Author: Dr. Joseph Brindley GENCOA and Dr. Anas Ghailane (AVALUXE)
Reactive Magnetron Sputtering
Reactive magnetron sputtering is a well-established PVD technique for depositing thin compound films on substrates. Common examples include:
· Flat-panel displays for televisions and cell phones
· Photovoltaic coatings on solar cells
· Optical coatings
· Decorative coatings on hardware and automotive components
· Solar insulating coatings on architectural glass
In reactive magnetron sputtering, sputtering of a target is conducted in the presence of a reactive gas (e.g., oxygen, nitrogen) that reacts with sputtered material and forms a compound film on the substrate. The most common reactively sputtered films are oxides and nitrides.
What is the Challenge?
The reaction between reactive gas and the sputtered material is known to cause process instabilities which means that the only stable operating area is in the presence of highly levels of reactive gas (excess gas). This leads to reduced deposition rates as the very high partial pressure of the reactive gas results in compound formation on the whole of the metal target surface (target poisoning). Sputter yields from poisoned targets are typically 3x to 5x less than from a metallic target.
Solution: Reactive Gas Controller
A reactive gas controller as GENCOA’s Speedflo provides automatic feedback control and high-speed gas control to help prevent a runaway situation leading to target poisoning. With such a device it is possible to control a reactive process in what would usually be an unstable region.
Gas control during reactive sputtering strongly influences the deposition rate and film properties of the compound being deposited. Reactive gases can trap the target in poisoned mode unless the partial pressures of the reactive gas(es) are individually monitored and controlled at high speed. The dynamics of a reactive sputtering system typically requires a closed-loop feedback speed of control in the 10’s of msec range. Active feedback control reacts and adjusts the reactive gas flow control valves within 1 msec. Depending upon gas line lengths and system size, the gas will then take typically between 10-100 msecs to enter the area in-front of the sputter target. With a closed loop feedback control time of <100msec most reactive processes can be maintained at high rate and with good control in the ‘transition’ region.
Operation of the process in the ‘transition’ region between the elemental and poisoned states of the target ensures that the metal target material is sputtered at high rate, and there is ‘just’ enough reactive gas present to ensure the correct stoichiometry of compound layer is formed on the substrate. Operating in this region is the ideal situation to achieve higher deposition rates and stable film properties.
Sensoring
An important part of the control system is the choice of ‘sensor’ to provide feedback from the process of the effect of the reactive gas changes. The sensor signal is the ‘input’ to the controller and a fast and stable input signal makes achieving good control more straightforward. For some metal and gas combinations, the sputter target operating voltage is a suitable input signal (works with Al and Si target materials with O2 and N2 gas). Using the target voltage simplifies the hardware and hence reduces the cost of the overall control solution.
In the graph above we see that the deposition rate decreases with increasing oxygen flow because of the coverage of the target with metal. The inflection point A is the point that should result in stoichiometric coating. The speedflo helps maintain the process at that point at which the deposition rate is relatively high.
The graph above shows an example of a process control at a given set point of a relevant sensor such as the target voltage. The actuator maintains the process in control through the mass flow controller.
Other material combinations require a gas or metal ‘plasma emission’ signal from the process area or from a remote plasma sensing head (gas only signal). The plasma emission is universal, in that any material or gas can be monitored and can be used to provide local sensing and gas control for very large deposition chambers (multi-channel sensing to tune uniformity over large areas). The plasma emission signal is ‘carried’ to the controller via a fibre-optic link, hence the signal is very fast – speed of light. However, the light intensity needs to be converted into a digital voltage signal for input into the control architecture. This fast conversion is typically via a narrow bandpass optical filter and a photomultiplier tube (PMT). The PMT detector method maintains the sub 1 msec signal processing which is critical for good feedback control.
An alternative method to convert the light signal is via a CCD type spectrometer. The spectrometer can provide multiple gas input signals, but the integration time is typically >50 msec, so the closed loop control speeds are much longer. This slower response speed and CCD array drift means spectrometers are typically only used for process development rather than industrial process control.
Many sputtering processes just use oxygen as the reactive gas, and hence another class of sensor comes into play – Lambda oxygen ‘sniffing’. Lambda sensors are used for automotive engine management to control the efficiency of the combustion. The lambda device converts the amount of group 16 gas present (in this case O2) into a proportional voltage output. With good control of the sensor temperature and adaption for use in a vacuum environment (see VacGasG16 type), this can be a convenient method to create a gas signal to use for successful reactive gas feedback control.
Benefits of Reactive Gas Control
· Much improved deposition rates are achieved (x 2.5 – 4 dependant on process*), for lower cost production
· Thin film stoichiometry and properties control – eliminates process drift
· More precisely controlled film uniformity, 1.5% over large areas (ideal uniformity for double low E glazing)
· More energy efficient process (energy consumption can be reduced by up to 70%) – a typical customer was able to reduce power applied to each cathode by 100kW/h leading to >30% energy savings. For some applications, energy usage can be lowered by as much as 70%
* Material properties of coated surface determines the maximum rate achievable, e.g., for architectural glass, too fast a deposition rate leads to a more metallic coating which can lead to future corrosion problems.