U.S. patent application number 11/436550 was filed with the patent office on 2006-09-14 for programmable element latch circuit.
Invention is credited to Greg A. Blodgett.
Application Number | 20060203580 11/436550 |
Document ID | / |
Family ID | 24569523 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060203580 |
Kind Code |
A1 |
Blodgett; Greg A. |
September 14, 2006 |
Programmable element latch circuit
Abstract
An antifuse latch device and method for performing a redundancy
pretest without the use of additional test circuitry is disclosed.
Conventional antifuse latch devices are designed such that a
redundancy pretest cannot be performed on the antifuse latch device
once the antifuses are programmed but rather requires additional
circuitry to map the appropriate address bits to test the redundant
row or column. The present invention adds a level translating
inverter to a conventional antifuse latch device, thus allowing the
antifuse latch device to simulate an unblown antifuse by isolating
the antifuse from the latch.
Inventors: |
Blodgett; Greg A.; (Nampa,
ID) |
Correspondence
Address: |
DICKSTEIN SHAPIRO LLP
1825 EYE STREET NW
Washington
DC
20006-5403
US
|
Family ID: |
24569523 |
Appl. No.: |
11/436550 |
Filed: |
May 19, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10372196 |
Feb 25, 2003 |
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11436550 |
May 19, 2006 |
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09640741 |
Aug 18, 2000 |
6553556 |
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10372196 |
Feb 25, 2003 |
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Current U.S.
Class: |
365/200 |
Current CPC
Class: |
H03K 19/1736 20130101;
G11C 29/24 20130101; G11C 17/18 20130101; G11C 29/02 20130101; G11C
29/027 20130101 |
Class at
Publication: |
365/200 |
International
Class: |
G11C 29/00 20060101
G11C029/00 |
Claims
1-42. (canceled)
43. A method of testing an electrical device comprising:
programming a fusible device into a programmed state during a
first-time interval; receiving a signal corresponding to said
programmed state at a first input of a latch circuit during a
second time interval; receiving a control signal at a second input
of said latch circuit, said control signal having a first state
during a third time interval and a second state during a fourth
time interval; and producing an output signal at an output of said
latch circuit, said output signal having a state dependent on said
programmed state during said third time interval, and said output
signal having a state independent of said programmed state during
said fourth time interval.
44. A method of testing an electrical device as defined in claim 43
wherein said second time interval encompasses said third and fourth
time intervals.
45. A method of testing an electrical device as defined in claim 43
wherein said fusible device comprises an antifuse device.
46. A method of testing an electrical device as defined in claim 43
wherein said programming said fusible device into a programmed
state comprises applying a programming voltage to said fusible
device.
47. A programmable device comprising: fusible means for storing a
data state; circuit means for reading said data state and producing
a data output dependent on said data state during a first time
interval; said circuit means being adapted to produce a data output
independent of said data state during a second time interval, said
second time interval being subsequent to said first time
interval.
48. A programmable device as defined in claim 47, wherein said
circuit means for reading said data state comprises an inverter
circuit.
49. A programmable device as defined in claim 48 wherein said
inverter circuit comprises a level translating inverter.
50. A programmable device as defined in claim 47 further comprising
a signal input adapted to receive a control signal, said control
signal having a first state corresponding to said first time
interval and a second state corresponding to said second time
interval.
51. A programmable device as defined in claim 47 wherein said
fusible means comprises an antifuse.
52. A programmable device as defined in claim 47 wherein said
fusible means comprises a flash memory cell.
Description
BACKGROUND OF THE INVENTION
[0001] I. Field of the Invention
[0002] The present invention relates generally to a device and
method for testing semiconductor electrical devices. In particular,
the present invention relates to simulating a programmable state of
a device when the device has already been programmed to another
programmable state.
[0003] II. Description of the Related Art
[0004] In order to ensure proper operation, semiconductor devices
are typically tested before being packaged into a chip. A series of
probes on a test station electrically contact pads on each die to
access the semiconductor devices on the die. For example, in a
semiconductor memory device, the probes contact address pads and
data input/output pads to access selected memory cells in the
memory device. Typical dynamic random access memory ("DRAM")
devices include one or more arrays of memory cells arranged in rows
and columns. Each array of memory cells includes word or row lines
that select memory cells along a selected row, and bit or column
lines (or pairs of lines) that select individual memory cells along
a row to read data from, or write data to, the cells in the
selected row.
