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Materials Science Engineering make a more Energy Efficient Fuel Cell

 

 

Hydrogen Fuel Cell

 

 

While renewable energy sources help to fight the effects of global warming, they do have their drawbacks.  Renewable energy cannot be produced as predictably as plants powered by oil, coal, or natural gas.  Ideally, alternative energy plants would be paired with a huge energy storage system that would store and dispense power.  Stanford School of Engineering is working to use reversible fuel cells to combat this storage issue.  Fuel cells use oxygen and hydrogen to create electricity; if the process were reversed, the fuel cell could be used to also store electricity.

"You can use the electricity from wind or solar to split water into hydrogen and oxygen in a fuel cell operating in reverse," said William Chueh, an assistant professor of materials science and engineering at Stanford and a member of the Stanford Institute of Materials and Energy Sciences at SLAC National Accelerator Laboratory. "The hydrogen can be stored, and used later in the fuel cell to generate electricity at night or when the wind isn't blowing."

Fuel cells are not a perfect solution.  The chemical reactions that cleave water into hydrogen and oxygen or join them together are not completely understood – at least not to the degree necessary to make utility-grade storage systems.  Chueh is working alongside researchers from SLAC, Lawrence Berkeley National Laboratory and Sandia National Laboratories to study the chemical reactions in fuel cells in a new way.  In an article published in Nature Communications, Chueh and his team describe how they observed the hydrogen-oxygen reaction in a specific type of high-efficiency solid-oxide fuel cell.  They also took atomic-scale photos of the process using a particle accelerator called a synchrotron.  This type of analysis is first-of-its-kind and help lead to more efficient fuel cells that could eventually allow for utility-scale alternative energy systems.

Electrons Role

In a traditional fuel cell, a gas-tight membrane separates the anode and cathode. Oxygen molecules are introduced at the cathode where a catalyst fractures them into negatively charged oxygen ions.  These ions then make their way to the anode where they react with hydrogen molecules to form the cell's primary "waste" product: pure water.  To perform these reactions, electrons also need to make the journey.  Normally, the electrons are drawn to the cathode and the ions are drawn toward the anode, but while the ions pass directly through the membrane, the electrons can't penetrate it; they are forced to circumvent it via a circuit that can be harnessed to run anything from cars to power plants.

Because electrons do the designated "work" of fuel cells, they are thought of as the critical functioning component. But ion flow is just as important, said Chueh.

"Electrons and ions constitute a two-way traffic pattern in many electrochemical processes," Chueh said.  "Fuel cells require the simultaneous transfer of both electrons and ions at the catalysts, and both the electron and ion 'arrows' are essential."

Electron transfer in electrochemical processes such as corrosion and electroplating is relatively well understood, Chueh said, but ion flow has remained unclear.  This is due to the environment where ion transfer may best be studied -- catalysts in the interior of fuel cells -- is not conducive to inquiry.

Solid-oxide fuel cells operate at relatively high temperatures.  Certain materials are known to make superior fuel cell catalysts.  Cerium oxide, or ceria, is particularly efficient.  Cerium oxide fuel cells can hum along at 600 degrees Celsius, while fuel cells incorporating other catalysts must run at 800 C or more for optimal efficiency.  Those 200 degrees represent a huge difference, Chueh said.  "High temperatures are required for fast chemical reactivity," he said.  "But, generally speaking, the higher the temperature, the quicker fuel cell components will degrade.  So it's a major achievement if you can bring operating temperatures down."

How Does It Work

While cerium oxide established itself strong catalysts for fuel cells, it is unclear why it works so efficiently.  What were needed were visualizations of ions flowing through catalytic materials.  But putting an electron microscope into the pulsing, red-hot heart of a fuel cell running at full bore isn’t exactly possible.  "People have trying to observe these reactions for years," Chueh said.  "Figuring out an effective approach was very difficult." 

In their Nature Communications paper, Chueh and his colleagues at Berkeley, Sandia and SLAC split water into hydrogen and oxygen (and vice versa) in a cerium oxide fuel cell.  While the fuel cell was running, they applied high-brilliance X-rays produced by Berkeley Lab's Advanced Light Source to illuminate the routes the oxygen ions took in the catalyst.  Access to the ALS tool and the cooperation of the staff enabled the researchers to create "snapshots" revealing just why ceria is such aFuel Cell superior catalytic material: it is, paradoxically, defective.  "In this context, a 'defective' material is one that has a great many defects -- or, more specifically, missing oxygen atoms -- on an atomic scale," Chueh said. "For a fuel cell catalyst, that's highly desirable."

Such oxygen "vacancies," he said, allow for higher reactivity and quicker ion transport, which in turn translate into an accelerated fuel cell reaction rate and higher power. 

"It turns out that a poor catalytic material is one where the atoms are very densely packed, like billiard balls racked for a game of eight ball," Chueh said. "That tight structure inhibits ion flow. But ions are able to exploit the abundant vacancies in ceria. We can now probe these vacancies; we can determine just how and to what degree they contribute to ion transfer. That has huge implications. When we can track what goes on in catalytic materials at the nanoscale, we can make them better -- and, ultimately, make better fuel cells and even batteries."

 

 

http://www.sciencedaily.com/releases/2014/07/140709095931.htm

 

 

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

New Class of Electronic Devices Could Come From 2-D Transistors

 

innovationEarlier this spring two separate research projects were building transistors made solely from two-dimensional (2-D) materials.  Argonne National Laboratory researchers described a transparent thin-film transistor (TFT) that they had created in the Nano Letters journal.  They used tungsten diselenide (WSe2) as the semiconducting layer, graphene for the electrodes and hexagonal boron nitride as the insulator.  A week later the ACS Nano journal published that researchers from the Lawrence Berkeley National Laboratory had also built an all 2-D transistor that took the shape of a field emissions transistor (FET).  The Berkeley Lab FET used the same materials for their electrode and insulator layers as Argonne’s TFT, but used molybdenum disulfide (MoS2) as the semiconducting layer.

