Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Charge transfer device
Reexamination Certificate
2002-09-25
2003-12-16
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Charge transfer device
Reexamination Certificate
active
06664576
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to semiconductor technology and device designs, and more particularly to an organic-based thin film transistor device and a method for producing the same.
2. Description of the Related Art
Thin film transistors (TFTS) with active organic layers and polymer-based electronic components are emerging as an inexpensive alternative to silicon-based TFTs for some applications. The use of organic and polymeric materials provides two main advantages. First, organic-based devices can be produced using a simpler and cheaper fabrication process, in contrast to the expensive equipment and processing associated with silicon processing. Second, it is possible to fabricate the devices on flexible plastic substrates, due to the ability to process organic materials at lower temperatures, and to the greater mechanical flexibility of organic-based components relative to inorganic materials such as silicon and conductive metals. However, despite considerable research and development effort, organic-based TFTs have not yet reached commercialization, at least in part due to relatively poor device characteristics of prior art organic TFTs.
Fabrication of an all-organic TFT requires various organic or organic/inorganic hybrid materials: semiconductors, insulators, and conductors. The conductor may be selected from conducting polymers such as polyaniline and poly(ethylene dioxide thiophene), and metal or graphite colloid particle-based inks. There are a variety of polymeric organic insulators that may be used, such as polyamide or PMMA for the semiconductor. Organic p-type (hole transporting) and n-type (electron transporting) materials are both known in the art and have been tested as the semiconductive channel in TFTs.
In general, circuitry using organic transistors have the potential of reduced power consumption and simplicity in the design. However, complementary circuitry using both organic N and P channels transistors are not common, for example, U.S. Pat. No. 5,625,199, the complete disclosure of which is herein incorporated by reference, teaches a technique to fabricate complementary circuits with inorganic n-channel and organic p-channel thin film transistors. Additionally, U.S. Pat. No. 5,936,259, the complete disclosure of which is herein incorporated by reference, describes a switch based on a thin film transistor design (TFT) using a fused ring organic compound as semiconductor. Furthermore, U.S. Pat. No. 5,804,836, the complete disclosure of which is herein incorporated by reference, describes an image processor design which operates on an array of polymer grid triodes. Similarly, prior art disclosures also teach a 5-stage ring oscillator using copper hexadecafluorophthalocyanide as the n-channel material and oligothiophenel oligothiophene derivative as the p-channel material.
Two popular structures of an existing polymer thin-film transistor are shown in FIGS.
9
(
a
) and
9
(
b
). These structures have two major disadvantages. First, there is a comer thinning problem due to topography, and second, the most sensitive portion of the body element is exposed to process induced contamination. The resulting devices have poor performance and inconsistent properties. Shown in FIG.
9
(
a
) is the first typical structure of the polymer transistor. The source
11
and drain
12
are first patterned. Then the body material
13
is deposited and patterned. The body
13
is a semiconductive polymer or oligomer, and it is applied to the surface of the source
11
and drain
12
islands by evaporation, spin-coating, dip-coating or printing, depending on the organic semiconductor used.
The body material
13
is patterned in one of three ways. The most common method is by evaporation of the semiconductive material through a shadow mask. The other two methods are printing (i.e., screen printing or inkjet printing) and using conventional lithography by first applying a protective coating over the semiconductor, then applying the photoresist, patterning, and etching. A brief thermal anneal may be needed, depending on which type of organic semiconductor material is used. The last step is applying a protective coating to the semiconductor to passivate the devices from contamination.
After patterning the body portion
13
, the substrate is wet cleaned. The body surface, especially in the channel region, deteriorates due to the unwanted chemical reaction. After a thermal treatment, the body element
13
becomes thin around the comers
16
,
17
of the source
11
and drain
12
due to reflow. Typically, semiconductors decompose before melting. The source/drain
11
,
12
to body contact area are significantly reduced as the result of the comer thinning
16
,
17
of the body element
13
. Then, the gate material
14
is deposited after a thin insulating polymer
15
is coated on top of the body element
13
and the exposed source
11
and drain
12
regions.
Another common structure of the polymer TFT structure is shown in FIG.
9
(
b
). The gate
14
is formed first, then an insulating polymer
15
is coated thereon. Again, the corner thinning problem presented at the corners
16
,
17
of the gate
14
causes the possibility of shorting of source
11
and drain
12
to the gate
14
. After the source
11
and drain
12
are formed, the body element
13
is formed. In this case, since the body to channel interface is not exposed to any chemical, the resulting transistor yield and performance is better than those of the first transistor.
In both of the bottom-contact devices shown in FIGS.
9
(
a
) and
9
(
b
), there is a well-documented problem with ensuring good contact between the electrodes and the organic semiconductor. One approach used to solve this problem has been to modify the surface properties of gold electrodes using thin self-assembled monolayers, which improves wetting of the electrode by the organic semiconductor and may also decrease the chance of delaminating. However, the topography of the bottom electrodes may still hamper film formation and reduce the contact area. Therefore, there is a need for a new and improved structure and method for forming a polymer thin film transistor, which does not have the problems inherent with the prior art devices.
SUMMARY OF THE INVENTION
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional thin film transistor devices, the present invention has been devised, and it is an object of the present invention to provide a structure and method for a polymer thin film transistor with contact etch stops.
Another object of the present invention is to provide a thin film p olymer transistor having a vertical channel, or a transistor whose channel is structured in the third dimension (or 3-D). Yet another object of the present invention is to provide a new polymer thin film transistor structure and method which will result in a high-performance device. Still another object of the invention is to provide a polymer transistor structure so that its base layer is always protected from being contaminated during all process steps. It is yet another object of the invention to insure that the material thinning problem, inherent in conventional devices, is completely eliminated.
In order to attain the objects suggested above, there is provided, according to one aspect of the invention a vertical polymer transistor structure having a first conductive layer, filler structures co-planar with the first conductive layer, a semiconductor body layer above the first conductive layer, a second conductive layer above the semiconductor body layer, and an etch stop strip positioned between a portion of the first conductive layer and the semiconductor body layer. The vertical polymer transistor structure has filler structures that are electrically isolated from the first conductive layer. The filler structures are made of the same material as the first conductive layer. The first conductive layer, the semiconductor body layer, and the second conductive la
Breen Tricia L.
Clevenger Lawrence A.
Hsu Louis L.
Wang Li-Kong
Wong Kwong Hon
Hoang Quoc
International Business Machines - Corporation
McGinn & Gibb PLLC
Nelms David
Trepp, Esq. Robert M.
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