Investigating the Metal-Plasma Interaction during the Patterning of MgZnO Alloys, Used for Compute and Memory Applications
Authors: Ghorbani Leila , Kundu Shreya , Lazzarino Frederic , De Gendt Stefan 
Published: February 2024

Magnesium Zinc Oxide (MgZnO) is a novel metal alloy being considered for advanced memory applications. When used as a dielectricum in transistors in dynamic random-access memory (DRAM) [1,2] or core elements in resistive random-access memory (RRAM) [3,4], MgZnO exhibits the potential to display low cutoff current [Ioff], high mobility [3], and low processing temperature [1]. Therefore, minimal damage-inducing patterning of the alloy for use in electrical nanoscale devices is needed.
During the etching process, MgZnO film may be chemically and physically damaged, impacting its electrical performance. Changes in chemical composition and diffusion of etchants into the material are some types of chemical damages observed during etching. These can impact carrier mobility within the material. Physical damage consists of modified roughness, profile distortion, and redeposition on the pattern sidewalls. These are all critical issues because carrier transport in transistor channels is significantly affected by interface roughness between the channel and gate insulator [5]. The etch process developed should preserve the chemical composition and roughness in MgZnO and provide good selectivity when patterning other layers of the stack and hard mask (HM). There are three methods for dry etching consisting of physical, chemical, and physical-chemical mechanisms [6]. Pure physical etching is not selective and pure chemical etching can be isotropic. Reactive ion etching (RIE) is used in this study to generate an etching mechanism that can provide an optimal combination of chemical and physical etching [6].
In this work, we carried out the etch investigation of MgZnO film using an RIE-transformer coupled plasma (TCP) system. 10 nm MgZnO films are deposited on a 50 nm SiO2 layer/Si substrate. The impact of different gas mixing ratios, RF power, DC substrate bias, and process pressure are studied. The etching of MgZnO thin films is carried out using Cl2/CH4/Ar mixture. Cl2 increases chemical reactivity by creating MgCl2 [7] and ZnCl2 [8] by-products that are easier to remove than breaking Mg-O and Zn-O bonds from the films. Moreover, the addition of CH4 produces organometallic volatile etch by-product – Zn (CH3)2 (boiling point: 46°C) [8]. Therefore, MgZnO is to be etched partly by physical and chemical mechanisms. These hypotheses are studied in detail by carrying out in-depth X-ray photoelectron spectroscopy (XPS) based composition analysis on individual ZnO and MgO surfaces subjected to the aforementioned chemistry (Fig.1(a, b)). In addition, by increasing the voltage and power, the etching performance of MgZnO is further improved due to increasing ion energy and the presence of more active species in the plasma via higher dissociation and ionization, respectively (Fig.1(c, d)). These investigations should enable a more systematic patterning study of MgZnO-based features at different scaled dimensions and densities.
References
[1] J. Hwang, et al, Materials Science and Engineering: B, 266 (2021) 115063.
[2] J. Y. Li et al. Materials, 10(2) (2017) 126.
[3] Z. Zhang et al., Materialia, 15 (2021) 101001.
[4] X. Chen et al., Applied Physics Letters, 104 (2014) 043508.
[5] M. Geiger et al., Advanced material interfaces, 7(10) (2020) 1902145.
[6] S. J. Pearton, et al, Plasma processes and polymers, 2(1) (2005) 16-37.
[7] K. H. Baik et al., Applied Surface Science, 183 (2001) 26-32.
[8] J. S. Park et al., Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 21 (2003) 800.

Figure 1