EEWeb Pulse - Issue 91 by EEWeb Magazines

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Plessey Semiconductors: The Future of Integrated Circuit Sensors

How this semiconductor company is implementing its Electrical Potential Integrated Circuit (EPIC) sensor to monitor heart rate and heart health.

University Nanofabrication Facility: A High-Level Overview BY ALEX TOOMBS WITH EEWEB An in-depth look at the integrated circuit fabrication process at the University of Notre Dame's Nanofabrication Facility.

Vacuum Transistors: Nanotechnology for the 21st Century

BY KAREN KOHTZ WITH EEWEB How this old technology is revamped on a nanoscale, and how it could affect everything from telecommunications to space travel.

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INTERVIEW

David

Gascon ´ Libelium is a wireless sensor company based in Zaragoza, Spain. The company’s award-winning sensor technology has been implemented in multiple “Smart City” applications across the globe. We spoke with David Gascón, the Co-Founder and CTO of Libelium, about their contribution to creating a network of devices, some surprising applications for their sensors, and Libelium's vision for the "Smart World" of the future.

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EEWeb PULSE Could you tell us a little about your background? I am a computer engineer. I studied at the University of Zaragoza here in Spain, where Libelium is based. Once I finished my degree, I started thinking about my career. Since my final project was related to distributed communications, I thought it would be interesting to get into wireless networks and wireless distributed communications. I thought that it was a useful area, but that it could be improved by sending local information in each frame. This is where sensors come in. When I started Libelium around five years ago, I started to think about how I can connect the real world with the Internet. Normally, when you think about the Internet, it was only related to people actually connected to it through computers or smartphones. Now, we are going one step forward where we are trying to get every object in our life inside of the Internet somehow. People ask me all the time what my work is and I always tell them that I try to expand the limits of the Internet so that not only people are connected, but the real world is connected through smart sensors. Adding sensors to objects in your everyday life enables the technology behind all of these interconnections and creates the Internet of things. The motivation for Libelium stemmed from the wireless distributed communications that I studied for my degree. What was the fundraising process like when you started Libelium? When we started, we had no venture capital or external investments—it was all self-funded with about 3,000 dollars on the table, which was everything we needed to get started. We had a really clear vision to build

the technology from the beginning and knew how to cope with the development of new products. We started using open source platforms such as Arduino to make our first prototypes. After that, we moved to manufacturing the products. If you want to make an original product from the beginning, you obviously need investment money to manufacture it. We had to come up with a platform where you can buy the product for 30 dollars and you can easily connect to it. The idea was simple. I think one of the key points was that we really didn’t need much more money to start coming up with technology. Waspmote was the result of three years of research around how to create a low-power sensor device. Waspmote consumes only 0.07uA (microamperes) in the sleep mode, which is extremely low and was one of our first milestones when we started researching. What products does Libelium offer? There is really a natural evolution to our product line. We started with Waspmote—the sensor platform. One of the things that differentiates this platform from the rest is that it’s a platform with an opensource API so that developers could easily make modifications to the libraries in order to implement it in virtually any application. We always say that this is a platform for developers, and in fact, you can connect your own sensors so that people can create an end product

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very easily. We say that Waspmote is a platform to help other companies create end products because Waspmote isn’t a vertical platform. The other very important thing about Waspmote is the modularity. Normally, when you are working with a sensor you say, “Okay, I’m going to buy a smart sensor platform and I’m going to use Zigbee or Bluetooth.” What we made with Waspmote was a platform that you could add the sensors and the radio on top, so you have a core base board. Afterwards, you can add whatever radio model you like by simply plugging it in. There are three steps to this; you take the Waspmote platform, you select the radio protocol, and finally, you choose the sensors. To be an open-source platform and to be really modular were two of the key decisions we made when we built it. Another product we offer is the Plug & Sense. The Plug & Sense is essentially Waspmote inside, but with a robust waterproof enclosure and with more than 60 different sensor probes available to connect. It’s really a vertical product. This is for companies who don’t need to develop their own products, they just need to use a final product that is easy to maintain and install. It’s a great platform for companies that don’t want to deal with a lot of electronics and just want to implement the product, turn it on, and start sending data. For this reason, when we started the development of the Plug & Sense, one of the things that we had in mind was to implement a technique called OTA (over the air programming), which allows developers to program the sensor nodes from the

