By Ann Steffora Mutschler
Driven by high-speed interfaces, the demand for TSVs and the complexities that new process nodes bring, older packaging technologies like wirebonding can’t keep up.

The latest and greatest flip chip technologies offer much more flexibility, but at a cost. As such, the package plays a larger role than ever in determining system specifications because, depending on the packaging technology used, certain system parameters are either limited or not. For these reasons packaging is being considered up front in the design process—even at the very earliest planning stages. Moreover, the cost of packaging comprises a non-trivial portion of the total system cost. Throw 2.5D and 3D ICs into the mix and things get really interesting.

These issues are breaking down the brick walls between the IC designer and the packaging designer, observed Brad Griffin, product marketing director for Cadence’s Allegro product line. “I think we have hit a point where if you don’t at least consider some of the aspects simultaneously you’re going to end up with a system that either doesn’t perform like you would like it to or has additional cost than you would like it to have.”

Shafy Eltoukhy, vice president of manufacturing operations at Open-Silicon, agrees. “Essentially when we talk to the customer initially we try to understand from them what their thermal requirements are, like power, the footprint of the package, the thickness of the package and the cost. Also, if it’s a consumer part it will be different than if it’s a networking part. The cost has a big impact on the selling price. There are a lot of factors that come in at the very beginning, that being the power dissipation, signal integrity, form factor of the package and so on.”

There’s a complicated matrix to designing a package. “In some cases, at the very beginning, you don’t know exactly what the die size is going to be because customers keep adding functions and the die size keeps changing in the early stage. Having all the requirements from the mechanical to power to this and that, as well as the layout or the floorplan of the die, is also changing up front. The package has to be part of the very early stage of the design,” he said.

The company ties the package to the floor planning and the requirement as far as the speed and other considerations. Package design engineers get on board very early in the process—even before designing the die itself—to look at it from the floor planning point of view. That includes where to put high-speed interfaces, how fast these are going to be, packaging technology, i.e. flip chip or wirebond, power expectations, and so on. “It’s very complicated and people have started putting a very good interface between the packaging and floor planning engineers,” Eltoukhy added.

Broadcom recently detailed its experience making early tradeoffs during Cadence’s CDN Live event in March. The presentation can be viewed here (registration required).

In this case, using Cadence technology under development, Broadcom enabled its PCB designer to look at what the package footprint should be to best match the components on the board. From there it drives up from the package footprint to the bump matrix for the chip. They were able to determine the ideal bump matrix for the chip to be able to match the package footprint. Then from the bump matrix that drove the I/O pad ring to get the I/Os and chips placed properly.

Then, when it comes to 2.5D design, packaging issues are actually more complex than what they will be with true 3D ICs, according to John Park, methodology architect for IC packaging and pathfinding technologies in the Systems Design Division of Mentor Graphics. “I believe—and many of my customers believe—it is more complex because you have this intermediate substrate in which the die connect through these small microbumps, and there’s this die-to-die connectivity that happens on some unknown number.”

Engineering teams really want to save costs, or at least make the tradeoff of cost and thermal, etc., Park added. “They want some global routing technology that allows them to say, ‘What if, on the interposer, I limit it to three metal layers? Does that mean I need to add two more layers to my package substrate or does it mean that I have to add eight more layers to my package substrate?’ In that case, maybe they say, ‘I’m going to add one more metal layer to my silicon interposer so that I can reduce the layer count of the package.’ Now the costing gets very complex because you have this new intermediate routing structure that sits in between the traditional die to package connectivity that greatly impacts cost, routability, signal quality—all these types of things.”

Although some of these issues won’t be a problem with 3D, don’t expect a mass exodus from 2.5D when 3D becomes mainstream because it may be more expensive, noted Cadence’s Griffin. “You’re going to have to get to a certain quantity before you’re going to realize the value in going to 3D IC, so there’s going to be still a fair amount of chips in the lower to mid quantities for which they want to realize the performance gains of going to a silicon substrate to connect things together but they can’t necessarily go to the expensive TSMC to be able to connect all these things together properly. [Systems companies] will rely on less-expensive silicon interposer technology; it will have a little less performance but cost-wise, it will just be more efficient for them. I think you’re going to see both for quite some time, but i think you’re going to see the very-high-quantity things move to 3D IC as much as possible.”

