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TFT LCD Display Technologies Ref. SE-03 TFT - LCD Display Mélanie Gaillet - Application Scientist - Thin Film Division Most displays in current use employ cathode ray tube (CRT) technology similar to that used in most television sets. The CRT technology requires a certain distance from the beam projection device to the screen to function. Using other technologies, displays can be much thinner and are known as flat-panel displays. Flat panel display technologies include light-emitting diode (LED), liquid crystal display (LCD) and gas plasma. LED and gas plasma work by lighting up dis- play screen positions based on voltages at different grid intersections. LCDs are categorized as non-emissive display devices, in that respect they do not produce any form of light. LCDs either pass or block light that is reflected from an external light source or provided by a back/side li- ghting system. LCDs require far less energy than LED and gas plasma technologies and are cur- rently the primary technology for notebook and other laptop computers. LCD Structure Introduction to liquid Crystal LCD devices consists of a nematic liquid crystal The molecules forming liquid crystals are of- sandwiched between two plates of glass. In detail, ten characterized by cigar-shaped or elonga- first a sheet of glass is coated with a transparent con- ted molecules. The direction of the ducting metal oxide film which acts as an electrode. elongation defines the long axis of the mole- This film can be patterned to form the rows and co- cules. The essential properties of a liquid lumns of a passive matrix display or the individual crystal are its optical and electromagnetic pixels of an active matrix display. These electrodes anisotropy. The manifestation of this property are used to set up the voltage across the cell neces- at the molecular level is that the long axis of sary for the orientation transition. the molecules tend to align in a preferred di- Next, a polymer alignment layer is applied. This layer rection, that is, they have orientational order. undergoes a rubbing process which leaves a series Depending on the type of this orientational of parallel microscopic grooves in the film. These order, there are a number of distinct phases grooves help align the liquid crystal molecules in a of liquid crystals materials : nematic, smectic preferred direction, with their longitudinal axes pa- and cholesteric. rallel to the grooves. The same sheet of glass is pre- For displays applications, the most useful are pared and once the display has been filled with the nematic and twisted nematic phases. The liquid crystals, crossed polarizers are applied to the nematic phase is characterized by long-ran- exposed glass surfaces. ge orientational order. The long molecular When an electric field is applied between the two axes possess a preferred orientation, so that electrodes, liquid crystal molecules will react in on the average they are positioned parallel such a way as to control light passage. to the director. The director field is easily dis- torted by electromagnetic fields or by surfa- ces that have been properly prepared. By introducing a nematic liquid between two surfaces with the alignment preparation per- pendicular to each other, a peculiar situation is achieved where the director is seen to ro- tate in a regular fashion from one plate to another as one progresses along the twist axis. This is known as the 90 degree twisted nematic phase that is widely used for liquid crystal displays. Display addressing Full TFT-LCD device characterization An LCD is made with either a passi- Non destructive characterization of the different parts of the ve matrix or an active matrix display TFT-LCD device was successfully carried out by Spectrosco- grid. The active matrix LCD is also pic Ellipsometry (SE). The ellipsometric data were collected known as thin film transistor (TFT) at an angle of incidence of 70° using the Jobin Yvon UVISEL display. The passive matrix LCD has a NIR (260-1700 nm). grid of conductors with pixels located at each intersection in the grid. A current is sent across two The UVISEL Spectroscopic Phase Modulated Ellipsometer is conductors on the grid to control light for any pixel. An ac- a unique instrument that provides significant advantages for tive