Process and apparatus for applying orienting layers to a substrate for alignment of liquid crystal molecules

A process of making an orienting layer on a substrate for aligning liquid crystal molecules, especially of a large-scale liquid crystal display, includes moving the substrate (14) at a distance of 50 to 100 mm from a target (11) past the target in a substrate motion direction (15) and providing the target with a length l in a direction perpendicular to the substrate motion direction and parallel to the substrate surface (S) at least equal to the width s of the substrate (14); sputtering material from the target (11) onto the substrate surface (S) by forming a gas plasma in a vacuum chamber containing the target; and orienting the target (14) relative to the substrate so that the cathode surface (CS) of the target is inclined at a surface inclination angle a of from 60.degree. to 85.degree. to the substrate surface so that the material is sputtered obliquely onto the substrate surface to form the orienting layer. Another target (12) may be provided for forming an insulating layer prior to forming the orienting layer

BACKGROUND OF THE INVENTION 
The present invention relates to a process and apparatus for applying 
orienting layers to a substrate for alignment of liquid crystal molecules, 
especially for large-scale liquid crystal displays by sputtering materials 
which tend to grow prismatic crystals on the substrate by plasma 
deposition. 
The orientation of liquid crystal molecules of a display cell in a 
predetermined direction is crucial for operation of predominantly liquid 
crystal displays, especially of twisted nematic liquid crystal 
displays(TN-LCDs) or also ferroelectric liquid crystal displays(FLCDs). 
Because of this orientation of the "liquid crystals" their optical 
anisotropy can produce a macroscopically observable effect. This 
orientation generally occurs by application of an anisotropically 
orienting layer to the inner surfaces of each display substrate, before it 
is glued to a display comprising a covering and base substrate. 
Subsequently the liquid crystal is filled into the display. The orienting 
layers cause a recrystallization of the liquid crystal layers in a 
predetermined alignment. 
Several different processes are already known for making these orienting 
layers, of which the most important are described here briefly with their 
advantages and disadvantages. SiO.sub.2, for example from an electron beam 
vapor source, can be deposited as an orienting layer in a vacuum chamber 
on the inner substrate surface of a display at a very small angle of 
inclination of about 5.degree. to the substrate surface. During growth of 
the SiO.sub.2 layer prismatic crystals obliquely inclined in the direction 
of the vapor source on whose surfaces the liquid crystal molecules are 
deposited are produced. Because of the strongly anisotropic orientation of 
the oblique prismatic crystals a very impressive and homogeneous 
orientation results. The inclination angle of the prismatic crystals can 
be influenced to a certain extent by the vapor deposition parameters and 
thus also the so-called "edge tilt angle" of the liquid crystal molecules 
can be varied, which has a decisive effect on the switching properties of 
the display. Further, the manufacturing process for the orienting layers 
and thus the orientation properties are very reproducible by this vapor 
deposition. Furthermore, the orientation of the liquid crystals is very 
uniform. The SiO.sub.2 -layer is chemically very stable and sensitive to 
UV-light and high temperatures. Also the liquid crystal does not combine 
chemically with the SiO.sub.2 layer and can not change, i.e. no or minor 
and extinguishable formation of transient images occurs (image sticking). 
Because of the comparatively large surface area and hydrophilic character 
of the SiO.sub.2 -prismatic crystals, on introduction of the liquid 
crystals into the display frequently a chromatic separation of the 
individual components of the liquid crystal mixture occurs, whereby the 
switching behavior of the display in the separated regions is changed. An 
additional disadvantage of this process is that the vapor deposition 
sources are usually point sources and thus deposition on a comparatively 
large surface in a uniform manner is not possible. The alignment of the 
prismatic crystals varies in the individual substrate regions, since the 
molecules issue or impinge on the substrate surface at different angles 
from the point source. The inhomogeneity of course can be reduced in one 
direction by arranging several vapor deposition sources in a line. Because 
of that, however, the vapor deposition performance of the individual 
sources must be carefully accurately regulated. Moreover a very large 
apparatus expense, and especially a comparatively large vacuum processing 
chamber with associated large scale pumps and air locks, are required, 
which can only be provided by a very costly special manufacturing process. 