While the fabrication of transparent TFTs made from 2-D materials could lead to flexible displays with super-high density pixels, the impact of an all 2-D FET could potentially have a broader impact.  FETs are nearly omnipresent, being used in computers, mobile devices, and many other electronic devices.

Issues with FETs prior to Berkeley Lab’s work has been that their charge-carrier mobility degrades because of mismatches between the crystal structure and the atomic lattices of the individual components, namely the gate, source and drain electrodes.  These mismatches result in rough surfaces and in some cases dangling chemical bonds.  The completely 2-D FET developed at Berkeley Lab eliminates this issue by creating an electronic device in which the interfaces are based on van der Waals interactions.  These interactions represent all the attractive or repulsive forces between molecules that are not covalent bonds, instead of covalent bonding.  "In constructing our 2D FETs so that each component is made from layered materials with van der Waals interfaces, we provide a unique device structure in which the thickness of each component is well-defined without any surface roughness, not even at the atomic level," said Ali Javey, a faculty scientist in Berkeley Lab's Materials Sciences Division.  He also said that the approach "represents an important stepping stone towards the realization of a new class of electronic devices."  By having interfaces based on van der Waals interactions instead of covalent bonding, it will be possible to reach a degree of control in material engineering and device exploration that has yet to be seen. 

 

http://spectrum.ieee.org/nanoclast/semiconductors/devices/transistors-made-from-2d-materials

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

Thin-Film Solar Cells May Be Toxic Free In The Future.

 

solar cellCadmium chloride is definitely not healthy to be around.  Its cadmium ions are extremely toxic, and can cause heart disease, kidney disorders, and many other health problems.  It is ironic that such a toxic substance is essential for the manufacturing of clean energy: thin-film cadmium telluride solar cells.  University of Liverpool researchers have discovered a way to work around this however.  They have found that the cadmium chloride can be replaced with magnesium chloride, a safe and inexpensive alternative that could help to decrease the cost and environmental impact of thin-film photovoltaics.  At approximately $0.50 per pound, magnesium chloride is hundreds of times cheaper than cadmium chloride.

This new poison-free process could allow thin-film solar cells to challenge the dominance of silicon photovoltaics, which currently account for approximately 90 percent of the world’s solar market.  There are some major drawbacks with silicon photovoltaics.  They do not particularly absorb light well, so modules require layers of very high purity crystals, each more than 150 micrometers thick.  The price of these silicon slabs is hindering the efforts to reduce the price of solar power.  Thin-film solar cells may be a solution.  By using semiconductors that absorb the sun’s rays more efficiently, similar results can be obtained with sheets of lower purity material that are only 2 micrometers thick.  This results in drastically lower manufacturing costs. 

The leading thin-film technology, which is a sandwich of cadmium telluride and cadmium sulfide (CdTe/CdS), makes up between 5-7 percent of the solar power market.  While the technology is nothing new, CdTe cells have been slow to take off.  However, their efficiency has risen above 20 percent in the lab in the last few years, now only trailing silicon by approximately 5 percent. 

“Now that the efficiency has improved, CdTe can compete commercially with silicon,” says Jonathan Major, a photovoltaics researcher at the University of Liverpool who developed the new magnesium chloride process.  When light hits the boundary region between CdTe and CdS in the cells, it excites electrons that are drawn into the CdS layer (an n-type semiconductor).  As the holes left behind by those electrons fall into the CdTe (p-type) layer, the separation of charge generates a current.  The two layers must be treated with a solution of cadmium chloride or an equivalent to make them function efficiently.  “This process is used by all the [manufacturing] plants,” says Major, and it requires specialized industrial waste processing facilities to handle the material.  The treatment has several effects, one being that the material’s chloride ions help to make a better junction between the two semiconductor layers.  Also, Chen Li at Oak Ridge National Laboratory in Tennessee found that chloride replaces some tellurium in the CdTe layer.  “That protects electrons and holes from unwanted recombination,” says Li, which allows current to flow more efficiently. 

Major’s team tested several chloride salts as replacements for cadmium chloride, and found that a vapor treatment of magnesium chloride achieved the best results.  Their cells were able to achieve efficiency levels of 13.5 percent, similar to control cells made using the conventional process.  They were also able to match on other factors, such as voltage, current density, and stability.  Other design improvements, such as thinning the CdS layer, increased cell efficiency to 15.7 percent.  While fume hoods and gas masks are required during the cadmium chloride process, magnesium chloride can be deposited using an airbrush.  

Major has already been in touch with the leading manufacturer of CdTe solar cells: First Solar, located in Tempe, Arizona.  First Solar manufactured the world’s largest solar photovoltaic power facility, Arizona’s Agua Caliente Solar Project, which has an installed capacity of 290 megawatts.

“The cadmium chloride treatment is to date a critical part of the CdTe solar cell manufacturing sequence,” says Raffi Garabedian, chief technology officer at First Solar.  “We apply a full and robust set of environmental, health, and safety controls in order to guarantee that we have no adverse impacts as a result of our manufacturing operation.”  Garabedian adds that, "Despite the cost of these controls, the cadmium chloride treatment step is not an major cost driver in our manufacturing process.”  That however is not what Major was told.  “Talking to them privately," says Major, "they said that cadmium chloride was the second biggest expense in their process.”

