NONDESTRUCTIVE CHARACTERIZATION OF MATERIALS BY EVANESCENT MICROWAVES

A microstrip quarter wavelength (lambda(g)/4) resonator in conjunction with a small probe is used to resolve objects with characteristic dimensions as small as a thousandth of the wavelength (lambda(g)/1000). The characteristic length for the decay of the evanescent waves at the tip of the probe was measured to be approximately 100-150 mum at a microwave frequency of 1 GHz (lambda(free) almost-equal-to 30 cm). We applied this technique to map microwave conductivity of metallic lines on glass and printed circuit boards, and to investigate conductivity variations across a silicon wafer. It was possible to detect holes in printed circuit boards that were covered with solder and were not detectable otherwise.