Laser Sputter Deposition

Laser sputter deposition is a thin film deposition technique for silicon wafers, which uses high-energy pulsed laser radiation to ablate or knock-off target molecules and deposit them as a thin film layer on a wafer substrate. This technique was first tried by Smith and Turner in 1965. Three years later, Breech and Cross wrote a paper on laser vaporization and atom excitation.

At that time, quality of laser sputter deposition was far behind than that of the layers deposited by other techniques like molecular beam epitaxy and chemical vapor deposition. In 1987 laser sputter deposition became an established technique when Dijkkamp and his group used laser deposition on high-temperature superconductors (YBa2Cu3O7). Their work marked the starting point for the industry to utilize laser sputter deposition for layers of high quality crystals.

Since then, all types of oxides, nitrides, or even the metallic systems, and polymers have been deposited through laser.

How does laser sputter deposition work?

In laser sputter deposition, the target material undergoes a number of energy conversions before it is deposited on the substrate. First, when the radiated laser pulse hits the target, the energy is transferred as an electronic excitation, and then converted to thermal, chemical and mechanical energy.

All these energy conversions cause the target to evaporate, ablate, create plasma, and exfoliate.

The whole process can be divided in the following stages:

a.      Laser Sputtering of the Target Material

Laser sputtering uses a high-energy excimer laser pulse with very short wavelength like the KrF laser at 248nm. As the high-energy laser pulse is directed towards the target material causing energy transfer between the free electrons and the atoms. This will result in an extreme increase in temperature and in the evaporation of a part of the target.

 b.      Expansion of Material through Plasma

The vaporized material will expand towards the substrate in the plasma located in parallel to the target material. This expansion is dependent on the background pressure. An increase in background pressure slows down the excited particles, providing a higher chance for the deposited material to resputter. Thus, the deposition rate will decrease and material’s stoichiometry will be altered.

 c.       Deposition of the Sputtered Particles

Once the sputtered particles start bombarding the substrate surface, they will cause some of the substrate’s atoms to be knocked-off of its surface. Deposition begins when the condensation rate, caused by the collision of the sputtered particles of the target material and the substrate, is high enough to obtain thermal equilibrium.  It is important to have of continuous flow of sputtered particles to maintain thermal equilibrium.

d.      Growth of the Deposited Material

There are three growth techniques that can be used in laser sputter deposition: step flow, layer by layer, and 3D growth. In step flow growth, once the atoms of the target material reach the substrate surface, they will diffuse. Step flow growth is usually used on substrates with high crystal miscuts since the atomic steps on the substrate surface are formed from miscuts on the crystal.

Layer-by-layer technique, on the other hand, continuously grows islands of material until the islands overlap each other, a phenomenon known as coalescence. During coalescence, there is a huge density of material on the crater of the substrate surface. This whole process is performed for every additional layer.

In 3D growth mode, islands of material are formed on top of each other. This technique is somehow similar to layer-by-layer except for the fact that the deposition does not appear in a layered fashion because materials roughen every time it is added on top of the island.

Growth Modes for Laser Sputter Deposition

Figure 1. Growth Modes for Laser Sputter Deposition: (a) Step Flow, (b) Layer-by-layer, (c) 3D Growth.

The Characteristics of Laser Sputter Deposition

In comparison with other deposition techniques, laser sputter deposition is much more flexible as shown by the following parameters:

Target Size

Since the target size of the laser beam on laser sputter deposition is small, it is possible for this technique to deposit complex compounds. Such capability is usually useful for research purposes, especially when the preparation for the target material using other techniques would be expensive, like in the case of superconductor films.

Target Material Type

A deposition process with different types of target material must be operated in an ultra high vacuum chamber with the laser source located outside the chamber.  Some of the materials that can be prepared using laser sputter deposition are listed in Table 1.

Material Type Compound Reference
High-temperature superconductors YBa2Cu3O7




Dijkkamp (1987)

Guarnieri (1988)

Foster (1990)

Shinde (2001)

Oxides SiO2 Fogarassy (1990)
Carbides SiC Balooch (1990)
Nitrides TiN Biunno (1989)
Ferroelectric materials Pb(Zr,Ti)O3 Kidoh (1991)
Diamond-like carbon C Martin (1990)
Buckminster fullerene C60 Curl and Smalley (1991)
Polymers Polyethylene, PMMA Hansen and Robitaille (1988)
Metallic systems 30 alloys/multilayer


Krebs and Bremert (1993)

Geurtsen (1996)

Table 1. Some of the materials prepared for the first time by laser sputter deposition.

