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MRAM Takes Center Stage at IEDM 2016

Monday, December 12th, 2016

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By Dave Lammers, Contributing Editor

The IEDM 2016 conference, held in early December in San Francisco, was somewhat of a coming-out party for magneto-resistive memory (MRAM). The MRAM presentations at IEDM were complemented by a special MRAM-focused poster session – organized by the IEEE Magnetics Society in cooperation with the IEEE Electron Devices Society (EDS) – with 33 posters and a lively crowd.

And in the opening keynote speech of the 62nd International Electron Devices Meeting, Seok-hee Lee, executive vice president at SK Hynix (Seoul), set the stage by saying that the race is on between DRAM and emerging memories such as MRAM. “Originally, people thought that DRAM scaling would stop. Then engineers in the DRAM and NAND worlds worked hard and pushed out the end further in the future,” he said.

While cautioning that MRAM bit cells are larger than in DRAM and thus more more costly, Lee said MRAM has “very strong potential in embedded memory.”

SK Hynix is not the only company with a full-blown MRAM development effort underway. Samsung, which earlier bought MRAM startup Grandis and which has a materials-related research relationship with IBM, attracted a standing-room-only crowd to its MRAM paper at IEDM. TSMC is working with TDK on its program, and Sony is using 300mm wafers to build high-performance MRAMs for startup Avalanche Technology.

And one knowledgeable source said “the biggest processor company also has purchased a lot of equipment” for its MRAM development effort.

Dave Eggleston, vice president of emerging memory at GlobalFoundries, said he believes GlobalFoundries is the furthest along on the MRAM optimization curve, partly due to its technology and manufacturing partnership with Everspin Technologies (Chandler, Ariz.). Everspin has been working on MRAM for more than 20 years, and has shipped nearly 60 million discrete MRAMs, largely to the cache buffering and industrial markets.

GlobalFoundries has announced plans to use embedded STT-MRAM in its 22FDX platform, which uses fully-depleted SOI technology, as early as 2018.

Future versions of MRAM– such as spin orbit torque (SOT) MRAM and Voltage Controlled MRAM — could compete with SRAM and DRAM. Analysts said today’s spin-transfer torque STT-MRAM – referring to the torque that arises from the transfer of electron spins to the free magnetic layer — is vying for commercial adoption as ever-faster processors need higher performance memory subsystems.

STT-MRAM is fast enough to fit in as a new memory layer below the processor and the SRAM-based L1/L2 cache layers, and above DRAM and storage-level NAND flash layers, said Gary Bronner, vice president of research at Rambus Inc.

With good data retention and speed, and medium density, MRAM “may have advantages in the lower-level caches” of systems which have large amounts of on-chip SRAM, Bronner said, due in part to MRAM’s smaller cell size than six-transistor SRAM. While DRAM in the sub-20nm nodes faces cost issues as its moves to more complex capacitor structures, Bronner said that “thus far STT-MRAM) is not cheaper than DRAM.”

IBM researchers, which pioneered the spin-transfer torque approach to MRAM, are working on a high-performance MRAM technology which could be used in servers.

As of now, MRAM density is limited largely by the size of the transistors required to drive sufficient current to the magnetic tunnel junction (MTJ) to flip its magnetic orientation. Dan Edelstein, an IBM fellow working on MRAM development at IBM Research, said “it is a tall order for MRAM to replace DRAM. But MRAM could be used in system-level memory architectures and as an embedded memory technology.”

PVD and etch challenges

Edelstein, who was a key figure in developing copper interconnects at IBM some twenty years ago, said MRAM only requires a few extra mask layers to be integrated into the BEOL in logic. But there remain major challenges in improving the throughput of the PVD deposition steps required to deposit the complex material stack and to control the interfacial layers.

The PVD steps must deposit approximately 30 layers and control them to Angstrom-level precision. Deposition must occur under very low base pressure, and in oxygen- and water-vapor free environments. While tool vendors are working on productization of 300mm MRAM deposition tools, Edelstein said keeping particles under control and minimizing the maintenance and chamber cleaning are all challenging.

