Ed’s Threads 080613
The last musings in this blog on Friday the 13th of June, 2008

Process integration drives the IC industry
The next 10 years will witness more changes in mainstream manufacturing technology for ICs than in the last 40 years combined. An industry based on “what have you done for me lately” can never rest on its laurels, and so innovation must continue, despite limits in 2D scaling. With rapidly escalating costs projected for 32nm node and smaller digital CMOS manufacturing, it is inevitable that IC companies look to analog, packaging, and heterogeneous integration to add relatively greater value for lower cost and risk. Unique process integration challenges at each fab will drive everything for the next ten years, as shown recently by presentations at the recent International Interconnect Technology Conference IITC., some of which I've already discussed in this blog.

What are the ramifications of all of these subtle changes? With basic “unit-process” building blocks fairly well established, it is likely that the only fundamentally new tools to be developed will be for metrology. The current generation of thin-film, lithography, and thermal processing tools are extremely productive and should continue to be used with modest evolutionary upgrades over the next 10 years. The only exception to this is probably EUV lithography, which is still under development but shows promise.

New integration of old processes can be seen in the evolution of barrier layers for Cu metal in dual-damascene structures. At IITC, Novellus showed that PVD Ti/TiN is the newest Cu barrier to replace Ta -- though Ti is the standard barrier for Al metal lines it was replaced by Ta when the industry first began using Cu lines. Though less expensive than Ta, Ti can react with both fluorine (in FSG dielectric) and Cu and so was not considered to be an acceptable barrier. However, now that FSG has been replaced with SiOC in many fabs, a 5nm thin TiN layer (formed with 50 at% N) capping on Ti stops Cu diffusion. An ultra-thin PVD Ti wetting layer on top of the TiN provides a good surface for ECD fill of Cu.

NEC has targeted high-reliability automobile MCUs, and examined PVD Ti/Ru barrier metal for these applications. After 7 hours at 350°C, Ti diffuses into the Ru, but no Cu diffuses through the Ti. PVD-Ru with (001) orientation can be removed by CMP using conventional slurry for Ta/TaN removal. The Ru barrier achieved 12% lower resistance than the conventional Ta type barrier for 70nm wide Cu lines, while time-dependent dielectric breakdown (TDDB) was unchanged. The Ru/Ti barrier shows 35× longer lifetime (T50) than Ta/TaN. Whatever process is finally developed will run for over 10 years with essentially no changes, as per the supply contracts to the auto industry. I toured NEC Roseville recently and saw old 200mm tools being installed with plans to run for >10 years using ≥150nm node processes.

Low-k air-gaps are also great examples of using old unit-processes in new ways, since CVD oxides (SiOx and SiOC) and spin-on polymers already well developed will be used, while established lithography and etching technologies will complete the integration. EDN's Ron Wilson covered many of the latest air-gap integration details from IITC in his fine blog, but the most important consideration is that no new manufacturing tools are needed to make them happen. Designs may need to be tweaked, and the process integration will be challenging, but this approach is relatively low-risk and may be integrated into older fab lines.

Further proving that “there is no more noise (…there is only signal),” IMEC’s Michele Stucchi at IITC examined the effects of inherent line-edge roughness (LER) and via misalignments on the Efield and electrical breakdown between wires. An enhanced electrical field between adjacent lines is induced by LER-induced reduction in spacing. Compared to a nominal electrical field between lines with zero LER, a probabilistic analysis results in enhancement to the field, which can be a factor >2 in 30nm spaced lines. For wires with 30nm spacing, Efield may be high enough to guarantee at least one bridge in 20μm of line length. For via misalignment of 10nm in standard dual-damascene structures with 30nm spacing, the field enhancement can be a factor >3.

An amazing example of tricky process integration to eliminate variability in lithography was shown by Matthew Breitwisch of IBM, in describing work with Macronyx on phase-change memory (PCM) technology. Ge2Sb2Te5 (GST) was the first material investigated which changes from a high-resistance amorphous structure to a low-resistance crystal at ~170°C. A PCM cell is made up of a variable resistor in series with an access device (which may be a diode, BJT, or FET), and current flux near one million A/cm2 is needed for a few nanoseconds to create the change. To concentrate the current in programming, uniform small pores are achieved using a key-hole (a.k.a., “pinch-off CVD”) process:

- blanket oxide, hardmask and nitride depositions,
- lithography,
- anisotropic etching through the dielectric stack,
- an isotropic oxide etch to create an undercut below the hardmask, and
- conformal polysilicon CVD to form consistent 43nm wide keyholes.

