Jaemo Im

Composition-control of magnetron-sputter-deposited (BaxSr1 - X)Ti1 + yO3 + z thin films for voltage tunable devices

Precise control of composition and microstructure is critical for the production of (BaxSr1−x)Ti1+yO3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high-frequency devices. Here, we present results on composition-microstructure-electrical property relationships for polycrystalline BST films produced by magnetron-sputter deposition, that are appropriate for microwave and millimeter-wave applications such as varactors and frequency triplers. Films with controlled compositions were grown from a stoichiometric Ba0.5Sr0.5TiO3 target by control of the background processing gas pressure. It was determined that the (Ba+Sr)/Ti ratios of these BST films could be adjusted from 0.73 to 0.98 by changing the total (Ar+O2) process pressure, while the O2/Ar ratio did not strongly affect the metal ion composition. Film crystalline structure and dielectric properties ...

Layered (BaxSr1-x)Ti1+yO3+z thin films for high frequency tunable devices

Low dielectric loss and high electrical breakdown fields, in conjunction with an application-specific permittivity response, are simultaneous requirements for (Ba{sub x}Sr{sub 1-x})Ti{sub 1+y}O{sub 3+z} thin films. This has proved problematic, in that synthesis conditions which tend to reduce dielectric loss also tend to reduce zero-bias permittivity and electric-field tunability of the permittivity, thereby necessitating undesired compromises in overall performance for some applications. In order to address this problem of simultaneous optimization, we fabricated BST thin films with very low dielectric loss, Ti-rich BST layers ((Ba+Sr)/Ti=0.73) at the electrode/BST interfaces, in series with a primary, high tunability, high permittivity BST layer ((Ba+Sr)/Ti=0.9) in the center of this layered BST structure. Planar capacitors fabricated from the BST films exhibit low dielectric loss (tan {delta}=0.005) simultaneously with high (76%) tunability at room temperature and high breakdown fields ({approx}4 MV/cm), compared with capacitors with a single BST layer (Ba+Sr/Ti=0.9). Post annealing of the layered BST films at 800 C further improved dielectric loss down to 0.003, while keeping the high tunability and high breakdown field of the as-deposited films.

(BaxSr1−x)Ti1+yO3+z interface contamination and its effect on electrical properties

Surface contamination and cleaning processes of (BaxSr1−x)Ti1+yO3+z(BST) and Pt films were investigated using in situ, real-time mass spectroscopy of recoiled ions (MSRI). MSRI analysis revealed that BST film surfaces exposed to atmospheric ambient are contaminated with carbon and hydrogen containing species, which could be removed by thermal decomposition/desorption in an oxygen ambient. Cleaning of the BST surface was accomplished by annealing at 500 °C in ⩾1 mTorr O2, resulting in complete elimination of these species. Similar contamination on Pt film surfaces could be eliminated with only 200 °C annealing in 5×10−4 Torr of O2. Annealing of the BST film surface in oxygen prior to deposition of the top Pt electrode results in a clean top Pt/BST interface that yields BST capacitors with much lower and more symmetric leakage current characteristics, and lower dielectric losses compared to BST capacitors with contaminated top Pt electrode/BST interfaces.

Studies of film growth processes and surface structural characterization of ferroelectric memory‐compatible SrBi2Ta2O9 layered perovskites via in situ, real‐time ion‐beam analysis

In situ, real‐time studies of layered perovskite SrBi2Ta2O9 (SBT) film growth processes were performed using a time‐of‐flight ion scattering and recoil spectroscopy (TOF ISARS) technique. These studies revealed two important features related to the synthesis of SBT films via ion‐beam sputter‐deposition, namely: (a) atomic oxygen originating from a multicomponent SBT target during the sputtering process is incorporated in the growing film more efficiently than molecular oxygen; and (b) the SBT surface appears to be terminated in an incomplete (Bi2O2)2+layer with a top surface of oxygen atoms, which may be responsible for the high resistance to polarization fatigue exhibited by Pt/SBT/Pt capacitors.

Transmission electron microscopy study of hydrogen-induced degradation in strontium bismuth tantalate thin films

The fabrication of nonvolatile ferroelectric random access memories based on SrBi2Ta2O9 (SBT) or other ferroelectric capacitors require exposure of these capacitors to processing gases mixtures including hydrogen. This results in a strong degradation of the capacitor electrical properties, mainly due to the interaction of hydrogen with the ferroelectric layer. Using surface analysis methods, we previously determined that the hydrogen-induced degradation of SBT capacitors might be partially due to the degradation of the near surface region of the SBT layer. It was also demonstrated that oxygen annealing after the hydrogen exposure results in the recovery of the degraded SBT surface layer and the electrical properties of the capacitor. We have now performed detailed cross sectional transmission electron microscopy studies of virgin, hydrogen, and oxygen annealed SBT/Pt/TiO2/SiO2/Si heterostructures. These studies combined microstructural imaging with a nanoscale compositional analysis of the SBT layer as a ...

