Determination of optical constants of thin films in the EUV.

The determination of fundamental optical parameters is essential for the development of new optical elements such as mirrors, gratings, or photomasks. Especially in the extreme ultraviolet (EUV) and soft x-ray spectral range, the existing databases for the refractive indices of many materials and compositions are insufficient or are a mixture of experimentally measured and calculated values from atomic scattering factors. Since the physical properties of bulk materials and thin films with thicknesses in the nanometer range are not identical, measurements need to be performed on thin layers. In this study we demonstrate how optical constants of various thin film samples on a bulk substrate can be determined from reflection measurements in the EUV photon energy range from 62 eV to 124 eV. Thin films with thickness of 20 nm to 50 nm of pure Mo, Ni, Pt, Ru, Ta, and Te and different compositions of NixAlx, PtTe, PtxMo, RuxTax, Ru3Re, Ru2W, and TaTeN were prepared by DC magnetron sputtering and measured using EUV reflectometry. The determination optical constants of the different materials are discussed and compared to existing tabulated values.

Influence of the Chalcogen Element on the Filament Stability in CuIn(Te,Se,S)2/Al2O3 Filamentary Switching Devices.

In this paper, we report on the use of CuInX2 (X = Te, Se, S) as a cation supply layer in filamentary switching applications. Being used as absorber layers in solar cells, we take advantage of the reported Cu ionic conductivity of these materials to investigate the effect of the chalcogen element on filament stability. In situ X-ray diffraction showed material stability attractive for back-end-of-line in semiconductor industry. When integrated in 580 µm diameter memory cells, more volatile switching was found at low compliance current using CuInS2 and CuInSe2 compared to CuInTe2, which is ascribed to the natural tendency for Cu to diffuse back from the switching layer to the cation supply layer because of the larger difference in electrochemical potential using Se or S. Low-current and scaled behavior was also confirmed using conductive atomic force microscopy. Hence, by varying the chalcogen element, a method is presented to modulate the filament stability.

Atomic Layer Deposition of Ruthenium with TiN Interface for Sub-10 nm Advanced Interconnects beyond Copper.

Atomic layer deposition of ruthenium is studied as a barrierless metallization solution for future sub-10 nm interconnect technology nodes. We demonstrate the void-free filling in sub-10 nm wide single damascene lines using an ALD process in combination with 2.5 Å of ALD TiN interface and postdeposition annealing. At such small dimensions, the ruthenium effective resistance depends less on the scaling than that of Cu/barrier systems. Ruthenium effective resistance potentially crosses the Cu curve at 14 and 10 nm according to the semiempirical interconnect resistance model for advanced technology nodes. These extremely scaled ruthenium lines show excellent electromigration behavior. Time-dependent dielectric breakdown measurements reveal negligible ruthenium ion drift into low-κ dielectrics up to 200 °C, demonstrating that ruthenium can be used as a barrierless metallization in interconnects. These results indicate that ruthenium is highly promising as a replacement to Cu as the metallization solution for future technology nodes.

Understanding the Dual Nature of the Filament Dissolution in Conductive Bridging Devices.

The formation and rupture of conductive filaments (CFs) inside an insulating medium is used as hardware encoding of the state of a memory cell ("1" - "0") in filamentary-based conductive bridging memories. Currently accepted models explain the filament erase (reset) as the subtraction of conductive metal atoms from the CF; however, they do not fully account for the rich set of phenomena experimentally observed during the reset. The details of the filament erase are unraveled on the nanometer scale by means of an atomic force microscopy-based tomography technique enabling the 3D observation of erased CFs. "Non-broken" and "broken" CFs are observed, whereby the increase in resistance originates, respectively, from a constriction point in the current path and from an interrupted CF. We demonstrate that their existence and morphology can be related to the specific formation history of the CF, and we identify the physical volume of the CF as being mainly responsible for the type of filament erase.

Combinatorial Study of Ag-Te Thin Films and Their Application as Cation Supply Layer in CBRAM Cells.

