The methods of deposition IGZO films have distinct classification. In this paper, we categorize them in physical and chemical type that representing the source of films is solid and liquid. Following the exposition will discuss them in detail.
2.3.1 Magnetron sputtering
The sputtering method is working in the glow discharging region which has higher energy and density of electrons. To put the substrate at anode and set the targets at cathode in the argon ambient, then the cations which accelerated by the electrical field bombard the target. At this time, the targets of atoms are leaved out and going to the substrate to form the IGZO films. The reason of the cations which is driving to target is the potential of plasma always higher than chamber, target and substrate.
Moreover, target connect with cathode will increase the potential difference between plasma and target. If setting a magnet under the target, there was an external magnetic field to increase the plasma density, so the
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cations which bombard the target will increase simultaneously. It is difficult to discharge with the DC power when the low conductivity materials or insulators were to be the deposition substrate. So, there have to use the RF power which the frequency needs to reach the grade of megahertz (usually 13.56MHz) to be the power supply. Using the self-bias phenomenon in the RF discharge, it can make the target potential will always in negative values to ensure the bombardment will almost continuously which is same capability as the DC discharge.
So far, the sputtering technique is one of the most common in deposition the IGZO films, including electrodes of flat display and energy efficiency windows. The generally properties of sputtering are described following:
(1) Widely scope of the process films such as metal, alloy and insulator.
(2) The films thickness can control by the apply power and process times.
(3) The stable, uniform and large area films can be obtained.
(4) Because of higher bombardment energy, so it can deposit the excellent adhesion and crystallization films.
(5) Long target lifetime, so it can operate at continuous and automatic long time process.
There is another magnetron sputtering method which getting high density plasma by RF-DC couple manner. In general RF magnetron sputtering, the self-bias of the target can change with the RF frequency and power, and it controls the ion energy of bombarding to the target.
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When the RF frequency increased, the self-bias of the target will decreased, the RF power increased, the self-bias of the target will also increase. However, in the RF-DC couple magnetron sputtering system, the RF power is mainly to generate high density plasma, and the DC power is to adjust the electric potential of the target, by doing that is easy to control the deposition conditions.
2.3.2 Pulsed laser deposition
Pulsed laser deposition (PLD) is a technology where a high energy density focusing pulsed laser beam is struck a target of the material that is to be deposited. The target is vaporized which deposits it as a thin film on substrate. This method can be applied on many materials, so it can deposit lots of thin films. But the growth rate of PLD is extremely slow;
therefore, it is not a mass production technology. This process can occur in ultra-high vacuum or in the presence of a ambient gas, such as oxygen which is commonly used when depositing oxide to fully oxygenate the deposited films.
The PLD basic machinery is simple relative to many other deposition techniques, the physical phenomena between laser and target interaction and the film growth are quite complex. The absorption energy is converted to electronic excitation and thermal energy resulting in evaporation and plasma formation when the laser pulse is absorbed by the target. The ejected varieties full of the surrounding vacuum including atoms, molecules, electrons, ions, clusters, particulates and molten globules, before depositing on the typically hot substrate.
19 different source of precursors are required for each element of the desired compound.
(2) It can be maintained the target composition in the deposited thin films. Because of the very short duration and high energy of the laser pulse, target material immediately toward the substrate, every component of the phase has an analogous deposition rate, so the thin films composition is almost unchanged.
(3) The energy associated with the high ionic content in laser ablation plumes and high particle velocities appear to aid crystal growth and lower the substrate temperature required for epitaxy.
(4) PLD is clean, low cost and capable of producing simply by switching several different targets.
There are also a heaps of advantages of PLD, these include:
(1) PLD brings difficulty to controlling thickness uniformity across the sample, but this problem can be overcome, to some extent, by scanning the laser beam on a larger size target.
(2) The plume of ablated material is highly forward directed, which causes poor conformal step coverage. It also makes thickness monitoring difficult.
