Stratos is an evolution of Malvern Panalytical’s market-leading thin film metrology software used in the semiconductor industry. A distinct feature of Stratos is the ability to analyze thin layers by using only bulk standards in the calibation. This is achieved by calibrating primarily using bulk standards. By doing so, one reduces the dependence on standards that are layered samples, which can be difficult to obtain or certify. Layered standards can be added to improve the calibration and are referred to as TAGS. Stratos is also able to reference specific TAGS for different sample types to optimize the calibration. Certified reference materials can still be used for ultimate accuracy (if available). Stratos also makes use of a step-by-step setup guide, called the Virtual Analyst, which simplifies and optimizes the setup of an application.
It suggests the best possible measurement conditions for a specific sample type. In the past this has been a manual, time-consuming and tedious process. An application of Stratos is the analysis of metallic nickel- tantalum thin films, used as Cu diffusion barriers in advanced interconnect applications. In this industry the determination of thickness of multilayers is critical.
This study was performed using a Zetium wavelength dispersive XRF spectrometer. It was configured with a 2.4 kW Rh anode, SST R-mAX tube and is a fully integrated system with a X-Y sample handler and state-of-the-art SuperQ software, featuring an advanced Fundamental Parameters algorithm. Zetium is engineered for excellence in terms of both analytical accuracy, precision and operational performance.
The samples analyzed in this study were provided by the MESA+ Institute of Technology and by the Faculty of Science and Technology (Department of Catalytic Processes and Materials) of the University of Twente (the Netherlands). Samples contained Ni and Ta layers deposited on a silicon substrate, as shown in Figure 1. Individual layers of pure metals were sputter-deposited on a Si(100) wafer with a diameter of 120 mm, with an individual layer thickness ranging from 7 to 20 nm. The samples were cut into 1x1 cm pieces to fit into a stainless steel cup with a 10 mm aperture.
Figure 1. Cross section of a sample with Ta-Ni coating on a Si substrate
Calibration setup with the Virtual Analyst
The calibration was set up using only bulk samples. These bulk samples were a mixture of pure element samples and setup samples from the Omnian package, Malvern Panalytical’s solution for standardless analysis. Based on the expected thickness and composition of the unknown samples, Malvern Panalytical’s Virtual Analyst facilitates the setup of the optimum measurement program to analyze multilayer samples. The best lines are chosen from an extensive list of measurement conditions possible with the Zetium instrument. Line selection is based on the response of a line with respect to changes in thickness and composition within the expected concentration and thickness range of the unknown samples.
Ni and Ta were measured for 150 s each, resulting in a total measurement time of 5 minutes per sample.
Results and discussion
The corresponding XRF results using bulk standards are compared with the nominal deposition values. The results are reported in Table 1 and demonstrate that Stratos is capable of accurately analyzing multilayer samples using only bulk
standards. Further improvement of the results are possible with the addition of in-type samples to the original calibration.
Table 1. Analysis results for a layered sample
In order to evaluate the instrument repeatability of the method, the same wafer fragment was analyzed ten times sequentially, with loading/unloading between each measurement. The results are reported in Table 2 and show excellent repeatability of this method.
Table 2. Results of measurement repeatability (10) on a wafer sample
With this excellent measurement repeatability for the thickness of the layers on the wafer, the homogeneity of the thickness across the wafer can be evaluated. Seven fragments were collected from several parts of the wafer and analyzed. Table 3 presents these results showing some deviation in thickness across the whole area of the sample, indicating the high sensitivity of Stratos to variation in thickness. With the instrument repeatability being four times lower, one can be confident in reliably determining homogeneity of this material.
Table 3. Thickness variation test
Dr. ir. Roald M. Tiggelaar, MESA+ Institute for Nanotechnology and Ing. J.A.M. Vrielink, Faculty of Science and Technology (Department of Catalytic Processes and Materials) from the University of Twente, who provided the samples for analysis in this data sheet.
This data sheet demonstrates that Zetium, in combination with Stratos performs accurate analysis of layer thicknesses of Ni-Ta thin films deposited on a Si substrate. The method established by using the Virtual Analyst is fast, non-destructive and permits the automatic selection of optimal measurement conditions for the analysis of thin films and multilayer samples.
Thanks to the excellent repeatability of the Zetium, the homogeneity of the thickness across wafers can be evaluated, making Zetium in combination with Stratos ideal for process control analysis.