PROBE THE INTERFACE

New tools for examining thin films and surfaces shed light on manufacturing procedures

You'd hardly expect a few measly layers of molecules to make much difference in the properties of macroscopic materials. But they do. Thin films and surface coatings lie at the heart of a host of applications ranging from common house paint and lubricants to chemical sensors and optics.

One of the keys to controlling the performance of devices that depend on thin films is controlling the film itself. And that, in turn, requires being able to probe the film in detail to determine its structure and composition. Earlier this month at Pittcon, researchers discussed some of the latest developments and applications in surface analysis and microscopic imaging techniques.

Patrick Chapon reported on glow-discharge methods used to study thin films on glass and other materials. Chapon, a product manager at instrument manufacturer Horiba Jobin Yvon, Longjumeau, France, noted that the technique can be used to examine surface layers and to obtain depth profiles of bulk or multilayer materials.

The glow-discharge process is similar to the principle of operation of a neon light tube, Chapon pointed out. Applying a voltage across a pair of electrodes at either end of a gas-filled tube forms a plasma, causing the gas to emit light of a characteristic color.

Similarly, in an argon-filled glow-discharge lamp, argon ions collide with a sample, which serves as one of the electrodes, and cause atoms to be dislodged from the sample surface. The analyte species become excited and then lose energy and emit light of characteristic wavelengths. As the atomic-scale sandblasting process digs a tiny crater in the sample, light from the sputtered atoms is analyzed in a spectrometer. In that way, element-specific data are collected layer by layer--providing a depth profile in just minutes.

Information of that type can be used by manufacturers to evaluate coating procedures. "It tells you why one layer is good or no good or why a certain process is working properly and another one isn't," Chapon said.

Earlier studies carried out by Chapon and coworkers demonstrated that the glow-discharge method was well-suited to studying numerous types of materials, including aluminum used in the aeronautics industry and nitride and carbide coatings used to harden machine tools. But coated glass presented special challenges because the material is nonconducting and the coatings are fragile.

Nanometer-thick layers of tin oxide are commonly applied to glass to provide protection against ultraviolet light and to control other properties. The coatings are generally deposited on flat sheets of glass, but for applications in architecture and the automobile industry, the coated glass often needs to be heated above 500 C to shape and curve each pane as needed. Unfortunately, the heat treatment can ruin the surface coating.

To study the effects of heat treatments, Chapon and coworkers pulsed the radio-frequency source used to form the plasma instead of running it continuously, which would have heated the insulating samples excessively during analysis and led to ambiguous results. The team found that so long as the sample temperature was well-controlled, the glow-discharge method was able to probe the glass-coating process intimately.

FOR EXAMPLE, they found that 250 C treatments cause sodium in the glass to begin diffusing into the tin oxide layer. And treatments at 500 C cause calcium (and even more sodium) to migrate to the surface, and tin to diffuse into the bulk glass.

William J. McCarthy, a product manager with Thermo Electron's molecular spectroscopy division in Madison, Wis., reported on a case study in which Fourier transform infrared (IR) microscopy methods and image-analysis tools were used to examine a polymer-based packaging material used in the food service industry that was failing quality-control tests.

Meat, fish, and other products are often wrapped in plastic films that are composed of multiple polymer layers, McCarthy noted. Typically, the films are made from combinations of polyethylene, polyamides, and other materials, and are designed for strength and to control diffusion of odors and water vapor.

By analyzing the IR micrographs with edge-enhancing algorithms and other newly developed image-processing methods, McCarthy and coworkers deduced that the film consisted of three distinct components stacked in seven layers. The study revealed a suspicious-looking micrometer-sized feature that turned out to be a clump of polymeric adhesive meant to hold the layers together. The "glue" had been applied improperly and was causing the film to delaminate and fail.

A handful of molecules sitting at a technologically important surface can mean the difference between products that work as designed and those that don't. Thanks to continued advances in surface probes, manufacturers are getting an ever better look at those critical interfaces.