The Pulsed Nature of Laser

Some complex compounds such as polymer-metals require different multilayered conditions for the deposition of each component. One example is the deposition of polycarbonate silver. This kind of deposition is not possible with other techniques because of the big gap between the energy requirements of the two components. But with laser sputter deposition, the pulsed nature of the laser allows the polycarbonates to be deposited at low laser fluence of 60mJ/cm2 and the silver crystals at much higher laser fluence, about 80 times higher at 5J/cm2.


With the continuous advancements in laser technology, laser sputter deposition has emerged in the industry as a competitive technique for depositing thin films of complex stoichiometry but because of the small target size usable, it is not practical for large-scale layer formation.

Common Cluster Beam Formation Techniques

Cluster ion beam is a deposition process where high quality films are formed through clusters of electrically charged ions. In IC manufacturing, cluster ion beam is used for film formation of dielectric and other wafer layers at low substrate temperature.

While there are different techniques to generate clusters, the criteria of identifying the appropriate type of technique typically depend on the requirement of the project (see Common Techniques Used to Form Cluster Ions). After considering several factors on generating cluster ions, the next phase is choosing the right method of extracting the clusters from the chamber and forming a high quality beam. This phase is critical to the efficiency of the whole cluster ion beam process, since using an improper method will disturb the flow of clusters and disperse the beam, hence, will cause some of the clusters to go astray.

The pressure gradient between the condensation chamber and the high vacuum region causes the cluster flow to accelerate as the clusters exit from the chamber. This acceleration is dependent on the pressure ratio:



P0 is the source pressure of the nozzle from the condensation chamber.

Pb is the pressure at the high vacuum region (background pressure).

Once this ratio exceeds the constant G, which is always less 2.1 for gases, the flow will be faster than the local speed of sound in the gas and the Mach number, M>1. This means the flow has become supersonic and shock waves may cause problems on the process. Shock waves are undesirable and, unfortunately, unavoidable in cluster beams. These shock waves causes beam scattering, which degrades the intensity of the beam. Shock waves manifest because there is a need for the cluster flow to adapt on the boundary conditions downstream. However, the propagation speed against the flow of the information on downstream conditions is the same as the local speed of sound, which means at M>1, it will be difficult to propagate the information upstream. Thus, when the cluster flow adapts with the boundary conditions, shock waves are formed in the cluster beam.

The following are four of the most common cluster beam formation techniques:

1.      Supersonic Jet Expansion

In a supersonic jet expansion system, the cluster flow accelerates in the nozzle at approximately M=1. As it moves away from the nozzle, the cluster will continue to accelerate and expand until it over expands. The over-expanded clusters are recompressed by a series of shocks generated in a barrel shock, which is located between the nozzle and the Mach disk. The Mach disk serves as the transition point of the flow from supersonic back to subsonic. It is situated perpendicular to the flow at a characteristic distance given by-

XM/d = 0.67(Pb/P0)1/2


XM/d is the correlation of the distance from the nozzle to the Mach disk in nozzle units

To steer clear of the scattering by the shocks, the extraction must occur before the beam reaches the Mach disk from the zone of silence. The zone of silence is the isolated region inside the barrel shock, which the information of the conditions outside cannot penetrate. A skimmer is used to pass the clusters through the center without any disturbance and to deflect the off axis molecules away from the axis.

2.      Campargue Source

This method operates at a relatively high background pressure of ≥10-2 mbar and uses Roots pumps to pump out the gas from the chamber to the beam. Moreover, its Mach disk is placed within the zone of silence, which turns the expansion as if it was in a perfect vacuum. The high background pressure has significantly lessened the required energy to pump the gas out of the chamber, which also equates to reduction of the system size. However, the scattering inside the skimmer has intensifies at high pressure since there are shock waves at the edge of the skimmer. This leads to a formation boundary layer at the skimmer opening, which also means reduction on the open channel through the skimmer. Thus, the design of the skimmer in Campargue source systems is very crucial. To meet the small external angle requirement of keeping away from the scattering outside the skimmer back in to the beam, and the large internal angle requirement of preventing the scattering inside the skimmer from the gas molecules, the skimmer opening must around 50°.

