Mask for patterning devices

Details Mask for patterning devices Mask for patterning devices

2001-03-12

2003-07-22

Young, Christopher G. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

Reexamination Certificate

active

06596443

ABSTRACT:

This application is related to concurrently filed Patent Application Ser. No. pending, attorney docket no. 10052/1201, which is incorporated by reference in its entirety.
FIELD OF THE INVENTION
The present invention relates to the field of semiconductor fabrication, and more particularly to an integrated mask and the use of such a mask to fabricate electrodes.
BACKGROUND OF THE INVENTION
Organic light emitting devices (OLEDs), which make use of thin films that emit light when excited by electric current, are becoming an increasingly popular technology for applications such as flat panel displays. Popular OLED configurations include double heterostructure, single heterostructure, and single layer, and a wide variety of organic materials, some of which are described in U.S. Pat. No. 5,707,745, which is incorporated herein by reference.
Flat panel displays typically include an array of picture elements, or pixels, deposited and patterned on a substrate. Such a pixel array is typically a matrix of rows and columns. In an OLED display, each OLED pixel includes an organic light emitting diode that is situated at the intersection of each column and row line. The first OLED displays, like the first LCD (Liquid Crystal Displays), have typically been addressed as a passive matrix (PM) display. This means that to cause a particular pixel to luminesce, electrical signals are applied to the row and column lines of that particular pixel. The more current that is pumped through each pixel diode, the brighter the pixel appears visually. One method of providing grayscale to the display is to vary the current level of the pixel.
In practice, a voltage is applied to a single row line, and a path for current flow is selectively provided at individual columns. This provides current flow through selected pixels on the single row, thus allowing current to flow causing each pixel in that row line to luminesce at the desired brightness. The next row line is then addressed, and once again, all the pixels on that row line are energized to produce the required brightness. The display continuously scans all the row lines sequentially, typically completing at least 60 scans of the overall display each second. In this way, flicker is not seen since the display is addressed fast enough, for typical observation conditions, that the pixels cannot be seen to be continuously turning on and off. Preferably, the magnitude of current flow through each column can be controlled, such that the brightness of the pixels can be controlled.
For OLEDs from which the light emission is only out of the bottom of the device, that is, only through the substrate side of the device, a transparent anode material such as indium tin oxide (ITO) may be used as the bottom electrode. Since the top electrode of such a device does not need to be transparent, such a top electrode, which is typically a cathode, may be comprised of a thick and reflective metal layer having a high electrical conductivity. In contrast, for transparent or top-emitting OLEDs, a transparent cathode such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745 may be used. As distinct from a transparent or bottom-emitting OLED, a top-emitting OLED is one which may have an opaque and/or reflective substrate, such that light is produced only out of the top of the device and not through the substrate.
The transparent cathode that is used in such a transparent or top-emitting device preferably has optical transmission characteristics such that the OLED has an optical transmission of at least about 50%. More preferably, the transparent cathode has optical transmission characteristics that permit the OLED to have an optical transmission of at least about 70%, still more preferably, at least about 85%. These requirements place significant limitations on the materials and thicknesses of the transparent cathode.
The transparent cathodes as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745 typically comprise a thin layer of metal such as Mg:Ag with a thickness, for example, that is less than about 100 angstroms. The Mg:Ag layer is coated with a transparent, electrically-conductive, sputter-deposited, ITO layer. Such cathodes may be referred to as compound cathodes or as TOLED (“Transparent-OLED”) cathodes. The thickness of the Mg:Ag and ITO layers in such compound cathodes may each be adjusted to produce the desired combination of both high optical transmission and high electrical conductivity, for example, an electrical conductivity as reflected by an overall cathode resistivity of about 30-100 &OHgr;/□ (ohms per square). However, even though such a relatively low resistivity may be acceptable for certain types of applications, such a resistivity may still be somewhat too high for a passive matrix array of OLED pixels in which the current that powers each pixel needs to be conducted across the entire array through the narrow strips of the compound cathode.
It is known to use bus lines to mitigate limitations on the electrical conductivity of a transparent electrode. In the context of a passive matrix array of OLEDs, the bus line is a thick electrically conductive strip that runs parallel to a transparent electrode, and which provides electrical conductivity in the direction of the electrode. For example, U.S. Pat. No. 6,016,033 to Jones et al. discloses the use of a bus line in an array of OLEDs. Because the bus line is made of a thick electrically conductive material, it does not transmit light, and unfavorably results in an inactive area on the array of OLEDs. Because it is desirable to maximize the active area of an OLED display, it is desirable to minimize the area of the bus line. The active area may be quantified by a “fill-factor,” which is the percentage of the area of an array that is active or that emits light. Because of the enhanced electrical conductivity that is provided by a bus line, a bus line may be used notwithstanding the disadvantageous inactive area.
The organic materials of an OLED are very sensitive, and may be damaged by conventional semiconductor processing. For example, any exposure to high temperature or chemical processing may damage the organic layers and adversely affect device reliability. As a result, the processes conventionally used to fabricate a thick metal feature such as a bus line may damage any organic layers that are already present.
One technique that may be used to protect the delicate organic layers of an OLED is an integrated mask through which layers may be selectively deposited during fabrication. The mask is “integrated” because it is left in place after fabrication, thus being integrated into the final device. Using an integrated mask is particularly desirable where the steps used to pattern material or to remove a mask have the potential to damage the device. Even where the integrated mask does not cover the delicate organic layers, the integrated mask protects the delicate organic layers by providing a patterning mechanism that does not require the patterning or removal of a mask once the organic layers are in place, i.e., the potentially damaging processes used to form the integrated mask are performed before the organic layers are present, and the potentially damaging processes used to remove a mask are not performed at all because the integrated mask is left in place. It is known to use an integrated mask to fabricate the top electrodes of an array of OLEDs, as disclosed in U.S. Pat. No. 5,701,055 to Nagayama et. al.
One problem that has been observed with conventional integrated masks is the shorting of adjacent electrode layers across the mask. It is known to use integrated masks having an overhang to mitigate shorting across the mask, as disclosed in U.S. Pat. No. 5,701,055 to Nagayama et. al. However, even with a conventional overhang, the process used to deposit the electrode must meet certain criteria to avoid shorting problems.
First, in order to avoid shorting problems, the “footprint” of the deposited material, defined as the surface area onto which significant material is deposited, should be sharply

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