Studies of ferroelectric film growth and capacitor interface processes via insitu analytical techniques and correlation with electrical properties

Abstract Precise control of composition and microstructure of multicomponent oxide thin films is critical for the production of ferroelectric and high dielectric constant thin film devices. In addition, the integration of film-based capacitors with semiconductor substrates, for device fabrication, requires good control of the composition and structureof the dielectric/substrate and top electrode / dielectric interfaces to control the capacitor properties. In order to understand the processes described above, we are using a variety of integrated complementary in situ analytical techniques including time-of-flight ion scattering and recoil spectroscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, and ex situ methods such as transmission electron microscopy, scanning force microscopy, and scanning electron microscopy. Examples of studies recently performed by our group that are reviewed here include: (a) effects of microstructure on the oxidation of Ti-Al layers that can be used in a dual functionality as a diffusion barrier and bottom electrode for integration of ferroelectric capacitors with semiconductors; (b) studies of the surface and dielectric layer/bottom electrode interface during growth of BaxSr1−xTiO3 films on Ir/TiN/SiO2/Si for fabrication of BST capacitors for DRAMs; and (c) studies of the effect of interface contamination and structure on the electrical properties of BST capacitors for high frequency devices.

Magnetron sputter-deposited multilayer (BaxSr1x)Ti1+yO3+z thin films for passive and active devices

Abstract High permittivity (BaxSr1−x)Ti1+yO3+z(BST) thin films are being investigated for integration into charge storage dielectrics and electric-field tunable elements for high frequency devices. For the latter application, it is desirable to have BST capacitors with high tunability and low losses. Therefore, we investigated the use of multilayer BST thin films consisting of very low dielectric loss BST/electrode interfacial layers ((Ba+Sr)/Ti = 0.73) sandwiching a high tunability, high permittivity primary BST layer ((Ba+Sr)/Ti = 0.9). BST capacitors with multiple layers of controlled composition can be effectively produced insitu by magnetron sputter deposition, using a single stoichiometric target and controlling the layer composition by changing the total process gas (Ar+O<2) pressure. The layered BST film capacitors exhibit simultaneous low loss (tan Δ = 0.005), high tunability (76%), high charge storage energy density (34 J/cm3), low leakage, and high dielectric breakdown (>2.8 MV/cm).

Study of Growth Processes in Ferroelectric Films and Layered Heterostructures via in Situ, Real-Time Ion Beam Analysis

Ferroelectric films can display a wide range of dielectric, ferroelectric, piezoelectric, electrostrictive, and pyroelectric properties. The potential utilization of these properties in a new generation of devices has motivated intensive studies on the synthesis, characterization, and determination of processing-microstructure-property relationships of ferroelectric thin films during the last five years. In addition, there has been an increased drive for integrating ferroelectric film-based heterostructures with different substrate materials to demonstrate devices which exploit the dielectric, ferroelectric, piezoelectric, electrostrictive, and pyroelectric properties of these materials. For example, the high dielectric permittivities of perovskite-type materials can be advantageously used in dynamic random access memories (DRAM),1-3 while the large values of switchable remanent polarization of ferroelectric materials are suitable for non-volatile ferroelectric random access memories (NVFRAM).1-3

Measurement of local magnetization in the buried layer of a pseudo-spin-valve submicron wire

A pseudo-spin-valve (PSV) wire [Au(3 nm)/Py(10 nm)/Cu(5 nm)/Co(10 nm)/Si] was patterned by e-beam lithography into two sections with different widths, connected by a narrow part that acts as a domain wall trap. The two sections have different magnetic shape anisotropies and thus different coercive fields. Since the sample has two different magnetic layers (soft and hard) and two different anisotropies (thick and thin widths), this patterned system has more than two magnetic configuration states depending on the applied field strength. To probe local magnetization from the two different sections, x-ray magnetic circular dichroism (XMCD) measurements were done on the PSV wire with a microfocused x-ray beam. Measurements were done on the buried hard layer, from which magnetic information cannot be obtained by surface-sensitive techniques. The XMCD experimental results are compared with micromagnetic simulations.

Studies of ferroelectric heterostructure thin films and interfaces, via in situ analytical techniques

Abstract The science and technology of ferroelectric thin films has experienced an explosive development during the last ten years. Low-density non-volatile ferroelectric random access memories (NVFRAMs) are now incorporated in commercial products such as “smart cards”, while high permittivity capacitors are incorporated in cellular phones. However, substantial work is still needed to develop materials integration strategies for high-density memories. We have demonstrated that the implementation of complementary in situ characterization techniques is critical to understand film growth and interface processes, which play critical roles in film microstructures and properties. We are using uniquely integrated time of flight ion scattering and recoil spectroscopy (TOF-ISARS) and spectroscopic ellipsometry (SE) techniques to perform in situ, real-time studies of film growth processes in the high background gas pressure required to growth ferroelectric thin films. TOF-ISARS provides information on surface processes, while SE permits the investigation of buried interfaces as they are being formed. Recent studies on SrBi2Ta2O9 (SBT) and BaxSr1-xTiO3 (BST) film growth and interface processes are discussed.