Professor Cogedell Des40A
Raw Materials of CPU
CPU is the abbreviation of Central Processing Unit. It is the electronic circuitry which carries out the instruction of programs. The basic raw materials used in a CPU are silicon, copper, aluminum, and various plastics. Some precious metal like gold and silver are also used for creating wires. Since CPUs are largely consumed in modern society, it is important for producers to consider energy inputs and environmental impacts of raw materials.
Silicon is the second most abundant element in the earth’s crust. It exists as silicon dioxide and as silicates. The Silicon dioxide appears as a major component in sand, quartz, and rock. Industry silicon is produced by carbothermic reaction processes. Silicon dioxide is reduced to elemental silicon by carbon under high temperature (~2000°C). Usually, silicon dioxide is provided by high-grade quartz. Carbon is provided by charcoal, petroleum coke, coal, and wood chips—or a blend depending on economic applicability. The purification occurs in a furnace which produces silicon with more than 90% purification.
The U.S. produced 410,000 tons of silicon in 2015 accounting for 50.62% of global production. The total silicon production includes 53%  ferrosilicon and 47%  pure silicon. The total estimated value is $1.14 billion . In the U.S, the majority of silicon is produced east of the Mississippi River. The U.S has several large silicon manufacturing companies, such as Dow Corning Corp, Globe Metallurgical, Inc. (GMI), and WVA Manufacturing, LLC. These producers also have manufacturing facilities in China, Argentina and South Africa. The world production of silicon metal was 8,100 tons in 2015. Of all of these, China is the largest producer. It produced 5,500 tons of silicon in 2015, accounting for 67.9% of global estimated production. Based on production weight, Russia is the second large producer followed by the United States, Norway, and then Brazil. In 2014, the U.S imported 270,000 tons of ferrosilicon and 141000 tons of metal silicon. China and Brazil are the leading sources of U.S silicon importation.
Silicon is the most important material used on CPU for its semiconducting performance. Pure silicon insulates because the four electrons in its outer orbital are bonded with covalent bonds. Electrons are tightly held and equally shared by the participating atoms so they cannot move freely to conduct electricity. However, if a silicon crystal is added with a little impurity, such as arsenic, boron, and phosphorus, its conductivity will be improved significantly; this process is called doping.
Doping transfers an insulated silicon crystal into a conductor. According to the kind of impurity, doping is classified into N-type doping and P-type doping. In n-type doping, elements with five outer electrons are added such as: arsenic, phosphorous, or antimony. When they are bonded with silicon’s four outer electrons, one extra outer electron is left free. Hence, electricity is conducted by negative charge. In p-type doping, gallium, boron, or aluminum are added. One of the atoms in the semiconductor lattice is replaced by an element with three valence electrons, which creates a hole. By accepting an electron from a neighboring atom, the hole can be passed, creating positive current. By combining n-type silicon and p-type silicon, a diode is formed, and a transistor is created by three silicon layers as an NPN or PNP order.
A silicon wafer is produced by Czochralski’s process. A silicon seed crystal is put in molten silicon. The melted mixture starts cooling and crystals begin to grow around the seed. Meanwhile, the seed is extracted and rotated in order to make temperature uniform. “Layers were grown by interdiffusion process between deposited atoms and substrates. When the appropriate substrates and deposition species are used, the silicides consist of smaller domains for all layers examined, and the shape and size of the domains were much affected by the silicide materials and their growth processes.” A circular shaped silicon crystal, which is also called ingot, is grown this way. The ingot is then sliced into wafers. The wafers are then ground and polished to reach flat, mirror like surfaces for depositing tiny copper wires. Usually, this process is not achieved by a CPU manufacturer. Instead, they buy wafers from other companies and create CPUs from them.
The wafer first goes through a process called photolithography. First, a layer of silicon dioxide is laid down on top of the wafer. Then, parts of the wafer are poured with photoresists which are chemicals sensitive to ultraviolet light, and then spun at a high speed to produce layers. A designed image of the mask is put on each layer to protect certain parts of it from the etching and ion implantation. The etching process creates binary patterns for transistors and the ion implantation forms circuits on the wafers. Then, the photo resist material is removed by a washing process. Lithography processes are largely needed in CPU production. It typically accounts for about 30  percent of the cost of manufacturing. And it determines a chip’s transistor speed and size for further machining. The photolithography process requires a lot of energy and toxic chemical reagents. Hydrochloric acid, sodium hydroxide, acetic acid, and nitric acid are used for etching and cleaning wafers.