INTERVIEW Cloud or Internet. There could be a hundred sensor nodes around a city and you can control each one of them individually through the Internet. The last product in this line is called Meshlium. This is essentially the gateway of the sensor network. Sensor nodes send the information to this gateway by using low consumption protocols such as 802.15.4, ZigBee or Bluetooth; then Meshlium takes this information and sends it to the Cloud by using TCP/IP sockets or through HTTP request via Ethernet, WiFi, or 3G. However Meshlium is not only a door to the Internet, it as a complete Linux machine which also integrates internal data bases and allows us to perfom complex sensing capabilities, such as smartphone detection, by monitoring WiFi and Bluetooth frames sent by iPhone and Android devices. What are some real-world applications for these products? One of the most important and most significant projects we are involved with is a smart cities sensor project. We deployed a 1,000-node network in the city of Santander in a project called “Smart Santander,” which is in northern Spain. This network measures a variety of parameters. For example, we installed a bunch of nodes underground to measure the automobile activity—not only how many cars are passing, but in terms of cars that are parking. We could say that 400 of the nodes are used in a smart parking application. They monitor the center of the city and they send information to the citizens—in real time, such as how many free parking spots are free in each street. You could get this information just by connecting with your smartphone. The other 600 nodes in the city measure air contamination and pollution—CO2 and NO2 levels—

A Waspmote unit implemented in a Smart City

and measure the noise levels in different streets, in order to create a real-time noise map of the city to see how the noise impacts the citizens. Also, luminosity sensors are used to create a smart lighting application where the streetlights turn on when the sunlight dims to a certain level. Another project we have in northern Spain is a smart agriculture network for vineyards so that you can measure in real-time how much water is absorbing into the crops and how much light is being reflected through

our ultraviolet sensors. The farmers can keep track of the water levels using moisture sensors and they can also measure danger to the crops if there is damaging frost at night. There is also a similar project concerning forest fire detection. We implemented 200 nodes throughout an entire forest with CO and CO2 sensors that monitor whether or not there is fire or smoke. During the Fukushima plant meltdown in Japan, there was a lot of radiation emitted. We then started thinking of

Libelium's sensors expand the limits of the Internet so that everything is connected through sensor networks. Visit www.eeweb.com

EEWeb PULSE how to measure the amount of radiation through sensors, and we created a new model of Waspmote, which introduced our radiation sensors and sent them over to Fukushima. These can be implemented in the gardens and windows that measure the radiation around you. This was a really beautiful project for us because we gave away these nodes for free and we realized that this way, people can measure the radiation without actually being there, which saves lives in the long run. After that, we created a Plug & Sense radiation model. Could you tell us a little more about Libelium? We are based in Zaragoza, Spain and there are around 35 people who work here. Zaragoza is a mediumsized city, which is based in between Madrid and Barcelona, so it’s really well connected. All of the people that work here are relatively young. My partner, Alicia Asín—the other cofounder of the company—is just 30 years old. The rest of the employee’s ages range from 25 to 35. It’s a really young team to be making not only software, but also hardware products. We are learning a lot about how the market worksand about how we can improve manufacturing—it's not the same to just make software. We are learning to cope with hardware designs and electronic devices and make them look nice. This last year, we managed to release a new product for each month of the year, so this gives you an idea of how hard our team is working—there is a lot of strength and movement, and there are a lot of dynamic people that work here. It’s a small company, but we move quickly.

