Dry Etching of Silicon is not just Trench Making
Deep Reactive Ion Etching (DRIE, or the Bosch process) enables the fabrication of high aspect ratios by repeating deposition and etching cycles. Every cycle can be broken down into three stages: (i) isotropic polymer deposition (driven by neutral radicals), (ii) strongly-directional polymer removal (driven by charged ions accelerated vertically), (iii) ion-driven (vertical) + radical-driven (isotropic) silicon etching.
Due to the action of the ions, the polymer etch rate (rpol) and the vertical component of the silicon etch rate (rsi,v) are essentially independent of the aspect ratio since the ions propagate mostly along the vertical direction due to the electric field.
Due to the action of the neutrals, the polymer deposition rate (dpol) and the isotropic component of the silicon etch rate (rsi,iso) are attenuated with increasing aspect-ratio since the radicals must diffuse randomly into the cavities. Overall, the etch rate per cycle is a complex function of dpol, rpol, rsi,v. Thus, in practice DRIE is difficult to understand and limited by the following three features:
Macroloading: The etch rate is inversely proportional to the total amount of exposed wafer area. It is due to the consumption of the etchant and its slow transport from regions far above the mask level. This can be accounted for by simply correcting the etch rate using the formula shown below image.
Microloading: Dependence of the etch rate on the density of features. Equally wide trenches located nearby are etched less deeply than similar trenches located farther apart. It results from the development of a lower concentration (C) of the etching species at feature-dense areas (e.g. Ca< C). This is due to increased local consumption of the etching species at the mask level and their slow replacement by diffusion transport from nearby regions.
Aspect Ratio Dependent Etching (ARDE = lag effect): The etch rate increases with the trench width. For a given trench, it decreases with etching time as the aspect ratio increases. It is due to concentration differences across wide/thin trenches (e.g. cP < c). This is due to larger consumption of the etching species at the perimeter surface of larger trenches.