Tag Archives: letter-mems-tech

A frequency comb source is a light source with a spectrum containing thousands of laser lines. The development of these sources has been revolutionary for fundamental science. It has allowed the construction of a link between the optical part of the electromagnetic spectrum and the radio frequency part. As such, it has allowed researchers to determine optical frequencies with an unprecedented precision. Amongst others, frequency comb light sources have been used in optical clocks enabling precise time keeping. The enormous impact of frequency comb light sources on science was highlighted in 2005, when the Nobel Prize for physics was awarded to Prof. T. Haensch and Prof J. Hall for their work on optical frequency metrology using frequency combs.

Lately, frequency combs have been used to target more real life applications. In several experiments, it has been shown that the specific properties of the sources can be used to do fast, high-resolution spectroscopy over a broad spectrum. However, traditional comb sources are not at the right wavelength spectrum for doing spectroscopy.

Ghent University, imec, the Max Planck Institute for Quantum Optics in Garching and the Auckland University in New Zealand have developed mid-infrared frequency combs, working in the mid-infrared molecular fingerprinting region of the electromagnetic spectrum. In this wavelength region, many molecules have specific absorption bands that can be used in spectroscopy to determine the presence and concentration of these molecules in samples. The researchers successfully realized the broad frequency combs, by combining the strong light-matter interaction in silicon with its broad transparency window. By fabricating so-called nanowire silicon photonics waveguides to confine the light in a very small area waveguide, they further enhanced the strong light-matter interaction allowing them to broaden the spectrum of the frequency combs into the mid-infrared. The achievements were possible through the use of a unique pump laser source, previously developed by ICFO, Spain. The results are an important step towards a small-footprint chip scale mid-infrared frequency comb source. Such sources could act as sensitive cheap gas sensors in the mid-infrared. These would be important for example for environmental monitoring for measuring air-pollution or in medical diagnostics as a cheap tool to do breath analysis. It is worth noting that the reported work has been the result of collaboration between three grants of the European Research Council (ERC), i.e. Multicomb, Miracle and InSpectra.

IRT Nanoelec, an R&D consortium focused on ICT using micro- and nanoelectronics, and CMP, which provides prototyping and low-volume production of ICs and MEMS, are launching a platform for multi-project-wafer, post-process 3D integration (3D-MPW).

The new and disruptive 3D configurations and assemblies created by this IRT Nanoelec/CMP initiative are designed to promote 3D integration.

This service, the first of its kind, extends CMP’s regular MPW offer by using mature 3D post-process technologies at wafer level from IRT Nanoelec. These technologies include through-silicon-vias (TSV, via last), fine-pitch vertical interconnects (micro pillar with solder) and specific finishing for 3D integration like under-bump metallurgy (UBM). These 3D modules will enable a wide panel of new, full 3D architectures, like multiple-die stacking with flip-chip, side-by-side heterogeneous integration, and 3D partitioning of different CMOS dies issued from CMP runs.

3D integration is highly complementary to traditional CMOS scaling, and has very strong potential in terms of size reduction, heterogeneous integration, miniaturization, performance improvements and, possibly, reduction of costs at the system level. The technology is now emerging in more and more applications, such as FPGA, 3D memories and MEMS, and involves wafer-level processing on dedicated runs.

The new platform provides for the first time access to post-process 3D technologies after regular CMOS MPW runs, for proof of concept, prototypes and/or small series production. This enables a large group of users to take advantage of cost division made possible, at silicon level, by the MPW regular services followed by post-process technologies. In addition, it allows 3D-MPW users to divide the cost of post processing. This benefits a large group of customers, such as universities, SMEs, research institutes and systems integrators, that usually do not have access to the 3D modules at large foundries.

These 3D post-process technologies require very limited re-design of existing chips, and will be initially used for specific CMOS nodes available at CMP. They may be extended in the future, depending on demand. CMP is responsible for supporting, checking and compiling the customer’s requests, while IRT Nanoelec, which has a very strong background in 3D integration – in particular through the institute CEA-Leti – will manage the 3D post-processing.

Today, at the 2015 International Solid State Circuits Conference (ISSCC), imec and Panasonic presented a transceiver chip for phase-modulated continuous-wave radar at 79GHz. This achievement demonstrates the potential of downscaled CMOS for cheap millimeter-wave (mm-wave) radar systems that can be used for accurate presence and motion detection.

