No edit summary
No edit summary
Line 35: Line 35:


===Vacuum Chamber===  
===Vacuum Chamber===  
 
VD is performed in a near ideal vacuum environment to prevent contamination. The relative size of the vacuum compared to the size of the vaporized material relative to the distance that precursor material must travel is enormous. The size of the vacuum is highly influential in PVD processes. I.E. Magnetron sputtering continually accelerates particles in a helical fashion. As time increase so does the velocity and energy proportional to velocity. The greater the energy the greater and higher volume of precursor material reacts more for a more efficient thin film deposit. 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. <ref name="seven"> Chiu. H, T. Department of Applied Chemistry: National Chioa Tung University  "Introduction to deposition" http://chiuserv.ac.nctu.edu.tw/~htchiu/cvd/deposition.htm(7),</ref>.
VD is performed in a near ideal vacuum environment to prevent contamination. The relative size of the vacuum compared to the size of the vaporized material relative to the distance that precursor material must travel is enormous. The size of the vacuum is highly influential in PVD processes. I.E. Magnetron sputtering continually accelerates particles in a helical fashion. As time increase so does the velocity and energy proportional to velocity. The greater the energy the greater and higher volume of precursor material reacts more for a more efficient thin film deposit. 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. <ref name="seven"> Chiu. H, T. Department of Applied Chemistry: National Chioa Tung University  "Introduction to deposition".http://chiuserv.ac.nctu.edu.tw/~htchiu/cvd/deposition.htm (7),</ref>


== Processing ==
== Processing ==

Revision as of 02:23, 26 November 2008

Template:MECH370

Introduction

Figure 1. Schematic of a PECVD Process [1]

Vapor Deposition is a processing method to lay a thin layer of a precursor material onto a substrate material to improve its mechanical and chemical properties. Vapor deposition is categorized into two major subdivisions of processing:

  • (1) W (CVD)
  • (2) W (PVD)

Vapor Deposition is an atomistic process in which aW material is vaporized from a solid or gaseous precursor material and transported in a vacuum in the form of excited atoms or molecules and deposited on a Wmaterial where it condenses and is deposited as a thin film. Vapor Deposition was originally ‘coined by the authors CF Powell, JH Oxley and JM Blocher Jr. in their 1966 book “Vapor Deposition” [2].

The fundamental theory behind vapor deposition roots from theoretical physics,chemistry and thermodynamics and the actual deposition process was developed with advancements in vacuum technology, electricity and magnetism, thermodynamics and fluid flow[2]. The fundamental theory Processes fundamental theory is an combination of many fields including statistic physics, chemistry, and electromagnetism.

Vapor Deposition processes are highly desired to achieve precise and thin coatings or multi-coatings on a material to enhance properties that are lacking. The application of thin films is ideal by means of deposition because it can be applied to substrates of numerous orientations and complex geometric shapes.

CVD and PVD is highly popular in the semiconductor industry to enhance the conductive and magnetic properties of metals without considerable cost. Deposition films in semiconductors are so thin that the process is extremely beneficial and economic. Deposited Material can be in the form of:

Physical Vapor Deposition

PVD utilizes the principals of thermodynamic by focusing concentrated forms of energy on a solid precursor material. This solid precursor material becomes excited through energy bombardment; Magnetic sputtering, lasers, Arc evaporation. Energy causes bonds to break in the crystal lattice structure and atoms becomes ionized as they are dislodge from the precursor material.[1]The ionized material is released and is transferred by pressure gradient to where it is deposited as a thin film on the substrate material.[3].

Chemical Vapor Deposition

CVD relies on chemical reactivity between ionized vaporized gas within the deposition chamber. Ionized gas is injected through control valves into the deposition chamber where a chemical reaction occurs between the precursor and substrate.Cite error: Closing </ref> missing for <ref> tag.

Processing

There are three major parameters which determine both the deposition density, rate and characteristics of the film

Figure 3. Fluid flow and its effects within depositon chamber [4]
  • Mass transport-Fluid flow and diffusion
  • Kinematic Reaction
  • Chemical Reaction Phase Chemistry

[4]


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.[5]

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.

Nucleation and Coating Growth

The Four stages of coating growth

  • (1)Initially nucleation of single atoms on the surface occurs.
  • (2)If the time of atom migration on the surface is great enough to meet another atom before being evaporated these atoms join together to form an island.
  • (3)As the energy required to evaporate one atom from the pair is higher than needed for a separate atom stable islands (nuclei) start to form on the surface.
  • (4)The islands W and the continual growth of the film takes place.

the link below has a detailed video of the process http://www.pvd-coatings.co.uk/theory-of-pvd-coatings-nucleation-and-coating-growth.htm

Thornton Zone model

Figure 4. A schematic of the Thornton growth zone model[1]

This model summarizes the relationship between the substrate temperature, kinetic energy of the ions and the deposition rate. This illustrates the relationship between the coating morphology and the deposition temperature and the pressure

It is important to know that substrate temperature and deposition rate affect coating density. If material is deposited with a low substrate temperature the condensed atoms do not have enough kinetic energy (mobility) to jump across lattice sites and reach positions of a lower Gibbs energy. A similar effect is also observed when high deposition rates are used.

Coating Characteristics

In practice when coating takes place parameters highly sought out to apply the material with the minimal Gibbs free energy possible and make the material ideal include:

  • 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:


W

Figure 3. An example of laser ablation

Pulsed W 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.

W

Figure 2. Magnetron in use

W 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 W 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.

W

Figure 3. Arc Evaporation Schematic

WW 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.

W

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.

W 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

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

[3]

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. [1].


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.

[6].

References

  1. 1.0 1.1 1.2 1.3 PVD-Coatings. "A theory of PVD coatigns", http://www.pvd-coatings.co.uk/theory.htm,(2), Cite error: Invalid <ref> tag; name "two" defined multiple times with different content
  2. Mark Allendorf, "From Bunsen to VLSI 150 Years of Growth in Chemical Vapor Deposition Technology", The electrochemical society IF3-98(1), 36-39
  3. 3.0 3.1 Azom - Materials "Physical Vapor Deposition (PVD) an introduction ", http://www.azom.com/Details.asp?ArticleID=1558(4),
  4. 4.0 4.1 TimeDomain CVD, Inc."CVD Process in its entirety",http://www.timedomaincvd.com/CVD_Fundamentals/Fundamentals_of_CVD.html (6),
  5. Mattox. D,M. 1998 William Publishing." Handbook of Physical Vapor Deposition(PVD) Processing ", .http://www.knovel.com.proxy.queensu.ca/web/portal/basic_search/display?_EXT_KNOVEL_DISPLAY_bookid=63(5),
  6. Tanaka, Yoichiro. "Sheild for Physical Vapor Deposition Sheild" Patent # 5, 824, 197(3), 36-39

<layout name="Project" />

Cookies help us deliver our services. By using our services, you agree to our use of cookies.