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Semiconductor: All about it, all inside it, and what all can it do... to change our lifes.

What is a Semiconductor?

Semiconductors are materials which have a conductivity between conductors (general metals) and nonconductors orinsulators (such as ceramics). Semiconductors are made from pure elements, typically silicon or geranium, or compoundssuch as gallium arsenide. In a process called doping, small amounts of impurities are added to pure semiconductors causing

large changes in the conductivity of the material.
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Due to their role in the fabrication of electronic devices, semiconductors are an important part of our lives. Imagine lifewithout electronic devices. There would be no radios, not TV's, no computers, no video games, and poor medical diagnosticequipment. Although many electronic devices could be made using vacuum tube technology, the developments insemiconductor technology during the past 50 years have made electronic devices smaller, faster, and more reliable. Think fora minute of all the encounters you have with electronic devices. How many have you seen or used in the last twenty-fourhours? Each has important components that have been manufactured with electronic materials.
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How is a Semiconductor Chip Made?

Wafer Fab Manufacturing

The process of manufacturing semiconductors or integrated circuits (commonly called ICs or chips) typically consists of

hundreds of steps, during which hundreds of copies of an integrated circuit are formed on a single wafer.

The processing of Silicon wafers to produce integrated circuits involves a good deal of chemistry and physics. In order to alterthe surface conditions and properties, it is necessary to use both inert and toxic chemicals, specific and unusual conditions,and to manipulate those conditions with both plasma-state

elements and with RF (Radio Frequency) energies. Starting with thin, round wafers of silicon crystal, in diameters of 150, 200,and 300mm, the processes described here build up a succession of layers of materials and geometries to produce thousandsof electronic devices at tiny sizes, which together

function as integrated circuits (ICs). The devices which now occupy the surface of a one-inch square IC would have occupiedthe better part of a medium-sized room 20 years ago, when all these devices (transistors, resistors, capacitors, and so on)were only available as discreet units.
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The conditions under which these processes can work to successfully transform the silicon into ICs require an absoluteabsence of contaminants. Thus, the process chambers normally operate under vacuum, with elemental, molecular, and otherparticulate contaminants rigorously controlled. In order to understand these processes, then, we will begin the study of semiconductor processing with an overview of vacuum systems and theory, of gas systems and theory, as applied specificallyto these tools, and of clean room processes and procedures.

Generally, the process involves the creation of 8 to 20, and frequently more, patterned layers on (and into) the wafer,

ultimately forming the complete integrated circuit. This layering process creates (interconnected) electrically active regionson the semiconductor wafer surface.

Silicon Wafer Manufacturing

Silicon Wafer Manufacturing

Semiconductor manufacturing begins with production of the wafer, i.e., a thin, round slice of a semiconductor material

varying in size 6 inches to 12 inches in diameter. The finished wafer is approximately 15 mil thick. The materials are primarilysilicon; however, gallium arsenide, silicon carbide, germanium and others undergo similar processes. Purified polycrystallinesilicon is created from sand, one of the most abundant materials available on our planet, is heated to a molten liquid. In aprocess similar to repeatedly dipping a wick in wax to make a candle, a small piece of solid silicon (seed) is dipped in moltenliquid. As the seed is slowly withdrawn (by mechanical means) from the melt, the liquid quickly cools to form a single crystalingot.
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This cylindrical crystal ingot is then ground to a uniform diameter. A diamond saw blade slices the ingot into thin wafers. Thecut wafers are then processed through a series of machines where they are ground (optically) smooth and chemicallypolished.

The wafers are now ready to be sent to the wafer fabrication area fab,where they are used as the foundation formanufacturing integrated circuits (ICs).

The heart of any semiconductor manufacturing business is the fab,where the integrated circuit is formed on the wafer. Thefabrication process, which takes place in an environmentally controlled clean room, involves a series of principle repetitivesteps described below. Typically, it takes from 10-30 days, and frequently much longer, to complete the fabricationprocess.

Thermal Oxidation--Wafers are pre-cleaned using high-purity deionized water and various low-particulate chemicals, amust for high-yield production. The silicon wafers are heated to approximately 1000 C and exposed to ultra-pure oxygen inthe oxidation furnace. Under carefully controlled conditions, a silicon dioxide insulator film of uniform thickness is formedon the surface of the wafer.

Patterning--Masking is used to protect one area of the wafer while working on another. This process is referred to asphotolithography. A photo resist, light-sensitive film is spin coated onto the wafer, giving it characteristics similar to aphotographic film. A (micro) aligner aligns the wafer to a glass mask and then projects an intense ultraviolet light throughthe mask, exposing the photo resist with the mask pattern, thereby transferring the image from the mask into the light-sensitive film.

Etching--The wafer image is then developed (like a photo negative). The exposed photo resist is chemically removed and

baked to harden the remaining photo resist pattern, which now is no longer light sensitive. It is then exposed to a chemicalwet solution or plasma (gas discharge) so that areas not covered by the hardened photo resist are etched away. Theremaining photo resist is now removed using either wet or plasma chemistry. The wafer is optically inspected to assurethat the image transfer from the mask to the top silicon layer is correct, and then goes on to the next step.
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Doping/Diffusion--Atoms with one less electron than silicon (such as boron) or one more electron than silicon (such asphosphorus) are introduced into the area exposed by the etch process, to alter the electrical character (conductivity) of thesilicon. These areas are called P type or N type, respectively, which reflects their conducting characteristics.

