A semiconductor device "laser fuse" or "laser fusible link" is a portion of a conductive layer, in many cases polysilicon, that is opened by the application of a laser. Laser fuses provide an effective way to alter the operation of a semiconductor device after it has been fabricated. One such alteration includes implementing redundancy schemes to replace defective portions of a integrated circuit with redundant portions (often referred to as "laser repair").
An important consideration in the fabrication of devices employing laser fuses is the thickness of the dielectric covering the fuse. When opening a laser fuse, the laser is applied, a portion of the laser fuse is vaporized, and the dielectric breaks open, allowing the vaporized laser fuse material to escape. In the event dielectric is too thick, the dielectric will not break open, and the vaporized material will not escape. Consequently, the laser fuse will remain electrically conductive. Because polysilicon is typically used as an initial layer of interconnect, for devices using polysilicon as the fuse material, two or more dielectric layers are subsequently deposited over the laser fuse resulting in an unacceptably high overall dielectric thickness. To reduce the amount of dielectric over the fuse it is known in the prior art to etch "fuse windows" into the dielectric layers covering the fuse. In order to reduce the number of fabrication steps required to manufacture the device, it is also known in the prior art to etch both fuse windows and "bond pad windows" in the same step. Bond pad windows are windows that are etched into the dielectric to expose the device bond pads. As is well known, the bond pads enable wire bonding between the device and a device package.
A potential drawback to simultaneous bond pad window and fuse window etching is the potential for fuse window over-etch. Referring now to FIG. 1a and 1b, an example of fuse window over-etch is illustrated. FIGS. 1a and 1b set forth a side cross sectional view of portions of a semiconductor device 10 that utilizes two layers of polysilicon and one layer of metal. FIG. 1a illustrates the device 10 prior to fuse window etch. The device is shown to include a laser fuse 12 composed of polysilicon 14 with a layer of silicide 16 formed thereon. The laser fuse 12 is fabricated on thermally grown field oxide 18, and covered with first dielectric layer 20. A second layer of polysilicon (not shown) is deposited and patterned on the first dielectric layer 20. A second dielectric layer 22 is then deposited. A metallization layer is deposited and patterned on the second dielectric layer 22. In FIGS. 1a and 1b, the metallization layer includes the bond pads of the semiconductor device. A portion of a bond pad 24 is shown situated on the second dielectric layer 22. A passivation layer 26 is deposited over the previous layers.
FIG. 1b illustrates the device 10 after an etch step that has resulted in fuse window over-etch. As shown in the figure, a bond pad window 28 has been etched through the passivation layer 26 to expose the bond pad 24. At the same time, a fuse window 30 has been etched over the laser fuse 12. The resulting fuse window 30, however, is over-etched. Rather than reducing the thickness of the dielectric/passivation layers over the laser fuse 12, all of the overlying layers have been removed, exposing, and partially etching, the laser fuse 12.
Fuse window over-etch commonly arises when fuse windows and bond pad windows are created simultaneously. To eliminate the possibility of residual passivation remaining on the bond pads 24, which can result in high resistance wire bonds or wire bonds of insufficient strength, it is known to over-etch the pad windows. In the event the etch rate is higher than expected (an acid bath was recently changed or maintained at too high a concentration) or the thickness of the passivation or dielectric layers (20, 22 or 26) was insufficient (due to process variation), the laser fuse 12 can be exposed within the fuse window 30. The exposed laser fuse 12 can be etched and/or subject to oxidation, both of which can increase its resistance, degrading the overall performance of the semiconductor device.
To reduce fuse window over-etch it is known in the prior art to etch the bond pad windows and fuse windows in two different etch steps. This increases processing time and process complexity, however.
Another prior art method to eliminate fuse window over-etch is illustrated in FIGS. 2a and 2b. The method includes forming a number of laser fuses 12, depositing a first dielectric layer 20, and then forming a second layer polysilicon (poly2) strip 32 over the laser fuses 12. A second dielectric layer 22 and passivation layer 26 are deposited over the poly2 strip. A fuse window 30 is etched into the passivation layer 26 and the second dielectric layer 22. The poly2 strip 32 functions as an etch barrier, preventing the underlying first dielectric layer 20 from being etched. This method has potential drawbacks, however.
As shown best in the top view of FIG. 2a, a first laser fuse 12a and an adjacent second laser fuse 12b have been opened during laser repair. A portion of the poly2 strip 32 has also been vaporized. As is best shown in the cross section of FIG. 2b, while both laser fuses (12a and 12b) are open, the first laser fuse 12a is shorted to the poly2 strip 32 via a first short 34, and the second laser fuse 12b is shorted to the poly2 strip 32 via a second short 36. One skilled in the art would recognize that shorting can also occur across an opened laser fuse 12 via the poly2 strip 32. Thus, while providing adequate protection from fuse window over-etch, the use of a poly2 strip 32 can produce shorts across otherwise opened laser fuses 12.
It would be desirable to provide a laser fuse structure that protects the laser fuse from over-etch without the drawbacks of prior art methods.