Design for Testability in VLSI

Design for testability (DFT) is an essential technique in VLSI chip design that improves the testability of the chip and helps to detect and isolate faults accurately. DFT has become increasingly important as the importance of VLSI chips has increased and the demand for higher chip density has risen. 

In this article, we will take a closer look at what DFT is, why it is important, and how it works.

What is Design for Testability (DFT) in VLSI?

Designers use DFT as a design technique to add circuitry to the chip, which improves the observability and controllability of internal nodes and increases the testability of all logic in the chip, making testing cost-effective. DFT detects manufacturing defects, such as process variation, transistor defects, and interconnect faults, that can cause a chip to fail. DFT helps to improve the quality of the chips produced and reduces the cost of manufacturing by making a chip more testable.

Also read about: Chip Design Verification

Why is Design for Testability important in VLSI?

There are several reasons why DFT is important in VLSI:

• Increased chip density: With the increasing density of VLSI chips, the number of transistors in a chip has grown significantly. This has made it more difficult to test all the components of a chip, as it is challenging to observe and control the internal nodes of the chip. DFT helps to address this problem by improving the testability of the chip.
• Manufacturing variability: Manufacturing variability is a significant issue in VLSI chip design. The process of manufacturing chips involves many complex steps, and the variability in these steps can result in significant differences in the final product. DFT helps to detect and isolate defects that may result from manufacturing variability.
• Cost of testing: Testing a chip can be an expensive process, particularly if the chip is complex. DFT helps to reduce the cost of testing by making the chip more testable. By improving the testability of the chip, DFT can help to reduce the time and resources required to test the chip.

How does DFT work?

DFT has various stages within the VLSI chip design process, including:

• Design stage: in the design stage, designers incorporate test structures to measure the testability of each component. These structures, such as scan chains, boundary scan, and built-in self-test (BIST) structures, enable easier chip testing. Designers use scan chains to capture the state of the flip-flops in the design, boundary scan to test the I/O pins of the chip, and BIST structures to test the chip’s functionality.
• Test generation: DFT speeds up the automatic test pattern generation (ATPG) process, which generates tests to detect faults in the chip. DFT makes the ATPG process more efficient, reducing the time and resources needed to generate tests.
• First silicon: during the initial prototype stage, the first silicon serves as a prototype of the chip, and defects are detected and addressed with appropriate diagnostics. DFT is used to rectify all process problems, model errors, and pattern errors.
• Chip production:in the chip production process, DFT is utilized to test the overall quality of the shipped product. To ensure that the product functions smoothly, chips undergo extensive checking and testing. DFT helps to detect and isolate faults in the chip, thereby enhancing the quality of the final product.
• Board-level test: testing the operational life of chips is the purpose of board-level testing. DFT is used to perform temperature tests to ensure that the chip is working correctly under different temperature conditions, thus improving its reliability.
• System-level test: to ensure the smooth operation of replaceable parts, we perform system-level testing. We use DFT to detect faults in the chip and improve the overall quality of the product.

Benefits of DFT in VLSI

DFT provides several benefits in VLSI chip design, including:

• Improved testability: by improving the testability of the chip, DFT helps to detect and isolate faults accurately. This improves the quality of the chips produced, reducing the cost of manufacturing.
• Reduced cost of testing: DFT helps to reduce the cost of testing by making the chip more testable. This reduces the time and resources required to test the chip, resulting in cost savings.
• Faster development cycles: by improving the testability of the chip, DFT can help to speed up the development cycle. This can help companies to bring new products to market more quickly.
• Easier diagnostics: DFT makes it easier to diagnose faults in the chip, reducing the time required to identify and fix problems.

How to Get DFT Services?

Semiconductor design companies offer DFT services to assist their clients in enhancing the testability of their VLSI chips. These services generally comprise the following:

• DFT consulting: collaborate with clients to identify sections of their chip design that could benefit from DFT. DFT consultants offer suggestions on the types of DFT structures that should be integrated into the design and assist clients in optimizing their designs for testability.
• DFT implementation: DFT implementation services involve adding DFT structures to a client’s chip design. This includes the design and implementation of scan chains, boundary scan, and built in self test (BIST) structures.
• ATPG generation: automatic test pattern generation (ATPG) is used to generate tests that can detect faults in the chip. DFT service providers can generate ATPG for their clients, reducing the time and resources required to generate tests.
• Test program development: test if the chip is a part of test program development. DFT service providers develop test programs for their clients, ensuring that the chip undergoes thorough testing before shipment.
• Failure analysis: identify the root cause of a chip failure. DFT service providers can perform failure analysis on behalf of their clients, helping them to identify and fix issues with their chip designs.

DFT services are important for semiconductor companies to ensure chip quality and reduce manufacturing costs. They can work with DFT experts to optimize chip designs for testability.

Consider incorporating DFT techniques in your VLSI chip design to improve testability and reduce the cost of manufacturing. Contact Aeliasoft to learn more about how they can help.

Key Takeaways

Integrated circuit (IC) development technology has witnessed a continuous improvement in reducing defect density and minimum feature size, leading to increasingly complex chip designs. Nonetheless, this complexity poses testing challenges since VLSI chips can have multiple internal circuit nodes that are not directly controllable or observable from the chip’s input/output pins. Testing these chips for faults can be a daunting task, with significant time and cost implications.

DFT techniques aim to reduce the cost and effort required to generate test vector sequences for VLSI chips. By designing chips for testability, the identification of faulty chips in the field can also be simplified. However, choosing a DFT technique involves evaluating the advantages of simpler test vector generation, higher fault coverage, and possibly reduced test application time against the disadvantages. No DFT technique is ideal for all situations.

Furthermore, the complexity of modern VLSI chips only exacerbates this issue. Additionally, the cost of testing can also be a significant concern, as it can represent a significant portion of the overall production cost. Moreover, the ever-increasing demand for higher performance and lower power consumption has made it even more challenging to ensure reliable and accurate testing of VLSI chips.