Near-field imaging of a transistor device fabricated by the 65nm-technology node with light of the terahertz spectral regime allows to investigate the local conductivity of the transistor components. Near-field microscopy with light of the terahertz spectral regime is sensitive to free carrier concentrations in the between 10^16-10^19cm^-3. Free carrier concentrations in the range between 10^19 and 10^20cm^-3 can be analyzed with light of the mid-infrared (wavelength between 9-11µm) spectral regime.

The near-field image of a transistor test structure recorded at a wavelength of 118µm (2.54THz) exhibits a high signal (shown in red) for the metallic components in the centre of the image. The Si substrate reveals also pronounced signal changes, which are related to the designed variation of the free carrier concentration in the substrate. A control image recorded at a wavelength of 11µm verifies that the THz signal changes on the Si area indeed stem from variations of the free carrier concentration. THz near-field imaging of a single transistor device at a frequency of 2.54THz yields again a high signal (shown in red) in the highly conductive sample areas. Particularly the sample structures of the gate, source and drain contacts exhibit a higher signal in the THz image compared to the Si substrate. The channel region reveals an intermediate signal level as the free carrier concentration is not as high as for the contacts. The image taken at an infrared frequency lacks sensitivity to such free carrier concentrations and highlights only the metallic components of the transistor.

Images taken in cooperation with Infineon Technologies, Munich (Dr. J. Wittborn)

Comparison of the complex near-field signal of doped nanostructures with model calculations allows to quantify the free carrier concentration at the nanometre scale without the need of electrical contacts. Even the investigation of the free carrier mobility can be expected by spectroscopic studies in an extended spectral regime with neaSNOM.