Lateral MIM device and method of production

A lateral MIM structure for use in liquid crystal and other optical display devices and method of manufacture, which eliminates deviations in element operating characteristics, especially for pixel electrodes, by utilizing top edges of the input signal conductor as active MIM element areas in combination with side conductor surfaces. The lateral MIM structure is made by electrically contouring insulation formed on a first conductor to have a lower resistance where it covers at least one side surface and top edge to allow MIM electrical interactions and than where it covers the remaining top surface so as to substantially prevent electrical interactions over this surface. A second conductor is then deposited on the insulation layer. This is accomplished by forming one insulation material on the top of the input conductor using a positive photoresist and etch-back procedure, followed by forming a second insulation layer over the first and the conductor side surfaces and top edges. Alternatively, the first insulation material is etched using a negative photoresist and etch-back, followed by forming the second insulation material only on top of the first conductor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to liquid crystal display devices and more 
particularly to MIM type nonlinear pixel driving elements and a method of 
manufacturing lateral MIM elements with substantially uniform operating 
characteristics. The invention further relates to a pixel driver element 
structure that provides improved operating characteristics in liquid 
crystal display devices. 
2. Description of Related Art 
Metal-insulator-metal (MIM) diodes are a common type of nonlinear element 
used to provide or control the driving voltage for signal conductors 
corresponding to individual pixels in liquid crystal displays and similar 
optical devices. A top view of a typical metal-insulator-metal (MIM) 
element is illustrated in FIG. 4 where it is shown being formed on a 
transparent element substrate 27 as a pixel driver element. A cross 
section taken along line I--I' extending through the same MIM element is 
shown in FIG. 5. Both of these figures show the use of a lateral (side 
surface) MIM structure in which only the side surface of an input signal 
conductor or electrode 21 is utilized as an input portion of the active 
pixel driver or control element. The MIM element is shown having a single 
output conductor 25 connected to one pixel electrode or conductor 26. 
In forming conventional lateral MIM elements, such as illustrated in FIG. 
4, all surfaces except the side surface 22 of the first signal conductor 
21, i.e., where the MIM diode conduction occurs, are covered with a layer 
of insulating material to provide an electrical barrier layer 23. The 
electrical resistance of the barrier layer 23 is made sufficiently large 
so that it does not allow any other portion of the conductor 21 to 
function as part of the MIM diode. A lower resistance, typically thinner, 
insulating layer 24 is disposed on the side surface of the conductor 21 
where the active MIM element is to be formed. 
As seen in FIG. 5, material forming a second, output signal, conductor 25 
for the nonlinear element is disposed on top of the insulating layers 24 
and 23, and connects to the corresponding pixel electrode 26. A MIM diode 
is formed in the region where the first conductor 21, the insulator 24, 
and the second conductor 25 overlap and becomes the pixel voltage driving 
element for the pixel electrode 26 for one pixel of an associated display 
device. An array of such MIM driver elements is generally formed across 
the surface of the transparent element substrate 27 to achieve the desired 
optical display size and control. 
This type of MIM structure, using known manufacturing techniques, allows 
the surface area of the MIM driver element to be made extremely small. 
This has proven to be an effective technology for increasing the density 
of pixel drivers and electrodes, and the manufacturing precision for 
liquid crystal display devices using MIM elements. 
Several materials found useful in manufacturing the conductors for MIM 
elements include tantalum, aluminum, gold, ITO, NiCr+Au, and ITO+Cr, while 
useful insulator materials include TaO.sub.x, SiO.sub.x, SiN.sub.x, 
SiO.sub.x N.sub.y, TaN.sub.x and ZnO.sub.x. The insulating layers are 
generally formed by thermal or anodic oxidation, or by sputtering. In 
addition to inorganic compounds, polyimides and other organic materials 
can also be used in forming insulating barrier layers. 
The most common structure used in manufacturing MIM driver elements is one 
that employs tantalum (Ta) for the first conductor 21, an oxide of 
tantalum (TaO.sub.x) for the insulator 24, and chrome (Cr) for the second 
conductor, resulting in a Ta/TaO.sub.x /Cr element structure. 
However, using the conventional lateral MIM structure and manufacturing 
technique results in deviations in the characteristics of many of the MIM 
driver elements within a given optical display device. Because the lateral 
MIM element is formed on the side surface of the first conductor, where 
overlapped by the insulator 24 and second conductor 25, the surface area 
of the MIM element is proportional to the film thickness of the first 
conductor 21 and the angle formed between the side surface 22 and the 
transparent substrate 27, i.e., the cross sectional shape or taper of the 
side surface. Therefore, the area of the MIM element is very sensitive to 
variations in the film thickness or the side angle of the first conductor. 
