The present invention belongs to the field of the fabrication of liquid crystal cells, on a silicon substrate, according to a technology generally designated by the acronym LCOS (Liquid Crystal On Silicon). It relates more especially to a process for fabricating such cells according to collective methods. In a known manner, a collective method of fabricating liquid crystal cells on a silicon substrate comprises the obtaining of a silicon wafer, on which has been formed a plurality of active matrix circuits, these circuits being disposed on the wafer according to a substantially orthogonal array.
The collective method then comprises the following steps, well known to the person skilled in the art:
The collective assembling of the silicon and glass supports requires that the respective sizes of the individual substrates be the same on both supports. In
Represented in
The silicon substrate 5 comprises an active zone 10 and a peripheral zone comprising a connection zone 11. The active zone 10 is situated inside the zone delimited by the sealing frame 7 and comprises the matrix of pixel elements. The connection zone 11 is situated outside the sealing frame and comprises contact pads (Pi).
The glass substrate 6 comprises a back electrode pattern 12, which defines a window through which the matrix of pixel elements is viewed. The former is positioned with respect to the silicon substrate in such a way as to clear the connection zone 11, in such a fashion as to allow the connection of the cell to a control device 13 of a display system, for example by means of a flexible printed circuit 14. Provision is generally made for the back electrode 12 to overhang the frame, in a zone 12a of the transparent substrate 6 overhanging with respect to the silicon substrate, making it possible to connect the back electrode to a control device of the cell, for example by means of a flexible printed circuit.
Each active matrix circuit is formed in a subdivision of the silicon wafer, that is generally called a “die” in the literature. As represented in
Each subdivision (or individual substrate, or active matrix circuit) comprises an active zone ZA, with the pixel elements, and a peripheral zone ZP, around the active zone, which comprises contact pads, P1, P2, P3, P4. These contact pads are situated in one and the same connection zone 101, in the example, on the upper horizontal edge. These pads are intended to receive the matrix addressing signals provided by an external control device 13 of the cell, for example by means of a flexible printed circuit 14 (
A sealing frame 102 is disposed around the active zone ZA. This frame, not completely closed, allows the assembly with a substrate carrying the back electrode, and the formation of a cavity between the two substrates, so as to receive the liquid crystals. In a known manner, after introduction of the liquid crystals, the opening 103 produced in the frame is closed.
The design rules devised to take account, in particular, of the tolerances on the fabrication equipment used (accuracies, alignments), impose certain minimum dimensions. For a given type of display screen, the dimensions of the corresponding active matrix circuit are known. For example, for HDTV type applications, the definition of the cell is 1920 pixels (horizontal) by 1080 pixels (vertical). With an on-silicon technology which gives a pixel area of 10×10 μm2, one has a corresponding cell active zone area of the order of 207 mm2. This is the functional area of the active matrix circuit. Around this zone there is a peripheral, nonfunctional zone, whose dimensions depend on the design rules, determined so as to have high reliability of fabrication, while taking account of the problems of tolerances of alignment, of thickness of deposition of adhesive seal (sealing frame) according to the technique employed (screen printing, syringe or dispenser), of thickness of the cuts, and the like. These design rules translate into minimum dimensions to be complied with, which condition the pitch of the placement array for locating the circuits on the silicon wafer.
More precisely, and referring to
Returning to the example of a liquid crystal cell for HDTV type applications, it was seen that the cell active zone area is of the order of 207 mm2, this corresponding to the share of functional area of the silicon subdivision.
The nonfunctional silicon area, corresponding to the peripheral zone ZP, and which is related to the dimensions c1 to c4, represents an area of the order of 151 mm2, i.e. 42% of the total area of silicon, when taking c1=c2=0.7 mm, c3=0.5 mm and c4=1 mm.
Thus, the proportion of the nonfunctional zones in the silicon with respect to the functional zones is fairly high.
Now, silicon is an expensive material. If it is possible to reduce the proportion of nonfunctional zones, to the benefit of the functional zones in the silicon, the cost of the liquid crystal cells emanating from this technology is significantly lowered.
An object of the invention is to reduce the proportion of the nonfunctional zones of the silicon substrates in liquid crystal cells, so as to obtain a reduction in the cost of fabricating these cells.
The idea on which the invention is based is to relocate the zone of connection of the active matrix circuit onto the glass substrate. The constraint related to compliance with the fourth dimension c4 can then be dispensed with. It is then possible to reduce the silicon area necessary for each cell.
More precisely, according to the invention, the sealing frame is disposed on each active matrix circuit of the wafer, so that a portion of the frame overlaps the contact pads. The frame comprises a seal and conducting elements disposed in the seal. These conducting elements ensure electrical continuity of the contact pads on the matrix with corresponding connection means made on the transparent support. These conducting elements are also spacers (shims) which guarantee the spacing between the two substrates.
Such as characterized, the invention therefore relates to a method of fabricating a plurality of individual liquid crystal cells, each comprising a first substrate comprising a back electrode and a second active matrix substrate, which are assembled with a sealing frame producing a cavity between the two substrates for liquid crystals, the first substrates being formed collectively on a transparent support, the second substrates being formed collectively on a silicon wafer, and comprising contact pads. According to the invention
A minimum silicon area is thus obtained, which makes it possible to gain around 10% of silicon area on a wafer. This is space freed up for the production of additional circuits on a silicon wafer. For example, on a wafer on which 15 rows of 9 active matrix circuits, i.e. 135 circuits, will be produced, one row of circuits will be gained, i.e. 9 circuits in the example.
