How sand is transformed into siliconchips:The amazing journey from sandcastle to Core i7 Presented by: Processor Surabhi Singh
• The deserts of Arizona are home to Intels Fab 32, a $3billion factorythats performing one of the most complicated electrical engineering featsof our time.•Its here that processors with components measuring just 45 millionths ofa millimeter across are manufactured, ready to be shipped out tomotherboard manufacturers all over the world.•It may seem an impossible transformation, but these complexcomponents are made from nothing more glamorous than sand. Such atransformative feat isnt simple. The production process requires morethan 300 individual steps.•An in-depth look at Intels manufacturing method and the whole processsummed up into 10 stages.
STAGE 1:CONVERTING SAND TOSILICON• Sand is composed of silica (also known as silicon dioxide), and is thestarting point for making a processor.•To extract the element silicon from the silica, it must be reduced. This isaccomplished by heating a mixture of silica and carbon in an electric arcfurnace to a temperature in excess of 2,000°C.•The end result of this process is a substance referred to as metallurgical-grade silicon, which is up to 97 percent pure.
Refinement: The silicon is ground to a fine powder and reacted withgaseous hydrogen chloride in a fluidized bed reactor at 300°C to give aliquid compound of silicon called trichlorosilane.Impurities such as iron, aluminium, boron and phosphorous also react togive their chlorides, which are then removed by fractional distillation. Thepurified trichlorosilane is vaporized and reacted with hydrogen gas at1,100°C so that the elemental silicon is retrieved.During the reaction, silicon is deposited on the surface of an electricallyheated ultra-pure silicon rod to produce a silicon ingot. The end result isreferred to as electronic-grade silicon, and has a purity of 99.999999 percent.
STAGE 2 : CREATING A CYLINDRICALCRYSTAL Although pure to a very high degree, raw electronic-grade silicon has apolycrystalline structure.To turn it into a usable material, the silicon must be turned into singlecrystals that have a regular atomic structure. This transformation isachieved through the Czochralski Process. Electronic-grade silicon ismelted in a rotating quartz crucible and held at just above its melting pointof 1,414°C.A tiny crystal of silicon is then dipped into the molten silicon and slowlywithdrawn while being continuously rotated in the opposite direction tothe rotation of the crucible. The crystal acts as a seed, causing silicon fromthe crucible to crystallize around it. This builds up a rod – called a boule –
STAGE 3: SLICING THE CRYSTAL INTOWAFERSSlicing : To maximize the surface area of silicon availablefor making chips, the boule is sliced up into discs calledwafers. The sharp edges of each wafer are then smootheddown to prevent the wafers from chipping during laterprocesses.Lapping & Etching: Now the surfaces are polished usingan abrasive slurry until the wafers are flat to within anastonishing 2μm (two thousandths of a millimeter). Thewafer is then etched in a mixture of nitric, hydrofluoricand acetic acids.The result of all this refining and treating is an evensmoother and cleaner surface.
STAGE 4: MAKING A PATTERNED OXIDE LAYER(A) The wafer is heated to a high temperature in a furnace. The surface layer of silicon reacts with the oxygen present to create a layer of silicon dioxide.(B) A layer of photoresist is applied.(C) The wafer is exposed to UV light through a photographic mask or film that defines the required pattern of circuit features.(D) The next stage is to develop the latent circuit image. This process is carried out using an alkaline solution. Those parts of the photoresist that were exposed to the UV soften in the solution and are washed away.(E) Hydrofluoric acid is now used to dissolve those parts of the silicon oxide layer where the photoresist has been washed away.(F) Finally, a solvent is used to remove the remaining photoresist, leaving a
STAGE 5: MOSFET DESIGNMOSFETs are the switches at the heart of processor designThe first step in creating a circuit is to create n-type and p-type regions. Below isthe method Intel uses for its 90nm process and beyond:A.The wafer is exposed to a beam of boron ions. These implant themselves andcreate areas called p-wells. These are, used in the n-channel MOSFETs.B.A different photoresist pattern is now applied, and a beam of phosphorous ionsis used in the same way to create n-wells for the p-channel MOSFETs.C.In the final ion implantation stage, another beam of phosphorous ions is used tocreate the n-type regions in the p-wells that will act as the source and drain of then-channel MOSFETs.D.Next, following the deposition of a patterned oxide layer , a layer of silicon-germanium doped with boron (which is a p-type material) is applied.
STAGE 6: ADDING GATES TO COMPLETE THE MOSFETS The first job is to produce a patterned oxide layer as described in Step 4.In this case, the oxide layer will have gaps only in the gate regions of theMOSFETs. The first part of the gate is a very thin insulating layer of silicon dioxide,deposited on the surface of the silicon between the source and the drain.This is done Using Chemical Vapor Deposition (CVD), a process thattakes place in a furnace filled with various gases that cause a chemicalreaction to take place on the surface of the silicon. To complete the MOSFET, a layer of silicon is applied over the top of thethin oxide layer to act as a conductor. Again, CVD is used, and the siliconis applied via an oxidation reaction.
