Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-02-25
2002-08-20
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C438S132000
Reexamination Certificate
active
06436585
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to semiconductor fabrication and more particularly, to an improved method and apparatus for forming electrical fuses by a photolithographic process using a novel mask structure.
2. Description of Related Art
In fabricating microelectronic semiconductor devices on a wafer substrate or chip, such as of silicon, to form an integrated circuit (IC), various metal layers and insulation layers are deposited in selective sequence. To maximize integration of device components in the available substrate area to fit more components in the same area, increased IC miniaturization is utilized. Reduced pitch dimensions are needed for denser packing of components per present day very large scale integration (VLSI), e.g., at sub-micron (below 1 micron, i.e., 1,000 nanometer or 10,000 angstrom) dimensions. Redundancy techniques are used in semiconductor fabrication to provide deliberate duplication of circuit components to decrease the probability of a circuit failure and thus increase circuit reliability. To offset defects that can occur in the circuitry, multiple copies of a given circuit component can be connected in parallel to achieve continued operation upon failure of a particular component. Each such multiple component can be provided with a set of fuses that can be blown to replace a failed component by a duplicate one during fusing operation of redundancy activation, e.g., at final IC chip testing.
Some ICs have conductive links between semiconductor devices that are coupled to fuses that can be laser cut (blown) after fabrication. Thus, in a dynamic random access memory (DRAM) circuit, fuses can protect transistor gate stacks from destruction due to inadvertent build-up charges. After IC fabrication, the fuses can be blown to permit the DRAM circuit to function as if the protective current paths never existed. Also, fuses are used to set the enable bit and the address bits of a redundant array element in a DRAM circuit. To replace a defective main memory array element within a main memory array, a redundant array is provided which has a plurality of fuses in a fuse decoder array. In replacing a defective main memory array element, individual fuses in the fuse array are blown to set their values to either a “1” or a “0” as required by the decoder circuit. During operation, the values of the fuses in the fuse array are loaded into the fuse latch array upon power up, and these values then decoded by the fuse decoder circuit during run time. This facilitates replacement of specific failed main memory array elements by specific redundant elements of the redundant array, all by well known techniques.
In particular, high density DRAMs are commonly designed with memory cell redundancy whereby the redundant memory cells avoid loss of an entire memory in the event that a minor number of memory cells fail to function. Redundant memory cell activation is effected by blowing fuses selectively placed throughout the memory. Blowing a set of fuses disables the defective memory cell and enables in its place a redundant working memory cell.
Fuses may also be incorporated in an IC of a semiconductor device module adapted for custom wiring operation, depending on the requirements of the end user. By blowing selective fuses in the circuit, the semiconductor device module can be customized for particular purposes. Fuse blowing is effected by heating the fuse to melt it, and creating an open circuit, e.g., to replace a defective memory cell or other component by a functional cell or different component. The fuse is usually of aluminum, copper or other highly conductive metal or metal alloy, and has a central portion or fuse segment (fuse link) of smaller cross sectional area than its ends (connector terminals) to reduce the energy needed to melt the fuse and create an open circuit condition. The melting (blowing) of fuses can be effected by a laser beam with a controlled beam width. This can result in laser-induced damage to the area beneath the fuse link, mainly due to absorption of laser energy. Alternatively, fuses can be electrically blown by applying a high current thereto for heating the fuse link by electrical power. To enable electrical fusing with voltages below about 10 volts in semiconductor devices, it is important that the cross sectional area reduction from the connector terminals to the electrical fuse segment (fuse link) be as large as possible, preferably greater than 5 or 10. However, the need for such large connector terminals limits the proximity of neighboring fuses. The voltage necessary to activate an electrical fuse is very sensitive to the geometry of the fuse. The shape of the fuse, its linearity, and the size of the connector terminals all impact the voltage needed to blow the fuse. It is, therefore, important that the fuse geometry be optimized.
An electrical fuse is basically an expendable overcurrent protective device having a circuit-opening fusible (meltable) conductive, e.g., metal or metallic material, fuse segment (fuse link) heated and destroyed by passing an overcurrent through it. The overcurrent heats the fuse link beyond the normal level of radiation loss of the generated resistance heat that keeps its temperature below that at which it melts. The fuse link resistance is particularly determined by the material of which it is made, its cross sectional area, its length, and its temperature. Like any electrically conductive wire (and apart from the material of which it is made and its temperature), of the length of a fuse link or connector terminal is doubled, its resistance is also doubled, whereas if its cross sectional area is doubled, its resistance is halved. In short, the resistance of a fuse link or connector terminal is directly proportional to its length and inversely proportional to its cross sectional area.
Some examples of the fabrication of semiconductor devices with fuse arrangements are shown in U.S. Pat. No. 4,635,345, U.S. Pat. No. 5,436,496, U.S. Pat. No. 5,313,424, U.S. Pat. No. 5,420,456, and Japanese Patent Document JA 403124047. The disclosure of these references are incorporated herein by reference.
Electrically activated fuses having tight pitches are well suited for use in semiconductors. However, the prior art has not been able to achieve an optimized electrical fuse design and method of fabrication permitting tight fuse pitches in semiconductor devices while enabling electrical fusing at voltages below about 10 volts, such as during diffusing operation of redundancy activation or custom wiring, i.e., without compromising desired pitch reduction. Moreover, the prior art has not been able to provide a fuse mask for use in lithographic semiconductor fabrication which may produce local areas having widths narrower than the allowed minimum design feature.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a fuse mask for use in lithographic semiconductor fabrication which may produce local areas having widths narrower than the allowed minimum design feature.
It is another object of the present invention to provide a method of making such a fuse.
A further object of the invention is to provide a method of using such a fuse to produce local areas, designed to be blown, having widths narrower than the allowed minimum design feature.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
SUMMARY OF THE INVENTION
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of making a photolithography mask for use in creating an electrical fuse on a semiconductor structure which comprises initially determining a pattern for a desired electrical fuse, with the pattern including a fuse portion of substantially constant width except for a localized narrowed region of the fuse portion at which the electrical fuse i
Brintzinger Axel
Einspruch Fred L.
Haffner Henning
Narayan Chandrasekhar
Thomas Alan C.
DeLio & Peterson LLC
International Business Machines - Corporation
Peterson Peter W.
Rosasco S.
Schnurmann H. Daniel
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