Infrared near-field imaging of implanted semiconductors: Evidence of a pure dielectric contrast

The miniaturization of the components in microelectronics requires nowadays the study of the charge distribution in semiconductors with a submicronic spatial resolution. This high resolution is essential in the metrology of integrated circuits. This local study may also be of importance for local characterization of conductivity in various solids such as superconductors whose properties can be spatially not homogeneous in ‘‘the normal state.’’ The free carrier density can be revealed by optical methods such as near and midinfrared reflection, transmission, and spectroscopic measurements. These optical measurements are becoming more popular, notably because of the soft interaction between the light field and the materials. The spatial resolution of these techniques however is physically limited by diffraction. For example, at l510 mm, the fundamental limit of spatial resolution is 5 mm, which is not enough to characterize carrier distribution in submicronic modern electronic devices. Recently, a reflection mode scanning near-field optical microscope ~SNOM! using an apertureless probe has been introduced, its use in the mid-IR region (l510.6 mm), in a dark field configuration, has allowed to achieve a l/600 optical resolution ~17 nm!. Figure 1 reminds the principle of this IR SNOM configuration. It consists on a grazing incidence of an IR light beam issued from a CO2 laser, in p polarization. The IR light is focused onto the surface sample above which an apertureless metallic tip ~tungsten! vibrates. Results of numerical calculations have shown that for p polarization, we observe a strong enhancement of the electric field intensity just below the tip apex. The tip is prepared by electrochemical erosion of a bent tungsten wire, this technique is detailed in Ref. 5. The radius of curvature of the tip end is smaller than 20 nm. The tip end may be viewed as a Rayleigh particle which scatters in local interaction with the sample surface, that is to say in the sample near-field. The tip vibration allows to perform lock-in detection at the vibration frequency ~typically 5 kHz! of the scattered light collected with a Cassegrain objective in the far-field. This permits one to get out, from the total intensity received by the detector, the small part which corresponds to light scattered by the local interaction between the Rayleigh particle and the optical/topographical properties of the sample. This small