STAGE 7: CONNECTING THE MOSFETS WITH COPPER TRACKSOnce all of this has been done, the wafer will contain billions of MOSFETs. In order forthem to work together as circuits, they need to be connected together to produce lots ofindividual chips, each of them still containing millions of MOSFETs. The process used byIntel is as follows:A.The initial state of the MOSFETs on the wafer.B.A layer of insulation (silicon dioxide) has to be applied to the wafer so that theinterconnecting tracks dont short all the MOSFETs.C.Hydrofluoric acid is used to etch holes in the silicon dioxide insulation.D.After that, trenches in the pattern of the required interconnection tracks areetched into the silicon dioxide through another photoresist layer.E.A top layer of copper is then applied by electroplating. The resultant metallicpins that protrude through the insulating layer are called vias.F.The wafer is now covered in a layer of copper. The final stage is to take this off.In a process called chemical-mechanical polishing, the excess copper is removed.
STAGE 8: COMPLETING THE CIRCUIT An insulating layer of silicon dioxide protects the MOSFETs. Holesetched through it permit connections to be made.Its not always feasible to wire up a circuit without wires crossing. If therewas just one rogue interconnection, any tracks that crossed would short.To avoid this, MOSFETs have more than one metallic layer, eachinsulated by another layer of silicon dioxide and connected using vias.
STAGE 9: SORTING THE GOOD CHIPS FROM THE BAD All being well, the wafer should now contain a couple of hundred dies(the official name for chips), but in reality, not all of them will workcorrectly.The next job is therefore to find out which dies are working, a task that iscarried out by a wafer probe. This piece of hardware uses pins that line upwith the contacts on a die, through which electrical signals can be passed toput the processor through its paces. Dies are categorized as functional or non-functional, but there mightalso be several examples of partially functional dies.Processors in which only some of the dies are working can still be sold as a
STAGE 10 : PACKAGING TO SURVIVE THE REAL WORLDWe might have a fully working die now, but, as it stands, its much toofragile to ship to a motherboard manufacturer. Furthermore, the die hashundreds or thousands of connections to the outside world, but its only a fewmillimeters square, making it far too fiddly for an electronics company tomake connectors for it.The final step, therefore, is to encase the bare chip into a package thatmost people would think of as a processor. The end result is that the die isfirmly attached to the package, and electrical connections are made betweenthe contacts on the die and the contacts on the package.A final test on the finished assembly is all thats needed before the processorcan be shipped to a manufacturer and ultimately used to power a computer.
Liquid Metal Makes Silicon Crystals at Record Low TemperaturesJan. 24, 2013 — A new way of making crystalline silicon, developed by U-Mresearchers, could make this crucial ingredient of computers and solar cells muchcheaper and greener.Silicon dioxide, or sand, makes up about 40 percent of Earths crust, but the industrialmethod for converting sand into crystalline silicon is expensive and has a majorenvironmental impact due to the extreme processing conditions.Recently, Maldonado and chemistry graduate students Junsi Gu and Eli Fahrenkrugdiscovered a way to make silicon crystals directly at just 180 F. And they did it bytaking advantage of a phenomenon you can see right in your kitchen.When water is super-saturated with sugar, that sugar can spontaneously form crystals,popularly known as rock candy."Instead of water, were using liquid metal, and instead of sugar, were using silicon,"Maldonado said.Maldonado and colleagues made a solution containing silicon tetrachloride and layeredit over a liquid gallium electrode. Electrons from the metal converted the silicontetrachloride into raw silicon, which then dissolved into the liquid metal."The liquid metal is the key aspect of our process," Maldonado said. "Many solid metalscan also deliver electrons that transform silicon tetrachloride into disordered silicon, butonly metals like gallium can additionally serve as liquids for silicon crystallizationwithout additional heat."
The researchers reported dark films of silicon crystals accumulating on thesurfaces of their liquid gallium electrodes. So far, the crystals are very small,about 1/2000th of a millimeter in diameter, but Maldonado hopes to improve thetechnique and make larger silicon crystals, tailored for applications such asconverting light energy to electricity or storing energy. The team is exploringseveral variations on the process, including the use of other low-melting-pointmetal alloys.If the approach proves viable, the implications could be huge, especially for thesolar energy industry. Crystalline silicon is presently the most-used solar energymaterial, but the cost of silicon has driven many researchers to actively seekalternative semiconductors."Its too premature to estimate precisely how much the process could lower theprice of silicon, but the potential for a scalable, dramatically less expensive andmore environmentally benign process is there," Maldonado said. "The dreamultimately is to go from sand to crystalline silicon in one step. Theres nofundamental law that says this cant be done."The study, which appears in the Journal of the American Chemical Society, wasfunded by the American Chemical Society Petroleum Research Fund.The university is pursuing patent protection for the intellectual property and isseeking commercialization partners to help bring the technology to market.