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INTERVIEW

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What is the future looking like for Libelium? One of the things that we have to cope with now is multiple integration with the Cloud systems. On the one side, we are the manufacturers of this sensor technology and in the middle there are the Cloud system providers. On the other end, there are the end users. One of the most important things we have to deal with in the coming years is making a a platform that can be easily integrated with the Cloud system. You can’t make these kinds of proprietary protocols because if you can’t talk to a web server and you don’t do an open implementation of the communication protocols, you won't really grow. Everyday, there are a bunch of

new Cloud companies created, so you have to be open in terms of compatibility and compliance. Over the next year, we have to be really compatible with Cloud systems so that when someone wants to create a new project, they will think of using the Libelium platform because it can

be easily integrated in any database, web server, and in any smart phone technology. We are clearly making this not only open-source API, but also the communication frame, so that any web developer can take our sensor frames and integrate in their own sensor and monitor applications. ■

For more information, visit:

www.libelium.com

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INTERVIEW

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SPECIAL FEATURE

The Future of Integrated Circuit Sensors

Plessey Semiconductors is a privately held electronic design and manufacturing company based in Plymouth, UK. Currently, Plessey is migrating from a silicon wafer foundry model with various process technologies available to customers who design their own integrated circuits, to an Integrated Device Manufacturer (IDM). After acquiring IP and other assets a few years ago, Plessey then acquired its own manufacturing facility in Plymouth in order to move towards building its own products. We spoke with Dr. Keith Strickland, the CTO of Plessey, about the restructuring and redirection of the company, their EPIC sensor technology, and the future market potential for its products.

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EPIC Technology After Plessey’s reincarnation back in 2009, the company set out to focus on becoming an IDM to leverage its base processing technology IP and expertise. Plessey is now forecasting explosive growth from a range of innovative products and components including their new Electric Potential Integrated Circuit (EPIC) sensor, which Plessey has been working to commercialize for a little over two years. In regards to the EPIC sensors, Dr. Strickland clarifies that it is “easiest to consider it as a modern-day electrometer that effectively acts as a voltmeter. It has an extremely high input impedance amplifier at its core, with various positive feedback techniques.”

Applications With these characteristics, the sensors are able to measure the minutest change in an electric field. “There’s an electric field all around us,” Strickland states, “which does vary considerably, but on average, it’s around 100 volts per meter.” This field can be easily disrupted, especially by the human body. In this situation, Strickland states, “The EPIC sensor can actually pick up on that disruption of the field and can also act in a more capacitive mode; so if you’re touching the sensor it can sense your body’s electrophysiology signals like your ECG from your heart.” The ECG (ElectroCardioGram) is a signal of electrical

activity of the heart usually attained through a resistive electrode attached to the surface of the skin. Plessey’s EPIC sensor is so good at detecting electrical disruptions that it can provide an effective, quality ECG reading from merely holding two sensors across the chest over your heart, or even from simply holding a sensor in each hand. Strickland says that a majority of Plessey’s applications are in early medical diagnoses or monitoring in the health and sports fitness areas but potentially one day replacing medical cardiology-level ECG measurements. Another potential application for this type of sensor is occupancy sensing and security systems, given that the sensor can pick up on disruptions Plessey’s EPIC in the electric fields in a room. sensor is so good Along similar at detecting eleclines, gesture rectrical disruptions ognition is anoththat it can provide er area in which an effective, qualPlessey has been ity ECG reading making some from merely holdbreakthroughs in. ing two sensors Through an array of EPIC sensors across the chest one can pick up over your heart, or electric field diseven from simply ruptions being holding a sensor made by hand in each hand. gestures and can determine what gestures are being made. “We have demonstrated the use of the sensors for controlling a mouse curser on a screen,” Strickland states.

