Tag Archives: interconnect

Nowhere Near Room Temp Superconductors

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On-chip metal interconnects limit IC speed in many advanced design today, and with signal delay proportional to the product of the resistance (R) of wires and the capacitance (C) of dielectric insulation, wires with R lower than that of copper (Cu) metal would significantly improve IC performance. We know of superconductors—materials with zero resistance to electrical current flow—but only at “critical temperature” (Tc) well below 77°K, and so there has been an ongoing quest by scientists to find a material with Tc above room temperature of 298°K.

Sadly, after 4 years and nearly 1000 materials tested, a team of 6 Japanese research groups led by Hideo Hosono from the Tokyo Institute of Technology found no room temperature superconductors. They did find 100 previously unknown superconductors with Tc <56°K, and they published crystal structures and phase diagrams of all materials studied to help other researchers avoid now known dead-ends (DOI: 10.1088/1468-6996/16/3/033503).

Other researchers continue to explore the possibilities of using one-dimensional (1D) carbon-based materials such as carbon-nano-tubes (CNT) or graphene as on-chip conductors. So far, there are extreme difficulties in controlling the growth of such 1D structures within interconnect patterns, and additional challenges with forming ohmic contacts between CNT and Cu lines across billions of connections in a modern IC. More science is seemingly needed to find new paths before the engineers can explore those paths to find better solutions. Meanwhile…for the next few years at least…expect Cu metal to be the continued choice for nearly all multi-level metal interconnects on chip.

—E.K.

Bottoms-up ELD of Cobalt Plugs

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As reported in more detail at Solid State Technology, during the IEEE IITC now happening in Grenoble, imec and Lam showed a new Electroless Deposition (ELD) cobalt (Co) process that is claimed to provide void-free bottoms-up pre-filling of vias and contacts. The unit-process is intended to be integrated into flows to produce scaled interconnects for logic and DRAM ICs at the 7nm node and below. Co-incidentally at IITC this year, imec and Lam also presented on a new ELD copper (Cu) process for micron-plus-scale through-silicon vias (TSV).

The bulk resistivities of metals commonly used in IC fabrication are as follows (E-8 Ω⋅m):
Cu – 1.70,
Al – 2.74,
W – 5.3, and
Co – 5.8.
Of course, the above values for bulk materials assume minimal influence of grain sizes and boundary layers. However, in scaled on-chip interconnect structures using in today’s advanced ICs, the resistivity is dominated by grain-boundaries and interfacial materials. Consequently, the resistivity of vias in 7nm node and beyond interconnects may be similar for Cu and Co depending upon the grain-sizes and barrier layers.

The melting temperatures of these metals are as follows (°C):
Al – 660,
Cu – 1084,
Co – 1495, and
W – 3400.
With higher melting temperature compared to Cu, Co contacts/plugs would provide some of the thermal stability of W to allow for easier integration of transistors and interconnects. Seemingly, the main reason to use Co instead of W is that the latter requires CVD processing that intrinsically does not allow for bottom-up deposition.

—E.K.

Moore’s Law is Dead – (Part 3) Where?

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…we reach the atomic limits of device scaling.

At ~4nm pitch we run out of room “at the bottom,” after patterning costs explode at 45nm pitch.

Lead bongo player of physics Richard Feynman famously said, “There’s plenty of room at the bottom,” and in 1959 when the IC was invented a semiconductor device was composed of billions of atoms so it seemed that it would always be so. Today, however, we can see the atomic limits of miniaturization on the horizon, and we can start to imagine the smallest possible functioning electronic device.

Today’s leading edge ICs are made using “22nm node” fab technology where the smallest lithographically defined structure—likely a transistor gate—is just 22nm across. However, the pitch between such transistors is ~120nm, because we are already dealing with the resolution limits of lithography using water-immersion 193nm with off-axis-illumination through phase-shift masks. Even if a “next-generation” lithography (NGL) technology were proven cost-effective in manufacturing— perhaps EbDW for guidelines combined with DSA for feature fill and EUV for trim—we still must control individual atoms.

We may have confidence in shrinking to 62nm pitch for a 4x increase in density. We may even be optimistic that we can shrink further to a 41nm pitch for a ~10x increase in density…but that’s nearing the atomic limits of variability. There are many hypothesized nanoscale devices which could succeed silicon CMOS in IC, but one commonality of all devices is that they will have to be electrically connected. Therefore, we can simplify our consideration of the atomic limits of device scaling by focusing on the smallest possible interconnect.