Regardless of cost implications, replacing toxic cadmium chloride is clearly a sensible move, as we may see more magnesium chloride used in the future.  

http://spectrum.ieee.org/energywise/green-tech/solar/thin-film-solar-cell-freed-from-toxic-processing

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

Integrated Circuit Design Changes Could Bring Back Vacuum Electronics

 

Vacuum TubeBy the mid 1970s, the only vacuum tubes you could find in western electronics were in certain kinds of specialized equipment.  Currently, vacuum tubes are pretty much a nonexistent technology, but that may change in the future.  Some changes to the fabrication techniques used in integrated circuit design could bring vacuum electronics back. 

NASA Ames Research Center has been working to develop vacuum-channel transistors.  While the research is still in the early stages, their prototypes hold great promise.  Vacuum-channel transistors have the potential to work 10 times as fast as ordinary silicon transistors and may have the ability to operate at terahertz frequencies.  They are also much more tolerant of heat and radiation.  To understand why these development may be possible, it will help to understand a little about the construction and functionality of vacuum tubes.  While the vacuum tubes that amplified signals in radios and televisions during the first half of the 20th century seem to not resemble the metal-oxide semiconductor field-effect transistors (MOSFETs) that are used in modern electronics, they do have similarities.  Both are three-terminal devices.  The voltage applied to one terminal, the grid for the vacuum tube and the gate for the MOSFET, controls the amount of current flowing between the other two (from cathode to anode in a vacuum tube and from source to drain in a MOSFET).  This allows both devices to function as an amplifier, or in some cases a switch.   How electric current flows in a vacuum tube compared to a transistor is very different however.  Vacuum tubes rely on a process called thermionic emission, where heating the cathode causes it to shed electrons into the surrounding vacuum.  The current in transistors however comes from the drift and diffusion of electrons between the source and the drain through the solid semiconducting material that separates them.

Solid-state electronics surpassed vacuum tubes due to their lower costs, smaller size, longer lifetimes, efficiency, ruggedness, reliability, and consistency.  However, when solely looking for a medium to transport charge, vacuum beats semiconductors.  Electrons are able to move freely through a vacuum, where they collide with the atoms in a solid state.  This process is called crystal-lattice scattering.  Also, vacuums are not susceptible to the kind of radiation damage that semiconductors are, and they produce less noise and distortion than solid-state materials. When only a few vacuums were needed to operate a radio or television, their drawbacks were not that significant.  However, as circuits became more complicated, it became obvious something needed to change.  For example, the 1946 ENIAC computer used 17,468 vacuum tubes, weighed 27 metric tons, and took up almost 200 square meters of floor space.  The transistor revolution ended these issues.  The great change in electronics occurred not so much because of the intrinsic advantages of semiconductors but because engineers had the ability to mass-produce and combine transistors in integrated circuits by etching a silicon wafer with the appropriate pattern.  As the technology progressed, more transistors could be put on a microchip, allowing the circuit design to become more complicated from one generation to the next. 

After over 40 years, the oxide layer that insulates the gate electrode of a typical MOSFET is only a few nanometers thick, and only a few tens of nanometers separate its source and drain.  While transistors can't get much smaller, the quest for faster and more energy-efficient chips moves forward.  One possible candidate to replace the traditional transistor is the vacuum-channel transistor.  This combines the best aspects of the vacuum tubes and transistors and can be made just as small and inexpensively as any solid-state device.  In a vacuum an electric filament is used to heat the cathode to allow it to emit electrons.  Vacuum-channel transistors do not require a filament or a hot cathode.  If the device is small enough the electric field across it is sufficient to draw electrons from the source by the field emission process.  Removing the inefficient heating element reduces the area each devices takes up and makes the new transistor more energy efficient. Current flows in the vacuum-channel transistors would be done the same as with traditional MOSFETs, using a gate electrode that has an insulating dielectric material, such as silicon dioxide, separating it from the current channel.  The dielectric insulator transfers the electric field where it's needed while preventing the flow of current into the gate.   

While the work being done with vacuum-channel transistors is in the early stages, developments could have a major impact on devices where speed is critical.  The first effort to create a prototype produced a device that could operate at 460 gigahertz, approximately 10 times faster than the best silicon devices.  This offers great promise for the vacuum-channel transistors to operate in the terahertz gap. 

 

http://spectrum.ieee.org/semiconductors/devices/introducing-the-vacuum-transistor-a-device-made-of-nothing

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

IEEE 2014 Medal of Honor Recipient Comes from the Field of Power Semiconductors

 

circuit boardB. Jayant Baliga, originally from the outskirts of Bangalone, India, is this year’s IEEE Medal of Honor recipient.  Science and engineering were a part of Baliga’s life from an early age.  His father, one of India’s preeminent electrical engineers, was chairman and managing director of Bharat Electronics Limited.  Baliga developed his interest in science, especially electrical engineering, by immersing himself in his father’s technical library.  Later he studied electrical engineering at the Indian Institute of Technology Madras.  While studying there Baliga found a subject that interested him even more, physics, but switching majors was not an option.  He decided to combine his interests and study semiconductors.  Wanting to avoid living under his father’s shadow, Baliga decided to continue his studies of semiconductors abroad at Rensselaer Polytechnic Institute (RPI) in Troy, New York.   

