Category Archives: Process Materials

By Pete Singer

Increasingly complicated 3D structures such finFETs and 3D NAND require very high aspect ratio etches. This, in turn, calls for higher gas flow rates to improve selectivity and profile control. Higher gas flow rates also mean higher etch rates, which help throughput, and  higher rates of removal for etch byproducts.

“Gas flow rates are now approaching the limit of the turbopump,” said Dawn Stephenson, Business Development Manager – Chamber Solutions at Edwards Vacuum. “No longer is it only the process pressure that’s defining the size of the turbopump, it’s now also about how much gas you can put through the turbopump.”

Turbopumps operate by spinning rotors at very high rates of speed (Figure 1). These rotors propel gases and process byproducts down and out of the pump. The rotors are magnetically levitated (maglev) to reduce friction and increase rotor speed.

Figure 1. Spinning rotors propel gases and process byproducts out of the pump.

The challenge starts with processes that have high gas flow rates, over a thousand sccm, and lower chamber pressures, below 100 mTorr.  Such processes include chamber clean steps where high flows of oxygen-containing gases are used to remove and flush the process byproducts from inside the chamber, through Silicon via (TSV) in which SF6is widely used at high gas flowrates for deep silicon reactive ion etch (RIE) and more recently, gaseous chemical oxide removal (COR) which typically uses HF and NH3to remove oxide hard masks.

However, the challenge is intensified with the more general trend to higher aspect ratio etch across all technologies.

Stephenson said the maximum amount of gas you can put through a maglev turbo is determined by two things: the motor power and the rotor temperature. Both of these are affected adversely by the molecular weight of the gas. “The heavier the molecule, the lower the limit. For motor power, if the gas flow rate is increased, the load on the rotor is increased, and then you need more power. Eventually you reach a gas flow at which you exceed the amount of power you have to keep the rotor spinning and it will slow down,” she said.

The rotor temperature is an even bigger limiting factor. “As gas flow rates increase, the number of molecules hitting the rotor are increased. The amount of energy transferred into the rotors is also increased which elevates the temperature of the rotor. Because the rotor is suspended in a vacuum and because it’s levitated, it’s not very easy to remove that heat from the rotor because its primary thermal transfer is through radiation,” she explained.

Pumping heavier gases, particularly ones that have poor thermal conductivity, cause the rotor temperature to rise, leading to what is known as “rotor creep.”Rotor creep is material growth due to high temperature and centrifugal force (stress).  Rotor creep deformation over time narrows clearances between rotor and stator and can eventually lead to contact and catastrophic failure (Figure 2).

Figure 2. Edwards pumps have the highest benchmark for rotor creep life temperature in the industry, due to the use of a premium aluminum alloy as the base material for its mag-lev rotors, combined with a low stress design.

Where it gets even worse are in applications where the turbopump is externally heated to reduce byproduct deposition inside the pump. Such a heated pump will have a higher baseline rotor temperature and significantly lower allowable gas flowrates than an unheated one. This becomes a challenge particularly for the heated turbopumps on semiconductor etch and flat panel display processes using typical reactant gases such as HBr and SF6.  “Those are very heavy gases with low thermal conductivity and the maximum limit of the turbopump is actually quite low,” Stephenson said.

The good news is that Edwards has been diligently working to overcome these challenges. “What we have done to maximize the amount of gas you can put into our turbopumps is to  ensure our rotors can withstand the highest possible temperature design limit for a 10 year creep lifetime.   We use a premium alloy for the base rotor material and then beyond that we have done a lot of work with our proprietary modeling techniques to design a very low stress rotor because the creep is due to two factors: the temperature and the centrifugal stress. Because of those two things combined, we’re able to achieve the highest benchmark for rotor creep life temperature in the industry,” she said.

Furthermore, the company has worked on thermal optimization of the turbopump platform. “That means putting in thermal isolation where needed to try to help keep the rotor and motor cool. At the same time, we also need to keep the gas path hot to stop byproducts from depositing. We have also released a high emissivity rotor coating that helps keep the rotor cool,” Stephenson said. A corrosion resistant, black ceramic rotor coating is used to maximize heat radiation, which helps keep the rotor cool and gives more headroom on gas flowrate before the creep life temperature is reached.

Edwards has also developed a unique real-time rotor temperature sensor: Direct, dynamic rotor temperature reporting eliminates over-conservative estimated max gas flow limits and allows pump operation at real maximum gas flow in real duty cycle while maintaining safety and lifetime reliability.

In summary, enabling higher flows at lower process pressures is becoming a critical capability for advanced Etch applications, and Edwards have addressed this need with several innovations, including optimized rotor design to minimize creep, high emissivity coating, and real time temperature monitoring.

To eliminate voids, it is important to control the process to minimize moisture absorption and optimize a curing profile for die attach materials.

