16 Chemical Vapor Deposition Process

Dr. Ayushi Paliwal and Dr. Monika Tomar

1. Introduction

Chemical deposition techniques play a very crucial role in the design and manufacture of several useful devices. These techniques are employed to produce high quality and high performance solid materials. They are mainly used to produce thin films.

These techniques help in overcoming several drawbacks which we observe in physical deposition techniques like poor conformality, low throughput, restricted directional variation and reduced compositional control.

This field is dominated by mainly two groups:

• The first group comprising Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD). Both these techniques comprise of a process in which gas phase is used to transport volatile molecules to the surface serving as substrate.
• The second group having technique known as Chemical Solution Processing or Sol-Gel Processing which uses liquid phase as the mass transfer media.

In both the processes, the basic technique employed is the same. The final material in both the cases is created by molecules of chemical compounds which are being served as precursors.

Precursor is one of the compounds that participate in the chemical reaction that produces another compound. These precursors are directed to the substrate surface and been chemically modified to obtain the desired film.

We’ll study about the two groups in a bit more detail in the following sections.

2. CHEMICAL VAPOUR DEPOSITION

In this process, the substrate (often referred to as wafer) which is placed inside a chamber is exposed to one or more volatile precursors which get deposited on the substrate to produce the desired film. Since there is a chemical reaction involved, so very frequently by-products are released which are removed by gas flow through the reaction chamber.

In micro-device fabrication, CVD is widely used to deposit materials in various forms such as: monocrystalline, polycrystalline, amorphous and epitaxial. The materials which are often deposited by CVD are: Sulfides, Oxides, Nitrides, carbides, silicides etc.

TYPES OF CVD:

CVD is divided into several categories based on different criteria which are as follows:

1. On the basis of OPERATING PRESSURE:

i) Atmospheric pressure CVD (APCVD)- Deposition is done at atmospheric pressure. This process has advantage of fast deposition at low temperature. But the major disadvantage is low purity.

ii) Low pressure CVD (LPCVD – Reduced pressure is required for good uniformity of film because it ensures the reduction of unwanted gas-phase reactions). The pressure ranges form 30 to 250 pa

iii) Ultrahigh Vacuum CVD (UHVCVD – The pressure is significantly reduced upto or less than 10−6 pa (∼ 10−8 Torr).

The CVD which is used prominently in modern world is either LPCVD or UHVCVD. The reduced pressure in the reaction chamber during LPCVD and UHCVD processes enhances the gas diffusivity, which reduces the unwanted gas phase reactions and improving coating uniformity.

2. On the basis of Physical Characteristics Of Vapour.

i) Aerosol assisted CVD (AACVD) – In this the precursors are transported to the substrate by the means of liquid/gas aerosol, which is generated with the aid of ultrasonic treatment. This technique is employed when we have non-volatile precursors.

ii) Direct liquid injection CVD (DLICVD) – The precursor in this CVD is in liquid form or solid dissolved in suitable solvent. This liquid precursor is injected into the vaporization chamber directed towards the injectors. The precursor vapors in the process are transported to the substrate just as in the case of classical CVD. This technique is being used in the case of solid or liquid precursors. This technique is also an aid in achieving high growth rates due to perfect control of precursor flow (control of the liquid flow). DLICVD also has advantage of Accurate control of doping level

3. On the basis of Plasma Methods.

i) Plasma Enhanced CVD (PECVD) – This process utilizes plasma to enhance reaction rate of the precursors. This allows the deposition at lower temperature, which is a very critical parameter in the manufacture of semiconductors. Lower temperatures also inhibit the deposition of organic coatings such as plasma polymers, which have been used for nanoparticle surface fictionalization.

ii) Microwave plasma assisted CVD (MPCVD) – This process utilizes a microwave plasma source.

iii) Remote plasma-enhanced CVD (RPECVD) – this is similar to PECVD except that in this wafer (substrate) is not directly placed in the plasma discharge region. This has a benefit that we can reduce the processing temperature down to room temperature.

