Recent Advances in Non-Contact Electrical Metrology for Manufacturing Needs and Development of WBG and UWBG Semiconductors

Corona-charge non-contact C-V (CnCV) tools introduced by Semilab SDI offer precise wafer level monitoring of electrical properties with cost effectiveness and time saving advantages especially important in production and development of wide bandgap materials. The CnCV measurement principle utilizes a corona charge, ΔQ, biasing of the surface and measurement of the voltage response, V, with a Kelvin probe. A non-contact capacitance-voltage, C-V, characteristic is generated using a differential capacitance, C = ΔQ/ΔV. In typical CnCV measurements the corona charge placed on the surface creates a Schottky type surface barrier. This is analogous to a gate bias, however it is achieved without fabrication of any gate structure or physical contact. Negligible minority carrier generation in WBG materials results in stable deposited charge in the dark, enabling reliable capacitance-voltage measurements. Doping determination and profiling is the primary application of CnCV. Initially intended for SiC epitaxial wafers, CnCV tools were successfully applied for characterization of GaN and AlGaN/GaN HEMT structures. The most recent applications include ultra-wide bandgap, UWBG, semiconductors such as β-Ga₂O₃. There are no high bandgap limitations of the CnCV technique thus the metrology can support the development of cubic boron nitride, c-BN, diamond and AlN.

The WBG industry welcomed the CnCV technique with cost savings and rapid feedback advantages over traditional C-V measurements on fabricated test structures. Drawing on experience from approximately 30 implemented tools, the industry provided feedback leading to recent advances highlighted in this work. Among the top priorities on the industry's wish list is increasing wafer testing throughput, driven by the demands of growing wafer mass production.
A breakthrough in CnCV measurement speed was achieved with a novel approach based on near-UV illumination-induced photoneutralization of corona charge. In this "Kinetic mode," short millisecond illumination pulses follow a single large corona charge dose, replacing the multi-step charge deposition required in standard CnCV. Doping determination relies on the discovery of a direct relationship between the photoneutralization time constant (τ_ph) and the dopant concentration [1]. By adjusting illumination intensity, τ_ph can be reduced to a fraction of a second, enabling fast dopant measurements that meet the industry requirement of 20 wafers per hour for typical 9-site wafer testing. Applications to WBG SiC and UWBG Ga₂O₃ have demonstrated Kinetic mode precision of 0.1%.

A new application of the Kinetic mode was recently developed to characterize AlGaN/GaN HEMT structures, including pinch-off voltage (V_P), AlGaN layer thickness, and two-dimensional electron gas density (2DEG). This method offers a 10-fold speed advantage over standard CnCV, reducing whole-wafer mapping times from hours to minutes.

Another key advancement addresses the industry's need for non-contact corona-based QUAD (Quality, Uniformity, and Defects) mapping to identify electrically active defects in SiC epi. A new die map analysis method incorporates die yield bin maps of defects classified by in-die depletion surface voltage values, enabling direct comparison with final device performance. Recent studies performed in collaboration with industrial partners demonstrated strong correlations between QUAD detected defects and the electrical properties of merged PiN Schottky diodes. These advances position QUAD as a unique complement to UVPL measurements for identifying electrically active defects that impair device functionality.

References:
1. M. Wilson, J. Lagowski, C. Almeida, B. Schrayer, and A. Savtchouk, “Semiconductor doping characterization method using photoneutralization time constant of corona surface charge”, US Patent 12154833B2, Nov. 26, 2024.