Technology News
11/01/2005
Turning blue is good news for solid-state lighting
Scientists have been trying to get an organic compound that emits a stable blue light with a lifetime that exceeds the previously thought 1000 hr lifetime barrier ever since Shuji Nakamura’s invention of the first inorganic blue LED in the mid-90s. The challenge with stability for blue emission, however, is that it requires a very high energy, and many failure mechanisms exist that make obtaining long-term reliability a daunting challenge, says Janice Mahon, VP of technology commercialization at Universal Display Corp. (UDC).
UDC’s recent announcement of a longer operating-lifetime phosphorescent-organic LED (PHOLED), >15,000 hr at 200candela/m2, was achieved with a deeper blue (at 474nm peak emission wavelength) as defined by CIE coordinates (0.16, 0.37). (CIE is a color mapping system first developed by the Commission Internationale de ľÉclairage.) The external quantum efficiency was reported to be 9.5% with a 22cd/amp luminous efficiency.
The efficiency achieved by UDC and its research partners at Princeton U. and the U. of Southern California was significant in that it was previously thought that using fluorescent OLEDs was the only way to commercialize the technology. “Our work on PHOLED technology has dispelled this myth by using novel materials and device structures to convert 100% of the excited states into triplets that radiate as usable light,” explains Mahon.
“One class of phosphorescent compounds are organometallic compounds that are typically doped into a host organic molecule,” Mahon says. “In PHOLEDs, all singlet excitons may be converted into the triplet state through intersystem crossing via the presence of a heavy metal atom. The triplet states can emit radiatively, enabling extremely high conversion efficiencies.” (An exciton comes about as the result of an electron binding with its hole, i.e., an electron/hole pair in a semiconductor. See figure.)
While a typical PHOLED may have 3-5 layers, one of the UDC device architectures uses five layers. “Two layers are for hole injection and transport; one layer is the emissive layer; and two layers are for blocking and electron transport,” says Mahon. Efficiency lifetime of the product is maximized due to the greater control of the charge balance afforded by the increased number of layers.
The company anticipates that the blue PHOLED breakthrough will bring it closer to an all-phosphorescent system - in combination with its red and green PHOLED technology - that can provide greater power efficiency for portable and large-area displays and for solid-state lighting applications. - D.V.
Researchers find new mechanism for particle growth in nanocomposites
A research team from the Georgia Institute of Technology and Drexel U. has discovered a new mechanism by which polymer materials used in nanocomposites control the growth of particles. Reported at the recent national meeting of the American Chemical Society, the findings could provide a new tool for controlling the formation of nanoparticles.
Growing particles within the confinement of polymer-based structures is one technique commonly used for controlling nanoparticle growth. After the particles form, the polymer matrix can be removed, or the resulting nanocomposite used for a variety of applications.
In a series of experiments, the research team found a strong relationship between the chemical reactivity of the polymer and the size and shape of resulting nanoparticles.
“We found that in the melt the key parameter influencing particle size is actually the type of interaction with the polymer. The molecular weight of the polymer and the synthesis temperature are almost insignificant,” says Rina Tannenbaum, an associate professor in Georgia Tech’s School of Materials Science and Engineering.
Tannenbaum and her colleagues created iron-oxide nanoparticles within polymer films of different types, including polystyrene, poly(methyl methacrylate), bisphenol polycarbonate, poly(vinylidene di-fluoride) ,and polysulfone. The polymeric matrix was then decomposed using heat, leaving the particles to be characterized using transmission electron microscopy.
“These polymers spanned a variety of functional groups that differed in the strength and nature of their interactions with the iron oxide particles and in their position along with polymer chain,” Tannenbaum says. “We found that the characteristic nanoparticle size decreased with the increasing affinity - the strength of the interaction - between the polymer and the iron-oxide particles.”
 The size and shape of iron oxide nanoclusters are controlled by interfacial interactions with polymers having various functional groups. (Image courtesy of Rina Tannenbaum) |
Specifically, iron-oxide particles formed in strongly interacting polymer media tended to be small (10-20nm in dia.) and pyramid-shaped, while those formed in weakly-interacting media tended to be larger (40-60nm in dia.) and spherical (see figure).
