Jean Van Den Steen

Electrical model of a liquid crystal pixel with dynamic, voltage history-dependent capacitance value

The voltage history-dependent nature of a liquid crystal pixel capacitance becomes an important issue in active matrix addressing as large storage capacitors are replaced with more intelligent circuitry such as in-pixel frame buffers. In this paper, a simple but flexible Spice macro model is introduced that allows accurate simulations of the electrical behaviour of a nematic liquid crystal pixel capacitance. The model correctly predicts voltage drops caused by the increasing dielectric constant when liquid crystal molecules align themselves to the electric field. An internal node of the macro model represents the average director orientation and can also be used to predict the optical response. In its basic embodiment, the model uses a first-order, low pass RMS filter to implement the dynamic behaviour of the pixel, which suffices to predict response delays and asymmetric rise and fall times. However, the model also supports more elaborated filters that offer more control over the simulated dynamic behaviour. A number of simulations are performed that illustrate the usefulness of the new model during the design of novel ‘smart’ pixel architectures.

34.1: Invited Paper: Microdisplays with High Pixel Counts

Recent developments in high-resolution microdisplays are discussed. The importance of resolution limiting factors such as diffraction of light, fringe fields, voltage requirements and stepper field size is shown. Difficulties related to stitching and large-area CMP are reviewed, Parallelism in the driver design is discussed and the first results of a 2560×2048 pixel microdisplay are presented.

Measurement methodology for vertically aligned nematic reflective displays

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.

Use of stitching in microdisplay fabrication

Microdisplays are considered to be an enabling technology for ultra-high resolution displays. Pixels can indeed be made very small using CMOS technology. Nevertheless, in multimillion pixel microdisplays, the die size becomes a limiting issue, because the optical field size of present day lithographic steppers, and consequently the resulting chip size, is limited to about 20x20 mm2. This paper shows how this limitation can be overcome using stitching techniques and discusses the resulting challenges in design as well as lithography. The partitioning of the schematics and the layout in stitchable modules is reviewed, as well as the definition of boundary conditions for these modules. The difficulties concerning design verification, and a virtual stitching procedure that can be helpful, are presented. From a technological point of view, the stepper job creation and the scribe line definition is described, as well as the positioning accuracy that is achieved. It is also shown that careful partitioning of the design can lead to a single stitchable mask set that allows the creation of microdisplays with several different resolutions. The paper is illustrated with results from the European Esprit project Mosarel, a project that has shown the feasibility of a 2560x2048 pixel microdisplay.

Silicon-based reflective polymer-dispersed LC display for portable low-power applications

Recently LCOS microdisplays are becoming available for personal IT applications, despite some problems which are less critical in poly silicon or amorphous silicon based displays. The most common problems which must be encountered are the polarization of the pixels and the light shielding of the silicon substrate. In this paper a methods proposed which solves the light shielding and pixel flatness problem. A non-critical back-end processing which can be applied outside the silicon foundry has been developed. The effectiveness of the light shielding on a working demonstrator display is shown. To avoid light losses caused by a polarization filter, a polymer dispersed LC has been chosen. By decreasing the cell gap we made the PDLC voltage compatible with a standard 3 micrometers CMOS process and its response fast enough to be used for video applications. It is shown that this choice is very suited in direct view and portable applications. The realized prototype has 3 bit grey levels and is video compatible and can be used in a number of applications, such as personal viewers, PDAs and data displays.