CA1039841A

CA1039841A - Hollow-space cell and method for its manufacture

Info

Publication number: CA1039841A
Application number: CA223,988A
Authority: CA
Inventors: Giorgio Barzilai; Paolo Maltese; Cesare M. Ottavi
Original assignee: Individual
Current assignee: Individual
Priority date: 1974-04-05
Filing date: 1975-04-07
Publication date: 1978-10-03
Also published as: DK143525B; ATA261175A; BE827641A; IL47108A; FR2266926B1; FR2266926A1; AT354533B; DE2435422A1; CH613052A5; SE418651B; DE2435422C2; IT1015905B; GB1507855A; DK143525C; NL7503952A; DK144375A; IL47108A0; SE7503883L

Abstract

HOLLOW-SPACE CELL AND METHOD FOR ITS
MANUFACTURE

ABSTRACT OF THE DISCLOSURE .-A method is disclosed for the manufacture of cells having a hollow space adapted to lodge a liquid crystal, such cells being used, for example, in various display devices. The essence of the method is to induce in the planar supporting members which make up the cell walls internal stresses to maintain the confronting cell surfaces pressed together with the interposition of appropriate spreaders. Such internal stresses are originated by imparting an appropriate curvature to the supporting members prior to uniting them and welding them together. Critical values for the radius of curvature are given, as well as for the spreader interspaces . -

