AVS2015 Session PS+SS+TF-FrM: Atomic Layer Etching (ALE) and Low-Damage Processes II

CMOS devices have continued to scale dimensionally following the implementation of FinFET transistors. Self-alignment of the source and drain contact to the gate has been presented as an integration solution starting at the 22nm technology node¹. This self-aligned contact (SAC) integration creates additional challenges and constraints on the etch process for the 10 nm node and beyond. Due to smaller feature dimensions, lithography overlay, and full contact wrap-around of the transistor fins, unprecedented etch precision is now required.

A novel approach for SAC oxide etching has been developed which addresses the many tradeoffs of this application using a directional atomic layer etch process (ALE) as reported by Hudson et al². Key trade-offs to enable contact etching capability of CDs as small as 10 nm include SAC spacer loss, lack of profile control, and contact not-opens. This cyclic SiO₂ ALE process repeats discrete unit process steps of fluorocarbon deposition and ion bombardment to achieve high selectivity of SiO₂ to Si₃N₄ while simultaneously addressing these tradeoffs. Oxide removal rates can be precisely controlled with minimal removal of Si₃N₄ films, enabling a highly selective etch process. Anisotropic, directional etch behavior superior to traditional SiO2 etch is enabled, creating vertical oxide profiles. This capability is highly desirable for SAC etch applications as it maintains the integrity of the gate electrode spacer during etch. Experimental results showing the ability of this oxide ALE process to eliminate tradeoffs is presented

[1] C. Auth, et al, “A 22 nm high performance and low-power CMOS technology featuring fully-depleted tri-gate transistors, self-aligned contacts and high density MIM capacitors,” accepted in VLSI Symp. Tech. Dig., Jun. 2012.

[2] E. Hudson, et al, “Highly Selective Atomic Layer Etching of Silicon Dioxide Using Fluorocarbons,” accepted in AVS 61^st International Symp. & Exhibition, Nov. 2014.

The Self-Aligned Contact (SAC) process has been widely adopted to achieve aligned narrow contacts between electrodes as the pitch shrinkage has progressed with the miniaturization of devices in high-scale integration. In SAC fabrication, it is important to achieve high selectivity of the interlayer insulator (SiO₂) over an etch stop film (SiN) to improve insulation tolerance between the contact plug and the wiring. Such high-selective etch processes have been realized by depositing fluorocarbon (FC) film selectively on the SiN film by using the composition difference between SiO₂ and SiN with fluorocarbon plasma, which protects SiN surface during SiO₂ etch. In order to minimize SiN loss, a balance of FC film thickness and the penetration depth of the ion energy on SiN become significant [1,2]. When trying to achieve improved SiN loss reduction, reducing ion energy is one effective solution, but a balance of ion energy flux and FC radical flux (E_i Γ _i / Γ _CF) breaks down, resulting in an excess amount of deposition, causing etch stop to occur. This is caused by the limited control margin of the ion energy flux over the FC radical flux ratio using conventional processes.

Atomic Layer Etching (ALE) concept has attracted great attention in recent years for its precise fabrication potential at the atomic level and its ability to solve this issue [3,4]. ALE method enables clear separation of ion energy flux and FC radical flux supply by controlling E_i Γ _i / Γ _CF dynamically using a new parameter of flux ratio respective to step time. Furthermore, specific control of the surface condition at each cycle is expected to be effective for precise fabrication. This report discusses the application of the ALE concept in the SAC process as a Quasi-ALE scheme with consideration for implementation to volume production. This scheme realized a dramatic improvement of SiO₂ etch performance with substantial reduction of SiN loss. Further analysis of the surface condition by XPS, SIMS, HR-RBS helped determine the mechanism of selectivity enhancement. Quasi-ALE technology is a promising weapon, corresponding to leading-edge processes of various fabrication requirements along with the miniaturization of devices, towards 10nm and beyond.

Reference

[1] T.Tatsumi, M.Sekine et al. JVST B 18(4), 2000

[2] M.Matsui, M.Sekine et al. JVST A 19(4), 2001

[3] D.Metzler, G.S.Oehrlein et al. JVST A 32(2), 2014

[4] M. Honda, AVS 61th Int. Symp. & Exhibit. (2014)

