Refrigeration – Storage of solidified or liquified gas – Including cryostat
Reexamination Certificate
2000-02-02
2001-10-16
Doerrler, William (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Including cryostat
Reexamination Certificate
active
06301903
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and method for activating electrically programmable fusible links in VLSI circuits, specifically fusible links having a reduced interlink spacing, without damaging the underlying substrate.
High density dynamic random access memories (DRAMS) are designed with memory cell redundancy. The redundant memory cells avoid the loss of an entire memory in the event that a minor number of memory cells are not functioning. Activation of the redundant memory cells is accomplished by fusible links which are strategically placed throughout the memory. Activation of a fusible link results in the disabling of the defective memory cell, while enabling in its place a redundant memory cell.
The process of “blowing” fusible links is implemented by heating the fusible link which is to be blown. The heated fusible link melts or evaporates, creating an open circuit for replacing the defective memory cells with a functional cell.
The fusible links are made of aluminum, copper and other high conductive metal or metal alloys. The conductive fusible link generally has a central width portion which is smaller than the ends, to reduce the amount of energy necessary to melt the fusible link to create an open circuit condition.
The melting of fusible links may be accomplished using a laser beam having a controlled beam width. Additionally, the fusible links may be opened by applying a high current thereto, heating the fusible links as a result of power dissipation in the fusible link which is sufficient to melt the fusible links.
Utilizing the laser for blowing the fusible links requires that two conditions be observed to avoid damage to the circuit. The first is to have a beam width which is sufficiently narrow to blow only a single fusible link without inadvertently blowing an adjacent fusible link. The second condition which must be observed is avoiding damage to the underlying silicon substrate which supports the fuse elements.
These requirements are at odds with each other in that while it is desirable to decrease the distance between fusible links, i.e., pitch, there is a corresponding reduction in the laser beam width which produces a high energy beam which may damage the silicon substrate. In order to further increase the density of DRAMS, it is therefore desirable to reduce the pitch between fusible links, while at the same time being able to blow the fusible links without damaging the underlying silicon surface or adjacent fusible links.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a method and apparatus for activating fusible links without damaging the underlying circuit substrate.
It is a further object of this invention to provide a method and apparatus for blowing fusible links having a reduced link pitch without damaging the underlying circuit substrate.
These and other objects of the invention are provided by a method and apparatus in accordance with the invention. A method is provided which will melt a fusible link having a reduced pitch without damaging the underlying silicon substrate. The process for blowing the fusible link reduces laser light absorption characteristics of the substrate. By reducing the absorption characteristics of the substrate, higher energy laser beams (with smaller wavelength) may be used, having a correspondingly smaller beam diameter for safely blowing fusible links on a tight pitch. The decreased beam width of the laser beam permits a decrease in the fusible link pitch and an increase in the laser beam energy density.
In accordance with a preferred embodiment, a substrate bearing the fusible links is supported in a chuck or fixture which is cooled to a temperature below ambient. As the cooling temperature decreases, the absorption characteristics of the substrate changes. As a result, it is possible to reduce the wavelength of the laser beam and corresponding beam width, i.e., spot size, to less than 1 &mgr;m. The substrate fixture may be cooled through a refrigeration unit having cooling coils in heat transfer contact with the fixture which in turn reduces the substrate temperature.
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Edelstein Daniel Charles
Narayan Chandrasekhar
Connolly Bove Lodge & Hutz
Doerrler William
Drake Malik N.
International Business Machines - Corporation
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