Brendan Foran

The effect of interfacial layer properties on the performance of Hf-based gate stack devices

The influence of Hf-based dielectrics on the underlying SiO2 interfacial layer (IL) in high-k gate stacks is investigated. An increase in the IL dielectric constant, which correlates to an increase of the positive fixed charge density in the IL, is found to depend on the starting, pre-high-k deposition thickness of the IL. Electron energy-loss spectroscopy and electron spin resonance spectra exhibit signatures of the high-k-induced oxygen deficiency in the IL consistent with the electrical data. It is concluded that high temperature processing generates oxygen vacancies in the IL responsible for the observed trend in transistor performance.

Grazing-incidence small angle x-ray scattering studies of phase separation in hafnium silicate films

Grazing-incidence small-angle x-ray scattering (GISAXS) and high-resolution transmission electron microscopy (HRTEM) were used to investigate phase separation in hafnium silicate films after rapid thermal annealing between 700 and 1000 °C. 4-nm-thick Hf–silicate films with 80 and 40 mol % HfO2, respectively, were prepared by metalorganic vapor deposition. Films of the two compositions showed distinctly different phase-separated microstructures, consistent with two limiting cases of microstructural evolution: nucleation/growth and spinodal decomposition. Films with 40 mol % HfO2 phase separated in the amorphous by spinodal decomposition and exhibited a characteristic wavelength in the plane of the film. Decomposition with a wavelength of ∼3 nm could be detected at 800 °C. At 1000 °C the films rapidly demixed with a wavelength of 5 nm. In contrast, films with 80 mol % HfO2 phase separated by nucleation and growth of crystallites, and showed a more random microstructure. The factors determining specific film...

Spectroscopic ellipsometry characterization of HfxSiyOz films using the Cody–Lorentz parameterized model

A parameterized, Kramers–Kronig consistent, Cody–Lorentz optical model is used to simulate the dielectric response of thin HfxSiyOz films. Optical constants are determined in the range 0.75–8.35eV. The Cody–Lorentz model has three specific differences when compared to the previously employed Tauc–Lorentz model: (1) weak exponential absorption below the band gap, (2) a modified joint density-of-states, and (3) a restriction on the e1(∞) parameter. These three differences allow the Cody–Lorentz model to have an improved fit to experimental data. As a result of a more accurate optical model for HfxSiyOz, we were able to identify an interfacial layer with thickness in close agreement with transmission electron microscopy measurements. Use of the Tauc–Lorentz model when fitting the same experimental data could not identify an interfacial layer. Results are also discussed in which the Cody–Lorentz model shows sensitivity to varying degrees of silicate composition.

Interfacial Layer-Induced Mobility Degradation in High-

Analysis of electrical and scanning transmission electron microscopy (STEM) and electron energy loss spectra (EELS) data suggests that Hf-based high-k dielectrics deposited on a SiO2 layer modifies the oxygen content of the latter resulting in reduction of the oxide energy band gap and correspondingly increasing its k value. High-k deposition on thinner SiO2 films, below 1.1 nm, may lead to the formation of a highly oxygen deficient amorphous interfacial layer adjacent to the Si substrate. This layer was identified as an important factor contributing to mobility degradation in high-k transistors.

k

Using the concept of metastable phase diagrams, we discuss the microstructure evolution during annealing of amorphous ZrO2–SiO2 and HfO2–SiO2 thin films for gate dielectric applications. These systems are characterized by a low solid solubility, a liquid miscibility gap and a kinetic barrier to the formation of the complex, crystalline silicate. We show that phase partitioning is expected for most compositions. Compositions within the metastable extensions of the spinodal are unstable and will spontaneously unmix in the amorphous phase upon heating. Hafnia- or zirconia-rich phases will subsequently crystallize to form HfO2 or ZrO2. Most compositions outside the metastable extensions of the liquid phase miscibility gap must phase separate above the crystallization temperature by nucleation of crystalline HfO2 or ZrO2 out of an amorphous silica-rich matrix. We present calculations of the metastable extensions of the miscibility gap and spinodal. The calculations predict that SiO2-rich compositions, investigated for gate dielectric applications, will show spinodal decomposition if they contain less than ~90 mol% SiO2 at the typical device processing temperature of 1000°C. Experimental studies of Hf-silicate films with three different compositions, between ~40 and 80 mol% HfO2 that lie inside and outside the miscibility gap, respectively, are presented. All three compositions show phase separation. Despite the different pathways of microstructure evolution, the final phase separated microstructures are similar. Experimental verification of the pathways that lead to these microstructures requires further studies.

