The radio frequency discharge plasma sources are widely utilized to prepare functional thin films and to etch insulated layers in semiconductor devices in microelectronic industry. Especially, a capacitively coupled plasma (CCP) is the most popular discharge because the equipment is very simple and almost maintenance free. However, there is a problem such as low-density plasma under low-gas pressure less than 10 Pa, that is, low processing rate. In this chapter, the production principle of conventional CCP and the special CCP with various electrodes and magnets is reviewed. The applications prepared by the special CCP system are also presented. Finally, the future plan including problems is described.
- RF plasma
- capacitively coupled plasma
- plasma processing
- hollow cathode discharge
- ring-shaped hollow plasma
- magnetized plasma
A large-scale integrated circuit (LSI), various sensors, and other semiconductor devices are essential for various electrical and electronic devices and automobile [1, 2]. In general, these devices are fabricated by plasma-processing techniques such as dry etching and plasma deposition using low-pressure plasma. The plasma processing is the most usually utilized chemical and physical processes in microelectronics fabrication for functional thin film preparation and dry etching of silicon-based films.
In the dry-etching processing, the silicon wafer patterned by the photoresist mask for LSI is exposed in plasma containing halogen molecules (e.g., CF4, CHF3, SF6) [3, 4, 5, 6, 7]. In the case of these gases, the fluorine atoms dissociated by an electron impact collision with the molecules react with the silicon surface to generate a volatile etch product like SiF4 so that the silicon wafer is etched with assistance of energetic ions produced in the plasma. For example, a high-density plasma sources have been developed to challenge patterning features less than 0.25 μm with high aspect ratios .
The plasma deposition has a sputtering deposition and a plasma-enhanced chemical vapor deposition (PECVD) method. The sputtering deposition is to impinge ions to the material target biased by a negative potential so as to sputter atoms from the target. The functional thin films such as transparent conductive oxide used in tableted computer and smartphone are deposited by sputtering method . The sputtering depositions are prepared by DC magnetron and radio frequency (RF) magnetron plasma sources . Various typed sputtering sources have been developed for the synthesis of high-quality thin films . Especially, RF magnetron plasma source has an advantage that the insulated films can be deposited on the various substrates compared with DC magnetron plasma source. The PECVD is to dissociate molecule by electron impact so as to deposit radicals .
In these plasma-processing techniques, capacitively coupled plasma (CCP) with parallel plates was widely utilized. The physics of CCP has been studied by many researchers [11, 12, 13, 14, 15, 16]. The sustaining mechanism of CCP is the electron heating process that the oscillating radio frequency sheath near the powered electrode plays an important role in electron acceleration as well as the collisional heating in the presence of electric fields and the emission process of secondary electrons from the electrode [11, 12, 13, 14, 15, 16]. However, the electron heating process cannot produce high-density plasma.
The requirement of these semiconductor devices with high-speed operation and high performance is increasing annually. In order to perform the demand, high-speed plasma processing, that is, a high-density plasma production, is important. However, CCP does not attain high-density plasma. Thus, the high-density CCP is required. In this chapter, the production principle of conventional CCP and the special CCP with various electrodes and magnets is reviewed.
2. Production principle of conventional capacitively coupled plasma
In order to perform the high-speed processing of the LSI semiconductor, high-density plasma is needed for CCP. The power balance of CCP in the form of a global model  is expressed as the following equation:
When only thermal electrons are lost to the boundary surfaces ,
In order to increase the plasma density at the typical frequency of 13.56 MHz, ingenious device is required. In the next section, the various ingenuities will be introduced.
3. Effect of high secondary electron emission oxide on high-density capacitively coupled plasma production
In general, plasma is generated by electrons with an energy higher than an ionization potential of target neutral gas . According to the ionization cross section  for noble gases of He, Ne, Ar, and Xe, their ionization energy ranges from 10 to 30 eV, while the energy is a few 100 electron volts when the ionization cross section becomes maximum. These electrons are effectively possible to ionize neutral gases through inelastic collisions. Then, it is expected to produce high-density plasma. In CCP discharge, it is also easy to generate a high voltage of a few hundred volts between the powered and grounded electrodes, that is, CCP can generate secondary electron emission (SEE) from the powered electrode. It was reported that magnesium oxide (MgO) electrodes have a high SEE coefficient which are a few 10 times higher than that of conventional metal electrodes such as aluminum .
In this section, the effect of SEE as the acceleration mechanism of electrons is proposed to solve the serious problem of CCP density. The RF breakdown voltage and plasma density are studied experimentally. As show in Figure 1(a), an RF power of 13.56 MHz was supplied to generate CCP between two electrodes of 20-mm diameter with a gap
Figure 2 shows RF breakdown voltage
Figure 3 shows plasma density
According to the scaling law under the assumption that the stochastic heating without the SEE effect in the main discharge mechanism in CCP , the plasma density can be expressed as
For MgO electrode, the plasma density is one order of magnitude higher than that for Al electrode. It is found that plasma density increases obviously with increasing
The reason why the plasma density for MgO becomes constant for
Figure 4 shows optical emission intensity of the Ne I line (588.1 nm) as a function of
4. Effect of structured electrodes on high-density capacitively coupled plasma production
In this section, it is described that structured electrodes can produce the high-density capacitively coupled plasma. One of the structured electrodes is a hollow cathode. The hollow cathode discharge  is applied into a production mechanism of CCP to attain high-density plasma.