Repeating the Above Steps--The thermal oxidation, masking, etching and doping steps are repeated many times until thelast "front end" layer is completed (all active devices have been formed).

Dielectric Deposition and Metallization--Following completion of the "front end," the individual devices are interconnected"backend" (like on a PC board) using a series of alternating metal depositions, dielectric films, with their respective

patterning. Current semiconductor fabrication includes as many as 5 to 7 metal layers for logic, and fewer for memory,separated by dielectric layers (insulators).

Passivation--After the last metal is patterned, a final insulating layer (passivation) is deposited to protect the circuit fromdamage and contamination. Openings are etched in this film to allow access to the top metal later by electrical probes andsubsequent wire bonds.

Electrical Test--An automatic, computer-driven test system checks for functionality of each chip on the wafer. Chips that donot pass the test are marked for automatic rejection. For simpler devices a mechanical probe is used.

Assembly--A diamond saw slices the wafer into single chips. Sizes can vary from 1 x 1 mm to 10 x 10 mm. The rejectedchips are discarded and the remaining chips are visually inspected under a high-power microscope before packaging.

Each chip is then assembled into an appropriate package that provides the contact leads for the chip. In one type of interconnect a wire bonding machine attaches wires, a fraction of the width of a human hair, to the leads of the package.The packaged chip is tested again prior to delivery to the customer. Alternative, the chip can be assembled in a ceramicpackage for certain high performance applications.

The Video Below show a complete step by step procedure of wafer Fab Manufacturing.


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Detailed steps of Preparation of the Silicon Wafer Media

Wafer products are measured at various stages of the process to identify defects inducted by the manufacturing process. Thisis done to eliminate unsatisfactory wafer materials from the process stream and to sort the wafers into batches of uniformthickness and at a final inspection stage. These wafers will become the basic raw material for new integrated circuits.

The following is a summary of the steps in a typical wafer manufacturing process.

Crystal Growth and Wafer Slicing Process

The first step in the wafer manufacturing process is the formation of a large, perfect silicon crystal. The crystal is grown froma seed crystal that is a perfect crystal. The silicon is supplied in granular powder form, then melted in a crucible. The s eed isimmersed carefully into the crucible of molten silicon, then slowly withdrawn.

The Czochralski method

Step 1: Obtaining the Sand The sand used to grow the wafers has to be a very clean and good form of silicon. For this reasonnot just any sand scraped off the beach will do. Most of the sand used for these processes is shipped from the beaches of Australia.

Step 2: Preparing the Molten Silicon Bath The sand (SiO2)is taken and put into a crucible and is heated to about 1600 degreesC just above its melting point. The molten sand will become the source of the silicon that will be the wafer.


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Step 3 : Making the Ingot A pure silicon seed crystal is now placed into the molten sand bath. This crystal will be pulled outslowly as it is rotated. The dominant technique is known as the Czochralski (cz) method. The result is a pure silicon cylinderthat is called an ingot.

Growth of Epitaxial Silicon

This step is done to provide a good clean surface for later processing. If a layer of Silicon is grown onto

the top of the wafer using chemical methods then that layer is of a much better quality then the slightly

An epitaxial reactor

This step is done to provide a good clean surface for later processing. If a layer of Silicon is grown onto the top of the waferusing chemical methods then that layer is of a much better quality then the slightly damaged or unclean layer of silicon in thewafer. The epitaxial layer is where the actual processing will be done. The diameter of the silicon ingot is determined by thetemperature variables as well as the rate at which the ingot is withdrawn. When the ingot is the correct length, it is removed,then ground to a uniform external surface and diameter. Each of the wafers is given either a notch or a flat edge that will beused later in orienting the wafer into the exact position for later procedures. In these two figures you can see a notch (above)

and flats. Flats in this image are exaggerated for clarity.

Step 4: Preparing the Wafers

After the ingot is ground into the correct diameter for the wafers, the silicon ingot is sliced into very thin wafers. This isusually done with a diamond saw. A diamond saw for cutting wafers Each of these wafers will then go through polishing untilthey are very smooth and just the right thickness.

Thickness Sorting

Following slicing, silicon wafers are often sorted on an automated basis into batches of uniform thickness to increaseproductivity in the next process step, lapping. During thickness sorting, the wafer manufacturer can also identifydefect trends resulting from the slicing process.


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Lapping & Etching Processes

Lapping removes the surface silicon which has been cracked or otherwise damaged by the slicing process, and assures a flatsurface. Wafers are then etched in a chemically active reagent to remove any crystal damage remaining from the previousprocess step.

Thickness Sorting and Flatness Checking

Following lapping or etching, silicon wafers are measured for flatness to identify and control defect trends resulting from thelapping and etching processes. Wafers are also often sorted on an automated basis according to thickness in order toincrease productivity in the next process step, polishing.

Polishing Process

Polishing is a chemical/mechanical process that smoothes the uneven surface left by the lapping and etching processes andmakes the wafer flat and smooth enough to support optical photo lithography

Final Dimensional and Electrical Properties Qualification

The wafers undergo a final test, performed in order to demonstrate conformance with customerspecification for flatness, thickness, resistivity and type. Process induced defect and defect trend information is used by thewafer manufacturer for yield and process management of the immediately preceding steps. Information regarding surfacedefects, such as scratches and particles, and defect trend information are used by the wafer manufacturer for yield andprocess improvement.


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Silicon Wafer Processing Steps...Continue in next blog..
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