Mm-wave radar technology is used in advanced driver assistance systems (ADAS) to improve safety in blurry conditions such as dust, fog and darkness, where image-based driver assistance systems lack robustness. It also offers longer range, higher precision and invisible mounting capabilities compared to ultrasound sensors. Imec’s 79GHz radar solution is based on advanced (28nm) CMOS technology, and it is an attractive alternative to the current SiGe-based technology as it offers a path to a low-power, compact and integrated solution. Moreover, at the expected high manufacturing volumes, CMOS technology is intrinsically low-cost.

Imec’s and Panasonic’s transceiver chip contains a control loop to suppress the spillover from the transmitter into the receiver without affecting the RF performance. With a power consumption of 260mW, the output power of the transmitter is 11dBm, while the RX gain is 35dB with a noise figure below 7dB and a TX-to-RX spillover suppression of 15dB. Thanks to the wide modulation bandwidth, the achievable depth resolution is 7.5cm.

“We are pleased with these excellent performance results on 28nm CMOS technology, and excited about the new opportunities they present for mm-wave radar systems, not only for automotive radar, but also for other applications such as smart homes, unmanned aerial vehicles (UAVs), robotics and others.” stated Wim Van Thillo, program director Perceptive Systems for the Internet of Things at imec. “This transceiver chip is an important milestone we have realized in our pursuit of a complete high-performance radar system fully integrated onto a single chip.”

Interested companies have access to imec’s CMOS-based 79GHz radar technology by joining imec’s industrial affiliation program or through IP licensing.

At this year’s Consumer Electronics Show in Las Vegas, the big theme was the “Internet of things” — the idea that everything in the human environment, from kitchen appliances to industrial equipment, could be equipped with sensors and processors that can exchange data, helping with maintenance and the coordination of tasks.

Realizing that vision, however, requires transmitters that are powerful enough to broadcast to devices dozens of yards away but energy-efficient enough to last for months — or even to harvest energy from heat or mechanical vibrations.

“A key challenge is designing these circuits with extremely low standby power, because most of these devices are just sitting idling, waiting for some event to trigger a communication,” explains Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering at MIT. “When it’s on, you want to be as efficient as possible, and when it’s off, you want to really cut off the off-state power, the leakage power.”

This week, at the Institute of Electrical and Electronics Engineers’ International Solid-State Circuits Conference, Chandrakasan’s group will present a new transmitter design that reduces off-state leakage 100-fold. At the same time, it provides adequate power for Bluetooth transmission, or for the even longer-range 802.15.4 wireless-communication protocol.

“The trick is that we borrow techniques that we use to reduce the leakage power in digital circuits,” Chandrakasan explains. The basic element of a digital circuit is a transistor, in which two electrical leads are connected by a semiconducting material, such as silicon. In their native states, semiconductors are not particularly good conductors. But in a transistor, the semiconductor has a second wire sitting on top of it, which runs perpendicularly to the electrical leads. Sending a positive charge through this wire — known as the gate — draws electrons toward it. The concentration of electrons creates a bridge that current can cross between the leads.

But while semiconductors are not naturally very good conductors, neither are they perfect insulators. Even when no charge is applied to the gate, some current still leaks across the transistor. It’s not much, but over time, it can make a big difference in the battery life of a device that spends most of its time sitting idle.

Going negative

Chandrakasan — along with Arun Paidimarri, an MIT graduate student in electrical engineering and computer science and first author on the paper, and Nathan Ickes, a research scientist in Chandrakasan’s lab — reduces the leakage by applying a negative charge to the gate when the transmitter is idle. That drives electrons away from the electrical leads, making the semiconductor a much better insulator.

Of course, that strategy works only if generating the negative charge consumes less energy than the circuit would otherwise lose to leakage. In tests conducted on a prototype chip fabricated through the Taiwan Semiconductor Manufacturing Company’s research program, the MIT researchers found that their circuit spent only 20 picowatts of power to save 10,000 picowatts in leakage.

To generate the negative charge efficiently, the MIT researchers use a circuit known as a charge pump, which is a small network of capacitors — electronic components that can store charge — and switches. When the charge pump is exposed to the voltage that drives the chip, charge builds up in one of the capacitors. Throwing one of the switches connects the positive end of the capacitor to the ground, causing a current to flow out the other end. This process is repeated over and over. The only real power drain comes from throwing the switch, which happens about 15 times a second.

