Paper PS+EM-WeM1
Study of High Selective Silicon Nitride Etching Mechanisms in Remote Plasmas: Impact of Wafer Temperature

Wednesday, October 24, 2018, 8:00 am, Room 104A

Nowadays in the Semiconductor industry, challenging applications often requires ultra-high selectivity etching processes. Wet processes are often used but have drawbacks and show limitations in high aspect ratio features. One alternative possibility is to use chemical downstream etching plasmas. In this work, NF₃/O₂ downstream plasmas are used to etch selectively Si₃N₄ towards SiO₂ in high aspect ratio patterns (over 100).

In NF₃/O₂ plasmas, we observe that the wafer temperature (T°) has a considerable (but non linear) impact on the etching selectivity. When T° is raised from 40°C to 100°C, the selectivity first drop and then increase again, with a marked minima at 70°C. Indeed, the etching rate of Si₃N₄ and SiO₂ have a different behavior with T°: while the SiO2 etch rate increase slowly and continuously with T°, the Si₃N₄ etch rate first drop between 40 and 70 °C and then increases again at higher T°. This effect is attributed to two mechanisms in competition, the etching led by atomic fluorine and surface passivation via oxidation. To better understand the nonlinear behavior of the Si₃N₄ etch rate, the etching mechanisms of Si₃N₄ as a function of T° was investigated by plasma (VUV absorption spectroscopy) and surface diagnostics (Ellipsometry,XPS and AFM).

Angular XPS analysis show that the Si₃N₄ surface oxidation is minimal at low T° (40°C). As the wafer T° is increased, the thickness of the oxidized layer also increases rapidly until it reaches its maximum (about 5 nm) at 70°C. This is attributed to an enhanced diffusion of the O atoms produce by the plasma in the Si₃N₄ material. At higher T° the thickness stays constant but the amount of O in the layer decreases. In the NF₃/O plasma, atomic fluorine are responsible for the etching of both Si3N4 and SiO2 but with a natural selectivity. Therefore, the surface oxidation of the Si₃N₄ surface during etching is going to slow down the nitride etch rate and the thicker this layer is the smaller the etch rate will be (F atoms must diffuse through this layer to reach Si₃N₄). Therefore, when the T° is increased the Si₃N₄ etch rate initially drops because the SiO_x layer at its surface becomes thicker. Above 70°C the layer thickness stays constant but its degree of oxidation is decreasing when T° is increased: this explain why the Si₃N₄ etch rate increases again above 70°C. Hence the nonlinear behavior of the selectivity is due to a competition between the etching (by atomic fluorine) and surface oxidation, which strongly depends on T°. We highlighted via our research an important change on the etching mechanism at 70 °C, explained by the rapid formation of a thick oxidized layer.