Extreme Ultraviolet Metrology

To verify and characterize the EUV transmission performance (at 58.4 nm) of our EUV thin film metal foil aluminum filters and their various coating configurations, a vacuum test chamber, designed for a Dual grating spectrometer (DGS~2005) was refurbished and reconfigured. This chamber was docked with our custom built SSC EUV monochromator (~30.4/58.4 nm, schematic on left below), that has a normal incidence grating with a high voltage (~ 5kV, 2.5 mA) helium gas discharge incident light source. This reflective grating inside monochromator is mounted on a servo motor mechanism which can be adjusted/rotated to focus one of the many diffracted orders (usually first) of the reflected light at the output slit (towards docked vacuum chamber). The Vacuum across the system was maintained using a serial combination of a mechanical and a Turbo pump at either ends of monochromator and DGS i.e. in total four pumps were used [~10e(-7) Torr].

Schematic of Monochromator setup and Vacuum test Chamber (Wieman, 2005)

The reflected light at the exit of the monochromator then strikes the first optical surface inside the vacuum test chamber (DGS), which is a 200nm/600nm free standing gold foil grating, followed by our EUV filters. Three such EUV filters could be mounted on a horizontal stage controlled by a mechanical translation mechanism. The filters are followed by two detectors mounted at zeroth order(fixed) and the first order of diffraction (movable by a vacuum motor, images below) by the 200/600 nm entrance gratings.

The detectors used are radiation hardened Opto-IRD Silicon photo diodes, which were previously calibrated (Response function ) at NIST Beam line-9, with a synchrotron light source. Their output is measured using a Keithly source meter which includes a calibrated pre-amp. The output data collected is the current measured (~nA) at each detector w.rt to the position of the movable filter. This way, a filter's surface was characterized for EUV transmission by scanning horizontally on a incident fixed light beam (scan image below).

Vacuum chamber for Dual Grating Spectrometer(DGS)

MONOCHROMATOR

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