FR2788349A1 - Production method for liquid crystal cell, comprising thinning by polishing at least one side plate enclosing liquid crystal by processing between two parallel supports, at least one abrasive - Google Patents

Abstract

The liquid crystal display (LCD) cell comprises two plates (1,3) enclosing the liquid crystal (2), where the plates in contact with the liquid crystal are provided with electrodes. The production method includes at least one stage of thinning at least one plate (1,3). The thinning stage is carried out by placing the liquid crystal cell between two supports or platforms (4,5), at least one with abrasive surface (15), putting the cell under pressure between the supports and setting the supports in motion relative and parallel to each other. At least one of the plates of the cell is subjected to machining operation by rectification. A wear plate can be joined to one of the plates (1,3) by an adhesive for the protection from wear. The procedure for joining two liquid crystal cells includes thinning one plate of each cell and joining by an adhesive on thinning sides. The procedure can include thinning a second plate of one of the cells, thinning one plate of a third cell, and joining by an adhesive on thinned sides. The procedure includes the formation of micro-lenses on at least one thinned side of the liquid crystal cell, where each pixel of the cell corresponds to a micro-lens. The thinned plate of the cell can be joined to a light-guiding plate comprising micro-guides in the form of regions of different index of refraction for guiding light to pixels. An assembly of joined cells can be used as a color filter. The procedure can include joining by an adhesive or molding of the cell onto a card containing electronic circuits and connection to electronic circuits. The thickness of one of the plates (1,3) is less than 200 micrometer. An assembly of cells can comprise the means for independent electrical control of different cells for an improved performance of the deflection angle and the number of grey levels. The liquid crystal cell assembly can contain a masking plate placed on the plate of thickness less than 500 micrometer for the purpose of micro-lithography.

Description

i

  PROCESS FOR PRODUCING A LIQUID CRYSTAL CELL

  LIQUID CRYSTAL CELL AND CHIP CARD INCORPORATING A

Such a cell

  The invention is applicable to the production of thin-film liquid crystal cells and its application to visualization, laser beam processing, UV photolithography and the display of information on

smart cards.

  Liquid crystal matrices (LCD) generally consist of two glass slides whose thickness is of the order of 1 mm and separated by a few micrometers using shims thick

  to limit the space occupied by the liquid crystal.

  Some of these "Active" type matrices are increasingly used for their good electro-optical performance. In these matrices, a MOS transistor is placed at each intersection between lines and columns, the latter being metallized. As a result, the ratio between the effective area for the electro-optical modulation and the total area of each pixel is considerably reduced, about 25% or less for steps of 50 μm for example. In the diagram of Figures 1 and 2, we therefore have the

  typical values: p = 50 μm, a = 25 μm and d = 1 mm.

  In areas such as visualization where the luminous efficiency is decisive for the system, the implementation of a microlens before each pixel a priori allows a significant gain in transmission. We calculate below the physical limitations due

  respectively diffraction and geometry.

  For a monochromatic source at the wavelength X, the diffraction widening at the focal plane is k.d / n.p. To transmit the main diffraction lobe, it is necessary to

  therefore respect the condition X.d / n.p <a / 2 be d <a.n.p / 2.X.

  d <1 mm for n = 1.5, p = 50 μm, a = 25 pmet X = 0.6 μm For a source characterized by a geometric extent "Es" with a beam of dimension D, so that the enlargement is contained in the opening of the pixel, it is necessary to respect the condition d <(anDVi) / (2 / Es) is Es (mm2sr) 10 50 100 dmx (pm) 500 230 165 In visualization and more particularly in projection with liquid crystal, a technique consists in use blades thinner than 500 μm with sources of very small geometric extent. The disadvantages of this solution are essentially technological because it is difficult to handle large plates (300 to 400 mm or more) and of such a small thickness during the many deposition operations, photolithography, etching, heat treatment, friction,

10 assembly and cutting.

  Moreover, this solution does not perfectly optimize the transmission of the flux of the source whose geometric extent increases when it

  accumulates a large number of hours of operation.

  For other applications where it is desirable to have a very large concentration of light, to effect over-resolution, for example, the only way is to further mask the electrode plane at the cost of a significant loss of light. flux. For example, with a source of 10 mm2sr with a thickness d = 500 pm, the minimum diameter of the spot

  focal length is 15 pm, or about one third of the pitch.