Unfortunately, with current production process technology, the film 
thickness is often inconsistently distributed across a substrate which 
results in deviations in the operating characteristics of MIM elements 
distributed across the surface. The inability to precisely control MIM 
element surface area during the production process leads to inadequate 
control of pixel display conditions throughout a liquid crystal or similar 
type display device, that is, in the final product. 
What is needed is a pixel voltage driver assembly using an array of MIM 
elements which provides substantially uniform device characteristics 
across the array, and, thus, across the optical device. It would also be 
beneficial if the MIM elements can be manufactured by a process of minimum 
complexity and in association with a variety of pixel driver applications. 
SUMMARY OF THE INVENTION 
In order to solve the problems encountered in the art, one purpose of the 
present invention is to provide a MIM device having improved 
reproducibility. 
An advantage of the invention is that liquid crystal displays and similar 
optical devices can be produced having increased uniformity in pixel 
element operating characteristics. 
Another purpose of the invention is to provide a liquid crystal display 
with improved operating characteristics and more uniform control response 
across an array of pixels. 
These and other purposes, objects, and advantages are achieved in a lateral 
MIM element for use on a substrate for liquid crystal display devices 
wherein a first conductor, an insulator, and a second conductor are each 
formed by sequential deposition on a transparent substrate. The insulator 
is electrically contoured to have a lower resistance over at least one 
side and top edge than over the remaining, top, first conductor surface, 
so that electrical interaction between the first conductor and any other 
conductors positioned on the second insulator is prevented. This is 
generally accomplished by making the insulator thicker over the top 
surface than on the top edges and sides of the conductor, although 
materials of differing electrical resistance can also be used. 
In a first embodiment, the insulator is made up of a first insulator formed 
on the side surfaces and top edges of the first conductor and a second 
insulator formed on the remaining area of the top surface, with the second 
insulator having a greater resistance, generally by being made 
sufficiently thicker than the first insulator, to prevent electrical 
interaction. The overlapping parts of the first conductor, the first 
insulator and the second conductor form a lateral MIM element for driving 
a pixel electrode in a liquid crystal display or similar optical device. 
In further aspects of the invention, the conductor consists of material 
chosen from the group of Ta, Al, Au, ITO, NiCr+Au, and ITO+Cr, while the 
first and second insulators consist of material chosen from the group of 
TaO.sub.x, SiO.sub.x, SiN.sub.x, SiO.sub.x N.sub.y, TaN.sub.x and 
ZnO.sub.x. In the alternative, the insulators are made from organic 
compounds. In a preferred embodiment, the first conductor is manufactured 
using tantalum film approximately 250 nm thick, the first insulator using 
a TaO.sub.x film about 30-60 nm thick, and the second conductor using 
chrome (Cr), resulting in a Ta/TaO.sub.x /Cr device structure. This 
results in a combined first and second insulator thickness on the order of 
300 nm or more. 
The lateral MIM element of the invention is manufactured by, forming a 
first conductor on a support substrate and then forming and contouring a 
barrier layer or insulation material over the conductor so that insulation 
resistance is appropriately greater on a top surface of the first 
conductor than on the side and top edges. In a preferred embodiment, 
insulation material is deposited over the first conductor and etched back 
using a positive resist material which is exposed from the underside of 
the conductor. This leaves an island of insulation material on a portion 
of the top surface of the conductor. A second layer of, the same or a 
different, insulation material is then deposited over the conductor, and 
the first insulation material. Therefore, the electrical resistance of the 
insulation is contoured by controlling the deposition thickness to be 
thicker on the top surface. Alternatively, if the resist is a negative 
resist, after development and etching the first insulation material will 
only be left on the first conductor sides and top edges, exposing the 
remaining top surface of the conductor. A second insulator or insulation 
material of greater electrical resistance or thickness would then be 
deposited on this remaining top surface and the resist then removed along 
with excess insulation material. 
Other objects and advantages together with a fuller understanding of the 
invention will become apparent and appreciated by referring o the 
following description and claims taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides a new lateral MIM element structure found 
useful as a pixel electrode voltage driver, and a method of manufacturing 
the structure. The invention provides a contoured insulation layer 
structure which improves control over the MIM active element area and 
improves reproducibility of MIM element operating characteristics, 
especially across an array of such elements. 
The overall structure for the pixel voltage driver of the present invention 
is illustrated in the cross-sectional view of FIG. 1, and the individual 
method steps used to manufacture the element are illustrated in FIGS. 2(a) 
through 2(d). 
