GB1583176A - Molecular alignment process and device for liquid crystals displays - Google Patents

Description

(54) MOLECULAR ALIGNMENT PROCESS AND DEVICE FOR LIQUID CRYSTAL DISPLAYS (71) We, TEXAS INSTRUMENTS IN CORPORATED, a Corporation organized according to the laws of the State of Delaware, United States of America, of 13500 North Central Expressway, Dallas, Texas, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to liquid crystal devices and to a method for making the same, and more particularly to liquid crystal devices of the type having surface-aligned liquid crystal molecules.

It is known that a preselected alignment of the molecules of liquid crystal material with the surface of a liquid crystal cell can be obtained by coating the cell surface with various mterials using a slant evaporation process, as described in British patent specification number 1,388,077. In such an evaporation process, certain materials comprising aluminum, gold, magnesium fluoride, carbon, tungsten oxide, or silicon monoxide are directionally vacuum deposited onto the cell surface using a vacuum deposition angle of approximately 10 to the plane of the surface being coated. The liquid crystal molecules are thereby aligned so as to have a tilt angle of about 30 to 450 in the absence of an applied electric field.

One problem with devices employing this alignment technique is that the viewing angle thereof is somewhat limited; except within a comparatively small angle, the general appearance and legibility of the displayed image of the device is quite sensitive to changes in viewing angle. While techniques have been developed for increasing the device viewing angle, suchtechni- ques themselves have introduced certain new problmes relating to the uniformity of appearance of the displayed image.

One alignment technique has been reported by W. Urbach, M. Boix, and E.

Guyon, "Alignment of Nematics and Smectics on Evaporated Films", Applied Physics Letters, Vol. 25, No. 9, November 1, 1974.

It was reported that by using a slant evaporation of about 30 to the plane of the substrate as opposed to the 10 used by Janning, in the above mentioned British patent specification the liquid crystal molecules are aligned substantially parallel to the substrate surface having a 0 tilt angle as opposed to the 30 to 45" tilt angle reported by Janning. However, a disadvantage of the 30 slant evaporation process is that the resulting zero alignment tilt angle, when used in a twisted nematic field effect display, results in different groups of molecules being rotated in different directions under the influence of an applied electric field.

The optical characteristics of these different groups of molecules with an applied electric field varies depending upon the angle of view thereof, and for any given viewing angle, the different groups of molecules display areas of different contrast, resulting in a displayed image having somewhat patchy appearance.

A molecular surface alignment technique directed at improving both the viewing angle and the appearance problems of such.

devices has been reported in U.S. Patent No. 3,967,883, issued on July 6, 1976, to Dietrich Meyerhofer and Alan Sussman.

Devices fabricated by this technique have utility, for example, in displays used for indicating the time of day and for electronic calculators.

In one embodiment reported by D.

Meyerhofer and A. Sussman, a first layer of silicon monoxide having a thickness of about 500 angstroms is vacuum deposited by a slant evaporation process as indicated in the aforesaid Janning patent but at an angle of about 30 to the surface of a cell substrate. Thereafter, a second layer of silicon monoxide having a thickness of only 5-10 angstroms is similarly deposited onto the first layer. The direction of growth of this second layer is at about right angles to the direction of growth of the first layer, and this second layer is deposited at about an angle of 10 to the substrate surface. Owing to the extreme thinness of the second layer, it is believed that this layer is quite nonuniform and randomly discontinuous in its coverage of the first layer, whereby portions of the first layer are exposed through the second layer.Because the two layers have been grown at right angles to one another, both layers are effective to induce alignment of the liquid crystal molecules in directions parallel to the plane including the direction of deposition of the second layer. As a result of this technique, the tilt angle of the molecules through the film or liquid crystal material averages out to be about 1" to 5".

In another embodiment reported, the sequence of the depositions is reversed; the first or bottom layer being deposited at a comparatively low angle, for example 10 , and the second or upper layer being deposited at a comparatively high angle, for example 30". The first or bottom layer has a reported thickness of about 500 angstroms, while the covering layer is made quite thin, for example about 5-10 angstroms. Again, the tilt of the molecules through the film of liquid crystal material averages out to be about 1" to 5 .

The embodiments described in the D.

Meyerhofer and A. Sussman patent require the slant evaporation of a layer having an average thickness of between 5-10 angstroms. This range imposes a problem in the manufacture of devices employing this technique. Due to the extreme thinness of this second layer, it is difficult to uniformly control such a layer from substrate to substrate, as the deposition time required is very short. Correspondingly, a large distribution range of thicknesses is produced, and a corresponding proportion of liquid crystal cells will have tilt angles outside the desired range.