The keyhole widths are controlled by the oxide etch and the poly CVD, and not by the lithography, such that equally sized pores form in the middle of line spaces of varying width…all without any new unit-processes.

With razor-thin process windows and systematic process-design interactions, each fab may have to optimize its own integrated process flow. If each fab runs a unique flow, then a mask-set run on one line will yield very differently on a second line, and this reality may create problems for companies trying dual-sourcing with foundries.

New materials and evolutionary upgrades to old materials will continue to support new integration schemes in fabs, while most of the tools will remain the same. However, OEM applications labs will run non-stop as fabs try new designs-of-experiments (DOE) for new integration schemes. It will be increasingly difficult to get process information out of fabs, since each line will have to be set up with unique tricks and integration schemes. Designers will have to really start innovating, as digital CMOS shrinks no longer guarantee lower cost and higher performance.

There will be many IC types that will probably never be designed at the 32nm node. A friend working on a graphics processing chip explained to me that they needed to go to 65nm to get a speed improvement for their target application, but their modeling shows that additional speed will now provide no user benefit. Going to 45nm adds huge design costs and manufacturing yield risk, and so they now plan to stick to 65nm but work on cost and power optimization in re-design. Their next 65nm chip may use 3D interconnects or integrated passives for improved performance, however, so innovation will continue… just in new directions. Such is the blessing/curse of “living in interesting times.”

Henceforth, my challenges will be elsewhere. This is my last blog post on my last day as a Technical Editor for Solid State Technology magazine; I am resigning to pursue other interests, first of which will be an immediate 8-week sabbatical. When I return, I will probably work for the semiconductor manufacturing industry, in an as-yet undetermined capacity that doubtlessly will be "interesting." I thank PennWell Publishing for years of rewarding employment, and wish the company well. Special thanks to James Montgomery, who diligently and masterfully copy edited most of these 76 postings.

—E.K.

Labels: 32nm, 3D, airgap, CMOS, copper, IITC, integration, interconnect, process, research

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080613: Process integration drives the IC industry

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Ed’s Threads 070608
Musings by Ed Korczynski on June 08, 2007

IITC 2007: Airgaps & chip-stacks
Airgaps and 3D-stacks were the big news from the 10th International Interconnect Technology Conference (IITC) recently held near the San Francisco airport. Two major new materials was presented—IBM showed rhodium (Rh) electro-chemical deposition (ECD) for ≤32nm contact plugs, and Fujitsu showed nano-clustered silicon (NCS) with low k=2.25 for a dielectric—but most new work involves the same materials combined in clever new ways. Airgap technology was covered in four oral presentations, three posters, and countless informal hallway discussions.

Dan Edelstein, IBM Fellow and manager of BEOL technology strategy at Yorktown Heights, NY, gave an invited talk on the many integration challenges for 32nm node interconnects, including resist poisoning from low-k outgassing, low-k damage removal, and the need for improved thin-film interfaces. “We need to keep adding innovation just to stay on the trend-line,” he commented. For example, the industry has historically seen chronically low SiCOH low-k adhesion on SiCHN barrier layers—regardless of equipment, CVD precursor, or plasma preclean—due to a carbon-rich initial deposition. Adding a diverter-valve to the tool allows for stabilized precursor flow before RF power is turned on, which eliminates the carbon-rich deposition and thus solves the adhesion issue. With subtle integration challenges such as these, IBM has chosen to add airgaps as a side-loop with no new materials, tools, or baseline processes. Airgaps drop k by ~35% for any given dielectric material, Edelstein noted, adding that IBM has “shown this on gapped SiOF and low-k SiCOH, and will do it next on ULK porous SiCOH.”

The IBM airgap process both removes and re-deposits some dielectric material, while most airgap approaches for logic chips rely on removal processes alone. The Crolles2Alliance (CEA-Leti, Freescale, NXP, and ST) uses SiO2 at line-levels and a polymer for the via-levels within the dielectric stack, then HF vapor or wet-etch-chemistries to remove the SiO2. NXP and Dow Chemical showed removal of a thermally degradable polymer (TDP) through a CVD SiOC cap layer to make ~30% airgaps at M2 as part of a keff ~2.5 to hit 32nm node specs.

