by: Sarah ShiehContact angle measurement (CA) is a
simple-to-adopt method for surface analysis related to surface
energy and tension.
Pages: S-8; March,
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
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.
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.
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
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