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Cold Plasmas

Low-temperature plasmas are often referred to as gas discharges, in which the plasma is generated electrically after supplying a gas with a sufficiently high voltage (ionization process of gas). The plasma is characterized by an electron concentration and an electron energy distribution. Free electrons gain energy via acceleration by the applied field. These energetic electrons lose energy by inelastic collisions with neutral species, causing excitation and ionization. The generated plasma consists of ions, electrons, and neutral species in both ground and excited states.

Low-temperature plasma treatment has been used in the last years as a useful tool to modify the surface properties of different materials by the interaction of energetic particles (ions, electrons, neutrals) and photons with surfaces exposed to plasmas. A great variety of plasmas originates from the possibility to vary their parameters: chemical composition, pressure, power, and electromagnetic field structure or reactor configuration. Consequently, plasmas are employed for different industrial applications (Fig. 1), such as cleaning and activation, etching, chemical functionalization and coating of any materials: metals, plastics, rubber, composites, glass, textiles, among others.

Industrial applications of plasmas
Industrial applications of plasmas
 

Black-silicon production process by plasma

The physical structuring of silicon is one of the cornerstones of modern microelectronics and integrated circuits. Regarding Silicon-based optical detectors and solar cells, the high reflectivity at input surface can hinder efficient light collection. Surface texturing is one way to minimize unwanted reflections and lately, the anti-reflection effect of the Si surface texturization has been particularly studied in view of light absorption enhancement in solar cells. The texturing of the Si surface was also proved to be effective in functionalizing it as a scaffold for biomedical applications, by providing selective antibacterial characteristics. In these fields of application, the realization of high-aspect-ratio vertical features on Si substrates is the crucial step in the fabrication process. For this purpose, laser-based manufacturing processes have been proposed and developed; wet chemical etchants can also be exploited to create anisotropic profiles, because they are cost-effective and easy to use, but not environmentally friendly. The use of dry processes instead of wet has been strongly supported in recent years, and dry plasma etching has become a conventional technology in microelectronics. In particular, halogen-based plasmas have been extensively used for Si etching. What is usually seen as an undesirable effect in plasma etching is surface roughening, which can usefully be controlled to induce Si texturing under certain experimental conditions. The Si texturization has been studied mainly in fluorine-based plasmas by exploiting the random automasking effect on surface during the etching. It has been demonstrated that the automasking effect is obtained if a random passivation is generated during the etching. The passivation is essential for protection of sidewalls and obtaining vertical features. C4F8 gas has been used as the reactant to produce a polymeric passivation layer which protects the trench sidewalls realized in the etching process by fluorine based gas such as SF6. Maruyama et al. reported a texturization process which employed SF6 plasma in the etching process and a mixture of O2 gas during the passivation step. It is worth mentioning that the latest process exploits a low-density plasma power and no polymeric passivation is needed, instead a growing SiOxFy layer is formed during the passivation step. In this experimental activity, we report on the development of a method, based on hydrogen in place of oxygen, for the formation of the passivation layer. The purpose of this research is the fabrication of nano-structures on Si substrates (See figures below), to be used either to enhance light absorption for opto-electronics and photovoltaic applications, or to provide patterned surfaces for bio-medical applications. The process is realized in a low-density Capacitively-coupled plasma RIE  reactor in CF4/H2 mixture.

Plasma textured Si wafer (top view)
Plasma textured Si wafer (top view)
Plasma textured Si wafer (cross-sectional view)
Plasma textured Si wafer (cross-sectional view)
Spectral reflectance of reference Si wafer
Spectral reflectance of reference Si wafer (A) and of plasma textured Si (B)

 

 

 

E. Vassallo et al., Thin Solid Films 603 (2016) 173-179

DOI: 10.1016/j.tsf.2016.02.008

 

Tungsten oxide thin film photo-anodes produced by RF plasma sputtering

Research on photocatalytic materials has attracted much interest as a sustainable method for water splitting, pollutant degradation and synthetic fuel production. The exposure of semiconducting oxides to light at photon energies higher than their band gap induces inter-band transition, resulting in generation of electron-hole pairs. The photogenerated electrons and holes can then react with species adsorbed onto the semiconductor surface. Among candidate semiconductor photocatalysts, TiO2 is perhaps the most widely studied because of its low cost, stability, nontoxicity, and vast range of applications. TiO2, however, absorbs only ultraviolet (UV) light due to its wide band gap (≈ 3.2 eV), causing the efficiency of solar energy conversion into chemical energy to be limited to ≈ 4% of the sunlight radiation. Therefore, a photocatalyst that can be activated with a broader range of wavelengths is highly desirable. In this context, tungsten oxide (WO3) featuring  a band gap in the range from 2.6 eV to 2.8 eV has been recently proposed as a semiconductor photocatalyst active in the visible (Vis) range, enabling efficient light absorption at λ < 500 nm. Moreover, photogenerated holes in WO3 can oxidize virtually a wide range of environmental pollutants, as recently reported. The photocatalytic activity depends on the electronic structure, which depends on the crystal structure and surface morphology. In the W-O system, many crystalline phases are possible due to both the tendency for sub-stoichiometric compounds formation and the variable crystalline arrangements. According to the literature, WO3 possible crystalline phases are triclinic, monoclinic, orthorhombic and tetragonal. The stability of each phase is influenced by the preparation methods. For example, nanocrystalline thin films growth conditions are critical, including type of substrate, temperature, and pressure. On the other hand nano-structured WO3 has turned out to have much higher photocatalytic activity compared to bulk WO3 photolectrodes. The small size of grains in a material can substantially influence optical properties, charge transport and electronic band structure. In fact apart from high surface area, which is fundamental for photocatalytic applications, nano-WO3 exhibits structures and morphologies with unique properties that do not exist in bulk. 

A variety of technological processes have been demonstrated to synthesize tungsten oxide, including electrochemical synthesis, laser technique, pulsed laser deposition and sol-gel technique, however, due to either the high thermal budget required or the excessively long time processing, these techniques are likely to be not scalable to industrial production. In this research activity, we report on WO3 fabrication process based on radio-frequency reactive plasma sputtering in diode configuration and the characterization of the structural, optical and photocatalytic properties of the deposited films. 

The experimental apparatus consists of a parallel-plate, capacitive-coupled system, made up of a cylindrical

stainless steel vacuum chamber with an asymmetric electrode configuration [1-2]. A powered electrode (water cooled) is connected to an RF power supply, coupled with an automatic impedance matching unit, while the other electrode, made up of stainless steel, is grounded. A 3-in diameter target of W (purity 99.9%) is placed on the powered electrode. The substrates are placed on the ground electrode at 4-6 cm away from the powered electrode. The sputtering power can be set up to 300 W (up to 1500 V of  DC self-bias voltage). The substrate can be heated up to 420 ° C. The substrate temperature is monitored by a thermocouple fixed directly on the substrate. No bias voltage is applied to the substrate holder. The process chamber is pumped to a base pressure below 1 × 10-5 Pa; high-purity gases Ar/O2 are introduced into the vacuum chamber through a mass flow controller in order to establish the desired working pressure (1.5-3 Pa). Ar plasma promotes the W target sputtering while O2 is added to induce chemical reactions with the atoms ejected from the target forming tungsten oxide.

 

Plasma sputtering system
Layout of a plasma sputtering system
 
 
 
H.R. Koenig, L.I. Maissel, IBM J. Res. Dev. 14 (1970) 168.
 
M. Pedroni et al., Thin Solid Films 616 (2016) 375–380.
 

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