Angstrom-level plasma etching precision is required by semiconductor manufacturing for the sub-14 nm technology node. Atomic layer etching (ALE), achieved by a series of self-limiting cycles, can precisely control the amount of reactant available and resulting etching depths. Recently, controlled etching of SiO₂ at the Angstrom-level based on steady-state Ar plasma, periodic injection of a defined number of fluorocarbon (FC) molecules, and synchronized plasma-based Ar⁺ ion bombardment has been demonstrated [1,2]. This novel ALE approach is achieved by deposition of a thin (several Angstroms) reactive FC layer on the material surface using pulsed FC flow. Subsequent low energy Ar⁺ ion bombardment removes the FC layer along with SiO₂ from the surface. The ion energies were selected to allow only chemical enhanced etching to take place without any physical sputtering, which enables a self-limited SiO₂ removal. We report on controlled etching of Si₃N₄ and SiO₂ layers of the order of one to several Angstroms using this cyclic ALE approach. The work was performed in an inductively coupled plasma reactor. Using SiO₂-Si₃N₄-SiO₂ multi-layer stacks on a Si substrate enabled precise evaluation of selectivity, self-limitation, and modification by in situ real time ellipsometry. Si₃N₄ etching and etch selectivity of SiO₂ over Si₃N₄ were studied and evaluated with regard to the dependence on FC surface coverage, precursor selection, ion energy, and etch step length. Surface chemistries of SiO₂ and Si₃N₄ were investigated by vacuum transferred x-ray photoemission spectroscopy (XPS) at each stage of the ALE process. The choice of precursor can have a significant impact on the surface chemistry and therefore the chemically enhanced etching characteristics.

The authors gratefully acknowledge financial support of this work from National Science Foundation (CBET-1134273) and Lam Research Corporation.

References:

[1] D. Metzler, R. Bruce, S. Engelmann, E. A. Joseph, and G. S. Oehrlein, "Fluorocarbon assisted atomic layer etching of SiO₂ using cyclic Ar/C₄F₈ plasma", J Vac Sci Technol A 32, 020603 (2014)

[2] E. Hudson, V. Vidyarthi, R. Bhowmick, R. Bise, H.J. Shin, G. Delgadino, B. Jariwala, D. Lambert, S. Deshmukh, “Highly selective etching of Silicon Dioxide Using Fluorocarbons”; AVS 61st International Symposium & Exhibition (2014);

The requirement for atomic scale etching is becoming more important with increasing miniaturization of semiconductor devices. A novel approach for oxide etching has been developed by Metzler et al [1]. Controlled etching of SiO₂ at the angstrom-level is based on steady-state Ar plasma and deposition of a thin reactive fluorocarbon layer enabled by precise, periodic C₄F₈ injection. High process stability is necessary for the success of this method and its use in mass production. Chamber wall interactions are crucial to the stability of this process. In this research, we studied the influence of chamber wall temperature and chamber wall chemical state on ALE process performance. The experiments were conducted in an inductively coupled plasma system excited at 13.56 MHz. The temperature of the quartz coupling window was measured with an infrared temperature sensor. In situ real time ellipsometry allows for film thickness measurements during the process. Plasma gas-phase chemistry was characterized by optical emission spectroscopy. We conducted the cyclic Ar/C₄F₈ SiO₂ ALE process using different initial chamber temperature, and chamber wall polymer coverage condition. It was found that although the polymer film thickness deposited in each cycle is constant, the etching behavior changed, likely related to a change in depositing species. Chamber wall temperature shows a clear effect on the CO and SiF emission and their relative ratio. When the surface of the quartz coupling window started to be covered by a fluorocarbon film, the overall CO and SiF intensity decreased while the CO/SiF emission peak ratio did not change. The relationship to observed etching behavior will be discussed.

The authors gratefully acknowledge financial support of this work from National Science Foundation (CBET-1134273) and US Department of Energy (DE-SC0001939).

References:

[1]D. Metzler, R. Bruce, S. Engelmann, E.A. Joseph, and G.S. Oehrlein, J Vac Sci Technol A 32, 020603 (2014)

In this presentation, we will describe two plasma technologies that could potentially reach that goal.

A new Technology is the so-called “Thin Layer Etching” technology. In the first step of the TLE technology, H2 or He Ions produced by a capacitive plasma induce modification of silicon based materials while in a second step the modified material is removed in an all dry NF3/NH3 remote plasma that form volatile products with the modified silicon based materials. Performance achieved by TLE for nitride spacer etching will be shown and compared to conventional ICP results.

Fast gas pulsing technology could also be a promising way to form ultra-thin reactive layer during plasma processing, allowing atomic precision etching to be achieved. This concept will be explained and discussed based on preliminary result of silicon etching in chlorine plasmas using atomistic simulation.

In the semiconductor industry, the use of atomic layer etching (ALE) makes it feasible to accurately control the critical dimensions to nanometer level or smaller. In ALE, the target substrate is first exposed to a reactive gas that passivates the surface, which is then followed by ion bombardment with energy below the sputtering threshold. It is critical to precisely control the ion energy and flux during the etching process to remove the topmost layer of the passivated surface without damaging the underlying substrate. Once the passivation layer is removed, the etch process stops. The passivation and etching steps are repeated until one has etched to the desired thickness. In contrast to conventional plasma etch processes, microfabrication using ALE promises high selectivity and low damage to the substrate.