  To our knowledge, there are currently no electro-optical components using stacks of several layers of liquid crystal although this perspective is very interesting. The reason is undoubtedly linked to the technological complexity commonly accepted for this

kind of components.

  Moreover, the realization of a liquid crystal screen comprising a stack of several cells (for color visualization for example) is difficult to achieve because of the significant thicknesses of the

glass slides of the cells.

  The invention solves these various problems.

  The invention therefore relates to a method for producing a liquid crystal screen comprising steps for producing a cell comprising two plates enclosing a liquid crystal, the faces of the plates in contact with the liquid crystal being provided with electrodes, characterized in that it then comprises a step of thinning at least one of said

plates.

  The invention also relates to a liquid crystal cell comprising two blades enclosing a liquid crystal, the faces of the blades in contact with the liquid crystal carrying electrodes, characterized in that the thickness of at least one of the two blades is order or less than 300 pm. The mechanical rigidity of state-of-the-art LCDs prevents

  integrate them on a smart card.

  The invention therefore also relates to a smart card comprising an electronic circuit, characterized in that it comprises a cell glued to one of the faces of the card or molded in the card,

  10 electrical connections connecting the cell to the electronic circuits.

  The different objects and features of the invention will appear

  more clearly in the following description given by way of example and

  in the appended figures which represent: FIGS. 1 and 2, liquid crystal screens such as those known in the art; FIG. 3, an example of the embodiment method of the invention; FIGS. 4a, 4b and 5, variants of the method of the invention; FIGS. 6a and 6b, a liquid crystal cell according to the invention provided on one side with a matrix of microlenses; FIGS. 7a and 7b, a stack of several liquid crystal cells according to the invention; - Figure 8, a trichromatic system using three liquid crystal cells; FIG. 9, an illustration of the angular field of a liquid crystal matrix; - Figures 10 and 11, systems using blades having waveguide structures; - Figure 12, a screen system using a touch control device; FIG. 13, the application of the invention to a phase and amplitude modulator; Figures 14a and 14b, alternative embodiments of the invention; FIGS. 15a and 15b, a liquid crystal matrix that can be used in microelectronic manufacturing technologies; FIGS. 16a and 16b, the application of the invention to a card

smart card type.

  Referring to Figure 3, we will describe an example of

  method of carrying out the invention.

  Having realized according to a known technique, a liquid crystal cell as shown in Figure 3 and having two blades 1 and 3, at least one of which is transparent, which enclose a liquid crystal 2. The two blades are sealed together and the cell thus forms a rigid set. According to the invention, at least one of the blades 1 for example is then

  grinded and polished to thin the thickness of the blade.

  According to Figure 3, the blade 3 of the liquid crystal cell is fixed, for example by gluing, to a support part 4 for handling the cell. The assembly is applied by the outer face of the blade 1 of the cell to a surface 15 of a workpiece 5. This surface 15 comprises an abrasive. The surface 15 is then animated by a displacement movement (of rotation for example) relative to the cell while a pressure is exerted on the part 4 so as to apply the cell against the surface 15. The blade 1 is thus thinned until reaching the dashed line 11. Then the support piece 4 can be removed. For example, if the support piece 4 had been fixed to the blade 3 by thermo-bonding; the support part 4 can be removed by heating the assembly or by attack

chemical glue.

  FIG. 4a shows a variant of the method according to which a wear part 7 is fixed to the blade 3. The assembly is placed between two machining surfaces 15 and 18 of two machining parts 5 and 8. A pressure is applied to the part 8 so as to clamp the cell between the two surfaces 15 and 18. The two machining parts are driven relative movement parallel to each other so that the machining surfaces. 15 and 18 cause thinning of the blade 1 and the wear part 7. The blade 1 is thinned until it reaches the dotted line 11 and the

  wear part 7 is thinned until reaching the dotted line 17.

  According to FIG. 4b, the liquid crystal cell is placed directly between the machining surfaces 15 and 18 of the parts 5 and 8 without the use of a wear part 7. Under these conditions, during machining both

  Blades 1 and 3 are thinned until reaching the dotted lines 11 and 13.

