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Testing Semiconductor Memories: Theory and Practice

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Over 30% of today's designs implements embedded memories and continuously consumes more and more die area. While embedded memory presents significant system performance and cost reduction advantages, it brings its own testing issues. Test vector style tests are not suitable for verifying embedded memory arrays because it is too costly. This is because the time spent in the manufacturing tester grows exponentially as the embedded memory die area increases. Sometimes it is impossible to create a set of vectors that can detect all possible types of memory defect.

Implementing embedded memory built in self-test (BIST) can alleviate these problems. In simplistic terms, memory BIST is an on-chip utility that enables the execution of a proven set of algorithmic style verification tests directly on the memory array. These tests can be executed at the design's full operating frequency to prove the memory array operations and identify errors caused by silicon defects.

Embedded memories are the most dense components within a system-on-chip (SOC), accounting for up to 90% of its real estate.1 Memories also are the most sensitive to process defects, making it essential to thoroughly test them in the SOCs.

Because memories are used as test vehicles for monitoring the silicon process and improving its yield, extracting additional diagnostic data to determine the causes of failures now is required in the testing strategy. In addition to diagnosis, many embedded memories are designed with built-in redundancy, which provides spare rows and columns that can replace failing locations. Redundancy enables the manufacturer to repair a number of otherwise defective devices to ensure maximum production yield.
Characteristics of today's SOC designs include the following:

1. Typically more than 30 embedded memories on a chip.
2. Memories scattered around the device rather than concentrated in one location.
3. Different types and sizes of memories.
4. Memories doubly embedded inside embedded cores.
5. Test access to these memories from only a few chip I/O pins.

A deep-submicron test strategy will have to handle all of these memory issues.
These issues are being addressed by the use of built-in self-test (BIST). BIST is the methodology of choice for testing embedded memories within SOCs. It offers a simple and low-cost means to test for failures of embedded memories without significantly impacting device performance.

While it has been used primarily for production pass/fail testing, BIST can be extended to provide the diagnostic data required for process monitoring and repair. Although the area overhead required by the BIST circuitry is increased, designing the diagnostic circuitry into the BIST provides many advantages in terms of time for both setup and test.

This book will take you intensively through all the memory BIST algorithms and ways to test them. I consider this as the bible for mbist testing.