Technologies / Silicon DRIE / DRIE for MEMS

Silicon and MEMS

Silicon is a popular material for fabricating Micro Electro-Mechanical Systems (MEMS) due to its abundance, mechanical strength, fatigue resistance, electrical properties and well-established fabrication techniques from the semiconductor industry.

Silicon Micromachining

3D MEMS structures are formed by masking the surface of the silicon wafer with photoresist, nitride or oxide, and then unmasked areas are etched away forming deep trenches or holes in the silicon.  Silicon can be wet etched using specific chemical solutions or dry-etched using XeF₂ gas or a plasma containing reactive Fl, Cl, Br or I ions. 

Anisotropic Silicon Etching

XeF₂ and most etchant solutions will etch silicon isotropically (1) where etching progresses at the same speed in all directions. 

When using KOH solution to etch monocrystalline silicon, Si <111> planes etch approximately 100 times slower than other crystal orientations. Therefore, etching a rectangular hole in a (100)-Si wafer will result in a pyramid shaped etch pit with 54.7° walls (2).  In addition to this dependance on crystallographic orientation of the wafer, other disadvantages of wet etching are "stiction" between moving parts of the MEMS structure which might result in device failure and safe disposal of waste solution.

Many MEMS structures require vertical features to be etched deep into the silicon (3).  While dry, anisotropic silicon etching is possible with conventional RIE or ICP, using a combination of plasma chemistry and directional ion bombardment, only relatively shallow depths of 5-10µm can be achieved.

Cryogenic etching can etch deep vertical features in silicon, but relies on extremely low temperatures (-100°C to -130°C) to suppress sidewall etching, which increases equipment cost of ownership, causes particles/contamination due to condensation and does not allow the use of photoresist masks due to thermal cracking.  This process also requires high ion bombardment to remove oxidation layer from the bottom of the feature, leading to low mask selectivity.

Deep Reactive Ion Etching (DRIE) with its many advantages over alternative technologies, has become the most popular technique for deep anisotropic etching of silicon MEMS.

Si airbag sensor (Courtesy of Robert Bosch GmbH)	Si microturbine (Courtesy of MIT)	Si MEMS optical switch (Courtesy of Sercalo Microtechnology Ltd)