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Silicon Technology Extensions shown at MRS Spring 2015

Monday, June 1st, 2015

By Ed Korczynski, Sr. Technical Editor, Solid State Technology/SemiMD

In the spring meeting of the Materials Research Society held recently in San Francisco, Symposium A: Emerging Silicon Science and Technology included presentations on controlling the structure of crystalline spheres and thin-films. Such structures could be used in future complementary metal-oxide semiconductor (CMOS) devices and in photonic circuits built using silicon.

Alexander Gumennik, et al., from the Massachusetts Institute of Technology, presented on “Extraordinary Stress in Silicon Spheres via Anomalous In-Fiber Expansion” as a way to control the bandgap of silicon and thus enable the use of silicon for photodetection at higher wavelengths. A silica fiber with a crystalline silicon core is fed through a flame yielding spherical silicon droplets via capillary instabilities. Upon cooling the spheres solidify and expand against the stiff silica cladding generating high stress conditions. Band gap shifts of 0.05 eV to the red (in Si) are observed, corresponding to internal stress levels. These stress levels exceed the surface stress as measured through birefringence measurements by an order of magnitude, thus hinting at a pressure-focusing mechanism. The effects of the solidification kinetics on the stress levels reached inside the spheres were explored, and the experimental results were found to be in agreement with a pressure-focusing mechanism arising from radial solidification of the spheres from the outer shell to the center. The simplicity of this approach presents compelling opportunities for the achievement of unusual phases and chemical reactions that would occur under high-pressure high-temperature conditions, which therefore opens up a pathway towards the realization of new in-fiber optoelectronic devices.

Fabio  Carta and others from Columbia University working with researchers from IBM showed results on “Excimer Laser Crystallization of Silicon Thin Films on Low-K Dielectrics for Monolithic 3D Integration.” This research supports the “Monolithic 3D” (M3D) approach to 3D CMOS integration as popularized by CEA-LETI, as opposed to the used of Through Silicon Vias (TSV). M3D requires processing temperature below 400°C if copper interconnects and low-k dielectric will be used in the bottom layer. Excimer laser crystallization (ELC) takes advantage of a short laser pulse to fully melt the amorphous silicon layer without allowing excessive time for the heat to spread throughout the structure, achieving large grain polycrystalline layer on top of temperature sensitive substrates. The team crystallized 100nm thick amorphous silicon layers on top of SiO2 and SiCOH (low-k) dielectrics. SEM micrographs show that post-ELC polycrystalline silicon is characterized by micron-long grains with an average width of 543 nm for the SiO2 sample and 570 nm for the low-k samples. A 1D simulation of the crystallization process on a back end of line structure shows that interconnect lines experience a maximum temperature lower than 70°C for the 0.5 μm dielectric, which makes ELC on low-k a viable pathway for achieving monolithic integration.

Seiji  Morisaki, et al., from Hiroshima Univ, showed results for “Micro-Thermal-Plasma-Jet Crystallization of Amorphous Silicon Strips and High-Speed Operation of CMOS Circuit.” The researchers used micro-thermal-plasma-jet (µ-TPJ) for zone melting recrystallization (ZMR) of amorphous silicon (a-Si) films to form lateral grains larger than 60 µm. By applying ZMR on a-Si strip patterns with widths <3 µm, single liquid-solid interfaces move inside the strips and formation of random grain boundaries (GBs) are significantly suppressed. Applying such strip patterns to active channels of thin-film-transistors (TFTs) results in a demonstrated field effect mobility (µFE) higher than 300 cm2/V*s because they contain minimal grain-boundaries. These a-Si strip pattern were then used to characteristic variability of n- and p-channel TFTs and CMOS ring oscillators. The strip patterns showed improved uniformities and defect densities, in general. A 9-stage ring oscillator fabricated with conventional TFTs had a maximum frequency (Fmax) of operation of 58 MHz under supply voltage (Vdd) of 5V which corresponds to a 1-stage delay (τ) of 0.94 ns, while strip channel TFTs demonstrated 108 MHz Fmax and τ decreased to 0.52 ns.

Ebrahim  Najafi, et al., from the California Institute of Technology, showed how “Ultrafast Imaging of Carrier Dynamics at the p-n Junction Interface” based on scanning ultrafast electron microscopy (SUEM) combines the spatial resolution of an electron probe with the temporal resolution of an optical pulse to enable unprecedented studies of carrier dynamics in spatially complex geometries. Observing the behavior of carriers in both space and time provides direct imaging of carrier excitation, transport, and recombination in the silicon p-n junction and the ability to follow their spatiotemporal behavior. Carrier separation on the surface of the p-n junction extends tens of microns beyond the depletion layer, as explained by a model using a ballistic-type transport. With the invention of SUEM, it should now be possible to study density profiles and electric potentials at surfaces and interfaces at the ultrafast time scale with the spatial resolution of the electron probe.

As a reminder, the Call For Paper for the MRS Fall 2015 meeting closes on June 18.


Blog review December 16, 2014

Tuesday, December 16th, 2014

Maybe, just maybe, ASML Holding N.V. (ASML) has made the near-impossible a reality by creating a cost-effective Extreme Ultra-Violet (EUV @ ~13.5nm wavelength) all-reflective lithographic tool. The company has announced that Taiwan Semiconductor Manufacturing Company Ltd. (TSMC) has ordered two NXE:3350B EUV systems for delivery in 2015 with the intention to use those systems in production. In addition, two NXE:3300B systems already delivered to TSMC will be upgraded to NXE:3350B performance. While costs and throughputs are conspicuously not-mentioned, this is still an important step for the industry.

The good and the great of the electron device world will make their usual pilgrimage to San Francisco for the 2014 IEEE International Electron Devices Meeting. Dick James of Chipworks writes that it’s the conference where companies strut their technology, and post some of the research that may make it into real product in the next few years.

The 4th Annual Global Interposer Technology Workshop at GaTech gathered 200 attendees from 11 countries to discuss the status of interposer technology. It has become the one meeting where you can find all the key interposer layers including those representing glass, laminate and silicon, blogs Phil Garrou.

Sharon C. Glotzer and Nicholas A. Kotov are both researchers at the University of Michigan who were just awarded a MRS Medal at the Materials Research Society (MRS) Fall Meeting in San Francisco for their work on “Integration of Computation and Experiment for Discovery and Design of Nanoparticle Self-Assembly.”

In order to keep pace with Moore’s Law, semiconductor market leaders have had to adopt increasingly challenging technology roadmaps, which are leading to new demands on electronic materials (EM) product quality for leading-edge chip manufacturing. Dr. Atul Athalye, Head of Technology, Linde Electronics, discusses the challenges.

ST further accelerates its FD-SOI ROs* by 2ps/stage, and reduces SRAM’s VMIN by an extra 70mV. IBM shows an apple-to-apple comparison of 10nm FinFETs on Bulk and SOI. AIST improves the energy efficiency of its FPGA by more than 10X and Nikon shows 2 wafers can be bonded with an overlay accuracy better than 250nm. Adele Hars reports.

Does your design’s interconnect have high enough wire width to withstand ESD? Frank Feng of Mentor Graphics writes in his blog that although applying DRC to check for ESD protection has been in use for a while, designers still struggle to perform this check, because a pure DRC approach can’t identify the direction of an electrical current flow, which means the check can’t directly differentiate the width or length of a wire polygon against a current flow.

At the recent IMAPS conference, Samsung electro-mechanics compared their Plated Mold Via Technology (PMV) to the well known Amkor Through Mold Via  (TMV) technology. The two process flows are compared. Phil Garrou reports.