BY RONGWEI ZHANG and VIKAS GUPTA, Semiconductor Packaging, Texas Instruments Inc., Dallas, TX

Polymeric die attach material, either in paste or in film form, is the most common type of adhesive used to attach chips to metallic or organic substrates in plastic-encapsulated IC packages. It offers many advantages over solders such as lower processing temperatures, lower stress, ease of application, excellent adhesion and a wide variety of products to meet a specific application. As microelectronics move towards thinner, smaller form factors, increased functionality, and higher power density, void formation in die attach joints (FIGURE 1), i.e. in die attach materials and/or at die attach interfaces, is one of the key issues that pose challenges for thermal management, electrical insulation and package reliability.

Impact of voids

Voids in die attach joints have a significant impact on die attach material cracking and interfacial delamination. Voids increase moisture absorption. If plastic packages with a larger amount of absorbed moisture are subject to a reflow process, the absorbed moisture (or condensed water in the voids) will vaporize, resulting in a higher vapor pressure. Moreover, stress concentrations occur near the voids and frequently are responsible for crack initiation. On the other hand, voids at the interface can degrade adhesive strength. The combined effect of higher vapor pressure, stress concentration around the voids and decreased adhesion, as a result of void formation, will make the package more susceptible to delamination and cracking [1].

Additionally, heat is dissipated mainly through die attach layer to the exposed pad in plastic packages with an exposed pad. Voids in die attach joints can result in a higher thermal resistance and thus increase junction temperatures significantly, thereby impacting the power device performance and reliability.

And finally, voiding is known to adversely affect electrical performance. Voiding can increase the volume resistivity of electrically conductive die attach materials, while decreasing electrical isolation capability. Therefore, it is crucial to minimize or eliminate voids in die attach joints to prevent mechanical, thermal and electrical failures.

Void detection

The ability to detect voids is key to ensuring the quality and reliability of die attach joints. There are four common techniques to detect voids: (1) Scanning Acoustic Microcopy (SAM), (2) X-ray imaging, (3) cross-section or parallel polishing with optical or electron microscope, and (4) glass die/slide with optical microscope (Fig. 1). The significant advantage of SAM over other techniques lies in its ability to detect voids in different layers within a package non-destructively. Void size detection is limited by the minimal defect size detected by SAM. If the void is too small, it may not be detected at all, depending on the package and equipment used. X-ray analysis allows for non-destructive detection of voids in silver-filled die attach materials. However its limits lie in its low resolution and magnification, a low sensitivity for the detection of voids in a thick sample, and its inability to differentiate voids at different interfaces [2]. Cross-section or parallel polishing with electronic microscope provides a very high magnification image to detect small voids, although it is destructive and time-consuming. Glass die or glass substrate with an optical microscope provides a simple, quick and easy way to visualize the voids.

Potential root causes of voids and solutions

There are four major sources of voids: (1) air trapped during a thawing process, (2) moisture induced voids, (3) voids formed during die attach film (DAF) lamination, and (4) volatile induced voids.

Freeze-thaw voids When an uncured die attach paste in a plastic syringe is removed from a freezer (typically -40oC) to an ambient environment for thawing, the syringe warms and expands faster than the adhesive. This intro- duces a gap between syringe and the adhesive. Upon thawing, the adhesive will re-wet the syringe wall and air located in between the container and adhesive may become trapped. As a result, voids form. This is referred as freeze-thaw void [3]. The voids in pastes may cause incom- plete dispensing pattern leading to inconsistent bond line thickness (BLT) and die tilt, thus causing delamination. Planetary centrifugal mixer is the most commonly used and effective equipment to remove this type of void.

Moisture induced voids

Die attach material contains polar functional groups, such as hydroxyl group in epoxy resins and amide group in curing agents, which will absorb moisture from the environment during exposure in die attach process. As the industry moves to larger lead frame strips (100mm x 300mm), the total number of units on a lead frame strip increase significantly. As a result, die attach pastes may have been exposed to a production environment significantly longer before die placement. After die placement, there could also be a significant amount of waiting time (up to 24 hours) before curing. Both can result in a high moisture absorption in die attach pastes. Moreover, organic substrates can absorb moisture, while moisture may be present on metal lead frame surfaces. As temperatures increase during curing, absorbed moisture or condensed water will evolve as stream to cause voiding. Voids can also form at the DAF-substrate interface as a result of moisture uptake during the staging time between film attach and encapsulation process. Controlling moisture absorption of substrates and die attach materials at each stage before curing and production environment are critical to prevent moisture induced voids in die attach joints.

Void formation during DAF lamination

One challenge associated with DAF is voiding during DAF lamination, especially when it is applied to organic substrates [FIGURE 1(d)]. There is a correlation of void pattern with the substrate surface topography [4]. Generally, increasing temperature, pressure and press time can reduce DAF melt viscosity and enable DAF to better wet lead frame or substrates, thereby preventing entrapment of voids at die attach process. If the DAF curing percentage is high before molding, then DAF has limited flow ability, and thus cannot completely fill the large gaps on the substrate. Consequently, voids present at the interface between DAF and an organic substrate since die bonding process. But if DAF has a lower curing percentage before molding, then DAF can re-soft and flow into large gaps under heat and transfer pressure to achieve voids-free bond line post molding [4].

