Richard P. Oleksak

Chemical and Structural Investigation of High-Resolution Patterning with HafSOx

High-resolution transmission electron microscopy (TEM) imaging and energy-dispersive X-ray spectroscopy (EDS) chemical mapping have been used to examine key processing steps that enable sub-20-nm lithographic patterning of the material Hf(OH)4-2x-2y(O2)x(SO4)y·qH2O (HafSOx). Results reveal that blanket films are smooth and chemically homogeneous. Upon exposure with an electron beam, the films become insoluble in aqueous tetramethylammonium hydroxide [TMAH(aq)]. The mobility of sulfate in the exposed films, however, remains high, because it is readily exchanged with hydroxide from the TMAH(aq) solution. Annealing the films after soaking in TMAH(aq) results in the formation of a dense hafnium hydroxide oxide material that can be converted to crystalline HfO2 with a high electron-beam dose. A series of 9 nm lines is written with variable spacing to investigate the cross-sectional shape of the patterned lines and the residual material found between them.

Aqueous-based synthesis of gallium tungsten oxide thin film dielectrics

An aqueous-based approach to the deposition of gallium tungsten oxide thin film dielectrics was developed where the metal content (Ga2−xWx/2O3−δ, x = 0.31–0.66) can be controlled by the initial precursor concentrations, while the oxygen deficiency (δ) can be controlled by post annealing. The spin-coated precursor converts to an amorphous oxide thin film with uniform distribution of metal cations after heating to 400 °C. The Ga2−xWx/2O3−δ band gap decreased with increasing W content from 4.9 eV for the undoped film (Ga2O3) to 4.6 eV for the film with the highest W content (Ga1.34W0.33O3−δ). The electronic properties of the dielectric films significantly improved with incorporation of W, where a decrease in leakage current and an increase in breakdown field strength from 2.9 MV cm−1 (Ga2O3) to 5.2 MV cm−1 (Ga1.34W0.33O3−δ) was observed. Prior studies have suggested that Ga2−xWx/2O3−δ thin films can form dense low-k dielectric materials, where the relative amount of W in the film can significantly modify the dielectric constant. However our studies found no significant decrease in the dielectric constant with W doping of Ga2O3. We found that annealing the films >400 °C in air resulted in the formation of sub-stoichiometric WOx and metallic W throughout the films which may complicate dielectric measurements.

Continuous synthesis of colloidal chalcopyrite copper indium diselenide nanocrystal inks

A continuous synthetic method in a micro-tubular reactor is introduced for synthesizing mono-disperse and solution-stable chalcopyrite colloidal copper indium diselenide nanocrystal (CuInSe2 NC) inks with potential scalability. It was found that the morphologies of the CuInSe2 NCs were dependent on the Cu/In/Se composition. The NC morphology changed from spherical to hexagonal to trigonal with increasing In or Se content, whereas trigonal morphologies synthesized at high temperature yielded chalcopyrite CuInSe2 NCs. A laboratory-scale photovoltaic device with 1.9% efficiency under AM1.5G illumination was also fabricated to verify the utility of these inks.

Characterization of high-resolution HafSOx inorganic resists

Inorganic resists are of considerable interest for advanced lithography at the nanoscale due to the potential for high resolution, low line width roughness (LWR), and high sensitivity. Historically inorganic resists suffered from low sensitivity, however approaches have been identified to increase sensitivity while maintaining high contrast. An aqueous precursor of Hf(OH)4-2x-2y(O2)x(SO4)y·qH2O (HafSOx) has been demonstrated with excellent sensitivity to EUV and electrons, while still obtaining high resolution and low LWR. In this work, we characterize both HafSOx precursor solutions and spin-coated thin films using high-resolution transmission electron microscopy (HR-TEM) with energy-dispersive X-ray spectroscopy (EDS) elemental analysis. HR-TEM of precursor solutions drop cast onto TEM grids confirmed the presence of nanoscale particles. HR-TEM cross sectional images showed that spin-coated HafSOx films are initially uniform in appearance and composition for thin (12 nm) films, however thicker (30 nm) films display segregation of species leading to multilayer structures. Regardless of film thickness, extended exposure to the high energy TEM electron beam induces significant migration of oxygen species to the Si interface. These species result in the formation of SiOx layers that increase in thickness with an increase in TEM electron beam dose. Sulfate is also very mobile in the films and likely assists in the significant condensation exhibited in completely processed films.

Interfacial Chemistry-Induced Modulation of Schottky Barrier Heights: In Situ Measurements of the Pt–Amorphous Indium Gallium Zinc Oxide Interface Using X-ray Photoelectron Spectroscopy

A method to understand the role of interfacial chemistry on the modulation of Schottky barrier heights for platinum and amorphous indium gallium zinc oxide (a-IGZO) interfaces is demonstrated through thermal processing and background ambient pressure control. In situ X-ray photoelectron spectroscopy was used to characterize the interfacial chemistries that modulate barrier heights in this system. The primary changes were a significant chemical reduction of indium, from In3+ to In0, that occurs during deposition of Pt on to the a-IGZO surface in ultrahigh vacuum. Postannealing and controlling the background ambient O2 pressure allows further tuning of the reduction of indium and the corresponding Schottky barrier heights from 0.17 to 0.77 eV. Understanding the detailed interfacial chemistries at Pt/a-IGZO interfaces may allow for improved electronic device performance, including Schottky diodes, memristors, and metal-semiconductor field-effect transistors.