5 Chemical Methods

2. Chemical Deposition Process

The Chemical Solution Deposition (CSD) Technique or Sol-gel processing is a convenient and versatile method to synthesize ceramics and glass materials in a wide variety of forms such as powders, thin film coatings, micro-porous inorganic membranes, fibers, monoliths and porous aerogel materials etc. [Li et al. (2004), Singh S.K. et al. (2006)]. CSD involves the preparation of homogeneous solution of desired compounds either by dissolving of metal-organic (or inorganic) salts in a desired polar or non-polar solvent. Starting chemicals are usually complex compounds of the relevant metal ions, acting as solutes and are called precursors. Thereafter, the prepared homogeneous solution was treated with suitable stabilizers for aging and capping agents to obtain the controlled desired shape. In CSD technique various combination of precursors-solvents and stabilizers have been employed in the production of thin films. Their unique properties and reactions affect the preparation process and determine the final product. Therefore essential features of precursor and solvent are highlighted below:-

Ideal features of compound to be used as precursors:

High metal content to minimize the volume change during the transformation from metal-organic solution to inorganic film.
High solubility in a common solvent with other starting compounds.
Chemically compatible with other compounds accordance to the formulation.
Cost effective to reproduce as the capital equipment requirements is small with no high vacuum systems requirement. The cost of precursors should not change this advantage.
Thermally decompose without evaporating, melting or leaving a carbon deposit.

Ideal features of solvents

High vaporization rates so that evaporates quickly at lower temperature as possible where vaporization depends on vapor pressure and also interaction between solvent and solute.
Careful selection of solvent in order to get a solution with high concentration of necessary components, proper viscosity, and surface tension.

Depending on the nature of the precursors, there are several preparation techniques available for the synthesis of materials by CSD technique. Some of the techniques are listed below:

2.1 All Alkoxide Method

Generally, alkoxide of each metal with desired composition are dissolved in an alcohol solvent thereafter addition of water to alkoxide solution results in hydrolysis which is followed by condensation, network formation and the eventual formation of polymeric gel/sol. Now a day’s advanced ceramics are multicomponent materials having two or more types of metal cations in the same lattice [Samuneva et al. (1993)]. Since, alkoxide precursors are mixed at the molecular level in the solution (bottom to top approach) therefore a high degree of homogeneity is expected. However, unequal hydrolysis and condensation rates of the used metal alkoxides leads to major problem in forming homogeneous multicomponent solution. It also results in phase separation during hydrolysis or thermal treatment, leading to higher crystallization temperatures or even undesired crystalline phases. The formations of multicomponent- alkoxides are hard to achieve thus forcing to choose an alternative method for the synthesis of multicomponent materials.

2.2 Modified Alkoxide-Salt Method:

The alkoxide-salt approach can overcome the aforementioned problems with all-alkoxide method need to form the multicomponent materials with desired composition. The term salt for the described process basically refers to carboxylates, nitrates, sulfates, carbonates, chlorides, and hydroxides. The alkoxides and the carboxylates fall in the group of organic derivatives of metals with metal-oxygen-carbon bonds while nitrates, sulfates, carbonates and chlorides comes in the category of inorganic derivative of metals. Certain species like carboxylate anions (RCOO)-, nitrates (NO3)- etc. are found to be versatile ligands capable of binding the metals in chelating or bridging modes. The metal carboxylates have the important feature of forming metal-metal bond [Chen et al. (1991)]. A metal acetate/nitrates works for most metals and are active source of metal oxide. The choice of solvent is also very important in alkoxide-salt approach as its influence can be multiple: it can stabilize intermediates and thus modify the reactions. Utilization of different derivatives of alcohols as solvent helps in further formation of the reactive species. The main advantage of the modified alkoxide-salt approach is formation of comparatively nonreactive side product in comparison to all-alkoxide approach. In all acetate-alkoxides sol, either choice of carboxylate salts (solutes) or alkoxide alcohol (solvents) holds bridging position and binds different metals together [Chen et al. (1991)]. These reactions are further controlled by adding complex agents known as stabilizers that react with metal precursors at a molecular level, giving rise to new molecular precursors of different structure, reactivity and functionality.