Description

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This invention relates to a hollow-space cell and to a method of making the cell.
The invention aims to provide a cell having the charac-teristic feature that the thickness of the hollow space can be made to great accuracy and remains substantially constant in spite of mechanical and thermal stresses the cell may be required to undergo.
Herein the term "cell" means a unit formed by at least two supporting members, which are generally thin relative to their transverse dimensions, for example slabs, having at least two confronting surfaces, kept spaced apart from one another by ~appropriate means, and having therebetween a hollow space. This -unit can be a part of a more intricate structure, such as a multiple cell containing a plurality of superposedly arranged hollow spaces.
It is known that cells have gained importance for a i number of applications in modern industry. More particularly, - this is true of the electronic industry, wherein liquid-crystal cells may provide solutions to a number of problems connected with the visualization of images and data.
Liquid crystal cells, as is well known, are cells in -the hollow space of which a layer of a liquid crystal is contained, ~
that is, a mesomorphic phase which, even though in the liquid state, - ~-still retains a few properties of the solid crystals and, more particularly, anisotropy.
In cells manufactured according to the conventional methods, two supporting members in the form of for instance glass -plates are welded together in the vicinity of their edges with ~`
resins or other cementing materials which are adapted to hold them ~
in the correct position and, often, hermetically to seal the ;
hollow space. The correct gap between the surfaces is obtained ~`~
by means of spacers, made of a variety of materials and a~anged .
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: . , - :. . - .-103984~
along the welded edges or being an integral part of the welding seam.
Recent studies have shown that, to the end of the satisfactory operation of a cell in general, and of a liquid-crystal cell in particular, it is necessary for the thickness of the hollow space to be kept, at every point, as constant as practicable in spite of variations of the ambient conditions.
In the specific case of the liquid-crystal cells in which, usually, a thin hollow space thickness is required, in the range from 1 and 50 microns, and wherein the supporting members are planar glass plates, it has been observed that, as the thickness is varied, undesirable changes can occur in operation, and/or even destructive phenomena, such as a gradual destruction of the alignment of the liquid-crystal molecules.
Now, in the cells made according to the conventional methods as outlined above, the supporting members become easily deformed when subjected to temperature differentials and/or mechanical stresses. In addition, in the case of liquid-crystal cells, it is necessary to have glasses with perfectly parallel surfaces in order that an even thickness may be obtained. By subjecting, for example, a cell having peripheral spacers to a force tending to compress the cell, and acting, for example, centrally in a direction perpendicular to`the surface of the support--ing members, the result is an unacceptable decrease of the thick-ness of the hollow space, as measured at the centre.
In order that such a deformation may become small, it is required that the supporting plates have a high stiffness, that is, a large wall thickness.
The situation has been improved in the past by using, rather than spacers located on the edges only, spacers positioned evenly in the hollow space. These spacers must be small enough and ~paced, so as to avoid any excessive diminution of the useful 103984~
surface of the cell and any disturbance in the cell operation.
Such spaced apart spacers can be made in a commercially accepta~le manner in several ways. A method which is already known involves spreading on one of the surfaces granules of an appropriate size.
Another method involves forming such granules by removing with appropriate ~ethods, for examplle by chemical etching, a layer of material from one or both surfaces, the removed material having a shape and a thickness equal to the hollow space one desired to obtain, with the exception of a few areas which are masked with appropriate means. `
- In such a case, inasmuch as the distance between the -resting points of the supporting members on the spacers is of the same order of magnitude as the thickness of the thinner slab, the compressive deformation are smaller than before, and are essentially due to the crushing of high spots on the spacers whereas, as regards the traction forces, the behaviour of the structure remains unaltered.
The behaviour as described above, however, will take place only in the ideal case in which the two confronting surfaces are exactly parallel to one another, for example perfectly planar.
In actual practice there will be extended areas where, by applying ~`
a compression force, the spacers will only contact the walls after a certain decrease of the thickness of the hollow space in those -areas, a decrease ~hich is comparable with the planarity error of the surfaces.
According to the present invention there is provided a hollow-space cell including at least one pair of planar members such as glass plates welded to one another along their edges, and between confronting surfaces of which there is formed a hollow space for receiving a liquid crystal, and in the hollow space, a plurality of spacers having the same thickness as the hollow spacè and in a preselected arrangement between the confronting 10398~a~
surfaces of the two members, wherein the members are internally stressed in such a way that their confronting surfaces are urged towards one another and into contact with the spacers. With the invention, the thickness of the hollow space is more accurately produced, the effect of any unevenness in the surfaces of the supporting members is diminished, and the cell offers a greater resistance to the mechanical and/or thermal stresses tending to cause variations of the thickness of the cell hollow space.
Stated another way, by virtue of the internal stresses generated in the two supporting members, the cell: 1. reacts to a compressive force acting on the supporting members like a cell equipped with scattered spacers, but with an improvement due to the fact that from the beginning any unevenness is partially compensated and any high spots between the spacers and surfaces are partially flattened, at least at the areas of mutual contact, owing to the permanent compression state caused by the aforesaid stresses, and 2. is responsive in a very favourable way to any tractive force since the aforementioned compression state counter-acts at least an initial deformation due to traction.
The invention also provides a method of making a hollow-space cell according to the invention, in which the two supporting members are prearranged with confronting surfaces defining the hollow space in which the spacers are provided in a preselected arrangement, welding the edges of the two supporting members to one another and internally stressing the two supporting members so that their confronting surfaces are urged towards one another and into contact with the spacers.
Preferably, in a cell of the invention, when the cell is opened by breaking the welding at the edges of the members, the hollow space takes the shape of a negative lens in which the difference between the curvatures of the two surfaces as considered on the same plane as the cross-sectional plane of the cell is more ~0398~
than 10 3 microns/sq. centimetre and/or the difference between the - maximum and minimum thickness of the hollow space is greater than 3 microns. In the method of the invention, it is preferred that said internal stresses are generated by imparting a curvature to at least one of the two supporting members and prearranging the members so that the hollow space initially takes the shape of a `
negative lens, whereafter, the two supporting members are com-pressed one against the other until the confronting surfaces contact the spacers, and the supporting members, maintained in this condition, are then welded together along their edges;
Thus, the cells of the present invention may also be characterized by the fact that, by virtue of the internal stresses ;
of the two supporting members, when welding seam(s) along the edges are broken, the hollow space takes the shape of a negative lens again. More exactly, th s negative lens shape as taken by the hollow space due to the disruption of the welding seam is distinguished and recognized since the difference between the curvatures of the two surfaces which confine the hollow space, `~
said curvatures being considered in the same plane as the sectional plane of the cell, is more than 10 3 micron/sq. centimetre.
Likewise, the abovementioned negative lens shape of the hollow ..; .
space when the cell is open is characterised by the fact that the difference between the maximum and the minimum thickness of the hollow space is more than 3 microns.
As will be appreciated, with the method as outlined above, there are obtained, in the supporting members of the completed cell, residual stresses which maintain the supporting members in a state of compression which concurrently contributes towards smoothing the high spots of the contact areas (spacers and peripheral edges) between the two supporting members and to reduce the effect of unevenness of the confronting surfaces.
These stresses, then, are the same as oppose the tractive forces ... - .~.. . . . . . ............. . . . .
. .