Then, the wafer is doped to be a transistor. Copper is filled into holes etched on the top insulation layer. Tiny copper wires are built in order to transfer electricity and finally the wafer is tested. “This level of testing includes not only traditional computational abilities, but also internal diagnostics along with voltage readings, cascade sequences (does data flow through as it should), etc.” After being tested, the wafer is then cut up into its individual chips. Individual chips are then packaged into plastic shells. “The package has functioned as both a carrier and as an enclosure.” This completes the manufacturing of a CPU. Some steps in the process cause environmental pollution. “The main environmental impact source evaluated by the IMPACT 2002 method is the etching process, following by thin film, diffusion, photolithography, and CMP processes.”
Copper is largely used in modern industry for its high thermal and electrical conductivity. Copper is usually found in nature as sulfate and oxide. Copper ores contain between 0.5 and 2.0 percent copper . According to Mineral Commodity Summaries published by U.S. Geological Survey, the world total copper production was 18,700,000 tons in 2014. Chile is the largest copper producing country followed by China. The United States produced 1,370,000 tons of copper in this year. 180,000  tons have been recycled from old scrap—equivalent to 10% of consumption. 590,000 tons of copper have been imported with 51% from Chile.
The copper ore is ground, concentrated, and slurried with water and chemical reagents.  Then, the concentrated copper is refined by pyrometallurgy or hydrometallurgy methods depending on the ore type and other economic reasons. Oxygen, iron, sulfuric acid, silica, and various organic compounds are chemicals which are often involved in copper refining. Waste products include tailings and slag. They contain lead, arsenic, and other toxic chemicals. If the wastes are not dealt with properly, they may cause health hazard to the neighboring communities. Copper processing produces air emissions such as sulfur dioxide and particulates containing iron, copper oxides and sulfates. The treatment methods are capturing particulates by adopting emissions control equipment and converting sulfur dioxide into sulfuric acid by single stage electrostatic precipitation.
Plastics are materials composing CPU packages and are used in insulating wires. Plastics are produced by oil and natural gas. First, crude oil or natural gas is converted to ethylene and propylene. Then through further processes, small hydrocarbon monomers are chemically bonded to form a long chain. Plastics are thermal and electrical insulators with high resistant to chemical corrosion. Hence, they are used to coat wires and crust chips in case of damage. Plastic waste is a global challenge. Lots of plastic waste is poured into oceans. As time pass by, plastics are broken into microplastics which are harmful to marine organisms. They are not biodegradable, so they may enter the food chain and be absorbed by human beings. What is more concerning is the large amount of plastic waste being dumped into landfills without any raw materials or energy being recycled.
In conclusion, a CPU is composed mostly of silicon, cooper, and plastics. In the U.S silicon recycling is nonexistent due to high labor cost. However, the highest energy expenditure happens during silicon purification, which means most energy is wasted once a CPU is scrapped. It’s important for a CPU designer to consider the cost of recycling the materials so that there is an economic incentive to do so. Otherwise companies will favor simply throwing away the materials.
 Emily K. Schnebele, “Silicon” 2014 Minerals Yearbook U.S. Department of the Interior U.S. Geological Survey, 2014, 1-3.
 Emily K. Schnebele, “Silicon” 2016 Minerals Yearbook U.S. Department of the Interior U.S. Geological Survey, 2016, 150-151.
 Greig, William J. Integrated Circuit Packaging, Assembly and Interconnections. New York: Springer, 2007. Print.
 “Copper mining” schoolscience Research Councils UK, March 6, 2016, <http://resources.schoolscience.co.uk/CDA/14-16/cumining/index.html>
 Mark Brininstool, “Copper” 2015 Minerals Yearbook U.S. Department of the Interior U.S. Geological Survey, 2015, 48-49.
 “Primary Metals” ILLINOIS SUSTAINABLE TECHNOLOGY CENTER March 6, 2016 <http://www.wmrc.uiuc.edu/main_sections/info_services/library_docs/manuals/primmetals/Chapter5.htm>
Professor Cogdell Des40A
Waste and Emissions of a CPU
Sustainable reuse and recycling of e-waste becomes more critical each year. As technology becomes more popular and accessible to the entire world, the rate of e-waste increases directly as a result. The heart of a computer is the central processing unit or CPU. Many of the products which we consider e-waste contain some sort of processor. To guard the environment from this ever increasing issue of accumulating e-waste, the potential for reuse and recycling in the lifecycle of a CPU’s waste must be met.
