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Description
Conventional four-point-probe (4PP) stations achieve high-accuracy sheet-resistance measurements but often lack the ability to perform mapping across an area of a sample. Commercial tools also feature a large (80-100 mil) probe spacing, which limits the spatial resolution of the sheet-resistance measurement.We retrofitted an R-θ-Z wafer stage with a 3-D-printed probe head and spring-loaded, 410 µm-diameter gold pins, controlled through a new Python automation stack, to build a wafer-mapper. While the gold probe pins work well on metal films, sheet-resistance measurements on silicon samples require the probe tips to mechanically pierce the native SiO₂ that spontaneously grows on silicon wafers. For device-grade specimens this mechanical scratching is undesirable,because it could lead to the introduction of mechanical defects. Initial experiments were conducted that applied high-voltage (≤ 105V), low-current (≤ 1 mA) pulses to break down the oxide electrically. However, tip deformation increased the effective contact area, raising the breakdown voltage beyond practical limits and preventing reliable contact formation, causing large variations in the mapping data. We therefore explored a contact-less eddy-current approach using a single-loop RF coil. The RF excitation signal was swept from 100 kHz to 6 GHz while its complex reflection coefficient ( S₁₁ ) was captured. The resulting resonance-splitting or “fan-out” of S₁₁ spectra correlates monotonically with the sheet resistivity of test wafers (1-140 Ω □⁻¹). LTSpice models of the coil-wafer system reproduced the measured trends, lending confidence that calibrated peak-tracking can yield quantitative resistivity maps. This work demonstrates the feasibility of a hybrid probe station that performs non-contact characterization of bulk silicon samples. In future iterations this characterization technique can also be applied to thin-film measurements. Key design lessons and an outline for refining the probe head and extraction algorithms are presented.
ContributorsStringer, Evan (Author, Co-author) / Goryll, Michael (Thesis director) / Celano, Umberto (Committee member) / Barrett, The Honors College (Contributor) / Electrical Engineering Program (Contributor) / Computer Science and Engineering Program (Contributor)
Created2025-05