As shown in the cross section of FIG. 1, a pixel driver element 10 is 
formed on a transparent substrate 1, on which is formed a transparent base 
film. Conductive material is deposited on the substrate and processed to 
form a first conductor 2, having side surfaces 3, upper or top edges 4, 
and a remaining top surface 5, which does not include the top edges 4. A 
material for forming a first insulator 6 is deposited on top of the first 
conductor 2, sides 3 and edges 4, and other desired portions of the 
display device substrate. A second insulator 7 is formed on the top 
surface of the first conductor 2, excluding the edges 4, and a second 
conductor 8 is then formed on top of the two insulators 6 and 7 followed 
by formation of the appropriate pixel electrode 9. Since the second 
insulator 7 is formed on the remaining part 5 of the conductor top 
surface, and is thicker or higher in resistance than the first insulator 
6, only the laminated portion where the first conductor 2, first insulator 
6, and second conductor 8 overlap functions as a nonlinear pixel driver 
element and none of the area covered or overlapped by the second insulator 
7. 
As seen in FIG. 1, the active portion of the MIM structure which makes up 
the pixel driver element 10 is formed on the top edges 4 of the first 
conductor 2 as well as the side surfaces 3. Therefore, the total, 
electrically active, surface area of the MIM element is the combined areas 
of the side surface 3 and the top edge 4, under the electrode or conductor 
8. 
As discussed above, a major problem with prior art MIM elements is 
deviations that occur in the surface area for any side surface 3; that is, 
the areas of the side surfaces 3 tend to vary due to deviations in film 
thickness during the manufacturing process. This is compounded by possible 
variations in the taper angle or shape of the first conductor sides 
relative to the substrate 1. However, it has been discovered that by using 
the MIM element structure of the present invention, the deviations in 
surface area of sides 3 can effectively be absorbed using the additional 
surface area of the upper edges 4. Therefore, as described below, the film 
thickness of the conductor material and any resist used in processing, 
shown in FIG. 2(b) as item 13, as well as the amount of exposure light for 
resist processing need only be optimized. As a result, deviations in the 
total active surface area of the MIM pixel driving element, which is now 
formed from the total of the side surfaces 3 and the top edges 4, are 
effectively decreased. 
The preferred production process used to manufacture an element substrate 
for a liquid crystal display device using the MIM structure of the 
invention is illustrated in the steps of FIG. 2 [2(a)-2(d)]. FIG. 2(a) is 
a cross sectional view showing the transparent substrate 1 after formation 
from a transparent base film of material such as TaO.sub.x on which is 
formed an electrode or conductor 11 from material such as, but not limited 
to, tantalum as the first conductor and an insulating film 12 made from 
material such as TaO.sub.x. The tantalum electrode 11 is generally formed 
by known deposition and photo-etching techniques, and the insulating 
TaO.sub.x film 12 is subsequently formed on top of the electrode 11 as a 
barrier layer by known techniques such as sputtering or chemical vapor 
deposition (CVD). In the illustrated embodiment of FIG. 2(a), the 
TaO.sub.x film 12 is also positioned on top of the transparent substrate 
1, but it will be readily apparent to those skilled in the art that the 
film 12 need only extend over the side surfaces of the electrode 11 for 
purposes of the immediate invention, and other areas or surfaces to be 
covered are dependent upon specific applications. 
A layer of optically sensitive resist or photoresist 13, here with positive 
type photo-sensitivity (part where light does not strike is remaining 
resist) is applied to the surfaces 16 and 17 where the electrode 11 is 
present as shown in FIG. 2(b). The photoresist material is subsequently 
exposed to optical radiation from a direction opposite the conductor 11 
surface of the transparent substrate 1, that is, from the rear surface of 
the transparent substrate 1, as shown by the directional arrows in FIG. 
2(b). As a result, only the positive photoresist above the electrode 11, 
which is not sufficiently transparent to allow passage of a significant 
amount of the exposure light, is not exposed while the remaining resist is 
exposed, since the thin TaO.sub.x layer 12 and other oxide or nitride 
films are substantially transparent to the exposure light. Therefore, when 
developed, only that portion of the photoresist 14 positioned on top of, 
or above, the electrode 11 remains as shown in FIG. 2(c). With this resist 
in place, etching of the insulation material is performed using known 
techniques until the side surfaces of the electrode 11 are exposed, 
resulting in the configuration shown in FIG. 2(d). 