Turned on

To make the transmitter more efficient when it’s active, the researchers adopted techniques that have long been a feature of work in Chandrakasan’s group. Ordinarily, the frequency at which a transmitter can broadcast is a function of its voltage. But the MIT researchers decomposed the problem of generating an electromagnetic signal into discrete steps, only some of which require higher voltages. For those steps, the circuit uses capacitors and inductors to increase voltage locally. That keeps the overall voltage of the circuit down, while still enabling high-frequency transmissions.

What those efficiencies mean for battery life depends on how frequently the transmitter is operational. But if it can get away with broadcasting only every hour or so, the researchers’ circuit can reduce power consumption 100-fold.

Newly developed tiny antennas, likened to spotlights on the nanoscale, offer the potential to measure food safety, identify pollutants in the air and even quickly diagnose and treat cancer, according to the Australian scientists who created them. The new antennas are cubic in shape. They do a better job than previous spherical ones at directing an ultra-narrow beam of light where it is needed, with little or no loss due to heating and scattering, they say.

In a paper published in the Journal of Applied Physics, from AIP Publishing, Debabrata Sikdar of Monash University in Victoria, Australia, and colleagues describe these and other envisioned applications for their nanocubes in “laboratories-on-a-chip.” The cubes, composed of insulating, rather than conducting or semiconducting materials as were the spherical versions, are easier to fabricate as well as more effective, he says.

Sikdar’s paper presents analysis and simulation of 200-nanometer dielectric (nonconductive) nanoncubes placed in the path of visible and near-infrared light sources. The nanocubes are arranged in a chain, and the space between them can be adjusted to fine-tune the light beam as needed for various applications. As the separation between cubes increases, the angular width of the beam narrows and directionality improves, the researchers say.

“Unidirectional nanoantennas induce directionality to any omnidirectional light emitters like microlasers, nanolasers or spasers, and even quantum dots,” Sikdar said in an interview. Spasers are similar to lasers, but employ minute oscillations of electrons rather than light. Quantum dots are tiny crystals that produce specific colors, based on their size, and are widely used in color televisions. “Analogous to nanoscale spotlights, the cubic antennas focus light with precise control over direction and beam width,” he said.

The new cubic nanoantennas have the potential to revolutionize the infant field of nano-electromechanical systems (NEMS). “These unidirectional nanoantennas are most suitable for integrated optics-based biosensors to detect proteins, DNA, antibodies, enzymes, etc., in truly portable lab-on-a-chip platforms of the future,” Sikdar said. “They can also potentially replace the lossy on-chip IC (integrated circuit) interconnects, via transmitting optical signals within and among ICs, to ensure ultrafast data processing while minimizing device heating,” he added.

Sikdar and his colleagues plan to begin constructing unidirectional cubic NEMS antennas in the near future at the Melbourne Center for Nanofabrication. “We would like to collaborate with other research groups across the world, making all these wonders possible,” he said.

Design features that contributed most to the improved performance include increased rotational speed, integrated rotor sleeves, and increased purge injection temperature.

BY MIKE BOGER, Edwards Vacuum, Tokyo, Japan

The use of high-k dielectric films deposited through atomic layer deposition, primarily in batch furnaces, has intensified, particularly in the manufacture of memory devices and high-k metal gates (HKMG) in logic devices. ALD uses a sequential purge and injection of the precursor gases to generate slow, but accurate growth of the films one atomic layer at a time. One of the precusors is typically a metal organic compound from a liquid source, commonly zirconium or hafnium-containing materials, followed by ozone to create the high-k film.

Wafers are usually processed in a furnace with batch sizes of 200 or more wafers. Reliability of the vacuum system is imperative to prevent contamination and consequent scrapping of the wafers. Unexpected failures can cause significant loss of work in process and process downtime. For example, if the vacuum pump seizes suddenly due to internal contamination by process by-products, the pressure in the pipe between the vacuum and furnaces rises, and there is a risk that powder deposited in the pipe will flow back into the furnace. This powder can not only contaminate wafers in the furnace, but also force a time-consuming clean-up that may remove the furnace from operation for a day or more.

The challenge

The mean-time-between-service (MTBS) for a vacuum pump used in semiconductor manufacturing varies greatly depending on the particular process it supports and the design of the pump. For the ALD processes considered here most failures caused process by-products can be grouped into four categories.

