3-D Integration Impact and Challenges

People want transistors—lots of them. It pretty much doesn’t matter what shape they’re in, how small they are, or how fast they operate. Simply said, the more the merrier. Diversity is also good. The more different the transistors, the more useful and interesting the product. And without any question, the cheaper the transistors, the better. So the issue is, how best to achieve as many diverse transistors at the lowest cost possible.

One approach is more chips. Placing a lot of chips close together on a small board will produce a system with many transistors. Another way is more transistors per chip. Keep on scaling the technology to provide more transistors in one or a few chips.

silicon chipThe third option combines these two approaches. Let’s have many chips with many transistors and end up with a huge number of transistors. However, there is a limit to this approach. It’s well understood that scaling is coming to an end. And placing multiple chips on a board can have a terrible effect on a system’s overall speed and power dissipation.

But there is an elegant and intellectually simple solution. Rather than connecting these chips horizontally across a board, connect them vertically, providing N times more transistors, where N is the number of chips stacked one above another. Such vertical, 3-D integration was first broached by William Shockley, co-inventor of the transistor at Bell Labs in 1947. Shockley described the 3-D integration concept in a 1958 patent, which was followed by Merlin Smith and Emanuel Stern’s 1967 patent outlining how best to produce the holes between layers. We now call these inter-layer holes through silicon vias (TSVs). Technology is still catching up to these 3-D concepts.

Three-dimensional integration offers exciting advantages. For example, the vertical distance between layers is much shorter than the horizontal dimensions across a chip. Three-dimensional circuits, therefore, operate faster and dissipate less power than their 2-D equivalent. A 3-D system is shockingly small, permitting it to fit much more conveniently into a tiny space. Think small portable electronics (e.g., credit cards).

But the most exciting advantage of 3-D integration isn’t the small form factor, higher speed, or lower power; it’s the natural ability to support many disparate technologies and functions as one integrated, heterogeneous system. Even better, each chip layer can be optimized for a particular function and technology, since the individual chips can each be developed in isolation. No more trading off different capabilities to combine disparate technologies on the same chip. Now we can use the absolute best technology for each layer and a completely different and optimized technology for a different layer. This approach enables all kinds of novel applications that until now couldn’t have been conceived or would have been cost-prohibitive.

Imagine placing a microprocessor plane below a MEMS-accelerometer plane below an analog plane (with ADCs) below a temperature sensor, all below a video imager (which has to be at the top to “see”). All of these planes fit together into a tiny (smaller than a fingernail) silicon cube while operating at higher speeds and dissipating lower power.

There are technical issues, including: how to best make the TSVs, how to construct the system architecture to fully exploit the system’s 3-D nature, how to deliver power across these multiple planes, how to synchronize this system to best move data around the cube, how to manage system design complexity, and much more.

Two issues rise to the top. The first is power dissipation (specifically, power density). When many transistors switch at a high rate within a tiny volume, the temperature rises, which can impair performance and reliability. I believe this issue, albeit difficult, is technically solvable and simply will require a lot of good engineering.

The real problem is cost. How do we mature this technology quickly enough to drive the costs down to a point where volume commercial applications are possible? Many companies are close to producing tangible 3-D-based products. Cubes of highly dense memory will likely be the first serious and cost-effective product. Early versions are already available. Three-dimensional integration will soon be here in a serious way with what will be a fascinating assortment of all kinds of exciting new products. You won’t have to wait too long.

3-D Printing with Liquid Metals

by Collin Ladd and Michael Dickey

Our research group at North Carolina State University has been studying new ways to use simple processes to print liquid metals into 3-D shapes at room temperature. 3-D printing is gaining popularity because of the ability to quickly go from concept to reality to design, replicate, or create objects. For example, it is now possible to draw an object on a computer or scan a physical object into software and have a highly detailed replica within a few hours.

