Classification And Theory Of PVD Coating Technology

- May 04, 2019-

Classification and theory of PVD coating technology

 

As a kind of special shape material, thin film can be amorphous, polycrystalline and monocrystalline. It can be made of simple elements or compounds, inorganic materials or organic materials.

 

Thin film technology includes physical vapor deposition (evaporation, sputtering, ion plating, arc plating, plasma plating) and chemical vapor deposition. The technology used in our factory is Physical Vapor Deposition(PVD).

 

One.Vacuum evaporation coating

Resistance heating evaporation and electron beam heating evaporation:

1. Basic principles:

A process in which the substrate or workpiece to be coated is placed in a high vacuum chamber and heated to vaporize (or sublimate) the film-forming material and deposited on the surface of the substrate or workpiece to form a thin film.

 

2. Type of evaporation source:

image

 (a) (b) (c) (d)

3. Factors affecting film quality:

A. The position of the substrate

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The proper placement of substrate is the precondition to obtain uniform film.

B. In order to ensure the mass of the membrane, the pressure shall be as low as Pr (Pa).

L represents the distance from the evaporation source to the substrate as L(cm).

C. Evaporation rate. When evaporation rate is small, gas molecules are immediately adsorbed on the deposited membrane atoms (or molecules), resulting in loose membrane structure, coarse particles and many defects. On the contrary, the membrane structure is uniform and compact, the mechanical strength is high, and the stress inside the membrane is large.

D. Under normal circumstances, when the substrate temperature is high, the kinetic energy of the adsorbed atoms increases accordingly, and the formed film is easy to crystallize and reduce the lattice defects. When the substrate temperature is low, there is not enough energy to supply the adsorbed atoms, so it is easy to form amorphous film.

 

Two.Magnetron sputtering coating

Magnetron sputtering is a new type of sputtering coating method developed on the basis of cathode sputtering in the 1970s. Because it effectively overcomes the fatal weakness of low cathode sputtering rate and the increase of substrate temperature caused by electrons, it has obtained rapid development and wide application.

 

1. Magnetron sputtering:

The phenomenon that the atoms on the target surface are hit by ion bombarding the target material is called sputtering. Sputtering film is realized when the atoms generated by sputtering are deposited on the surface of the substrate (workpiece).

Basic principles of magnetron sputtering:

Magnetron sputtering was in splash zone and a magnetic field perpendicular to the direction of the electric field, in the orthogonal electric intensity and magnetic field B E electron motion equation, the electronics in the form of cycloidal wheel along the target surface to the perpendicular to the direction of the E and B is parallel, thus greatly extended the electronic itinerary, increased the electron collisions with gas molecules, improve the ionization efficiency. So the secondary electron magnetic field under the control of the track, it can be all used for ionization energy, when the energy exhausted, only is absorbed by the anode (chassis). The following figure:

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These electrons are accelerated by the electric field and gain energy, and then collide with the atoms or molecules of the gas, even if they ionize, so that the plasma can be maintained.

 

Magnetron sputtering is to control the movement of electrons by adding runway magnetic field to the target surface, extend their travel around the target surface, and improve the plasma density, so the sputtering coating rate is greatly improved.

 

Secondary electron yield:

Secondary electron yield refers to the number of secondary electrons per ion bombarding the target. Theoretical analysis shows that the secondary electron yield of the metal target is independent of the ion energy when the ion energy is less than 500eV(actually less than 1000eV).

 

Sputtering Yield:

Magnetron sputtering has a working voltage of 200~500V, which determines that the maximum ion energy of the target is 500eV, and the accelerated argon ion is perpendicular to the target.

 

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Interaction between incident ions and materials:

The interaction between energy-carrying ions and the target surface results in:

A. Surface particles: sputtering atoms, backscatter atoms, desorption impurity atoms, and secondary electrons.
B. Surface physicochemical phenomena: cleaning, etching and chemical reactions.
C. Point defects, line defects, hot studs, collision cascades, ion implantation, amorphous states, and compounds in the material surface layer.

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Sputtering Techniques:

 

Sputtering technology can be divided into:

 

A. Diode sputtering by dc glow discharge;

B. Tripole sputtering by arc discharge of hot wire;

C. Rf sputtering using rf discharge;

D. Magnetron sputtering control of glow discharge using enclosed runway magnetic field.

 

2 magnetron sputtering cathode structure:

At present, magnetron sputtering devices for industrial use mainly use rectangular planar magnetron sputtering cathode (figure a). Generally, the target material size used has two specifications: VT machine: length width thickness (450.5 120 6)mm; ZCK machine: 460 100 6. Cylindrical magnetron sputtering cathode is also gradually used in production (figure b). Compared with them, the utilization rate of plane target material is only 20-30%, that is, the utilization rate is low.

