**2.2. Plasma**

In a vacuum system, plasma can be produced by introducing the process gas and applying the power. The process gas can be underwent ionization, excitation/relaxation, and dissociation under the power. Therefore, energetic ions, electrons, light (from deep vacuum ultraviolet (VUV) to infrared (IR)), and highly reactive radicals are produced in the plasma [29, 30]. In semiconductor processing, plasma technology can be used for ion implantation, etching, and deposition. The ion implantation processing is achieved by the energetic ions. The etching processing involves both physical and chemical reactions, which are related to the energetic ions and the highly reactive radicals, respectively. The deposition processing only relies on the highly reactive radicals for chemical reaction.

To produce the plasma, three main reactors are used: capacitively coupled plasma (CCP), inductively coupled plasma (ICP), and remote or downstream plasma (RP or DSP) [31]. The energy transfers are through capacitive coupling by parallel electrodes, inductive coupling by a coil, and microwaves for CCP, ICP, and RP systems, respectively. In the CCP and ICP systems, light from VUV to IR, energetic ions, electrons, and highly reactive radicals are presented. In the RP reactors, however, the plasma generation region is usually separated from the processing region. Additionally, a grid between the plasma and the substrate is used for charge neutralization, and a special measure is designed to minimize the photon flux. As a result, only reactive radicals or dissociated molecules or atoms can reach the surface of the wafer. This minimizes the damage from light and/or high-energy species. Due to the absence of ions, the RP reactors cannot provide patterning etching.

In the ICP systems, there are two applied RF power: one is source power (top power), and the other is bias power (bottom power). Therefore, plasma density and ion energy can be controlled separately. Additionally, the ICP system has the highest plasma density with 1011–1012 electrons/cm3 [31]. The plasma density of CCP system is 109 –1010 electrons/cm3 . The RP system has the lowest plasma density. Due to anisotropic etching property provided by ion bombardment, ICP and CCP systems are usually used for pattern etching. Since dielectric films are very sensitive to ion bombardment and ICP reactors lack passivating species required by typical dielectric etching, CCP reactors are mostly used for dielectric patterning etching. On the other hand, ICP reactors are often used for conductor patterning etching due to the etching rate consideration. To avoid damage by ion bombardment and UV light irradiation or no need anisotropic etching in the plasma process, RP reactors are the best choice. So, cleaning and resist stripping processes during semiconductor fabrication can be done by RP reactors.
