Objective lens for producing a radiation focus in the inside of a

Abstract

Objective lens for producing a radiation focus in the inside of a specimen. Many laser measuring methods for sensing charge carrier density or the distribution of a potential in the inside of an integrated circuit (IC) of microelectronics are based on what is referred to as 'backside-probing' technique, whereby the laser radiation (LA) is focused into the plane of the voltage-carrying components (SK) from the backside of the component using a conventional microscope objective. Since the irradiation occurs through the substrate (SU), a pronounced spherical aberration arises that limits the spatial resolution to approximately 2 through 4 .mu.m. For producing a sub-.mu. probe in the substrate (SU), a lens is arranged on the polished backside (RS) of the integrated circuit (IC), this lens being composed of a silicon base plate (GP, refractive index of n.sub.1), a sphere (KU, refractive index of n.sub.2 <n.sub.1, radius of r.sub.2) lying in a recess of the base plate (GP), and a hemispherical silicon shell (KS, refractive index of n.sub.1, outside radius of r.sub.1, inside radius of r.sub.2). Given a suitable selection of the refractive indices and of the radii, the lens and the substrate (SU) form an optical unit acting as a 2-index Luneburg lens that focuses an incident, parallel ray beam (LA) at a point (LF) lying in the inside of the substrate (SU).