Ultra Deep Reactive Ion Etching of High Aspect-Ratio and Thick Silicon Using a Ramped-Parameter Process

This paper reports an advanced deep reactive ion etching (DRIE) process for realizing ultra-deep (>500- $\mu \text{m}$ ) and ultra-high aspect-ratio (UHAR) silicon structures (AR > 40 for 1-mm through-trench etch, $\text {AR}\approx 80$ for 500- $\mu \text{m}$ through-trench etch, and AR > 20 for 500- $\mu \text{m}$ through-hole etch), with straight sidewalls across a wide range of feature sizes. The challenges of making such structures are overcome by continuously ramping critical parameters of the Bosch DRIE process throughout the process, including the 380-kHz bias power during etch step, the etch/passivation step duration, and the chamber pressure. The masking material capable of enduring the long DRIE process is also discussed; 10- $\mu \text{m}$ and 25- $\mu \text{m}$ wide trenches are etched to a depth of >750- $\mu \text{m}$ and >1000- $\mu \text{m}$ , respectively, in 1-mm-thick silicon wafers with straight sidewall profiles and flat trench bottoms. Deeper trenches are expected to be etched beyond a 1-mm thick wafer with thicker and/or higher selectivity masking materials. We have also demonstrated etching of circular holes of diameters as small as 25- $\mu \text{m}$ to a depth of >500- $\mu \text{m}$ , and potentially with 10–15 $\mu \text{m}$ diameter holes. This advanced DRIE process offers opportunities for applications ranging from through-silicon via in 3-D CMOS integration to emerging micro- and meso-scale microelectromechanical system applications that demand ultra-deep and UHAR DRIE. [2017–0298]