Of the many facets of the CPU, I will highlight the silicon input and the chemicals used in the etching process. Silicon plays the crucial role of the semiconductor in a CPU. This allows voltage to transmit within the CPU. The CPU manufacturers outsource the production of silicon ingots which will later be turned into the wafers on which transmitters are made. The specifics behind making silicon ingots is not readily known since it is a proprietary process . However, at the end of the process, an ingot of very pure silicon is produced which is then used by the CPU manufacturer to slice into wafers. Once these wafers are polished, they are ready to be etched. The etching lays the foundation for the many transistors that will be made from a single 300mm diameter wafer . The top layer of the silicon wafer is oxidized forming Silicon Dioxide which acts as an insulator. By applying an array of chemicals to partially dissolve the Silicon Dioxide, an etched out base remains for the copper ions to be deposited . The wafer is then sliced further into various dies which are ranked based on their location on the wafer. Generally the outer dies are of poorer quality while the inner dies are used for military and medical grade electronics. These individual dies are then packaged into their casing which consumers then purchase . The final product is then tested and “binned” based on its performance. The binning process is where the final clock speed and cache size is determined . While there are many variations in architecture like “x86, ARM, DRAM, SoCs, ASICS,” the difference “is found only in the wiring” while the other steps in the production of a CPU are virtually identical . This is useful to note so that we can make broader generalizations about the waste of CPUs regardless of their particular architecture.
Adapting the calculations from the “Viability of recycling semiconductors in Intel Processors” source, I can calculate the amount of silicon used in CPUs from the EPA’s 2009 survey . In 2009, 72,750,000 computers were sold in the United States (from Table 2). The high-end Intel i7 processor was released in 2009 with a die size of 263mm2 . Since the trend of processor die size is downward and generally smaller as price increases, we can use this area with confidence that our calculations for silicon quantity will be based on the smaller scale dies. Generally the thickness of a silicon wafer slice is 1-2mm . If we take the low end of this thickness and given a silicon density of 2.65g/cm3, one processor die would have about “.697g of high purity silicon” . This results in about 50,700kg of silicon per year being produced for CPUs in desktops and laptops in the US.
In IBM’s PELM (product end-of-life management) program, they were able to recycle 76% of the “IBM IT equipment put on the market in 2014”, with only 0.5% “sent to landfills or incinerated” . I will use this number along with the fact that the silicon in a CPU must undergo a series of steps to remove contamination which results in about 1.5% silicon lost to purification . Together, there is a roughly 2% loss in silicon from reclaiming and purifying a CPU’s silicon. That means 49,686kg of silicon can be reclaimed from our previously calculated value. From this calculation, recycling the silicon should be a plausible solution for CPU silicon waste. The problem is how to incentivize companies to implement this final step in the lifecycle of a CPU. It appears that Intel does not have a take-back program for recycling their processors . This is troubling since they currently have a market share of about 80% in the microprocessor market . Generally consumers don’t buy a CPU alone, unless they plan on building their own computer. Intel most likely depends on companies like Dell and HP to provide their own take-back program for the pre-built computers containing Intel CPUs [10, 11]. However, the push must be for all major companies to provide end-of-life options for their products.
The etching process requires many corrosive and hazardous chemicals that need to be properly treated before disposal. Acids like Sulfuric Acid, Hydroflouric Acid, Nitric Acid, and Phosphoric Acid are used for etching and cleaning wafers . The corrosive wastes are treated in on-site “elementary acid neutralization systems” . Once the pH adjusts to between 6 and 10, the waste can then be discharged to a public sewage facility. Hydroflouric Acid is the key component in etching away the Silicon Dioxide for the copper wiring to be placed. It must be treated in “fluoride precipitation systems” where the fluoride is separated from the water thus neutralizing the hazard and forming calcium fluoride cakes that will be disposed in landfills . This stage in the lifecycle of a CPU is where most hazardous chemicals are used. It is up to the chip manufacturer to abide by chemical regulations so that they are disposed of properly.