[0005] In a test procedure, predetermined data or voltage values
are typically written to selected row and column addresses that
correspond to certain memory cells, and then the voltage values are
read from those memory cells to determine if the read data matches
the data written to those addresses. If the read data does not
match the written data, then the memory cells at the selected
addresses likely contain defects and the semiconductor device fails
the test.
[0006] Many semiconductor devices, particularly memory devices,
include redundant circuitry on the semiconductor device that can be
employed to compensate for certain detected failures. As a result,
by enabling such redundant circuitry, the device need not be
discarded even if it fails a particular test. For example, memory
devices typically employ redundant rows and columns of memory cells
so that if a memory cell in a column or row of the primary memory
array is defective, then an entire row or column or partial row or
column of redundant memory cells can be substituted therefor,
respectively.
[0007] Substitution of one of the redundant rows or columns is
conventionally accomplished by blowing selected antifuses in a bank
of antifuse latch devices to select redundant rows or columns to
replace defective primary rows or columns. Each bank represents a
memory address. If a given primary row or column in the array
contains a defective memory cell, antifuses in the bank of
antifuses are blown such that the bank of antifuses produces a
binary output matching the defective address. An antifuse is a
capacitive device that may be blown by applying a relatively high
voltage across it which causes the dielectric layer in the antifuse
to break down and form a conductive path. A blown antifuse will
conduct current while an unblown antifuse will not conduct current.
For example, if the defective primary row or column has an 8-bit
binary address of 00100100, then the appropriate antifuses in a
bank of 8 antifuses are blown to store this address. The individual
antifuses are generally contained in antifuse latch devices which
generate a digital value or signal indicating whether the antifuse
is blown or unblown and may be arranged in groups of 8, each group
of 8 defining the address fuses for one antifuse bank.
[0008] When an address in the memory device is accessed, a compare
circuit compares an incoming address to addresses stored in the
antifuse banks to determine whether the incoming address matches an
address containing a defective memory cell. If the compare circuit
determines such a match, then it outputs a match signal to a row or
column decoder. In response, the row or column decoder causes an
appropriate redundant row or column to be accessed, and ignores the
defective primary row or column in the array.
[0009] After antifuses have been programmed to store an address of
a defective primary row or column, testing often occurs where it
would be beneficial if the antifuse latch device could maintain a
state other than that programmed. For example, an antifuse latch
device may physically have a blown antifuse, but need to simulate
an unblown state to test different configurations of the redundant
rows or columns.
[0010] Redundant elements are typically tested by assigning a
pretest address to each antifuse bank. This pretest address is hard
coded so that each memory device has the same redundant pretest
address, and the test program is therefore valid for every device.
Additional test circuitry is required on the memory device to
achieve this hard coding of the pretest addresses. As the number of
redundant elements increases, the amount of test circuitry required
to define the pretest addresses also increases. For ease of
testing, it is desirable to have the ability to test redundant
elements using pretest addresses even after the elements have been
programmed to repair defective memory elements, e.g. after a repair
address has been programmed into the antifuse bank.
[0011] Traditionally, in a pretest test mode, the antifuse bank is
forced to output a match in response to a pretest address
regardless of the state of the antifuse latches. The forced match
is accomplished through the use of test circuitry, which bypasses
the antifuse latch versus input address compare circuitry, and
therefore is not accurate in terms of address to match signal
delay. In order to generate the match signal, addresses must be
decoded and logically combined with the pretest test mode signal to
determine when to force the match. This pretest address decoder
increases in size as the number of redundant elements on a memory
device increases because more address combinations are required to
provide enough unique pretest addresses. Providing sequential
pretest addresses require an even larger pretest address decoder
since more address terms are required for each bank. For example,
in the prior art, it has been sufficient to use just one address
term ANDed with the pretest signal to decode a redundant element
fuse bank. A0 high would enable bank 0, A1 high would enable bank
1, A2 high would enable bank 2, etc. To avoid enabling multiple
fuse banks, only address 0, 2, 4, etc. would be valid using this
methodology, and the maximum number of unique fuse bank pretest
addresses is limited to the number of address inputs to the device.
