Glancing angle deposition (GLAD) is a thin film deposition technique in which a substrate is placed at an angle to the direction of the coating vapor flux, while that substrate is rotated or manipulated to control the film’s characteristics and growth. It is primarily used as an electron-beam thermal evaporation process, but it is also sometimes used with sputtering coatings.
GLAD is a refinement of oblique angle deposition (OAD). Oblique means slanted less than 90 degrees. In OAD, the substrate is stationary and placed at an oblique angle to the direction of deposition flux so as to control columnar and helical nanostructure growth.
Glancing angle deposition involves the application of two variables, both the angle of incident of the vapor flux as well as the rotation of the substrate. This enables exceptional process control for the creation of highly desirable thin film features such as optical reflectiveness or wettability.
What is the angle of deposition?
The angle of deposition is defined by how many degrees the angle of the vapor coating flux varies from a 90-degree perpendicular deposition. It is the angle of the direction of the vapor flux subtended from a 90-degree perpendicular angle.
The angle of deposition has several very significant effects on how thin films using GLAD are created due to line-of-sight shadowing and substrate surface diffusion.
The angle of deposition and columnar growth
When the substrate is tilted at an angle to the arriving coating material, columnar growth occurs faster on the surface that is tilted towards the direction of the incoming vapor flux. During the initial stages of film formation, atomic nuclei randomly condense on the substrate surface.
As other incoming atoms condense around these atomic nuclei and begin to grow into columns, line-of-sight shadowing prevents the subsequent incoming atoms from impinging on the area directly behind these initial nuclei.
The result is known as competitive columnar growth, in which the side of the column on the side of the incoming deposition flux, grows faster than the opposite side in the shadow. Tall columns grow at the expense of smaller columns due to self-shadowing.
As taller columns continue to grow, they shadow the shorter columns until they are completely cut off from the coating flux, and growth continues on the tallest columns only. Tilting the angle of deposition with GLAD controls the porosity of columnar growth and its surface area.
By precisely controlling the angle of deposition, films can be created with extremely high sensitivity to light reflectivity for optical sensors, or wettability for creating self-cleaning surfaces.
Effects of rotation of the substrate
Thin film structures produced by GLAD are categorized by; 1) The deposition angle, and 2) The substrate rotation which can either be relatively stationary, changing at discrete time intervals, or continuously rotating where the substrate is in constant motion to control the dynamics of the thin film deposition.
With GLAD, if the substrate is rotated 180 degrees during deposition, what was formerly the uphill, faster growing side of the column now becomes masked in the shadow of what was formerly the slower growing side. This self-shadow or self-masking as a result of rotating the substrate allows for the growth of nano-structures never before possible with extremely high process control.
By rotating the substrate during deposition, columnar growth is easily modifiable due to this self-aligning from the shadowing effect. Columns can be sculpted into a variety of shapes and morphologies including slanted and vertical columns, helices, spirals, C-shaped, S-shaped and hollow core columns.
For example, when the substrate is rotated 180 degrees during deposition for a fixed amount of time, then reversed to the original position, it produces what from a cross section looks like a zig-zag pattern. The angle of deposition is the diagonal angle of a column until the rotation is reversed and the slant goes in the opposite direction. When the rotation of the substrate is more gradual it can produce spiraling helical structures.
Electron-beam evaporation systems are usually preferred for GLAD because they operate at low pressure and the coating material can be placed at a further distance from the substrate. Thermal evaporation systems don’t require a background gas to operate so the columnar growth can be more easily controlled.
Being primarily a thermal evaporation technique, GLAD is compatible with a very large range of coating materials including MgF2, SiO2, Si, TiO2, and ITO.
In addition to the angle of deposition and the substrate rotation, GLAD columnar growth is also influenced, as with any physical vapor deposition process, by the temperature of the substrate, deposition rate, vacuum pressure, gas composition, and types of coatings and substrates.
GLAD is used for a wide range of applications, but to date has primarily been used in optical devices where the angle of reflectivity can be precisely controlled by the angle of deposition for high-quality optical sensors.
Self-cleaning technologies are another area where GLAD has become the thin film deposition process of choice due to the porosity of the film and surface area being able to be controlled by the deposition angle.
There are many examples of self-cleaning processes in nature such as the leaves of plants or butterfly wings. Self-cleaning surfaces are the result of the surface contact angle or wettability, which is the ability of a liquid to spread over a surface. It is measured by the angle formed between the surfaces of a liquid droplet with the solid surface.
The self-cleaning property of a high surface energy GLAD coating where water droplets naturally spread-out, results in dust particles being more easily removed by the rolling motion of water droplets across the surface, rather than bunching up and concentrating.
GLAD is utilized to create surfaces with superior light transmission qualities like self-cleaning solar panels and windowpanes because the shape and in-plane alignment of columns can be easily modified.
Other applications include the creation of nanowires for nano electrical arrays, pressure sensors, humidity sensors, fuel cells, battery charge storage and electrochemical supercapacitors.
The combination of being able to control with two stepper motors, both the angle of deposition and the rotation of the substrate, makes glancing angle deposition or GLAD a highly flexible physical vapor deposition technique with extremely precise process control.
Semicore’s CAPOS all-in-one thermal evaporation or sputtering system
GLAD films can be created with Semicore’s breakthrough CAPOS thin film deposition system, the PVD Industry’s first cost effective “Open-platform” design, ideal for both precision R&D and batch production.
Flexible enough that it can be used for either sputtering or evaporation coatings, this platform is also used for Semicore’s CAPOS-CT series, as a highly versatile “Cluster Tool” platform that can be configured with multiple Process Modules (PM) and cassette-to-cassette operation.
For more detailed information on these units’ specifications please download the PDF.
Matt Hughes is President of Semicore Equipment Inc, a leading worldwide supplier of sputtering equipment for the electronics, solar energy, optical, medical, military, automotive, and related high-tech industries. Please let our helpful support staff answer any questions you have regarding What is Glancing Angle Deposition and how to implement the best techniques and equipment for your specific Thin Film Deposition Thermal Evaporation & Sputtering needs by contacting us at firstname.lastname@example.org or by calling 925-373-8201.