How to Measure Microwave Photoconductive Response in a Display

TFT industry requires characterization techniques for process optimization to get an enhanced throughput of fabrication. Utilizing the Microwave Photoconductive Decay (µ-PCD) and Microwave Photoconductive Response (µ-PCR) metrology allow the electrical properties of the active layer to be analyzed straight after layer deposition or later on during the process, ahead of forming the devices themselves.

This early-stage, non-destructive, non-contact, characterization technique can be employed in instances of InGaZnO (IGZO) and Low-temperature Poly-Silicon (LTPS)-based TFT panel processes.

Metrology Overview

Excitation by UV-Laser

Thin films of IGZO and LTPS (the active layer of a TFT), can be characterized by observing the excited charge carrier behavior in these materials. The recombination behavior monitoring can supply a fingerprint of the layer electrical properties by exciting the charge carriers.

For excitation, a pulsed UV-laser (349 nm) is utilized: electron-hole pairs in the material are produced by the pulse. The recombination can occur through different physical phenomena, which are usually Auger-processes or trap-assisted. Conclusions to physical properties of the material can be drawn by calculating the recombination of the charge carriers, and so the electrical quality and homogeneity can be established.

Microwave Detection of Transient Behavior

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

The free charge carrier concentration in the sample is adjusted by the laser excitation changes, so the microwave reflection will experience a large adjustment. A transient decay can be measured as the charge carriers recombine through different physical phenomena, which shows the relaxation of the sample in the end.

Nonlinear curve fitting is employed to acquire the different characteristic carrier lifetimes of the material.

Evaluation of the Microwave Response

The nature of the transient curve reflects on the charge carrier electrical properties. The transient is made up of different decay components which can be correlated with different time constants.

These are called the charge carrier lifetimes, which are widely employed quality parameters in the TFT and semiconductor industries. In µ-PCR, the response amplitude is also measured. These figures can be utilized to optimize the TFT fabrication process to attain the best device performance.


  • Stabilized laser source – feedback control unit is used to give long-term stability to the laser and reduce short-term pulse noise
  • Optimized for flat panel application
  • Adjustable transient analysis method in order to detect advanced electrical properties of the sample
  • IQ-detection of response – complex-amplitude transient detection reduces phase-noise of the measurement

This information has been sourced, reviewed and adapted from materials provided by Semilab Semiconductor Physics Laboratory.

For more information on this source, please visit Semilab Semiconductor Physics Laboratory.


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