3.      Fenn Type Free Jet

Fenn type free jets also use high background pressure (Pb ≥10-4 mbar) but its Mach disk is located far from the nozzle or sometimes it is not present at all, which allows smooth transition from the expansion to the molecular flow. Unlike the Campargue source, the skimmer’s design is much less crucial. The typical optimum skimmer opening angle is around 30° and its location on the system depends on the beam’s angular divergence. 

4.      Mixed Beams, velocity slip

By introducing small concentrations of heavier gas molecules, the heavy molecules in mixed beams will be able to flow at the same acceleration as the lighter ones and most likely will gain kinetic energy of several electron volts. However, there is a possibility for velocity slip to occur, where the heavier molecules may not have enough acceleration when its weight becomes too much greater than the lighter molecules at low source pressure. This usually happens at regions with large pressure gradient like the subsonic region of the condensation chamber’s nozzle and the supersonic region of the high vacuum region. Velocity slip can bring in dependency of energy on mass, which means it is crucial when mass-selecting the beam.

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Common Techniques Used to Form Cluster Ions

Due to their high catalytic activity in solutions as well as their usefulness in preparation of dispersed metal catalysts, clusters, also known as colloids, had been a very important aspect in science since its discovery in 18th century. A cluster is an atom or molecule ensemble whose size is in between a molecule and bulk solid. In IC manufacturing, clusters of electrically charged ions are used to form high quality films of dielectrics, metals, and other wafer layers at low substrate temperature. By ionizing and energizing the gas atoms that are condensed into clusters, this process, known as cluster ion beam, induces interactions between the cluster ions and the target atoms. The simultaneous interaction of the ions and the target atoms produces non-linear sputter and film formation.

Some important features of cluster ion beam are-

  • Formation of macro aggregates or cluster. It has been considered that it is difficult, if possible, to generate clusters of solid material at room temperature. But with cluster ion beam, large clusters of few hundreds to thousands of atoms are formed through pure evaporation expansion process.
  • Accelerating the atoms with high potential energy helps cluster ion beam in film deposition at low temperature.

The clusters are formed from the abrupt decrease in temperature of the gas vapor.  One common method is by using an evaporation cell, where the evaporant is heated up until its vapor expands [adiabatically] to more than 100 to 10-3 or 10-7 mbar. The adiabatic expansion will suddenly cool down the vapor, thus, forming clusters of atoms. The clusters are ionized by electron bombardment and accelerated by high potentials. Both the neutral clusters, at 0.1 eV, and ionized clusters, at few eV higher than the neutral, are diffused to the substrate and get deposited as a film of pure and excellent adhesion.

Six Common Cluster Formation Techniques

Figure 1. Six Common Cluster Formation Techniques: (a) supersonic expansion, (b) inert gas condensation, (c) laser vaporization, (d) electrical arc discharge, (e) ion bombardment, (f) liquid metal ion source.

There are different techniques that can be employed to generate clusters depending on instances of the requirement such as the need for free clusters, or the deposited ones. Size, material type, and boiling/melting temperature also influence the type of method for creating the clusters. Six of the common cluster formation methods are:

1.      Supersonic Expansion

In supersonic expansion, a stream of gas is expanded into a vacuum from a nozzle. Clusters are formed if there is an adequate collision before the vapor reaches the complex part of the beam and its adiabatic cooling ceases. More likely, the generated clusters will have up to a few hundred of atoms, low translational temperature and speed distribution, but high vibrational temperature, which means there is a greater chance for the atoms from the escaping clusters to evaporate. To support cluster production, the expansion of the beam is constrained in a conical nozzle. This will not only increase the number of collisions, but will also produce larger clusters and improve the center-line flux.

Supersonic expansion is best suited for metals with low boiling points because of the difficulty in vaporizing such materials through heat. Clusters produced from metal expansion in an oven contain only a small number of atoms. Thus, applications requiring clusters from metals prefer supersonic expansion wherein a heated metal vapor at a partial pressure of 10 to 100 mbar is mixed with an inert gas (e.g. argon) at a stagnation pressure of several atmospheres. During the adiabatic expansion, the temperature of the inert gas will lower rapidly, assisting metal vapor cooling through collisions.

