Surface engineering: Adding a surface layer with reactive magnetron sputtering with high quality and efficiency: GENCOA Optix and Speedflo.
- Anas Ghailane
- 1 day ago
- 8 min read
Updated: 4 hours ago
Dr. Anas Ghailane, Engineer at Avaluxe International GmbH
Introduction:
In order to make the surface of an engineering product capable of withstanding loads in service whether mechanical (wear) or electrochemical (corrosion), surface engineers use different approaches to enhance the properties of the surface. We distinguish between heat treatment of the surface (heating and quenching), thermochemical diffusion (example: carburizing, nitriding) or mechanical treatment (for example shot peening). These approaches usually raise the hardness from 8 GPa to 12 GPa.
In applications such as machining (milling, drilling), a hardness higher than 15 GPa is required for sufficient abrasion resistance. A high-speed steel offers a hardness of 8 GPa. To reach a hardness higher than 15 GPa, surface engineers use the approach of adding a surface layer that is several nanometers to few micrometers thick. To deposit such a thin layer, vapor deposition processes are used. These include Physical vapor deposition (PVD) and chemical vapor deposition (CVD).
In both PVD and CVD, a vapor of the material to be deposited is generated in a first step. In a second step, the vapor is then transported, under vacuum, towards the surface of the product to be coated, that in the final step condenses over that surface and grows as a coating.
The main difference between PVD and CVD is that in PVD the vapor is created by physical mean and in CVD the vapor is created by chemical mean. Therefore, PVD has advantages in this regard as it does not require chemical gas precursors which are corrosive and toxic and lead also to harmful chemical byproducts. Moreover, the PVD requires lower temperatures (200 – 450 °C) whereas in CVD (800 – 1000°C) is required.
Among the physical methods to generate vapor, we distinguish between heat (Thermal evaporation) or electric arc (electric arc evaporation) or energetic particle bombardment (magnetron sputtering).
In this article, we will talk about the magnetron sputtering process and its features and how optical emission spectroscopy and close loop feedback control enhance throughput and maintain coating quality.
1. Magnetron Sputtering
Before creating the vapor to be deposited, a deposition chamber is evacuated to reach a vacuum between 10-6 to 10-5 mbar. There are four reasons why a vacuum is necessary:
- Avoid contamination from atmospheric gas molecules.
- Allow efficient evaporation (high sputtering rate).
- Allow ionization of a working gas, so that its atoms are used as bombarding species.
- Allow the vapor of the coating material to be transported towards the substrate.
Inside the vacuum chamber, two electrodes are placed parallel to each other. The cathode is negatively biased, and the anode is positively biased.
After introducing argon gas, which is chosen because it is inert and inexpensive, a high negative voltage (300 – 1000 V) is applied to the cathode, with respect to the body of the chamber which is grounded. This is the anode. The substrate can be also slightly negatively biased with respect to the chamber, or isolated from the chamber (floating) or grounded.
The negatively biased cathode emits electrons by electric field emission. These electrons have a kinetic energy equal to the electrical energy provided. This energy allows them to knock off electrons from the outer shell of the argon gas atoms and leaves the argon atoms positively charged. These cations are then accelerated to the negatively biased cathode and eject as a result atoms out of the cathode. The process is called sputtering, and the cathode is called the target. If it is conductive, a direct current voltage can be applied. If it is not, a radio frequency alternating current can be applied.
In order to enhance the probability of ionization so that the target species are also ionized, which has a high benefit in obtaining a coating with enhanced properties, a pulsed signal with high peak current is applied. This allows a very high current for a short time and gives enough time for the target to cool down. This technology is called HiPIMS.
Behind the cathode, there are permanent magnets that allow the creation of a magnetic field perpendicular to an electric field. These arrangements allow the electrons to keep orbiting the vicinity of the target which increases the ionization probability and the efficiency of the process since it allows a low voltage to be applied to ionize argon (800 – 1000 V) compared to 3000 V without magnetron.
The ionized gas present in the chamber now is called plasma. This is the fourth state of matter from which stars are made. They emit light when the electron ejected recombines with a cation to produce a neutral atom. An electron with an energy lower than the ionization potential of argon can excite the atom, emitting light as well.
3. Optical emission spectroscopy: Analyzing light from the plasma, GENCOA Optix
If we measure the light intensity emitted from argon plasma, we will know the quantity or the mass of the argon gas present in the chamber. In magnetron sputtering, we can introduce to the chamber, in addition to the working gas argon, a reactive gas, such as nitrogen, oxygen, acetylene, methane, to make compound coatings such as metal nitrides, oxides and carbides.
By measuring the light intensity of each element and separating the intensities with respect to their wavelength using a spectrometer, we obtain a light spectrum that gives the gas composition in our deposition chamber. GENCOA Optix can detect all gases present in chamber that emits light within a wavelength range [200 – 850 nm]. Also, the Optix software can calculate the partial pressure of each element.
