Effect of carbon enrichment induced by photoresist on highly selective SiO2 etching

Abstract

We propose two models to discuss the behavior of the selective etching of SiO2 to the underlying Si3N4 with changing wafer surface temperatures. For this investigation, three specimens, SiO2, Si3N4, and poly-Si, which are nonpatterned, photoresist-patterned, and poly-silicon-patterned, respectively, have been etched in a surface wave plasma system equipped with an electrostatic chuck for wafer temperature control. The coolant temperature, which controls the wafer temperature, has been changed from −20 to 50 °C. For the nonpatterned wafer, the etch rates of SiO2, Si3N4, and poly-Si increase and the selectivities decreases with wafer temperature. However, for the samples patterned with either photoresist or poly-Si, the etch rates of SiO2 decrease with wafer temperature. The temperature rise also leads to an enhancement of selectivity of SiO2/Si3N4, and the steeper profile angles. The presence of a masking layer, even for the poly-Si-patterned samples, results in a different etching behavior. This is because the sticking probability of the polymer precursor becomes smaller on the sidewall of the profile with the temperature increase. Therefore the thickness of polymer on the sidewall of the contact hole decreases, and the thickness of polymer on the bottom increases as the wafer temperature goes up. Comparing photoresist-patterned samples with poly-Si-patterned ones, we can corroborate the role of the photoresist mask layer, which provides a higher carbon-to-fluorine ratio at the near surface. The carbon enrichment accelerates more steeply the etch rate decrement of the substrate layer. In summary, there are two main contributions attributed by the substrate temperature: changing the sticking coefficient of the fluorocarbon precursor and enhancing the photoresist erosion.