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Vapor Deposition of thin Films

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Introduction

File:Mech370 CVD.jpg
Figure 1. Schematic of a PVD Process.

Vapor Deposition is a method of material processing. The two major subdivision of Vapor Deposition processing include: (1) Chemical Vapor DepositionDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. (Plasma Enhanced) (2) Physical Vapor DepositionDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. (PVD)

Vapor Deposition is an atomistic process in which precursorDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. material is vaporized from a solid or gaseous source in the form of atoms or molecules and transported in vapor form through a vacuumDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. or low pressure environment to a substrateDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. where it condenses and is deposited usually as a thin film. Physical vapor deposition utilizes thermodynamic and physical properties of each materials in order to deposit materials-such as physical bombardment- whereas CVD relies on chemical reactivity of precursors to deposit thin films. Vapor Deposition was originally ‘coined by the authors CF Powell, JH Oxley and JM Blocher Jr. in their 1966 book “Vapor Deposition” [1].

The process was developed alongside vacuum technology and is a direct result of studying gaseous chemical interaction with electricity and magnetismDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.[2]. The Processes fundamental theory is an combination of many fields including statistic physics, chemistry, and electromagnetism.


Vapor Deposition processes are so highly desired to achieve unique properties in materials because films can be applied to substrates of numerous orientations and complex geometric shapes. It is highly popular in the semiconductor industry for just such manufacturing constraints.

Deposited Material can be in the form of:

  • PolycrystallineDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.
  • AmorphousDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.
  • epitaxialDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.

Theory

Deposition Rates

Four factors affecting deposition rates

  • Chemistry of reaction; Intermediate steps, by products and activation energy
  • Reaction mechanisms
  • Composition of deposit(stoiciometry)
  • Structure of deposit(geometric arrangements of atoms)

Vacuum Environment

PVD is performed in a low pressure vacuum. The relative size of the vacuum compared to the distance vaporized material has to travel is a relatively enormous distance between the vaporization source and the substrate before collision occurs. The vacuum allows for control of the amount of input material during processing. System is made up of a deposition chamber, introduction chambers a vacuum pumping system, exhaust system, gas inlet system, and interconnected system of tubes. In addition fixtures and tooling to hold and move the substrates are important to the system design.

Processing

There are three specific subdivisions that drive the vapor deposition process

  • Mass transport-Fluid flow and diffusion
  • Kinematic Reaction
  • Chemical Reaction Phase Chemistry

Below, each of these steps will be discussed and the electrochemical, fluid and thermodynamics properties will be described. A Schematic of the Deposition Chamber can be found in figure (x)

Mass Transport

Precursor material enters the chamber by force flow. Gas follows streamlines as a result of force flow diffuses within the chamber. The material driven by concentration gradient within the chambers. The input gases diffuse through the boundary layers and come into contact with substrate. After chemical reactions occur, a recombination of molecular byproducts occur and a de-absorption occurs as the excess material flows out of the chamber

Energy Reactions

Precursor material reacts with substrate as a result of an external force. Enormous amounts of energy are used to physically break bonds, distort lattices and embed vaporized material onto and into the substrate causing a film to forcibly be deposited. Often laser pulses, magnetic sputtering and arc evaporation are two examples of how this process works. Lasers are used to break bonds and become ejected from material. Magnetic sputtering utilized magnetic fields to attract ionized precursor material to substrates at high velocities which are forcibly embedded and arc evaporation use high voltage to melt material into charged plasma which attracts vaporized material, than quickly cools.


Chemical Reaction

Temperature, gas concentration and kinetics are all control mechanisms for chemical interactions between precursor-substrate material. Dependant on the specific materials, the precursor comes into contact with the substrate and absorption of molecules on the surface occurs. The gas simultaneously diffuses through the boundary layer and decomposition of precursor molecules occurs and converts into a solid film.

Coating Characteristics

Films are generally only a few microns thick and deposited at relatively slow rates. Films are generally:

  • Fine grained
  • Impervious
  • High purity
  • Enhance properties of material it coats.

