Advances in thin film coatings have had a profound, if largely invisible, impact on virtually all areas of our daily lives. The improvements in thin film coatings are made possible by both widening the range of materials that can be used to create coatings, as well as improving the technology to deposit thin film layers. Modern methods vary based on application, but all center on depositing layers of material that range from a few atoms to a few microns. One of the most popular methods for creating thin films is sputter coating. Read on to find out more about the technology of thin film coatings and how the latest advances in vacuum can make this process more efficient, more precise, and more reliable.
Anti-Reflective/High Reflectivity Coatings
When light moves from a medium with one refractive index, to a medium with a differing refractive index (such as air to glass), some of that light is reflected rather than transmitted. Layering coatings allows for selectively filtering light based on wavelengths. For example, layering coatings help shield our eyesight by from harmful UV rays (<400nm) in eyeglasses/sunglasses. It also has allowed for modern fluorescent microscopy by providing a myriad of possible fluorescent dye combinations. The award of the 2008 Nobel Prize in Chemistry for the discovery and development of green fluorescent protein (GFP) was aided by the kind of filters described here. Chroma Technology is one company offering custom and prototype filters created using sputtered thin film optical coatings.
Transparent Conductive Oxide
Transparent Conductive Oxide (TCO) coatings allow for nearly invisible electrically conductive coatings to be applied. Commonly used materials are Indium Tin Oxide (ITO), and more recently, Aluminum Zinc Oxide (AlZnO). While rarely “seen”, TCO coatings are encountered daily in the form of liquid crystal displays (LCD), touch screens, and even solar cells and photovoltaics. Conductive coatings also enable some unique sensor and strain monitoring technologies.
Diamond-like Carbon Coatings
Diamond-like carbon coatings (DLC) are becoming less exotic and more accessible as companies like Duralar technologies develop coating instruments specifically designed to fabricate DLC coatings. These coatings are so named, as they provide many of the same beneficial properties of actual diamonds: resistance to abrasion (either mechanical or chemical), hardness, and lubricity. These high performance coatings are becoming commonly used for many mechanical processes that formerly required large amounts of lubricants (typically fossil fuel based) to prevent abrasive wearing. Automotive components and firearm components often have DLC coatings applied. DLC, much like synthetic diamonds, are typically applied using Chemical Vapor Deposition (CVD). CVD differs from the sputter coating more commonly used for the other thin films mentioned. DLCs are also used in microelectronics, medical devices, displays, and sensors as well.
Biocompatible Hard Coatings
Sputtering has also been useful in the creation of biocompatible coatings. One of the challenges for medical device manufacturers is finding materials that have the needed properties (often resistance to chemical, biological, and mechanical wear) and do not stimulate an immune response (commonly termed “rejection”). 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 include Titanium Nitride (TiN) and variations thereof.
Coating to Create Thin Films
There are many methods for depositing thin film layers, such as those described above. One of the most popular processes is known as sputter coating or “sputtering” for short (technically a form of physical vapor deposition). The common usage of the word “sputter” means to eject or spray; for thin film coatings, it is an apt description for the process of sputter coating as well. Sputter coating works by “spraying” molecules from a target onto a suitable substrate while under extremely low pressure (vacuum conditions). The target material is bombarded by ions of an inert gas, commonly Argon. While sputter coating always operates as described above, there are some variations in how the bombardment occurs that influence system design.
In Ion Beam Sputtering (IBS), ions are generated in a dedicated ion source and accelerated toward the target. In magnetron sputtering, the target is also referred to as the “cathode” because a large negative voltage is applied to create a plasma from the inert gas being injected around it. The ions in the plasma are pulled toward the target by the electromagnetic field. In the ions’ kinetic energy knocks some of the target’s molecules free, ejecting them away from the target. Some of the ejected target molecules arrive at the substrate to which they form a very strong bond. Sputtering techniques typically require initial chamber evacuation, followed by addition of an inert gas from which the plasma is created (most often Argon). 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, Argon (or similar) gas input, and potentially reaction gas inputs. While research and development facilities may create their own sputtering systems, many commercial options also exist.
The Role of Vacuum Control in Sputter Coating Systems
The performance of sputter coating systems depends on many factors, with process gas partial-pressures and overall system pressure being among the most important. Low latency, high accuracy, cost effectiveness, and fast control are generally foremost among the design objectives. Alicat differential pressure-based laminar flow measurements allow for a very fast, accurate control of the input gases. Alicat mass flow controllers are able to achieve control as quick as 30ms, and can report multiple parameters back to the system (volumetric and mass flow, pressure, and temperature) up to 1000 times/second. Accuracy is 0.6% of reading for most mass flow measurements, which ensures the proper amounts of gas are delivered. Alicat mass flow devices can also input gases based on “true mass” (actual weight of gas molecules delivered) or quantify the number of moles for stoichiometric calculations. The latest technology means that control of gas inputs (especially Argon for plasma generation) is achieved quickly and easily.
Control of the vacuum level within the sputtering chamber is also an essential component of the sputter coating process. Alicat’s new Conductor Series allows for one platform to measure and control the vacuum level within the chamber, integrating a separate vacuum gauge, vacuum controller, and throttle valve. The Alicat Conductor Series orchestrates the components to ensure the necessary vacuum is achieved making the sputtering chamber ready to begin thin film deposition. As the deposition process begins, introduction of additional gases, as well as changes in temperature, can effect chamber conditions and subsequent process control. Throughout the process, the Alicat instrument works to ensure that the pressure level in the sputtering chamber remains in harmony with your process control requirements.Conductor-series vacuum controllers
The Alicat Conductor Series, as well as our share the same easy-to-use digital and analog interfaces (or choice of supported industrial protocols). This bridges the gap commonly found between mass flow and vacuum control systems, allowing process controls to be easily accessed from a common interface. Alicat systems also feature an innovative display, allowing for “at a glance” understanding of sputtering chamber conditions.
All Alicat instruments, including the new Conductor Series can be custom-built to your precise specifications and ship in days, not weeks. These products integrate into your systems and replace multiple components increasing the responsiveness of your system and improving efficiency. Our sensors operate with high accuracy even at low ranges, down to 10 millitorr. Talk to an engineer today to find out how the world’s fastest flow controller company can help make your vacuum coating process more precise and efficient.Conductor-series vacuum controllers
Mic Chaudoir, Ph.D. – Market Manager, Alicat Scientific
Nicholas Day- Engineering Manager, Chroma Technology Corp.