Etching the complex materials stack is even harder. Chemical RIE is not practical for MRAMs at this point, and using ion beam etching (IBE) presents challenges in terms of avoiding re-deposition of material sputtered off during the IBE etch steps for the high-aspect-ratio MTJs.

For the tool vendors, MRAMs present challenges as companies go from R&D to high-volume manufacturing, Edelstein said.

A Samsung MRAM researcher, Y.J. Song, briefly described IBE challenges during an IEDM presentation describing an embedded STT-MRAM with a respectable 8-Mbit density and a cell size of .0364 sq. micron. “We worked to optimize the contact etching,” using IBE etch during the patterning steps, he said. The short fail rate was reduced, while keeping the processing temperature at less than 350°C, Song said.

Samsung embedded an STT-MRAM module in the copper back end of the line (BEOL) of a 28nm logic process. (Source: Samsung presentation at IEDM 2016).

Many of the presentations at IEDM described improvements in key parameters, such as the tunnel magnetic resistance (TMR), cell size, data retention, and read error rates at high temperatures or low operating voltages.

An SK Hynix presentation described a 4-Gbit STT-MRAM optimized as a stand-alone, high-density memory. “There still are reliability issues for high-density MRAM memory,” said SK Hynix’s S.-W. Chung. The industry needs to boost the TMR “as high as possible” and work on improving the “not sufficiently long” retention times.

At high temperatures, error rates tend to rise, a concern in certain applications. And since devices are subjected to brief periods of high temperatures during reflow soldering, that issue must be dealt with as well, detailed by a Bosch presentation at IEDM.

Cleans and encapsulation important

Gouri Sankar Kar, who is coordinating the MRAM research program at the Imec consortium (Leuven, Belgium), said one challenge is to reduce the cell size and pitch without damaging the magnetic properties of the magnetic tunnel junction. For the 28nm logic node, embedded MRAM would be in the range of a 200nm pitch and 45nm critical dimensions (CDs). At the IEDM poster session, Imec presented an 8nm cell size STT-MRAM that could intersect the 10nm logic node, with the MRAM pitch in the 100nm range. GlobalFoundries, Micron, Qualcomm, Sony and TSMC are among the participants in the Imec MRAM effort.

Kar said in addition to the etch challenges, post-patterning treatment and the encapsulation liner can have a major impact on MTJ materials selection. “Some metals can be cleaned immediately, and some not. For the materials stack, patterning (litho and etch) and clean optimization are crucial.”

“Chemical etch (RIE) is not really possible at this stage. All the tool vendors are working on physical sputter etch (IBE) where they can limit damage. But I would say all the major tool vendors at this point have good tools,” Kar said.

To reach volume manufacturing, tool vendors need to improve the tool up-time and reduce the maintenance cycles. There is a “tail bits” relationship between the rate of bit failures and the health of the chambers that still needs improvement. “The cleanup steps after etching are very, very critical” to the overall effort to improving the cost effectiveness of MRAM, Kar said, adding that he is “very positive” about the future of MRAM technology.

A complete flow at AMAT

Applied Materials is among the equipment companies participating in the Imec program, with TEL and Canon-Anelva also heavily involved. Beyond that, Applied has developed a complete MRAM manufacturing flow at the company’s Dan Maydan Center in Santa Clara, and presented its cooperative work with Qualcomm on MRAM development at IEDM.

In an interview, Er-Xuan Ping, the Applied Materials managing director in charge of memory and materials technologies, said about 20 different layers, including about ten different materials, must be deposited to create the magnetic tunnel junctions. As recently as a few years ago, throughput of this materials stack was “extremely slow,” he said. But now Applied’s multi-cathode PVD tool, specially developed for MRAM deposition, can deposit 5 Angstrom films in just a few seconds. Throughput is approaching 20 wafers per hour.

Applied Materials “basically created a brand-new PVD chamber” for STT-MRAM, and he said the tool has a new e-chuck, optimized chamber walls and a multi-cathode design.

