X-Ray Diffraction Analyzer Provides Accuracy, Automation

Because germanium is such a large atom, producing this heterogeneous layer epitaxially puts considerable strain in the layer — it is a "strained heterolayer." The layer must be that thin, otherwise it will relax when it is subjected to processing and will not provide the required electrical performance. It must remain strained, and to do so, it must be thin.

Philips Analytical (Almelo, Netherlands) has developed an X-ray metrology platform called the DCD Pro — referring to double crystal diffractometry — to cope with this problem. Initially, the company was asked to solve the problem of determining layer thickness and germanium content. Typically, the layer is composed of 10-15% germanium. This means that it is not just a matter of taking one measurement at the wafer's center, but of measuring the layer's homogeneity across the entire wafer.

Initially, the platform was designed to measure blanket depositions and not structured product wafers. Later, the original measurement requirement was enlarged by the addition of an extra, or box layer (box in the sense that there was a certain germanium content throughout the layer), and a graded layer was added to one side of this structure. The graded layer went from 15% down to 0% in germanium content, across a thickness of ~300 Å. The additional information required had to show the grade's thickness and from that determine the 15 to 0% slope. Additionally, measurements had to be done on product wafer test structures, and the whole system had to be Class 1 cleanroom compliant.

	The DCD Pro X-ray metrology platform is designed to measure the SiGe content of extremely thin films in test areas 400 × 400 µm, using a 1 mm 2 X-ray spot size. (Source: Philips Analytical)

The platform uses the vision system to locate the test area, find its central part where the X-ray beam is applied, and optimize the resulting signal. The system can be taught to step across the structured wafer and measure various sites across it. It evolved as a measurement platform originally intended for blanket deposition measurement to one for selective epitaxy — entirely different results are obtained. The new system's capabilities will be of considerable assistance to the process development engineer, and it is fully automated with an internal cassette system that enables it to automatically load 25 wafers, and then measure as many sites as required on each.

While it still cannot be labeled a true in-line metrology tool, for an X-ray system it is quite advanced, particularly in terms of automation. Presently, the platform is only available in a 200 mm, one-cassette configuration; however, work is proceeding on a 300 mm, multicassette system.