SemIsrael Blogs

Todd Burkholder and Kendall Hiles, Siemens EDA

In the dynamic landscape of semiconductor manufacturing, where innovation frequently outpaces established methodologies, the imperative for robust validation techniques has always been critical. The advent of 3D integrated circuits (3D ICs)—which involve the stacking of multiple active device layers or chiplets—introduce novel manufacturing challenges and reliability concerns that make verification and validation even more critical.

To help them navigate this difficult terrain, the semiconductor industry has two specialized mechanisms: test vehicles and daisy chain testing. This article delves into the foundational role of these validation mechanisms, exploring their implementation and how they contribute to quality assurance in the arena of 3D IC designs. It underscores why proactive, rigorous testing through test vehicles is not merely an option but a strategic necessity for mitigating risk and ensuring the successful realization of advanced packaging technologies.

Test Vehicles Are Not Just for Cars

A test vehicle is a purpose-built structure designed not for commercial sale or direct product functionality, but solely for evaluating and validating specific aspects of a manufacturing process or a new technology. Unlike a product intended for an end-user function, a test vehicle acts as a dedicated test bed, allowing engineers to probe the limits of materials, processes, and designs under controlled conditions. This approach is paramount when introducing novel elements into the semiconductor fabrication flow—such as new bump and ball connections, the integration of components on unconventional organic substrates, or the embedding of chips directly within laminate materials.

Consider a scenario where a manufacturer aims to implement an embedded chip within a complex laminate structure—perhaps for a cutting-edge wearable device. Before committing to the mass production of millions of such devices, which entails substantial financial investment and potential for catastrophic delays, a test vehicle is deployed. This vehicle would specifically test the process of creating the cavity, accurately placing the chip, and ensuring robust electrical connectivity within the laminate. By validating these critical manufacturing steps with a test vehicle, companies can identify and rectify potential issues early in the development cycle.

Electrical Connections: Check!

Within the broader scope of test vehicles, daisy chain testing stands out as a fundamental technique for assessing the integrity of electrical interconnections, particularly in advanced packaging configurations like 3D ICs.

A daisy chain is an electrical circuit designed to serially connect multiple points, such as solder bumps or micro-bumps, in a continuous path. This allows for a simple, yet highly effective, electrical resistance measurement across the entire chain. While the test vehicle validates the entire manufacturing process, the daisy chain specifically verifies the electrical connectivity and mechanical integrity of the interconnects, such as bumps and balls, within the subsystem.

Figure 1. 3D view, with the die chains shown in green.

(Source all images: Siemens Digital Industries Software)

To illustrate, consider a flip-chip device, a common advanced packaging technique where a semiconductor die is inverted and connected to a substrate via an array of solder bumps. As these assemblies undergo thermal cycling (repeated heating and cooling), the differential thermal expansion between the die and the substrate can induce significant stress, particularly at the corner bumps. This stress can lead to micro-cracks or opens in the interconnects. Now, your manufacturer might guarantee the reliability of connections for an 80,000-pin chip, but what happens if you want to create a design that pushes this limit to 100,000 pins? How can you ascertain the reliability of these additional, higher-density connections?

A daisy chain test vehicle addresses this dilemma by creating a continuous electrical path that traverses these critical interconnects. For instance, on the substrate, a trace might connect pin 1 to pin 2; on the die, a trace connects pin 2 to pin 3; then back to the substrate from pin 3 to pin 4, and so on, forming a literal chain through each bump and ball. The ends of this chain are then routed to a ball grid array (BGA) pad, which connects to an external tester. By measuring the total electrical resistance across this chain, any open circuit will result in an infinite resistance, immediately indicating a failure.

For more granular fault isolation, test vehicles often incorporate multiple, shorter daisy chains, or include “taps” at intermediate points along a longer chain, routing these taps to separate BGA pads. If a main chain fails, resistance checks on these sub-chains or taps can pinpoint the approximate location of the open circuit. This capability is invaluable for subsequent physical failure analysis, enabling engineers to precisely locate the defect through techniques like cross-sectioning or scanning acoustic microscopy, thus accelerating root cause identification and process improvement.