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(3) There is an intrinsic splashing associated with laser ablation itself, which produces droplets or big particles of the target material on the substrate surface. This is particularly serious as it will result in device failure from an industrial perspective.
2.3.3 Spray pyrolysis
Deposition IGZO thin films by spray pyrolysis has been used for a long time. The deposition material can use solid or liquid source, according to the previous statement of the definition of the deposition method, it may be categorized to the physical type, but it is similar to the CVD method, so we still categorizing it in chemical manner.
Spray pyrolysis is the most in common uses in the pyrolysis manners.
The precursor solution is pulverized as affine mist via a spray nozzle and a carrier gas at high pressure in spray pyrolysis. The so produced mist condenses on a preheated substrate, and is instantly pyrolysed (spray pyrolysis). The process can be conducted in one or more pulses to obtain uniform films. Spray pyrolysis is suitable for substrate with complex geometry, and can be used for a variety of oxide materials. Although the first impression of spray pyrolysis is simple to do, but is concerns at least seven parameters, including heater temperature, carrier gas flow rate, gap distance, solution drop size, solution component, solution flow rate and substrate velocity through the heater.
There is different reaction with increasing the substrate temperature when the solution drops leave from nozzle to the substrate. From figure 2-7, in process A, the solution drop sprinkled on the substrate, vaporizes,
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then leaves a dry precipitate in which decomposition occurs; in process B, the solvent evaporates before the solution drop arrives at the surface and the precipitate bombards upon the surface where decomposition occurs;
in process C, the solvent vaporizes as the solution drop accesses the substrate, then the solid melts and vaporizes, its vapor diffuses to the substrate to undergo a heterogeneous reaction there; in process D, at the hugest temperatures, the metallic compound vaporizes before it arrives the substrate and the chemical reaction takes place in the vapor phase.
Apparently, we hope not to happen to the process A and D, because it will cause rough and viscosity thin films. So, select the appropriate substrate temperature and make the uniform and equal size of droplet will help the reaction perfectly.
The advantages of spray pyrolysis are summarized below:
(1) The spray pyrolysis can be easy and cheap.
(2) Substrate with complex geometries can be coated.
(3) Leads to uniform and high quality coatings.
(4) Low crystallization temperatures.
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integrated network of either discrete particles or network polymers.
Typical precursors are metal alkoxides and metal chlorides, which undergo various forms of hydrolysis and poly-condensation reactions.
In the chemical procedure, the sol (or solution) gradually evolves number of ways. The simplest method is to allow time for sedimentation to occur, and then pour off the remaining liquid. Centrifugation can also be used to accelerate the process of phase separation.
Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significant amount of shrinkage and densification. The rate which the solvent can be removed is ultimately determined by the distribution of porosity in the gel. The ultimate microstructure of the final component will clearly be strongly influenced by changes imposed upon the structural template during this phase of processing.
Afterwards, it will be necessary that thermal treatment or firing process in order to favor further poly-condensation and enhance mechanical properties and structural stability via final sintering, densification and grain growth. One obvious advantage of using this method as compared to more traditional processing technology is to
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achieve densification at a much lower temperature.
The precursor sol can be cast into a suitable container with the desired shape or used to synthesize powders to deposit on a substrate to form a film. The sol-gel approach is a low-cost and low-temperature technique that allows the control of the chemical composition of fine products. Even small quantities of dopants, such as organic dyes and rare earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. It can be used in ceramics processing and manufacturing as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes.
2.3.5 Dip coating
Dip coating is a conventional method of deposition thin films for research purpose. Uniform films can be applied onto planar substrate.
Spin coating is used more often for industrial processes. The process of dip coating is putted the substrate in the deposition solution first, and then pull up the substrate in regular speed, after that the successful thin film will obtained by drying and annealing. This way of deposition thin film is one of the most common used in sol-gel method.
There are many properties of dip coating manner:
(1) It can be deposited on the irregular surface or double-faced substrate.
(2) Few nanometers of thin films can be acquired.
(3) Simple operation, but usually unstable.
(4) Unfit to high viscosity fluid.