Active solid-state devices (e.g. – transistors – solid-state diode – Gate arrays – With particular signal path connections
Reexamination Certificate
1999-08-02
2001-05-01
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Gate arrays
With particular signal path connections
C257S529000, C257S758000
Reexamination Certificate
active
06225652
ABSTRACT:
BACKGROUND
1. Field of Invention
This invention relates to laser fuses for use in integrated circuits and more particularly to laser fuse structures which allow increased packing density in an integrated circuit.
2. Related Art
Laser fuses are in common use in the semiconductor industry as circuit elements for customizing alterations of individual integrated circuits (ICs) for the purpose of repair or reconfiguration. During laser configuration, specific fuses are blown open by a targeting laser beam resulting in a desired pattern of blown and not blown fuses as required by the repair or configuration scheme. Using laser fuse-based circuits, it is possible for defective memory bits to be swapped out of a memory array, for custom functions to turned on or off in an application-specific IC (ASIC), and for serial numbers to be written to individual ICs. Despite their usefulness, laser fuses are typically used sparingly in an IC due to the large layout area cost of each fuse, each fuse taking up the area of many transistors. This large area may be generally attributed to the long wavelength of light employed by lasers, the less controllable optical and positioning subsystems used in commercial lasering systems compared with wafer lithography systems, and the need to space off unrelated circuitry away from the laser fuse to avoid collateral damage during fuse blowing.
FIGS. 1
,
2
A, and
2
B illustrate the large area requirements of conventional laser fuses.
FIG. 1
shows the physical layout of an array
10
of conventional laser fuses
100
, configured to connect perpendicular lines
110
and
120
, as required to implement a laser configurable cross-point junction between orthogonal signal buses. Lines
110
are formed from an upper conductive layer, while lines
120
are formed from a lower conductive layer, these layers being separated by an intermediate insulating layer (not shown). A selection of interconnections between lines
110
and
120
is implemented by either blowing or retaining laser fuses
100
.
FIG. 2A
shows a detailed view of one portion of array
10
of
FIG. 1
containing one fuse
100
. Laser fuse
100
has a fuse body
210
, which is typically the same width (
FIG. 2A
) or narrower (
FIG. 2B
) than connection terminals
220
of laser fuse
100
. The term fuse body refers to that portion of the fuse structure which is irradiated by the laser beam and removed during lasering. If a disconnection is desired between line
110
and line
120
, a laser is directed at an intended beam blast area
230
overlapping fuse body
210
, blowing the fuse to effect the desired disconnection. A relatively high laser energy is required to blow fuse
100
, and thus connection terminals
220
must be made long enough to protect connection nodes
240
from thermally conducted heat damage during lasering. This additional length adds to the Y-direction pitch as measured between lines
110
. Additionally, since the laser beam typically has a radial Gaussian energy distribution, increasing the beam energy tends to laterally spread the beam blast area. Thus, adjacent lines
120
must be spaced off a greater distance in the X-direction to avoid collateral beam damage which might cut into these lines. As a consequence of the X-direction and Y-direction layout requirements, the packing density of conventional laser fuses
100
on an IC surface is relatively low, as is most evident when trying to lay out structures which use a large number of fuses such as a laser configurable cross-point junction between two wide signal buses. The result is an undesirable lower density and/or larger size IC.
Laser fuse designs must also take into account the heat transfer effects that occur during and immediately after fuse blowing. A principle design objective is that the greatest portion of the laser energy goes into heating the fuse body and the lowest portion go into heating surrounding or underlying structures. This minimizes damage to nearby structures, and also lowers the energy required to blow the fuse, thus allowing use of a less powerful and therefore smaller diameter laser beam. This design objective is ideally satisfied by having the fuse body be maximally thermally isolated from other structures. The glass insulator enclosing the fuse body approximates this requirement as it provides good thermal insulation as well as electrical insulation, but the electrical terminals of the fuse are problematic. The materials typically used to form the electrical terminals (either metals or polysilicon) have a high thermal conductivity when compared with the glass insulation, and thus form an undesirable thermal path for heat to escape the fuse body and cause damage to nearby structures, while making it harder to blow the fuse by sapping thermal energy out of the blast zone.
In conventional laser fuse designs, this heat conduction problem is minimized by implementing long fuse connection terminal nodes
220
between the laser blast area and the fuse connection nodes to reduce the heat transferred to the connection nodes. A number of inventions have further addressed the problem of thermal management. For example, in Lou et al., U.S. Pat. No. 5,729,042, entitled “Raised Fuse Structure For Laser Repair”, a pedestal structure is disclosed to improve the thermal flow characteristics, and in Shiozaki et al., U.S. Pat. No. 4,682,204, entitled “Fuse Element For Integrated Circuit Memory Device”, a corrugated surface is used under the fuse terminals to increase their effective thermal length, both of which are incorporated by reference in their entirety.
Any technique that can reduce the amount of energy required to blow a fuse is also found useful. A well-known technique is to cover the fuse body with a thin layer of glass so as to form a bomb-vessel enclosure of the fuse body. This results in a more uniform vaporization of the fuse and in lower energy requirements when compared with open-top fuses which may splatter and tend to form connective stringers unless shot with high energy or multiple pulses. For example, in Fischer, U.S. Pat. No. 4,853,758, entitled “Laser-Blown Fuses”, a fabrication process is disclosed that reduces the energy required to blow a fuse by controlling the thickness of the overlying thin glass layer, and in Gilmour et al., U.S. Pat. No. 5,760,674, entitled “Fusible Links With Improved Interconnect Structure”, a distinct intermediate interconnect level is used to space off the fuse body from the electrical terminals of the fuse, both of which are incorporated by reference in their entirety. Note that the lateral interconnections used by Gilmour et al. provide thermal isolation, but still require significant layout space.
Accordingly, it is desirable to have a laser fuse structure which allows increased packing density of laser fuse elements on a IC surface both through minimized layout dimensions and through improved thermal management techniques.
SUMMARY
The present invention provides a laser fuse structure and array using a vertical via to connect the laser fuse body with an underlying conductive line, wherein the fuse body is on one conductive layer and is directly above one or more connection vias to a lower conductive layer. Because the via connection terminals are located directly below the fuse body instead of off to one side, the lateral dimensions of the fuse are reduced, which allows more laser fuse structures to be placed in an array. As a result, less chip surface area is required for the placement of such fuses, thereby increasing the packing density of ICs using such fuses.
A further advantage of the present invention is that the via may be optimized to control the amount of thermal conduction away from the fuse body, thus reducing the amount of laser blast heat transferred out of the fuse body into the surrounding circuitry. In one embodiment of this invention, the horizontal cross-section of the via should be made small so as to minimize the unwanted thermal conduction away from the along this terminal connection. Also, forming the via from a material with a lower therm
Chen Tom
Clear Logic, Inc.
Crane Sara
Sjkerven, Morrill, MacPherson LLP
LandOfFree
Vertical laser fuse structure allowing increased packing... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Vertical laser fuse structure allowing increased packing..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Vertical laser fuse structure allowing increased packing... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2492488