The prepared solutions using above mentioned methods can be used in various ways for the preparation of thin films structure. To obtain the layered thin film structures, coating of prepared homogeneous solution on a substrate can be done using the spin coating or dip coating techniques to obtain an inorganic oxide thin film. Whereas, the post deposition thermal treatment is required for the crystallization of as deposited amorphous films [Singh et al. (2008)]. In the dip coating technique, the uniformity and the thickness of the film is controlled by the optimization of various parameters like rate at which the film is pulled out of the gel, time for which the substrate was kept in the solution and the viscosity of the solution. While in the case of spin coating technique, the uniformity and thickness of the film are controlled by the spinning rate, time and the viscosity of the sol. However, sol gel derived powders can also be obtained further either by precipitation or high temperature treatment which can be used in the formation of thick discs and pellets. These pellets can be further utilized as targets for the deposition of thin films using PLD and sputtering.

2.3 Spin Coating

Preparation of a highly cross-linked solid thin film, onto substrates can be achieved by mean of hydrolysis and condensation of the molecular precursor using the spin coating technique. The substrate is placed on the chuck table and held by vacuum (~10-2 mTorr), while rotating. Once the substrate is fixed on the chuck the filtered precursor solution is drop casted on the substrate using the pipette in measured quantity and allowed to spin at a particular rpm in spin coater. Typical spining speed is ranging from 500 to 6000 rpm, depending on the properties of the fluid as well as the substrate. To achieve a required thickness of film along with uniformity the combination of spin speed and time are selected and optimized. In the spin-up stage the liquid flows radially outward by mean of centrifugal force acting outwards giving rise to the formation of uniform thin film.

In general, higher spin speeds and longer spin times are observed to form thinner films. The spin coating process involves a large number of variables that tend to cancel and average out during the spin process and it is best to allow sufficient time for this to occur. A separate drying step is sometimes required after the high-speed spin step to further dry the film. This can be advantageous for thick films since long drying times may be necessary to increase the physical stability of the film before handling. A firing process is necessary to remove the residual organic solvents and organic groups in the deposited film and to convert the organic precursor film into an inorganic film. Because of the large volume shrinkage accompanying the densification process during firing, cracking of the films was a serious problem. To prevent cracking, a multistage deposition process is adopted. The spin on layers was pyrolysed in air at 300 °C for five minutes on a preheated hot plate. Thicker films were prepared by repeating the deposition and pyrolyzation cycle. Multiple coatings were required to achieve a required thickness. Finally a post deposition heat treatment is provided in air required for further densification, grain growth and to obtain highly crystalline structure of thin films.

APPLICATIONS:

Used in ceramics manufacturing processes.
Sol- gel derived materials have various applications in optics, electronics, biosensors etc.
Products manufactured with this process include various ceramic membranes for microfiltration, ultrafitration and reverse osmosis.

The advantages of CSD the technique over other deposition techniques are:

Provides excellent adhesion between the substrate and the top coat.
No deterioration of substrate surface.
Helps in obtaining the desired shapes of materials into complex geometries.
High purity products can be obtained by choosing the high purity precursors.
Low temperature sintering capability, usually 200-600°C.
Simple, economic and effective method to produce high quality film coatings. The disadvantages of CSD technique are as:
Weak bonding, low wear-resistance and high permeability.
Difficult to control the porosity of materials.
Limit of the maximum coating thickness as high value of thickness may result in cracking.
Not suitable for bulk production.

2.4) Electroplating

Electroplating is the application of a metal coating to a metallic or other conducting surface by an electrochemical process. Electroplating is both an art and a science. Although based on several technologies and sciences, including chemistry, physics, chemical and electrical engineering, metallurgy, and perhaps others, it retains in some ways the aspects of an art. In fact, of course, all the sciences have elements of art which can be learned only by experience; all the reading of textbooks on chemistry will not produce a chemist. No text on electroplating will produce an expert electroplater; there is no substitute for experience and what is somewhat inelegantly termed know-how.

2.5) Plasma enhanced chemical vapor deposition (PECVD)

This is the CVD process which uses plasma (cold plasma) to deposit thin films from a gas state to a solid state on a substrate. The reaction involved in this process occurs after the creation of plasma of the reacting gases. This plasma is created by application of RF (AC) frequency or DC discharge between two electrodes.

The space between the electrodes is filled with the reacting gases. Through this process, we can deposit films on substrates at lower temperatures than that of standard CVD.

UNDERLYING PROCESS – Plasma is that state of gas in which a significant percentage of atoms or molecules are ionized. Plasmas with low fractional ionization are of great use in the material processing because electrons in such plasma are very light as compared to atoms and molecules such that the energy exchange between the electron and the neutral gas being used for the creation of plasma is very insignificant.

