The sensitivity of a high-rate photon-counting optical communications link depends on the performance of the photon counter used to detect the optical signal. In this paper, we focus on ways to reduce the effect of blocking, which is loss due to time periods in which the photon counter is inactive following a preceding detection event. This blocking loss can be reduced by using an array of photon counting detectors or by using photon counters with a shorter inactive period. Both of these techniques for reducing the blocking loss can be employed by using a multi-element superconducting nanowire single-photon detector. Two-element superconducting nanowire single-photon detectors are used to demonstrate error-free photon counting optical communication at data rates of 781 Mbit/s and 1.25 Gbit/s.
The use of coherent radiation from undulator beamlines has been used to directly measure the real and imaginary parts of the index of refraction of several metals1. Here we extend the same interferometric technique to slightly higher energies, and measure the indices of refraction of silicon and ruthenium, essential materials for extreme ultraviolet (EUV) lithography. Both materials are tested at-wavelength (13.4 nm.) Silicon is also measured about its L2 (99.8 eV) and L3 (99.2 eV) absorption edges. This measurement technique is currently being expanded further to soft X-ray wavelengths.
Phase Shift Masks (PSM) for Extreme Ultraviolet Lithography (EUVL) have the potential for extending the lithographic capability of EUVL beyond the 32-nm node. The concept of EUV PSM structures can be implemented either by adding absorber structures on top of the Mo/Si multilayers (additive approach) or by partial etching into the multilayers (subtractive approach). Among many technical challenges, evaluating optical constants of absorber materials is the most important issue particularly for PSM by the additive approach (PSM-ADD), while the etch stop capability and the etch selectivity with vertical sidewall profile are the main technical challenges for PSM fabricated by the subtractive method (PSM-SUB). For fast turn around of PSM development, the indirect optical constant evaluation through material analysis is a useful metrology technique. The optical constant of TaSiN extracted from Rutherford Backscattering Spectroscopy (RBS) data agrees well with that obtained from the direct measurement by transmission interferometric technique. For PSM-SUB, the concept of embedding B4C and NiFe etch stop layer (ESL) is verified by measuring reflectivity of the ESL embedded substrates and demonstrating good etch stop capability.
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