Volatile induced voids

Voids in die attach joints are generally formed during thermal curing since die attach pastes contain volatiles such as low molecular weight additives, diluents, and in some cases solvents for adjusting the viscosity for dispensing or printing. To study the effect of outgassing amounts on voids, we select three commercially available die attach materials with a significant difference in outgassing amounts using the same curing profile. As shown in FIGURE 2, as temperature increases, all die attach pastes outgas. DA1 shows a weight loss of 0.74wt%, DA2 3.1wt% and DA3 10.62wt%. Once volatiles start to outgas during thermal curing, they will begin to accumulate within the die attach material or at die attach interfaces. Voids begin to form by the entrapment of outgassing species or moisture. After voids initially form, voids can continue to grow until the volatiles have been consumed or the paste has been cured enough to form a highly cross- linked network. FIGURE 3 shows optical images of dices assembled onto glass slides using three die attach materials. As expected, DA1 shows no voids for both die sizes of 2.9mm x 2.9mm and of 9.0mm x 9.2mm, due to a very low amount of outgassing (0.74wt%). DA2 shows no voids for the small die size, but many small voids under the die periphery for the large die. Large voids are observed for DA3 for both die sizes since it has a very large amount of outgassing (10.62wt%). DA2 also shows voids even with a medium die size 6.4mm x 6.4mm [FIGURE 3(g)]. Differential Scanning Calorimetry (DSC) was used to further study the curing behaviors of DA2 and DA3, as shown in FIGURES 4 and 5. Comparing FIGURE 4 with FIGURE 5, it is interesting to observe the difference in thermal behavior of the two die attach materials. For DA2, as curing starts, the weight loss rate becomes slower, while the weight loss rate for DA3 accelerates as curing starts. It is very likely that the outgassing species in DA2 is reactive diluent, which has a lower weight loss rate when the reaction starts. But for DA3, outgassing is a non-reactive solvent, and possibly with other reactive species. The non-reactive solvent has a boiling point at 172.9oC, as verified in the DSC. Heat generated in the curing process accelerates evaporation of the solvent. The continuous, slow release outgassing amount during ramp and curing at 180oC explains the formation of small voids in DA2, while fast evaporation of solvent accounts for large voids in DA3. To reduce or eliminate voids during thermal curing, a simple and the most common approach is to use a two-step (or multi- step)cure.Thefirststepisdesignedtoremovevolatiles, followed by a second step of curing. With the first step at 120oC for 1h to remove more volatiles, DA2 shows significantly less voids for a die size of 6.4mm x 6.4mm [FIGURE 3(h)].

Ideally, the majority (if not all) of volatiles should be removed prior to the gelation point, which is defined as the intersection of G’ and G’’ in a rheological test. Because the viscosity of die attach, materials increases dramatically after their gelation point. A higher amount of volatiles released after gelation point (or later stage of curing) are more likely to form voids. Therefore, the combined characterization of TGA and DSC, as well as rheological test, provides a good guideline to design optimal curing profiles to minimize or eliminate voids.

Summary

This article provides an understanding of void impact in die attach joints, the techniques to detect voids, voiding mechanisms, and their corresponding solutions. To eliminate voids, it is important to control the process to minimize moisture absorption and optimize a curing profile for die attach materials. TGA, DSC and Rheometer are key analytical tools to optimize a curing profile to prevent voiding. In addition, many other properties such as modulus, coefficient of thermal expansion (CTE), and adhesion need to be considered when optimizing curing profiles. Last but not least, it is crucial to develop die attach materials with less outgassing and moisture absorption without compro- mising manufacturability, reliability and performance.

References

1. R.W.Zhang,etal., “Solving delamination in lead frame-based packages,” Chip Scale Review, 2015, pp. 44-48.
2. L. Angrisani, et al., “Detection and location of defects in electronic devices by means of scanning ultrasonic microcopy and the wavelet transform,” Measurement, 2002, Vol. 31, pp. 77-91.
3. D. Wyatt, et al., “Method for reducing freeze-thaw voids in uncured adhesives,” 2006 US 11/402,170.
4. Y. Q. Su, et al., “Effect of transfer pressure on die attach film void perfor- mance,” 2009 IEEE 11th Electronic Packaging Technology Conference, pp. 754-757.

RONGWEI ZHANG is a Packaging Engineer, and VIKAS GUPTA is an Engineering Manager, Semiconductor Packaging, Texas Instruments Inc., Dallas, TX.

If your laptop or cell phone starts to feel warm after playing hours of video games or running too many apps at one time, those devices are actually doing their job.

Whisking heat away from the circuitry in a computer’s innards to the outside environment is critical: Overheated computer chips can make programs run slower or freeze, shut the device down altogether or cause permanent damage.

As consumers demand smaller, faster and more powerful electronic devices that draw more current and generate more heat, the issue of heat management is reaching a bottleneck. With current technology, there’s a limit to the amount of heat that can be dissipated from the inside out.

Researchers at the University of Texas at Dallas and their collaborators at the University of Illinois at Urbana-Champaign and the University of Houston have created a potential solution, described in a study published online July 5 in the journal Science.

