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EEWeb Pulse - Issue 91

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Interview with David Gascón – Co-Founder and CTO of Libelium; Plessey -The Future of Integrated Circuit Sensors; University Nanofabrication Facility - A High-Level Overview; Vacuum Transistors - Nanotechnology for the 21st Century; RTZ – Return to Zero Comic
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  • FEATURED ARTICLE

  • EEWeb PULSE TABLE OF CONTENTS

    3Visit www.eeweb.com

    David Gascn CO-FOUNDER & CTO OF LIBELIUM

    A conversation with the CTO of Libelium about the company's award-winning sensor technology and how it's creating a vision for "Smart Cities" of the future.

    An in-depth look at the integrated circuit fabrication process at the University of Notre Dame's Nanofabrication Facility.

    RTZ - Return to Zero Comic

    Featured Products

    4

    13

    20

    28

    University Nanofabrication Facility:

    Vacuum Transistors: Nanotechnology for

    32

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

    14Plessey Semiconductors: The Future of

    BY ALEX TOOMBS WITH EEWEB

    Integrated Circuit Sensors

    A High-Level Overview

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

    the 21st Century

  • EEWeb PULSE INTERVIEW

    4 EEWeb | Electrical Engineering Community

    DavidGascon

  • EEWeb PULSE INTERVIEW

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    DavidGasconLibelium is a wireless sensor company based in Zaragoza,

    Spain. The companys award-winning sensor technology has been implemented in multiple Smart City applica-

    tions across the globe. We spoke with David Gascn, 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.

  • EEWeb PULSE INTERVIEW

    6 EEWeb | Electrical Engineering Community

    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 investmentsit 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 didnt 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 Waspmotethe sensor platform. One of the things that differentiates this plat-form from the rest is that its a platform with an open-source API so that devel-opers could easily make modifications to the librar-ies in order to implement it in virtually any applica-tion. We always say that this is a plat-form for develop-ers, and in fact, you can connect your own sensors so that people can create an end product

    very easily. We say that Waspmote is a platform to help other companies create end products because Wasp-mote isnt a vertical platform.

    The other very important thing about Waspmote is the modularity. Nor-mally, when you are working with a sensor you say, Okay, Im going to buy a smart sensor platform and Im going to use Zigbee or Bluetooth. What we made with Waspmote was a platform that you could add the sen-sors 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 es-sentially Waspmote inside, but with a robust waterproof enclosure and with more than 60 different sensor probes available to connect. Its really a vertical product. This is for companies who dont need to develop their own products, they just need to use a final product that is easy to maintain and install. Its a great plat-form for companies that dont 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 develop-ment of the Plug &

    Sense, one of the things that we had

    in mind was to implement

    a technique called OTA (over the air program-

    ming), which allows developers to program the sensor nodes from the

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    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 gate-way 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 activitynot 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 citizensin 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 pollutionCO2 and NO2 levels

    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

    A Waspmote unit implemented in a Smart City

    Libelium's sensors expand the limits of the Internet so

    that everything is connected through

    sensor networks.

  • EEWeb PULSE INTERVIEW

    8 EEWeb | Electrical Engineering Community

    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 medium-sized city, which is based in between Madrid and Barcelona, so its really well connected. All of the people that work here are relatively young. My partner, Alicia Asnthe other co-founder of the companyis just 30 years old. The rest of the employees ages range from 25 to 35. Its 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 manufacturingit'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 workingthere is a lot of strength and movement, and there are a lot of dynamic people that work here. Its a small company, but we move quickly.

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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 cant make these kinds of proprietary protocols because if you cant talk to a web server and you dont 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

  • EEWeb PULSE INTERVIEW

    11Visit www.eeweb.comCopyright 2013, Silicon Frameworks, LLC

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  • EEWeb PULSE SPECIAL FEATURE

    14 EEWeb | Electrical Engineering Community

    The Future ofIntegrated Circuit

    Sensors

  • EEWeb PULSE SPECIAL FEATURE

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    The Future ofIntegrated 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.

