Organic light emitting devices, which make use of thin film materials 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, as described in PCT Application WO 96/19792, which is incorporated by reference.
OLED's may be fabricated using shadow mask technology. However, it is difficult to accurately align multiple layers of deposited material using shadow masks, and the masks tend to clog. Moreover, it is difficult to fabricate features smaller than about 300 microns using a shadow mask, whereas OLEDs smaller than about 100 microns by 100 microns, and possibly smaller than about 10 microns by 10 microns, are preferred for a high resolution, full color flat panel display.
An array of 20 micron.times.20 micron polymer LEDs has been fabricated using direct photoablation with the 193 nm. emission of an eximer laser. S. Noach et al, Appl. Phys. Lett. 69, 3650, Dec. 9, 1996. While this dimension is suitable for a high resolution display, the low speed of laser photoablation are undesirable for commercialization.
Photolithographic patterning involves the use of a photoresist to create patterns in a material deposited on a substrate, and can be used to pattern materials and fabricate devices on a submicron scale, much smaller than can be achieved with shadow mask technology. Photolithographic patterning is also well suited to commercialization because it can be used to quickly fabricate large panels. However, the organic materials used to fabricate OLEDs may be quickly degraded from exposure to deleterious substances such as water, solvents, developers, and even atmospheric conditions. In particular, many of the chemicals used in photolithographic processing, such as solvents and developers used to wash away photoresist, may rapidly degrade such organic materials. Great care should be taken to ensure that the organic materials are not exposed to deleterious substances during the patterning of top electrodes and afterward.
U.S. Pat. No. 5,294,870 to Tang discloses the use of a series of parallel walls formed by photolithography prior to deposition of an organic EL layer such that photolithographic patterning steps or wet chemistry are not required after the organic EL medium is deposited.
It is known to use a multi-layer photoresist system with an overhang to deposit materials with sloped edges. W. R. Runyan & K. E. Bean, Semiconductor Integrated Circuit Processing Technology, p.560, Addison-Wesley, 1990. It is also known to use a photoresist system with an overhang to deposit small features, and to facilitate the lift-off of photoresist after material has been deposited in applications such as the fabrication of narrow gate gallium arsenide transistors.