Authors: Mic Chaudoir, Ph.D. and Nicholas Day, Engineering Manager, Chroma Technology Corp.
Improving vacuum control for sputter coating
Accessibility to a wide range of materials and advances in thin film deposition technology have led to significant improvements in thin film coatings. This has had a profound impact on many areas of daily life, from medical settings to laboratories to the automotive industry.
Read on to learn about thin film coating technologies and how advances in vacuum are improving process efficiency, precision, and reliability.
Types of coatings
Anti-reflective/high reflectivity coatings
When light moves between mediums of different refractive indices, such as from air to glass, some of that light is reflected rather than transmitted. Layered coatings can selectively filter the light based on wavelength. For example, layered coatings help shield our eyes from harmful UV rays (1oo-400 nm) in eyeglasses/sunglasses. Chroma Technology offers custom and prototype filters created using sputtered thin film optical coatings.
They have also contributed to the myriad of fluorescent dye combinations used in modern fluorescent microscopy, including the green fluorescent protein (GFP). The scientists who discovered and developed GFP were awarded the 2008 Nobel Prize in Chemistry and this protein is a staple in many labs.
Transparent conducting oxide
Transparent conducting oxide (TCO) is used to apply nearly invisible, electrically conductive coatings. Commonly used materials are indium tin oxide and aluminum zinc oxide.
While rarely “seen,” TCO coatings are encountered daily in the form of liquid crystal displays (LCDs), touch screens, and even solar cells and photovoltaics. Conductive coatings are also used for unique sensor and strain monitoring technologies.
Diamond-like carbon coatings
Diamond-like carbon coatings (DLCs) are becoming increasingly common due to technological developments by companies like Duralar Technologies. These coatings provide the many beneficial properties of actual diamonds, including resistance to abrasion (either mechanical or chemical), hardness, and lubricity.
DLC coatings are used for many mechanical processes that formerly required large amounts of lubricants (typically fossil fuel based) to prevent abrasive wearing. They are often used to coat components used in automotives, firearms, microelectronics, medical devices, displays and sensors.
Biocompatible hard coatings
One of the challenges for medical device manufacturers is finding materials that are resistant to chemical, biological, and mechanical wear while simultaneously not stimulating an immune response. Many materials that would normally be well suited cannot be used, due to their biological reactivity.
In many cases, materials that stimulate an immune response can be shielded and made biocompatible with appropriate hard coatings. Biocompatible hard coatings can have the added benefit of preventing unwanted cell adhesion. Typical coatings utilize variations of titanium nitride (TiN).
Coating techniques for creating thin films
There are many methods for depositing thin film layers. One of the most popular processes is known as sputter coating or “sputtering” for short. Sputter coating works by “spraying” molecules from a target onto a suitable substrate while under extremely low pressure vacuum conditions. To do this, the target material is bombarded by ions of an inert gas.
While sputter coating always operates as described above, there are some variations in how the method of bombardment:
- In ion beam sputtering, ions are generated in a dedicated ion source and accelerated toward the target.
- In magnetron sputtering, a large negative voltage is applied to the target (or “cathode”) to create a plasma from inert gas. An electromagnetic field then pulls the plasma ions toward the target, causing them to collide with the target and knock some of the molecules free. Some of these ejected target molecules then collide with and bond to the substrate.
The role of mass flow and vacuum control in sputter coating systems
Sputtering techniques typically require initial chamber evacuation, followed by the addition of an inert gas (often argon) from which the plasma is created. In reactive sputtering, additional gas (often oxygen or nitrogen) is added to create reaction products deposited onto the substrate surface. Creating the appropriate chamber conditions then requires coordination of vacuum levels, inert gas input, and potentially reaction gas inputs. While research and development facilities may create their own sputtering systems, many commercial options also exist.
In sputter coating systems, it is critical to maintain process gas partial-pressures and overall system pressure. Low latency, high accuracy, cost effectiveness, and fast control are generally foremost among the design objectives.
Mass flow control
Alicat mass flow controllers have several features allowing them to quickly, accurately control gas inputs like argon for plasma generation:
- Control response times as quick as 30 ms
- Multi-parameter reporting of volumetric and mass flow, pressure, and temperature to the system up to 1000 times/second
- Accuracy of 0.6% of reading
- Input gases based on “true mass” (actual weight of gas molecules delivered) or quantify the number of moles for stoichiometric calculations
Maintaining the vacuum within the sputtering chamber is also essential. This can be challenging due to the introduction of additional gases and changes in temperature that affect chamber conditions and subsequent process control.
A typical pressure control setup is demonstrated in the diagram below, with a separate vacuum gauge, vacuum controller, and throttle valve.
The IVC-Series pressure controller can be used to upgrade these setups, improving process efficiency and reducing space and necessary parts. Each controller measures and controls the vacuum level within the chamber, integrating a separate vacuum gauge, vacuum controller, and throttle valve. Throughout the process, the instrument maintains the pressure level in the sputtering chamber with high accuracy even at low ranges, down to 10 millitorr.
Alicat vacuum controllers are available with easy-to-use digital and analog interfaces, or your choice of supported industrial protocols. This bridges the gap commonly found between mass flow and vacuum control systems, allowing easy access to process controls from a common interface. Alicat systems also feature an integrated backlit display, allowing for an “at a glance” understanding of sputtering chamber conditions.
All Alicat instruments can be custom-built to meet your precise specifications. These products are easy to integrate into pre-existing systems, simplifying setups and increasing responsiveness and efficiency. Contact an applications engineer to discuss your process needs.