In-Situ X-Ray Reflectivity Enables Finer Deposition and Etch Control

Alexander E. Braun, Associate Editor -- Semiconductor International, 3/1/1999

X-ray techniques play important roles in thin-film research. Sometimes, small-angle X-ray reflectivity is the only nondestructive analysis method available for opaque or metal films and multilayer structures. However, current thin-film analysis X-ray systems are designed for ex-situ measurements performed after the film deposition process.

Recent work at the Moscow State Institute of Electronics and Mathematics with the Moscow Research Institute of Vacuum Techniques by Drs. A. Baranov, P. Kondrashov and I. Smirnov has resulted in the creation of an in-situ X-ray reflectivity monitoring system (Fig.1).

Since X-ray interference does not only occur when the angle of incidence changes but also when film thickness alters (assuming identical angles and wave-lengths), by recording the reflection coefficient's timing it becomes possible to use film thickness variation to arrive at results as accurate as those provided by the angle of incidence method. This is crucial, since it enables film deposition information (thickness, density, surface and interface roughness) to be tracked in-situ during processing, in real-time.

In-situ X-ray reflectivity seems ideally suited to monitor and control the deposition process. It enables simultaneous determination of thickness, deposition rate, roughness and density. Use of such a system could alter current process methodologies such as the production of multilayer metallic structures for spin valves. The absence of a reliable way to monitor film parameters during deposition has led equipment manufacturers to focus on equipment stabilization to ensure that factors such as material sources, gas evacuation and inlet systems do not drift. Thus, except for directly measuring film deposition, everything is tracked: pressure, gas flow, sources, temperature, etc., with the film not being directly measured until deposition completion.

Fig. 1. Vacuum chamber with system for in-situ X-ray monitoring of thin-film parameters during deposition: 1-X-ray source, 2-detector, 3-RF-generator, 4-window, 5-plasma, 6-substrate, 7-substrate holder, 8-vacuum chamber.

Fig. 2. Time trace of reflectivity intensity during the deposition of six carbon layers: 1, 3, 5 ­ RF-PACVD; 2, 4, 6 ­ DC magnetron sputtering. Thickness of each layer: 1-15 nm, 2-15 nm, 3-13 nm, 4-13 nm, 5-12 nm, 6-13 nm.

The development of multilayer structures is time-consuming, requiring much testing to develop a recipe. Next-generation devices will require a means by which to track deposition. In-situ X-ray reflectivity can provide a real-time trace (Fig. 2) that enables the operator to estimate the dynamics and stability of the process. When the X-ray monitoring system is coupled to the equipment, it becomes possible to produce films of a given thickness and halt the process in the event of a failure or if the film's density or roughness deviate from established parameters. In-situ X-ray reflectivity can control more than just deposition and could also be used for etch control.

The monitoring of multilayer non-periodic structures might also benefit from this technique. The reflection coefficient's angle of incidence is very complex due to the superposition of reflections from the different layers, which sometimes defy interpretation. In-situ measurements display these interference patterns sequentially, enabling readings of one forming layer at a time.

In-situ X-ray reflectivity appears to be a serious alternative to ellipsometry and PULSE technology. Its principal advantage is direct measurement of layer thickness and deposition rate. It is a nondestructive control method sensitive to sub-angstrom thickness and surface roughness variations that is not limited by the material studied, number of layers or vacuum technology requirements. Because the wavelength used is comparable to the interatomic distance, it is possible to measure film thicknesses down to a few angstroms with fraction-of-an-angstrom accuracy.