One of the holy grails in design is a chip that can diagnose itself and then repair its own failed elements--the autonomic chip. To accomplish this, designers must embed some form of repair mechanism and redundant logic within the chip. To do this economically, the mechanism must not require any new materials, any new processes, or any additional masks.
The repair technology also must work even if the chip is marginally functional. Ideally, the programming time would be short and totally contained on the chip. An autonomic chip would then be able to diagnose, test, and repair itself in the fab, at wafer level, in the package, and multiple times in the field.
The eFUSE, developed by the Semiconductor Research and Development Center at IBM Microelectronics, Systems and Technology Group, could lead to such a chip. This novel fuse structure uses electromigration, which used to be a problem for designers.
With the proper biasing, however, electromigration can cause a thin layer of metal silicide to "break." This creates an open circuit in a reasonable amount of time--about 200 µs. Unlike laser and electrical fuses, which splatter particles of metal or metal vapor when "blown," the eFUSE is a clean approach since the metal merely "migrates" from one place to another.
The designers had to turn back the clock, because modern copper metallization schemes are electromigration-resistant. So, they sandwiched a metal silicide between a nitride overlayer and a high-resistance polysilicon line (see the figure). Driving a current through the silicide raises the temperature and sets up a gradient.
The silicide electromigrates, but the current is sustained only as long as the polysilicon is hot, intrinsic, and conductive. This forces electromigration of the silicide to completion, turning off the current. Everything then cools down, and the link is left with the high resistance of the polysilicon line.
IBM has applied this scheme to embedded DRAM arrays to repair defective rows as well as adjust bias voltages. The technology has many additional applications, too. With it, chips can be optimized for power and performance. It also allows the integration of more autonomic functions, helps in supply-chain management, provides chip identification during authentication, and enters IDs for RFID tags. It's even useful in logic-compatible WORM implementations.
For details, contact Subramanian Iyer at [email protected].