As shown in FIG. 2(d), an insulating or barrier layer 15, made from the 
TaO.sub.x film, is, or remains, deposited on the top surface 17 of the 
electrode 11. This material becomes the second insulator which is used to 
create the requisite barrier layer 7. The side surfaces 16 and the top 
edges 17 are the surfaces of the electrode 11, but when etching is 
performed, the TaO.sub.x film 12 where the resist 14 is present on the top 
is not etched and only the other parts of the TaO.sub.x film 12 shown in 
FIG. 2(c) are etched as shown in FIG. 2(d). The etching rate of the 
TaO.sub.x material used for the insulation film 12 is constant, and, 
therefore, etching reaches the side surfaces 16 and the top edges of the 
conductor 11 at the same time when the TaO.sub.x film 12 is etched. 
An important factor in this MIM manufacturing process is controlling the 
position of the photoresist material 14 remaining after the exposure and 
developing steps. A portion of the exposure light contains a diffraction 
component 18 as shown in FIG. 3, which is directed inward of the electrode 
11 shadow through interaction with the material 12. This transfers a 
significant amount of energy into a portion 19 of the photoresist 
otherwise shielded by the electrode 11, and part 19 of the positive resist 
present on top of the electrode 11 is sensitized by the exposure light 
during the exposure step shown in FIG. 2(b). Therefore, the position of 
the resist 20 remaining after development is generally slightly narrower 
than the top surface 17 of the electrode 11 and can be controlled by 
appropriately setting the exposure conditions in advance. Variations in 
the conductor or electrode 11 size are automatically compensated for by 
using this back side exposure technique. 
An exemplary embodiment for pixel control elements having little deviation 
in element surface area can be achieved by employing an approximately 250 
nm thick tantalum film for the electrode 11 with a TaO.sub.x film about 
300 nm thick for the insulator or barrier layer 12. Positive photoresist 
14 is deposited as an approximately 1300 nm thick film and exposed from 
the rear surface of the substrate 1 using an illumination energy of about 
70 mj/cm.sup.2. This typically results in exposing about 600 nm of the top 
conductor edges. 
After formation of the barrier or insulation layer 15, another insulating 
film in the form of a thin TaO.sub.x film is formed as the first insulator 
on the side surfaces 16 and exposed top edges 17 of the electrode 11. This 
is typically accomplished by anodic or thermal oxidation after obtaining 
the configuration shown in FIG. 2(d). Chrome is then deposited in 
preparation to form both the second conductor 8 and the pixel electrode 9 
(FIG. 1), and the appropriate conductor and pixel electrode patterns are 
then formed by techniques such as photo-etching, producing the element 
shown in FIG. 1. 
Using the previous exemplary embodiment, the second TaO.sub.x film would be 
deposited as a thin film on the order of 60 nm thick, followed by a layer 
of chrome (Cr) for the second conductor and, generally, ITO for the pixel 
electrode. 
Alternatively, if the photoresist is a negative resist, after initial 
development and etching insulation material will only be left on the first 
conductor sides and top edges, exposing the remaining top surface of the 
conductor. The second insulation material of greater electrical resistance 
or thickness is then deposited on this remaining top surface and the 
resist removed along with excess second insulation material to provide the 
structure of FIG. 1. 
The resulting structure employs not only the side edge of the first 
conductor but also a portion of the outer top surface edge of the 
conductor for the active MIM element area. If the size of the area used 
along the top edge 4 of the first conductor 2 is chosen appropriately, 
this additional area is much larger than anticipated variations in the 
film thickness for the film used for the first conductor. Therefore, any 
film thickness variations are effectively absorbed in terms of their 
impact on the active area of the MIM element and a great degree of 
element-to-element reproducibility and uniformity is achieved across an 
array of such elements. Variations in the conductor size are also 
compensated for by using the back side exposure process. 
In contrast to element substrates that used prior art lateral (side 
surface) MIM elements for the pixel element, MIM elements produced using 
the inventive method now have substantially uniform element-to-element 
operating characteristics, thus solving many of the problems encountered 
in large surface area liquid crystal or display and similar devices. 
While the invention has been described in conjunction with several specific 
embodiments, it is evident to those skilled in the art that many further 
alternatives, modifications, and variations will be apparent in light of 
the forgoing description. For example materials other than tantalum are 
useful for forming MIM conductors, and many materials have been found 
useful for manufacturing the insulators. At the same time, the disclosed 
MIM structure may be useful for application to voltage driver elements 
other than for liquid crystal display devices. Thus, the invention 
described herein is intended to embrace all such alternatives, 
modifications, applications and variations as may fall within the spirit 
and scope of the appended claims.