Improved composition metrology in compound semiconductors using the Neptune system

Instruments used

EDAX^® Neptune (EDS-WDS) System

Background

Compound semiconductors have unique material properties that enable their use in a wide range of electronic and optical device applications, from high-frequency (microwave) and high-power electronics to light-emitting and light-sensitive devices. A compound semiconductor is composed of chemical elements belonging to two or more different groups in the periodic table, e.g., III–V; frequently, scientists and engineers utilize alloying of three elements to optimize electrical and optical properties for a specific application. However, this requires precise and accurate control of the fabrication conditions to produce high-quality materials of uniform composition, as even a small change in growth conditions can lead to significant fluctuations in the compound semiconductor’s composition, dramatically impacting the yield of viable devices.

Methods to determine material composition quantitatively and with high accuracy at the micro- and nano-scale are required by manufacturers for quality assurance purposes and to optimize conditions during process development. This presents a difficult analytical challenge as commonly used methods for compositional analysis—such as energy dispersive x-ray spectrometry (EDS) in the scanning electron microscope (SEM)—run into fundamental limits that preclude their use: due to the high spatial resolution requirements in compound semiconductor technologies, the use of intermediate-to-low accelerating voltages is required to confine the interaction volume within the sample being analyzed. However, at low accelerating voltages, often only the x-ray L lines are excited from elements used in compound semiconductors meaning that peak overlap is common in EDS, especially for materials that contain elements of similar atomic number, making quantitative compositional metrology uncertain with EDS alone. However, wavelength dispersive spectrometry (WDS) has superior spectral resolution compared to EDS (up to 15x better), enabling x-ray lines of elements used in compound semiconductor materials to be resolved, therefore permitting quantitative analysis with high confidence. Furthermore, WDS has a significantly higher peak-to-background ratio than EDS, aiding in the identification and quantification of even trace elements.