Si1-xGex Film Analysis

Mapping the Ge concentration of Si_1-xGe_x with the 2830 ZT

Introduction

Si_1-xGe_x technology has spread rapidly in the semiconductor industry. It offers substantial advantages like higher IC speed and reduction of noise, especially for high frequency applications, while conventional Si processing routes can be used for fabrication. Si_1-xGe_x brings about new metrology challenges with respect to process control, like mapping of the Ge homogeneity of deposited films. X-ray fluorescence spectrometry (XRF) provides a fast and non-destructive solution in this respect, yielding high precision results.

Instrumentation

A Malvern Panalytical 2830 ZT XRF wafer analyzer equipped with a fixed Ge channel for measuring the Ge-Kα signal. Details of the measurement conditions are presented in Table 1.

Theoretical considerations

For a Si_1-xGe_x film, the intensity of the emitted Ge-Kα radiation depends on the thickness, the Ge concentration, and the (concentration dependent) film density. For calibration purposes, the influence of these three parameters can be combined via multiplication. Division through the Si_1-xGe_x molar mass and multiplication by Avogadro’s number yields the atomic area density for Ge in atoms per cm²:

The above relation makes it possible to investigate the Ge homogeneity of Si_1-xGe_x process films, based on the intensity of the Ge-Kα signal.

Table 1. Measurement conditions

Procedure

A calibration line was set up using Si_1-xGe_x reference films with thickness between 4330 and 7270 Å and composition (x) between 0.05 and 0.25, as determined by X-ray diffraction. Fundamental parameter correction was applied to ensure validity over the complete concentration range. To demonstrate the reproducibility of the XRF analysis, a 15-cycle repeated measurement was performed with loading and unloading of the wafer between each cycle. In addition, the Ge homogeneity of a Si_1-xGe_x film was mapped.

Results

Figure 1 shows the calibration of the Ge-Kα signal against Ge concentration, expressed in 1016 atoms per cm². This calibration allows the investigation of the homogeneity of unknown Si_1-xGe_x films in terms of the atomic area density of Ge. If desired, the results can be expressed in other units without any difficulty. In addition, the analysis can easily be combined with other metrology techniques, yielding an even more detailed picture of the investigated films. For instance, X-ray diffraction and/or reflectivity measurements can be used to determine layer thickness and density. These data can be used in the XRF analysis, yielding the actual Ge concentration of the film. The results presented in Figure 2 and Table 2 illustrate the high reproducibility of the analysis. A typical example of a wafer map is shown in Figure 3.

Figure 1. Calibration of the Ge-Kα signal against the atomic area density 

Figure 2. Reproducibility experiment for a single spot on a Si_1-xGe_x film with an atomic area density of 5.022*1016 Ge at/cm². The dotted lines represent ±2 sigma confidence limits.

Figure 3. Wafer map of a Si_1-xGe_x film, spot size 10 mm, 130 spots. 

Conclusion

Malvern Panalytical’s 2830 ZT enables high precision analysis of the homogeneity of Si_1-xGe_x films, using the Ge-Kα signal as a reliable measure for the Ge atomic area density. Measurements can be performed in a broad Ge concentration range and can therefore easily be adapted to customer requirements. The analysis can readily be combined with other metrology techniques.