Modeling and Optimization of Wet ALE Process
Authors: Bassett Derek , Abel Kate 
Published: December 2024

Atomic layer etching (ALE) has become an important process for semiconductor device manufacturing due to its unique ability to control surface etching with single molecular level control. ALE first began in the laboratory over thirty years ago, and in the past ten years it has matured into an important part of the semiconductor manufacturing process with several solutions available on the market for high volume manufacturing.
Most ALE processes have been developed in a vapor or rarified gas environment, often using a thermal or plasma process to facilitate the required chemical reactions. (1) In recent years there has also been work on digital etching with liquid chemistry, including ALE processes for copper, cobalt, ruthenium, and noble metals (2-5).
With wet ALE processes relatively unexplored compared to the longer studied vapor phase processes, there has not been much work done to understand how the interaction of fluid flow and the chemical reaction kinetics can affect a wet ALE process. Digital wet etch processes generally require two different solvents for the ligand deposition/adsorption and subsequent etching steps, and it’s important to minimize any mixing of the two liquids: otherwise the etch process ceases to be digital and instead becomes a continuous etch process, with resulting problems in roughness and local nonuniformity.
In this paper we will present results of a series of simulations to model the sequentially switching liquid flow and surface reactions of an ALE process: both in a simplified flow cell system and in center dispense over a spinning wafer.
The ALE reactions are modeled assuming Langmuir isotherms: one for the ligand deposition and another for the subsequent etching step. The kinetic equation for a pseudo first order heterogeneous surface adsorption isotherm is as follows:

dq/dt = k(qe
- q)
Where q is the surface adsorption density, t is time, k is the reaction rate constant, and qe
is the equilibrium concentration of the adsorbate. qe
is determined using the standard Langmuir isotherm:

qe = q∞
(K cA
/(1 + K cA
))
Where q∞
is the maximum possible surface adsorbate concentration, cA
is the concentration of adsorbate in the solvent, and K is the Langmuir adsorption equilibrium constant. A similar set of equations are used for the desorption in the etching step, with different values for the rate constant k and equilibrium surface density qe
.
An example of the spinning wafer liquid dispense simulation is shown in Figure 1. This is an axisymmetric simulation, where liquid is being continually dispensed at 1 L/min onto the center of a wafer spinning at 1000 RPM, with the liquid switching to a different solvent at t = 0. The figure shows the volume fraction in the liquid film at 0.1s as the displacing fluid flows from the center of the wafer at the left to the edge of the wafer at the right. The volume fraction at the bottom boundary, which represents the wafer surface, shows that the transition from one solvent to another is not instantaneous, and that there will be some mixing between the two.
Our analysis and results will also include dimensional analysis of the chemical reaction and fluid flow system, showing how parameters like reaction rate constants, spin speed, dispense flow rate, flow velocity, and solvent switching frequency can affect the etching performance in terms of etching uniformity and surface roughness for any generalized wet digital etch system.

Kanarik, K. J.; Lill, T.; Hudson, E. A.; Sriraman, S.; Tan, S.; Marks, J.; Vahedi, V.; Gottscho, R. A. Overview of Atomic Layer Etching in the Semiconductor Industry. JVST A 2015, 33 (2), 020802.

Netzband, C.; Arkalgud, S.; Abel, P.; Faguet, J. Wet Atomic Layer Etching of Copper Structures for Highly Scaled Copper Hybrid Bonding and Fully Aligned Vias. In 2022 IEEE 72nd Electronic Components and Technology Conference (ECTC); 2022; pp 707–711.

Abel, P. Wet Atomic Layer Etching Using Self-Limiting and Solubility-Limited Reactions. US10982335B2, April 20, 2021.

Abel, P. Methods for Wet Atomic Layer Etching of Ruthenium. US11802342B2, October 31, 2023.

Abel, P. Methods for Wet Etching of Noble Metals. US20230121246A1, April 20, 2023.

Figure 1