Vapor Depostion Methods

There are numerous methods to deposit thin films on substrate materials. Some of the more popular ones include:


Pulses Laser DepositionDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.

Figure 3. An example of laser ablation

Pulsed laserDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. deposition (PLD) is the use of a high power laser rotated and focused at a target in a vacuum chamber. The material will absorb this energy and lattice bonds will be broken. Surface atoms are disassociated and ejected in the form of an {WP|ablation}{WP|plume}. Layered plums travel at high speeds in this chamber and impinge on the surface of a rotating substrate. The plume and substrate make contact at high impact energies particles react on the surface, adhere and compress leaving a deposit of a thin film on the surface. Laser pulses continue to ablate more product and the film thickens. It is important to note that implantation and sputtering can occur during this process to a small degree.

Magnetic SputteringDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.

Figure 2. Magnetron in use

MagnetronDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. Sputtering is a processing method that is used to coat virtually any material of any shape and orientation. Sputtering is the removal of atomized material from a solid due to energetic bombardment of its surface layers by ionized or neutral particles at high impact velocities. Magnetic Sputtering is performed in near vacuum environmental conditions. During this particles bombardment, a controlled flow of inert gas is introduced to raise the pressure to allow the magnetronDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. to operate. A high negative voltage source is applied to the substrate material that attracts positive ions at high speeds. The impact energy, if greater than the binding energy of the lattice site create an oscillation withing the crystal plane and cause a recoiling effects; thus resulting in a sputtering effect from the surface atoms. The magnetic field within the system traps secondary electrons. The electrons flow in a helical path around a magnetic line and thus more ionizing collision occurs with the inert gas in the system.

Arc EvaporationDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.

Figure 3. Arc Evaporation Schematic

ArcDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.evaporationDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. processing uses electricity to deposit precursor material. A high current low voltage arc is connected to a microscopic cathode. This shorted current source is highly energetic emitting area known as a cathode spot. This concentrated temperature, generally extremely high results in a high velocity (10 km/s) of vaporized cathodic material that is removed from its location creating microscopic molten chips at the surface. A plasma with ionized ions, and neutral particles are released. A reactive gas is introduced and evaporation will occur. This gas will fill the chip defect spots left by the cathode and a film will be deposited.

Plasma Enhanced CVDDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD.

Plasma Enhanced Chemical Vapor Deposition is mainly used for the deposition of dielectric films and passivation films like silicon oxide or nitride at low temperature. This type of process is driven by the heating of a gas or plasma rather than the sequestered material. This process is ideal when dopants are relatively low.

PlasmasDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. are formed usign a radio frequency generator. These reactive ions are subdivided into 'thermal' and 'cold' varieties.

  • Thermal- High enough energy in particles to separate electrons from atoms
  • Cold - Temperatures lower than ionized energies

Cold Plasmas are most effective below atmospheric pressure and are more practical in vacuum technologies. Plasmas adhere to substrates efficiently and have high growth rates.

Ions are continually reacting with film material already exposed to plasma, a new positive charge on the substrate film exists.


Other Popular methods

  • Atmosphere pressure chemical vapor deposition
  • PhotoChemical vapor deposition
  • Chemical vapor infiltration
  • Chemical beam epitaxy

Applications

Current

  • Fibre Optics and telecommunications
  • Semiconductors(major use)
  • Aerospace
  • Automotive
  • Surgical/Medical
  • Dies and molds for all manner of material processing
  • Cutting tools

Future

  • Nano and biotechnology
  • Space exploration equipment
  • Immunology


  • Endless uses

Process and Material Improvment

Physical Application Improvments

Damage from particle bombardment exists. Damage is constrained to the top layer of material due to the application of the films. The films do cover these imperfections however they application of films can have a high influence on the mechanical properties of material.


Photons beams can also damage materials. If applied with greater energies than 10 eV electrons in the conduction band of many silicon dioxide materials can exist and create trapped holes. Thus gates can damaged in semiconductors.

