# Patent

## Title
Built-in-self-test (BIST) engine configured to store a per pattern based fail status in a pattern mask register

## Abstract

A BIST engine configured to store a per pattern based fail status during memory BIST run and related processes thereof are provided. The method includes testing a plurality of patterns in at least one memory device and determining which of the plurality of patterns has detected a fail during execution of each pattern. The method further includes storing a per pattern based fail status of each of the detected failed patterns.

## Background
FIELD OF THE INVENTION
The invention relates to a built-in-self-test (BIST) engine and, more particularly, to a BIST engine configured to store a per pattern based fail status during memory BIST run and related processes thereof.
BACKGROUND
Memory BIST (built-in self-test) is an important tool for testing memories (including finding/diagnosing and repairing defects within those memories). As more memory is integrated into chips, thorough BIST test and repair is a requirement in order to ensure reasonable product quality/reliability levels. To improve BIST quality oftentimes more test patterns are run as part of manufacturing test. Total test time can take many millions of cycles when all necessary test patterns are included.
Memory BIST is designed to operate as many memories as possible simultaneously, while still avoiding false failures due to over test. False failures may be due to exceeding the power specification for a certain chip design, amongst other fail modes. For a given chip design, this might be a small subset, whereas for other chip designs this could include virtually all memories. In addition, memory BIST should be able to test with some margin compared to the normal functional application in order to produce good SPQL (shipped product quality level) while minimizing impacts to yield. Also, high quality memory test and repair via extremely thorough test needs to be balanced against test time. Test time can be a major cost component of chips (sometimes reaching into the multi-minute range).
During manufacturing test, the BIST engine executes multiple test patterns to test the connected memories. If a fail is detected during the execution of a pattern, the fail information is collected by the BIST engine. If more fails are detected in any of the subsequent patterns, the new fail information is accumulated with the old fail information in the BIST engine. At the end of the test, the BIST has the information whether any of the memories associated with it has failed. But, it is not possible in such methodologies to know the distribution of fails on a per-pattern basis.
SUMMARY
In an aspect of the invention, a method comprises testing a plurality of patterns in at least one memory device and determining which of the plurality of patterns has detected a fail during execution of each pattern. The method further comprises storing a per pattern based fail status of each of the detected failed patterns.
In an aspect of the invention, a method comprises reusing a pattern mask register of a BIST engine to store fail information at a bit location for a pattern that detected a fail.
In an aspect of the invention, a method comprises: issuing an execution command to a BIST engine to run patterns which are not masked by a pattern mask register; enabling the pattern mask register for data collection; after completion of the pattern, determining whether there is a FAIL for the tested pattern and, if so, enable FAIL collection; and updating a corresponding pattern bit in the pattern mask register, which will store the pattern that caught a FAIL.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention.
FIG. 1 shows a BIST engine which implements processes in accordance with aspects of the present invention.
FIG. 2 shows an illustrative, non-limiting example of a pattern mask register which can be implemented in the BIST engine of FIG. 1.
FIG. 3 shows a flow diagram implementing the processes in accordance with aspects of the present invention.



