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Posts Tagged ‘interconnects’

New Tungsten Barrier/Liner, Fill Processes Reduce Resistance and Increase Yield

Friday, June 3rd, 2016

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By Pete Singer, Editor-in-Chief

Today’s most advanced chips pack two billion transistors on a die size of 100 mm2. Considering transistors are three terminal devices, that equates to six billion contacts to those transistors, which connect to 10-15 Layers of stacked wiring. Although the wiring is copper, the contacts at the transistor level and the so-called local interconnect level just above the contact level are made of tungsten (Figure 1). Although tungsten has slightly higher resistance than copper, the danger of copper contamination killing the transistor is such that tungsten is still used.

Figure 1. The contact (black area) is the first, smallest, most critical connection between the transistor and interconnect wiring. Source: TECHINSIGHTS

Two looming problems are that contact resistance is going up, to the point where it will soon be higher than that of the transistor (Figure 2). Yield is also at risk since just one bad contact can cause entire portions of the chip to fail. “Not only are there a lot of these contacts, they’re very challenging to make because they are so small and getting even smaller with each node,” said Jonathan Bakke, Global Product Manager, Transistor and Interconnect Group, Applied Materials.

Figure 2. At the 10nm node and beyond, contact and plug resistance is expected to rise exponentially and dominate.

Applied Materials recently launched two new products aimed at reducing contact resistance and improving yield in tungsten contacts. The Applied Endura® Volta™ CVD W product results in a new tungsten-based material that serves as both a barrier and a liner, enabling the lower resistance W fill to be three times wider than in traditional process flows. The end result is an increase of up to 90% in contact resistance. The Applied Centura® iSprint™ ALD/CVD SSW (seam-suppressed tungsten) product achieves bottom-up gap fill in tungsten contact CVD processes, reducing seams and voids, which increases yield.

The traditional process flow to from a contact has been to deposit a layer of titanium to form a silicide layer by reacting with the silicon, followed by a TiN barrier. This barrier film prevents the diffusion of fluorine into the silicon of the transistor from the tungsten hexafluoride (WF6) used to deposit the subsequent tungsten contact fill. Because tungsten doesn’t grow directly on TiN, a seed layer of W is typically deposited by ALD before the WF6 CVD bulk fill.

Two challenges associated with this approach is that the barrier and liner have not scaled – they have been made as thin as possible, but they’ve reached a limit. The TiN barrier is typically around 30-40Å and the liner film another 20Å. As a result, the volume of the overall plug made of the more desirable, lower resistance W is reduced. “The TiN and tungsten based liner are both high resistance layers. The more volume they occupy, the more they contribute to resistance,” Bakke said.

The second challenge is that, because the W CVD process results in a conformal fill, where all sides grow at the same rate, a seam is often formed in the middle of the contact. Or, even worse, the top closes before the W completely fills the contact hole, resulting in a void. Both seams and voids can be exposed or breached during the subsequent chemical mechanical planarization (CMP) step. “The contacts or local interconnects are becoming much smaller with each node and they’re getting more challenging to fill with low resistance material and without seams or voids,” Bakke said. Figure 3 shows common problems with resistance and yield.

Figure 3: Barriers and liners don't scale, leaving less room for low resistance W fill. Seams and voids can cause yield problems.

Seams and voids can lead to yield problems such as overly high contact resist or even open contacts. If even a few of the 6 billion contacts on a chip fail, there can a big impact on yield. One study (Figure 4), shows that even at the 20nm node, one defect in a billion can lead to a yield loss of 15% or more. “This tells you that you really have to have perfect gap fill. If one contact goes, it can knock out an entire portion of the device and make it inoperable,” Bakke said.

Figure 4. Source: Nvidia

Enter the Applied Endura® Volta™ CVD W and the Applied Centura® iSprint™ ALD/CVD SSW (seam-suppressed tungsten).

