Compound semi-conductor analysis with correlative ramen imaging

Doping and topographic variation visualised by Thomas Meyer, Judith Beer and Damon Strom

Semiconductors are the materials from which the engines of the information age are built, and their advancement is among the most vital endeavors in technology. The first step in their production generally involves crystal growth and sectioning into thin wafers. The wafers are then altered using methods such as doping to give them specific electronic properties. Access to the subtlest details of these chemical and structural modifications on the sub-micrometer scale is crucial in new device development and final product quality control.

Based on inelastic light scattering by molecules that produces unique energy shifts, Raman spectroscopy can quickly identify material components. In Raman imaging, a spectrum is acquired at each pixel by scanning the sample, which provides local chemical information. Confocal Raman imaging features a beam path that strongly rejects light from outside the focal plane for generating depth scans and 3D measurements.

Raman microscopy is a powerful tool for semiconductor research that can nondestructively acquire high-resolution, spatially-resolved information to determine the chemical composition of a sample, visualize component distribution, and characterize properties such ascrystallinity, strain, stress or doping.

This is particularly valuable for compound semiconductors, which often consist of multiple elements and complex structures.The measurements below demonstrate the insight that correlative Raman imaging can provide to researchers investigating stress, doping and topographic variation in a large-area wafer measurement, and evaluating a Frank-Read source in a 3D correlative Raman and photoluminescence imaging experiment.

Topographic Raman imaging of a SiC Wafer

To meet the challenge of maintaining nanoscale-precision across the surface of a 150 mm (6 inch) diameter Silicon Carbide (SiC) wafer, a WITec alpha300 Raman system was used. This example was outfitted. with a large-area scanning stage and a TrueSurface profilometry module to compensate for topographic variations.

Raman imaging revealed alterations in the doping-sensitive A1(LO)-mode at 960 rel. cm-1 of the Raman spectrum (Fig. 1A) for a region within the wafer (Fig. 1B). Compared to the bulk wafer area (red), this region contained a higher doping concentration (blue). The sensitivity of the system enabled the detection of minimal shifts of the E2(high) mode at 776 rel. cm-1 which is sensitive to material stress and strain. In comparison to the overall wafer, more central regions were exposed to compressive stress while distal regions were subjected to relatively higher tensile stress (Fig. 1C). TrueSurface compensated for height variations within the sample and allowed the recording of the wafer’s topography and warpage (Fig. 1D) simultaneously along with the Raman spectral information.

Correlative Raman-pl imaging of a frank-read source in GaN

A a possible origin of stress in crystals, including crystalline semiconductors, is a Frank-Read source (FR-source). The term describes the dislocations and repeating wrapping patterns that result from deformations and alterations in a crystal lattice. These can be detected and located with Raman imaging.

In photoluminescence (PL), photons excite electrons, then fall back to a ground state and re-emit a photon at a longer emission wavelength which is characteristic for each material.

For semiconductors, the PL-emitted light can serve as an indicator of its bandwidth, as the energy of excited electrons is reduced to the bandgap minimum in the relaxation process.

Here, a WITec alpha300 Raman microscope equipped with the TrueComponent Analysis software feature was used to generate a high-resolution 3D map of an FR-source in GaN (Fig. 2A). The obtained Raman spectra were automatically analyzed to detect spectral differences and identify components. Three different components were found for GaN: The relatively relaxed GaN (red) and two stressed forms (blue, green) within the FR source. Next, PL signals were analyzed, and the visualized emission peak position (Fig. 2B) shows a different PL fingerprint for the FR-source compared to the overall sample, confirming the alterations in its semiconducting properties.

Summary

The examples shown demonstrate the utility of Raman imaging for characterizing compound semiconductors. The alpha300 Raman system set up for large-area scanning measured doping, stress and topography in a 150 mm SiC wafer and another alpha300 Raman microscope carried out a correlative Raman-PL measurement of GaN that visualized its composition and stress states in three dimensions.

Researchers in semiconductor development rely on detailed, conclusive investigations such as these to achieve a comprehensive understanding of their materials and manufacturing processes. The WITec alpha300 line of Raman microscopes offer precise, versatile tools that can help accelerate their rate of advance.

Thomas Meyer, Judith Beer and Damon Strom are with Oxford Instruments WITec.

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