In the following statistics, I have included the most relevant products to the lifecycle of a CPU from the EPA’s original survey. In 2009, 38% of computers and 8% of mobile devices were recycled, up from 5% and 2% respectively in 2006 . This meager increase in recycling is made more striking when compared to the much greater increase in product sales during these years. In 2006, there was a net product sales of 224,820,000 units of those listed in Table 2. In 2009, there was a net product sales of 288,550,000 units. This means in 2006 there were 29,613,600 units recycled and in 2009 there were 44,933,000 units recycled. While almost 64 million more units were sold only about 15 million more were recycled. It seems e-waste recycling habits are not increasing to match our consumption habits. A large part of this number is due to mobile devices. Consider that these years encapsulate the beginning of an upwards trend in smartphones with the iPhone being released in 2007 . The take-away from all of this is that our recycling habits for e-waste will have to increase greatly to balance out the increasingly saturated market of electronics that contain CPUs. An updated survey has not been released by the EPA, but in the coming years, this will be an important comparison to return to so we can determine if the e-waste problem is being handled properly.
E-waste is growing at an annual rate of 5% in million metric tons (Figure 1). The majority of which comes from North America and Eastern Asia. STEP, which stands for “solving the e-waste problem”, is an organization dedicated to educating producers and the public about the growing e-waste problem and possible solutions. Their first main idea is implementing EPR (extended producer responsibility) systems. This means that the producer, instead of the consumer, is responsible for the end-of-life management of a product. STEP believes the producer should implement a take-back system which at its core aims to incentivize design with an emphasis on reuse and recycling knowing that the product will come back to the company . This will help promote their second main idea of a circular economy (Figure 2). With resource depletion as a constant threat to our collective rates of consumption, STEP predicts that as resources become scarce and prices increase, companies will have a greater incentive to recycle their products and products in the same categories . This will create a cycle that is more sustainable than the current condition.
Proper disposal of e-waste is a difficult process that cannot be achieved by consumers alone. In my research for the waste of CPUs, it was difficult to find dedicated articles to my subject. Generally the CPU is abstracted in favor of referring to computers and mobile devices. If producers can offer take-back programs for their products then society will be headed towards a sustainable circular economy where e-waste is seen as a resource rather than a waste.
 "Electronics Waste Management in the United States Through 2009." EPA. N.p., May 2011. Web. 05 Mar. 2016.
 "CPU Shack - CPU Collection Museum - How a CPU Microprocessor Is Made." CPU Shack - CPU Collection Museum - How a CPU Microprocessor Is Made. N.p., n.d. Web. 06 Mar. 2016.
 "From Sand to Hand: How a CPU Is Made | Chips | Geek.com." Geekcom. Rick Hodgin, 9 July 2009. Web. 02 Feb. 2016.
 Toepelt, Bert. "Core I7: Blazing Fast, O/C Changes." Tom's Hardware. N.p., 2 Nov. 2008. Web. 6 Mar. 2016.
 "Viability of Recycling Semiconductors in Intel Processors." - Appropedia: The Sustainability Wiki. N.p., 24 Mar. 2013. Web. 06 Mar. 2016.
 "Product Recycling Programs." IBM Product Recycling Programs. N.p., n.d. Web. 06 Mar. 2016.
 "Intel and the Environment." Intel. N.p., n.d. Web. 6 Mar. 2016.
 Kay, Roger. "Intel And AMD: The Juggernaut Vs. The Squid." Forbes. Forbes Magazine, 25 Nov. 2014. Web. 06 Mar. 2016.
 McCann, Duncan, and Annelaure Wittmann. "E-waste Prevention, Take-back System Design and Policy Approaches " Home. N.p., 13 Feb. 2015. Web. 6 Mar. 2016. (p22 and p12)
 "Dell Recycling." Dell. N.p., n.d. Web. 07 Mar. 2016. <http://www.dell.com/learn/us/en/uscorp1/dell-environment-recycling?s=corp>.
 "Product Return and Recycling | HP® Official Site." Product Return and Recycling | HP® Official Site. N.p., n.d. Web. 07 Mar. 2016.
 "Hazardous Waste Management in the Semiconductor Industry." SESHA - Scholarships. N.p., 7 Oct. 2003. Web. 6 Mar. 2016. <http://www.semiconductorsafety.org/scholarships/>.
 "Macworld Expo Keynote Live Update." Macworld. N.p., n.d. Web. 08 Mar. 2016.
