Measurement methodology for vertically aligned nematic reflective displays

Abstract

We developed a measurement method for the characteristics of microdisplays specifically aimed at vertically aligned nematic reflective cells. It allows determination of contrast ratio and cell gap, and gives good estimates for the pretilt angle and the elastic surface-coupling constant. The set-up consists of a laser source, high quality polarisers, a beamsplitter mirror, a quarter-wave plate and a sensitive photodiode. A model for the polarization changes in the light caused by each component allows the extraction of the initial phase retardation induced by the cell and gives a first estimate of the thickness. Simulation of the director configuration in liquid crystals is then used to enhance the accuracy by taking into account the properties of a real LC cell. Matching of the simulation and the measurements yields the required values together with a calibrated simulation model. 1.INTRODUCTIONLCoS devices are serious candidates for becoming the dominant lightvalve technology in many applications 1 . The vertically aligned nematic LC mode is ideally suited for LCoS applications, offering high performance in terms of contrast, response time and driving voltage requirements 2 . As these devices become more common, a measurement method to determine characteristics and ensure quality of production will be needed. For transmissive devices, especially employing twisted LC configurations, display metrology methods are thoroughly investigated and standardised 3 . Publications concerning measurements for reflective devices are not so common and most are aimed at LC modes employing twist 4 . METHODOLOGYThe display characteristics of interest typically include the contrast ratio, the response speed, cell-gap uniformity and some information about the liquid crystal orientation angles. The methodology described concentrates on contrast ratio and cell gap, giving information on orientation angles and anchoring energy as a side effect. Response speed measurements are very straightforward and thus not discussed. OPTICAL SET-UPDue to the size of a typical LCoS device, illumination with a laser spot is most convenient if one wants to scan the active area to detect non-uniformities. The laser is also the ideal source of light to measure the inherent contrast ratio of the cell. The set-up is an on-axis configuration. Figure 1 schematically shows the arrangement of the optical elements in our measuring device. The laser source is a HeNe-laser emitting virtually non-polarised light at 543 nm. After being deflected by a mirror, the beam is linearly polarised by a Glan-Thompson polariser. These polarisers offer high extinction ratios in the crossed configuration (a ratio of more than 10 5 is easily achieved), which will be needed in view of the expected high contrast of the cell itself. The light is then sent through a broadband dielectric beamsplitter and projected onto the display under test. The reflected beam again encounters the beamsplitter, where a part of the light is * dcuypers@elis.rug.ac.be; phone +32 9 264 3404; fax +32 9 26...

Pre‐tilt angle and cell‐gap measurement of vertically aligned non‐twisted liquid‐crystal displays

“…A methodology for this was reported and used in Ref. 8. The light coming from the display is sent through a quarter-wave plate before being passed through the analyzer.…”

“…A single-wavelength measuring method using a rotated wave plate developed for the vertically aligned mode was presented in Ref. 8. Like most of these methods, the determination of the two parameters remains somewhat interdependent, leading to a loss of accuracy.…”

Assembly technology for the manufacture of LCOS panels

“…The incidence angle is defined as the angle between the average material deposition direction on the substrate during the evaporation process and the substrate normal. Pre-tilt angles were measured directly on the actual cells using a phase-retardation technique 17 ; this methodology gives very accurate results.…”