Current memory devices may require 3 or 4 address terms be ANDed
together to generate the required number of pretest addresses.
Another drawback of this method is that the tester requires a large
memory space for the pretest addresses even though only a few of
the addresses are actually valid. Thus, there exits a need for an
antifuse latch that can be temporarily programmed for test purposes
to a state which is independent of the programmed state and which
can ideally be programmed into sequential pretest address
states.
SUMMARY OF THE INVENTION
[0012] The present invention relates to a device and method for use
in memory devices employing redundant rows and/or columns. The
present invention provides an antifuse latch device which may
perform a redundancy pretest using the real time operational signal
path. The circuit of the present invention implements a level
translating inverter to control a voltage to the gate of a
transistor; the transistor having a source terminal connected to
the output of an antifuse. The level translating inverter causes
the circuit to simulate an unblown antifuse (or state of other
similar programmable elements) by not supplying a voltage
sufficient to drive the gate of the transistor receiving a signal
from a physically blown antifuse. A blown antifuse can also be
simulated by rendering the FA (fuse address) signal high to supply
a low signal to the latch, where the physical antifuse is
unblown.
[0013] Thus, the present invention provides more reliable results
for a redundancy pretest, as the speed of signal propagation as
well as the functionality of other components in the signal path
can be observed during the test mode. The present invention also
eliminates the need for some of the circuitry used solely for the
redundancy pretest.
[0014] The device and method of operation of the invention are also
applicable to semiconductor memory devices with other types of
programmable elements, i.e. fuses, flash cells, etc., as these
elements have similar programmable functionality as an antifuse.
For example, a memory device may employ fuses for adjusting the
output level of a voltage regulator, or for configuring the device
for one of a plurality of device operating modes. Possible
operating modes include but are not limited to SDRAM latency, data
path width, delay locked loop control for DDR DRAM, etc;
[0015] Many other semiconductor devices have fuse options which
change the operational characteristics of the device in accordance
with the fuses that are blown. Voltage levels, timing delays,
input/output configurations, etc. can all be programmed with fuses.
The present invention allows for these options to be fully tested
without actually blowing the fuses, or adding additional test
latches to override the programmed state of the option fuses. Also,
the present invention allows for devices with options programmed to
be tested as though only a default set of the options had been
blown. In this manner, programmed devices which might otherwise be
incompatible for parallel testing are again made compatible. For
example, parts which are programmed to be 32 Meg.times.8 can be
forced to a 64 Meg.times.4 input/output configuration using the
actual antifuse latch which is programmed to determine the final
device configuration.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing and other advantages and features of the
invention will become more apparent from the detailed description
of exemplary embodiments provided below with reference to the
accompanying drawings in which:
[0017] FIG. 1 is an illustration of a conventional antifuse latch
device;
[0018] FIG. 2 is an illustration of an improved antifuse latch
device in accordance with an exemplary embodiment of the present
invention; and
[0019] FIG. 3 illustrates a processor system employing a memory
device containing the antifuse latch device of FIG. 2.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] Understanding a conventional antifuse latch device used in
memory devices, depicted in FIG. 1, is necessary to fully
comprehend the present invention, as the present invention improves
upon the circuit of FIG. 1. FIG. 1 illustrates a conventional
antifuse latch device used in memory devices to determine
assignment of redundant rows or columns for addresses having
defective primary rows or columns.
