Electrospray droplet sources for thin film deposition

Electrospraying utilises electrical forces for liquid atomisation. Droplets obtained by this method are highly charged to a fraction of the Rayleigh limit. The advantage of electrospraying is that the droplets can be extremely small, down to the order of 10’s nanometres, and the charge and size of the droplets can be controlled to some extent be electrical means. Motion of the charged droplets can be controlled by electric field. The deposition efficiency of the charged spray on an object is usually higher than that for uncharged droplets. Electrospray is, or potentially can be applied to many processes in industry and in scientific instruments manufacturing. The paper reviews electrospray methods and devices, including liquid metal ion sources, used for thin film deposition. This technique is applied in modern material technologies, microelectronics, micromachining, and nanotechnology. Introduction Electrospraying is a method of liquid atomisation by electrical forces. The atomiser nozzle is usually made in the form of metal capillary, which is biased by a high voltage. The shear stress on the liquid surface, due to the established electric field, causes elongation of a jet and its disintegration into droplets. The droplets obtained by this method can be extremely small, in special cases down to nanometers. The advantage of the electrospray is that droplets are highly charged, up to a fraction of the Rayleigh limit. The Rayleigh limit [1] is the magnitude of charge on a drop, which overcomes the surface tension force that leads to the drop fission. This charge is given by the following equation: QR 1⁄4 2pð16rle0rÞ ð1Þ in which rl is the liquid surface tension, e0 is the electric permittivity of the free space, and r is the droplet radius. The charge and size of the droplets can be controlled to some extent by adjusting the liquid flow rate and the voltage applied to the nozzle. Charged droplets are self-dispersing in the space due to mutual Coulomb repulsion, that results in the absence of droplet’s agglomeration. The expansion of a spherical cloud of charged droplets is given by the Eq. [2]: 1 cd dcd dt 1⁄4 2Cce0cdE 2 ds ggql 1⁄4 cd sexp ð2Þ in which cd is the initial mass concentration of the droplets, gg is the gas dynamic viscosity, ql is the liquid density, and Cc is the Cunningham slip correction factor. Eds is the electric field on a single droplet surface charged to the magnitude Qd: Eds 1⁄4 Qd 4pe0r ð3Þ In Eq. (2) sexp is time constant of the expansion of the cloud: A. Jaworek (&) Institute of Fluid Flow Machinery, Polish Academy of Sciences, Fiszera 14, Gdańsk 80-952, Poland e-mail: [email protected] J Mater Sci (2007) 42:266–297 DOI 10.1007/s10853-006-0842-9 123 Electrospray droplet sources for thin film deposition A. Jaworek Received: 23 August 2004 / Accepted: 17 October 2005 / Published online: 28 November 2006 Springer Science+Business Media, LLC 2006 sexp 1⁄4 ggql 2Cce0Eds ð4Þ The mass concentration of the droplets due to the cloud expansion decreases reciprocally with time: cd 1⁄4 cd0 1 t sexp ð5Þ The motion of the charged droplets can be easily controlled (including deflection or focusing) by an electric field. The deposition of a charged spray or solid particles on an object can be more effective than for un-charged one [3]. An apparatus for electrospraying is very simple and cheap. Main shortcoming of electrospraying, limiting its widespread use in industry, is its low throughput. To overcome this problem, the multi-nozzle or slit-nozzle systems [4–11] were proposed. Mechanical spraying by rotary [12–14] or pneumatic atomisers [15–17] with grounded nozzle and high voltage induction electrode can also be used for production of large amount of charged spray. However, the charge of the droplets produced by this method is one order of magnitude lower than the Rayleigh limit. One of the most important features characterising any spray system is the mode of spraying. There were many spraying modes discovered and discussed in the literature [18–25], but they can be categorised into two main groups: • The first group, which is characteristic in that only fragments of liquid are ejected directly from the meniscus at the capillary outlet. These fragments can be in the form of regular large drops, fine droplets, or elongated spindles at the moment of their detachment. • In the second group, the liquid is elongated into a fine jet, which disintegrates into droplets due to its instability. It was observed that the jet could be smooth and stable or could move in a regular way: rotate around the capillary axis or oscillate in its plane [22]. Sometimes a few jets on the circumference of the capillary can be formed. Charged sprays found application in many fields, including painting or thin film deposition. The physical and chemical methods for thin and thick films deposition from gaseous, solution, molten or solid state were reviewed by Altenburg et al. [26] but only minor attention was devoted to the electrospraying. Recently, Choy [27] reviewed of the current and potential development of chemical vapour deposition processes and their application to film deposition, with only brief presentation of electrospraying. Therefore, there is a need for presentation of electrospray applications in the thin film technology with a summary of the benefits it offers in this field. The purpose of this paper is to outline electrospray devices, including liquid metal ion sources, used for thin solid film deposition. The advantages that electrospray has over other methods of metal or ceramic film production are also pointed out. All the relevant details of the available technical data regarding fundamental experiments and laboratory demonstrations on the electrostatic method of thin film deposition are summarised in the tables. Thin solid film deposition Introduction Thin solid films are used to improve surface properties of mechanical elements or in scientific or measuring instruments, and in electronic devices. Electrostatic deposition is the process of depositing a material on a substrate by electrical forces. Initially, electrospray was used to produce thin layers of radioactive materials, such as aor b particle sources, or targets prepared for activation in particle accelerators or nuclear reactors. Recently, electrospray was used for thin film deposition in nanotechnology and nanoelectronics. There are several methods used for thin layer deposition on a substrate: 1. casting of a solution or colloid suspension on a substrate, followed by solvent evaporation, 2. cathode spraying, applicable for metal layers preparation, 3. condensation of vapours of a material on the substrate, 4. radio-frequency sputtering, 5. laser ablation, 6. chemical vapour deposition, 7. physical vapour deposition, 8. microwave plasma coating, 9. flame-assisted vapour deposition, 10. electrodeposition of the layer by electrolysis, used for metals deposition, 11. electrospraying. Large amount of material is lost to the chamber walls when cathode spraying, chemical vapour deposition, or vapour condensation is used. When a solution or suspension of a material to be deposited is sprayed J Mater Sci (2007) 42:266–297 267