The MRAM-optimized PVD tool does not have an official name yet, and Ping said he refers to it as multi-cathode PVD. With MRAM requiring deposition of so many different metals and other materials, the Applied tool does not require the wafer to be moved in and out, increasing efficiency. The shape and structure of the chamber wall, Ping said, allow absorption of downstream plasma material so that it doesn’t come back as particles.

For etch, Applied has worked to create etching processes that result in very low bit failure rates, but at relatively relaxed pitches in the 130-200nm range. “We have developed new etch technologies so we don’t think etch will be a limiting factor. But etch is still challenging, especially for cells with 50nm and smaller cell sizes. We are still in unknown territory there,” said Ping.

Jürgen Langer, R&D manager at Singulus Technology (Frankfurt, Germany), said Singulus has developed a production-optimized PVD tool which can deposit “30 material layers in the Angstrom range. We can get 20 wafers per hour throughputs, so I would say this is not a beta tool, it is for production.”

Jürgen Langer, R&D manager, presented a poster on MRAM deposition from Singulus Technology (Frankfurt, Germany).

Where does it fit?

Once the production challenges of making MRAM are ironed out, the question remains: Where will MRAM fit in the systems of tomorrow?

Tom Coughlin, a data storage consultant based in Atascadero, Calif., said embedded MRAM “could have a very important effect for industrial and consumer devices. MRAM could be part of the memory cache layers, providing power advantages over other non-volatile devices.” And with its ability to power on and power off without expending energy, MRAM could reduce overall power consumption in smart phones, cutting in to the SRAM and NOR sectors.

“MRAM definitely has a niche, replacing some DRAM and SRAM. It may replace NOR. Flash will continue for mass storage, and then there is the 3D Crosspoint from Intel. I do believe MRAM has a solid basis for being part of that menagerie. We are almost in a Cambrian explosion in memory these days,” Coughlin said.

Applied Materials Releases Selective Etch Tool

Wednesday, June 29th, 2016

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By Ed Korczynski, Sr. Technical Editor

Applied Materials has disclosed commercial availability of new Selectra(TM) selective etch twin-chamber hardware for the company’s high-volume manufacturing (HVM) Producer® platform. Using standard fluorine and chlorine gases already used in traditional Reactive Ion Etch (RIE) chambers, this new tool provides atomic-level precision in the selective removal of materials in 3D devices structures increasingly used for the most advanced silicon ICs. The tool is already in use at three customer fabs for finFET logic HVM, and at two memory fab customers, with a total of >350 chambers planned to have been shipped to many customers by the end of 2016.

Figure 1 shows a simplified cross-sectional schematic of the Selectra chamber, where the dashed white line indicates some manner of screening functionality so that “Ions are blocked, chemistry passes through” according to the company. In an exclusive interview with Solid State Technology, company representative refused to disclose any hardware details. “We are using typical chemistries that are used in the industry,” explained Ajay Bhatnagar, managing director of Selective Removal Products for Applied Materials. “If there are specific new applications needed than we can use new chemistry. We have a lot of IP on how we filter ions and how we allow radicals to combine on the wafer to create selectivity.”

FIG 1: Simplified cross-sectional schematic of a silicon wafer being etched by the neutral radicals downstream of the plasma in the Selectra chamber. (Source: Applied Materials)

From first principles we can assume that the ion filtering is accomplished with some manner of electrically-grounded metal screen. This etch technology accomplishes similar process results to Atomic Layer Etch (ALE) systems sold by Lam, while avoiding the need for specialized self-limiting chemistries and the accompanying chamber throughput reductions associated with pulse-purge process recipes.

“What we are doing is being able to control the amount of radicals coming to the wafer surface and controlling the removal rates very uniformly across the wafer surface,” asserted Bhatnagar. “If you have this level of atomic control then you don’t need the self-limiting capability. Most of our customers are controlling process with time, so we don’t need to use self-limiting chemistry.” Applied Materials claims that this allows the Selectra tool to have higher relative productivity compared to an ALE tool.

