Evaluation of Gallium Phosphide Substrate for Solid Immersion Lens

The solid immersion lens (SIL) has been critical for extending the useful life of optical microscopy for semiconductor failure analysis. By matching the refractive index of the Si device under test (DUT) the Si SIL allows the collection of light at wide angles that would otherwise be internally reflected inside the DUT, thereby increasing numerical aperture and resolution by a factor of 3.5. Increasingly smaller semiconductor feature sizes require shorter laser wavelengths to get adequate resolution from optical microscopy [1] - [3]. Si SILs present limitations for shorter wavelengths since they do not transmit light below 1100nm. A replacement material is needed with high transmittance at shorter wavelengths and a high refractive index close to that of Si (~3.5). This paper focuses on GaP (Gallium Phosphide) with good transparency down to 600nm and high refractive index (3.3), but traditionally poorer optical quality than Si. We optically characterized GaP substrates and fabricated GaP SILs from those substrates for use in a confocal laser scanning microscope and were able to achieve the theoretical resolution limit when coupled with an optimized backing objective.