Thin film deposition is the process of creating and depositing thin film coatings onto a substrate material. These coatings can be made of many different materials, from metals to oxides to compounds. Thin film coatings also have many different characteristics which are leveraged to alter or improve some element of the substrate performance. For example, some are transparent; some are very durable and scratch-resistant; and some increase or decrease the conductivity of electricity or transmission of signals.
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Thin film deposition is an important manufacturing step in the production of many opto-electronic, solid state and medical devices and products, including consumer electronics, semiconductor lasers, fiber lasers, LED displays, optical filters, compound semiconductors, precision optics, microscopy & microanalysis sample slides, and medical implants. There are a few different technologies and methods that can be used to apply thin film coatings, and an array of tools and equipment that can be used to streamline or enhance the thin film deposition process.
There is no one-size-fits-all, perfect thin film deposition system or method. Your technique and configuration of choice depends on the performance and production requirements that are unique to your application.
Physical vapor deposition (PVD) describes a group of thin film deposition techniques that involve vaporizing a solid material in a vacuum, then depositing that material onto a substrate. Coatings created in this manner are highly durable, and resistant to scratching and corrosion. PVD is useful in the production of devices ranging from solar cells to eyeglasses to semiconductors.
The benefits of PVD are numerous, and include the creation of a hard coating that is resistant to corrosion and scratching. PVD also creates thin films that can tolerate high temperatures. Denton’s PVD systems can be integrated with in-situ controls that allow for feedback and stability throughout the process.
Potential drawbacks of PVD include cost, as these methods may require a larger investment than other thin film deposition processes. The cost can also vary among PVD methods themselves. For example, evaporation is a lower-cost type of PVD, while ion beam sputtering is a rather costly option. Magnetron sputtering is more expensive than other methods, but has better scalability.
A variety of techniques fall under the PVD umbrella. For a comparison of these method types, read: Which PVD Method to Use: Magnetron Sputtering v. Evaporation.
Chemical vapor deposition (CVD) describes a group of thin film deposition techniques in which a substrate is placed into a vacuum chamber, two chemical precursors are heated, which causes them to vaporize. When they meet on the substrate surface, a chemical reaction occurs to form a high-performance thin film coating. CVD is useful in creating coatings for a wide variety of applications including medical devices, automotive components and silicon wafers.
Benefits of CVD methods include the ability to use these processes on a wide variety of substrates, as well as the ability to coat intricate or complex topographies. Thin films created through CVD also typically maintain their bonds well in high-stress environments.
Some drawbacks may include limitations in size, which is dependent on the size of the vacuum chamber, as well as that most methods typically require a higher temperature to drive the chemical reactions. It can also be difficult to mask the surface of the coating.
There are a variety of CVD methods:
In this process, which takes place at normal, or atmospheric, pressure and a lower temperature than other methods, the substrate is exposed to at least one volatile precursor. The precursor(s) react on the surface of the substrate to deposit the thin film. It can be used to deposit doped and undoped oxides, and the deposition is fairly quick. Thin films produced by this method are low-density and have moderate coverage.
In low-pressure CVD, heat is used to break down a precursor gas inside the chamber where the reaction will take place. This causes the reactive gas to react with the substrate when it is injected into the chamber, and this reaction creates the thin film coating. Low-pressure CVD is commonly used for the deposition of materials including polysilicon and silicon nitride, and can be useful for batch processes. Coatings created with this process are more uniform and feature fewer defects, but the process requires a higher temperature which can limit the materials available to use.
In ultrahigh vacuum CVD, the substrate is exposed to precursor gases in an ultrahigh vacuum (<10^{-6} Pa). These precursors then react and deposit onto the substrate, forming the thin film.
While atomic layer deposition falls under the CVD umbrella, it differs in that precursor materials are kept separate during the reaction. In this process, the reaction occurs due to sequential pulsing of precursor vapors—one atomic layer is formed during each pulse. Pulses are repeated until the thin film reaches its desired thickness. Benefits of atomic layer deposition include high quality defect-free coating, as well as improved thickness uniformity.
Plasma-enhanced CVD is a lower-temperature alternative to standard CVD, and is often used in the production of electronic devices. One common application for PIB-CVD coatings is to protect these devices from corrosion. It can create, for example, high-quality silicon dioxide (SiO2) film at 300°C to 350°C as opposed to the temperature range of 650°C to 850°C required by standard CVD to create similar films. In plasma-enhanced PIB-CVD, a pair of reactive gases are excited to create a plasma. This causes a chemical reaction that results in the thin film being deposited onto the substrate.
