One way to modify the physical and chemical properties of metals is plasma etching. The technique is particularly popular in the digital world for the fabrication of integrated chips.
People have been using concentrated acids to etch copper made printed circuit boards for years. But with the introduction of plasma etching system in late 80s, PCB manufacturers started relying on this technique.
Plasma etching, as the name implies, is a technique of etching in which plasma is used as an etchant instead of strong acids. Plasma is the fourth state of matter, formed by ionizing gas particles, through radio frequency or heating. In order to understand the process of plasma etching, it is important to understand the working of a plasma etching system. The system consists of two symmetrical electrodes for the generation of radio frequency and a ground electrode on which the sample to be etched is placed. There is a gas inlet trough which etchant enters the system and there is an outlet for plasma. As the gas enters the system, voltage is applied to partially ionize gas particles.
Usually the frequency of power generated is 13.6MHz, which is considered as a standard plasma forming frequency worldwide. Radio frequency is utilized to excite gas electrons and change their state. In order to etch a material with plasma, the etching system generates high speed pulses of plasma. Depending on the type of etching (dry or wet), plasma consists of either ions or radicals. The process of plasma etching also results in the production of volatile chemical compounds, formed as by-products during the chemical reaction between the plasma and the material to be etched. It takes some time for the plasma atoms to embed at the printed circuit board.
Modes of plasma etching system vary with the type of the material to be etched, nature of etchant and etching type. For example for dry etching, the system operates on capacitive coupling of radio frequency. Other factors which influence the generation of plasma and mode of plasma generator are temperature and pressure. Slightest variations in the operating pressure can change the collision frequency of electrons considerably. Moreover, to sustain the state of plasma, it is very to maintain the operating pressure.
So, plasma etching can effectively improve the quality of fabrication of integrated circuits. Moreover, it can also be used for treating delicate semiconductor wafers and a wide range of substrates.