acting upon the cell. 1~3984~
As outlined above, the characteristic feature of the cells according to the present invention resides in the residual stresses which remain in the plates or supporting members which define the hollow space. In addition to the method consisting in starting from one or two convex plates, other methods can be adopted, such as the following: a) subjecting the plates, in the course of approaching and sealin~ the peripheral edges of the plates, to a compressive stress acting upon the outermost layers of the plates and parallel thereto, ox b) forming on the outer surfaces of the two plates, in correspondence with the areas of mutual welding, grooves wherein additional material is forcibly introduced, whose presence and permanence generates the above mentioned stresses.
The construc~ion of the cell according to-the present invention, in addition to affording better performance, avoids rigorous specification of the starting surfaces, making it possible to accept starting surfaces of a quality which is less strictly specified, as will be shown in the ensuing practical examples.
As a matter of fact, the geometrical unevennesses of the surfaces do not directly give rise, as in the conventional technique, to unevennesses in the thickness of the hollow space, but mainly originate unevennesses in the mutually exchanged compressive stresses.
In addition, an outstanding aspect of the present invention is to permit the manufacture of a low cost cell of the desired shape and with the desired hollow space thickness.
The features and advantages of the present invention will become more clearly apparent from the ensuing description in connection with the accompanying drawings, wherein:
Figures 1 and 2 are diagrammatical showings of the two glass plate supporting members of a cell according to the present ~ ` ' ' ' , ,, ' ,' .

in~ention, before and after their pressing together and hermetic sealing, respectively.
Figure 3 shows the stress-strain plot characteristic of cells according to the present invention.
Figure 4 and 5 are views similar to Figures 1 and 2 of a modification of the construction of cells made according to the present invention.
Figures 6 and 7 show cross-sections of further embodiments of cells according to the invention.
Referring now to Fig. 1, the numerals 10 and ll indicate two supporting plates, usually of glass. In the plate 10, in correspondence with the surface which is intended to confront the other plate 11, there is formed a cavity 12, which provides in the cell the hollow space for lodging the liquid crystal and in which there are also provided spacers 13, in addition to a peripheral ridge 1~ extending all along the perimeter of the hollow space.
Both the cavity 12 and the spacers twhich are evenly distributed) are made with the photoetching method, as known in other fields, but unusual for the cells, for removing with a chemical etchant a layer having the shape and the size which are desired with the tolerances which may be obtained with such a procedure.
As clearly seen in Fig. 1, the plates 10, 11 used as the starting members, are curved with a cylindrical curvature and are arranged with the convex faces confronting one another.
Once the processing operations on the individual plates have been completed, these are matched by urging together, under an appropriate pressure, the diverging edges until both the plates become planar and then carrying out at this stage the peripheral welding 30, so that a cell such as shown in Fig. 2 is thus obtained.
The cell according to the present invention is character-ised in addition by stress-strain plots of the kind shown in Fig.

:lV39841 3, which plots are obtained by applying perpendicularly to the surfaces two equal and opposite forces, F (Fig. 2), which for the purposes of Fig. 3 are positive when acting towards one another, and measuring the reduction, L, of the thickness of the hollow space, for example by interpherometric methods of volume measure-ment.
A characteristic feature of such plots is that, for small deformations, the ratio of the deformation to the stress (and so to forces F) is that which would obtain with a structure in which the two supporting members were bound to one another both via the spacers and at the welding seams, whereas, for forces which are sufficiently great in the direction of traction, (i.e.
negative F) the ratio of the deformation (now negative L) to the stress corresponds to that in a structure in which the two supports are bound to one another only at the welding seams. All the deformations as listed above are of the elastic and reversible kind.
In this connection, a plausible explanation of the behaviour of the cells according to the present invention seems to be a distribution of the internal stresses such as that assumed in Fig. 2.
In the structure of Fig. 2 the welding seam 30 on the inner corners of the edges of the plates 10, 11, which can be mechanically symbolized as a hinge, subjects the plates, (which, if let free, would tend to take the convex shape as shown in Fig.
1), to tractive forces as shown by the arrows 15 with respect to plate 10. The outer layers of the material of the plates are stretched, whereas the inner layers, in contact with the hollow space, are compressed. In the regions of the spacers there are acting between one plate and the other the compressive force as shown by the arrows 16, on plate 10 whose resultant is equal and' opposite to that of the tractive forces 15.

