AN900 Datasheet, PDF (12/15 Pages) STMicroelectronics – INTRODUCTION TO SEMICONDUCTOR TECHNOLOGY

INTRODUCTION TO SEMICONDUCTOR TECHNOLOGY
Most of the early semiconductor devices were made with PMOS technologies because it was
easier to obtain stable manufacturing process with this technology. As higher speeds and
greater densities were needed, new devices were implemented with NMOS. This was due to
the higher speed of N-channel charge carriers (electrons) in silicon and also to the progress in
the control of silicon doping. But CMOS technology has begun to see widespread commercial
use in memory devices: it allowed the use of very low power devices. At the beginning, CMOS
were slower than NMOS devices. Today, CMOS technology has been improved to produce
higher speed devices.
2.2 FABRICATION OF A TRANSISTOR
The fabrication begins with a slice of single crystal silicon, uniformly doped P-type.
The wafer is oxidized in a furnace to grow a thin
layer of silicon dioxide (SiO₂) on the surface. Sil-
icon nitride is then deposited on the oxidized
wafer in a gas phase chemical reactor. The wafer
is now ready to receive the first pattern of what is
to become a many layered complex circuit. The
first pattern defines the boundaries of the active
regions of the integrated circuit, where transis-
tors, capacitors, diffused resistors and first level
interconnects will be made.
The patterned wafer is then implanted with boron
atoms. Boron will only reach the etched zones of
the silicon substrate, creating P-type doped
areas that will electrically separate active areas.
Wafer is oxidized again and the thick oxide only
grows in the etched areas due to silicon nitride’s
properties as an oxidation barrier.
The remaining silicon nitride layer is removed.
Now that the areas for active transistors have
been defined and isolated, the transistor types can be determined. The wafer is patterned and
implanted with dopant atoms.The energy and dose at which the dopant atoms are implanted
determines much of the transistor’s characteristics.
The transistor types defined, the gate oxide of the
active transistors are grown in a high temperature
furnace. The gate oxide layer is then masked and
holes are etched to provide direct access to buried
contacts where needed. A polycristaline silicon
layer is deposited on the wafer. The gate layer is
then patterned to define the actual transistor gates
and interconnect paths.
12/15