  • Corrosion – Attack on the metal components of the pump results in the opening of clearances leading to loss of base vacuum. Depending on the location of corrosion, the oxidation of the metal may actually generate powder that can cause seizure of rotating elements.
  • Plating – The deposition of metal compounds on the surface of internal components fouls internal mechanism clearances, causing the pump to seize.
  • Powder ingestion – Powder that enters the pump can jam rotating elements, leading to seizure.
  • Condensation – Compounds in the pumped gas stream transition from a gaseous to a solid phase within the pump, depositing on internal surfaces and eventually leading to loss of clearance and seizure.

Monitoring of pump operating conditions, such as input power, current, and running temperature, can provide an indication of the health of the pump. Events that lead to failure are generally gradual in nature. Advance notice periods can be measured in days. However, failures of vacuum pumps on high-k ALD processes often happen suddenly with little to no indication of distress prior to seizure.

A typical example of a vacuum pump used on a high-k ALD process is shown in FIGURE 1. This pump was used in a full production environment and consisted of a 1,800 m3h-1 mechanical booster mounted above a 160 m3h-1 dry pump. In this case, the pump exhibited a strong spike in running power, approximately 20 times normal, and was immediately removed for inspection. Significant deposition is evident in the booster (Fig. 1 left) and also in the last stage of the dry pump (Fig. 1 right). Evidence of the loss of clearance that caused the spike in input power is observed as a shiny area on the rotor lobe. In operation this pump was exposed to TEMAH (hafnium-containing liquid precursor), TMA (aluminum-containing liquid precursor), and ozone for producing HfO2 and TMA Al2O3. It was exchanged after 1,200 hours of use.

ALD 1-A ALD 1-B

 

FIGURE 1. A picture of a disassembled pump after 1,200 hours of use on a high-k ALD process showing the deposition in the booster (left) and loss of clearance in the last stage of the dry pump (right). 

FIGURE 2 provides another example of a pump that was removed due to detection of a spike in input current. In this case, the booster, second stage, and final stage of the pump are shown. Although the process was nominally the same (deposition of HfO2 and Al2O3), the deposition pattern is different. In this case, the booster and early stages of the dry pump show signs of a thin coating of a material that exhibits a green iridescent sheen. The final stage of the pump has a brown powder accumulation, but of a lighter color than that shown in Fig. 1.

FIGURE 2. Pictures of a disassembled pump that was removed for inspection after only 457 hours due to a large current spike detected during operation. In order, the pictures show the booster, second stage of the dry pump, and the final stage of the dry pump.

FIGURE 2. Pictures of a disassembled pump that was removed for inspection after only 457 hours due to a large current spike detected during operation. In order, the pictures show the booster, second stage of the dry pump, and the final stage of the dry pump.

In both of the examples shown in Figs. 1 and 2, the service interval of the pump was short and below the user’s expectations. In these cases, which are representative of all the pumps used on this process, the user was forced to exchange pumps frequently to minimize the risk of wafer loss. Other customers had similar experiences. TABLE 1 lists the films deposited and the preventative maintenance service intervals implemented by four customers. Analysis of serviced pumps suggested that processes depositing zirconium oxide were more challenging for the pump.

Screen Shot 2015-02-10 at 5.30.54 PM

Analysis

To better understand the reliability improvement challenge, a sample of the deposited material from a failed pump was analyzed. The results of the analysis, shown in FIGURE 3, revealed deposits rich in carbon and metal oxides, consistent with metal-organic precursors. The rate of oxide deposition appeared to be higher than that which would occur through pure ALD mechanisms, suggesting some chemical vapor deposition (CVD) or decomposition of the gases being pumped.

FIGURE 3. Analysis of the deposition within a failed pump showing hafnium, oxygen, and carbon components.

FIGURE 3. Analysis of the deposition within a failed pump showing hafnium, oxygen, and carbon components.

A survey of literature [1], [2], [3], [4] revealed that the typical reactants used in high-k ALD can react at high pressure and at low temperature without the need for external energetic activation. This suggests that even if there were no CVD or decomposition of gases within the pump, ALD-like films can still be deposited on the internal surfaces of the pump.

