AVS2010 Session PS2-MoA: Atmospheric Plasma Processing and Micro Plasmas

The dielectric barrier discharge (DBD) is recognized as a promising tool of thin films deposition on various substrates at atmospheric pressure. Emerging applications including encapsulation of flexible solar cells and flexible displays require large scale low costs production of transparent uniform dense layers with low level of surface defects. Unfortunately the common operational mode of the atmospheric pressure DBD is filamentary, resulting in strong spatial non-uniformity of plasma chemistry and affecting the quality of the deposited films. Sustaining of the filament-free non-thermal plasma over the large area substrate at atmospheric pressure remains a challenging task especially considering the discharge in ambient air [1].

In present contribution the diffuse dielectric barrier discharge in atmospheric pressure air was applied for the thin film deposition on polymeric web in industrially relevant roll-to-roll configuration. The silica-like film deposition was performed using the admixture of hexamethyldisiloxane precursor to air flow. Discharge diagnostics was realized by means of fast ICCD imaging; time resolved optical emission spectroscopy and electrical characteristics analysis. ICCD discharge imaging confirms plasma uniformity in a microsecond time scale, while at nanosecond time scale shows fast propagating lateral ionization waves. Morphology and composition analyses, performed by means of AFM, ATR-FTIR and XPS methods, indicate that the process results in ultra-smooth films (roughness comparable to initial substrate roughness) and shows the possibility to synthesize carbon-free layers.

Large-scale, low-pressure plasmas play an essential role in the manufacturing of integrated circuits that are now ubiquitous in consumer electronics. In recent years, new challenges have arisen for these top-down approaches to materials processing. Future electronic devices will incorporate nanoscale materials such as nanoparticles, carbon nanotubes, and silicon nanowires that cannot be fabricated by current plasma technology because of limitations associated with photolithography. In addition, emerging applications in sensors, energy, and medicine require materials that must be prepared from the “bottom-up”. The aim of our research is to develop a new class of plasmas, termed “microplasmas”, for nanomaterials synthesis.

Microscale plasmas or microplasmas are a special class of electrical discharges formed in geometries where at least one dimension is less than 1 mm. As a result of their unique scaling, microplasmas operate stably at atmospheric pressure and contain large concentrations of energetic electrons (1-10 eV). These properties are attractive for a range of nanomaterials applications. Vapor-phase metal-organic precursors can be dissociated at ambient conditions (i.e. room temperature and atmospheric pressure) to homogeneously nucleate metal¹ and alloyed² nanoparticles. The formation of metal nanoparticles in the gas phase allows direct introduction of these materials as catalysts for carbon nanotube and silicon nanowire growth³. Recently, we have also coupled microplasmas with liquids or polymeric films to nucleate nanoparticles from metal ions⁴. In this talk, I will discuss these topics in detail, highlighting the advantages of microplasma-based systems for the synthesis of well-defined nanomaterials.

1. W-H. Chiang and R. M. Sankaran, “Microplasma synthesis of metal nanoparticles for gas-phase studies of catalyzed carbon nanotube growth,” Appl. Phys. Lett., Vol. 91, 121503 (2007).

2. W-H. Chiang and R. M. Sankaran, “Synergistic effects in bimetallic nanoparticles for low temperature carbon nanotube growth,” Adv. Mater., Vol. 20, 4857 (2008).

3. W-H. Chiang and R. M. Sankaran, “Linking catalyst composition to chirality distributions of as-grown single-walled carbon nanotubes by tuning Ni Fe nanoparticles,” Nat. Mater., Vol. 8, 882 (2009).

4. C. Richmonds and R. M. Sankaran, “Plasma-liquid electrochemistry: Rapid synthesis of colloidal metal nanoparticles by microplasma reduction of aqueous cations,” Appl. Phys. Lett., Vol. 93, 131501 (2008).

Micro-discharges can be produced in a variety of different configurations and using a variety of different technologies. The properties of these micro-discharges have been examined with increasing interest during the past decade. There has also been intense interest in new fabrication techniques for the production of micro-discharges. In this paper we present the fabrication technology used to make micro-discharge “reactors” on a silicon (Si) substrate along with selected plasma properties. For the fabrication of these reactors we have used 4” Si wafers and standard cleanroom facilities. The fabrication technology used is compatible with standard CMOS device fabrication and the micro-discharge reactors fabricated can be used to produce either DC or AC discharges. The micro-discharges operate well at high-pressure. They were given planar cathodes and ring-shaped anodes separated by a SiO₂ dielectric with a thickness of approximately 5-6 μm rather than the much more common ~100 μm. The micro-discharge reactors can consist of either a single hole or multiple holes and we have built devices with holes from 25 to 150 microns in diameter. The micro-discharge measurements were obtained for inert gas, DC plasmas between 100 and 1000 Torr. Although we used only a single ballast resistor, many hole micro-discharges were obtained because the cathode surface area of each micro-discharge was limited. This also acts to limit the discharge power. In addition, we will report on current leakage and parasitic sparks during high pressure operation. Finally, we discuss the spread of the micro-discharge over the anode and cathode surfaces as well as the life time of the micro-discharge reactors.

