DRIE (Deep Reactive Ion Etching)

Notes

The alternating cycles leave characteristic scalloped sidewalls whose roughness must be controlled for optical or fluidic uses.

Overview

DRIE (deep reactive ion etching) is a plasma etch process that cuts deep, steep-sided holes and trenches into silicon, often tens to hundreds of micrometres deep and many times deeper than they are wide. It was developed for MEMS and is also used for through-silicon vias and deep trench structures. The dominant production method is the Bosch process, which alternates short etch and passivation steps.

How it works

In the Bosch process, patented by Robert Bosch GmbH from work by Franz Laermer and Andrea Schilp in the early 1990s, the tool switches gases every few seconds. An SF6 plasma etches silicon, largely in all directions; then a C4F8 plasma coats every surface with a thin, Teflon-like polymer. In the next etch step, ions accelerated towards the wafer sputter the polymer off the trench floor but not the sidewalls, so etching continues downwards while the walls stay protected. Repeating the cycle many times builds a deep, nearly vertical trench.

Each cycle leaves a small ripple, called scalloping, on the sidewall. Faster gas switching shrinks it: Samco reports scallops below 5 nm with 0.1 second switching while keeping an etch rate of about 2 micrometres per minute. Smooth walls matter for TSVs, which must be lined and filled with copper.

Other challenges include profile control (bowing, taper and tilt near the wafer edge) and notching where the etch reaches the buried oxide of a silicon-on-insulator wafer, caused by charging. The main alternative, cryogenic DRIE, chills the wafer to around minus 110 degrees Celsius so that sideways etching slows in a continuous process, but masks can crack at those temperatures and the Bosch process remains the recognised production technique.

Examples

• Etching a capacitive accelerometer through the silicon device layer of an SOI wafer, stopping on the buried oxide.
• Forming through-silicon vias for 3D wafer-level packaging before they are filled with copper.
• A tapered 400 micrometre deep hole etched with the Bosch process for a specialised MEMS structure.
• Silicon parts for high-end mechanical watches, which DRIE can cut in almost any outline.

Why it matters

DRIE is a core process for silicon MEMS, such as capacitive accelerometers etched through the silicon layer of SOI wafers, and for the vias that connect stacked dies. Etch process engineers, MEMS process integration engineers and TSV process engineers tune it, and its depth uniformity, sidewall quality and tilt decide whether moving parts behave as designed and whether vias line up through a stack.

Related terms

• Bulk micromachining
• TSV
• MEMS
• Scalloping

Where it appears on the Hub

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