As a master student studying under Sorab K. Ghandhi, Baliga worked on gallium arsenide semiconductors.  During his Ph.D. work he investigated a technique he could use for growing indium arsenide and gallium indium arsenide semiconductors, a process now known as metal-organic chemical vapor deposition.  This research was extremely dangerous, as the compounds involved would detonate when exposed to air.  Ghandhi was not detoured, and counseled his student to build a reaction vessel that was “really tight”.  After earning his Ph.D. in 1974, Baliga hoped to acquire a research position with IBM or Bell Laboratories.  However, with only a student visa, Baliga was not able to get an interview with either institution.  A fellow graduate student at RPI, who was also working for General Electric Research Laboratory, told him about a position investigating power devices.  Baliga was not thrilled with the possibility of working with power devices, believing that all the interesting work had already been done.  With no other options, Baliga applied and got the job.

Baliga’s early work for GE involved thyristors-semicondcutor devices, which are now mostly used for handling extremely high voltages.  During his studies, Baliga thought it may be possible to get them to work like regular transistors, which can be switched on and off on command.  GE had the need for energy-saving variable-frequency motor drives, and Baliga designed a thyristor-like device that combined attributes of MOSFETs and bipolar transistors.  At this time these semiconductors had not been combined.

Baliga’s colleagues shared his idea with GE’s chairman and CEO, Jack F. Welch Jr., and in 1981 Welch traveled to GE’s research center to be briefed on the new transistor concept.  The meeting went well and within a year the team was fabricating wafers with the new design.  Originally the device was named the “insulated-gate rectifier,” in an attempt to distinguish it from ordinary transistors.  Later Baliga changed with name to insulated-gate bipolar transistor (IGBT) as to not confuse application engineers.

insulated gate bipolar transistor

The IGBT was successful in avoiding catastrophic “latch up” – the thyristor-like continuation of current flow after a transistor is turned off.  However, it was still switching off too slowly to be used for variable-frequency motor drives.  Known methods of upping the speed of a transistor would ruin this type of MOS device.  Baliga created a way to speed up the IGBT: electron irradiation.  While this method had been used on bipolar power rectifiers, it damaged the MOS device.  Baliga figured out a way to apply enough heat to repair the damaged done to the MOS structure while keeping the speed boost.

After one of GE’s investments went badly, Welch decided to sell off GE’s entire semiconductor business in 1988, leaving Baliga’s expertise useless to the company.  While Baliga was ensured he would have a position in management, his heart was still in science.  With other offers holding little promise, and the academic activity in power devices nonexistent in the United States, Baliga chose to create his own research program.  In 1988 Baliga moved to North Carolina State, where he has taught and done research for 25 years now.  Recently, President Obama visited to announce the creation of the Next Generation Power Electronics Innovation Institute and a $140 million US grant to the university that Baliga and his team at the university’s Future Renewable Electric Energy Delivery and Management Systems Center helped to win.

One of the goals of the new institute is to speed the development of MOSFETs and other power devices made with wide-bandgap semiconductors.  In the future wide-bandgap MOSFETs should be cheap and reliable enough to replace IGBTs.  Baliga is ok with this potential outcome.  While he’s the creator of the silicon IGBT, a narrow-bandgap device, Baliga has always supported wide-bandgap devices as well.  While developing the IGBT at GE, he was also creating the first wide-bandgap power semiconductor, a gallium arsenide rectifier.  During this time he also created a way to calculate from basic theory what semiconductor types function best for power devices.  This expression is now known as Baliga’s figure of merit, and highlights the potential of silicon carbide and other wide-bandgap semiconductors.  The challenge, and what Baliga and his students are actively pursuing, is a way to make these devices cheap enough to compete with silicon. 

As always, please feel free to comment below and let the bloggers at Glew Engineering know if there is a specific topic you’d like us to blog about in the future.

 

Schneider, David. (2014, May). The Power Broker. IEEE Spectrum, 52-58.

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

Series on Semiconductor Processing and ICs, Part 14: Inventions that Lead to the Modern Integrated Circuit

 

Phone SwitchboardBelow is Glew Engineering’s 14th article in the series on ICs and semiconductor processing.  These articles are written for those that are not technical specialists in the semiconductor field.  Below we highlight some of the crucial inventions that lead to the common integrated circuit.

Many of the devices that make up today’s integrated circuits were invented long before the technology was available to mass-produce them.  Rectification, photoconductivity, and other basic semiconductor properties were discovered prior to 1900, although they were not fully understood at that time.  By the mid 1930s, simple devices based on these properties were available.  During this time the physics behind the behavior of metal/semiconductor contacts began to be understood.  Much of this understanding was based on the work done by William Shockley and Nevill Mott.  World War II put much of the initial semiconductor work on pause, particularly at Bell Telephone Laboratories where an effort was underway to find a solid-state device for switching telephone signals.  Shortly after the end of the war work resumed and a major breakthrough was seen in December 1947 when a point contact transistor was demonstrated.  The work that followed resulted in the bipolar transistor and resulted in the Nobel Prize in physics for John Bardeen, Walter Brattain, and William Shockley in 1956. 

Renewed interest in semiconductors was seen in the 1950s, when it became apparent that the reliability issues associated with the new transistor structures were related to surface effects.  In an experiment performed in 1953, Brattain and Bardeen found that the surface properties of semiconductors could be controlled by exposure to oxygen, water, or ozone ambient.  Other experiments over the next few years led to the first high-quality SiO2 layers grown on Silicon (Si) substrates. 

The first point contact transistors in 1947 were built in polycrystalline germanium.  Shortly after that, the device was demonstrated in silicon and in single-crystal material.  These developments had significant impacted on integrated circuit of the future.  Single crystals provided uniform and reproducible device characteristics, leading to the ability to integrate millions of identical components side by side on a chip.  Many of the developments associated with developing single crystal source material belong to Gordon Teal of Bell Labs. 