Heart Chart Everytime your heart beats, it emits an electrical pulse in order to pump blood. The ECG reads this signal to measure your heartbeat and heart health. 16

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Plessey’s Future According to Dr. Strickland, Plessey is committed to “expanding into other markets, with the base technology lending itself to other types of sensors and thus the opportunity to provide an integration of multi sensing systems.” Plessey’s foundry facilities are available to its customers to design CMOS image sensors; “it’s a technology that we know is fit for purpose and we are looking at how we might integrate that with further integrated sensor technology,” Strickland tells us.

SPECIAL FEATURE

Plessey’s handheld ECG monitor utilizes a unique disruptive electric potential sensing technology to detect an ECG signal.

is used to control lighting systems.” As the company changes drastically by becoming more product-centric, it seems that at the end of the day, their foundation in innovation remains as strong. Visit Plessey’s website for application notes, videos, demonstration kits and more:

www.plesseysemiconductors.com

Plessey’s other goal is to integrate such sensor technology into their LED business—sometime in the next quarter, its first line of GaN on Silicon LEDs will be introduced. “Our interest is moving more up that food chain into LED modules and systems,” Strickland says, “and ultimately in smart lighting applications where sensor technology

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rsity

ation Lab:

evel Overview Alex Toombs Electrical Engineering Student

n in-depth look at the IC fabrication rocess at the University of Notre Dame's anofabrication Facility.

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ntegrated circuits have come a long way over the past few decades, from the very first chip working in a deserted TI lab during the summer of 1958 to the current core i7 processors from Intel. As the circuits that make up the electronics we use today grow ever more complex, better processes and technologies are required in order to improve and fabricate wafers that can serve the purposes that we need. A clean room with all equipment required for CMOS chip fabrication can cost upwards of $10 million at the low end, with newer, smaller-feature processes costing exponentially more. This makes IC fabrication a process with a high entry barrier as large, expensive labs are required produce working chips in most cases, in addition to employing highly skilled workers to both operate and repair the equipment in the lab. These factors contribute to the high overall costs of produced wafers. Despite these costs, some universities are able to afford clean rooms and the equipment required for CMOS processes, fewer still allowing undergraduate students access to the labs. As an electrical engineering student at the University of Notre Dame, I was fortunate to be able to get a chance to learn about the clean room through a semester-long course. Students enrolled in the IC Fabrication course, taught by Dr. Greg Snider, are Entering a allowed access to the $25 milclean room can lion Notre Dame be a daunting Nanofabrication Facility. The Faexperience, cility offers equipand intimidatment for almost every step of the ing in the level process required of preparation to produce sound chips that encode it requires. the Notre Dame Fight Song in 9 bits. Being able to produce my own working sound chip has given me insight into the workings of a university-level clean room, tying together all that I have learned as an undergrad student. Entering a clean room can be a daunting experience, and intimidating in the level of preparation it requires. The clean rooms at the Notre Dame Nanofabrication Facility (NDNF) are segregated into class 10000, 1000, and 100 areas, corresponding to the allowable number of particles per cubic foot of air. Specifically, these levels

