PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITION

Plasma-enhanced chemical vapor deposition (PECVD) is a process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by RF (AC) frequency or DC discharge between two electrodes, the space between which is filled with the reacting gases. Plasma deposition is often used in semiconductor manufacturing to deposit films conformably (covering sidewalls) and onto wafers containing metal layers or other temperature-sensitive structures. PECVD also yields some of the fastest deposition rates while maintaining film quality (such as roughness, defects/voids), as compared with sputter deposition and thermal/electron-beam evaporation, often at the expense of uniformity. Silicon dioxide can be deposited using a combination of silicon precursor gasses like dichlorosilane or silane and oxygen precursors, such as oxygen and nitrous oxide, typically at pressures from a few millitorr to a few torr. Plasma-deposited silicon nitride, formed from silane and ammonia or nitrogen, is also widely used, although it is important to note that it is not possible to deposit a pure nitride in this fashion. Plasma nitrides always contain a large amount of hydrogen, which can be bonded to silicon (Si-H) or nitrogen (Si-NH); this hydrogen has an important influence on IR and UV absorption, stability, mechanical stress, and electrical conductivity.
Silicon Dioxide can also be deposited from a tetraethoxysilane (TEOS) silicon precursor in an oxygen or oxygen-argon plasma. These films can be contaminated with significant carbon and hydrogen as silanol, and can be unstable in air. Pressures of a few torr and small electrode spacings, and/or dual frequency deposition, are helpful to achieve high deposition rates with good film stability.
High-density plasma deposition of silicon dioxide from silane and oxygen/argon has been widely used to create a nearly hydrogen-free film with good conformability over complex surfaces, the latter resulting from intense ion bombardment and consequent sputtering of the deposited molecules from vertical onto horizontal surfaces.