A simulation of the vapor pressure of TEMAH (one of the precursors used) within the pump was conducted, assuming a mass flow rate of 0.2 mg min−1 for TEMAH. The simulation results were compared to the measured vapor pressure of TEMAH to determine if there was any risk of TEMAH condensing within the vacuum pump. The results, shown in FIGURE 4, suggest that there are sufficient safety margins in the actual conditions. The TEMAH will stay in vapor form while it travels through the pump, even if the actual flow varied by an order of magnitude from that assumed. Moreover, the pump temperature could be reduced substantially without risk of condensing TEMAH within the pump.

FIGURE 4. Vapor pressure of TEMAH (0.2 mg/min with 14 slm of nitrogen) and simulated vapor pressure of TEMAH in the dry pump, inlet to outlet.

FIGURE 4. Vapor pressure of TEMAH (0.2 mg/min with 14 slm of nitrogen) and simulated vapor pressure of TEMAH in the dry pump, inlet to outlet.

A number of pumps were inspected, a large majority of which were pumps exchanged prior to seizure. Unfortunately, although powder was evident in the final stages of all pumps, not all pumps had powders of the same color. Moreover, as seen in the middle photograph of Fig. 2, some pumps and boosters were relatively clean exhibiting just a green sheen of deposition.

None of the observations, other than powder in the final stage of the dry pump, were consistently repeatable, suggesting that factors upstream of the pump were also contributing to short service intervals. Powder loading varied between pumps and within the pumps, although the heaviest deposition was always located in the final stages of the dry pump. It is normal for the most deposition to occur near the exhaust of the pump because of the generally increased temperature of the exhaust gas and the increase in vapor pressure of the materials being pumped.

A diagram of the dry pump stages from inlet to outlet is shown in FIGURE 5, where the sleeves are also shown. Consistently, the final stage shaft sleeve, which is located between the 4th and 5th stage of the pump, was the weakest link in the design. Deposition would collect on the sleeve’s surface. Resulting friction between the sleeve and the stator would cause the components to heat, expand, and finally seize the pump.

FIGURE 5. Schematic of the dry pump mechanism showing inlet (1st stage) to outlet (5th stage). Rotor sleeves are shown in green.

FIGURE 5. Schematic of the dry pump mechanism showing inlet (1st stage) to outlet (5th stage). Rotor sleeves are shown in green.

FIGURE 6 shows the sleeves from between three stages of a pump exchanged for service. Another example is shown in the right side picture of Fig. 1. The sleeves are steel with a PTFE coating, giving them a green color. Evidence of the deposition is clear in the shaft sleeves on the right side of the picture.

FIGURE 6. Picture of sleeves in an exchanged pump showing deposition on the outer surfaces.

FIGURE 6. Picture of sleeves in an exchanged pump showing deposition on the outer surfaces.

Extending pump service intervals

Inconsistencies in powder deposition that suggested variations in upstream conditions were ultimately traced to condensation in the gas lines to the process chamber. The amount of condensed liquid and the length of the flow step in the ALD cycle affected the amount of deposition. When the user took care to avoid condensation, a much more consistent pattern of deposition was observed within the pump.

For any particular dry pump, the two most convenient elements that can be adjusted are the nitrogen purge and the temperature of the pump. Adding purge, or changing the location of the purge, can affect the partial pressure of the gases being pumped. Purge can also affect the temperature of the gas being pumped. In this case the purge flow was already 76 slm and further increase could have affected the downstream gas abatement device.

Experiments to extend the MTBS focused on the pump running temperature. Temperature changes within the pump can dramatically affect the propensity of the pumped gases to condense on the internal surfaces of the pump as well as the rate of reactions of any gases being pumped. However, varying the pump temperature from 140°C to nearly 180°C made any appreciable change to the service interval.

Finally, two pumps with designs that differed significantly from the original pump were evaluated. Additionally, new pump A provided significantly greater capacity at higher inlet pressures than new pump B, at the expense of greater power consumption. The results are shown in TABLE 2.

Screen Shot 2015-02-10 at 5.32.47 PM

New Pump A was initially installed with a temperature set point of 130°C. It was removed after six months for inspection prior to failure. New Pump B was tested with a temperature set point of 110°C. It was removed after six months prior to failure. A comparison of the internal condition of the Original Pump and New Pump B is shown in FIGURE 7.

FIGURE 7. Pictures comparing the third stage of the original pump and New Pump B showing the different deposition patterns.

FIGURE 7. Pictures comparing the third stage of the original pump and New Pump B showing the different deposition patterns.