of cleanliness, which are orders of magnitude better than the billions of particles in the same volume available outside, are designed to improve chip yield on large wafers containing many integrated circuit dies. Depending upon the â&#x20AC;&#x153;dirtinessâ&#x20AC;? of the room you are working in, more protection may be needed. After suiting up with the proper protective gear in the proper order, you pass through the airlock and end up in the cleanroom. Our process used 100 mm silicon wafers, which started as lightly p-type. As there were slight variations among the wafers we received, we started out by characterizing each wafer. This was a minor but necessary part of the process that we had to complete before processing began. The box of wafers our group was processing is pictured in Figure 1. The next step consists of cleaning the wafers using a process known as an RCA clean. This takes place at the beginning of processing, and before the wafers enter any of the oxidation or metal anneal furnace tubes. RCA cleaning requires three heated baths that selectively remove contaminates from the surface of the wafer, followed by a bath of buffered hydrofluoric acid. The three baths are designed to remove organic compounds, strip oxides, and remove any ionic contamination. Between each bath, a full rinse with deionized water removes any remaining chemicals before the wafer cartridge enters the next bath. Any particles that are on the wafer can destroy imaged features on the wafer or introduce dopants where unintended, altering the electrical characteristics of the devices unintentionally. This step requires additional protective equipment, as the baths use highly concentrated acids at temperatures of seventy degrees Celsius or more, and hydrofluoric is lethal if even a palm-sized amount of liquid is exposed to human skin. After an RCA clean, the first step is to apply photoresist to a wafer. Photoresist is a term for a variety of chemicals with organic compounds in them that change thicknesses when exposed to light of certain wavelengths, much like chemicals in photographs that are exposed under camera light. These chemicals are crucial for the process of lithography, which exposes patterns to wafers in order to define the layers that eventually become devices on the wafer. Photoresist generally requires an adhesion promoter, such as HMDS, a solvent that is vacuum exposed to all wafers in order to increase the adhesion between the resist and the silicon wafer surface. Spinning photoresist requires a recipe that spins individual wafers at high RPMs, ensuring an even distribution without any particles on the wafers. IC fabrication is a constant war against particles, the smallest of which can destroy devices. A picture of the

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Figure 1: Wafer Box, 100 mm Silicon Wafers

TECH ARTICLE

Figure 2: NDNF Photoresist Spinner, 100 mm Wafers

NDNF photoresist spinners is shown in Figure 2. The red After the wafer has been exposed by the stepper, photostreaks in the bowl of the photoreresist remains only on certain areas sist spinner are the leftover resist of the wafer, with patterns etched that spins off the wafer, requiring down to the substrate where light Any particles that acetone to clean off. hit the wafer. This is so that patterns may be etched into the substrate, are on the wafer After the photoresist is spun and ensuring that any patterns in the baked upon the wafer, the wafer can destroy imaged photoresist can be transferred must be exposed to the pattern depermanently, constructing the patfeatures on the signed for that layer of the device. terns that eventually constitute the wafer or introduce Many devices require upwards of device. Etching is a blanket term six or seven layers, meaning that for a process that can use acids dopants where the process must be repeated each like HF, reactive ion etching (RIE), unintended, altertime, and that a new mask must or other methods in order to selecbe designed. The NDNF is fortutively remove parts of the substrate. ing the electrical nate enough to have a commerOur lab most often uses RIE, in a characteristics cial stepper, which automatically loadlocked device from Plasmacan expose wafers once they are therm. The RIE and its loadlock of the devices unmanually loaded. Sometimes, alignare pictured in Figure 3. intentionally. ments must be adjusted manually to ensure that the wafer is aligned RIE processes use several toxic with the maskâ&#x20AC;&#x201D;within a tolerance gases, such as sulfur hexafluoride, of several microns. Because of this, our first lithography in a pressurized container in order to selectively remove was designed to etch only alignment marks onto each die. areas of the wafer. The photoresist mask is kept on in Visit www.eeweb.com

EEWeb PULSE order to ensure that only the areas desired are actually etched. Still, due to the high selectivity of the gases, processes must be designed carefully to run only as long as needed. This is especially true of early processes, where the silicon and photoresist are the only materials in the chamber. These processes take anywhere from one to ten minutes, depending upon what is being etched and how deep of an etch is needed. Plasma is created from a source in the machine, which can be observed through the chamber terminal during operation; an example picture of this plasma is shown in Figure 4. After the process is run, wafers are removed, now with patterns permanently etched in. These wafers can have a number of things happen after, though they typically are sent to another facility in California for dopant implantation. Dopants introduce holes or electrons at certain locations, giving the desired electrical characteristics for device operation. Ion implantation introduces many dopants at very fine areas, acting like a machine gun on the surface. The NDNF does not have the equipment for this process, so we contract another company in order to do the work. For these steps, the photoresist is