The Crolles2Alliance also showed some of the integration tricks needed to use porous ULK dielectrics at the 32nm node. Different plasmas may seal pore surfaces to provide barrier properties for long-term reliability: CH4 adds C, NH3 substitutes N for C leading toward SiON compositions, and He/H2 plasmas retain near original stoichiometry. Though Cu bulk resistivity is only ~2.2 µOhm-cm, for 60nm line widths it is ~2.9 and increases with reducing widths. CMOS32 uses 50nm Cu line widths for M1, requiring a self-aligned barrier (SAB) <4nm for EM performance, an ALD barrier and thin-Cu seed for filling, and either a CuSiN or CoWP cap layer.

NEC research labs showed that direct ECD of Cu without a Cu-seed layer provides larger grain size and higher Cu(111) orientation. Damascene structures were first sealed with TiN, then either Ta/Cu or Ru layers were deposited. The TiN barrier layer is definitely needed beneath Ru to block Cu diffusion into the dielectric. Ru PVD using DC magnetron sputtering with Ar gas at room temperature produces high orientation of Ru(002). Since Ru(002) is hexagonal-close-packed, it matches well with the preferred Cu(111) face-centered-cubic orientation such that 40%-50% can be grown directly on Ru in dual-damascene structures. Some day, metal line specifications may include not just dimensions and resistivity, but grain orientation and size-distribution too.

Ibaraki U. and Hitachi presented research showing that higher chemical purity leads to lower resistivity in Cu lines. Increasing both the Cu anode purity from 4N to 9N along with the CuSO4·5H2O purity from 3N to 6N reduced line resistance by 21% in 50nm wide lines, with all other process parameters held constant. The high-purity process increased the average grain size from 70 to 74nm, and significantly reduced the oxygen content in the final annealed Cu lines to <1 wt% from the previous 3-4 wt%.

Based on first principles of thermodynamics, an alloy of Cu/Mn can be annealed to result in self-segregation of Mn to the dielectric/Cu barrier. One fundamental advantage of this process is that no barrier is formed at the bottoms of vias, which minimizes resistance. Toshiba’s R&D; group tested self-aligned Mn barriers with 244-via-chain structures and found one-third the resistance compared to Cu vias using the standard Ta barrier.

Georgia Tech and U. of New Mexico researchers showed that a 60% increase in the total number of wire levels is sufficient to account for ~5x increase in the resistivity of wires. Careful routing and a logical hierarchy seem to go a long way, but eventually the industry must get serious about 3D ICs using chip-stacks.

Patrick Leduc of CEA-Leti provided an overview of the main challenges to realizing high density 3D ICs: bonding with ±1µm alignment at T<400°C, Si thinning to <15µm, and through-silicon via (TSV) diameters <3µm. Thermal management issues may not be too difficult—assuming each transistor contributes 0.7W to a 50 W/cm2 average—since bulk silicon acts as an efficient heat spreader and the metal lines conduct well.

Freescale’s Scott Pozder explained that EDA software tools may be the current biggest limitation to 3D integration, since standard tools cannot even account for metal levels on multiple chips. If you explicitly design for 3D, then models show that multiplicative yield-losses can be avoided or eliminated.

There were ~480 conference attendees this year (plus several hundred additional folks running evening supplier-seminars and exhibit booths). Among the attendees with whom I enjoyed discussions were (in alphabetical order) Al Bergendahl, Chris Case, Paul Feeney, Terry Francis, Mike Fury, Xiao Hu Liu, Steven Luce, Satya Nitta, Mike Shapiro, and a special appearance by casually retired Mike Thomas.

—E.K.

Labels: airgap, copper, dual-damascene, IITC, interconnect, low-k, self-aligned barrier, ULK

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070608: IITC2007 airgaps & chip-stacks

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Ed’s Threads 070525
Musings by Ed Korczynski on May 25, 2007

Intel-IBM fab hype-war and truths
It has been said that the first casualty of war is the truth…even more so in a hype-war. An interview appearing on a popular electronics industry Web site is the most recent battlefield in this ongoing hype-war between the world’s semiconductor manufacturing heavyweights. As tradeoffs in fab technology become more fundamental at 45nm nodes and beyond, different companies choose to deploy similar technologies in different ways. The truth is inherently complex and thus a bit complex to describe, and can die under the assault of hype.

Among fundamental choices today we find the following: double-patterned dry or single-patterned wet lithography for critical layer patterning, and use of porous or airgap low-k dielectric for on-chip multi-level interconnects. Intel has chosen double-patterned dry lithography and non-airgap low-k dielectrics. IBM has chosen single-patterned wet-lithography and airgap dielectrics.