Researchers at the University of Texas at Dallas and their collaborators have created and characterized tiny crystals of boron arsenide, like the one shown here imaged with an electron microscope, that have high thermal conductivity. Because the semiconducting material efficiently transports heat, it might be used in future electronics to help keep smaller, more powerful devices from overheating. The research is described in a study published online July 5, 2018 in the journal Science. Credit: University of Texas at Dallas

Bing Lv (pronounced “love”), assistant professor of physics in the School of Natural Sciences and Mathematics at UT Dallas, and his colleagues produced crystals of a semiconducting material called boron arsenide that have an extremely high thermal conductivity, a property that describes a material’s ability to transport heat.

“Heat management is very important for industries that rely on computer chips and transistors,” said Lv, a corresponding author of the study. “For high-powered, small electronics, we cannot use metal to dissipate heat because metal can cause a short circuit. We cannot apply cooling fans because those take up space. What we need is an inexpensive semiconductor that also disperses a lot of heat.”

Most of today’s computer chips are made of the element silicon, a crystalline semiconducting material that does an adequate job of dissipating heat. But silicon, in combination with other cooling technology incorporated into devices, can handle only so much.

Diamond has the highest known thermal conductivity, around 2,200 watts per meter-kelvin, compared to about 150 watts per meter-kelvin for silicon. Although diamond has been incorporated occasionally in demanding heat-dissipation applications, the cost of natural diamonds and structural defects in manmade diamond films make the material impractical for widespread use in electronics, Lv said.

In 2013, researchers at Boston College and the Naval Research Laboratory published research that predicted boron arsenide could potentially perform as well as diamond as a heat spreader. In 2015, Lv and his colleagues at the University of Houston successfully produced such boron arsenide crystals, but the material had a fairly low thermal conductivity, around 200 watts per meter-kelvin.

Since then, Lv’s work at UT Dallas has focused on optimizing the crystal-growing process to boost the material’s performance.

“We have been working on this research for the last three years, and now have gotten the thermal conductivity up to about 1,000 watts per meter-kelvin, which is second only to diamond in bulk materials,” Lv said.

Lv worked with postdoctoral research associate Sheng Li, co-lead author of the study, and physics doctoral student Xiaoyuan Liu, also a study author, to create the high thermal conductivity crystals at UT Dallas using a technique called chemical vapor transport. The raw materials — the elements boron and arsenic — are placed in a chamber that is hot on one end and cold on the other. Inside the chamber, another chemical transports the boron and arsenic from the hot end to the cooler end, where the elements combine to form crystals.

“To jump from our previous results of 200 watts per meter-kelvin up to 1,000 watts per meter-kelvin, we needed to adjust many parameters, including the raw materials we started with, the temperature and pressure of the chamber, even the type of tubing we used and how we cleaned the equipment,” Lv said.

David Cahill and Pinshane Huang’s research groups at the University of Illinois at Urbana-Champaign played a key role in the current work, studying defects in the boron arsenide crystals by state-of-the-art electron microscopy and measuring the thermal conductivity of the very small crystals produced at UT Dallas.

“We measure the thermal conductivity using a method developed at Illinois over the past dozen years called ‘time-domain thermoreflectance’ or TDTR,” said Cahill, professor and head of the Department of Materials Science and Engineering and a corresponding author of the study. “TDTR enables us to measure the thermal conductivity of almost any material over a wide range of conditions and was essential for the success of this work.”

The way heat is dissipated in boron arsenide and other crystals is linked to the vibrations of the material. As the crystal vibrates, the motion creates packets of energy called phonons, which can be thought of as quasiparticles carrying heat. Lv said the unique features of boron arsenide crystals — including the mass difference between the boron and arsenic atoms — contribute to the ability of the phonons to travel more efficiently away from the crystals.

“I think boron arsenide has great potential for the future of electronics,” Lv said. “Its semiconducting properties are very comparable to silicon, which is why it would be ideal to incorporate boron arsenide into semiconducting devices.”

Lv said that while the element arsenic by itself can be toxic to humans, once it is incorporated into a compound like boron arsenide, the material becomes very stable and nontoxic.

The next step in the work will include trying other processes to improve the growth and properties of this material for large scale applications, Lv said.

A Tokyo Institute of Technology research team has shown copper nitride acts as an n-type semiconductor, with p-type conduction provided by fluorine doping, utilizing a unique nitriding technique applicable for mass production and a computational search for appropriate doping elements, as well as atomically resolved microscopy and electronic structure analysis using synchrotron radiation. These n-type and p-type copper nitride semiconductors could potentially replace the conventional toxic or rare materials in photovoltaic cells.

Thin film photovoltaics have equivalent efficiency and can cut the cost of materials compared to market-dominating silicon solar panels. Utilizing the photovoltaic effect, thin layers of specific p-type and n-type materials are sandwiched together to produce electricity from sunlight. The technology promises a brighter future for solar energy, allowing low-cost and scalable manufacturing routes compared to crystalline silicon technology, even though toxic and rare materials are used in commercialized thin film solar cells. A Tokyo Institute of Technology team has challenged to find a new candidate material for producing cleaner, cheaper thin film photovoltaics.