By Cheryl Ajluni

At the 46th Design Automation Conference in San Francisco last month, attention turned to a discussion of how to extend the momentum of Moore’s Law into the next decade. One plausible solution, according to Philippe Magarshack, the general manager of Central CAD & Design Solutions at STMicroelectronics, is 3D stacking for complex System-on-Chips (SoCs).

The concept of 3D stacking or integration technology is not new. In fact, 3D stacking of dies has been successfully demonstrated and is currently being commercially employed in some embedded domains (e.g., stacking DRAM memory on CPU cores). A recent 3D IC report from Yole Développement suggests that by 2012, the number of 3D IC-processed wafers could surpass 10 million units, driven in part by handset, wireless and computing applications. Given the intense interest and work going into developing 3D integration technology, this prediction seems just about right—assuming, of course, that a few challenges can first be met.

Exploring the third dimension

Very simply put, 3D integration consists of stacking integrated circuits and connecting them vertically so that they behave as a single device. A 3D chip is therefore just a stack of multiple device layers with direct vertical interconnects tunneling through them. So what’s the big deal about 3D integration?

Today’s semiconductor chips face extreme pressure to achieve increased performance, while reducing their size and accommodating lots of new functionality. When these factors coalesce in traditional 2D chips, longer interconnects result. In SoCs, longer interconnects translate into reduced speed and increased power consumption.

A key benefit of 3D integration is that it can reduce the length of interconnects. Additionally, it provides higher transistor density, faster interconnects and heterogeneous technology integration, with potentially lower power, cost and faster time-to-market. As Matt Nowak, director of engineering in the VLSI technology group of Qualcomm’s CDMA technology division, pointed out in a DAC 2008 presentation, the 3D approach “achieves extremely high densities, allowing us to use heterogeneous technologies and reduce form factor. The key is that it enables the use of new differentiating technologies to build new architectures that cannot be built in existing technologies.”

Eyeing recent developments

Up to this point, most efforts in 3D integration have focused on developing different fabrication techniques for stacking multiple device layers and forming the vertical interconnects. Much of the work has been done through collaborations with academia, industry organizations and government-sponsored laboratories around the world. One of the key technologies to come out of this research is a next-generation interconnect technology known as Through-Silicon Via (TSV). The TSV is a vertical electrical connection that passes completely through a silicon wafer or die to produce multilevel chips with an optimum combination of cost, functionality, performance, and power consumption. By using TSV technology, 3D ICs can pack greater functionality into a smaller footprint and realize shorter critical electrical paths, resulting in faster operation.

Some of the other developments to come out of ongoing 3D integration research were recently recognized at the Electronic Components and Technology Conference. Sandia National Laboratories presented details of its W TSV process, which is said to provide a suitably low-resistance metal with a coefficient of thermal expansion close to Si, a via fill that is conformal, and can be readily integrated into IC fabrication. IMEC introduced a novel process for die-to-wafer bonding (using Cu-Cu bonds) of its 3D SIC technology and a scalable TSV technology for 3D wafer-level packaging. Its TSV technology is designed for 3D structures where interconnects are fabricated after standard CMOS processing.

SEMATECH also is focusing its 3D research on TSV technology, particularly for implementation. The industry organization is actively working to bring together partners from across industry—chipmakers, equipment and materials suppliers, assembly and packaging service companies—to make 3D TSV suitable for high-volume manufacturing (Figure 1).

Figure 1. In contrast to the 2D-SoC or 3D System-in-Package, 3D TSVs offer a cost-effective way to achieve high density and performance, while also being able to integrate non-CMOS products with CMOS. The SEMATECH 3D project is based on cost modeling to assure products will be both manufacturable and affordable.