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Obviously with the forces lS and 16, there is actually intended a schematic showing exemplifying the forces for unit thickness of the cross section, as given by the resultants of the pressure along the corresponding lines of application in the section. It is thus apparent that a characteristic feature of the cells according to the preferred embodiment of the Figs. l and 2 is that, by breaking the seals, the two plates tend to resume the original curvature again. In Figs. 4 and 5 there is shown a modification of the preferred embodiment in which the starting plates have a spherical curvature. It is possible in such a way to impart to the starting plates the desired curvature radius while respecting the tolerance ranges which are of vital importance. To make the cells according to the present invention, it is preferred to start from planar glass plates which have the desired degree of planarity, that is plate of surface quality within such a range as not to interfere at a subsequent time with the thickness of the hollow space. Then, with methods which are known, there is imparted to these plates a spherical curvature with a radius of at least 5 metres, preferably with a radius of between 20 and 80 metres. After or prior to imparting the above mentioned curvature, one forms on the planar plates the spacers in the desired number and arrangement, with methods which are conventional in this field.
The plates with the confronting convex surfaces, are then pressed together and~sealed along the edges as described for the previous embodiment.
In the structure of Fig. 6 the welding along the edges as effected by means of the material 17, which is mechanically comparable to a rigid restrained end, and the median surface of the plates would not change their shape if the plates were left free. The surface of the edge, on the contrary, would tend to assum~ the shape by the dashed line. This is because in this _ g _ .

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- : : .

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case the welding material 17 is so applied that the outer layers of the plates are subjected by the material 17 to compression as represented by the force 18, whilst the internal layers undergo tensile stresses due to the forces 19. The elastic reaction of the plates 10, 11, causes the presence of the compression forces 20 at the spacers and of the counteracting forces 21, acting along the edges.
In the structure of Fig. 7, also if the plates 10, 11 are left free, the median surfaces do not change their shape. The plates are welded together by the layers 22 at their edges and at the centre and are provided with grooves filled with a material 23, in the region of the welds, such as to apply compression forces 24 in the external layers of the plates.
As a consequence, the compression forces 26 at the spacers and the balancing forces 25 and 27 at the welded edges are originated.
According to the method of preparation of the cells which is also a part of the present invention, one starts from supporting members having at least two surfaces to be placed in mutually confronting relationship, to which are applied the appropriate spacers or of which one or both is etched to form the cavities of the desired depth separated by the spacers. The cavities may be of differing depths if a variable thickness hollow space is desired. Subsequently, the surfaces are matched by imparting thereto a pressure or a stretching force which is adapted to the shapes of the starting surfaces so that the support-ing members are deformed elastically and the surfaces contact one another via the spacers. The edges are then welded and possibly other preselected positions are welded so that, upon releasing the force system, the surfaces remain in contact through the spacers and stresses remain in the interior of the supporting members and of welding seams so as to originate a compression .: . . - . - ~ . - .
.:

lV398~
state of the surfaces. Such welding seams can also fulfil the requirement of hermetically sealing the hollow space.
As an alternative, or in conjunction with the above steps, the internal stresses of the structure are impressed, either wholly or in part, after or during the welding of the two supporting members, by the agency of plastic creep of all or of a part of the materials of which the supporting members are made.
The supporting members to be used must be constituted by materials, such as glass, having such mechanical properties as to be able to undergo stresses while retaining the internal stresses, under operative conditions and consistently an adequate service life of the cell, without being additionally deformed, broken or otherwise impaired during the service life~
The spacers can be also of a number of materials, they must have such thickness, deformability and arrangement as to give rise to the formation of the expected hollow space. It has been found that for obtaining hollow spaces having thickness less than 5 microns, the distribution of the spacers should be such that no point of the hollow space is spaced more than 4 mm from a spacer ;
or from the perimeter of the hollow space.
The surfaces forming the cell walls can be bored or drilled for filling up the cell.
The methocl and the cells as described in the present invention find a particular but not exclusive application in the electronic industry.
With reference to Fig. 2 a cell can be obtained accord-ing to the structure as diagrammatically shown in the Figure, as follows. The starting material is glass nominally planar plates of a thickness of 3 mm and with surfaces to be confrontingly positioned having unevenness in the range of 1 micron so as to give rise at any point and along any cross-sectional line of the surface to curvatures of less than 0.1 micron/sq. centimetre.

