Ed’s Threads 20080331
Musings by Ed Korczynski on March 31, 2008

MRS meeting covers nanostuff and microthings
Over 4000 researchers were in San Francisco last week for the annual Materials Research Society (MRS) spring meeting, to discuss advances in materials for electronics, energy, health, and transportation. Over 40 technical session run in parallel, with >10 sessions of interest to the semiconductor manufacturing industry at any given time. Theory and results for new IC memory cells, extensions of CMOS logic, and future quantum-dots and nano-rods were shown. Graphene still seems like a possible replacement for silicon in ICs.

In his Kavli plenary lecture in nanoscience, Prof. A. Paul Alivisatos of UC-Berkeley described recent work by his group and others on transformations in nanocrystals. Chemical transformations can be used to obtain complex nanocrystalline structures though sequential chemical operations. In an example, CdSe reacted with Ag+ to form Ag2Se which could then be combined with Cd2+ to completely reverse back to CdSe, while the volume of the nanoparticle was completely preserved. Such cation exchanges can occur in semiconductor nanorods and hollow spheres with shape preservation, but when shapes do transform their final forms are currently difficult to predict.

Much of the new materials work is targeted toward finding nanoscale structures which can switch between two measurable states to function as memory cells. Two of the newer random-access memory (RAM) cell types under development are phase-change RAM (PRAM) and resistive RAM (ReRAM). With Numonyx now officially launched to commercialize PRAM along with Flash, there were many papers looking at manufacturing process flows to optimize the deposition and programming of the antimony-telluride (SbTe) family of “calcogenide” materials which undergo thermally-assisted transitions between crystalline and amorphous phases. Independent of the MRS meeting, materials supplier ATMI recently announced co-development plans with Ovonyx for calcogenide CVD precursors.

ReRAM using metal-oxides as switching elements comes in two fundamentally different variations: one-time programmable through the growth of nano-metallic-filaments, and reversible through ionic transport between electrodes. ReRAM materials may be used in PRAM-like cells, or also used as the switching element in cross-bar architecture arrays. HP Labs, US NIST, and Hokkaido University all showed advances in hybrid circuits built using cross-bar arrays.

For extensions of CMOS logic, with a somewhat clear path forward in new materials for high-k and metal gates, a lot of research now centers on doping technologies. G. Lansbergen et al. (B3.7) from TU Delft (The Netherlands) along with Purdue (USA), University of Melbourne (Australia), IMEC (Belgium), and Caltech (USA) showed the ability to work with a single Arsenic dopant atom in a p-MOS finFET; their experiments represent the first evidence of the ability to engineer the quantum state of a single-donor electron by surface gate control. While single-ion doping is way beyond today’s fab specs, more precise control is needed for the placement of often <100 atoms for channels and contacts.

Wilfried Vandervorst of IMEC showed that Laser Spike Anneal (LSA) which is essentially “diffusion-less” calls for re-integration from prior rapid-thermal annealing (RTA) schemes where lateral diffusion is significant. Due to the very low thermal budgets needed to form ultra-shallow junctions (USJ), LSA is more subject to pocket dopant fluctuations than spike anneals. Random dopant fluctuations must be controlled, along with structural variations on gate cross-sections which appear as undercuts and footing. LSA helps equivalent oxide thickness (EOT) scaling for gate dielectrics by elimination of a 2-3Å thick re-growth layer. However, to ensure reliability in gate stacks, an RTA step can be added after LSA to improve the situation somewhat. Looking forward to embedded SiGe, LSA so far induces junction leakage and defects gliding along certain crystalline planes which unfortunately relaxes desired strain. LSA for embedded SiC, however, avoids SiC relaxation which improves the strain retention in nMOS. Gate profile control is critical for diffusion-less USJ, which may mean gate-last integation schemes will be easier to integrate.

Karuppanan Sheker, of SemEquip, presented on how to use cluster-carbon implants to improve the Si:C layer formation. There is ~2% limit to how much C can be substituted in silicon lattice. At the VLSI Technology Symposium 2007, IBM showed [C]sub of 1.65% with mono-atomic C implants and pre-amorphizing implants (PAI). Using clustered carbon eliminates the need for the PAI and provides [C]sub >2%. The source is two benzene rings in the form of C14H14, which upon striking a silicon crystal in the 6-10keV implant energy range automatically induces amorphization with depth of 20nm-40nm. The greater the amorphous layer thickness the higher the percentage C which can be substitutionally incorporated.