Contamination

Byproduct contamination within the deposition chamber results from poor internal ventilation. Byproduct material gets caught in valves crystallizes around valves and other moving parts which lead to machine upkeep and longer downtime. Material also crystallizes after it coats these moving parts and than reenters where the precursor material is converting to a film and contaminates the process leading to lower efficiency. The crystalline byproducts that deposit inside the entire vacuum system need to be addressed.

  • It reduces the conductance of the vacuum pumping line, which slows down the process and reduces the system's throughput.
  • It can cause drift in process vacuum gauges, which in turn causes the process to degrade from its original performance.
  • It can become a source of particle generation from back-flow of the deposited material into the process chamber

Here are a few suggestions to Improve such retardants

  • Install ventilation components to prevent negative feedback of material between deposition chamber and inflow
  • Install throttling valves to equalize velocity and increase pressure to prevent negative feedback
  • Install some sort of heating device to prevent material from crystallizing and contaminating films

Airfoil and Electron Beam gun/Laser to PVD Process

Airfoils- A method for applying a thin film as a coating to a substrate in which the thermal conductivity of the ceramic material is reduced or lowered by up to 10%. Thermal barrier coatings having improved performance characterisitics with the aid of an electron beam PVD having lower thermal conductivity.

Electron Beam guns are located within the apparatus, using a material of ytrria-stabilized zirconia. A high energy source to is available to melt and vaporize the ceramic material. An airfoil is position at a critical distance 'x' and disperse vapors, redirect chamber internal forces and speed up the application process. A portion of film is deposited on air foils, however vaporization of the ingot material

Thermal conductivity decreases the further the precursor ingots are located from the film substrate. Advantages include having thinner coatings capable of withstanding the same temperature gradients than by prior practices. Manufacturing process can stay the same simply adjusts the thickness variations to obtain desired variation. Two electron beams(in vacuum) or laser outside vaccum shoot inside windows 

Theory is based up thermal conductivity where: k is thermal conductivity p is density c is thermal diffusivity Cp is specific heat

k = p*c* cp

further more steady state equation: j is heat flux j = k*(dTemp/thickness)

steady state, k(thermal conductivity) is same k1/t1 = k2/t2; based on ratio of thickness to density of material.

Efficiency put airfoils near ingots to uniform thickness. Based on thermal diffusivity being constant, densities of coatings stay constant also and the thickness decreases as the desntiy incraeases. [2].


Shield for Sputtering Effieciency

A target shield with a concave curved surface directed away from the the intended target can affect the magnetic field lines that intersect and improve the vertical directionality of the sputtered material. A high aspect ration opening in the substrate be be filled at low pressure.

This concave plate placed in the orthogonal direction to the magnetic and electric field lines do not reach the shield baed on its shape areduce numebr of ions lost from plasma by impacting the shield.THis curved shield protecs the sidewalls from deposition without interfering with the target. Few electron impacs with the shiled result in fewer electrons lsot from the plasma thus allowing plasma to be obtained at very low pressures. The concave portion of the shielf bent inward to the susceptor is bent veritcally go protect deposition against the bottom wall also. allowing more ions to impact substrated in the verticle direction. Film is improved and magnetic field lines become more horizontal near center of target.

[3].

Carbon Nanotubes

Carbon nanotubes are a relatively new technology composed of carbon isotopes -- cylindrical carbon molecules 50,000 times thinner than a human hair -- have properties that make them potentially useful in nanotechnology, electronics, optics and reinforcing composite materials. With an internal bonding structure rivaling that of another well-known form of carbon, diamonds, carbon nanotubes are extraordinarily strong and can be highly efficient electrical conductors.

References

  1. Mark Allendorf, "From Bunsen to VLSI 150 Years of Growth in Chemical Vapor Deposition Technology", The electrochemical society IF3-98(1), 36-39
  2. Joseph D. Rigney, David J. Wortman, "Physical properties of thermal barrier coatings using electron beam-physical" Google Patents #6620465(2), 36-39
  3. Tanaka, Yoichiro. "Sheild for Physical Vapor Deposition Sheild" Patent # 5, 824, 197(3), 36-39

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