These processes have only a very limited applicability to the making of 
large scale displays. 
The most widely used methods for making of the orienting layers in the 
industrial manufacture of liquid crystal displays utilize organic layers 
such as polyimides, polyvinylalcohols or other plastic films which are 
flung or centrifuged on the substrates in a liquid phase. After a suitable 
drying and hardening process this plastic film which is a few nanometers 
thick is rubbed or ground with a carbon fiber or velvet brush in one 
direction. Because of that, microscopic comparatively small tracks or 
tears are produced in the layer and an orientation of the organic 
molecules and/or molecular chains occurs. The resulting micro-mechanical 
and/or molecular anisotropy of the film causes an orientation of the 
liquid crystals later filled into the display. The advantage of this 
process is that the size of the substrate surface plays no role or has no 
effect. This process is also comparatively economical; however it is only 
reproducible to a limited extent. The result of the orientation of the 
molecules depends on a very large number of process parameters such as the 
rubbing pressure of the velvet or carbon fiber brushes, rubbing strength, 
polymerization, and crystallization degree of the plastic film and its 
chemical properties as well as the surface reactions of the film with 
water from the air or from solvents. These process parameters can be 
optimized only with difficulty. Furthermore, the plastic layers are only 
comparatively poorly stable relative to temperature changes or UV light. 
With ferroelectric liquid crystals there is also the disadvantage that the 
organic molecules of the orienting film combine vigorously chemically with 
the organic liquid crystal molecules and thus are rotated with the liquid 
crystal molecules on application of an electric field or hold them in 
their position and thus make the required rotation of molecules difficult. 
The result is a so-called "burning-in" of written-in or stored images 
("image sticking") or a monostable switching behavior of the normally 
bistable switching ferroelectric liquid crystal. 
Besides the brushing or rubbing of plastic film an orientation of polymer 
films with the aid of linearly polarized UV-light is also known. The 
polymerization is direction-dependent so as to provide the required 
anisotropy because of the direction-dependence of the polarized light. The 
production of microstructure in a plastic layer by impression or stamping 
techniques or photolithography has already been tried. The disadvantage of 
comparatively poor long-term stability of the plastic layers relative to 
temperature changes and UV light exposure and the chemical reactivity of 
the plastic molecules with the liquid crystal molecules however cannot be 
eliminated by this process. 
Also, already there have been sporadic suggestions regarding the 
possibility of an inclined sputtering or deposition of material with low 
pressure plasmas, which lead to prismatic growth. Aluminum nitride is a 
particularly suitable material for this type of deposition and leads to 
prismatic growth in a nitrogen atmosphere when the deposition is at 
comparatively large impingement angles to the substrate surface. However, 
when a comparatively large substrate surface must be sputtered from a 
point-like target difficulties which are similar to those encountered with 
vapor deposition result. Indeed large-surface area targets can be used 
here, whereby comparatively fewer inhomogeneities result because the 
sputtering processes are more diffuse in comparison to vapor deposition. 
However, these methods also simultaneously reduce the directional 
anisotropy required for the orientation of the prismatic crystals. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process of sputtering 
an orienting layer on a substrate for liquid crystal molecules, which has 
a more uniform anisotropic orientation and a comparatively higher chemical 
and UV stability than orienting layers made by currently known processes, 
especially for large-scale substrates. 
It is another object of the present invention to provide an apparatus for 
performing the aforementioned process. 
According to the present invention, the process of forming an orienting 
layer on a substrate for aligning liquid crystal molecules, especially of 
a large-scale liquid crystal display, comprises the steps of: 
a) moving the substrate closely past at least one target in a substrate 
motion direction and providing the at least one target with a length in a 
direction perpendicular to the substrate motion direction and parallel to 
the substrate surface at least equal to the width of the substrate; 
b) sputtering material from the at least one target onto the substrate 
surface by forming a gas plasma in a vacuum chamber containing the at 
least one target; and 
c) orienting the at least one target relative to the substrate so that a 
cathode surface of the at least one target is inclined at a surface 
inclination angle to the substrate surface such that the material is 
sputtered obliquely onto the substrate surface from the target. 