DETAILED DESCRIPTION
The invention relates to a built-in-self-test (BIST) engine and, more particularly, to a BIST engine configured to store a per pattern based fail status during memory BIST run and related processes thereof. In more specific embodiments, a pattern mask register can be re-used to store test results of individual test patterns that have completed diagnostics. In this way, it is possible to generate and access test pass/fail results for particular patterns from a BIST system used to test memory devices.
By implementing the present invention, e.g., providing a structure and method to collect the failure information on per-pattern basis using the existing resources of the BIST engine, it is now possible to know which patterns fail in which Process-Voltage-Temperature (PVT) corner so that the test flow can be optimized to run patterns which are detecting fails in different PVT corners. Moreover, if, in a particular PVT corner, it is observed that the fails are being detected by only a few handful patterns out of the whole pattern suite then, by implementing the systems and processes described herein, the test flow can be optimized to exclude the patterns, which are not detecting any fails, from executing. Also, if it is observed that across PVT corners only a subset of patterns are capturing the fails, then the test flow can be updated accordingly to include only those patterns for execution. Additionally, if a given pattern does find fails but does not find any unique fails (a fail that does not fail any other pattern), then it would be possible to disable that pattern, as all fails found by that pattern will still be found by some other pattern.
In more specific embodiments, the BIST engine contains a pattern mask register where each bit corresponds to a pattern in a pattern suite. When a bit corresponding to a particular pattern is set to ‘1’ in the pattern mask register, the pattern is “masked” and is skipped during the BIST execution. However, when a fail is reported during the execution of a pattern, the pattern mask register is re-used to capture the fail information, e.g., the fail information can be stored at the bit location for the pattern. For example, during the execution of a test pattern, if a fail is detected then a single bit “fail status” register is set to ‘1’. After the execution of the pattern, the value of the fail status register is written to the particular bit in the pattern mask register which corresponds to the current pattern. Thus, the pattern mask register bits are re-used to capture the fail information. At the end of the BIST run, the pattern mask register is read out to determine which patterns have detected a fail. For example, a bit in the pattern mask register set to ‘1’ implies that the corresponding pattern has detected a fail, whereas a value of ‘0’ implies that no fails were detected during the execution of the particular pattern. A value of ‘0’ can also imply that the pattern was masked.
FIG. 1 shows a BIST engine in accordance with aspects of the present invention. The BIST engine 100 includes logic 105 comprising AND gates 114 and 116 and OR gates 110, in addition to a bit fail status register 112. The bit fail status register 112 is initialized to ‘0’ at the beginning of the BIST run, as well as before the start of any subsequent pattern execution. The BIST engine 100 further includes demultiplexer 120 and a pattern mask register 122. As described further herein, the pattern mask register 122 not only is used to control whether each individual pattern is to be masked so that it is not run and skipped, it can also store the failed bit which their corresponding patterns have detected a fail. That is, the pattern mask register 122 can be used to collect per pattern fail information in order to aid in the extensive characterization process. In embodiments, the pattern mask register 122 can be re-used to store fail information across all BIST/memory types, and across all chips in the production test flow
The demultiplexer 120 is a device which takes a single input signal, e.g., a FAIL (‘1’) and selects one of many data-output-lines for populating respective bit locations of the pattern mask register 122. In embodiments, the demultiplexer 120 is controlled by an FSM (finite state machine) 118. For example, the FSM 118 will instruct the demultiplexer 120 to input a ‘1’ (FAIL) or a ‘0’ (PASS) into a certain location in the pattern mask register 122, where each bit will represent a PASS/FAIL indication corresponding to a particular pattern in the pattern suite (e.g., pattern mask register) as shown in the example of FIG. 2.
In operation, a FAIL (‘1’) of any memory (SBIO) will be passed through OR gate 110 to the fail status register 112. The FAIL (‘1’) will feed back to AND gate 114 of the logic 105 until the pattern is complete, such that when a fail is captured into the fail status register it will stay there until the end of the pattern (i.e. a “sticky latch”), at which time AND gates 114, 116 will be brought to ‘0’, e.g., deactivated, so that a FAIL (‘1’) will no longer be stored in the fail status register 112. In parallel with this, the FAIL (‘1’) will be moved into a bit position in the pattern mask register, which corresponds to the pattern that was just completed. In this way, when a FAIL is detected during the execution of a pattern, the fail status register 112 is set to ‘1’ and this ‘1’ stays in the fail status register until the completion of the pattern. At the end of the pattern, the value of the fail status register 112 is written into the bit of the pattern mask register (e.g., control register) 122 by demultiplexer 120, wherein the location corresponds to the current pattern being executed. In embodiments, the FSM 118 will instruct the demultiplexer 120 to input a ‘1’ (FAIL) into a certain location in the pattern mask register 122, where each bit will correspond to a particular failed pattern in the pattern suite (e.g., pattern mask register) as shown in the example of FIG. 2. It should also be noted that the pattern mask register 122 will be set to ‘0’ for any masked pattern (which is not tested).
At the end of the BIST run, when all patterns have been executed, the pattern mask register 122 will have only those bits set to ‘1’, for which their corresponding patterns have detected a fail. Thus, at the end of the BIST run, reading out the pattern mask register 122 will provide the fail status on a per pattern basis. In embodiments, the failing pattern information can be shifted out of the pattern mask register 122 using an existing shuttle register interface.