There are several other kinds of CVD processes such as Atomic layer CVD, Combustion CVD, Hot filament CVD, Hybrid Physical-CVD, Metal organic-CVD, rapid thermal-CVD, vapor phase epitaxy and Photo-Initiated CVD.

The most frequently used CVD techniques in the modern times are LPCVD and PECVD. So let us learn about them more in the forthcoming sections.

2.1 Low pressure chemical vapor deposition (LPCVD)

It is that chemical vapor deposition technique in which heat is used to initiate the reaction of precursor gas onto the substrate (solid). In this deposition of thin films on semiconductor substrate occurs usually ranging from a few nanometers to many micrometers.

The reaction occurring is responsible for the formation of solid phase material. Low pressure ensures that no unwanted gas phase reaction occurs and also the uniformity across the substrate enhances.

Mode of Operation – It can be performed in either hot or cold walled quartz tube reactor.

Hot walls operation has an advantage that it allows batch processing and hence a better throughput. They also increase thermal uniformity and hence as a result, uniform films are obtained.

But it has a disadvantage also. In this process, the deposition also occurs on the hot furnace walls which bound us to frequently clean the chamber and to replace the tube so as to avoid particle contamination in the subsequent deposition. Cold walled reactors have an advantage on this part because they are low maintenance as there is no deposition on the chamber walls.

In LPCVD, tube is evacuated to low pressures, in the range of 10 mTorr to 1 Torr. Now when the tube is under vacuum, it is heated up to the required deposition temperature (which is the temperature at which the precursor gas starts decomposing).

Temperatures which are attained in LPCVD generally lie in the range 425-900°C depending upon the process and the reactive gases being used. After attaining the desired temperature, gas in inserted into the tube. This gas then diffuses in the tube and reacts with the surface of the substrate which leads to formation of the solid phase material. Excess gas if any is then pumped out of the tube and goes through the abatement system.

The main reason for using LPCVD in place of APCVD is the ratio of the mass transport velocity and the velocity of reaction on the surface. It is often observed that, LPCVD films are more uniform, lower in defects and exhibit better step coverage than the films produced by conventional PECVD and PVD techniques.

During APCVD the ratio of the mass transport velocity and the velocity of reaction on the surface is close to one as the two velocities are of the same order of magnitude. The velocity of the mass transport depends mainly on the reactant concentration, diffusion, and thickness of the border layer.

When the pressure is lowered during LPCVD, the diffusion of the gas decreases proportionally to the reciprocal of the pressure. The pressure for LPCVD is usually around 10-1000 Pa while standard atmospheric pressure is 101,325 Pa. If the pressure is lowered from atmospheric pressure to about 100 Pa the diffusion will decrease by almost 1000. This means that the velocity of mass transport will decrease meaning the substrates can approach more closely and the deposited films show better uniformity and homogeneity.

Despite of having so many advantages, it suffers from a disadvantage also which is that it requires higher temperatures. Requirement of high temperature limits the types of substrate and other materials which can be present on the samples.

APPLICATIONS – LPCVD is mostly employed to deposit polysilicon (used for gate contacts), silicon nitride and silicon dioxide (used for isolation). Most LPCVD films are with good conformal step coverage and offer good sidewall protection.

2.2 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 could be maintained at a very high equivalent temperatures (tens of thousands of kelvins) equivalent to several electron volts average energy whereas the neutral atoms 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:

Oxides and Nitrides of Silicon (SiOx, SiNx and SiOxNy) 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

Silicon Carbide (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.

DISADVANTAGES:

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.

3. ATOMIC LAYER DEPOSITION

ALD is a process in which film is grown by repeated exposure of alternate gaseous species (referred to as precursors) on the substrate. Unlike the case of CVD, all the precursors are not present in the chamber simultaneously, but are inserted in a series of sequential and non-overlapping pulses. In each of these pulses, the precursor reacts with the surface in such a limiting way that the reaction self stops when all the reactive sites present at the surface are occupied.