Transistors

Changes in the composition of atomic layer deposited, uncapped hafnium dioxide films, as a function of anneal temperature, have been evaluated by several advanced analytical techniques including; x-ray reflectivity, high-resolution transmission electron microscopy, and medium energy ion scattering. It is shown that such measurements of the high-k/Si interface layer are inconclusive and may be misinterpreted to suggest the presence of an HfxSi1−xO2 (x∼0.5) transition layer. It is also demonstrated that high-temperature anneal of uncapped films may result in the formation of voids which propagate through the dielectric layer into the silicon substrate. Trends associated with defect generation, interfacial oxide growth, and the low probability of material intermixing during anneal processing are discussed.

Application of Metastable Phase Diagrams to Silicate Thin Films for Alternative Gate Dielectrics

We investigate oxygen vacancy ordering in epitaxial (La0.5Sr0.5)CoO3−∂ thin films grown by sputter deposition on (001) LaAlO3 and (001) SrTiO3. After annealing at 500 °C under oxygen partial pressures greater than those used during deposition, films transform to a long-range oxygen vacancy ordered structure with orthorhombic symmetry. Observed orientation variants of the oxygen vacancy ordered structures are different for the two substrates. We discuss the relationship between film stress due to lattice and thermal mismatch with the substrate, and vacancy ordering.

Physicochemical properties of HfO2 in response to rapid thermal anneal

High-resolution transmission electron microscopy (HR-TEM) has been used as the ultimate method of thickness measurement for thin films. The appearance of phase contrast interference patterns in HR-TEM images has long been confused as the appearance of a crystal lattice by nonspecialists. Relatively easy to interpret crystal lattice images are now directly observed with the introduction of annular dark-field detectors for scanning TEM (STEM). With the recent development of reliable lattice image processing software that creates crystal structure images from phase contrast data, HR-TEM can also provide crystal lattice images. The resolution of both methods has been steadily improved reaching now into the sub-Angstrom region. Improvements in electron lens and image analysis software are increasing the spatial resolution of both methods. Optimum resolution for STEM requires that the probe beam be highly localized. In STEM, beam localization is enhanced by selection of the correct aperture. When STEM measurement is done using a highly localized probe beam, HR-TEM and STEM measurement of the thickness of silicon oxynitride films agree within experimental error. In this article, the optimum conditions for HR-TEM and STEM measurement are discussed along with a method for repeatable film thickness determination. The impact of sample thickness is also discussed. The key result in this article is the proposal of a reproducible method for film thickness determination.

Impact of stress on oxygen vacancy ordering in epitaxial (La0.5Sr0.5)CoO3−∂ thin films

Electron energy-loss spectroscopy combined with high-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy was used to investigate the chemistry of interfacial layers in HfO2 gate stacks capped with polycrystalline Si gate electrodes. To interpret the energy-loss near-edge fine structure (ELNES) obtained from the interfacial layers, reference spectra were obtained from single crystal hafnium silicate (HfSiO4), monoclinic HfO2 powder, and amorphous SiO2. No bulk-like silicate bonding could be detected in the ELNES of Si L2,3 and O K edges recorded from layers at the Si substrate interface. Compared to bulk SiO2, the interfacial ELNES showed additional features that were caused by overlap of signals from Si, HfO2, and SiO2, despite a relatively small electron probe size of ∼3A. HAADF showed that interfacial roughness caused the projected thickness of nominally pure SiO2 (within the detection limit of the method) to be as small as ∼5A in many locations.

Thin Dielectric Film Thickness Determination by Advanced Transmission Electron Microscopy

We have demonstrated a uniform, robust interface for high-k deposition with significant improvements in device electrical performance compared to conventional surface preparation techniques. The interface was a thin thermal oxide that was grown and then etched back in a controlled manner to the desired thickness. Utilizing this approach, an equivalent oxide thickness (EOT) as low as 0.87 nm has been demonstrated on high-k gate stacks having improved electrical characteristics as compared to more conventionally prepared starting surfaces.