matrix has a transistor located at each pixel intersec- display applications when compared to conventional ellip- tion, requiring less current to control the luminance of a someters. Its technology is the most suitable for accurate thin pixel. For this reason, the current in an active matrix display film measurement on transparent substrates as the software can be switched on and off more frequently, improving the includes advanced capabilities for automatic correction of screen refresh time. backside reflections. Polymer / ITO / Glass 0.900 0.900 0.800 0.800 0.700 0.700 Polymer 210 Å 0.600 0.500 0.600 1280 Å Is Ic ITO 0.400 0.500 0.300 0.400 Glass 0.200 0.100 0.300 0.000 1 1.5 2 2.5 3 3.5 4 4.5 Photon Energy (eV) The 1st part of the device consists of a glass substrate TFT device design covered with an ITO layer and a polymer layer. The most common TFT design called inverse staggered ITO is a semi-transparent conducting material which structure is presented below. This structure presents the ad- exhibits absorption in the FUV and NIR and is known vantages of a simple fabrication process and a high elec- to be inhomogeneous due to the deposition method tron mobility. or post-treatments. A graded layer model has to be In the TFT array fabrication process the first step consists of taken into account in the model. gate and storage-capacitor electrodes construction with Refractive index and thickness of each layer were ac- 2000-3000 A of a metal such as aluminum, chromium, curately characterized from 0.75 to 4.5 eV. tantalum or tungsten layer deposition. Then a triple layer of silicon nitride and amorphous silicon is deposited using PECVD. Graded ITO Optical Properties 2.6 1.1 2.4 1 2.2 0.9 2 0.8 1.8 0.7 n 1.6 0.6 k 1.4 0.5 1.2 0.4 1 0.3 0.8 0.2 0.6 0.1 1 1.5 2 2.5 3 3.5 4 4.5 Photon Energy (eV) ito_top.dsp (n) ito_bottom.dsp (n) ito_top.dsp (k) ito_bottom.dsp (k) Polymer Optical Properties TFT structure 1.56 24 Å Roughness 0.600 0.600 0.400 1.55 n-a-Sin-a-Si 514 Å 0.400 0.200 0.200 1.54 Is 0.000 Ic a-Si 1838 Å 0.000 n 0 k -0.200 1.53 -0.200 -0.400 Sin 4095 Å -0.400 -0.600 1.52 1 1.5 2 2.5 3 3.5 4 4.5 Glass 0.7 mm Photon Energy (eV) 1.51 Characterization of the TFT structure is presented be- 1 1.5 2 2.5 3 3.5 4 4.5 Photon Energy (eV) low. Optical constants depend strongly on process condi- Nematic liquid crystal tions. Ellipsometry is sensitive to the effect of dopants 0.700 as it induces changes in optical properties. 0.200 0.600 Glass 1 mm 0.100 0.500 a-Si and n-a-Si Optical Properties Is 0.000 Ic Liquid Crystal 20 µm 0.400 3.5 5 -0.100 Glass 1 mm 0.300 4.8 3 -0.200 0.200 4.6 2.5 1 1.5 2 2.5 3 3.5 4.4 Photon Energy (eV) 4.2 2 n k The second part of 4 1.5 the device consists of a nematic liquid crystal confined 3.8 between two plates of glass. The characterization of 3.6 1 this structure requires advanced modeling functions 3.4 0.5 such as anisotropy and double backside correction. 3.2 The liquid crystal is homeotropic and presents uniaxial 3 0 1 1.5 2 2.5 3 3.5 4 4.5 anisotropy with N axis (perpendicular to the sample Photon Energy (eV) plane). a-Si.dsp (n) n-a-Si.dsp (n) a-Si.dsp (k) n-a-Si.dsp (k) Liquid Crystal Optical Properties with ne > no SiN Optical Properties s - Printed in France - 09/2003 1.66 0.022 2.34 1.64 0.02 2.31 1.62 0.018 1.6 2.28 0.016 1.58 2.25 0.014 n 1.56 0 k n 2.22 0.012 k 1.54 0.01 2.19 lly binding under any circumstance 1.52 0.008 2.16 1.5 0.006 1.48 2.13 0.004 1.46 2.1 0.002 1 1.5 2 2.5 3 3.5 4 4.5 Photon Energy (eV) 2.07 0 1 1.5 2 2.5 3 3.5 4 4.5 LC_ne.dsp (n) LC_no.dsp (n) LC_ne.dsp (k) LC_no.dsp (k) Photon Energy (eV) This document is not contractua Conclusion Phase Modulated Spectroscopic Ellipsometry is an excellent technique for the highly accurate characterization of complete TFT-LCD device. The technique allows the determination of film thickness, optical properties but also more complex proper- ties such as graded or anisotropic layers and effect of dopants. In the flat panel industry the pressure to reduce manufacturing costs is and reliable metrology tools are required to control the different steps of the process. Spectroscopic ellipsometry is a non destructive technique which presents advanced capa- bilities and proven reliability tailored for qualification and on-line production control..
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