Help: tool support needed!

While ongoing research and development is absolutely critical to the success of 3D integration, perhaps one of the greatest challenges it faces is tool support in terms of design techniques and methodologies. Without it, engineers have virtually no efficient way to exploit the technology’s benefits. Tool support is especially critical when it comes to 3D integration because vertical stacking tends to increase thermal resistances, further exacerbating temperature-induced problems that can negatively affect system reliability, performance and leakage power. The use of 3D also will significantly complicate the typical design flow.

The key, of course, lies in creating a standardized design environment and methodology for physical design of 3D chips that could support a range of different tools. Having the tools integrated in one place would make it easier for designers to explore and make architectural decisions and then, to hand those decisions off to next stages in the design process.

3D IC integration is still in its infancy and, as a result, tools developed today for one specific application (e.g., stacked memory) may not be suitable for heterogeneous integration tomorrow. Nevertheless, there are some tools available now, with more in development. Some of these tools include:

3D PathFinding

Javelin Design Automation. 3D PathFinding provides a detailed 3D flow for accurate performance/power/cost estimates that can be used for rapid design exploration and optimization of 3D stacked ICs. Developed in collaboration with IMEC and Qualcomm, the solution extends Javelin’s existing PathFinding methodology and j360 Silicon PathFinder physical design prototype platform to support virtual chip design (Figure 2).

Figure 2. Javelin’s 3D PathFinding solution allows the designer to assess the impact of various 3D interconnect strategies throughout the IC design and fabrication process, in a matter of just a few hours or days. Silicon process engineers can use it to fine-tune their technology to the system architecture specifications.

MAX-3D, R3Integrator, R3CAD, and R3Artist; R3Logic

These tools, developed through work conducted as part of research programs sponsored by the Defense Advanced Research Projects Agency (DARPA), enable 3D IC design and analysis (Figure 3). MAX-3D is a 3D mask layout tool whose technology file includes all properties of stacking process, wafer orientations, bond materials, via electrical/material properties, and also incorporates 2D foundry design kits. R3Integrator is used for die/interposer/package co-design with TSVs. R3CAD is a java-based, multi-platform tool for 3D design research and prototype study and R3Artist is an embedded 3D layout editor (Figure 3).

R3Logic is currently collaborating with STMicroelectronics and CEA-LETI to develop a full 3D design flow for 3D heterogeneous system and system-in-package design.

Figure 3. R3Artist features single and multiple wafer technologies, integrated material properties database and solid model extraction, including dielectric layers.

3DCACTI

3DCACTI estimates the optimum access times and power dissipation of a cache using 3D IC technology for a given number of active device layers and by partitioning device layers for various technology nodes. Based on the estimation, it searches for the optimized configuration that provides the best delay, power and area efficiency trade-off according to the cost function for a given number of different 3D partitions.

3D Magic and PR3D, Massachusetts Institute of Technology

3D Magic is a comprehensive layout methodology for 3D circuit-layout editing and extraction with MAGIC, an open source layout editor developed by UC Berkeley. PR3D is a placement and routing tool for standard cell design in 3D. Both tools were developed through MIT’s Interconnect Focus Center Research Program. MIT also developed SysRel (System-Level IC Reliability) for assessing the interconnect reliability of 3D ICs from a thermal-aware perspective at the circuit-layout level.

Conclusion

With the pressure on traditional 2D chips mounting, 3D integration has begun to establish itself as a viable means of breathing more life into Moore’s Law. It certainly touches on all the hot buttons in the industry today, namely low power, cost and time-to-market. The challenge will be in ensuring that these benefits are realized in a timely and efficient manner. 3D-specific design tools and methodologies are coming to meet this challenge head on. In the meantime, the tools available now and the groundwork for future tools and methodologies being laid by industry organizations, academia and commercial companies alike, will go along way in ensuring 3D integration plays a critical role in the future of the semiconductor industry.