Abstract
In high aspect ratio structures such as 3D NAND stacks, high-k oxides or certain metals need to be removed from sidewalls and even non-line-of-site locations where access via reactive ion etching would be challenging or impossible. To achieve etching in these locations, thermal processes need to be applied that lack any intrinsic directionality and are therefore isotropic. Thermal Atomic Layer Etching is one option to facilitate this kind of process. Therefore, it has gathered significant attention recently and is now being explored in academia and industry.
In this work, we demonstrate isotropic Atomic Layer Etching on metallic molybdenum using a three-step process that involves oxygenation of the metal surface, a conversion reaction using boron-trichloride followed by a hydrogen-fluoride clean step. We measured the etch rate per cycle to be around 10 Å with low temperature dependence in the range between 150°C and 250°C.
Furthermore, we examined the effectiveness of various oxygenation methods on the surface of metallic molybdenum films. These methods included thermal oxygenation as a function of temperature, oxygenation by a remote plasma and in-situ plasma fluorination. We established that an oxygenation level of more than 70% as measured with XPS is needed in order for the ALE process to work. Levels in this range could be achieved via in-situ plasma oxygenation as well as with a remote plasma. Thermal oxygenation, on the other hand, required substrate temperatures in access of 400°C to oxygenate at the 70% level.
In this study we also show an example of molybdenum ALE in an application that involved the removal of this metal from a 50:1 aspect ratio 3D NAND structure. We measured etch rates on vertical and horizontal surfaces as well as the depth dependence in removal rate of molybdenum.