Due to the intrinsic 2D resolutions limits of optical lithography, leading IC fabs now use multi-patterning (MP) litho flows where sacrificial thin-films must be removed to create the final desired layout. Due to litho limits and CMOS device scaling limits, 2D logic transistors are being replaced by 3D finFETs and eventually Gate-All-Around (GAA) horizontal nanowires (NW). Due to dielectric leakage at the atomic scale, 2D NAND memory is being replaced by 3D-NAND stacks. All of these advanced IC fab processes require the removal of atomic-scale materials with extreme selectivity to remaining materials, so the Selectra chamber is expected to be a future work-horse for the industry.

When the industry moves to GAA-NW transistors, alternating layers of Si and SiGe will be grown on the wafer surface, 2D patterned into fins, and then the sacrificial SiGe must be selectively etched to form 3D arrays of NW. Figure 2 shows the SiGe etched from alternating Si/SiGe stacks using a Selectra tool, with sharp Si corners after etch indicating excellent selectivity.

FIG 2: SEM cross-section showing excellent etch of SiGe within alternating Si/SiGe layers, as will be needed for Gate-All-Around (GAA) horizontal NanoWire (NW) transistor formation. (Source: Applied Materials)

“One of the fundamental differences between this system and old downstream plasma ashers, is that it was designed to provide extreme selectivity to different materials,” said Matt Cogorno, global product manager of Selective Removal Products for Applied Materials. “With this system we can provide silicon to titanium-nitride selectivity at 5000:1, or silicon to silicon-nitride selectivity at 2000:1. This is accomplished with the unique hardware architecture in the chamber combined with how we mix the chemistries. Also, there is no polymer formation in the etch process, so after etching there are no additional processing issues with the need for ashing and/or a wet-etch step to remove polymers.”

Systems can also be used to provide dry cleaning and surface-preparation due to the extreme selectivity and damage-free material removal.  “You can control the removal rates,” explained Cogorno. “You don’t have ions on the wafer, but you can modulate the number of radicals coming down.” For HVM of ICs with atomic-scale device structures, this new tool can widen process windows and reduce costs compared to both dry RIE and wet etching.

—E.K.

Applied Materials’ Olympia ALD Spins Powerful New Capabilities

Monday, July 13th, 2015

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By Ed Korczynski, Sr. Technical Editor

Applied Materials today unveiled the Applied Olympia ALD system, using thermal sequential-ALD technology for the high-volume manufacturing (HVM) of leading-edge 3D memory and logic chips. Strictly speaking this is a mini-batch tool, since four 300mm wafers are loaded onto a turn-table in the chamber that continuously rotates through four gas-isolated modular processing zones. Each zone can be configured to flow any arbitrary ALD precursor or to exposure the surface to Rapid-Thermal-Processing (RTP) illumination, so an extraordinary combination of ALD processes can be run in the tool. “What are the applications that will result from this? We don’t know yet because the world has never before had a tool which could provide these capabilities,” said David Chu, Strategic Marketing, Applied’s Dielectric Systems and Modules group.

Fig.1: The four zones within the Olympia sequential-ALD chamber can be configured to use any combination of precursors or treatments. (Source: Applied Materials)

Figure 1 shows that in addition to a high-throughput simple ALD process such that wafers would rotate through A-B-A-B precursors in sequence, or zones configured in an A-B-C-B sequence to produce a nano-laminate such as Zirconia-Alumina-Zirconia (ZAZ), almost any combination of pre- and post-treatments can be used. The gas-panel and chemical source sub-systems in the tool allow for the use up to 4 precursors. Consequently, Olympia opens the way to depositing the widest spectrum of next-generation atomic-scale conformal films including advanced patterning films, higher- and lower-k dielectrics, low-temperature films, and nano-laminates.

“The Olympia system overcomes fundamental limitations chipmakers are experiencing with conventional ALD technologies, such as reduced chemistry control of single-wafer solutions and long cycle times of furnaces,” Dr. Mukund Srinivasan, vice president and general manager of Applied’s Dielectric Systems and Modules group. “Because of this, we’re seeing strong market response, with Olympia systems installed at multiple customers to support their move to 10nm and beyond.” Future device structures will need more and more conformal ALD, as new materials will have to coat new 3D features.

