| A scan chain refers to a series of flip-flops (or registers) connected in a linear sequence that allows for testing and debugging of integrated circuits. During the testing process, test data is shifted into the scan chain and propagated through the circuit. By analyzing the output of the scan chain, engineers can identify faults within the circuit.
The scan chain is critical for diagnosing issues in advanced technology nodes, particularly when dealing with functional and logic hard failures. Techniques like Laser Voltage Imaging (LVI) are often used to debug issues related to the scan chain by visually mapping active components that toggle at specific frequencies, allowing for the precise localization of defects.
The key steps of scan-chain-based testing are:
- Scan Chain Introduction:
- Flip-flops in the design are modified to serve dual roles:
- As regular flip-flops during normal operation.
- As "scan cells" during test mode. These scan cells allow the design to be divided into smaller, more manageable segments of combinational logic.
- Test Pattern Generation:
- Automatic Test Pattern Generation (ATPG) software generates test patterns based on fault models. These patterns are designed to detect potential faults within the segmented logic. The ATPG tool ensures high fault coverage, typically around 95% or more.
- Scan Chain Operation:
- The scan cells are linked together into "scan chains," which act as shift registers when the circuit is in test mode. External Automatic Test Equipment (ATE) uses these scan chains to load test pattern data into the device under test (DUT).
- Test Execution:
- After loading a test pattern, the design is switched back to functional mode, where the test response is captured over one or more clock cycles. The design is then returned to test mode, and the captured test response is shifted out while the next test pattern is shifted in.
- Result Analysis:
- The ATE compares the captured test response with the expected response data. Any discrepancies indicate potential defects, which are logged for further investigation.
- Partial Scan (Optional):
- Not all registers may be connected to a scan chain in some designs, a practice known as partial scan.
Examples
of circuits which have or have not scan chains involved:
- NAND Flash Memory:
- Scan Chain Usage: NAND flash memory typically does not require scan chains for fault analysis. NAND flash memory primarily uses methods like Error Correction Codes (ECC), built-in self-test (BIST), and other error detection and correction techniques during manufacturing testing and field use. Fault analysis in NAND focuses more on issues like charge retention, read/write errors, and endurance, rather than using scan chains to test internal logic.
- Fault Analysis Methods: For NAND, fault analysis often involves techniques like:
- Parametric Testing: Checking the electrical characteristics of the memory cells.
- Error Correction: Analyzing and correcting bit errors through ECC.
- Built-in Self-Test (BIST): Automating the testing process to identify defective blocks or pages.
- Focus Areas: The focus is on ensuring data integrity, reliability, and endurance rather than debugging complex logic circuits, which is where scan chains would typically be used.
- DRAM Memory:
- Scan Chain Usage: DRAM, unlike NAND, may incorporate scan chains, especially in more complex designs or advanced technology nodes where logic circuits within the memory controller or other integrated logic require testing. Scan chains can be used to test and debug the logic that controls the memory arrays, such as row/column decoders, sense amplifiers, or other control logic.
- Fault Analysis Methods: For DRAM, fault analysis may involve:
- Memory BIST (MBIST): A common technique for testing the memory arrays and associated logic.
- Parametric and Functional Testing: Ensuring that the memory operates correctly across various operating conditions.
- Scan Chains: Used in the testing of logic circuits within the memory, helping to identify faults in the control logic or other integrated circuits that manage the DRAM operation.
- Focus Areas: DRAM testing focuses on the integrity of the memory cells, speed, power consumption, and the correct operation of logic circuits. Scan chains help in diagnosing issues within the logic parts of DRAM chips.
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