Newer finFET architectures, which may first be used for SRAM arrays, require unique integration flows. Mark van Dal, NXP-TSMC Research Center, showed that when implants into fins amorphized the silicon material, the re-crystallization in complex fin shapes results in scattering and other sources of variability. The exact reason for the device degradation is not known, but using either BF2 or B+Ge implants (both of which induce amorphization) result in more transistor variability. At fin widths of 1µm there is no difference, but for fins <0.1µm wide the effect is clearly seen. When non-amorphizing B implants are used, no device performance degradation is observed.

— E.K.

Labels: CMOS, finFET, graphene, materials research, nano, PRAM, ReRAM

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080331: MRS meeting covers nanostuff and microthings

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Ed’s Threads 070413
Musings by Ed Korczynski on April 13, 2007

MRS meeting specs the future
The Materials Research Society (MRS) spring meeting was in San Francisco April 9-13, and the near- and far-term possibilities for process technology in our industry were presented to a record numbers of attendees. Researchers showed results from the world’s leading labs for electronic materials development: CMOS high-k gate dielectrics, nano-imprint lithography, organic semiconductors, quantum dots, and nano-tubes. It’s like sipping from a firehose, unless you’re interested in just one of the 36 parallel sessions.

Sachin Joshi of UT-Austin showed that hybrid-orientation technology (HOT) silicon wafers based on the MEMC direct silicon bonding (DSB) approach contain inherent defect-rich junctions between orientations. Shallow-trench isolation (STI) regions 60-140nm wide may be used to eliminate these defects, though this seriously limits circuit density, he pointed out. Non-silicon channels will probably also limit density, so their use will probably be limited to RF and mixed-signal applications in small portions of chips.

Arief Budiman from Stanford analyzed the grain orientation in submicron damascene copper lines using the synchrotron x-ray beam from the ALS Berkeley Lab. This very bright x-ray source and submicron spot-size (0.8 x 0.8µm) allows for resolution of crystal bending/stress as well as the dislocation density. Starting with large single grains spanning across the width of the line (“bamboo structure”), Budiman’s group observed clear directionality of EM-induced plasticity and thus the orientation of slip-planes. They found that <112> orientations were most susceptible to plastic deformation, so any grains with such orientations that line up with the induced EM-stress will deform. Grain orientation controls plasticity, which in turn influences EM degradation mechanisms and circuit reliability.

An analysis of the influence of microstructure on void formation in failed copper interconnects, from Intel's Sadasivan Shankar, revealed that voids first nucleate at triple-boundaries caused by stress-induced de-cohesion at copper interfaces. These voids can be easily pinned by a grain boundary, which provides a fast diffusion path for the void to grow across the width of a line or via. “It almost unzips the grain-boundary,” he commented. A 2D model developed with Brown and UT-Austin accounts for current flow and stress, diffusion along surfaces and interfaces, void migration, and the interaction of voids and grain boundaries.

Duane Boning, the MIT professor who created one of the first useful pattern-density step-height CMP models in the 1990s, showed progress on new physically based models. By explicitly including pad properties—elastic response (including lateral coupling across the pad), slurry transport, and average asperities—he showed how chip-scale uniformity can now be predicted.

Roland Rzehak of Qimonda in Dresden, Germany, provided both an overview and details of inexplicable CMP removal-rate variations using ceria-slurries. A counter-intuitive “slow start phenomena” slows the removal rate for the first minute of pattern planarization to be ~2.5X lower than that for blanket films. Ceria particles may initially adsorb in trenches to take some of the pressure load. However, Qimonda observes additional non-uniformities implying influences of pattern pitch, the pad material, and possibly effects from chemical additives to the slurry.

MRS meetings also cover wilder technologies like superconductors, neuro-prosthetic interfaces, and “the nature of design using nature’s portfolio” like the self-assembly of sea-shells or the nano-hairs of gecko feet. Materials scientists and engineers continue to explore the structure-property relationships of the physical world, and confirm that there is indeed “still plenty of room at the bottom.”

— E.K.

Labels: electronics, materials research, modeling, nano, self-assembly, silicon

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070413: MRS meeting specs the future

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