Applied Science and Convergence Technology 2019; 28(5): 131-138
Published online September 30, 2019
Copyright © The Korean Vacuum Society.
Tae Hyung Kima and Geun Young Yeoma,b,*
aDepartment of Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
bSKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
For plasma-based deposition techniques, high ionization through high-density plasma plays an important role in improving the deposited film. Various deposition methods such as high-power impulse magnetron sputtering and ion-beam sputtering have been developed for physical vapor deposition technology and are still being studied. Further, studies have been carried out to control the characteristics of the deposited films by directly controlling plasma characteristics using an inductively coupled plasma (ICP) antenna; this is called an ICP-assisted magnetron sputter (ICP MS) technology. The ICP MS method exhibits the characteristics of low resistance, high materials density, and low stress because it can perform processes at low temperatures and with high-density ion bombardment at low energies. Using the ICP-assisted sputter technology, the crystal structure of a thin film can be also controlled. In recent years, devices and thin films that are becoming finer are required to have a low resistance property in a thin film and a low temperature process because of the possible thin-film thermal damage. In this review, some of the important aspects of the ICP MS technology, which can solve problems such as low resistance, high density thin film, and low temperature process, are discussed.
Keywords: Sputter, Inductively coupled plasma, Inductively coupled plasma magnetron sputter, Ionization
Sputter or sputtering refers to the physical removal of atoms from the surface of a material by the collision of energetic particles on the surface of the material. The removal of the atoms from the surface is achieved by momentum transfer between the atoms released and the atoms on the surface adjacent to these atoms. In addition to the sputtering phenomena, various interactions such as surface-neutralization, secondary electron emission, ion implantation, and radiation damage occur when energy-bearing particles collide with the surface of a material. Sputter deposition is one of the most representative physical vapor deposition (PVD) methods, and it is one of the most widely used deposition methods in semiconductor and display fields. Sputter deposition technology was discovered in 1852 by Grove using direct current (DC) to discharge plasma in a vacuum tube for the deposition. These DC diode sputtering devices had disadvantages in that plasma discharge was difficult to achieve at low pressure, and it had a low deposition rate. Thus, many studies focused on improving the sputtering equipment using several approaches. A magnetron sputtering apparatus was developed to assist DC glow discharge using a magnetic field to confine the energetic electrons emitted from the cathode near the cathode surface using an E × B field (electric field vertical to magnetic field).
For deposition or etching equipment that use a plasma discharge method, properties of the plasma are important; among these properties, the characteristics of the ions in plasma have the highest influence on the deposited film. However, the characteristics of the ions in plasma are usually dominated by the ions of the inert sputtering gas while the effect of the ions of the sputtered materials is low. Over the last few years, various ionized sputtering techniques have appeared that can achieve a high degree of ionization of the sputtered atoms . Magnetron sputtering achieves a higher plasma density compared to sputtering without a magnetic field for the same input power. However, if very high power is continuously applied, the target or the deposited thin film may be damaged by the high plasma energy [2–5]; further, there is a power limitation that can be applied to the target. High-power impulse magnetron sputtering, which uses pulse powers composed of extremely high power densities on the order of kW· cm−2 in short pulses (impulses) of tens of microseconds at a low duty cycle (on/off time ratio) of < 10 % can overcome this limitation [6–9]. However, the deposition rate decreases as the sputtering on-time becomes shorter. Ion beam sputtering (IBS) is another deposition method wherein an ion beam gun is mounted inside a sputter device [10–12]. The ions generated from the ion gun collide with the target or the thin film on the substrate to improve the characteristics of the thin film. The IBS method has an advantage in that it can change the sputter ion incident angle, and the ion flux and energy can be varied separately. However, there is a drawback that the cost of the ion gun is high and large-area processing is difficult owing to the limitation of the deposition area caused by the small beam size. In addition to these two methods, there are other PVD methods that use high ionization; for example, self-sustained sputtering magnetron, which uses highly ionized sputtered target atoms as the plasma ions for self-sputtering that can be obtained under high power conditions, and hollow cathode magnetron sputtering, which uses a hollow-shaped cathode to increase plasma density by additionally confining the high-energy electrons near the cathode [13–18].
