AVS 71 Session EL2-TuA: Spectroscopic Ellipsometry Novel Methodologies

Thermal dry etching uses gas-phase reactants in a vacuum and physicochemical reactions based on thermal activation, providing isotropic material removal for lateral patterning without line of sight. Thermal dry etching covers atomic layer etching (ALE) and spontaneous etching. ALE is defined by self-limiting reactions, separated by purge steps. These half-reactions modify and sequentially volatilize a thin film surface, thereby removing material digitally one ultra-thin layer per cycle. Conversely, spontaneous etching is characterized by a sustained reaction of a thin film surface with one etchant only, thereby removing a targeted material with a continuous etch rate.

This invited talk reviews exemplary studies of thermal ALE and spontaneous etching, utilizing in situ spectroscopic ellipsometry (iSE) to reveal thickness changes, self-limiting behavior, synergy between half-reactions, and selectivity between different materials. An iSE instrument (J.A. Woollam Co.) acquired ellipsometric spectra for 5 s at the end of reactant purge steps. Interference enhancement enabled thickness precision of ±0.01 Å.

Al₂O₃ thermal ALE using sequential hydrogen fluoride (HF)/trimethylaluminum (TMA) exposures exhibited a linear etch per cycle (EPC) at 275℃. After initial fluorination, consecutive HF exposures gave virtually no Al₂O₃ thickness loss. This self-limiting behavior corresponded to ideal ALE synergy because all material removal resulted solely from a favorable interaction of the HF/TMA sequence and no etching occurred by either HF or TMA alone. SiO₂ thermal ALE using sequential TMA/HF exposures likewise exhibited a linear EPC at 275℃. Consecutive HF exposures displayed negligible SiO₂ thickness loss, especially after eliminating H₂O during the fluorination step. This self-limiting behavior revealed near-ideal synergy for SiO₂ ALE.

SiN_x thermal ALE using sequential TMA/HF exposures discovered no ALE synergy because consecutive exposures of HF alone caused predominant SiN_x spontaneous etching. This difference between near-ideal versus no ALE synergy obtained great inherent selectivity between major SiN_x versus minor SiO₂ spontaneous etching using anhydrous HF vapor at 275℃. Using anhydrous HF at temperatures >150℃ also discovered facile spontaneous etching of single-crystalline, poly-crystalline, and amorphous Si films with high selectivity compared to SiO₂ retention.

In contrast, co-adsorbing polar molecules with anhydrous HF had a drastic effect. Co-dosing NH₃+HF at 275℃ obtained exceptional selectivity for rapid SiO₂ versus negligible SiN_x spontaneous etching. Similarly, co-adsorbing dimethylamine with HF at 200℃ enabled substantial SiO₂ spontaneous etching.

We investigate the transient dielectric function (DF) of Germanium at very high electron-hole pair densities using time-resolved spectroscopic ellipsometry. By employing a pump-probe technique, we explore the evolution of the critical points near the L-valley on a femtosecond time scale. Through modeling the DF of the material under different carrier concentrations, we analyze the impact that the photo-induced phenomena, such as phase-filling and many-body effects, have on the material's optical properties.

Ellipsometry and direct transmission/reflection FTIR spectrometry are versatile techniques for measuring the optical constants and thicknesses of arbitrary stacks of thin films. The self-referencing nature of ellipsometry allows high sensitivities and low noise, and the parallelized nature of FTIR spectroscopy allows convenient and fast measurements, but these techniques are insufficient to measure extinction coefficients (κ) lower than ~1x10^-2 in thin samples. When κ cannot be readily measured with ellipsometry and FTIR spectroscopy, it may be interpolated between regions of measurable κ with Kramers-Kronig consistent oscillator models. However, in low-loss regimes, different oscillator models can result in κ differing by orders of magnitude; for example, for UV-visible-NIR ellipsometry on ~200-nm-thick Si₃N₄ membranes, we observed diverging fits for κ at wavelengths longer than ~300 nm using three different oscillator models (Cauchy-Urbach, Tauc-Lorentz, and Cody-Lorentz) that all fit the ellipsometry data.^[1]

We propose the use of an additional direct measurement of absorptivity at a low-loss wavelength using self-referencing photothermal common-path interferometry (PCI). PCI is a sensitive absorption-measurement technique wherein a thermal lensing effect caused by absorption of a chopped high-powered pump laser in a sample is observed through the distortion of a co-incident low-powered probe laser. PCI has been previously used to measure losses in materials for LIGO interferometer mirror coatings and other optical components,^[2] but conventional PCI does not allow calibration of the measurement of freestanding membranes, and is more suited for supported thin films.^[1] Our self-referencing PCI makes use of monolayer graphene deposited on a freestanding membrane to create a high absorptivity reference sample that is similar in photothermal characteristics to the sample being tested, but whose loss can be measured with ellipsometry to allow calibration of the PCI measurement and thus accurately measure the low-loss sample.

We measured the imaginary part of the refractive index, κ, for Si₃N₄ (~1.9x10^-7) and SiN_x (~6.8x10^-5) at 1064 nm using self-referencing PCI. Using this data, we have assembled comprehensive optical models of Si₃N₄ and SiN_x from UV-Vis-NIR-MIR ellipsometry and FTIR spectroscopy. More broadly, our approach of merging several different spectroscopic techniques can be translated to other low-loss materials and allows the creation of highly accurate broadband materials datasets for optical simulation and design.

[1]. D.Feng, et al, arXiv:2404.04449, 2024.

[2]. J. Steinlechner, et al, Classical Quantum Gravity 2015