According to one aspect of the present invention there is provided a method for the fabrication of a molecular alignment substrate for a liquid crystal display device including the steps of: depositing a first alignment layer having a thickness of 50 to 500 by slant evaporation onto a planar substrate surface at an angle less than 15 to said surface and depositing a second alignment layer by slant evaporation onto said first alignment layer at an angle to said surface between 20 and 45" and with a direction substantially perpendicular to the evaporation direction of said first layer the thickness of said second layer being from one to five times as great as that of said first layer, whereby the induced molecular alignment of a liquid crystal in contact with the fabricated substrate is intermediate those which would be obtained using the first and second alignment layers alone.

The molecular alignment substrate may be formed from a substrate having a smooth planar surface. This substrate may be mounted on a substrate holder in a bell jar by any suitable means. Material to be deposited on the substrate may be heated by a heater element to evaporation temperature. The evaporation material to be deposited by evaporation may be silicon monoxide, silicon dioxide, gold, aluminium, carbon, tungsten oxide, platinum, magnesium fluoride, or chromium. However, other materials may be used for either or both of the two alignment layers. The heater element may be connected to heater electrodes which in turn are connected to a power source.

In operation a suitably high vacuum of about 2 x 10-5 torr may be provided within the enclosure of the bell jar utilizing known tecniques. The evaporant material may be heated to its evaporation temperature by passing a current through the heater electrodes thereby producing evaporation beams which travel to the surface of the substrate for deposition thereon.

The invention also provides liquid crystal display devices using molecular alignment substrates fabricated according to the above recited aspect of the invention. The molecular alignment substrates may be used as at least one of a pair of substrates with the liquid crystal material sandwiched in between them. Where a display device uses a pair of such alignment substrates they may be oriented so that one alignment substrate produces an alignment in the liquid crystal molecules which is transverse to the alignment of the molecules produced by the other substrate.

These display devices may be used, for example, in electronic watches, in which case the display is driven by a suitable watch chip, or they may be in an electronic calculator and driven by a calculator chip coupled to a keyboard.

A method embodying the invention will now be desribed in detail by way of example only with reference to the accompanying drawings in which: Figure 1 is a perspective view indicating the deposition of a first alignment layer; Figure 2 is a perspective view indicating a second deposited layer and subsequently completed molecular alignment substrate; and Figure 3 is a cross-section of an assembled twisted nematic liquid crystal display.

Referring to Figure 1 a planar substrate 3 has a first layer 7 of silicon monoxide thereon. A first evaporation layer is deposited along a direction 6 which lies in an x-z plane and makes a first angle of about 5 to the substrate. However, any angle of incidence of less than 15 is also suitable. The first evaporation layer is deposited for approximately 45 seconds which results in a layer of about 60 angstroms thick. This first deposited layer may be of any thickness between about 50 and 500 . The first layer 7 is shown in Figure 1 in a highly idealized form. The actual topography of the first layer 7 is a great deal more complex.

The idealized tetrahedron structure as indicated by element 7 of Figure 1 results from random nucleation of the evaporation followed by shadowing of the surrounding substrate area during subsequent evaporation.

Upon completion of the first layer, a second layer is subsequently deposited on top of the first layer, as indicated in Figure 2. This second layer is now deposited along a new direction 11 which lies in a z-y plane which is perpendicular to the x-z plane which contained the evaporation direction 6 of the first layer. In addition, the new evaporation direction makes a second angle of incidence 0 with the substrate 3 of 30".

However, any angle of incidence of between 20 and 45" may be employed. The second layer which is deposited on top of the first layer is deposited for about 30 seconds which results in a covering layer of about 250 angstroms. The second layer may be of any thickness between 50 and 500A with the ratio of the thickness of the second layer to the first layer is in the range of about 1-5 (1 S T30/T5 S 5). This second layer when deposited on top of the first layer results in a topography illustrated in a highly idealized form by the tetrahedron structure 9 in Figure 2. The actual topography is a great deal more complicated.The tetrahedron structure produced as a result of the second evaporation step on top of the first evaporation layer has a smaller apex angle than the tetrahedron structure deposited during the first slant evaporation step.

The deposition thickness varies according to the inverse square of the distance between the substrate and the evaporant source, and is proportional to the cosine of the deposition angle as measured from the normal of the substrate. For this reason, the deposition rate during the 30 slant evaporation step is about 6 times the deposition rate of the 10 slant evaporation step.

Because the two layers have been grown at right angles to one another, in the directions shown, both layers are effective to induce alignment of the liquid crystal molecules in directons parallel to the x-z plane of Figure 2.

