AN900 Datasheet, PDF (3/15 Pages) STMicroelectronics – INTRODUCTION TO SEMICONDUCTOR TECHNOLOGY

INTRODUCTION TO SEMICONDUCTOR TECHNOLOGY
Passivation
Back-lap
Wafers are sealed with a passivation layer to prevent the device from contamina-
tion or moisture attack. This layer is usually made of silicon nitride or a silicon oxide
composite.
It’s the last step of wafer fabrication. Wafer thickness is reduced (for microcontroller
chips, thickness is reduced from 650 to 380 microns), and sometimes a thin gold
layer is deposited on the back of the wafer.
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 rep-
resenting 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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