Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Fusible link or intentional destruct circuit
Reexamination Certificate
1999-04-29
2001-07-31
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Fusible link or intentional destruct circuit
C365S096000
Reexamination Certificate
active
06268760
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to integrated fusible link status detection methods.
BACKGROUND: POLYSILICON FUSE PROGRAMMING CIRCUITS
Polysilicon fuses form one of several structures used as a nonvolatile trim for analog and digital circuits. Previously, polysilicon fuse circuits have taken a form typically as shown in
FIG. 4
, where a small current source
400
(typically 1 microampere) and threshold detector (e.g. comparator
402
) have been used to detect whether a fuse F
1
is intact or blown. This method has the disadvantages of: (1) requiring a continuous current flow (for an unblown fuse), and (2) allowing the fuse to appear as unblown if even a small amount of regrowth occurs.
In a typical case of a circuit employing a 1 microampere current source and 1 V threshold detection, any resistance value less than approximately 1 megohm will appear to be a good fuse. The typical resistance of an unblown integrated fuse is around 100 Ohms. The disadvantage of continuous current flow has been bypassed previously by reading the fuse conditions at chip reset and latching them into volatile logic, and then powering down the current source using active logic.
A related choice, which affects the requirements for detecting whether the fuse has be completely blown, is the way in which the fuse is blown. The conventional method for blowing fuses (whether poly, metal or zener) is by adding probe pads to each end of the fuse, and directly applying a voltage, through probes, to blow the appropriate fuses during probing. This need for probe pads adds significantly to the chip area. Another method is to “blow” the fuse using a laser during the electrical probe operation. This method has additional cost associated with combining a laser with the tester. Optimally, it would be desirable to be able to blow the fuses when the chip was in the package. This would provide for improved in-package voltage precision, using the same serial interface that was used for normal circuit operation.
Polysilicon Fuse Status Detection Circuit With Serial Interface Fusing
The present application discloses a polysilicon fuse programming and status detection circuit with zero operating current and higher reblow capability where filament regrowth occurs. This is achieved by using a feedback connection, where the output logic state which indicates a blown state is fed back to a pull-up transistor which constantly applies voltage to one end of the blown fuse. Thus if regrowth occurs the fuse will be reblown. At the same time, the logic state of the fuse is always directly available as a logic level, without any special requirements for latching and powering down the fuse-reading circuitry. The circuit provides in one embodiment, for example, a pair of transistors which are both used to provide current through the fuse during blowing. In this embodiment control of one transistor controls both, so that when the signal to blow the fuse is removed, both circuits are turned off (unless filament regrowth occurs, and then one of the transistors automatically turns on and reblows the filament). Some embodiments also disclose a serial interface fusing technique for blowing the fuse while the circuit is enclosed in packaging.
The disclosed inventions, in various embodiments, provide a zero operating current, static logic polysilicon fuse-state detector circuit, and solve the problems presented by conventional methods where excessive current is used in polysilicon fuse arrays, and false detections occur due to filament regrowth.
REFERENCES:
patent: 5442589 (1995-08-01), Kowalski
patent: 5579266 (1996-11-01), Tahara
patent: 5731760 (1998-03-01), Ramirez
Marshall Andrew
Summerlin Travis
Brady III Wade James
Lam Tuan T.
Nguyen Hiep
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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