Metrology for MTJ Layer Thickness & Composition

MTJ measurement: A key WDXRF measurement capability

An important structure of MRAM is the MTJ structure that contains light elements, such as B and Mg. Analysis of these elements are a specialty area of WDXRF. EDXRF and optical methods cannot analyze those light elements.

MTJ analysis concept

For the simultaneous analysis of MgO and CoFeB layers, the analysis model shown below at right is used (thicknesses of repeated layers are consolidated).

XRF systems are not well suited to distinguish the spectrum from different layers containing the same element. For the MTJ structure, the systems cannot separate two CoFeB layers. in this case, therefore, CoFeB layers should be analyzed as one layer. (The effects of absorption and excitation are minimal since the layers are thin.)

MgO thin film analysis by WDXRF tool

A Ta layer is often used as a cap and/or barrier around the MTJ structure. In this case, the Ta-Mz peak appears near Mg-Kα but it is definitely separated using a WDXRF system. This is one of the significant features of a WDXRF system, whose resolution capability is high. With an EDXRF system, a small Mg-Kα peak can be buried by the high background of Si-Kα and Ta-Mz.

For thin film multilayers, the FP method is generally used because it can calculate the absorption by other layers and the composition differences. FP calibrations are correlations between theoretical and measured intensities. It is possible to analyze with a few standard samples and apply other structures, such as single layers and/or pure metals.

MgO thin film analysis by WDXRF (AZX 400)

MgO thin film analysis by WDXRF (WaferX 310)

CoFeB thin film analysis by WDXRF

For the CoFeB layer, compositions are often analyzed, not just the thicknesses. In this case, measurements of B-Kα are required. However, when B-Kα is measured with stacked samples, spectra from Pt, Ta, Ru and W (if used, and possibly other elements) overlap it. Therefor, dummy single layer smaples of CoFeB should be used for the analyses.

CoFeB thin film analysis by WDXRF (AZX 400)

CoFeB thin film analysis by WDXRF (WaferX 310)

Analysis of stacked MRAM

For the analysis of stacked MRAM samples, the analysis model should set as shown below with summed thickness of repeated layers.