[0021] The antifuse latch device 95 receives an operating voltage
Vcc at a source of a PMOS transistor 96. The PMOS transistor 96 is
coupled through two PMOS transistors 102, 104 connected in parallel
to an input of an inverter 106. The input of the inverter 106 is
coupled to a ground through two NMOS transistors 110, 112 connected
in series. This circuitry forms a latch to output a discrete "1" or
"0." Gates of the PMOS transistor 102 and the NMOS transistor 110
receive a read fuse signal (RDFUS*). The RDFUS* signal is an active
low signal which is normally high to render the PMOS transistor 102
non-conductive and the NMOS transistor 110 conductive. The input of
the inverter 106 is also coupled to a first terminal of an antifuse
114 through two NMOS transistors 116, 118. A gate of the NMOS
transistor 116 receives a signal DVC2F which is normally slightly
greater than one-half Vcc and maintains the NMOS transistor 116 in
a conductive state. A gate of the NMOS transistor 118 receives a
boosted voltage Vccp (that exceeds Vcc) and maintains the NMOS
transistor 118 in a conductive state. A junction between the NMOS
transistors 116, 118 receives a bank select signal (BSEL*) through
an NMOS transistor 120 having a gate receiving a fuse address
signal (FA). A second terminal of the antifuse 114 receives a
programming signal CGND which is at ground potential in normal
operation.
[0022] The antifuse latch device 95 is programmed during
manufacture of a memory device after a test to determine which
primary rows or columns of addresses in the memory device are
defective. Additionally, the redundant rows and columns are tested
using a separate circuit only used for testing the redundant
elements. Such testing of redundant elements is typically not
performed using antifuse latch device 95.
[0023] During programming, the BSEL* signal is brought low and the
CGND signal is raised to about ten volts. Selected antifuses, such
as the antifuse 114, are blown when the fuse address signal (FA) is
brought high to render the NMOS transistor 120 conductive to allow
current to flow through the antifuse 114 and the NMOS transistors
118, 120. The current breaks down the dielectric layer in the
antifuse 114. If the antifuse 114 is to remain unblown the FA
signal is kept low such that the NMOS transistor 120 prevents
current from flowing through the antifuse 114. The signals BSEL*,
FA, and CGND are used only during the manufacture of the memory
device to program antifuse circuits. During operation of the memory
device, the fuse add signal FA is held low to render the NMOS
transistor 120 non-conductive, and the common ground signal CGND is
coupled to ground through a transistor (not shown).
[0024] The antifuse latch device 95 indicates whether the antifuse
114 is blown or unblown with an output signal at an output of the
inverter 106. The antifuse circuit 95 is read by an active low
pulse in the RDFUS* output signal to generate the signal. When the
RDFUS* signal is brought low, the PMOS transistor 102 is rendered
conductive to couple Vcc to the first terminal of the antifuse 114
through the transistors 96, 102, 116, 118. If the antifuse 114 is
unblown and thus remains non-conductive, the antifuse 114 is
charged. The voltage at the input of the inverter 106 is allowed to
rise with the voltage on the terminal of the antifuse 118 because
the NMOS transistors 116, 118 are ON. When the voltage rises above
a threshold voltage of the inverter 106, the inverter 106 outputs a
low signal to indicate that the antifuse 114 is unblown. Gates of
the PMOS transistor 104 and the NMOS transistor 112 are connected
to the output of the inverter 106 so that the transistor 104
latches the signal at the output of the inverter 106 and the
transistor 112 is switched OFF when the RDFUS* signal is brought
high at the end of its pulse to turn ON the transistor 110.
[0025] If the antifuse 114 is blown such that it conducts current,
then the input of the inverter 106 is held at substantially zero
volts despite Vcc being applied to the input of the inverter 106
through the PMOS transistors 96, 102. When the RDFUS* signal is
brought high, the input of the inverter 106 will remain low and its
output will be high. As a result, the PMOS transistor 104 is turned
OFF and the NMOS transistor 112 is turned ON to latch the output of
the inverter 106 high. The signal at the output of the inverter 106
thereby indicates the state of the antifuse 114 and provides one
digit of an address of a defective row or column.
[0026] While the above operation of antifuse latch device 95
illustrates how the antifuse circuit 95 is programmed, antifuse
latch device 95 may not be used for a redundancy pretest once the
antifuse is programmed because the pretest address will generally
not match the programmed address and is not practical to alter the
pretest addresses for each device being tested. Antifuse latch
device 95 has no method to simulate an unblown antifuse where
antifuse latch device 95 physically contains a blown antifuse
114.
