**5. Inkjet printing**

Inkjet printing is a relatively novel process compared to industrial printing and other thin film coating technologies. Inkjet printing involves delivering of a small volume of a fluid material, typically in the picoliter to nanoliter range, onto the substrate surface [9]. Inkjet printers comprises of three basic parts, the motion stage, control systems attached with the print heads, and vision system. The printer heads are connected directly to the cartridge filled with solutions. Inkjet printing systems typically require three mechanical degrees of freedom, one rotational (θ stage) and two translational (X and Y stages) to generate 2D patterns align with previously printed patterns and realize facilely stacked structures. The vision system is employed for substrate alignment and to observe ejected droplets in flight to monitor the droplets. Whereas, the control systems is used to optimize the stage and the printer head temperature, which can directly influence the substrate temperature and the dropping velocity, respectively [10].

Two different approaches are used to translate ink on the substrate surface: thermal and piezoelectric approach as depicted in **Figure 5**. The print cartridges consist of a series of tiny chambers and each chamber is attached with a heater over the nozzle. In case of thermal printing approach, a pulse of current is applied to the heater which leads to a rapid vaporization of the ink and build up pressure which push the droplet of ink out of nozzle [11]. The internal temperature creates a bubble in the cartridge and facilitates the single droplet formation of ink out of the nozzle via volume expansion. The negative pressure inside the cartridge after the droplet is ejected, draws new ink inside the reservoir [10]. In piezoelectric approach, the ink is ejected out from a nozzle through a sudden quasi-adiabatic reduction of the chamber volume via piezoelectric action. In the initial state, the ink in the printer head is in equilibrium state. Upon applying a voltage pulse signal to the piezoelectric element, the ink is ejected out due to the volume expansion inside the printer head. As the kinetic energy overcome the threshold, an in-flight droplet is generated and flights towards the target surface. In the sequentially applied opposite voltage, the ink in the printer head refills, and the whole process is repeated.

#### **Figure 5.**

*Schematic of inkjet printing (a) thermal printing. (b) Piezoelectric printing.*

Several thousand droplets are delivered to substrate in every second and pinter head also moves at the same time. Thermal printing is simple in design and low cost approach, however, it is confined to vaporizable inks to form bubble. The elevated operating temperature makes it inappropriate for polymer-based printing. Therefore, piezoelectric printing is widely used than thermal printing [12].

Ink viscosity and surface tension are crucial parameters during inkjet printing process. The low value of viscosity is required to allow the ink to fill the chamber as well as nozzle and the high surface tension enough to hold the ink in the nozzle without dripping [13]. To create droplets via thermal nozzles, the inks must be heatcompatible and sensitive to the volume contraction/expansion depending on the temperature. The piezoelectric nozzles contain a piezoelectric film along the wall of a reservoir. The deformation of the film generates the mechanical volume expansion in response to applied voltage pulses and as result the ink is ejected in response to the pressure generated by the piezoelectric element [10]. Typical inks used with a piezoelectric printing require a viscosity of 0.5–40 cp and a surface tension of 20–70 dyne/cm as the piezoelectric transducer only generates limited power, and printing high viscosity ink is difficult [14]. The piezoelectric nozzles have relatively better resolution and require lower temperature, which enables more precise operation to deposit thin film and do not suffer from ink degradation concerns and temperature-sensitive solvent choice. However thermal nozzles are typically less expensive and widely used in commercial printers [10].

Stable drop formation without satellite droplets after ejection from the nozzles is also important to obtain well-defined printed patterns on a substrate. The droplet velocity and volume are strongly depend on the pulse width and amplitude. The size of the droplets increases linearly with the size of the nozzle. However, the fine droplets generates high resolution and high surface morphology of final printed film. Usually nozzle size, droplets shape and fly direction are determined by the manufacture company and these parameters depend on the composition of the ink, especially on the solvent. Surface tension and viscosity are the primary physical properties that determine the shape and droplet-tail of in-flight droplets, and satellite droplet formation [10].

#### **6. Summary**

Spin coating, dip coating and spray coating are very common techniques and widely used to deposit thin films in research laboratories as well as industries. The

#### *Thin Film Deposition: Solution Based Approach DOI: http://dx.doi.org/10.5772/intechopen.94455*

spin coating is one of the important route for lab scale due high reproducibility and its suitability over a wide viscosity range. It is a quick and easy approach to grow uniform film at small scale from a few nanometers to a few micrometers in thickness. However, it is not suitable for scale up in industry due to its high material consumption and the restriction to large area. Dip coating is a commonly used method for mirror coating and dye processing as it can provide easy and fast deposition thin films over a large area. The key advantages is the large area processing and the thickness of film can be controlled by withdraw speed and viscosity of solution. Dip coating requires a high volume of coating solution and large tanks. Despite of long deposition life (several months) only about 20% of the solution can be used. The spray coating technique is able to access a broad spectrum of fluids, and offers the opportunity to tune the system to deposit any kinds of solution and obtain the desired film thickness. It is reproducible, and have great potential for large scale production. Inkjet printing is a powerful and cost-effective technique for deposition of liquid inks with high accuracy. The special characteristics offered by inkjet printing includes additive patterning, reducing materials consumption, non-contact deposition, low cost and capability of large area. Moreover, inkjet printing is capable of deposition a given material on a substrate that has pre-existing patterns, where contamination or damage of patterns would be induced with other deposition processes. However, to deposit materials on substrates, the solution or ink must be compatible with the print head and the viscosity should within a specific range.