CPU: Embodied Energy
Every chip is mostly composed of 99% pure crystalline silicon. These small chips go through a very intricate process of production in order to reach that level of purity. The many parts that go into the life-cycle of a processor can be broken down into the raw materials, the embodied energy, and the wastes and emissions. Here, I will be focusing solely on the energy aspect of its life-cycle, this includes the energy used in order to mine and separate silicon, purify, refine, and cut it, and the energy consumed via transportation of the product around the globe. Although there are multiple steps the processor has to go through from start to finish, I had found that most of the energy was concentrated within refinement and fabrication. A computer chip is a very high fidelity product and requires an inordinate amount of effort to produce for its small size.
Silicon can be converted from sand, as Intel does in the deserts of Arizona, here in the US, where fabrication is done within the country. To extract the element silicon from the silica residing within the sand, it needs to be placed within a furnace that is heated to over 2,000 degrees Celsius (“How sand is transformed into silicon chips”). However, the majority of silicon mining and production comes out of China, one of the countries with the richest silicon ore. Silicon PV modules produced in China during 2010 accounts for 45% of the worldwide market. 90% of them were exported to Europe and North America. Production of silicon in china amounts to 84,000 tons in 2011, ranking it first in the world. The amount of energy consumed during silicon ore mining comes out to 0.07 kilowatt hour per kilogram of silicon. This includes the industrial explosives used during mining, the transportation of silicon rocks, and the factories breaking down the rocks using mechanical crushers (Dong Yang).
The electricity consumed here is measured roughly 10.7 megajoules of primary energy per kilowatt hour of electricity. This is the worldwide average value for fuel consumption. Areas with lower power efficiency numbers much higher: China being 12 MJ. The primary energies consumed breaks down to 69% gas and coal, 19% hydro power, 9% nuclear power, and only 3% geothermal, solar, wind, and biomass power (Boyd).
The most rigorous process comes after the ore is extracted. In order to convert the raw silicon into something pure enough to hold transistors, it needs to be refined. Raw silica is to be twice refined into ingots. The first step of creating Metallurgical Silicon requires 13 kilowatt hour per kilogram of silicon. It is then converted to trichlorosilane, requiring 50 kWh per kg. The trichlorosilane is converted to polysilicon, requiring further 250 kWh per kg of silicon. The polysilicon is then crystallized into ingots, taking another 250 kWh per kg of silicon, then finally cut into silicon wafers, requiring 240 kWh per kg of silicon. The process chain from silica to wafers takes 2127 kWh per kilogram of silicon outputted, with a silicon yield of only 9.5%. The low yield is due to the losses occurring within each step in the refining process, increasing the total energy cost per kg of silicon outputted alongside the intense consumption of energy from the process itself (Boyd). “Only 43% of the pure silicon crystal used in the process becomes part of the chip” ("Computer Chip Life Cycle - The Environmental Literacy Council").
In order to manage the facilities of production, another 1.5 kWh per centimeter squared area of silicon produced is consumed. This power is put towards water cooling, exhaust flow, water distribution, airflow, vacuum pumps, and any other factors to go alongside the actual production within the factories. This amount was actually cut down by about 0.6 kWh over the past few years due to rising energy costs and efficiency improvements. The World Semiconductor Council had also set emission reduction goals that had played a factor towards reducing the energy consumption (Boyd).
After processing, the wafers are cut and packaged in a separate facility. The range varies between distributors and companies for distance travelled. The raw product needs to be moved to a fabrication facility, shipped over to a facility to be cut and packaged, then shipped out to warehouses or distributors. The transportation factor gets a little muddy after that due to the huge number of variables residing in consumer distribution. The amount of energy consumed by plane and truck transportation from mining to packaged shipment comes out to 2.7 MJ per ton of freight shipped per mile for trucks and 0.38 MJ per ton-mile for planes (Boyd). The assembly and packaging of the chips had remained largely unchanged over the years and is assumed as a 5.9 MJ constant (Oliver).
After the chip is manufactured, it is then obviously put to use. This could range from processors residing in home computers, to the smaller processors residing within mobile phones. The power consumed by an average computer chip had risen from 14 watts of a Pentium processor to 140 watts for an i7 processor per MIPS over the last 15 years. MIPS being million instructions per second, a general unit of measurement for computing performance (Boyd).