1~)39E~4~
The plates, measuring 10 by 10 cms, are subjected to such a heat treatment as to induce therein a curvature of 1 micron/sq.
centimetre, almost uniformly since it is much greater than any curvatures due to initial unevenness.
On the convex surface of one plate photoetching is carried out to remove a square cavity of 9 by 9 cms, 3 microns - deep, leaving projections, which are the spacers, extending from the bottom of the cavity and having a height equal to the cavity depth, the projections being of cylindrical configuration and with a diameter of 0.1 mm and located on the apexes of the theoretical lattice of 1 by 1 millimetres in the interior of the cavity.
; There may then be deposited on the convex surfaces electrically and chemically active thin layers using methods which do not cause additional deformation in the glass so that the desired configurations are provided in the layers.
The two convex surfaces are then matched and, by impressing an even pressure of 1 kilogram/sq. centimetre, all the spacers are brought into contact with the other plate.
From the edges to the interior a small amount of epoxy resin is caused to seep and is allowed thoroughly to polymerise so as to prevent any subsequent displacements.
Upon releasing the pressure applied to the plates, the inner surfaces are parallel to within 0.2 micron, as confirmed by the figures of interference fringes under sodium monochromatic light.
The same result can be obtained by applying a pressure of 2 kilograms per square centimetre before and after the cement-ing step, limited to a S mm wide strip along the entire periphery of the cell.
Another example for making the structure of Fig. 2 is as ~ollows.

~ 12 -, -- : ' . , , 1~39l~1 The same operations as described in the previous case are repeated, with the exception of the glass curving, but for cementing, instead of the epoxy resin, a glass paste melting at about 550 C is used.
During the thermal cycle which is required for welding the glass, the cell is subjected to a pressure over its entire surface and simultaneously to forces tending to shrink the outer `~ layers of the plates. The forces are obtained by applying to the outer cell surfaces two metal blocks which are surfaces, rigid and have an expansion coefficient higher than that of glass. The blocks are pressed against the plates at 570C, allowed to cool down to 440C, while still maintaining the cell under a pressure of 2 kilograms/sq. centimetre. This fact produces a plastic creep of the plate glass which gives rise, with cooling, to the internal stresses of Fig. 2 and to the same thickness or evenness of the hollow space of the previously described embodiment.
The structure of Fig. 7 can be obtained as follows.
The starting supporting members are flat plates having the same thickness and surface properties as in the preceding examples.
To obtain a 53 x 73 cm structure, plates of correspond-ing size are cut and one of them is provided with a 50 x 7 cm cavity, 3 microns deep, with spacers at the apexes of an ideal lattice of-5 mm, as in the preceding cases.
Any required layers are deposited onto the plates, without inducing any deformation.
A 3 microns deep layer of epoxy resin is coated on one plate as a strip of 1.5 cms width, both along the edges and also over its surface so as to form a lattice of strips 10 cms apart, this having some interruptions allowing communication between the 10 x 10 cms squares into which the plate surface is thus divided.
The plates are matched and pressed together under 2 kgs/

sq. inch pressure, whilst the resin is completely polymerised.