By the mid 1950s, both grown junction and alloy junction bipolar transistors were commercially available.  Germanium was still the dominant material used at this time.  While these junctions were useful components, the technologies used to build them were not extendible to multitransistor integrated circuits.  Exposed junctions were present on the semiconductor surface but no way to interconnect multiple devices was available.  Part of the solution was provided by the invention of gas phase diffusion processes at Bell Labs.  This led to the commercial availability of diffused mesa bipolar transistors by 1957. 

The next major breakthrough came with the invention of the planar process by Jean Hoerni of Fairchild Semiconductor.  This process relied on the gas phase diffusion of dopants to produce N- and P-type regions, as well as the ability of SiO2 to mask these diffusions.  This major advancement was largely responsible for the switch from germanium to silicon.  One final invention was necessary to allow for modern IC technology.  That was the ability to integrate multiple components on the same chip and to interconnect them to form a circuit.  Jack Kilby of Texas Instruments and Robert Noyce of Faichild Semiconductor invented the integrated circuit in 1959.  By combining P- and N-type diffusions and SiO2 passivation layers, many types of devices including transistors, resistors, and capacitors are possible in modern IC structures. 

Since 1960, the basic technologies used to manufacture integrated circuits have not changed.  There have however been significant improvements to depositing, etching, diffusing, and patterning.  While these changes have been evolutionary, they have not necessarily been revolutionary.  The rapid evolution over the last 50 years has been enormous and we should expect many more developments in the years to come.

Jim Plummer, one of the co-authors of the text Silicon VLSI Technology: Fundamentals, Practice and Modeling, earned his PhD degree in Electrical Engineering from Stanford University in 1971.[i]  From 1971-1978, Plummer was a member of Stanford's research staff in the Integrated Circuit Lab.  After working as an associate professor at Stanford, Plummer became a professor of electrical  engineering in 1983.  Plummer has worked in a variety of areas involving silicon devices and technology.  His early work focused on high-voltage ICs and high-voltage device structures.  With the assistance of his team, Plummer made a crucial contribution to integrated CMOS logic and high-voltage lateral DMOS devices on the same chip and demonstrated circuits operating at several hundred volts.  His work led to several power MOS device concepts such as the IGBT which have become important power switching devices.

We hope you enjoyed this overview crucial inventions leading to the integrated circuits we see today.  As always, please feel free to leave a comment below and let the bloggers at Glew Engineering know if there is a specific subject matter that you would like us to cover in the future.

 


[i] https://profiles.stanford.edu/jim-plummer

Plummer, J. D., Deal, M. D., & Griffin, P. B. (2000). Silicon VLSI Technology: Fundamentals, Practice and Modeling. New Jersey: Prentice Hall.

 

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

Series on Semiconductor Processing and Integrated Circuits, Part 13: Packaging

 

semiconductor chipThis is Glew Engineering’s 13th article in the series on ICs and semiconductor processing.  These articles have been written for those that are not technical specialists in the semiconductor field.  Below we will focus on one of the final steps of semiconductor processing: packaging.

In the past semiconductor packaging has not been as sophisticated as the wafer fabrication process, but the VLSI/ULSI era in chip density has forced the advancement of chip packaging technology and production automation.  High-density chips result more bonding pads and more electrical connections, requiring a cleaner package and process.  Below we will discuss the four functions of a semiconductor package, as well as the five common parts of a package.

Semiconductor Package Function

The four basic functions performed by a semiconductor package are: a substantial lead system, physical protection, environmental protection, and heat dissipation.  The primary function of the package is to allow connection of the chip and the circuit board or directly to an electronic product.  Due to the thin and fragile metal system used to interconnect the components on the chip surface, this connection cannot be made directly.  Fragile wires are protected by a substantial electrical lead system that serves as a connection of the chip to the outside world.  Secondly, the package provides physical protection to the chip against breakage, particulate contamination, and abuse.  Protection is attained by securing the chip to a die-attachment area and surrounding the chip, wire bonds, and inner packaging leads with an appropriate enclosure.  Packaging protects the chip from chemicals, moisture, and gases that could potentially interfere with the chip’s function.  Lastly, the package enclosure materials serve to draw the heat away from most chips, and a packaging material is often chosen based on its thermal management properties. 

Common Semiconductor Package Parts

1. Die-attachment area

2. Inner leads

3. Outer leads

4. Chip/package connection

5. Enclosures  

The die-attachment area is in the center of every package and may have an electrical connection that serves to connect the back of the chip to the rest of the lead system.  This area must be absolutely flat in order to support the chip in the package.  The metal lead system chip is continuous from the die-attach cavity to the printed circuit board or electronic device.  The system inner connections, or inner leads, are usually the narrowest portion of the lead system.  The leads become wider and finally end outside of the package.  This portion is called the outer leads.  The chip is electrically connected to the lead system of the package with bonding wires, balls, or other on-chip connectors.  The die-attach area, bonding wires, and inner and outer leads make up the electrical parts of the package.  The other portion is the enclosure or body.  This is the area that provides the protection and heat-dissipation functions.  The completeness of the seal of the enclosure is broken into two categories: hermetic and non-hermetic.  Hermetic sealing creates a package that is impervious to the penetration of moisture and other gases.  Hermetic seals are necessary to use on chips that will operate in harsh environments.  Metal and ceramic enclosures are preferred materials used for hermetically sealed packages.  Non-hermetically sealed packages are often enough for the average consumer application, such as computers and entertainment systems.  Non-hermetic packages are composed of epoxy resins or polyimide materials and are often referred to as plastic packages.

semiconductor packagingPackages now include two and a half, and three-dimensional packages.  These include many new developments such as thru silicon vias (TSV), package on package (POP), and other advances.  IC designers stack multiple dice in the same package, thereby decreasing distance, and increasing signal speed and density.  Modern memory packages may have 16 or more die stacked on top of each other. 

