Static information storage and retrieval – Read/write circuit – Testing
Reexamination Certificate
1998-03-20
2001-09-04
Le, Vu A. (Department: 2824)
Static information storage and retrieval
Read/write circuit
Testing
C365S185230, C365S230060
Reexamination Certificate
active
06285608
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor memory devices. More particularly, the present invention relates to a method and apparatus for using the supply voltage as a power source for bitline stress and multicolumn testing in semiconductor memory devices.
BACKGROUND
Electronic information handling or computer systems, whether large machines, microcomputers or small and simple digital processing devices, require memory for storing data and program instructions. Various memory systems have been developed over the years to address the evolving needs of information handling systems. Often, an information handling system will employ a variety of memory technologies, which may be grouped or distinguished, for the purposes herein, as mass storage devices and semiconductor memory.
Mass storage devices are generally nonvolatile and primarily used for information not frequently accessed. Mass storage devices are sometimes called “moving-surface memory” because they take the form of discs and tapes. Such mass storage devices have large capacities, great flexibility and low cost. Of course, moving-surface memories require relatively high power to operate and are prone to mechanical failures over the life of the devices. Also, such mass storage devices are referred to as “serial” or “sequential” memories, from which data is available only in the same sequence as it is originally stored. Consequently, such mass storage devices are hampered by longer access times that creates an inconvenience every time they are used.
In contrast, semiconductor memory is usually the most rapidly accessible memory and, thus, the one from which most instructions and recently-used data are stored. In a semiconductor memory, often the time required for storing and retrieving information generally is independent of the physical location of the information within the memory. Semiconductor memory typically store information in a large array of cells. A group of cells are electrically connected together by a bitline, or data line. An electrical signal is used to program a cell or cells. The electrical signal on the data line is controlled by the bitline driver circuit. Accordingly, a semiconductor memory device may include several groups of cells, each coupled together with a data line operated by a bitline driver. The electrical signal on the data line is supplied by a programming signal provided on semiconductor memory devices. The programming signal must supply a large amount of current on the data line.
Prior to shipping, a manufacturer will test its semiconductor memory devices. Among the tests performed include a bitline stress test. Generally, the bitline stress test is used to observe the effect of the electrical signal on the data line on cells not intended to be programmed. In the bitline stress test, a higher voltage than usual is applied to all of the cells in a group. During such a test, a leakage current is produced from the cells, and the total leakage current during a test can be quite substantial. Thus, in order to perform this test, a larger than usual amount of current is required. Other tests require that multiple groups of cells are programmed at the same time. These tests require a large amount of current which is itself multiplied by the number of groups programmed in parallel.
The large amount of current required for these tests can have a deleterious effect on the circuitry of the memory device. For example, in flash memory devices, a significant type of semiconductor memory device, the source of the programming signal is often on the opposite side of the chip from the bitline driver. A wide line able to supply large currents is required to travel a relatively great distance in order to provide the programming signal to the bitline driver. The amount of current carried by the line and the distance the current is required to travel effects electromigration issues that contribute to signal loss in the line and unwanted electrical interference in the memory device. Also, many of the relatively delicate circuitry elements are adversely affected by the large amounts of currents during testing of the device.
SUMMARY OF THE INVENTION
The present invention relates to a method for using a supply signal, rather than a programming signal, to test bitline stress and multicolumn programming in semiconductor memories. Also, the present invention relates to a memory device which makes use of the supply signal to test bitline stress and multicolumn programming. The present invention, in various aspects, employs the high currents as in the prior art. The present invention, however, provides for a power source that is proximate the relevant circuitry. Thus, the problem of electromigration encountered in the prior art is reduced.
One aspect of the present invention is a method for testing bitline stress in a memory device having a bitline driver. The bitline driver controls the voltage on the bitline, or data line. The method in this aspect includes the step of generating a programming signal and a supply signal. Both the programming signal and the supply signal are suitable for powering the memory device. The supply signal is provided to the bitline driver during the test-programming of the memory device. Further features of this aspect of the invention include providing a nonvolatile memory device having a plurality of cells associated with the data line and the bitline driver.
Another aspect of the present invention is a memory device suitable for testing bitline stress in the above-described manner. A programming signal and a supply signal provide power to the memory device. The memory device includes a bitline driver circuit which provides an output to a data line. The circuit isolates the programming signal from the data line, and the supply signal is placed in electrical communication with the data line.
The present invention in its various aspects includes several advantages over the prior art. Among these advantages is that the supply signal provided to the bitline driver is set with a larger degree of precision than in the prior art. This larger degree of precision provides for more accurate testing procedures than in the prior art. Also, the present invention makes use of the structure preferred for general operation of the device, and does not require a modification to the size of the lines providing the programming signal for the limited purpose of testing the device, as in the prior art. The lines providing the supply signal are typically larger than the lines providing the programming signal. The present invention employs the existing and larger supply signal lines. Furthermore, the use of the larger capacity supply signal, as compared with the capacity of the programming signal, greatly reduces a drop in voltage across the circuit elements follower as a result of the large currents flowing through them.
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Le Vu A.
Micro)n Technology, Inc.
Schwegman Lundberg Woessner & Kluth P.A.
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