(a) This is a copper and Copper Nitride. (b) Theoretical Calculation for P-type and N-type Copper Nitride. (c) Direct Observation of Fluorine Position in Fluorine-doped Copper Nitride. (a) An image of thin film copper plates before and after reacting with ammonia and oxygen. Copper metal has been transformed to copper nitride. (b) Copper insertion for an n-type semiconductor and fluorine insertion for a p-type semiconductor. (c) Nitrogen plotted in red, fluorine in green, and copper in blue. Fluorine is located at the open space of the crystal as predicted by the theoretical calculation. Credit: Advanced Materials

They have focused on a simple binary compound, copper nitride that is composed of environmentally friendly elements. However, growing a nitride crystal in a high quality form is challenging as history tells us to develop gallium nitride blue LEDs. Matsuzaki and his coworkers have overcome the difficulty by introducing a novel catalytic reaction route using ammonia and oxidant gas. This compound, pictured through the photograph in figure (a), is an n-type conductor that has excess electrons. On the other hand, by inserting fluorine element in the open space of the crystal, they found this n-type compound transformed into p-type as predicted by theoretical calculations and directly proven by atomically resolved microscopy in figures (b) and (c), respectively.

All existing thin film photovoltaics require a p-type or n-type partner in their makeup of a sandwich structure, requiring huge efforts to find the best combination. P-type and n-type conduction in the same material developed by Matsuzaki and his coworkers are beneficial to design a highly efficient solar cell structure without such efforts. This material is non-toxic, abundant, and therefore potentially cheap–ideal replacements for in use cadmium telluride and copper indium gallium diselenide thin film solar cells. With the development of these p-type and n-type semiconductors, in a scalable forming technique using simple safe and abundant elements, the positive qualities will further bring thin film technology into the light.

By Paula Doe, SEMI

New metrology and inspection technologies and new analysis approaches made possible by improving compute technology offer solutions to finding the increasingly subtle variations in materials and subsystems that meet specifications but still cause defects on the wafer. More collaboration across the supply chain is helping too.  SEMICON West programs on materials and subsystems will address these issues.

New metrology approaches needed to deal with process margin challenges

As device process margins shrink and subtler materials variations cause unwanted variations,  the need for better monitoring of both surface and sub-surface material variations is driving a trend towards “metro-spection” – the convergence of metrology and inspection. “Device process margins have eroded to the point that traditional metrology strategies and techniques are no longer viable for controlling yield and parametric performance,” says Nanometrics Vice President Robert Fiordalice, who will speak in the materials program at SEMICON West. “Limited sampling capability, low throughput, insufficient sensitivity or the destructive nature of the techniques can often become problems. What’s more, deviations in material characteristics are not always determined by the initial quality of the material, but often arise from variations during the integration of the materials.”

“Device process margins have eroded to the point that traditional metrology strategies and techniques are no longer viable for controlling yield and parametric performance.” – Robert Fiordalice, Nanometrics

One new type of inline tool or line monitoring technology is Fourier Transform Infrared (FTIR) spectroscopy, traditionally used in quality control or tool characterization. Better sensitivity and higher throughput now enable rapid analysis and feedback for on-the-fly detection of subtle deviations in film properties that may compromise device performance or yield.

More advanced analytics will help extract new information from old metrology

More expensive metrology may not be required to identify subtle variations in in-spec materials that cause wafer defects. Today’s advanced compute capabilities now enable more sophisticated analysis of existing data and the identification of small but significant variations in raw materials and finished goods.

The figure of merit (FoM) values presented in certificate of analysis (CoA) reports miss subtle variations in raw material properties. Of particular note is the reduction of molecular weight distributions to a mean, and standard deviation, whereas variations in the tails are associated with pattern defects. Advanced compute capabilities now allow the industry to step beyond the FoM in favor of more holistic measures, enabling predictive analysis of resist chemical variations associated with specific pattern defects. Source: JSR Micro

“We often don’t need to find a new measure, but just a new way of looking at what we measure now,” says Jim Mulready, vice president of global quality assurance at JSR Micro. Mulready will speak in the SEMICON West program on materials defectivity issues. “The certificate of analysis reduces multiple measurements to a single figure of merit. But if we ignore all that raw data, we miss a chance to learn.  One of our sayings in quality is ‘Customers don’t feel the average, they feel the variation.’ In many electronic materials, the quality of the raw material can have a big impact on the final performance, but the types of analysis needed to look at the tails of the distribution of these measures (such as molecular weight) in detail used to be really hard to do. Now it’s becoming increasingly straightforward and affordable.”

 “We often don’t need to find a new measure, but just a new way looking at what we measure now.” – Jim Mulready, JSR Micro

Mulready says tools now available in the data processing sector enable the identification of subtle variations in materials that can cause defects on the wafer. These tools use methods like detailed subtractions of chromatography curves of polymer raw materials or analysis of tails of distributions of molecular weights. “Our job now is to drive these kinds of more sophisticated data analysis back into our chemical supply chain as well,” says Mulready. “We must work more closely with our suppliers to integrate their raw materials into our products. The reason the JSRs of the world exist is as a safety valve to reduce the variation from the chemical industry before it gets to the fab.”