The plasma used for processing is operated at pressures between millitorr to few torr. Since the electrons are very light so they coud be maintained at a very high equivalent temperatures (tens of thousands of kelvins) equivalent to several electronvolts average energy whereas the neutral atom remain at the ambient temperature. These highly energetic electrons are efficient to induce many processes that would otherwise be very difficult to process at low temperatures, such as dissociation of precursor molecules and the creation of huge amount of free radicals.

Another benefit of deposition within a discharge arises from the fact that electrons are more mobile than ions. Due to this, plasma becomes the most positive among the things it is in contact with. The difference of the voltage between plasma and the objects in its contacts generally occurs across a thin sheath region. Ionized atoms or molecules diffuse to the edge of the sheath region. They feel an electrostatic force and are accelerated towards the neighboring surface.

Thus, all the surfaces which are exposed to the plasma get energetic ion bombardment. The potential across the sheath that surrounds an electrically-isolated object is around 10–20 V, but much higher sheath potentials can be achieved by adjustments in reactor geometry and configuration. Thus, films can be exposed to energetic ion bombardment during deposition. This bombardment often leads to increase in density of the film, and help to remove contamination thus improving film’s electrical and mechanical properties.

When high-density plasma is used, the ion density is high enough such that significant sputtering of the deposited film occurs; this sputtering can be used to help in making the film plain and fill troughs or holes.

There are two parallel electrodes in the system – a grounded electrode and an RF-energized electrode. This capacitive coupling between the electrodes excites the reactant gases into plasma, which then induces a chemical reaction and results in the reaction product which is then deposited at the substrate.

The substrate is placed at the grounded electrode is generally heated to 250° C to 350° C, depending on the film that needs to be deposited. In comparison CVD requires 600° C to 800° C. The lower temperature requirement is very essential in some cases where high temperatures such as in CVD may damage the device being fabricated.

APPLICATIONS – It has many important applications in material deposition. It has been used commercially to deposit following films:

SiO_x, SiN_x and SiO_xN_y deposition for a wide range of applications including photonics structures, passivation, hard mask, etc.
Amorphous silicon (a-Si:H)
Tetraethyl orthosilicate (TEOS SiO2) with conformal step coverage, or void-free good step coverage
SiC
Diamond-like carbon (DLC)

ADVANTAGES:

Low operating temperature.
Uniform coating of different shapes. Good step coverage.
High packing density.
Film characteristics as a function of depth. Less stress.

DRAWBACKS:

Precursors are toxic and so are the byproducts.
Equipment is not economical.
Capacity is limited.
Contamination occurs from precursor and carrier molecules.
Stoichiometry is hard to obtain.

SUMMARY:

The chemical deposition techniques of various kinds have been discussed.
The advantages of chemical techniques over physical techniques have been told.
The modern day chemical techniques like LPCVD and PECVD have been explained and their advantages over other conventional CVD techniques are discussed.
Importance of ALD technique in the deposition of thin films with atomically controlled thickness has been elaborated.
The Sol-gel technique is explained by a step by step process. Also its economic importance has been described.

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REFERENCES

“Low Pressure Chemical Vapor Deposition – Technology and Equipment”. Crystec Technology Trading GmbH.
Crystec Technology Trading GmbH, Plasma Enhanced Chemical Vapor Deposition – Technology and Equipment
Tavares, Jason; Swanson, E.J.; Coulombe, S. (2008). “Plasma Synthesis of Coated Metal Nanoparticles with Surface Properties Tailored for Dispersion”. Plasma Processes and Polymers. 5 (8): 759.
Schropp, R.E.I.; B. Stannowski; A.M. Brockhoff; P.A.T.T. van Veenendaal; J.K. Rath. “Hot wire CVD of heterogeneous and polycrystalline silicon semiconducting thin films for application in thin film transistors and solar cells”. Materials Physics and Mechanics. pp. 73–82.
The Material Science of thin films” by Milton Ohring.
“Coatings on Glass” (volume 6) by H. K. Pulker.
“Handbook of thin film Technology” by Frey, Hartmut, Khan and Hamid R.
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“Deposition Technology for films and coatings” by Rointan F. Bunshah.
“Vacuum technology” A. Roth (North-Holland Publishing Company, Amsterdam) 1982