From a description of the embodiment presented, it is clear where the differences and the improvements over the D.

Meyerhofer and A. Sussman patent lie. By employing a moderately thin first layer and a heavier deposited said second layer as contrasted to the teachings of D. Meyerhofer and A. Sussman, the applicant has discovered that substantially thicker layers may be employed to achieve the desired results of his invention. It is the ability to employ heavier deposited layers of about 60A or greater that allows for the suitability of this process for mass production and eliminates the difficult process of depositing layers of about 10 angstroms in a reproducible manner.

In the assembly of display devices, each of two substrates may be provided with two slant evaporated layers, and the two substrates assembled together in known fashion to provide a cell having a general appearance such as that shown in Figure 3. Figure 3 represents a cross-section of a twisted nematic display having dual polarizers. The liquid crystal display as indicated in Figure 3 comprises optically transmissive substrates 15 each having an optically transmissive and electrically conductive coating on a face thereof as indicated by element 17. These elements are insulated by an encapsulating dielectric layer 19. The slant evaporated alignment layers are deposed on the dielectric layer so as to contact the liquid crystal material confined therein.The pair of alignment surfaces 9 are oriented so that the direction of the nematic directors are twisted at an angle of about 90" from each other forming a twisted display The cell is sealed by means of a frit 23. The use dual polarizers 21 completes the liquid crystal cell structure.

By employing the alignment structure as shown in Figure 2 and Figure 3, liquid crystal molecules are oriented so as to have a tilt angle of about 1" to 5 , as measured by a conoscopic technique.

This molecular alignment process results in twisted nematic displays with improved contract versus viewing angle performance and improved performance for time shared multiplexed operations in the context of frit sealed displays.

A display including a molecular alignment substrate fabricated by a method according to the present invention may be used in an electronic watch or in an electronic calculator.

The method of fabricating molecular alignment layers disclosed herein is not limited to the application of twisted nematic displays. Rather, the molecular alignment method herein disclosed is applicable to any liquid crystal device requiring a molecular alignment tilt angle of about 1" to 5 from the substrate surface.

While a particular embodiment of this invention has been disclosed herein, it will be understood that various modifications may become apparent to those skilled in the art without departing from the scope of the invention.

WHAT WE CLAIM IS: 1. A method for the fabrication of a molecular alignment substrate for a liquid crystal display device including the steps of: depositing a first alignment layer having a thickness of 50 to 500 A by slant evaporation onto a planar substrate surface at an angle less than 15 to said surface, and depositing a second alignment layer by slant evaporation onto said first alignment layer at an angle to said surface between 20 and 45" and with a direction substantially perpendicular to the evaporation direction of said first layer, the thickness of said second layer being from one to five times as great as that of said first layer, whereby the induced molecular alignment of a liquid crystal in contact with the fabricated substrate is intermediate those which would be obtained using the first and second alignment layers alone.

2. A method as set forth in claim 1 wherein said first and second layers comprise gold, silicon dioxide, silicon monoxide, carbon, tungsten oxide, chromium, magnesium fluoride, aluminium or platinum.

3. A method as set forth in claim 1 wherein the thickness of said first layer is 60 A and of said second layer is 250 A.

4. A method of forming a molecularly aligned liquid crystal device including the step of: sealing liquid crystal material between two substrates, at least one of which is fabricated according to any one of claims 1 to 3, so that the liquid crystal material is in contact with said at least one substrate.

5. A liquid crystal display device formed by a method according to claim 4.

6. A method according to claim 4 wherein each substrate is fabricated according to any one of claims 1, 2 and 3 and the substrates are so oriented that the molecular alignment induced by one substrate is in a direction transverse to the molecular align ment induced by the other substrate.

7. A liquid crystal display device includ ing a liquid crystal material sealed between and in contact with two substrates each one fabricated by a method according to any one of claims 1, 2 and 3 wherein the substrates are so oriented that the molecular alignment induced by one substrate is in a direction transverse to the molecular alignment in duced by the other substrate.

8. A liquid crystal display including a device according to claim 5 or claim 7 sandwiched between a pair of polarizers.

9. A digital electronic watch including electrical time-keeping circuit means connected to a liquid crystal display device according to claim 5 or claim 7.

10. An electronic calculator including electronic calculating circuit means connected to keyboard means and a liquid crystal display device according to claim 5 or

Claims (1)

1. claim 7.

   11. A liquid crystal display device according to claim 5 or claim 7 and substantially as herein described with reference to the accompanying drawings.

   12. A method according to claim 1 and substantially as herein described with reference to the accompanying drawings.