FIG. 3 shows a flow diagram implementing the processes of the present invention. Prior to discussing the processes of the flow diagram, it needs to be recognized that the processes can be implemented with multiple BIST engines. For example, there are different types of BIST engines used for different types of memories, e.g., 1-port, 2-port memories or TCAMs. Using the processes of the present invention, during standard production test, it is now possible to obtain pattern specific fail information for 1-port, 2-port or TCAM specific patterns during normal production testing, without requiring any additional test patterns or test time. Also, some BISTs are connected to larger memories and some are connected to smaller memories. By implementing the processes of the present invention it is now possible to obtain a distribution of fails by pattern, across different types and sizes of memory. Thus, the re-use of the pattern mask register enables access to information about fail distribution on a per-pattern basis for each group of memories sharing a BIST instance and across different types of memory. It is important to be able to collect this information during production testing, because it allows the test pattern characterization and resulting optimization to be performed over an extremely large volume of test data. The processes described herein also allows the test program to be modified so as to reduce test-time by eliminating patterns which, based upon extensive characterization, are determined to be unnecessary as they might never or only very rarely, catch a unique fail.
Referring now to FIG. 3, at step 300, each BIST engine is issued an execution (run) command and the pattern mask register is enabled for data collection. By issuing the execution command, the BIST engine can begin executing the test patterns which are not masked by the pattern mask register. In embodiments, all BIST engine instances on a chip can be run in parallel.
At step 305, a determination is made as to whether the pattern is enabled; in other words, a determination is made as to whether the pattern is masked, in which case the pattern is skipped during the BIST execution. If the pattern is masked, then the processes proceed to the next pattern, at step 310. By way of example, a pattern has been masked in the pattern mask register 122 with a value of ‘1’ in its corresponding bit. It should also be noted that while the BIST engine 100 skips the pattern, the corresponding bit in the pattern mask register will be set to ‘0’.
If the pattern is not masked, then the processes proceed to step 315. At step 315, the pattern is run. After completion of the pattern, the processes continue to step 320, where a determination is made as to whether there is a FAIL for the tested pattern and, if so, the FAIL collection is enabled. Accordingly, when a FAIL is seen in a particular pattern of an SBIO, this information is sent to the corresponding BIST engine where it is latched and stored in the pattern mask register 22, at step 325. Thus, at the end of each pattern the particular FAIL information is used to update the corresponding pattern bit in the pattern mask register, which will store the fact that that the particular pattern caught a FAIL. The fail status latch is then reset to “0” prior to moving to the next pattern. This happens in all BIST engine instances where a fail is discovered.
At step 330, a determination is made as to whether all of the patterns have been completed, e.g., tested. At the end of the total BIST run, when all the unmasked patterns for all BIST instances have been executed, the failing pattern information, now stored in the pattern mask registers of each BIST, can be shifted from the BIST engines out to the tester using a BIST shuttle readout register, at step 335. At step 340, the process can analyze the data to determine which patterns have failed.
In embodiments, prior to the next BIST run, the pattern mask registers for all BIST instances are re-programmed in order to select which patterns would be masked for the next BIST run. Alternatively, it may be advantageous to not re-program the mask register, and let the failing pattern data for the previous BIST run(s) dictate which patterns would be masked for the remaining BIST run(s). This allows a mechanism to let the logic circuit decide which patterns to disable based upon previous BIST failing pattern data, without needing to analyze this externally (outside the test environment). In this way, the next BIST run would only focus on running patterns which have not yet failed. At the end of a bunch of BIST runs, it would be simple to now read out which patterns have never failed across the multiple BIST runs.
As should now be understood, the collected information can help in faster debug/verification. For example, in known systems in order to determine which patterns had failed, it would be necessary to mask all but one pattern at a time for each test. Once all of the tests are completed for all of the unmasked patterns, it would then be possible to determine which patterns failed, as only one pattern per test would be executed. This is very time consuming and programming intensive for test pattern generation. In comparison, the processes of the present invention can run tests on all patterns, store the failure in the pattern mask register and the read out all of the failures therefrom.
In this way, it is possible to speed up the debug process during initial hardware bring-up. It is also possible to more quickly isolate problem areas in the array and design experiments faster, with more precise target to isolate possible root causes. For example, a pattern that is intended to test for row-to-row short circuiting, might show up as failing quite often, and can help steer the failure analysis team to look for a short circuiting defect in a certain part of the circuit. This feature is useful for obtaining failing pattern information across all parts during standard production test and thus helps in creating a statistical data of the pattern effectiveness and thus can help to optimize the manufacturing test flow, by allowing the intelligent disablement of certain patterns, which are shown to very rarely, if ever, catch a unique fail (e.g., a fail that is only caught by that one pattern).
The processes and systems described herein are also useful in eliminating the need for testing patterns, where there is no unique fail found for the particular pattern. For example, if a same fail is found for several patterns, it can be implied that the fail is not unique to any single pattern. By determining that the fail is not unique to any particular pattern, some patterns can be now skipped or masked effectively increasing test speed and efficiency.
The structure(s) as described above is used in integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.
The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

