Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing...
Reexamination Certificate
1999-11-04
2002-06-18
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
C359S016000, C359S016000, C428S001400
Reexamination Certificate
active
06407797
ABSTRACT:
FIELD
The present invention relates to a method for manufacturing a polymer coated long duration optical memory device having applications in ferroelectric liquid crystal materials.
BACKGROUND
Ferroelectric liquid crystal devices (FLCD) are widely used in spatial light modulators, light shutters, optical inter-connects, optical switches, optical gates for optical computing, high definition television terminals, electro-optic modulators etc. The optical addressed spatial light modulators has a great interest due to their applications in optical data processing, image amplification, incoherent to coherent image converters, wavelength converters, optical correlators etc. The surface bistability in ferroelectric liquid crystal materials in which the thickness of the cell is smaller than the pitch value of material, has a great potential from the application point of view. The fast switching bistable electro-optic devices based on ferroelectric liquid crystal material comprises a well aligned thin (1-3 &mgr;m) layer of FLC sandwiched between two optically flat polished glass plates having transparent electrically conducting electrode patterns thereon. The two glass plates, are peripherically sealed and the electrical connections are taken out from the substrates. The display fixed with crossed polarizers on outer faces of both glass plates is connected with very large scale integrated (VLSI) electronic modules to be used as an information display for displaying electrical or optical data.
At present ferroelectric liquid crystal displays or devices are being prepared by
1. shearing the plates just below the smectic A-smectic C* (Sm C*−Sm A) transition temperature
2. by applying magnetic field
3. temperature gradient from spacer edge
4. depositing thin films of silicon monoxide on the supporting substrates with oblique angle of evaporation
5. polymer treated rubbed plates and
6. post treating the zig-zag defected sample with an ac voltage of 20 Vpp for few hours.
PRIOR ART
It is known in the state of the art that FLCD's are being prepared by shearing the two glass plates at a temperature just below the Sm A-Sm C* phase transition. Smectic layers get aligned by applying shear and that layer structure is retained in Sm C* phase of liquid crystal to get an uniform defect free alignment of ferroelectric liquid crystals, reference may be made to N. A. Clark & S. T. Lagerwall, U.S. Pat. No. 4,563,059; N. A. Clark, and S. T. Lagerwall, Appl.Phys Lett, Vol.36, 899 (1980). The drawback of shearing technique is to get an uniform alignment over a large area because of the difficulty in applying uniform shear and to maintain a uniform temperature through out the display. The technique thus cannot be used as a commercially viable one.
It is also known in the state of art that the FLCD can be prepared by aligning the FLC molecules by magnetic field. Ferroelectric liquid crystal film encapsulated between transparent conducting glass plates is cooled slowly to Sm C* phase from its isotropic phase in strong uniform magnetic field. Reference may be made to A. M. Biradar, S. Wrobel and W. Haase, as described in Phys.Rev. A Vol. 39, 2693 (1989). The drawback in this process is to get uniform magnetic field over the large area and it takes long preparation time. Another known process in the state of the art is to grow smectic A phase under a temperature gradient as described by K. Ishikawa et. al; in Jpn. J. Appl. Phys. Vol.23, L211-213, (1984). The nucleation of the smectic A phase is initiated with the aid of spacer edge. The temperature gradient is obtained by using an Indium tin oxide (ITO) electrode properly etched as a local heater. This technique is very cumbersome and the maximum obtainable macrodomain is about one square millimeter in size.
