Description
Abstract
X-ay Diffraction (XRD) mapping is a non-destructive metrology technique that enables the reconstruction of warpage induced on a Silicon wafer through thermo-mechanical stress. Here, we mapped the wafer's warpage using a methodology based on a series of line scans in the x and y directions and at different 90-degree rotations of the same sample. These line scans collect rocking curves from the wafer's surface, recording the diffraction angle (ω) deviated from the Bragg angle due to surface misorientation. The surface warpage reflects in XRD measurements by inducing a difference between the measured diffraction angle and the reference Bragg angle (ω − ω0) and rocking curve broadening (FWHM). By collecting and integrating the rocking curves (RCs) and FWHM broadening from the whole surface and multiple rotations of the wafer, we could generate 3D maps of the surface function f(x) and the angular misorientation (warpage). The warpage exhibits a convex shape, aligning with optical profilometry measurements reported in the literature. The lab-based XRDI has the potential to be developed to map the wafer's warpage in a shorter time and in situ, as can be perfectly performed in Synchrotron radiation source.
Fig. 1: The schematic of the FR4 sample showing the rocking curve collection procedure using x-ray diffraction mapping. b). The 3D map of the collected angular offset recorded in both x and y directions and integrated in a single 3D map. c) Warpage map extracted from integrating the vertical and horizontal scans performed in multiple 90 degrees. The wafer surface shows a convex warpage.
Developing real-time or semi-real-time metrology methods to measure the thermal and mechanical stress in the packaged wafer is highly dependent on the accuracy of the scanning methods [1]. Lab-based XRDI scans must be further developed to automate scanning the wafer in multiple rotations, integrate the data captured in different directions, and reduce the number of black spots in the diffraction angle obtained from the corners of the wafer. The fabrication process of a complete packaged integrated circuit (IC) is complex, involving various materials with distinct coefficients of thermal expansion (CTEs), such as wafer bond pad, wafer attach adhesive/epoxy glue, moulding compounds, and Cu-filled through-silicon-vias [2].
Thermal stress and wafer warpage commonly occur in the packaged chip during thermal processing steps due to the CTE mismatch of these materials. In this work, we present the development of a comprehensive X-ray diffraction mapping technique to collect information about the warpage induced on surface of a Silicon wafer. We performed line scans from the left to the right edges on surface of the wafer and complemented them with line scans at multiple 90-degree rotations.
The collected rocking curve data were processed and input into a modelling procedure to extract the wafer warpage and surface displacement with high resolution. Warpage induced a curvature in the silicon crystal planes, leading to variations in the angular positions across the distorted silicon wafer and, consequently, a broadening of the rocking curve. The spatially resolved RC map was generated by integrating a series of RCs collected at different positions across the silicon wafer along the x-direction.
References
[1] A. Toda and N. Ikarashi, Japanese Journal of Applied Physics, 49 (2010) 04DB03.
[2] X. Xin, N. E. Gorji, M.-L. Tseng, IEEE Trans. Components Packaging & Manuf. Technol., 14 (2024) 1164.