As shown by Hoofman, et.al, in the pages of SST last year, there are many different airgap process flows, which can produce many different airgap structures. Airgaps may be complete or partial between lines, and this is one of the more fundamental parameters to consider in integrating the structures into real chips. Thus, airgap1 is not airgap2 (General Semantics suggests the use of “indexing” to remind us of the essential distinctions between members of any conceptual set).

Using “complete airgap” flows and removing all dielectric between lines to achieve the absolute minimum capacitance does indeed create the two general problems articulated by Mark Bohr in the recorded interview: an expensive critical-lithography mask to ensure proper via landings, and copper electromigration sensitivity. However, the IBM flow uses airgaps only in the middle of line spaces while leaving dielectric material on the sidewalls of copper lines.

The IBM airgap process exposes parts of some metal sidewalls during the tricky three-stage gap etch, but the subsequent dielectric CVD process re-coats the exposed sidewall areas prior to “pinching off” the tops of gaps. With adequate sidewall dielectric in place, via landing and electromigration issues can be minimized if not avoided. Intel’s Bohr certainly understands airgap integration issues far better than I do, but because he’s not in a position to comment on how his competitor’s process might work, he accurately and properly expressed Intel’s results in the interview, and the generic disadvantages of approaches not currently taken by Intel.

However, a statement summarizing the interview reads thusly: “Mark Bohr, Intel senior fellow, says his company looked at air-gap technology like IBM recently introduced, and dismissed it as costly and inefficient.” That's a misinterpretation. The truth is that Intel never panned IBM’s airgap technology—Bohr answered a specific question about IBM’s technology with a generic answer about non-IBM technology. Of course, Bohr can very reasonably say that he has no knowledge if IBM’s technology beyond what is muddied in the official press release. Since IBM’s marketing spun the technology truth to the point of grandiose hype, it provides easy opportunity for Intel to comment on the hype instead of the truth. (Incidentally, further information on the general concept of self-assembled nanotechnology for lithographic masking applications can be found in the most recent issue of Microlithography World.)

The IBM hype was that no new lithography is needed. Intel counters that a generic process flow for airgaps requires critical-lithography steps. The truth is that the IBM flow does use an additional lithography step for each airgap level, but it’s non-critical and the mask generation has been automated as a button in the EDA deck. Critical-lithography steps for 45nm node processing can be 10X more expensive than non-critical patterning steps. If the IBM airgap process required critical lithography for each level then it might add 20%-25% to the cost of each chip, but with non-critical lithography it might add only 5%-8%.

For most people in the world who lack experience in fab processes, subtle complexities get lost in translation and details are distorted or lost. The truth about nanometer-era fab processes is that they are all tough to develop with inherent integration trade-offs. With Intel and IBM now going down divergent paths, it’s truly difficult to assess a fab technology “leader” in anything but hype.

— E.K.

Labels: airgap, fab, IBM, IC, Intel, interconnect, low-k, self-assembly, ULK

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070525: Intel-IBM fab hype-war and truths

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Ed’s Threads 070504
Musings by Ed Korczynski on May 04, 2007 (updated June 12, 2007 to correct details of the IBM airgap etch process, which had erroneously referred to the third-step being RIE, when it is wet as confirmed by both D. Edelstein and S. Nitta)

IBM adds airgaps for faster chips
Airgaps have long been considered as structures to increase the speed of on-chip IC interconnects, though no one had developed manufacturing-worthy process flows. Only in the last year have companies such as Philips (now NXP) shown overviews of likely airgap manufacturing processes, though without production commitments. Now IBM has invented a new variation on airgaps that uses a self-assembling polymer mask layer as part of the process flow, and claims this can be a simple drop-in addition that adds only ~1% to chip cost for each dielectric layer gapped. Thus for an advanced multilevel interconnect, a ~5% cost adder should provide 35% faster chips or 15% less power consumption.

Circuit speeds are limited by the dielectric constant (k) of the insulating material surrounding metal lines, so the industry's Roadmap has focused on ever lower k dielectric materials. Unfortunately, materials engineering for a new dielectric material is difficult and expensive, and despite tremendous efforts and many false-starts over the years, the entire world has now settled on SiCOH by CVD as the lone dielectric material (k~3) that provides acceptable cost, yield, and reliability. So-called ultralow-k (ULK, aka “extreme low-k”) films are merely k~3 SiCOH with the addition of ~20%-40% by volume of nanopores to reach k~2.4. More nanopores cannot be added without degrading yield and reliability, so the only practical way to get to k~2 is to incorporate a single large pore with clever processing as an “airgap.”

