Tomoko Sekiguchi

Nitrogen Distribution and Chemical Bonding State Analyses in Oxynitride Film by Spatially Resolved Electron Energy Loss Spectroscopy (EELS)

Nitrogen distribution and chemical bonding states in a multilayer (SiO2/Si3N4/SiOxNy(oxynitride)/Si) are analyzed using spatially resolved electron energy loss spectroscopy (EELS). Multiple spectra of different points in a selected area are obtained simultaneously, and core-loss intensity and chemical bonding states are analyzed at each point. Nitrogen is accumulated both at the oxynitride and silicon interface and in the vicinity of oxide surface, and the nitrogen peak full widths at half-maximum (FWHM) are about 2 nm and 3.2 nm, respectively. The nitrogen-accumulated interface layer consists of two regions: the upper region of the interface may have N–O bonds and the lower region consists of mainly nitrides. The EELS has spatial and energy resolutions of about 1 nm and 0.1 eV, respectively.

Scalability of TiN/HfAlO/TiN MIM DRAM capacitor to 0.7-nm-EOT and beyond

We have proposed guiding principle of material selection of electrode/dielectric combination for MIM DRAM capacitors by theoretically taking the tunneling barrier height into account. Accordingly, we found that phase-controlled HfO2 (HfAlO) with TiN electrode is promising. TiN/HfAlO/TiN MIM capacitors with an ultra-thin Al2O3 on the bottom TiN electrode were fabricated and an EOT of 0.7 nm with a leakage current of 80 nA/cm2 was successfully achieved.

Photoresist cross-sectional shape change caused by scanning electron microscope-induced shrinkage

Abstract. Change in the cross-sectional profile of a photoresist (PR) pattern due to shrinkage was evaluated to investigate the mechanism of electron beam-induced shrinkage. A scanning transmission electron microscope (STEM) was used to observe the cross-sectional profiles of PR lines after atomic-layer deposition of metal oxide and carbon deposition on the sample surface. A HfO2 thin layer enhanced the profile contrast in the STEM measurements without blurring the edge, which enabled the precise cross-sectional measurement of the PR patterns. We found interesting features associated with shrinkage from the detailed profile change obtained using this method, such as a rounding of the pattern top, a necking of the sidewall profile, a rounding of the foot in the pattern on the organic underlying layer, and voltage-independent sidewall shrinkage under a large electron beam dose. These behaviors along with the results from a Monte Carlo simulation are discussed. Consequently, these observations experimentally clarified that the elastic deformation effect and the impact of the secondary electrons emitted from the spaces around the pattern into the sidewall are important to interpret the change in the shape of the pattern induced by shrinkage.

Precise measurement of photoresist cross-sectional shape change caused by SEM-induced shrinkage

The mechanism of photoresist shrinkage induced by electron-beam (EB) irradiation was studied. A precise cross-sectional profile of a photoresist pattern was obtained by a scanning transmission electron microscope (STEM) after atomic layer deposition of HfO2 on the sample patterns. Photoresist lines and spaces fabricated with positive-tone development and negative-tone development were exposed to an EB with much higher dose than a practical dose (to accelerate shrinkage intentionally). The obtained STEM images of the patterns before and after EB irradiation show that the shrinkage of the negative-tone-developed patterns is smaller than that of the positive-tone-developed patterns. This observation is explained by the fact that negative-tone-developed photoresist molecules do not contain protection groups, whose volatilization caused by EB irradiation is one of the origins of shrinkage. Another finding is that the EB irradiation causes top-rounding and necking of the pattern profile as well as linewidth slimming. The rounding of the pattern top profile suggests that the pattern’s shape was elastically deformed. In addition, EB irradiation only onto the spaces caused sidewall shrinkage and a necking profile, although no electrons were irradiated directly onto the pattern. These phenomena are considered to be due to the electrons scattered from the spaces to the pattern sidewall. Finally, a Monte Carlo simulation of electron scattering showed that the distribution of the deposited EB energy on the pattern surface corresponds to the above-described change in pattern shape. Consequently, these observations and simulation results clarify the importance of the effect of elastic shape change and the impact of the electrons scattered from the underlying layer onto the sidewall in the mechanism of photoresist shrinkage.

Theoretical Screening of Candidate Materials for DRAM Capacitors and Experimental Demonstration of a Cubic-Hafnia MIM Capacitor

To screen candidate materials for dynamic random-access memory capacitors, the tunneling probability at a constant equivalent oxide thickness (EOT) of metal-insulator-metal (MIM) capacitors was theoretically maximized according to a tradeoff between permittivity and band offset. As a result, it was found that cubic HfO2 with a TiN electrode is a promising candidate. TiN/Al-doped HfO2/TiN MIM capacitors were fabricated by inserting Al2O3 layers for phase control of HfO2 and for suppression of TiN oxidation. The fabricated capacitors exhibit leakage current of 80 nA/cm2 at 1 V and EOT of 0.7 nm. Moreover, the main leakage current was estimated to originate from oxygen vacancies.

Band Engineering of Rutile TiO2 by Cobalt Doping in Ru/Rutile-TiO2/Ru Capacitor aiming 40-nm DRAM and Beyond

A Feasible fabrication process of Ru/rutile-Co-doped TiO2/Ru capacitor for 40-nm DRAM and beyond is developed. As-deposited Ru lower electrode is found out to crystallize Co-doped TiO2 overlayer into rutile having relative permittivity beyond 90 owing to lattice matching. Furthermore, we predicted doping of element having small ionic radius, such as Co, increase Schottky barrier, leading to decrease in leakage current. Temperature dependence of leakage current of RIR-Co-doped-TiO2 capacitor and band structure analysis with X-ray photoelectron spectroscopy shows that leakage current is determined by balancing between thermionic current and tunneling current through Schottky barrier. By doping of Co, the thermionic current decreases, whereas the tunneling current increases. With calculation using theoretical dependence of amount of doped element on leakage current, optimum percentage of Co is estimated as 0.3-0.6 % to achieve a sufficiently low leakage-current. This technology opens possibility to utilize 40-nm DRAM and beyond.

Two-Dimensional Boron Analysis in Borophosphosilicate Glass Film Using Transmission Electron Microscope with Imaging Filter

Two-dimensional boron concentration mapping of borophosphosilicate glass (BPSG) film has been performed using a transmission electron microscope equipped with an imaging filter. The atomic ratio of boron to silicon is obtained using core-loss electrons of boron K and silicon L23 spectra. Boron concentration increases at the BPSG/phosphosilicate glass (PSG) interface, which is consistent with the result obtained by secondary ion mass spectrometry (SIMS).