A DC sputtering device has disadvantages in that it has a low deposition rate and it is difficult to achieve plasma discharge at low pressure. Thus, studies focused on improving the sputtering apparatus using various approaches. In particular, a magnetron sputtering apparatus was developed to assist DC glow discharge using a magnetic field to confine energetic electrons emitted from the cathode near the cathode surface using an E × B field (electric field vertical to magnetic field).
As shown in Fig. 1, the magnetron sputter device uses a magnetron to increase the ionization rate of electrons. The N and S poles are installed such that electrons emitted from the target spiral around the target continuously, and the plasma generating gas constantly causes a collision. The collision probability is proportional to the travel distance of the electron, and consequently, the helical motion around the target increases the travel distance of the electrons and ionization probability. This leads to increased ion bombardment of the target, which provides higher sputtering rates, and consequently, higher deposition rates at the substrate.
There are only a few differences in the design between a conventional magnetron and an unbalanced magnetron. However, the difference in performance between the two types of magnetrons is very significant. In a conventional magnetron, plasma is strongly confined to the target region. A region of dense plasma typically extends ~60 mm from the target surface. In an unbalanced magnetron, the outer ring of magnets is strengthened relative to the central pole. In this case, not all field lines are closed between the central and outer poles in the magnetron, but some are directed toward the substrate, and some secondary electrons follow these field lines. Consequently, the plasma is no longer strongly confined to the target region but is also allowed to flow out toward the substrate. Thus, high ion currents can be extracted from the plasma without the need to externally bias the substrate. Earlier studies had shown that in some magnetron designs, not all field lines closed in on themselves . However, it was Windows and Savvides who first appreciated the significance of this effect when they systematically varied the magnetic configuration of an otherwise conventional magnetron [20–22]. They and other researchers have subsequently shown that substrate ion current densities of 5 mA/cm2 and higher, i.e., approximately an order of magnitude higher than that for a conventional magnetron, can be routinely generated when using an unbalanced magnetron [22–24]. A comparison between the plasma confinement obtained in different magnetron modes is shown schematically in Fig. 2.
Thus, in addition to providing a high flux of coating atoms (compared to a basic sputtering source), an unbalanced magnetron acts as a very effective ion source. Furthermore, the ion current drawn at the substrate is directly proportional to the target current. The deposition rate is also directly proportional to the target current. Therefore, unlike other ion-plating processes [25,26], the ion-to-atom arrival ratio at the substrate remains constant with an increasing deposition rate [27–29].
Radio frequency (RF) ICPs have been widely studied for over 130 years. The basic concept for generating an ICP stems from Faraday’s law,
Among various ionization PVD methods, ICP MS is a magnetron sputter (MS) method that uses an ICP in addition to the near the cathode. An ICP antenna is mounted near a magnetron source to discharge the ICP plasma, thereby improving the characteristics of the deposited film by directly increasing plasma ionization and bombardment during sputter deposition. These allow more energetic ions from the plasma formed from the plasma gas and the particles emitted from the target to reach the surface of the substrate as compared to a typical sputter device, as shown in Fig. 3. Various studies have been carried out to control the characteristics of the deposited thin films by applying ICP additionally during sputtering. Through magnetron sputtering assisted with ICP, high-quality thin films can be obtained using a low temperature process, and other material properties such as the structure of the deposited thin films can be controlled [33–57].
