Matthew Wormington

High resolution X-ray diffraction using a high brilliance source, with rapid data analysis by auto-fitting

Abstract In a production environment in particular, fast data collection and analysis, which are also highly reliable, are desirable. Measurement can be speeded up by increasing the diffracted intensity, thus reducing the time required to measure it reliably. Increased intensity with a smaller beam footprint at the sample have been achieved in a double-crystal diffractometer by the use of a novel ellipsoidal mirror working by total external reflection, positioned before the reference crystal. To optimise the performance of the mirror and provide high brightnesses, an X-ray source with a very small focal spot is required. Such a high brightness source has been made that uses electromagnetic focusing of the electron beam onto the target. Rapid data analysis is achieved by the use of an auto-fitting program that employs a genetic algorithm and the full dynamical theory of X-ray diffraction. Choice of an appropriate error function produces a deep global minimum while the genetic algorithm avoids convergence on local minima. From the model that produces the best fit, samples parameters such as layer thickness and alloy composition are extracted with quantified goodness of fit.

Asymmetric Relaxation of SiGe in Patterned Si Line Structures

High resolution X‐ray diffraction (HRXRD) measurements were performed using a commercially‐available X‐ray metrology tool, the BedeMetrix™‐L, on small test pads containing arrays of SiGe line structures selectively deposited in Si recesses with various window dimensions. Reciprocal space maps (RSMs) were performed in two orthogonal 〈110〉 directions in order to determine the lattice parameter parallel and perpendicular to the lines. With narrow lines, asymmetric relaxation effects were seen: the SiGe was fully strained along the long dimension of the lines while there was significant relaxation along the short dimension of the lines. The magnitude of the relaxation increased significantly for lines with short dimension below about 1 μm. We show how to determine the lattice parameters, and hence the strain of the SiGe in the [110] and [−110] directions, the Ge composition and the relaxation initially using RSMs, but with an extension to measurements more suitable for in‐fab metrology.

Accuracy and Repeatability of X‐Ray Metrology

All metrology tools must make a trade‐off between data quality and wafer throughput. Moreover, most X‐ray metrology may be performed on regions inside the scribe lines. This paper discusses the appropriate choices of trade‐offs for throughput, repeatability and spot size, choosing examples from silicon‐germanium composition and thickness metrology. The repeatability varies from 0.7% to 0.1% 1σ with data collection cycles between 20 s and 1000 s. We show that for most of the parameters determined by X‐rays the metrology is absolute, and that both accuracy and tool matching is achieved by traceable calibration of the X‐ray wavelength and the angle encoders on the tools. Tool matching achieved by this absolute approach to metrology is typically 0.2% in thickness and 0.05% in composition for Si‐20%Ge layers ∼ 50 nm thick. For 30 nm metal or dielectric layers measured within a 100 mm scribe line, 0.65% 1σ repeatability is achieved at 150 measurement points/hr and 0.33% at 75 sites/hr.