In many semiconductor manufacturing processes, a plasma is used, e.g. in sputtering, deposition or in dry etch prcesses. An important point here is that the plasma is not heated. Therefore wafers, which were already metallized, can be processed in plasma processes.
Plasma is also called the fourth state of matter or fourth aggregate state. An aggregate state is a qualitative condition of materials, which depends on the temperature and pressure. The three states solid, liquid and gaseous one encounters in everyday's life. If temperature is low, every atom in a solid is fixed at one point. Attractive forces prevent them from moving. At absolute zero (-273.15 °C) substances enter into no reaction. With increasing temperature, the particles start to oscillate, and the bonds of the atoms are unstable. If the melting point is reached, a substance transforms from the first to the second aggregate state: ice (solid) is transformed into water (liquid).
The gravitational forces in liquids are still present, but the particles are able to move and have no fixed places like in solid state, the particles are adapted for example to a predetermined shape. If the temperature increases further, the bonds are completely broken, the particles move independently of one another. At the boiling point of a substance it transforms from second to third state: water (liquid) turns into water vapor (gaseous).
While the volume of solids and liquids is constant, gaseous substances take the existing space completely, the particles are distributed evenly throughout the room.
Each substance has a specific melting and boiling point. Silicon melts at 1414 °C and passes into gaseous state at about 2900 °C. If one introduces even more energy to a substance, the collisions between the particles will strike out electrons from the outermost electron shells. Now there are free electrons and positively charged ions: the plasma state is reached.