  Practical achievements have shown that a thickness much less than 300 μm, for example 200 μm, was obtained on a blade of a 2.2 inch diagonal screen 1024 × 1024 pixels without deterioration since a normal electrooptical operation was found after electrical reconnection and screen test. Thinning at 60 pm + 3 pm

  was also obtained by this method.

  This thickness may not be the ultimate limit yet. It must be considered on the one hand that the thickness shims are very close together, at the pitch of the pixels for example. On the other hand, it is noted that the thinning operation is done in the presence of the liquid crystal, it contributes

  the mechanical strength of the glass slides.

  Using the method of FIG. 4a, thicknesses of 150 μm to + μm have been obtained on matrices 1, 4 "with an integrated control circuit, and such a method is well suited for the production of large series, since several matrices can be The thinning of materials other than glass as counter-electrode or active matrix can be envisaged: Silica, Germanium, Silicon, BSO, plastic ... in order to produce electrooptical modulators in optical bands other than visible : Ultraviolet (UV), Infrared (IR) Bands 1, 2 or 3 or with

  different mechanical properties (plastic).

  In the foregoing, the machining of the blade (s) may be completed by polishing with the aid of a surface covered with an alumina fabric. Figure 5 shows another method for thinning the two plates of an LCD cell. For this purpose, a first plate 1 is thinned, for example that carrying the counter-electrode of the cell. This

  thinning is done for example as shown in Figure 3.

  Then, stick a shim 9 to the plate 1 which has been thinned.

  Then we place the assembly between the machining parts 5 and 8 so as to

  thin the second plate 3 of the LCD cell. Finally, remove the hold 9.

  Once the LCD matrix is thinned on one or both sides, the invention provides for producing a matrix of microlenses aligned and glued. A more elaborate technique consists in directly replicating the microrelief on the thinned blade of the cell using a photo-polymer or thermo-crosslinkable polymer. Microlens matrix and LCD cell are then perfectly integrated facilitating their implementation in

  very compact multilayer stacks in particular.

  The two methods of assembly are conceivable: Transfer of an existing microlens matrix 20 in contact with the LCD matrix on the side of the counterelectrode plate 3

  thinned, spatial fit and collage (Figure 6a).

  Molding microlenses 20 on a photocrosslinkable polymer layer deposited on the outer face of the blade of

against electrode 3 (Figure 6b).

  In both cases, it is necessary to precisely align the two arrays, at a fraction of a pixel of the LCD cell near. For this, a positioning machine similar to that used to superpose masks on a silicon wafer during the development of an integrated circuit

can be used.

  In the case of the embodiment of FIG. 6a, a collimated light source makes it possible to display the foci of the different microlenses that must be adjusted in the center of the pixels of the LCD cell. Once this positioning is obtained, a pressure is applied to immobilize the two parts together during the thermally stimulated bonding or by

ultraviolet.

  In the case of molding (FIG. 6b), the radius of curvature R of the microlens for a focal length f in the medium of index n is R = (n-1) * f / n, ie approximately R = f / 3 with glass (n = 1.5). For a pupil 4) of microlens, the thickness to be engraved is then e = VI / 8R is e = 3 pm for 4) =

40 μm and a focal length of 200 μm.

  The possibility of thinning the counter-electrode and / or the matrix of an LCD matrix also makes it possible to envisage multilayer assemblies with

  liquid crystal offering many advantages over the state of the art.

  We explain below the principles of such assemblages of several

LCD cells.

  The following method describes how to make such an assembly of two LCD cells to obtain a component as shown in FIG. 7a. The plates (3, 3 ') of the two cells carrying the counter-electrodes are first thinned. Then, the electrical connections of the active matrices located on the plates 1, 1 'are made. The two plates 3, 3 'are then joined and positioned in such a way as to make the pixels of the two cells coincide. Then, the assembly is carried out by gluing the two plates

3 and 3 '.

  FIG. 7b shows the assembly of three LCD1 cells,

LCD2, LCD3.

  Such an assembly will be done for example by following the following method Thinning of the plate 3 of the counter-electrode of the

LCD1 cell.

  To Electrical connection of the active matrix of this LCD1 cell. * Thinning of the plate 3 'of the counter-electrode of the

LCD2 cell.