In this work, we investigate binary Ag-Te thin films and their functionality as a cation supply layer in conductive bridge random access memory devices. A combinatorial sputter deposition technique is used to deposit a graded Ag(x)Te(1-x) (0 < x < 1) layer with varying composition as a function of the position on the substrate. The crystallinity, surface morphology, and material stability under thermal treatment as a function of the composition of the material are investigated. From this screening, a narrow composition range between 33 and 38 at% Te is selected which shows a good morphology and a high melting temperature. Functionality of a single Ag(2-δ)Te composition as cation supply layer in CBRAM with dedicated Al2O3 switching layer is then investigated by implementing it in 580 µm diameter dot Pt/Ag(2-δ)Te/Al2O3/Si cells. Switching properties are investigated and compared to cells with a pure Ag cation supply layer. An improved cycling behavior is observed when Te is added compared to pure Ag, which we relate to the ionic conducting properties of Ag2Te and the preferred formation of Ag-Te phases.

Three-dimensional observation of the conductive filament in nanoscaled resistive memory devices.

The basic unit of information in filamentary-based resistive switching memories is physically stored in a conductive filament. Therefore, the overall performance of the device is indissolubly related to the properties of such filament. In this Letter, we report for the first time on the three-dimensional (3D) observation of the shape of the conductive filament. The observation of the filament is done in a nanoscale conductive-bridging device, which is programmed under real operative conditions. To obtain the 3D-information we developed a dedicated tomography technique based on conductive atomic force microscopy. The shape and size of the conductive filament are obtained in three-dimensions with nanometric resolution. The observed filament presents a conical shape with the narrow part close to the inert-electrode. On the basis of this shape, we conclude that the dynamic filament-growth is limited by the cation transport. In addition, we demonstrate the role of the programming current, which clearly influences the physical-volume of the induced conductive filaments.

Switching mechanism and reverse engineering of low-power Cu-based resistive switching devices.

In the recent past, filamentary-based resistive switching devices have emerged as predominant candidates for future non-volatile memory storage. Most of the striking characteristics of these devices are still limited by the high power consumption and poor understanding of the intimate resistive switching mechanism. In this study, we present an atomic scale study of the filament formation in CuTe-Al2O3 by using a conductive scanning probe tip to analyse the shape and dimensions of the filament. Filaments studied were either created within a normal device or locally formed while using the tip as the top electrode. We demonstrate that it is possible to create with C-AFM a filament with a signature identical to a device (i.e. two orders of magnitude resistance window, 10(4) s retention operating at 5 µA). This is obtained by a dedicated material and resistance selection for the conductive tip. The filamentary mechanism of fully processed devices is demonstrated and observed by C-AFM. Filaments created with C-AFM can be repeatedly cycled and the ON state presents a 20 nm highly conductive spot which can be repeatedly turned into a poorly conductive path in the OFF state.

Ultrathin NiGe films prepared via catalytic solid-vapor reaction of Ni with GeH(4).

A low-temperature (225-300 °C) solid-vapor reaction process is reported for the synthesis of ultrathin NiGe films (∼6-23 nm) on 300 mm Si wafers covered with thermal oxide. The films were prepared via catalytic chemical vapor reaction of germane (GeH4) gas with physical vapor deposited (PVD) Ni films of different thickness (2-10 nm). The process optimization by investigating GeH4 partial pressure, reaction temperature, and time shows that low resistive, stoichiometric, and phase pure NiGe films can be formed within a broad window. NiGe films crystallized in an orthorhombic structure and were found to exhibit a smooth morphology with homogeneous composition as evidenced by glancing angle X-ray diffraction (GIXRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Rutherford back-scattering (RBS) analysis. Transmission electron microscopy (TEM) analysis shows that the NiGe layers exhibit a good adhesion without voids and a sharp interface on the thermal oxide. The NiGe films were found to be morphologically and structurally stable up to 500 °C and exhibit a resistivity value of 29 µΩ cm for 10 nm NiGe films.

Influence of carbon alloying on the thermal stability and resistive switching behavior of copper-telluride based CBRAM cells.

We report the improved thermal stability of carbon alloyed Cu0.6Te0.4 for resistive memory applications. Copper-tellurium-based memory cells show enhanced switching behavior, but the complex sequence of phase transformations upon annealing is disadvantageous for integration in a device. We show that addition of about 40 at % carbon to the Cu-telluride layer results in an amorphous material up to 360 °C. This material was then integrated in a TiN/Cu0.6Te0.4-C/Al2O3/Si resistive memory cell, and compared to pure Cu0.6Te0.4. Very attractive endurance (up to 1 × 10(3) cycles) and retention properties (up to 1 × 10(4) s at 85 °C) are observed. The enhanced thermal stability and good switching behavior make this material a promising candidate for integration in memory devices.