When introducing the working gas and the reactive gases, the pressure in the chamber is usually between 10-3 to 10-2 mbar. A residual gas ionizer that uses a quadrupole and separates the elements according to their masses, will not work beyond 10-6 mbar and cannot be used during the coating manufacturing phase.
GENCOA Optix, relying on optical emission, and not requiring a filament, works in a pressure range between [10-6 mbar – 0.5 mbar]. It is therefore suitable to be used as a gas composition monitoring device during deposition which offers several advantages such as:
- Preventing non-conformity of the chemical composition of the coating. Acting fast before multiple production cycles has been executed. An unwanted gas composition in the chamber can occur, for example, due to the presence of a leak, or polluted gas bottle, or an uncalibrated mass flow controller.
- Generating a good basis vacuum down to 1.10-5 mbar or below is time-consuming because of the presence of water vapor. Increasing the temperature allows the removal of water vapor to be faster. The Optix can be used to tune temperature and time of water vapor outgassing.
- Outgassing can also be used to remove adsorbed hydrocarbons, following a deposition where a high amount of acetylene for example has been used. The Optix can be used here also to optimize the outgassing process.
- Optix can be used not only in reactive magnetron sputtering but also in reactive arc evaporation, and thermal evaporation.
- It can also be used in target manufacturing processes to monitor quality, such as hot isostatic pressing.
- The Optix can also be used as a sensor to deliver reliable input signal, to a close loop feedback controller, an essential device in reactive magnetron sputtering.
2. GENCOA Speedflo, a close feedback controller for reactive magnetron sputtering
Most of the interesting coatings in the industry of products that require wear resistance, or other physical, chemical or electrochemical properties are compounds. For example, TiN, TiAlN, TiC, CrN etc are used as hard coating to bring about abrasion resistance. They provide also, by their chemical inertness, a physical barrier against corrosive agents and increase corrosion resistance. Also, in addition to oxides, they can be used as decorative coatings.
These coatings are made, as mentioned in section 3, using reactive magnetron sputtering. In this process, the reactive gas reacts with the metal atom that has been ejected from the target, on all surfaces. The surface is needed to take out the excess energy and allow chemical bonding to occur between the two species.
When the reaction occurs on the substrate surface, this is the purpose. When it occurs inevitably on the target surface, some process parameters exhibit instabilities. These are the target voltage, the sputtering rate and therefore the deposition rate, and finally the reactive gas partial pressure.
Instability means that for one value of a reactive gas flow rate, two values of the three mentioned parameters are possible. Meaning that the repeatability of the process will be compromised. One value is higher than the other, by mostly 2 to 3 times. This is illustrated in figure 1.
Therefore, by choosing to operate the process at the highest value of the deposition rate, the process can be run with high throughput at every production cycle.
In order to stabilize the process, a close loop feedback control is used. Here every millisecond, the output signal (either target voltage, sputtering rate, or partial pressure) is fed to the controller that compares it to the desired set point and generates an error signal, that, through a pseudo derivative feedback control algorithm (PDF Controller), generates a control signal. The control signal communicates the suitable reactive gas flow rate to the mass flow controller, to keep the process in control.
The time that the gas takes to reach the target vicinity is 500 ms. So being below that time in feeding the output signal to the controller for correction, enables reliable process control.
4. Choosing the sensor for Speedflo process control
The choice of a suitable sensor will also contribute to obtaining a good tracking signal for the setpoint. We have three different signals that we can have a look at: The target voltage, the metal amount ejected from the target and the partial pressure of the reactive gas.
In industrial coating processes, rotation of the substrate table is used. Here, in the case of target voltage, for one set point that is chosen, two gas flow rates are possible. The reason for this is the possible variation of the plasma potential, when the substrate is driven over the target. The two gas flow rates, however, will have different effects on the deposition rate and partial pressure, and the process can therefore not be controlled.
Thus, a more suitable sensor will be an optical fiber, placed in the vicinity of the target, that measures the light emitted from exited metal atoms sputtered from the target. The light generates the output signal, by passing through an optical band pass filter of the desired metal to be monitored and using a photomultiplier.
Since the Optix is also an optical emission spectrometer, its output signal generated is also most suitable. The Optix can fed the partial pressure of the reactive gas to the Speedflo.
Conclusion
The use of the Optix is twofold: plasma gas composition monitoring, offering the advantages mentioned in section 3; and a reliable sensor for the Speedflo, that keeps the process stable, and with high throughput, as explained in section 4.
The three images below show respectively: The Speedflo, the Speedflo mini and the Optix.
For more information, please contact Dr. Anas Ghailane by sending an email to aghailane@avaluxe-technologies.de
1-4 plasma emission input
HiPIMS sensor option
1 plasma emission input
HiPIMS sensor option
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