| The optical circuit-analysis (OptiCA) system (or PICA---picosecond imaging circuit analysis), based on infrared photon emission from the switching transistors in CMOS chips, is an efficient non-invasive testing method for the new generation VLSI CMOS circuits. We have developed a new type of detector based on ultra-thin superconducting NbN films, the so-called NbN superconducting single-photon detector (SSPD), and implemented it in the OptiCA system.; The first-generation SSPDs had a simple structure of a 0.2-mum-wide and 1-mum-long NbN stripe with a thickness of about 10 nm. They counted photons but suffered from a low quantum efficiency (QE). The second-generation SSPDs were fabricated with meander-structured NbN stripes covering an active area of either 4 x 4 mum2 or 10 x 10 mum 2 which increased the coupling of the incident light to the detector. The stripes had the width of about 200 nm and the thickness of 10 nm. These SSPDs have shown greatly improved performance compared to the single-striped detectors, and were characterized by GHz counting rates of visible and near-infrared photons and negligible dark counts. The time-resolved measurement showed that the SSPD could respond to pulses with ∼100-ps time delay, corresponding to a repetition rate of 10 GHz. A photoresponse time delay phenomenon in the SSPDs has been investigated and discussed.; Finally, the third-generation SSPDs, with the stripe thickness of 3.5 nm, were fabricated and characterized. The most important feature of these SSPDs was the greatly improved QE. They exhibit an experimentally determined QE ranging from 5% at the 1.55-mum wavelength up to 20% at 0.4-mum wavelength, a low timing jitter (about 18 ps) and a high sensitivity (noise equivalent power on the order of 10-18 W/Hz1/2 at 1.3 mum wavelength).; The second- and third-generation SSPDs are presented in detail in this work and they have been used for VLSI CMOS circuit testing with the OptiCA system. The IR photon emissions from both PMOS and NMOS transistors were clearly resolved with greatly improved timing resolution (timing histograms have the full-width-at-half-maximum of ∼60 ps). The signal acquisition time with SSPDs was much shorter as compared with other detectors. |