The maximum amount of material deposited on the surface after a single exposure to all of the precursors (known as an ALD cycle) can be only estimated by the nature of precursor-surface interaction. To grow uniform and high precision films on large and complex substrates, one needs to vary the number of cycles.

ALD is considered to be one of very good deposition methods which have the potential to grow very thin, conformal films with controlled thickness and composition at the atomic level.

UNDERLYING PROCESS: In a typical ALD process, a substrate is exposed to two reactants A and B in a sequential way such that no overlapping is there. Unlike other techniques such as chemical vapor deposition (CVD), where thin film grows in a steady-state process, in ALD each reactant reacts with the surface in a self-limited way. The reactant molecules can react only with a limited number of reactive sites on the surface.

As soon as all such sites are consumed in the reactor, the growth stops. The remaining reactant molecules are ejected out and only then reactant B is inserted into the reactor. By this kind of alternating exposures of A and B, a thin film is deposited.

This process is shown in the figure below. Therefore, when describing an ALD process one defines both dose times (the time a surface is being exposed to a precursor) and purge times (the time in between doses for the precursor to empty the chamber) for each precursor. The dose-purge-dose-purge sequence of a binary ALD process constitutes an ALD cycle. ALD processes are described in terms of their growth per cycle.

A basic schematic of the Atomic Layer Deposition (ALD) process is shown in the figure below. Now what’s happening in the diagram is explained as follows: In Frame A, precursor 1 (in blue) is added to the reaction chamber containing the material surface to be coated by ALD. After precursor 1 has adsorbed on the surface, any excess is removed from the reaction chamber. Precursor 2 (red) is added (Frame B) and reacts with precursor 1 to create another layer on the surface (Frame C). Precursor 2 is then cleared from the reaction chamber and this process is repeated until a desired thickness is achieved and the resulting product resembles Frame D.

In ALD, enough time must be given in each reaction step so that a full adsorption density is achieved. After this happens, process reaches its saturation. This time depends mainly on two factors: the precursor pressure, and the sticking probability. Therefore, the rate of adsorption per unit of surface area can be expressed as:

=   ∗

Where R is the rate of adsorption, S is the sticking probability, and F is the incident molar flux. Nonetheless, a key characteristic of ALD is that S will change with time. As more and more molecules react with the surface, this sticking probability will decrease until reaching a minimum value of zero once saturation is attained.

APPPLICATIONS: ALD can be used in various applications including,

High-k gate oxides

Storage capacitor dielectrics

Pinhole-free passivation layers for OLEDs and polymers Passivation of crystal silicon solar cells

High aspect ratio diffusion barriers for Cu interconnects Adhesion layers

Organic semiconductors

Highly conformal coatings for microfluidic and MEMS applications Other nanotechnology and nano-electronic applications

Coating of nanoporous structures

Fuel cells, e.g. single metal coating for catalyst layers Bio MEMS

ADVANTAGES: ALD produces film of atomically controlled thickness. Also, the growth of different multilayer structures is forthright. Due to the sensitivity and precision of the equipment, it is an asset to those in the field of microelectronics and nanotechnology in producing small, but efficient semiconductors. ALD is usually run at lower temperatures. The lower temperature is beneficial when working with fragile substrates, such as biological samples. Some precursors that are thermally unstable can also be used as long as their decomposition rate is relatively slow.

DISADVANTAGES: High purity of the substrates is very important, and so high costs will follow. Though this cost is not much relative to the cost of the equipment needed but one needs to perform several trial runs before finding out the apt conditions that favored their desired result. So it is not much economically viable. Also ALD is a very slow process and can be effectively used for substrates used in microelectronics and nanotechnology because their thick atomic layers are not needed. There are some chemical limitations as well. Precursors which are used should be volatile but not subject to decomposition. So this imposes a limitation on the substrates that can be used.

