A Selective Etching Process for Chemically Inert High-k Metal Oxides
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A Selective Etching Process for Chemically Inert High-k Metal Oxides Katherine L. Saenger, Harald F. Okorn-Schmidt, and Christopher P. D'Emic IBM Research Division, T.J. Watson Research Center, Yorktown Heights, NY 10598, U.S.A. ABSTRACT Once annealed, high-k metal oxides such as HfO2 and Al2O3 can be extremely difficult to etch by wet chemical methods. Here we describe how ion bombardment at relatively low energy (a few hundred eV) can be used to make exposed regions of annealed HfO2 films etchable in aqueous HF-based solutions. HfO2 layers, 2-5 nm in thickness, were deposited by atomic layer chemical vapor deposition (ALCVD) on Si substrates, annealed at 700 oC, and subjected to selected-area ion bombardment supplied by an oxygen plasma in a reactive ion etching tool. Etch times (as indicated by time to "dewet") were examined as a function of HfO2 thickness, the power and time of oxygen plasma treatments, post-oxygen-plasma anneals, and wet etch chemistry. Strategies for etching thicker films and additional data provided by electrochemical open circuit potential (OCP) measurements will also be discussed. INTRODUCTION High-k metal oxides such as HfO2 and Al2O3 are being investigated as gate dielectrics for advanced CMOS devices due to the expected limitations of SiO2 at equivalent oxide thicknesses (EOT) less than 1 nm. However, integration of these metal oxide films into devices requires the existence of selective and controllable patterning processes. Conventional wet etches typically have no effect on these films, which are extremely chemically inert once they have been annealed, and reactive ion etching (RIE) processes, while effective, typically lack the selectivity required to prevent damage to underlying layers (such as silicon). The etch method described here is shown schematically in Fig. 1. It comprises two basic steps: a selectively applied ion (or other charged particle) bombardment process to "damage" the metal oxide (MOx) film followed by an aqueous HF-based wet etch to selectively remove the regions of the film that have been damaged. For this work, the ion bombardment is provided by a relatively mild oxygen plasma treatment whose effects are largely localized to the top ~3 nm of the bombarded surface, a thickness roughly comparable to the thickness of the film being etched. Blanket MOx
Selected-area ion bombardment
Damaged MOx
Patterned MOx
Si
A. DAMAGE
B. WET ETCH
Fig. 1. Schematic of etch process showing damage (A) and wet etch (B) steps. It has long been known that ion implantation (I/I) damage can increase the wet etch rate of SiO2 films [1, 2]. Recently this approach has been used to enable the etching of chemically
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inert HfO2 and Al2O3 metal oxide films [3, 4]. However, the low energy, plasma-based ion bombardment of the present work may often be preferred to the higher ion energies typically used for implantation if (i) the metal oxide films are thin (i.e., of a thickness comparable to the "damage depth") and/or (ii) the ion implantation would induce unacceptable subs
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