Evaluation of the effectiveness of H2 plasmas in removing boron from Si after etching of HfO2 films in BCl3 plasmas

Abstract

Etching of high dielectric constant (“high-k”) materials in BCl3-containing plasmas leaves behind a boron residue on underlying Si or SiO2 surfaces during the over-etching period. Boron is a p-type dopant and therefore it is best if it is removed prior to subsequent processing. We have investigated the effectiveness of H2 plasmas in removing this boron-containing layer. Following etching of HfO2 thin films in a high-density BCl3 plasma, including a 60 s overetch period, samples were transferred under vacuum to a UHV chamber equipped with x-ray photoelectron spectroscopy (XPS). After observing areal density B-coverages of ∼1×1015cm−2 (equivalent of ∼1monolayer), the samples were transferred back to the plasma reactor for exposure to the H2 cleaning plasma, and then reexamined by XPS. Optical emission spectroscopy was used to monitor B deposition on and removal from the plasma chamber walls. B deposition on the reactor walls during BCl3 plasma exposure reached saturated coverage in ∼2min. Following this, the H2 plasma removed half of the B on the walls in 90 s and 90% in 320 s. B was rapidly removed (<5s) from BCl3 plasma over-etched Si surfaces provided that the walls were first cleaned in the H2 plasma, with the Si sample held in the UHV chamber during the chamber cleaning process. Conversely, it took much longer (∼180s) to remove all detectable B on the sample surface if the sample and the reactor chamber walls were cleaned in the H2 plasma at the same time. Apparently B is transported from the walls to the Si surface in this latter case. Etching rates of 22 and 1700Å∕min were measured for SiO2 and Si, respectively, in the H2 cleaning plasma. The fast etching of Si allows still shorter cleaning times, or preferably dilute H2 plasmas to be effective in removing B with less Si removal, provided the chamber walls are first cleaned in an H2 plasma.