Very high frequency plasma reactant for atomic layer deposition
Graphical abstract
Introduction
High-permittivity (high-k) dielectrics have been explored as an alternative gate insulator to conventional SiO2 in metal-oxide-semiconductor-field-effect-transistor (MOSFET) technology [1], [2], [3]. Among various high-k deposition techniques, atomic layer deposition (ALD) is favored due to its excellent process controllability for extremely thin high-k films [4]. Plasma-enhanced ALD (PE-ALD) using plasma reactants has been spotlighted because of several advantages over conventional thermal ALD using gas reactants, such as high growth per cycle (GPC) and a wide process window induced by the high chemical reactivity of plasma radicals in the reactant [5], [6], [7], [8]. High-k dielectrics produced via PE-ALD have been widely used for dynamic random access memory (DRAM) capacitor insulators and MOSFET gate dielectrics [9], [10], [11].
Some of the plasma sources that generate ions such as the direct plasma and the remote plasma may negatively influence the electrical properties of the high-k films in MOSFETs because ion bombardment by the energetic ions can generate defects in the films. For instance, plasma radicals mechanically penetrate films and disrupt chemical stoichiometry, leading to deterioration of the electrical properties of the oxide [12], [13], [14]. In addition, the highly energetic plasma radicals usually generate a thick interlayer between the PE-ALD high-k films and Si substrate. Since the formation of an interlayer changes a designed k value and affects the electrical properties in the high-k stack, the deposition of an interlayer-free high-k is desired. Generally, the interlayer in the PE-ALD is thicker than that in thermal ALD, since the energetic species of PE-ALD, such as plasma ions and/or radicals, can react with substrate atoms farther from the surface than those accessible with thermal ALD [15], [16]. Researchers have devoted significant effort to minimize the negative effects of plasma on film quality and device properties. In the attempts for a PE-CVD, a pulsed plasma was used in place of the continuous plasma to reduce the ion bombardment period on the surface by shortening the total plasma exposure time [6], [17], [18], [19]. In other reports, plasma was remotely generated and transferred to the deposition chamber, so that the substrates were less affected by ion bombardment than in the case of direct plasma exposure [20], [21].
Very high frequency (VHF) plasma was used in place of the commonly-used 13.56 MHz radio frequency (RF) plasma in the report on plasma-enhanced chemical vapor deposition (PE-CVD) processes [22], [23], [24], [25]. The VHF plasma typically has a frequency in the range of 60–150 MHz, so that the plasma density of VHF plasma is higher than that of RF plasma, and the kinetic energy of ions and/or radicals on the surface in VHF plasma is usually lower than that in RF plasma [26]. Compared to RF plasma, the negative effects of plasma-on-film properties are less significant in the VHF plasma process due to its low-ion kinetic energy and high plasma density [26], [27]. For amorphous Si deposition, the VHF PE-CVD process shows several advantages over RF PE-CVD, such as high film quality and high growth rate, rendering higher production yield and lower cost [27]. Therefore, VHF plasma is expected to be superior to RF plasma as a reactant for thin film deposition processes in the context of higher GPC and lower ion damage [28], [29], [30]. Although a low-damage and interlayer-free PE-ALD process has great importance for industrial purposes as well as scientific understanding, there has been no report on PE-ALD using VHF plasma as well as PE-CVD oxide using VHF plasma so far. Since the key mechanism of CVD growth is quite different to that of ALD growth, the application of VHF plasma to PE-ALD is not simple but the process conditions, such as plasma matching, should be carefully controlled. Furthermore, the conventional thermal ALD and RF PE-ALD have well met the requirements of thin film deposition so far, however, the current real nanodevices fabricated in the range of few nanometers require much better quality and precise control in thin film deposition. Therefore, we employed a new plasma source for novel PE-ALD to achieve high quality dielectric layers for the next generation nanodevices.
For this study, we conducted a comparative study of growth characteristics and film properties by varying plasma process parameters. Al2O3 was selected due to its applicability for a wide range of applications, such as a high-k dielectric for MOSFET gate oxides [31], a high-density moisture barrier for an organic light emitting diode (OLED) [32], and surface passivation for silicon solar cells [33]. Al2O3 films were grown using 13.56 MHz for RF and 60 for VHF PE-ALD with plasma generating powers of 100, 200, and 300 W. The effects on plasma were studied in terms of ALD growth characteristics and film properties such as roughness, density, interlayer thickness, conformality, chemical composition, and electrical properties.
Section snippets
Materials and methods
We used a commercial ALD chamber (ALD5008 of SNTEK Co.). The ALD system has a double showerhead for improved uniformity. A plasma matcher controls the variable capacitor to optimize the forward and reflected power, delivering constant power to the process chamber. O2 plasma of the capacitively-coupled plasma (CCP) type was generated between the showerhead connected to the plasma matcher and the substrate, which was grounded during the reactant exposure step. For PE-ALD, plasma reactant was
Results and discussion
Fig. 1(a) and (b) show the plots of plasma density and electron temperature at various plasma frequencies with increasing plasma power from 100 W to 500 W. As shown in Fig. 1(a), the plasma density increased almost linearly for both frequencies with increasing plasma power. With increasing input power, the electric field between source and substrate was increased due to increased plasma voltage, so that the acceleration of electrons was increased. Therefore, it caused the increase of collision
Conclusions
In summary, the growth characteristics and film properties of PE-ALD Al2O3 using RF and VHF plasma reactant were systematically investigated. Compared to RF plasma, VHF plasma significantly reduced plasma damage due to its lower electron temperature. Low-energy ions in the VHF plasma effectively reduce the negative effect of ion bombardment on the Si substrate, so that the resulting PE-ALD Al2O3 films have beneficial dielectric properties such as interlayer-free PE-ALD, high dielectric
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Acknowledgments
This work was supported by the Industrial Strategic Technology Development Program (10041926, Development of high-density plasma technologies for thin-film deposition of nanoscale semiconductors and flexible display processing) funded by the Ministry of Knowledge Economy (MKE, Korea) and by the MOTIE (Ministry of Trade, Industry & Energy (10053098)) and KSRC (Korea Semiconductor Research Consortium) support program for the development of future semiconductor devices.
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