Method and Apparatus for Stress Testing a Semiconductor Memory

1. Field of the Invention

This invention relates in general to semiconductor memories and, more specifically, to stress testing of such memories.

2. State of the Art

As shown in FIG. 1, a portion 10 of a conventional Dynamic Random Access Memory (DRAM) (not shown in its entirety) includes sense amplifiers 12 and 14 shared by a pair of sub-arrays 16 and 18. During some read operations within the sub-array 16, an isolation signal ISO.sub.-- 2 activates isolation NMOS transistors 20 so logic bits on bitlines 22 within the sub-array 16 can be sensed by shared sense amplifiers 12 and 14. Meanwhile, another isolation signal ISO.sub.-- 3 deactivates isolation NMOS transistors 24 so signals on bitlines 26 within the sub-array 18 do not reach the shared sense amplifiers 12 and 14 during the read operations. Of course, some read operations in the sub-array 18 are performed in a complementary manner, with the isolation NMOS transistors 24 in an activated state and the isolation NMOS transistors 20 in a deactivated state.

As described in U.S. Pat. No. 5,339,273 to Taguchi, one method for stress testing the conventional DRAM involves reading a logic bit from one of the sub-arrays 16 and 18, for example, while both the isolation NMOS transistors 20 and the isolation NMOS transistors 24 are activated. This loads a memory cell (not shown) outputting the logic bit with approximately twice the normal bitline capacitance C.sub.bitline When the memory cell is weak, its stored charge q cannot overcome the doubled bitline capacitance C.sub.bitline to produce a voltage V.sub.sense at one of the sense amplifiers 12 and 14 that is of sufficient magnitude to be sensed (i.e., V.sub.sense =q .div.C.sub.bitline, so that when C.sub.bitline doubles and q remains constant, V.sub.sense is cut in half). As a result, the logic bit may be misread, thereby identifying the memory cell as being weak. The row or column (not shown) containing the weak memory cell can then be replaced with a redundant row or column (not shown), or the DRAM can be scrapped.

Unfortunately, the Taguchi method described above does not work with sense amplifiers 28 and 30 positioned outside the sub-arrays 16 and 18, because these amplifiers 28 and 30 are each connected to only one of the sub-arrays 16 and 18. As a result, memory cells (not shown) connected to the sense amplifiers 28 and 30 through bitlines 32 and 34 and isolating NMOS transistors 36 and 38 (activated by isolation signals ISO.sub.-- 1 and ISO.sub.-- 4) cannot be loaded with double their normal bitline capacitance, and thus cannot be stress tested, using the described Taguchi method.

Therefore, there is a need in the art for an apparatus and method in a semiconductor memory, such as a DRAM, for stress testing memory cells associated with sense amplifiers connected to only one sub-array within the memory.

An apparatus in accordance with the present invention stress tests a semiconductor memory, such as a Dynamic Random Access Memory (DRAM), by connecting a sense amplifier of the semiconductor memory to at least two pairs of complementary bitlines within the same memory array, such as a sub-array, of the semiconductor memory through two pairs of isolation switches activated at substantially the same time by activating circuitry of the apparatus. The apparatus thus provides for the stress testing of memory cells associated with sense amplifiers connected to only one array or sub-array within a semiconductor memory. The apparatus also provides an alternative to the conventional Taguchi method described above for stress testing memory cells associated with sense amplifiers connected to more than one array or sub-array within a semiconductor memory.

In additional embodiments of the present invention, a semiconductor memory, an electronic system, a semiconductor wafer, and a DRAM device incorporate the inventive apparatus described above.

In an inventive method for stress testing a semiconductor memory, a sense amplifier of the semiconductor memory is switchably coupled to at least two pairs of complementary bitlines in the same memory array of the semiconductor memory at substantially the same time.

FIG. 1 is a block diagram and circuit schematic of a portion of a conventional Dynamic Random Access Memory (DRAM) to which a conventional stress testing method is applied;

FIG. 2 is a block diagram and circuit schematic of a portion of a DRAM including circuitry for performing stress tests on the DRAM in accordance with the present invention;

FIG. 3 is a circuit schematic showing circuitry for initiating a test mode in the stress testing circuitry of FIG. 2;

FIG. 4 is a block diagram of an electronic system incorporating the DRAM of FIG. 2; and

FIG. 5 is a diagram of a semiconductor wafer incorporating the DRAM of FIG. 2.

As shown in FIG. 2, a portion 40 of a Dynamic Random Access Memory (DRAM) (not shown in its entirety) includes isolation NMOS transistors 42, 44, 46, 48, 50, and 52 connecting sense amplifiers 54, 55, 56, 57, and 58 to complementary pairs of bitlines 60, 61, 62, 63, 64, 65, and 66 within sub-arrays 68 and 70. Although the present invention will be described with respect to a DRAM, it will be understood by those having skill in the field of this invention that the invention is applicable to other semiconductor memories as well. Also, it will be understood that although the invention will be described in association with a semiconductor memory having multiple sub-arrays, it is equally applicable to semiconductor memories containing a single memory array.