A process has been developed for the Endura – Applied’s platform for metal deposition, including PVD and CVD – to deposit a tungsten-based CVD film that serves as both the barrier layer and the liner layer. At around the 30Å thickness that would be typical of just the barrier, and it’s as effective a barrier as TiN. “We’re doing materials engineering to create the first new liner for tungsten plug in 10 years,” Bakke said. This means more of the volume of the contact consists of the lower resistivity W fill (Figure 5). “You can actually triple the tungsten fill width at the 15 nm node. You get a lot more low-resistance material in there. Beyond that, it’s a simpler process flow, by removing the one layer, the liner,” Bakke added.

Figure 5

Figure 6 shows how the new W-based barrier/liner compares to the standard flow. The tungsten-based film is 75% lower in resistitivity that the TiN (left). At thicknesses which are relevant for the 10nm node, an 80% reduction in total stack resistivity is seen (right).

Figure 6

Perhaps even more important is the contact resistance, as shown in Figure 7, which charts contact resistance vs critical dimension. “By the time you’re getting to the 10 and 7nm node thicknesses, you actually have a big drop in resistivity up to about 90% reduction in resistance at the 7nm node thicknesses,” Bakke explained.

Figure 7

One reason why plug resistance is becoming more important is indicated by the orange line in Fig. 7, which shows silicide contact resistance. “For a long time, the silicide was the big contributor to the transistor contact total resistance. Manufacturers spent a lot time trying to decrease that resistance as they scaled. There’s a cross-over point (blue line) where the plug starts become of higher resistance than the contact. We need to focus on bring the plug resistance back down so it’s not the major contributor to the total resistance,” Bakke said.

Figure 8 shows the end result, with a clean interface between both the tungsten and underlying tungsten layer. “The Volta W adheres very well to dielectric sidewalls. And the W fill is able to deposit on the Volta W and give good gap fill performance,” said Bakke. “It’s also able to survive all the post-processing steps, such as CMP and deposition of copper.”

Figure 8. Degas, clean and Volta W are integrated in the Endura platform.

The Applied Centura® iSprint™ ALD/CVD SSW process uses a “special treatment” after the liner (or barrier/liner in the case of Volta W) to suppress growth on the field and induce growth in a bottom-up fashion (Figure 9). This bottom-up growth eliminates seams and voids. “Because you have a more robust fill, you get an improved yield because you don’t breach the contact or local interconnect during the CMP step,” Bakke said. “This is the first bottom-up tungsten CVD in high volume manufacturing,” he added.

Figure 9. Bottom-up fill is shown in a diagram (top) and in an actual structure.

Bakke wouldn’t say what the special treatment was, but a patent search revealed a possible approach, involving activated nitrogen where the activated nitrogen is deposited preferentially on the surface regions.

New Applied PVD system targets TiN hardmasks for 10nm, 7nm chips

Tuesday, May 19th, 2015

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By Jeff Dorsch, Contributing Editor

Applied Materials today introduces the Applied Endura Cirrus HTX PVD, a physical vapor deposition system for creating titanium nitride hardmask films that could be used in fabricating 10-nanometer and 7nm chips.

“Titanium nitride is the metal hardmask of choice,” harder than copper and nearly as hard as diamond, says Sree Kesapragada, Applied’s global product manager for Metal Deposition Products.

“Patterning plays key role in defining the interconnect,” Kesapragada says. “Perfect via alignment is critical for device yield. Hardmask ensures the perfect via alignment critical for yield.”

The hardmasks created with the Endura Cirrus HTX TiN system strike the required balance between neutral stress and film density hardness, he asserts. The TiN hardmask, meant to resist the erosion of etching, helps ensure that via etches land where they are supposed to, and not too close to neighboring vias, which can creates shorts.

Metal hardmask layer manages alignment errors.

Applied has worked with customers at multiple sites in developing the new PVD system over the past two to three years, according to Kesapragada. He emphasizes that the Cirrus HTX TiN system offers “precision control over TiN crystal growth,” as the process chamber is “designed for tensile high-density TiN films.” The new PVD system enables high density, tensile films thanks to a high level of ionization during deposition made possible by a high frequency source.