  Thinning of the plate 1 'of the active matrix of the LCD2 cell. * Positioning and gluing of the LCD2 cell against the LCD1 cell with the plate 1 'of the active matrix LCD2 glued to the

plate 3 of LCD1.

  25. Electrical connection of the active matrix of the LCD2 cell.

  Thinning plate 3 "of the counter-electrode of the

LCD3 cell.

  Positioning and gluing the LCD3 cell against the LCD2 cell.

  * Electrical connection of the LCD3 cell.

  The encapsulation of refractive components (microlenses) or polarizing can be made between the LCD cells to constitute

more elaborate functions.

  Thin counter electrode plate LCD matrices with microlenses such as those depicted in FIGS. 6a, 6b can be used in a three-plane RGB projector (FIG. 8). The postponement or molding of the microlenses near the pixels (approximately 100 μm) makes it possible to center the collimated beam from the source of the light with a large angular field of the order of 10 on either side of the axis. . A gain in luminous efficiency of the order of 1.5 can thus be obtained with the sources and

  LCD cells generally used (Es between 100 and 100 mm2sr).

  The angular field of the LCD matrices: 8a = + na / 2h (FIG. 9).

  Such a configuration has the effect of increasing the aperture required at the lens of the same amount 8c o an additional cost for this component which represents a large part of the cost of the entire projector. Also, to further increase the gain in light efficiency or to reduce the cost, the invention provides to use a matrix of microguides 40 and 41 on each side of the matrix so as to channel the luminous flux.

  in the useful area of the pixel as described in Figure 10.

  The opening of a microguide delimited by the indices n1 and n2 is ao = + n2 arcos (n1 / n2), ie 35 for n. = 1.52 (polyacrylate) and n2 = 1.63

(polysulfone) for example.

  To facilitate the realization of visualization systems projection or direct vision, the thinning technique can also be used to stack 3 LCD cells. It is then necessary to provide a pixel-to-pixel imaging system (at least in the case of projection) with microguides and to insert subtractive polarizers between each matrix (Magenta "M", Cyan "C" and Yellow "J"). "). Figure 11 shows how

  these different elements could be arranged.

  The cells LCD1, LCD2, LCD3 are in "Twisted" or "Parallel X / 2" mode to control the rotation of the plane of polarization with respect to the three subtractive polarizers M, C and J with the voltage applied to each pixel. After crossing the stack, a color image

is obtained.

  A variant of this structure is to use cells

  Successive LCDs to increase gray levels as needed.

  The method for producing such an assembly may be the following: Thinning of the counter electrode plates of the cells

LCD1, LCD2, LCD3.

  Positioning and gluing microguide arrays on

thinned faces.

  At thinning of plates of active matrices of cells

LCD1 to LCD3.

  * Bonding input polarizers (P), Magenta (M), Cyan (C) and

Yellow (J) each to a cell.

  Positioning and gluing the three elements to form the RGB block as shown in Figure 11. The thickness of a

together can be less than 3 mm.

  The invention is applicable in direct display systems with touch-type functions "keyboard" or tablet digitizer. In these systems, the generally large thickness of the glass introduces a parallax defect. Thinning, at a thickness h, of the counterelectrode plate 3 makes it possible to eliminate this defect as the

shows figure 12.

  The parallax 6 defect depends on the angle of incidence i of the

  "pen" and is expressed by the following relation: 6 = h tgr with sini = nsinr.

  For example, for an incidence i = 45 in glass (n = 1.5), the following values can be obtained: hpm I 1 200 5001 00 1 100 6 m 53 107 265 583 The invention is also applicable to the production of a modulator of phase and amplitude by the association of two cells LCD1 and LCD2. It can be used in an optical correlator for example. A two-layer configuration such as that detailed in FIG. 7a is perfectly adapted to this function. Figure 13 shows the directions

  polarizer and liquid crystal cells.

  The generation of a coherent image is made from a plane wave incident on the spatial modulator. The thickness h of the counter electrode plates can thus be estimated by evaluating the diffraction of a pixel over the distance 2h. Let "hmax" be the maximum thickness, "a" the side of the useful area of the pixel, the condition on the thickness so that the main diffraction lobe is contained in the following pixel is: hmax, = na2 / 4X or hma , = 400 pm for a = 30pm and = 0.85 pm, or hmax = 100 pm for a = 15 pm at the same x For better coupling between the two LCD cells, a matrix of microlenses can be placed at the interface to provide pixel-to-pixel imagery. These microlenses must then consist of a medium of index n2 greater than the index n of the glass and have a focal length f

  such that h = 2f to image a pixel of LCD1 on LCD2.