: '' , - . - . - ;. , : .
-The thus obtaine~ cell, upon the pressure being released, is not uniform since only few spacers are in mutual contact.
U-shaped grooves are then cut in the outer surfaces o~
both plates, by means of a diamond wheel, at the welding zones, the grooves being 1.2 mm deep and 1 mm wide.
Within said grooves the material 23, which may be aluminium, conveniently in the form of a wire, is inserted, and is then pressed by means of a press. The residual compression of the aluminium is such as to originate the system of stresses as shown in Fig. 7 and a thickness uniformity of the hollow space within 0.1 micron all over the surface.
The structure of Fig. 4 can be obtained starting from plates which have been curved according to a spherical curvature.
The starting material is planar glass plates of the thickness of 3 millimetres and having a size of 68 by 35.5 millimetres. On the surface of one of the two plates there is formed by photo-etching a centered hollow space of 58 by 20 milli~etres, having a depth of 3 microns, and with spacers starting from the bottom of the hollow space and having a height equal to the hollow space depth, of a cylindrical shape and with a diameter of 0.05 milli-metres, placed on the apexes of an ideal lattice of 2 by 2 millimetres as drawn in the inside of the hollow space. The thus treated plates undergo such a heat treatment as to produce therein a spherical curvature corresponding to a radius of 51 metres.
There may then be deposited on the convex surfaces electrically and chemically active thin layers, but without deform-ing the glass and the desired configurations are thus obtained on the layers.
The plates are matched by impressing a uniform pressure along the edges until the spacers are in contact and the epoxy ~`~
res~n which is caused to seep from the edges towards the interior along a preselected distance is allowed to polymerise.

': ' ' - :. -- ' :

1'~)398~1 ~
sy releasing the pressure applied to the plates, the inner surfaces remain parallel within a tolerance of 0.1 to 0.6 micron, as confirmed by the figures of the interference fringes in monochromatic light (sodium yellow light).

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Claims

WHAT WE CLAIM IS:-

1. A hollow-space cell including at least one pair of planar supporting members such as glass plates welded to one another along their edges, and between confronting surfaces of which there is formed a hollow space for receiving a liquid crystal, and, in the hollow space. a plurality of spacers having the same thickness as the hollow space and in a preselected arrangement between the confronting surfaces of the two members, wherein the members are internally stressed in such a way that their confronting surfaces are urged towards one another and into contact with the spacers.

2. A hollow-space cell according to claim 1, wherein when the cell is opened by breaking the welding at the edges of the members, the hollow space takes the shape of a negative lens in which the difference between the curvatures of the two surfaces as considered on the same plane as the cross-sectional plane of the cell is more than 10-3 microns/sq. centimetre and/or the difference between the maximum and the minimum thickness of the hollow space is greater than 3 microns.

3. A hollow-space cell according to claim 1 wherein the thickness of the hollow space is not greater than 5 microns and every point in the hollow space is not more than 4 mm from the nearest spacer or from the perimeter of the hollow space.

4. A hollow-space cell according to claim 1, 2 or 3, wherein grooves are provided on the free surfaces of the two supporting members, the grooves corresponding to the welding zones, and insert material is forced into and seated in said grooves to generate said stresses.

5. A method of making a hollow-space cell according to claim 1, in which the two supporting members are prearranged with confronting surfaces defining the hollow space in which the spacers are provided in a preselected arrangement, welding the edges of the two supporting members to one another and internally stressing the two supporting members so that their confronting surfaces are urged towards one another and into contact with the spacers.

6. A method according to claim 5, wherein said internal stresses are generated by imparting a curvature to at least one of the two supporting members and prearranging the members so that the hollow space initially takes the shape of a negative lens, whereafter the two supporting members are compressed one against the other until the confronting surfaces contact the spacers, and the supporting members, maintained in this condition, are then welded together along their edges.

7. A method according to claim 6 wherein both the supporting members are initially curved and are matched with their convex surfaces in confronting relationship.

8. A method according to claim 7, wherein said supporting members have an initial spherical curvature with a radius of curvature of at least 5 metres.

9. A method according to claim 8, wherein the radius of curvature is between 20 and 80 metres.

10. A method according to claim 5, wherein the supporting members are initially parallel and the stresses are induced by means of grooves formed in the free outer surfaces of both supporting members at the welding zones, and wedging insert material into said grooves.

11. A method according to claim 5, wherein the supporting members are initially planar and are welded together along their edges so that the frame formed by the welding material induces in the outermost surface layers of the supporting members a compression parallel to the external surface of the same supporting members.

12. A method according to claim 5 wherein the hollow space and the spacers are obtained by chemical etching more particularly with the photoetching methods for planar plates.

13. A method according to claim 5 wherein the spacers are formed with a thickness not greater than 5 microns and are distributed so that no point of the hollow space is spaced more than 4 mm from a spacer or from the perimeter of the hollow space.