The invention also relates to a liquid crystal cell, with a glass substrate carrying the back electrode and a silicon substrate comprising an active matrix circuit. According to the invention, the second substrate has a cutout corresponding to the contour of the sealing frame. The cell comprises means of connection of the active matrix circuit that are relocated onto the glass substrate, and overhang with respect to the silicon substrate, and a sealing frame which assembles the two substrates overlapping the contact pads of the silicon substrate and a portion of the relocated connection means and comprising a seal and conducting elements disposed in the seal.
Other advantages and characteristics of the invention will become more clearly apparent on reading the description which follows, offered by way of nonlimiting indication of the invention and with reference to the appended drawings, in which:
a diagrammatically represents a placement array for locating active matrix circuits on a silicon wafer;
b represents a grid square of a corresponding array of back electrode circuits on a transparent support;
a diagrammatically represents a placement array for locating active matrix circuits on a silicon wafer according to the invention;
b represents a grid square of a corresponding array of back electrode circuits on a transparent support;
a to 7d illustrate various modes of embodiment of the conducting elements according to the invention.
A liquid crystal cell obtained by applying the principle of fabrication according to the invention is illustrated in
Compared with a cell of the state of the art as represented in
The sealing frame 7 is disposed in such a way as to overlap the contact pads Pi of the active matrix circuit, on the silicon substrate and a portion P′i opposite of the connection means 20 relocated onto the transparent substrate 6. The sealing frame is made from a seal material, such as silicone gel for example. Conducting elements 7a are disposed in the seal. Various processes for making these conducting elements may be used, and will be detailed later. Through these conducting elements 7a, the electrical continuity is ensured between each contact pad Pi of the active matrix circuit and a corresponding element P′i of the relocated means of connection 20. Through these conducting elements 7a, the spacing between the two substrates is also defined: these conducting elements are also spacers.
In a general manner, it may be noted that spacers E are generally provided over the whole perimeter of the frame. According to the invention, in the connection zones, these spacers are then embodied by the conducting elements 7a. Elsewhere may be disposed the spacers customarily used, such as balls or fibers of silica. However, elements of the same nature as the conducting elements 7a may equally well be used as spacers E.
By using connection means relocated onto the transparent substrate according to the principle of the invention, the silicon substrates can be cut along cutting lines which follow the contour of the frame, while complying with the design rules. This is what is represented in
Stated otherwise, if x1 and y1 denote the distance separating two adjacent vertical and horizontal cutting lines, in the array of
As far as the transparent support is concerned, and as represented in
The disposition of the conducting elements 7a in the seal of the sealing frame 7 (
Thus, in a cell according to the invention, the control signals of the circuits placed on the silicon substrate travel exclusively through the transparent substrate, across the opposed contact zones, linked by the conducting elements of the sealing seal.
The invention furthermore makes it possible to dispose contact pads optionally on several edges, this perhaps being beneficial for the design of the active matrix circuit itself, for the disposition of the conducting lines with respect to the active elements. It is thus possible to dispose contact pads Pi on an edge, and tags Pj on another edge. Such is the case for the cell represented in
With relocated connection means, and a sealing frame overlapping the contact pads of the active matrix circuit on the silicon substrate, the nonfunctional zone of the silicon substrate may be reduced. Preferably, the cutting lines therefore correspond to the contour of the sealing frame, to within the design constraints (c3).
As may be seen in
The method of fabrication therefore comprises a step of cutting the silicon wafer into active matrix individual substrates 5 and the transferring and the assembling of each of these silicon substrates onto a corresponding transparent substrate.
Before transfer and assembly, a layer of polyimide is deposited and then rubbed away on the circuits of the transparent substrate and on each of the individual silicon substrates, on the active matrix circuit, and this will allow the alignment of the liquid crystals which will be injected, in the microstriations thus formed.
After cutting of the silicon substrates, and assembly onto the transparent support, with a corresponding transparent substrate, the glass support can thereafter be cut according to the customary techniques. The liquid crystal is introduced according to any known method, then the openings in the frames are plugged. The individual liquid crystal cells are obtained.
In practice, a reduction in the silicon area of each active matrix substrate is obtained, which makes it possible to produce around 10% of extra circuits on each wafer. The costs of fabricating the liquid crystal cells are thus reduced.
Represented in
In a first embodiment represented in
When the two substrates are assembled the one to the other, the silicone gel, or the adhesive which forms the material of the seal 30 is pressed, so that the ball comes directly into contact on each side on the substrates.
The diameter of the balls thus defines the gap between the two assembled substrates, that is to say the size of the cavity.
For cells with a small gap, less than 2 microns, the use of balls as conducting elements and as spacers is no longer suitable.
Represented in
Preferably, these tags will be made on the silicon substrate, on the contact pads, by any suitable technique (photoetching). The seal may be deposited thereafter, on the silicon substrate, overlapping these tags, or on the transparent substrate. As indicated previously, when the two substrates are assembled the one to the other, the silicone gel, or the adhesive which forms the material of the seal is pressed, so that the conducting tag comes directly into contact on each side on the substrates.
Another embodiment is represented in
In
In
Here again, and in the two variant embodiments of the resin tag, when the two substrates are assembled the one to the other, the silicone gel, or the adhesive which forms the material of the seal is pressed, so that the resin tag furnished with its conducting layer comes directly into contact on each side on the substrates.
In all cases, the conducting elements 7a which ensure electrical continuity between the contact pads of the silicon substrate and the connection means relocated onto the transparent substrate, also ensure the function of spacers: they fix the gap between the two substrates, and hence the gap of the cavity.
In the other parts of the frame which do not overlap connection zones, there are also spacers E (
Finally, it will be noted that each transparent substrate will have a suitable shape after cutting, allowing connection of the back electrode according to any known technique.
Number | Date | Country | Kind |
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0216360 | Dec 2002 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP03/50944 | 12/4/2003 | WO | 6/20/2005 |