Microdischarges have gained the interest of the plasma community for the 15 past years. Among them, Micro Hollow cathode Discharges (MHCDs) have the very interesting property to operate at atmospheric pressure in a stable non thermal regime. We have studied the ignition and extinction of such microdischarges. Our samples are made in alumina, covered by a 5 µm thick Nickel layer on both sides and drilled by a laser process. We made experiments in helium and argon at a pressure between 100 and 1000 Torr. To initiate the plasma, we increase the discharge voltage linearly and slowly (20 sec) until the voltage breakdown was reached. During the microplasma ignition, a high current pulse as high as several tens of microamps appeared before a stable and constant lower value was obtained. We will compare these current pulses to those obtained in the so-called self pulsing regime. We will also show electrical and optical measurements carried out to characterize the phenomenon. Finally, we will show the discharge current and voltage temporal evolution at the very last moments of the microplasma, just before its extinction. The physical mechanism of the ignition and extinction will be discussed to explain the measured waveforms.

The Center for Plasma-Material Interactions (CPMI) at the University of Illinois at Urbana-Champaign has developed large-scale microwave-induced atmospheric plasma sources for use in various manufacturing applications. The microwave source employed has a working frequency at 2.45 GHz, and a maximum input power of 6 kW. Plasma sources of three different configurations have been developed in order to tailor the plasma configuration to various specific applications. A cold plasma torch head has the ability to generate an atmospheric plasma with a temperature range from room temperature (20°C) to more than 1,000 °C. A thermal plasma torch has been developed such that the temperature range extends to 2,000 °C. A linear line source suitable for production line integration has the ability to sustain a 20-centimeter long atmospheric plasma. Various gas compositions (He, Ar, N₂ and O₂) are used to reveal the functionalizations of different radicals and particles. OES system has been used to analyze critical characteristics such as electron density (n_e~10¹⁴cm^-3) and temperature (T_e~1eV), plasma temperature (T_g~300-2,000K) under different operating conditions and results of material processing correlated to those measurements so that a selectable and repeatable material process can be obtained. Hydrophilicity tests on polymer substrates reveal that the “cold” atmospheric plasma has the ability to modify the surface energy within seconds of exposure at a relatively low flux of incident particles, without deforming bulk material substrates; while the “thermal” atmospheric plasma is used to assist in Nd:YAG laser ablation (f=100Hz, P_av = 2.0W at 266 nm, 12.5W at 532nm, and 32.5W at 1064nm) of metal or ceramic materials, to provide a means to deposit high quality contamination free films on substrate with better lamination at a relatively higher deposition rate (~5,000 nm/min) in comparison to traditional PVD methods.

This presentation includes the diagnostic studies of an atmospheric pressure plasma jet (APPJ) and the use of this plasma jet to perform ZnO thin film deposition. The APPJ under investigation is sustained by a pulsed power supply with a repetitive power frequency up to 25 kHz using N₂ or O₂ as the plasma gas. The assessment of how the ambient air influences the plasma characteristics and how it can effectively be minimized are the focus of the diagnostic work. To minimize the ambient air influence, the exit of the jet is shielded with a glass tube with the inside diameter ranging from 3 to 5 cm . The exit of the tube is covered by a metal plate and leaving a gap of 0.5 to 3 mm. When the N₂ plasma is used, the visible jet length is much longer with such an arrangement. The effective area within which the jet is treated increases by more than a factor of two, as confirmed by the contact angle measurement made on the treated glass surface. In O₂ plasmas, the intensity of atomic oxygen emission (777.4 nm) increases by more than one order of magnitude with the presence of the glass tube. When photoresist is etched using this oxygen plasma jet, the jet with the presence of the glass tube shows a increase in the etching rate by more than 50 % than the case without the presence of glass tube.

The use of this APPJ to perform ZnO thin film deposition is studied as the second part of this presentation. ZnO thin films are deposited on a silicon wafer by spraying nebulized zinc-containing salt solutions, namely ZnCl₂ and Zn(NO₃)₂, into the downstream of the plasma jet. Preliminary studies show that by using N₂ plasmas, a better quality film can be obtained comparing with using O₂ plasmas. The film quality is found to be sensitive to the plasma conditions. With properly adjusted process parameters, dense and smooth films can be deposited with a rate higher than 75 nm/min. Improvement of the electrical conductivity and the study of the photoluminescence properties of the film are currently underway.