[0027] The present invention provides a modification to the
antifuse latch device of FIG. 1 to allow a redundancy pretest
before or after antifuses 114 have been programmed by providing for
the simulation of an unblown or blown antifuse using the same
circuitry that will be used in the actual operation of the memory
device. FIG. 2 illustrates an exemplary embodiment of the present
invention. Antifuse latch device 195 includes a level translating
inverter 202 having an output connected to the gate of NMOS
transistor 118. The level translating inverter 202 passes Vccp to
the gate of NMOS transistor 118 when its input 204 is a low signal
and passes ground to the gate of NMOS transistor 118 when its input
204 is high.
[0028] In operation, the state of the antifuse 114 can be simulated
during a pretest after antifuse 114 had been programmed. By sending
a high signal level to input 204 of the level translating inverter
202 and driving the RDFUS* signal low, where all antifuse latch
devices 195 have a common level translating inverter 202 circuit
and a common RDFUS*, NMOS transistor 118 is shut off and antifuse
114 is isolated from the latch formed by inverter 106 and
transistors 96, 104, 110, and 112. Then, by driving BSEL* low,
where BSEL* is common for all antifuse latch devices 195, and by
driving FA high for particular antifuse latch devices 195, to
selectively program targeted antifuse latch devices 195, a blown
state is simulated. Thereafter, RDFUS* is driven high.
[0029] A blown antifuse is simulated because with BSEL* low and FA
high, a low value is passed through NMOS transistors 120 and 116 to
the input of inverter 106 overriding the logic high at the input of
106 because the drive strength of the series NMOS transistors 120
and 116 is greater than the drive strength of the PMOS transistors
96 and 104. Similarly, an antifuse latch device 195 will simulate
an unblown antifuse if FA is never driven high.
[0030] Even after all FA are driven back low, the antifuse latch
device 195 will retain the desired programmed state as long as the
fuse read signal RDFUS* is held high. For an entire bank of
antifuse latch devices 195, where the bank has a common level
translating inverter 202, a common bank select signal BSEL* and
individual FA lines per antifuse latch device 195, the entire bank
can be cleared and then programmed in one cycle by driving the
appropriate FA lines high with BSEL* low. In this manner, banks of
redundant elements can be assigned sequential pretest address
without the need of a pretest address decoder. Also, most of the
fuse latch as well as the address comparator can be used in the
redundant element pretest sequence providing identical timing to
the normal operation of the memory device. Once testing is
completed, the antifuse latch device 195 may resume its programmed
state by driving input 204 low, which renders NMOS transistor 118
conductive, no longer isolating the antifuse 114.
[0031] Typically, each redundant element antifuse bank will also
have an enable fuse to enable that particular bank to be active.
The present invention allows for the enable fuses to be
individually programmed regardless of the programmed state of the
enable antifuse. For example, all enable fuses can be cleared to
the unblown state allowing the device to be tested as though no
repair had been done.
[0032] FIG. 3. illustrates a simplified processor system 402 which
may employ memory devices containing the redundant row/column
pretest method and circuitry of the present invention. Processor
system 402 includes central processing unit (CPU) 412, RAM and ROM
memory devices 408, 410, input/output (I/O) devices 404, 406,
floppy disk drive 414 and CD ROM drive 416. All of the above
components communicate with each other over bus 418. The RAM memory
device 408 may use the FIG. 2 antifuse latch device 195 for
programming, testing and real time operation of redundant rows or
columns. RAM 408 and CPU 412 may also be integrated together on a
single chip.
[0033] It is to be understood that the above description is
intended to be illustrative and not restrictive. Many variations to
the above-described device and method will be readily apparent to
those having ordinary skill in the art. For example, as mentioned
above, the above device and method may be employed with any type of
programmable element, such as a fuse or flash cell, etc., where one
or more programmed states must be simulated for testing. The type
of logic implemented will vary based upon the type of programmable
element used.
[0034] For the purposes of this disclosure, antifuse, laser fuse,
electrical fuse, etc. are interchangeable terms. For example, a
fuse latch used in conjunction with laser fuses can be modified in
accordance with the teachings of the present invention to
accomplish the benefits of the invention for an antifuse latch.
[0035] Accordingly, the present invention is not to be considered
as limited by the specifics of the particular device and method
which have been described and illustrated, but is only limited by
the scope of the appended claims.
* * * * *