Switching of phase change memory (PCM) materials between crystalline and amorphous phase with electrical pulses and optical properties make it an important candidate for storage class memory and neuromorphic computing. However, PCM materials can be sensitive to air exposure during integration, therefore in-vacuo RIE and encapsulation is important to provide the required oxygen diffusion barrier. Low temperature SiN deposition can be used for low thermal budget integration schemes provided a good film conformality is achieved and damage or etching to the PCM elements is mitigated. In this work, ammonia- (NH3-) free, plasma enhanced chemical vapor deposition (PECVD) SiN films deposited at 40°C (microwave plasma) and 200°C (inductively coupled plasma), are compared and wet etch rates and optical properties are evaluated. NH3-free SiN films were deposited using SiH4, N2, H2, and Ar as source gases. Tuning the plasma parameters during encapsulation we observed simultaneous selective etching of GST and controlled SiN film deposition. Hydrogen and argon addition to the plasma mixture provided the main control knob for in-situ GST trimming during deposition, avoiding any type of elemental or structural damage to the GST films.
Phase Change Memory (PCM) materials can be damaged during plasma exposure leading to changes in phase transition behavior. Etch-induced damage and crystallization properties of GeSbTe (GST) were evaluated as a function of substrate temperature, plasma chemistry, and plasma exposure time. Enhanced damage formation is related to selective elemental depletion and non-volatilized etch residue retention in the near surface region. These experiments validate literature findings that crystallization time increases with reduction in film thickness for GST samples capped with a thin SiO2 film, indicating the presence of a modified layer which serves as an interface layer material. A direct comparison of passivating properties of hydrofluorocarbon and hydrocarbon on GST can be more conclusive with a fine tuning of film thickness and an evaluation of total residue retention with depth profiling.
Plasma etch residue formation and its removal from silicon nitride (SiN) films deposited at 200ºC, 480ºC and 700ºC is explored. X-Ray Photoelectron Spectroscopy (XPS) measurements showed that SiN contains more nitrogen (N) and less oxygen (O) with increasing deposition temperature. SiN films were etched in an Inductively Coupled Plasma (ICP) reactor in a halogen/hydrofluorocarbon (H:HFC) gas mixture; the carbon (C) containing species in the resulting residue films were studied as a function of the H:HFC ratio in the plasma. Post-plasma etch cleaning methods of the SiN surface were compared, these included: wet treatment with diluted hydrofluoric acid (dHF), sputtering with argon (Ar) plasma, and combined dHF and Ar plasma. After etch, Secondary Ion Mass Spectroscopy (SIMS) and XPS data showed formation of fluorocarbon (FC) films on SiN. FC film thickness after etch was estimated from XPS to reach up to 2 nm. Ultimately the SiN etch rate was shown to drop with increasing deposited C thickness while the lower nitrogen content in the SiN film (i.e. 200ºC) led to higher etch rate, which is in good agreement with literature. Ar plasma sputter turned out to be the most effective way of cleaning C residues: C surface content after Ar sputter was reduced to or below the reference data (unetched sample). In terms of wet treatment, an optimized chemistry was identified (AltChem) and post-RIE cleaning was more efficient than dHF in reducing C surface concentrations.
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