During normal read operations on bitlines 60 within the sub-array 68, for example, a test signal TEST.sub.-- 1 deactivates the isolation transistors 42 to isolate the sense amplifier 56 from the bitlines 60. At the same time, an isolation signal ISO.sub.-- 2 activates the isolation transistors 50 so the sense amplifier 54 can sense a logic bit on the bitlines 60, and another isolation signal ISO.sub.-- 3 deactivates the isolation transistors 52 to isolate the sense amplifier 54 from the bitlines 64 in the sub-array 70.

The status of isolation signals ISO.sub.-- 1, ISO.sub.-- 2, ISO.sub.-- 3, and ISO.sub.-- 4, and the status of test signals TEST.sub.-- 1 and TEST.sub.-- 2, during all normal read operations within the sub-arrays 68 and 70 is summarized in the following table:

During stress testing of the DRAM, memory cells (not shown) associated with the bitlines 62 within the sub-array 68, for example, may be stress tested in accordance with the present invention in the following manner. The isolation signal ISO.sub.-- 1 activates the isolation transistors 50 to allow the bitlines 62 to be sensed by the sense amplifier 56. At the same time, the test signal TEST.sub.-- 1 activates the isolation transistors 42 to connect the bitlines 62 to the bitlines 60. As a result, memory cells associated with the bitlines 62 are loaded with twice their normal bitline capacitance, allowing for the identification of weak memory cells through misreads resulting from the doubled capacitance load in the same manner as previously described.

In general, stress testing of any of the bitlines 62, 63, and 66 occurs as summarized in the following table.

The isolation transistors 42, 44, 46, and 48 are shown in FIG. 2 as being associated with sense amplifiers 56, 57, and 58, which are themselves each associated with only one of the sub-arrays 68 and 70. The embodiment of the present invention depicted in FIG. 2 is thus illustrated as providing a stress testing capability to bitlines 62, 63, and 66 which cannot be provided using the conventional Taguchi method previously described. However, it should be understood that additional isolation transistors (not shown) could also be provided so that, during a stress test of the DRAM, the sense amplifier 54, for example, can be connected to the bitlines 60 and 64 in accordance with the conventional Taguchi method, or can be connected to bitlines 60 and 62. The present invention thus provides an alternative means for conducting stress tests using sense amplifiers shared by two or more sub-arrays.

As shown in FIG. 3, activating circuitry 70 provides an active (i.e., high) test signal TEST.sub.-- 1 when a stress test mode signal TEST.sub.-- MODE* activates a PMOS transistor 72, the complement ISO.sub.-- 1* of the isolation signal ISO.sub.-- 1 activates another PMOS transistor 74, and the isolation signal ISO.sub.-- 2 activates still another PMOS transistor 76. The activating circuitry 70 provides an inactive (i.e., low) test signal TEST.sub.-- 1 when the stress test mode signal TEST.sub.-- MODE*, the complement signal ISO.sub.-- 1*, or the isolation signal ISO.sub.-- 2 activates any one of a group of NMOS transistors 78. Other activating circuitry (not shown) provides the test signal TEST.sub.-- 2, with the test signal TEST.sub.-- 2 being active (i.e., high) when the stress test mode signal TEST.sub.-- MODE* is active (i.e., low), the isolation signal ISO.sub.-- 3 (FIG. 2) is inactive (i.e., low), and the complement ISO.sub.-- 4* of the isolation signal ISO.sub.-- 4 (FIG. 2) is active (i.e., low), and with the test signal TEST.sub.-- 2 being inactive (i.e., low) when either the stress test mode signal TEST.sub.-- MODE* is inactive (i.e., high), the isolation signal ISO.sub.-- 3 is active (i.e., high), or the complement signal ISO.sub.-- 4* is inactive (i.e., high).

As shown in FIG. 4, an electronic system 80 includes an input device 82, an output device 84, a processor device 86, and a memory device 88 that incorporates a DRAM 90 that includes the DRAM portion 40 of FIG. 2. Of course, it should be understood that any one of the input, output, processor, and memory devices 82, 84, 86, and 88 may incorporate the present invention.

As shown in FIG. 5, a DRAM 92 that includes the DRAM portion 40 of FIG. 2 is fabricated on the surface of a semiconductor wafer 94. The wafer 94 may comprise a sliced wafer of silicon, or may comprise any one of a wide variety of substrates, including, for example, a Silicon-on-Sapphire (SOS) substrate, a Silicon-on-Insulator (SOI) substrate, or a Silicon-on-Glass (SOG) substrate.

Although the present invention has been described with reference to particular embodiments, the invention is not limited to these described embodiments. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods that operate according to the principles of the invention as described.