Smaller, faster, and better are always the primary goals for the development of innovative semiconductor devices. Smaller, faster and more efficient – are the most important factors in semiconductor developments. However, these demands are becoming more and more difficult as it comes to 32nm technology and beyond. Since the dimensions of the components are so close to the physical limit, new processes and new materials must be applied to IC manufacturing.
The extra complexities involved with these new developments bring new variables into the production processes. Consequently, new metrology tools are required to monitor more closely and in more detail in order to prevent severe yield loss. Among those new developments, the epitaxial silicon-germanium process is one of the most important changes.
There are many ways to improve the speed of electric devices by changing the circuit design. But ultimately the average propagating speed of electrons in a specific material defines the speed of a device. In other words, the fundamental task is to improve the electron mobility in the devices. By changing the base material of the channel in the transistor structure from pure silicon to epitaxial silicon, germanium (SiGe) alloy, the electron mobility can be improved because of the different band structure.
To control the electronic properties of this SiGe channel, the thickness and the Ge content of the layer must be accurately adjusted during the process. The most effective non-contact and non-destructive metrology to monitor both parameters is High-Resolution X-Ray Diffractometry (HRXRD). Furthermore, an HRXRD metrology tool needs to provide a low cross-sectional beam to illuminate the small pads produced with the selective epitaxial growth (SEG) process.
Patternrecognition system is essential to assure the measurement location.
Results
The Bruker AXS D8 FABLINE metrology system was used to determine the structure of the SiGe patterned thin film. The x-ray beam size can be as small as 50 x 50µm to optimise the measurement on the small SiGe pad.
After a wafer was loaded into the D8 FABLINE, a series of pattern recognition procedures was executed according to a set measurement recipe. As soon as the test pad is located, the HRXRD measurement starts followed by the analysis of all data – all steps fully automated. The analysis results gave a thickness of the SiGe layer of 56.19nm and a Ge composition of 18.48%. On top of the SiGe layer is a Si cap layer of 27.56nm. In a Silicon Fab, these structure parameters will be used for the process control purpose to assure the product quality.
Conclusion
Bruker D8 FABLINE was used to determine the layer structure of the SiGe thin film. Featuring 100µm cross-sectional x-ray beam and pattern recognition, it is optimised to measure the layer structure of smaller test pad. The D8 FABLINE is designed for fully automated and cleanroom compatible fab environment. It complies with the SEMI standards. The D8 FABLINE is a perfect solution in-line quality control.
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