Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2000-03-17
2001-02-20
Pham, Long (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S072000, C257S088000, C257S098000, C257S099000, C438S029000
Reexamination Certificate
active
06191433
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to organic light emitting diode (OLED) structures and more particularly to an OLED display device and a method for patterning cathodes to fabricate the device.
BACKGROUND ART
Organic light emitting diodes (OLEDs) are electroluminescent (EL) devices that emit light generated by radiative recombination of injected electrons and holes within one or more organic EL layers of the OLEDs. OLEDs have electrical and optical characteristics which are attractive for operation within pixel-addressed displays. For example, OLEDs operate at low voltages and are relatively efficient. In addition, OLEDs can be fabricated into thin, lightweight display devices. Furthermore, OLEDs can be designed to emit light of different colors to create color display devices.
In
FIG. 1
, an OLED display device
10
in accordance with the prior art is illustrated. The OLED display device includes a substrate
12
, an anode layer
14
, an organic electroluminescent (EL) region
16
, and a cathode layer
18
. The substrate may be transparent or opaque. Thus, the display device may be configured to emit light through the substrate, or through the cathode layer. If the substrate is transparent, the substrate may be made of silica or plastic. However, if the substrate is opaque, the substrate may be made of Si, plastic or a flexible metal foil. The anode layer is typically made of a transparent conducting material, such as Indium Tin Oxide (ITO), while the cathode layer is typically made of a conducting metal with a low work function, such as Ca or Mg. The anode layer and the cathode layer are patterned, so that individual pixels of the display device can be addressed. The organic EL region is composed of at least one organic or polymer layer.
Although the relative positions of the anode and cathode layers
14
and
18
may be inverted, conventional OLED display devices have typically been configured with the anode layer located between the cathode layer and the substrate, as illustrated by the OLED display device
10
of FIG.
1
. The reason for this preference is that the cathode layer is typically made of a low work function metal, which is difficult to pattern, easily oxidized, and not amenable to lithography. Nevertheless, various OLED devices having the alternative orientation of the two layers are known.
U.S. Pat. No. 5,608,287 to Hung et al. describes an OLED display device that includes a cathode layer disposed on a substrate with an anode layer formed over the EL region and the cathode layer. The cathode layer is formed of either metal silicides, such as rare earth silicides, or metal borides, such as lanthanum boride and chromium boride, having a work function of 4.0 eV or less. The composition of the cathode layer is described to provide protection from atmospheric corrosion during fabrication. Another OLED display device that includes a cathode layer disposed directly on a substrate is described in U.S. Pat. No. 5,424,560 to Norman et al. In this device, the cathode layer is formed of low work function material, such as heavily doped diamond, or a conductive metal incorporating Ce, Ca or the like.
Although these known OLED display devices operate well for their intended purpose, what is needed is an OLED display device having a patterned cathode layer positioned below the anode layer and a method of fabricating the display device that eases the process for patterning the cathode layer.
SUMMARY OF THE INVENTION
An OLED display device and a method of fabricating the device utilize a patterned layer of conductive pads formed over a substrate to fabricate a cathode layer without the need to subsequently pattern the cathode layer to create individually addressable cathodes. The design of the OLED display device is such that the cathode layer is positioned below the anode layer. The OLED display device may be configured to emit light through the substrate or through the top layer, i.e., the anode layer.
In a first embodiment, the OLED display device includes the substrate, an optional dielectric layer, the patterned layer of conductive pads, the cathode layer, an EL region, and the anode layer. In this embodiment, the conductive pads, which are formed over the underlying dielectric layer have a cross-sectional profile with sharp edges, so that the overlying cathode layer will not be contiguous. Portions of the cathode layer are formed on the conductive pads, while remaining portions of the cathode layer are formed over exposed areas of the dielectric layer between the pads. Thus, the conductive pads function as an “in situ” shadowing mask that patterns the cathode layer when it is formed.
The cross-sectional profile of the conductive pads may be trapezoidal, such that the portions of the cathode layer deposited over the dielectric layer are not in contact with the conductive pads. Thus, the conductive pads are not electrically shorted by these portions of the cathode layer. In an alternative configuration, the cross-sectional profile is rectangular. In this configuration, the portions of the cathode layer deposited over the dielectric layer may be in contact with the conductive pads. However, the cathode layer is sufficiently thin to prevent or substantially limit any lateral conduction between the conductive pads via the portions of the cathode layer deposited over the dielectric layer. Preferably, the thickness of the cathode layer is at most 10 nm, while the separation distance between two adjacent conductive pads is at least 1 &mgr;m.
In a second embodiment, the conductive pads are configured such that their profile does not have sharp edges. Consequently, the cathode layer that is formed over the conductive pads is contiguous. This is significant since the sharp edges of the conductive pads may detrimentally affect the connectivity of the top electrodes, i.e., the anode layer. In this embodiment, the cathode layer is made of a composite material, which includes cathode components and non-conducting components. The composite material allows the resulting cathode layer to have the desired characteristics to effectively inject electrons vertically into the EL region, while limiting lateral conduction between the conductive pads.
In a preferred embodiment, the cathode layer is formed by co-evaporating the cathode components and the non-conducting components. The desired characteristics of the cathode layer can be achieved by tuning the rates of evaporations. In an alternative configuration, the spaces between the conductive pads may be filled with dielectric material prior to the deposition of the composite material which forms the cathode layer. This provides a planar surface on which subsequent layers of the OLED display device are deposited.
REFERENCES:
patent: 5424560 (1995-06-01), Norman et al.
patent: 5608287 (1997-03-01), Hung et al.
patent: 5739545 (1998-04-01), Guha et al.
patent: 5902688 (1999-05-01), Antoniadis et al.
patent: 5953585 (1999-09-01), Miyaguchi
patent: 5965281 (1999-10-01), Cao
patent: 5972419 (1999-10-01), Roitman
Antoniadis Homer
Roitman Daniel B.
Agilent Technologie,s Inc.
Pham Long
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