The Many Faces of Test Vehicles

The responsibility for creating test vehicles is often shared between different entities within the semiconductor ecosystem. Manufacturers, including outsourced semiconductor assembly and test providers and foundries, frequently develop their own test vehicles to validate their proprietary processes and guarantee performance specifications. For example, if a manufacturer claims the ability to reliably produce 12-micron line and space features with an 80,000-pin die, they will likely employ internal test vehicles to substantiate these claims and qualify their processes.

However, as technology advances and customers push the boundaries of what is currently manufacturable, the onus for test vehicle development can shift. When a customer designs a product with a 120,000-pin die, exceeding the manufacturer’s standard guarantees, that customer may need to commission or design their own test vehicles. In such cases, the “die” within the test vehicle is typically a “dummy die”—an inert silicon piece featuring only a redistribution layer with the necessary electrical connections for the daisy chain or other test structures, rather than active circuitry. This dummy die mimics the physical characteristics of the actual product die without the complexity or cost of fabricating functional transistors.

Layout engineers, who are responsible for the physical design of integrated circuits and packages, are the primary architects of these test vehicles. They translate the specific validation requirements into physical designs, ensuring that the test structures accurately represent the critical features and potential failure modes of the target product. It is common for large customers to develop numerous test vehicles—sometimes 10 to 12 for every product—to comprehensively evaluate various aspects of manufacturing—from interconnect reliability to thermal performance—across different process variations. This collaborative and often iterative process between manufacturers and customers is essential for lowering the risk of advanced packaging innovations.

Going Beyond Simple Daisy Chains

The “Wild West” nature of the 3D IC space, where companies are rapidly innovating new integration schemes, necessitates test vehicles that incorporate increasingly sophisticated structures that go beyond basic connectivity.

One critical application involves the validation of silicon bridges used for die-to-die interfaces. In many 3D IC designs, a silicon bridge—a small silicon interposer—is employed to connect multiple chiplets (e.g., an ASIC or HBM) on an organic substrate. This bridge is typically flipped, with its pins oriented upwards, allowing the chiplets to index and connect precisely to it. The manufacturing process for aligning and bonding these components is highly complex. A test vehicle for this scenario would include a dummy silicon bridge and dummy chiplets, all designed with daisy chain structures that traverse the critical interconnects. This allows engineers to quickly verify the physical and electrical integrity of the bridge connections without the need for functional, live chips that would require complex at-speed testing. If a manufacturing process change is introduced, a quick run with this test vehicle can confirm its efficacy without risking expensive functional prototypes.

Furthermore, test vehicles can be equipped with structures that simulate real-world operational stresses and detect subtle manufacturing defects. For instance, while a basic daisy chain measures resistance, advanced test vehicles often include:

• Heaters: These are resistive patterns embedded within the substrate that, when energized, generate localized heat. By incorporating heaters, engineers can simulate the thermal stresses experienced by interconnections under operational load. This allows for accelerated lifetime testing and the identification of interconnect failures caused by coefficient of thermal expansion mismatches under conditions that mimic actual device operation.
• Comb Structures: These interdigitated patterns are designed to detect capacitive discharge. They are highly sensitive to residual contaminants—such as flux residue or other process byproducts—that might be left on the device after mounting or cleaning processes. These residues can lead to leakage currents or even short circuits, compromising device reliability. By measuring the capacitance or leakage current across these comb structures, manufacturers can assess the cleanliness and integrity of their assembly processes.
• Stacked Via Chains: The ability to stack vias directly on top of each other across multiple layers is crucial for achieving high routing density in 3D ICs. However, stacked vias are highly susceptible to manufacturing misregistration, when successive layers are not perfectly aligned. Test vehicles address this by incorporating multiple daisy chains, each with a controlled degree of via misregistration. For example, one chain might have perfectly aligned stacked vias, while subsequent chains have vias intentionally offset by 5 microns, 10 microns, or even 15 microns. By testing these chains, engineers can determine the maximum allowable misregistration before connectivity is lost, establishing critical design rules and process windows.

Figure 2. Complex via stacking and heaters.

These advanced structures provide a comprehensive suite of tools for probing the limits of manufacturing processes, enabling engineers to qualify new materials, optimize process parameters, and ensure the robust performance of their 3D IC designs.