The apparatus for performing the process according to the invention 
includes a vacuum chamber in which the at least one target is arranged and 
a palette on which the substrate is moved past the at least one target. 
The cathode surface is advantageously inclined to a considerable extent in 
relation to the substrate surface. 
For large-scale glass substrates for displays so-called vertical in-line 
sputtering devices for sputtering on thin layers are currently used. In 
this type of apparatus, the large glass substrate is suspended on a 
vertical palette, with which it is moved in a horizontal direction 
comparatively closely past a target extending in a vertical direction 
during the sputtering process in a vacuum chamber. The sputtering plasma 
is present between the target cathode and the palette and/or the shields 
of the target. A uniform layer growth may be achieved over the entire 
substrate surface in this device. The uniformity in the motion direction 
of the palette is achieved by the uniform motion of the substrate relative 
to the cathode over the entire substrate width. The homogeneity in the 
direction vertical or perpendicular to the motion direction of the 
substrate is achieved by the longitudinal extent over the entire substrate 
height. Furthermore, shielding baffles at the ends of the target can be 
provided which eventually compensate for the eventual sputtering 
inhomogeneities in this region. 
In the apparatus according to the invention, the target surface is inclined 
relative to the substrate surface at an angle which is advantageously 
between 60.degree. and 85.degree.. The apparatus according to the 
invention may therefore be formed by modifying a currently known 
in-line-sputtering apparatus without extensive rebuilding and 
reengineering. Thus orienting layers can be sputtered on substrates with a 
comparatively high surface layer uniformity, also over a comparatively 
large surface area. 
In preferred embodiments of the apparatus according to the invention, the 
spacing between the substrate and the target is comparatively small, so 
that the volume of the vacuum chamber can be small. Advantageously, the 
substrate can be spaced from 50 to 100 mm from the target as it is moved 
by it. This corresponds to from one to three times the mean free path of 
the molecules in the process gas so that the sputtered-on molecules do not 
lose their motion direction because of random collisions prior to reaching 
the substrate surface. The target cathode can be suitably shielded by 
shields to prevent plasma formation outside of the target surface and to 
limit the sputtering or deposition direction to the predetermined angular 
range on the substrate surface. The grown crystal layer has a well-defined 
orientation because of the atoms arriving from one side, i.e. a crystal 
lattice is formed so that it is oriented in a single direction. This 
directional anisotropy of the oriented layer later directs the liquid 
crystals inside of a completed and filled display in the desired way. The 
rate of deposition or sputtering is sufficient in spite of the narrow 
impingement angle. Furthermore, a lameliar-like shield can be arranged in 
front of the target, which catches the molecules which are inclined too 
much or too little and which would affect the orientation of the growing 
crystal lattice in an undesirable manner. Further, a magnetic field can be 
produced in front of the cathode surface of the target whose circular 
field lines extend in planes perpendicular to the surface of the cathode 
and parallel to the motion direction of the substrate. Because of that, in 
a known way the rate of sputtering can be increased while maintaining 
plasma power at the same level. Because the field lines have no components 
along the target, the orientation of the growing crystal lattice in the 
motion direction of the substrate is additionally aided. In the method 
according to the invention, high frequency alternating current plasmas may 
be produced which allow a deposition or sputtering of nonconducting and 
transparent materials such as quartz(SiO.sub.2) on the substrate, as these 
have particularly desirable properties for the orienting layer. 