2.      Inert Gas Condensation

This technique is similar with supersonic expansion in a way that both use nozzle. But for the case of inert gas condensation, the clusters are formed prior the expansion in the high vacuum region. A flowing stream of cold inert gas cools down the metal vapor, which over saturates the vapor and condenses it into clusters. The clusters then exits through the nozzle to the high vacuum region where adiabatic expansion occurs. The cold inert gas creates low internal temperature clusters, which, some scientists say, drives the negligibility of re-evaporation. The size of clusters formed from inert gas condensation usually ranges from dimers to approximately 105 atoms, with a controlled rate of growth. On Sattler’s prototype source in 1980, clusters with a few hundreds of atoms of antimony, bismuth, and lead are produced at a detected count rate of 10 per second. To date, the count rate of clusters from this technique has improved to 1010 up to 1011 per second.

One weak point of supersonic expansion is that when the large clusters cannot keep up with the velocity of the carrier gas to maintain a kinetic energy in the beam to less than 1 eV. Inert gas condensation is often used on metal clusters whose evaporation temperature is higher than the attainable point of supersonic expansion.

3.      Laser Vaporization

Laser vaporization generates clusters by vaporizing the material in a pulse-laser and entailing the vapor to a pulsed flow of cold inert gas, in the same manner as in the inert gas condensation. However, this technique employs a greater stagnation pressure than in the inert gas condensation, which intensifies adiabatic cooling during the expansion in the high vacuum area. Laser vaporization can produce clusters that have a few hundred atoms of almost any metal, even the refractory metals.

4.      Electrical Arc Discharge

Also known as PACIS (Pulsed Arc Cluster Ion Source), this technique is a derivative of laser vaporization method. But instead of indirectly vaporizing the material with a pulsed-laser, an electrical arc is used to directly vaporize it. This produces clusters containing about fifty atoms intensified to up to a few ångstrom. Ten percent of these clusters have already been ionized by the electrical arc, thus, eliminating a separate stage for cluster ionization.

5.      Ion Bombardment

By bombarding the material with heavy high-energy ions, ion bombardment or sputtering technique ‘knocks-off’ clusters of atoms from the material. This produces hot clusters of positive, negative, and neutral charges, which eventually cool down as they travel down the target. However, due to the high-energy spread of the ‘knocked-off’ ions, usually up to 30 eV, the cluster ions are having a trouble to smoothly land on the target. Hence, this technique is best suited for small-sized clusters because the intensity of distribution falls away with cluster size.

6.      Liquid Metal Ion Source

Liquid metal ion source (LMIS) are often used to form multiply charged clusters of metals with low melting point. A capillary filled with a liquid metal is charged with an electric field, which thrusts the liquid metal to the Taylor cone. The field at the tip of the cone is usually very strong that it may ripped-off the cluster ions. There is also a possibility for Coulomb explosion to happen since the multiply charged clusters are mutual. Similar to ion bombardment, clusters generated by liquid metal ion source also experiences high kinetic energy spread, making it difficult for the ions to attain soft-landing on the target.

The determination of the appropriate cluster formation method for a specific project relies on some criteria such as the parameters listed in Table 1. This summarizes the relative performance, based on the best case, of each technique.

Cluster Formation Technique Cluster Size Beam Energy Spread Cluster Internal Temperature Cluster Material Cluster Charge Beam Intensity Cost
Supersonic expansion <100 lowvaries with size high alkali metals neutrals high rational
Inert gas condensation 2 to >105 low<0.1eV low metals with melting points up to Ag/Fe neutrals high0.6 to 40 nA rational
Laser vaporization <200 low low any metal neutrals high
Electrical arc discharge <50 low low any metal neutrals and ions high2Å per pulse rational
Ion bombardment <10 to 20 20 to 30eV high any metal neutrals and ions 10 nA rational
Liquid metal ion source <100 10 to 50 eV or100 to 200 eV high metals with melting points up to Au Ions 20 nA at 2m rational

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Deposition Rate of Chemical Vapor Deposition

Chemical Vapor Deposition (CVD) is the process used to deposit thin film of solid material in various applications like fabrications of novel powder, fiber, preforms of ceramic composites, coatings for corrosion and wear resistance, and synthetic diamond. It is the most widely used technique in IC microfabrication for the oxide and nitride layers of the wafers. CVD process grows the film by chemically combining the material to an organic reactant and transports the chemical precursor to the target surface, which is energized either through heat, ion, or photon. The energy in the target drives the chemical reaction between the surface and the precursor to break the chemical precursor and incorporate the material to the growing film on the surface. (see Reaction Mechanisms of Chemical Vapor Deposition)

