Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2000-11-16
2003-01-14
Booth, Richard (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
C438S099000
Reexamination Certificate
active
06506616
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related in general to the field of semiconductor devices and processes and more specifically to the method for fabricating photolithographically defined encapsulated organic light-emitting diodes.
DESCRIPTION OF THE RELATED ART
Commercial light emitting diodes (LEDs) typically constitute a p-n junction of inorganic, doped semiconducting materials such as gallium arsenide (GaAs) or aluminum gallium arsenide (AlGaAs). At these junctions between the doped layers, recombination of electrons and holes results in interband emission of light.
In contrast, organic materials are difficult to dope. P-N junctions are not stable and light emitting diodes are therefore designed on p-i-n structures, where the emissive layer in which the charge recombination occurs is nominally intrinsic. Due to the softness of the organic lattice, carriers tend to be polaronic in nature and recombine to form relatively localized molecular excited states (excitons), which then lead to luminescence by the organic material, providing organic light-emitting diodes (OLEDs).
Recently, OLEDs have drawn much attention, especially for emissive display applications. Since OLEDs can be fabricated on any smooth surface, such as silicon wafers, and at low (<100° C.) temperatures, they are also very promising for many optoelectronic applications. Electroluminescent devices have been constructed using multi-layer organic films. Basic structure and working are described in “Electroluminescence of Doped Organic Thin Films” (J. Appl. Phys., vol. 65, pp. 3610-3616, 1989) by C. W. Tang, S. A. VanSlyke, and C. H. Chen. The review article “Status of and Prospects for Organic Electroluminescence” (J. Materials Res., vol. 11, pp. 3174-3187. December 1996, by L. J. Rothberg and A. J. Lovinger) describes various OLED device structures in the form of stacks of thin layers with carrier injection and transverse current flow. For example, the stack may be a transparent substrate (for instance, glass), a transparent anode (for instance, indium-tin oxide, ITO) , a hole transport layer (for instance, TPD), an emissive layer which also is an electron transport layer and in which electron-hole recombination and luminescence occur (for instance, Alq3), and a cathode (a metal with low work function, for instance, magnesium or a magnesium-containing alloy such as Mg:Ag). “TPD” is N,N′-diphenyl-N,N′-bis(3-methylphenyl)1,1′biphenyl-4,4′diamine. “Alq3” is tris(8-hydroxy) quinoline aluminum.
The schematic energy level diagram exhibits discontinuities between the emitter and the hole transport layer. The discontinuity is greater for electron transport to the hole transport layer than the discontinuity in the opposite direction; consequently, holes from the hole transport layer inject into the emitter and recombine with electrons to form excitons, which in turn excite the emitter to luminesce.
A different approach using siloxane self-assembly techniques, has been described in U.S. Pat. No. 5,834,100, issued on Nov. 10, 1998 (Marks et al., “Organic Light-Emitting Diodes and Method for Assembly and Emission Control”).
In addition to the OLEDs, many related devices such as organic laser diodes, photodetectors, etc. may be realized using organic semiconductors. For many applications such as on-chip interconnects, laser diodes are preferred over LEDs. Laser action has been demonstrated in polymeric organic films, but only by employing optical pumping (for instance, “Laser Emission from Solutions and Films Containing Semiconducting Polymer and Titanium Dioxide Nanocystals”, Chem. Phys. Lett., vol. 256, pp. 424-430, 1996, by F. Hide, B. J. Schwartz, M. A. Diaz-Garcia, and A. J. Heeger; “Lasing from Conjugated-Polymer Microcavities”, Nature, vol. 382, pp. 695-697, by N. Tessler, G. J. Denton, and R. H. Friend; “Semiconducting Polymers: a New Class of Solid-State Laser Materials”, Science, vol. 273, pp. 1833-1836, 1996, by F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, M. R. Andersson, Q. Pei, and A. J. Heeger). Inadequate charge injection is the main roadblock in achieving an organic-based solid-state laser from electrically pumped organic films.
In their paper “Enhanced Electron Injection in Organic Electroluminescence Devices using an Al/LiF Electrode” (Appl. Phys. Lett., vol. 70, pp.152-154, 1997), L. S. Hung, C. W. Tang, and M. G. Mason disclose the beneficial effects of inserting an inorganic dielectric layer (LiF, thin enough for electron tunneling, 0.5 to 1.0 nm) between the metal cathode (Al) and organic material.
The energy bands of Alq3 are bent downwards by the contact with LiF, thus substantially lowering the electronic barrier height of the Alq3-Al interfaces and enhancing the electron injection. The operating voltage is reduced and cathode metals of higher work function can be used. Further, the devices employ a thin (15 nm) buffer layer at the anode (ITO), comprised of CuPc (copper phthalocyanine). The hole transport layer is NPB (N,N′-bis(1-naphthyl)-N,N′diphenyl-1,1′-biphenyl-4,4′-diamine). Alq3 is the emissive as well as electron transport layer.
Methods for fabrication and characterization (such as film thickness, and light intensity and wavelength) have been described in “Characterization of Organic Thin Films for OLEDs using Spectroscopic Ellipsometry” (F. G. Celii, T. B. Harton, and O. F. Phillips, J. Electronic Materials, vol. 26, pp. 366-371, 1997). The organic materials may be amorphous or polycrystalline discrete molecular, or may be polymeric. Polymer layers differ from discrete molecular layers because they are typically not fabricated by vacuum vapor deposition, but rather by spin coating from an appropriate solvent. The polymeric layers may also be deposited (either by vapor deposition or by spin coating) as pre-polymer layers and then converted either thermally or photochemically to the active form. Spin coating, spin casting, or melt techniques have the advantage of large area coverage and low fabrication cost.
In U.S. patent application Ser. No. 09/156,166, filed on Sep. 17 1998 (Celii et al., “Organic Light Emitting Diodes”), to which the present invention is related, an OLED is provided with dielectric barriers at both the anode-organic and cathode-organic interfaces. Increased carrier injection efficiencies and increased overall OLED efficiency plus lower voltage operation are thus enabled.
One of the major difficulties in fabricating OLEDs and organic laser diodes (OLDs) is that many solvents used for cleaning or photolithography dissolve the organic layer of the OLEDs and OLDs. As a result, OLEDs are currently fabricated by using shadow masks. Use of shadow masks may be acceptable for certain applications, but the majority of applications, especially those requiring small OLEDs, will require a fabrication process based on photolithography. In addition, since exposure of the organic layer to moisture (or oxygen) may degrade the light-emitting characteristics of the organic material, the organic layer needs to be encapsulated. Although an encapsulation process by reactive ion etching of the organic layer with an aluminum mask layer has been reported in the literature (C. C. Wu, J. C. Sturm, R. A. Register, and M. E. Thompson, “Integrated Three-Color Organic Light-Emitting Devices”, Appl. Phys. Lett., vol. 69, pp. 3117-3119, 1996), the mask layer was patterned by a shadow mask, not by a photolithography process.
An urgent need has therefore arisen to conceive structure and fabrication methods of electrically pumped organic laser diodes based on photolithography techniques suitable for miniaturization and high process yield. Preferably, this concept should be based on fundamental design solutions flexible enough to be applied for different diode and laser product families and a wide spectrum of material and assembly variations. Manufacturing should be low cost and the devices stable and reliable. Preferably, the innovations should be accomplished using established fabrication techniques and the installed equipment base.
SUMMARY O
Celii Francis G.
Jacobs Simon J.
Kim Tae S.
Booth Richard
Brady III W. James
Lattin Christopher
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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