  The liquid crystal of the LCD1 cell serving as a phase modulator is aligned parallel to the polarization direction of the polarizer P. For the LCD2 cell serving as an amplitude modulator, the liquid crystal is parallel (twisted nematic) or at 45 ( electrically controlled birefringence) of the polarization direction of the polarizer P. This mainly concerns the phase or amplitude apodization of the beams, their 1D or 2D deflection and the two-dimensional wavefront correction. For apodization, the configuration described in FIG. 13 may be suitable as it is but may also be supplemented with a third amplitude-modulating LCD cell to increase the number of gray levels to 10 or 12 bits, for example, knowing that a matrix LCD is limited to 8 bits. For the deflection function, a blased phase structure, ramp 0-2it at N levels allows efficiency lIlo = sin2 (n / N) / (7c / N) 2. To obtain N levels, only M liquid crystal layers are needed if the phase levels of each layer are in geometric progression of reason 2, the relation between N and M is N = 2M. The table below shows how efficiency changes with the number of layers:

M 2 3 4 5

N 4 8 16 25

  1 / 10o 81% 95% 98.7% 99.5% Thus, the phase variations for a stack with 3 layers for example are respectively 7, t / 2 and n / 4. With an operating wavelength at x = 1.06 μm and a liquid crystal of An = 0.1, it is necessary to

  practice thicknesses of 5 pm, 3 pm and 2 pm respectively.

  The spacing between the different liquid crystal layers is as previously governed by the diffraction law: hma ,, = na2 / 4; for example, hm ,, ax = 320 pm for a = 30 pm and X = 1.06 pm. A thickness of 150

pm is therefore very well adapted.

  The deflection angle of such a component is cc = k X / A for a laser beam at the wavelength X and a period 0-2 kt for the length A. With k = 2 (order 2), X = 1.06 μm, a period A of three pixels at a pitch of 40 μm or 120 μm, the angle ax is 1. The number of points depends on the number of pixels addressed, a 1000x1000 bidirectional deviation with a typical active matrix. The response time with a nematic structure is around 1 Oms (imposed by the thickest layer, 5 pm for example). Figures 14a and 14b show such devices

  derivatives of the embodiments described above.

  The variants of this structure are the following Use of ferroelectric liquid crystals to reduce the response time to less than 100 ps but it is necessary in this case to use an electrode comb addressing. The two deviation axes can be obtained but by multiplying the

number of layers per 2.

  * Initial doubled structure to process unpolarized beams. * The structure of Figure 14a is simpler to achieve because it does not use matrices microguides but its assembly process is more complex. Efficiency is limited by the opening rate of the active matrix. This structure is better

  suitable for comb control of transparent electrodes.

  * A transverse electric field control can be used advantageously in the UV or IR bands where it is

  difficult to make electrodes sufficiently transparent.

  The structure of Figure 14b is preferable in this case for

  do not overly penalize the opening by the metal electrodes.

  The UV band between 300 and 365 nm is still widely used in the field of microelectronics and photolithography in general. The definitions targeted with this spectral range are of the order of 0.5 μm. The possibility of thinning the counter-electrode blade of an LCD matrix opens up very interesting possibilities for placing the microlenses very close to the pixels (see FIGS. 15a and 15b). This results in extremely small focal tasks. A microlens of diameter OD = 40 μm, for example placed at the distance h = 100 μm, corresponds to a diffraction spot of + 2.5 X, ie less than 1 μm. Under the same conditions, a divergence beam + 1 generates a focal task of only + 1.7 μm. It is therefore conceivable to place in front of each microlens a diaphragm of 10 diameter 4 μm without losing too much luminous flux. Such an electro-optic modulator used in place of a mask in a 10X UV photorépéteur then offers an over-resolution factor of 10x10 for a pixel pitch on the 40 μm LCD matrix. It is thus possible to inscribe on the sample to be irradiated spot less than 0.5 pm at a rate of 100 million per mechanical position if the matrix has a million pixels and piezoelectric actuators 40 pm stroke. Figure 15 details the configuration of such an LCD modulator controlled by two piezoelectric actuators (Sx, 8y). The invention is also applicable in the card technique

  integrated circuits also called smart cards.