Continued collaboration with equipment suppliers required as well

While the industry has been talking about the need for tighter collaboration between materials suppliers and equipment manufacturers for years, it still doesn’t always happen. “The material supplier and the equipment maker are tied together like kids in a three-legged race when we deliver an integrated system for consistent on-wafer performance,” says Cristina Chu, TEL/NEXX director of strategic business development, another speaker in the materials program.  “When we introduce changes to the tool hardware, we need to make sure it doesn’t upset the system. Similarly, we need the material supplier to send a bottle over when a new chemistry formulation is under development. If a new chemistry runs into problems in the field, it will take much more time for both of us to fix it at the customer site. The toolmaker can provide a slightly different perspective on applications, while being more objective than a customer on how the formulation performs compared to earlier versions.”

“The material supplier and the equipment maker are tied together like kids in a three-legged race when we deliver an integrated system for consistent on-wafer performance.” – Cristina Chu, TEL/NEXX

Regular and ongoing collaboration between chemistry suppliers and toolmakers enables the highest quality system solution to reach the customer. Chu notes that her team tries to maintain consistent collaborations with material suppliers across changes in organizations as the business environment changes. “For consistent on-wafer capabilities, we need a consistent collaboration process with chemistry suppliers. We need to meet with materials providers at a regular cadence throughout their development process. We need to check back with them as we scale up results from the coupon to the wafer level and to work out the kinks in the integrated solution together. The quality and consistency of our combined performance at the customer depends on ensuring the quality and consistency of our development and evaluation process as well.”

Fabs and subsystems suppliers look to pilot data sharing program to improve process margins

With ever tighter process margins, subtle variations in parameters that don’t appear in the specifications are also compromising results on the wafer, and neither the fab nor the supplier alone has the full information needed to improve performance. To help, a SEMI standards group is developing a protocol for a pilot program to standardize and automate some data sharing.

“In order for engineers to have constructive conversations about how to improve performance, we all need to exchange more information.” – Eric Bruce, Samsung Austin

The fab knows that performance is best with a particular parameter value, and knows when performance fluctuates,  but often faces a black box problem with no way of knowing what exactly is wrong. In the rush to get the tool back up, the fab engineers may not get around to emailing the supplier about the issue for some time. The subsystems supplier, on the other hand, may know the cause of the variation,  but likely has no way of knowing the critical parameters or ideal target valuesfor the fab’s process..  “In order for engineers to have constructive conversations about how to improve performance, we all need to exchange more information,” says Eric Bruce, Samsung Austin diffusion engineer, and co-chair of the SEMI standards effort working on the issue, who will speak in the subsystems program at SEMICON West.

A potential solution could be to create a standard and automated process to share particular data, agreed to in the purchasing contract, whereby the subsystems supplier shares more information about their parameters with the fab, and the fab in return gives feedback on what parameters work best to drive improved performance. The best place to start will likely be on parts that do not contain core yield-related IP, but where usage and lifetime information is useful.

“We’re looking for people to participate in a pilot program to work together with suppliers to try sharing some information to improve performance,” says Bruce. “There’s a lot of this sharing in the backroom anyway, but this could make it fast and automated, and make everyone’s engineering job a lot easier.”

As silicon-based semiconductors reach their performance limits, gallium nitride (GaN) is becoming the next go-to material to advance light-emitting diode (LED) technologies, high-frequency transistors and photovoltaic devices. Holding GaN back, however, is its high numbers of defects.

This material degradation is due to dislocations — when atoms become displaced in the crystal lattice structure. When multiple dislocations simultaneously move from shear force, bonds along the lattice planes stretch and eventually break. As the atoms rearrange themselves to reform their bonds, some planes stay intact while others become permanently deformed, with only half planes in place. If the shear force is great enough, the dislocation will end up along the edge of the material.

As silicon-based semiconductors reach performance limits, gallium nitride is becoming the next go-to material for several technologies. Holding GaN back, however, is its high numbers of defects. Better understanding how GaN defects form at the atomic level could improve the performance of the devices made using this material. Researchers have taken a significant step by examining and determining six core configurations of the GaN lattice. They present their findings in the Journal of Applied Physics. This image shoes the distribution of stresses per atom (a) and (b) of a-edge dislocations along the <1-100> direction in wurtzite GaN. Credit: Physics Department, Aristotle University of Thessaloniki

Layering GaN on substrates of different materials makes the problem that much worse because the lattice structures typically don’t align. This is why expanding our understanding of how GaN defects form at the atomic level could improve the performance of the devices made using this material.

A team of researchers has taken a significant step toward this goal by examining and determining six core configurations of the GaN lattice. They presented their findings in the Journal of Applied Physics, from AIP Publishing.

“The goal is to identify, process and characterize these dislocations to fully understand the impact of defects in GaN so we can find specific ways to optimize this material,” said Joseph Kioseoglou, a researcher at the Aristotle University of Thessaloniki and an author of the paper.

There are also problems that are intrinsic to the properties of GaN that result in unwanted effects like color shifts in the emission of GaN-based LEDs. According to Kioseoglou, this could potentially could be addressed by exploiting different growth orientations.

