Profile Reflectometer Research Articles

Simultaneous measurement of six layers in a silicon on insulator film stack using visible-near-IR spectrophotometry and single-wavelength beam profile reflectometry

Visible-near-IR spectrophotometry was combined with single-wavelength beam profile reflectometry to measure all six layer thicknesses of a silicon on oxide film stack. The spectrometer or beam profile reflectometer alone is insufficient to solve such a complicated film stack. Development of a robust measurement recipe required a new global optimization method working in parameter spaces of up to 12 parameters. The recipe obtained was applied to a 50-site mapping of the wafer. We found excellent agreement between our optical measurement results and scanning electron micrograph (SEM) data for all thicknesses.

Simultaneous measurement of six layers in a silicon on insulator film stack using spectrophotometry and beam profile reflectometry

We performed simultaneous measurements of all the layer thicknesses in six layer silicon on oxide film structures. Visible-near-infrared spectrophotometry was combined with beam profile reflectometry to produce enough information to discriminate between potential solutions; the spectrometer or beam profile reflectometer alone is insufficient to solve such a complicated film stack. Development of a robust measurement recipe required a new global optimization method working in parameter spaces of up to 12 parameters. The recipe obtained was applied to a 50 site mapping of the wafer. We found excellent agreement between our optical measurement results and scanning electron micrograph data for all thicknesses.

Reflectometry on ITER

Reflectometry will be used on ITER to measure the density profile in the main plasma and divertor regions, and to measure the plasma position and shape in order to provide a standby reference for the magnetic diagnostics in long pulse discharges. The high temperatures of the ITER core and the resultant significant relativistic downshift of the second-harmonic electron cyclotron absorption imply that both low-field side O-mode and high-field side lower cut-off (X−l mode) systems are required to access the full plasma profile. A low-field side upper cut-off (X−u mode) system will also be required for measurements of the scrape-off layer. For measurements of the plasma position and shape, an O-mode system is optimum due to the large range of magnetic field along the plasma periphery and the wide range of possible plasma configurations achievable on ITER. A robust real-time calibration technique of the whole transmission line is required. It is likely that an accurate estimate of the position of the plasma will require the simultaneous use of signals from the profile reflectometer. For the divertor, profiles with peak densities in the range 1019–1022/m3 are to be measured with a target resolution of 3 mm. The large density range will necessitate the use of more than one system. Installing these reflectometers on ITER incurs additional difficulties such as the routing of the millimetre wave radiation around the complicated first wall and divertor structures and design of antennas able to operate through the first wall and blanket.

Development of a fast solid-state high-resolution density profile reflectometer system on the DIII-D tokamak

A new fast-sweep, broadband frequency-modulated reflectometer on the DIII-D tokamak has produced routine, reliable density profiles with high spatial (⩽1 cm) and temporal resolution (∼100 μs). The system utilizes a solid-state microwave source and an active quadrupler, covering the full Q-band (33–50 GHz) and providing high output power (20–60 mW). The rf source frequency is linearized using an arbitrary function generator and the temperature of the source is actively controlled to reduce rf frequency drifts. The system hardware allows the rf frequency to be swept fullband in 10 μs, but, due to digitization limits, the sweep time used currently is 75–100 μs. The reliability of the reconstructed profiles was improved by a combination of fast frequency sweep, which reduces density fluctuation effects on the measurements, and advanced signal analysis based on digital complex demodulation, which improves phase accuracy. The fast-sweep system has resolved fast-changing edge density profiles during edge localized modes with unprecedented resolution.

A swept two-frequency microwave reflectometer for edge density profile measurements on TFTR

The edge density profile can play a significant role in determining the plasma confinement and the coupling of the ion cyclotron resonance frequency (ICRF) heating power to the plasma. To experimentally measure the edge density profile in the Tokamak Fusion Test Facility (TFTR), a two-frequency microwave reflectometer is being built. This reflectometer will operate in a swept two-frequency configuration between 91 and 118 GHz using the extraordinary mode. The frequency separation between the two microwave signals will be held constant while the signals are swept across the frequency band. By measuring the differential phase delay between these two signals, the density profile can be reconstructed. Two-frequency profile reflectometry is discussed and results of modeling of this type of reflectometer measurement for TFTR are shown. Finally, the design of the TFTR edge profile reflectometer microwave system is described.