Small-conductive-particle detection with a microwave resonant cavity

The recent miniaturization and increase in the complexity of electrical devices have increased the influence of impurities on the device performance, especially the influence of small conductive particles with a size on the order of micrometers. Therefore, the inspection of devices during the manufacturing process has become important. In this study, changes in the resonance properties of a microwave cavity were used to detect a metal particle. The decrease in the resonance frequency for a 100 μm stainless steel sphere was about 0.1MHz, which was less than 1 ppm of the resonance frequency used. This frequency change is too small to detect directly in the output of a particle detector. Transmission measurements (the scattering parameter S21) at a frequency that was few megahertz higher than the resonance frequency where the S21 changes sharply were used to magnify signal changes resulting from the introduction of a small conductive particle. A resonance frequency of 13.3 GHz and a frequency 3MHz higher were chosen for obtaining measurements in the present study. By applying a filtering technique, it was shown that the minimum detectable sphere size is around 50 μm. This sensitivity was experimentally shown to be uneven along the long axis of the cavity. The unevenness was related to the electrical field strength of the standing wave at the resonance frequency. For practical industrial inspection of devices, multiple frequencies should be employed in view of the present discussion.