Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-01-21
2001-06-12
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S549000, C324S765010, C324S762010, C327S525000, C365S225700
Reexamination Certificate
active
06246243
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of semi-fusible links, and particularly to semi-fusible link systems used for parameter trimming in integrated circuits.
2. Description of the Related Art
“Fusible links” have been used in integrated circuits (ICs) for many years, as a means of trimming one or more IC parameters. A fusible link is characterized as having “intact” and “blown” states, providing a very low resistance when intact and an open circuit when blown. For example, an operational amplifier's offset voltage might be trimmed using a number of fusible links connected in series with a number of resistors, with the links and resistors arranged such that the resistance between the amplifier's offset input and ground can be adjusted by blowing open the appropriate links.
A number of approaches have been used to create fusible links. For example, an “anti-fuse”, i.e., a link that can be driven from an “open” state to a short circuit, is created by forcing a high current through a zener junction to trigger a reverse breakdown, which melts the metal contacts together across the junction to form a short. These “zener zap” links typically operate at 25-35 volts and require currents that would be destructive to a reasonably-sized transistor made with the same process. For this reason, zener zaps are typically programmed using dedicated bond pads for each link.
Minimum width metal tracks have also been used as fusible links, with a current forced through the metal to melt the link open. As with the zener zaps, the high currents required to open such links typically require dedicated bond pads for each link.
Fusible links have also been made from polysilicon (“poly”). Poly links are superior to either zener zaps or metal links in that the power required to blow open a poly link can be provided by an active circuit device, such as a reasonably large n-channel MOSFET. The ability to blow the links with active devices removes the need for the bond pad-per-link requirement, such that many links can be programmed by, for example, a serially-loaded shift register which requires only 2 bond pads. However, poly links do require a considerable amount of chip space to accommodate the large active devices needed to provide the programming current.
In addition to the above-noted problems, the substantial currents (referred to herein as the “programming currents”) needed to blow poly, metal, or zener zap links may damage the semiconductor. In particular, a die's thin glass passivation layer, which protects the circuit from mechanical or chemical damage, can rupture or crack during link blowing. This may allow moisture to penetrate to the chip's surface and allow corrosion to begin.
SUMMARY OF THE INVENTION
A programmable “semi-fusible” link system is presented which avoids the difficulties noted above. The system enables a number of links to be programmed without requiring dedicated bond pads, using currents which present little danger to the semiconductor and are generated with active devices of modest size.
As with a conventional link, a semi-fusible link has “intact” and “blown” states. However, while a conventional link is typically shorted when intact and open when blown, a semi-fusible link has a first resistance when intact, and a second, higher resistance when blown, which is accomplished by forcing a predetermined current through the link. A preferred semi-fusible link is a thin film resistor, which, when made as short and thin as practical, has a typical resistance of about 1k-8k &OHgr; when intact and about 20k-80k &OHgr; when blown.
Each of the links in a semi-fusible link system is connected to a respective programming circuit which, when activated, provides the programming current needed to blow the link. The links are also connected in series with respective current sources, and to respective threshold detectors connected to monitor the voltage across their respective links. Each threshold detector provides a logic output which indicates whether its link is intact or blown based on the magnitude of the link voltage. The logic output can then be used to, for example, drive a transistor switch which switches a known resistor into or out of a circuit, or be read out via a bond pad or a serial interface.
A circuit employing the present semi-fusible link system typically includes a number of such link/programming circuit/current source/threshold detector groups, with the current sources activated in turn by a logic decoder which responds to a fuse select signal. A “disable” circuit is also included which is activated when all desired links have been blown, which prevents the occurrence of any additional programming.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
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Analog Devices Inc.
Brown Glenn W.
Hamdan Wasseem H.
Koppel & Jacobs
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