AVS 69 Session PS2-ThA: Plasma Modeling and Characterization

With the recent advancements in semiconductor processes reaching the nanoscale, research is underway to enhance the uniformity of plasma in plasma etching reactors. The non-uniformity of the etch process is noticeable at the edge of the wafer due to inhomogeneous electrical characteristics. We investigate a mechanism to control the plasma sheath above the wafer edge for a uniform etching process over the dielectric focus ring by changing the electric field and ion flux uniformly across the wafer surface using a two-dimensional particle-in-cell simulation parallelized with a GPU. An appropriate waveform on the electrode inserted inside the focus ring changes the sheath oscillation and ion flux to improve the ion energy and angular distributions (IEADs) to achieve a better etch rate.

In recent years, several countries and semiconductor manufacturing companies have announced targets for net-zero carbon emission by 2050. Plasma etch processes are responsible for a high percentage of emission for chip manufacturing especially in high aspect ratio (HAR) dielectric etch. Such an etch process involves employment of high global warming potential (GWP) fluorocarbon gasses such as C4F8, CH2F2. In the past decades, Air Liquide R&D has developed multiple alternative chemistries in HAR applications with very low GWP. However, while the gas entering the plasma etch chamber may be high or low GWP, it is difficult to predict the emission gases post plasma due to the complexities of the breakdown and recombination processes within the plasma.

In a Capacitively Coupled Plasma (CCP) discharge, the processing rates and uniformity of the wafer depend on key parameters such as the ion flux, ion energy distribution function (IEDF), and plasma homogeneity. The non-Maxwellian nature of the IEDF requires a kinetic treatment, which can be achieved with Particle-In-Cell (PIC) simulations.

In this work, we develop a procedure to control the plasma uniformity and its dynamics using a weak magnetic field with a 2D cylindrical axisymmetric model. The present investigation takes leverage of PIC modeling and uses the explicit EDIPIC-2D code [1]. A detailed analysis of the sheath structure, the ion flux, and IEDF at the wafer will be performed. Additionally, we will also report recent examples and progress regarding 3D PIC modeling using the in-house LTP-PIC code [2].

¹ https://github.com/PrincetonUniversity/EDIPIC-2D

² T. Charoy, et al, “2D axial-azimuthal particle-in-cell benchmark for low-temperature partially magnetized plasmas,” Plasma Sources Sci. Technol. 28(10), 105010 (2019).

Acknowledgments:

This Research was funded by the US Department of Energy through CRADA agreement with AMAT.

Effect of feed gas content and substrate temperature on RIE of SiN_x with Ar/C₄F₆/O₂plasma

Xue Wang¹, Ryan J. Gasvoda², Eric A. Hudson³, Prabhat Kumar *³, Sumit Agarwal *¹

¹Colorado School of Mines, Chemical and Biological Engineering, Golden, CO, USA

²Lam Research Corporation, Tualatin, OR, USA

³Lam Research Corporation, Fremont, CA, USA

In reactive ion etching (RIE) with fluorocarbon-based plasmas, the etch selectivity of SiO₂ relative to SiN_x is controlled by tuning the F to C ratio in fluorocarbon feed gas, and by tuning the ion energy. Previously, we showed that the substrate temperature is one potential process knob to control the etch selectivity for SiO₂ over SiN_x during C₄F₆/Ar plasma RIE. Our in situ optical diagnostics show that increasing the substrate temperature during RIE of SiN_x from 70 to 120 °C creates an etch-stop layer and lowers the etch rate of SiN_x, with no noticeable effect over the same temperature range during RIE of SiO₂. In situ infrared spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS) show that the fluorocarbon layer on the SiN_x surface is more defluorinated at 120 °C, likely forming a graphitic etch stop layer which lowers the etch rate.

In this work, we explore the substrate temperature dependence on RIE of SiN_x as a function of O₂ dilution of a C₄F₆/Ar plasma. Using in situ four-wavelength ellipsometry, we measured the steady-state etch rate of SiN_x over 200 s. In our experiments, we varied the O₂ to C₄F₆ flow ratio over the range of 0.29 to 1.75. Consistent with previous studies, the etch rate of SiN_x increased with increasing O₂ to C₄F₆ flow ratio in the feed gas at both substrate temperature of 70 and 120 °C. High-resolution spectra of the C 1s and F 1s regions were measured for SiN_x surfaces after 200 s of RIE with ex situ XPS for O₂ to C₄F₆ flow rate ratios of 0.29, 0.75, and 1.75. Analysis of the XPS data shows that addition of O₂ results in a thinner CF_x layer, which enhances the etch rate of SiN_x. More interestingly, we found that the etch rate of SiN_x is higher at 120 °C than that at 70 °C at low O₂ to C₄F₆ ratios (< 1.25). This temperature dependence flips for O₂ to C₄F₆ ratios >1.25. We speculate the thickness of CF_x layer and mixing layer are influenced by the substrate temperature, which leads to this temperature dependent etch behavior.

Oral Presentation Requested

In semiconductor and display manufacturing, Perfluorocarbon (PFC) gases are widely used for cleaning post-etching and deposition. With the adoption of advanced patterning like Double Patterning Tech (DPT) and Quadruple Patterning Tech (QPT), PFC gas consumption is rising. However, PFC gases are chemically stable, leading to a high Global Warming Potential (GWP). Reducing PFC gas emissions is essential due to their long-lasting global climate impact.

Transitioning to alternative gases requires understanding ion and active species distribution and plasma density, critical factors dependent on the plasma source. This study focuses on the comprehensive characterization of ion and active species in C4H2F6-Z gas and compares it to the conventional process gas, CHF3. Additionally, an essential plasma parameter, plasma density, is measured using a cut-off probe. Experiments in uniform chambers with Capacitively Coupled Plasma (CCP) and Inductively Coupled Plasma (ICP) sources reveal changes in ion and active species with power, pressure, gas ratio, and pulsing frequency.

Comparative data between the conventional and new gases are discussed. This research contributes to the development of alternative precursors to reduce the impact of global warming.

In the manufacturing processes of 3D NAND, the high aspect ratio contact (HARC) etching process, which is one of the most critical steps, has encountered a significant challenge. HARC, typically performed at room temperature, has become increasingly difficult to achieve the desired high aspect ratio while maintaining high productivity. This challenge is expected to become harder when considering devices with highly stacked alternating layers of silicon-containing materials, such as SiO₂ and SiN. Therefore, cryogenic HARC technology has emerged as a promising solution to overcome this challenge, as it offers advantages in terms of productivity and better etch profile. Consequently, we conducted cryogenic aspect ratio etching of SiO₂ at CF₄/H₂/Ar plasma in a cryogenic reactive ion etch system. Overall, our results revealed that cryogenic aspect ratio etching of SiO₂ showed a higher etch rate and a higher aspect ratio under the experimental conditions. With these conditions, we conducted the cryogenic aspect ratio contact etching of SiO₂ for the comparison with the etching of SiO₂ at RT as well.