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Mahboobin 10:00 L04 Disc laimer—This paper partially fulfills a writing requirement for first year (freshman) engineering students at the University of Pittsburgh Swanson School of Engineering. This paper is a student, not a professional, paper. This paper is based on publicly available information and may not provide complete analyses of all relevant data. If this paper is used for any purpose than these authors’ partial fulfillment of a writing requirement for first year (freshman) engineering students at the University of Pittsburgh Swanson School of Engineering, the user does so at his or her own risk. REPLACING SILICON: CARBON NANOTUBE TRANSISTORS Dillon Axarlis ([email protected]) CARBON NANOTUBES AS A REPLACEMENT FOR SILICON Carbon Nanotubes are small cylinders made up of graphite with a wall width of one atom. Many uses for carbon nanotubes are being researched, but the primary one is to replace silicon as a transistor in computer chips. Silicon is quickly becoming outdated and it will soon be at the limits of its performance. Replacing silicon transistors with carbon nanotubes will lead to faster more efficient computers that require less energy and as a result are better for the environment. Carbon nanotube transistors will also help create more powerful supercomputers to help scientists combat society’s everyday problems. However, there are some problems with carbon nanotubes that up until recently needed to be solved. Even after these problems are solved, a way to mass produce carbon nanotubes will have to be found. Despite these challenges, it is imperative that carbon nanotubes replace silicon as a material in transistors. THE LIMITS OF SILICON Moore’s Law is a function that dictates the speed at which an affordable CPU increases in power. Created by CoFounder of Intel Gordon Moore in 1965, Moore’s Law has held out for 50 years with one revision by Moore himself in 1975 changing the time limit from one year to every two years. In its current form, Moore’s Law states that the number of transistors in an affordable CPU will double every two years [1]. The problem with the computer industry is, however, that Moore’s Law is dying. As computers keep getting faster and faster, the limit of Moore’s Law creeps closer and closer. In order to double the number of transistors on a chip the size of the transistor has to decrease at an equal rate. Current CPU manufactures are running into problems manufacturing these transistors however. In wake of delaying its latest chipset from 2016 too late 2017, Intel has effectively killed Moore’s Law. Furthermore, Intel has declared that improvements in current transistor design can only continue for 5 more years. The problem is that current transistors are made up of silicon [2]. Although silicon has been at the forefront of transistor design, As transistor size decreases, electron mobility decreases. This causes an increased power consumption and a larger chance of faulty transistors. In order to continue the development of CPUs a new material University of Pittsburgh, Swanson School of Engineering 1 10.04.2016 with a higher electron mobility must be found and scientists have less than five years to find it [3]. One of these proposed materials is carbon nanotubes. Carbon nanotubes have many advantages over silicon with the major benefit being size [4]. WHAT ARE CARBON NANOTUBES? What exactly are carbon nanotubes? Carbon nanotubes are a cylindrical fiber of pure carbon graphite. These nanotubes are extremely small at a diameter of 0.7-50 nm. Comparatively, the current silicon transistor is 14nm and the projected minimum for a silicon transistor is 5nm. Carbon nanotubes have other advantages over silicon. These include the fact that the hexagonal pattern of bonded carbon atoms creates a phenomenon known as electron delocalization. This means that electrons can flow freely through carbon nanotubes with no impedance. This solves the electron mobility problems of silicon transistor. Carbon nanotubes high electron mobility also means that CPUs utilizing them will require less voltage resulting in a computer will have much lower power consumption and generate less heat [5]. Although carbon nanotubes may sound wonderful, there are many road blocks engineers face before they can be used in the cheap affordable CPUs listed under Moore’s Law. THE CHALLENGES WITH CARBON NANOTUBES In order to make carbon nanotubes a viable replacement for silicon the challenges with transforming the nanotubes to transistors must be overcome. Development on carbon nanotubes began over 25 years ago with a paper written by Sumio IIjima in 1991. If carbon nanotubes have been out for so long than why haven’t they replaced silicon yet? This boils down to three problems with making a carbon nanotube a transistor. These problems are a mixture problem involving the impurities of carbon nanotubes, an electrical resistance problem that occurs when trying to interface carbon nanotubes with regular conductors, and an alignment problem when trying to place an arrangement of carbon nanotubes on a computer chip [6]. The Mixture Problem When carbon nanotubes are manufactured two types of carbon nanotubes are produced in a seemingly unavoidable mixture. The problem with this is that one of the two types of carbon nanotubes created is not suitable for the production of transistors. The two types of carbon nanotubes created are a metallic carbon nanotube and a semi conductive carbon nanotube. Fortunately, as of April of 2015, a small team at the University of Illinois has devised a way of separating the metallic carbon nanotubes from the semi conductive ones. The process laying out carbon nanotubes of a sheet of metal. Than the carbon nanotubes were covered with an organic material. Because the metallic carbon nanotubes conduct electricity, when the sheet was charged with an electric current, the metallic carbon nanotubes heated up and burned of the organic material. This caused only the metallic carbon nanotubes to be exposed to an etching compound that destroyed the metallic nanotubes leaving only pure semiconducting carbon nanotubes behind. The method devised by the University of Illinois is relatively inexpensive making it perfect for the large scale manufacturing of carbon nanotube transistors [6]. The Electrical Resistance Problem With the problem of separating carbon nanotubes solved scientists moved onto the problem of getting carbon nanotubes to interface with other materials. It would be the researchers