Technical synopsis and benefits of plasma etching method
In integrated chips fabrication, chemical etching refers to the process of removal of metal traces from the substrate. The etching agent dissolves metal, leaving behind the print of circuit layout. There are two kinds of etching; wet etching and dry etching.
Wet etching involves the use of concentrated acids as enchants. On the other hand dry etching is a technique in which no liquid chemical is used. One of the most commonly used etchants in chemical free etching is plasma.
Configuration of a Plasma Etching System
The general configuration of a plasma producing system involves two metal electrodes, a power supply and a ground electrode. Voltage is applied between the two symmetrical metal electrodes in order to produce an electric field of high intensity. The standard radio frequency for plasma production is 13.6MHz. When the gas enters a plasma etching system, gas electrons receive energy and become excited. Gas turns into plasma when all the particles are ionized. The material to be etched is placed in the plasma etching system. Ionized particles react with the material and remove all the metal traces, leaving the rest of the wafer intact. See more on www.tantec.com about Plasma Etching.
Process of Plasma Etching
The process of plasma etching consists of the following steps:
- Plasma generation- plasma is generated from a particular gas in the plasma production chamber.
- Diffusion- reactive plasma particles are diffused to the surface of the circuit board to etch the metal away.
- Absorption- ionized particles with high energy are absorbed into the surface of the material being etched.
- Chemical reaction- as the reactive particles in the plasma come into contact with the metal, a series of chemical reactions occur in which volatile by-products are formed.
- Desorption- by-products formed during the chemical reaction between plasma and metal are desorbed. Most of these by-products are diffused into the mixture of reactive gases present in the plasma etching system.
Following considerations must be kept in mind while etching a material with plasma:
- Plasma selectivity- plasma selectivity means the ability of plasma to remove only the targeted material without changing the properties of the substrate or any other material on the circuit board/IC. The selectivity of plasma depends on the type of reactive species selected for the plasma production.
- Undercutting- although the by-products formed during plasma reaction with the material are volatile, all the traces of these by-products should be immediately removed from the material being etched. Otherwise, these by-products can react with the substrate, which is to be left intact. The phenomenon of consumption of substrate or material protected with the mask during the chemical reaction with etching by-products is called as undercutting.
- Etching direction- etching direction is very important for keeping the material protected by the etching mask intact. In most cases those etchants are selected which follow only vertical direction for etching. This way the etchant will follow the pattern of the protective mask on the substrate or wafer.
Plasma etching is one of the most reliable techniques of etching and IC fabricators are using it on commercial scale.