Whereas the basic function of a daisy chain, measuring resistance, provides an immediate pass/fail indication for connectivity, the true power of test vehicles lies in their ability to provide diagnostic information that informs process improvements and establishes reliable manufacturing windows.

For example, the use of heaters allows engineers to understand how interconnects behave under thermal stress, which is a primary driver of failure in advanced packaging. If corner bumps are consistently failing under simulated operational temperatures, the data from the heated test vehicle can guide adjustments to underfill materials, bonding processes, or package design to mitigate these stresses. Similarly, comb structures provide direct feedback on the efficacy of cleaning protocols, helping to identify and eliminate sources of contamination that could lead to latent defects or early field failures.

Figure 3. Heaters help show how interconnects behave under thermal stress.

The ability to stack vias directly, rather than having to “dog-bone” (offset) them around obstacles, offers immense benefits in terms of routing density and overall package size reduction. However, each layer in a multi-layer substrate has its own registration tolerances, and accumulating these tolerances can lead to misaligned vias. By designing test vehicles with chains of vias intentionally misaligned by varying degrees, manufacturers can empirically determine the maximum permissible misregistration before electrical continuity is compromised. This data is invaluable for setting robust design rules and ensuring that production yields remain high even with tight pitch requirements. These insights are critical for pushing the boundaries of what is manufacturable while maintaining high reliability.

Don’t Go It Alone: EDA Tools Are Here with More to Come

Despite the sophisticated capabilities of current test vehicles, the rapid evolution of 3D IC technology continues to present new challenges and opportunities for innovation in validation methodologies. One significant area for improvement lies in the automation of test vehicle design, particularly for daisy chain structures.

Currently, it is not uncommon for design engineers to manually create daisy chain netlists. Because a single electrical net in a design tool typically connects all pins with the same name, creating a serial daisy chain requires breaking the continuous path into discrete, uniquely named segments (e.g., Fred_1, Fred_2, and Fred_3). This manual process, often involving spreadsheets and painstaking netlist manipulation, is prone to human error, time-consuming, and inefficient. As 3D IC designs become increasingly complex with hundreds of thousands or even millions of interconnects, manual daisy chain generation becomes an unsustainable bottleneck.

Fortunately, the industry is moving towards more intelligent EDA tools that can automate the creation of these complex test structures. These tools leverage advanced algorithms to generate daisy chain patterns, comb structures, and stacked via arrays directly from design specifications, ensuring accuracy and significantly reducing design cycle time. Furthermore, these tools can integrate with simulation environments to predict the electrical and thermal performance of the test structures, providing a more comprehensive validation strategy.

Figure 4. Comb structures help detect capacitive discharge.

As 3D IC technology matures, the demand for even more integrated and predictive test vehicle solutions will grow. This includes the development of multi-physics simulation capabilities that can accurately model the interplay of thermal, mechanical, and electrical stresses within complex 3D structures, guiding the design of more effective test vehicles. The future of 3D IC validation will undoubtedly involve a tighter integration of design, simulation, and physical testing, ensuring that the promise of advanced packaging can be realized with unwavering reliability and manufacturability. For those still relying on manual methods, the message is clear: leverage the evolving capabilities of EDA tools to streamline test vehicle design and accelerate the path to robust 3D IC products.

Test Driving Reliability and Performance

Ultimately, test vehicles serve as an economical and efficient means to validate critical manufacturing steps without the prohibitive cost and complexity of using functional product dies. They allow for the isolation of specific process variables and the precise characterization of their impact on reliability.

This targeted approach is essential for ensuring that 3D IC designs meet the rigorous quality and performance standards demanded by the industry, preventing costly recalls and maintaining customer trust.

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Todd Burkholder is a Senior Editor at Siemens DISW. For over 30 years, he has worked as editor, author, and ghost writer with internal and external customers to create print and digital content across a broad range of high-tech and EDA technologies. Todd began his career in marketing for high-technology and other industries in 1992 after earning a Bachelor of Science at Portland State University and a Master of Science degree from the University of Arizona.

Kendall Hiles is a distinguished Senior 3D IC Product Specialist at Siemens EDA, bringing four decades of experience in electronic design automation. His unique perspective stems from his experience working across multiple technological transitions in the semiconductor industry, from traditional IC design to modern 3D IC implementation.