiO3 film epitaxially grown on Si (100) substrate by MBE, showing the formation of an atomically abrupt crystalline interface between the SrTiO3 and Si[23]

Fig. 9 (a) HRTEM and (b) HAADF-STEM images from an epitaxial Si/LaAlO3 interface showing an interface reconstruction where every third La column is missing at the interface. (c) Interface models based on the HAADF-STEM images[25]

Fig. 10 Z-STEM images of the HfO2 gate dielectrics grown by atomic layer deposition on Si substrates and their interfaces with TiN electrodes and silicon, annealed at (a) 800℃ and (b) 900℃[26] The dashed lines are a guide to the eye to indicate the approximate position of the interfacial layer. Note the roughening of interfaces after annealing at 900℃

Fig. 11 Three-dimensional locations of single Hf atoms in a Si/SiO2/HfO2 stacked device[30] (a) A schematic view diagram of the Si/SiO2/HfO2 stacked device with a SiO2 layer shown in red and the alternative dielectric, HfO2 shown in yellow color, where S and D representing the source and drain, respectively. (b) Three ADF images of the stack is represented by slice views in planes of x-y, x-z, and y-z, respectively. One isolated single Hf atom is marked the white circles. This representation demonstrates that the Hf atom is located inside the TEM sample. (c) A 3D reconstruction of part of the HfO2/SiO2/Si interface structure showing single Hf atoms. The Si substrate was color coded in gold, whereas the HfO2 film is marked solid yellow. Single Hf atoms with different positions within the interface layer are coded separately in green, black, red, and blue colors, respectively

References 30

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