Achieving more repeatable coatings with faster, higher precise pressure control
Background on Vacuum Coating
Vacuum coating process control engineers create a vacuum (pressures below atmosphere) in a sealed chamber. They remove atmospheric gases which would interfere with the desired reactions between the substances in the process. Into the vacuum, they introduce other gases and materials—things like vaporized metals, ionized molecules or plasma-reactive compounds—to bind them with the surfaces of other materials.
A simplified description of a system has two key features: first, a vacuum chamber plumbed to several stages of vacuum pumps, with a throttle valve between the chamber and the pumps. The throttle valve regulates how hard the pumps draw, as well as providing a strong shut-off to prevent leakage.
Second, gas sources, which are selectively introduced into the emptied chamber through mass flow controllers (meters with integrated flow control valves). These gases are used to bring the coating environment to appropriate operating levels of density.
Some gases are used as non-reactive ‘neutral’ gases, to keep the chamber pressure at an ideal sub-atmospheric level without affecting the concentrations of process chemicals. Others are reactive gases, which are an integral part of a process, such as oxygen for an oxidation coating. In some processes, non-reactive gases are used as carriers to spread and dilute the reactive materials. Reactive materials may be a vaporized metal which has been ionized through melting and electrical field manipulation. Still other gases may be used to “purge” or cleanse the system of the last traces of the previous phase.
Coating mechanisms vary. Some are fairly passive—for example, allowing the vapor to settle slowly on the substrate. Others involve bonding films to substrates through plasma arcing the vapor cloud. Or the process may require charging the substrate electrically, to attract specific ions.
Uneven coating quality and poor repeatability, resulting from fluctuations in vacuum chamber pressure.
Cause and context
In the rarefied atmospheres of vacuum coating systems, pressure fluctuations arise—and small changes count for a lot. From the interaction of control components and asynchronous operation of those elements—non-reactive gas flows, reactive gases being introduced, and vacuum pumps turning on and off—all may affect the pressure and balance of the ingredients.
More subtly, transient fluctuations due to poor timing of on-off switching, or long settling times in reaching a flow set point may throw a chamber’s delicate balance of chemicals off, affecting evenness, adherence and thickness of the coatings. Substandard coatings cause cost overruns on targets, process gases and substrates. Control and settling times exceeding 100 ms of valves and flow controllers can, depending on the application, have a qualitative effect on film thickness and uniformity, and repeatability of the process.
Gain a well-controlled coating environment using an Alicat Pressure Controller with an External Sensor (PC-EXTSEN-D). It controls the pressure of the vacuum chamber through a controlled introduction of non-reactive gas. The chamber pressure is measured by an external vacuum gauge, directly connected to the process chamber. The pressure measurement is read by the pressure controller via analog connection. Reactive gases are introduced to the process chamber through Alicat Mass Flow Controllers (MFCs). As the Alicat Pressure Controller detects changes in the chamber pressure from the inflow, it adjusts the level of non-reactive gas to regulate the process pressure and create optimal deposition conditions. Faster control response times help to maintain optimal deposition results, reducing fluctuations.
Both the pressure and flow control instruments use a common control interface, standardizing commands and programming. Serial or analog communications can be used to control the instruments. Through either the remote or the onboard interfaces, the MFCs can be reset to flow new gases while retaining the NIST-traceable accuracy specification accorded to the instrument. Changing gases is a matter of a simple selection, without wasted hours spent calculating K-factors and without affecting accuracy.
Instruments with low- and micro-flow rates provide unique flexibility. Drop-in replacements for competitive MFCs fit the SEMI footprint and can be matched in plumbing connections, as well as factory-wired to the same pin connections.
Normally, changes in the flow rate of reactive gases will disturb the system pressure and result in inconsistent coatings.
Because of its rapid response time and high repeatability, Alicat pressure controllers maintain the chamber pressure precisely at the desired level, while the system control manipulates the reactive gases independently.
Coating system integrators who have adopted this method report that the faster control response time of Alicat instruments substantially improve their results by providing a well-controlled coating environment, with repeatable results.
Additional benefits are realized through the instruments’ high accuracy and ease in changing fluids.
- Repeatability: ±0.08% Full Scale
- Operating range: 0.5% to 100% Full Scale / 200:1 Turndown
- Typical response time: 100 ms (Adjustable)
- Accuracy at calibration conditions after tare: up to ±(0.4% of reading + 0.2% of full scale) NIST-traceable
- Repeatability: ±0.2% of Full Scale
- User selectable 98 gases at specified accuracy