3-D printing with liquid metals: a line of dollsMost 3-D printers currently pattern plastics, but printing metal objects is of particular interest because of metal’s physical strength and electrical conductivity. Because of the difficulty involved with metal printing, it is considered one of the “frontiers” of 3-D printing.
There are several approaches for 3-D printing of metals, but they all have limitations, including high temperatures (making it harder to co-print with other materials) and prohibitively expensive equipment. The most popular approach to printing metals is to use lasers or electron beams to sinter fine metal powders together at elevated temperatures, one layer at a time, to form solid metal parts.

Our approach uses a simple method to enable direct printing of liquid metals at room temperature. We print liquid metal alloys primarily composed of gallium. These alloys have metallic conductivity and a viscosity similar to water. Unlike mercury, gallium is not considered toxic nor does it evaporate. We extrude this metal from a nozzle to create droplets that can be stacked to form 3-D structures. Normally, two droplets of liquid (e.g., water) merge together into a single drop if stacked on each other. However, these metal droplets do not succumb to surface-tension effects because the metal rapidly forms a solid oxide “skin” on its surface that mechanically stabilizes the printed structures. This skin also makes it possible to extrude wires or metal fibers.

This printing process is important for two reasons. First, it enables the printing of metallic structures at room temperature using a process that is compatible with other printed materials (e.g., plastics). Second, it results in metal structures that can be used for flexible and stretchable electronics.

 

Stretchable electronics are motivated by the new applications that emerge by building electronic functionality on deformable substrates. It may enable new wearable sensors and textiles that deform naturally with the human body, or even an elastic array of embedded sensors that could serve as a substitute for skin on a prosthetic or robot-controlled fingertip. Unlike the bendable polyimide-based circuits commonly seen on a ribbon cable or inside a digital camera, stretchable electronics require more mechanical robustness, which may involve the ability to deform like a rubber band. However, a stretchable device need not be 100% elastic. Solid components embedded in a substrate (e.g., silicone) can be incorporated into a stretchable device if the connections between them can adequately deform.

Using our approach, we can direct print freestanding wire bonds or circuit traces to directly connect components—without etching or solder—at room temperature. Encasing these structures in polymer enables these interconnects to be stretched tenfold without losing electrical conductivity. Liquid metal wires also have been shown to be self-healing, even after being completely severed. Our group has demonstrated several applications of the liquid metal in soft, stretchable components including deformable antennas, soft-memory devices, ultra-stretchable wires, and soft optical components.

Although our approach is promising, there are some notable limitations. Gallium alloys are expensive and the price is expected to rise due to gallium’s expanding industrial use. Nevertheless, it is possible to print microscale structures without using much volume, which helps keep the cost down per component. Liquid metal structures must also be encased in a polymer substrate because they are not strong enough to stand by themselves for rugged applications.

Our current work is focused on optimizing this process and exploring new material possibilities for 3-D printing. We hope advancements will enable users to print new embedded electronic components that were previously challenging or impossible to construct using a 3-D printer.

Collin Ladd (claddc4@gmail.com)  is pursuing a career in medicine at the Medical University of South Carolina in Charleston, SC. Since 2009, he has been the primary researcher for the 3-D printed liquid metals project at The Dickey Group, which is headed by Michael Dickey. Collin’s interests include circuit board design and robotics. He has been an avid electronics hobbyist since high school.

Collin Ladd (claddc4@gmail.com) is pursuing a career in medicine at the Medical University of South Carolina in Charleston, SC. Since 2009, he has been the primary researcher for the 3-D printed liquid metals project at The Dickey Group, which is headed by Michael Dickey. Collin’s interests include circuit board design and robotics. He has been an avid electronics hobbyist since high school.

Michael Dickey (mddickey@ncsu.edu) is an associate professor at the North Carolina State University Department of Chemical and Biomolecular Engineering. His research includes studying soft materials, thin films and interfaces, and unconventional nanofabrication techniques. His research group’s projects include stretchable electronics, patterning gels, and self-folding sheets.

Michael Dickey (mddickey@ncsu.edu) is an associate professor at the North Carolina State University Department of Chemical and Biomolecular Engineering. His research includes studying soft materials, thin films and interfaces, and unconventional nanofabrication techniques. His research group’s projects include stretchable electronics, patterning gels, and self-folding sheets.