A multiyear development effort to create a manufacturable airgap process was led by IBM fellow Dan Edelstein, program manager for low-k CVD BEOL, who provided Solid State Technology and WaferNews with exclusive insight into how they achieved these remarkable results. He explained that unlike previously known airgap process flows, the IBM approach starts with a standard dual-damascene copper and SiCOH dielectric process that has been in production for years. Airgaps are formed using a multi-step etch, using a hardmask patterned with either self-assembling monolayers or standard lithography depending upon the geometry of the interconnect.

Unfortunately, IBM's press release touting the airgap achievement is so grossly hyped that it’s caused severe misunderstanding throughout most press reports on this process. The new technique "skips the masking and light-etching process,” says the official release. “Instead IBM scientists discovered the right mix of compounds, which they pour onto a silicon wafer with the wired chip patterns, then bake it.”

In reality, while self-assembly can be used to make an array of nominally 20nm holes by spin-coating and baking, these holes merely pattern the hardmask that is used to etch the gaps into the dielectric, explained Edelstein. A non-critical lithography step is used to block out circuit areas that do not need gaps, he said. The self-assembly layer is not even used to pattern the hardmask used to make airgaps at upper levels of the interconnect. “At some point in the hierarchy it becomes more viable to use lithography instead of self-assembly,” he said.

While IBM doesn't use airgaps for the first level of metal, they could be used at any of the higher levels within the hierarchical interconnect stack, Edelstein noted. “Most chips won’t need air-gaps on all levels, but perhaps on half,” he said.

No matter the level, a special three-step etch process to form gaps with narrow top openings is the key to this process (see figure). “We etch a narrow channel down so it will cap off, then deliberately damage the dielectric and etch it so it looks like a balloon,” he explained. “You have a big gap with a drop in capacitance and then a small slot that gets pinched off.”

Starting with dual-damascene copper lines/vias and SiCOH single-phase dielectric, the essential IBM airgap process flow is as follows:

1) Deposit hardmask;
2) Spin-coat an imaging layer; either special new diblock polymer or standard photoresist;
3) Create holes using either the self-assembly properties of the diblock or standard lithography;
4) Block out circuit areas to not be etched using non-critical photolithography;
5) Transfer holes from the imaging layer to the hardmask;
6) Etch three-step sequence—first an anisotropic RIE to form deep openings into SiCOH, then plasma damage of the column sidewalls, then isotropic wet etch to remove most of the remaining SiCOH underneath the hardmask;
7) Strip hardmask; and
8) PECVD of the next SiCOH dielectric level to cap the gaps with a classic “pinch-off” shape.

Since the self-assembling mask layer is not aligned to the underlying interconnect structures, and since the block-out mask is “non-critical” to save costs, the hardmask will inevitably expose the tops and sides of some metal lines to RIE. Consequently, the SiCOH etch chemistry needs to have excellent selectivity so as to not attack copper and any metallic barrier layers. Edelstein says that they’ve been able to work with standard gas precursors for this critical RIE step.

The new airgap process is an optional loop off of the standard flow, so designers can choose to use airgaps at any of the levels in the on-chip interconnect hierarchy—and IBM also has developed an automated algorithm for making the block-out mask. “As a customer you can turn on the air-gap option for any level on any chip. We can put the gaps in independent of any incoming design,” Edelstein told WaferNEWS. The ability to add air-gaps as a “drop-in” to an existing on-chip interconnect process flow minimizes risks, and explains the company’s confidence that this flow will be used in manufacturing by 2009.

While the diblock polymer is only one part of this airgap process, it is a significant addition. Chemists at IBM Almaden Research reportedly developed this material for broad applications in fabs—it’s like a standard photoresist in terms of handling and dispensing, it has a wide process window, and IBM has detected no shelf-life problems for up to one year.

Using self-assembly in coordination with lithography opens up new possibilities in general for integrated process flows, so look for news of additional applications in coming years. “We hope that we can use directed self-assembly to get to other device features,” said Edelstein. “This is just the tip of the iceberg.”

— E.K.

Labels: airgap, fab, IBM, IC, interconnect, low-k, self-assebly, ULK

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070504: IBM add airgaps for faster chips

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