4. CHEMICAL SOLUTION PROCESSING OR SOL GEL PROCESSING:

SOL-GEL is a chemical solution process used to produce ceramic or glass material in the form of thin films, fibers or powders. This method is generally used for deposition of oxides specially oxides of silicon and titanium. The term sol-gel refers to a process in which solid nanoparticles dispersed in a liquid (a sol) gather together to form a continuous three-dimensional network extending throughout the liquid (a gel). A sol is a colloidal (the dispersed phase is so small that gravitational forces do not exist; only Van der Waals forces and surface charges are present) or molecular suspension of solid particles of ions in a solvent. A gel is a semi-rigid mass which is formed when the solvent from the sol begins to evaporate and the particles or ions which are left behind start to join together in a continuous network.

UNDERLYING PROCESS – The precursors that are used in sol-gel process are typically metal alkoxides and metal chlorides, which undergo hydrolysis (this is a process where a chemical compound is broken down by reacting with water).The sol then proceeds towards the formation of an inorganic network which contains a liquid phase called gel. Then formation of metal-oxo (M-O-M) and metal-hydroxo (M-OH-O) polymers starts in the solution. These form because formation of metal oxides involves connecting the metal centre with these groups. Then the drying process starts which remove the liquid phase from the gel thus forming a porous material. Then thermal treatment may be given to enhance polycondensation and the mechanical properties. The precursor can be used to either deposit thin films (by dip coating or spin coating) or to synthesis powders.

An overview of sol-gel process can be represented pictorially as:

APPLICATIONS:

It is used in ceramics manufacturing processes, as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes.

Sol-gel derived materials have various applications in optics, electronics, energy, space, (bio) sensors, medicine (e.g. controlled drug release) and separation (e.g. chromatography) technology. One of the more important applications of sol-gel process is to carry out Zeolite synthesis.

Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicalite sol formed by this method is very stable.

Other products manufactured with this process include various ceramic membranes for microfiltration, ultrafiltration, nanofiltration, pervaporation and reverse osmosis.

ADVANTAGES:

It produces thin bond-coating to provide excellent adhesion between the metallic substrate and the top coat.

It produces thick coating to provide corrosion protection performance. It can easily shape materials into complex geometries in a gel state.

It can produce high purity products because the organo-metallic precursor of the desired ceramic oxides can be mixed and dissolved in a specified solvent and hydrolyzed into a sol, and subsequently a gel, the composition is highly controllable.

It can have low temperature sintering capability, usually 200-600°C.

It can provide a simple, economic and effective method to produce high quality coatings.

DISADVANTAGES:

Despite its advantages, sol-gel technique possesses some industrial limitations which are, weak bonding, low wear-resistance, high permeability, and difficult controlling of porosity.

Generally, the limit of the maximum coating thickness is 0.5 μm when the crack-free property is a critical requirement.

The trapped organics with the thick coating quite often result in failure during thermal process.

The present sol-gel technique is substrate-dependent, and the thermal expansion mismatch limits the wide application of sol-gel technique.

5. SUMMARY:

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

Weblink-

• https://pdfs.semanticscholar.org/e6d1/3653af3b06951e8d5899fbeaf454862ba62d.pdf
• (http://www.sigmaaldrich.com/technical-documents/articles/material-matters/chemical-deposition.html)
• (http://www.ttu.ee/public/m/Mehaanikateaduskond/Instituudid/Materjalitehnika_instituut/mtx9010/ Korgtehnoloogia_Deposition_methods.pptx)
• (http://lnf-wiki.eecs.umich.edu/wiki/Low_pressure_chemical_vapor_deposition)
• https://www.oxford-instruments.com/products/etching-deposition-and-growth/plasma-etch-deposition/pecvd