When engineering even-smaller structures using ALD, thermal budgets inherently decrease to prevent atomic inter-diffusion. Compared to thermal ALD, Plasma-Enhanced ALD (PEALD) functions at reduced temperatures but tend to induce impurities in the film because of excess energy in the chamber. The ability of Olympia to do RTP for each sequentially deposited atomic-layer leads to final film properties that are inherently superior in defectivity levels to PEALD films at the same thermal budget:  alumina, silica, silicon-nitride, titania, and titanium-nitride depositions into high aspect-ratio structures have been shown.

Purging (from the tool) pump-purge

Fab engineers who have to deal with ALD technology—from process to facilities—should be very happy working with Olympia because the precursors flow through the chamber continuously instead of having to use the pump-purge sequences typical of single-wafer and mini-batch ALD tools used for IC fabrication. Pump-purge sequences in ALD tools result in the following wastes:

*   Wasted chemistry since tools generally shunt precursor-A past the chamber directly to the pump-line when precursor-B is flowing and vice-versa,

*   More wasted chemistry because the entire chamber gets coated along with the wafer,

*   Wasted cleaning chemistry during routine chamber and pump preventative-maintenance,

*   Wasted downtime to clean the chamber and pump, and

*   Wasted device yield because precursors flowing in the same space at different times can accidentally overlap and create defects.

“Today there are chemistries that are more or less compatible with tools,” reminded Chu. “When you try to use less-compatible chemistries, the purge times in single-wafer tools really begin to reduce the productivity of the process. There are chemistries out there today that would be desirable to use that are not pursued due to the limitations of pump-purge chambers.”

—E.K.

Applied Materials Q4 Report

Friday, November 14th, 2014

By Jeff Dorsch, contributing editor

Applied Materials reported in a fourth quarter earnings call  that its long-pending merger with Tokyo Electron Ltd. may not close until the first quarter of 2015.

Gary Dickerson, Applied’s president and CEO, told analysts that the combination with TEL has just received unconditional approval from Germany’s competition authority. Without disclosing details, he added that the regulatory process is potentially pushing closing of the transaction into the new year.

Applied and TEL previously said that they expected to complete the mega-merger during the second half of calendar 2014. Shareholders of both companies have approved the transaction, leaving the process in the hands of antitrust regulators in several countries.

Last July, the two companies announced that the combined company will be called Eteris B.V.

Applied also reported today the financial results of its fiscal fourth quarter and fiscal year ended October 26. In Q4, Applied posted net income of $290 million on revenue of $2.26 billion, compared with the year-ago figures of $183 million in net income and revenue of $1.99 billion.

For the full year, Applied posted net income of $1.1 billion on revenue of $9.07 billion, compared with fiscal 2013’s net income of $256 million on revenue of $7.51 billion.

Applied had orders of $2.26 billion in the fourth quarter and of $9.65 billion in the fiscal year.

For its fiscal first quarter, the company is forecasting its net sales will be flat to up 5 percent from the fiscal fourth quarter.

Blog Review: December 2, 2013

Monday, December 2nd, 2013

Phil Garrou completes his look at various packaging and 3D integration happenings from Semicon Taiwan, including news from Disco, Namics and Amkor. Choon Lee of Amkor, for example, predicted a silicon interposer cost of 2.7-4$/cm sq (100 sq mm) and expectations of organic interposer costs at 50% cost reduction.

Dynamic resource allocation can significantly improve turnaround time in post-tapeout flow. Mark Simmons of Mentor Graphics blogs about recent work that demonstrated 30% aggregate turnaround time improvement for a large set of jobs in conjunction with a greater than 90% average utilization across all hardware resources.

The MEMS Industry Group blog reflects on the trend toward sensor fusion and the role that hardware approaches such as FPGAs and microcontrollers will play in moving the technology forward.

44 years ago, the internet was born when two computers, one at UCLA and one at the Stanford Research Institute, connected over ARPANET (Advanced Research Projects Agency Network) to exchange the world’s first “host-to-host” message. Ricky Gradwohl of Applied Materials celebrates the “birthday” with thoughts on how far the internet has come.