Etching: Wet and Dry Physical or Chemical Etching Etching is the process where unwanted areas of films are removedby either dissolving them in a wet chemical solution (Wet Etching)or by reacting them with gases in a plasma to form volatileproducts (Dry Etching). Resist protects areas which are to remain.In some cases a hardmask, usually patterned layers of SiO2 or Si3N4, are used when theetch selectivity to photoresist is low or the etching environmentcauses resist to delaminate. This is part of lithography - pattern transfer. Etch Rate Removed thickness per Time (nm/min) or (A/min) Dependent on:
Etchant solution Etchant concentration Temperature Agitation Density and microstructure of the material Porosity Impurities Ex: High concentration of Phosphorous speeds up SiO2 Etching Ex: CVD SiO2 is etched faster than thermal Oxidized SiO2 Etch rate uniformity Etching Selectivity, Sfm Isotropy Isotropic Etching Aniotropic Etching Wet Chemical Etching Wet etches: - are in general isotropic
(not used to etch features less than 3 m) - achieve high selectivity for most film combinations - capable of high throughputs - use comparably cheap equipment - can have resist adhesion problems - can etch just about anything Example Wet Processes For SiO2 etching
- HF + NH4F+H20(buffered oxide etch or BOE) For Si3N4 - Hot phosphoric acid: H3PO4 at 180 C - need to use oxide hard mask Silicon - Nitric, HF, acetic acids - HNO3 + HF + CH3COOH + H2O Aluminum - Acetic, nitric, phosphoric acids at C - CH3COOH+HNO3+H3PO4 Dry Etching What is a plasma (glow discharge)?
A plasma is a partially ionized gas made up of equal parts positivelyand negatively charged particles. Plasmas are generated by flowing gases through an electric ormagnetic field. These fields remove electrons from some of the gas molecules.Theliberated electrons are accelerated, or energized, by the fields. The energetic electrons slam into other gas molecules, liberatingmore electrons, which are accelerated and liberate more electronsfrom gas molecules, thus sustaining the plasma. 50 ~ 100 C Dry Etching General Types
Plasma Etching 13.56 MHz (or a few KHz) Isotropic Ion Milling/Sputtering Ar+ Anisotropic Poor selectivity Deep Etches Reactive Ion Etching Combination of Physical and Chemical Etching Sputtering Reactive Ion Etching (RIE)
Combination of chemical and physical etching Directional etching due to ion assistance. In RIE processes the wafers sit on the powered electrode.Thisplacement sets up a negative bias on the wafer which acceleratespositively charge ions toward the surface.These ions enhance thechemical etching mechanisms and allow anisotropic etching. Wet etches are simpler, but dry etches provide better line widthcontrol since it is anisotropic. High Density Plasma etchers:
High Density Plasma etchers (HDP) Electron Cyclotron Resonance Plasma Etchers: ECR Room Temperature without thermal activation. Etch and Deposition. Less Ion-Bombardment induced damages. Inductively Coupled Plasma: ICP Transformer Coupled Plasma: TCP Surface Wave Coupled Plasma: SWP Electron Cyclotron and Synchrotron Radiation