Sol Gel Process
What is Sol-Gel and how is it used?
The sol-gel process is a method for creating solid materials from small molecules. It’s commonly used to deposit single- or multicomponent oxide coatings on glass or metal surfaces.
There are two main coating techniques:
- Spin coating – for one-sided coatings
- Dip coating – for double-sided coatings
Both methods are widely applied in manufacturing thin films.
A defining feature of sol-gel processing is that it begins with a liquid sol: a dispersion of solid particles (1–100 nm) in water or an organic solvent. Through the sol-gel transformation, this liquid is converted into a solid gel by forming a 3D network of nanoparticles. Controlled heat treatment in air then converts the gel into a dense oxide-ceramic material.
How Is a Sol-Gel Created?
A sol-gel is formed by condensing a solution of metal oxide precursors into a three-dimensional network. The result is a bi-phasic system, where a fluid fills the spaces inside the solid network.
Drying this gel in a controlled way produces porous solids with unique thermal, mechanical, optical, and chemical properties.
While early sol-gels were mostly silica-based (produced by condensing silanols, SiOH), the process has evolved into a flexible synthesis method. Today, sol-gel materials can be made from almost any transition metal, and even as composite materials.
Spin Coating Sol-Gel Steps
One-Sided Sol-Gel Coating with a Spin Coater
Spin coating is ideal for applying thin, uniform layers on flat substrates. While often used for photoresist coating in semiconductor manufacturing, POLOS® spin coaters are also popular for:
- Spin-coating polymer thin films (e.g., PDMS, PMMA, block copolymers)v
- Low-cost sol-gel ZnO film deposition (often on glass substrates)
One of spin coating’s main advantages is natural thickness uniformity. This means that during spinning, the liquid layer evens out and stays consistent, provided viscosity remains stable across the surface.
Typical Spin Coating Program for Thin Polymer Films
To control thin film thickness with sol-gel spin coating
- Dissolve or disperse the polymer in a solvent to create the sol
- Drop the sol onto the substrate surface
- Spin at the desired speed and duration. For example, a few drops at 3,000 rpm for 30 seconds can produce a fine, uniform layer.
POLOS® offers special chucks for a wide range of substrates, compatible with or without vacuum. Models like the SPIN150x and SPIN200x are perfect for these methods and new applications are discovered by our customers every day.
Some applications of sol gel materials:
| Application | Sol-Gel Material |
| Optical fibers | High purity doped silica gel films for optical fiber precursors |
| Protective optical coatings | Abrasion resistance silica gel coatings on plastic substrates |
| Anti-reflective optical coatings | Laser windows, smart windows |
| Thermal insulation for windows | Aerogel window spacers, solar collector coatings |
| High Temperature Refractory Insulation | Ceramic foams |
| Chemical Sensors | Thin film NOx sensors, sol-gel coated crystal oscillators |
| Catalysts and Adsorbents | Silica alumina solid acid catalysts, high surface area catalyst supports, Silica gel desiccant |
| Ceramic membranes | Sol-gel molecular sieves, antibacterial filters |
| Abrasives | Alumina abrasives |
| Dental sealants and fillers | Hydroxyapatite |
(source: http://www.mdpi.com/journal/materials/special_issues/sol-gel-technique-materials)