There was very little regarding the recycling or waste disposal of CPUs, at least in terms of the energy consumed at this stage of its life-cycle. There was an article discussing the possibility of reusing the chip, however, due to the nature of progressing technology, this is only practical when the chip is only using less than 100 KJ per day, as newer tech is more powerful and would be more efficient with the energy required to power it. Should the chip be reused, assuming a 3 year life span over the course of 9 years, the energy consumed amounts to 7.60 MJ as opposed to the 19.78 MJ required to replace it (Oliver).
The total energy consumed by a single chip amounts to about 2 gigajoules of energy. This is the equivalent of 200 gallons of gasoline (Oliver). The bulk of the power in the chip’s more recent life-cycle comes from the use of the chip, amounting to 1593 MJ per cut die, where the fabrication and transportation presides in the rest. In 1995, the use was but 151 MJ, while the energy required for production more than doubles that of today (Boyd). These chips have a high environmental impact relative to their weight. A microchip requires more than 300 times the amount of energy needed to produce an automobile relative to its size. One car is the equivalent to 9 or 10 computers ("Computer Chip Life Cycle - The Environmental Literacy Council”).
There were some missing pieces to my research regarding the life cycle of the processor. The most notable being the energies residing within the vast amount of chemicals required in the refinement and etching process, and the energies required to recycle or dispose of electronic waste. There was some information regarding the chemicals within a couple of the listed articles, but the sheer number of them made it a little impractical for follow-up research. There were conflicting articles discussing the disposal of chips, most seemingly theoretical than practical, stating that the high concentration of silicon makes the chip easy to salvage, but there was few evidence suggesting that chips were recycled at all. This may be mostly due to consumer and distributor negligence and the small size of the product itself.
It was generally difficult to find information about energy that is not tied to the general use of the chip within computers or the use of the silicon within other products such as solar paneling. It was also tricky to navigate the jargon and technical aspects regarding this subject, requiring further reading and researching in order to learn about the multiple parts of the processor and the many types of chips throughout history that existed out in the world. A majority of my information came from a single source: the Sarah Boyd article. It was one of the few sources that I was able to find that went into great detail about energy within detailed steps of the chip’s life cycle.
1. Boyd, Sarah B. Life-cycle Assessment of Semiconductors. New York, NY: Springer, 2012. Print.
2. Chandramouli Venkatesan. “Comparative Carbon Footprint Assessment of the Manufacturing and Use Phases of Two Generations of AMD Accelerated Processing Units.” September 2015. AMD. Web. <http://www.amd.com/Documents/carbon-footprint-study.pdf>
3. "Computer Chip Life Cycle - The Environmental Literacy Council." The Environmental Literacy Council. N.p., n.d. Web. 2016. <http://enviroliteracy.org/environment-society/life-cycle-analysis/computer-chip-life-cycle/>.
4. Dong Yang, et. al. “Life-cycle assessment of China's multi-crystalline silicon photovoltaic modules considering international trade.” Cleaner Production. 94 (2015): 35-45.Intel. 2014 Corporate Responsibility Report. Web. <http://csrreportbuilder.intel.com/PDFFiles/CSR_2014_Full-Report.pdf>.
5. "From Sand to Hand: How a CPU Is Made | Chips | Geek.com." Geekcom. Rick Hodgin, 9 July 2009. Web. 02 Feb. 2016.
6. Garlapati, Vijay Kumar. "E-waste in India and Developed Countries: Management, Recycling, Business and Biotechnological Initiatives." Renewable and Sustainable Energy Reviews 54 (2016): 874-81. Web.
7. Graham Templeton. “What is silicon, and why are computer chips made from it?” EXTREMETECH June 22, 2015 Feb 02 2016. <http://www.extremetech.com/extreme/208501-what-is-silicon-and-why-are-computer-chips-made-from-it>
8. “How sand is transformed into silicon chips” Techradar. May 24 2009 Feb 02 2016.http://www.techradar.com/us/news/computing-components/processors/how-sand-is-transformed-into-silicon-chips-599785>
9. Oliver, John Y., Rajeevan Amirtharajah, Venkatesh Akella, Roland Geyer, and Frederic T. Chong. "Life Cycle Aware Computing: Reusing Silicon Technology." Computer 40.12 (2007): 56-61. Web. <http://people.cs.uchicago.edu/~ftchong/papers/computer07.pdf>.
10. Siddharth Jain. “A comparative assessment of the carbon footprint of AMD Fusion products with the previous generation products.” Web. <https://www.amd.com/Documents/APU%20Carbon%20Footprint%20white%20paper%20FINAL%201%2021%2011.pdf>