The researchers used computational analysis via molecular dynamics and density functional theory simulations to determine the structural and electronic properties of a-type basal edge dislocations along the <1-100> direction in GaN. Dislocations along this direction are common in semipolar growth orientations.

The study was based on three models with different core configurations. The first consisted of three nitrogen (N) atoms and one gallium (Ga) atom for the Ga polarity; the second had four N atoms and two Ga atoms; the third contained two N atoms and two Ga core-associated atoms. Molecular dynamic calculations were performed using approximately 15,000 atoms for each configuration.

The researchers found that the N polarity configurations exhibited significantly more states in the bandgap compared to the Ga polarity ones, with the N polar configurations presenting smaller bandgap values.

“There is a connection between the smaller bandgap values and the great number of states inside them,” said Kioseoglou. “These findings potentially demonstrate the role of nitrogen as a major contributor to dislocation-related effects in GaN-based devices.”

Gases and engineering company The Linde Group, a supplier of electronic materials, is investing in the expansion of existing products to improve business continuity planning (BCP), while adding new products with improved purity to meet the growing needs of sub-10nm semiconductor factories and advanced flat panel manufacturers.

Expanded capacity of fluorine/nitrogen mixtures
Linde has expanded capacity for fluorine/nitrogen mixtures at Medford, Oregon for etching and chamber cleaning applications.

  • This allows both low- and high-pressure fluorine and nitrogen mixture production.
  • On-site high-purity fluorine production minimizes third-party supply issues.
  • The product line is expanding to include fluorine/argon mixtures in place with tri-mix       capability(fluorine/argon/nitrogen) later in 2018.
  • This facility complements fluorine mixture production at the Linde Alpha, New Jersey facility.

New precursors to meet customer requirements
New elements of innovation continue to emerge in CVD, ALD, and ALE precursors such as high-volume supply capabilities, process solutions to deliver quality in our advanced precursors and an applications lab to support new materials development. Linde is developing deposition precursors and etch gases: silicon precursors, digermanium mixtures, high K and metal gate precursors, isotope gases and etch gases such as CF3I (trifluoroiodomethane) and custom fluorinated silane.

“Linde recognizes that our customers continue to make investments in new processes and technologies, and we are committed to investing with them for the materials they will require now and in the future,” states Matt Adams, Head of Sales and Marketing for Linde Electronics and Specialty Products.

Linde Electronics will be exhibiting at the SEMICON West tradeshow in San Francisco July 10-12. Its focus will be on the quality, expertise, commitment and environmental leadership that Linde Electronics brings to the semiconductor industry through such offerings as electronic specialty gases, on-site solutions, materials recycling and recovery and SPECTRA® nitrogen plants.

SEMICON West is the annual tradeshow for the micro-electronics manufacturing industry. All visitors are welcome to visit Linde in booth number 5644 in the North hall in the Moscone Center in San Francisco.

Researchers at Tokyo Institute of Technology have developed flexible terahertz imagers based on chemically “tunable” carbon nanotube materials. The findings expand the scope of terahertz applications to include wrap-around, wearable technologies as well as large-area photonic devices.

Carbon nanotubes (CNTs) are beginning to take the electronics world by storm, and now their use in terahertz (THz) technologies has taken a big step forward.

The CNT THz imager enabled clear, non-destructive visualization of a metal paper clip inside an envelope. Credit: ACS Applied Nano Materials

Due to their excellent conductivity and unique physical properties, CNTs are an attractive option for next-generation electronic devices. One of the most promising developments is their application in THz devices. Increasingly, THz imagers are emerging as a safe and viable alternative to conventional imaging systems across a wide range of applications, from airport security, food inspection and art authentication to medical and environmental sensing technologies.

The demand for THz detectors that can deliver real-time imaging for a broad range of industrial applications has spurred research into low-cost, flexible THz imaging systems. Yukio Kawano of the Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of Technology (Tokyo Tech), is a world-renowned expert in this field. In 2016, for example, he announced the development of wearable terahertz technologies based on multiarrayed carbon nanotubes.

Kawano and his team have since been investigating THz detection performance for various types of CNT materials, in recognition of the fact that there is plenty of room for improvement to meet the needs of industrial-scale applications.

Now, they report the development of flexible THz imagers for CNT films that can be fine-tuned to maximize THz detector performance.

Publishing their findings in ACS Applied Nano Materials, the new THz imagers are based on chemically adjustable semiconducting CNT films.

By making use of a technology known as ionic liquid gating[1], the researchers demonstrated that they could obtain a high degree of control over key factors related to THz detector performance for a CNT film with a thickness of 30 micrometers. This level of thickness was important to ensure that the imagers would maintain their free-standing shape and flexibility, as shown in Figure 1.

“Additionally,” the team says, “we developed gate-free Fermi-level[2] tuning based on variable-concentration dopant solutions and fabricated a Fermi-level-tuned p?n junction[3] CNT THz imager.” In experiments using this new type of imager, the researchers achieved successful visualization of a metal paper clip inside a standard envelope (see Figure 2.)

The bendability of the new THz imager and the possibility of even further fine-tuning will expand the range of CNT-based devices that could be developed in the near future.

Moreover, low-cost fabrication methods such as inkjet coating could make large-area THz imaging devices more readily available.