Reaction-Rate Limited Deposition

The two factors which affect the rate of film growth or the flux of the material in a CVD process are gas diffusion and surface process. As defined by Fick’s Law, the material’s flux to the substrate is a function of gas diffusion coefficient, concentration gradient of the layer, length of the surface for that will be deposited, and Reynold’s number – a dimensionless gas constant. Mathematically, the flux of the material is

 Fl = D Δc/2L 3√(Re           Equation 1

Since Reynold’s number is directly proportional to gas velocity, film growth rate is, therefore, also dependent on the square root of gas velocity. Moreover, gas velocity and gas flow rate is proportional to each other in a fixed volume reaction chamber.  Thus, film growth rate can be expressed as a function of gas flow rate, which is outlined in figure 1. The plot manifests the square root dependence of growth rate on flow rate as the former logarithmically increases with the latter.

However, there is a point where the growth rate saturates and becomes independent of the flow rate, which means no matter how much the flow rate is increased it cannot anymore affect the film growth rate. Once this happens, reaction rate takes over the control on deposition as demonstrated in the reaction-rate limited regime or the Arrhenius plot in figure 2 where growth rate is exponentially dependent on temperature. For a thermally driven surface reaction, the film growth rate can be mathematically modeled by:

R = Roe-Ea ⁄ kT            Equation 2


Ro is the frequency factor

Ea is the activation energy in electron Volt (eV), which is presumed to be approximately equal to the slope of the Arrhenius plot in Figure 2

T is temperature in Kelvin (K)

Film growth rate as a function of gas flow rate

Figure 1. Film growth rate as a function of gas flow rate (photo courtesy of book title here)

Film growth rate as a function of temperature

Figure 2. Film growth rate as a function of temperature (photo courtesy of book title here)

Vertical and Horizontal Wafer Stacking

Figure 3. Vertical and Horizontal Wafer Stacking (photo courtesy of book title here)

In a practical application, reaction-rate limited allows low pressure chemical vapor deposition (LPCVD) to stack the wafers vertically with very minimal spacing in between since the rate of reactant transport holds lesser importance (see Figure 3).  The diffusivity, D, of the reactants in a LPCVD reactor of ~1 Torr is magnified to 1000 times than its value at atmospheric pressure, which increases the arrival rate of the reactants to the substrate to one order of its current magnitude. Thus, the rate limiting step dominates the surface reaction control.

Mass-Transport Limited Deposition

On the other half of figure 2, excessive increase in temperature banishes the effect of reaction rate on the growth rate. At this regime, reaction rate cannot exceed anymore the rate at which the reactant gases are transported on the surface, no matter how high the temperature is. For this phase of rate-limiting reaction, which is known as the mass-transport limited deposition, growth rate is approximately equal to the square root of gas velocity.

Aside from the derived flux equation from Fick’s law (equation 1), the flux of the material, without considering its diffusion through the layer, can also be expressed as

Fl = h(CG-CS)          Equation 3


h is the mass transfer coefficient

CG is the reactant concentration at bulk of gas and

CS is the reactant concentration at substrate surface.

Moreover, the mass transport on a motionless layer in a CVD process is deduced to proceed by diffusion. Mathematically, this assumption is

Fl = D ( (CG-CS) / δ)          Equation 4

Equating equations 3 and 4, yields to

h (CG-CS) = D ( (CG-CS) / δ)

h = D / δ          Equation 5

For mass-transfer limited deposition D = 1. Thus, growth rate in this regime is

R = h = 1/δ = √U          Equation 6

Unlike in the reaction-rate limited regime where temperature owns the main control on growth rate, temperature is less important in mass-transport limited since its level does not limit the deposition rate. Applications for mass-transport limited like the atmospheric pressure chemical vapor deposition (APCVD) operate with the wafers stacked horizontally such that the flux of the reactant species is equally distributed to every corner of the wafer as well as of the other wafers.

Flow Stability

Uniform deposition requires stability of the flow in a CVD reaction chamber, which greatly depends on its laminar development before reaching the susceptor. As predicted by Schlichting, the flow entrance length for a full velocity profile is given by the equation

IF = 0.04HRe          Equation 7


H is height of the flow channel

Re is Reynold’s number

However, the thermal entrance length for a fully developed radial profile is seven times longer than its velocity entrance length.

