Reactive Ion Etching (RIE) Etching Basics
A disadvantage of wet etching is the undercutting caused by the isotropy of the etch. The purpose of dry etching is to create an anisotropic etch - meaning that the etch is uni-directional. An anisotropic etch is critical for high-fidelity pattern transfer.
RIE etching is one method of dry etching.
The figure below shows a diagram of a common RIE setup. An RIE consists of two electrodes (1 and 4) that create an electric field (3) meant to accelerate ions (2) toward the surface of the samples (5).
The area labeled (2) represents plasma that contains both positively and negatively charged ions in equal quantities. These ions are generated from the gas that is pumped into the chamber. At BYU we use O2 and CF4 gasses for our etches. In the Diagram CF4 has been pumped into the chamber, making a plasma with many Fluorine (F-) Ions
The Fluorine ions are accelerated in the electric field. cause them to collide into the surface of the sample. A hard mask is used to protect certain areas from etching, exposing only the areas desired to be etched.
The figure below shows a photoresist mask on silicon dioxide. The etching ions are accelerated into the etching region, where they combine with silicon dioxide and then are dispersed. Because the electric field accelerated ions toward the surface, the etching caused by these ions is much more dominant than the etching of radicals - ions traveling in varied directions, so the etching is anisotropic.
Etch Chemistries of Different Etch Processes
|Material Being Etched||Etching Chemistry|
|Deep Si trench||HBr/NF3/O2/SF6|
|Shallow Si trench||HBr/Cl2/O2|
|Poly Si||HBr/Cl2/O2, HBr/O2, BCl3/Cl2, SF6|
|Al||BCl3/Cl2, SiCl4/Cl2, HBr/Cl2|
|W||SF6 only, NF3/Cl2|
|WSi2, TiSi2, CoSi2||CCl2F2/NF3, CF4/Cl2, Cl2/N2/C2F6|
|Si02||CF4/O2, CF4/CHF3/Ar, C2F6, C3F8,C4F8/CO, C5F8, CH2F2|
|Si3N4||CF4/O2, CHF3/02, CH2F2, CH2CHF2|
Table taken from Semiconductor Devices - Physics and Technology by S.M. Sze. (pg. 440)