What is Thin Film Deposition?

What is Physical Vapor Deposition (PVD)?

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.

Evaporation

There are multiple types of evaporative deposition:

• E-beam evaporation (electron beam): In this process, a highly-charged electron beam evaporates the target material. The evaporated material is then deposited onto the substrate, and the atoms formed create the thin film. This process is often used for optical thin films such as solar panels, glasses and architectural glass.
• Ion assisted deposition (IAD): This process produces films with less scatter than typical evaporation. For a deeper dive into IAD, download our infographic.
• Thermal evaporation: In this simpler form of PVD, a resistive heat source heats the target material until vapor pressure is produced. The evaporated material then coats the substrate, forming the thin film. It is used to deposit metals such as silver and aluminum for OLEDS, solar cells and thin film transistors.

Magnetron Sputtering

Magnetron sputtering is a versatile, plasma-based coating technique. In this process, magnetically confined plasma is created near the surface of the target material. Ions from that plasma collide with the target material, and the atoms ejected from those collisions are what are “sputtered”, or deposited onto the substrate to create the thin film. It is often used for metallic or insulating coatings for optical and electrical purposes.

Pulsed Laser Deposition (PLD)

Pulsed laser deposition is a type of evaporation process that uses laser pulses to evaporate the target material. This produces a plume of plasma that then deposits onto the substrate, forming the thin film. While optimizing this process can require more time and effort than other methods, due to the many variables at play, its benefits include high deposition rates and a streamlined process, as well as no filaments for easier maintenance.

Common applications include:

• Fiber optic lasers for telecom and wireless comm systems
• Fiber lasers for metal cutting
• Consumer electronics
• Laser diodes for 3D sensing
• LED/OLED displays

Common coatings used across this industry are:

• AR/HR coatings
• Optical filters
• TCO coatings
• Indium bump coatings
• DLC coatings

AR/HR coatings

Anti-reflectivity/high reflectivity coatings are both used to change an optic’s degree of reflectivity, but in opposite ways. An AR coating increases the transmission of light waves to reduce surface reflections. An HR coating increases surface reflectance to create mirrors that reflect all light of a given wavelength.

AR coatings are commonly used in eyeglasses, photographic lenses and solar cells, and any application in which reflection would diminish performance or user experience. Laser applications use both AR and HR coatings.

Common deposition methods:

• Ion assisted deposition
• Magnetron sputtering
• Ion beam deposition

Indium bump

Indium bump deposition is used primarily in focal plane arrays for IR imaging. As feature sizes get smaller, manufacturers need a more effective solution than indium bump electroplating. Indium bump deposition offers high shear strength, good lift-off and collimation control for such purposes.

Maximum yield indium bump deposition is achieved with a cooled substrate stage for temperature control, and collimation control that enables direct deposition. When both temperature and geometry are optimized, so is the crystal structure, enabling high shear strength and improving performance.

Common deposition methods:

• E-beam evaporation
• Thermal evaporation

Transparent conductive oxide coatings (TCO)

These are transparent, electrically conductive film coatings that are commonly used in electronics. They are particularly common in displays. They offer improved precision and higher yield, and typically have a high transmittance and low resistivity. Specific applications for TCO coatings include photovoltaics, liquid crystal displays, OLEDs and touchscreens.

Common deposition methods:

• Magnetron sputtering

Optical filters

Optical filters are similar to AR/HR coatings in that they are used to modify light waves within an optical system. The difference here is that it’s for the purpose of filtering light through the optic, not just to change reflectance on the surface.

Very precise coatings are required to accurately control reflectance, transmittance, absorbance and resistance. Filters are commonly used in photolithography, image enhancement, image splitting, color match imaging and Raman spectroscopy.

Common deposition methods:

• Ion assisted deposition
• E-beam evaporation
• Magnetron sputtering

Diamond-like carbon coatings (DLC)

DLC coatings are integral to consumer electronics, particularly the display market, where they are used to protect displays and LEDs. These coatings offer improved hardness, chemical and scratch resistance, high thermal conductivity and electrical resistivity.

With DLC coatings, product life in the end-use application is extended; commonly used on displays, sensors and flexible semiconductors. Displays also require other protection for durability and to extend product lifetime. For example, hydrophobic coatings are needed to make touchscreens water resistant and resistant to wear from constant contact with skin oil.

Common deposition methods:

• PE-CVD

Metallization

One of the most common methods for coating semiconductors, metallization involves depositing a metal film over a dielectric film (or layers of both) onto a semiconductor wafer. Adhesion layers and conduction layers are deposited to create gate stacks, allowing the device to supply an electric current or provide an electronic signal. Metallization requires good adhesion between the thin film coating and the wafer, otherwise performance will be adversely affected.

Common deposition methods:

• Magnetron sputtering
• E-beam evaporation

Medical Applications

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

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.

Common deposition methods:

• Magnetron sputtering for conformal coatings for wear resistance/color coding
• PE-CVD on TiN coatings to enhance durability

Thin Film Characteristics/Performance

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.

Film Thickness

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.

Film Uniformity

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.

Reflectivity

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.

Grain size

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

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.

Plasma emissions monitor

A PEM is often used to improve throughput and yield while ensuring that performance-related properties do not fall outside of the desired setpoints. PEM involves monitoring reactive gas flow during the deposition of oxide and nitride films. It prevents target poisoning by maintaining the gas intensity at a constant setpoint. A PEM is a good option for applications like high-rate optical film deposition, process quality control, metallurgical research and process control, and process R&D.

Optical monitoring system

An OMS is ideal for precision optics applications where feedback is needed as an optical film is produced. Similar to a PEM, an OMS offers excellent repeatability and yield. The system monitors an optic’s properties by observing its reflection or transmission through a substrate or witness chip. Based on the reflection, the OMS will signal the end of the deposition when the desired optical thickness has been reached. It’s a desirable control for applications like AR/HR coatings, laser facets, optical sensors and filters, band pass filters, IR filters and coatings, neutral density filters, metallization and beam splitters.

Quartz crystal monitoring

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.

Ellipsometer/reflectometer

Ellipsometers and reflectometers can be used during the thin film deposition process to provide feedback for Statistical Process Control. These in-situ controls are good options for monitoring curvature, stress or reflectivity.

Process Software

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.