4.1. Multi-hollow electrode
The effects of a multi-hollow cathode discharge and a high SEE are applied to capacitively coupled plasma to produce high-density plasma [25, 26]. Figure 5(a) and (b) shows the experimental apparatus and construction of the multi-hollow electrode, respectively. As shown in Figure 5(b), one plate has 35 holes with 5-mm diameter and 15-mm length, and these holes lay on a concentric circle. In order to emit secondary electron emission from the electrode facing the multi-hollow electrode, the other electrode is biased by the voltage of low frequency of 1 MHz. The plate is called as the substrate electrode.
Figure 6 shows plasma density and electron-neutral mean free path as a function of Ar gas pressure. Here, plasma density was estimated by ion saturation current density of a negatively biased probe because plasma density is proportional to ion saturation current . The electron-neutral mean free path
The effect of SEE to attain high-density plasma production was examined by biasing the substrate electrode. Figure 7 shows plasma density as a function of substrate biasing voltage
4.2. Ring-shaped hollow electrode
In the previous subsection, the effect of multi-hollow electrode on high-density capacitively coupled plasma was described . In this subsection, the ring-shaped hollow electrode was tried to produce high-density capacitively coupled plasma. The high-density plasma in the trench diffuses toward the downstream region, and then the radial profile of plasma density becomes uniform at a certain axial position. The influence of trench width and gas pressure on plasma density and its profile is examined, comparing the case of a conventional flat electrode.
It is very important to accelerate electrons in the trench by moving RF cathode sheath for producing the high-density plasma with a hollow cathode effect. To satisfy the hollow cathode effect, it is required that the hollow trench width
Figure 10(a) and (b) show typical images of plasma emission near the RF electrode for the ring-shaped hollow electrode and the flat electrode, respectively. As shown in Figure 10(a), a high intensity of plasma emission is observed near the ring-shaped hollow trench for the ring-shaped hollow electrode. The hollow cathode effect is attained in the trench. On the other hand, the conventional flat electrode shows a uniform glow plasma on the whole electrode.
Figure 11 shows plasma density
Figure 12 shows plasma density as a function of Ar gas pressure for
4.3. Magnetized ring-shaped hollow cathode discharge
In a low pressure of 1 Pa, it is difficult to produce plasma using only hollow cathode discharge . In this subsection, in order to attain high-density plasma in the low-gas pressure, the combination of hollow cathode discharge and magnetic confinement with magnets is proposed. The addition of a magnetic field is one candidate for performing discharge in the low-gas pressure. It is easy to magnetize electrons under the low-gas pressure.
Figure 13 shows the construction of a ring-shaped hollow cathode for (a) the NS-NS arrangement, (b) the NS-SN arrangement, and (c) the NS arrangement of permanent magnets. The neodymium magnets were used. Three arrangements of permanent magnets were investigated. The NS-Ns arrangement is that two NS magnets with 7 × 10 mm2 in cross section and 10 mm in length are positioned at both walls of a hollow trench and six couples of the NS-NS magnets are used as shown in Figure 13(a). In the NS-SN arrangement, NS and SM magnets are mounted as shown in Figure 13(b). As shown in Figure 13(c), six NS magnets with 10 × 15 mm2 in cross section and 6 mm in length are set at the bottom of a hollow trench for the NS arrangement. The outside of magnets is covered by iron yokes with 1 mm in thickness as shown in Figure 13.
Figure 14 shows two-dimensional distributions of magnetic field lines at (a) NS-NS, (b) NS-SN, and (c) NS arrangements of permanent magnets, respectively. Here, the area enclosed by dashed lines is the hollow trench. As shown in Figure 14(b), for the NS-SN arrangement, it is clear that the profile of the magnetic field lines is quite different from the other arrangement. The magnetic field lines show a cusp profile in the trench. The NS-SN arrangement is the best profile for magnetic confinement of electrons.
Figure 15 shows plasma density as a function of gas pressure at
In these chapters, in order to improve the plasma density in CCP, various typed CCP discharges have been presented. In Section 3, it is indicated that RF electrode with a high secondary electron emission oxide of MgO is effective to produce high-density capacitively coupled plasma. The plasma density for MgO electrode increases drastically with increasing RF voltage compared with the metal electrode of Al. In Section 4, it is described that the structured electrode plays an important role to improve the plasma density. This mechanism is ascribed by the hollow cathode effect.
The radio frequency capacitively coupled plasma source is widely utilized in the semiconductor fabrications. However, the source has a serious problem, although it has some merits such as simple structure and maintenance free. In this chapter, some solutions are introduced by adding simple methods. The first method is the high secondary electron emission electrode. The second method is multi-hollow and ring-shaped electrodes. The third method is magnetized ring-shaped electrode. All methods attained high-density plasma production. Especially, the ring-shaped electrode with magnets performed high-density plasma with 1011 cm−3 under a low-gas pressure. These methods will be useful to advance capacitively coupled plasma for microelectronic technology.
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