  The possibility of making thinned liquid crystal cells

  allows to design a smart card equipped with a liquid crystal display.

  Thus, as shown in FIGS. 16a and 16b, an LCD liquid crystal cell whose plates carrying the electrodes have been thinned, is glued on the CA card or molded in the card. CO electrical connections are then made to connect the LCD cell to the electronic circuits of the PU chip themselves connected to a power source (battery polymer film ...) SE. Information contained in the electronic circuits of the chip can therefore be read according to the use that one wishes to make. A battery, such as a thin-layer battery,

  can be provided and allows autonomous operation of the display.

Claims (19)

  1. A method of producing a liquid crystal screen comprising steps for producing a cell comprising two plates (1, 3) enclosing a liquid crystal (2), the faces of the plates in contact with the liquid crystal being provided with electrodes, characterized in that it then comprises a step of thinning at least one of said plates (1 or 3). 2. Method according to claim 1, characterized in that the thinning step is done by placing the cell between two plates (4, 5), at least one of which comprises an abrasive surface, and then pressurized between the two trays and relative displacement of the two trays   parallel to each other.   3. Method according to claim 2, characterized in that before placing the cell between the two plates, at least one of the plates   of the cell is the object of a grinding machining operation.   4. Method according to claim 2, characterized in that the surfaces (15, 18) of the two trays in contact with the cell are abrasive.   5. Method according to one of claims 2 or 4, characterized in   that before placement of the cell between the two plates, a wear part (7) is glued to one of the two plates to protect against wear.   6. Method according to one of claims 2 or 4, characterized in   what is expected the thinning of a plate of a first cell (LCD1), then the thinning of a plate of a second cell (LCD2)   then gluing the two cells by their thinned plates.   7. Method according to claim 6, characterized in that it then provides: - the thinning of the second plate of one of the cells; Thinning a plate of a third cell (LCD3); the bonding of the third cell, by its thinned plate, to the second thinned plate.   8. Method according to one of claims 2 or 4, characterized in   what is expected is the thinning of a plate of at least a first, a second and a third cell (LCD1 to LCD3) then the stacking and the bonding said cells.   9. Method according to one of claims 2 or 4, characterized in   it provides a step of producing lenses (20) on at least one of the thinned plates of a cell (LCD). 10. Process according to claim 9, characterized in that   each pixel of the cell (LCD) corresponds to a lens.   11. Method according to one of claims 2 or 4, characterized in   at one of the thinned plates of a cell (LCD) is contiguous a 1o guide plate (40, 41) having areas of different indices and   to guide the light to the pixels of the cell.   12. Method according to claim 11, characterized in that several sets of cells (LCD1, LCD2, LCD3) and plates of   guide (40, 41) are contiguous to each other.   13. Method according to one of claims 8 or 12, characterized   in that before stacking said cells (LCD1 to LCD3) a filter of   particular color (M, C, J) is associated with each cell.   14. Method according to one of claims 2 or 4, characterized in   it comprises a step of bonding or molding said cell on or in a card comprising electronic circuits (PU) and connection   from the cell to the electronic circuits energy source (SE).   15. Liquid crystal cell comprising two blades (1, 3) enclosing a liquid crystal (2), the faces of the blades in contact with the liquid crystal carrying electrodes, characterized in that the thickness of one   at least two blades is of the order or less than 200 μm.   16. A liquid crystal cell according to claim 15, comprising a plurality of stacked liquid crystal cells of which at least one blade of each cell is thinned, a color filter being associated with each cell. 17. A liquid crystal cell according to claim 15, characterized in that it comprises a plurality of stacked cells as well as means for independently electrically controlling the different cells so as to obtain an improvement of the deflection angle performance, and the number of gray levels.   18. Smart card comprising an electronic circuit, characterized in that it comprises a cell according to claim 15, glued to one of the faces of the card or molded in the card, electrical connections   connecting the cell to the electronic circuits.   19. A liquid crystal cell according to claim 15, characterized in that it comprises a masking plate contiguous to the blade thickness less than 500 pm, the assembly being intended for microlithography.