at IBM who would figure out how to solve this problem. A problem with too much electrical resistance and carbon nanotubes may seem confusing. Nanotubes by themselves have amazing electrical properties. However, it is when the metallic contacts required to connect the transistors are added to a large electrical resistance occurs. This resistance makes traditional carbon nanotube transistors slower than their silicon counterparts. IBM found a solution to this problem by changing the orientation of a nanotube transistor. Traditionally the contacts for a nanotube transistor are attached to the top of the nanotube. By attaching the contacts to the ends rather than the ends of the carbon nanotube, IBM found that they can negate the electrical resistance issue found on nanotube transistors smaller than 10nm. This method allows transistors to be made as small as the technology limits without having to worry about the contact size. It also allows IBM to make many transistors out of one carbon nanotube further decreasing chip size. With this problem solved, IBM believes that it is on track to deliver carbon nanotube based computers by 2020 [7]. However, in order to mass produce carbon nanotube based CPUs the transistors must be able to be efficiently arranged on a computer chip. The Arrangement Problem With the ability to make carbon nanotube transistors made up of pure semiconducting nanotubes, scientists and engineers focused on arranging the nanotube transistors onto a single wafer. In order to do this the University of Wisconsin developed a process called dose-controlled, floating evaporative self-assembly [6]. This process involves placing thin strips of carbon nanotubes on a substrate and submerging it is a solvent. The substrate is the vertically removed from the solvent and the substrate is baked to evaporate of any remaining solvent. This process results in a densely packed group of carbon nanotubes [8]. University of Wisconsin’s carbon nanotube transistors performed flawlessly and was able to achieve current that was 1.9 times that of its silicon cousin. For the first time, carbon nanotubes outperformed silicon and the had done it by an impressive margin. However, the process that the University of Wisconsin used to arrange the transistors is not ready for mass production. Until a way to mass produce these carbon nanotube transistors for use in integrated circuits and CPUs, carbon nanotubes will not be available for both commercial and consumer use [9]. WHAY ARE CARBON NANOTUBE TRANSISTORS IMPORTANT? Although carbon nanotubes are not yet able to be mass produced, carbon nanotubes are important for the future of society. On the small scale, carbon nanotubes lead to improvements of our everyday lives. Carbon nanotubes promise faster more energy efficient computers, cell phones and electronic devices. Carbon nanotube transistors are a huge leap forward in the green initiative. As the human race becomes more dependent on computers, more and more energy is used to support these devices. By creating electronic devices with longer battery lives and less power consumption we reduce our dependence on power plants that burn fossil fuels. More efficient computers also help extend the battery life on computer heavy electric cars such as the Tesla Model S [9]. Another strong benefit of Carbon nanotubes is their use in the high power computing industry. Some of the main uses of supercomputers are to understand climate change, develop better batteries and super conductors, and improve drugs and medications. With the current technology, the power of supercomputers barley increases as the years go by. With carbon nanotubes more powerful super computers can be made to help increase the speed at which scientists can discover new ways to combat climate change [2]. With few alternatives to silicon there is a distinct need for carbon nanotubes to take its place in circuit chips. The time on the clock is ticking as silicon is expected to reach its limit by 2019 [6]. Fortunately, IBM thinks that it can commercially produce carbon nanotube transistors by 2020 [7]. CARBON NANOTUBES WILL CHANGE THE FUTURE Silicon is becoming outdated and is an inefficient way to tackle the problems of the future. Carbon nanotubes are its most prominent replacement. The challenges that go along with replacing silicon with carbon nanotubes are being overcome as we speak and it is just a matter of developing a way to mass produce carbon nanotube transistors. By developing carbon nanotubes, we create faster more efficient computers that help us solve modern problems and lead to a better cleaner earth. SOURCES [1] “Moore’s Law.” MemeBridge Accessed 10.30.16 http://www.mooreslaw.org/ [2] T. Simonite. “Moore’s Law Is Dead. Now What?” MIT Technology Review. 5.13.16. Accessed 10.30.16. https://www.technologyreview.com/s/601441/moores-lawis-dead-now-what/ [3] S. Anthony “7nm, 5nm, 3nm: The new materials and transistors that will take us to the limits of Moore’s law.” ExtremeTech. 7.26.13 Accessed 10.30.16 https://www.extremetech.com/computing/162376-7nm5nm-3nm-the-new-materials-and-transistors-that-will-takeus-to-the-limits-of-moores-law [4] L. Wood. “Carbon nanotubes in a race against time to replace silicon.” ComputerWorld 11.10.15. Accessed 10.30.16. http://www.computerworld.com/article/3002260/emergingtechnology/carbon-nanotubes-in-a-race-against-time-toreplace-silicon.html [5] “What Are Carbon Nanotubes?” Nanocomp Technologies, INC. Accessed 10.30.16 http://www.nanocomptech.com/what-are-carbon-nanotubes [6] K. Murane. “Carbon Nanotubes Are Getting Closer To Making Our Electronic Devices Obsolete.” Forbes 9.8.16 Accessed 10.30.16. http://www.forbes.com/sites/kevinmurnane/2016/09/08/carb on-nanotubes-are-getting-closer-to-making-our-electronicdevices-obsolete/#26a2d40943bb [7] M. Orcutt. “IBM Reports Breakthrough on Carbon Nanotube Transistors.” MIT Technology Review. 10.1.15 Accessed 10.30.16 https://www.technologyreview.com/s/541921/ibm-reportsbreakthrough-on-carbon-nanotube-transistors/ [8] Y. Joo, G. Brady, M. Arnold, et al. “Dose-Controlled, Floating Evaporative Self-assembly and Alighnment of Semiconducting Carbon Nanotubes from Organic Solvents.” ACS Publications 2014 pp. 1-7 [9] J. Fimgas. “Carbon nanotube transistors promise faster, leaner processors.” Engadget. 9.5.16 Accessed 10.30.16 ACKNOWLEDGMENTS This paper is inspired by a man who I only know as Demonseed Elite who first introduced me to the topic of carbon nanotubes and helped facilitate my interest in the world of modern computing solutions.