  • EEWeb PULSE SPECIAL FEATURE

    16 EEWeb | Electrical Engineering Community

    After Plesseys 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.

    activity of the heart usually attained through a resistive electrode attached to the surface of the skin. Plesseys 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 Plesseys 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 in the electric fields in a room. Along similar lines, gesture rec-ognition is anoth-er area in which Plessey has been making some breakthroughs in. Through an array of EPIC sensors one can pick up electric field dis-ruptions being made by 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.

    EPIC Technology

    With these characteristics, the sensors are able to mea-sure the minutest change in an electric field. Theres an electric field all around us, Strickland states, which does vary considerably, but on average, its 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 youre touching the sensor it can sense your bodys electrophysiology signals like your ECG from your heart. The ECG (ElectroCardioGram) is a signal of electrical

    Applications

    Everytime your heart beats, it emits an elec-trical pulse in order to pump blood. The ECG reads this signal to measure your heart-beat and heart health.

    Heart Chart

    Plesseys EPIC sensor is so good at detecting elec-trical disruptions that it can provide an effective, qual-ity ECG reading

    from merely hold-ing two sensors across the chest

    over your heart, or even from simply holding a sensor

    in each hand.

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    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 oppor-tunity to provide an integration of multi sensing systems. Plesseys foundry facilities are available to its customers to design CMOS image sensors; its 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.

    Plesseys other goal is to integrate such sensor technology into their LED businesssometime 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

    Plesseys Future

    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 Plesseys website for application notes, videos, demonstration kits and more:

    Plesseys hand-held ECG monitor utilizes a unique

    disruptive electric potential sensing

    technology to detect an ECG signal.

    www.plesseysemiconductors.com

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  • EEWeb PULSE TECH ARTICLE

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    Alex ToombsElectrical EngineeringStudent

    Fabrication Lab:High-Level Overview

    A University

    An in-depth look at the IC fabrication process at the University of Notre Dame's Nanofabrication Facility.

    An in-depth look at the IC fabrication process at the University of Notre Dame's Nanofabrication Facility.

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    Alex ToombsElectrical EngineeringStudent

    Fabrication Lab:High-Level Overview

    A University

    An in-depth look at the IC fabrication process at the University of Notre Dame's Nanofabrication Facility.

    An in-depth look at the IC fabrication process at the University of Notre Dame's Nanofabrication Facility.

  • EEWeb PULSE TECH ARTICLE

    22 EEWeb | Electrical Engineering Community

    Integrated 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 Fab-

    rication course, taught by Dr. Greg Snider, are allowed access to the $25 mil-lion Notre Dame Nanofabrication Facility. The Fa-cility offers equip-ment for almost every step of the process required to produce sound chips that encode 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. Depend-ing upon the dirtiness 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, fol-lowed 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 concen-trated 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 lithog-raphy, 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

    Entering a clean room can be a daunting experience, and intimidat-ing in the level of preparation it requires.

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    Figure 1: Wafer Box, 100 mm Silicon Wafers Figure 2: NDNF Photoresist Spinner, 100 mm Wafers

    NDNF photoresist spinners is shown in Figure 2. The red streaks in the bowl of the photore-sist spinner are the leftover resist that spins off the wafer, requiring acetone to clean off.

    After the photoresist is spun and baked upon the wafer, the wafer must be exposed to the pattern de-signed for that layer of the device. Many devices require upwards of six or seven layers, meaning that the process must be repeated each time, and that a new mask must be designed. The NDNF is fortu-nate enough to have a commer-cial stepper, which automatically can expose wafers once they are manually loaded. Sometimes, align-ments must be adjusted manually to ensure that the wafer is aligned with the maskwithin a tolerance of several microns. Because of this, our first lithography was designed to etch only alignment marks onto each die.

    After the wafer has been exposed by the stepper, photo-resist remains only on certain areas of the wafer, with patterns etched down to the substrate where light hit the wafer. This is so that patterns may be etched into the substrate, ensuring that any patterns in the photoresist can be transferred permanently, constructing the pat-terns that eventually constitute the device. Etching is a blanket term for a process that can use acids like HF, reactive ion etching (RIE), or other methods in order to selec-tively remove parts of the substrate. Our lab most often uses RIE, in a loadlocked device from Plasma-therm. The RIE and its loadlock are pictured in Figure 3.