We hope you enjoyed this brief overview of semiconductor packaging functions and the parts.  As always, please feel free to leave a comment below and let the bloggers at Glew Engineering know if there is a specific subject matter that you would like us to cover in the future.

 

Van Zant, P. (2000). Microchip fabrication, a practical guide to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill.     

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

Light Sensors Could Improve Medical Devices and Security Imaging

 

smartphoneThe implications of new and developing medical technologies in society today are vast.  Everyday new methods are discovered that will help us to track and monitor our health and wellness more effectively.  With the rising costs of insurance and the recent changes to the healthcare field many are looking for more affordable ways to maintain our health management. Here is an article about a particularly promising new development to medical devices that could have positive effects on healthcare.

Researchers at the University of Surrey have developed a new type of light sensor that could allow for medical and security imaging via low cost cameras.  Using these light sensors, near infrared light can be used to perform non-invasive medical procedures, such as measuring oxygen level in tissues and detecting tumors.  This technology is also already widely being used in security systems and for quality control in the food industry.   

The new light sensor is able to detect the full spectrum of light, from ultra-violet (UV), to visible and near infrared light. It is the near infrared light that is most useful in this application. This new technology would allow surgeons to “see” inside tissues to find tumors prior to surgery and also be useful to consumer products such as cameras and mobile devices that could soon be equipped with night imaging options.  This is useful for capturing quality photos in the dark or allowing parents to monitor their child’s blood or tissue oxygenation level using only their Smartphone, which could then be linked to healthcare professionals.[i]

The technology behind these new and exciting advances involves C60 Nanorods and Inorganic Photodoping[ii] and was developed by researchers at University of Surrey. One dimensional single-crystal nanorods of C60 possess unique optoelectric properties which makes them perfect for use in manufacturing low-cost large-area flexible photoconductor devices.  Researchers were able to enhance the photosensitivity of the C60 nanorods by using a very low photodoping mechanism. Photodoping is used to solve a naturally occurring problem.  When a C60 nanorod photoconductor device is irradiated with photons most of the excited electrons are concentrated in trap states rather than being available to contribute to the  photocurrent as a whole,  which reduces the responsivity and decreases the photosensitivity overall. To counteract this and to improve the photo-oxidation stability and extend the sensitivity of the device, researchers then introduced a photodopant. This fills the traps that naturally occur when C60 suffers degradation.  These photodopants must form a type-II heterojunction so that when the composite is photoexcited the photodopants provide the additional electrons needed to fill the C60 traps states. This encourages the photosensitivity needed to detect the full spectrum of light.

Typically when photodopant devices are introduced the donor to acceptor ratio is very high and the devices rely primarily on the absorption and conductivity of the donor material. In this case in order to achieve the photosensitivity required for the use of these light sensors in a medical setting, the photodopants introduced enhance the photosensitivity of the acceptor material (i.e. the C60 nanorods) and not the donor material.  To validate that the photodoped C60 nanorods truly experienced an enhancement of photosensitivity, the research team University of Surrey compared the responsivity spectrum of all the photodoped devices to that of an undoped C60 nanorod device.  When the tests were done and comparisons were read results showed that responsivity was increased in the photodoped devices by up to two orders of magnitude. This reinforced the idea that via an ultralow doping mechanism and careful selection of dopants and photodopants, both mobility and the photoconductivity of such a trap rich molecular organic semiconductor can be enhanced by many orders of magnitude.   

While this technology may not be implemented and readily available to all within the next year, chances are that within the next decade these light sensors could be a standard part of all medical practices’.  This could conceivably greatly enhance preventative care and drive down costs of major procedures.  It also has major implications for security and military personnel as well.  It is appropriate to note that without the availability of semiconductor processing this technology would never have been plausible. 

We hope that you found this overview of the article useful and interesting.  Please feel free to comment below and let the bloggers at Glew Engineering know if there is a specific topic you’d like us to blog about in the future.

 


[i] University of Surrey. (2014, May 23). New sensor could light the way forward in low-cost medical imaging. ScienceDaily. Retrieved May 23, 2014 from www.sciencedaily.com/releases/2014/05/140523082928.htm

[ii] Rinku Saran, Vlad Stolojan, Richard J. Curry. (23 May 2014). Ultrahigh Performance C60 Nanorod Large Area Flexible Photoconductor Devices via Ultralow Organic and Inorganic PhotodopingScientific Reports, 2014; 4 DOI: 10.1038/srep05041

 

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

Series on Semiconductor Processing and Integrated Circuits, Part 12: Integrated Circuit Types

 

Integrated CircuitThis is Glew Engineering’s 12th article in the series on ICs and semiconductor processing, written for those who are not technical specialists in this field.  While the majority of this series has focused on semiconductor processing, herein we highlight general circuit families and functions of integrated circuits (ICs).

A solid-state integrated circuit is comprised of a number of separate functional areas.  Each chip, regardless of its intended function, has an input and encode section where the incoming signals are coded into a form the circuit can understand.  The majority of the circuit area contains the circuitry required to perform the circuit function, either memory or logic.  Once the data is manipulated by the circuit, it goes to a decode section where it is changed into a form that is usable by the machine’s output mechanism.  The output section transmits the data to the outside word.  