Based on the experimental results, Tannenbaum and Assoc. Prof. Nily Dan of Drexel’s Department of Chemical Engineering charted the relationship between average particle size and the reactivity of the polymer interface. That information should help other scientists as they attempt to regulate the growth of nanoparticles using polymer reactivity. The researchers stated that the control mechanism should be broadly applicable to other particles and polymeric materials.
Liquid-cooling technique uses microfluidic channels on chip backs
A new technique for fabricating liquid cooling channels onto the backs of high-performance ICs could allow denser packaging of chips while providing better temperature control and improved reliability. Developed at the Georgia Institute of Technology, the wafer-level fabrication technique includes polymer pipes that will allow electronic and cooling interconnections to be made simultaneously using automated manufacturing processes.
The low-temperature technique, compatible with conventional microelectronics manufacturing processing, allows fabrication of the microfluidic cooling channels without damage to ICs. The on-chip microfluidic technique was described at the eighth annual IEEE International Interconnect Conference.
 Close-up of microchannels on the backside of a silicon wafer. (Photo courtesy of Georgia Tech Research News) |
“This scheme offers a simple and compact solution to transfer cooling liquid directly into a gigascale integrated (GSI) chip, and is fully compatible with conventional flip-chip packaging,” says Bing Dang, a graduate research assistant in Georgia Tech’s School of Electrical and Computer Engineering.
Calculations show the system, which can have either straight-line or serpentine microchannel configurations, should be able to cool 100W/cm2. Heat removal capacity depends on the flow rate of the coolant and its pressure, with smaller- diameter microchannels more efficient at heat transfer.
Dang expects the technology to be used first in high-performance specialty processors that can justify the cost of the cooling system. So far, the researchers have demonstrated continuous liquid flow on a chip for several hours without failure, but additional testing is still needed to confirm long-term reliability, he adds.
UV curing adds mechanical strength
Ultraviolet (UV) curing has emerged as a way to add mechanical strength to low-k dielectrics, according to some researchers at the recent IITC (International Interconnect Technology Conference). UV not only is proving more effective than e-beam curing, which sometimes causes damage because of high energy electrons, but also appears to have potential to solve other problems as well.
Strengthening may be due to loose methyl groups crosslinking to silicon-based molecules under UV radiation, according to Kevin Durr, curing product line manager for Axcelis, with a 50-100% improvement for porous and 25-50% for dense dielectrics.
Takeshi Furasama of Renesas explains that standard SiOC was converted to high-modulus material through UV curing to convert terminating methyl groups into network components, boosting Young’s Modulus from 10-21 GPa for 90nm dielectrics with a k value of 3.
The UV curing techniques came from efforts by Axcelis to extend the applications for UV technology initially used to photostabilize Novalac-based resists after the move to chemically amplified resists. At first, according to Durr, low-k developers were looking only for mechanical strengthening, but now they see UV curing as potentially offering many other improvements, such as lowered film stress, more uniform films, better plasma resistance, and poragen removal without lower k values.
Durr explains that some developers are finding that by tailoring the chemical bonds to provide UV-active sites within the dielectric material, superior results and faster curing can be accomplished. - B.H.
Wafer tool inspects at different wavelengths
A new wafer inspection tool from KLA-Tencor comes with “superset capability” that allows it to inspect at UV, DUV, and visible wavelengths as needed. “Different defects and materials require different wavelengths, and the best one isn’t predictable,” says Keith Wells, VP of programs engineering at KLA-Tencor’s Wafer Inspection Division.
The 2800 Series brightfield inspection tool employs a mercury-xenon lamp for illumination and images the wafer on a proprietary CCD detector using a patented broadband catadioptric objective lens with 0.9NA. The unique lens was needed to avoid aberrations that would have limited the resolution and field size for broadband imaging. The sensor chip is designed for backside illumination through a thinned substrate, thereby avoiding the strong DUV absorption of the top layers.
That same DUV absorption on the wafer is one problem overcome by the flexible illumination strategy. Using a lamp with a filter rather than a laser, the detector has to wait longer to catch enough photons to form an image. The image sensor is synched to the wafer stage motion, allowing the same point on the wafer to continue to add light to the corresponding pixel of the image while that pixel moves across the sensor. This time-domain integration (TDI) strategy improves both the resolution and signal level of the image. - M.D.L.
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