Figure 3: Plasmatherm RIE and Wafer Loadlock

still left on the wafer as a mask to prevent stray dopants from disrupting other devicesâ&#x20AC;&#x2122; electrical conductivity. Sometimes, another lithography will be needed before an implantation is done. In these cases, a Plasmatherm device called the PVA can strip off all organic photoresist compound without affecting the etched substrate. Eventually, dopants need to be activated with an oxidation furnace. Oxidation furnaces are large devices that range from 400 C to 1200 C, depending upon process, and they are used to drive in dopants, activate implantations, grow silicon dioxide layers, and anneal metal to the wafer before packaging. Our oxidation furnaces required that we did an RCA clean before entering the wafers, as the furnace tubes were the cleanest environments in the lab. These oxidations usually took a total time of between two and seven hours, which meant many late nights in the lab. However, implant activation was perhaps the most important step of our process, defining the electrical characteristics. At the end, after close to a dozen lithographies and many other steps, we were left with sound chips that encoded

Figure 4: Plasma in RIE Chamber Window

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TECH ARTICLE

Figure 5: Alignment Marks of 100 mm Wafer, 10x Magnification the Notre Dame Fight song. Each die also contained a few other devices; Ohmic contacts, ring oscillators, and inverters. We used a four-point probe system to test each device before continuing to dice and package the wafers. Figure 5 shows a shot of one of our wafers under 10x magnification. While university fabrication labs are very different in their design and goals than a commercial lab, I feel I am very fortunate to have had the opportunity to work in one nonetheless. As a semiconductor and nanotechnologyfocused electrical engineering student, classes such as IC fabrication tie my major together for me in a way that other classes cannot. Seeing a working inverter or sound chip at the end of the day is an ecstatic feeling that brings together many hours of obscure learning and careful processing. Hopefully, my experiences in the Notre Dame Nanofabrication Facility can help in your own engineering edification.

About the Author Alex Toombs is a senior electrical engineering at the University of Notre Dame, concentrating in semiconductor devices and nanotechnology. His academic, professional and research experiences have exposed him to a wide variety of fields; from financial analysis to semiconductor device design; from quantum mechanics to Android application development; and from low-cost biology tool design to audio technology. Following his graduation in May 2013, he will be joining the cloud startup Apcera as a Software Engineer.

To read more articles by this author or to comment on this article, visit his EEWeb Profile.

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A's Ames Research Center talk about their work ology, the benefits of vacuum transistors over solid urprising new applications for this technology.

yappan Woo Han

In the past, vacuum tubes were widely used for amplifying electrical signals, but have long been replaced by other devices such as MOSFETs, which require lower power and less cost to fabricate. Researchers Jin-Woo Han, M. Meyyappan, and Jae Sub Oh however, last year completed research in which they were able to use silicon technology to create a gate insulated vacuum channel transistor, which offers higher frequency and power than other commonly used devices. You can read about their research in vacuum nanoelectronics in their article: Vacuum Nanoelectronics: Back to the Future? I interviewed M. Meyyappan and Jin-Woo Han from NASAâ&#x20AC;&#x2122;s Ames Research Center about their research, and we discussed the superiority of vacuum tubes, possible applications of their nanoscale vacuum transistor, and the challenges that stand between the technology and widespread market availability. Visit www.eeweb.com

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Superior Technology: Higher Speeds, Power, and Frequency There are several ways in which vacuum tubes could be used to create superior devices. Jin Woo Han explained that in widely-used silicon technology, electrons have to travel through a silicon lattice, which puts a limitation on how fast electrons can travel, as there are obstacles in the path of the electrons that cause them to bounce back and forth. In a vacuum however, such obstacles do not exist, and higher speeds, power, and frequencies can be reached. By making everything out of silicon except for the silicon-conducting channel, which would be left as a vacuum, the researchers were able to have the best of both worlds — they eliminated the limitation and the speed of electrons, but kept costs down with silicon technology.

