METROLOGY OVERVIEW:

Excitation by UV-laser

Thin films of IGZO and LTPS – i.e. the active layer of a TFT – can be characterized by monitoring the excited charge carrier behavior in these materials. By exciting the charge carriers, the recombination behavior monitoring can provide a fingerprint of the layer electrical properties.

A pulsed UV-laser (349 nm) is used for excitation: the pulse creates electron-hole pairs in the material. The recombination can happen through different physical phenomena, which are typically trap-assisted or Auger-processes, etc. By measuring the recombination of the charge carriers, we can draw conclusions to physical properties of the material, thus the electrical quality and homogeneity can be evaluated.

Microwave detection of transient behavior

The charge carrier recombination can be detected by means a non-contact microwave metrology system. A tunable ~22-26 GHz microwave field is radiated onto the sample surface using a compact waveguide antenna. Tuning can compensate sample effects and is done automatically. The source of the field is a Voltage Controlled Oscillator (VCO). Microwave local signal and reflection from the sample is measured.

The laser excitation changes the free charge carrier concentration in the sample, thus microwave reflection will then suffer great change. As the charge carriers recombine through different physical phenomena, a transient decay can be measured, which in the end shows the relaxation of the sample. Nonlinear curve fitting is used to obtain the different characteristic carrier lifetimes of the material.