Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-06-19
2001-03-27
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S130000
Reexamination Certificate
active
06208398
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to the alignment of liquid crystals, and in particular to the homeotropic alignment of liquid crystals on porous materials.
2. Related Art
Liquid crystal is a substance that behaves like both a liquid and a solid. Although the molecules in liquid crystals move past each other relatively easily, similar to molecules in the liquid, all the molecules in a microscopic neighborhood in a liquid crystal are oriented in a similar manner as a solid crystal. Liquid crystals do not melt directly to the liquid phase but instead, first pass through a paracrystalline stage in which the molecules are partially ordered. In this stage, the liquid crystal is a cloudy or translucent fluid but has some of the optical properties of a solid crystal.
There are several classes of liquid crystals. These classes include nematic, various kinds of smectic phases, and cholesteric. Each is characterized by a different spatial arrangement of the molecules and is designated by the alignments of their molecules. Typical nematic types of liquid crystal have rodlike elongated molecules oriented parallel to one another without a layer structure. Although nematic liquid crystals have little positional ordering or layering, nematic liquid crystals have strong orientational ordering. In smectic liquid crystals, the rodlike molecules are positioned in a parallel manner with respect to one another, thereby forming a layer. Nevertheless, within the formed layer, only a small periodic patterns exists. The cholesteric types of liquid crystals have their rodlike molecules parallel to one another. The molecules are arranged in a helical or spiral fashion.
The alignment of the molecules in the particular liquid crystal is very important in producing properly functioning devices utilizing liquid crystals. The spatially varying orientation of the liquid crystal molecules can be affected by external stimuli such as electric or magnetic fields, temperature, and mechanical stress. This gives rise to useful optical effects such as polarization guiding and variable (and controllable) phase retardation or scattering. Liquid crystals are used to construct displays used in digital watches, calculators, miniature television sets, as well as large, flat projection screens, liquid-crystal computer displays for portable computers, and other items.
In addition, liquid crystals are being used increasingly in optical devices. Proper alignment of liquid crystals used in optical devices is very desirable. These devices include optical information storage and processing systems, optoelectronic neuromorphic systems, liquid crystal displays, and electrically programmable diffractive optical elements and beam shaping devices.
There are two main types of liquid crystal alignment, namely homogeneous liquid crystal alignment and homeotropic liquid crystal alignment. Homogeneous liquid crystal alignment is the most common alignment technique presently used. This type of alignment is typically used in twisted nematic liquid crystals for consumer electronics and other related products. A locally averaged direction of the elongated molecules' (is a unit vector referred to as a “director”) is aligned parallel to the surface of the substrate.
In homeotropic liquid crystal alignment, the director is aligned perpendicular to the surface of the substrate. In other words, the elongated rodlike molecules have their long axis perpendicular to the surface of the substrate. Current methods for providing homeotropic alignment of liquid crystals include spinning and baking chemical surface coupling agents, such as silane compounds, on the substrate or glass.
For example, certain chemicals, such as octadecyltriethoxysilane (OTS), provide homeotropic alignment of the neighboring liquid crystal molecules when spun on a glass substrate or on indium-tin-oxide (ITO).
However, current alignment methods of liquid crystals are not suitable for all devices requiring homogeneous and homeotropic alignment, including conventionally fabricated VLSI integrated circuits, such as analog spatial light modulators and smart pixel arrays.
Therefore what is needed is a new method for achieving homeotropic alignment of liquid crystals, particularly on semiconductor dies and wafers, optoelectronic integrated circuits and the like. What is also needed is a cost effective and practical homeotropic alignment technique of liquid crystals for various optical devices and semiconductor die and wafer devices that does not require rubbing the semiconductor die or performing oblique evaporations. What is further needed is a method for creating hybrid aligned nematic cells on semiconductor dies and wafers, optoelectronic integrated circuits and the like. What is further needed is a homeotropic alignment method compatible with aluminum electrodes allowing liquid crystal devices to be fabricated on the surface of integrated circuits.
Whatever the merits of the above mentioned systems and methods, they do not achieve the benefits of the present invention.
SUMMARY OF THE INVENTION
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention is a new method of homeotropic alignment of liquid crystals, and is embodied by new hybrid aligned nematic cell devices. The new homeotropic alignment method of liquid crystals can be achieved in the invention on porous materials, such as porous anodic aluminum oxide.
Specifically, an alignment film, such as a porous film, is formed on a substrate made of or coated with aluminum by anodizing it in an acidic environment, such as an acidic electrolyte under suitable conditions. This forms a layer of porous aluminum oxide on the surface of the substrate. The pores of the aluminum oxide are elongated and have their long axis approximately normal or perpendicular to the surface of the substrate. The diameter of the elongated pores can vary depending on the anodization process and acidic electrolyte used.
Next, liquid crystal, such as nematic liquid crystal, is put in contact with the surface of the substrate to eventually achieve homeotropic alignment of the liquid crystal. This alignment is achieved by a combination of the interaction between the aluminum oxide's pore walls and of the elastic behavior of nematic liquid crystals. Within the aluminum oxide pores, the liquid crystal molecules tend to align so that the elastic deformation energy of the embedded liquid crystals is minimized. This energy is minimized when the director, (the locally averaged direction of the long axis), is parallel to the walls of the pores, or perpendicular to the surface of the substrate. This homeotropic alignment is carried to some extent to the bulk of the nematic film by bulk elasticity.
In addition, the present invention is embodied by hybrid aligned nematic (HAN) cells for producing analog spatial light modulators and smart pixel arrays on integrated circuits. The HAN liquid crystal cells are built using a bottom substrate inducing homeotropic alignment of liquid crystals and a top substrate inducing homogeneous alignment of liquid crystals. The locally averaged orientation of the long axis of the liquid crystal molecules varies smoothly from homogeneous alignment on the top substrate to homeotropic alignment on the bottom substrate. The bulk of the nematic film is affected by both substrates.
In a reflective cell, the top substrate, such as a cover glass plate, is coated for example with a transparent electrode and a material suitable to induce homogeneous alignment of the liquid crystal molecules. The bottom substrate is a porous material, such as aluminum, and is partially anodized. The remaining aluminum under the oxide is used as an optical mirror and as an electrode for applying a voltage across the cell, thereby defining pixels. Competition is created between the homogeneous alignment induced by the cover plate and the homeotropic ali
Drolet Jean-Jacques P.
Psaltis Demetri
Scherer Axel
California Institute of Technology
Michaelson & Wallace
Parker Kenneth
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