In this presentation, we discuss the properties of ALE using results from molecular dynamics (MD) simulations. The simulation procedure is conceptually similar to those described in previous publications [1,2]. In this study, a crystalline Si(100)-(2x1) or amorphous surface (made by low energy Ar⁺ ion bombardment) was generated and equilibrated at room temperature. The bottom layers were fixed in space, and the periodic boundary conditions were applied laterally to remove the boundary effect. The ions are modeled as energetic neutrals. The surface was passivated by repeated bombardment with low energy Cl atoms at normal incident, which was followed by Ar⁺ or Cl⁺ ion bombardment to remove the passivation topmost layers. The Berendsen scheme is used between ion/neutral impacts to remove the energy from the surface region and cool the surface layer to room temperature. The Stillinger Weber (SW) type potentials are used for Si-Si, Si-Cl and Cl-Cl interactions. The Ar-Si and Ar-Cl interactions were modeled using Moliere potentials. The leap-frog form of Verlet algorithm was used to numerically integrate the Newton’s equation of motion. The MD is applied to study several variants of the ALE process. The fundamental properties of Si etching are also investigated for both bare and Cl-passivated Si surfaces with several ions including Ar⁺, Cl⁺ and Cl₂⁺. These fundamental studies are used to interpret our layer-by-layer ALE experiments in our laboratory.

Reference

[1] N. A. Kubota, D. J. Economou and S. J. Plimpton, J. Appl. Phys. 83, 4055 (1998).

[2] B. A. Helmer and D. B. Graves, J. Vac. Sci. Technol. A. 16, 3502 (1998).

III-N quantum dots (QDs) gain media have generated great interest because of their desirable properties such as low threshold and temperature independence due to the discrete nature of the density of states. A uniform and high-density two-dimensional (2D) array of an isolated QD structure is required when considering applications in visible wavelength such as white LED. In general, size distribution, uniformity, and high-density are trade-offs when using a conventional self-assembly method; therefore, we have developed a technique that integrates a bio-template with neutral beam etching (NBE) process.

In this work, quantum nanodisks (QNDs) were fabricated from InGaN/GaN single quantum well (SQW) by using a bio-template and NBE. We developed a damage-less, top-down fabrication process for achieving high density of QNDs such as 2 x 10¹¹ cm^-2 embedded in 10 nm in diameter and 20 nm high nanopillars. The fabricated QNDs have great potential for fabricating quantum optoelectronic devices because of controllable diameter and thickness.

The InGaN/GaN SQW wafer was grown on a 2-inch c-plane sapphire substrate by metal-organic vapor phase epitaxy (MOVPE). The structure consisted of a 1μm-thick GaN buffer layer, 3nm-thick In_0.1GaN and a 10nm-thick GaN capping layer. We used ferritins modified with polyethylene glycol (PEG ferritins) that include a metal oxide core for the etching mask. Oxygen annealing in vacuum was used to remove the ferritin protein shell at 350˚C, at chamber pressure of 32 Pa. Therefore the 7 nm diameter iron core was remained on the surface. Then hydrogen radial treatment, hydrogen passivation and NBE etching process were performed. At first, hydrogen radical treatment was realized to remove the surface oxide at chamber pressure of 32 Pa at 350˚C. Subsequently, hydrogen passivation was done to avoid any re-oxidation during the process. Finally, SQW was etched completely to form nanopillars using 40 sccm Cl₂ at a chamber pressure of 0.1 Pa, with a substrate temperature of 100˚C, ICP power of 800 W and bottom electrode bias power of 10W. As a result, InGaN/GaN 10 nm in diameter and 20 nm high nanopillars could be fabricated. The etching profile was confirmed by Transmission electron microscopy (TEM).

After etching, we measured the photoluminescence (PL) and time-resolved PL (TRPL) to observe the quantum confinement energy levels. According to the PL measurements, we found an energy shift of 1.25 eV, from 2.9eV for SQWs to 2.75eV for QNDs. Although these measurements are still on-going now, we will clearly analyze and discuss the phenomena related to this shift in energy in the near future.

With lower E_i and controllable reactivity, these plasmas are promising to etch sub-nm-thick stacked materials. However, the interactions between reactive plasmas and surfaces are so complex that the efficient development of new processes can require numerical simulations. Therefore, we develop Molecular Dynamics (MD) simulations to understand the impact of various plasma technologies on the interactions between ultrathin Si films and Cl₂ plasmas under a wide range of plasma conditions. They help to understand the precise role of E_i in plasma-surface interactions, the relationship between the flux/energy of reactive species bombarding the surface and its structural/chemical modifications.

In this study, MD simulations - coupled with experiments - are performed to quantify modifications (plasma-induced damage, etch rate) of Si films after exposition to various Cl₂ plasma conditions, simulated by bombarding the substrate with both ion and neutral species. All simulations show the formation of a stable SiCl_x reactive layer and a constant etch yield (EY) at steady state. The key plasma parameter to control the etching of ultrathin Si layers is E_i, which lowers both the damaged layer thickness and EY when it is decreased. The neutral-to-ion flux ratio (Γ) is the 2^nd key parameter: its increase reduces the damaged layer thickness while the etch rate grows. While maintaining Γ constant, the neutral dissociation rate and the ion composition do not influence significantly the etching process. Etching simulations of a simple Si pattern are then compared to the etching of blanket silicon, focusing on phenomena such as ion channeling, passivation/damage of pattern sidewalls and top pattern erosion.