Radiation Immunity

that they won’t get destroyed by radiation… and the same thing goes for the military. The distance the military goes is smaller, but they also have different types of radiation, some in common with NASA, to worry about.” “Spending a lot of time packaging and preparing the device that you and I have access to already and putting it in space is not only time-consuming,” he went on, “it is also an expensive proposition. That’s why military electronics and space electronics are far more expensive than the electronics that everybody else buys.” However, he continued, because there is no semiconductor in a vacuum transistor, and no charge transport (which would be detrimental to device operation under conditions in space) the vacuum transistor is “inherently immune to space radiation.” Essentially that means that, with a vacuum transistor, NASA and the military would be able to use chips and new technology when everyone else does, rather than having to “wait 5 more years to package it, and then send it up.”

In a vacuum an electron shoots from one place to another, and the electrons travel at a much, much higher speed. What that means for telecommunications is higher frequency and higher bandwidth.

Another way in which vacuum tubes are superior to other technologies, M. Meyyappan explained, is that they are unaffected by radiation. This means that vacuum tube transistors could be very beneficial to both space and military industries. “It’s no secret,” he said, “that anything that is used in space is a few generations — two, three, or even four generations — behind the electronics that you and I buy. That is because it takes many years for the electronics to be space-qualified; they have to be packaged in such a way

The vacuum transistor would also be immune to high temperatures, and could be put close to things like a jet engine, which is very hard to do with current technology. Because of the higher speeds and frequencies that can be achieved, the vacuum transistor could also have telecommunications applications, Jin Woo Han explained.

A vacuum transistor (left) vs. a solid transistor (right)

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Vacuum Tubes. Photo Credit: Stefan Riepl

Applications in Telecommunications “Electrons can go at pretty much an unlimited speed in a vacuum compared to silicon,” M. Meyyappan continued. “In silicon there is a lattice like a matrix, and so it’s like watching a pinball machine. The electron gets bounced around because it hits a few obstacles — like in a pinball machine. For that reason it’s slower than a vacuum, which is not like a pinball machine. In a vacuum an electron shoots from one place to another, and the electrons travel at a much, much higher speed. What that means for telecommunications is higher frequency and higher bandwidth.”

Challenges I asked what challenges stand in the way of market-scale production of nanoscale vacuum tubes, and M. Meyyappan explained that lifetime testing of the devices still is yet to be done, and also a 95% production yield must be met (which means only 5% of wafers would be discarded) before anyone would consider large-scale production. “It’s not like we have demonstrated industrial scale,” he said, “We have fabricated on a wafer… but the expectation these days is a 95% yield and nobody has even tried on that scale.” However, because the technology is silicon-based, industrial scale manufacturing is very doable. “There is not a whole lot to invent,” he said. “We are not talking about fancy new materials like carbon nanotubes, where you have to learn something — everything is still made in

a silicon fab, using a silicon-fab line, and using silicon processing. In that sense, it is not anything very hard.”

Unconventional Uses & the Forbidden Gap Not only could vacuum tubes revolutionize military and space electronics, but there could be other, less conventional, uses, Jin Woo Han pointed out. If you could utilize a vacuum nanoelectronic array, for instance, you could analyze the performance of electrons and “utilize [the device] as a gas sensor or a gas detector.” What that means, is the possibility of completely unconventional applications. Because the device operates at a terahertz frequency (between 1 terahertz and 30 terahertz) at what is called the forbidden frequency, and because there are no commercial devices that operate at that frequency, the device could have many unconventional applications. “Since the vacuum nanoelectronics and vacuum nanotransistors can perform at a terahertz frequency,” M. Meyyappan said, “[they]could be used for things like spectroscopy and imaging, and could be a replacement for the kind of x-ray-based scanning (as seen in airport security systems) that have negative connotations in terms of health and so forth. You wouldn’t have to worry about that with the terahertz frequency, and it could also be used for identifying drugs, contrabands, and those kinds of things. There are lots of unconventional applications for this type of device.” ■

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