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| WAFER FABRICATION (FRONT-END) |
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Identical integrated circuits, called die, are made on each wafer in a multi-step process. Each step adds a new layer to the wafer or modifies the existing one. These layers form the elements of the individual electronic circuits.
The main steps for the fabrication of a die are summarized in the following table. Some of them are repeated several times at different stages of the process. The order given here doesn't reflect the real order of fabrication process.
PhotoMasking
This step shapes the different components. The principle is quite simple (see drawing on next page). Resin is put down on the wafer which is then exposed to light through a specific mask. The lighten part of the resin softens and is rinsed off with solvents (developing step).
Etching
This operation removes a thin film material. There are two different methods: wet (using a liquid or soluble compound) or dry (using a gaseous compound like oxygen or chlorine).
Diffusion
This step is used to introduce dopants inside the material or to grow a thin oxide layer onto the wafer. Wafers are inserted into a high temperature furnace (up to 1200 ° C) and doping gazes penetrate the silicon or react with it to grow a silicon oxide layer.
Ionic Implantation
It allows to introduce a dopant at a given depth into the material using a high energy electron beam.
Metal Deposition
It allows the realization of electrical connections between the different cells of the integrated circuit and the outside. Two different methods are used to deposit the metal: evaporation or sputtering.
Initially, the silicon chip forms part of a very thin (usually 650 microns), round silicon slice: the raw wafer. Wafer diameters are typically 125, 150 or 200 mm (5, 6 or 8 inches). However raw pure silicon has a main electrical property: it is an isolating material. So some of the features of silicon have to be altered, by means of well controlled processes. This is obtained by "doping" the silicon.
Dopants (or doping atoms) are purposely inserted in the silicon lattice, hence changing the features of the material in predefined areas: they are divided into “N” and “P” categories representing the negative and positive carriers they hold. Many different dopants are used to achieve these desired features: Phosphorous, Arsenic (N type) and Boron (P type) are the most frequently used ones. Semiconductors manufacturers purchase wafers predoped with N or P impurities to an impurity level of.1 ppm (one doping atom per ten million atoms of silicon). There are two ways to dope the silicon. The first one is to insert the wafer into a furnace.
Doping gases are then introduced which impregnate the silicon surface. This is one part of the manufacturing process called diffusion (the other part being the oxide growth). The second way to dope the silicon is called ionic implantation. In this case, doping atoms are introduced inside the silicon using an electron beam. Unlike diffusion, ionic implantation allows to put atoms at a given depth inside the silicon and basically allows a better control of all the main parameters during the process. Ionic implantation process is simpler than diffusion process but more costly (ionic implanters are very expensive machines). |
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