Paper PS+NS+SS+TF-ThM1
Strategies to Control the Etch per Cycle During Atomic Layer Etching of SiO₂ and SiN_x

Thursday, November 2, 2017, 8:00 am, Room 23

Decreasing device dimensions and the incorporation of increasingly complex 3D architectures place new constraints on conventional plasma processing techniques. One method to address the limitations of conventional etching is atomic layer etching (ALE) which can provide low damage and atomic-scale etch control. ALE has been extensively studied for a variety of materials, including Al₂O₃, HfO₂, Si, and Si-based dielectrics. In this study, we have explored the atomistic‑level details of an SiO₂ and SiN_x ALE process consisting of a hydrocarbon-containing precursor dose, CF_x deposition from a C₄F₈/Ar plasma, and an Ar plasma activation step in which the CF_x film is activated and the underlying substrates are etched. In this study, we used in situ attenuated total reflection Fourier transform infrared (ATR‑FTIR) spectroscopy and in situ four-wavelength ellipsometry during ALE to monitor the surface reactions and film composition as well as the net film thickness during the deposition and etching steps.

Sequential cycles of ALE of SiO₂ show a drift in the etch per cycle (EPC) with increasing cycle number. We attribute the drift in EPC is from excess CF_x that is liberated from the reactor walls in the Ar plasma step. This increase in the EPC occurs even though the infrared spectra confirm that the CF_x deposition onto the SiO₂ film is reproducible from cycle to cycle. To minimize the drift in EPC, Ar plasma half‑cycles of twice the length are employed, which allows for the removal of CF_x from the reactor walls during each cycle, thus creating more reproducible chamber wall conditions.

To further control the EPC, a hydrocarbon precursor prior to the start of ALE retards the EPC. A broad feature centered at ~1,400 cm^-1 builds up on the surface with increasing hydrocarbon dose frequency and cycle number, which is assigned to a carbonaceous film of CH_xF_y. The film acts as a blocking layer which prevents the activation of CF_x at the CF_x/SiO₂ interface and thus limits SiO₂ etching. No graphitic carbon buildup is observed. However, increasing the Ar plasma half‑cycle length limits the buildup of the CH_xF_y film and increases the EPC. Using the same baseline processing conditions as ALE of SiO₂, ALE of SiN_x leads to a carbonaceous film buildup of both CH_xF_y and nitrile species at ~2,225 cm^‑1 which accumulates over cycle number and eventually leads to an etch stop. A longer Ar plasma half-cycle limits the accumulation of the CH_xF_y film and the EPC drift. The addition of a hydrocarbon precursor retards the EPC in a similar fashion as observed on the SiO₂ film.