## Claims

1. A method, comprising:
testing a plurality of patterns in at least one memory device;
determining at least one of the plurality of patterns detects a fail in the at least one memory device during execution of the at least one of the plurality of patterns on the at least one memory device;
storing, in a pattern mask register of a built-in-self-test (BIST) engine, a per pattern based fail status of each of the at least one of the plurality of patterns that detects the fail;
masking, in the pattern mask register of the BIST engine, patterns other than the at least one of the plurality of patterns that detects the fail; and
skipping the execution of the patterns that are masked in the pattern mask register of the BIST engine.

2. The method of claim 1, wherein the plurality of patterns are fail specific for 1-port, 2-port, or TCAMs.

3. The method of claim 2, wherein the pattern mask register is reused to store the fail status of each of the plurality of patterns that has detected the fail.

4. The method of claim 2, wherein the fail status is stored at a bit location of the pattern mask register which corresponds to each of the plurality patterns that has detected the fail.

5. The method of claim 4, wherein a bit of the pattern mask register that corresponds to the fail status is written to the pattern mask register at a location corresponding to each of the plurality patterns that has detected the fail after the execution of each of the plurality patterns.

6. The method of claim 5, further comprising reading out the pattern mask register to determine each of the plurality of patterns that have detected the fail at the end of a BIST run.

7. The method of claim 6, wherein the bit set to ‘1’ in the pattern mask register corresponds to each of the patterns has detected the fail, and the bit set to ‘0’ corresponds to each of the patterns has not detected the fail during the execution of each of the plurality of patterns.

8. The method of claim 7, further comprising, for each of the plurality of patterns, masking the pattern in response to the bit corresponding to the pattern being set to ‘0’ and skipping the pattern during BIST execution.

9. The method of claim 2, wherein the pattern mask register further controls that each of the plurality of patterns is to be masked and skipped during a BIST run in response to a bit corresponding to the fail status of each of the plurality of patterns being set to ‘0’.

10. The method of claim 2, wherein a bit fail status register is initialized to ‘0’ at a beginning of a BIST run and set to a value of ‘1’ upon detection of the fail, and at an end of the BIST run of each of the plurality of patterns, the value of the fail status register is written into a bit of the pattern mask register, wherein each bit of the pattern mask register corresponds to a particular failed pattern in a pattern suite.

11. The method of claim 10, wherein a location of the value within the pattern mask register corresponds to a current pattern of each of the plurality of patterns being executed.

12. The method of claim 1, wherein during a BIST run, a FAIL (‘1’) of the at least one memory device passes through an OR gate to be stored at a fail register and fed back to an AND gate until a pattern being executed is complete,
wherein at an end of a run of the pattern being executed, the fail register no longer stores the FAIL (‘1’) of the at least one memory device,
wherein, in parallel with the fail register no longer storing the FAIL (‘1’), and the FAIL (‘1’) is moved into a bit position in the pattern mask register which corresponds to the pattern being executed, and
wherein at an end of the BIST run, after all patterns have been executed, the pattern mask register having only bits set to ‘1’, for corresponding patterns that have detected a fail on a per pattern basis such that each bit in pattern mask register corresponds to a particular failed pattern in the pattern suite for the at least one memory device.

13. A method comprising:
reusing a pattern mask register of a BIST engine to store fail information at a bit location for a pattern that detected a fail in at least one memory device;
masking, in the pattern mask register of the BIST engine, the pattern that detected the fail; and
skipping the execution of the pattern that detected the fail.

14. The method of claim 13, wherein during execution of the pattern, in response to a fail being detected, a single bit, corresponding to a location of the pattern which detected the fail in the pattern mask register, is set to ‘1’.

15. The method of claim 13, further comprising:
during a BIST run, a FAIL (‘1’) of the at least one memory device passes through an OR gate to be stored at a fail register and fed back to an AND gate until the pattern being executed is complete,
wherein at an end of a run of the pattern being executed, the fail register no longer stores the FAIL (‘1’) of the at least one memory device,
wherein, in parallel with the fail register no longer storing the FAIL (‘1’), the FAIL (‘1’) is moved into a bit position in the pattern mask register which corresponds to the pattern being executed; and
wherein at an end of a BIST run, the pattern mask register is read to determine which pattern has detected the fail on a per pattern basis such that each bit in pattern mask register corresponds to a particular failed pattern in the pattern suite for the at least one memory device.

16. The method of claim 13, wherein the pattern mask register controls whether the pattern is to be masked and skipped during a BIST run in response to a bit corresponding to the pattern being set to “1”.

17. A method comprising:
issuing an execution command to a BIST engine to run unmasked patterns;
enabling a pattern mask register for data collection;
after completion of the pattern, determining whether there is a FAIL for the tested pattern in at least one memory device and, if there is a FAIL, then enable FAIL collection;
updating a corresponding pattern bit in the pattern mask register, which stores the pattern that detected a FAIL, wherein the updating comprises:
masking, in the pattern mask register, the pattern that detected the FAIL; and
skipping the execution of the pattern that detected the FAIL.

18. The method of claim 17, further comprising determining all patterns have been tested and, if all patterns have been tested, then the information of the pattern that that detected the FAIL stored in the pattern mask register is shifted to a tester using a BIST shuttle readout register.

19. The method of claim 17, wherein prior to a next BIST run, the pattern mask register for all BIST instances is programmed to select which pattern is masked for a next BIST run and not programmed such that a failed pattern for the previous BIST run dictates which pattern is masked for a remaining BIST run.

20. The method of claim 17, further comprising determining the pattern is masked, in which case the pattern is skipped during the BIST run, and a next pattern is tested.