SEMI today announced the formation of the SEMI Electronic Materials Group (EMG), a new collaborative technology community that combines the former Chemical & Gas Manufacturers Group (CGMG), the Silicon Manufacturers Group (SMG) and other SEMI member segments to better serve the interests of the electronics materials industry. The group is open to SEMI Members involved in materials manufacture, distribution and services throughout the microelectronics industry.

“Materials companies are the linchpin of innovation – enabling advances in technology across the microelectronics value chain – from sand to smartphones,” said Bart Pitcock, vice president and general manager, North America for KMG Electronic Chemicals and chair of the EMG Americas Chapter. “We are pleased to build out this SEMI platform to drive program collaboration, information exchange, issues management and communication to materials industry stakeholders including customers and policymakers.”

Electronic materials have played an increasingly important role in technology innovation as electronics move from IT-centric to ubiquitous computing across consumer, industrial and data management markets. The market size for wafer fabrication materials (US$ 28 billion), semiconductor packaging materials (US$ 19 billion), and electronics assembly materials (US$ 20 billion) reflects the critical importance of materials to the growth and expansion of the worldwide electronic manufacturing ecosystem.

To help manage growing interdependencies across the microelectronics supply chain, the EMG now represents all materials makers, aligning with the SEMI mission to serve members across the microelectronics design and manufacturing industries.

As the first SEMI technology community, the Silicon Manufacturers’ Group was instrumental in the evolution of SEMI and the industry, defining standards for silicon wafers, the substrate on which semiconductors are built.

“Members of the former Silicon Manufacturers’ Group are pleased to join forces with other companies that provide the critical materials that enable the worldwide electronics manufacturing industries,” said Neil Weaver, director, Product Development and Applications Engineering of Shin-Etsu Handotai America. “We see great value and mutual purpose in working with others in the electronics materials community to advance our common interests.”

The EMG will continue its mission to facilitate collective efforts on issues related to the microelectronics materials industry that are more effectively addressed by an industry association than by individual companies.

“We are pleased with the unanimous affirmation of the new community by SEMI regions and member segments worldwide,” said Tom Salmon, vice president of Collaborative Technology Platforms at SEMI.

An international collaborative research group including Tokyo Institute of Technology, Universite PARIS DIDEROT and CNRS has discovered that CO2 is selectively reduced to CO[1] when a photocatalyst[2] composed of an organic semiconductor material and an iron complex is exposed to visible light. They have made clear that it is possible to convert CO2, the major factor of global warming, into a valuable carbon resource using visible light as the energy source, even with a photocatalyst composed of only commonly occurring elements.

This is CO2 reduction using a photocatalyst combining carbon nitride and an iron compl. Credit: Osamu Ishitani

In recent years, technologies to reduce CO2into a resource using metal complexes and semiconductors as photocatalysts are being developed worldwide. If this technology called artificial photosynthesis can be applied, scientists would be able to convert CO2, which is considered the major factor of global warming and is being treated as a villain, into a valuable carbon resource using sunlight as the energy source.

Complexes and inorganic semiconductors containing precious and rare metals such as ruthenium, rhenium, and tantalum have been used in highly active photocatalysts reported so far. However, considering the tremendous amount of CO2, there was a need to create new photocatalysts made only with elements widely available on Earth.

Professor Osamu Ishitani, Associate Professor Kazuhiko Maeda, research staff Ryo Kuriki and others of Tokyo Tech, with the support of JST (Japan Science and Technology Agency)’s Strategic Basic Research Programs (CREST Establishment of Molecular Technology towards the Creation of New Functions) for international collaborative research projects, performed collaborative research with the research group of Professor Marc Robert of Universite PARIS DIDEROT and CNRS. As a result, by fusing carbon nitride, an organic semiconductor, with a complex made of iron and organic materials and using it as a photocatalyst, they succeeded in turning CO2 into a resource at high efficiency under the condition of exposure to visible light at ordinary temperature and pressure.

By combining the organic semiconductor carbon nitride[3], made of carbon and nitrogen, with an iron complex and using it as a photocatalyst, they found that they could reduce carbon dioxide (CO2) to carbon monoxide (CO) at high efficiency. This photocatalytic reaction progresses when exposed to visible light, which is the major component in the wavelength band of sunlight. The carbon nitride absorbs visible light and drives the migration of electrons from the reducing agent to the iron complex, the catalyst. The iron complex uses that electrons to reduce CO2 to CO. The turnover number[4], the external quantum efficiency[5], and the selectivity[6] of CO2 reduction–performance indicators for the formation of CO–reached 155, 4.2%, and 99%, respectively. These values are almost the same as when precious metal or rare metal complexes are used, and about ten times more than photocatalysts reported so far using base metals or organic molecules.

This research was the first to demonstrate that CO2 can be reduced into a resource efficiently using sunlight as the energy source, even by using materials which exist abundantly on Earth, such as carbon, nitrogen, and iron. Tasks remaining are to further improve their function as a photocatalyst and to succeed in fusing them with oxidation photocatalysts which can use water, which exists abundantly on Earth and is inexpensive, as a reducing agent.