High film desnity is needed to prevent erosion, and a neutral-to-tensile stress is needed for pattern fidelity. CVD/ALD films have tensile stress, but are low density. Traditionally deposited TiN films have good density, but compressive stress.

The formation of “islands” of TiN crystals is almost like chemical vapor deposition, “layer by layer,” Kesapragada says, “in a PVD chamber.”

In the process chamber, the first of its kind, titanium atoms are reactively sputtered in a nitrogen-based plasma, allowing for tunable composition, according to Applied. This chamber can be used for high-volume manufacturing of semiconductors with 7nm features, covering two process-node generations, Kesapragada says.

There is also “very established integration” with chemical mechanical planarization equipment, he adds.

Applied is the market leader in TiN PVD systems, with more than 200 systems shipped, according to Kesapragada. Those PVD systems have more than 700 process chambers, he adds.

The Endura Cirrus HTX TiN PVD system is being formally introduced this week at the IEEE’s 2015 International Interconnect Technology Conference in Grenoble, France.

Intel Announces “New Interconnect” for 14nm

Tuesday, September 2nd, 2014

By Dr. Phil Garrou, Contributing Editor, Solid State Technology

Intel has just announced that Embedded Multi-die Interconnect Bridge (“EMIB”) packaging technology will be available to 14nm foundry customers. Claiming it is a “…lower cost and simpler 2.5D packaging approach for very high density interconnects between heterogeneous dies on a single package.” [link]

Intel released the following description “Instead of an expensive silicon interposer with TSV (through silicon via), a small silicon bridge chip is embedded in the package, enabling very high density die-to-die connections only where needed. Standard flip-chip assembly is used for robust power delivery and to connect high-speed signals directly from chip to the package substrate. EMIB eliminates the need for TSVs and specialized interposer silicon that add complexity and cost.”

It is highly likely that this is tied to the issuance of patent application publication US 2014/0070380 A1 published March 13 2014.

In simplified form interconnect bridges (“silicon glass or ceramic”) are embedded in a laminate substrate and connected with flip chip as shown below.

Bridge Interconnect as described in recent Intel patent.

A cross section of the package is more revealing showing connections through the laminate and connections through the bridge substrate (316) which would be TSV in the case of a silicon bridge substrate. The underside of the bridge substrate (314) may be connected to another bridge substrate for further interconnect routing as shown below.

While there is no silicon interposer, there do appear to be TSV in the embedded interconnect substrate as shown below. While removing complexity from the IC fabrication by eliminating TSV from the foundry process, the packaging operation becomes much more complex.

Since the 2.5D interposer has been reduced in size to the interconnect bridges this may reduce cost, but will increase signal length vs a true 3D stack or a silicon interposer 2.5D.

Further details will be discussed in a future IFTLE blog.

Intel EMIB Module in Cross Section

Blog review April 22, 2014

Tuesday, April 22nd, 2014

Pete Singer blogs that it’s difficult to make interconnects much smaller without introducing significant increases in resistivity. At the upcoming IITC/AMC joint conference in May, many papers focus on new materials that could lead to reduced resistivity and enable further interconnect scaling. Most notably, graphene and CNTs provide an interesting alternative to copper.

Phil Garrou continues his analysis of the IMAPS Device Packaging Conference with a look at the presentations made by Flip Chip International and SUSS (the use of lasers in the manufacturing of WLP); GLOBALFOUNDRIES, Amkor and Open Silicon (a 2.5D ARM dual core product demonstrator which consists of 2 ARM die on a high density silicon interposer); Corning (results of multiple glass interposer programs) and Namics (underfill products for FC BGA and FC CSP).

Extending Copper Interconnect Beyond The 14nm Node

Thursday, June 20th, 2013

Fabricating interconnects is one of the most process-intensive and cost-sensitive parts of manufacturing. To find out more about what’s changing in this area and why it’s so important, click here.