Contact angle describes the shape of a liquid droplet resting on a solid surface. When drawing a tangent line from the droplet to the touch of the solid surface, the contact angle is the angle between the tangent line and the solid surface.

If a liquid with well-known properties is used, the resulting interfacial tension can be used to identify the nature of the solid. This technique is extremely surface sensitive, with the ability to detect properties on monolayers.

How the Technique Works
When a droplet of liquid rests on the surface of a solid, the shape of the droplet is determined by the balance of the interfacial liquid/vapor/solid forces. CA can be used to detect the presence of films, coating, or contaminants with a surface energy different from that of the underlying substrate.

When a droplet of high surface tension liquid is placed on a solid of low surface energy, the liquid surface tension will cause the droplet to form a spherical shape (lowest energy shape). The measurement provides information regarding the bonding energy of the solid surface and surface tension of the droplet. Because its simplicity, CA has been broadly accepted for material surface analysis related to wetting, adhesion, and absorption.

Industry Applications
CA helps chemists determine the properties of detergents, surfactants, coatings, adhesives, etc. The semiconductor industry uses this technique to qualify wafer cleanliness, hexamethyldisiloxane (HMDS) process control, photoresist/developer studies, chemical mechanical planarization (CMP) process development, surface modification process development, and quality control.

Administering the Measurement
Traditionally, CA utilizes a sample stage to hold the substrate, a syringe to apply a droplet of liquid, a light source to illuminate the droplet, and a set of optics for magnifying the image for observation. (An instrument such as a goniometer typically is used.)

After placing a droplet of liquid (1-25 µL) onto the surface, the outline of the droplet is studied through the magnifier. The operator positions the tangent line from the droplet to the touch of the surface. A protractor within the optics then provides a reading of the contact angle.

Unfortunately, operator subjectivity often interferes with the accuracy of the technique. Modern contact angle systems, however, adopt precision optics and charge-couple device (CCD) cameras with image processing hardware and software to enhance the performance of contact angle analysis, making it easier, quicker, and more precise.

A droplet of liquid is dispensed onto the substrate surface (manually or automatically), and a CCD camera reveals the profile of the droplet on the computer screen. Software calculates the tangent to the droplet shape and the contact angle. Data and the image are collected, analyzed, and saved on computer.

Surface preparation has become very important in a variety of industries. In many cases, surface cleanliness is a vital concern prior to finishing or processing. Modern contact angle instruments with computerized video processing offer a reliable, accurate, and repeatable method to quantify cleanliness levels of surfaces.

Sarah Shieh is Director of Operation, Equipment, AST Products (Billerica, MA).