Three Circuit Types

Circuit types fall into three broad categories: logic, memory, and logic and memory (microprocessors).  Logic circuits perform a specified logical operation on the incoming data.  For example, pushing the key for the letter “B” on a keyboard will result in the letter B appearing on the computer screen.  Memory circuits are designed to store and give back data in the same form in which it is entered.  Selecting the (pi) key on a calculator activates the memory portion of the circuit where the value of pi (3.14159…) is stored.  The remembered value, 3.14159, is then displayed on the screen.  The third type of circuit combines both logic and memory in a microprocessor circuit.  The world’s first single chip microprocessor was introduced to the public by Intel in 1971.[i]  This microprocessor allowed for the design of powerful personal electronics such as computers, digital watches, and one-chip calculators to name a few.  While microprocessors have been nick named a “computer on a chip”, they are not truly a computer.  Even the simplest computers require large amounts of memory, which microprocessors don’t have.  Below we will further discuss the differences between the three types of circuits.      

Logical Circuits

Analog-Digital Logic Circuits

Logical circuits fall into two categories: analog and digital.  Analog circuits were the earliest logic circuits developed.  Process engineer Dave Talbert and designer Robert Widlar created the first commercially successful analog IC in 1964.[ii]  Analog circuits have an output that is proportional to the input, where digital circuits have a predetermined output in response to a variety of inputs.  A dimmer light switch is an example of an analog device.  Turing the control varies the voltage, which varies the brightness of the light.  A traditional on-off switch is a digital device where only two levels of brightness are available: on or off. 

Memory Circuits in More Detail 

In the early 1970s industry forecasters saw their predictions become a reality when solid-state memories finally surpassed core memory.  While core memory offered lower costs, solid circuits were smaller, more reliable, and faster. 

Nonvolatile Memories

A nonvolatile memory device is one that doesn’t lose its stored information when power is lost.  Nonvolatile memory circuits include ROM, PROM, EPROM, and EEPROM.  In integrated circuits, the ROM (Read Only Memory) design is the principal nonvolatile circuit.  The function of this circuit is to give back precoded information that is designed into the chip memory array section during fabrication.  PROM stands for programmable read only memory.  Each memory cell is connected into the circuit through a fuse.  Users program the PROM into their memory circuit requirements by blowing fuses at the unwanted memory cell locations.  Once programming is completed, the PROM is changed to a ROM and the information is permanently coded into the chip.  EPROM (erasable programmable ROM) allows for information stored in the ROM to be changed without having to replace the whole chip.  The erasable feature is built in by using MMOS (memory MOS) transistors.  EEPROM (electronically erasable PROM) allows for memory to be programed and reprogramed while the chip is in the socket of the machine.  Programming and erasing takes place by pulses from the outside that place charges in selected memory cells or drain the charges away. 

Volatile Memories

While nonvolatile memory provides protection against power loss, it is often slow, not very dense, and does not have write capability.  Volatile memory is used to produce fast and high-density memory circuits.  RAM (random-access memory) is one type of circuit used for high-density memory storage.  “Random” refers to the ability of the computer to directly retrieve any information stored in the circuit.  DRMAs (dynamic random access memory) come in two designs: dynamic and static.  Information is stored on DRAMs by a charge built up in the capacitor.  However, this charge drains rapidly and the memory information must be re-inputted, or refreshed, on a constant basis.  The goal of DRAM IC design is small-cell design for high-density and closely spaced components with small and thin parts for speed.  SRAM (static random-access ram) memories are based on a cell design that does not need to be refreshed.  Once the information is on the chip, it will stay as long as the power remains on.  This is done with a cell containing several transistors and capacitors.  Information can be read and written with a SRAM cell much faster than with a RAM design since transistors can be switched faster than capacitors can be charged and drained.  The downside to SRAM is that they are less dense than DRAMs. 

We hope you enjoyed this brief review of various integrated circuit types.  As always, please feel free to leave a comment below and let the bloggers at Glew Engineering know if there is a specific subject matter that you would like us to cover in the future.



[i] http://inventors.about.com/od/mstartinventions/a/microprocessor.htm

[ii] http://www.computerhistory.org/semiconductor/timeline/1964-analog.html

Van Zant, P. (2000). Microchip fabrication, a practical guid to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill

 

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 

Series on Semiconductor Processing and Integrated Circuits, Part 11: Basic Semiconductor Circuit Components

 

integrated circuitThis article is our 11th in a series intended as an overview for those who are not technical specialists in the semiconductor processing field.  The following is a brief over view of some of the basic structures that make up a semiconductor devices, or integrated circuits (IC): resistors, capacitors, diodes, transistors, fuses, and conductors.  These components are made in the course of standard semiconductor processing and are typically contained in an IC.

Resistors

Resistors limit current flow by using dielectric materials or high-resistivity portions of a semiconductor wafer surface.  In the semiconductor industry, resistors are formed from isolated sections of the wafer surface, doped regions, and deposited thin films.  The value of a resistor is (in ohms) a function of the resistivity, a material property of the resistor, and its physical dimensions, and can be expressed in the following relationship:

R = ρ L/A                                                                    (1)

 

In the above equation, ρ is resistivity, L is length of resistive region, and A is cross-sectional area of the resistive region.

 resistor

Resistor

A majority of the resistors in integrated circuits are formed by a sequence of oxidation, masking, and doping operation.  A typical resistor is in the shape of a dumbbell, with the square ends acting as contact regions and the long skinny region serving the resistor function.  After doping and reoxidation, contact holes are etched in the square ends to contact the resistor into the circuit.  A resistor is a two-contact, no-junction device.  The term no-junction device means that the current flows between the contacts without crossing an N-P or P-N junction.  The junctions serve to confine the current flow in the resistive region.  Doped resistors can be formed during any of the doping steps during semiconductor processing.  Those formed by ion implantation have more controlled values than those in diffused regions.  Resistors generate heat according to the ohm’s law and the power law:  V= IR and P = IV, yielding P= I2R.  The voltage and current in a resistor do not have a time lag relative to each other, unlike capacitors and inductors.

