Characterization of TFT and LTPS TFT-LCD Display Panels by Spectroscopic Ellipsometry

Characterization of a-Si panels.

Three layers of silicon and a top native oxide were used to describe accurately the device. The sensitivity of the ellipsometry technique allows the characterization of materials with similar optical constants (around 0.1), such as those exhibited by the high and low deposition rate (HDR/LDR), and doped a-Si materials.

The non-destructive characterizations of a-Si and Low Temperature Poly Silicon TFT-LCD panels were successfully carried out by spectroscopic ellipsometry. The ellipsometric data were collected at an angle of incidence of 70° across the spectral range 1.5-5 eV using the HORIBA Jobin Yvon UVISEL spectroscopic ellipsometer.

Spectroscopic ellipsometry was used to characterize both thicknesses and optical constants of TFT-LCD devices. Moreover the grain size of p-Si materials was investigated during the study.

The optical constants of silicon materials depend strongly on process conditions. The optical properties of amorphous silicon layers are generally calculated using the Tauc Lorentz or new amorphous dispersion formula included in the materials library of the DeltaPsi2 software.

Poly crystalline layers are often modelled by a mixture of c-Si and a-Si using the Effective Medium Approximation. It allows to determine the material composition inside the layer and hence the crystallinity. 

Spectroscopic ellipsometry is an excellent technique for the highly accurate characterization of TFT-LCD display panels based on a-Si and  TPS technologies. Owing to the sensitivity of the UVISEL spectroscopic ellipsometer and the advanced modeling features included in the DeltaPsi2 software it is possible to detect in a multistack different a-Si layers processed by various methods. Moreover the spectroscopic ellipsometry measurements allow determination of the grain size of p-Si films and illustrate the ability to characterize the crystallinity of silicon with high accuracy.

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