Medical implants and similar devices often require coatings for protection. Coating specs need to be very precise and particular to meet medical regulations and standards, such as those specified by the FDA. Safety and health is of the utmost importance in this market so consistent performance needs to be ensured for every device and coating. Implants will face exposure to human tissue and other biological substances/materials, so they need to be protected from degradation without harming the person.
Common coating solutions for the medical market include DLC and biocompatible hard coatings.
Biocompatible hard coatings are often made up of titanium nitride, titanium aluminum nitride, chrome nitride or titanium carbo-nitride. They are used to increase the durability/lifetime of biomedical implants, and help with dimensional stability for stimulation electrodes, such as those used in implantable neural prostheses leads.
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Common deposition methods:
There are a number of different properties that can affect thin film performance. The properties that manufacturers need to focus on vary depending on specific application needs. Precision optical coatings often require a very tight precision and repeatability, for instance, whereas good adhesion is an important requirement for high-volume metallization coatings.
This refers to the thickness of the thin film coating. Your thin film coating needs to fall within the right thickness parameters for your specific application in order to ensure performance.
Some applications require a very fine, precise coating, for example, to allow for the right degree of light transmission. Other applications require a thin film coating for durability and protection, so they can be a little bit thicker and may not need to be so precise.
Thickness also has big implications for next-gen products. Flexible displays are starting to mature on the market for consumers but they require thinner film coatings so that they can bend as needed.
Uniformity is one of the most important properties across all applications and coatings. If thickness distribution is not even, it could inhibit durability and performance for the entire film. Some substrates also have a more complex topography, with bumps and vias present, but the thickness also needs to remain consistent across the surface.
Uniformity affects other important production factors such as yield and cost of ownership, along with thin film performance. Better thickness uniformity means more usable parts are produced, but the costs of achieving uniformity with a particular system or configuration must also be considered and weighed.
The degree of reflectance is particularly important for AR/HR coatings used in laser applications. A thin film’s reflectivity determines if it will act as a mirror and reflect light back out, or allow light to pass through as in an optical filter or laser diode.
Whether it’s reducing the glare on a pair of glasses or a VR headset, or filtering out visible light in a smartphone’s face recognition software, reflectivity is a very important property to control for in many applications in the opto-electronics and optics spaces.
Solid metals are composed of individual crystallites or particles, and the size of these particles is referred to as grain size. Grain size itself determines many other properties specific to the metal, most notably its density. High energy and higher migration between particles leads to a smaller grain size, while lower energy and lower migration leads to a larger grain size.
Density is inversely proportional to the grain size, so for applications requiring a high density, you want a coating that has more migration between particles and a smaller grain size. By controlling the grain size and density, you can also improve the thin film’s wear resistance and strength.
Yield is determined by the number of usable parts that are produced by your thin film deposition system. It’s largely determined by your configuration’s precision and repeatability. Reaching a high yield is also impacted by performance requirements. If your coating requires very tight specs, you’ll need to prioritize precision to hit within those specs in order to raise your yield.
It’s important to consider exactly what an application requires in order to determine what an acceptable yield is. For example, your application may only require a spec to fall within +/- 5% of a certain setpoint. If you improve precision beyond that, it’s not going to significantly raise your yield, so it may not be worth the investment.
In-situ controls provide feedback on the deposition process and thin film formation by observing it in real time. Controls allow you to make adjustments during the deposition process so you can ensure the coating is hitting the necessary specs. Thanks to these critical adjustments, you gain better process control and more precise thin film coatings.
There are several different types of in-situ controls:
QCM differs from an OMS because it is used to measure the physical thickness of a film, not the optical thickness. QCM involves indirect monitoring of a separate chip, not the actual substrate, so it requires calibration within the tool. The QCM provides a linear change in crystal frequency to signal the film thickness on the crystal. It’s a cost-effective control that is best applied when there are broad specifications on optical properties of metal and optical films.
Software is an integral part of any thin film deposition system. Reliable software allows you to execute critical process steps with accuracy and efficiency. If you’re using a front-end option or scaling your system up for high-volume manufacturing, having a software package that enables automation is integral.
Your software should be able to handle your specific throughput and production requirements while also delivering precision and performance. Important features in a software package, particularly for high-volume or demanding applications, include a recipe builder, automated scheduler and on-tool, customizable charting capabilities.
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