4nmPitchDevice_TheorySo what is the smallest possible electrical interconnect? So far it would be a Single-Walled Carbon NanoTube (SWCNT) doped with metals to be conducting. The minimum diameter of a SWCNT happens to be 0.4nm, but that was found inside another CNT and the minimum repeatable diameter for a stand-alone SWCNT is ~1nm. So if we need three contacts to a device then the smallest device we can build with atoms would be a 3nm diameter quantum dot. As shown in the figure at right, if we examine a plan-view of such a device we can just fit three 1nm diameter contacts within the area.

Our magical device will have to be electrically isolated and so some manner of dielectric will be needed with some minimal number of atoms. Atomic Layer Deposition (ALD) of alumina has been proven in very tight geometries, and 3 atomic layers of alumina takes up ~1nm so we can assume that spacing between devices. A rectangular array would then result in ~16nm2 as the smallest possible 3-terminal device that can be built on the surface of planet Earth.

Note that a SWCNT of ~1 nm diameter theoretically could carry ~25 microAmps across an estimated 5kOhm internal resistance [(ECS Transactions, 3 (2) 441-448 (2006)]. I will leave it to someone with a stronger device physics background to comment as to the suitability of such contacts for useful circuitry. However, from a manufacturing perspective, to ensure electrical contacts to billions of nanoscale devices we generally use redundant structures, and doubling the number of SWCNT contacts to a 3-terminal device would call for ~8 nm pitch.

However, before we reach the 4-8nm pitch theoretical limits of device scaling, we will reach relative economic limits of scaling just one device feature such as a transistor gate. Recall that there are just 22 silicon atoms (assuming silicon crystal lattice spacing of ~0.3nm) across a ~7nm line, and every atom counts in controlling device parameters. Imec’s Aaron Thean recently provided an excellent overview of scaled finFET technologies, and though the work does not look at packing density we can draw some general trends. If we assume 41nm pitch and double fins with 20nm gate length then each device would use ~1,600 nm2.

Where are we now? Let us consider traditional 6-transistor (6T) SRAM cells built using “22nm node” logic process flows to have minimal area of ~100,000 nm2 or ~16,000 nm2 per transistor. At IEDM2013 (9.1), TSMC announced a “16nm node” 6T SRAM with ~70,000 nm2 area or ~10,000 nm2 per transistor.

IBM recently announced that 6 parallel 30nm long SWCNT spaced 8nm apart will be developed as transistors for ICs by the year 2020. Such an array would use up ~1440 nm2 of area. Again, this is at best another 10x in density compared to today’s “22nm node” ICs.

Imec held another Technology Forum at SEMICON/West this year, in which Wilfried Vandervorst presented an overview of innovations in metrology needed to continue shrinking device dimensions. His work with Scanning Spreading Resistance Microscopy (SSRM) is extraordinary, showing ability to resolve 1-2nm conductivity variations in memory cell material. Working with Resistive RAM (ReRAM) material using a 2nm diameter probe tip as the top contact, researchers were able to show switching of the material only underneath the contact…thus proving that a stable ReRAM cell can be made with that diameter. If we use cross-bar architectures of that material we’d be at a 4nm pitch for memory, coincidentally the same pitch needed for the densest array of 3-terminal logic components.

IC SCALING LIMITATION

Pitch / “Node”

Transistor nm2

Scale from 22nm
193nm lithography double-patterning

124nm / “22nm”

16000

1
Atomic variability (economics)

41nm / “7nm”

1600

10
Perfect atoms (physics)

4nm

16

1000

The refreshing aspect of this interconnect analysis is that it just doesn’t matter what magical switch you imagine replacing CMOS. No matter whether you imagine quantum-dots or molecular memories as circuit elements, you have to somehow connect them together.

Note also that moving to 3D IC designs does not fundamentally change the economic limits of scaling, nor does it alter the interconnect challenge. 3D ICs will certainly allow for greater number of devices to be packed into a given volume, so mobile applications will likely continue to pull for 3D integration. However, the cost/transistor is limited by 2D process technologies that have evolved over 60 years to provide maximum efficiency. Stacking IC layers will allow for faster and smaller devices, though generally only with greater costs.

Atoms don’t scale.

Past posts in the blog series:

Moore’s Law is Dead – (Part 1) What defines the end, and

Moore’s Law is Dead – (Part 2) When we reach economic limits.

The final post in this blog series (but not the blog) will discuss:

Moore’s Law is Dead – (Part 4) Why we say long live “Moore’s Law”!

—E.K.