    RIE processes use several toxic gases, such as sulfur hexafluoride,

    in a pressurized container in order to selectively remove areas of the wafer. The photoresist mask is kept on in

    Any particles that are on the wafer

    can destroy imaged features on the

    wafer or introduce dopants where

    unintended, alter-ing the electrical characteristics

    of the devices un-intentionally.

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    24 EEWeb | Electrical Engineering Community

    order to ensure that only the areas desired are actually etched. Still, due to the high selectivity of the gases, pro-cesses 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 implanta-tion. 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 Figure 4: Plasma in RIE Chamber Window

    still left on the wafer as a mask to prevent stray dopants from disrupting other devices 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

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    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 nanotechnology-focused 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 AuthorAlex 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 semiconduc-tor 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 com-ment on this article, visit his EEWeb Profile.

    Figure 5: Alignment Marks of 100 mm Wafer, 10x Magnification

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    28 EEWeb | Electrical Engineering Community

    Interview withM. Meyyappanand Jin Woo Han

    Nanoelectronics:Vacuum

    Karen KohtzAssistant Editor, EEWeb

    Researchers from NASA's Ames Research Center talk about their work with vacuum nanotechnology, the benefits of vacuum transistors over solid transistors, and some surprising new applications for this technology.

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    29Visit www.eeweb.com

    Interview withM. Meyyappanand Jin Woo Han

    Nanoelectronics:Vacuum

    Karen KohtzAssistant Editor, EEWeb

    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 NASAs 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.

    Researchers from NASA's Ames Research Center talk about their work with vacuum nanotechnology, the benefits of vacuum transistors over solid transistors, and some surprising new applications for this technology.

  • EEWeb PULSE TECH ARTICLE

    30 EEWeb | Electrical Engineering Community

    Superior Technology: Higher Speeds, Power, and FrequencyThere 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 ImmunityAnother way in which vacuum tubes are superior to other technologies, M. Meyyappan explained, is that they are unaf-fected by radiation. This means that vacuum tube transistors could be very beneficial to both space and military industries. Its no secret, he said, that anything that is used in space is a few generations two, three, or even four genera-tions behind the electron-ics 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

    that they wont 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 put-ting it in space is not only time-consuming, he went on, it is also an expensive proposition. Thats why military electronics and space electronics are far more expensive than the electronics that everybody else buys.

    However, he continued, because there is no semicon-ductor in a vacuum transistor, and no charge transport

    (which would be detrimental to device operation under conditions in space) the vacuum transistor is inher-ently immune to space radia-tion. Essentially that means that, with a vacuum transis-tor, 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.

    The vacuum transistor would also be immune to high tem-peratures, and could be put close to things like a jet engine, which is very hard to do with current technol-ogy. Because of the higher speeds and frequencies that

    can be achieved, the vacuum transistor could also have telecommunications applications, Jin Woo Han explained.

    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 telecommuni-cations is higher

    frequency and higher bandwidth.

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

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    Applications in TelecommunicationsElectrons can go at pretty much an unlimited speed in a vacuum compared to silicon, M. Meyyappan con-tinued. In silicon there is a lattice like a matrix, and so its like watching a pinball machine. The electron gets bounced around because it hits a few obstacles like in a pinball machine. For that reason its 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.

    ChallengesI asked what challenges stand in the way of market-scale production of nanoscale vacuum tubes, and M. Meyyap-pan 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. Its not like we have demonstrated industrial scale, he said, We have fabricated on a wafer but the expecta-tion these days is a 95% yield and nobody has even tried on that scale.

    However, because the technology is silicon-based, in-dustrial 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 GapNot only could vacuum tubes revolutionize military and space electronics, but there could be other, less conven-tional, 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 uncon-ventional 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 unconven-tional 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 con-notations in terms of health and so forth. You wouldnt 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.

    Vacuum Tubes. Photo Credit: Stefan Riepl

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