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Figure a figure b

 

Figure a is a kind of magnetic field produced by permanent magnet track rectangular plane magnetron sputtering cathode, the target material in contact with the pole shoe. Outside of target material along the N pole shoe, on the center of a S pole shoe, N and S pole shoe respectively subjected to reverse polarity of strontium ferrite or ndfeb permanent magnets. Put a permeability of pure iron back connect the other end of the permanent magnet, namely to produce magnetic field of the magnetic circuit of runway.

 

Figure b is a cylindrical hollow magnetron cathode, which is a cathode target with a magnet placed in a cylindrical target, with N and S poles well arranged, water cooling and dynamic sealing.
The function of the pole shoe: to form a closed magnetic circuit with very small magnetic resistance.

 

At present, we commonly used permanent magnetic materials: barium ferrite (BaO · 6F1e2O3), strontium ferrite (SrO · 6F1e2O3), ndfeb permanent magnet.
Magnetron sputtering electrode:
Practical magnetron sputtering electrodes have the following four basic structures:

 

 image

 

(a) coaxial cylinder; (b) flat type; (c) cone (S gun) type; (d) plane or cylindrical hollow type
1 - the substrate; 2 - target material; 3 - shield

 

3 sputtering process:
Drawing system diagram of magnetron sputtering coating machine:

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Sputtering process parameters:

The relationship between target voltage u of sputtering and target current density J is as follows: uJ=K1
Where K1 is the allowable value of target power density, a constant.

 

The target current density can be determined according to the selected target voltage and the allowable target power density.

 

Reduce the Ar pressure is helpful to improve the deposition rate, and to improve coating adhesion and film density. Magnetron sputtering Ar pressure is usually chosen as 0.5 Pa, the impedance of the gas discharge with the decreasing of Ar pressure rises. Magnetron sputtering, can adjust the Ar pressure appropriately, make the target power density and voltage, respectively, at the same time close to its target value and the best value. Therefore, improve the process principle of deposition rate is: as close as possible to the target power density value; The target voltage is as close to the optimal value as possible.

 

A. Sputtering of pure metal film:
In physical vapor deposition, both evaporation and sputtering are suitable for pure metal films, but the rate of evaporation is higher.
At present, the target materials used are: Al, Ti, Cu, Cr, etc

B. Sputtering of alloy film:
Among the physical vapor deposition techniques, sputtering is the most suitable for the deposition of alloy films. Sputtering methods include multi-target sputtering, Mosaic target sputtering and alloy target sputtering.
The target materials used at present include AlTi, ZrTi, CuTi and so on.

C. Sputtering of compound film:
Compound film usually refers to the film layer formed by the mutual combination of metallic elements with C, N, B, S and other non-metallic elements. The plating methods include dc sputtering, rf sputtering and reactive sputtering.

 

1. Dc sputtering compound film must be used,For example, conductive compound targets such as SnO2, TiC, MoB and MoSi2 are usually made by powder metallurgy, which is very expensive.The plating of ITO transparent conductive film is an industrial application of dc sputtering compound film.

2. Rf sputtering is not limited by whether the target is conductive or not. It can be metal or insulated ceramic target.

3. Reactive sputtering is when the metal target sputtering, at the same time to the coating chamber into the gas containing the required non-xin elements. TiC(black) USES Ti target, and the working gas is Ar+C2H2 or Ar+CH4.

 

In reactive sputtering, the injected reaction gas not only reacts with the film atoms deposited on the workpiece to form a compound film, but also reacts with the target material to form a compound on the target surface, which can make the stripping rate of the target material and correspondingly reduce the coating rate even by an order of magnitude, which is easy to cause target poisoning.

 

At the beginning of the compounds in the process of sputtering, only into the pure Ar, then gradually increase the reaction gas (C2H2 or N2, etc.), at the beginning of the reaction gas just pass through, sputtering rate change is not large, when reaction gas reaches a certain limit, the sputtering rate presents the obvious change, and then continue to increase the reaction gas, sputtering rate showed a trend of steady again. It was found that the direction of the inverse process in a certain range between the curve of the misalignment, appear "hysteresis curve" image. This is called "target poisoning curve". See below:

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Target poisoning curve
Measures to prevent target poisoning:
. Enhance extraction rate of the vacuum system;
Reduce the reaction gas.
Isolate the reaction gas from the target.

 

Examples of sputtering compound films are as follows:

Membrane material
artifacts
function
TiN,
High speed steel bit and milling cutter
Hard wear-resisting
Stainless steel case and strap
The gold decoration
Ceramics and tiles
The gold decoration
ITO
Transparent conductive glass
Transparent conductive
SiO2
Transparent conductive glass
Prevent sodium ion diffusion
Al2O3
Integrated circuit silicon chip
Insulation passivation
MgF2
Optical lens
Minus the anti-reflection
TiC
Stainless steel phone case and parts
decoration

 

Magnetron sputtering ion plating technology:

After the 80 s, connect the bias of magnetron sputtering is called the magnetron sputtering Ion Plating, hereinafter referred to as sputtering Ion Plating (Sputtreing Ion Plating, abbreviation SIP). Our factory currently include the use of equipment made of Plating film, namely the use of the technology.