Since ICP MS is a plasma-based deposition system, it is very important to analyze and understand the plasma characteristics generated during the deposition. A typical ICP MS system is as shown in Fig. 4, which was investigated by Setsuhara
Higher deposition rate during sputter deposition for higher throughput is one of the most important factors for lower process cost. Improved sputtering using ICP in ICP MS is confirmed by several studies [34–36]. This is because, when the ICP RF power is added and increased during ICP MS, the current of the target is increased because of the increased plasma density near the target by the additional ionization of the gas by the ICP, which increases the deposition rate. The study by Lee
Over the last decade, numerous research studies on sputter deposition have been carried out for flexible displays or electronic devices. Among these studies, many are related to low temperature deposition of materials to perform the deposition process on substrates such as polymer substrates for flexible displays [37–40] and silicon substrates for nanoelectronics. For flexible displays, the substrates are damaged at higher deposition temperature (e.g., polymer substrates), and silicon substrates for nanoelectronics are damaged by rapid diffusion as the size of the electronic device becomes smaller. Therefore, various studies were conducted to obtain high-quality thin films by using a sputter processes at low temperatures [41–45]. Materials such as indium tin oxide, IGZO, and Al-doped ZnO (AZO), which are well known as transparent conducting oxide (TCO) materials, are generally deposited at high temperatures during sputter deposition, or they are annealed at high temperatures after deposition at low temperatures to obtain improved material properties of the thin film [46–48]. However, because the polymer substrate cannot be processed at a high temperature, a lower temperature TCO deposition process condition is required. Sahu
In various thin film deposition studies using the ICP MS system, there are many reports which show that the controlled ICP applied during MS improved the characteristics of the deposited films [43–57]. For example, using controlled plasma for TCO films, the resistance decreased, the mobility increased, and the transparency improved by the addition of the controlled ICP under the same magnetron sputter conditions [34,44,49]. The cause of the improved thin film characteristics was investigated, and the results showed that improved material properties are related to the change in the thin film structure and the density of the thin film caused by the changes in the characteristics of ion species in the plasma to the thin film [42,44,49,50]. In addition, some researchers found changes in mechanical properties such as hardness or stress and the changes in electrical properties using ICP MS [50–52].
The changes in the composition and binding energy of the components in the thin film are some of the factors that directly affect the change in the chemical properties of the deposited thin films. In general, the composition of the deposited thin film during the reactive sputter deposition of compounds such as oxides or nitrides can be changed by varying the composition ratio of the target during the magnetron sputtering of the compounds or by changing the reactive gas percentages such as oxygen and nitrogen in the gas mixture. However, during ICP MS, because the ICP changes the dissociated radical concentrations of nitrogen or oxygen mixed with Ar, the composition of the compound thin film can be changed without changing the reactive gas percentages during reactive sputtering [41,42,45,53]. ICP MS also changes the binding energy of each component in the deposited thin film by changing the bonding characteristics during the deposition because of the increased ion density in the plasma in addition to the changes in the radical characteristics of the reactive gas in the plasma [44,49,57]. Matsuo
Thin films deposited by various methods including sputter deposition improve the chemical and electrical properties; similar improvement are also observed in mechanical characteristics such as hardness, adhesion strength, and wear resistance, and changes in mechanical characteristics caused by ICP MS have also been investigated [50–52]. In the case of Lim
Film structure is closely related to the physical, chemical, and electrical characteristics of the deposited thin film; therefore, a technique in forming a required film structure is an important deposition technique. Further, many studies identified a change in the structure of thin films by ICP MS [51,53–57]. Shin
Vanadium dioxide (VO2) is increasingly attracting interest because of its metal–insulator transition phenomena that show remarkable changes in resistivity at temperatures around 68 °C [53,60]. Nihei
Studies using ICP MS for improving deposited films were reviewed. The studies showed that high-quality thin films can be obtained at low temperature conditions owing to the high-ionization characteristics of the plasma, using ICP MS. The improved thin film properties are related to the increased ion bombardment effect or ion species control due to the high plasma density. The ICP assistance during magnetron sputtering was useful not only for changing the chemical and electrical properties but also for controlling the crystal structure of the deposited thin film. The thin film with higher material density obtained via ICP assistance during the magnetron sputtering can be applied to electronic devices and thin film materials requiring mechanical properties such as high wear resistance and low stress characteristics. Recently, this ICP MS technology has been applied to new fields such as graphene synthesis at low temperatures . Even though various advantages can be obtained using ICP assistance during magnetron sputtering, there are some issues that needs to be improved; for example, a particle issue where the deposition material builds up on the ICP antenna. However, there is a possibility that ICP MS deposition technology can be a solution to solving problems such as achieving higher electrical resistance of the metal thin film for extremely small devices and film thickness, and minimizing damage due to high process temperatures.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2018R1A2B6001019).
Comparison of characteristic differences between conventional magnetron sputter and ICP assisted sputter system.
|Conventional MS||ICP MS|
|Working pressure||Low (< 5 mTorr)||Very low (< 1 mTorr)|
|Film property control||None||Possible|
Deposition conditions for samples labeled S1–S8.
|No.||ICP (W)||RF Power (W)||O2 flow (sccm)||Ts (°C)||Total pressure (Pa)||Film thickness (nm)|