Analog IP Under Scanner: A Deep Dive into Post-Silicon Bug Avoidance

Introduction:

Every semiconductor product expects Analog IP(s), such as DDR/PCI/DP Phy, to be flawless. However, due to performance demands, IP(s) developed on the latest process nodes often undergo concurrent development with the SOC, making the SOC(s) vulnerable to analog IP bugs. These bugs can incur significant costs in terms of money, time-to-market (TTM), and even brand value. ?

This article analyzes post-silicon bugs, their sources, general debugging procedures, and avoidance strategies. It also includes real-life experiences and advice, primarily aimed at Analog IP architects, technologists, and leaders. The objective is to trigger discussions and insights within the Analog IP community. ?

Understanding Post-Silicon Bugs

Nature of Silicon Bugs:

Post-silicon bugs are design or process flaws that prevent silicon from meeting specifications, including power, performance, reliability, yield, and DPM (defects per million). The scope of these bugs, their solutions, fixing costs, and overall impact are extensive. ?

Prominent Reasons for Post-Silicon Bugs:

Insufficient validation during the pre-silicon design phase is a primary cause of these bugs. While design and validation engineers invest significant effort in uncovering issues before tape-out, the analog IP team dedicates even more time to validation than design itself. Other contributing factors include process excursions and unclear specifications. ?

Debug Methodology and Tools:

Due to the diverse nature of silicon bugs, a standard debug procedure isn't always feasible. Each bug has unique failure mechanisms, signatures, and occurrences. The initial debug steps involve capturing failure signatures and electrical/functional waveforms using logic analyzers or oscilloscopes. If the signals aren't readily observable, techniques like FIB (Focused Ion Beam) or LADA (Laser-Assisted Device Alteration) may be necessary. Constructing the failure mechanism based on architecture and supporting it with waveform capture is crucial. Reproducing the failure in the pre-silicon design environment and gathering evidence from various sources are essential for informed decision-making and design changes. ?

Solution Space

Design Change:

A design change is the most direct but expensive solution, often requiring a new stepping, which can be costly. It's typically considered a last resort unless no other solutions are available or the silicon stepping is already planned. ?

Robust Workarounds:

Workarounds through firmware/BIOS changes or product engineering can be more cost-effective. Logic features often have safety nets that can be used for workarounds. Product engineering tests can also be modified to capture faulty parts. ?

Taking an Errata (No Fix):

In some cases, the impact of not fixing the bug may be acceptable, especially if the yield loss or specification violation is minimal. This requires thorough communication with customers, including detailed information about the failure, occurrence, impact, and the rationale for not implementing a fix. ?

Impact of Post-Silicon Issues

Post-silicon issues can have significant consequences for product teams, including:

• Cost: R&D work for fixing bugs and new stepping can amount to millions of dollars. ?
• Time-to-Market: New steppings introduce delays in tapeout, fabrication, and post-silicon work, potentially pushing back schedules by one or two quarters. ?
• Brand and Reputation: Late silicon bugs can generate negative publicity, damaging the company's brand and reputation.
• Product Competitiveness: Risky or expensive fixes may lead to compromises in performance, reliability, or product specifications, impacting competitiveness.

Prominent Categories/Sources of Silicon Bugs

Categorizing the sources of silicon bugs can be challenging due to overlapping boundaries. However, identifying high-risk areas is crucial for targeted validation efforts. Common sources include:

• Analog Design (Performance Bugs): Analog circuits, consuming a significant portion of the IP area, are prone to silicon miscorrelation, leading to failures in meeting performance, power, or reliability specifications. Oversights in validation can also contribute to bugs in this area.
• Digital Design (Functional Bugs): While smaller in area, the digital part of analog IP contains numerous features and transistors. The interface between analog and digital circuits is particularly susceptible to bugs due to the lack of industry-standard specifications and the architectural dependency of verification strategies.
• Reliability-Related Issues: These can occur in both analog and digital areas, but the probability is higher in analog circuits due to their high-performance requirements and larger design area. Reliability issues often manifest over time and may be discovered months or even years later, sometimes in the field. Process tweaks or changes in operating conditions are common fixes.
• Process Excursion: Process shifts in the latest process nodes can lead to silicon bugs. Temporary excursions may require aggressive HVM testing and temporary yield impacts, while permanent shifts may necessitate design changes.

Silicon Bug Avoidance: A Mindset Change

Achieving bug-free silicon requires a cultural and mindset shift, with every team member and stakeholder taking responsibility. While there's no guaranteed solution, the following actions can contribute significantly:

IP Management:

• Include at least one test vehicle (test chip) based on post-silicon validation in the IP development cycle.
• Plan for two test chips if both the process and architecture are new to the IP.
• Ensure independent testing of ecosystem components before integration.

Architects:

• Prioritize architectural simplicity and avoid unnecessary complexity.
• Incorporate safety nets and emergency-based features in the architecture.
• Adhere to industry standards and implement features completely within the IP to avoid integration challenges.

Analog Design Team:

• Focus on rigorous validation of analog circuits, considering all potential scenarios.
• Pay close attention to the analog-digital boundary, employing techniques like AMS simulations for validation.
• Incorporate redundancy in design features, multiple solutions for risky paths, and BIOS programmability to enable workarounds.
• Ensure proper observability and testability to facilitate the detection and isolation of bugs.

Logic Design and Verification Team:

• Implement redundancy in logic features and algorithms to enable workarounds.
• Employ diverse verification techniques, including co-simulation, formal property verification, and emulation.
• Develop intelligent BMODs for analog design to enable effective verification of the digital part.

Physical Design and Mask Design:

• Thoroughly validate physical design for timing path completeness and mask design for reliability issues.

Conclusion:

Post-silicon bugs in analog IP present a formidable challenge in semiconductor development. This article has explored the sources of these bugs, their impact, and strategies for mitigation. By fostering a culture of bug avoidance, prioritizing architectural simplicity and redundancy, and implementing rigorous validation techniques, we can strive towards robust, bug-free silicon. While the complexity of analog design makes complete eradication of bugs an elusive goal, a collective commitment to proactive measures and continuous improvement can significantly enhance the reliability and performance of analog IP, ultimately leading to successful semiconductor products.

Disclaimer:

It should be noted that the article covers a pattern of post-silicon issues and cannot be used as a definitive playbook. The information shared is based on personal experience and discussions with industry experts and should not be attributed to any past or present employers.