Manufacturing of Microprocessor Farooq Ahmad Shah
Preliminary Discussion• Integrated circuit: An electronic circuit designed to perform some function, in which the electronic components (transistors, resistors, diodes, capacitors, etc.) are miniaturized, built into a small "chip" made of silicon and interconnected through tiny strands of aluminum. More commonly referred to as "microchips", or "IC chips―• The chip found inside of a "musical" birthday card contain only a few dozen components, whereas a Pentium processor today contains over 800 million transistors! When an integrated circuit is extremely complex, it is usually known as a microprocessor.
Preliminary Discussion (contd.) • Microprocessors are manufactured in clean rooms, the ambiance of which cleaner than a hospital operating room. – The air in the clean room is air conditioned to a temperature of 21C (70F) and 45% relative humidity. – The air is passed through a high-efficiency particulate air (HEPA) filter to capture particle contaminants – workers must be covered from head to toe in specialized garments, called bunny suits, designed to trap particles.
Outline of the processing stepsSequence of processing steps in the production of integrated circuits: (1)pure silicon is formed from the molten state into an ingot and then slicedinto wafers; (2) fabrication of integrated circuits on the wafer surface; and(3) wafer is cut into chips and packaged.
Silicon Ingot• To provide uniform electrical properties throughout, the silicon must be almost totally pure and a perfect crystal. Ingot diameter 300mm(12 in) and up to 3m(10 ft) longThe Czochralski process for growing single-crystal ingots of silicon: (a) initialsetup prior to start of crystal pulling, and (b) during crystal pulling to formthe boule.
Silicon Ingot (contd.)Grinding operations used in shaping the silicon ingot: (a) a form ofcylindrical grinding provides diameter and roundness control, and (b) a flatground on the cylinder.
Silicon Wafers Wafer slicing using a diamond abrasive cutoff saw.
Silicon Wafers (contd.)Two of the steps in wafer preparation: (a) contour grinding to round thewafer rim, and (b) surface polishing.
IC Fabrication • Repeated sequential steps (200 times or more). • Layering: Adding new material on top of or into the silicon like dopants, insulators, conductors. • Photolithography: By exposing a light sensitive chemical (photoresist) through a mask, the desired pattern is transferred onto the current top layer. • Etching: Removes the material to give the desired pattern.
IC Fabrication (contd.)AN EXAMPLE: How can you create a p-type region in a n-type doped siliconregion? Ions of arsenic are bombarded using ion implantation to create a n-type doped region. Layer of SiO2 is grown using oxidation. Layer of photoresist is applied. • photoresist is an organic polymer sensitive to light radiation in a certain wavelength range; the sensitivity causes either an increase or decrease in solubility of the polymer to certain chemicals.
IC Fabrication (E.g. contd.) Expose photoresist using appropriate lithographic mask. Develop the photoresist. Etch photoresist and silicon dioxide. Implant boron Remove silicon dioxide
IC Fabrication (contd.)(A) A p-type wafer (silicon doped with Boron) has a epilayer of n-type (silicon doped with Phosphorous orArsenic)(B) A mask is used to implant Silicon Dioxide, for the insulator(C) Acceptor atoms (Boron) are diffused into the window in the Silicon Dioxide(D) Using another mask additional Silicon Dioxide is grown. and donor atoms (elements like Arsenic withexcess electrons) are implanted.(E) Another mask is used to grow additional Silicon Dioxide. Another mask is then used to implantevaporated Aluminum or Copper for the contacts. This is a Bipolar Junction Transistor (BJT).
Packaging• Wafer Testing: a computer-controlled needle probes contacts the chip connection pads and a series of DC tests are carried out to indicate short circuits and other faults; the failed chips are marked with an ink dot; these defects are not packaged.• Chip Separation: A thin diamond-impregnated saw blade is used to perform the cutting operation. The sawing machine is highly automatic and its alignment with the ‘‘streets’’ between circuits is very accurate.• Die Bonding: Automated handling systems pick the separated chips and place them for on the die. Epoxy is applied to the base of the chip.• Wire Bonding After the die is bonded to the package, electrical connections are made between the contact pads on the chip surface and the package leads.• Video
Ion Implantation First a gas containing the desired dopant is ionized by bombarding it with electrons, producing charged atoms. This process will also produce ions of other types, are screened out. By carefully adjusting the field, the desired ions are given the correct arc to exit the mass spectrometer. Ions that are heavier will curve too wide and ions that are lighter will curve too sharply to escape. The dopant ions are then accelerated through a very strong electric field and fired at the wafer. A patterned layer of photoresist or other material is used to block the dopant atoms from areas where they are not needed. In areas of silicon that are exposed, the ions crash into the surface at high rates of speeds.The total dose of dopant depends upon how many ions are fired into the silicon. Wafer mustbe annealed to allow the doper atoms and silicon atoms to position themselves in their properplaces.
PhotolithographyPhotolithography, also known as opticallithography, uses light radiation to expose acoating of photoresist on the surface of thesilicon wafer; a mask containing therequired geometric pattern for each layerseparates the light source from the wafer,so that only the portions of the photoresistnot blocked by the mask are exposed. Themask consists of a flat plate of transparentglass on to which a thin film of an opaquesubstance has been deposited in certainareas to form the desired pattern. Thicknessof the glass plate is around 2 mm (0.080 in),whereas the deposited film is only a fewmm thick—for some film materials, less than1 mm.The mask itself is fabricated bylithography, the pattern being based oncircuit design data,usually in the form of digital output fromthe CAD system used by the circuitdesigner.
EtchingEtching is usually done selectively, by coating surface areas that are to be protectedand leaving other areas exposed for etching. The coating maybean etch-resistant photoresist, or it maybe a previously applied layer of material suchas silicon dioxide.Wet Chemical Etching involves the use of an aqueous solution, usually an acid, toetch away a target material. The etching solution is selected because it chemicallyattacks the specific material to be removed and not the protective layer used as amask.Dry Plasma Etching uses an ionized gas to etch a target material. The ionized gas iscreated by introducing an appropriate gas mixture into a vacuum chamber. and usingradio frequency (RF) electrical energy to ionize a portion of the gas, thus creatinga plasma. The high-energy plasma reacts with the target surface, vaporizing thematerial toremove it.
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