Capacitors

A capacitor is a device that stores an electrical charge.  A junction capacitor is formed at every junction in a semiconductor device.   When a voltage is applied across any junction, carriers on each side move away from the junction, leaving a depleted region that acts as a capacitor.  The value of this junction capacitance must be taken into account when the circuit is designed. 

 capacitor

One of the issues with oxide-metal capacitors is their large area.  Trench capacitors help to solve this problem by creating a capacitor in a trench etched vertically into the wafer surface.  Etching is done either isotropically with wet techniques or anisotropically with dry techniques.  The trench sidewalls are oxidized and the center is filled with deposited polysilicon.  Stacked capacitors offer another alternative to conserving surface area.  The need for small high dielectric capacitors for dynamic random access memory (DRAM) circuits has driven this development.  The storage portion of a DRAM cell is a capacitor and can be in planar, cylindrical, or fin shaped.  Similarly, FLASH memory has large numbers of capacitors.  The voltage and current in a capacitor have a time lag relative to each other.

Diodes

A diode is a two-region, two-contact, device separated by a junction, which either allows current to easily pass or acts as a current block.  A diode is the simplest junction device. 

 

DIODE

Voltage polarity, called biasing, determines which function the diode performs.  When the current voltage is the same as the diode region, the diode is in forward bias and the current flows easily.  When the polarities are reversed, the diode is reverse-biased and the current is blocked.  Diodes are used in circuits to steer the current around the circuit and are usually formed along with transistor doping steps.  In MOS circuits, most of the diodes are formed with the source-drain doping step.

Transistors

A transistor is a three-contact, three-part, two-junction device that performs as a switch or an amplifier.  A NPN transistor is where a P-type region lies between two N-type semiconductors.  A PNP transistor is just the opposite, where an N-type region lies between two P-type semiconductors.  

 transistor

Transistors

There are two common types of transistors: bipolar (amplifier) and MOSFET (switch).  A bipolar transistor has three terminals: the base, collector, and emitter.  The current flows from the emitter region, through the base and into the collector.  Bipolar transistors feature fast switching speeds, which are governed by a variety of factors including base width.  The shorter an electron, or hole, has to travel, the less time it will take.  To achieve fast speeds, the transistor is maintained in the on position, requiring that the base always have power.  This will lead toward a buildup of heat in the transistor and will eventually affect the circuit. 

On the other hand, a MOSFET transistor is a field effect device.  It has three terminals: source, gate, and drain.  The middle terminal (gate) does not require current to turn it on, but instead an electrical field.  The field effect is due to a voltage applied across an insulator (gate oxide) on top of the gate, the middle terminal.  The gate turns the transistor “on” or “off.” (Note that off is not used as a preposition at the end of the last sentence, so the usage meets with the approval of persnickety grammarians.)  MOS is an acronym for metal oxide semiconductor, because the gate stack was originally a metal trace applying a voltage to the gate oxide, which communicated with the underlying semiconductor.

Fuses and Conductors

A fuse is a short piece of conducting material (wire) that is designed to melt and separate in the event of excessive current.  Semiconductor devices are especially sensitive to over voltages and over currents and require fast acting protection.  Fuses used in semiconductor devices have specially designed necks that allow for rapid melting.  Once the fuse melts, the entire circuit opens and current is prevented from flowing through the components.[i]

Metals such as copper, aluminum, and silver generally make good conductors. Conductors are materials that obey Ohm’s law, have low resistance, and can carry electrics currents without dissipating a lot of power.  Conductors help to ensure that a majority of a signal’s power reaches its destination instead of over-heating the circuit.   

Metal traces or conductors in an IC that are placed next to each other and separated by insulating (dielectric) material form inadvertent capacitors, and slow down the circuit operation.  The product of the resistance (R) of the conductor and the capacitance (C) of the diode structure, is know is the RC time constant.

We hope that you found this review of circuit components helpful.  Please feel free to comment below and let the bloggers at Glew Engineering know if there is a specific topic you’d like us to blog about in the future.



[i] http://dokor.com/Bussmann/Data/2_Fuse_Characteristics.pdf

Van Zant, P. (2000). Microchip fabrication, a practical guid to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill.

For more information on Glew Engineering Consulting visit the Glew Engineering website, blog or call 800-877-5892 or 650-641-3019. 
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Linear v Novellus (Semiconductor Equipment)

  
  

After 8 long years, Novellus finally rid itself of the lawsuit with Linear Technology. Irell and Manella LLP, for whom Glew Engineering has worked in the past, took no prisoners in the unanimous jury verdict announced yesterday in favor of their client Novellus.  The jury consisted of 12 men and women in Santa Clara, CA, the heart of the silicon valley.  Certainly good news for Novellus' legal team, as well as their bottom line. Congratulation to Jonathan Kagan Esq. and his colleagues.  Now both sides can get back to what they do best - making chips and chip equipment.

Novellus' also shipped their 1000th Vector PECVD tool in February? Considering the tool's throughput and uptime, there may be as many chips out there by now with Novellus' dielectric films as those of any semiconductor equipment manufacturer. See the details at: 

http://ir.novellus.com/releasedetail.cfm?ReleaseID=441840

 

Semiconductor Equipment, Glew Engineering

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