Design and Manufacturing of GaAs Aspherical SILs

A solid immersion lens (SIL) is an indispensable tool for semiconductor failure analysis. SILs can dramatically enhance the optical resolution limit by increasing the numerical aperture (NA) of the objective lens. Normally SILs are made of Si because the semiconductor wafer material consists of Si. Si has a very high refractive index (about 3.5) and it makes the resolution limit 3.5 times higher than that of normal lenses. However, recently the technology node of cutting-edge devices became too small for Si-SILs. To overcome the resolution problem, short wavelength lasers have been introduced to get a higher resolution.[1], [2], [3] Unfortunately Si is only transparent for wavelengths longer than 1100nm. GaAs and GaP on the other hand are transparent at shorter wavelengths than Si, while their refractive index is equivalent to that of Si. Designing and manufacturing aspherically shaped SILs is however needed to make it possible to use these materials as SILs, unless the backing objective is redesigned and optimized for the SIL. We have designed and manufactured a GaAs-SIL with aspherical shape and demonstrated diffraction limited optical imaging quality while using a backing objective lens with no aberration correction.