Improved Process Gas Control Optimizes Reactive Sputtering

Customized valve response algorithms in flow controllers produce better films, with more control of the target’s transition state.

Reactive Sputtering

Coating a knee joint

Physical Vapor Deposition (PVD) is a thin film coating process used to produce coatings of pure metals, metal alloys and ceramics in the 1-10m range. PVD is an umbrella term used to describe a couple of different thin film deposition techniques and one of which is Reactive Sputtering. The following are a few key terms that are commonly used when discussing Reactive Sputtering:

  1. Sputter: This word is derived from the Latin word ‘Sputare’, meaning to “emit saliva with noise”. In this case, it just means ‘to emit or release’.
  2. Substrate: The material which is being coated with a thin film of the target material.
  3. Target: The pure metal, alloy or ceramic which is either the sole or one of the components of the deposited thin film.
  4. Vapor cloud: A cloud of atoms that are displaced or sputtered from the target.
  5. Inert gas: Gas which does not take part in the chemical reaction required to form thin films but is used to generate a plasma that activates the reactive gas and maintains chamber pressure.
  6. Reactive gas: Gas that in the presence of plasma, reacts with the target material to form thin films on the substrate.
  7. Target poisoning: A phenomenon where due to an increase in the partial pressure of the reactive gas, the chemical reaction happens at the target instead of the vapor cloud or the substrate, which leads to thin film deposition on the target.

Reactive Sputtering CVD coating diagram

Reactive sputtering is used to deposit a coating on a substrate by introducing a reactive gas, such as O2 or N2, into a plasma typically generated using an inert gas, such as Argon. The ionized Argon is accelerated towards the negatively charged target. These Argon ions bombard the target, sputtering target material in to the chamber and the target material accelerates towards the substrate. The reactive gas which is activated by the plasma then chemically reacts with the target material, either in transit to the substrate or on the surface of the substrate and forms a compound which forms the thin film. The resultant oxide, nitride or carbide coating (dependent on the reactive species used) imparts properties such as low emissivity, high impact strength and temperature resistance, excellent abrasion resistance and durability to the substrate that is coated.

Reactive Sputtering deposition is very commonly used in manufacturing and R+D processes, such as:

  • Flat-panel displays for televisions and cell phones
  • Photovoltaic coatings on solar cells
  • Optical coatings on sunglasses
  • Decorative coatings on hardware and automotive components
  • Insulating coatings on architectural glass

Alicat mass flow controllers are compatible with the most common reactive gases used in reactive sputtering. Those gases—and some of their typical compounds—are:

  • Oxygen (O2) – creating oxides such as Al2O3, SiO2, TiO2, ITO (Indium Tin Oxide)
  • Nitrogen (N2) – producing nitrides such as TiN, ZrN, CrN
  • Acetylene (C2H2) or Methane (CH4) – for diamond-like carbon films

Controlling the sputtering process

A lot of different variables are involved in ensuring an efficient sputtering deposition process but undoubtedly, one of the most crucial factors in achieving an efficient and high-quality deposition process is the control on the amount and distribution of the reactive gas in the vacuum chamber. When working with a combination of inert and reactive gases, partial pressure of the reactive gas is one of the variables that affects the characteristics of the thin film the most and partial pressure is directly proportional to the flow rate of the gas.

Using an open loop control to modulate the amount and timing of reactive gas flows, deposition rates and film properties can surely be optimized with precision to a certain extent but to implement a much tighter control over the deposition process Residual Gas Analyzers (RGA) are used with Alicat mass flow controllers. RGAs monitor the partial pressures of gas species involved in the process. The partial pressure readings from the RGA can be used to command an appropriate flow set point from the Alicat flow controllers, hence making it a closed loop process. Since the reactions involved in Reactive Sputtering are electrochemical and may take place in milliseconds of exposure of the target and substrate, your mass flow controllers need to react with a lightning fast response time.

Alicat mass flow controllers provide an indication response time of 10ms and in vacuum applications a typical control response time of 50-100ms. Alicat devices are equipped with a field-adjustable PID algorithm which provides you with an increased control over your system performance.

Avoiding Target Poisoning

Another important variable in determining deposition quality is the condition of the target during the sputtering process. Increasing reactive gas flow to the process speeds the chemical reaction, but can also cause full coverage of the target or “target poisoning”. When target poisoning occurs, the process can be negatively affected. A decreased deposition rate, as well as unwanted changes in vacuum and voltage levels can occur which can damage both the target and the substrate. Closed-loop process control systems are used to maintain the “transition” state of the target and avoid poisoning.

Reactive sputtering target poisoning

Closed-loop control is also more flexible than flow control coating, allowing for multi-gas and multi-zone process control. Alicat MFCs provide field-adjustable PID programmability for coating experts to attain the fastest response speed of any mass flow controller, improving process stability and coating chamber conditions. PID and PDF+ algorithms optimize control by altering the rate and manner of response control commands. Multiple PID algorithms are available, allowing the user to generate the best possible flow control response to any process control signals.

Alicat MFCs are compatible with mechanical and electrical connections used on existing sputtering tools, and are often used as a means to upgrade or update process controls. Many digital and analog interfaces are available, including RS-232 and RS-485. They can be combined with protocols such as DeviceNet and EtherNet/IP or EtherCAT.

Alicat mass flow controllers include the Gas Select™ feature. With it, you can easily switch gases without the need for K-factor calculations or other compensation, because each MFC contains a database of gas properties covering the full range of operating pressures and temperatures. Combining Gas Select with an expansive 0.01-100% control range, users greatly reduce the number of different MFCs necessary to meet the demands of reactive sputtering.

Alicat’s Applications Engineers can help you find the best mass flow controllers for your reactive sputtering application. Contact them using our online chat system, call in, send us a question on a web form or email info@alicat.com.

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