Four differences in the new pump design are believed to have contributed to improved reliability:

  • 180% increase in rotational speed (180%) resulting in less residence time of the pumped gases.
  • Reduced operating temperature. Although many semiconductor processes benefit from a hot pump, this ALD process does not.
  • No rotor sleeves. The rotor sleeve in the new pumps was integrated with the rotor element itself. This not only removed the necessity for a coating, but appeared to strengthen the mechanism.
  • Heated purge. The purge in the new pumps is warmed to within 95% of the stator temperature to prevent cooling effects and reduce the chance of spontaneous condensation of gases.

Subsequent experience with a large number of pumps and customers has confirmed the advantages provided by the new pump design. New pump B is the recommended pump for this application with fixed service intervals varying between 4 and 6 months depending on the specific characteristics of the process supported.

Conclusions

Deposition of high-k materials using ALD is a widely used technique for today’s transistor and memory structures. At early introduction of the process in high volume manufacturing, pump reliability became a key concern. Careful analysis and cooperation with customers resulted in extending the service interval of the pumps from one to up to six months, an achievement that significantly reduced operating expenses and production losses due to wafer contamination and equipment downtime caused by unexpected pump failures. Analysis of the pump condition and test results showed that, more than temperature or purge, a different pump design provided the greatest improvement in service intervals. Design features that contributed most to the improved performance include increased rotational speed, integrated rotor sleeves, and increased purge injection temperature.

References

1. J. M. et al., “Impact of Hf-precursor choice on scaling and performance of high-k gate dielectrics hf-based high-k materials,” ECSTrans., p. 59, 2007.
2. X. L. et al., “Ald of hafnium oxide thin films from tetrakis (ethylmethylamino) hafnium and ozone,” J. of ECS, vol. 152, 2005.
3. H. Furuya, “Formation of metal oxide film,” Sep 2008, patent application: US20080226820 A1.
4. Y. S. et al., “Atomic layer deposition of hafnium oxide and hafnium silicate thin films using liquid precursors and ozone,” J. Vac. Sci. Tech. A, vol. 22, 2004.

A novel approach to growing nanowires promises a new means of control over their light-emitting and electronic properties. In a recent issue of Nano Letters, scientists from the U.S. Department of Energy’s Lawrence Berkeley National Lab (Berkeley Lab) demonstrated a new growth technique that uses specially engineered catalysts. These catalysts, which are precursors to growing the nanowires, have given scientists more options than ever in turning the color of light-emitting nanowires.

The new approach could potentially be applied to a variety of materials and be used for making next-generation devices such as solar cells, light emitting diodes, high power electronics and more, says Shaul Aloni, staff scientist at Berkeley Lab’s Molecular Foundry, a DOE user facility, and lead author on the study.

Since the early 2000s, scientists have made steady progress in cultivating nanowires. Initially, early nanowire samples resembled “tangled noodles or wildfire-ravaged forests,” according to the researchers. More recently, scientists have found various conditions lead to the growth of more orderly nanowire arrays.

For instance, certain substrates on which the nanowires grow create conditions so that the nanowire growth orientation is dictated by the substrate’s underlying crystal structure. Unfortunately, this and other approaches haven’t been foolproof and some nanowires still go rogue.

Moreover, there is no simple way to grow different types of nanowires in the same environment and on the same substrate. This would be useful if you wanted to selectively grow nanowires with different electronic or optical properties in the same batch, for example.

“At the Molecular Foundry we are aiming to develop new strategies and add new tools to the bag of tricks used for nanomaterials synthesis,” says Aloni. “For years we were searching for cleverer ways to grow nanostructures with different optical properties in identical growth conditions. Engineering the catalyst brings us closer to achieving this goal.”

The researchers focused on nanowires made of gallium nitride. In its bulk (non-nanoscale) form, gallium nitride emits light in the blue or ultraviolet range. If indium atoms are added to it, the range can be extended to include red, essentially making it a broad-spectrum tunable light source in the visible range.

The problem is that adding indium atoms puts the crystal structure of gallium nitride under stress, which leads to poorly performing devices. Gallium nitride nanowires, however, don’t experience the same sort of crystal strain, so scientists hope to use them as tunable, broad-spectrum light sources.

To achieve their control, the team focused on the catalysis which guide the nanowire growth. Normally, researchers use catalysts made of a single metal. The Berkeley team decided to use metallic mixtures of gold and nickel, called alloys, as catalysts instead.