IF = 0.28HRe          Equation 8

The characteristic of the flow of the gaseous reactants in a CVD process can be measured through a dimensionless gas constant known as Knudsen number (Kn). Knudsen number is defined as the ratio of the average distance that a molecule travels before colliding with another molecule or the molecular mean free path (λ) to the flow field length (L), which, in wafer fabrication’s case, is the size of the device structure. Knudsen number classifies the gas flow as:

  • continuum if Kn<0.01
  • slip if Kn is in between 0.01 and 0.1
  • transition if 0.1<Kn<10
  • free molecular for Kn>10

The reactant flow on the substrate usually falls on the transition or free molecular classification. As for the λ, the typical λ in a CVD process ranges from 0.1 microns to >100 microns at 100 Torr. But since the trend for the integrated circuits is to shrink up to the nanometer range, the λ may not be enough to attain uniform thickness over the whole process. For the industry to overcome this challenge on uniformity, the dominance of the molecular flow must be maintained by operating on very low pressure chemical vapor deposition (VLPCVD).

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Reaction Mechanisms of Chemical Vapor Deposition

Chemical vapor deposition (CVD) is a process used to grow a film of solid material by transporting a chemical precursor in its vapor phase and decomposing it through heat. Typically, a material in its solid form is vaporized and diluted with an organic reactant, which will assist the material’s surface mobility. The chemical combination is transported to the heated surface where it decomposes, leaving the material on the heated surface to migrate to the growing film. This technique is widely used in IC microfabrication as a deposition method for the layers comprising a semiconductor wafer, specifically the oxide and nitride layers. It significantly helped transistor shrinking by establishing very thin film deposits in the substrate.

CVD is also used for fabrications of novel powder, fiber, components of dense structural parts, preforms of ceramic composites, coatings for corrosion and wear resistance, and synthetic diamonds. Basically, CVD is used in deposition of film in surfaces that other methods cannot achieve. Over other deposition methods, CVD is very versatile, and can form pure and dense films at low temperature and low atmospheric pressure. It can deposit a wide range of materials in a number of orientations such as monocrystalline, polycrystalline, epitaxial, uniaxial, and amorphous.  These materials can be a metallic element like tungsten and aluminum, or non-metallic like carbon, or even a compound such as fluorocarbon, nitride, carbide, oxide, etc.

Reaction Mechanism

In a typical CVD process, the actual deposition process takes place in a reactant chamber. Figure 1 details the phases of reactions in CVD, which starts at mass transport of the reactants to the chamber (a). This produces film precursors and undesirable by-products during the gas phase reaction (b). Then the film precursors move towards the surface (c) where it is adsorbed (d).  The selectively occurring adatoms reacts with the surface and migrates the film forming material to the growth sites (e). Once, at the growth site, the material will be incorporated to the growing film (f) whereas both the by-products and volatile reactants are desorbed (g) and removed from the chamber through vacuum and exhaust systems (h). Some by-products are treated appropriately before releasing it to the atmosphere to eliminate any danger it can introduce to the environment, especially the toxic, flammable, or corrosive gases.


Figure 1. Reaction Mechanism of a CVD Process.

Since the materials involved in CVD is in gaseous form, this process requires volatile and stable precursors. The precursors usable for CVD can be categorized as halides, hydrides,  metal organic compounds,  alkyls,  alkoxides, carbonyls, dialylimides,  and diketonates, to name a few. In general, only a single element from the precursor compound is deposited during the CVD process, while the rest of it is volatized. For instances that the precursor provides two or more elements, the transport mechanism is faster and simpler since it requires lesser amount of reactants to deposit the material.

Another important aspect of every CVD system is the energy source (thermal, electron or laser) needed to ignite the chemical reaction of the film precursors with the target surface. Among these energy sources, the most widely used is the thermal energy. In a CVD process with a thermal energy source, the chamber is first heated to a certain temperature. Once the diffused precursors reached the heated surface, equilibrium shifts towards the material to be deposited, which has lower temperature.