Alternatively, metal oxide or metal nitride layers can be sputtered on the 
substrate by forming a direct current plasma and by adding to the plasma a 
reactive gas such as oxygen or nitrogen (reactive sputtering of oxides, 
nitrides or the like). The thin quartz layers with well-defined crystal 
orientation have a particularly good orienting effect especially with 
ferroelectric liquid crystal displays with cell thickness of about 1.5 
micrometers. This type of quartz layer has a very good long-term stability 
and resistance to UV radiation. By selection of sputtering parameters such 
as pressure, temperature, sputtering power, and angle of impingement of 
the sputtered material, as well as selection of the reactive gas 
components, the layer properties can be varied over a wide range in a 
predetermined way. 
Additional parallel sputtering targets can also be present in the apparatus 
according to the invention so as to provide several different layers prior 
to application of the orienting layer on the substrate. Particularly, an 
insulating layer made from a sputtered-on transparent dielectric is 
usually necessary on the image point electrodes because of the short 
circuiting probability between the rows and columns due to the required 
small cell spacing, especially in large-scale ferroelectric liquid crystal 
displays. In an apparatus with multiple targets, the insulating layer can 
be applied first in one machine cycle and then the orienting layer can be 
sputtered on, so that machine and process costs can be reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1 and 2 show a vacuum chamber 10 with a rectangular transverse 
cross-section with two targets 11 and 12. A substrate 14 arranged 
horizontally on a palette 13 is moved past these targets 11 and 12 in the 
direction of the arrow 15. Both targets 11 and 12 are rectangular, as 
shown in FIG. 2 and have a length l which is at least equal to the width s 
of the substrate 14 in a direction perpendicular to its direction of 
motion 15 and parallel to the substrate surface, S'. The cathode surface 
CS of the target 11 is inclined to a substantial extent at an inclination 
angle a relative to the surface S' of the substrate 14. A plasma 16 is 
formed in front of the cathode surface CS of the target 11, from which 
particles arrive on the surface S' of the substrate 14, as shown in FIG. 1 
by the arrows 17. The sides of the target 11 are shielded by the shields 
18 and 19 so that no plasma can form laterally to the target cathode 11. 
The shields 18 and 19 form an outlet for the particles 17 which is narrow 
enough so that particles impinge at the desired predetermined narrow 
angles on the surface S' of the substrate 14. Because of the narrow 
impingement angle range and because of the length l of the target 11, an 
extremely uniform orienting layer with anisotropic alignment properties is 
distributed uniformly over the entire substrate surface S' and is produced 
by a uniform motion of the substrate 14 past the target 11. A second 
target 12 with a noninclined target cathode surface can be used to make, 
e.g., an insulating layer prior to application of the orienting layer on 
the substrate. 
Thus two layers may be sputtered one after the other on the substrate 14 
during cyclic operation of the apparatus shown in FIGS. 1 and 2. The 
apparatus shown in FIGS. 1 and 2 may be obtained by modifying a commercial 
in-line-sputtering apparatus with two parallel targets of the same type as 
target 12 with horizontal sputtering devices. One of the targets is then 
replaced with a target with cathodes inclined to a substantial extent and 
provided with suitable shields. 
FIG. 3 is a detailed cross-sectional view of a target 11', in front of 
which a magnetic field is provided, whose circular field lines 20 extend 
in a plane parallel to the drawing plane and which, because of that, 
assists in the orientation of growing prismatic crystals. These field 
lines extend up to the vicinity of the substrate 14'. 
By "closely passing" in relation to the motion of the substrate past the 
targets is meant at a distance so the desired orienting layer is deposited 
with the desired oblique impingement of the plasma material on the 
surface. 
In the embodiment shown in the drawing the distance d between the substrate 
surface S' and the target 11 can be from 50 to 100 mm and the surface 
inclination angle a can be from 60.degree. to 85.degree.. 
The targets 11 and 12 can each comprise a target plate. 
While the invention has been illustrated and described as embodied in a 
process and apparatus for applying orienting layers to a substrate for 
alignment of liquid crystal molecules, it is not intended to be limited to 
the details shown, since various modifications and structural changes may 
be made without departing in any way from the spirit of the present 
invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention. 
What is claimed is new and desired to be protected by Letters Patent is set 
forth in the appended claims.