In the study, the researchers found that the gallium-nitride nanowire growth orientation strongly depended on the relative concentration of nickel and gold within the catalyst. By altering the concentrations in the alloy, the researchers could precisely manipulate, even on the same substrate in the same batch, the orientation of the nanowires.

“No one had used bi-metalic catalysts to control growth direction before,” says Tevye Kuykendall, scientist at Berkeley Lab’s Molecular Foundry. Kuykendall says the mechanism driving the new growth process is not fully understood, but it involves the different tendencies of gold and nickel to align with various crystallographic surfaces at point where nanowires start to grow.

The researchers also showed that depending on the growth direction chosen, different optical properties were observed thanks to the crystal surfaces exposed at the surface of the nanowire. “One of the things that make nanostructures interesting, is that the surface plays a larger role in defining the material’s properties,” says Aloni. This leads to changes in optical properties not seen in larger-bulk materials, making them more useful.

Aloni says the team will next focus more on the chemistry of the different nanowire surfaces to further tailor the nanowire’s optical properties.

At next week’s SPIE Photonics West 2015, imec will present a new set of snapshot hyperspectral CMOS image sensors featuring spectral filter structures in a mosaic layout, processed per-pixel on 4×4 and 5×5 ‘Bayer-like’ arrays.

Imec’s hyperspectral filter structures are processed at wafer-level on commercially available CMOS image sensor wafers, enabling extremely compact, low cost and mass-producible hyperspectral imaging solutions. This paves the way to multiple applications ranging from machine vision, medical imaging, precision agriculture to higher volume industries such as security, automotive and consumer electronic devices.

“Imec’s latest achievements in hyperspectral imaging emphasize how our promising technology has become an industrially viable solution for a number of applications,” said Andy Lambrechts, program manager at imec. “The new mosaic architecture, and extended spectral range, brings unique advantages compared to our previously announced hyperspectral linescan sensors for applications in which scanning would not be practical. It enables spectral imaging in a truly compact, tiny form-factor, that can even be scaled to handheld devices. From the technology standpoint, we have now successfully demonstrated linescan and tiled sensors, in which spectral filters cover many pixels, to mosaic sensors, in which filters vary from pixel to pixel. At the same time, the spectral range is extended and now covers down to 470nm.”

The newly developed mosaic sensors feature one spectral filter per pixel, arranged in mosaics of 4×4 (16 spectral bands) or 5×5 (25 spectral bands) deposited onto a full array of 2 Million pixels 5.5µm size CMOSIS CMV2000 sensor. Two versions of the mosaic hyperspectral image sensors have been developed:

  • one 4×4 mosaic with 16 bands in the 470-630nm (visible range)
  • one 5×5 mosaic with 25 bands in the 600-1000nm range (Visible – NIR range)

“Imec’s hyperspectral imaging sensors (100bands linescan, 32bands tiled and 16/25bands mosaic designs) are off-the-shelf, commercially available engineering sample sensors that we developed to address the fragmented machine vision market and to trigger interest for this unique technology from potential end-users in other industries,” explained Jerome Baron, business development manager at imec. “We also offer customized spectral filtering solutions for companies that are already familiar with the technology and interested in developing proprietary solutions with a specific performance in terms of speed, compactness, spatial versus spectral resolution, bands selection, or cost.”

Located at booth 4635 at SPIE Photonics West, imec will demonstrate the 3 different versions of these hyperspectral image sensors. First engineering samples have been manufactured and now available for evaluation to early partners.

Researchers at aBeam Technologies, Lawrence Berkeley National Laboratory and Argonne National Laboratory have developed a technology to fabricate test patterns with a minimum linewidth down to 1.5nm. The fabricated nanostructures are used to test metrological equipment. The designed patterns involve thousands of lines with precisely designed linewidths; these lines are combined in such a way that the distribution of linewidths appears to be random at any location. This pseudo- random test pattern allows nanometrological systems to be characterized over their entire dynamic range.

lawrence berk micro2 lawrence berk micro1

TEM images of the test pattern with linewidths down to 1.5nm. The width of the lines was designed to form a pseudo-random test pattern; the pattern is used to characterize metrological instrumentation. The scale bar on the top image is 50nm. 