Deposition Rate

The rate of deposition or the flux of the material is greatly affected by gas diffusion and surface processes.  Flux defines the amount of substance that will flow through an area at a given time.   As postulated in Fick’s first law, flux will always flow from the area with higher concentration to the area with lower concentration. It relates the flux of the material to the diffusivity of the gas, and the concentration of the layer separating the surface target surface and the bulk flow. Mathematically, the flux of the depositing material is given by:

Fl = D(dc/dx)                          Equation 1


Fl is the diffusion flux of the material in amount of substance per unit area per time (mol/m2.s);

D is the diffusion coefficient in square of length over time (m2/s);

dc is the concentration of the layer in amount of substance per unit volume (mol/m3); and

dx is the boundary layer thickness (m).

The boundary layer thickness, which is a function of the distance x along the substrate and can computed from

δ(x)=√(ηx/ρU)                          Equation 2

Moreover, the average boundary layer in a surface is modeled by Prandtl as:

δ = 1/L ∫0L δ(x)dx= 2/3 [L√(η/ρUL)]                          Equation 3


η is gas viscosity in pressure and time (Pa·s);

ρ is gas density in units of mass per volume (kg / m3);

U is gas stream velocity in unit of length over time (m / s); and

L is the length of the surface being deposited on (m);

is a reverse of dimensionless gas constant called Reynold’s number Re. In fluid motion, Reynold’s number is the ratio of the inertial forces to the viscous effects for a given flow motion. It is defined as:

Re = ρUL/η                          Equation 4

Substituting equation 4 in equation 3 yields to:

δ = 2L/(3√( Re))                          Equation 5

Lastly, substituting equation 5 in equation 1 will simplify Fick’s Law equation to:

Fl = D  Δc/2L 3√( Re)                          Equation 6

This demonstrates that aside from diffusion coefficient and concentration gradient, the deposition rate of a material also depends on the square root of the gas velocity, gas viscosity, and gas density.

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Molecular beam epitaxy

Molecular beam epitaxy (MBE), a technique used to form an epitaxial growth, is very often used in semiconductor industry as deposition method of thin films on wafer substrates due to its efficiency, controlled doping characteristic, repeatability, and uniformity. But before immersing to MBE, let’s take a quick look on its mother form the epitaxial deposition.

Epitaxial Growth Deposition

In 1960s, Bell Laboratories developed a technique to deposit materials in an orderly fashion at a very low speed; and therefore, allowing an accurate control on the deposit’s thickness and dopant concentration. This technique is now known as epitaxial growth deposition.

Epitaxial growth deposition grows a single crystal film over a substrate by arranging the atoms on top of the substrate.  The same structure of atom arrangement were grown layer-by-layer; hence, giving the film a well-ordered crystal formation that matches the crystalline structure of the substrate

Types of Epitaxial Growths

Epitaxial growth can be categorized as either homoepitaxial or heteroepitaxial depending on the type of material grown on the substrate.

A homoepitaxial growth has a film of the same material as the substrate (i.e. Si on Si growth).  This forms a film purer than the substrate and provides the ability to dope the layers independently from the substrate.

A heteroepitaxial growth, on the other hand, has a film of different material than the substrate (i.e. AlAs on GaAs or GaAs on Si growth). The difference on the materials used leads to unmatched lattice formation which will either stress or relax the growth. This will affect some properties of the film such as electrical, optical, thermal and mechanical. Though may be a misfit, this property of heteroepitaxy is used as an advantage on optoelectronic and band gap engineering applications.

Lattice formation

Figure 1. Lattice formation: (a) homoepitaxial; (b) heteroepitaxial

Processes to Form Epitaxial Growths

Deployment of epitaxy can be performed in a number of ways two of which are physical and chemical vapor deposition.

Chemical vapor deposition (CVD) decomposes the gaseous chemicals through heat at temperature lower than the melting point of the substrate. Since most of the candidates as wafer film deposits are difficult to transport through gas (because of low vapor pressure), CVD chemically attaches a compound with a high vapor pressure to the material. The attached compound will aid the surface mobility of the material. At raised temperature the material-compound bond will be easily broken, leaving the material on the substrate, while the compound pressure will be pumped away by vacuum.

Physical vapor deposition (PVD), on the other hand, uses the vapor formation of the material in deployment of thin film into the substrate. The material is either heated to its vaporization point like in thermal evaporation or its ions are knocked-off from it like in sputtering. Another PVD technique wherein the material’s atom diffuses to the growing film in an ultra high vacuum (UHV) environment is the Molecular Beam Epitaxy (MBE).

Chemical vapor deposition

Figure 2. Chemical vapor deposition: (1) material element is bonded with an organic compound; (2) composition is transported to the substrate; (3) heat breaks apart their bond, leaving the element free to bond to the growing film; (4) organic compound evaporates.

