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.

Laser sources played an important role in the introduction of ellipsometry to the semiconductor industry. This method was largely displaced by spectroscopic methods. Over the past five years, there has been a resurgence of interest in laser ellipsometry particularly for high precision measurements of ultra-thin dielectric films. Advances in laser, phase modulation, optical-design, and electronic instrumentation enable significant improvement in precision and stability relative to previous generations. The industrial requirements and operating principle for an exemplar of this new class of systems will be reviewed. Examples of measurements on thin films including high-k gate dielectrics shall be provided.