Another process of making an uniform homogeneously aligned display is to deposit thin films of silicon monoxide at a pressure of 1×10
−5
torr, on the supporting transparent conducting glass plates with oblique angle of evaporation of 5°, 10° and 15° as described by Bawa et. al. in Appl. Phys. Lett. Vol.57, p.1398 (1990). The alignment of ferroelectric liquid crystal molecules is in the plane of evaporation but tilted with respect to the substrates depending upon the thickness of SiO
2
to support the surface alignment. The display shows good bistability and is defect free. However, in this case the contrast is poor because of the high tilt angle of smectic layers (30°). Oblique deposition of SiO
2
has also been studied for different angles of evoparation as described by Ouchi et. al. in Jpn. J.Appl.Phys, Vol.27, L725 (1988) and L. A. Goodman, in IEEE. Trans. Electron Devices ED-24, 995 (1977) and has been extensively used with nematic liquid crystals. Substrates deposited at 60° angle with the normal, yield low (0°) pretilt surfaces as described by Y. Takanishi et.al; in Jpn. J. Appl. Phys; Vol. 28, L48 (1988).
Another known useful process in the state of the art is to align ferroelectric liquid crystal homogeneously, to rub unidirectionally the polyimide/polyamide treated transparent conducting plates as described by J. S. Patel et.al; in Ferroelectrics, Vol. 59, 137, (1984). Good alignment can be achieved by this technique over a large area. This method has been used to prepare the bistable devices as claimed by Yoshihara et al., in U.S. Pat. No. 5,568,299 and Tsuboyama et.al. in U.S. Pat. No. 05,013,137. However, the occurrence of zig-zag defects as described by M. A. Handschy and N. A. Clark; in Ferroelectrics, Vol. 59, 69 (1984) and line defects as described by Ishikawa et.al; in Jpn. J. Appl. Phys; Vol. 23, L 666 (1984) does not give rise to uniform contrast due to the presence of defect boundaries. Another drawback in this technique is that the charges are accumulated at the interface between the polymer and FLC material which destroys the bistability of the FLC device as described by Chieu and Yang, in Appl. Phys. Lett; Vol. 56, 1326 (1990).
Another known process in the state of art to get an uniform defect free sample is to post-treat the sample with AC field for a long time. The zig-zag type of defects which appears in a polyimide/polyamide treated rubbed glass plates can be removed by applying an alternating electric field of suitable frequency as detailed by S. S. Bawa et.al; in Jpn. J. Appl. Phys. Vol. 28, 662, (1989). The device can be obtained with large uniform area with good contrast and shows a bistability (memory effect) for few seconds. However, the long post-treatment timing makes the process to be a commercially non-viable one.
OBJECTS
The main object of the present invention is to provide a method for the preparation of a polymer coated long duration optical memory device having applications in ferroelectric liquid crystal materials which obviates the drawbacks of hitherto known processes as described in Table-1.
TABLE-1
Glass
Coating of known composition
Memory
substrate
(Polymer & Thickness)
(ref.)
1.
ITO coated
300 A° thick with
Bistable memory with
glass substrate
polyamide (1,4-butylene
few m.sec. memory
terephthalate)
Ref. APL, 47, 1278 (85)
2.
ITO coated
600 A° Polyimide resin
AC stabilized effect
glass substrate
memory
Ref.JJAP, 27, 1115 (88)
3.
ITO coated
polyimide coating
AC stabilized memory
glass substrates
Ref.JJAP, 33, 209 (94)
4.
ITO coated
Polyamide coating
few hours memory
glass substrates
(Nylon 6/6 or 6/9)
Ref.JJAP, 34, 3602 (95)
Ferroelectrics, 211, 9 (98)
Another object of the present invention is to provide a process for making an optical memory device having a high contrast display.
Still another object of the present invention is to provide an optical memory device having a microsecond switching time.
Yet another object of the invention is to provide a process for preparing an optical memory device using the homogeneous alignment of liquid crystal materials
Another object of the present invention is to provide a process for preparing a memory device by deposition of thick polymer coating on glass substrates.
Yet another object of the present invention is to provide an optical memory devi
Bawa Sukhwant S.
Biradar Ashok M.
Chandra Subhas
Haridas Ereuvassi P.
Chung David
Council of Scientific & Industrial Research
Parker Kenneth
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