The test pattern contains alternating lines of silicon and silicon-tungsten, this results in a pretty good contrast in the metrological systems. The size of the sample is fairly large, apprx. 6×6 microns, and involves thousands of lines, each according to its designed width. Earlier, aBeam and LBNL reported the capability of fabricating 4nm lines and spaces using e-beam lithography, atomic layer deposition, and nanoimprint.

Dr. Sergey Babin, president of aBeam Technologies said, “The semiconductor industry is moving toward a half-pitch of 11nm and 7nm. Therefore, metrology equipment should be very accurate, at least one order of magnitude more accurate than that. The characterization of metrology systems requires test patterns at a scale one order smaller than the measured features. The fabrication was a challenge, especially for such a complex pattern as a pseudo-random design, but we succeeded.”

Dr. Valeriy Yashchuk, a researcher at the Advanced Light Source of LBNL continued: “When you measure anything, you have to be sure that your metrological system produces accurate results, otherwise what kind of results will you get, nobody knows. Qualifying and tuning metrology systems at the nanoscale is not easy. We designed the test pattern that is capable of characterizing nano-metrology systems over their entire dynamic range, resulting in the modulation transfer function, the most comprehensive characteristic of any system.”

The test pattern is to be used to characterize almost any nano-metrology system. Experiments were performed using a scanning electron microscope (SEM), atomic force microscope (AFM), and soft x-ray microscopes. A part of an ideal test-sample and its SEM microscopy image is shown below. The image includes imperfection in the microscope and needs to be characterized.

The power spectral density of the sample is flat; the spectra of the image has a significant cut-off at high frequencies; this is used to characterize the microscope over its dynamic range and show the degradation of the microscope’s sensitivity as soon as the linewidth becomes smaller.

This project was led by Dr. Sergey Babin, president of aBeam Technologies, Dr. Valeriy Yashchuk of Lawrence Berkeley National Laboratory and Dr. Ray Conley of Argonne National Laboratory. This work was supported by the Department of Energy under the contract #DE-SC0011352 in the framework of a STTR project.

Imec, Medtronic, Ghent University and their project partners today announced the launch of the CARDIS project. Together they will develop and validate an early-stage cardio vascular disease detection platform using integrated silicon photonics. The project is supported by the recently launched European Union’s Horizon 2020 Framework Programme for Industrial leadership in Information and Communication Technologies (H2020). The project’s overarching goal is the investigation and demonstration of a mobile, low-cost device based on a silicon photonics integrated laser Doppler vibrometer. The concept will be validated for the screening of arterial stiffness, detection of stenosis and heart failure in a clinical setting.

Early identification of individuals at risk for cardio vascular disease (CVD) allows early intervention for halting or reversing the pathological process. This drives the CARDIS project team to develop a mobile, low-cost, non-invasive, point-of-care screening device for CVD. Assessment of arterial stiffness by measurement of the aortic pulse wave velocity (aPWV) is included in the latest ESC/ESH guidelines for CVD risk prediction. Besides aPWV, early identification of arterial stenosis and cardiac contraction abnormalities can be used to improve CVD risk classification. To date, there are no tools available to screen a large population set at primary care level on these parameters, and individuals that are considered to be at low or moderate risk too often go undiagnosed.

The CARDIS research activities include:

  • The investigation, design and fabrication op the optical subsystems and components.
  • The integration of the subsystems and building of a multi-array laser interferometer system.
  • The development of a process flow scalable to high volumes for all subsystems and their integration steps.
  • The investigation and development of the biomechanical model for translating optical signals related to skin-level vibrations into underlying CVD physiological events.
  • The validation of the system in a clinical setting.

Over the next three and a half years, CARDIS will be managed by imec, through imec’s associated laboratory located at Ghent University (Photonics Research Group in the Department of Information Technology). Medtronic Bakken Research Center (Netherlands) will be responsible for the scientific and technical coordination of the project. Other industrial, academic and clinical partners will bring their expertise to the project: SIOS Messtechnik (Germany), University College Cork Tyndall (Ireland), INSERM (France), Queen Mary University of London (United Kingdom), Universiteit Maastricht (Netherlands), Ghent University and Fundico (Belgium).

Interested to learn more about the potential of silicon photonics? Imec is exhibiting at next week’s SPIE Photonics West in San Francisco (booth 4635) and organizing a workshop and demonstration session on Silicon Photoncis together with MOSIS (February 10-11).