1. Manufacturing process of decorative coating (TiN or TiC) by magnetron sputtering ion plating technology:

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2. PVD coating process:

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Abstract: according to the requirements of the film, the level of vacuum plays a crucial role in the quality of the film. For the products produced by our factory, the vacuum degree before film forming is required to reach 5.0 10-3pa (pumping time is about 30-60 minutes).

 

Pumping heating: when the vacuum degree is reached (e.g., 2.0 10-2pa), start heating and open the rotating frame.

 

Objective: to reduce or eliminate the adsorbed gas on the surface of the products and the vacuum chamber by baking, so as to improve the quality and performance of the film to meet the requirements, but it must be noted that:

 

A. In the true range, the heating can be turned on, which can prevent the surface of the goods from being oxidized.
B. The turntable must be opened when heating is started.

 

Target cleaning (also known as point target): the target can only be opened and cleaned when the vacuum degree reaches a certain range (the required range of the products produced by our factory is 7.0 10-3~5.0 10-3pa).

 

Objective: to remove the adsorbed gas and clean the coating on the surface of the target.

 

Ion cleaning: artifacts after preheating treatment, the surface will still be there is some dirt, may also have slight oxide layer, ion cleaning is to remove the dirt and surface oxidation layer is one of effective methods. For the Ar gas filling the high pressure in vacuum chamber, artifacts and negative bias caused by glow discharge at the same time, by ionization of Ar ion under the action of electric field, the high-energy bombarding artifacts, and achieve the dirt on the surface of the workpiece splash out, clean and the purpose of the activation on the surface of the cargo.

 

Film formation: when the pressure of working gas argon reaches a certain level, the target is opened and an appropriate amount of reactive gas is added for sputtering, and finally the required film is obtained. At present, nitride film, oxide film and carbide film are obtained through nitrogen (N2), oxygen (O2), methane (CH4), acetylene (C2H2), carbon monoxide (CO) and other gases.

 

Matters needing attention in film forming process:

1. Is the Ar flow and pressure normal?
2. Before opening the target, give bias voltage, start the rotating frame, and check whether there is a short circuit in the cargo.
3. The target voltage, target current, pressure and bias current should be paid attention to during the film forming process.

 

Cooling: high temperature will be generated during the film forming process, so as to avoid the film layer from stress caused by the temperature difference between the inside and outside of the vacuum chamber. After the film forming, proper cooling is required before the film is released.
Vacuum chamber . Cargo unit and the vacuum chamber cleaning.

 

Related parameters of magnetron sputtering ion plating:

Magnetron sputtering workpiece has three types of electrical connection: grounding, suspension and bias.
The coating device is usually a vacuum chamber housing ground as the anode, and the provisions of zero potential.
Suspension is the process of insulating the workpiece from the anode (housing) and cathode and suspending it in plasma.
Bias is to add tens of volts to hundreds of volts of negative bias on the workpiece, when the bias is zero that is grounded.

 

1. Ion arrival ratio:

In ion plating, the effect of incident ions on the structure and properties of the film mainly depends on the ion energy and the ion flux.
In ion plating, the energy obtained by the incident ion on each deposited atom is called the energy gain value.
Ea = Ei (ev)
Type of Ei is the energy of the incident ion (ev), I/Φ Φ for ions to reach than a.

 

2. Bias and current:

The practical process parameters of ion plating are the bias voltage and current density of the workpiece.At present, our factory in the plating TiN or TiC process, the bias control added in -100~-400V, bias current in 2~6A or so.

 

3. Pulsed sputtering:

Pulsed sputtering generally USES rectangular wave voltages.

Pulse period is T, the time of target sputtering in each cycle is t-delta T, delta T is the positive pulse time (width) added to the target.V- and V+ are the voltage amplitude of negative and positive pulses added to the target, respectively.

 

4. Abnormal cases during operation:

At present, the main models used in the production are: vt-1200, SVS and COM continuous coating machine, zck-1500 and other different types of target equipment.

Abnormal phenomenon
The consequences
a.
The shield of the car is not well when the target is washed
The surface of the goods was contaminated, resulting in the film of the goods after the explosion coating
B
Bias short circuit occurs during ion cleaning
Function test NG after film forming (rejected)
C
During the film forming process, the reaction gas flow rate is too large (e.g., C2H2), resulting in target poisoning
After the products come out of the furnace, the film coating or color uneven phenomenon is obvious
D
Target cooling water is not opened during target opening
Equipment damage, serious cause casualties

IKS PVD,vacuum coating machine,contact:iks.pvd@foxmail.com

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