Physical Vapor Deposition Techniques

Figure 3. Physical vapor deposition techniques: thermal evaporation (a) and sputtering (b)

Molecular Beam Epitaxy (MBE)

Molecular Beam Epitaxy

Figure 4. Molecular Beam Epitaxy

The choice of epitaxial growth technique depends on the kind of epitaxial structure required for the device application and the needs of production. Like in mass production, MBE may not be the main option because compared to other techniques, MBE has lower growth rate. However, for cases where material purity and the accuracy on control of the film thickness and doping profile are more important than production yield MBE is usually the choice in manufacturing.

MBE is an ultra high vacuum (UHV) technique used in growing high quality epitaxy. Its process is somewhat very simple: the material is heated by the effusion cell, the flux of atoms are then transported to the UHV environment where they will travel directly towards the heated substrate. Once on the substrate, the atoms will diffuse on the growing film and bonds with it. Each element is transported separately by a controlled beam, providing a more controlled doping concentration for every element. The beam flux can be turned on or off through a shutter or valve.

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PVD: Sputtering and Thermal Evaporation

Any wafer fabrication involves deposition of layers of materials, which will serve as a conductor, an insulator, or a buffer. Thin film metal layers like aluminum interconnects are deposited in the wafer substrate through Physical Vapor Deposition (PVD) – a process which uses vacuum deposition technique.

One of the earliest PVD techniques is the thermal evaporation. In thermal evaporation, the metal is heated to its vaporization point. It will then evaporate to the wafer to form the thin film.  However, since evaporated materials tend to be directional, thermal evaporation provides poor coverage.

The poor coverage problem in thermal evaporation was solved when sputtering emerged. Sputtering is one of the most widely used PVD techniques by the semiconductor industry in deployment of thin film metal layers over the wafer substrate. It injects the metal layer in the substrate by ‘knocking-off’ the metal atoms out of the metal material and bombards the ‘knocked-off’ ions on to the wafer. This grants sputtering a better coverage of deposition than thermal evaporation.

Physical Vapor Deposition Techniques

Figure: Physical vapor deposition techniques: (a) thermal evaporation and (b) sputtering

Following are the main differences of thermal evaporation and sputtering:

1. Coverage Area

As mentioned, sputtering has a larger coverage area than thermal evaporation because of the nature of their distribution method. The vapors used in thermal evaporation are directional; while the ion bombardment of the sputtering is ‘rain-like’.

2. Deposition Rate Control Factors

In thermal evaporation, deposition rate can be controlled by the amount of heat supplied on the material (vaporization point). On the other hand, sputtering controls its rate through gas pressure, temperature, the potential difference between the material, which acts as the cathode of system and the wafers placed on the system’s anode.

3. Deposition Rate

When talking about deposition rate, the more controlled the rate is (lower number of layers/second), the better. Sputtering can trim down its deployment of metal layers up to one atomic layer per second. Whereas thermal injection can only control it to hundreds or thousands of atomic layers per second.

4. Choice of Material

Sputtering has a wider range of choices for material than thermal evaporation.

5. Decomposition of Material

The uniformity of decomposition and erosion of the material in sputtering makes it more efficient than thermal evaporation.

6. Equipment Cost

Operating using sputtering will cost more than thermal evaporation because the latter only needs a vacuum chamber with precise thermometers; while the former requires twice or thrice of the energy used in thermal deposition to excite the ion of the material.

7. Surface Damage

Surface damage has higher possibility in sputtering. Its ion particle bombardment can induce damage in the substrate.



Coverage Area poor coverage large coverage
Deposition Rate Control Factors temperature only gas pressure,temperature, andpotential difference between the material and the wafer
Deposition Rate thousand atomic layers per sec one atomic layer per sec
Choice of Material Limited wide range of variety
Decomposition of Material High Low (uniform)
Equipment Cost Cheaper more expensive
Surface Damage very low ion particle bombardment can induce damage

Considering the pros and cons between these two techniques, sputtering will clearly prevail over thermal evaporation for industrial applications. Thermal evaporation, on the other hand, is most of the time preferred in laboratory experiments.

Aside from being the main choice for thin film deposition in semiconductor industry, sputtering is also used in fabrication of compact disks, large area displays, and other deposition process on blades and gears.

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