2, Univ. Grenoble Alpes, CEA, LETI, Grenoble, , France
3, Institut de Physique de Rennes, CNRS-UMR 6251, Université Rennes-1, Rennes, , France
Plasma etching, a commonly used method for the patterning of III-V materials for electronic and optoelectronic devices, is known to induce defects. These defects and the associated strain modify the material band structure, which may in turn impact device performance and stability. Plasma induced defects have been extensively studied on blanket substrates, much less on pattern sidewalls.
We have developed a battery of tools and methods for the three-dimensional characterization of patterns. Atomic force microscopy (AFM) on a tilted sample enables the measurement of line edge roughness (LER) at all heights along sidewalls with a resolution in the nanometer range, while auger electron spectroscopy (AES) provides information about surface chemistry. Based upon the work of Cassidy on polarized photoluminescence, we have also developed a polarized cathodoluminescence technique to measure strain anisotropy in the surface plane with a much improved resolution (around 100 nm). These measurements are enabled by the specific configuration of our Attolight instrument, which does not include a fiber optics connection. To our knowledge, no previous work on polarized cathodoluminescence for strain measurement has been reported. By combining the obtained in plane strain anisotropy with the global strain derived from the energy shift of the emission, we are able to extract the strains in the directions across patterns and perpendicular to the wafer. Cathodoluminescence is also used to measure emission intensity.
These techniques, and others, are applied to InP lines, in order to correlate the luminescence intensity with the observed strain and LER, as well as structural and chemical modifications. To evaluate the influence of plasma conditions, two chemistries and two temperatures are selected: Cl2 / CH4 and CH4 / H2 at 100°C and 200°C. To evaluate the influence of the edge roughness, two masking strategies are used, leading to LER of 6 to 12 nm. To evaluate the total impacted depth, 1, 3 and 6 μm wide lines are patterned. Damage depth is assessed by wet etching the patterns in H2O2 / HCl and/or annealing them at 350°C to recrystallize in volume.
Within an InP line, we observe a compressive strain in the direction across the line and a tensile strain in the direction perpendicular to the wafer, both extending about 2 μm from the edges. These measurements are compared to those obtained by μRaman spectroscopy and X-Ray diffraction, and correlated to structural defects observed by TEM and chemical modifications obtained by AES.
In conclusion, we have developed a series of tools, including AFM and AES on a tilted sample, to characterize patterned sidewalls. We have also developed a new high resolution strain measurement technique based upon cathodoluminescence. These techniques, and others, were used to understand the modifications induced by patterning in InP structures, with the aim of improving plasma parameters and ultimately device performance and stability.