Effect of SF₆ and C₄F₈ Flow Rate on Etched Surface Profile and Grass Formation in Deep Reactive Ion Etching Process

Deep reactive ion etching (DRIE) is one of the most important methods of etching silicon and involves numerous interdependent parameters that affect the resulting etching quality and device functionality. Some of the issues in the etching process include bottom grass formation, notching effects, and isotropic etching. The influence of SF₆ and C₄F₈ flow rates, as well as etching and passivation cycle duration, on the etched sidewall profile and grass growth is discussed in this paper. The combination of a 300 sccm SF₆ flow rate and a 100 sccm C₄F₈ flow rate produces a flat bottom surface with no grass formation. The creation of grass begins in the corners of the structures as the SF₆ flow rate increases while the C₄F₈ flow rate remains constant (100 sccm). The flow rate of C₄F₈ was varied from 50 to 200 sccm (with SF₆ at 300 sccm), with lower flow rates affecting the profile of the etched sidewalls and higher flow rates causing grass development. The etch/passivation cycle ratio was also discovered to be important in preventing grass growth. The clamping or loading of the 4-inch wafer had cooling issues that affected the surface profile of the structures and demonstrated that insufficient cooling of the substrate during etching led to the production of isotropic etching of structures and resolved using lubricant media for proper heat dissipation. The process recipes were optimized to achieve flat smooth bottom surface and straight vertical sidewalls with no defects.