Study of Copper Surface Preparation by Sequential Atomic Layer Wet Etching and Laser Annealing Treatments
Authors: Iwasaki Akihisa , Akanishi Yuya , Mazzamuto Fulvio , Kesters Els , Le Quoc Toan , Holsteyns Frank 
Published: February 2020

Atomic scale surface preparation such as atomic layer etching, cleaning, modifying and coating becomes increasingly important for device scaling at sub-10 nm technology nodes in order to meet minimized process variations and its highly selective process requirements [1-2]. In the fully self-align via process, a topography is created by recess etching the underlying metal (copper) layer using chemical mixtures. In addition to a high compatibility with the materials exposed, one of the important requirements is to control copper surface recessing such that the change in its roughness is kept at a minimum. The fundamental understanding and control of the polycrystalline surface reactions and interactions at the atomic scale are needed. In this paper, we will demonstrate the copper recess process for atomic layer wet etching (ALWE) and combination with laser thermal annealing (LTA) for surface smoothening. For copper recess, hydrogen peroxide (H2O2) and an acid (HF, aqueous hydrogen fluoride) are used at room temperature in a two-step wet etching cycle to remove 1 nm of copper oxide. In the first step of the cycle, copper is oxidized by diluted H2O2 to form a copper oxide layer that is self-limited to a thickness of 1 nm for processing time ranging from 5 to 120 s. The second step of the cycle consists of removing this oxide layer with diluted HF with a controlled dissolved oxygen concentration <50 ppb on a wet cleaning tool. Under these conditions, unoxidized copper is not etched [3]. However, polycrystalline copper is etched preferentially along the grain boundaries, which resulted in an increase of surface roughness. These two steps are repeated until the desired etch depth is obtained. For surface smoothening process, excimer laser thermal annealing is performed. This technology has garnered great interest in semiconductor processing as it enables ultrafast annealing with very limited thermal budgets [4]. The process enables heating, melting and subsequent solidification of copper surface resulting in grain boundary repairing by crystalline regrowth without changing copper crystallinity structure Cu (111) neither the low-k characteristics [5]. The copper phase formation was linked to laser energy density indicating there is an optimum process window for that material system. It will be shown that an optimized ALWE can enable self-limited Cu oxide formation and highly selective oxide etching to unoxidized Cu, and the following LTA treatment can produce a uniform lower copper resistivity with a remarkably smoother surface.
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